Nanosponges for Environmental Remediation 3031410769, 9783031410765

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Shikha Gulati   Editor

Nanosponges for Environmental Remediation

Nanosponges for Environmental Remediation

Shikha Gulati Editor

Nanosponges for Environmental Remediation

Editor Shikha Gulati Department of Chemistry, Sri Venkateswara College University of Delhi Dhaula Kuan, Delhi, India

ISBN 978-3-031-41076-5 ISBN 978-3-031-41077-2 (eBook) https://doi.org/10.1007/978-3-031-41077-2 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland Paper in this product is recyclable.

Dedicated to My beloved Parents

Preface

Nanosponges, a three-dimensional porous structure with holes at its core, have recently been recognised as the best adsorbents for a range of environmental contaminants. When compared to other regularly utilised environmental restoration modalities, the use of nanosponges can be considered as a cost-effective strategy with low energy and time demands. Because of its cone-like form, high surface area, simplicity of synthesis of host–guest complexes, and existence of many functions among the numerous readily available nanomaterials, nanosponges are the most cutting-edge material for adsorption applications. In order to remove toxins from the environment, further study should be done on these namomaterials with distinctive physicochemical properties, topologies, and strongly cross-linked three-dimensional networks. The number of studies using nanosponges has significantly increased during the last ten years. Numerous obstacles have been removed as a result, and new study fields have formed. The modern developments in nanosponges are compiled in the following book: Nanosponges for Environmental Remediation. This book explores the complete chemistry of nanosponges as well as methods for chemical synthesis and modification, characterisation, and uses of nanosponges in environmental remediation. Thus, this book examines the most recent findings and research in the use of nanosponges in environmental applications, showing the extent to which the community has adopted these novel materials. The book offers the methodologies underlying recent breakthroughs in addition to offering an overview of recent research by significant groups in the area, giving both seasoned researchers and those just entering the subject a stable environment. This book will also highlight the difficulties in light of earlier indicators of advancement and the need for additional study, as well as specifics regarding the current cutting-edge technology and future views with a multidisciplinary approach. The aim of this book is to give non-specialist readers, whether in academia or industry, a thorough grasp of a topic where fresh research is emerging and

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is of interest to a larger scientific audience. Therefore, it brings me great pleasure to be able to compile a book with multiple authors that features noteworthy contributions from the best research laboratories in the field of nanosponges for environmental cleanup. The 20 chapters describe significant discoveries and significant breakthroughs that have come about as a result of these possibilities. Nanosponges and their various varieties are introduced in Chapters “Introduction to Nanosponges” and “Different Types of Nanosponges Used in Environmental Remediation”, with an emphasis on their cutting-edge potential in environmental applications. Then, in Chapter “General Synthetic Routes for Various Nanosponges”, the many techniques for creating various kinds of nanosponges are discussed, as well as the chemical alterations that can be made to them in order to use them for a variety of environmental applications. Additionally, a number of characterisation techniques have been used to understand the characteristics of the manufactured nanosponges. As a result, chapter “Characterization Techniques for Nanosponges” briefly discusses different methods of characterising nanosponges and the materials on which they are based. One of the key types of cyclodextrin-based nanosponges, as well as their synthesis, functionalisation techniques, and application for water, soil, and aquifer remediation, were covered in Chapters “Introduction to Cyclodextrin-Based Nanosponges”–“Application of Cyclodextrin-Based Nanosponges in Soil and Aquifer Bioremediation”. Additionally, Chapter “Removal of Organic and Inorganic Contaminants from Water Using Nanosponge Cyclodextrin Polyurethanes” covers the topic of removing organic and inorganic contaminants from water using nanosponge cyclodextrin polyurethanes. The Metal-Organic Framework Sponges, their synthetic and functionalisation strategies, and applications for toxic or greenhouse gas adsorption and water remediation are discussed in chapters “Introduction to Metal-Organic Framework Sponges and Their Synthetic and Functionalization Strategies”–“Metal-Organic Framework Sponges for Water Remediation”. In Chapter “Introduction to Sponge-Like Functional Materials from TEMPO-Oxidized Cellulose Nanofibers”, new sponge-like functional materials made from TEMPO-oxidised cellulose nanofibres are introduced, and their use in cleaning up the environment is discussed. Metal oxide nanosponges, another significant type of nanosponges, are featured in chapters “Synthesis and Application of Types of Metal Oxide Nanosponges in Water Treatment” and “Synthesis and Application of Metal and Metal Oxide-Based Nanosponges as Sensors”, where a brief discussion of their synthesis, use in water treatment, and potential as sensors is provided. Polymer-based nanosponges and cellulose-based nanosponges for water remediation are presented in chapters “Polymer-Based Nanobiocomposite as a Filter Nanosponge for Wastewater Remediation” and “Cellulose-Based Nanosponges for Wastewater Remediation”. In chapters “Application of Nanosponges for Aquifer Bioremediation”–“Environmental Applications of Nanosponges (NSPs) to Clean up Oil Spills”, a thorough discussion of the application of nanosponges for aquifer bioremediation, removal of hazardous dyes from wastewater, and cleanup of oil spills is provided. Conclusions and future outlooks on nanosponges round out Chapter “Concluding Remarks and Future Perspectives of Nanosponges in Environmental Remediation”, allowing academics interested in this topic to plan their future research and development. To

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help the reader through the dense literature, the contributing authors have chosen sources, enabling a broad audience to learn about the topic. If readers who are investigating nanosponges find this book useful, it will make me really happy as an editor. Dhaula Kuan, Delhi, India

Dr. Shikha Gulati

Acknowledgments

I give thanks to God, the Almighty, for his blessings in my life and for giving me the ability to finish the book. Next, I would like to extend my sincere gratitude to Prof. C. Sheela Reddy, Principal, Sri Venkateswara College, University of Delhi, for her warm encouragement, unselfish support, constant willingness to assist, and facilitation of an acceptable environment in the college. Her advice was both kind and fruitful. She also provided a well-equipped ICT lab and library, which were really helpful to me while working. I want to thank my beloved parents, my daughter, and all of my mentors for their inspiration and support. Without the contributions of all the renowned authors, this book would not have been possible to write. In addition, many people have generously and unintentionally helped me finish this book; therefore, I would like to extend my sincere gratitude to them. We appreciate the assistance and support of the Springer crew. Last but not least, I want to thank Springer Nature Switzerland for letting us publish this book. Dr. Shikha Gulati

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About This Book

I was inspired to write the book Nanosponges for Environmental Remediation by the developing field of nanosponges. An international and interdisciplinary team of top experts in the domains of nanosponges synthesis, functionalisation, characterisation, and application for environmental remediation have come together to write this book. The authors offer a distinctive viewpoint on nanosponges research that gives the reader a firm grounding in fundamental approaches and explains how to think about nanosponges. Researchers new to nanosponges can use this book as a starting point for their theoretical or practical study. The book contains 20 chapters that cover a range of topics, including the usage of nanosponges for environmental remediation as well as their synthesis and functionalisation. The potential of nanosponges in various fields is currently expanding at a rapid rate because to the large range of options they provide in terms of structure, integration of active species, and postsynthetic alterations, among other things. Establishing future research goals with a focus on significant unique characteristics in this class of materials as a whole is the main goal. The book will be useful to scientists and researchers who are interested in nanosponges and environmental cleanup.

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Contents

Introduction to Nanosponges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dorothy Sachdeva, Naveen Goyal, Anoushka Amar, and Shikha Gulati Different Types of Nanosponges Used in Environmental Remediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shikha Gulati, Asvika Nigam, and Sanjay Kumar

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General Synthetic Routes for Various Nanosponges . . . . . . . . . . . . . . . . . . . Lakshita Chhabra, Anoushka Amar, Shikha Gulati, and Rajender S. Varma

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Characterization Techniques for Nanosponges . . . . . . . . . . . . . . . . . . . . . . . Pragya Malik, Durgesh Nandini, and Bijay P. Tripathi

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Introduction to Cyclodextrin-Based Nanosponges . . . . . . . . . . . . . . . . . . . . Gianluca Utzeri, Dina Murtinho, and Artur J. M. Valente

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Synthesis, Functionalization Strategies and Application of Different Types of Cyclodextrin-Based Nanosponges for Water Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 Naveen Goyal, Dorothy Sachdeva, and Udupa Sujit Application of Cyclodextrin-Based Nanosponges in Soil and Aquifer Bioremediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 Shefali Shukla, Bulbul Sagar, and Sarthak Gupta Removal of Organic and Inorganic Contaminants from Water Using Nanosponge Cyclodextrin Polyurethanes . . . . . . . . . . . . . . . . . . . . . . . 169 Chetna Gupta, Parul Pant, and Sachender Mishra Introduction to Metal–Organic Framework Sponges and Their Synthetic and Functionalization Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 Preeti Bhatt, Abhay Srivastava, and Subinoy Rana

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Contents

Application of Metal–Organic Framework Sponges for Toxic or Greenhouse Gas Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 219 Abhay Srivastava, Preeti Bhatt, and Subinoy Rana Metal–Organic Framework Sponges for Water Remediation . . . . . . . . . . 247 Gyanendra Kumar, Mohd Ehtesham, Satendra Kumar, Bachan Meena, Gobind Ji Rai, and Dhanraj T. Masram Introduction to Sponge-Like Functional Materials from TEMPO-Oxidized Cellulose Nanofibers . . . . . . . . . . . . . . . . . . . . . . . . 263 Pooja, Tarisha Gupta, Madhav Dutt, and Laishram Saya Synthesis and Application of Types of Metal Oxide Nanosponges in Water Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 Archa Gulati and Ajeet Kumar Synthesis and Application of Metal and Metal Oxide-Based Nanosponges as Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 Vijay Beniwal, Naveen Sharma, and Jyoti Jain Polymer-Based Nanobiocomposite as a Filter Nanosponge for Wastewater Remediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335 Shikha Gulati, Aashleshaa Mishra, Manan Rana, and Nabeela Ansari Cellulose-Based Nanosponges for Wastewater Remediation . . . . . . . . . . . . 355 Laishram Saya, Ratandeep, Bikaramjeet, and Pooja Application of Nanosponges for Aquifer Bioremediation . . . . . . . . . . . . . . 383 Shikha Gulati, Himshweta, Manan Rana, Nabeela Ansari, and Shalu Sachdeva Nanostructured Sponges for the Removal of Toxic Dyes from Wastewater . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407 Gunjan Purohit, Manish Rawat, and Diwan S. Rawat Environmental Applications of Nanosponges (NSPs) to Clean up Oil Spills . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425 Yamini, Vikrant Singh Rao, Neeraj Mishra, and Sanjay Kumar Concluding Remarks and Future Perspectives of Nanosponges in Environmental Remediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 449 Shefali Shukla, Ankita Sangwan, Nandini Pabreja, and Shikha Gulati

About the Editor

Dr. Shikha Gulati (M.Sc., Ph.D.) is working as Assistant Professor of Chemistry in Sri Venkateswara College, University of Delhi. She has expertise in nanomaterials, inorganic chemistry, nanosponges, Metal-Organic Frameworks, green chemistry, catalysis, and analytical chemistry. Dr. Shikha’s multiple publications, as well as chapters in numerous other books, speak to her research prowess and strong writing abilities. She has also published a number of research papers in reputable international journals. In numerous universities all throughout India, her writings are cited for various undergraduate courses. The Young Researcher Award 2020 was also given to Dr. Gulati for her efforts in the area of nanotechnology. Her expertise in inorganic chemistry, nanomaterials, and nanosponges has greatly benefited this work.

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Abbreviations

2-MIB 3D 3-D AA AC AG AGS APF BASF BC BDC BET BIM BJH BNC BOD BP5 BPA BPDC BPE BPEB bPEI BTC BTEX CA CAFOs CANS CCS CD CDNS CDs

2-methylisoborneol Three-dimensional Three-dimensional Acrylic acid Activated carbon Arabinogalactan Arabinogalactan sulphate Aminophenol/formaldehyde Badische Anilin und Soda Fabrik Biofibre-reinforced biocomposites Benzenedicarboxylate Brunauer–Emmett–Teller 1-benzylimidazole Barrett, Joyner, and Halenda (BJH) Bacterial nanocellulose Biological oxygen demand 1,2,3,4-butanetetracarboxylic acid Bisphenol A 4,4' -biphenyl dicarboxylate ligands 1,2-bis(4-pyridyl)ethene 1,4-bis[2-(4-pyridyl)ethenyl]benzene branched polyethylenimine Benzene-1,3,5-Tricarboxylate Benzene, toluene, ethylbenzene, and xylene Citric acid Concentrated Animal Feeding Operations Cellulose acetate nanosponge CO2 capture and storage Cyclodextrin Cyclodextrin-based nanosponge Cyclodextrins xix

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CFCs CGTases CMC CMP CNC CNF CNP CNTs CO2 CR CSD CTS CTS-gPAA/MMT CuAAC CUS CV CyCaNSs CYT DBPs DCM DCP DDD DDE DDT DEA DEX DFPS DLS DMA DMF DMSO DNA DOC DPC DRIFTS DSC DTG DVB EBT ECH ED EDC EDS EDTA EDX

Abbreviations

Chlorofluorocarbons Cycloglycosyl transferase amylases Carboxymethyl cellulose 4,4' -Bis(chloromethyl)-1,1' -biphenyl Cellulose nanocrystal Cellulose nanofibres Cellulose nanoparticles Carbon nanotubes Carbon dioxide Congo red Cambridge Structure Database Chitosan Chitosan-g-poly(acrylic acid)/montmorillonite Cu-catalysed azido-alkyne cycloaddition Coordination unsaturation sites Crystal violet Cyclodextrin–calixarene nanosponge Cytoxan Disinfection by products Dichloromethane Dichlorophenol Dichlorodiphenyldichloroethane Dichlorodiphenyldichloroethylene Dichlorodiphenyltrichloroethane Diethyl formamide Dexamethasone 4,4' -Difluoro diphenyl sulphone Dynamic light scattering Dimethylacetamide Dimethyl formamide Dimethyl sulphoxide Deoxyribo nucleic acid Dissolved organic carbon Diphenyl carbonate Diffuse reflectance infrared Fourier transform spectroscopy Differential scanning calorimetry Differential thermogravimetry Divinyl benzene Eriochrome Black T Epichlorohydrin Electron diffraction Endocrine-disrupting compounds Energy-dispersive X-ray spectroscopy Ethylene diamine tetraacetic acid Energy-dispersive X-ray

Abbreviations

ELISA ENMs EPI FA FESEM FET FIB FMOFs FPS FTIR GAC GC-MS GI Tract GO GWP HC HCFCs HCPSN HDI HKUST-1 HMDS HMW HNT HNT-CD HOBP HPBCD HPBCD-NP HPLC HRMAS HRTEM ICDD ICP-MS IRMOF JCPDS k-CG LC LDH LMW MAF MB MBA MCD: VI MFM MG MIL

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Enzyme-linked immunosorbent assay Engineered nanomaterials Epichlorohydrin Folic acid Field emission scanning electron microscopy Field-effect transistor Focused ion beam Functional Metal-Organic Frameworks 4,4' –difluorodiphenylsulphone Fourier transform infrared spectroscopy Granular activated carbon Gas chromatography-mass spectrometry Gastrointestinal tract Graphene oxide Global warming potential Hydrocarbons Hydrochlorofluorocarbons Hyper-cross-linked polystyrene nanosponges Hexamethylene diisocyanate Hong Kong University of Science and Technology Hexamethyldisilazane High molecular weight Halloysite nanotubes Halloysite−cyclodextrin nanosponges 2-hydroxy-4(octyloxy)-benzophenone Hydroxypropyl-β-cyclodextrin (Hydroxypropyl-β-cyclodextrin nonylphenol High-performance liquid chromatography High-resolution mass spectrometry High-resolution-transmission electron microscopy International Centre for Diffraction Data Inductive coupled plasma-mass spectrometry IsoReticular Metal-Organic Frameworks Joint Committee on Powder Diffraction Standards k-carrageenan Line caps Layered double hydroxide Low molecular weight Metal azolate frameworks Methylene Blue N, N' -Methylene-bis-acrylamide Methacrylic-βCD with 1-vinylimidazole (VI) Manchester Framework Materials Malachite green Materials Institute Lavoisier

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MIP MLD MN-PCDP MNPs MNSs MO MOF MOF-S MOG MPH MRI MS MTBE MWCNT NBC NC NCFs NCW NFM NFs Ni–Co2–O HNSs NM NMR NOM NP NPEs NPs NS NSSs NTch OXY PAAm PAHs PAI PAN PBDEs PCBs PCL PCP PCR PCS PDI PFCs PL PLA

Abbreviations

Molecular imprinting Million litres per day Magnetic nanoparticles porous β-CD polymer Magnetic nanoparticles Magnetic nanosponges Metal oxide-based nanosponges Metal-Organic Framework Molecular Organic Framework Sponge Microorganisms Melphalan Magnetic resonance imaging Mass spectrometer Methyl tert-butyl ether Multiwalled carbon nanotube Nanobiocomposite Nanocellulose Nanocellulose fibres Nanocellulose whiskers Nanofibre membranes Nanofibres Nickel–cobalt oxide-based hollow nanosponges Nanomaterial Nuclear magnetic resonance Natural organic matter Nonylphenol Nonylphenol ethoxylates Nanoparticles Nanosponge Nanostructured sponges Nanotechnology Oxyresveratrol Polyacrylamide Polycyclic aromatic hydrocarbons Poly(acrylamide-co-itaconic acid) 1-(2-pyridylazo) 2-napthol Polybrominated diphenyls Polychlorinated biphenyls Polycaprolactone Porous coordination polymers Polymerase chain reaction Photon correlation spectroscopy 1,4-Phenylene diisocyanate Perfluorocarbons Photoluminescence Polylactic acid

Abbreviations

PLP PM PMA PMDA pMWCNT-βCD PNB PNF Ppm PQ PRX PSD PSE PSI PSM PSP PULL PVA PXRD py qm RE RES RhB RPM RS SAED SALE SALI SBU SC-XRD SDC SDS SEM SERS SF SFNF SLI SO SPE SPR TC TCE TCL TDI TEM

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Paliperidone Particulate matter Pyromellitic anhydride Pyromellitic dianhydride Phosphorylated multiwalled carbon nanotube-β cyclodextrin Polymer nanobiocomposites Polymer nanofibre Parts per million Paraquat Piroxicam Post-synthetic deprotection Post-synthetic exchange Post-synthetic insertion Post-synthetic modification Post-synthetic polymerisation Pullulan Polyvinylalcohol Powder X-ray diffraction Pyridine Maximum sorbed amount Removal efficiency Resveratrol Rhodamine B Rounds per minute Raman spectroscopy Selected area electron diffraction Solvent-assisted ligand exchange Solvent-assisted ligand incorporation Secondary Building Unit Single-crystal X-ray diffraction 4,4' -stilbene dicarboxylate Sodium dodecyl sulphate Scanning electron microscopy Surface-enhanced Raman scattering Safranin O Silk fibroin nanofibres Sequential linker installation Safranin Solid-phase extraction Surface plasmon resonance Tetracycline hydrochloride Trichloroethylene Terephthaloyl chloride Toluene-2,4-diisocyanate Transmission electron microscopy

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TEMPO TFCs TFN TFP TGA THMs TOC TOCN TOCNF TOUS-CNFs TPP TSS TTSB ATR UiO UiO-66 USEPA UV VOC WHO XRD ZIF ZIF-8 ZVI β-CD β-CDMOFs β-CD-P5

Abbreviations

2,2,6,6-Tetramethylpiperidinyloxyl Thin-film composites Tetrafluoroterephthalonitrile Tetrafluoroterephthalonitrile Thermogravimetric analysis Trihalogen methanes Total organic content TEMPO-oxidised cellulose nanofibrils 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO)-oxidised cellulose nanofibres Transparent, Outstanding, Uniform, and Sustainable Cellulose Nanofibres Triphenyl phosphine Total suspended solids Attenuated total reflectance Universitetet i Oslo University of Oslo United States Environmental Protection Agency Ultraviolet Volatile organic compound World Health Organisation X-ray diffraction Zeolitic imidazolate frameworks Zeolitic imidazolate Framework-8 Zero valent iron β-Cyclodextrin Metal-Organic Frameworks β-cyclodextrin/pillar[5]arene

Introduction to Nanosponges Dorothy Sachdeva, Naveen Goyal, Anoushka Amar, and Shikha Gulati

Abbreviations BIM BPA bPEI CA CD CDNS DMF DMSO EBT EDTA EPI FIB HDI HKUST MOF MOF-S PAAm PL PSM py SBU TDI

1-Benzylimidazole Bisphenol A Branched polyethyleneimine Citric acid Cyclodextrin Cyclodextrin nanosponges Dimethyl formamide Dimethyl sulphoxide Eriochrome Black T Ethylene diamine tetraacetic acid Epichlorohydrin Focused ion beam Hexamethylene diisocyanate Hong Kong University of Science and Technology Molecular organic framework Molecular organic framework sponge Polyacrylamide Photoluminescence Post-synthetic modification Pyridine Secondary building unit Toluene-2,4-diisocyanate

D. Sachdeva · N. Goyal Materials Research Centre, Indian Institute of Science, Bangalore, India A. Amar · S. Gulati (B) Department of Chemistry, Sri Venkateswara College, University of Delhi, Delhi 110021, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Gulati (ed.), Nanosponges for Environmental Remediation, https://doi.org/10.1007/978-3-031-41077-2_1

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1 Introduction Nanotechnology is the understanding of nanomaterials in detail in terms of their structure, properties, and applications. It combines the knowledge of engineering, biology, chemistry, and physics. An important aspect of nanotechnology includes the synthesis of nanomaterials whose at least one of the dimensions is between 1 and 100 nm range. They can form 0D materials whose all three dimensions are in the nanometre range, 1D materials which have two dimensions in the nanometre regime, and 2D materials whose only one dimension is in the nanometre regime. The world is on the verge of a serious environmental catastrophe that will cost us a fortune. The current state of our environment is completely deteriorating. Global environmental issues are escalating, and we must adopt a viewpoint to address calamities beforehand with fresh ideas and strategies. Nature takes millions of years to clean up the pollution in air, water, and soil. Most of the environmental pollution is primarily brought up by industries and automotive exhaust emissions [1]. Many different perspectives are being used to study the direct and indirect effects of nanotechnology on environmental pollution [2]. Figure 1 shows different pollutants in water, air, and soil which affect the health of living beings [3]. Air pollution contains various toxic gases including NH3 , H2 S, SO2 , NO(x) , and volatile organic compounds (VOCs). Some gases like CH4 and CO2 are responsible for the greenhouse effect causing an increase in the earth’s temperature and melting of the glaciers. It is high time to prevent the further increase in pollutants and remove them from the environment sustainably. Among the illnesses brought by the aforementioned pollutants are pulmonary problems including asthma, malignancies, cardiac disorders, skin issues, and CNS dysfunctions [4]. Flocculation, condensation, sand filtration, froth floatation, and activated carbon adsorption are some of the long-used techniques. These, however, have their own drawbacks, such as the production of non-recyclable chemicals, ineffective metal ion scarping, and requires excessive energy input. Industries have tried to incorporate lightweight and highly durable raw materials, like the use of semiconductors in manufacturing technology resulting in the reduction in the release of two million tonnes of carbon compounds and saving billions of dollars in the energy field and results in the reduction of air pollution, but semiconductors have their own disposal issues [5]. Similar, to air pollution, water pollution is another environmental concern. Polluted water contains certain impurities such as hazardous organic compounds, heavy metals, soluble dyes, etc. These impurities cause several health issues in terms of cardiovascular disorders, respiratory diseases, allergic reactions, damage to the liver, etc. So, there is a dire need to use such substances which can adsorb these impurities from the wastewater. Nanotechnology looks to be an emerging solution to these issues [6]. Amongst different kinds of nanomaterials, nanosponges are one type that could be employed for a variety of applications due to their porous nature, large surface area, selectivity, and stability. The field of nanosponges is growing drastically, which can be clearly

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Fig. 1 Different kinds of pollutants and nanomaterials for their remediation. Reprinted from Del Prado-Audelo et al. [3]

seen from the total number of publications and citations that have come in the year span of 2001 to May 2023 from the plot shown in Fig. 2. Nanosponges are a novel category of cross-linked polymers consisting of particles with nanosized cavities. Polymers are long-range polyesters cross-linked with small molecules which act as hooks to tie the polymers together. The nanosponge usually lies between 50 and 100 nm with a diameter of less than 4 μm. Nanosponges are used for the encapsulation of substances and result in increased stability, reduction in side effects, and enhanced flexibility. Nanosponges have the potential application for pollution management, mitigation and to enhance the ability of traditional environmental clean-up methods [7]. The main aim of this chapter is to provide an overview of the nanosponges. In the starting, the structure and properties of nanosponges have been introduced along with their advantages and disadvantages in comparison to other materials. The complete classification of these nanosponges has been provided along with the methods to synthesize them. Characterization techniques are also highlighted. In the later part, various environmental applications such as wastewater purification, harmful gas adsorption, sensors, and cleaning of oil water spills have been discussed.

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Fig. 2 Plot depicting citations and Number of publications of nanosponges from Web of Science core collection from 2001 to May 2023

2 Structure and Properties of Nanosponges Nanosponges are hyper-cross-linked polymers of solid nanoparticles having cavities in the nano-regime. Nano-porous hydrogels, nano-porous membranes, and nanoporous particles make up the three main categories of nano-porous structures. The difference in porosity and size is the thin line that separates nanosponges from the nanoparticles. Nanoparticles are measured in nanometers, whereas nanosponges are typically less than 5 μm in size and have pores that are measured in nanometers. It has frequently been stated that nanosponges are porous nanoparticles or microparticles. Direct visualization of their 3D structure is challenging but important to understand their properties. Spectroscopic studies predict certain aspects of structures in terms of functional group present, hydrogen and carbon environment, elemental composition, etc. but a detailed 3D view can only be possible using microscopic studies. However, normal transmission electron microscopy images can only provide projected twodimensional images of a 3D object. To overcome this, Grunert et al., used crosssectional microscopic analysis using focused ion beam (FIB) techniques to visualize the 3D structure of gold nanosponges. Figure 3a, b shows the SEM images of gold nanosponges on the substrate with coarse and fine-pore sizes. Figure 3c, d represents their cross-sectional image obtained after cutting slices of nanosponges using FIB techniques [8]. Cross-sectional images clearly depict the presence of pores not only on the outer surface but are present inside the core also. Due to the large internal surface areas and cavity volumes of these nanosponges, these carriers have lately been used for separation, adsorption, and catalysis. The exterior surface of nanosponges is porous which helps them to adsorb specific chemical substances depending upon the size of the pore formed for their removal.

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Fig. 3 SEM images of a coarse and b fine-pored gold nanosponges on the substrate and their cross-section images (c) and (d), respectively. Reprinted from Grunert et al. [8]

Nanosponges have a tiny mesh-like structure that entraps ingredients within them. Due to this property, they are used as a carrier for certain molecules, or entrap to remove them. The salient features of nanosponges are listed below. (1) The size of nanosponges is usually less than 5 μm with the variable polarity of the cavities. Depending upon the ratios of the polymer and cross-linker used for the formation, the size, and polarity can be adjusted. (2) Nanosponges can be formed as in any crystalline or para-crystalline form by varying the reaction conditions. (3) Nanosponges are non-toxic, porous, insoluble in organic solvents, and have high thermal stability up to 300 °C [9]. (4) Nanosponges are reliable over a range of pH from 1 to 11 [10]. (5) The three-dimensional structure of nanosponges encapsulates, transport, and release various molecules. Depending upon the functional groups they can be targeted to specific sites [11]. (6) Simple thermal desorption, solvent extraction, microwaves, and ultrasound method can be used to duplicate them. (7) Nanosponges can form inclusion or non-inclusion-based complexes when they interact with different molecules.

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(8) Nanosponges can also acquire magnetic properties by including magnetic particles during formation. Owing to these properties nanosponges offer certain advantages for various applications than other nanomaterials as listed below.

2.1 Advantages of Nanosponges Over Other Nanomaterials (1) Nanosponges are amphiphilic in nature and hence can solubilize both hydrophilic and hydrophobic substances. (2) The major advantage includes control over the specific target of the desired pollutant. (3) Nanosponges have minimum side effects if consumed by aquatic life during water treatment. (4) Nanosponges have very high tunability than other nanomaterials, i.e., control over the structure of the particle and the size of the cavity. By specifying the amount of cross-linkers and polymers used, the degree of cross-linking varies affecting the properties of nanosponges like adsorption capacity and selectivity. (5) Nanosponges are cost-effective and easily scalable for commercial production. (6) Nanosponges are biodegradable, so their degradation is not a major concern in this case.

2.2 Disadvantages of Nanosponges Over Other Nanomaterials (1) Nanosponges are suitable for the encapsulation of only small particles with a molecular mass of less than 500 g/mol [12]. (2) Adsorption capacity is largely affected by the degree of crystallinity [12].

3 Classification of Nanosponges Nanosponges are classified as Titanium Based nanosponges, Carbon coated metallic nanosponges, hyper cross-linked polystyrene nanosponges, silicon nanosponge particles, β-Cyclodextrin based nanosponges, Metal–Organic Framework sponges (MOFS), metal oxide nanosponges, and cellulose-based nanosponges (Fig. 4). The βCyclodextrin based nanosponges are further subdivided by varying the cross-linkers used as carbonate-based, carbamate-based, polyamidoamine-based, ester-based, and hybrid nanosponges.

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Fig. 4 Classifications of nanosponges

3.1 Titanium-Based Nanosponges TiO2 is an important compound for photochemical and photoelectrochemical properties. It acts as a sanitizer for the environment. Hybridizing TiO2 with metal nanoparticles, titanium-based nanosponges could be synthesized which has been used in various applications like photocatalysis, solar cells, gas sensors, biomedicine, and for water purification. Different techniques including sol–gel, anodization, hydrothermal and atomic-layer deposition could be used for their synthesis. Yu et al., synthesized TiO2 /Ag nanosponge composite using the surface sol– gel method as shown in Fig. 5. Ag nanoparticles are uniformly dispersed on TiO2 nanosponges which showed very high photocatalytic activity [13]. Polystyrene nanospheres are functionalized by a well-defined coating of TiO2 over their surface to make core–shell nanoparticles, which are then converted into TiO2 nanosponges just by calcinating in a tube furnace. This is followed by the phase change from the anatase to the rutile phase [14]. Fernandez-Domene et al. synthesized different morphology of TiO2 -based nanosponges consisting of different phases anatase–rutile with Reynolds number greater than zero. The formation of these nanosponges results in higher surface area and higher conductivity as compared to

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Fig. 5 Schematic representation of the synthesis of TiO2 /Ag hybrid nanosponges. Reprinted with permission from Yu et al. [13]. Copyright (2012) American Chemical Society

nanotubes. Also, the density of the donor species decreases with increasing Reynolds number, i.e., nanosponges are less defective than nanotubes. As a result, nanosponges show much higher photoelectrochemical activity as compared to nanotubes due to low recombination probability [15]. TiO2 nanosponges are further functionalized by doping Li+ cations. This results in decreasing resistance and increasing conductivity of TiO2 nanosponges. The flat band density and the donor site density increase in the case of doping due to more defects engineered and leads to enhancement of their photocatalytic activity for photodegradation of various pollutants [16].

3.2 Carbon-Coated Metallic Nanosponges An inert adsorbent called activated carbon is mostly used for the removal of various pollutants from water and industrial gas-phase streams by a simple adsorption process. The novel metallic nanoparticles which have a very thin layer of activated carbon all over their surface form three-dimensional metallic nanosponges called carbon-coated metallic nanosponges. The metal-impregnated fibres are allowed to

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heat at a very high temperature until the carbon is vaporized and the metal sintered and get connected to another metal resulting in the formation of nanosponges. The nanosponges are synthesized in the form of a tube-like structure using the carbonized fibre as a template resulting in a high surface-to-volume ratio. Nanoparticles synthesized earlier have only a two-dimensional active surface area due to the inactive substrate on which it lies. But in the case of nanosponges, the three-dimensional active surface area is available which improves the adsorption tendency of pollutants [17]. This is further used as a catalyst, and for strengthening of polymers [18].

3.3 Hyper Cross-Linked Polystyrene Nanosponges Polystyrene is a biodegradable material, easily recyclable, and inert in air. Hyper cross-linked polymers are a type of nano-porous materials made up of polystyrene polymer and consist of a variety of present and prospective uses, including drug administration, chromatographic separation, gas sorption and separation, and heterogeneous catalysis. Davankov et al. synthesized intramolecularly hyper cross-linked nanosponges by chloromethylation and heating of ethylene dichloride solution with SnCl4 and linear polystyrene. In the dilute solution of polystyrene coils rigid intramolecular bridges are introduced which results in the formation of spherical nanosponges. They have a large inner surface area and are strongly swell for linear polystyrene [19]. These are synthesized in porogen by suspension of styrene, divinylbenzene, and vinyl benzyl chloride. Chloromethylated polystyrene further undergoes Friedel–crafts reaction to form cross-linkage nanosponges. Pore size, volume, and surface area are greatly affected by the type of porogen used. As shown in Fig. 6A, in the case of cyclohexanol large pore sizes are formed with low specific surface area. Whereas when toluene is used as a porogen, a smaller pore size is generated as shown in Fig. 6B, since this porogen has good thermodynamic compatibility which leads to phase separation at a much later stage leading to retaining the microgel bead’s structure. When a mixture of cyclohexanol and toluene is used as co-porogen, cyclohexanol induces fast phase separation resulting in the formation of macropores, while toluene induces micropores. As a result, these co-porogen’s lead to a high number of meso and macropores as shown in Fig. 6C [20].

3.4 Silicon-Based Nanosponges Silicon-based nanosponges are another class of nanosponges having a porous structure and high activity. The development of electrochemical sensors and biosensors has made use of materials with silica chemistry known as silicon-based nanosponges as potential electrode modifiers. Metallurgical-grade silicon powder is used for the formation of silicon nanosponges particles with silicon particle sizes between 1 and 4 μm range because of the etching process. Uniform pore distribution between

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Fig. 6 Pore formation in Chloromethylated polystyrene and hyper cross-linked polystyrene using porogen such as Cyclohexanol, Toluene, and a mixture of both as represented by scheme A, B, and C, respectively. Reprinted with permission from Liu et al. [20]. Copyright (2008) Elsevier

the nanocrystals is observed within the entire nanosponge particle [21]. Chadwick et al. used electrochemical etching in hydrofluoric acid solution to form silicon nanosponges. The impurities present at the surface of silicon particles lead to the formation of porous structure. The mechanism of pore nucleation and formation gives the composition of porous silicon nanosponges [22]. The etching of silicon substrate can also be done using ultrasonic waves to synthesize ordered pore structures. Grinding and ball milling of the powdered silicon and further etching will also result in the porous structure. Chemical etching has also been used but the etching mechanism in this case is not clearly understood till now, it creates disordered pores which are difficult to analyze [23]. Yu and co-workers synthesized aluminosilicate nanosponges with kaolinite structure [Al2 Si2 O5 (OH)4 ]. The reaction scheme is shown in Fig. 7a. These nanosponges are then applied for the removal of cationic dye, methylene blue as well as anionic dye, azorubine. As the surface area of the nanosponges is found to be higher compared to nanoplates, therefore their sorption is also much higher than nanoplates as depicted in Fig. 7b [24]. They have also found applications in biosensors, and optoelectronics.

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Fig. 7 a Synthesis of aluminosilicate nanosponges with the SEM image of the final product, b Plot between zeta-potential and pH value showing that nanosponges are more favourable than nanoplates due to higher surface area. Reprinted with permission from Golubeva et al. [24]. Copyright (2021) American Chemical Society

3.5 β-Cyclodextrin Nanosponges This is one of the major classes of nanosponges that are easily synthesized and have huge applications. Cyclodextrin (CD) are oligosaccharides having D-glucopyranose bonded by 1,4-gylcosidic linkages produced by enzymatic hydrolysis of starch. This CD undergoes copolymerization with cross-linkers to form Cyclodextrin Nanosponges (CDNS). It has a cone-shaped structure having a hydrophilic outer surface and hydrophobic inner cavities containing C–H groups and ethereal oxygen [25]. By varying the type of cross-linker used it is further subdivided into urethane, ester, carbonate, and ether-based nanosponges. (a) Urethane-based nanosponges Diisocyanates including hexamethylene diisocyanate (HDI) and toluene-2,4diisocyanate (TDI) are employed as cross-linkers during the synthesis of Urethane CDNS. The synthesis of these nanosponges is performed in an aqueous medium which adsorbs the organic molecules including p-Nitrophenol at very low concentration [26]. (b) Ester or acid-based nanosponges Dianhydrides and carboxylic acids including ethylene diamine tetraacetic acid (EDTA), butane tetracarboxylic acid, or citric acid (CA) is used as crosslinkers for these CDNS. The mechanical properties, elasticity, and stiffness of these CDNS depend upon the ratio of the cross-linkers used, e.g., the 1:6 ratio of EDTA dianhydride has the highest degree of cross-linking as compared to lower ratios [27]. (c) Carbonate-based nanosponges Active carbonyl groups including diphenyl carbonate, triphosgene, and 1,1' carbonyldiimidazole are employed as cross-linkers for the synthesis of these CDNS.

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Similar to ester-based CDNS, the mechanical properties, elasticity, and stiffness of these CDNS depends upon the ratio of the cross-linkers used [28]. (d) Ether-based nanosponges Epichlorohydrin (EPI), bisphenol A (BPA) digylcidyl ether, or ethylene glycol digylcidyl ether are used as cross-linkers for the synthesis of these CDNS. These CDNS can be soluble in aqueous or basic medium depending upon the reaction conditions. The adsorption properties of CDNS are governed by the degree of cross-linking [29].

3.6 Metal–Organic Framework Nanosponges Metal–Organic Frameworks (MOF) are highly crystalline inorganic–organic hybrid materials with porous structures showing sponge-like properties. Generally, MOF having a diameter between 0.5 and 1 nm shows good adsorption capacity and can be termed MOF sponges (MOF-S) [30]. MOF consists of an inorganic cluster of metal ions that are linked to the organic linkers resulting in a cage-like structure. Due to variable pore size, dimensionality, and chemical environment, a diverse range of MOFs are being synthesized. MOF has a very high surface area and a highly porous nature which makes it a suitable candidate for catalysis for various applications. Reticular synthesis consists of metal ions and organic linkers which form a crystalline network known as secondary building units (SBU). This SBU assembles and directs the formation of 3D MOF-S as represented in Fig. 8. The structure and chemical properties of MOF-S can be varied and optimized for specific applications. Further, pore size can be altered by tweaking the organic linker and functionalization can be altered to form MOF-S for improving its properties. MOF-S have specific functional groups which are highly specific and selective and hence can be used for the sensing of metals, gases, or small molecules [31]. MOF-S are very effective in cleaning the environment by removing toxic gases specifically without disturbing the other substances.

Fig. 8 Schematic representation of synthesis and functionalization of MOF

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3.7 Metal Oxide Nanosponges The porous metal oxide nanosponges are another class of nanosponges. These possess high stability, large surface area, greater mass transfer rate, high electron mobility, and smaller particle size. The semiconductor band gap, surface energy, and magnetic properties of these materials rely on the type and size of metal oxide which could be varied according to the application required. Mono, bi, and polymetallic oxide nanosponges are being synthesized for various applications. Due to aggregation problems in mono-metal oxide nanosponges, they are not used for industrial applications due to the lesser surface area. However, bi and polymetallic oxide nanosponges enhance the physical and chemical properties and therefore could be used for various applications. Liu et al., fabricated a Cu–Zn–TiO2 nanotube array to remove nitrate from groundwater with a removal rate of 97.5%. Varying the temperature has a drastic effect but pH change does not affect the removal of nitrate [32]. Zururi et al., synthesized titania nanosponges highly selective for hydrogen gas. Due to the threedimensional interconnected structure of metal oxide nanosponge, sensors based on these structures show ultrahigh chemical sensitivity [33].

3.8 Cellulose-Based Nanosponges Cellulose-based nanosponges are an emerging class of nanosponges that combines the properties of both cellulose and porous materials. By recycling scraps of paper to prevent the generation of greenhouse gases, cellulose insulation contributes to a reduction in carbon emissions. By using cellulose insulation, one may reduce the volume of wastepaper that ends up in landfills as well as the carbon footprint. Cellulose nanofiber materials have been employed in a lot of commercial items because of their durability and strength as compared to polymer-based materials like plastics, which are potentially hazardous to the environment. These porous materials have shown their application in adsorption, separation, and as a catalyst. The porosity in the structure could be induced either during the reaction or post-synthesis. Chelating molecules like MOFs, thiols, or amino groups could be used to functionalize cellulose-based nanosponges having applicability in removing heavy metals, and dyes and adsorbing harmful gases from the environment. Fiorati et al., synthesized cellulose-based nanosponges by reducing the amount of branched polyethyleneimine (bPEI) which is toxic to the environment. To compensate for the amount of bPEI, citric acid (CA), non-toxic for aquatic life, was used. This replacement does not alter the adsorption capacity of heavy metals [34]. Riva et al., prepared nanosponge using (2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) and oxidized cellulose nanofibers, bPEI and CA. Different dyes including Brilliant Blue R, Naphthol Blue Black, and Orange II Sodium Salt, are adsorbed efficiently using this nanosponge [35].

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4 Methods of Synthesis of Nanosponges The synthesis of nanosponges requires polymer and cross-linkers. Polymers include cyclodextrins, polystyrenes, polyurethanes, polyesters, etc. Crosslinkers include molecules that have bifunctional groups such as dicarbonyl, diisocyanates, and dicarboxylate compounds. Various methods used for the synthesis of nanosponges are discussed below. (a) Solvent Method The polymer is mixed with the desired solvent in a polar aprotic solvent like dimethyl sulphoxide (DMSO), and dimethyl formamide (DMF). This mixture is then transferred to a specific molar ratio of polymer to cross-linker between 4 and 16 depending upon the application required. This follows the heating of the reaction between 10 °C and the solvent’s reflux temperature for 1 h to 48 h. The cross-linkers used for this method are dimethyl carbonate and carbonyl diimidazole [36]. The final product was then transferred to a large amount of distilled water after cooling to room temperature followed by vacuum filtration and Soxhlet extraction using ethanol. Finally, the product is dried in a vacuum and ground in the mechanical mill to form a fine powdered product [37]. (b) Ultrasound Assisted Method This method consists of a polymer and cross-linkers which when subjected to Ultrasonic waves form nanosponges without any solvent at 90 °C for 5 h. The polymer was mixed with a cross-linker in a specific molar ratio. The excess reagent was removed with the help of washing with water and further purified by Soxhlet extraction using ethanol and then the final product is dried in a vacuum and kept in a desiccator at room temperature. In general, spherical uniform-sized nanosponges are formed using this method. The rate of formation of nanosponges via this method is quite high. Most cross-linkers used in this case are diphenyl carbonate and pyromellitic anhydride [38, 39]. (c) Melt Method This is a simple melting method in which the precursors, cross-linkers, and polymers are taken in a finite ratio and stirred continuously while heating at 100 °C for 5 h. The reaction mixture is then allowed to cool to room temperature. Further, the product is washed with specific solvents for the removal of excess precursors and by-products formed during the reaction [40]. (d) Microwave Method The polymer and cross-linker are dispersed in a microwave-active solvent like diphenyl formamide in a fixed proportion. These are subjected to microwave radiation at a specific temperature, time, and pressure. Upon formation of the final product, the solvent is removed, and the product is washed with water and further purification is done by Soxhlet extraction using ethanol. The final purified product was dried at 60 °C and kept at room temperature for further use [41].

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5 Factors Affecting Nanosponges Various factors can affect the formation and application of nanosponges as illustrated below. Polymer and cross-linker: The kind of polymer employed influences the formation of nanosponges, and the diameter of the cavity formed. By fluctuating the degree of cross-linking and the nature of the cross-linker used, the hydrophobic or hydrophilic nature of nanosponges can be tuned. Hydrophobic nanosponges are synthesized by diisocyanates, diphenyl carbonate, pyromellitic anhydride, or carbonyl di-imidazoles as a cross-linker which act as a carrier for water-soluble moieties [38]. Using EPI as a cross-linker, β-CD results in the formation of hydrophilic nanosponges which are used for the removal of acidic dyes namely Eriochrome Black T (EBT). β-CD forms π–π interactions with aromatic rings of EBT and electrostatic between positively charged β-CD and negatively charged EBT [42]. Temperature: It contributes as a major factor in the complexation of nanosponges during their synthesis and adsorption mechanism. At higher temperatures, the stability constant of the nanosponges decreases due to lower nanosponges-guest molecule interaction forces. Therefore, the temperature should be optimized and maintained fixed during the whole reaction [43]. Reaction condition: Different methods of preparation of nanosponges have their unique effect on the structure and properties of nanosponges which could be used for specific activities. Reaction conditions like stirring speed, pressure, pH, and the atmosphere surrounding the reaction affect their formation. Degree of substitution: The position, number, and nature of substituents greatly affect the complexation of nanosponges. The position of the substituent will depend upon the reaction condition. This results in a change in the physiochemical properties due to different position occupancy of the functional groups. With the same degree of substitution for HP-β-CD, their physiochemical properties differ due to the different positions of the hydroxypropyl group on the CD moiety. The number of substituents is directly related to the degree of cross-linking. Many substituents will result in a higher degree of cross-linking and this yields nanosponges with more pores due to more interconnections. The type of substitution governs the nanosponges formation as β-CD exists in different forms with different functional groups on the surface. And further different types of complexes are formed by varying the type of cross-linker used during synthesis [38, 44]. Molecular entity: For the encapsulation of moieties by nanosponges certain basic characteristics are required. The molecular weight should lie between 100 and 400 g/ mol, the melting point should be less than 250 °C, the solubility should be less than 10 mg/mL in water, and should have less than 5 condensed rings to be easily trapped by the cavities. The solvent medium also plays a major role in the interaction of

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cavities with targeted compounds. In hydrophilic solvent, organic guest molecules will be transferred to the hydrophobic cavities whereas organic solvent will tend to remove organic molecules from the cavities. These interactions between host and guest molecules will depend upon polarity, size solvent system, and structural property [38, 45].

6 Characterization Techniques of Nanosponges Nanosponges synthesized using various above-mentioned processes are characterized using multiple techniques. Brief detail of characterization techniques with their particular aim is mentioned in Table 1.

7 Applications of Nanosponges Literature is flooded with numerous applications of various types of nanosponges. Major applications already established in the biological field are drug delivery, absorption of toxins, biocatalyst carriers, sustainable delivery systems, protection against photodegradation, and antiviral applications. Recently, these nanosponges have been explored for environmental applications also and receiving huge success in this regard. Environmental applications such as wastewater treatment, adsorption of poisonous gases, sensors, and cleaning up of oil spills have been discussed in the following section.

7.1 Wastewater Treatment Water is a necessity of daily life routine. Waste from various industries is directly put into the water which deteriorates its quality and makes it difficult for the aquatic life to survive [52]. So, there is a huge requirement for the development of adsorbent material which effectively removes impurities from the wastewater by adsorbing them. Activated carbon is one such material, which removes the organic pollutants from the water bodies. One of the classes of nanosponges, CDNS is found very effective for wastewater treatment. They are cost-effective and applicable for numerous pollutants removal like heavy metals, dyes, drugs, pesticides, organic pollutants, and phenols. They can form host–guest interaction via complex formation with pollutant due to the presence of a hydrophobic cavity present inside CDNS. Due to the availability of surface-active sites, they can undergo ionic interactions on the surface of CDNS. The presence of pores in CDNS helps to diffuse due to the hydrophilic nature of CDNS [25].

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Table 1 Brief description of the aim of various techniques employed for the characterization of nanosponges No.

Characterization techniques

Aim

1

Solubility studies

To study inclusion complexation for determining the solubility of the molecules/pollutants using High-Performance Liquid Chromatography [46]

2

Microscopic studies

To study the morphology and surface topography of nanosponges using Scanning electron microscopy, and Transmission electron microscopy [8, 33]

3

Particle size and polydispersity

To calculate the mean diameter and polydispersity index dynamic light scattering with a particle size analyzer is used [38]

4

Porosity

To find the number of nanochannels and nanocavities [12]

5

Zeta potential

To calculate the electric potential between two layers of fluids within the dispersed particles. It indicates the stability of the colloidal dispersion [47]

6

Swelling

Nanosponges are dipped in solvent to measure the amount of swelling quantity [48]

7

X-ray diffractometry

To estimate the formation of host–guest complexes in a solid state [38, 49]

8

Saturation state interaction

Using UV-spectroscopy the absorption capacity in a saturated state can be determined [47]

9

Raman spectroscopy

Employing various stretching and binding modes to understand the molecular structure [40]

10

Thin layer chromatography

In this, the Rf value is taken to know about the complex intermediate formed between the particular pollutant and nanosponges [50]

11

Thermo-analytical methods

To determine the thermal decomposition of the nanosponges. The change may include melting, evaporation, decomposition, or oxidation due to the formation of complexation [51]

Heavy metals discarded from the industries could be removed using aromatic cross-linked β-CD by different interactions like hydrophobic inner-cavity, host– guest, hydrogen bonding at the surface, and Van der Waal interactions. Depending upon CD derivative employed, and the ratio of CD to cross-linker being used, the adsorption of pollutants will change. Cu2+ ions could be removed from wastewater using the CA-modulated CD. At lower pH values, the removal efficiency is quite low but on increasing the pH value i.e., slightly acidic medium the removal efficiency increases due to charging at the surface site and decreasing active functional groups [53]. He et al., found that at pH values greater than 3 the adsorption capacity of Cu2+ , Cd2+ , and Pb2+ increases as the aromatic group of the polymer attains negative charge which could easily attract positively charged metal ions by electrostatic

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interactions. The highest removal efficiency was found for Pb2+ ions (196 mg g−1 ) followed by Cu2+ (164 mg g−1 ) and Cd2+ (136 mg g−1 ) [54]. Sikder et al., synthesized Chitosan iron nanoparticle carboxymethylated β-CD for the extraction of copper and chromium ions from the polluted water with the adsorption capacity of 250 and 142.8 mg/g, respectively [55]. This process of adsorption is highly exothermic and reduces these metal ions at the expense of the oxidation of iron. Textile industries are a major contributor to synthetic dyes, leather, and printing industries. They block the pathway of sunlight into the water bodies by the accumulation of dye on the surface resulting in the inhibition of plant growth, reduction in oxygen level, and ceasing photosynthesis. For the removal of EBT, β-CD has been used due to the formation of π–π interactions among the β-CD and aromatic rings of the EBT accompanied by the electrostatic interactions [42]. Methylene Blue, a commonly used dye could be removed from wastewater using Tetrafluoroterephthalonitrile as a cross-linker and on undergoing copolymerization of β-CD with pillar [5] arene forms host–guest interaction resulting in the formation of a cavity with high surface area and removal efficiency [56]. Liu et al., synthesized β-CD-magnetic graphene hybrid for the removal of Rhodamine 6G dye with 90% efficiency even after several cycles [57]. Agricultural waste is directly dumped into water bodies resulting in the contamination of water bodies. Pesticides are directly or indirectly accumulated in the food chain which affects human health. Using EPI as a cross-linker, Liu et al., used CDNS to remove atrazine, benalaxyl, butylene fipronil, and simazine pesticides from the wastewater via the generation of host–guest complexes, physical adsorption at the surface and by swelling in an aqueous medium [58]. CD when undergoes polymerization with 1-benzylimidazole (BIM) forms a β-CD-BIMOTs accompanied by the formation of βCD-BIMOTs-TDI by further polymerization with toluene diisocyanate. The resultant complex has high removal capacity, large pore size, and increased thermal stability [59]. Zhou et al., used triphenylmethane-4,4' ,4'' triisocyanate for the functionalization of β-CD to remove 2,4-Dichlorophenol pesticide as compared to phenol or 2-chlorophenol [60]. Salazar and co-workers synthesized magnetic β-CD for the removal of dinotefuran from water bodies as shown in Fig. 9a, with the maximum sorption of 4.5 × 10–3 mmol/g and are efficient up to eight cycles of adsorption and desorption [61]. Martwong et al., removed paraquat from wastewater using polyvinyl alcohol as a cross-linker in the presence of CA. This nanosponge has shown a removal efficiency of 94.5% with an adsorption capacity of 90.3%. The schematic for the adsorption–desorption mechanism is depicted in Fig. 9b [62]. Wastewater discarded from pharmaceutical industries consists of large amounts of drugs which increases the oxygen demand and increases the pH value of water. When EPI is used as a cross-linker for the removal of ibuprofen, it shows a removal efficiency of 85% [63]. Moulahcene and co-workers, employed CA cross-linker for polymerization of α, β, and γ-CD for the effective removal of progesterone. The maximum adsorption capacity was found to be of α-CD: CA polymer with 95% capacity [64]. Yu et al. [65], used β-CD: EDTA for adsorption of ciprofloxacin drug from the wastewater showing 327 mg/g adsorption capacity at pH range 4–6.

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Fig. 9 a Magnetic β-CD used for the removal of dinotefuran and back recovery, Reprinted from Salazar et al. [61]. b Schematic showing the adsorption and desorption of paraquat by magnetic β-CD. Reprinted from Martwong et al. [62]

These nanosponges can undergo functionalization to further improve the properties of the CDNS. These can form magnetic nanocomposites, hydrogel, or hybrid CDNS. For the removal of Ni2+ , Cu2+ , Cd2+ , and Hg2+ carboxymethylated β-CD has been used. To further enhance the removal efficiency of Cu2+ ions, Badruddoza and co-workers, hybridized CDNS with Fe3 O4 nanoparticles [66]. Methacrylate β-CD when reacted with acrylamide undergoes copolymerization to form β-CDpolyacrylamide (β-CD-PAAm) which is basically nanosponge-based hydrogel. As compared to PAAm gel, this has a larger pore size and higher swelling ratio which is being used for the removal of dyes like phenolphthalein and bisphenol A [67]. For the elimination of Pb2+ and Co2+ ions from the polluted water, Taka et al., used hybrid carbon nanotubes/CD/Ag doped TiO2 nanosponges. The adsorption capacity of lead and cobalt ions was found 35.86 and 7.8 mg g−1 , respectively [68].

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7.2 Gas Adsorption Our environment is a combination of biotic and abiotic components. Various health issues are generated by air pollution. Chronic Obstructive Pulmonary Disease, asthma, cough, and respiratory diseases are common illnesses found in human beings due to bad air quality. The rate at which various gases, fine solid particles, and liquid aerosols are released into the environment exceeds the ability of the environment to dissipate, dilute, or absorb them. The burning of fossil fuels and industries generates CO2 , the main component of greenhouse gas. Other components include methane, nitrogen dioxide, and fluoro compounds. The greenhouse gases present in the atmosphere absorb solar radiation resulting in global warming. For this reason, additional agents are needed to adsorb them and to reduce the overall percentage of CO2 in the air. In the last few years, various methods are being employed for the removal of these gases like ionic liquids, organic polymers, zeolites, and MOFs-based nanosponges. MOF sponges can be used for environmental remediation by adsorption, storage, or purification of harmful gases. They are being used for the removal of harmful gases like hydrogen sulphide, sulphur dioxide, carbon dioxide, ammonia, and methane. Figure 10 shows the schematic representation of different gas pollutants adsorbed by modified MOF-S [69]. As compared to the conventional methods for gas adsorption stronger interactions for H2 S can be achieved through MOFs with metal ions that possess unsaturated coordination sites while retaining their structure. Further functionalization of MOF could be done for specific activities by changing their physio-chemical properties and modulating their structure. Prefunctionalization of MOF could be done by imparting pre-modified ligands. Here the functional groups are inserted into the aromatic rings of the organic linkers. For amino functional groups on the aromatic ring, the selectivity for specific gas increases. Rada et al., showed a higher adsorption of CO2 using titanium-based MOF, NH2 -MIL-125 (Ti) than the unfunctionalized MIL-125 (Ti) [70]. This pre-functionalization could also hinder the formation of MOF. There is a limitation to the addition of functionalized linker to form MOF. So, for further modifying MOF, organic linkers are modified after linkage. This is referred to as a post-synthetic modification (PSM). The basic criterion is that the MOF structure will remain intact during the reaction. The commonly used methods for PSM of MOF-S are covalent, non-covalent, and capturing. The carboxylate groups of MOF-S could be modified using covalent bonding. In the late 1900’s, the first report on PSM on MOF-S was done by Kim and Lee [71, 72]. PSM could also be employed for the formation of functionalized materials with specific properties. Till today, PSM is considered a trustworthy method to functionalize MOF-S for various properties and applications. Non-covalent PSM involves the incorporation of organic molecules containing metal atoms onto the SBU unit of MOF. Other than these various methods are deployed for PSM like insertion, deprotection, polymerization, and exchange for desired properties of MOF [73]. Various nanosponges have been employed for the selective adsorption of the gases, a few examples are discussed below.

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Fig. 10 Schematic representation of various gas pollutants adsorbed by modified MOF-S. Reprinted from Augustus et al. [69]

(1) H2 S gas: H2 S gas has a high risk for human health. It combines with Fe3+ in the body and blocks oxygen and results in catastrophic effects. Nickerl et al., synthesized UiO-67, Cu2+ -bipyridine functionalized MOF adsorb H2 S with the adsorption capacity of up to 7.8 wt% [74]. Liu et al., pyridine (py) MOF using Ni2+ metal like Ni(py)AlF5 and Ni(py)NbOF5 for the absorption of H2 S gas. The pyridine complex of Ni showed much more activity as compared to Mg complexed systems [75]. (2) SO2 gas: SO2 is generated in the atmosphere from the burning of non-renewable energy sources, and could react with water molecules to form sulphuric acid which causes irritation in the eyes and throat, and damages the respiratory system. Using amine, aromatic groups, or alkynes different kinds of MOF are being synthesized like MOF-5, MOF-177, or MOF-199. Depending upon the type of MOF, the cavity size and surface area values differ. Carter et al. [76], reported zirconium-based Manchester Framework Materials 601 as the highest adsorber for sulphur dioxide having 12.3 mmol g−1 adsorption capacity at STP. (3) CO2 gas: The greenhouse gas CO2 is a prime concern for global warming. Various adsorbents are being employed for the removal of CO2 from the atmosphere like activated carbon and zeolites. Zeolitic imidazolate frameworks have a very high adsorption capacity for CO2 showing that polar moieties are highly selective for CO2 . Dybtsev et al. [77], synthesized MOF functionalized with

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Mn2+ ion, a highly stable and porous compound used for the selective sorption of CO2 . (4) NH3 gas: Ammonia is another harmful gas that can cause lung damage and even cause death when exposed at a high rate. Various metals are used for the functionalization of MOF like zinc, copper, cobalt, nickel, and magnesium. Petit et al. [78], used HKUST-1 (Hong Kong University of Science and Technology) for the elimination of ammonia gas having 12.1 mmol g−1 adsorption capacity of NH3 at 1 bar pressure for only 2 h. (5) Methane gas: Depletion of the ozone layer is a result of methane production in the atmosphere. The major advantage of MOF over other adsorbents is that they are highly selective for methane even in humid conditions. Stoeck et al. [79] used MOF functionalized with Cu2+ ions and linked with carbazole for methane adsorption with a capacity of 308 mg g−1 at STP.

7.3 Sensors A sensor is a tool that accepts a stimulus or signal and reacts to the stimulus with an electrical signal. The output circuit transmits some forms of electrical signals, such as current or voltage. In essence, a sensor is a device that detects many signal kinds, including physical, chemical, and biological impulses and converts them into an electric signal. The basic properties of metal and metal-oxide semiconductors are being used for sensor purposes mainly in electrochemical sensors. Numerous research is conducted on semiconductor metal oxide as they are a promising material for solid-state gas sensors. Earlier, nanobelts, nanotubes, and nanorods were applied for gas sensing applications [80–82]. But recently three-dimensional crosslinked metal oxide polymer nanosponges have tremendous applications because of the presence of high surface area and porous nature, they increase the kinetics of chemical reactions and could be employed for gas detection. For the sensing of hydrogen gas, Zhao et al. synthesized ZnO nanosponges and decorated their surface using Pd metal nanoparticles. A film of ZnO was deposited on a glass substrate having uniform width of about 13 nm and porosity of about 73%. They found that on increasing temperature from 20 to 80 °C, the ZnO layer is more conducting, but the Pd layer resistance increases due to the scattering of electrons and degrades the Pd layer due to thermal stress. As a result, the collective combination of both reduces the response as well as recovery time. These nanosponges show a response time of 41 s for 2% hydrogen at 20 °C whereas at 80 °C it decreases to 0.3 s with a recovery time of 18 s [83]. Zuruzi and co-workers synthesized TiO2 nanosponges for the detection of hydrogen gas. This three-dimensional sponge-like structure is formed of interconnected nanowires and nanowalls. As nanosponges are formed by Ti films, the morphology of the nanosponges is affected by the concentration of Ti ions. Thin films with high Ti, concentration results in the formation of thin nanowalls along with small nanopores and nanowires. As a result, for 1 ppm of H2 , the response time

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of TiO2 nanosponges was found to be 417 s. However, for 4000 ppm of H2 , the response time was 814 s [33]. Feng et al., synthesized novel Pd nanosponges simply by dealloying the amorphous Pd for the detection of non-enzymatic glucose. Because of the presence of a high surface area of these porous nanosponges, the reaction rate is high as both interior and exterior surfaces are available for reaction. These Pd nanosponges are highly stable and have a high selectivity and sensitivity toward glucose detection up to 2 μM [84]. Sharma et al., encapsulate Au nanoparticles within ZnO nanosponges by fraternization of ZnO nanostructures with HCl and Au nanoparticles at room temperature. The sponge-like structure consists of a wurtzite hexagonal structure of ZnO which is used for sensors by photoluminescence (PL) emission. They predicted that ZnO: Au nanosponges could potentially be beneficial for the sensor selectivity and sensitivity of bio-sensor applications, based on the observed blue shift of the PL emission peak [85]. Yang and co-workers synthesized Co-MOF for the detection of H2 O2 . These nanosponges modified with glassy carbon electrodes showed high electrochemical activity towards H2 O2 with high sensitivity of 83.10 μA mM−1 cm2 at low concentrations between 5 and 9 μM. A cyclic voltammetry plot is shown in Fig. 11a in 0.1 M solution of sodium hydroxide at 20 mVs−1 scan rate. The solution without H2 O2 showed a reduction peak at − 0.4 V corresponding to the reduction of Co3+ to Co2+ , whereas 1 mM and 2 mM H2 O2 solution showed a sharp increase in the reduction current which is due to the reduction of H2 O2 by the metal ion of Co-MOF. On varying the applied potential at a constant concentration of H2 O2 , a typical I–t graph is plotted in Fig. 11b. At − 0.2 V potential the current response is low and on decreasing the applied potential the current response increases, but at − 0.6 V there is a huge increase in the noise, so − 0.4 V is considered as an optimum value of applied potential [86]. The SEM image of the Co-MOF nanosponges having 10 μm size, used to detect H2 O2 is shown in Fig. 11c.

7.4 Clean Up Oil Spills The way crude oil and tar stick to the dirt and soil, they pollute the environment and are hard to remove. Hazardous pollutants seeping into the soil can harm the health of animals and humans who intake crops grown in this soil. The technologies used to remove these poisons from hazardous waste currently have a poor track record and are costly. Engineers at Cornell University created 20 nm long nanosponges with a hydrophobic inner and a hydrophilic outside that can selfassemble in water. These particles are small enough to move swiftly through sand and dirt without getting stuck. These nanosponges were injected into the sand that was tar- and phenanthrene- and polycyclic aromatic hydrocarbon (PAH)-contaminated at the bottom of a steel columnar section. They saw that the nanosponges were clearing the dust quite well as they traveled upward in the column. Phenanthrene

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Fig. 11 a Cyclic voltammogram of Co-MOF in 0.1 M NaOH solution with and without H2 O2 at 20 mVs−1 scan rate, b Amperometry response of Co-MOF in 1 mM H2 O2 from − 0.2 to − 0.6 V range, c SEM image of Co-MOF. Reprinted with permission from Yang et al. [86]. Copyright (2015) Elsevier

was drawn from the sand grains and drawn into the core of the sponge by the waterinsoluble parts of the nanosponge [87]. The “pump and treat remediation,” which entails pumping contaminated groundwater to the surface, cleaning it of contaminants, and then injecting it back into the earth, is intended to be improved using this technique, according to researchers. These nanosponges may gather pollutants more effectively than digging holes in the ground. Pollutants that have been removed by washing might be reintroduced to the soil to continue cleansing. Oils when released unintentionally into the water bodies, like in marine or onshore, result in oil pollution. Onshore activities like domestic household oil terminals receiving oil, industrial activity producing oily wastewater, and oil tank cleaning of docked ships contribute the majority to oil pollution [88]. These activities could lead to oily seawater if the oil is discharged into the nearby sea. Additionally, compared to pollution on land, marine oil pollution had a greater negative effect on the ecosystem. This is caused by the existence of ocean surface currents and the presence of living organisms both above and below the ocean’s surface. While live marine species suffer by ingesting food that has been polluted by oil and by oil attaching to their bodies, which restricts their ability to move about in order to survive, these currents accelerate the spread of the oil across a greater region of the sea. Nanosponges are one such material that can be used effectively for the sorption of oils. The oil sorption mechanism of nanosponges is as follows:

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Capillary Action: Unaided liquid flow into small spaces is a surface phenomenon known as capillary action. There is resistance in this flow against outside forces like gravity. Micro-scale fibers absorb oil via the mechanism of capillary action is demonstrated by polypropylene fibrous mats [89]. Low oil sorption capacities are the result of the large voids that form between the micro-scale fibers, which have a diameter of about 20 μm. Adsorption: When oil molecules stick to the surface, they form an oily film that causes oil adsorption to occur [90]. In the initial stages of oil adsorption on polystyrene nanosponges, the Van der Waal interactions are active. Absorption: The oil that needs to be retrieved back is mixed with the sorbent materials for absorption. Few sorbents that are available for use in responding to oil spills are absorbents, but the majority are adsorbents. Nanosponges produced by various polymers have varying adsorbing and absorbing properties. They have been created using a variety of polymers, including cellulose acetate, polyurethane, polystyrene, and polyvinyl chloride [91]. On the other hand, research has been done on the production of nanosponges using polymers with various molecular weights of the same material [92]. To increase the absorption tendency of nanosponges, preparation of polymer solutions using polymers with different molecular weights should be carried out in order to investigate the effects of the polymer solution’s viscosity on its conductance, the surface tension of the produced sponge, and its oil sorption capacities.

8 Conclusion and Future Perspectives Research on nanosponges has increased considerably over the last two decades since their role is indispensable for the development of nanotechnology. Nanosponges are an excellent choice for cleansing the environment and enhancing the ability of the traditional clean-up methods due to their highly porous nature, high surface area, highly selective, and specificity. They encapsulate the molecules, increasing flexibility and stability. The presence of both hydrophobic and hydrophilic moieties causes them to have a variety of structural domains and solve the issue of solubility. They circumvent the limitations of traditional approaches used for environmental remediation since they are easy to synthesize and functionalize. This chapter explores nanosponges, including their types, characteristics, synthesis, and characterizations. The final section covers some of their uses including water purification, gas adsorption, and sensors. Nanosponges have been utilized in various environmental applications and exploring the impacts of the synthesis process, particle size, porosity, crystallinity, and degree of crosslinking offers tremendous opportunities. Environmental application of these nanosponges is still at an early stage and more efforts must be made in this direction. Furthermore, creating a high-yield, affordable, reproducible process that can be quickly modified for mass manufacturing, is encouraged. Though

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nanosponges are easy to form the key disadvantage is the presence of solvent residues or the by-products formed during the reaction which are a significant contributor to increasing the amount of waste in the environment. So, it is important to find methods that generate fewer residues or convert these residues into less harmful compounds so that these have minimal impact on environmental pollution.

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Different Types of Nanosponges Used in Environmental Remediation Shikha Gulati, Asvika Nigam, and Sanjay Kumar

Abbreviations CA CANS CD CDNS CNS CyCaNSs EPI FIB MOF MOF-S NS

Cellulose acetate Cellulose acetate nanosponge Cyclodextrin Cyclodextrin nanosponges Cellulose-based nanosponge Cyclodextrin-calixarene nanosponge Epichlorohydrin Focused ion beam Metal-organic framework Metal-organic framework sponges Nanosponges

1 Introduction Anthropogenic activities like industrial, agricultural, and other activities are mostly responsible for the increase in environmental pollution levels in recent years [1]. New and eco-friendly solutions need to be found as soon as possible as the environment is deteriorating at an alarming rate and this will cost us a fortune. Numerous harmful S. Gulati · S. Kumar (B) Department of Chemistry, Sri Venkateswara College, University of Delhi, Delhi 110021, India e-mail: [email protected] A. Nigam Department of Chemistry, School of Natural Sciences, Shiv Nadar University, Greater Noida, Dadri 201314, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Gulati (ed.), Nanosponges for Environmental Remediation, https://doi.org/10.1007/978-3-031-41077-2_2

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gasses, such as NH3 , H2 S, SO2 , NO(x) , and volatile organic compounds, are found in air pollution. The greenhouse effect, which raises the earth’s temperature and causes glaciers to melt, is brought on by gases like CH4 and CO2 . It is imperative to halt the spread of pollution and responsibly remove it from the environment. Asthma, cancer, cardiac disorders, and skin conditions are only a few of the maladies brought on by the aforementioned pollutants. Water pollution is a problem for the environment, just like air pollution. Water that has been contaminated may contain contaminants including heavy metals, soluble dyes, toxic chemical compounds, etc. These contaminants contribute to a variety of health problems, including cardiovascular diseases, respiratory conditions, allergic responses, liver damage, etc. Therefore, it is imperative to use compounds that can remove these pollutants from wastewater. In this regard, novel technologies, such as the combination of nanotechnology and bioremediation, are urgently needed to speed up the cost-effective remediation process to remove harmful pollutants from the environment faster than traditional remediation approaches. Many studies have demonstrated that nanoparticles have unique characteristics, such as enhanced catalysis and adsorption, as well as higher reactivity. Further, the use of microbes and their extracts as potential, ecologically acceptable catalysts for manufactured nanomaterial is currently widespread. Thus, the combination of these two technologies, together known as nano-bioremediation, has the potential to drastically change environmental remediation in the long run since it is more efficient, safe, ecologically friendly, inexpensive, and green. In general nanosponges (NSs) are a particular kind of nanoparticle that are hollow sponge-like structures with multiple cavities that can hold payloads that are nanoscale in size. These manipulable nanoporous polymers were given the term “nanosponges” because of their nanometer-sized pores. NSs are colloidal polymeric hyper cross-linked structures. As NSs create inclusion complexes with lots of active molecules, this is made possible. An inclusion compound, often referred to as an inclusion complex (Fig. 1), is a type of chemical complex in which the “host” chemical molecule includes a cavity that can fit the “guest” compound. When creating inclusion complexes, NSs are known to have high solubilizing power [2]. As a result, these can accept environmental pollutants into their cavity and aid in the process of environmental restoration. This chapter discusses the different types of nanosponges, methods of synthesis, modes of action, and their applications in environmental remediation. There are several different NS types that have been shown to be highly beneficial to environmental remediation. They each have different synthesis processes, uses, and adsorption mechanisms. Among these various kinds of nanosponges are Cyclodextrins and their types (α, β, γ CDs), Titanium and Silicone based NS, Hyper-linked polystyrene-based NS, Cellulose based nanosponges, Metal ion based nanosponge, Polymer Nanosponge, Glycopolymer Nanosponges, Polyol functionalized mesoporous nanosponges, Inorganic–organic nanosponges, Pyromellitic and Citrate nanosponges, Modified and Non-modified cellulose acetate nanosponges, Cyclodextrin-Calixarene nanosponges, Carbon- coated nanosponges, Metal oxide nanosponges, MOF- based nanosponges.

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Fig. 1 The formation of an inclusion complex in water [3]. Copyright (2017) Elsevier

2 General Structure of Nanosponges Hyper-cross-linked solid nanoparticle polymers with cavities are known as nanosponges. The three primary types of nano-porous structures are nano-porous hydrogels, nano-porous membranes, and nano-porous particles. Most commonly, Nanosponges are thought to be nanoporous particles. The narrow line separating nanosponges from nanoparticles is determined by differences in size and porosity. Nanosponges are typically less than 5 μm in size and have pores that are measured in nanometers, whereas nanoparticles are measured in nanometers. The claim that nanosponges are nano-porous nanoparticles or microparticles has been made frequently. It is difficult to directly visualize their 3D structure, yet it is crucial to comprehend their features. Certain characteristics of structures, such as the presence of functional groups, the environment around hydrogen and carbon, and the elemental makeup, are predicted by spectroscopic research, etc., but microscopic research is the only way to get a detailed 3D image. However, the projected two-dimensional views of a 3D object that can be obtained from standard transmission electron microscopy images are limited. To get around this, Grunert et al. used focused ion beam (FIB) techniques to visualize the 3D structure of gold nanosponges using cross-sectional microscopic examination. Recent advances in separation, adsorption, and catalysis have been made possible by the use of nano-sized carriers, which have high interior surface areas and pore volumes. Due to their porous outer surface, nanosponges can adsorb a variety of chemical compounds depending on the size of the pore created to remove them. Ingredients are trapped inside the microscopic mesh-like structure of nanosponges. They can either entrap molecules to remove them or serve as a carrier for specific molecules thanks to their characteristic.

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3 Different Types of Nanosponges Several types of Nanosponges that are used for environmental remediation are discussed in this section.

3.1 Cyclodextrin-Based Nanosponges Because of their unusual and amphiphilic structure, cyclodextrins (CDs) are remarkable compounds. The first three parts of the oligosaccharide are made up of 6, 7, or 8 glucopyranose units and are referred to as α-, β-, or γ-cyclodextrin, respectively. They

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are natural oligosaccharides created by α-(1,4)-linked glucopyranose units. At water– solid interfaces, cyclodextrin cavities offer a hydrophobic environment and hence produce a significant affinity to organic molecules. Although polymeric cyclodextrins and organic guest molecules have a formation constant (K) that is more than eight orders of magnitude greater than molecular cyclodextrin systems in water, the process is nonetheless totally reversible in organic solvents like ethanol. According to these findings, which were assessed by ion-trap mass spectrometry and UV–visible spectroscopy, dangerous organic pollutants may be reduced to parts-per-trillion levels in water by these polymers. A family of cyclic glucopyranose oligomers known as cyclodextrins (Fig. 2) are created by enzymatic reactions on hydrolyzed starch. They feature distinctive toroidal forms that produce cavities with well-defined cone shapes. Depending on the number of oligomers, these cavities range in size from 5 to 9 in diameter and 8 in-depth. Different tiny chemical compounds with a geometry suitable for their tubular cavities can be included in cyclodextrins. The majority of the inclusion is dependent on hydrophobic interactions between the guest chemical molecules and the host cyclodextrin. In fact, it was discovered that cyclodextrins and long alkyl chains may form inclusion complexes. These aromatic compounds included substituted benzenes. The absorption or separation of different chemical agents can be based on the interactions between cyclodextrins and organic molecules. Cyclodextrins are soluble in water and some organic solvents, though. Additionally, the range of their inclusion formation constant (K) is only between 10 and 1000. Cyclodextrins cannot be used directly to separate organics from water because of their solubility in water. Cyclodextrins are frequently immobilized on solid particles as a stationary phase for enantiomer separations to get around the solubility problems [4]. β-cyclodextrin It has been shown that β-cyclodextrin and its variants are effective at removing heavy metal ions from water. For many years, activated carbon has been employed in this manner. However, there are significant drawbacks to activated carbon, including low selectivity, issues with regeneration, and delayed, non-specific uptake. The use of beta-cyclodextrin has been made in order to address these drawbacks. Beta-CD creates inclusion complexes by adsorbing heavy metal ions from wastewater. BetaCD’s hydrophilic outside and hydrophobic interior help to create complexes with heavy metal ions. Studies revealed that β-CD and its derivatives were more effective at absorbing, separating, and detecting heavy metal ions, pointing to potential uses for their removal [5]. Generally speaking, organic solvents do not make beta-CDs soluble; however, this is not always the case. There are several ways to make Beta-cyclodextrinbased nanosponges such as approaches for interfacial phenomena, hyper-cross-linked cyclodextrin, ultrasound-assisted manufacturing, and emulsion solvent evaporation. Like its alpha and gamma cousins, beta-CD is highly soluble in water. It cannot be used to directly purify water due to its solubility. From this point on, beta-CD must be

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Fig. 2 Pictorial representations of a chemical structure of a cyclodextrin molecule and the different types of cyclodextrin molecule [3]. Copyright (2017) Elsevier

modified. The beta-CD is typically crosslinked using the appropriate reagents. The process of copolymerizing the hydroxyl of CD with two or more functional groups on other substances, such as epoxide, is known as cross-linking. The most popular crosslinking agent for bulk polymerization is epichlorohydrin (EPI) [6]. As a result, branched beta-cyclodextrin polymers with low water solubility are produced. The resulting polymer becomes insoluble in water if the level of crosslinking is high enough, making them suitable for use in water treatment. α-cyclodextrin Alpha-cyclodextrins (α-CDs) are cyclic oligosaccharides that include six 1,4-linked d-glucose chains. The glucose chair conformation of the CDs causes them to be arranged as a hollow truncated cone, which gives the CD inner cavity a hydrophobic nature in contrast to the hydrophilic external surface. Moderate water solubility, tastelessness, odorlessness, thermal stability (up to 200 °C), and stability in both alkaline and acid solutions are among the functional qualities of -CDs [7].

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γ-cyclodextrin One of the three popular types of cyclodextrin is γ-cyclodextrin. Eight glucose units make up the molecule, which has the appearance of a hollow, truncated cone with a hydrophilic exterior and a hydrophobic interior. The molecule is more preferred than its α and β counterparts due to the higher amount of glucose units, and it can form inclusion bodies with steroids and macrocycles. The molecule has a solubility of 232 g/L, 25 °C due to γ-Cyclodextrin flexibility and non-coplanar structure. It is non-toxic, biodegradable, and beneficial to the environment.

Modified CD Nanosponges The CD nanosponges have been modified with a range of functional groups and nanomaterials to improve their characteristics and effectiveness for the removal of more complicated water contaminants. The most common chemical modifications of nanosponge CD insoluble polymers which have been used for water treatment include modification of CD with ionic liquid, copolymerization of CD with agricultural products (beetroot fibers), carbon nanotubes (CNTs), nanofibers, alginates beads, carbon spheres, dendrimers; the use of nanoparticles or nanocatalysts; the addition of titanium, silicon, and incorporation into membrane creating composites of biological and inorganic materials. The papers discussed in this review mostly centre on the chemical alterations of insoluble polymers in CD nanosponges with functionalized carbon nanotubes, nanocatalysts (TiO2 ), or nanoparticles (Ag) [8].

3.2 Titanium-Based Nanosponges An essential substance for photochemical and photoelectrochemical properties is TiO2 . It serves as an environmental sanitizer (Fig. 3). Titanium-based nanosponges could be created by combining TiO2 with metal nanoparticles and have been employed in a variety of applications, including gas sensors, photocatalysis, solar cells, biomedicine, and water filtration are a few examples. Their synthesis could be accomplished using a variety of methods, such as sol–gel, anodization, hydrothermal, and atomic-layer deposition. TiO2 is functionalized onto the surface of polystyrene nanospheres to create core– shell nanoparticles, which are then transformed into TiO2 nanosponges by simple calcination in a tube furnace. This is followed by the phase transformation from the anatase to the rutile phase [10]. With a Reynolds number greater than zero, Fernandez-Domene et al. created several morphologies of TiO2 -based nanosponges made up of various anatase–rutile phases. When compared to nanotubes, these nanosponges have a larger surface area and greater conductivity. Also, the density of the donor species decreases with

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Fig. 3 Synthesis of titanium-based nanosponges [9]. Copyright (2012) American Chemical Society

increasing Reynolds number, i.e., nanosponges are less defective than nanotubes [11]. Due to their low recombination probability, nanosponges exhibit substantially higher photoelectrochemical activity than nanotubes. By doping Li+ cations, TiO2 nanosponges are further functionalized. As a result, resistance is reduced and conductivity is increased. Due to more designed defects in the case of doping, the flat band density and the density of the donor sites both rise, which improves their photocatalytic activity for the photodegradation of diverse contaminants [12].

3.3 Silicone-Based Nanosponges Silicon-based nanosponges are another class of nanosponges having a porous structure and high activity. The development of electrochemical sensors and biosensors has made use of materials with silica chemistry known as silicon-based nanosponges as potential electrode modifiers. A metallurgical grade silicon powder is used to make silicon nanosponge particles, which are created by scratching nearly 1–4 micronsized particles. To create silicon nanosponges, Chadwick et al. used electrochemical

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Fig. 4 Schematic illustration of Silicone nanoparticle-based anode. Creative Commons

etching in a solution of hydrofluoric acid. A porous structure is created as a result of the impurities that are found on the surface of silicon particles. The process by which pores form and the composition of porous silicon nanosponges results from the formation [13]. Ultrasonic waves can also be used to etch silicon substrates in order to create organized pore architectures. The porous structures are also produced by grinding and ball milling the powdered silicon and by further etching. Chemical etching has also been utilized, but the mechanism has not yet been fully clarified; as a result, disordered pores that are challenging to analyze are produced [14]. Al2 Si2 O5 (OH)4 aluminosilicate nanosponges with kaolinite structure were created by Yu and colleagues (Fig. 4). These nanosponges are then applied for the removal of cationic dye, methylene blue as well as anionic dye, azorubine. As the surface area of the nanosponges is found to be higher compared to nanoplates, therefore their sorption is also much higher than nanoplates [15]. In the fields of sensors, catalysts and medicines, photosensitizers, adsorbents, explosives, and electrodes for fuel cells, the very porous silicon NS serves as a carrier material. Additionally, it serves as a precursor for high surface area ceramic materials like SiNa and SiC.

3.4 Hyper-Linked Polystyrene-Based Nanosponges Polystyrene is a biodegradable material, easily recyclable, and inert in air. Hyper cross-linked polymers are a type of nano-porous materials made up of polystyrene polymer and consist of a variety of present and prospective uses, including drug administration, chromatographic separation, gas sorption and separation, and heterogeneous catalysis. Individual polystyrene coils were suspended in diluted solvents, and then massive volumes of stiff intramolecular bridges were added, causing these coils to strongly constrict and eventually form spherical NSs. The NS solutions had high rates of sedimentation, low viscosity, and high diffusion. These NSs had increased interior

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surface areas and strong swell for linear polystyrene [16]. By using the concepts of size exclusion chromatography, the hyper-cross-linked NS was utilized in the proper separation of inorganic electrolytes. Tissue scaffolds have been made of hyper-crosslinked polystyrene NS, cyclodextrin-based NS, and cyclodextrin-based NS.

3.5 Cellulose-Based Nanosponges An ecotoxicological assessment based on a biological effect-based approach is presented in order to promote the application of nano-adsorbent materials for the removal of heavy metal ions from seawater and prevent any potential negative effects on marine life. Artificial seawater that had been contaminated with ZnCl2 was treated using recently created eco-friendly cellulose-based nanosponges. A nanostructured cellulose sponge suitable for removing heavy metal ions from water is created by cross-linking 2,2,6,6-tetramethylpiperidine-1-oxyl-oxidized and ultrasonicated cellulose nanofibers in the presence of branched polyethyleneimine. CNS has a 2D sheet-like shape that is characterized by a microporosity that can be seen using scanning electron microscopy and a nanoporosity that can be seen through a small angle neutron scattering examination that is done in-depth. The environmental safety (ecosafety) of CNS is achieved by utilizing an eco-design approach capable of combining life cycle assessment and environmental concerns. More recently, it was discovered that the adsorption efficiency of CNS from seawater is better than from freshwater [17].

3.6 Metal Ion-Based Nanosponge By utilizing ammonia borane as a reducing agent in water, metal nanosponges were produced using a straightforward reduction technique. A thorough understanding of the nanosponge generation mechanism in the solution state was attained by using various amine-boranes with varying reduction abilities. These metal nanosponges differ from their bulk counterparts in terms of their hydrogen sorption properties and catalytic activity.

3.7 Polymer Nanosponge Tannic acid covalently cross-linked with -cyclodextrin via a condensation reaction can be used to create polymer nanosponges with high selectivity/sensitivity for removing lead from wastewater, and the phenolic hydroxyl groups of tannic acid have the ability to bind lead effectively, forming stable structures.

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3.8 Glycopolymer Nanosponges Fischer glycosylation, click reaction, and cross-linking reaction were used to create glycopolymer nanosponges from monosaccharides and beta-cyclodextrin in order to remove boron from water. Significant gains in adsorption rates and removal abilities may be caused by secondary bonding, such as van der Waals forces and hydrogen bonds, between the integrated saccharides and the adsorbates, offering functional candidates for water treatment and seawater desalination.

3.9 Polyol Functionalized Mesoporous Nanosponges In various processes, including water treatment, boron extraction, and saltwater desalination, quick removal of boric acid is crucial. Ion-exchange resins, although being the most popular boron-specific adsorbents, are frequently criticized due to their low adsorption capacity and sluggish adsorption rate. Mesoporous nanosponges with cis-diols functionalized on the surface are synthesized using a copper-free method, and they are utilized to quickly remove organic micropollutants and boric acid from water. In order to control the surface morphology and porosity structure of cyclodextrin (CD)-scaffolded nanosponges, various cross-linking agents are applied. The Staudinger reaction is then conducted to generate amine groups on the primary face of CD for subsequent reaction with d-(+)-gluconic acid δ-lactone, which successfully immobilizes high-density cis-diols on the surface for efficient chelating with boric acid. While the time to establish equilibrium adsorption is drastically shortened from up to 2 days to tens of minutes (up to 60 times faster), functional polymer nanosponges have demonstrated equivalent boron adsorption capacity to commercial resins. These porous polymers quickly absorb bisphenol A, and equilibrium adsorption can be reached in under two minutes. As a result of the secondary bonding, which simultaneously reduces the adsorption time and increases the adsorption capacity, the presence of immobilized polyols suggests a synergistic impact.

3.10 Inorganic–Organic Nanosponges Inorganic–organic nanosponges are produced by microwave irradiation methods using halloysite clay (halloysite nanotubes) and organic cyclodextrin derivatives in a solvent-free environment. Rhodamine B and other cationic/anionic dyes are removed using these nanosponges as nanoadsorbents. Notably, electrostatic interactions and the pH of the solution might affect the adsorption process. These hybrid nanosponge materials allow for the highly selective adsorption of cationic dyes, which have higher adsorption efficiencies than anionic dyes.

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3.11 Pyromellitic and Citrate Nanosponges By reacting β-cyclodextrin and line caps with pyromellitic dianhydride in dimethyl sulfoxide, pyromellitic nanosponges are created. At a metal concentration of 500 ppm, pyromellitic nanosponges show greater retention potentials than citrate nanosponges. However, at low metal concentrations (50 ppm), these citrate and pyromellitic nanosponges show better retention potentials (94%). In contrast to the other created nanosponge, the citrate nanosponge may selectively adsorb sizable amounts of heavy metals in the presence of interfering salts from seawater.

3.12 Modified and Non-Modified Cellulose Acetate Nanosponges It has been found to be effective to employ dithizone-modified cellulose acetate nanosponges as an adsorbent in a batch process to remove Pb(II) and Cd(II) ions from contaminated waters. Recently, noxious compounds have been removed from water using cellulose acetate (CA) adsorbent derivatives such as silver-loaded cellulose acetate hollow fiber membranes, cellulose acetate/polyethyleneimine blend microfiltration membranes, epoxy functionalized poly (ether-sulfone) incorporated CA ultrafiltration membranes, and so on. Lead and cadmium ions have recently been adsorbed to cellulose. Even more effective than unmodified CANSs are acetate nanosponges that have been treated with dithizone, a substance that can be strongly complex with lead and cadmium ions. The proposed approach exhibits considerable potential for rapid and efficient Pb(II) and Cd(II) adsorption from aqueous solutions via a simple process.

3.13 Cyclodextrin-Calixarene Nanosponges Cyclodextrin-calixarene nanosponges (CyCaNSs) have been utilized as sorbents to remove Pb2+ ions from aqueous solutions. The adsorption studies were carried out on solutions without and with the addition of background salts, under varied operational settings, taking into account that the removal treatments may involve polluted waters with different characteristics. According to Fontana et al., Lo Meo et al., and Massaro et al., it is produced by co-polymerizing cyclodextrin and calixarene derivatives, which are connected by 1,2,3-triazole linker units. These materials (referred to as CyCaNSs from here on) gain from the presence of two distinct co-monomers with somewhat complementary supramolecular host abilities as well as from their viable chemical post-modification, which makes it simple to add additional functionalities, like amine or carboxyl groups, to the polymeric network [18].

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3.14 Carbon-Coated Nanosponges By using a straightforward adsorption technique, activated carbon, an inert solid adsorbent material, is frequently utilized to remove different contaminants from water and industrial gas-phase streams. The unique metallic nanoparticles were completely covered in a very thin coating of activated carbon. Carbon-coated metallic nanosponges are a type of three-dimensional metallic nanosponges. The metal-impregnated fibers are heated to a very high temperature, where they remain until the carbon is vaporized, the metal is sintered, connects to another metal, and forms nanosponges. Using the carbonized fiber as a template, the nanosponges are created in the shape of tubes with a higher surface-to-volume ratio. The inert substrate on which the previously synthesized nanoparticles are located causes them to only have a two-dimensional active surface area. However, the threedimensional active surface area of nanosponges is available, improving the adsorption propensity of pollutants [19]. Additionally, this is utilized as a catalyst or to strengthen polymers [20].

3.15 Metal Oxide Nanosponges Another type of nanosponges is those made of porous metal oxide. These have a higher mass transfer rate, a larger surface area, high electron mobility, and smaller particle sizes. The magnetic characteristics, surface energy, and semiconductor band gap of these materials are dependent on the type and size of metal oxide, which might vary according to the needed use. For a variety of uses, mono, bi, and polymetallic oxide nanosponges are being created. Mono-metal oxide nanosponges have aggregation issues, therefore because of their smaller surface area, they are not used in industrial applications. Bi- and polymetallic oxide nanosponges, however, improve the physical and chemical properties and can be employed in a variety of ways. Using a Cu–Zn–TiO2 nanotube array, Liu et al. eliminate nitrate with a 97.5% removal rate from groundwater. Temperature changes have a significant impact, whereas pH changes have no impact on nitrate elimination [21]. Titania nanosponges that are extremely selective for hydrogen gas were created by Zururi et al. Metal oxide nanosponge’s three-dimensional linked structure gives rise to sensors with extremely high chemical sensitivity [22].

3.16 Metal–Organic Framework Nanosponges The extremely crystalline inorganic–organic hybrid materials known as Metal– Organic Frameworks (MOF) have porous architectures that resemble sponges. MOF sponges are typically MOF with a diameter between 0.5 and 1 nm that exhibit good

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Fig. 5 Schematic representation of synthesis and functionalization of MOF. Creative Commons

adsorption capability (MOF-S) [23]. The MOF is made up of an inorganic cluster of metal ions connected by organic linkers to form a cage-like structure. A wide variety of MOFs are being synthesized as a result of the pore size, dimensionality, and chemical environment variations. MOF is a good option for catalysis in a variety of processes due to its extremely large surface area and highly porous character. Secondary building units (SBUs), a crystalline network created by the reticular synthesis of metal ions and organic linkers, are one type of crystalline structure. This SBU coordinates and assembles the three-dimensional MOF-S structure as depicted in Fig. 5. The building and MOF-S’s chemical characteristics can be changed and tailored for certain uses. Additionally, the organic linker can be tweaked to change the pore size, and the functionalization can be changed to create MOF-S, which has improved characteristics. As a result of their unique functional groups’ high levels of specificity and selectivity, MOF-S can be utilized to detect metal ions, gasses, and tiny molecules [24]. By precisely eliminating hazardous gasses from the environment without affecting other chemicals, MOF-S are particularly successful at purifying the air.

4 Applications of Different Types of Nanosponges in Environmental Remediation Recently, these nanosponges have been explored for environmental applications also and receiving huge success in this regard. Environmental applications such as wastewater treatment, adsorption of poisonous gasses, sensors, and cleaning up of oil spills are presented here.

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Serial No.

Applications

Method

(1)

Wastewater treatment

CDNS is found very effective for wastewater treatment. They are cost-effective and applicable for numerous pollutants removals like heavy metals, dyes, drugs, pesticides, organic pollutants, and phenolic compounds. They can form host–guest interaction via complex formation due to the presence of a hydrophobic cavity present inside CDNS. Due to the presence of active sites on the surface, they can undergo electrostatic interactions on the surface of CDNS. The presence of pores in CDNS helps to diffuse due to the hydrophilic nature of CDNS

(2)

Adsorption of poisonous gases

In the last few years, various methods are being employed for the removal of these gases like ionic liquids, organic polymers, zeolites, and MOFs-based nanosponges. MOF sponges can be used for environmental remediation by adsorption, storage, or purification of harmful gases. They are being used for the removal of harmful gases like hydrogen sulphide, sulphur dioxide, carbon dioxide, ammonia, and methane

(3)

Sensors

Earlier, nanobelts, nanotubes, and nanorods were applied for gas sensing applications. But recently three-dimensional cross-linked metal oxide polymer nanosponges have attracted great attention due to their high surface area and porous nature, they increase the rate of a chemical reaction and could be used for the detection of gas. ZnO and TiO2 NSs are used to detect gases

5 Conclusion The numerous uses of NS have created a wide range of opportunities for environmental remediation. Due to their very porous nature, high surface area, high selectiveness, and specificity, nanosponges are a great option for cleaning the environment and improving the effectiveness of the classic clean-up methods. The molecules are encapsulated, which increases flexibility and stability. They feature a diversity of structural domains and are soluble because they contain both hydrophobic and hydrophilic moieties. As they are simple to synthesize and functionalize, they get around the restrictions of conventional techniques employed for environmental cleanup. There are several different NSs, which we have looked at in this chapter. We can select the type of NS based on the requirements and anticipate positive outcomes because these various forms of NSs have distinct properties, such as adsorption rates, formation constants, and much more. Additionally, it is urged to develop a high-yield, reasonably priced, reproducible technique that can be easily changed for mass manufacturing. Although it is simple to create nanosponges, their main drawback is the presence of solvent remnants or reaction byproducts, which significantly contribute to the growth of waste in the environment. Therefore, it’s critical to develop processes

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that either produce fewer residues or transform them into less dangerous molecules to reduce their negative effects on the environment.

References 1. Utzeri, G., Matias, P. M. C., Murtinho, D., & Valente, A. J. (2022). Cyclodextrin-based nanosponges: Overview and opportunities. Frontiers in Chemistry, 10, 859406. 2. Baglieri, A., Nègre, M., Trotta, F., Bracco, P., & Gennari, M. (2013). Journal of Environmental Science and Health—Part B Pesticides, Food Contaminants, and Agricultural Wastes, 48(9), 784–792. 3. Leudjo Taka, A., Pillay, K., & Yangkou Mbianda, X. (2017, March 1). Carbohydrate polymers. Elsevier. 4. Li, D., & Ma, M. (2000). Clean Products and Processes, 2, 112–116. 5. Xia, Y. (2008). Preparation and adsorption of novel cellulosic fibers modified by β cyclodextrin. Polymers for Advanced Technologies, 19(4). 6. Yuan, C., Bai, Y.-X., & Jin, Z.-Y. (2013). Preparation and analysis of cyclodextrin derivatives. In Cyclodextrin chemistry. 7. Câmara, A. K. F. I., de Souza Paglarini, C., Vidal, V. A. S., & dos Santos, M., et al. (2020). Meat products as prebiotic food carrier. Elsevier BV. 8. Taka, A. L., Pillay, K., & Mbianda, X. Y. (2017). Nanosponge cyclodextrin polyurethanes and their modification with nanomaterials for the removal of pollutants from waste water: a review. Carbohydrate Polymers. 9. Golubeva, O. Y., Alikina, Y. A., & Khamova, T. V., et al. (2021, November 1). Inorganic chemistry. American Chemical Society. 10. Guo, L., Gao, G., Liu, X., & Liu, F. (2008). Preparation and characterization of TiO2 nanosponge. Materials Chemistry and Physics, 111, 322–325. 11. Fernández-Domene, R. M., Sánchez-Tovar, R., Sánchez-González, S., & García-Antón, J. (2016). Photoelectrochemical characterization of anatase-rutile mixed TiO2 nanosponges. International Journal of Hydrogen Energy, 41, 18380–18388. 12. Blasco-Tamarit, E., Muñoz-Portero, M.-J., Sánchez-Tovar, R., Fernández-Domene, R. M., & García-Antón, J. (2018). The effect of Reynolds number on TiO2 nanosponges doped with Li+ cations. New Journal of Chemistry, 42, 11054–11063. 13. Chadwick, E. G., et al. (2013). Compositional characterisation of metallurgical grade silicon and porous silicon nanosponge particles. RSC Advances, 3, 19393–19402. 14. Chadwick, E. G., Beloshapkin, S., & Tanner, D. A. (2012). Microstructural characterisation of metallurgical grade porous silicon nanosponge particles. Journal of Materials Science, 47, 2396–2404. 15. Golubeva, O. Yu., Alikina, Y. A., Khamova, T. V, Vladimirova, E. V., & Shamova, O. V. (2021). Aluminosilicate nanosponges: Synthesis, properties, and application prospects. Inorganic Chemistry, 60, 17008–17018. 16. Davankov, V. A., Ilyin, M. M., Tsyurupa, M. P., Timofeeva, G. I., & Dubrovina, L. V. (1996). From a dissolved polystyrene coil to an intramolecularly-hyper-cross-linked “nanosponge.” Macromolecules, 29, 8398–8403. 17. Liberatori, G., Grassi, G., Guidi, P., Bernardeschi, M., Fiorati, A., Scarcelli, V., Genovese, M., Faleri, C., Protano, G., Frenzilli, G., Punta, C., & Corsi, I. (2020). Nanomaterials, 10(7), 1–20. 18. Cataldo, S., Lo Meo, P., Conte, P., Di Vincenzo, A., Milea, D., & Pettignano, A. (2021). Carbohydrate Polymers, 267. 19. Lian, K. (2017). Nanoparticles, nanosponges, methods of synthesis, and methods of use. Preprint. 20. Lian, K., & Wu, Q. (2009). Carbon-encased metal nanoparticles and sponges, methods of synthesis, and methods of use. Preprint.

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21. Liu, F., et al. (2016). Fabrication and characterization of a Cu-Zn-TiO2 nanotube array polymetallic nanoelectrode for electrochemically removing nitrate from groundwater. Journal of the Electrochemical Society, 163, E421. 22. Zuruzi, A. S., MacDonald, N. C., Moskovits, M., & Kolmakov, A. (2007). Metal oxide “nanosponges” as chemical sensors: Highly sensitive detection of hydrogen with nanosponge titania. Angewandte Chemie, 119, 4376–4379. 23. Moghadam, P. Z., et al. (2017). Development of a cambridge structural database subset: A collection of metal-organic frameworks for past, present, and future. Chemistry of Materials, 29, 2618–2625. 24. Goyal, N., & Rai, R. K. (2022). Recent progress in the synthesis and electrocatalytic application of metal–organic frameworks encapsulated nanoparticle composites. In S. Gulati (Ed.), Metalorganic frameworks (MOFs) as catalysts (pp. 731–764). Springer Nature Singapore. https:// doi.org/10.1007/978-981-16-7959-9_27

General Synthetic Routes for Various Nanosponges Lakshita Chhabra, Anoushka Amar, Shikha Gulati, and Rajender S. Varma

Abbreviations APF CD DPC HPTEM NSs

Aminophenol/formaldehyde Cyclo Dextrin Diphenyl carbonate High-resolution-transmission electron microscopy Nano Sponges

1 Introduction There are various pollutants such as organic dyes, heavy metal ions, and phenolic chemicals that are prevalent in the environment and can be adsorbed using nanosponges (NSs) [6], a three-dimensional porous structure with holes at its centre. The usage of NSs can be viewed as a cost-effective method with low energy and time requirements when compared to other often deployed environmental remediation technologies. Because of their cone-like structure, high surface area, ease of formation of host–guest complexes, and presence of various functionalities, cyclodextrinbased nanosponges—among the many available nanosponges—have garnered enormous attention and have attained the status of cutting-edge substances for adsorption. L. Chhabra · A. Amar · S. Gulati (B) Department of Chemistry, Sri Venkateswara College, University of Delhi, Delhi 110021, India e-mail: [email protected] R. S. Varma (B) Department of Chemistry, Centre of Excellence for Research in Sustainable Chemistry, Federal University of São Carlos, São Carlos, São Paulo 13565-905, Brazil e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Gulati (ed.), Nanosponges for Environmental Remediation, https://doi.org/10.1007/978-3-031-41077-2_3

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Further research ought to be undertaken on these polymers with unique physicochemical characteristics, topologies, and strongly cross-linked three-dimensional networks to remove not only microbiological contaminants but other pollutants from the environment as well. Furthermore, the surface functionalization of these nanosponges can significantly improve their environmental remediation attributes. In general, NSs can be manufactured by combining organic or inorganic substances [3]. The typical procedure for creating NSs entails dissolving the selected cyclodextrin (CD) in a suitable solvent (often an aprotic organic solvent), adding a catalyst if necessary, followed by addition of a crosslinker [12] while continuously stirring or sonicating the mixture [3]. If the cross-linker is liquid and capable of solubilizing the CDs, melt polymerization can also be carried out. It may occasionally be necessary to raise the temperature in order to start the crosslinking process. Precipitation polymerization or a sol–gel method can be used to describe the complete reaction, and both can result in the development of monolithic blocks or, in the latter instance, newly created polymers that are precipitated from the liquid phase [12]. In addition to these processes, various synthesis pathways for NSs have been investigated. In the described CD-based NSs interfacial polymerization, two immiscible solutions—one created by combining CDs with an alkaline solution and another by blending the selected cross-linker with a chlorinated solvent—are strongly mixed and agitated together [3]. Another example is the production of CD-based NSs by straightforward dehydration process. In the presence of adequate catalyst, the CD and the cross-linker— typically an acid with two or more carboxylic groups—are dissolved in water. The surplus water used initially as a solvent and the water released from the cross-linking condensation reaction is subsequently removed by heating and under low pressure in some instances [10, 29]. The majority of CD-based nanosponges (CD-NSs) documented in the litrature are prepared using step-grow polymerization processes; other manufacturing techniques, including rapid polymerization, require multistep processes because it may be necessary to derivatize the deployed CD’s first [5, 25]. This chapter offers an overview of several methods employed for the synthesis of NSs such as the solvent method, ultrasound-assisted method, melt method, bubble electrospinning, synthesis by the use of microwave radiation from hyper-crosslinked β-cyclodextrin, emulsion solvent diffusion method, and the Quasi emulsion solvent method.

2 Components Deployed in the Synthesis of Nanosponges Polymers and cross-linkers are the main components that are required to synthesize NSs. There are several materials that have produced promising results and are capable of being utilized to create NSs, depending on the type of NSs desired and the necessary amount of cross-linking. Due to its impact on the desired application and encapsulation, the amount of crosslinking is a crucial component of NSs. The

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Table 1 Components utilized in the synthesis of nanosponges [22] Polymer

Copolymer

Crosslinker

Polar solvents

Hyper crosslinked Polystyrene Cyclodextrin (alkoxy carbonyl cyclodextrins)

• Poly(valerolactone allyl valerolactone) • Poly(valerolactone allylvalerolactone oxypanedione)

• Carbonyl diimidazole • Carboxylic acid dianhydrides

• Ethanol • Dimethylacetamide

Methyl-β-Cyclodextrin Ethylcellulose

Diaryl carbonates

Dimethylformamide

Hydroxy propyl β-cyclodextrin

Dichloromethane

Polyvinyl alcohol

Poly-valerolactone

Di isocyanates

Eudragit RS100

Glutaraldehyde

Acrylic polymer

• Pyromellitic anhydride • 2,2-bis(acrylamide) Acetic Acid

examples of the substances that were deployed in the preparation process of NSs are presented in Table 1 [22].

2.1 Polymers Hyper cross-linked polystyrenes, cyclodextrins, alkyloxy carbonyl cyclodextrins, 2-hydroxy propyl cyclodextrins, and copolymers such as poly (valerolactone-allyl valerolactone) and poly (valero lactone-allyl valerolactone) are among the polymers deployed to fabricate NSs [18].

2.2 Crosslinkers Diphenyl carbonate, diaryl carbonates, diisocyanates, pyrrolidine, carbonyl diimidazoles, epichloridrine, glutaraldehyde, carboxylic acid dianhydrides, 2,2bis(acrylamido)acetic acid, and dichloromethane are some of the crosslinkers utilized to create Nano sponges [21].

3 General Methods for Synthesis of Nanosponges (NSs) There are several procedures that can be employed to synthesize NSs and they are briefly discussed here (Fig. 1).

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1.Solvent method 1.Quasi emulsion solvent method

1.Emulsion solvent diffusion method

1.Ultrasoundassisted method

Methods of Synthesis of NSs

Synthiesis from hyper crosslinked βcyclodextrin

1.Bubble electrospinning

1.Melt method

1.Microwave radiation method

Fig. 1 Different methods for the synthesis of nanosponges (NSs)

3.1 Solvent Method This process exploits the use of suitable solvents, such as the polar aprotic solvents like dimethylformamide and dimethyl sulfoxide, which were then carefully mixed with the polymer (Fig. 2). The optimal crosslinker/polymer ratio has been 4:1 to which the aforementioned mixture was applied. Following the mixing, the combination was allowed to react for 48 h at a temperature range of 10 °C and up to the solvent’s reflux temperature. After the completion of reaction, the solution was cooled to the ambient temperature [16]. To extract the product from the ensued solution, excessive amounts of distilled water was added and the product was subsequently recovered under vacuum filtering. The resulting product was vacuum-dried and then mechanically milled to create a uniform powder [15].

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Fig. 2 A Solvent method B Ultrasound-assisted synthesis C Emulsion solvent diffusion method D Melting method. Reprinted with permission from Jain et al. [7]. Copyright 2020 Elsevier

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3.2 Ultrasound-Assisted Method The ultrasonic technology with polymer combination has been used in the ultrasoundassisted method of synthesis for NSs (Fig. 2). Without the use of a solvent, polymer crosslinking occurred via the use of ultrasonic vibrations. Using this technique, NSs were created by combining crosslinkers and polymers without the need for a solvent while being sonicated. This technique generated spherical, uniform-sized NSs [1]. In a flask, the crosslinker and polymer were combined in a certain molar ratio. The flask was heated to 90 °C by submerging it in a water-filled ultrasonic bath, 5 h of sonicating time was adequate in most cases. After allowing the entire mixture to cool down, the final crude product was separated. To remove the non-reacted polymer, the product was rinsed with water. It then underwent purification using a protracted Soxhlet extraction process with ethanol. The finished product was vacuum-dried and kept at 25 °C for further use [1, 15].

3.3 Melt Method In the melting process, the crosslinker and the polymer are combined by melting wherein the components are all thoroughly blended together (Fig. 2). The acquisition product is washed multiple times with an appropriate fluid to collect NSs. The product is cleaned, the waste polymer and unreacted chemicals are removed, and it is then segregated into NSs. Further experimentation for drug encapsulation is performed using such pure NSs [14].

3.4 Emulsion Solvent Diffusion Method This approach employed both diffused and continuous phases wherein NSs were generated by mixing ethyl cellulose and polyvinyl alcohol in various ratios or amounts (Fig. 2). Solunke et al. prepared the NSs by dissolving the target medication and ethyl cellulose that make up the dispersed phase in 20 mL of dichloromethane. 150 mL of the continuous phase (aqueous) was then added with some polyvinyl alcohol (PVA). The blend was then agitated for about two hours at a speed of 1000 rpm. The final NSs product was gathered via filteration and was dried in an oven at 400 °C [20].

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3.5 Bubble Electrospinning A strong-voltage source, a grounded collector, a syringe, and a pump for the syringe are the main components needed in a traditional and common electrospinning arrangement. But the volume of production of nanofibers is one of the main limitations that restrict their uses. Polyvinyl alcohol is an additional polymer that can be utilized in bubble electrospinning where the solution (10%) is prepared by adding distilled water to it. This solution is then heated between 80 and 90 °C for two hours to create a single-phase mixture. The solution of polymer was then allowed to reach room temperature before being employed to create nanoporous fibres [24].

3.6 Microwave Radiation Method This straightforward method of microwave (MW) irradiation synthesis of cyclodextrin nanosponges (CD NSs) greatly reduced the reaction time. When NSs were created using MW radiation, the reaction time was four times faster than when it was prepared using the conventional heating techniques. A uniform distribution of particle sizes and consistent crystallinity were also the hallmark of this process. In order to compare the advantages of MW-assisted heating to traditional heating during the synthesis of CDbased NSs, Singireddy et al. [19] performed a research study. It was revealed that the NSs produced by MW synthesis were extremely crystalline, exhibited higher levels of complexity, and had a limited size distribution, according to the results of highresolution transmission electron microscopy (HRTEM). During MW-assisted heated conditions, the response times for all processes were significantly shortened, and the outcomes of the reactions were enhanced. [19]The advantage of employing radiation from microwaves for synthesis is that it delivers direct energy to the desired polar molecules, thus providing energy to the molecules rather then heating the container’s walls or the liquid (solvent) next to the reactant molecules. A MW synthesizer was employed by Zainuddin et al. [26] to create para crystalline-CD, where diphenyl carbonate (DPC) was used for cross-linking [2].

3.7 Preparation of NSs from Hyper Crosslinked β-cyclodextrin Based on the crosslinker utilized in a neutral or acidic structure, NSs can be combined; they comprise solid components that have undergone modification in their crystalline form to show their dissolvability with unique structures (Fig. 3) [23]. They are utilized to increase the liquid dissolvability of medications when aqueous solution are not adequate for solubilization [4].

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Fig. 3 Cyclodextrins linked with cross-liking polymer to form cyclodextrin nanosponge. Reprinted with permission from Sherje et al. [17]. Copyright 2017 Elsevier

3.8 Quasi Emulsion Solvent Method The polymer in this case was used to assemble the NSs in various computations. The interior tier is prepared and applied to a very dissolvable level using Eudragit RS 100 which elicited a reaction and degraded at 35 °C when subjected to ultrasonication [13]. This intrinsic procedure, which is used in the polyvinyl alcohol-containing outer phase, functions as an emulsifying operator. The mixture is mixed at 1000–2000 rpm for three hours at room temperature before being dried for 12 h in an air-warmed oven at 40 °C [22].

4 Important Functionalization Strategies for Nanosponges The NSs functionalized with metal oxide species have garnered significant attention. The “empty” pores of nanoparticles are creatively transformed into “sponge-like” 3D nanospace by filling them with resoles rich in metallic anchors, which effectively fit high-content metallic elements while retaining their strongly dispersed status throughout the production process. This contrasts to directly altering metal species onto nanoparticles as performed through conventional approaches. This method can be applied to functionalize various single and binary metal compositions using highly distributed metals [27]. The well distributed copper species have been created using this method which demonstrate outstanding Fenton-like catalytic activity and improved cancer cell inhibition capabilities. These discoveries offer fresh perspectives and practical methods for creating metal-functionalized nanomaterials for cutting-edge applications [27]. Due to their monodispersity, simplicity of synthesis, and abundance of functional groups, nanostructured resoles have been increasingly popular in recent years as templates for the fabrication of complex nanostructures [9, 28]. The 3-aminophenol/ formaldehyde (APF) nano-resoles outperformed other resoles in terms of their capacity to accommodate metal ions, resulting in metal-based nanostructures with adaptable morphologies and compositions [8, 11]. This may be explained by the APF

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framework’s numerous microporous metal binding sites, which enable the immobilization of metal ions as precursors into “sponge”-like APF resoles. This unusual behavior is anticipated to be put to creative use as a novel method for the functionalization of metal oxide species onto nanomaterials. Thus, a unique 3D nanosponge (3D-NS) enabled approach has been devised to simultaneously functionalize metal oxide species with high dispersity and high content. To absorb metal ions, fractal silica nanoparticles (3D-NS@SiO2 ) are packed with sponge-like resoles in their 3D nanopores. Fractal silica nanoparticles functionalized with highly scattered sub-2 nm metal oxide species are produced after calcination in the air to eliminate resoles. It is shown that the existence of fractal silica supports and N-riching resoles are two essential elements; the absence of any factor results in aggregated metal oxide species [27]. This 3D-NS permitted metal modification method allows for the production of fractal silica nanoparticles functionalized with strongly dispersed metal oxide species and various metal arrangements, such as transition metal oxide species, lanthanide metal oxide species, and binary metal oxide species. Highly distributed copper species made with this technique exhibit exceptional Fenton-like catalytic activity and improved cancer cell inhibition capabilities as a proof of concept [27].

5 Conclusion Cyclodextrin-based nanosponges (CD-NSs) are insoluble, highly cross-linked 3D network polymers deployed in several scientific and technological fields. Crosslinking CDs bring significant benefits to CD-NSs relative to the respective native CDs employed. In general, CD-NSs are able to form complexes with a wide variety of molecules. This is due to the presence of interstitial spaces among CDs, which can host more hydrophilic guests. A further advantage, deriving from the use of NSs, is represented by the polymer network that surrounds the cavities and hampers the diffusion of entrapped guest molecules, thus promoting slower release kinetics. No less important is the fact that NSs are insoluble, hence they can be easily recovered from aqueous media and recycled more effectively.

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Characterization Techniques for Nanosponges Pragya Malik, Durgesh Nandini, and Bijay P. Tripathi

Abbreviations CD CD CMC CYT DEX DLS DPC DSC DTG ED EDS FESEM FTIR HDI HOBP HRMAS ICDD JCPDS MPH

Circular dichroism Cyclodextrin Carboxy methyl cellulose Cytoxan Dexamethasone Dynamic light scattering Diphenyl carbonate Differential scanning calorimetry Differential thermogravimetry Electron diffraction Energy dispersive X-ray spectroscopy Field emission scanning electron microscopy Fourier transform infrared Hexamethylene diisocyanate 2-Hydroxy-4(octyloxy)-benzophenone High resolution mass spectrometry International centre for diffraction data Joint committee on powder diffraction standards Melphalan

P. Malik · B. P. Tripathi (B) Department of Materials Science and Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India e-mail: [email protected] P. Malik · D. Nandini (B) Centre for Fire, Explosive and Environment Safety, Defence Research and Development Organisation, Timarpur, Delhi 110054, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Gulati (ed.), Nanosponges for Environmental Remediation, https://doi.org/10.1007/978-3-031-41077-2_4

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MS NMR NS PLP PMA PMDA PRX PXRD RS SEM SERS TCL TEM TFN TGA TPP UV XRD

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Mass spectrometer Nuclear magnetic resonance Nanosponge Paliperidone Pyromelettic anhydride Pyromellitic dianhydride Piroxicam Powder X-Ray diffraction Raman spectroscopy Scanning electron microscopy Surface-enhanced Raman scattering Terephthaloyl chloride Transmission electron microscopy Tetrafluoroterephthalonitrile Thermal gravimetric analysis Triphenyl phosphine Ultraviolet X-Ray diffraction

1 Introduction Nanosponges are nanoporous sponge-like structures formed by the three-dimensional crosslinking of polymers having cavities in the nanoscale. In general, the size of the nanosponges lies in the range of 10–25 μm with a cavity size of 5–300 μm [1, 2]. Most common examples of nanosponges are based on titanium, silicon, polystyrene, and cyclodextrin [3]. In 1998, DeQuan Li and Min Ma devised the term cyclodextrin nanosponges while working on beta-cyclodextrin crosslinked by diisocyanates [4]. Cyclodextrin nanosponges are mesh-like structures formed by crosslinking of cyclodextrin polymer which is an oligomer of d-glucopyranose units joined together by α-1,4 linkages, having a characteristic toroidal structure with appropriate crosslinkers, as depicted in Fig. 1 [5]. They exhibit an excellent tendency to encapsulate molecules of variable size, both hydrophilic and lipophilic, depending on the cavity size, kind of cyclodextrin, crosslinker, and the degree of crosslinking involved in the synthesis [6]. Nanoporous polymers have a processing advantage over inorganic materials since they can be transformed into powders, granular solids, thin films, and potentially smart membranes. The versatility of these materials makes it possible to employ them in various applications and formats [1]. They have the potential for chemical modifications, offering the chance of decorating them with other nanoparticles to obtain tunable improved properties. Recent literature suggests that they are biologically safe, biodegradable, non-toxic, and economically attractive and hence have found immense applications in targeted drug delivery, among others [8]. Owing to

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Fig. 1 Chemical structure of β-cyclodextrin molecule and β-cyclodextrin nanosponge. Reprinted with permission from Alongi et al. [7]. Copyright 2011 Elsevier

their chemically attractive structure, biological safety, and tunable properties, they have emerged as promising structures with applications in environmental remediation, wastewater treatment, drug delivery, cosmetics, catalysis, agriculture, textiles, and flame retardants. The versatile applications of nanosponges call for an increasing demand for analytical tools for the characterization of nanosponges so as to gain a deeper insight into the chemical structure and understanding of their mechanism of action. Researchers have been actively employing the use of particle size analyzers, Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance spectroscopy (NMR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, X-ray diffraction studies, and thermal analysis techniques to characterize the nanosponges as listed below in Table 1 [6].

2 Size Determination and Topography Analysis The average diameter and morphology of the nanosponge prepared and its void is determined using particle analyzers like dynamic light scattering (DLS), SEM, and TEM. In comparison to light microscopy, electron microscopy (EM) provides a much higher resolution and magnification range. The magnification range for SEM and TEM, for instance, lies in the range of 10 to 500,000 and 2000 to 1 million,

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Table 1 Various characterization techniques for nanosponges and their role in structure elucidation Characterization technique Information derived SEM

Determination of size and morphology

TEM

Determination of size and morphology with higher magnification and resolution than SEM

DLS

Particle size and zeta potential

XRD/PXRD

Determination of crystal structure, defects, purity, and lattice parameter

TGA/DSC

Indicate the thermal stability, glass transition temperature, melting point, exothermic and endothermic processes

UV–Visible spectroscopy

Absorbance/transmittance

Raman spectroscopy

Vibrational dynamics, polymorphism

NMR spectroscopy

To confirm the synthesis and monitor the progress of formation by identification of characteristic peak

FTIR spectroscopy

Identification of functional group and change in the characteristic bond peak

Circular dichroism

Spectroscopic analysis of chiral molecules

respectively, allowing the characterization of structures from micro- to nanoscale. Also, the negative charge on the electrons assists in interacting with the atoms. EM techniques produce high-energy electron beams, which are incident on the specimen in a vacuum chamber [9]. SEM is an imaging technique that involves scanning the material surface of the sample and its interaction with the incident electron beam to produce an image of the sample. The shape, size, and size distribution of the nanoparticles can be interpreted. TEM works on the transmittance of a beam of electrons through the sample to yield a highly resolved two-dimensional image to examine the internal structure [10]. Although the working principle of SEM and TEM is quite different, they produce similar data [11]. However, there are a few points of difference. Firstly, while SEM provides a resolution of 1 nm, TEM can resolve up to 0.1 nm [12]. Secondly, the incident electron energy employed in SEM lies typically in the range of 1–30 keV and that for TEM in 80–300 keV [9]. DLS is an important technique for evaluating the diffusion behavior of molecules in solution which consequently helps in calculating the hydrodynamic radii/particle size and the diffusion coefficient. It primarily studies the Brownian motion that originates from the collisions of the solvent molecules and relates this with the particle size. This Brownian motion is dependent on size, temperature, and solvent viscosity. As the diffusion of the molecules is evaluated over time, their size can be obtained [13]. Brownian motion results in changes in the distance between the scattering centers with time, causing constructive and destructive interferences in the intensity of the scattered light. This results in the variation of scattering light intensity with time, yielding information about the translational diffusion coefficient and particle radii. These values are, however, influenced by the movement of the particle in the medium. Hence, the diameter of the particle revealed may include solvent and other

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molecules which surround the particle and move with it; therefore, it is referred to as the hydrodynamic diameter or radius. The technique reports the results in the plot of the intensity of the scattered radiation with a particle size distribution, which can be converted to contributions per volume/number using MIE theory, provided the refractive indices of the particles must be known. DLS size assessments of nonspherical particles must be carried out precisely. The hydrodynamic radius is determined by the translational diffusion coefficient using the Stokes–Einstein equation, which calculates the predicted diameter for spherical objects. The diameter is always approximated to that of a sphere diffusing in the same medium at the same velocity, regardless of the shape of the particle. Modifications in the translational diffusion coefficient due to alterations in the particle’s form lead to fluctuations in the predicted diameter. Furthermore, polydispersity is another important point of consideration. For a monodisperse system, particle size distribution is narrow, and average particle diameter can be evaluated. For polydisperse systems, different algorithms need to be applied in order to obtain the mean diameter. DLS can be used to study particle stability as well. One of the applications of the method is the ability to establish the minimal concentration of a stabilizing agent required to prevent particle aggregation. Repeated studies may reveal aggregation, coagulation, or even sintering after thermal treatment, which would lead to a larger average diameter due to a variety of parameters, including preparation time and reagent concentration. It is also possible to study how various types of molecules and particles in solution create nanoscale complexes with DLS [14]. Guo and coworkers developed titanium-based nanosponges and examined the morphology using JEM-2000EX TEM and JEOL JSM 6700F SEM. They used functionalized polystyrene nanoparticles as templates for the synthesis of titania nanoparticles and hollow microspheres and determined the particle size using TEM. A dark ring of titania, about 4 nm in size, encompassing the functionalized polystyrene nanoparticles of diameter 154 nm, was reported. Further, calcination was done at different temperatures to obtain nanosponges. The particle size varied depending on the calcination temperature, and the average outer and inner diameter was found to be 154 nm and 100 nm, respectively, by SEM analysis [15]. Salazar et al. developed cyclodextrin-based nanosponges inclusion compounds in combination with gold nanoparticles to encapsulate anti-tumor drugs, namely melphalan (MPH) and cytoxan (CYT). They confirmed the formation of the inclusion compounds (ICs) with LEO VP1400 analytical SEM. SEM micrograms of the CD nanosponges revealed porous morphology, while that of the two drugs, melphalan, and cytoxan showed rectangular crystals of 100 μm size and rods of 1 μm respectively. Moreover, on drug encapsulation, the surface morphology was altered, indicating the inclusion of the drug molecules in the cavities. Homogeneous morphology of the nanosponges was observed along with a particle size of 1 μm accompanied by the absence of the drug crystals as seen earlier. The morphology was retained even after the deposition of gold nanoparticles, as evident from SEM images. Additionally, EDS (energy dispersive X-ray spectroscopy) analysis results were interpreted. They provided information about the elemental analysis of NS-MPH-AuNP and NSCYT-AuNP. The graphs confirmed the presence of C, N, O, and Cl in NS-MPH, C,

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N, O, P, and Cl in NS-CYT, and that of Au nanoparticles confirming the attachment of ICs to the gold nanoparticles. TEM analysis suggested the distribution of gold nanoparticles on the inclusion compounds in the form of aggregates or isolated. The micrograms also indicated three components, namely, spherical nanoparticles, an organic phase, and a nano bar-shaped component. The nano bars can be correlated to the crystallization of the drugs. In addition to the above size determination techniques, the group also performed DLS studies to estimate the hydrodynamic diameter and Z-potential of the AuNPs and ICs. The results showed that the hydrodynamic diameter of the AuNPs, NS-MPH-AuNP, and NS-CYT-AuNP was 19 ± 2 nm, 633 ± 43 nm, and 618 ± 38 nm, respectively. Further, the Z-potential of AuNPs decreased upon association with the MPH and CYT ICs from − 46 mV to − 31 mV and − 35 mV, respectively, indicating stabilization. Lastly, the polydispersity index values of the three above-mentioned systems pointed towards stability and homogeneity of the systems with broad particle size distribution [8]. Farrell et al. synthesized silicon nanosponge particles using metallurgical-grade silicon powder. Each silicon nanosponge particle is composed of multiple nanocrystals containing voids between them and among the nanosponge structure. This distinctive property enables them for used in catalysis, drugs, sensors, explosives, photosensitizers, ceramics, and electrodes as carrier material. The TEM micrograms of the prepared nanosponges showed nanocrystals and pores disposed between them and in the entire nanosponge. The average diameter of the pores was in the range of 2–8 nm, as calculated from nitrogen adsorption isotherm data with the help of the Barret-Joyner-Halenda scheme. Also, the lattice fringes observed corresponded to the Si (1 1 1) plane and indicated the crystallinity of the nanocrystals [16]. Krishna and team were amongst the first to report the formation of a noble metal porous nanosponge from a simple yet kinetically controlled reduction process with sodium borohydride in the absence of a capping agent. The synthesis of the porous materials is the simple room temperature reduction of metal salt in water. They characterized the gold nanosponges using FESEM, which produced images showing the particle size to be 200 μm and thickness 12 μm. The low and high magnification FESEM images are presented in Fig. 2. The porous highly networked structure was confirmed by the higher magnification images. The thickness of the ligaments which make up the interconnected network was observed to be in the range of 20–50 nm. This was further confirmed by TEM analysis which reported the ligament thickness to be between 50 and 150 nm. Furthermore, similar porous structures were obtained for silver, platinum, and palladium also. TEM images revealed the size of the pores and ligaments in the range of 50–100 nm and 50–80 nm, respectively for the silver sponge network. For platinum-based porous structures, the size of the network was found to be around 30 nm and that of the particles to be around 5 nm. The particles retained their networked morphology even after sonication [17]. The use of cellular nanosponges in the inhibition of the SARS-CoV-2 virus was reported by Zhang et al. in 2020. They targeted the host cells in place of the causing agent as a new approach to drug development. The infectivity of the coronavirus depends on the protein receptors present in the target cells. The nanosponges prepared

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Fig. 2 Low (a) and high (b) magnification FESEM images of Au nanosponges. Reprinted with permission from Krishna et al. [17]. Copyright 2010 American Chemical Society

from human cell-derived membranes were used as a substitute for the protein receptors. They developed two kinds of nanosponges, based on the literature available for SARS-CoV-2, namely, human lung epithelial type-II nanosponge (Epithelial-NS) and human macrophage nanosponges with poly(lactic-co-glycolic acid) (PLGA) as the nanoparticle core. It was hypothesized that the coronavirus would bind with the nanosponges and lose its ability to target the protein receptors for its cellular entry, inhibiting its activity. The group characterized the hydrodynamic radius of the nanosponge and its zeta potential by DLS using Malvern ZEN 3600 Zetasizer. The surface potential of the epithelial-NS and macrophage NS was found to be − 32.2 ± 1.5 and − 30.0 ± 0.9 mV, respectively, which was comparable with the source cells and less negative than that of the PLGA core [18].

3 X-Ray Diffraction An X-ray beam is an electromagnetic wave with an electric field that vibrates perpendicular to the direction of motion at a constant frequency. As a result of this fluctuation in the electric field, electrons experience a sinusoidal change with respect to time at the same frequency. New electromagnetic waves, i.e., X-rays, are produced due to the periodic acceleration and deceleration of the electrons. This phenomenon of scattering of X-rays by electrons is termed Thomson scattering [19]. X-Ray diffraction is a useful technique for the characterization of the crystal structure of materials, including nanoparticles. Single crystal and powder XRD data aids in resolving the structure by the shifts observed in the position of peaks to indicate the crystal orientation, lattice parameter, purity, defects in the crystals, and stress. The technique is based on Bragg’s law and Scherrer’s equation to determine the position and width of the diffraction peaks, respectively. The traditional XRD methods have

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been established for large particles with more diffracting planes and fewer surface effects, and hence ambiguity arises if the traditional procedures can be applied to nanomaterials owing to their predominant surface phenomenon [12, 20]. However, the downside is that the XRD peaks for amorphous materials are excessively broad for particles having size less than 3 nm. The benefit of XRD techniques is that they produce statistically representative, volume-averaged readings. The composition of the particles can be ascertained by comparing the position and strength of the peaks with the reference patterns available in the International Centre for Diffraction Data (ICDD), known formerly as the Joint Committee on Powder Diffraction Standards, (JCPDS) database [21]. The intensity of diffracted X-rays is affected by the atomic species as well as the atomic arrangement. While estimating the diffraction of X-rays from a crystal, the knowledge of the atomic scattering factors, a measure of the ability of the X-rays to scatter per atom, needs to be considered. An atom’s nucleus does not scatter X-rays since it is comparatively bulky in comparison to an X-ray photon. The ability of an atom to scatter is solely determined by the quantity and dispersion of its electrons [22]. There are several ways of measurement of the intensity of a scattered X-ray beam from crystalline materials, each method has a distinct benefit. The most popular technique involves using a diffractometer to detect the X-ray diffraction intensity as a function of the scattering angle (also known as diffraction angle) from a powder sample [23]. X-ray powder diffraction, a modification of XRD for single crystals, is a method of analyzing polycrystalline materials that contain numerous tiny, randomly oriented crystals. Powders prepared by crystallites with selected orientations can also be processed using this technique. Usually, powders of polycrystalline materials with crystallite sizes of a few micrometers or less are the subjects of research. Since it is expected in this case that for crystallographic planes having interplanar distance dhkl , there is always a sizable portion of correctly oriented crystals that comply with Bragg’s law, all reflections that satisfy the |Fhkl | /= 0 requirements will be detected experimentally, where Fhkl is the structure factor [24]. The application of XRD in the characterization of nanosponges may be explained in the following ways. Firstly, the absence of the appropriate drug peak in X-ray diffractograms shows that the drug has been encapsulated in NS. By identifying changes in 2θ values in physical mixtures, pure drugs, and NS, this approach may verify the crystalline and amorphous characteristics of these nanosystems, as shown in diverse NS formulations. Additionally, variations in the intensity of diffractogram peaks provide insight into the NS crystal structure. The crystalline and paracrystalline constitution of NS, therefore, affects their solubility, a crucial quality for therapeutic products. The approach can also be used to reveal porosity, which is a notable parameter. It has been noted in the literature that the lyophilization procedure causes certain formulations to lose their crystal character, which may be verified by PXRD [19]. Guo et al. studied the crystallization structure of the titanium-coated functionalized polystyrene nanosponges using Rigaku wide-angle X-ray diffractometer with Cu Kα radiation of wavelength 1.541 Å. Pure titania nanoparticles exhibited anatase structure below 600 °C, transitioned to rutile in between 600 and 800 °C, and formed

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rutile structure completely above 800 °C. However, TiO2 -coated particles demonstrated an increase in the transition temperature from anatase to rutile by 200 °C. As per the Scherrer equation, the average crystal size was found to be 36 nm at 800 °C when calculated by analysis of (1 0 1) peak [15]. Salazar and colleagues studied the drug loading of two anti-tumor drugs in CDbased nanosponges. They performed the X-ray powder diffraction analysis of the individual drug molecules and the prepared inclusion compounds. They observed the absence of the strong diffraction peaks present in the XRPD of drug molecules accompanied by the presence of a widened peak. This reflected the encapsulation of the drug molecules in the voids of the nanosponges [8]. Krishna and co-workers confirmed the polycrystalline nature of the noble metalbased nanosponges using electron diffraction (ED) which indicated that the nanonetworks arose due to the fusion of nanoparticles. Peaks corresponding to the cubic phase were also evident from the PXRD patterns of all the metal nanosponges [17]. Golubeva et al. fabricated inorganic aluminosilicate nanosponges with kaolinite structure [Al2 Si2 O5 (OH)4 ] in acidic, alkaline, and neutral pH without using organic crosslinking agents or polymers. They confirmed the attachment of aluminosilicate to the kaolin structure by XRD. Figure 3 shows the XRD patterns obtained in HCl, HF, NaOH, and water. The XRD patterns suggested that all samples created over a 12-day synthesis process in neutral, alkaline, and acidic environments were crystallized. Aluminosilicate gels which were hydrothermally treated in HF and HCl for 3 and 6 days, respectively, produced samples with poor crystallization. Alkaline media, on the other hand, facilitated a higher degree of crystallization with spherical morphology (~500 nm diameter) more quickly than acidic or neutral media after 3 days of synthesis. The characteristic XRD peaks of kaolin appear at 7.14 and 3.57 Å. The patterns exhibited strong peaks at 2θ = 11.8–12, 20.07, 24.9, 35.1, 38.1, 55.08, and 62.6 °C corresponding to the planes (001), (100), (002), (110), (003), (210), and (300) respectively. The position of the reflections of the poorly crystallized samples indicated the assignment of the aluminosilicate to the kaolinite group. Also, the X-ray phase analysis of the samples synthesized in HF for 3 and 6 days showed the presence of an impurity phase, bayerite [25].

4 Thermal Behavior Analysis Thermal analysis techniques include the study of the physical behavior of the substance and its degradation products as a function of temperature. The thermal methods differ in the measurement of properties and the applied temperature program. Various thermal methods of analysis include thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), differential thermal analysis (DTA), and microthermal analysis [26]. TGA measures the weight loss of a substance with respect to temperature. It is employed for determining differences in the weight loss of the NS matrix and finding evidence of cross-linking between moieties and NS formulations. Differences

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Fig. 3 XRD patterns of the various samples prepared in different conditions: a in HCl (pH 2.6), b in HF (pH 2.6), c in NaOH (pH 12), and d in water (pH 7). Reprinted with permission from Golubeva et al. [25]. Copyright 2021 American Chemical Society

in weight losses are the major determiners used to assess the thermogravimetric curves of both amorphous and crystalline polymers utilized in the preparation of NS. Additionally, it has been demonstrated that due to an increase in thermal stability, small molecule-loaded NS should behave differently thermogravimetrically than the molecule that is free [19]. The atmosphere employed in the experiment may be inert, reactive, or oxidizing which may be changed while the measurement is being made. TGA curve is plotted between the change in mass (%) and temperature. A complimentary plot is the first derivative curve with temperature, pointing at the rate of mass change, known as the differential thermogravimetric plot (DTG). Loss or gain of mass is evident as steps in the TGA curve or peaks in the DTG curve and can happen in various ways as depicted in Fig. 4. The nature of the gaseous products generated in the TGA can be researched by combining the TGA with a mass spectrometer (MS) or a Fourier transform infrared spectrometer (FTIR). The MS or FTIR can trace the evolution patterns of several substances and are distinctive for substances. Through spectrum interpretation and comparison with database reference spectra, the spectra obtained can be used to characterize the drug or class of substances. Thus,

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Evaporation of volatile content, moisture, drying, adsorption and desorption Oxidation of metals, oxidative decomposition of organic substances

Causes of changes in mass during thermal analysis

Formation of gaseous products due to pyrolysis, carbonization, and thermal decomposition in inert atmosphere Chemical reactions eg. decarboxylation, condensation etc. Loss or uptake of water molecule Curie transition - change in magnetic properties of the ferromagnetic materials with temperature

Fig. 4 Various ways in which mass loss or gain for a sample can occur during thermal analysis [27]

decomposition paths can be studied. TGA-MS is highly specific with the benefit of the detection of an extremely small amount of substance. Further, TGA-FTIR offers high chemical specificity and can characterize substances by their functional groups [27]. DSC is a thermoanalytical tool used to assess the effect of temperature changes on the chemical or physical characteristics of nanocarriers and their constituent materials. The underlying principle of DSC involves comparing the heat flow rates of the polymers, cross-linkers, active molecules, and their nanosponges upon heating or cooling at the same rate. Peaks in the differential heat flow are caused by variations brought on by the evolution or absorption of heat. The enthalpy change is often directly proportional to the peak area. The direction of the peak typically indicates thermal events, both endothermic and exothermic in the temperature range of − 120 to 600 °C [19]. Exothermic processes like adsorption, crystallization, oxidation, and polymerization are indicated as an increase in heat flow, while endothermic reactions are depicted as a decrease in the heat flow with temperatures, like dehydration, decomposition, and reduction [26]. DSC is a precise analytical technique for determining the melting and decomposition points. Due to solid-to-solid phase transition and liquid crystal intermediates, this approach has a good possibility of detecting a phase transition besides the melting point. Additionally, it gives a quantitative estimation of the heat of transitions, reactions, first-order thermodynamic variations like fusion, and second-order thermodynamic variations like the glass transition temperature [19]. A sample and reference are placed in sample holders in the instrument, and the difference in heat flow between the two is measured. A small sample quantity is transferred to the sample pan to form a uniform layer at the bottom. The commonly used reference is an aluminium pan, while the platinum, gold, stainless steel, graphite, and glass ampoules are other options. The measurement is usually done in an inert atmosphere of helium or nitrogen. However, air and oxygen may be purged to study

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the oxidation process. Purging of the gas aids in eliminating moisture while oxygen assists in transferring heat to the pan. Salazar and co-workers also evaluated the thermal behavior of the drugencapsulated nanosponges and their inclusion compounds (ICs) using TGA. They observed that the major degradative process for the drugs MPH and CYT occurred around 250 °C and 271 °C, respectively, while 65.1% weight loss for the nanosponges occurred at 345 °C. For the ICs, two degradation steps were observed. The first occurred at 230 °C corresponding to the degradation of the drugs and the second at 350 °C relating to the degradation of the nanosponges [8]. For the removal of boric acid and other organic micropollutants, Liao and Zhang fabricated cyclodextrin (CD) based nanosponges with cis-diols functionalized on the surface and different crosslinkers—tetrafluoroterephthalonitrile (TFN) and terephthaloyl chloride (TCL) and reactant ratios to regulate surface morphology. The thermal stability studies of the different CD polymers were performed by thermogravimetric analysis in a nitrogen atmosphere by heating the sample at a rate of 10 °C/ min in a range of 50–800 °C. The initial degradation temperature of TCL crosslinked polymers was higher than TFN crosslinked polymers by almost 100 °C. Moreover, the TCL crosslinked polymers exhibited higher weight loss than TFN crosslinked polymers after degradation. The higher reactivity of TCL renders a higher degree of crosslinking in poly(β-CD-TCL), which might be the reason for higher weight loss. Also, the high stability of the C–F bond over the C–O bond also points towards more retention of TFN crosslinked CD nanosponges. The degradation temperature and weight loss or retention amount are tabulated in Table 2 [28]. Golubeva and team developed a single-step hydrothermal method of synthesis of inorganic aluminosilicate nanosponges with kaolinite structure [Al2 Si2 O5 (OH)4 ] in acidic pH of 2.26 at 220 °C without the use of organic crosslinking agent or polymers. They performed the thermal analysis (TG + DSC type) of the fabricated samples using NETZSCH STA 429 CD device by heating a pellet of aluminosilicate samples in the temperature range of 40–1100 °C at a rate of 20 °C per minute in dynamic air atmosphere. The TG, DSC, and IC (ion current) curves are obtained and analyzed further. The analysis of the obtained curves revealed that samples with various morphologies, including spongy, spherical, and mixed synthesized in HF medium, have roughly the same TG/DSC pattern linked to the release of adsorbed water, accompanied by an endothermic effect in the region of 100–110 °C, and removal of constitutional water, accompanied by an endothermic effect in the region of 490 °C, as well. Thermal analysis and mass spectrometry results for samples with spongy, spherical morphologies, as well as a sample with mixed morphology Table 2 Comparative thermal analysis data of the cyclodextrin-based TCL and TFN crosslinked polymers Parameter

poly(β-CD-TCL)

poly(β-CD-TFN)

Degradation temperature

~270 °C

~170 °C

Weight loss/retention

~85% weight loss

~35% weight retention

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synthesized in HF medium, show that the water content of the various samples varies, with nano-spongy morphology samples having the least amount of water [25].

5 UV–Visible Spectroscopy Absorption spectroscopy is based on the absorbance or transmittance of incident radiation by the solution [26]. The Ultraviolet–Visible absorption spectral range is 190–900 nm in the electromagnetic spectrum. The linear relationship between absorbance and the concentration of the solution renders the tool helpful in quantitative estimation. Moreover, it is an attractive tool for the characterization of the electronic and optical properties of a substance [29]. The absorption of the UV–Visible light causes electronic transitions in the molecule from the lower energy ground state to the higher energy excited level. As there are multiple quantized energy levels in a molecule, a specific excitation corresponds to a specific wavelength of radiation of that particular quantized level, exhibiting a single discrete line in the spectrum. Because the transitions are at the electronic level, this spectroscopy is also termed electronic spectroscopy. Compounds with conjugated bonds, delocalization, and aromatic rings absorb in this region and are hence characterized using the technique. The UV–Visible spectrum is recorded based on Lambert–Beer’s law. It states that the fraction of incident radiation absorbed by the sample does not depend on the intensity of the incident radiation. Beer’s law suggests that the absorbance by a sample varies proportionally with the amount of the substance. Lambert–Beer’s law combines the observations of the two laws and is expressed in Eq. 1 [30]. A = log10 I0 /I = εlc

(1)

where A is the absorbance or optical density, ε is the molar absorptivity or the molar extinction coefficient, l is the path length of the sample in cm, c is the concentration of the sample, I 0 is the intensity of the incident radiation, and I is the intensity of the transmitted radiation [30]. In UV–Vis spectroscopy, a chromophore is a group that is responsible for absorption in the UV–visible region. In addition, an auxochrome is a group that alters the wavelength and intensity of absorption. The presence of an auxochrome may cause a hyperchromic effect, that is the increase in the intensity of absorption, εmax . On the contrary, a hypochromic shift is a decrease in the intensity of absorption, which can be attributed to the introduction of a group that disturbs the conjugation in the chromophore. The Bathochromic or red shift occurs when the absorption maximum shifts to a longer wavelength due to an auxochrome or solvent effect. Whereas the hypsochromic or blue shift takes place when the shift is towards a shorter wavelength [30]. UV–Visible spectrophotometry is a straightforward, quick, economically attractive, and useful technique for optimizing and analyzing nanosponges in solution.

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Encapsulation in NS causes a significant alteration in the absorption spectra of the guest molecules that results in band widening or bathochromic shift. The transfer of a guest moiety to the nanosponge channels from an aqueous medium can shift the peaks of the original UV–visible absorption spectra of the drug, depending on the drug’s chromophore location, establishing the encapsulation of the drug in the nanosponge. However, this approach lacks specificity as a result of the existence of interfering moieties in the cavity. As a result, it does not offer any conclusive evidence that drug-loaded NS is formed. It is possible to see variations in the bathochromic or hypsochromic shifts in the maximum absorption of the active moieties as well as an increase or decrease in the intensity of the UV–visible spectrum as a result of NS production [19]. To confirm the presence of nanoparticles on the surface of the organic compounds, UV–Visible spectroscopy was performed by Salazar et al. In the absorption spectra of the inclusion compounds conjugated with gold nanoparticles, the characteristic plasmon band of gold nanoparticles at 565 nm was observed suggesting the stability of the nanoparticles. A bathochromic shift may possibly correspond to the aggregation of the nanoparticles on the crystal surface as also indicated by TEM micrograms and the broadening of the absorption spectra [8]. UV–vis absorption spectra of all of the noble metal nanosponge samples prepared by Krishna et al. displayed nearly flat absorption across the whole visible spectrum and in the IR region pertaining to the surface plasmon resonance for extended network nanostructures. Size and shape-dependent surface plasmon resonances in Ag and Au nanoparticles were carefully evaluated. One major plasmon band, for instance, had been observed in spherical Ag and Au nanoparticles at wavelengths of 420 and 520 nm, respectively [17]. Figure 5 shows the UV spectra of the metal nanosponges and their images in powder and pellet form. Alongi and others investigated the photooxidation of polypropylene by UV lamp and the interaction between β-cyclodextrin and two UV stabilizers, 2-hydroxy4(octyloxy)-benzophenone (HOBP) and triphenyl phosphite (TPP). The presence of an amine-based antioxidant already existing in the polyolefin with a characteristic absorbance of 211 and 232 nm was evident in the UV spectra of polypropylene and NS-containing polypropylene. The shift of the absorbance towards a higher wavelength may be possibly due to this inhibitor affecting the photooxidation process of the prepared formulations. In addition, the HOBP-based polypropylene formulation showed the characteristic absorbance of the aromatic group at 232, 262, and 303 nm [7].

6 Raman Spectroscopy The spectroscopic technique was named after its inventor C.V. Raman. It is a relatively fast, easy, non-destructive, and efficient technique [31]. Based on the inelastic scattering of the monochromatic incident beam of light, in accordance with the Raman effect, that is, the frequency of the scattered radiation produced on interaction with

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Fig. 5 a UV–Vis spectra of the metal nanosponges b NS in powder form c NS in pellet form. Reprinted with permission from Krishna et al. [17]. Copyright 2010 American Chemical Society

the vibrating molecules is different from the frequency of the incident radiation. On interacting with the molecules of the sample, the incident monochromatic radiation scatters in all directions. Rayleigh scattering occurs when the frequency of a considerable portion of this dispersed radiation is equal to that of the incident radiation. However, the frequency of a small portion of the scattered radiation is different from that of the incident radiation. Stokes and anti-Stokes lines are observed in the Raman spectrum when the frequency of incident radiation is higher and lower, respectively, than the frequency of scattered radiation. Stokes bands appear to be more intense than anti-Stokes bands as they represent the transitions from the lower to higher energy states. They are hence characterized in conventional Raman spectroscopy. Moreover,

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fluorescing samples are used to estimate anti-Stokes bands because fluorescence interferes with Stokes bands [32]. The frequency of the incident radiation does not affect the size of Raman shifts. However, it affects Raman scattering. A prerequisite for the substance to be Raman active is the change in polarizability during molecular vibration. In a typical Raman spectrophotometer, the measurement angle for scattered radiation is a right angle to the radiation incident. For the purpose of dissolving the sample, water is an excellent choice as it causes low Raman scattering. Glass is generally utilized for optical components like mirrors, lenses, and sample cells [32]. It has been used extensively to characterize nanostructures. For instance, carbonbased materials can be characterized by Raman spectroscopy to illustrate the creation of nanostructures. The ratio of the intensities of the D and G bands (ID /IG ) is one metric used to assess the level of disorder. Since the ID /IG ratio for the nanopolyhedrons is 1.15, it suggests that there is no long-range order and that the nanostructures have formed. Although the technique is not particularly damaging, it has a smaller cross-section compared to the absorption and emission processes, which makes it more difficult to characterize diluted solutions or nanoscale structures like extremely thin sheets. The Raman signal can be increased by a factor of up to 107 by the surface-enhanced Raman scattering (SERS) spectroscopy method by combining these systems with metallic nanostructures, which are thin films formed on roughened metal surfaces and solutions diluted in metal colloids. In order to investigate specific qualities (adsorption, electron transfer, stability, etc.), control mechanisms for chemical surface and nanoparticle production are necessary for the advancement of a wide variety of nanostructured materials. SERS spectroscopy can therefore be used to characterize a variety of nanostructured materials when they are combined with metal nanoparticles, deposited on roughened metal surfaces, covered in evaporated metal films, and dipped in metal ion solutions to induce chemical reduction with laser excitation [31]. Raman spectroscopy (RS) is a versatile tool for the analysis of a variety of materials, overcoming most of the drawbacks of the existing spectroscopic methods. It is applicable for both qualitative (by monitoring their frequency) and quantitative purposes (by measuring the intensity) [31]. Noble metal-based nanosponges prepared by Krishna and colleagues were also investigated for their SERS activity. Nanosponges of silver and gold were found to strengthen the Raman signals of Rhodamine 6G dye even at lower concentrations of 10−6 M. As discussed in the above section, the nanosponges also exhibited broad surface plasmon absorption in the visible and IR region, due to which the SERS is not constrained to only a certain excitation wavelength, as opposed to the case of Au and Ag nanoparticles. Additionally, these nanostructures can be easily deposited onto porous substrates with a porosity of a few micrometers and utilized in biofiltration to separate, identify, and eliminate hazardous bacteria and viruses. E. coli bacteria were used to study the antibacterial activity of a silver spongy network placed on a Whatman filter paper. The considerable antibacterial effect of the nanosilverdeposited Whatman filter paper was demonstrated by the formation of the inhibitory

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Table 3 Comparison of the modes observed in the dry and water-treated sample of CDNS with the CD to PMA molar ratio 1:4 Modes in the dry sample 3100–3077 2961

cm−1

(broad band)

cm−1

3004 cm−1 (bump) 2917

cm−1

Modes in hydrated sample 3111 cm−1 No observable change in intensity and position Significant decrease in intensity 2926 cm−1

zone on the silver nanosponge-embedded filter paper in the bacterial growth medium [17]. Mele and team developed β-cyclodextrin based nanosponges from CD and pyromellitic anhydride by changing the cyclodextrin crosslinker ratio (1:2, 1:4, and 1:8). Preliminary examination of the Raman spectra of the three samples clearly displayed a bump characteristic of disordered systems at 15–30 cm−1 , which may be due to the collective vibrational modes and a broadened line at 5 cm−1 pertaining to the quasielastic vibration. As the degree of crosslinker increased, changes in the low-energy vibrational dynamics were noted. Further, a line at 90 cm−1 was seen in the spectrum of the CDNS with a 1:8 molar ratio, which was absent in the other two samples. They also investigated the effect of hydration on the sample with a molar ratio of 1:4 by recording the spectrum of the dry and water-treated NS in the energy region 2800–3150 cm−1 . The shifts and changes in the intensity of the significant lines observed upon hydration are tabulated in Table 3. These alterations suggest that on the hydration of the dry samples, structural changes occur in the nanosponge with polymer to crosslinker ratio of 1:4. Moreover, the shift in the 2917 cm−1 vibration mode depicts the hardening of the bond [33]. Swaminathan et al. developed β-cyclodextrin nanosponge with a crosslinker like diphenyl carbonate and loaded it with a model drug, dexamathasone (DEX). They observed the Raman spectrum for the encapsulated drug and noted that the characteristic peaks of the individual drug molecule present at 1620, 1480, 1440, 950, and 680 cm−1 were significantly displaced or merged on complexation. As reported, the group was among the first few to record and interpret the Raman spectra of drugs and polymers [34].

7 Nuclear Magnetic Resonance (NMR) NMR is a spectroscopic technique that utilizes the magnetic properties of the atomic nucleus to derive structural information. Magnetically active nuclei resonate at a characteristic frequency in the radio frequency range of the electromagnetic spectrum in the presence of a strong magnetic field. Information about the molecule can be derived with mild variations in this resonance frequency. The resonant frequency is also determined by the chemical environment and position of the atom within the

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molecule. This is because the bonding electrons encompassing the nuclei create their own local magnetic field that affects the external magnetic field. This difference, on the order of one part per million, is known as the chemical shift, δ. The chemical shift helps in the identification of the different atoms and provides detailed information about the molecule based on the predictable effects of electronegative atoms, unsaturated groups in the vicinity, and molecular symmetry [35]. NMR spectroscopy revolves around the quantum mechanics of nuclear spin angular momentum, denoted by the nuclear spin quantum number I. It can have different values depending on the atomic and mass number of the nuclei as demonstrated in Table 4. A point of note is that nuclei belonging to the second category are NMR inactive [36]. Certain examples of research groups that used NMR as a characterizing technique for nanosponges have been discussed in the following section. Pivato and coworkers synthesized β-cyclodextrin nanosponge hydrogels by crosslinking with pyromellitic anhydride for drug delivery of the anti-inflammatory drug piroxicam. They studied the translational dynamics of the drug molecules in the synthesized and optimized hydrogels by 1 H high-resolution magic angle spinning (HR-MAS) NMR spectroscopy. HR-MAS NMR spectroscopy plays a pivotal role in obtaining the NMR spectra of molecules trapped in confined soft matter as the nature of the sample changes from non-viscous, isotropic water solution to soft highly viscous hydrogel-like materials. With the help of HR-MAS, the host–guest interactions can be studied along with the dynamic properties of the small drug molecules that are mobile in the nanoporous polymer structure. The approach has an attractive property of decoupling the spectra of the entrapped drug and the polymer which is filtered out while retaining a requisite degree of mobility, resulting in a high-resolution spectrum. Figure 6 shows the molecular structure of the drug piroxicam. Table 4 Relation between nuclear spin quantum number and atomic & mass number Mass number Atomic number Nuclear spin quantum number Examples Odd

Odd or even

Half integral (1/2, 3/2 …)

Even

Odd

Integral (1, 2, 3 …)

Even

Even

Zero

Fig. 6 Molecular structure of piroxicam salt

1 H, 13 C, 19 F, 17 O, 31 P, 35 Cl 1 6 9 8 17 15 2 H, 14 N 1 7 16 O, 12 C 8 6

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Additionally, 1 H high-resolution NMR spectroscopy was explored to examine the structure, aggregation behavior, and dynamics of the small molecules dissolved in D2 O. As the concentration of piroxicam increased in water, slight shielding of all the aromatic protons and the N-Methyl protons was observed. While most protons showed an upfield shift in the range of 0.03–0.06 ppm, the aromatic proton H8 showed a difference of 0.63 ppm. This shift may indicate the aggregation behavior which is expected in aromatic drug compounds in an aqueous solution owing to the formation of π–π dimers or oligomers. The HR-MAS spectrum of the PRX CDNS was similar to the 1 H high-resolution spectrum. Also, the HR-MAS spectra recorded at varying concentrations of the drug showed similarities for the lower concentration samples (0.003 and 0.006 M) than for 0.15 M sample of H6, H8, and N-Me. This implied that the effective concentration of the drug in the gel phase was less than the previously employed concentration of 0.15 M. The relative downfield shift of protons, H3 and H7 and upfield shift of H8 in the gel phase may be attributed to the formation of the drug-CD complex by the encapsulation of the drug in the hydrophobic cavity of cyclodextrin monomers in CDNS [37]. Moin et al. designed cyclodextrin-based nanosponges crosslinked by dimethyl carbonate using hot-melt compression method and loaded three drugs onto it, namely caffeine, paracetamol, and aceclofenac. They established the loading of the drug on the prepared nanosponges by using 1 H NMR. The spectra obtained for cyclodextrin and the entrapped drugs exhibited significant changes in the chemical shift values, with a pronounced effect on the protons in the cavity. From this inclusion of drugs into the hydrophobic cavity of the nanosponges may be concluded. Two protons showed an upfield shift in the drug-loaded NSs due to magnetic anisotropy, indicating that they were located near the p-electron cloud of the aromatic nucleus [38]. Pushpalatha and the team synthesized cyclodextrin-based nanosponges with diphenyl carbonate (DPC) and pyromellitic dianhydride (PMDA) for curcumin delivery. They recorded the 13 C spectra of DPC and PMDA nanosponges in deuterated DMSO solvent. The peak located at 155.13 ppm in the 13 C NMR spectrum of DPC nanosponge corresponds to the carbonyl group of DPC which is involved in crosslinking of cyclodextrin molecules. Additionally, the peaks at 168.57 and 168.60 ppm in the spectrum of PMDA nanosponge can be correlated with the ester carbonyl group their presence may be attributed to the crosslink between the hydroxyl group of cyclodextrin and PMDA. Lastly, the peak at 167.61 ppm was assigned to the carbonyl carbon of the carboxylic acid group of the crosslinked cyclodextrin molecules [39]. Rao et al. synthesized β-cyclodextrin (CD) based nanosponges (NS) by crosslinking via carbonate bonds to incorporate the drug telmisartan. The 13 C NMR spectra of a few preparations, including CD and NS, were collected. The appearance of the peak at 155 ppm in the NS spectra proved that NS was created from CD. The experimental value aligned with the predicted spectrum produced by ACD/ Chemsketch [40].

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8 Fourier Transform Infrared Spectroscopy Infrared spectroscopy is based on the interaction of electromagnetic radiations with molecular vibrations. The spectrum is a plot of the transmission of IR radiation with respect to wavelength. (IR chap) In terms of wavelength, the range of IR is 2.5– 25 μm, while it is 4000–400 cm−1 in terms of wavenumber. IR radiation encompasses the stretching and bending vibrational frequencies of a molecule. The frequency which matches the natural frequency of the bonds is absorbed by the molecule. The prerequisite for a bond to be IR active, i.e., absorb IR radiation is that it must have a dipole moment that changes at the same frequency as the incident radiation frequency for energy transfer. It is a commonly used technique used to identify molecules as a fingerprint. Secondly, the structural information of the molecule can also be elucidated since each bond (N–H, O–H, C–C, S–H, etc.) has a specific absorption frequency range which changes only for a special type of bond/attachment [41]. Fourier transform-based infrared spectrometers have been in use for a long time now for mid-IR instrumentation. FT-type instruments have an advantage over the conventional dispersive type as they provide a better signal-to-noise ratio and improved detection limits, making them an attractive choice [26]. The vibrational modes of the polymers, cross-linkers, and active molecules undergo a shift from their initial positions during cross-linking, causing the broadening or absence of their characteristic bands. Fluctuations in peak intensities and shifts in their wave numbers may also occur, suggesting interactions between the polymers and either the cross-linker or one another. These modifications may result from the restricted stretching vibration in these compounds brought on by the crosslinking process involving the polymer, cross-linker, and medication. Additionally, cross-linking and/or complexation in NS causes interatomic bond weakening. Along with notable selectivity and sensitivity, the other key benefits of this analytical method include low cost, extensive diffusion, and a quick and simple spectrum gathering process [19]. FTIR can assist in analyzing the sample structure and indicate the presence of new bond formation. Golubeva et al. synthesized inorganic aluminosilicate nanosponges in acidic, basic, and neutral media to optimize the reaction conditions to obtain the porous structures described above. They analyzed the structure of the sample and the assignment of aluminosilicate on kaolinite with FTIR by scanning the samples in the range of 4000–350 cm−1 at a resolution of 4 cm−1 (Fig. 7). Similar spectra for aluminosilicates and raw kaolinite were obtained. Characteristic bands of hydroxyl groups in kaolinite minerals, present on the octahedral surface layers, appeared at 3695 and 3620 cm−1 in the spectra of all aluminosilicate samples except for those treated hydrothermally in HCl and HF for 3 and 6 days. These hydroxyl bonds are known to form weak hydrogen bonds with the Si–O–Si atoms. These bands are characteristic of natural kaolinite with platy morphology. For the samples with spongy or thin layer morphology, the absence of the bands may be because of the small particle size, absence of lamellar morphology, and difficulty in forming hydrogen bonds between the layers [25].

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Fig. 7 a FTIR spectra of the samples b the O–H stretching. Reprinted with permission from Golubeva et al. [25]. Copyright 2021 American Chemical Society

Alongi et al. examined the interaction between β-cyclodextrin and two UV stabilizers, 2-hydroxy-4(octyloxy)-benzophenone (HOBP) and triphenyl phosphite (TPP) during the photooxidation by UV lamp of polypropylene. They characterized the nanosponges, stabilizers, and their binary combinations by FTIR spectroscopy. Although the spectra of nanosponges were weakly resolved and characterized by low intensity, they were strikingly comparable to that of single-cyclodextrins. Because of the carbonate carbonyl group bridges linking the cyclodextrin molecules, the symmetrical and asymmetrical vibration modes of methylene groups at 2920 cm−1 , stretching vibration of the hydroxyl, ether, and carbon skeleton group at 3300, 1160, and 1026 cm−1 , respectively could be interpreted. Additionally, a typical band at 1760 cm−1 was observed which can be attributed to the carbonyl group of carbonate bridges linking cyclodextrin units. The FTIR spectrum of the HOBP and TPP-based pair exhibited perfect overlapping of the peaks of the individual components with distinguishable bands of both the constituent [7]. To investigate the interactions between paliperidone (PLP), an anti-psychotic drug, and nanosponges, FTIR analysis was conducted by Sherje and co-workers. They developed PLP-loaded β cyclodextrin-based nanosponges crosslinked by carbonyldiimidazole. The IR spectrum revealed the shifting, possible merging, or loss of several characteristic PLP peaks. Some peaks of lower intensity were also observed. The interaction of PLP with nanosponges is suggested by the considerable shift in the IR bands of PLP at 3294.57, 2935.42, 1733.77, 1537.55, 1270.72, and 1131.26 cm−1 corresponding to O–H stretch, =C–H stretch, C=C stretch, C=N stretch, C-H stretch, and C-F stretch respectively. [42]. Deshmukh et al. aimed to produce impregnated lysozyme onto surface-active nanosponges by damaging bacterial cell walls to maintain its conformational stability by the catalysis of the hydrolyzing process of 1,4-β-linkages between N-acetyld-glucosamine and N-acetylmuramic acid residues in peptidoglycan layer encompassing the bacterial cell membrane, and to control the release of calcium in hypocalcaemia condition. Various β-cyclodextrin based nanosponges were synthesized with

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carbonyl diimidazole. Calcium carbonate and carboxymethyl cellulose (CMC) were added to enhance the loading efficiency and ease the synthetics method. The FTIR spectra of cyclodextrin showed asymmetric and symmetric stretching of v[OH] and [CH2 ] corresponding to absorption bands at 3399.75 cm−1 and 2923.46 cm−1 , respectively. The absorption band lying in the range 1000 and 1200 cm−1 can be related to the stretching of C–O because of primary and secondary alcohol. CaCO3 containing sample displayed absorption bands at 1084, 878, 1455, 713, and 1430 cm−1 , which, respectively, corresponded to vibrations of the symmetric stretching, symmetric bending, asymmetric stretching, asymmetric bending, and C–O stretching vibrations. The presence of CMC revealed two rather faint bands at 2980 and 2830 cm−1 , related to the asymmetric and symmetric C-H stretching modes. The nanosponges characteristically represent the carbonyl group between two cyclodextrin molecules by a peak at 1753 cm−1 . Lysozyme showed two bands at 1670 and 1530 cm−1 that are characteristic of the amide bands. The location of the two amide bands did not change with lysozyme immobilization on nanosponges, suggesting no conformational changes in the secondary structure of the protein. Additionally, it was noted that the presence of lysozyme, its concentration, and the method employed did not alter CaCO3 or nanosponges in any way [43].

9 Circular Dichroism Circular dichroism spectroscopy is a chiroptical method, a chiral variant of absorption spectroscopy. While non-chiral spectroscopic methods use unpolarized electromagnetic radiation, chiroptical methods care applicable only to chiral samples and derive information from the conventional spectroscopic techniques. It is a sensitive tool capable of studying the three-dimensional states of matter with significant selectivity. Circular dichroism is the most popular chiroptical method among other methods like optical rotatory dispersion, Raman optical activity, circularly polarized luminescence etc. The extent of absorption of left and right circularly polarized light is different in chiral molecules. This difference is measured by CD spectroscopy and is expressed as ΔA(ν) = AL (ν) − AR (ν)

(2)

where ΔA(ν) is the circular dichroism, a dimensionless quantity; AL (ν) and AR (ν) are the absorbance of the left and right circularly polarized light, respectively [44]. The signals in the CD spectra are however 3–5 orders of magnitude weaker than the absorption peaks in the parent spectroscopy. Therefore, there is a need for enhanced and specialized instruments for measurement. Chiroptical spectroscopies are more intensity oriented, while parent spectroscopies are directed toward the position of the signal [44].

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Molecules with chromophores that absorb in the UV–visible region give rise to electronic circular dichroism (ECD). While vibrational circular dichroism is produced by absorption in the IR region [45]. Cyclodextrins have chiral centers, but they, however, do not absorb light in the UV–visible region. Guest molecules on the other hand are optically active, and their inclusion in cyclodextrin induces the circular dichroism spectra [3]. A substance must have both chirality and electron-optical absorption in order to have a circular dichroic absorption. The majority of guest molecules, including pnitrophenol, absorb light but are not chiral, unlike chiral cyclodextrin, which does not. However, upon combination of the cyclodextrin and the guest component resulting in the formation of a new complex with chirality and optical absorption, the induced circular dichroism becomes visible. A circular dichroic signal was seen at 405 nm in the spectra with fine structures for the cyclodextrin-hexamethylene diisocyanate (HDI) polymer with nitrophenol intercalation prepared by Min Ma and DeQuan Li. A single positive circular dichroic absorption was present in the majority of cyclodextrin inclusion complexes in the solution, which showed that polar nitrophenols are parallel to and loosely arranged inside the cyclodextrin cylindrical cavities. On the other hand, negative CD absorptions implied that the nitrophenols were parallel to the cylindrical chambers of the cyclodextrin. The circular dichroism absorption was split or subject to the Cotton effect due to the following causes. Firstly, stacking caused the electric transition moment of intramolecular charge transfer inside nitrophenol (λmax 405 nm) to interact with other dipoles. When the angle between the transition moments was higher than 90°, the resulting Cotton effects typically had a positive direction. Secondly, due to a close match between the guest and host, lengthy substitutions on the cyclodextrin torus that lengthen the nanochannels appeared to have beneficial Cotton effects. These findings aligned with the strongly cross-linked, threedimensional structures connected by guest molecule-filled cyclodextrin nanochannels. It also suggested that these channels allow guest molecules to move from an aqueous solution into the solid of the polymer, producing a favourable fine Cotton effect at 405 nm [4].

10 Conclusion Through this review, the authors have explained the need for the various characterization techniques that can be employed for nanosponges. The role of each method is described and will guide the researchers to understand the purpose and process. Emphasis has also been laid on the implementation and adaption of each technique by several research groups working worldwide. The study of their work will guide future scientists in assessing the most suitable technique to be employed by them for their desired nanostructure. With their diverse applications, nanosponges have proven to be promising structures and hence improved sought-after characterization techniques are an absolute necessity for the research front to move forward.

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Acknowledgements The authors express their gratitude to Mr. Arvind Kumar, Director, CFEES, DRDO, Delhi, for providing the necessary administrative, infrastructural, and financial assistance to publish this work. Pragya Malik acknowledges CFEES, DRDO, for a junior research fellowship.

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Introduction to Cyclodextrin-Based Nanosponges Gianluca Utzeri, Dina Murtinho, and Artur J. M. Valente

Abbreviations 1,4AP 2,2AAA AIBN APM BET CD CDI CDNS CGTase CTA CuAAC DEET DMC DMF DMSO DPC DSC EDTA EPB EPI

1,4-Disacryloylpiperazine 2,2-Bisacryloamidoacetic acid Azobisisobutyronitrile Pyromellitic anhydride Brunauer-Emmett-Teller analysis Cyclodextrin Carbonyldiimidazole Cyclodextrin-based nanosponge Cyclodextrin glucosyl transferase Citric acid Cu(I)-catalyzed azide-alkyne cycloaddition N,N-Diethyl-3-toluamide Dimethylcarbonate Dimethylformamide Dimethyl sulfoxide Diphenylcarbonate Differential scanning calorimetry Ethylenediaminetetraacetic acid Epiclon B-440 Epichlorohydrin

G. Utzeri (B) · D. Murtinho · A. J. M. Valente CQC—IMS, Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal e-mail: [email protected] D. Murtinho e-mail: [email protected] A. J. M. Valente e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Gulati (ed.), Nanosponges for Environmental Remediation, https://doi.org/10.1007/978-3-031-41077-2_5

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Fourier-transform infrared spectroscopy Hydroxypropyl-β-CD 1,6-Hexamethylenediisocyanate 4,4-Methylenediphenyl diisocyanate Nuclear magnetic resonance Nanosponge Polychlorinated biphenyls Scanning electron microscopy Toluene diisocyanate Transmission electron microscopy 2,3,5,6-Tetrafluoroterephthalonitrile Thermogravimetric analysis

1 Cyclodextrin The development of cyclodextrins has, in the opinion of these authors, three important milestones. The first data, from the late 19th century, are due to the Frenchman Antoine Villiers and his studies on the effect of microorganisms, in particular Bacillus amylobacter (Clostridium butyricum), on potato starch, having observed the respective fermentation in the presence of these microorganisms. The main reaction products, which Villiers observed in the form of crystals, were made up of dextrins (a term in illo tempore a degradation product of starch [1, 2]) and later were referred as “cellulosines” [3, 4]; being the first references to the molecules that would later be known as cyclodextrins. Around 15 years after the first references to cellulosines, Franz Schardinger— an Austrian chemist and bacteriologist—isolated Bacillus macerans (Aerobacillus macerans) and found that its action on potato starch produces crystalline dextrins similar to those described by Villiers [5, 6]. Schardinger developed protocols that allowed the synthesis and purification of 2 different dextrins, labelled as α- and β-dextrins [7]. The importance of Schardinger’s work lies in the isolation of the microorganism capable of generating the extracellular enzyme CGTase (cyclodextrin glucosyl transferase) that hydrolyzes starch-forming dextrins [8], as well as being the first to hypothesize that dextrins are cyclic polysaccharides. The relevance of Schardinger’s work was recognized and for more than a century dextrins were described as “Schardinger dextrins” [9, 10]. Although Schardinger dextrins, afterward also named cycloamylases [11], have always received attention from researchers [12, 13], the awakening to the unique properties and multiple applications only occurred in the last quarter of the twentieth century with the crucial contributions of Wolfram Saenger and József Szejtli. Among other things, the decisive contribution of these researchers has been to unveil the crystal structure of cyclodextrins [14] and to understand the mechanisms of inclusion complex formation [15, 16] (originally described by Cramer et al. [17]). Szejtli also

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Fig. 1 Pictoral representation of the most common native cyclodextrins and their physical properties

had an outstanding role in the large-scale use of cyclodextrins, which continues to this day. In structural terms, cyclodextrins are macrocyclic oligosaccharides with a hollow truncated cone shape following the rigid 4 C1 chair conformation of the glucopyranose units linked by α(1-4) ether bonds. The well known native cyclodextrins, called α-, β- and γ-cyclodextrins, consist of 6, 7 e 8 monomeric units, and have outer diameters of 13.7, 15.3 e 16.9 Å, and cavity volumes equal to 174, 262, e 427 Å3 , respectively [18, 19] (Fig. 1). Nevertheless, in aqueous solution, the structure of cyclodextrins undergoes significant distortion or, although less frequently, exhibits a collapsing conformation [20]. The number of water molecules that can be located in the cavity of cyclodextrins depends on their volume; thus, considering the volume of one water molecule equal to 30 Å3 [21] 5, 8, and 14 water molecules can be expected to be present in α-, βand γ-cyclodextrins, respectively. However, whereas water molecules in the presence of β- and γ-cyclodextrins exhibit similar behavior to that in liquid water, the water molecules in α-cyclodextrin are less hydrogen bridge bonded, as can be inferred from heat capacity measurements carried out by Brignner and Wadsö [22]. Another crucial aspect to consider is the solubility of the cyclodextrins. This property does not follow a particular trend, with higher solubility of α- and γ-CD and relatively low solubility of β-CD [18]. The solubility of CDs is justified by the presence of hydroxyl groups on both rims of the structure, either on the larger rim (wider rim) (secondary hydroxyls) or on the smaller rim (narrow rim or edge). The occurrence of intramolecular hydrogen bonds between the secondary hydroxyls of β-CD is pointed out as a possible justification for the low solubility of this cyclodextrin [23, 24]. In fact, such drawback is overcome when using, for example, hydroxypropyl-β-cyclodextrin. Nonetheless, unlike the outside, the inside of cyclodextrins exhibits hydrophobic characteristics, since it is lined with hydrogen atoms and oxygen glycosidic bridges, giving it a Lewis base character. The amphiphilicity associated with the donut-like structure and water solubility of cyclodextrins allows the formation of highly stable supramolecular adducts, essentially of host-guest type, with a wide range of hydrophobic or moderately

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hydrophobic compounds [25–27]. In thermodynamic terms, host-guest supramolecular adducts are highly stable due to the gain in enthalpy and entropy upon complexation. The former occurs mainly due to hydrophobic interactions, whilst the latter occurs as a consequence of dehydration of the guest as well as the cyclodextrin cavity leading to a positive entropic variation [28]. This capacity of cyclodextrins to react and produce supramolecular host-guest adducts had (and still has) a significant impact on the development of methodologies and formulations in a wide variety of areas [27, 29, 30], such as pharmaceuticals [31, 32] and food and beverages [33– 35]. Such a success is justified due to the possibility of cyclodextrins being used to modify, improve or control the properties of guest compounds as, for example, to increase solubility and bioavailability [36–38], enantiomeric differentiation [39, 40], to control the volatility and antioxidant properties [41–44], to modify surfactant micellization properties [25], permeation and preservation properties in food packaging [45] and catalysis [46, 47]. The versatility of cyclodextrins and the ease in forming supramolecular compounds in solution, led to the synthesis of polymeric materials containing cyclodextrins, either as building blocks or as pending groups, by grafting CD in polymers [21, 48–50]. Major applications of these materials include environmental remediation and biomedical and pharmaceutical areas [51–54].

2 Cyclodextrin-Based Nanosponges Cyclodextrin-based polymers emerged with the need to develop materials that combine the advantages of cyclodextrins, namely the possibility of establishing inclusion complexes with numerous molecules, with the facility of removal and recycling after use, since the inclusion process can be reversible. The obtainment of crosslinked and water-insoluble materials, in addition to the possibility to introduce new functionalities in CDs, specifically through the use of different crosslinkers, has allowed the development of a smorgasbord of new materials with applications ranging from pharmaceuticals to environmental remediation. Hence, by a careful choice of the type of crosslinker, it is possible to prepare materials with different degrees of hydrophilicity, depending on the intended application. Although the denomination cyclodextrin nanosponge was only introduced in 1999 by Li and Ma [55, 56] many other researchers have previously synthesized hyper reticulated materials that fall into this category. The first polymers using cyclodextrins as monomers were described in 1965 by Solms and Egli and resulted from the reaction of epichlorohydrin with native cyclodextrins with, giving rise to crosslinked materials with glyceryl groups linking the CDs [57]. Since this first work until today, many other crosslinked cyclodextrin-based polymers have been developed including hydrogels, foams, sponges, and particles, among others. Defined as a 3D-supramolecular structure, nanosponges are highly reticulated, nanoporous, crystalline, or paracrystalline polymers, forming colloidal dispersions in aqueous media. As will be mentioned in this chapter, these can be synthesized in the form of powder or gels, as a function of the crosslinker class and the molar

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ratio linker-substrate. The latter can be divided into two major categories: inorganic, such as magnetic-iron, titanium oxide, silver- or silica-based nanoparticles, and organic, such as polystyrene, cellulose, starch, or cyclodextrins. For the preparation of nanosponges, a great variety of synthetic strategies can be applied: from the most traditional wet chemistry via multi-steps or one-pot reactions usually with a higher solvent volume consumption, prolonged times, and high temperatures, that typically results in amorphous products; microwave or ultrasonic-assisted processes with reduced solvent volumes and times; mechanical synthesis processes (dry-chemistry) where the use of solvent is minimal or absent, with minimal reaction times, low temperatures and higher crystallinity of the products. Thus, the high degree of freedom in choice the of synthetic pathways allows to design of materials with specific physicochemical properties. However, it must be specified that the usage of inorganic substrates foresees higher costs both in reagents and energetic terms since the synthesis is usually based on multi-steps processes [58], besides the need for a support structure, mostly of polymeric nature [59]. Additionally, in the last years, the application of inorganic nanomaterials has been the subject of discussion due to their toxicity and persistence (their size allows them to enter animal and vegetal cells, where they can accumulate) [60]. These aspects are two considerable disadvantages in biomedical and environmental remediation fields, for example. Those drawbacks are significantly reduced when using polymeric substrates, especially natural polymers that present low toxicity [61] and high biodegradability; beyond being abundant in nature and, therefore, with a low cost/efficiency ratio. Excellent examples are the cyclodextrin-based nanosponges (CDNSs) obtained from α-, β-, γ-, hydroxypropyl-, their derivatives, or even their mixtures as substrate. CDNSs demonstrate superior performance when compared to native cyclodextrins in increasing the availability and shelf life of active ingredients (e.g. pharmaceuticals or phytopharmaceuticals) and also ameliorating their sustained release. When used in pharmaceutical applications, CDNSs must comply with some specific requirements, mainly regarding biocompatibility and biodegradability. The non- formation of toxic degradation products under physiological conditions as well as the excretion mechanisms after use are aspects of special relevance. In this context, the shape and dimensions of the particles and the composition of the NSs are parameters that can affect their biocompatibility and biodegradability. Some cell viability studies have demonstrated that CDNSs are safe for clinical usage [62]. Cyclodextrin nanosponges are characterized by having a nanoporous and highly crosslinked structure capable of hosting smaller, hydrophobic molecules in the CD cavity and interacting with more hydrophilic or larger molecules in the pores formed by the crosslinking. They are usually prepared by polymerization methods, using solvents capable of dissolving the CDs, such as DMF or DMSO. References are also found about the use of bulk polymerization methods, in which the reaction is carried out only using the CDs and the crosslinkers, that the liquid can act as both solvents and reactants. In both methods, it is possible to perform the reactions using ultrasound or microwave irradiation which, in general, allows the use of lower temperatures and/ or the reduction of reaction times [63]. Characteristics such as a synthetic process with some degree of randomness due to the existence of three reactive groups for

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each glucose unit of the cyclodextrin, that allows a multitude of combinations, as well as the somewhat random nature of the crosslinking process, combined with the formation of insoluble materials, generally make difficult the structural characterization of CDNS. Nonetheless, it is possible to obtain structural information using spectroscopic methods (e.g. FTIR, Raman, UV-vis), solid-state NMR, thermal analysis (e.g. DSC, TG), X-ray diffraction, microscopy techniques (e.g. SEM, TEM), porosimetry methods (e.g. BET), zeta potential, etc. [64]. Moreover, the loading and release capacity are two essential properties where the amount of available CDs in the polymeric network is of great importance. Hence, direct and indirect techniques have been applied to determine the cyclodextrin content, i.e. its availability, such as elemental analysis or inclusion complexes analysis with model compounds that have high formation constant (e.g. phenolphthalein or methyl orange, among others). The presence of domains with different degrees of hydrophilicity in polycyclodextrins overcomes the main limitations of pure cyclodextrins, enlarging their application to a wider range of lipophilic and hydrophilic compounds with higher molecular weights, as well as heavy metals [65]. Herein, their ability to interact with heavy metals makes them excellent substrates for the inclusion of metal nanoparticles [66], improving their performance in bioimaging, as sensors, and as catalysts. The range of applications is additionally enlarged if several manageable variables in the synthesis process are considered [67], such as the class of crosslinkers, the degree of crosslinking, the type of free functional groups acting as active sites [68], etc., on which the physicochemical properties of CDNSs depend (including amphiphilic behaviour, response to temperature or pH, mechanical properties, available surface area, porosity, and size distribution). Thus, if we consider the unique ability of the CDs to form host-guest compounds, it is quite logic that the main applications of the CDNSs are in the pharmaceutical/biomedical field, followed by environmental application, analytical chemistry [69], cosmetic, food, and catalysis. In this chapter, for the sake of clarity, CDNSs will be divided according to the class of crosslinkers used to prepare the nanosponges (Fig. 2) and greater emphasis will be given to the prepared nanomaterials, to the detriment of applications. It should be stressed that the main focus of this chapter does not include applications.

2.1 Principal Classes of Crosslinking Agents Epoxides Epichlorohydrin is the most widely used crosslinking monomer for the preparation of CD-based polymers, even for historical reasons, and allows to obtain polymers with high cyclodextrin incorporation and considerably high molecular weights [70]. These polymers are obtained in a single step using basic catalysis and heating and, depending on the stoichiometry and reaction conditions, water-soluble, gels or highly crosslinked polymers, and therefore insoluble, can be prepared [71].

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Fig. 2 Scheme of the classes of linkers commonly used in cyclodextrin-based nanosponges

Changing other reaction parameters such as base concentration (usually NaOH) and temperature also allows the modeling of the properties of the obtained polymers [48]. The crosslinking reaction is well studied and involves the first step the opening of the epoxide ring, by reaction with a deprotonated hydroxyl group of the cyclodextrin, and the formation of an alkoxide. The alkoxide, through an SN2 -type reaction, may give rise to a new terminal epoxide by elimination of the chloride ion, which can react with the alkoxy group of another CD (Fig. 3). The final hydrophilic, amorphous polymer contains ether and hydroxyl groups in its structure. Substitution can occur at any of the available hydroxyl groups of the CD glucopiranose ring. The hyper-reticulated polymers are mostly used in chromatographic applications while gels have been widely explored for environmental remediation. Another important aspect that must be considered in this type of material that influences the final application is the porosity. Usually, CD-EPI polymers exhibit low porosity and low surface areas (typically 1 to 10 m2 g−1 ) [72]. However, it is possible to prepare materials with higher porosity and larger surface areas by using organic solvents instead

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Fig. 3 Cyclodextrin-based nanosponges crosslinked with epichlorohydrin (EPI)

of water to undertake the reaction or by using co-crosslinkers with a more rigid structure (e.g. tetrafluoroterephthalonitrile) [73, 74]. There are other approaches that allow modeling the properties of the obtained polymers, for example through the addition of other reagents, besides EPI, during or after the polymerization reaction. The use of compounds such as glycidyl trimethylammonium chloride, which introduce ionic groups in the polymers, chloroacetic acid, chloroalkylamines, among others, confer more diversified properties and functionalities to the obtained materials [75]. Although the use of EPI crosslinked CDs is widely described in the literature, namely for the removal of environmental pollutants [71, 75] (e.g. dyes, metals, aromatic compounds), for biomedical applications [70] (e.g. increased bioavailability and controlled release of drugs), food science [52], among others, not all articles mention whether or not these are nanoporous materials. Most of the CD-EPI nanosponges described in the literature were synthesized using CD:EPI molar ratios of 1:10 and NaOH as catalysts, with the reaction stopped near the gelation point. Some papers report that the obtained polymers present average pore size between 2 and 5 nm, pore volumes of 1.3–14 × 10−3 cm3 g−1 and low surface areas (0.005–11 m2 g−1 ) [74, 76, 77]. With regard to particle size, the mentioned values can be quite different, ranging from just a few nanometers to values in the order of hundreds, depending on the synthetic method and also on the type of CDs used in the preparation of the polymers [78–80]. The percentages of CD incorporation in the final polymer can range from approximately 40 to 70% [76, 78, 79, 81]. As for applications, CD-EPI nanosponges have been employed in the increase the physiological availability of some drugs (e.g. ethionamide [78, 81], repaglinide [82], temoporfin [79]) and herbicides (e.g. nicosulfuron [80]), in pollutant removal [71, 74, 76, 83], in flame retardance [84] and also in catalysis [77].

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Recently, a more complex and planned study envisages the synthesis of nanosponges crosslinked with epichlorohydrin as support for ferromagnetic nanoparticles with improved biocompatibility and excellent haemocompatibility, responsive to external magnetic fields (e.g. nuclear magnetic resonance) and decorate on the surface with pending folic groups, as tumor cell recognition sites [85]. The use of ethers other than epichlorohydrin (e.g. 1,4 butanediol diglycidyl ether) for the synthesis of nanosponges has also been referenced [86].

Carbonyls The use of crosslinkers such as diphenyl and dimethylcarbonate, carbonyl diimidazole, or triphosgene allows the introduction of a carbonyl function linking the CD rings, through the formation of carbonates. Amorphous or semi-crystalline polymers can be obtained, depending on the conditions used in the reaction. In-mass and ultrasonic-assisted reactions allow to induce crystallinity and solution reactions lead to the formation of more amorphous structures [87]. These nanosponges can be prepared with different average particle diameters, from about 200 nm to values close to 1 μm [88]. Crosslinked CDs with carbonyl groups are hard polymers that exhibit some hydrophobicity and high thermal stability (up to about 300 °C). These materials show a characteristic band in FTIR, around 1700–1750 cm−1 , due to carbonyl groups [89, 90]. Generally, the crosslinking reaction occurs through a nucleophilic attack from the oxygen of a hydroxyl group of the CD to the carbonyl group of the different crosslinkers, with the formation of a tetrahedral intermediate, followed by elimination of phenol, methanol, or imidazole (depending on the crosslinking agent) and formation of the carbonyl group again. A second attack from another hydroxyl group of another CD to the carbonyl group leads to the formation of carbonate groups and thus to the crosslinking of the CDs (Fig. 4). Carbonate nanosponges were developed by Trotta et al. and the first published papers refer to their use for the increase of drug solubilization efficiency (dexamethasone or flurbiprofen) and for the removal of chlorinated aromatic compounds from water. These materials were synthesized by reaction of β-cyclodextrin with carbonyldiimidazole (solution polymerization) or diphenylcarbonate (bulk polymerization) [90–92]. Subsequently, NS of α- β- and γ-cyclodextrins crosslinked with carbonyldiimidazole (CDI) were also synthesized and used for oxygen delivery. The NS had surface areas of 40–50 m2 g−1 , an average diameter of 400–550 nm, and a zeta potential of − 30 mV [93]. Since these pioneering works, many others have been published using CDs and derivatives crosslinked with carbonyl compounds and used mainly in pharmaceutical applications (increased bioavailability and controlled release of drugs, including cancer drugs) [62, 64, 67, 94, 95]. Biocompatibility studies have shown that these NSs are non-toxic and show no appreciable degradation during

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Fig. 4 Scheme of the synthesis of carbonyl-cyclodextrin-based nanosponges

gastrointestinal transit, so they are considered safe for concentrations up to 2 g kg−1 for drug delivery [96]. CDNSs using carbonyldiimidazole as crosslinker are usually synthesized using β-CD, DMF or DMSO as a solvent, and different ratios of β-CD: CDI, usually 1:2, 1:3, 1:4, 1:6, or 1:8. The reaction occurs at temperatures varying from 100 °C to DMSO reflux (189 °C) The particle sizes obtained are in the range 300–600 nm and the zeta potentials are negative (between −10 and −30 mV). The polymers show high thermal stability and no thermal degradation is observed up to 320 °C [97–99]. The preparation of CDNS-CDI using an interfacial polymerization method in which β-CD is dissolved in a basic aqueous solution and CDI in dichloromethane has also been described. The NSs obtained in this way present smaller average particle sizes (~100–250 nm) than those obtained by solution polymerization [100]. More recently the use of a ball-mill to prepare CDNS-CDI in the absence of solvents has also been described [101, 102]. CDNS-CDI were used to increase biological availability, solubility, and durability of drugs (piroxicam [97], resveratrol [103], atorvastatin calcium [99], sulfamethoxazole [104], econazole nitrate [105], among others), controlled-release of drugs, including for cancer treatment (e.g. tamoxifen [98], L-DOPA [106], erlotinib [107], paclitaxel [108], bortezomib [109]), antimicrobial activity [100], encapsulation of antioxidants (polyphenols [110], kynurenic acid [102]) and as a carrier for herbicides (ailanthone) [111]. As an alternative to carbonyldiimidazole, diphenylcarbonate (DPC) is widely used to obtain crosslinked CDNS with carbonyl groups. In this case, diphenylcarbonate and CD are usually fused in the absence of solvent by heating to about 100 °C. CD:DPC ratios of 1:2, 1:4, 1:6, 1:8, 1:10 are commonly used, obtaining polymers with average particle size between ~ 100-1000 nm and zeta potentials between − 10 and − 30 mV [112–115]. The use of solvents [116] or ultrasound (to promote the in-mass reaction) has also been described, obtaining in the latter case particles with an average diameter of approximately 650 nm and zeta potentials of

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− 15 to − 24 mV [117]. Microwave irradiation was recently used for the synthesis of CDNS- DPC and particles with an average size close to 200 nm, using a CD:DPC ratio of 1:6, were obtained [118, 119]. The increase in bioavailability of drugs (nicosulfuron [80], dithranol [113], nifedipine [117], rilpivirine [118], domperidone [119], giseofulvin [120], antirestenotic agent DB103 [121]), the transport of therapeutic agents (phenylethylamine and 2-amino-4-(4-chlorophenyl)-thiazole and gold nanoparticles [112]), cancer therapy (camptothecin [122, 123], diosmin [115], curcumin [124]) and the encapsulation of food additives [116], essentials oils [114], antioxidants (ferulic acid [125] and limonene [126]) have been referred as applications of these kind of NSs. Dimethylcarbonate (DMC) can also be used for the preparation of crosslinked CDNS with a carbonate function. References to the use of this monomer are scarce in the literature, however, the preparation of these nanosponges is performed using dimethylformamide as solvent at reflux temperature. Particles with an average diameter of 487 nm were produced and these nanosponges were used for the release of curcumin [127]. The synthesis of CDNS-DMC through a hot-melt synthesis has also been reported. NSs were synthesized using different proportions of dimethyl carbonate and particles with an average diameter within approximately 200–280 nm were prepared and used for the encapsulation of paracetamol, aceclofenac, and caffeine in order to increase their solubility for application in combined therapy [128].

Anhydrides/carboxylic acids/acid chlorides Poly-anhydrides/carboxylic acids and acid chlorides can be joined in the same group of crosslinkers due to the resulting ester functional group that is obtained. However, NSs with lower chemical stability are obtained if compared to those with urethane groups. These three classes of monomers, together with the class of epoxides, are the most widely used for the synthesis of soluble and insoluble cyclodextrin- based polymers due to the simplicity of the reaction, often without prior modification of the CDs. Herein, only insoluble polymers it will be discussed. Generally, the ester-CDNSs exhibit a high-water sorption capacity, due to the contribution of the ester groups to the formation of hydrogen bonding, which is inversely proportional to the degree of crosslinking. Within the anhydride class and in general, pyromellitic anhydride (APM) is one of the most common monomers used for cyclodextrins polymerization. Generally, increasing the molar ratio of CD-APM allows to preparation of hydrogels (1:3) to powders (1:4, 1:6, and 1:8), as expected, due to the higher crosslinking degree, although with a reduction in the surface area. However, minimum particle size and PDI (polydispersity index) were obtained for CD-APM 1:6 ratio with the largest pore size (242 nm—technically outside the definition of nanosponge) [129]. The hydrogel CD-APM presents a high-water absorption capacity (830%) [130]. CDNSAPM has been applied as a diclofenac-selective fluorescent sensor [131] and, for the first time, in 2014, as a transport and permeation system for diclofenac through the

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skin [132]. CDNS-APM sensors have also been developed by multi-surface functionalization with anti-IgG and loaded with a peroxidase enzyme as a colorimetric and/ or electrochemical detection probe [133]. Differently, CDNS- APM has been applied as a nanosystem for the transport of N,N-diethyl-3-toluamide (DEET—insecticide) [134]. CDNS-APM prepared with different molar ratios were compared as transport systems for rosuvastatin (hepatic sterol synthase inhibitor). Increasing molar content of APM implies a considerable increase in pore size, which does not fit in the definition of nanosponges, although the drug loading capacity is promising [129]. CDNS-APM achieves an encapsulation efficiency of curcumin of ≈98% [135]. Other examples of anhydride monomers are phthalic and nitrophthalic anhydride. It was observed that higher molar ratios and reaction temperatures led to polymers with higher molecular weight (> 100 kDa), higher crosslinking degree, and the number of active sites, which play a key role in the flocculation process of Cu2+ ions [136]. Considering carboxylic acids, the reactions are mainly carried out at high temperatures. The usage of naphthalene dicarboxylic acid as a crosslinker led to a polymer whose particle size increases with the variation of time and temperature. This NS was applied to increase the solubility/stability and hence the applicability of Salvia officinalis essential oil [137]. Phthalic acid and succinic acid were used in the acid or acyl chloride forms, overcoming the previous functionalization of cyclodextrin and, obtaining polymers with a certain degree of crystallinity. By increasing the molar content of the crosslinking agent, higher efficiency of the reaction and crosslinking degree are obtained, with a homogeneous distribution of particle size. The two NSs showed a superior complexation of curcumin (≥ 60%) when compared to pure cyclodextrin (≈30%) [138]. The effect of the molar ratio and temperature on the synthesis of CDNSs with succinic acid was investigated via thermal analysis of the polymers, both in dried and hydrated forms. An increase in thermal stability was obtained using higher molar content of NaH, to deprotonate the CD, and temperature [139]. Succinic acid and Epiclon B-4400 (EPB) were also used as polyacid monomers for the inclusion of magnetic iron nanoparticles via chelation between carboxylate groups. The polymers were successfully applied to remove rhodamine B and methylene blue from aqueous solutions and reused for up to 5 cycles [140]. The preparation of βCD-based polymers with increasing molar content of EPB showed a reduction in pore size with an increase in surface area, thermal stability, and wateruptake capacity. The most crosslinked polymer also shows the highest ability to solubilize curcumin [141]. A study of diffusion of ibuprofen was carried out using CDNS-EDTA in molar ratio 1:4 and 1:8 as a delivery system [142, 143]. The same group of research demonstrated that a complete structural characterization of CDEDTA hydrogels can be obtained by FTIR and Raman analysis [144]. CDNS-EDTA shows a superior capacity for 5-fluorouracil solubilization when compared with pure β-CD and CDNS-DPC [145]. Another study compares hydroxypropyl-β-CD (HβCD) nanosponges synthesized using APM, DPC, and citric acid (CTA); the crosslinking with APM and CTA is achieved in the presence of a catalyst. The three polymers are used as transport systems for the anticancer drug naringenin [146]. CDNS-APM and CDNS-CTA exhibit good water-swelling ability as well as high interaction with Cu2+ and Zn2+ ions due to the presence of the carboxylate chelating group [147,

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148]. Recently, a synthetic process was developed that considers the use of CTA as a crosslinking agent and choline chloride as a co-solvent for the formation of a natural deep eutectic solvent. Three molar ratios, temperatures, and reaction times were tested, observing that the molar ratio plays a key role in the polymer properties, controlling the number of active groups and, consequently the removal efficiency of caffeine, molybdenum blue, and methyl orange [149]. Maleic acid has also been used as a monomer and was observed an enhancement in the thermal stability of CDNSs by decreasing the molar content of the acid and increasing the reaction temperature [150]; the monomer has also been used in the anhydride form [151]. Less common is the usage of 2,2-bisacryloamidoacetic acid (2,2AAA) as a crosslinking agent, which was compared with 1,4-disacryloylpiperazine (1,4AP) for the synthesis of NSs with α-, β- and γ-cyclodextrins. α- and γ-CDs crosslinked with 2,2AAA and 1,4AP, respectively, result in soluble polymers. On the other hand, for the insoluble polymers, the water-swelling ability decreases in the order γCD-2,2AAA < βCD-2,2AAA < αCD2,2AAA < βCD-1,4AP < αCD-1,4AP, probably due to greater structural flexibility of the aliphatic monomers when compared to the cyclic ones. [152]. Moreover, hydrogels of βCD-2,2AAA exhibit good thermal stability, with a water-swelling capacity of 1400% [153]. Terephthaloyl chloride was used as a crosslinking monomer in the preparation of nanosponges used as a polymeric support for tannic acid, which considerably increases Pb2+ uptake and presents high selectivity [154].

Isocyanates The usage of isocyanides as crosslinking monomers led to the formation of urethane linkages, that present a high chemical stability. Moreover, the high reactivity of the isocyanide groups results in a high yield of the reaction with the –OH groups of the cyclodextrins, obtaining thickly crosslinked and rigid structures. It is even possible to obtain polymers with similar physicochemical properties through different synthesis routes, without or in the presence of catalysts, by wet route (using very variable temperature range and reaction time), or by mechanochemical route (in the presence or not of inorganic particles). Similarly to anhydrides, by changing the molar ratio of CD-monomer, a gel or a powder product can be synthesized. The firsts are usually responsive to temperature variations, although with low-water sorption capacity [155–158]. At a constant molar ratio CD-crosslinker, the effect that aliphatic or aromatic isocyanides and chain size have on the surface area, porosity, CD content, and availability [159–161] was evaluated. It was observed that they mainly depend on the degree of rotational freedom of the crosslinker, stiffness, and steric hindrance (Fig. 5). By increasing the molar content of the crosslinkers, a decrease in particle size [157] and thermal stability [162] was observed. For instance, isocyanate monomers, such as 1,6-hexamethylene diisocyanate (HDI), 4,4-methylenediphenyl diisocyanate (MDI) and toluene diisocyanate (TDI), among others, have been successfully used in the synthesis of nanosponges for the removal of azo dyes and aromatic amines [163], as well as nitrosamines from model and real water samples from South Africa [164]; and as solid-phase for extraction processes [165]. CDNS-HDI were applied to

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Fig. 5 Representation of isocyanate-cyclodextrin-based nanosponges and effect of the different linkers in some physical properties of the nanosponges

remove ibuprofen from water and sewage [166] and as polymeric support for different Au nanoparticle clusters to evaluate their catalytic properties for the 4-nitrophenol reduction [167]. Epichlorohydrin and HDI as crosslinkers were compared at different molar ratios, showing similar complexation abilities for beta-carotene. However, CDNS-EPI is more efficient to enhance the carotene solubility [168]. Cyclodextrin derivatives can also be used to advantageously modulate the amphiphilic character of the polymer; for example, the use of heptakis(2,6-di-O-methyl)-β-CD crosslinked with diisocyanates is highly performant for the polychlorinated biphenyls (PCBs) removal of [169].

Amines/amides The usage of polyamines as crosslinkers in the synthesis of nanosponges needs the previous modification of cyclodextrins with better-leaving groups, such as tosylates or halogens, through mono-, di- or per- substitutions. Through correct synthetic planning, it has been possible to obtain doubly responsive materials, for example to pH or glutathione, using a mixture of monomers as crosslinkers, which were applied for the controlled release of doxorubicin [170]. Another interesting example is the use of aminated-CDNSs in their pure form or as a support structure for silver nanoparticles sensitive to pH variations, that were applied as sorbent for 11 compounds with different physicochemical properties [171] or as catalysts in the reduction of nitroarenes and the oxidation of anilines [172]. Recently, amino-CDNSs were successfully applied in the removal of a pure active ingredient and a commercial formulation of pesticide. The effect of the use of α- or β-cyclodextrin and the

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Fig. 6 Cyclodextrin-based nanosponges crosslinked with 2,3,5,6-tetrafluoroterephthalonitrile

aliphatic chain length of the diamines used as crosslinkers has been evaluated [173, 174]. Chain reaction polymerization is another technique used for the synthesis of nanosponges, with the use of highly reactive monomers, such as methylene bisacrylamide and acrylic acid. The nanosponges exhibit high chemical and thermal stability, high water-swelling capacity, ameliorate solubilization ability of dexibuprofen, and non-toxicity in ex-vivo studies [175].

Halogenated compounds Halogenated compounds, such as cyanuric chloride or 2,3,5,6tetrafluoroterephthalonitrile (TFTPN), can be used as crosslinkers for the preparation of CDNSs, through nucleophilic substitution reactions (Fig. 6). The polymerization reaction of β-CD with 2,3,5,6-tetrafluoroterephthalonitrile, described by Alsbaiee et al., when carried out in a mixture of THF:DMF 9:1, at 80 °C in the presence of potassium carbonate, allows obtaining the respective polymers in 45% yield. The reaction was done using several β-CD:TFTN ratios and surface areas between 35 and 263 m2 g−1 (the highest value is obtained for a β-CD:TFTN ratio of 1:3) and pore size ranging from 1.8 to 3.5 nm was obtained. If the reaction was carried out using NaOH at 60 °C, a non-porous polymer was obtained, with a smaller surface area (6 m2 g−1 ) and a lower water absorption capacity (86% versus 265%). Changing some reaction parameters such as base addition rate or initial monomer concentration led to higher reaction yields (up to 67%) [176]. More recently, other authors report the preparation of a polymer of βCD-TFTN 1:3.3, synthesized under similar conditions to those previously mentioned, with yields close to 40%. Analysis of the polymer by FTIR shows a band at 2252 cm−1 , characteristic of CN bonds. The specific surface area obtained was 55.2 m2 g−1 , total pore volume of 0.108 cm3 g−1 , and average pore diameter of 7.88 nm. This material was used for the solid phase extraction of aromatic amines [177]. CDNS-TFTPN has shown good results in the removal of several organic pollutants [73], in the separation and concentration of trace quinolones from wastewater [178], in the removal of a wide variety of organic micropollutants [179], and as a binding agent for monitoring endocrine disrupting chemicals [180].

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The reduction of nitrile groups to amine, using BH3 S(CH3 )2 , allowed the synthesis of another material, which has a high affinity for the removal of anionic micropollutants, in particular, anionic perfluorinated alkyl substances [181]. Reacting a mixture of β-CD and heptakis-(6-deoxy-6-azido)-β-cyclodextrin with TFTN, followed by reduction of the azide to amine and reaction with d-(+)-gluconic acid δ-lactone, allowed to prepare a NS that was used in the removal of boric acid and bisphenol A. The obtained NS presented a surface area of 9.11 m2 g−1 , and pore size < 8 nm before modification with d-(+)-gluconic acid δ-lactone [182]. The crosslinking reaction of β-CD with cyanuric chloride has also been described and a polymer containing triazine rings in its structure was prepared. The reaction was carried out using different percentages of β-CD-cyanuric chloride, in the presence of NaOH and benzyldimethylhexadecyl ammonium chloride, in an acetonitrile/water mixture. Under these conditions, the obtained polymers have a low surface area (4.5 m2 g−1 ). FTIR bands at 1720, 842, and 790 cm−1 confirm the presence of C=N bonds (first one) and aryl chloride in the final polymer. TGA analysis proves that these polymers are stable up to at least 250 °C. These polymers were used in the removal of benzene, bisphenol A, 2-naphthol, 2-cholor-biphenyl, and dibutyl phthalate with good removal efficiencies [183].

3 Miscellaneous and Other Crosslinkers A very interesting study on the application of CDNSs in-vitro and in-vivo, has evaluated the performance of 4 different polymers obtained with 4 different classes of monomers: toluene diisocyanate, carbonyldiimidazole, pyromellitic dianhydride, and citric acid. It was observed that the nanosponges showed decreasing thermal stability in the order: anhydride > epoxy > cyanate ≈ carbonate, similar size in water and tendency to aggregate in biological fluids, with exception of NS-urethane. The last one also shows a better sorption capacity of indole in water as well as in biological matrices, followed by NS-epoxy > NS-carbonate > NS-anhydride [184]. Another study used 1,6-hexamethylene diisocyanate, carbonyldiimidazole, and pyromellitic dianhydride in a molar ratio of 1:8 CD:crosslinkers. Two different synthetic processes were considered, namely bulk condensation and interfacial condensation. It was observed a pronounced effect of the synthetic process on particle size distribution 400–500 nm and 50–200 nm, respectively, and all polymer formulations did not exhibit toxicity at the limits studied [96]. In addition to the classes of crosslinkers mentioned above, it must be referred the synthesis of nanosponges via Click Chemistry methods, namely by reaction of azido-cyclodextrins with compounds containing terminal alkynes, in the presence of copper (CuAAC), originating triazoles. NSs of this type were synthesized by reacting heptakis-(6-deoxy)-(6-azido)-β-CD with tetrakis(25,26,27,28-propargyloxy)-(5,11,17,23-tert-butyl)-calix[4]arene using different ratios of CD:calixarene. Materials with mass yields between 72 and 96%, average pore sizes between 1.9 and 3.9 nm (pore size decreases with the increase in the

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percentage of crosslinker), surface areas between 3.2 and 8.1 m2 g−1 and pore volumes between 0.009 and 0.026 cm3 g−1 were prepared. The occurrence of vibrational modes at 1239 and 1015 cm−1 , confirm the formation of the triazine ring. This NSs were used for the sequestration of nitroarenes and for the release of bioactive molecules (quercetin and silibinin) [185, 186]. In addition to the aforementioned compounds, the usage of heptakis-(2,3dimethoxy)-(6-deoxy)-(6- azido)-β-CD and tetrakis-propargyloxy-calix[4]arene for the synthesis of different NSs have also been described. Post-modification of NSs was also performed by reduction of the unreacted azide groups to amine or by modification of the triazine formed ring with sodium carboxylate groups. These materials were used for the removal of p-nitroaniline, Bromocresol Green and Pb2+ from water [187, 188]. Another crosslinking strategy mentioned in the literature consists of using CDs modified with vinylic monomers that can be crosslinked by reaction with thiols or by Michael polyadditions [152]. For example, heptakis-6-(tert-butyldimethylsilyl)2-allyloxy-β-cyclodextrin was reacted with an inorganic halloysite clay modified with 3-mercaptopropyltrimethoxysilane, in the presence of AIBN, herein, obtaining an organic-inorganic hybrid nanosponge that was used in the absorption of cationic dyes [189].

4 Conclusion Cyclodextrin-based nanosponges have unique characteristics mainly due to the amphiphilic properties of cyclodextrins, as well as their ability to form supramolecular adducts, associated with the high surface areas of nanosponges. These and other properties can be modeled either with the use of different cyclodextrins, or even mixtures of cyclodextrins or with an appropriate choice of crosslinkers. The latter can play an important role in increasing the functions and applications of nanosponges. Although five different generations of nanosponges have been developed, from nanosponges with tunable swelling properties to nanosponges that may be able to permeate the cellular membrane, there is a long way to go in the development of nanosponges by using green synthesis approaches and new crosslinkers, as well as in the development of nanosponges with dual or multifunctions. With regard to the applications of nanosponges, there is still much to explore in order to take full advantage of the properties of nanosponges, in areas such as food packaging and catalysis. For these reasons, cyclodextrins remain one of those molecules that, although old, still have a promising future in research laboratories and industry. Acknowledgements The authors acknowledge Fundação para a Ciência e a Tecnologia (FCT), the Portuguese Agency for Scientific Research for the financial support through projects UIDP/00313/ 2020. Gianluca Utzeri thanks FCT for the PhD grant SFR/BD/146358/2019.

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Synthesis, Functionalization Strategies and Application of Different Types of Cyclodextrin-Based Nanosponges for Water Treatment Naveen Goyal, Dorothy Sachdeva, and Udupa Sujit

Abbreviations AA BIM BPA CD CDNS CMP DCP DFPS DLS DMF DMSO EBT EDTA EPI GO HDI MBA MCD: VI NP

Acrylic acid Benzylimidazole Bisphenol A Cyclodextrin Cyclodextrin nanosponges 4,4' -Bis(chloromethyl)-1,1' -biphenyl Dichlorophenol 4,4' -Difluoro diphenyl sulfone Dynamic light scattering Dimethyl formamide Dimethyl sulfoxide Eriochrome Black T Ethylene diamine tetraacetic acid Epichlorohydrin Graphene oxide Hexamethylene diisocyanate N,N' -methylene-bis-acrylamide Methacrylic-βCD with 1-vinylimidazole (VI) Nitrophenol

N. Goyal (B) · D. Sachdeva · U. Sujit Materials Research Centre, Indian Institute of Science, Bangalore 560012, India e-mail: [email protected] D. Sachdeva e-mail: [email protected] U. Sujit e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Gulati (ed.), Nanosponges for Environmental Remediation, https://doi.org/10.1007/978-3-031-41077-2_6

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Polyacrylamide Photon correlation spectroscopy 1,4-Phenylene diisocyanate Polydispersity index Toluene-2,4-diisocyanate Tetrafluoroterephthalonitrile 5,5' ,6,6' -tetrahydroxy-3,3,3' ,3' -tetramethylspirobisindane

1 Introduction Water is among the unavoidable necessities of life, but increment in the worldwide residents has increased demand for freshwater resources. Therefore, wastewater treatment is the utmost necessary chore for this generation. This wastewater is a collective contribution from various impurities such as heavy metals, drugs, ligands, water soluble dyes, and pesticides; removing all of these pollutants is tricky and time-consuming [1]. Various methods have been deployed to treat wastewater. The traditional methods used for water treatment include physical, chemical, mechanical, and biological methods. A brief description of some of these methods is illustrated below. (1) Physical methods include physical forces to remove contaminants e.g., the sedimentation process to settle down the heavy particles by gravitational pull [2], the flotation method induces air bubbles to remove particles from the water [3] and flow equalization for basic water treatment by varying flow and temperature at a certain duration [4]. (2) Chemical methods include the ion exchange of soluble impurities by various reagents to remove contaminants [5], and chemical precipitation of heavy metals to form insoluble precipitates [6]. (3) Mechanical methods include screening to remove gross pollutants [7], and filtering to remove organic materials [8]. (4) Biological methods include disinfection which inhibits microbial growth by destroying the structure within the microbes [9], and dechlorination uses activated charcoal or a reducing agent to kill the undesired bacteria from the wastewater [10]. (5) Ultrafiltration method for deletion of colloidal, macromolecular matter, and suspended materials from the wastewater by passing through a transmembrane with low pressure. This method was found potent for deletion of heavy metal ions [11]. (6) Reverse Osmosis removes low molecular weight organic compounds and minerals by allowing only purified water to move across a semi-permeable membrane [12].

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(7) Nanofiltration process is used for the elimination of heavy metals from the unpurified water. It could be considered just the intermediate process of ultrafiltration and reverse osmosis [13]. (8) Electrodialysis method applies an electric potential between the two electrodes, which results in the ion exchange through a selective membrane using either cation exchange or anion exchange membrane to remove heavy metal ions or hydrophilic impurities [14]. (9) Adsorption process removes the contaminants from the wastewater by physisorption or chemisorption process on the surface of the adsorbent. It is eco-friendly and reusable [15]. These traditional methods have various drawbacks, some of which are listed below. (1) (2) (3) (4) (5) (6)

Low selectivity towards a particular contaminant. Lower reaction kinetics. Resulting large number of sediments. Require high capital and operational cost. Membrane filtrations generally require high pressure. Ineffective against vat dyes.

Among all conventional methods, the adsorption process is commonly used for wastewater treatment due to its reusability, better kinetics, and higher efficiency. Cyclodextrins (CD) are one such adsorbents that are sustainable and eco-friendly to be used for wastewater treatment. CDs are oligosaccharides generated through enzymatic hydrolysis of starch and contain d-glucopyranose units bonded by 1,4 glycosidic bonds. These include both hydrophobic and hydrophilic moieties, which help them to dissolve in most solvents and hence increase their applications. These CDs have found applications in protein and drug delivery [16], food industry [17], and wastewater treatment [18]. CDs undergo copolymerization with the specific cross-linkers to form nanosponges which are favoured due to the greater number of hydroxyl groups. In 1998, DeQuan and Min Ma coined the term CD-based nanosponges (CDNS) for the first time. These nanosponges are porous 3D structures with hydrophilic properties which are very stable under broad pH range and temperature. Even the low solubility molecules can be accommodated in these CDNS. With an increase in the degree of cross-linking the hydrophilicity decrease, which could be used in sewerage purification. CDNS has capacity to interact with various pollutants such as organic dyes, phenols, heavy metal ions and pesticides via host–guest interaction, hydrogen bonding, electrostatic forces and van der waals interactions. Owing to these capabilities, CDNS are rising stars in terms of cleaning waste water by adsorption process. Not only for adsorption purposes, these CDNS also used for drug delivery, electro-catalyst and as a gas-sensor for specific selectivity [19–21]. This chapter deals with the synthetic strategies of CDNS and various functionalization methods to enhance cross-linking in these nanosponges. Among functionalization processes, chemistry of hydrogels, magnetic composites and hybrid

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nanostructure have been discussed in detail. Employment of these nanosponges for the water purification application is illustrated by providing numerous examples of removing contaminants from the waste water followed by conclusing and future remarks.

2 Structure The CDNS are porous 3-dimensional nanostructures with cyclic oligosaccharides linked with bifunctional cross-linking agents. CDs are classified based on amount of glucopyranose units as 6, 7, and 8 glucopyranose units are called α, β, and γ-CDs, respectively. The glucopyranose unit is present in the chair form, which results in the generation of a truncated cone shape structure as shown in Fig. 1a, b. The hydroxyl group of each glucopyranose is present on the outer surface of the cone, one with a primary –OH group of 6th position at the narrow edge while secondary –OH group of 2nd and 3rd positions at the wider edge, which makes the whole outer surface of the cone as hydrophilic which enhance the interaction with polar compounds. In contrast, the inner cavity contains C-H groups and ethereal oxygen which makes the core of CD hydrophobic. The hydrophilic nature of the outward surface of the CDs makes them water soluble through hydrogen bonding. To synthesize CDNS, CDs have to cross-link with various chemical reagents called cross-linkers. Generally, the cross-linkers used are ethylene glycol, epichlorohydrin (EPI), citric acid, ethylenediaminetetraacetic acid (EDTA), chitosan, diglycidyl ether or divinyl sulfone. The chemical structure of some of these cross-linkers employed for the generation of CDNS are displayed in Table 1. These cross-linkers act as an

Fig. 1 a Chemical structure of β-CD with the inner cavity, b truncated cone-like structure

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electrophilic center for the nucleophilic hydroxyl group of CDs present on the outer surface. The reaction scheme for the synthesis of CDNS using CD and cross-linker is shown in Fig. 2. Depending upon the type of cross-linkers used, the properties of CDNS vary drastically. Depending upon amount of CDs and the extent of crosslinking, size of the cavity varies. The smaller number of CDs and a high cross-linking degree, result in a smaller cavity size. Depending upon the reaction conditions like temperature, pH, and reaction time, CDNS with varying structures can be obtained. For example, smaller cavities are favored in highly acidic conditions whereas large cavities could be obtained by providing alkaline conditions. CDNS are divided into four generations namely first, second, third, and fourth generation.

2.1 First-Generation CDNS These are formed by a simple wet chemistry combination of CD with a cross-linker. Based on type of linkers used, these are classified into 4 subtypes depending upon the cross-linkers as urethane, carbonate, ester, and ether nanosponges, as illustrated below. Urethane-based nanosponges: Urethane or ethyl carbamate CDNS uses diisocyanates being hexamethylene diisocyanate (HDI) and toluene-2,4-diisocyanate (TDI) for the synthesis. These nanosponges require an aqueous medium for inclusion formation rather than an organic medium. As compared to activated charcoal, these nanosponges adsorb some organic moieties, namely p-nitrophenol (NP) even at very low ppb concentration levels which perhaps elucidated by the adsorption and diffusion of organic molecules through the surface to the bulk nanosponges [22]. These nanosponges have diverse applications for the removal of organic compounds, volatile compounds, and biological compounds like aromatic amino acids and bilirubin. The nanosponges adsorb about 24% of aromatic amino acids as compared to ~2% of branched-chain amino acids [23]. Carbonate-based nanosponges: Carbonate CDNS requires active carbonyl group cross-linkers like diphenyl carbonate, 1,1' -carbonyldiimidazole, and triphosgene. Depending upon the ligand, the reaction temperature varies from room temperature to high temperature. These CDNS also has a low surface area like Urethane CDNS (2 m2 /g) [24]. As compared to activated charcoal, these carbonate CDNS show high absorbance to eliminate chlorinated tenacious organic pollutants from wastewater, CDNS (~99.5%) removes pollutants much more efficiently as compared to activated charcoal [25]. The extent of cross-linkers binding in carbonate CDNS is characterized by combining Raman and IR spectroscopy. Rossi et al. have investigated that the mechanical properties of CDNS is highly dependent on the ratio of cross-linkers used. Even stiffness and elastic properties of CDNS depend upon the moles of cross-linkers used and not on the type of CD used [26].

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Table 1 Structures of different cross-linkers used to form β-CDNS Carbamates HDI

TDI

Carbonyls Diphenyl carbonate

1,1' -Carbonyldiimidazole

Triphosgene

Acids/esters EDTA

Butane tetracarboxylic dianhydride

Pyromellitic anhydride

Citric acid

Ethers EPI (continued)

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Table 1 (continued) Carbamates Ethylene glycol digylcidyl ether

Bisphenol A digylcidyl ether

Fig. 2 Synthesis of CDNS by reaction of β-CD and cross-linkers

Ester/acid based nanosponges: Ester CDNS requires dianhydrides and carboxylic acid groups like EDTA, butane tetracarboxylic dianhydride, pyromellitic anhydride, citric acid, etc. In contrast to Urethane and carbonate CDNS, the ester/acid CDNS adsorbs water and form hydrogels. With an increased ratio of cross-linkers the water adsorption decreases. Similar to carbonate CDNS, combining Raman and IR spectroscopy with quantum computational study, the degree of cross-linking by detecting the vibrations between pyromellitic CDNS can be predicted. When the ratio of CD: pyromellitic is 1:6, it shows the maximum cross-linking and on decreasing the ratio to 1:8, or 1:10 the cross-linking tends to decrease due to steric hindrance [27]. Similarly, in the case of EDTA dianhydride, 1:6 has the highest degree of cross-linking [28]. Similar to carbonate CDNS, the mechanical properties, stiffness, and elastic properties of these CDNS also depend upon the molar ratio of cross-linkers used. These show versatile functions due to their high solubility and stability in water during photo-catalysis in the case of derivates of benzoporphyrin. In 2005, Martel et al. prepared CDNS using citric acid and polyacrylic acid as cross-linkers. Polycondensation reaction simply involves the heating of reactants between 140 and

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170 °C in low pressure or in atmospheric conditions depending upon the type of product required. The product could be a soluble or insoluble polymer [29]. Skiba and Lahiani-Skiba [30] use the melt polycondensation method by simply heating CDs and citric acid in temperature ranging from 140 to 150 °C in a sodium phosphate medium. These CDNS are very good adsorbents for pharmaceutical materials from the water body and could be used over multiple cycles [30]. The EDTA CDNS and 1,2,3,4-butanetetracarboxylic acid CDNS help to adsorb heavy metal cations from the water bodies [31]. Pyromellitic nanosponges form a complex with a variety of metal cations of s-block and transition metals [32]. Ether-based nanosponges: Ether CDNS requires EPI, ethylene glycol digylcidyl ether, or bisphenol A digylcidyl ether as cross-linkers. Depending upon the reaction condition, these can form either soluble or insoluble polymers in aqueous medium or in basic condition. The magnitude of cross-linking and the chain length governs the adsorptivity of CDNS. The adsorption process follows Freundlich isotherm which shows that adsorption occurs only at the monolayer. Though EPI is toxic, still its CDNS is used in biomedical applications as an anti-inflammatory or antifungal drug. EPI CDNS has high adsorptivity for organic compounds like halogenated or aromatic pollutants, so these are used as stationary phases in chromatographic separation. These are also used to remove flavours in food industries. Contradicted to the abovementioned CDNS, this ether-based CDNS posses larger surface area of 35–263 m2 / g when prepared using tetrafluoroterephthalonitrile (TFP) with potassium carbonate acting as a catalyst [33].

2.2 Second-Generation CDNS To improve the functionality of first generation CDNS some special moieties are added to form second generation CDNS. They could be added either during the addition of cross-linkers, before adding cross-linker, or after adding cross-linker to functionalize nanosponges. Kawano et al. [34] formed hydrophobic nanosponges using heptakis(2,6-di-O-methyl)-CD and 1,4-phenylene diisocyanate (PDI) as crosslinker to remove polychlorobiphenyls in the apolar solvent. Fluorescent dyes like rhodamine or fluorescein is used to functionalize EPI CDNS. Lembo et al. [35] functionalize negatively charged CDNS by adding fluorescein isothiocyanate. These nanosponges are further studied for in-vitro studies and show high antiviral activity [35]. Zohrehvand et al. [36] added 2-naphthol to EPI CDNS in a basic medium to form fluorescent CDNS. Deshmukh et al. [37] used these secondgeneration CDNS as antibacterial agents by modifying the carboxylate group and introducing lysozyme.

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2.3 Third-Generation CDNS This class of nanosponges includes nanosponges that change their behaviour according to environmental conditions like temperature, pH, oxidation, or reduction condition. These can be utilized for specific drug delivery applications e.g., releasing drug into the target only by a specific pathway or for obtaining specific signals from the environment. Calixarene and CD undergo co-polymerization to form pH-sensitive nanosponges [38].

2.4 Fourth-Generation CDNS These are prepared by molecular imprinting techniques which are highly selective for specific molecules. Using the polymerization method, defined interactions are formed between the template and the monomer unit using cross-linkers. Later, after removing the template, the left-out cavity is responsible for selectivity. For the formation of such a cavity, the template must contain the hydrophobic part to form host–guest compounds with CDs. Kyzas et al. formed a molecularly imprinted polymer using TDI using different templates which shows good binding for dyes than chitosan [39].

2.5 Functionalization of CDNS Hydrogels Hydrogels have attracted great attention in the last few years as these are employed for the selective adsorption of heavy metals from the unpurified water. Song et al. found that methacrylate-based β-CDs undergo free-radical copolymerization with acrylamide monomer to form a 3D network of β-CD-polyacrylamide (β-CD-PAAm) called hydrogels. These hydrogels result in the formation of large pore-size with a high swelling ratio of 29.4 as compared to PAAm gel and are found effective in the removal of dyes such as phenolphthalein and bisphenol A (BPA). These hydrogels can be recycled by using methanol and can be employed further for 5 cycles [40]. Huang et al. [41] synthesized β-CD-based hydrogel via microwave irradiation using acrylic acid (AA) and N,N' -methylene-bis-acrylamide (MBA) which is used to remove Cd2+ , Pb2+ , and Cu2+ from the wastewater. Magnetic Nanocomposites Iron oxide nanocomposites are used for further functionalization of CDNS to improve their removal efficiency. As carboxymethylated β-CDs are used for the extraction of heavy metals like Cd2+ , Ni2+ , Cu2+ , and Hg2+ by interaction with the carboxyl group of these metals. Using the same substrate, Badruddoza et al. [42] synthesized an

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adsorbent by functionalizing it with Fe3 O4 nanoparticles which results more hydroxyl and carboxyl groups, enhances the removal efficiency for Cu2+ ions. Kong et al. [43] synthesized Fe3 S4 nanoparticles based β-CDs and used for deletion of Pb2+ ions from the unpurified water. As a consequence of electrostatic interaction and PbS precipitation, the adsorption capacity realized for the deletion of Pb2+ was 256 mg/g [43]. Hybrids Improvement in the adsorption performance by creating Hybrid nanosponges has been seen in the litrertuatre [44]. Cheng et al. [45] decorated β-CDs on graphene oxide (GO) which is highly stable in polar solvents. These complexes are used for the removal of various dyes like rhodamine B, methyl orange, and even methylene blue by nearly 100%. This membrane has specific adsorption for Cu2+ ions being permeability of 5 × 10−2 mmol m−2 h−1 [45]. The combined effect of the functionalization groups showed a much higher adsorption capacity. Lui et al. synthesized 3D porous hydrogel of β-CD/chitosan functionalized using graphene oxide is used for extracting methylene blue dye from wastewater. This shows a high adsorption capacity of 1134 mg/g for methylene blue and possibily recycled by simple filtration process [46]. Taka et al. [47] synthesized hybrid carbon nanotubes/CD/Ag doped TiO2 nanosponges for removing Co2+ and Pb2+ ions from wastewater. On increasing the pH of the solution, the adsorption capacities were found to be enhanced. The observed values for adsorption capacities for Co2+ and Pb2+ were found to be 7.8 and 35.86 mg/ g, respectively [47]. The introduction of multiple cross-linkers makes fast adsorption and improves the adsorption capacity. Wang et al. [48] copolymerize β-CDs with 5,5' ,6,6' tetrahydroxy-3,3,3' ,3' -tetramethylspirobisindane (TTSBI) and 2,3,5,6-TFP. The adsorption capacity of BPA was found to be 502 mg/g without any degradation up to 5 cycles. The epoxy group of GO reacts with the amino group of Amβ-CD which results in increased adsorption capacity of BPA. Figure 3 shows the schematic for the synthesis of such functionalized nanosponges [49].

3 Synthesis Method Synthesis of CDNS has been achieved using various approaches such as melt, solvent, microwave, ultrasound, and mechanochemical method, summarized in Table 2. In the melt method, cyclodextrin and other cross-linking agents are mixed, heated to a specific temperature, and stirred until a homogenous mixture is obtained. In the solvent method, a polymer–solvent mixture is excessively added to the solution containing organic cross-linkers dissolved in dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) [50]. In the Ultrasound method, the ultrasound waves create cavitation bubbles in the solution, which can generate localized high temperatures

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Fig. 3 Schematic diagram of the synthesis of β-CD based GO hybrid nanosponges

Table 2 Summary of various synthetic methods of CDNS Synthesis method

Energy

Time (h)

Temperature (o C)

Melt method

Thermal energy

4–5

100–130

Microwave

Microwave radiation

0.5–2

80–100

Ultrasound

Ultrasound radiation

4–5

90–100

Mechanochemical

Mechanical energy

0.5–2

120–180

Solvent method

Thermal energy

1–48

10—reflux temperature of the solvent

and pressures, leading to enhanced mixing, acceleration of the reaction kinetics, and improved reproducibility [51]. While in the microwave method, the mixture is exposed to microwave irradiation for a specific duration, typically a few minutes for cross-linking cyclodextrin to organic linkers, the obtained products usually are highly crystalline in nature with narrow size distribution [52]. The mechanochemical method is an emerging technique for the synthesizing nanosponges that incorporates the use of mechanical forces, namely grinding, milling, or mixing, to initiate and accelerate chemical reactions.

4 Characterization of Cyclodextrin-Based Nanosponges (1) Adsorption capacity: Adsorption capacity is the most important property and is determined using the following equation: Adsorption capacity = where Co Initial concentration of the pollutant.

(Co − Ct ) ∗ V m

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Ct Concentration of pollutants at time t. V Volume of water used in litres. m Mass of adsorbent used. When the adsorption of the pollutants attains equilibrium at constant temperature then adsorption isotherms are plotted to study the adsorption mechanism. The data is either fitted in a linearized manner or a non-linearized manner using Langmuir and Freundlich isotherm. For Langmuir isotherm: qe = qm K ∗

Ce 1 + KCe

For Freundlich isotherm: qe = KC1/n e where Ce qe qm K

concentration of pollutant solution at equilibrium adsorption capacity at equilibrium adsorption at any time depends upon adsorption energy

Using these two isotherms, the type of adsorption and the mechanism of adsorption can be predicted. Rising the amount of adsorbent, the adsorption either increases or decreases depending upon the type of pollutants. The adsorption capacity increases with an increase in adsorbent until saturation is reached, after which even on increasing the amount of adsorbent, adsorption will not take place due to the absence of any active sites. With an increase in contact time, the adsorption will increase, until equilibrium is attained. At equilibrium, most of the adsorbent sites are occupied therefore there is no further increase in the adsorption with time. So, CDs with lower equilibration time are preferred for water treatments. (2) Morphology and elemental distribution: Scanning Electron Microscopy (SEM) are powerful imaging techniques that allow for visualizing the surface morphology of the synthesized nanosponges. It provides high-resolution images revealing the nanosponges’ size, shape, and structure. Transmission Electron Microscopy (TEM) is another imaging technique that allows for the visualisation of the internal structure of nanosponges using bright-field and dark-field imaging. It provides high-resolution images that reveal pores’ size, shape, and distribution within the nanosponges. Since various elements are contained in the nanosponges, the mapping of the individual elements can be obtained by STEM using an EDX detector [53].

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(3) Surface area and pore volume: BET method is widely used for determining the surface area of materials. BET analysis involves measuring the adsorption of gas molecules onto the surface of the nanosponges at various relative pressures and then fitting the data to a mathematical model to determine the surface area and pore volume. (4) Average diameter and polydispersity: Dynamic light scattering (DLS), is a commonly used technique to determine the average diameter and polydispersity of nanoparticles, including nanosponges. The sample solution is loaded into a PCS instrument, a laser beam is directed through the solution, and the scattered light is measured at different angles. The intensity autocorrelation function of the scattered light is recorded and analysed using the Stokes–Einstein equation to determine the average diameter of the nanosponges. The polydispersity index (PDI) can be calculated from the distribution of particle sizes obtained from the PCS data. A PDI of zero indicates that all particles are the same size, while a PDI of one indicates that the particle size distribution is highly polydisperse [54]. For hydrodynamic diameter measurements, PCS assumes all particles are spherical and considers the dispersion medium’s effective viscosity, temperature, and refractive index [55]. (5) Zeta potential: It is used to estimate the electrostatic potential difference between the surface of a particle, such as nanosponges, and the surrounding fluid medium. Zeta potential measurements can provide information about the stability and surface charge of the nanosponges, which can impact their behaviour and interactions with other molecules [56]. (6) Thermal analysis: Thermoanalytical methods, includes differential scanning calorimetry (DSC), differential thermal analysis (DTA), and thermogravimetric analysis (TGA). DSC is a technique used to measure the heat flow associated with thermal transitions in materials. It can be used to determine nanosponges’ thermal stability, glass transition temperature, and melting point. TGA is a technique used to measure the weight changes of materials as they are heated or cooled. It can be used to determine nanosponges’ thermal stability and decomposition behaviour. Dynamic Mechanical Analysis (DMA) is another technique used to measure thermal stress. Also, it can be used to determine the viscoelastic properties of nanosponges as a function of temperature. Thermal conductivity measurements can be used to determine the heat transfer properties of nanosponges. Overall, thermal analysis studies can provide valuable information about the thermal properties of nanosponges, which can be used to optimise their synthesis, processing, and performance in various applications [57]. (7) Water uptake capacity: The water uptake capacity of nanosponges is a crucial property that affects their performance in various applications, including drug delivery and environmental remediation. To determine it, the specimens are first dried under controlled conditions to remove any moisture. The dry weight (Wd ) of the nanosponges is then measured. Next, a known quantity of nanosponges (Wd ) is added to a measured volume of distilled water (Vw ) and stirred gently. The mixture can equilibrate at a specific temperature for a set period. The

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nanosponges adsorb water and reach equilibrium, and the excess water is removed by filtration or centrifugation. The swollen nanosponges are then weighed (Ws ). The water uptake capacity is calculated as the ratio of water adsorbed (Ws − Wd ) to weight to the dry nanosponges (Wd ), multiplied by 100 [50].

5 Wastewater Treatment Water is a necessity for day-to-day life. It is of utmost importance to have highquality water for the on growing population. The industrial water is dumped into the nearby water body which causes harm to the aquatic life. So, it is a high need to improve the quality of water and the materials used for water purification. To remove pollutants two-step process could be followed including adsorption of solvents, oil, and organic compounds on the surface of activated charcoal followed by ion exchange to remove inorganic pollutants. Nowadays CDNS are considered to be suitable for this application as they have a very high affinity for the adsorption of these waste molecules. They are quite cheap as compared to activated charcoal which shows similar activity but at a higher cost [58]. They can undergo three different mechanisms such as: (1) Host guest complex formation due to the hydrophobic cavity formed by CDNS. (2) Diffusion through the pores owing to hydrophilic nature of the surface. (3) Ionic interaction because of the presence of surface active sites of CDNS. During complexation, various interactive forces come into the picture like host– guest complexation, van der Waal and hydrogen bonding between CDs and the guest molecules. Figure 4 shows the host–guest complex formation when a pollutant is being adsorbed by the inner cavity of CDNS. Depending upon different synthesis conditions various types of CDNS are formed according to their need. Numerous pollutants are found in wastewater including heavy metals, dyes, pesticides, phenolic compounds, and drugs. Different CDNS were synthesized for specific activities which are discussed in detail below section. Table 3 shows the adsorption capacity and removal efficiency for various pollutants.

5.1 Heavy Metal Ions Heavy metals directly coming from industries, are a major concern for water bodies. They have a direct impact on living beings by accumulation leading to mental disability, liver, and kidney diseases, and damage to cardiovascular, haematological, endocrine, immunological, and reproductive systems. Heavy metals like Cr3+ , Fe3+ , Co2+ , Ni2+ , Cu2+ , Zn2+ , Hg2+ , and Pb2+ could be eliminated from the unpurified water via complexation, exchange of ions, or electrostatic interactions (Table 3). A wide

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Fig. 4 Reaction scheme showing host–guest interaction mechanism of β-cyclodextrin and pollutant Table 3 Summary of removal of various pollutants from wastewater using CDNS Nanosponges

Pollutants

Adsorption capacity (mg/ g) /removal efficiency (%)

References

βCD-citric acid

Cu2+

45.15

Anceschi et al. [59]

β-CCWB

Cr6+

206.0

Huang et al. [60]

βCD-citric acid PEG

Zn2+

97.7

Liu et al. [61]

βCD-hydrogel

Cu2+ Pb2+ Cd2+

116.4 210.6 98.9

Huang et al. [60]

βCD-chitosan-Fe

Cu2+ Cr6+

250 142.8

Sikder et al. [62]

βCD-Fe3 O4 -GO

Malachite green

740.74

Wang et al. [63]

βCD-4,4' -bipyridine

Congo red Methyl orange

323 370

Li et al. [64]

βCD-DFPS

2-Naphthol

99%

Wang et al. [65]

βCD-dicyclohexylmethane-4,4' -diisocyanate

Testosterone

98–100%

Manaf et al. [66]

βCD:EPI

Phenanthrene

80%

Celebioglu et al. [67]

βCD-1,4 butanediol diglycidyl ether

Ciprofloxacin

90%

Rizzi et al. [68]

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variety of heavy metals could be removed using aliphatic or aromatic cross-linkers with β-CDs. The removal of heavy metals includes various processes like host–guest interactions, hydrophobic inner cavities, hydrogen bonding at the periphery, and van der Waals forces. As a result, heavy metal forms a chelating ring with the CDs which stabilizes it. Depending upon the type of CD derivative used, the adsorption for the same metal will change. For example, Chen et al. [69] found that Cu2+ is a commonly found metal in wastewater and could be removed using a citric acid derivative of CDs but will have a different effect at different pH values. For pH 1, there is no adsorption of Cu2+ which could be explained due to electrostatic interactions. At pH 6, the efficiency to remove Cu2+ increases drastically due to surface charge and decreasing active functional groups [69]. Anceschi et al. [59] showed that the modified version of this polymer with polyvinyl alcohol could also be used to remove Cu2+ from unpurified water with an adsorption capacity of 45.15 mg/g. The fibres of β-CDs are cross-linked by thermal treatment after spinning at 160 °C to make them hydrophobic [59]. The graphene oxide derivative of CDs could be used to remove Cr6+ . Magnetic CDs are formed using ferric oxide as a cross-linking agent. The better removal efficiency was achieved at lower pH values. This is because, at lower pH, zeta potential becomes positive which favours the electrostatic interactions between the adsorbent and Cr6+ , while on increasing the pH, the surface contain negative charges which reduces the electrostatic interaction and therefore decreases the adsorbent activity of the polymer [70]. The removal of heavy metals could be explained by complexation. Huang et al. synthesized low-cost β-CD-chitosan modified walnut shell biochar (β-CCWB) for the adsorption of Cr6+ which follows pseudo-second-order kinetics and Freundlich adsorption isotherm with an adsorption capacity of 206.0 mg/g. The rate-determining step was found to be the chemisorption of metal ions which is directly dependent on the concentration of active sites [60]. By using poly-carboxylic acid as a cross-linker, metals like Co2+ , Cr2+ , Ni2+ , and 2+ Zn will be adsorbed from the water. With a small amount of adsorbent of 2 g/L, a large amount of inorganic compounds 50 mg/L could be removed [71]. He et al. [72] showed that Cu2+ , Pb2+ , and Cd2+ showed better adsorption capacity when pH is more than 3 because the aromatic groups attain a negative charge which could attract metal ions. The maximum adsorption capacity of Cu2+ , Pb2+ , and Cd2+ was 164, 196, and 136 mg/g, respectively when the initial concentration was 200 mg/ L. This adsorption process is endothermic and spontaneous which depends upon the pH values, electrostatic interactions, and distribution of different metal ions. The order for the removal of metals ions was observed as Pb > Cu > Cd [72]. Liu et al. [61] combined citric acid and polyethylene glycol cross-linkers onto β-CDs and the resulting polymer was further cross-linked with chitosan using glutaraldehyde to form a complex that helps to remove Zn2+ from sewage water with the maximum adsorption capacity of 97.7 mg/g [61]. Metals could also be removed using ion exchange on the surface of adsorbents. This follows the Freundlich adsorption isotherm with uneven surface adsorption.

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Huang et al. [41] synthesized β-CD-based gel that adsorbs Cu2+ , Pb2+ , and Cd2+ with maximum adsorption capacities of 116.4, 210.6, and 98.9 mg/g, respectively. This hydrogel attains a 3D structure when metal ions are adsorbed by it with a high degradation efficiency of 79.4% in 3 weeks [41]. Sikder et al. [62] showed the adsorption capacity for Cu2+ and Cr6+ as 250 and 142.8 mg/g, respectively using Chitosan-Fe nanoparticle-carboxymethyl β-CDs. This process involves both the adsorption as well as the reduction of Cu2+ to Cu (0) and Cr6+ to Cr3+ at the expense of the oxidation of Fe nanoparticles from Fe (0) to Fe3+ . The adsorption process was found to be exothermic and endothermic for Cu2+ and Cr6+ , respectively [62]. Kong et al. [43] showed 256 mg/g adsorption capacity of Pb2+ at pH 6 using CD-Fe3 S4 nanoparticles. The removal of Pb2+ could be explained by the formation of galena precipitate and surface adsorption. It is also used for the removal of Zn2+ , Cd2+ , and Cu2+ as well as with efficient adsorption capacities [43]. Zhao et al. used rice straw biochar to functionalize CDNS to remove Cr6+ and 2+ Pb with a maximum removal capacity of 197.2 mg/g and 131.2 mg/g, respectively [73, 74].

5.2 Dyes A recent report by Chandanshive et al. [75] states that around 70 million tons of dyes are fabricated all over the world with ten thousand tons alone from clothing industries. The clothing industry produces a major part of synthetic dye along with other printing, leather, and cosmetics industries. The dyes which are not bound effectively to the fabric are discharged into the nearby water bodies without any pre-treatment causing ecotoxicological threats to the living organisms. Dye is a significant pollutant of the environment that enters the food chain and directly affects human and aquatic health as they are toxic, carcinogenic, and hazardous. Dyes contaminate aquatic life and the quality of water bodies by accumulation on the surface which blocks the sunlight from entering the water bodies which impairs photosynthesis, inhibits plant growth, and increases the chemical oxygen demand. Generally, dyes are water-soluble, so to eliminate them from water bodies is a typical task. Several CDs are utilized for this purpose as described below. The first CD derivative used for the deletion of Eriochrome Black T (EBT) is formed using EPI as a cross-linker with an adsorption capacity of 261.1 mg/g. The adsorption mechanism includes pi–pi interaction between the benzene rings of EBT and β-CDNS followed by the electrostatic interactions linking the adsorbent and the dye [76]. Copolymerization is one of the methods used to synthesize nanosponges for dye removal. Methylene Blue is an important compound in the dye and drug industries. When TFP is used as a cross-linker, copolymerization of two monomer units βCD and pillar [5] arene generates a cavity for the supramolecular interaction which

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further increases the surface area of nanosponges for the adsorption with a removal efficiency of 78% [77]. Adsorbents with magnetic properties are used quite readily as they have a high separation capacity for dyes from aqueous waste. Wang et al. [63] synthesized Fe3 O4 based β-CD on graphene oxide nanocomposite which showed a high adsorption capacity of 740.74 mg/g for Malachite Green and could be regenerated easily for repeated usage. Similarly, Liu et al. [78] showed the removal of Rhodamine 6G dye using a magnetic graphene hybrid of β-CDs. Firstly, magnetic graphene oxide is prepared followed by functionalization with β-CDs. Even 90% of the efficiency was retained even after 6 cycles [78]. Immobilized adsorbents have high efficiency for the removal of dyes from the unpurified water by electrostatic interactions. Crini and Peindy showed that carboxylic group-based CDs nanosponges showed very high adsorption capacity for Basic Blue 9 dye [79]. Jia et al. [80] removed Crystal violet dye using poly (Nisopropylacrylamide-co-methacrylic acid) as a cross-linker. This dye has a very high adsorption efficiency of 1253.8 mg/g which could be explained via host–guest and electrostatic interactions [80]. EDTA based β-CDNS were employed for the removal of Crystal violet, MB, and Safranin dyes by electrostatic interactions between the dye and the carboxylic group on EDTA with a removal efficiency of 90% [79]. Free radical copolymerization at specific pH is used for the removal of rhodamine B and Congo red dye using methacrylic-βCD with 1-vinylimidazole (VI) (MCD: VI). Qin et al. [81] explained the removal of these dyes by electrostatic interactions and pi-pi interactions between the dye and the nanosponges, which show different activity at different molar ratios. Congo red shows a maximum adsorption capacity of 1.12 mg/g for a higher molar ratio in comparison to rhodamine B whose maximum adsorption capacity is 366 mg/g at a lower molar ratio [81]. Li et al. [64] performed a Menshutkin reaction to synthesize benzyl-βCD hyperlinked with 4,4' -bipyridine for the removal of anionic dyes like Congo red, and methyl red. Both of them followed Langmuir adsorption isotherm and pseudo-second-order rate equation which showed the maximum adsorption capacity as 323 and 370 mg/ g for Congo red and methyl red, respectively. These CDNS has high useability and could be used at least 5 times [64].

5.3 Pesticides Pesticide is a chemical compound used to terminate pests in agriculture and domestic life. It is a major concern for today’s aquatic life and human health. Pesticides used in crop production are directly involved in the food chain and have severe effects on the environment. Nowadays nanosponges have been used for the removal of pesticides and their intermediates from the water bodies. Romita et al. [82] extract the atrazine pollutant from water using α, β, and γ-CD using EPI as a cross-linker. α and β-CDNS showed almost similar removal efficiency

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around 60%. Due to the presence of H-bonding and van der Waals interactions, inclusion complexes formed with the host–guest interactions between the polymer and the atrazine, following a pseudo-second-order kinetics model [82]. Liu et al. [83] formed porous CDNS and used it for the removal of atrazine, benalaxyl, butylene fipronil, and simazine using EPI as a cross-linker. The adsorption interactions include inclusion complexes, swelling in water, and physical adsorption on the surface. The three CD polymers namely βCD:EPI, HPβCD:EPI, and randomly methylated βCD:EPI combines in a specific mass ratio of 40:30:30 to form a multiplex [83]. As formed β-CD:EPI and γ-CD:EPI nanosponges were further oxidized using KMnO4 by Wang et al. and found that the pore size, swelling ratio, hydrophilicity and degree of cross-linking all increases with oxidation but the amount of CD available and the surface area decreases. For the removal of hydrophobic compounds like butachlor, fipronil, flufiprole, and benalaxyl, γ-CD:EPI is used while for hydrophilic compounds like atrazine, bromacil, pretilachlor, and fenamiphos β-CD:EPI is used [84]. Utzeri et al. [85, 86] synthesized β-CDNS by 1,6–hexane diamine and 1,12– dodecane diamine as two cross-linkers. Both of them showed comparable particle size while 1,6–hexane diamine-based β-CDNS showed high surface area, and pore size along with high hydrophilicity. This is used for the removal of imidacloprid with a removal efficiency of >90%. When HDI is used as a cross-linker, the removal efficiency was quite low but with the formation of composite gel using 10% of carbon, the elimination efficiency enhances to 81% [85, 86]. Raoov et al. [87] first synthesized insoluble ionic liquid polymer linking CD with 1-benzylimidazole (BIM) and formed β-CD-BIMOTs, which on further polymerization with TDI linker formed βCD-BIMOTs-TDI. It has a high pore size and high thermal stability. It showed high removal capacity for pesticides containing phenols as functional groups [87]. Zhou et al. [88] synthesized β-CD polymer using triphenylmethane-4,4' ,4'' triisocyanate for the removal of 2,4-Dichlorophenol (DCP). These polymeric nanosponges were found to have a high affinity specifically for the mentioned pollutant in comparison to phenol or 2-chlorophenol. So, even with the low surface area, this polymer is highly effective even when compared to activated charcoal. This polymer showed stability even after 5 cycles of adsorption [88]. Wang et al. [89] synthesized CDNS using DFPS, CMP, and DCP as the crosslinkers. The adsorption of DCP by β-CD:DFPS was found to be higher than 80% [65]. Later, Huang et al. [89] increased the removal efficiency of pesticides like 2NP, 2,4-DCP, 2,6-DCP, 2,4,6-trichlorophenyl, and BPA by increasing the amount of cross-linking in β-CD:CMP, the porous nature and phenyl contents [89]. Recently, hybrid CDNS have also been used for the removal of pollutants. Martwong et al. [90] used poly(vinyl alcohol) and citric acid for cross-linking between β-CD for the removal of hazardous agrochemical paraquat with a removal efficiency of 94.5%.

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5.4 Drugs Pharmaceutical industrial waste is one of the biggest concerns for the environment with severe side effects. When disposed to water bodies, an increase in biological oxygen demand and highly variable pH is observed. β-CDNS are quite effective for the removal of drug molecules from water bodies. The commonly found drug molecules in the water bodies are Ibuprofen, Testosterone, Progesterone, Androsterone, Ciprofloxacin, and Propranolol hydrochloride. Fenyvesi et al. [91] cross-linked β-CD with EPI which is used for the removal of various types of drug molecules like cholesterol, ibuprofen, diclofenac, ethynyl estriol, naproxen, β-estradiol, estriol, and ketoprofen. These nanosponges show a removal efficiency of 85% towards drugs like ibuprofen [91]. Moulahcene et al. [92] found that α, β, and γ-CD using citric acid as a cross-linker was effective for the removal of progesterone. Amongst all, α-CD:citric acid polymer showed the maximum adsorption capacity of 95% with high surface area and less swelling ability [92]. Yu et al. [93] found that the antibiotic, ciprofloxacin could be removed from the wastewater using β-CD:EDTA with an adsorption capacity of 327 mg/g in the pH range 4–6, in this case, the adsorption process was found highly dependent on the concentration of the counterions. Manaf et al. [66] synthesized CDNS using dicyclohexylmethane-4,4' diisocyanate as a cross-linker with three different molar ratios as 1:1, 1:2, and 1:3 to check the removal efficiency for testosterone, etiocholanolone, epitestosterone, androsterone, 5α–androstane-3α, 17β-diol and 5β-androstane-3α,17β–diol from the human urine. The polymer having a molar ratio of 1:1 was found to be the most effective for drug removal [66]. Rizzi et al. [68] synthesized CDNS using 1,4 butanediol diglycidyl ether as a cross-linker with α, β, and γ-CD with the removal efficiency of 90% selectively for Ciprofloxacin. Also, β-CD with TFP as a cross-linker could be deployed for the extraction of various kinds of drugs. The properties of these polymers depend upon the pH, and the molar ratio of the cross-linkers used. The incorporation of phenolate groups to the polymer unit depends upon the rate of change of pH and the initial concentration of monomer. Some of the drugs which could be removed using these nanosponges are ethynyl oestradiol, propranolol hydrochloride, carbamazepine, and chloroxylenol [94]. Varan et al. [95] synthesized different types of nanosponges using different crosslinkers as pyromellitic dianhydride, toluene diisocyanate, citric acid, and carbonyl diimidazole. They were used for the removal of indole with the decreasing removal efficiency order as toluene diisocyanate > citric acid > pyromellitic dianhydride > carbonyl diimidazole [95].

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6 Conclusion and Future Remarks Due to its unique physicochemical characteristics, such as the capacity to encapsulate a variety of non-polar pollutants in the inner core and electrostatically interact with polar impurities onto the hydrophilic surface, CDNS has been found as a state-of-theart adsorbents than other commonly used materials for adsorption of contaminants in the wastewater. Since conventional methods of wastewater treatment deal with certain challenges, such as excess residue formation, higher operational cost, lower removal efficiency, and selectivity, CDNS successfully overcomes all these barriers by virtue of lower synthesis and operating cost, higher selectivity, and easy functionalization. In the current chapter, more emphasis has been given to the synthetic method and functionalization of these nanosponges for wastewater treatment applications. The last part of this chapter combines the implementation of various CDNS for the effective elimination of impurities from the contaminated water. CDNS has already received huge success in wastewater treatment applications, but more elaborative studies in terms of the adsorption mechanism and their kinetic model are required to address the critical challenges associated with adsorption isotherms, the role of various parameters like pH, contact time, dosage of adsorbent, involvement of multiple reagents, and temperature. To further improve the efficacy of these nanosponges, the focus should be on easy fabrication and optimization processes, which are of low cost and better reusability. Simple and effective functionalization of CDNS has to be achieved with specific groups to enhance their selectivity for specific pollutants. The environmental impact of CDNS should be explored to ensure their sustainable nature, biocompatibility, and their use at the commercial level.

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Application of Cyclodextrin-Based Nanosponges in Soil and Aquifer Bioremediation Shefali Shukla, Bulbul Sagar, and Sarthak Gupta

Abbreviations 2-MIB BET BTEX CD CGTases CDNS CuAAC DBPs DLS DMSO DPC EDC FTIR GC/MS GAC HNTs HNT-CD HPBCD HPBCD-NP MN-PCDP MTBE NOM

2-Methylisoborneol Brunauer-Emmett-Teller Benzene, toluene, ethylbenzene and xylene Cyclodextrin Cycloglycosyl transferase amylases Cyclodextrin-based nanosponges Cu-catalyzed azido-alkyne cycloaddition Disinfection by products Dynamic light scattering Dimethyl sulfoxide Diphenyl carbonate Endocrine disrupting compounds Fourier transform infrared Gas chromatography–mass spectrometry Granular activated carbon Halloysite nanotubes Halloysite–cyclodextrin nanosponges Hydroxypropyl-β-cyclodextrin Hydroxypropyl-β-cyclodextrin nonylphenol Magnetic nanoparticles porous β-CD polymer Methyl tert-butyl ether Natural organic matter

S. Shukla (B) Department of Chemistry, Sri Venkateswara College, University of Delhi, Delhi 110021, India e-mail: [email protected] B. Sagar · S. Gupta Department of Chemistry, Indian Institute of Technology Delhi, Delhi 110016, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Gulati (ed.), Nanosponges for Environmental Remediation, https://doi.org/10.1007/978-3-031-41077-2_7

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NP NPEs PAHs PAN PBDEs PCBs SEM SERS SPE TEM TGA USEPA XRD

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Nonylphenol Nonylphenol ethoxylates Polyaromatic hydrocarbons 1-(2-Pyridylazo) 2-napthol Polybrominated diphenyls Polychlorinated biphenyls Scanning electron microscopy Surface-enhanced Raman spectroscopy Solid phase extraction Transmission electron microscopy Thermogravimetric analyzer United States Environmental Protection Agency X-ray diffraction

1 Introduction In synthetic modern technology, nanomaterials are employed to boost the process of environmental remediation. These nanomaterials, particularly, nanosponges, have been explored extensively by a group of scientists and researchers working in various sectors. Cross-linked polymeric solids with greater surface area, well-defined nanosized pores, and high thermal stability are some of the well-known characteristics of nanosponges [1]. These nanomaterials have high molecular weight and have found several uses in the pharmaceutical industry, heavy metal absorption, the removal of organic contaminants and colors from soil and water, aquifer bioremediation, and many other fields. Nanosponges are porous structures that can be used for the purpose of absorbing or binding particular molecules or substances. These materials are extremely adaptable, and they can be used in a variety of settings, such as industrial processes, medical procedures, and environmental clean-up, to absorb or adsorb various materials effectively and efficiently. Nanosponges are currently one of the most researched artificial materials that have been proven successful in lowering the concentrations of various pollutants and neutralizing their adverse effects. Due to their porous structure, nanosponges have a huge surface area that improves their capacity for adsorption. These 3D structures can be employed widely to accept a wide range of guest molecules through the processes of adsorption and absorption due to their distinct morphologies. Besides these being used for drug delivery in the pharmaceutical industries, these nanosponges may be used in agriculture to increase the fertility of the soil by captivating and removing chemical entities like pesticides, herbicides, etc. Soil remediation is one of the urgent requirements of present times. As there are multiple sources from which different contaminants reach the soil surface and are responsible for polluting thousands of sites across the world. The major sources of soil pollutants include urban activities, agricultural waste, and industrial wastes. These contaminants are not only harming the environment but are hazardous

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Fig. 1 Different techniques of bioremediation

to all forms of life. In aquifer remediation, these porous materials are used for eliminating organic/metallic pollutants from aquifers as many compounds are not retained by the soil colloids and hence have a tendency to water infiltration [2–4]. Thus, the use of nanosponges for the remediation of soil and wastewater holds great potential and there is much research still ongoing to optimize their properties, improve their stability, and ensure their safety for various applications. The present chapter provides a brief overview of applications of these cyclodextrins, surface-modified cyclodextrins, and cyclodextrin-based nanosponges with a special emphasis on their usage in soil bioremediation and wastewater remediation. Bioremediation utilizes naturally occurring microorganisms, such as fungi and bacteria, to degrade or alter contaminants into less harmful substances. These microorganisms can break down organic compounds or convert toxic chemicals into non-toxic forms through various metabolic processes. Some of the biological treatment techniques include bio-augmentation, bioventing, phytoremediation, biosparging and bioslurping (Fig. 1).

2 Cyclodextrins: A Novel Class of Supramolecular Encapsulating Hosts Cyclodextrins (CDs) are vibrant molecules having an amphiphilic structure that can be synthesized from the starch degradation products obtained by using cycloglycosyl transferase amylases (CGTases), the enzymes produced by various bacilli [5]. Due to the chair conformation of glucopyranose, the structure of CDs is a truncated cone or torus [6] with a hydrophobic inner central cavity and hydrophilic outer surface (Fig. 2a). The hydrophilic behavior of the outer cone is explained by the presence

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Fig. 2 a Chemical structure of CD, b 3D structure of CD, c CD cavity holding drug molecule to form inclusion complex. Reused with permission from Sherje et al. [8]. Copyright Elsevier 2017

of the hydroxyl group of the glucopyranose moiety at the outer edge where the primary and the secondary hydroxyl groups are positioned at the narrow edge and the wider edge respectively (Fig. 2b) [7]. Cyclodextrins have high reactivity due to the presence of a hydroxyl functional group which can be easily substituted or eliminated and hence provides a platform where their functionality can be easily modified by substituting the hydroxyl groups with other functionalities to synthesize CD derivatives.

2.1 Formation of Inclusion Complexes Modern technology uses CDs or modified CDs that are capable of forming inclusion complexes. Hydrophobic interactions between the host cyclodextrin and the guest organic molecule are responsible for the formation of host–guest complexes or the inclusion complexes (Fig. 2c). The driving force in the complex formation includes the removal of water from the cavity, hydrophobic interactions, hydrogen bonding, and weak electrostatic interactions and, van der Waals interactions [9]. The interactions among CDs and the guest organic molecules may be used as a foundation for the absorption or separation of diverse organic molecules. The central cavity of CD is lipophilic and hence can entrap suitable-sized molecules reversibly [10]. The inclusion complex formed exists in equilibrium with a free guest molecule, free CDs, and supra molecules of inclusion (Fig. 3). Free CD + Free guest ↔ inclusion complex (or Host–Guest conjugate). To better understand the inclusion mechanism, one should consider their thermodynamic parameters mainly free energy change (ΔG), enthalpy change (ΔH), and entropy change (ΔS). Hydrophobic interactions are entropy-driven (negative ΔH and large positive ΔS) whereas Van der Waal interactions are characterized by negative ΔH and negative ΔS. For this type of interaction driving force is the removal of water molecules from the cavity [10]. The value of binding constant ‘K’ which is also known as ‘affinity constant’ or ‘association complex’ is responsible for the

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Fig. 3 Formation of inclusion complex with an organic host molecule in the hydrophobic cavity of the cyclodextrin. Reused from Carneiro et al. [11]. Copyright MDPI 2019

formation and dissociation of the inclusion complex. A stable inclusion complex will have a higher value of ‘K’, apart from the affinity complex stability of the complex also depends upon the size of the pour and the adjustment of the guest molecule in the cavity [12]. An inclusion complex is formed when both the cyclodextrin and guest are in a molecular state, so the ability of complexation depends upon the inherent solubility (S0 ) of guest molecules and binding constant, the greater the intrinsic solubility greater will be the complexation, similarly greater the binding constant (K) greater will be the complexation (complexation efficiency—KS0 ) [13]. An increased solubility of the guest can affect the rate of inclusion, this is true for most of the cases, except for some of the organic solvents like ethanol. In ethanol, the solubility of the guest is increased and also there is an increase in the competition between the guest molecule and solvent ethanol to dock in the cavity of the host [14].

3 Use of Cyclodextrins in Soil Bioremediation The most environmentally friendly way of decontaminating the soil is through biodegradation where the microorganisms present in the soil are degrading the contaminants. These microorganisms require optimal conditions of temperature, pH, moisture content, and the presence of nutrients for their stimulation and functioning. Other factors like physiochemical properties and age of the contaminants present in the soil and soil properties are also the governing factors in the bioremediation of soil.

3.1 Bioavailability of Contaminants As most of the organic contaminants are hydrophobic, these are not soluble in water. These contaminants adhere to the surface of organic matter present in the soil and hence are very less bioavailable to the microorganisms for bioremediation. Such

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organic pollutants stay in the soil for a longer duration and can only be removed by physical or chemical methods. Conventionally, organic solvents or surfactants were employed to extract these pollutants from the soil but both these remedial methods have their own adverse effect on the environment [15]. Organic solvents pose serious concerns to the environment and are associated with other adversities like toxicity and non-biodegradability whereas surfactants are known to cause emulsions which are very difficult to remove. Therefore, there is an utmost need to look for a material that is efficacious in removing these pollutants and poses minimal or no ill effects to the environment.

3.2 Soil Bioremediation Through Cyclodextrins The natural process of bioremediation is very slow [16], and the use of other methods like organic solvents and surfactants has many after-effects and environmental constraints as previously discussed. Therefore, an urgent need for such materials which can accelerate the process of bioremediation and help in the speedy recovery of soil, was observed. In the quest for alternate materials for soil bioremediation, cyclodextrins were explored and documented as the greener alternatives to the conventional agents used for the removal of organic contaminants. These supramolecules have been shown to enhance the solubility of a wide range of organic contaminants. These CDs were able to capture the organic contaminants, like polyaromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), polybrominated diphenyls (PBDEs), nonylphenol (NP) and nonylphenol ethoxylates (NPEs), etc., and form the inclusion complex and help in the bioremediation of soil [17, 18]. Because of the formation of these inclusion complexes, these contaminants were made more bioavailable for microbial action. Many studies based on microbial soil bioremediation using CDs have been done and have shown the potential of cyclodextrins or their derivatives as the environmentally benign solution to the problem of environmental pollution. Table 1 provides examples of different studies which have been carried out using CDs in the bioremediation of soil.

4 Cyclodextrin Based Nanosponges Since cyclodextrins have a hydrophobic inner cavity, these are able to form inclusion complexes with small hydrophobic molecules through hydrophobic and weak electrostatic interactions. The formation of inclusion complexes based on hydrophobic interactions indicates that the cyclodextrin cavity is encapsulated either by the nonpolar molecule or by the non-polar part of the molecule, but there may also be another type of complexation which are not the inclusion complexes. In these types of complexes guest molecules are attached to cyclodextrin at the exterior [28] where depending upon the size of the guest and CDs one or two or more CDs can attach to one

Polybrominated diphenyl ether, Polychlorinated biphenyls, and metals (like Pb, Cu, and Ni) Organochlorine pesticides (like DDT) Nonylphenol, nonylphenol ethoxylates

Trifluralin Herbicides (diuron)

Petroleum hydrocarbons, trichloroethylene, pentachlorophenol Naphthalene, fluorene, pyrene, etc. 2,4-dinitrotoluene Diesel oil, mineral oil Aliphatic hydrocarbons

Methyl-β-cyclodextrin

α-Cyclodextrin

2-hydroxypropyl-β-cyclodextrin

β-cyclodextrin, HPBCD (hydroxypropyl-β-cyclodextrin)

2-hyoxypropyl-β-cyclodextrin

Cyclodextrin

2-hydroxypropyl-γ-cyclodextrin

Carboxymethyl-β-cyclodextrin

Randomly methylated-β-cyclodextrin

β-cyclodextrin

1

2

3

4

5

6

7

8

9

10

Soil contaminants

Cyclodextrin type

S. no

Hauser and Matthes [24]

Villaverde et al. [23]

Lara-Moreno et al. [22]

Sánchez-Trujillo et al. [21]

Morillo et al. [20]

Chen et al. [19]

Reference

They are nearly completely degraded after 3 months of exposure

No degradation of diesel after using CD, and high degradation in soil with mineral oil (after 3 months)

Low polarity concomitants can be removed

Bardi et al. [27]

Gruiz et al. [26]

Jiradecha [25]

Extraction capability depends upon the size of PAHs Sánchez-Trujillo and CDs hydrophobic capability et al. [21]

Along with cyclodextrin advanced oxidation processes are also needed

The key factor is that cyclodextrin has the ability to form an inclusion complex with different hydrophobic guest molecules

The high hydrophobicity of TFL is responsible for the formation of the inclusion complex

The solubilities of NPs can be increased up to 500 times due to the formation of the (hydroxypropyl-β-cyclodextrin nonylphenol) HPBCD-NP inclusion complex

CDs allow contaminants to dissociate from the inclusion complex

Removal is done by ultra-sound-assisted soil washing

Remark

Table 1 Selected studies on the use of different types of cyclodextrins for soil bioremediation

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guest molecule or one or two or more guest molecules can form the complex with a CD unit [29]. Loftsson et al. has depicted that drug–CD complexes can polymerize to form aggregates in aqueous solutions where the drug molecules get stabilized through hydrophobic interactions and electrostatic interactions (like dipole–dipole, Van der Waals, dispersion forces, etc.) inside their structure just like CD inclusion complex, to form non-inclusion complexes [30–34]. The adsorbate (here, a pollutant from a gaseous or liquid solution) is adsorbed on the surface of the adsorbent (solid) during the adsorption process, i.e. CD based nanosponges. ‘Cyclodextrin nanosponge’ term was first introduced in 1998 by DeQuan Li and Min Ma and they proved their utility in water purification [35]. Cyclodextrin (CD) based nanosponges are highly porous materials having amorphous or crystalline structure with spherical shape, and expansion properties. A large variety of nanosponges have been designed by various groups of researchers to alter their morphological and functional characteristics [1]. These supramolecular structures offer magnificent opportunities for removing organic and inorganic pollutants from water because of their high potential as sorbents for extracting heavy metal ions from aqueous solutions. In the past decade, these CD nanosponges have undergone a lot of alterations to improve their functionality and potential for removing a large variety of water contaminants. Various alterations in CDs include the use of ionic liquid for water treatment [36], polymerizing with agricultural products [37], combining them with dendrimers [38] addition of elements like Ti, Si, and Ag, etc. [39]. The pharmaceutical contaminants such as carbendazim, diclofenac, sulfamethoxazole, and furosemide present in wastewater are removed effectively because of efficient host–guest complexation in the hydrophobic cavity of the cyclodextrins [40]. Up to 90% of the pollutants can be removed by these nanostructures because of good adsorption capacity. The presence of a hydrophilic exterior and hydrophobic pocket make these nanosponges a very potential candidate for various industrial and biomedical applications. Due to their amphiphilic nature, these nanostructures are used pharmacologically as delivery vehicles to load and deliver certain drug molecules in biological systems. Their tuneable pore size makes it possible to prevent bacteria to penetrate the nanosponges and hence they act like a self-sterilizer. So, various groups of researchers are working in the direction of modification of nano-sponges to explore their productive applications in a variety of fields. Some of the domains where currently nanosponges are being used have been shown in Fig. 4.

5 Use of Cyclodextrin-Based NanoSponges in Waste Water Purification One of the most important elements for the existence of every living species is having access to clean drinking water. However, access to clean water has become more difficult in the modern era for a number of reasons, including environmental

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Fig. 4 Applications of cyclodextrin-based nanosponges. Reused from Utzeri et al. [1]. Copyright Frontiers 2022

water pollution from sewage, industrial effluents, chemicals, household trash, agricultural waste, pesticides, population growth, climate change, and many others. To overcome this challenge, water purification has become a critical issue around the globe to deal with. Treating industrial wastewater has always been a challenge because industrial effluents not only produce a bad odor, color, and taste but are also hazardous and toxic. The presence of heavy metals pollutants like lead, cobalt, and chromium; organic compounds like polychlorinated compounds, endocrine disrupting compounds (EDC), and natural organic matter (NOM) and pathogens such as bacteria, viruses, protozoan parasites are highly toxic and are responsible for various health issues [41]. Currently, there are several water treatment techniques available, but each one comes with its own limitation. Sometimes the cost associated with the technique is too high that it can’t be operated in every region. Besides, the techniques are effective for treating only a specific type of contaminant-biological, organic, or inorganic, but in the wastewater, all of them coexist. For example, in the purification technique using ion exchanges, the removal efficiency and separation selectivity are high, but the cost of synthesizing and regenerating resin limits its applicability. Other techniques like photocatalysis can simultaneously remove organic and metal pollutants, but they can be used only against a limited number of pollutants. Techniques like adsorption and reverse osmosis come with a low operational cost, but their selectivity is too low [42, 43]. To overcome the challenges posed by these conventional techniques, the use of nanotechnology has been explored and adopted for wastewater treatment.

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5.1 Formation of Nanosponges: Use of Crosslinkers for Linking of Cyclodextrin Molecules The glucopyranose moiety in cyclodextrins has a chair conformation that forms a hydrophobic pocket and hydrophilic exterior but their pocket cannot form stable inclusion complexes with hydrophilic and high molecular weight molecules [34]. Also Because of the solubility of CDs in water, they may not be employed directly in the separation of many organic pollutants in water. So, there is a dire need to modify the interior or surface characteristics to widen their applicability in various fields. Therefore, to triumph over the solubility issues, the CDs are regularly immobilized on solid particles as a desk-bound phase for organic pollutant separations [44, 45]. To accomplish these goals, a monomer is often used as a crosslinker to polymerize the CDs to form ‘Cyclodextrin Nanosponges’ (Fig. 5). For example- cyclodextrin-based polyester nanosponges are widely used for varied applications where polyesters are mixed with a crosslinker in a solution to form biodegradable nanosponges. Copolymerizing CDs with other monomers modifies the surface characteristics and ensures the significant applications of cyclodextrin-based nanosponges [46]. Some of the crosslinkers used in the production of nanosponges from cyclodextrin are listed in Table 2 [8]. These crosslinkers influence the properties of cyclodextrin polymers. Wise use of the crosslinkers may result in the formation of either water-soluble or water-insoluble polymers [8]. Wate-insoluble nanosponges are either liner or cross-linked polymers with higher water sorption capability. β-CD molecules when reacted with epichlorohydrin, one or more hydroxyl group of cyclodextrin form polymeric resin [47]. The β-cyclodextrin epichlorohydrin polymer is derivatized further with 1-(2-pyridylazo)-2-napthol (PAN) and can be used to captivate cobalt [48]. For the synthesis of Pyrometallic anhydride based sponges, dianhydride and CD are mixed in the DMSO (dimethyl sulfoxide) solvent with some organic base like trimethyl amine, the reaction was found to be exothermic and hence could be carried out at room temperature. The β-cyclodextrinPAN polymer can be synthesized by carrying polymerization of β-cyclodextrin with 1-(2-pyridylazo) 2-napthol which is used to synthesize electrorheological materials of an inclusive complex. The CD-based copolymers can be used for the adsorption of organic contaminants as they are amphiphilic in nature [49]. In recent years, cyclodextrin-calixarene hyper-reticulated co-polymers have been made by taking advantage of a well-known click chemistry reaction, the Cu-catalyzed AzidoAlkyneCycloaddition (CuAAC), which involves two different heptakis-6-azidocyclodextrins and two different propargyloxy-calix[4]arenes. Through the synthesis of a number of pre-and post-modified materials, the impact of chemical modifications on the physicochemical characteristics and the absorption capacities of cyclodextrincalixarene nanosponges (CyCaNSs) was observed. The reactants, azide, and alkyne, were reacted in various ratios to produce the substance with an excess of functional groups, which was then subjected to chemical transformation. Triazole group addition enhances proton binding capacity in polymers with low hydrophilicity by

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Fig. 5 Cyclodextrin nanosponges: cyclodextrin molecules linked via crosslinker reused with permission from Sherje et al. [8]. Copyright Elsevier 2017

Table 2 Cross linkers used for cyclodextrin-based nanosponges Type of crosslinker

Examples

Dicarboxylic acid chlorides

Glutaryl chloride, adipoyl chloride, sebacoyl chloride, terephthaloyl chloride

Acid anhydrides

Glutaric anhydride, maleic anhydride, phthalic anhydride

Diisocynates

Hexamethylene diisocyanate, isophorone diisocyanate, polydiisocyanate

Alkyl dihalides

Dichlorodimethyl siloxane, dichloromethane

Dicarboxylic acids

Succinic acid, 2,3-naphthalene dicarboxylic acid, 2,2-bis(acrylamido) acetic acid

Chlorhydrins

Epichlorohydrin

Reused with permission from Sherje et al. [8]. Copyright Elsevier 2017

forming intra-network hydrogen bonds. The post-modification with ionizable groups made the materials more sensitive to pH conditions [50].

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6 Aquifer Bioremediation Through Cyclcodextrin: A Newer Approach 6.1 Aquifer Bioremediation: A Background In the older times, a lot of open space, as well as plentiful groundwater and other resources, were available in usable form. Groundwater that is highly mineralized makes up around 1.69% of the entire amount of groundwater and doesn’t need any processing before usage. As the water passes through the ground, it gets filtered and becomes more mineralized. As a result of urbanization and an increase in human activity, various industrial effluents and other waste materials permeate through the earth, reach the water table, and contaminate the groundwater. Some chemicals, such as benzene, toluene, the three xylene isomers (BTEX), and methyl tert-butyl ether (MTBE), permeate the groundwater and pose major problems. Infiltration of several heavy metals, including mercury, chromium, cobalt, cadmium, lead, and arsenic, can lead to chronic diseases. These contaminants percolate through the ground, pose a threat to human life and wildlife, badly impact the aquifer, and pose major issues. To extract and remove the contamination a wide range of technologies have been employed. The conventional technique of remediation is to plow up unfit soil and transport it from a suitable location. There are certain drawbacks like the risk associated with handling, excavation, carrying away hazardous material, and finding new landfill sites for disposal. While toxins continue to exist in landfills, which also require ongoing monitoring and upkeep, this strategy only offers a temporary fix. Also, high-temperature incineration and various types of chemical decomposition are some of the technologies that are currently being used. These can be useful in lowering the levels of a range of contaminants but have some limitations, principally their cost for small-scale applications, technological complexity, and the lack of public acceptance, especially for incineration may increase the exposure of contaminants to both the labors at the site and the local residents [51]. Therefore, those techniques which appropriately eliminate the pollutants provide superior and permanent solutions to this problem. One of the most effective solutions that can quickly and affordably remove or treat different kinds of pollutants is called aquifer bioremediation. Aquifer bioremediation refers to the process of using biological organisms to clean up or reduce the contamination of groundwater or aquifers. Aquifers are subterranean layers of porous rock or sediment that store and transfer water. They can be contaminated by a variety of substances such as pesticides, fertilizers, industrial chemicals, and petroleum products. According to a report by the United States Environmental Protection Agency (USEPA), for soil and groundwater remediation, bioremediation contributes to 24% of the remediation technologies used [52–54]. Groundwater remediation can be done by using chemical, physical, and biological, and the amalgamation of these technologies may be employed to remove the pollutants or contaminants [55, 56].

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6.2 Use of Cyclcodextrin-Based Nanosponges in Aquifer Bioremediation In present times, one of the major problems is leaching of contaminants from soil to the groundwater. As many contaminants are not retained by the soil, therefore they are able to reach the underwater table through water infiltration. Triclopyr (3,5,6Trichloro-2-pyridinyloxyacetic acid), is an auxinic type herbicide used against annual and perennial broad-leaf weeds and woody plants. It has been shown to interfere with the normal growth process of the plant [57]. The herbicide has a very high water contaminating potential. In recently conducted experimental studies on different animals, it has been shown that the short exposure to the herbicide has resulted in tremors and lethargy whereas exposure over a prolonged period may even lead to kidney or liver damage. Cyclodextrin-based nanosponges have nanosized pores and have a limited capacity to bind the triclopyr molecule through inclusion complexation. But these nanosponges could stick to the cell membranes and hence affect the kinetics of the degradation process of the Triclopyr molecule. Therefore, these nanosponges help in the bioremediation of the organic contaminant [3].

7 Use of Nanosponges in the Removal of Metal Ions Heavy metals are those metallic elements that have relatively high density and are toxic even at extremely low concentrations e.g. Pb, Cr, Cu, Cd, Ni, and Zn [58]. Because of their non-biodegradability and potential biological and ecological risks, the presence of these heavy metals in soil and water bodies has drawn a lot of attention. Among all of these heavy metals, Pb displays the usual high toxicity and could cause major illnesses like anemia, renal dysfunction, cancer, brain damage, and even death [59]. Experimental data depicted that when β-cyclodextrin nanosponges cross-linked with tannic acid (Fig. 6), via a condensation reaction, it can selectively capture Pb2+ ions from wastewater where the hydroxyl group of phenol present in tannic acid forms a stable complex with Pb2+ ions as compared to the other metal ions. Within a short duration of 3 min, Tannic acid-linked nanosponges were able to remove around 81% of Pb2+ ions with the adsorption saturation capacity of 136.8 mg g−1 at an initial concentration of 200 mg/L. The entire adsorption process was completed within a time of 50 min due to high adsorption sites present in tannic acid [60]. In another report, Cataldo et al. studied the Pb2+ sorption potential of four cyclodextrin-calixarene nanosponges (CyCaNSs), NS1-NS4, which were already been synthesized and reported in the literature before [50, 61]. In the study by Cataldo et al. [62], it was documented that out of the four nanosponges, only NS3 and NS4, which were functionalized with amino and carboxyl groups respectively, deciphered good adsorption capacity and affinity towards the Pb2+ ions at pH = 5.0 [62]. Taka et al. [63] synthesized nanosponge biopolymer using amidation reaction, cross-linking polymerization, and sol–gel method. It was phosphorylated with

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Fig. 6 β-cyclodextrin nanosponges cross-linked with tannic acid. Reused with permission from Yang et al. [60]. Copyright Elsevier 2022

carbon nanotube-cyclodextrin and silver-doped titania. It was documented that these novel nanosponges biopolymers could efficiently remove Pb2+ and Co2+ metal ions from synthetic and mine effluent samples. The adsorption of both these ions, Pb2+ or Co2+ , onto the surface, was found to be dependent on the pH of the solution, adsorbent dosage, initial concentration, contact time, and temperature. Ion exchangeelectrostatic interactions and multilayer heterogeneous adsorption mechanisms were found to be responsible for adsorption for both Pb2+ and Co2+ , with an overall process to be an endothermic process. Using this nanosponge biopolymer composite, it was possible to remove up to 99.30% of Pb2+ and 95.05% of Co2+ from wastewater samples where the reaction was driven by a pseudo-second-order kinetic model. The best removal capabilities for Pb2+ and Co2+ from mine effluent samples were found to be 35.86 mg g−1 and 7.812 mg g−1 , respectively [63]. Conventionally, lead and cadmium ions have been removed from wastewater using a variety of methods, including chemical precipitation, flocculation-coagulation, ion exchange, adsorption, evaporation, biosorption, and membrane filtration [64, 65]. But each of these methods comes with its own limitations such as high energy consumption, complicated operations, and non-selectivity. Dithizone-modified cellulose acetate nanosponge has been prepared from waste photographic tapes, as an adsorbent, to get rid of Pb2+ and Cd2+ ions from contaminated water. In contrast to dithizone-modified cellulose acetate nanosponge, which increased efficiency to 99.5 and 95.5%, respectively, nonmodified cellulose acetate nanosponges could only adsorb about 2.0% of Pb2+ and 1.5% of Cd2+ from aqueous solutions of 50 mg L−1 and 10 mg L−1 , respectively.

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The dithizone-modified adsorbent’s maximum adsorption capabilities for lead and cadmium were reported to be 787 mg g−1 and 195 mg g−1 , respectively [66].

8 Use of Nanosponges in Removal of Organic Pollutants It is critical to remove organic pollutants from the environment because these pollutants contaminate the soil, aquifers, water table, and surface water bodies through runoff and water infiltration. Although some impurities may not be hazardous, may frequently cause undesirable issues like color, aroma, and foul taste. Other contaminants like trihalomethanes, haloarenes, and other chlorinated organic compounds are examples of the potentially dangerous disinfection by-products (DBPs) that can be produced even by the naturally occurring organic matter (NOM) in water. The majority of these chemical compounds are poisonous and constitute a substantial hazard to human health even at very low concentrations [67, 68]. β-cyclodextrin NSs have the potential to encapsulate the organic contaminants by forming complexes throughguest–host interaction. For better adsorption of organic pollutants, the nanomaterials are synthesized with a pore size of 0.7–1.2 nm, and also the hydrophobic cavity of CD provides a better platform for strong affinity bonding to the organic pollutants [35]. For the elimination of boron from water Liao et al. synthesized glycopolymer nanosponges using monosaccharides and β-cyclodextrin via Fischer glycosylation, click reaction, and cross-linking reaction. Better adsorption rates and removal capacities are produced by interactions such as Van der Waals forces and hydrogen bonds, opening up more possibilities for seawater desalination and wastewater treatment [69]. Bifunctional isocyanate linkers were used in the synthesis of nanosponge cyclodextrin polyurethanes, which were used in the selective removal of organic contaminants from water over a broad concentration range. Solid phase extraction (SPE) was used to extract water samples with low amounts of contaminants, and then GC/MS (Gaschromatography–mass spectrometry) was used to analyze the results. The Organic pollutants under investigation were chlorinated disinfection by-products (DBPs) and 2-methylisoborneol (2-MIB). For the removal of organic contaminants, the CD-based nanopolymers showed excellent absorption efficiency (>99%), which was determined to be much superior to granular activated carbon (GAC). These polyurethanes have additional benefits over GAC, such as the ability to be recycled repeatedly without losing effectiveness and these polymers could also quench the taste and odor-causing compounds [70]. Similarly, an organic compound like pnitrophenol from aqueous streams can be removed by nanosponges designed using βcyclodextrin and hexamethylene diisocyanate as the cross-linking agent. The adsorption process was studied under varied conditions and it was found to be unaffected by temperature conditions but dependent on the type of cross-linking agent concentration used. It was reported that these nanosponges had high porosity and rigidity with the maximum adsorption energy and capacity of 1.837 L mg−1 and 1 mg g−1 respectively [71]. Different detection approaches are needed to successfully monitor

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organic micropollutants in water because the majority of persistent organic pollutants cannot produce robust SERS (Surface-Enhanced Raman Spectroscopy) signals. In a recent study, the magnetic nanoparticles with porous β-CD polymer (MN-PCDP), called mesoporous nanosponge were designed to capture organic micropollutants with 90% removal efficiency in a period of 1 min. These magnetic nanoparticles had numerous advantages like selective adsorption with an enrichment factor up to ~1000. Additionally, MN-PCDP could reduce matrix interference and was found to be applicable to a wider range of sensing devices, including fluorescence, Raman, and infrared spectroscopes, with cost-effective, easy, rapid, flexible, and portable detection [72]. In order to capture lipophilic polycyclic aromatic molecules dispersed in water, alkylated hyperbranched polymers were designed. The inclusion formation constants for these nanopores were found to be 2.0 × 108 –6.3 × 106 M−1 for pyrene, 1.2 × 107 – 1.6 × 106 M−1 for fluoranthene, and 3.8 × 106 –4 × 105 M−1 for phenanthrene. The chemical structure of the parent polymers and the polycyclic aromatic chemicals used to determine the loading capabilities, for example, it was observed to be 6–31 mg g−1 of polymer for fluoranthene, 15–54 mg g−1 for phenanthrene, and 6–35 mg g−1 for pyrene. After the treatment of wastewater with these novel films, the level of pyrene and fluoranthene that remained in the sample ranged from 1 to 30 ppb and 50 to 70 ppb for the relatively more water-soluble phenanthrene. These films could be regenerated by treating them with acetonitrile [73].

9 Use of Nanosponges in Removal of Dyes Today dyes are used in many industries like paper, ink, and textiles, amongst which the textile industry uses the most amount of water and creates the largest pollutants in water bodies, making the water unfit for use. Though there are many techniques to eliminate dyes from water, the water solubility of these dyes poses the biggest challenge. Functionalized cyclodextrin-based nanosponges have effectively been able to remove dyes from the wastewater. In a report, novel nanosponges were prepared by cross-linking 1,2,3,4-butane tetracarboxylic acid and β-CD using poly(vinyl alcohol) to eliminate cationic organic dyes. The maximum adsorption was found to be 120.5, 92.6, and 64.9 mg g−1 with their reusability performance of 94.1%, 91.6%, and 94.6% for paraquat, safranin, and malachite green respectively. Mechanistically, strong electrostatic interactions between the cationic charge of the pollutant and the anionic charge of 1,2,3,4butanetetracarboxylic acid, host–guest interaction in the cavity of β-CD capturing the contaminant molecule in the cross-linked structure, and hydrogen bonding between hydrogen atoms of poly(vinyl alcohol) and nitrogen of organic pollutant molecules are responsible for the uptake of these organic dyes from wastewater. These green, environmentally friendly adsorbent nanosponges had significant potential for removing cationic organic contaminants from water [74].

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A recent work by Li et al. [75] synthesized a promising environment-friendly Cyclodextrin-based nanosponges (CDNS) in a single step by the solvothermal method using β-cyclodextrin (β-CD) and diphenyl carbonate (DPC) for discharge of dyestuff from wastewater. The synthetic CDNS were characterized by different techniques such as FTIR (Fourier transform infrared), XRD (X-Ray Diffraction), and TGA (Thermogravimetric Analyzer). Adsorption was investigated under a variety of conditions, including adsorbent quantity, the molar ratio of β-CD to DPC, pH, time, and starting concentration. Because of their structural differences, the testing results showed that Basic Red 46 and Rhodamine B dyes had maximal adsorption capabilities of 101.43 mg g−1 and 52.33 mg g−1 respectively. Both the dyes used followed the Langmuir monolayer adsorption model and the pseudo-second-order model [75]. In a study by Massaro et al. hybrid of Inorganic–organic nanosponge, hybrids were prepared using halloysite clay and cyclodextrin derivatives (HNT-CDs) under solventless conditions using microwave irradiation. Halloysite nanotubes (HNTs) are alumino-silicate clay that can be easily functionalized. They have a hollow tube-like structure with siloxane groups on the exterior and aluminol groups on the interior [76]. HNTs are naturally available, inexpensive, widely accessible, and have biocompatible [77, 78] and eco-compatible structures [79]. HNT-CDs as shown in Fig. 7, have hyper-reticulated structure, with the features of both HNT and cyclodextrin, as characterized by various methods such as FTIR (Fourier transform infrared), TGA (Thermogravimetric Analyzer), BET (Brunauer–Emmett–Teller), TEM (Transmission electron microscopy), SEM (Scanning Electron Microscopy), DLS (Dynamic light scattering). HNT-CDs nanosponge hybrids were utilized as nanoadsorbents, for the removal of Rhodamine B dye and some other cationic and anionic dyes. The results demonstrated that the electrostatic interactions and pH of the solution both had an impact on the adsorption process. Experimental results also revealed that the adsorption process followed the Freundlich isotherm model. Compared to anionic dyes, cationic dyes from aqueous solutions produced the best results. Thus, it can be concluded that this new nanomaterial has characteristics shared by cyclodextrin and halloysite and therefore, can be used in the bioremediation field [80].

10 Conclusion and Future Prospects Environmental remediation through Cyclodextrins and Cyclodextrin based ‘Nanosponges’ is a novel approach that has brought a lot of excitement amongst the various groups of researchers and their results have motivated them to further travel towards the exploration of the supramolecule. Recently these supramolecules and nanostructures have come into the limelight because of their extraordinary properties of large surface area, tunable functionality, and high thermal stability. All these properties have made them one of the recent and most explored molecules in the field of bioremediation of soil, aquifer bioremediation, and remediation of wastewater. In

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Fig. 7 Halloysite–cyclodextrin nanosponges. Reused with permission from Massaro et al. [80]. Copyright American Chemical Society 2017

the current chapter, we have covered a comprehensive review of the various modifications proposed by different research groups in designing these supramolecules and nanomaterials keeping in mind the types of pollutants to be removed. The use of nanosponges is a budding topic and is still in the early developmental stage where these structures can find application in diverse fields. For example. • Rational designing of the Nanosponges where structural defects and morphologies are better controlled is still under study. • The introduction of functional groups which act as crosslinkers need to be explored more so that the nanosponges may host a large variety of naturally occurring/ synthetic/semi-synthetic molecules. • Softwares for the computational modeling of these nanosponges need to be developed which would provide insight into the potential application of the library of designed structures. The use of CDs or derivatives in bioremediation is still at the developmental stage and has to go through multiple stages of scaling up the experiments before it actually finds application at a larger scale. Nevertheless, these Nanoarchitectures have shown their potential in the domain of efficient elimination of pollutants from the environment. These structures are expected to illustrate extraordinary performance in the near future and also in the diversified field of agrochemistry, biomedicine, cosmetics, bioremediation, and catalysis.

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Removal of Organic and Inorganic Contaminants from Water Using Nanosponge Cyclodextrin Polyurethanes Chetna Gupta, Parul Pant, and Sachender Mishra

Abbreviations AFM Ag ASTM Cd CD CNTS Co DBTDL DMF DMSO ECH EDC EGDGE GAC HMDI MWCNTS NaClAc NF NOM Pb PHPZC pMWCNTS

Atomic force microscope Silver American society for testing and materials Cadmium Cyclodextrin Carbon nanotubes Cobalt Dibutyltin dilaurate Dimethylformamide Dimethylsulfoxide Electron cyclotron heating Endocrine disrupting chemicals Ethylene glycol diglycidyl ether Granular activated carbon Hexamethylene diisocyanate Multi-walled carbon nanotubes Sodium salt of chloroacetic acid Nanofiltration Natural organic matter Lead PH of the point of zero charge Phosphorylated mwcnts

C. Gupta · P. Pant (B) · S. Mishra Department of Chemistry, Hansraj College, University of Delhi, Delhi 110007, India e-mail: [email protected] C. Gupta e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Gulati (ed.), Nanosponges for Environmental Remediation, https://doi.org/10.1007/978-3-031-41077-2_8

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Polysulfones Propane sultone Thermogravimetric analysis Titanium dioxide Titanium tetraisopropoxide Ultimate tensile strength

1 Introduction Any physical, chemical, biological, or radioactive substance or matter in water which is hazardous to health is referred to as a “contaminant.” These contaminants have been broadly divided into two categories: organic and inorganic contaminants. Organic pollutants are the chemical compounds that cause contamination (by carbon pollutants), such as polychlorinated, endocrine-disrupting chemicals (EDC), and natural organic matter [1, 2]. Compounds of inorganic by-products that result from radiation, noise, heat, or light are known as inorganic pollutants. Examples of inorganic contaminants include lead, mercury, chromium, cadmium, arsenic, and aluminum. These contaminants affect the quality of water in diverse ways. It’s a well-known fact that water plays a pivotal role in many areas of human life, including health, food, economy, and energy [3]. Today, water purification and sanitation are the most important global issues. Currently, the main focus is the removal of contaminants from industrial wastewater, surface water, and groundwater. Heavy metals and organic compounds are both extremely toxic and hazardous to human health, even at very low concentrations. In addition to organic and inorganic pollutants described above, pathogens, such as bacteria, viruses, and protozoan parasites, are another source of many health problems (waterborne diseases). Ion exchange, adsorption and photocatalysis, coagulation, flotation, reverse osmosis, ozonation, membrane filtration absorption processes, chemical precipitation, and electrochemical techniques are some of the techniques used to clean wastewater at the micro levels. These techniques, however, have a number of limitations, including the fact that they are either specifically designed for organic or inorganic pollutants (including heavy metals), whereas pathogenic microorganisms, organic contaminants, and inorganic contaminants coexist in the environment of wastewater [3–5]. The development of nanotechnology adsorption procedures was done to address the problems caused by the limitations of the aforementioned conventional cleanup techniques. This is an outcome of their effectiveness, affordability, and simplicity. Additionally, they don’t produce waste, therefore they may be recycled using a suitable desorption procedure. The introduction of nanotechnology and further research to develop adsorbent nanomaterials are the reason for easy and efficient wastewater treatment. Adsorbent

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nanomaterials, also known as nanosorbents, are nanostructured materials with pores that range in size from 1 to 100 nm and can be used to adsorb pollutant molecules, such as metal nanoparticles, carbon nanotubes, and nanosponge cyclodextrins [6]. This shift from conventional ways to modern ways for the removal of organic and inorganic contaminants has made water purification efficient now. Table 1 depicts various water treatment techniques and their advantages and disadvantages.

2 Nanosponges as an Effective Tool for the Removal of Organic and Inorganic Contaminants from Water In comparison to granular activated carbon (GAC), cyclodextrin (CD) polymers exhibit superior absorption efficiency (>99%) with respect to organic contaminants. Additionally, these CD polymers’ effectiveness in the treatment of wastewater was understood, and further developments of Cyclodextrin polymers were carried out which resulted in a better alternative as Nanosponge Cyclodextrin polyurethanes. The ability to absorb a group of organic pollutants from wastewater at very low concentrations has been demonstrated by the nanosponge cyclodextrin insoluble polymers, primarily polyurethanes, through host–guest interactions. Cyclodextrinbased Nanosponges (0.7–1.2 nm) exhibit a strong affinity for absorption of hazardous organic contaminants from wastewater because of the hydrophobic surrounding presented by Cyclodextrin cavities. Additionally, cyclodextrin-based nanosponges with a maximum adsorption capacity of 2 mg g−1 can be used to quickly remove contaminants from water (90%). Carbendazim, diclofenac, sulfamethoxazole, and furosemide are a few pharmaceutical pollutants that these nanostructures have the capacity to efficiently eliminate from water [21–23]. Similarly, a cross-linking oligomerization of cyclodextrin using phosphorylated multi-walled carbon nanotubes (MWCNTs) is also used to create polymeric nanobiocomposites with multi-functionality. This step is followed by a sol–gel step for the incorporation of silver (Ag) and titanium dioxide (TiO2 ) nanoparticles. These composites can further be used as possible filter nanosponges to remove contaminants such as harmful bacteria and organic/inorganic compounds from water [24].

2.1 Cyclodextrins, the Wonder Molecules Cyclodextrins are effective molecular chelating agents that are employed in a variety of industries, such as polymer synthesis, pharmaceuticals, cosmetics, food processing, chemicals, flavours, textiles, fermentation, catalysis, and environmental protection. A. Villiers, first reported cyclodextrins (CDs) in 1891. CDs are cyclic oligosaccharides made from enzymatic processes by treating starch with Bacillus macerans

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Table 1 Various conventional methods for water treatment Strategies

Limitations/challenges

Advantages

References

Ion exchange method

• High cost • Renewal of resins • Secondary pollution toxicity/ organic contamination from resins • Organic matter adsorption • Possibility of bacterial or chlorine contaminants

• High risk of removal • Fast kinetics • High capacity of treatment processes • Suitability from removing nitrates and arsenic from water

[7, 8]

Adsorption

• Low selectivity • High cost of adsorbents • Hard separation/isolation

• Ease of processes • High capacities for binding metals • Reusability • High efficiency/ efficacy

[9]

Electrochemical technique

• High cost • High energy consumption

• • • •

[10, 11]

Photocatalysis

• Long duration time • Restricted appliances • Preparative process of photocatalysts • Possibility of toxicity and Hazardous effects caused by inorganic coagulants

• Less harmful/toxic [12] by-products • Concurrent elimination of organics and metals

Flocculation/ coagulation

• Formation of large amounts of toxic sludge • Not suitable for removing heavy metals or emerging contaminants

• Improvements in water [13, 14] clarity to decrease turbidity

Chemical precipitation

• Not suitable for the purification • Low cost of water with a dilution of • Simplicity heavy metals at a low level • Sludge production • Additional operative cost for the removal of sludge

[15]

Flotation

• High cost of operation and maintenance • High cost of initial capital

[16]

High selectivity/ Flexible operations Scalability High separation

• A higher rate of overflow • Formation of concentrated sludge • Higher discernment • Higher proficiency • Minimal period of detention

(continued)

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Table 1 (continued) Strategies

Limitations/challenges

Advantages

References

• Requirement of small spaces • High selectivity • Requirement of low pressure

[17, 18]

• Not suitable for small organic • Seawater desalination pollutants • Organic/inorganic pollutant removal • High cost, energy, and pressure • Removal of bacteria from wastewater

[19, 20]

Membrane filtration • Complicated processes techniques • Low permeate flux • High cost of membranes • High costs of operation and maintenance • Deterioration of productivity • Frequency of membrane regeneration difficulties Reverse osmosis technique

(Bacteria) amylase. There are three different types of natural cyclodextrins i.e. α-CD, β-CD, γ-CD. α-CD, β-CD, γ-CD has six, seven, and eight glucose units respectively. These glucose units are connected by α-(1, 4) glucosidic bonds to form a ring. These three natural CDs—which vary in ring size and solubility are widely used due to their high reactivity [25, 26]. For all extraction purposes, β-CD has given better results when compared with α-CD, β-CD and γ-CD hence it has been exclusively used for wastewater purification purposes. The generalized structure of β-CD is given in Fig. 1. Edge of Primary Hydroxyl groups Edge of secondary Hydroxyl Groups

Hydrophobic Cavity

Fig. 1 Generalized active structure of β-CD

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2.2 Various Methods for Synthesis of CD’s The conversion of CD polymers is carried out by modification of β-CD to β-CD polymer (Fig. 2). Nanosponges Cyclodextrin polyurethanes are insoluble nanoporous polymers. The Cyclodextrin polyurethanes polymers are very useful in taking up the organic and inorganic contaminants from water (Table 1). Even cyclodextrin polyurethanes are comparatively more efficient in the extraction of undesirable ions like Cd2+ from water. Some methods of preparation of cyclodextrin polymers are listed below (Fig. 3). Various Cyclodextrin Various cyclodextrin polymers have been prepared using the following procedures which involve either the process of Deprotonation, condensation, or dehydration by using crosslinking reagents. CD polymers are created by using the procedures given below [22, 26–30]: (i) Crosslinking with bi- or multifunctional reagents. (ii) Polymerization of acrylic monomers containing pendant cyclodextrin units. (iii) linking to a polymeric backbone by covalent bonding or physical entrapment in different polymeric chains. Table 2 depicts CD polymers formed by using bi- or multifunctional reagents for crosslinking and their applications. Further modification of cyclodextrin polyurethane polymers is carried out to prepare cyclodextrin nanosponges.

β-CD POLYMER

β-CD

Fig. 2 CD to β-CD polymer

Methods of CDs polymerised using oxyanion

cyclodextrin polymers

Dehydration of CDs with a diacid (dicarboxylic acid) or diol

Fig. 3 Methods of preparation of cyclodextrin polymers

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Table 2 CD polymers created by using bi- or multifunctional reagents for crosslinking Crosslinking agent

Polymer obtained

Applications

ECH + polycarboxylic acid

Microparticles

Inclusion complexes (naringin)

ECH or EGDGE 3

Water soluble

ECH in the presence of NaClAc 1 or PS Ionic polymers 2

Drug delivery systems Ion exchange chromatography

ECH or EGDGE 3

Water soluble

Drug delivery systems

Dicarboxylic acid dihalides

Water soluble

Selective cigarette filters

ECH in the presence of NaClAc 1 or PS Ionic polymers 2

Ion exchange chromatography

N-methylol acrylamide

Water insoluble

Column chromatography

ECH

Block polymers Microparticles

Chromatographic separations

Diacids Diesters

Concentration of flavor

Dihalohydrocarbon diisocyanates

Removing of target organic compounds

Hexamethylene diisocyanates

Absorption of cholesterol

1 = Sodium salt of chloroacetic acid 2 = Propane sulfone 3 = Ethylene glycol diglycidyl ether

2.3 Preparation of Cyclodextrin Nanosponges A general methodology for the preparation of Cyclodextrin Nanosponge involves the usage of polar aprotic solvent like dimethylsulfoxide (DMSO) or dimethylformamide (DMF), wherein the primary hydroxyl group of the monomer CDs interacts with an excess of a cross-linker agent with a reaction span of 1–48 h [31] (Fig. 4). Reaction temperature ranges from 10 °C to the solvent reflux temperature. The reaction’s development can be examined by the IR spectroscopy method. The cross-linker peak’s disappearance from the reaction mixture serves as a marker for the completion of the reaction, and the appearance of mainly CO, NH (CO) groups and other functional groups, depending on the cross-linker type used, serves as a sign of the linker’s successful incorporation into the polymer. The dried and ground nanoporous cyclodextrin insoluble polymer is a white powder, granular solid, or film. Depending on the type of cross-linking agent employed, the cyclodextrin nanosponge polymer’s average surface area has been found to vary from 1.6 to 3.5 m2 g−1 [1, 13, 22, 26, 27, 30–32].

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β-CD POLYMER

β-CD NANOSPONGE

Fig. 4 Modification of β-CD polymer to β-CD nanosponge

β-CD NANOSPONGE

β-CD POLYURETHANE NANOSPONGES

Fig. 5 Synthesis of cyclodextrin polyurethane nanosponges

2.4 Synthesis of Cyclodextrin Polyurethane Nanosponges The cyclodextrin polyurethane nanosponges are very efficient in water purification. The voids of cyclodextrin are the real centers for the sorption of impurities. They are mainly obtained by doping with other materials to make them more efficient, and they can easily be prepared. Their hydrophilic nature improves because of the blending of these components which makes water permeability easy and high ejection rate of metal ions for example ions like Cd2+ , etc. A modified phase inversion approach is employed for the creation and assessment of cyclodextrin (CD) polyurethane composite membranes (Fig. 5). For example, to create nanofiltration (NF) membranes for the targeted elimination of Cd2+ ions out of the water at an appropriate pH of 6.9, beta-CD polyurethane was mixed with PSf at a concentration ranging from 0 to 10%. The resulting blended matrix-CD polyurethane/PSf membranes had increased water permeability, more Cd2+ rejection (up to 70%), and were more hydrophilic. The mixed matrix membranes, on the other hand, appeared rougher, less permeable, absorbed more water, and had lower mechanical stability. Additionally, PSf and -CD polyurethane exhibit hydrogen bonding interactions among them according to FT-IR measurements, suggesting how much the compatibility of the material is. Mixed matrix membranes appear to have increased water flux, according to the Dead-end filtration test. This shows that CD polyurethane and PSf can be used to provide functional nanofiltration with enhanced flux without affecting its rejection ability [33]. The following are the experimental methods for the preparation of cyclodextrin polyurethanes and modified cyclodextrin polyurethanes. (i) Preparation of CD polyurethanes Adams et al. reported the preparation of CD polyurethane by employing the following procedure. DMF (50 ml) and β-CD (5.00 g, 4.4 mmol) were put together while being stirred in a round bottom flask followed by the drop-by-drop addition of HMDI (0.70 ml, 4.4 mmol). 2 mL of DBTDL was added and the reaction was allowed to continue at room temperature under normal conditions. Excess solvent was removed and the polymer

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O HO

OH

Hexamethylene diisocynate

HO

O

C

N

NH

H

O

C O

DMF+Catalyst(DBTDL) 0H

0H

-CD

CD polyurethane

Scheme 1 β-CD polymerization process

obtained (as precipitate) was washed with acetone. The yield of the product was around 96% CD polyurethane, which is soluble in water but insoluble in DMF [33] (Scheme 1). (ii) Preparation of modified bio-nanosponge cyclodextrins pMWCNT-CD and pMWCNT-CD/Ag-TiO2 The preparation of insoluble, modified polymer nanocomposites based on bionanosponge cyclodextrin is discussed below. The main reason for this preparation is to enhance the filtration output of the CD polymers and the following diagrams depicts the formation of the modified nanosponges. The following are the steps to prepare these polymers: STEP 1: Preparation of CD Polyurethane by polymerization process (as described above) [1,38] reaction conditions are given below in Scheme 2. STEP 2: Multi-walled carbon nanotube functionalization: Using acid treatment, MWCNTs were functionalized (or oxidized) [34, 35]. STEP 3: Phosphorylation of MWCNTs: The first step involved chlorinating the oxidized MWCNTs with excessive amounts of oxalyl chloride. Then, to produce phosphorylated MWCNTs (pMWCNTs), an amidation procedure was used to phosphorylate the chlorinated MWCNT [34, 36]. STEP 4: Preparation of the nanosponge polyurethane pMWCNT-CD nanocomposite: Hexamethylene diisocyanate (HMDI) was used as a crosslinker agent during the polymerization of pMWCNTs with CDs, as described by Mamba and colleagues [37]. 1 g of β-CD was employed, and the weight percentage of pMWCNT in relation to it was 5% [34]. O HO

OH

OCN-(CH2)6-NCO

HO

O

NH

H

DMF, 75 C,24 hrs

Scheme 2 Preparation of CD polymers

N

C O

O

O

0

0H

C

0

N H

NH

C O

O

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STEP 5: Bio-nano Sponge polyurethane pMWCNT-CD/Ag-TiO2 nanocomposite preparation: The sol–gel method has been used for the synthesis wherein pMWCNTs-CD polymer (obtained in Step 4 above) was dispersed in 2-propanol was then mixed with TTIP. After swirling the mixture for 10 min, an aqueous solution of silver nitrate (pH 2) was added with constant stirring. After 24 h of vigorous stirring at room temperature, the resulting solution was eventually dried to produce pMWCNT-CD/Ag-TiO2 [37] (Schemes 3, 4 and 5).

Scheme 3 Functionalization of MWCNTs by acid treatment

Scheme 4 Phosphorylation of MWCNTs

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O OC HO

OH

C

P N

OCH 2CH 3 OCH 2CH 3 O

H

pMWCNT,DMF

OH

O

HMDI, 700C 0H

179

OC

C O

HN

N H

0

Scheme 5 Polymerization of pMWCNTs with β-CDs

3 Mechanism Involved in the Removal of Pollutants by Cyclodextrin Polyurethanes Adsorption Mechanism There are various possible mechanisms followed for the removal of pollutants by Cyclodextrin polyurethanes but the most effective mechanism is the adsorption mechanism. Adsorption is the process by which an adsorbate (pollutant in liquid or gaseous solutions) adheres to the surface of an adsorbent (solid), such as CNTs or CD nanosponges that have been treated with nanomaterials. The removal of large polar organic, inorganic, or heavy metal contaminants is therefore typically done by the adsorption approach [38]. According to Chiban et al. [39] and Ihsanullah et al. [40], the mechanisms of adsorption of these heavy metals or inorganic contaminants were found to be complex. Those that have been most extensively reported include electrostatic attraction, physical adsorption (primarily caused by van der Waal forces), adsorption-precipitation, and chemical interaction (that occurs between functionalized CNTs and metal ion pollutants). It was discovered that the adsorption mechanism also entailed the absorption process, which involved the formation of inclusion complexes (host–guest complexes) between the organic (guest) molecule and the CD cavity (host). The development of these host–guest complexes involves noncovalent connections, such as van der Waal forces. Based on their affinity (size and polarity) towards one another, the CD host and the organic guest molecule interact hydrophobically [41]. While it was discovered that during the adsorption process, the hydrophilic molecules (heavy metal ions, big polar organic pollutants, and inorganic pollutants) had been attached to the hydrophilic end of CD by the creation of covalent bonds or chemical contact [1].

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4 Factors Affecting the Adsorption of Polar and Big-Size Pollutants (i) The pH of the solution Because the pH impact mostly depends on the type of pollutant to be removed and the type of adsorbent to be utilized, pH is one of the primary factors impacting the adsorption process [39, 40]. Additionally, it was observed that the neutralisation of surface charge results in reduced pollutant adsorption at pH values below the pH of the point of zero charge (pHPZC). In contrast, high adsorption occurs when the pH of the pollutant solution exceeds pHPZC (43, 44 M). The pH of the desorbing solution has been shown to have an impact on the adsorbent renewal or desorption process [39, 40]. For instance, it has been observed that the desorption of Pb2+ is high at lower pH values (2.5) and low at higher pH values (>5) when HNO3 is used as the desorbing solvent and an adsorbent comprising modified CNTs [39, 40, 42]. (ii) Pollutant’s initial concentration The type of adsorbent utilized also affects how the initial concentration affects the adsorption mechanism. Mamba et al. have shown that the CD nanosponge polymer modified with functionalized CNTs performed significantly better than functionalized CNTs at low concentrations (10 ppm). For instance, the polymer was shown to remove lead, cobalt, and 4-chlorophenol in amounts of 68%, 67%, and 87%, respectively [34]. (iii) Adsorbent dosage Studies by Gupta et al., Ihsanullah et al. [40], and Rao et al. indicated that an increase in the amount can either decrease or improve the adsorption capacity depending also on the type of contaminant to be removed. According to studies by Gupta et al., Ihsanullah et al. [40], Rao et al., and others, the absorption of Pb2+ increased with an increase in adsorbent dosage while the uptake of Co2+ decreased with an increase in adsorbent dosage. (iv) Contact time According to experimental observations it has been found [39, 42] that, as contact time increases, the proportion of adsorption increases as well until equilibrium is attained. (v) Competing ions It has been discovered that the adsorption process for systems with multiple components is complex and may depend on aspects including pH, ionic radius, the existence of unoccupied active sites on the adsorbent, and electronegativity [39, 42].

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(vi) Temperature Due to the fact that temperature rises as a function of adsorption capacity (43, 44) temperature is thought to be an endothermic process. For instance, the absorption of Pb2+ is an endothermic process because the metal ions rapidly diffuse through the solution at high temperatures, which causes the solution’s viscosity to diminish [40]. In another body of literature, it was found that the exothermic reaction caused by a rise in temperature led to a decrease in the adsorption of metal ions pollutants [39, 42].

5 Preparation of Cyclodextrin Polyurethane Membrane Cyclodextrin polyurethanes become more effective when they are converted to membranes. The Cyclodextrin polyurethanes are low-fiber membranes which makes them widely useful in wastewater treatment. The method of preparation of cyclodextrin polyurethane membrane has been reported by Adams et al. [33]. They prepared Cyclodextrin polyurethane membrane by dissolving cyclodextrin polyurethane in DMF at different Concentrations ( 5 wt%, 8 wt% and 10 wt%) of polysulfones at 80 °C for 2 h. This resulted in 20% of polymer in DMF (desired concentration). A 200 nm thick Membrane was obtained on a glass plate by following the general procedure of membrane preparation (Fig. 6). Finally, the membrane was sandwiched between two sheets of unprinted paper for storage.

Dissolved β-cyclodextrin polyurethane in DMF at polysulfones.

20% polymer in DMF

12hrs

Product was allowed bubbles removed

Membrane obtained

The membrane was sandwiched between two sheets of unprinted paper for storage.

Fig. 6 Schematic process for the preparation of cyclodextrin polyurethane membrane

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6 Characterization and Analysis of β-Cyclodextrin Polyurethane Membrane Various techniques are employed to check the performance of the synthesized membrane so as to can get an overview of the characterization and shortcomings of the membrane (Fig. 7). FTIR Analysis, scanning electron microscopy, thermogravimetric analysis, Atomic force microscopy, and tensile strength test are used to check the formation of the membrane and the efficiency of the cyclodextrin polyurethane membrane by varying various parameters. The following analysis techniques are used to check the efficiency of doped membranes: A. FT-IR examination of CD polyurethane (doped with PSF) Presence of the NHCO stretching vibration peak (1546 cm−1 ) , OH group peaks in the IR spectra of β-CD polyurethane and β-CD monomer (3312 cm−1 and 3295 cm−1 , respectively) and disappearance of the isocyanate group peak, disappearing indicates that the polymerization activity has ended [28]. B. Scanning electron microscopy A scanning electron microscope is an instrument that uses the reflected electrons and produces an image of the test solution. The electrons passed interact with the test solution’s atoms and various signals are produced giving data regarding the composition and topography of the test solution. With the increase in the concentration of Fig. 7 Variuos techniques used to check the efficiency of doped membranes FT-IR

Tensile strength test

Atomic force microscopy

CYCLODEXTRIN POLYURETHANE MEMBRANE'S ANALYSIS FOR CHARACTERIZATION AND EFFICIENCY CHECKING Porosity, contact angle and water content

Scanning electron microscopy

Thermogravi metric analysis

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beta-CD polyurethanes, direct visibility of bigger and highly distinct circle-shaped patterns was observed. C. Thermogravimetric analysis The cross-linked structure of the membrane gives more stability to the membrane and the presence of crosslinking in the membrane can be cross-checked by doing comparative studies of thermogravimetric analysis. D. Porosity, contact angle, and water content It has been observed that the increase in beta-cyclodextrin polyurethane levels in the preparation procedure leads to an increase in the overall efficiency (porosity, contact angle, and water content) of the membranes. E. Atomic force microscopy It is observed that when the amount of β-CD polyurethane added to the composite membranes increases, the shape of the surface changes. F. Tensile strength test A membrane’s tensile strength is crucial because it reflects the material’s physical toughness and durability and serves as a gauge of its mechanical strength. When the ultimate tensile strength (UTS) of the doped membranes was evaluated. PSf membranes showed elevated UTS values in comparison to composite membranes. According to the findings, adding β-CD polyurethane to composite membranes reduces the ultimate tensile strength of PSF membranes [33].

7 Experimental Studies Done to Prove the Efficacy of β-CD Cyclodextrin Polyurethane Nanosponges Doped with Other Materials in Wastewater Treatment Adams et al. prepared polysulfone (PSf) and β-CD polyurethane composite membranes by employing a modified phase inversion technique. It was observed that the β-CD polyurethane blended polysulphone nanofiltration membrane obtained at 0–10% P-Sf concentration was highly selective for the removal of Cd2+ ion removal at a pH of 6.9 [34].

8 Conclusion Nanosponges are an appealing candidate for absorbing and eliminating organic and inorganic pollutants and contaminants because of their high porosity, ease of functionalization, simplicity, and cost-effectiveness in comparison to other nanosystems/

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materials (such as dyes, heavy metals, and pharmaceuticals) from wastewater. The unique physicochemical properties like non-toxicity, biocompatibility, bioabsorption, complexation behaviors, production at a cheaper rate, and ease of surface Functionalization) and high efficiency of Cyclodextrin-based Nanosponges makes them responsible for the removal of a large variety of pollutants. Due to the complexity of pollutants and the coexistence of pathogens, organic and inorganic pollutants, and functionalized carbon nanotubes (f-CNTs), cyclodextrin polymers, and nanocatalysts (TiO2 , Ag), it has been demonstrated that using these materials alone is not very effective. In order to concurrently and effectively remove inorganic, organic, and microbiological contaminants from wastewater to acceptable levels, it is necessary to design a water treatment technique. For the in-depth analysis of the efficiency of these nanosponges, further research is required to know their activity against pathogens. These CD polyurethane nanosponges are recyclable which gives them an edge over others. Scientists are working on various parameters to further enhance the pollutant removal efficiency of doped CD polyurethane nanosponges.

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Introduction to Metal–Organic Framework Sponges and Their Synthetic and Functionalization Strategies Preeti Bhatt, Abhay Srivastava, and Subinoy Rana

Abbreviations ATR BASF BDC BET BJH BPDC BPE BPEB BTC DEA DMA DMF DRIFTS EDS FT-IR HKUST IRMOF MIL MOF MOF-S

Attenuated total reflectance Badische Anilin und Soda Fabrik Benzenedicarboxylate Brunauer, Emmett, and Teller Barrett, Joyner, and Halenda 4,4' -biphenyl dicarboxylate ligands 1,2-bis(4-pyridyl)ethene 1,4-bis[2-(4-pyridyl)ethenyl]benzene Benzenetricarboxylate Diethyl formamide Dimethylacetamide Dimethyl formamide Diffuse reflectance infrared fourier transform spectroscopy Energy dispersive spectroscopy Fourier transform infrared Hong Kong University of Science and Technology IsoReticular metal–organic frameworks Materials Institute Lavoisier Metal-organic frameworks Metal-organic framework sponges

P. Bhatt · A. Srivastava · S. Rana (B) Materials Research Centre, Indian Institute of Science, Bangalore, India e-mail: [email protected] P. Bhatt e-mail: [email protected] A. Srivastava e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Gulati (ed.), Nanosponges for Environmental Remediation, https://doi.org/10.1007/978-3-031-41077-2_9

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Porous coordination polymers Post-synthetic deprotection Post-synthetic exchange Post-synthetic insertion Post-synthetic modification Post-synthetic polymerization Powder X-ray diffraction Solvent-assisted ligand exchange Solvent-assisted ligand incorporation Secondary building unit Single-crystal X-ray diffraction 4,4' -Stilbene dicarboxylate Sequential linker installation Thermogravimetric analysis Universitetet i Oslo X-ray diffraction Zeolite imidazolate framework

1 Introduction With the rapid development of nanotechnology, sponge-like materials have gained wide attention. Sponge-like materials are essentially referred to as soft, porous structures used for the adsorption of small molecules. Materials with pore dimensions in the nanometre range with the ability to adsorb and desorb guest molecules are called nanosponges. The development of cyclodextrin-based sponge-like materials led to the coining of the term “nanosponge” for the first time in the 1990s [1, 2]. These sponge-like materials can either be of organic or inorganic origin with nano-sized cavities like cyclodextrins [3], hydrogels [4], metal oxides [5], and metal–organic frameworks (MOFs). MOFs are a family of highly crystalline materials possessing sponge-like properties due to their ultra-porous structure and ability to adsorb/desorb a variety of molecules. In the literature, there is no clear distinction between MOFs and MOF sponges as such. However, there is a general consensus that MOF structures with pore diameter typically in the 0.5–1 nm range [6] exhibit good sponge-like behaviour in adsorbing small molecules or gases and can be called MOF sponges (MOF-S). However, in practice, the terms MOF and MOF-S are used interchangeably. The term porous coordination polymers (PCPs) is also used in the context of MOFs due to their similarity with the extended polymeric framework formed by the coordination of metal ions and organic linkers [7]. So, MOFs or MOF-S are inorganic–organic hybrid networks composed of two components: (i) A metal ion, which is present at the nodes, (ii) an organic linker, which is attached to the metallic components like arms producing a cage-like structure (Fig. 1). The inorganic metal centres are linked

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Fig. 1 Illustration of MOF formation

through multi-chelating organic linkers to generate consistent and continuous 2D or 3D structures, referred to as secondary building units (SBUs). At present, there is no accepted nomenclature for MOF structures. The name given to a MOF frequently includes the name of the lab where the new substance was found and a number (i.e. MIL-53, Materials Institute Lavoisier; NOTT-202, University of Nottingham; NU-125, Northwestern University; HKUST-1, Hong Kong University of Science and Technology; UiO-66, University of Oslo, etc.) or by their type and sequence of synthesis (IRMOF-1, MOF-74, MAF-4, ZIF-8, ZIF-90, CPL-2, COF-1 etc.) [8]. MOF-like structures were reported starting from the 1960s by Tomic and others [9]. However, the name metal–organic framework for the first time was coined by Omar Yaghi in 1995 when the first robust and durable MOF structure, designated as MOF-5, was synthesized by Yaghi et al. consisting of Zn4 O(BDC)3 SBUs [10]. Since then, over 20,000 structures of various MOFs have been developed, reported and investigated made with distinct metals and organic ligands [11]. A few well known MOFs and their corresponding metal ions and organic linkers are given in Fig. 2. Yaghi et al. [12] has popularized the concept of reticular synthesis and design in the context of MOF structures. Reticular synthesis is a top-down design concept, and is the process of assembling the MOF precursors, i.e., metal ions and organic ligands into molecular building blocks to form extended crystalline networks also called secondary building units (SBUs). These ordered structures are held together through strong bonds between the precursors. SBUs are responsible for directing the assembly process into the 3D MOF-S structures. Careful selection of Secondary building units (SBUs) and linkers allows the development of MOF-S having tailormade pore sizes, shapes, and functionality for specific applications [13]. Hence, the structural, magnetic, catalytic, electrical, and optical properties are highly tunable. Also, their synthesis is quite inexpensive, relatively easy, and in highly pure and crystalline form. This makes them an attractive material for use as nano sponges including applications in drug delivery [14], catalysis [15], gas storage and separation [16], and analyte sensing [17]. MOF-S are advantageous for applications in gas storage and gas separation. Compared to their traditional counterparts like activated carbons and zeolites, MOFS have advantageous structural features like high pore surface areas and volumes. Thus, the emergence of MOFs shows great promise in energy-oriented smart applications such as hydrogen [18–21], acetylene [22, 23] and methane [24–26] storage,

190 Fig. 2 Different MOF-S structures and their constituting metal and linker design. Reprinted with permission from Ref. [11]

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carbon dioxide capture [27, 28], and nitrogen oxide adsorption [29]. A vital step in the synthetic chemical industry is the separation of mixtures, and the necessary separation and purification of essential commodities consume about 15% of global energy consumption. MOFs have attained considerable attention in the separation of gaseous mixtures [30, 31]. Heterogeneous catalytic materials [32] are crucial for manufacturing industrial chemicals. They are highly favoured in industrial processes due to their durability and lower operational costs, particularly because it is simpler to recover/separate them from the products, allowing for the simplification of chemical processes. MOFS make an excellent family of heterogeneous catalysts because of their tailorable structural and chemical properties. They can be systemically optimized to carry out specialized catalytic reactions [33, 34]. The advantage of MOF-S catalysts is the presence of uncoordinated metal centres and readily accessible organic ligands. MOF-S based materials have been used as a catalyst or as a catalytic support for a number of organic transformations [35] including Friedel–Crafts alkylations and acylations [36–38], Knoevenagel condensation [39–41], Diels–alder reaction [42– 44], aldol condensation [45], CO oxidation [46, 47], coupling reactions [48–50], aldehyde cyanosilylation [51, 52], etc. MOF-S are effective tools for biomedical applications, particularly for drug delivery, due to their porous architecture, wide surface area, high pore volume, tuneability, and simple surface functionalization. The high porosity offers significantly high drug loading capacity and selective transport. Synthetic functionalization strategies can be altered to develop the MOF sponges or adjust the pore size of the MOF-S to improve loading capacity and enable controlled release. Therefore, MOF-S have been used to deliver diverse categories of therapeutics, including small molecules [53, 54], gasotransmitters [55], proteins [56], nucleic acids [57, 58], viruses [59], and cells [60]. Recently, various researchers have also been examining the potential of MOF-S as chemical sensors. To attain molecular recognition, MOF structures require specific functionalities. Since it is easier to modify MOF-S using post-synthetic modification to add specific functional groups responsive to the analyte of interest; MOF-S based sensors ensure high specificity and selectivity. So, MOF-S have been explored for sensing a variety of analytes at very low detection limits, including gases [61, 62], small molecules [63, 64], and metal ions [65]. In this chapter, we are going to look at various procedures for the synthesis of MOF-S and the reaction parameters affecting the synthesis process. We also will highlight the most widely used techniques for the characterization of MOF-S. Finally, we will explore the various strategies to functionalize MOF-S for imparting the desirable characteristic and properties to the developed MOF-S.

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2 Synthesis of Metal–Organic Framework Sponges Numerous synthetic methods have been employed for synthesizing MOF-S. The most conventional method of MOF-S synthesis is a solvothermal method. Other synthetic routes include electrochemical, microwave-assisted, sonochemical, template-based, and mechanochemical methods which are considered to be greener methods. The choice of the synthesis protocol depends upon a variety of factors such as the solubility of the precursors, the desired characteristics such as surface area and pore dimensions; and optimal yield by avoiding any undesirable chemical side-reactions.

2.1 Solvothermal Methods The most typical synthetic procedure used to create MOF-S is solvothermal synthesis [66]. The solvothermal process needs organic solvents such as methanol, ethylene glycol, and toluene as the reaction medium. Water can occasionally also be employed as a reaction solvent, and the process of synthesis of MOF-S is known as hydrothermal synthesis. It involves dissolving the metal salt or constituent organic linker and creating metal–ligand complexes using high-boiling, polar solvents at high temperatures (Fig. 3a). During solvothermal and hydrothermal processes, the energy needed to start and promote synthesis reactions is often provided by ordinary electric heating in several dozen hours. The formed SBUs are essentially metal–ligand clusters having a clearly defined geometry that gives the resultant MOF structure its stiffness and directionality. For instance, consider the formation of MOF-5, also called IRMOF-1 [10]; in the reaction protocol, the organic ligand, BDC and metal salt, Zn(NO3 )2 are solubilized in dimethylformamide (DMF) solvent, and heated to 100 °C in a sealed container. Under such high-temperature conditions, the BDC linker and Zn2+ ions react to generate octahedral SBUs; Zn4 O clusters coordinated by six BDC ligands. These octahedral SBUs are then joined by BDC bridge linkers forming an extensive 3D porous cubic lattice. The reaction mixture gives rise to the colourless, block-shaped crystals of the as-synthesized MOF.

2.2 Electrochemical Methods HKUST-1, with a composition of [Cu3 (BTC)2 ] was the first ever MOF synthesized using the electrochemical method by BASF in 2005 [67]. The electrochemical method is a green synthetic approach where the synthesis is carried out without any metal salts or corrosive anions like nitrate and preventing the production of toxic byproducts. This makes the electrochemical synthesis approach environment friendly [68]. The key step of MOF-S synthesis includes the production of metal ions by a

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Fig. 3 Schematic of MOF synthesis methods: a solvothermal method, b electrochemical method

metal salt’s reaction with acid. Instead of directly using metal salts, the electrochemical technique of MOF-S utilizes electrons generated via the electrochemical process as a metal ion source which are then passed into the reaction mixture. The reaction mixture is an electrolyte solution containing dissolved organic linker molecules. The anodic dissolution of a metal cathode is connected to an electrochemical process. Normally, MOF-S are created when an electric current passes through an electrochemical cell made up of a metal cathode submerged in an organic linker solution (Fig. 3b).

2.3 Microwave (MW)-Assisted Methods MW-assisted synthesis makes it simpler to produce green nanomaterials, especially nanoporous materials. So, the MW-approach has been employed for the synthesis of MOF-S materials successfully [69] (Fig. 4a). The batch procedures used for the synthesis of porous nanomaterials are usually very lengthy and time-consuming. The main advantage of MW-based synthesis is that the reaction rates are 1000-fold higher than the conventional heating procedures. The rate enhancement is mostly due to either thermal or kinetic effects as MW-irradiation can activate the molecules that possess a dipole moment. As a result, activation energy is transferred in a fraction of a nanosecond, raising the molecules’ instantaneous temperature (Ti ). Ti depends on the microwave power input, i.e., the higher the power intensities of the input

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Fig. 4 Schematic for MOF synthesis methods a microwave-assisted synthesis, b mechanochemical synthesis

MW, the higher and more stable the Ti . Numerous studies have enhanced the understanding of the phenomenon responsible for rapid synthesis via MW, such as (i) faster and more consistent heating of the reaction mixture, (ii) changes in the coordination of reacting species, (iii) superheating effect, (iv) generation of localized hot spots, and (v) improvement of the solubility of the precursors. MIL-100 (Cr), composed of chromium and H3 BTC linkers, was the first MOF reportedly prepared in a microwave oven within 4 h, instead of 4 days with a traditional electrical heating system in 2005 [70]. Mingyan and co-workers produced iron-containing MOFs, MIL88B and NH2 -MIL-88B, using a rapid MW-assisted solvothermal procedure [71]. Another lanthanide-based MOF have been developed by Lin et al. using MW-assisted approach [72].

2.4 Mechanochemical MOF Synthesis Mechanochemical synthesis offers a solvent-less pathway for the fabrication of MOFs and desired building blocks. The solid−solid reaction method achieves quantitative yields and eliminates the requirement of time-consuming processes. The first solvent-free synthesis was documented in a 2006 publication including synthesis of [Cu(INA)2 ] by 10 min of ball milling [73]. Mechanochemical reactions rely on reagents, typically solids, directly absorbing mechanical energy during milling or grinding processes. The energy needed to start chemical reactions in a ball-milling process is typically provided by friction and impact between the balls and the reactants. To trigger a chemical reaction, a high ball impact is required, otherwise, only elastic deformations can take place. The high-energy grinding leads to structural stress, bond breaking, and the formation of highly reactive radicals. The exposure of reactive atomic layers promotes chemical reactions at the interface of solid

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reactants. Mortar and pestle were initially used to perform mechanochemical reactions (Fig. 4b). However, ball mills and grinders that operate automatically are most commonly used today. With these tools, mechanosynthesis can be performed in a well-defined and reproducible manner. The most well-known MOFs, including MOF5, ZIF-8, HKUST-1, MIL-101, and UiO-66, have been successfully synthesized using the mechanochemical method [74].

2.5 Sonochemical Synthesis Sonochemical synthesis has become a commonly used synthetic technique, which employs ultrasound (US) effect and acoustic cavitation for reaction [75] (Fig. 5). Compared to traditional approaches, which were primarily limited by their requirement for higher reaction temperatures and prolonged response times, the utilization of US in MOF-S synthesis has narrowed the gap towards achieving lower particle sizes with gentler reaction conditions. In a comparative kinetic study performed by Haque et al. [76] using Fe-MIL-53 MOF, the average reaction time decreased in the order: Ultrasound > Microwaves > Solvothermal methods. The increased reaction rate was attributed to an increase in the pre-exponential factors instead of a decrease in activation energy as calculated from the Arrhenius equation [76]. The faster reaction rates in this method are attributed to a phenomenon known as acoustic cavitation. It is a process that involves the development and collapse of bubbles sequentially in a region known as a “hot spot”. These localized regions host very high temperatures exceeding 5000 °C, pressures beyond 1000 atm and heating/cooling rates above 1010 K/s. This results in intense localized heating, extremely high pressure conditions, and very brief lifetimes, thus generating a sufficient amount of energy to make it possible for processes that were previously challenging to carry out using other techniques to be completed with US irradiation, even at a macroscopic level at room temperature. In 2008, Qiu et al. pioneered the use of the sonochemical method for the synthesis of (Zn3 BTC2 )·12H2 O [77], and subsequently that same year, MOF-5 [78] and ZnBDC [79] were also synthesized by application of US irradiation.

Fig. 5 Schematic for the sonochemical synthesis of MOF

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2.6 Other Methods Other synthesis methods in addition to the aforementioned commonly used strategies, include microemulsions, microfluidics, and template strategies. Microemulsions involve the immobilization of nano-sized water droplets onto the organic phase using surfactants. The microemulsions’ micelles serve as nanoreactors and regulate the crystal formation kinetics. For the precisely controlled synthesis of MOF structures with various sizes and diverse morphologies, microemulsions offer significant prospects [80, 81]. Han and co-workers [82] conducted a systematic investigation to comprehend the influence of subregions and microstructures of microemulsions on the dimensions and morphology of the synthesized La-BTC MOFs [82]. Template-based methods can develop novel and useful MOF-S that might be difficult to synthesize by conventional strategies [83]. The major advantages of templated synthesis of MOF-S are: (a) Novel MOF-S with uncommon topologies or structures that are inaccessible by direct synthesis are possible to synthesize through the usage of templates; (b) Efficient for the synthesis of hierarchically porous MOF-S, that have both micropores and mesopores and even macropores [83]. Species used as templates include a variety of molecules: (i) various types of small organic molecules such as organic solvents [84], amine compounds, carboxylic acids, aromatic compounds including N-heterocycles, ionic liquids, and surfactants [85]; (ii) coordination compounds; (iii) polyoxometallates and polystyrene spheres [86]; (iv) Biomacromolecules including proteins; (v) block-co-oligomers; (vi) MOF structures to give rise to hierarchically porous MOFs; and (vii) various substrates like graphene oxide, metallic plates or meshes, Al2 O3 ceramics or even filter paper have been employed in MOF synthesis in recent years. Microfluidic synthesis of MOFs is desirable to meet industrial and commercial requirements of continuous, faster, and reliable synthetic processes [87]. The microfluidic system allows fine tailoring of physicochemical properties of MOFs by manipulating the flow of reagents in microchannels. Synthesis of various MOFs has been achieved by successfully employing microfluidics (Fig. 6) [88].

Fig. 6 Microfluidic synthesis of MOF schematic. Reproduced with permission from Ref. [87]

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3 Impact of Reaction Parameters on Synthesis of MOF Sponges The synthesis of MOF structures depends on many parameters related to synthesis temperature, the solvent used, the pH of the reaction medium, and the stoichiometric ratio of the reactants [89]. These synthesis parameters play a pivotal role in deciding the physicochemical properties of MOF-S structures. Other factors include the composition of the building blocks, which involve organic linkers and metal ions, and the kinetics of the MOF crystallization process, including nucleation and crystal growth.

3.1 Effect of Choice of Solvent The selection of the solvent used in the reaction is a key parameter that influences the MOF-S formation. The coordination behaviour of the components, organic linkers, and metal nodes is affected by the solvent both directly and indirectly. The solvent molecules can directly coordinate with metal ions or can be trapped inside the MOF pores as guest molecules acting as structure-directing agents [90]. Moreover, the degree of deprotonation of the carboxylate organic linkers can be modulated by the careful selection of solvents. For instance, amine-based solvents like DMF, DMA, and DEA can modulate the basicity of the reaction medium and thereby influencing the deprotonation of carboxylate linkers. Pachfule and co-workers [91] have demonstrated that the synthesis of two fluorinated copper based-MOFs under solvothermal conditions utilizing two different solvents, DMF and DEF, results in the formation of two structurally distinct MOFs (Fig. 7). Li et al. [92] have shown the influence of three solvents DMF, DMA, and DEF on the generation of coordination architecture of a Cd (II) MOF-S. The diverse coordination capability of the three solvents with the Cd (II) metal ion leads to three distinct structures [92]. The mixed solvent system can also significantly impact the MOF architecture. Forster’s group studied the influence of a mixture of the system of solvents in the formation of a Mg-based MOF employing a mixture of four solvents DMF, methanol, ethanol, and water. Since the coordination ability of water with Mg metal ion is much higher than the other three polar solvents, the mixture of the solvent system affected the formation of the coordination networks [93].

3.2 Effect of pH The acidity or alkalinity of the reaction medium has a profound impact on the MOF synthesis process. The pH of the reaction mix influences the protonation/

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Fig. 7 Effect of two solvents DMF and DEF on MOF structure. Reprinted with permission from Ref. [91]

deprotonation state of the organic ligand. The coordination modes of the polycarboxylate ligand are guided by the Hard-Soft-acid-base (HSAB) principle. Hence, the protonation state of the ligand governs the structure of the final product. Higher pH favours the formation of interpenetrating networks whereas the lower pH of the reaction medium gives rise to the formation of simple non-penetrating frameworks. Luo et al. [94] explored the formation of Cobalt(II) frameworks with H3 BTC as one of the ligands. Depending upon the protonation state of the H3 BTC, Co(II) salt formed three different frameworks [94]. Volkringer and co-workers [95] showed the effect of pH in an aluminum-based MOF; MIL-121. They demonstrated that pH of the reaction mix affects the bonding of aluminum in the inorganic network. At lower pH, aluminum is connected through µ2 -hydroxo corner of AlO6 octahedra; while at higher pH AlO6 octahedra are connected by the common edge of two µ2 -hydroxo groups (Fig. 8) [95].

3.3 Effect of Temperature Temperature, pressure, and time are the three physical factors governing the MOF crystal and growth formation process [96]. The solvothermal synthesis of MOF, typically involves the reaction of MOF precursors in a closed reaction chamber and at high temperature. The higher temperature speeds up the crystallization process by enhancing the solubility of the reactants. Hence, the solvothermal synthetic method is the method of choice when the organic ligands are not soluble at ordinary temperatures. Bernini and colleagues investigated the effect of temperature by preparing holmium(III) succinate hydrate frameworks under hydrothermal and non-hydrothermal conditions. Their work demonstrated that hydrothermal synthesis favoured the formation of more stable MOF structures [97]. Deng et al. [98] examined the impact of reaction temperature on the supercapacitor performance of two Ni(II) MOFs (Fig. 9). Conducting the reaction at two distinct temperatures 120 and 180 °C resulted into the formation of two distinct MOF structures due to different assembly

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Fig. 8 Influence of pH on the formation of aluminum based MIL-121 MOF. Reprinted with permission from Ref. [95]

processes. Different structures led to the different electrochemical performances of the two MOFs [98].

Fig. 9 Impact of reaction temperature on the structure and electrochemical characteristics of two Ni (II) MOFs. Reproduced with permission from Ref. [98]

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Fig. 10 Effect of molar ratio on crystal shape and size, SEM of MOF-5 at zinc nitrate to the terephthalic acid ratio a 1:2, b 1:1, and c 2:1. Reproduced with permission from Ref. [100]

3.4 Influence of Molar Ratio of Reactants Stoichiometry of the MOF precursors has a direct impact on the final MOF structure. Luan and co-workers [99] observed the effect of the ratio of the reactants in the self-assembly process of molecular braids. At a precisely modulated ligand-to-metal molar ratio, three distinct copper-based coordination networks formed [99]. Wang et al. [100] showed the effect of the stoichiometric ratio of metal ions to the ligands in the synthesis of MOF-5 (Fig. 10). When the metal-to-ligand ratio (zinc nitrate to terephthalic acid) was 1:2, it led to the generation of sheet-shaped geometry with dimensions 1–6 µm. However, the molar ratio of 1:1 led to the formation of prismaticshaped crystals of size 5–10 µm, while 2:1 formed cubic morphology with larger crystals [100].

4 Characterization of Metal–Organic Framework Sponges 4.1 Structural Characterization Fourier-Transform Infrared Spectroscopy The molecular structure, molecular symmetry, nature of interactions, and bondstrength can be probed through vibrational spectroscopy. This technique is useful for the characterization of (i) organic linkers of MOF-S, (ii) determination of hydroxyl coverage, (iii) phase purity of synthesized MOF-S, (iv) investigation of the interaction between guest molecules and host MOFs. IR spectroscopy is one of the mostly commonly utilized vibrational spectroscopy technique for analyzing MOF-S structures. Various data acquisition modes including transmission FTIR, DRIFTS, and ATR are used in the practice. For probing solid-state structures and host–guest interactions, transmission FTIR and DRIFTS are more commonly employed. However, it should be emphasized that it is crucial to combine various other complementary

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theoretical and experimental methodologies in addition to vibrational spectroscopy in order to characterize MOFs and investigate their adsorption and reactive performance [101].

Single Crystal X-ray Diffraction (SC-XRD) Single-crystal X-ray diffraction is regarded as the gold standard for the structural elucidation of crystalline materials [102]. It allows for determining the internal structure of MOF crystal, including lattice parameters such as cell dimensions, bond length, angles, and site-ordering. SC-XRD can be used to determine the exact location of atoms and calculation of bond lengths and angles. Crystals should typically be more significant than 5–10 mm, which can be difficult for some MOF families to achieve. Also, SC-XRD only offers insights into the structure of an individual crystal which does not accurately reflect the structure of the bulk material. Therefore, to determine the identity and phase purity of a MOF-S compound, further bulk characterization techniques must be used, such as powder X-ray diffraction.

Powder X-ray Diffraction (PXRD) Although a more challenging process, PXRD patterns are handy in determining the bulk crystallinity of MOF-S samples in case MOF single crystal growth is not feasible. A variety of information can be derived from the powder XRD pattern, including the determination of unit cell dimensions and assessment of phase purity. PXRD not only provides important information on the MOF microstructures, including crystal density, crystal size, and degree of crystallinity but also allows the detection of impurities and defects. So, PXRD is an effective technique for MOF-S characterization making it possible to achieve the target structure and to modulate particle properties to the desired application profile [103].

Solid-State NMR Spectroscopy (SS-NMR) SS-NMR is mostly employed to probe the local chemical environment inside a MOF [104]. It’s a powerful technique for MOF-S characterization and is a complementary technique to XRD in the structure determination of MOF-S. The identity of a particular functional group or oxidation state of the metal inside a MOF-S can also be determined using SS-NMR. It can provide the following information about MOF-S: (i) non-equivalent metal sites; (ii) the disorder around metals; (iii) local geometry in a metal cluster; (iv) molecular dynamics and diffusion of guest molecules and effects of activation of MOF-S and adsorption of molecules in the proximity of metal ions on the local environment of the metallic node; and (v) in situ phase changes in MOF-S. 1 H, 13 C, 14/15 N, 17 O, and 19 F-SSNMR is used for characterization of organic linkers. The various characterizations techniques of MOF-S structures are depicted (Fig. 11).

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Fig. 11 Characterization techniques for MOF-S

4.2 Morphological Characterization Scanning Electron Microscopy (SEM) SEM characterization of MOF reveals information regarding crystal size, morphology, and elemental composition [105]. Since most of the MOFs have insulating properties, difficulty in image acquisition or image artifacts may appear due to sample charging. Sample charging maybe avoided by coating the sample with an electron-conducting substance (such as gold) to lessen charge buildup from the electron gun. The accelerating voltage of the incident electron beam also impacts the image acquisition of MOF-S crystals. Although higher accelerating voltage can significantly improve image resolution, but may result in loss of surface details and may cause structural damage due to local heating effects. EDS-coupled SEM can provide both quantitative and qualitative data about the elemental distribution in MOF-S. However, care needs to be exercised in the selection of the coating material, as metals of interest peaks may be masked with peaks from that of the coating or even the presence of impurities in the coating material. For example, Osmium coatings frequently contain yttrium, generating inaccuracy in quantitative measurement.

Transmission Electron Microscopy (TEM) MOFs are typically too sensitive to be scanned in the standard TEM/STEM imaging mode due to knock-on damage, radiolysis, or heating induced by high-energy electrons. So, numerous techniques have been developed to minimize the effect of electron beam-induced heating and damage to the sample due to the knock-on effects of the electron beam [106]. For instance, TEM images were successfully acquired for

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MOF-5 by using a TEM installed with a holder for cooling purposes operating at 80 kV. Increasing accelerating voltage can also lessen radiolysis-related damage. However, only a few categories of relatively stable MOFs can be characterized through TEM before damage occurs attributed to the variable electron tolerance of MOFs.

4.3 Thermal Stability TGA is one of the mostly commonly used methods to determine the thermal stability and accessible pore volume of MOF-S. Thermogravimetric weight loss analysis provides insights into the thermal stability of the material and enables the quantification of the proportion of volatile components entrapped inside the MOF-S pores. This is done by monitoring the weight change occurring as the sample is heated at a uniform rate. A carrier purge gas species (e.g., N2 , air, or O2 ) controls the sample environment. In the TGA experiment of a typical MOF sample, the weight loss profile usually has three zones. The first phase corresponds to desolvation at a comparatively low temperature (3 mmol H2 S g–1 MOF [47]. Ni2+ -pyrazine (py) frameworks like Ni(py)NbOF5 and Ni(py)AlF5 that contain anionic inorganic pillars also absorb H2 S and have shown remarkable discrimination and transport capabilities of the polymer

Fig. 4 1% H2 S/99% CH4 (gold) and 1% H2 S/10% CO2 /89% CH4 (blue) challenge mixtures were evaluated at 20 °C to determine the H2 S saturation capabilities of the different materials. Reprinted with permission from Ref. [46]

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membrane towards H2 S [48]. These Ni-py materials have shown better stability in the presence of H2 S as well as H2 O.

3.3 Sulfur Dioxide Sulfur dioxide (SO2 ) is a toxic gas and a major air pollutant produced mainly from burning fossil fuels that contain sulfur compounds like coal or oil. SO2 can react with water in the respiratory system, and generate sulphuric acid, which is highly corrosive to the tissues in the respiratory system. High levels of SO2 exposure can result in irritation and harm to the nose, eyes, and throat. It may also impact the cardiovascular system, causing chest pain, heart palpitations, and other related health issues [49]. The primary constituent of acid rain is sulfur dioxide which possesses the potential to harm plants, and animals and can erode buildings and monuments. Along with ecological benefits, the complete removal of SO2 is also important before interaction with downstream catalysts for other gases in order to prevent poisoning [50, 51]. Here, we discuss some significant instances of MOF which set the benchmark for the adsorptive removal of SO2 . MOF-5, isoreticular MOF (IRMOF-3), MOF199, MOF-177, and IRMOF-62 were designed for capture of SO2 gas. These MOFs possess pores with variety of functional groups including amine, alkyne, and aromatics groups; and feature very large pore volumes ranging from 0.75 to 1.59 cm3 g−1 and have a very wide range of BET surface area 1264–3875 m2 g−1 . Interestingly, MOF-74 (Zn) underwent an irreversible color change upon SO2 adsorption, which was attributed to SO2 interacting with the five-coordinate of Zn(II) centre species in MOF-74 [31]. Researchers substituted different metal ions, such as Co, Mg, and Ni, for zinc and observed that MOF-74 (Mg) displayed better SO2 adsorption capability than MOF-74 (Co), however, it was discovered that for all MOF-74 series, the introduction of water had a negative impact on the stability of the frameworks and SO2 adsorption (Table 1). Table 1 SO2 capacities for selected porous materials MOF

Surface area (m2 g−1 )

Pore volume (cm3 g−1 )

SO2 capacity (mmol g–1 )

Reference

MOF-5

2205

1.22

< 0.016

[31]

IRMOF-3

1568

1.07

0.094

MOF-177

3875

1.59

0.016

MOF-199

1264

0.75

0.500 < 0.016

IRMOF-62

1814

0.99

MOF-74

632

0.39

3.030

FMOF-2

378

–

2.19

[52]

SIFSIX-1-Cu

–

–

11.01

[53]

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The ability of MOFs to adsorb SO2 can also be regulated by the organic linker. For instance, compared to UiO-66, UiO-66-ox (an UiO-66 derivative incorporates unbound carboxylic acid groups) exhibits an eightfold increase in the SO2 adsorption capacity [54]. For reversible gas adsorption, MOFs’ stability after SO2 adsorption is also essential. However, MOFs have the ability to switch back and forth between narrow pores and enlarged pores in response to guest inclusion. For instance, using the fluorinated ligand 2,20-bis(4-carboxyphenyl) hexafluoropropane with FMOF2 (Zn) shows pronounced lag effect in the adsorption–desorption of SO2 process consistent with a transition in structure, and P-XRD provided evidence for the structural flexibility of FMOF-2 [52]. Tan et al. [55] reported a MOF by using a benzene-1,4-dicarboxylate linker with Ni2+ , that has shown a very high uptake of SO2 (~9.97 mmol g−1 ) at room temperature and 1.1 bar and found out that SO2 is weakly adsorbed in two domains of pore and confirmed it by using in-situ infrared spectroscopy [55]. The zirconium-based MFM-601 was recently reported to have the highest SO2 absorption capacity of 12.3 mmol g−1 , at 298 K and 1 bar, and the binding regions for adsorbed SO2 molecules were identified in MFM-601 (Zr) utilizing in situ synchrotron X-ray diffraction techniques [56].

3.4 Nitrogen Monoxide Nitrogen monoxide, commonly referred to as nitric oxide (NO), is generated naturally in the body and acts as a naturally occurring signaling molecule within the body that regulates blood pressure and immune function. Elevated levels of NO can result in the production of nitrite and nitrate, which may build up in the body and can be responsible for respiratory distress, and neurological diseases [57]. NO is mainly produced from a variety of industrial processes, more than 50% total emissions of NO originate from automobile industries and electricity generation facilities [58]. NO interacts with O3 to form NO2 in the stratosphere, which is responsible for ozone depletion. Reducing the NO emissions is essential to protect the ozone layer and mitigate the effects of climate change [59]. MOFs with accessible metal coordination sites have predominantly been investigated for the adsorption of NO because NO strongly binds with metal ions and that can be confirmed by infrared spectroscopy. For instance, HKUST-1 (a MOF having Cu2+ metal ion) adsorbs 9 mmol NO g–1 MOF and ν(NO) band is appeared at 1887 cm−1 which corresponds to Cu2+ -NO bond in complexes [60, 61]. Ligand functionalization also matters in the case of NO adsorption. For example, the introduction of NH2 groups on trimesate linkers enhanced NO adsorption capacity in the case of amino-functionalized HKUST-1 in comparison to only HKUST-1 [62]. Some biocompatible MOFs have also been reported for delivery and adsorption of biologically active NO molecule [63]. The MOF-74 series is a well-explored class of MOFs for NO adsorption primarily because of its high concentration of accessible metal sites. By passing humid air through the MOFs, NO is completely detached from the MOF-74 and the starting material is completely recovered [64]. Iron-based MOFs

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have also been explored for NO adsorption, for instance, Fe-MOF-74 demonstrates an adsorption capacity 6.21 mmol g−1 at 7 mbar and room temperature [65].

3.5 Volatile Organic Compounds Volatile organic compounds (VOCs) refer to a class of organic chemicals that evaporates readily at ambient temperatures. Chemicals like petrol, solvents, paints, cleaning products, and perfumes are examples of VOCs that can be either naturally occurring or synthesized by humans [66]. VOCs affect both the ecological systems and human well being, and lead to the creation of smog, tropospheric ozone, and many carcinogens which can cause many health issues [67]. Benzene, toluene, ethylbenzene, and xylenes are a few examples of aromatic hydrocarbons that collectively make up a significant portion of the VOCs in the urban environment [68]. Rich organic content in MOFs offers an inherent benefit in the adsorption of aromatic VOCs. However, there are certain difficulties with using MOFs to adsorb VOCs in the air [69, 70]. MIL-101 is a MOF having unsaturated Cr metal sites with a carboxylate linker, that is well explored for VOCs adsorption because of its high surface area, easy tunability, and excellent chemical and thermal stability of the adsorbent material [71]. To examine the adsorption capabilities of MIL-101, different VOCs including toluene, n-hexane, butanone, dichloromethane, methanol, and n-butylamine were selected and it turns out MIL-101 exhibited the highest affinity for n-butylamine among the tested VOCs, while it displayed the least affinity for n-hexane [72]. Using the bis-pyrazolate as a linker, Galli et al. reported flexible MOFs for selective adsorption of thiophene gas from the mixture and observed that the adsorption capacity was 0.34 g of thiophene material per gram. The performance of the MOF was not affected even in 60% humidity, it can be utilized in real-world applications [73]. Zeolitic imidazolate framework, ZIF-8 is also very well explored for adsorption of organic vapours and gases due to exceptional thermal resilience and significantly spacious pore structure [74]. The most recent accomplishments mentioned in this chapter support the application of MOF sponges for the removal of toxic gases. However, to interact with such corrosive and reactive chemicals, the MOF sponges must exhibit outstanding durability by maintaining their structure, surface area, and adsorption capacity when exposed to dry and humid gas environments throughout repeated cycles. Although a significant barrier to the widespread use of MOF sponges continues to be their potentially high production costs, future developments in this field will make it possible for MOF sponges to cross these hurdles and to live up to their potential as designer multifunctional materials for a variety of applications.

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4 Greenhouse Gas Adsorption by MOF Sponges The whole globe is grappling with the serious problem of global warming [75]. The amplified emissions of greenhouse gases resulting from the combustion of fossil fuels, such as petroleum and coal, for energy generation, are among the primary contributors to global warming [76]. The separation and collection of greenhouse gases are one of the most critical problems which we are facing nowadays [77]. The effectiveness of MOF sponges in absorbing greenhouse gases like chlorofluorocarbons, methane, carbon dioxide, nitrous oxide, and hydrofluorocarbons in a number of studies [78–82]. Here, we look at some of the greenhouse gases and the MOFs that have been used to adsorb and capture them.

4.1 Carbon Dioxide Carbon dioxide (CO2 ) is a significant greenhouse gas responsible for trapping heat in the Earth’s atmosphere, leading to global warming. It is generated through various human and natural activities, including the combustion of fossil fuels, deforestation, and certain industrial processes [83]. According to some research findings, over the past 50 years, the atmospheric concentration of CO2 in the atmosphere has increased from roughly 310 ppm to over 380 ppm [84]. Zeolites, carbon molecular sieves, and activated carbon have all undergone substantial research as CO2 gas adsorbents [85, 86]. MOF sponges also have drawn considerable attention because of their distinctive characteristics like surface area, pore size, and high thermal resilence for CO2 capture and sequestration (CCS) because MOFs could offer an immediate solution that enables the continued utilization of fossil fuels until renewable energy technologies mature [83, 87, 88]. CUK-1 is a MOF, having pyridine-2,4-dicarboxylic acid, and unsaturated cobalt metal sites, that demonstrates preferential adsorption capability for CO2 over methane. Even after prolonged exposure to humid air, the CUK-1 compound was stable and resilient to endure a multiple sorption measurements without undergoing any sample modifications [89]. Dybtsev et al. reported Mn2+ metal-based MOF which shows permanent porosity, excellent thermal resilence, and high selective sorption for CO2 over other gases. MOFs having open metal sites have also been shown better adsorption and separation for CO2 gas. For instance, carborane-based MOFs have been studied for the adsorption of CO2 both with unsaturated metal co-ordination and without them, and observed better selectivity in the case of MOF having unsaturated metal sites [90]. Many more MOFs with open metal co-ordination sites have also demonstrated significant CO2 /CH4 or CO2 /N2 selectivities [91–93]. MOFs provide the possibility to tailor the functionality by utilizing simple organic chemistry. These tuning can be done both before and after (direct assembly and post modification) the synthesis of self-assembly [94]. For instance, Yaghi and co-workers tuned the adsorption capacity of the well-known structure of ZIF for CO2 adsorption

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Fig. 5 a CO2 isotherms of the ZIFs with different mixed links at 298 K. b Calculated selectivities for CO2 /CH4 , CO2 /N2 , and CO2 /O2 . Selectivities of the computed CO2 /CH4 dependency on surface area is shown in the inset. Reprinted with permission from Ref. [95]

by altering the imidazole linker, and observed better CO2 uptake in the case of polar functionality in ZIFs. These findings show that highly polar groups are advantageous for achieving high CO2 selectivities and high CO2 absorption (Fig. 5) [95]. Introduction of some functionalities can be difficult in a direct assembly approach, in that situation post modification approach can be beneficial for incorporating desired functionalities [96]. For instance, Farha et al. developed a variety of cavity-modified MOFs by substituting various different pyridine ligands in place of coordinated solvents [96]. Another example of post-synthetic MOF modification is reported by Bae et al. [97] py-CF3 modified MOF which demonstrated better selectivity for CO2 than the parent MOF. Incorporating the lithium ions into MOF by chemical reduction of MOF using lithium based reducing agent or exchanging the hydroxyl proton with Li+ can also increase the selectivity for CO2 in dimide-based MOFs [98, 99]. For instance, Bae et al. [100] recently demonstrated that the CO2 /CH4 selectivity is greatly increased when lithium ions are incorporated into MOFs using either of the two techniques. These MOFs have opened a totally new avenue for a sustainable future by capturing CO2 , which contributes maximum in greenhouse gases.

4.2 Methane After carbon dioxide, methane (CH4 ) ranks as the second-most significant greenhouse gas and its ozone depleting potential is more than 20 times greater compared to CO2 [101]. Methane can be generated from both natural processes such as and humanmade sources such as the decomposition of organic matter in waterlogged soil, landfills, and human activities like agriculture, and the use of fossil fuels [102]. Methane accumulation in the atmosphere leads to global warming and climate change, which can lead to the potential impacts of elevated sea levels, more frequent and severe extreme weather events, and disturbances to ecosystems and biodiversity [103].

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MOF sponges hold promise for methane as adsorbent material because of the total capacity of methane adsorption as well as its selective adsorption [7]. In terms of how methane adsorbs in MOFs, there are two regions where it can interact. The primary and major region is where methane binds with coordinatively unsaturated metal ions (open metal sites) and the second site is situated within regions characterized by heightened van der Waals potential pockets, but the interaction here is weak and majorly influenced by pore size only [104]. DUT-49, a MOF derived from copper metal ions and carbazole linker, has shown the highest capacity 308 mg g−1 at room temperature for adsorbing methane [105]. This MOF exemplifies a pressureenhancing material, as it enhances methane adsorption capacity under high-pressure conditions [106]. A metal azolate MOF called MAF-38 is generated by using metallic zinc, 1,3,5benzenetricarboxylic acid and 4-(1H-pyrazol-4-yl) pyridine as organic linkers. This MOF has shown considerable adsorption capacity of 246.4 mg g−1 at 65 bar [107]. MOF-210 is an example of IRMOF, made up of Zn4 O tetranuclear cluster with biphenyl-4,4' -dicarboxylate organic ligand having a maximal pore size of 48.3 Å and BET specific surface area of 6240 m2 /g, one of the largest surface area for a crystalline substance that has ever been reported [108]. The significant contribution of surface area in the field of methane storage at high pressures is made clear by MOF-177’s, which has shown an outstanding adsorption capacity of 221 mg g−1 at 100 bar [109]. Selectivity for methane adsorption over other gases in humid condition is the most desirable quality which MOF sponges can provide because most of the porous material collapse in the presence of humidity. The best MOF materials for the separation of methane in the presence of water have been screened and catalogued by Rogacka et al. [110]. Here, we have discussed the utilization of MOFs in the process of adsorption and separation of methane gas, and highlighted the advantages of MOFs over other porous materials.

4.3 Nitrogen Dioxide Nitrogen dioxide is a greenhouse gas that contributes to climate change and causes many premature deaths. The main source of NO2 is human activities, which include burning fossil fuels, transportation, and various industrial processes [111]. NO2 is responsible for the formation of photochemical smog, that depletes the ozone layer and accelerates global warming [112]. It’s crucial to lower NO2 emissions as well as capture the generated NO2 in order to lessen the damaging effects of greenhouse gases [113]. UiO-66 and UiO-67, these two MOFs having tetravalent Zirconium with carboxylate linker, absorb 7.9 wt% and 7.3 wt% NO2 , respectively under dry NO2 of 1000 ppm [114]. Further, amine functionalization of the carboxylate linker of UiO66 enhances the NO2 uptake by five times which is confirmed by 1 H NMR and IR

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spectroscopy [115]. Adsorption of NO2 is irreversible in this kind of Zr-based framework and leads to the partial collapse of the framework by a single cycle only [114]. Al-based MOF, named MFM-300, has shown reversible NO2 adsorption behaviours. MFM-300 demonstrated a considerable adsorption capacity of 14.1 mmol NO2 g–1 MOF throughout five cycles without losing its structural framework. MFM-300 (Al) exhibits exceptional selectivity by removing lower concentrations of NO2 [115].

4.4 Fluorocompounds Fluorocompounds such as hydrofluorocarbons (HFCs), chlorofluorocarbons (CFCs), perfluorocarbons (PFCs), hydrochlorofluorocarbons (HCFCs), and other fluorinated compounds are used extensively in industry as refrigerants, fluoropolymers [82]. However, due to their detrimental impact on the environment and ozone layer, these compounds have garnered interest from all around the world [116]. The majority of fluorocompounds are greenhouse gases that have an extremely high GWP lead to the depletion of the ozone layer. Cryogenic separation is the conventional approach that is most frequently employed for large-scale fluorocompound recycling but this process requires a lot of energy [117]. In contrast, gas separation using adsorption is more efficient in terms of energy and economy. The application of MOF sponges in gas storage opens enormous possibilities because of flexibility and easy-to-tuneable functionalities. R22 (chlorodifluoromethane) is a popular hydrochlorofluorocarbon (HCFC) refrigerant that belongs to the class of fluorocarbons and is commonly utilized in refrigeration and air conditioning systems for residential purposes [118]. Within the group of MOF adsorbents, MAF-X13 has the greatest ability to adsorb R22, with MAF-X10, MIL-101(Cr), and LIFM-26. The adsorption capacity of MAF-X13 was observed at 13.5 mmol g−1 for R22 adsorption which was higher than MAF-X10’s adsorption capacity (Fig. 6). The reason behind higher capacity of MAF-X13 is its pore size and pore volume. These MOF adsorbents also demonstrate higher thermal and chemical stability [119]. LIFM-26, a very robust perchlorinated MOF having linear dicarboxylate linker with iron metal sites, exhibits a considerable BET surface area of 1513 m2 g−1 and demonstrates an excellent adsorption capacity of fluorocarbons [120]. A MOF reported by Zheng et al. [121] named Ni-BPP (BPP is a molecule composed of biphenyl with para-COOH and 3,30-dioxido-4,40-biphenyldicarboxylate) has a higher capacity for adsorbing dichlorodifluoromethane or CFC-12 [121]. Chen et al. [19] used fluorinated carboxylate and fluorinated trigonal tetrazolate linkers to form two mesoporous MOFs. Among them, the most porous fluorinated MOF created to date is MOFF-5, which is composed of a tetrazolate ligand and Cu2+ metal sites and it has shown a remarkable capacity for CFCs capture [19]. Here, we have explored the application of MOFs in the adsorption and separation of fluorocarbons which lead to the greenhouse effect and emphasized the superiority of MOFs compared to other porous materials. In conclusion, MOFs sponges

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Fig. 6 a Crystal structure of MAF-X10, MAF-X12, MAF-X13, b the packing diagram along the c axis, trimeric Fe3 O cluster, and tcdc2 coordination mode of LIFM-26, c crystal structures of NiDOBDC, d MIL-101 and e fluorocarbons. Reprinted with permission from Refs. [82, 120, 121]

have emerged as a promising material for the adsorption of greenhouse gases due to their high surface area, tunable pore size, and selective adsorption behaviour. MOFs offer high selectivity, fast adsorption kinetics, and recyclability over other adsorbents. MOFs have the potential to be scaled up for industrial applications, such as CO2 , CH4 , and CFCs gases separation and adsorption. MOF sponges open avenues for the development of effective and sustainable solutions for reducing greenhouse emissions.

5 Factors Affecting the Adsorption Performance of MOF Sponges MOF sponges have the advantages to tune their chemical property by changing metal ions and ligands for selective adsorption of a particular gas. MOF sponges’ adsorption performance is affected by a number of parameters, including.

5.1 Pore Size and Structure The selectivity and capacity for various gases are governed by the pore size and structure of MOF sponges. While MOFs that have bigger pore sizes can encapsulate larger gas molecules, those with smaller pore sizes often offer higher selectivity for

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smaller gas molecules. This is so that gas molecules must fit within the pore of MOF sponges to be adsorbed. The connectivity of linkers and geometry have also an impact on how well MOF sponges operate as adsorbents. More intricate pore frameworks in MOFs may increase their selectivity for a particular gas.

5.2 Chemical Composition of MOFs The affinity of MOF sponges for particular gases can be altered by their chemical composition. The adsorption behaviour of the MOF sponges can be tuned by the use of different metals and linkers to make MOFs with various chemical characteristics. Chemical composition not only controls the affinity for particular gas but also it affects the pore size, pore volume and stability of MOF sponges as well.

5.3 Surface Area The amount of gases that MOF sponges can adsorb has a direct correlation with their surface area. MOF having a high surface area means high pore volume, which provides more possibilities for gas adsorption. Surface area of MOF sponges can also be tuned by changing the synthesis parameters such as metal ligands stochiometry, solvents, pressure, or temperature.

5.4 Humidity The ability of MOF sponges to adsorb gases from the air can be dramatically affected by humidity. By competing with gas molecules for adsorption sites, water vapor may reduce the MOF sponges’ capacity and selectivity for adsorption. This is due to the possibility of water molecules occupying the same pore by competing gas molecules, which causes a reduction in the effective surface area open to gas adsorption.

5.5 Adsorption Kinetics Adsorption kinetics, which quantifies the rate at which gas molecules adhere to the surface of the MOF sponge, is impacted by a number of variables, including the gas concentration, pressure, temperature, and the MOF sponge’s surface characteristics. Higher adsorption capacity is typically a result of faster adsorption kinetics, although this can also mean lower selectivity.

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6 Comparison of MOF Sponges with Other Adsorbents In comparison to conventional adsorbents like activated carbon, zeolites, and silica gel, MOF sponges provide a number of advantages and some disadvantages. Here are some of the key differences.

6.1 Surface Area The main advantage that MOF sponges provide is a higher surface area when compared to other conventional porous materials. The higher surface area means that MOFs can adsorb more gas molecules per unit mass, leading to higher adsorption capacity, and surface area can be adjusted by modifying the organic linkers in MOFs. Depending on the exact MOF structure and synthesis conditions, the specific surface area per gram of MOF sponges might range from 100 to 1000 m2 g−1 . This is significantly greater than other common adsorbents, like activated carbon and zeolites, which typically have surface areas of tens to hundreds of square meters per gram and hundreds to thousands of square meters per gram respectively.

6.2 Selectivity Compared to other adsorbents like activated carbon or zeolites, MOF sponges are more selective for some molecules. This is possible because MOF sponges’ pores could have been engineered to have particular sizes and shapes that allow them to selectively adsorb molecules of a particular size or shape. MOF sponges can be particularly designed for toxic and greenhouse gases’ adsorbent material, which is not possible for other porous materials.

6.3 Stability MOFs generally maintain their stability, when exposed to extreme conditions like high temperature, high pressure, acid or basic environments and exposure to humidity. This is because of the MOF structure’s strong coordination bonding between the organic linkers and metal centers, that can tolerate a variety of environmental circumstances. Whereas in extreme conditions, other adsorbents, like activated carbon or zeolites, may deteriorate or lose their adsorption capacity. MOFs frequently maintain their stability over time also, they can be used repeatedly without losing their adsorption capacity significantly. This is because of the

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stability of MOFs structure and by simply heating or purging with inert gas, the surface of the MOF can be restored to reuse. MOFs are selective for the guest gases to adsorb; MOFs maintain their stability in the presence of other gases and contaminants and they do not get collapse.

6.4 Cost Cost is the only factor that puts MOFs at a disadvantage relative to other adsorbents like silica gel, zeolites, or activated charcoal. The price of the starting materials and the complexity of the MOF synthesis process are the causes of this. Synthesis and characterization of MOFs required sophisticated instruments, which hinders the process of scaling up. However, it is important to remember that MOFs cost’ will continue to go down as more effective synthesis techniques emerge.

6.5 Applications The area of MOF’s application is very wide in comparison to other adsorbents. MOFs can be used for various applications ranging from gas separation, purification to gas storage. MOFs have found utility in numerous processes, including catalysis, drug administration, sensing, storage of energy, environmental cleanup, and biomedical imaging.

7 Challenges in the Field of MOF Sponges MOF sponges provided an alternative for the adsorption of polluting gas over other adsorbents like silica gel, activated carbon, or zeolites. There are still some questions left to be addressed, such as selectivity, bio-compatibility, cost-effectiveness, and reproducibility. The design of MOF sponges needs to be very selective, and it is difficult to customize a MOF to selectively adsorb a particular gas since different gases have distinct physicochemical properties. Most MOFs collapse in humid conditions, so we need to move forwards toward the synthesis of robust MOFs. Scaleability is another issue that we are facing nowadays, there is no established method of synthesis for MOFs in large quantities to meet the demand for capturing toxic and greenhouse gases. MOFs are also relatively expensive over other adsorbents, hence, it is significantly important to develop economival approaches for producing MOFs.

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8 Summary and Future Perspectives We have outlined an alternative and impactful approach for society to clean and save our environment from toxic and greenhouse gases using MOF sponges, in this chapter. MOF sponges are an excellent choice for capturing and storing these hazardous gases owing to their etraordinary blend of elevated porosity, adjustable chemical features, immense surface area, and very high pore volume. The present state of research on MOF sponges for the adsorption of greenhouse gases and toxic gases has been examined in this book chapter, which also highlights the substantial advancements that have been accomplished recently. A brief introduction to MOF sponges, various methods of synthesis, and characterization techniques is also discussed. High selectivity toward target gases is one of the key advantages of MOF sponges. Certain gases can be selectively adsorbed while excluding others by modifying the porosity and chemical composition of MOFs. This selectivity is essential for realworld applications because it makes it possible to efficiently capture target gases while reducing the adsorption of other atmospheric gases. Despite the excellent stability in harsh conditions, MOFs have certain limitations and drawbacks also which have been deliberated in the chapter. In conclusion, MOF sponges can provide a major advancement in the direction of a sustainable future by adsorbing toxic and greenhouse gases, still, there are issues that need to be resolved, including selectivity, stability, scalability, cost-effectiveness, and regeneration. Further optimization is needed for MOF sponges for practical applications, and to develop robust and cost-effective MOF sponges for the future.

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Metal–Organic Framework Sponges for Water Remediation Gyanendra Kumar, Mohd Ehtesham, Satendra Kumar, Bachan Meena, Gobind Ji Rai, and Dhanraj T. Masram

Abbreviations MOFs DMF BTC AC MB RhB TEM 3-D PCPs SBU CSD UiO-66 HKUST-1 MOF-5 MIL-101 ZIF-8

Metal–organic frameworks Dimethylformamide Benzene-1,3,5-Tricarboxylate Activated Carbon Methylene Blue Rhodamine-B Transmission Electron Microscopy Three-dimensional Porous Coordination Polymers Secondary Building Units Cambridge Structure Database University of Oslo Hong Kong University of Science and Technology Metal Organic Framework Material Institute of Lavoisier Zeolitic imidazolate Framework-8

G. Kumar · G. J. Rai Swami Shraddhanand College, University of Delhi, Alipur, Delhi 110036, India G. Kumar · B. Meena · D. T. Masram (B) Department of Chemistry, University of Delhi, Delhi 110007, India e-mail: [email protected] M. Ehtesham Jamia Millia Islamia (Central University), Delhi 110025, India S. Kumar Dayanand Subhash National (P.G.) College, Unnao 209801, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Gulati (ed.), Nanosponges for Environmental Remediation, https://doi.org/10.1007/978-3-031-41077-2_11

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1 Introduction of MOFs and MOF Sponges Porous solids are essential adsorbent materials and are broadly used in the industrial applications. These are classified as zeolites, activated carbon, and mesoporous silica [1–3]. Metal–Organic Frameworks (MOFs) are growing three-dimensional (3-D) structures with metal cations (zinc, copper, nickel, and zirconium) and organic linkers such as bidentate or tridentate carboxylic acids or N-containing aromatics. The basic structure is formed by coordination bonds between the metal cation and organic likers [4–7]. Metal cations influence the size as well as shape of MOFs. Whereas, organic linkers can increase the porosity and surface area. MOFs are highly crystalline material and formed one, two, or 3-D structures, and are porous in nature. The property of MOFs is specified by the choice of the organic linker and metal ion. They have ultra-porous nature and high surface area, which makes them efficient in drug delivery, catalysis, and storage of gas [8]. Being a porous material, it is called porous coordination polymers (PCPs) with infinite lattice unites and was developed from secondary building units (SBU) metal cation salt and polydentate organic linker. Yaghi et al. was identified these porous coordination polymers named MOF [8b]. In recent decades, MOFs have highly defined crystallinity and unique properties, having an extremely large surface area (10,000 m2 g−1 ), they are remarkable-elevating their potential applications in catalytic as well as drug delivery [9, 10]. According to the Cambridge Structure Database (CSD) were found over 800,000 MOFs [11]. There are different types of MOFs are reported, which is given in Fig. 1. MOFs are reported in the 1990s and are called coordination polymers. In 1995, Omar Yaghi et al. and their groups have synthesized and called the metal–organic framework [11a]. These are ordered networks of metal units and connected by organic linkers. MOFs have attractive chemical structures and high surface areas. However, organic linkers are long-chain molecules having multiple coordination sites, and metal cation serve as a core of the coordination bond and it binds with the lateral regions of organic linkers. Overall, the most common method to synthesize the MOFs, organic linkers and metal cations are mixed and heated in a suitable solvent for hours to days until they settle into an orderly structure. Ian Williams groups have reported a structure consisting of copper-based clusters as well as benzene tricarboxylic linkers known as HKUST-1, developing large pore volumes as well as high surface areas [11b]. Ian Williams groups have reported a structure consisting of copper-based clusters with benzene tricarboxylic linkers known as HKUST-1, having high surface areas and pore volumes. In 1994, Makoto Fujita and their groups were given MOFs are used as catalytic and wastewater treatments. Afterward, Yaghi et al. was reported MOF-5 have found high surface area than zeolites. Moreover, MOFs are peculiar micromaterial products that use a sponge-like network of ions of metal or clusters joined via organic linkers to trap specific gas molecules inside its pores. Over the last ten years, MOFs is an interesting material because their exceptional properties like extremely high porosity, muti-functional tenability, with simple synthesis make them suitable for an extensive list of applications [13–15]. There are extremely porous, hydrophobic, but low-density MOFs

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Fig. 1 Illustration of various MOFs [12] (Adapted with the permission Ref [12])

are utilised in the treatment of sewage [16–18]. MOFs frequently have a few flaws, namely a lack of physical strength and poor processing ability [19, 20]. Furthermore, crystal-like frameworks are often found as insoluble powders and are brittle. A lot of studied MOFs are also hydrolytically unstable, resulting in irreversible structural deterioration within a short time in humid air [21, 22]. MOFs may be incorporated onto or inside unique substrates to create an affordable, formable, and chemically inert material in the case of disappointing results, that could expand their practical uses [23, 24]. New contaminates are accumulating in the environment at a rising rate, exposing human life in hazards by toxicity water sources both directly and indirectly. In order to reduce the difficulties caused by water contamination, researchers have invested a lot of effort into developing new techniques, improving old ones, and providing sustainable solutions [25]. The adsorption and also photodegradation are both of the greatest environmentally benign methods, which utilised in water removing pollutants. These technologies have a number of advantages since the designs needed for cleaning wastewater are basic, inexpensive, and simple to operate [26, 27]. In these applications, MOFs are porous materials as well as crystalline structure, and are very unique class of material. Since their properties may be transformed, MOFs are thought to be highly suitable for application in wastewater treatment methods. The synthesis of MOF-based nanocomposites as over water remediation could be

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further improved using a number of strategies, such as hybridising other kinds of nanoparticles, like doping, organic ligand modification, and post-modifying MOFs via adding active functional groups [25]. MOFs are able to be hybridised with inorganic nanoparticles such carbon nanotubes (CNTs) [28], graphene oxide (GO) [29], and activated carbon [30] to enhance their morphological, structure stability as well physicochemical properties for water purification. For the removal of malachite green (MG), for instance, The ZIF-8/GO/CNT hybrid has shown extraordinary adsorption efficacy, stability as well as reusability [31]. Presently, a number of technologies have been used to succeed in wastewater remediation, among which are limited to adsorption [32] and catalysis [32–34]. Nowadays, insufficient drinking water, which is made worse by water pollution, is now a severe threat to human survival. Water remediation are employs filtration through membranes, adsorption, coagulation, oxidation, and biological techniques, represents a potential way to address this problem [32]. Membrane technology has become increasingly important in recent years for water remediation due to its benefits in compact footprints, straightforward operation, environmentally benign removal of pollutants, and efficient removal of wastes. According to this regard, extremely hydrophilic surfaces have been created using MOFs, one of the newly popular porous materials [35]. By virtue of unique porous 3-dimensional frameworks (MOFs), which an exclusive kind of nanoporous material to attracted numerous interests for water remediation. They offer outstanding qualities such an exceptional surface area, stability, and good porosity, having a framework that can be modified [36].

2 Synthesis Strategy Since MOFs were first discovered to be synthesised by numerous methods, including electrochemical, microwave, sonochemical, and solvothermal, have already been explored in research field. The synthesis of MOFs depends heavily on unreacted molecules, organic compound, and inorganic compounds. Purification and activation are two crucial stages that must come during the formation of MOFs since impurities might lower the material’s adsorption capacity and performance [37, 38]. The MOF network along with guest molecules (in air) can sometimes collapse when MOF materials need to be heated in order to be activated. Figure 2 lists many synthetic pathways for the synthesis of MOFs. Additionally, summary of the synthesis methodologies is given in detail: 1. Conventional Method The conventional approaches include together solvothermal and also nonsolvothermal techniques. Within the solvothermal approach, MOFs are created by slowly heating sealed tubes or vials with regular electric heating solvents like ethanol, methanol, formamides, acetones, as well water are denoted by the suffix “solvo” [39].

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Fig. 2 Successfully implemented methods and synthetic conditions for the development of MOFs

2. Hydrothermal Method Inorganic materials are frequently synthesised using the hydrothermal synthetic methodology [40, 41]. The starting materials are transformed into solutions using chosen solvents, after which the solutions are placed into hydrothermal kettles as well as heated to 100–200 °C. Excellent properties can be found in the materials created in hydrothermal environments [42, 43]. The water phase reaction in a nonequilibrium hydrothermal systems can typically form porous nanomaterials due to the fact that the hydrothermal kettle within the scheme maintains a particular self-generated pressure over the reaction phase [44]. Further, hydrothermal strategy is less corrosive to apparatus, and there is little inhibitor of downstream fermentation [45]. 3. Microwave-Assisted Method The microwave approach has been extensively used for MOFs synthesis. The substrate and the proper solvent are mixed together in a Teflon vessel, which is then sealed and heated for the specified amount of time under a microwave. It was first noted in 2005 that microwave treatment has a quicker reaction time towards the formation of MOFs. In addition, microwave-assisted approaches can produce reduced-particle MOFs or narrow-sized crystals compared to conventional ovens, along with even the crystal’s size might be modified during synthesis [46, 47]. Jhung et al. was synthesized MIL-100 using a microwave oven in a similar way to the solvothermal process.

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4. Electrochemical Method In the previous scenario, electrochemical reactions occur immediately on the electrode surface to synthesise the required MOFs, whereas in the latter, an electrochemical reaction is a phase in the MOF synthesis pathway. There are two categories of electrochemical techniques: cathodic electrosynthesis as well as anodic dissolution. Such electrochemical preparation procedure is a common method that enables tracking the operation of deposited film through modifications regarding the required voltage or current to avoid film consistency. Further, this method has a quick reaction time and mild conditions. Under normal circumstances, it can be resulted out, and the valuable equipment is not too difficult [48]. At the moment, electrochemical preparation methods can be divided into electrophoretic deposition, cathodic electrodeposition, and anodic electrodeposition methods [49, 50]. In 2006, Mueller et al. was first time synthesized Cu-BTC by the electrochemical process. 5. Sonochemical Method The reaction occur with the metal ion source and organic ligand is given energy by ultrasonic waves. The sonochemical approach might be speed up crystallisation and development during the crystal growth process. It is possible to prepare ZIF-8 layers using a sonochemical method [51, 52]. 6. Mechanochemical Method Since energy can be transferred directly within solid-phase reactants and only a small quantity of solvent (rather than, solvent-free environments) is needed to aid mechanochemical reactions referred to liquid-assisted grinding. Mechanochemical synthesis has attracted more attention due to its greener method instead of solutionbased routes. Additionally, these reactions typically require less time and can occur at room temperature. The mechanochemical synthesis typically entails the ball-milling or grinding combination of solid precursors within a ball-miller, which presents a chance to use insoluble metal sources. The zeolite imidazole frameworks (ZIF-8), in general ZIF-8 with cobalt, and ZIF-67 have been synthesised by this method [53, 54]. 7. Continuous-Flow Spray-Drying Method The continuous-flow spray-drying procedure is a cost-effective and environmentally friendly scheme for obtaining MOFs on vast scales for industrial exploitation while continuous synthesising MOFs in the form of NPs, composite materials, as well as spherical structures. This technique combines spray-drying and even continuousflow processes. Various MOFs family members including UiO-66 as well as Fe-BTC/ MIL-100 are synthesised through this method [55].

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3 Application of MOFs in Water Remediation The MOFs are highly crystalline constituents with exceptional surface area, porosity, and intrinsic chemical as well as physical properties. MOFs have extraordinary material and have a variety of applications in catalysis, [56] sensors, [57] CO2 storage, [58] hydrogen storage, [59] photocatalysis, [60] drug delivery, [61] biomedical application, [62] dye adsorption, [63] which is shown in Fig. 3. Among these the heterogeneous catalyst is an essential role in the chemical industry by leading to both economic and environmental sustainability. Nowadays, researchers are mainly focused on environmental protection to relies greatly on catalytic advancements and wastewater treatment. Resolved the past few years, a variability of techniques, including ion exchange, electrochemical-based procedures, solid-phase extraction, and precipitation, have been used to remove toxic materials from wastewater resources [64]. Methyl orange (MO), a dye, was adsorbed out of water and removed using a MOF for the first time in 2010. The stability of MOF is needed for effective use as a water filtration material. A variety of pollutants, including HMIs, poly-fluoroalkyl

Fig. 3 Different potential applications of MOFs

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compounds, dyes, medicines, and plasticizers, have been suggested for removal using MOFs and MOF composites [65]. MOFs have used in versatile fields, including water purification, adsorption, contaminant removal, catalysis, biocatalysis, nanomedicine, biosensors, energy source creation, fuel cells, and environmental applications. To comprehend the synthesis process and the prospective uses for Cello MOFs, the majority of the essential understanding related to both materials will be covered. Cello MOFs was used to degrade organic contaminants in water by chemical and photocatalysis, as well as by adsorption of colours, metal ions, oils, medicines, antibiotics, and pesticides [66]. One of the major environmental issues that the world is currently facing water contamination. Common contaminants can be largely categorised into two groups: inorganic substances, such as radioactive metal ions, oxyanions, inorganic acids, cations; and organic pollutants, like organic dyes, pharmaceuticals, personal care items, herbicides, as well as pesticides. Because these organic pollutants have high operating costs, it is not possible to implement them globally, as would be necessary to effectively address this global issue. Moreover, MOFs have emerged as a very promising material choice for water clean-up. Due to the reputation of the earliest MOF generations’ water-stability concerns, they were initially hardly ever used in the field [67]. By regulating the ratio of linkers and the hydrolysis conditions, MOF pore diameters in the 9.3–13.6 nm range emerged. The flawed mixed-linker MOF had improved catalytic as well as adsorptive characteristics. Additionally, interpenetration in COFs/MOFs can improve their stability and have an impact on how they are used for water remediation, according to a mixed bi metal Co–Ni–MOF that showed twofold interpenetration [68]. For the actual execution of water clean-up, the use of a safer metal centre was much preferred. With the extra feature of exceptional stability, the Zr-based UiO-66 MOFs offer appropriate substitutes for non-toxic and ecologically friendly metals [69]. The use of MOF/ CNM composites in numerous applications, such as removing pollutants from water (from organic chemicals and heavy metals), air purification, electrical storage of energy, transformation technology, and biological applications, has received a great deal of research. There are silent limited papers focused on the synthetic strategic pathway of cellulose to Cello MOFs coupled with their in-depth uses in the field of wastewater remediation, despite the continued use of various MOF/cellulose hybrids of diverse eco-friendly applications. In this context, innovative MOFs, like bio-MOFs through biocompatible metals as well as linkers like amino acids, biomolecules, etc., may be researched in the future with the intention of improving water quality. Worldwide, the MOF/cellulose-based hybrid synthesis and there in water remediation application sector are still in their infancy and face a number of obstacles, but they assist as a platform designed for continued development with a variety of possibilities [70]. To validate its viability in wastewater clean-up, the MOF’s recycling and regeneration characteristics underwent four continuous cycles of testing. In this chapter, MOFs were also considered as adsorbents for water remediation that provide simple separation and ongoing recycling. Although in theory photodegradation is preferable because it gets rid of the mostly pollutant as well as eliminates the need for additional processing, the adsorption method still has many advantages over photocatalysis [71]. In addition,

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several of the available technologies used for selenium elimination only work for the adsorption of selenite rather than selenate. Even though there haven’t been many attempts to get over the constraints listed, they have shown how exceptional of a potential MOFs are for treatment of wastewater. This work might be viewed as a limitless latent of MOF composites towards remediation water, even though additional studies utilising MOFs with better crystallinity under the basic circumstances used to the monolith would be highly useful. The number of MOFs used in treatment of wastewater remains low despite these significant advancements. For instance, by utilising various synthetic techniques has been feasible to significantly increase the numeral MOFs through high environmental constancy, which was one of MOFs’ primary limitations and a barrier to their usage for water clean-up just a few years ago. All things considered; significant progress has been complete in the creation of MOFs for eradication of hazardous class from water. Further, water remediation by MOF technology is still in its infancy when compared to other applications of MOFs based material like separation as well as gas storage [72]. Significant environmental and health problems result from the enrichment of these contaminants hip the environment beyond allowable limits. As a result, remediating contaminated water has gained international attention. Although using different MOFs-based composites for remediation of water produces desirable results, moreover, MOFs and their derivatives are restricted by a number of restrictions. It is still difficult to create effective methods and sorbents with increased removal efficiency and improved degrading capabilities [73]. MOFs are well known for their numerous uses in a wide range of industries, including water filtration, adsorption, sensing, catalysis, and drug delivery. It is well known that water or moisture can have a sensitively negative impact on the structure and form of the majority of MOFs. Therefore, a thorough assessment of MOF stability with respect to variables connected to these property changes is necessary. Furthermore, strong stability of water is required intended for the commercialization and growth of wider uses of this intriguing material, is surprisingly uncommon in MOFs in their early phases [74–76].

4 Conclusion This is necessary to synthesis of MOFs-based absorbents with appropriate structural properties and long-term water a stable state. To validate water-stable MOFs that are especially their corresponding compounds, their ability to function must be thoroughly investigated. The invention of water-stable MOFs that are for use in water cleaning is still under exploration. This will aid in overcoming the technical difficulties brought on by the instability. The mechanism of the adsorption process has to be clarified by additional study. This knowledge is essential for creating adaptable MOFs with high removal efficiencies. Due to their numerous benefits, MOFs are promising materials for eliminating contaminants from water. Large pore volumes and specific surface areas indicated good adsorption capability; numerous active sites

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allowed for chemical adsorption of contaminants; large-scale production of MOF; and some of them having high water stability. As a result, MOFs are frequently regarded as superior adsorbent materials for the remediation of water contamination. The elimination of organic contaminants from aqueous solutions has utilised a number of MOFs. Hazardous particle removal from wastewater has been investigated using MOFs. Although a more straightforward separable design is one of the limitations that must be overcome for widespread applications of MOFs are regarding the elimination of noxious substances.

5 Future Prospectus This chapter’s goal is to give a thorough summary of the new studies within the utilisation of MOFs to enhance the adsorptive and photocatalytic eviction of organic harmful substances from wastewater that have been documented in the literature. Additionally, this study briefly describes MOF synthesis technologies. Finally, the problems and prospects for large-scale MOF manufacture are examined. The three-dimensional (3D) organic–inorganic composite known as MOF is remarkable because it has very porous nanostructures are filled with groups of metal ions as well as organic linkers. Due to its carefully studied components, including its tunable pore networks, adequate adsorption sites, and other promising features, MOF has emerged as a magnetic material of choice for intellectuals along with researchers over the last few decades. Studies from earlier decades have also revealed a thorough examination of MOF materials, showcasing its excellent performance in catalysis, adsorption, and water harvesting. However, compared to MOF-based NMs, conventional catalysts as well as adsorbents like clay minerals, activated carbon, zeolites, etc., have very less adsorption as well pollutant removal capacities. As a result of the expanding population and increased water consumption, many processing methods have been developed to remove contaminated human-caused pollutants from wastewater from industries before eliminating it from biological ecosystems. As a result, many researchers have been interested in developments in the methods and studies utilised to create photocatalytic composites. Given that this industry produces enormous amounts of wastewater with a broad range of chemical interpretations, industrial wastewater, like fabric wastewater, is a particular concern.

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3. Hoffmann, F., Cornelius, M., Morell, J., & Fröba, M. (2006). Silica-based mesoporous organicinorganic hybrid materials. Wiley, 45(20), 3216–3251. 4. Tranchemontagne, D. J., Tranchemontagne, J. L., Keeffe, M. O., & Yaghi, O. M. (2009). Secondary building units, nets and bonding in the chemistry of metal–organic frameworks. Chemical Society Reviews, 38(5), 1257–1283. 5. Yuan, D., Zhao, D., Sun, D., & Zhou, H. C. (2010). An isoreticular series of metal-organic frameworks with dendritic hexacarboxylate ligands and exceptionally high gas-uptake capacity. Angewandte Chemie, 49(31), 5357–5361. 6. Czaja, A. U., Trukhan, N., & Müller, U. (2009). Industrial applications of metal-organic frameworks. Chemical Society Reviews, 38(5), 1284–1293. 7. Zhao, D., Yuan, D., Sun, D., & Zhou, H. C. (2009). Stabilization of metal-organic frameworks with high surface areas by the incorporation of mesocavities with microwindows. Journal of the American Chemical Society, 131(26), 9186–9188. 8. (a) Faust, T. (2016). MOFs move to market. Nature Chemistry, 8, 990–991. (b) Yaghi, O. M., & Li, H. (1995). Hydrothermal synthesis of a metal-organic framework containing large rectangular channels. Journal of the American Chemical Society, 117(41), 10401–10402. 9. Chen, H., Qiu, Q., Sharif, S., Ying, S., Wang, Y., & Ying, Y. (2018). Solution-phase synthesis of platinum nanoparticle-decorated metal-organic framework hybrid nanomaterials as biomimetic nanoenzymes for biosensing applications. ACS Applied Materials & Interfaces, 10(28), 24108– 24115. 10. Ko, N., Go, Y. B., Aratani, N., Choi, S. B., Furukawa, H., Ko, N., Go, Y. B., Aratani, N., Choi, S. B., Choi, E., Yazaydin, A. O., Snurr, R. Q., O’Keeffe, M., Kim, J., & Yaghi, O. M. (2010). Ultrahigh porosity in metal-organic frameworks. Science, 329(5990), 424–428. 11. Moghadam, P. Z., Li, A., Wiggin, S. B., Tao, A., Maloney, A. G. P., Wood, P. A., Ward, S. C., & Fairen-Jimenez, D. (2017). Development of a Cambridge structural database subset: A collection of metal−organic frameworks for past, present, and future. Chemistry of Materials, 29(7), 2618–2625; (a) Yaghi, O. M., Li, H. (1995). Hydrothermal synthesis of a metal-organic framework containing large rectangular channels. Journal of the American Chemical Society, 117(41), 10401–10402; (b) Bordiga, S., Regli, L., Bonino, F., Groppo, E., Lamberti, C., Xiao, B., Wheatley, P. S., Morris, R. E., & Zecchina, A. (2007). Adsorption properties of HKUST-1 toward hydrogen and other small molecules monitored by IR. Physical Chemistry Chemical Physics, 9(21), 2676–2685. 12. (a) Lee, Y. R., Kim, J., Ahn, W. S. (2009). Synthesis of metal-organic frameworks: A mini review. Korean Journal of Chemical Engineering, 30, 1667–1680. (b) Yang, S. J., Choi, J. Y., Chae, H. K., Cho, J. H., Nahm, K. S., Park, C. R. (2009). Preparation and enhanced hydrostability and hydrogen storage capacity of CNT@MOF-5 hybrid composite. Chemistry of Materials, 21, 1893–1897. 13. Xiao, Z. Y., Wang, Y. T., Zhang, S. Y., Fan, W. D., Xin, X. L., Pan, X. L., Zhang, L. L., & Sun, D. F. (2017). Stepwise synthesis of diverse isomer MOFs via metal-ion metathesis in a controlled single-crystal-to-single-crystal transformation. Crystal Growth & Design, 17, 4084–4089. 14. Li, H. J., Li, Q. Q., He, Y. L., Zhang, N., Xu, Z. Q., & Wang, Y. (2018). Facile fabrication of magnetic metal-organic framework composites for the highly selective removal of cationic dyes. Materials, (11) 744. 15. Kim, D., Kim, D. W., Buyukcakir, O., Kim, M. K., Polychronopoulou, K., & Coskun, A. (2017). Highly hydrophobic ZIF-8/carbon nitride foam with hierarchical porosity for oil capture and chemical fixation of CO2 . Advanced Function Materials (27), 1700706. 16. Lei, Z. W., Deng, Y. H., & Wang, C. Y. (2018). Multiphase surface growth of hydrophobic ZIF-8 on melamine sponge for excellent oil/water separation and effective catalysis in a knoevenagel reaction. Journal of Materials Chemistry A, 6, 3258–3263. 17. Wang, R., Xu, H. J., Zhang, K., Wei, S. Y., & Wu, D. Y. (2019). High-quality Al@Fe-MOF prepared using Fe-MOF as a micro-reactor to improve adsorption performance for selenite. Journal of Hazardous Materials, 364, 272–280. 18. Mon, M., Bruno, R., Ferrando-Soria, J., Armentano, D., & Pardo, E. (2018). Metal–organic framework technologies for water remediation: Towards a sustainable ecosystem. Journal of Materials Chemistry A, 6, 4912–4947.

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Introduction to Sponge-Like Functional Materials from TEMPO-Oxidized Cellulose Nanofibers Pooja, Tarisha Gupta, Madhav Dutt, and Laishram Saya

1 Introduction Over time, the progressive growth of industrialization and anthropogenic activities has resulted in the contamination of hazardous substances in landfills, industries, manufacturing sites, and private properties. It disturbs the ecological balance and poses potential hazards to human health and the environment, which causes a global concern that must be addressed [1–4]. Undoubtedly, one of the main challenges faced by society toward sustainable development today is environmental degradation. There is an ongoing exploration of new technologies to address air, water, and soil contamination. Hazardous pollutants include particulate matter (PM), heavy metals, dyes, organic compounds, pesticides, poisonous gases, industrial effluents, and oil spills [5]. Industries that utilize wet processing and discharge wastewater containing harmful contaminants like dyes, chemicals, and heavy metals pose a significant threat to the environment and all living organisms that depend on these water bodies. Heavy metals including Cu, Zn, Pb, As, Ni, Cr, and Cd are particularly problematic as they are pervasive sources of water and soil pollution. The World Health Organization (WHO) identified them as major threats. Even though nuclear power is promoted as a green technology, it can also have detrimental environmental impacts in addition to conventional industrial and agricultural practices [6]. The process of environmental remediation is crucial in reviving the natural surroundings by eliminating harmful pollutants or contaminants from various mediums, such as soil and water. The reduction of hazardous substances safeguards the environment Pooja · M. Dutt · L. Saya (B) Department of Chemistry, Sri Venkateswara College (University of Delhi), Dhaula Kuan, New Delhi 110021, India e-mail: [email protected] T. Gupta Discipline of Chemistry, Indian Institute of Technology Gandhinagar, Palaj, Gandhinagar, Gujarat 382355, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Gulati (ed.), Nanosponges for Environmental Remediation, https://doi.org/10.1007/978-3-031-41077-2_12

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Fig. 1 Various approaches for environmental remediation [5] (CC-BY-4.0)

and public health. Environmental remediation can use a variety of materials and approaches (listed in Fig. 1), as pollutant capture and degradation can be difficult due to the mixture of compounds and their low reactivity [5]. There is a rising need for ecologically friendly and sustainable materials [7]. Nanomaterials have shown promise in effectively addressing environmental remediation as adsorbents and catalysts [6]. Recent studies have focused on utilizing engineered nanomaterials to develop innovative environmental remediation technologies [3, 5]. Nanomaterials have higher reactivity and greater efficiency in comparison to their larger analogues due to their increased surface-to-volume ratio. Additionally, nanomaterials can be modified with functional groups that target specific pollutants, providing unique surface chemistry not found in traditional approaches. The physical properties, chemical composition, and morphology of nanomaterials can be intentionally adjusted to enhance their remediation performance. The tunable parameters and surface modification chemistry provides an advantage and make nanomaterials a superior option for environmental remediation compared to conventional methods [5]. Various types of nanomaterials, such as inorganic, carbon-based, and polymeric materials, are employed to eliminate environmental contaminants. These innovative materials have proven to be effective in the purification of pollutants such as heavy metals, dyes, organic compounds, pesticides, and volatile organic compounds (VOCs). Additionally, adsorption and photocatalytic reduction processes are frequently used to reduce the concentration of pollutants in the environment,

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specifically for organic pollutants, while the adsorption or reduction process is utilized for heavy metal ions [8]. The creation of nanoscale materials has advanced significantly, and the word “nanoremediation” was first used to describe the development and use of nanotechnology in the environmental area. Nanoremediation is a recently developed approach for environmental cleanup. It is highly efficient, cost-effective, and sustainable in terms of its environmental, social, and economic impacts. Given the wide range of nanomaterials available, it is crucial to carefully select the one that offers optimal performance and sustainability. Ideally, materials that use natural substances or repurposed waste as a precursor should be considered top contenders for this purpose [3]. Sponge-based porous materials are highly versatile and have a wide range of applications in chemo-catalysis, isolation-purification, resin exchange, and energy storage due to their exceptional structural properties. These properties include high surface area, porosity, low density, and good interaction with guest molecules both on their surface and within their internal network [9]. Efforts have been made to develop functional sponge-based materials in the field of environmental remediation due to their recyclability, cost-effectiveness, environmental friendliness, and ease of fabrication. Nanosponge is a modern material made up of tiny particles usually made of a synthesized carbon-containing polymer, with a narrow cavity of a few nanometers, which can be filled with small amounts of matter or toxins in a targeted manner [10, 11]. These are useful in environmental applications, where they can clean up ecosystems by purifying water or removing metal deposits [12]. Due to their small size, nanosponges can move quickly through substances like water, making it easy for them to locate and eliminate unwanted materials. Compared to microsponges, nanosponges are more advantageous as their smaller size causes less disruption to the system, reducing the risk of any adverse effects. Specifically, cyclodextrin-based nanosponges can be seen as a cost-efficient approach that requires less energy and time than other commonly used water treatment methods. These polymers have distinct physical and chemical properties, as well as highly interconnected threedimensional structures, and should be studied further for their potential in eliminating pollutants and microbial contaminants from wastewater simultaneously [13, 14]. Cyclodextrin nanosponges possess numerous advantages when used for environmental remediation, as their three-dimensional (3D) structure allows for pollutant removal percentages similar to those achieved with activated carbon. Although activated carbon production is expensive, Cyclodextrins and their derivatives are becoming increasingly affordable and easy to functionalize, resulting in a broad range of well-structured adsorbent materials [15]. Liberatori et al. demonstrated the evidence to support the ecologically safe use of cellulose-based nanosponges for extracting heavy metals from seawater [16]. Carbon Nanotubes (CNT)-based sponge materials are a promising option for environmental applications such as sorption, filtration, and separation, due to their lightweight nature, high porosity, and large surface area [17]. Liu et al. developed an inexpensive and energy-efficient method for water purification that can effectively remove organic pollutants, which are a major

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cause of environmental problems. They used conductive nanomaterials such as CNTs to enhance the electro-oxidation process, resulting in the creation of a conductive nanosponge filtration device. By integrating electrochemistry, the performance of the sponge-based device was significantly improved, allowing for the efficient adsorption and oxidation of organic compounds in aqueous solutions. The use of CNTs in this device not only serves as excellent electrocatalysts for the degradation of pollutant but also as conductive additives that increase the conductivity of polyurethane sponges [18]. There has been a noticeable uptick in the frequency of oil leaks and a corresponding surge in the rise of industrial wastewater contaminated with oil. These pose a grave threat to both the aquatic ecological system and our living conditions, as well as resulting in significant economic losses. To address this issue, adsorption or filtration is considered one of the most efficient methods for removing oily contamination from water among all available oil pollution treatment techniques [19, 20]. Therefore, the development of new strategies has been significantly focused on oil–water separation [21, 22], resulting in the emergence of various materials such as graphene/nanotubes sponges, carbon nanofibers aerogels, and polyurethane sponges coated with polymers [23]. Although commercial sponges including cellulose-based, graphene-based, and polyurethane sponges possess good surface areas and high absorption capacities, their hydrophilic/oleophilic properties render them unsuitable for oil–water separation. To enable their use in this application, modifications have been made to the sponge matrix by incorporating low surface energy materials and multiple levels of microscopic surface roughness, thereby altering their wettability to hydrophobicity or lipophilicity [24, 25]. Additionally, sponges have a low density that aids in oil penetration by reducing the barrier layer that forms from direct contact with water, which has a greater density than oil [26]. Therefore, the use of natural fibers and sponges as matrixes or supports has generated considerable interest due to their abundance, low cost, and renewability. Nanocellulose is among the many natural resources that have been used [23, 27, 28]. Cellulose, a biopolymer abundant in nature, is produced in vast quantities through photosynthesis, with an estimated annual production of 700 billion tons. Cellulose is a crystalline polysaccharide that is mainly derived from wood or cotton. It is known for its versatile properties such as renewability, biodegradability, and ease of structural modification [2]. Although cellulose has been studied for use in water purification adsorbents and membranes, its applications have been limited due to its low internal porosities, and small surface area of micro-sized cellulose materials. Consequently, nano-sized cellulose, particularly ultrafine and eco-friendly nanocellulose, has emerged as a potential alternative for wastewater treatment and other flexible applications due to its superior properties. Due to their unique structure, sustainability, excellent mechanical properties, and abundance, cellulose nanofibers (CNFs) have enormous potential as environmentally-friendly materials in modern times [2]. In various research fields, the creation of new bio-polymeric materials utilizing cellulose as the primary constituent is currently a popular area of focus. This is due to the widespread availability, biocompatibility, and desirable mechanical properties of

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these materials. However, despite these advantageous characteristics, the application of cellulose in developing advanced scaffolds is hindered by its inherent tendency to establish hydrogen bonds between the hydroxyl groups of a particular chain with adjacent oxygens. This leads to the formation of highly crystalline and insoluble structures. Over the past few years, multiple endeavours have been dedicated to the functionalization of natural cellulose to enhance its solubility and alter its physical and chemical properties to meet particular requirements for various applications [29]. TEMPO mediated wood cellulose oxidation has potential as a breakthrough nanotechnology, contributing to sustainable societies and eco-friendly systems reliant on renewable resources [30]. TEMPO oxidation is considered a highly effective and energy-efficient pretreatment for converting plant cellulose fibers into nanofibers. The process involves creating electrostatic repulsion between cellulose fibrils, which helps prevent the formation of strong inter-fibril hydrogen bonds. Furthermore, this process does not cause any oxidation within the cellulose crystallites [31]. These TOCNFs are prepared through selective oxidation of wood cellulose using TEMPO in presence of NaBr or NaClO environment [29]. Their preparation and characterization are shown in Fig. 2. The production of nanocellulose fibers using TEMPO-mediated selective oxidation is a promising methodology for achieving individual fibril dispersibility in a pure aqueous medium and reducing the cost of landfill disposal associated with

Fig. 2 Preparation of TOCNF [29] (CC-BY-4.0)

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synthetic polymers. This is due to the high density of sodium carboxylated moieties on the fibril surfaces. This nanomaterial offers excellent mechanical strength and highly reactive moieties (COONa+ ) which make it ideal for producing a range of mechanically robust functional composites, including hydro/aerogels [32, 33]. Since there has been a growing interest in developing oil–water separation materials using environmentally friendly substances as the base. Numerous studies have been conducted on cellulose sponges with super hydrophobic and super lipophilic properties. One such study by Halim et al. involved a TOCNF to create a super lipophilic and underwater superoleophobic material. The resulting sponge had a separation efficiency of up to 99% with only the use of gravity [26]. Additionally, studies have shown that the combination of TEMPO-mediated oxidation and homogenizing mechanical treatment can lead to the production of uniform cellulosic nanofibers. This approach leads to the formation of nanofibers with a width of 3–4 nm and a length of 2–3 μm from bleached wood pulp [30]. Recently, there has been extensive research on TOCNF-based adsorbents in the field of environmental remediation. Apart from this, these engineered nanostructured materials can be used in various fields such as energy storage, smart materials, drug delivery and healthcare, food packaging, and sensing applications due to their versatility. Moreover, their eco-friendly nature makes them suitable for sustainable and eco-safe nanoremediation, which addresses concerns associated with the use of nanotechnology [2, 34, 35]. This chapter deals with the utilization of TOCNFs for high-performance spongelike functional materials. These materials are environmentally friendly alternatives to synthetic polymers and are intended for safe and sustainable environmental nanoremediation.

2 Cellulose Nanofibers (CNFs) CNFs are nano-scale fibers belonging to an emerging class of nanomaterials. These are derived from natural cellulose fibers, the most abundant natural polymer on Earth [36]. Cellulose nanofibers have several interesting properties that make them useful for a variety of applications. These fibers are extremely small, with diameters typically ranging from 5 to 20 nm and lengths ranging from a few micrometers to several millimeters.

2.1 Definition and Properties of CNFs The properties of CNFs are influenced by various factors, including the source of cellulose, the method of isolation and the degree of fibrillation [36–38]. CNFs have a high aspect ratio, which makes them very strong and stiff [39]. In fact, they are

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stronger than steel on a weight-to-weight basis. CNFs also have a high surface areato-volume ratio, making them useful in various applications, such as water filtration, energy storage, and as reinforcement in composites. Additionally, CNFs are renewable, biodegradable, and non-toxic, which makes them an attractive alternative to synthetic materials. CNFs have unique physical, chemical, and mechanical properties that make them attractive for various applications, such as reinforcement materials, high-strength paper products, membranes, and biomedical devices [40]. They are also lightweight, biodegradable, and renewable, making them an environmentally friendly alternative to many synthetic materials. Overall, cellulose nanofibers have the potential to play an important role in developing sustainable and high-performance materials for a wide range of applications. There are several methods of preparing CNFs from cellulose fibers, including mechanical, chemical, and enzymatic methods. Mechanical methods involve the physical breakdown of cellulose fibers into nanofibers using mechanical forces such as grinding, high-pressure homogenization, and microfluidization. Chemical methods involve chemicals to dissolve the cellulose fibers and then regenerate them into CNFs. Some of the common chemical methods for preparing CNFs include acid hydrolysis, basic hydrolysis, and ionic liquids. Acid hydrolysis involves treating the cellulose fibers with acid to break down the fibers into nanofibers. Other chemical methods include oxidation and TEMPO-mediated oxidation. Enzymatic methods involve enzymes to break down cellulose fibers into CNFs. Enzymatic methods are considered to be more environmentally friendly than mechanical and chemical methods [36, 41, 42].

2.2 Types of CNFs and Their Sources CNFs can be produced from various sources, including wood, plants, bacteria, and animals, based on which they are categorized: (i) Wood-based CNFs: Wood-based cellulose nanofibers are one of the most commercially available CNFs. Wood is the most abundant natural source of cellulose, and its fibers have been used for papermaking for centuries. Wood-based CNFs are usually produced from wood pulp using mechanical or chemical treatments. Mechanical treatment involves the fibrillation of wood pulp fibers by high-intensity refining. In contrast, chemical treatment involves chemicals to remove lignin and hemicellulose from wood fibers, followed by mechanical fibrillation. The most common chemicals used for wood-based CNF production are sulfuric acid and sodium hydroxide. Wood-based CNFs have several advantages, including a high aspect ratio, excellent mechanical properties, and low production cost. However, the production process is energyintensive and generates a large amount of waste. The use of wood-based CNFs is limited to non-food applications due to the risk of contamination [30, 41].

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(ii) Plant-based CNFs: Plant-based CNFs are produced from various plant sources such as cotton, flax, hemp, jute, and sisal. Unlike wood-based CNFs, these are usually produced by chemical treatment followed by mechanical fibrillation. The chemical treatment involves the use of chemicals such as sodium hydroxide, hydrogen peroxide, or sodium chlorite to remove the lignin and hemicellulose from plant fibers. These have several advantages, including low production cost, high aspect ratio, and excellent mechanical properties. Plant-based CNFs have potential applications in the food industry due to their biocompatibility and biodegradability [41]. (iii) Bacterial-based CNFs: Bacterial-based CNFs are produced from bacterial cellulose, a type of cellulose produced by some bacteria such as Acetobacter xylinum. Bacterial cellulose has a unique structure and properties compared to plant and wood-based cellulose. It has a higher degree of polymerization, crystallinity, biocompatibility, and aspect ratio. Their production involves the fermentation of bacteria in a nutrient-rich medium followed by mechanical fibrillation. They also have potential applications in the biomedical field, such as wound healing, tissue engineering, and drug delivery [41, 43]. (iv) Animal-based CNFs: Animal-based CNFs are produced from various sources, such as shrimp, crab, and lobster shells, as well as silk fibers. Animal-based CNFs are usually produced by chemical treatment followed by mechanical fibrillation. The chemical treatment involves the use of chemicals such as potassium hydroxide, sodium hypochlorite, or hydrogen peroxide to remove the chitin and protein from animal fibers. Animal-based CNFs have several advantages, including a high aspect ratio, excellent mechanical properties, and biocompatibility. They also have potential applications in the food industry as natural food additives and in the biomedical field for drug delivery [36].

2.3 Preparation of CNFs Through TEMPO Oxidation One of the most popular methods for producing CNFs is the TEMPO oxidation method as shown in Fig. 3 [44]. The selective oxidation of the C6 primary hydroxyl groups on the cellulose fibers involves TEMPO and a co-oxidant (NaOCl or NaBr). The oxidation leads to carboxylate group formation on the cellulose fibers, which makes them highly water-soluble. This selective oxidation creates a negatively charged surface on the cellulose fibers, which allows the fibrillation of cellulose fibers into nanofibers through mechanical shearing or homogenization. The TEMPO oxidation method is relatively simple and can be performed in aqueous solutions at room temperature. The process involves several steps, including pre-treatment of the cellulose fibers, TEMPO oxidation, and mechanical disintegration [30, 31, 37, 45, 46]. The process of preparation of cellulose nanofibers through TEMPO oxidation involves the following steps:

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Fig. 3 Preparation of NaOH/urea-treated TEMPO-oxidized cellulose using a two-step process [47] (CC-BY-4.0)

1. Preparation of cellulose pulp by treating raw cellulose material with alkali and acid to remove impurities. This step is important to improve the access of primary alcohol groups on cellulose fibers to TEMPO oxidant. 2. The cellulose pulp is then oxidized with TEMPO, NaOCl, and NaBr in a buffer solution at a controlled pH and temperature. This step selectively oxidizes the C6 primary hydroxyl group to a carboxyl group, making the cellulose more water-soluble and easier to disperse. 3. The oxidized cellulose suspension is then homogenized under high-pressure to break down the cellulose fibers into highly water-soluble nanofibers with a high aspect ratio. 4. The resulting CNF suspension is filtered and washed to remove residual chemicals. 5. The purified CNFs are then dried to obtain a powder or film. In short, the TEMPO oxidation method is a simple and effective way to prepare CNFs. The method involves the selective oxidation of the primary alcohol groups on the cellulose fibers using TEMPO and a co-oxidant, followed by mechanical disintegration to produce highly water-soluble and highly uniform CNFs. These CNFs possess excellent characteristics including remarkable biocompatibility, high aspect ratio, and exceptional tensile strength, making them a highly promising material for environmental remediation applications [2, 30, 48].

3 Sponge-Like Functional Materials Sponge-like functional materials are versatile materials with a sponge-like structure with interconnected pores and a large surface area. These materials are also known for their high mechanical strength due to their unique structure. The interconnected porous network provides structural support, while the large surface area and porosity allow for efficient mass transport. Another important property of sponge-like functional materials is their ability to adsorb molecules.

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3.1 Definition and Properties of Sponge-Like Materials The high surface area and porosity of these materials make them excellent adsorbents for a wide range of molecules, including pollutants, dyes, and gases. These materials have garnered considerable interest in recent years because of their possible uses in different areas, including catalysis, energy storage, biomedical engineering, and environmental remediation [49]. Several methods for synthesizing sponge-like functional materials with tailored properties include sol–gel, hydrothermal, and template synthesis. The sol–gel approach is majorly used to prepare sponge-like functional materials. It involves the hydrolysis and condensation of metal alkoxides or other precursors to form a gel network. The resulting gel is typically dried and calcined to remove the organic components and produce a porous material. The pore size and porosity of the material can be regulated by changing the concentration of the precursor solution and the surfactant. Hydrothermal synthesis involves the use of high temperatures and pressure to promote the growth of crystals. Template synthesis involves the use of a sacrificial template that is removed after the material has formed [50]. Sponge-like functional materials have a diverse array of applications due to their distinct characteristics. Therefore, these have been investigated for various applications [51]. For example, porous carbon materials have been used as electrodes for supercapacitors due to their high surface area and good electrical conductivity. However, Metal–organic frameworks (MOFs) have been extensively studied for gas storage and separation applications due to their high surface area and tunable pore size [52]. Some of the most promising applications of these materials are discussed below: (i) Sponge-like functional materials are attractive for applications in energy storage due to their enormous surface area and porosity. These materials can be used as electrodes in batteries and high-performance supercapacitors to increase their energy storage capacity. (ii) Sponge-like functional materials are also attractive for catalytic applications owing to their large surface area and porosity. These materials can be used as catalyst supports or as catalysts themselves. The substantial surface area and porosity allow for efficient mass transport and provide a large surface area for catalytic reactions. (iii) Sponge-like functional materials can be used for environmental remediation due to their ability to adsorb pollutants. These materials have been used to remove water, air, and soil pollutants such as heavy metals and dyes [3, 4, 53, 54]. (iv) Sponge-like functional materials are also promising for gas separation applications. The large surface area and high pore volume of these materials make them attractive for separating gas mixtures based on their size and polarity. Sponge-like materials have been used to separate CO2 from flue gas and to purify natural gas.

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(v) Sponge-like functional materials have potential applications in the biomedical field as well. These materials can be used as drug delivery vehicles, wound dressings, and tissue engineering scaffolds. The porous structure of these materials allows for efficient drug delivery to target tissues and the regeneration of damaged tissues. This is due to their ability to absorb and release drugs in a controlled manner. Porous scaffolds have also been investigated for tissue engineering applications, as they can provide a porous structure for cell attachment and growth.

3.2 Types of Sponge-Like Materials and Their Applications in Environmental Remediation We will focus on some of the commonly used sponge-like materials and their applications in environmental remediation. These are listed as follows: (i) Activated carbon sponge: Activated carbon sponge is a highly porous material having a large surface area and excellent adsorption capacity. It is commonly used to remove organic pollutants such as pesticides, pharmaceuticals, and dyes from water and air. It has also been used for the removal of heavy metals including lead (Pb) and mercury (Hg) from water. Activated carbon sponge has shown promising results in removing emerging contaminants such as microplastics from water [55]. (ii) MOF sponge: MOF sponge is a sponge-like material made of metal ions and organic ligands. It has high porosity and tunable pore sizes, making it suitable for removing contaminants such as heavy metals, organic pollutants, and gases from water and air. MOF sponge has also shown promising results in removing radioactive elements such as uranium and plutonium from water [52]. (iii) Graphene sponge: Graphene sponge is a highly porous material made of graphene sheets. It has a large surface area and excellent adsorption capacity, making it suitable for removing various pollutants such as organic compounds, hazardous heavy metals, and radioactive elements from water and air. Graphene sponges have also been used for the removal of oil spills from water [56, 57]. (iv) Polymer sponge: A polymer sponge is a sponge-like material made of polymer chains. It has high porosity and tunable pore sizes, making it suitable for removing contaminants such as organic pollutants and heavy metals from water. A polymer sponge has also been used for the removal of microplastics from water [5]. (v) Zeolite sponge: Zeolite sponge is a type of sponge-like material made of aluminosilicate minerals. It has high porosity and tunable pore sizes, making it suitable for removing contaminants such as heavy metals, organic pollutants, and gases from water and air. Zeolite sponge has also been used to remove radioactive elements such as Cesium and Strontium from water [58].

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(vi) Carbon nanotube sponge: A carbon nanotube sponge is a type of sponge-like material made of carbon nanotubes. It has high porosity and a large surface area, making it suitable for removing various contaminants such as organic pollutants and heavy metals from water. Carbon nanotube sponge has also been used for the removal of oil spills from water [5]. (vii) Chitosan (CS) sponge: CS sponge is a type of sponge-like material made of CS, which is a natural polymer derived from chitin. It has high porosity and tunable pore sizes, making it suitable for removing various contaminants such as heavy metals, organic pollutants, and dyes from water. CS sponge has also been used for the removal of bacteria and viruses from water [57]. (viii) Cellulose sponge: A cellulose sponge is a type of sponge-like material made of cellulose fibers. It has high porosity and tunable pore sizes, making it suitable for removing contaminants such as heavy metals and organic pollutants from water. Cellulose sponges have also been used for the removal of microplastics from water [53, 57]. To sum up, sponge-like materials display great potential in environmental remediation due to their large surface area, tunable pore sizes, and high capacity for adsorbing contaminants. Further research is needed to optimize their performance and scalability for practical applications. The development of sponge-like functional materials will continue to be an area of active research in the coming years due to their potential for creating innovative solutions to current challenges in various fields. Further research is needed to explore their potential and optimize their properties for specific applications.

3.3 Advantages of CNFs to Produce Sponge-Like Materials for Environmental Remediation CNFs are renewable, biodegradable, and non-toxic materials that have gained significant attention for various applications, including environmental remediation. The unique properties of CNFs including large surface area, high aspect ratio, and good mechanical strength, make them suitable for producing sponge-like materials for environmental remediation. This chapter will discuss the advantages of using CNFs to produce sponge-like materials for environmental remediation. (i) CNFs possess a high surface area-to-volume ratio, which results in a large adsorption capacity. Several studies have shown that CNF-based sponge-like materials can efficiently remove heavy metals, organic pollutants, and dyes from wastewater as shown in Fig. 4 [59, 60]. (ii) CNFs are derived from natural sources, such as wood pulp, agricultural waste, and cotton, which are inexpensive and abundant. CNFs can be easily extracted using mechanical or chemical methods, which makes them cost-effective compared to other adsorbent materials [61, 62].

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Fig. 4 Nanocellulose in water treatment [61] (CC-BY-4.0)

(iii) CNFs are biodegradable and non-toxic, making them environmentally friendly. Unlike synthetic adsorbents, CNFs-based sponge-like materials do not leave harmful residues after use, and they can be disposed of easily without causing harm to the environment [57, 61]. (iv) CNFs can be easily modified to enhance their sorption properties. Functional groups can be introduced onto the CNFs surface through chemical modification, improving their selectivity and affinity towards specific pollutants [63, 64]. (v) CNFs-based sponge-like materials exhibit high mechanical strength, making them durable and resistant to mechanical stresses during adsorption. This property is crucial for the development of practical adsorbents for large-scale applications [57, 61]. In conclusion, CNF-based sponge-like materials offer several advantages for environmental remediation, including high adsorption capacity, low cost, abundance, biodegradability and non-toxicity, easy modification, and high mechanical strength. These properties make CNF-based sponge-like materials a promising alternative to conventional adsorbent materials for environmental remediation.

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4 Preparation of Sponge-Like Functional Materials from TOCNFs for Environmental Remediation Using TEMPO-mediated oxidation, wood celluloses may be transformed into individual nanofibers 3–4 nm broad and a few microns in length with a high aspect ratio. Utilizing TEMPO-mediated oxidation, significant quantities of C6 -carboxylate groups are selectively produced on each cellulose microfibril without affecting the initial crystalline structure and width of wood celluloses. The novel cellulose-based nanofibers created by the TEMPO-mediated oxidation of native cellulose fibers have the potential to be used in high-tech areas as new bio-based, ecologically benign nanomaterials.

4.1 Methods of Preparing Sponge-Like Materials from TOCNFs for Environmental Remediation The various methods that are discovered for the preparation of sponge-like materials are as follows: (i) Preparation of TOCNFs dispersed in water: After reaching carboxylate concentrations exceeding 0.8 mmol/g in bleached hardwood and softwood kraft pulps, mechanical treatment was performed on the oxidized cellulose or water slurries. This was done using a double-cylinder-type homogenizer, an ultrasonic homogenizer, or a domestic mixer. The treatment typically lasted for 2–10 min. As a result, transparent and viscous dispersions were observed. Using transmission electron microscopy (TEM), the dispersions were observed as individual TOCN of 3–4 nm broadness and a few microns length. Wood cellulose fibers that were previously 20–40 mm wide have now been broken down into their smallest components, which are either cellulose chains composed of 30–40 cellulose molecules or individual CNFs with high aspect ratios (exceeding 100). This is the first time that such small components of wood cellulose fibers have been achieved. By using TEMPO to facilitate oxidation, other natural celluloses were also transformed into individual nanofibers. The initial celluloses used as the beginning ingredients produced a variety of nanofibers in high yields [30]. (ii) Preparation of Chitin/TEMPO-CNFs hydrogels: Chitin and TEMPO-CNFs were combined with 8 weight percent of NaOH, 4 weight percent of urea, and 88 weight percent of water to create hydrogels, which were then refrigerated (20 °C) for four hours. The icy substance was warmed and vigorously stirred in a comfortable climate. Four rounds of chilling and reheating were completed to produce a dense and viscous solution. The percentage of chitin and TEMPOCNFs composition was held at 100/0, 90/10, 80/20, 70/30, 60/40, and 50/50, respectively. The total dry material in each hydrogel was kept at 2.0 g. Once a homogeneous solution was obtained, 0.2 mL of epichlorohydrin (ECH) was

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added to the hydrogel solution and agitated at 23 °C for 30 min. The solution was then maintained inside an oven at 40 °C [65]. (iii) CNFs by sonocatalysed-TEMPO-oxidation: In order to achieve 0.1% uniformity, 0.3 g of the oxidized cellulose sample was dissolved in 300 mL of water. Using an industrial mixer, the slurry was homogenized for a total of 20 min. In order to avoid overheating the mixer and the suspension, a 15-s pause was provided after every 45 s of mixing. After that, the suspension that had undergone mechanical treatment was centrifuged at 10,000 g for 10 min to separate the supernatant from the partly and fully fibrillated portions. Three 40 mL oxidized samples from the effluent were dried at 105 °C to determine the output of nanocellulose. The dry weights of the samples were then calculated. The mixture was kept at 4 °C for additional analyses, like microscopy [66]. Direct conversion of raw wood to TOCNFs is also discovered [31]. (iv) Preparation of nanocellulose scaffolded MOFs (MOFs@NC): Composites made of MOFs and nanocelluloses (NC) work together synergistically to produce special characteristics while integrating the innate benefits of each component. NC can function as a carrier with a heavily reticulated network while MOFs fully show high surface area and porosity in NC scaffold, giving significant active sites in multiple magnitudes of pores. Both in-situ and ex-situ synthesis methods can be used to create MOFs@NC. The term “in-situ synthesis” describes the initial immobilization of MOFs precursors (metal or binder) at NC and the subsequent inclusion of organic linkers to finish the fabrication of MOFs. In comparison, ex-situ synthesis suggests that MOF crystals are created before MOFs@NC are created. The mixture can be created by either submerging the moulded NC structure in a suspension of MOF or by combining prepared MOF and NC directly with ultrasonic stirring [52]. There are various other methods for preparing sponge-like materials as shown in Fig. 5. These include the Double-emulsion solvent evaporation method, the Quasiemulsion solvent diffusion method, the Liquid–liquid suspension polymerization method, and Oil in the oil emulsion solvent diffusion method. Some novel methods are also used, such as the preparation of sponges from bPEI-TOCNF nanocomposite [35], Lyophilization, electrohydrodynamic atomization, and Nucleic acid self-assembly [9].

4.2 Factors Affecting the Properties of Sponge-Like Materials for Environmental Remediation As we know, sponge particulates are porous materials with many beneficial characteristics, such as large porosity, elastic deformation, capillary action, and 3D response for environmental remediation, all of which are directly linked to their sponge-like structure and makeup. Additionally, these very appealing benefits make nano scaffolds a potential foundation for environmental remediation and biomedicine use.

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Fig. 5 Preparation methods of sponge particulates. a Double emulsion solvent evaporation method, b quasi-emulsion solvent diffusion method, c liquid–liquid suspension polymerization method, and d oil in oil emulsion solvent diffusion method. Reprinted with permission from Ref. [9] Copyright 2021 Elsevier B.V.

Here, we go over the factors affecting the properties of sponge particles and why they are better for environmental use. The interior pore diameters and volumes of sponge particulates range from 10 to 800 nm and 0.1 to 0.3 cm3 /g, respectively. There are numerous linked gaps in sponge particulates. Because of their high porosity, sponge particulates make excellent repositories for components with high loading capacities because they have a lot of accessible surface area and inner space to serve as host systems. Sponge particulates with a large surface and volume ratio produced by multiple holes enable for enormous accessible inner space with more absorption sites on both exterior and internal surface area to the adsorbed particles for environmental remediation purpose [9]. The production of CS hydrogel required careful fluid selection because both the gelation process and the hydrogel’s biocompatibility can be greatly impacted. However, if CS is dissolved in a polyprotic acid, the sol/gel transformation fails. Additionally, CS hydrogel prepared with lactic acid as opposed to hydrogel made with other acids has been found to show lower toxicity and higher histocompatibility. So, to guarantee an effective sol/gel transition, lactic acid was selected as the solvent of CS due to its minimal toxicity, resorbability in the body, and other factors. TOCNF content also affects various properties of nanofibers [67].

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4.3 Characterization Techniques for Sponge-Like Materials for Environmental Remediation Various characterization techniques are employed for the analysis of sponge-like materials. All these techniques are equally important for the examination of these materials. Some characterization methods are listed below: (i) Determination of Gelation Time: The test tube inversion technique is used to calculate the gelation duration [52]. In a nutshell, 1 mL of CS/TOCNF hydrogel was introduced to a tube in a liquid condition at room temperature, and the tube was kept at 37 °C in a water bath. Every 30 s, it was removed, rotated, and examined. The duration of the gelation process was determined from the moment the hydrogel ceased to flow. Another measure used to assess gelation was a colour change. The CS hydrogel without TOCNF had the greatest gelation time, whereas the CS hydrogel containing 0.8% of TOCNF had the shortest one. These findings showed that TOCNF addition may accelerate the gelation rate of the CS hydrogel. The rate of gelation increases with TOCNF content in the hydrogel [67]. (ii) Degradation Behaviour of CS/TOCNF Hydrogels: One particular hydrolytic enzyme for metabolising CS in humans is Lysozyme. It may be found in a variety of bodily tissues and fluids. Lysozyme degradation of CS/TOCNF hydrogels was quantified. After being submerged in PBS containing Lysozyme for 7, 14, and 35 days, morphological alterations and hydrogel erosion were noticed. After 35 days of incubation, the majority of the CS/TOCNF hydrogels disintegrated. However, CS hydrogel has decomposed less than other samples. According to the results, adding TOCNF speeds up the deterioration of the CS hydrogel [67]. (iii) Analysis of Surface Structure of CS/TOCNF Hydrogels: Scanning electron microscopy (SEM) was used to examine hydrogel microstructures. Following a 15-min fixation with 2% glutaraldehyde, an alcohol series (50–100%) was used to dehydrate hydrogels. Finally, they spent 10 minutes submerged in hexamethyldisilazane (HMDS). Then, using a sputter coater, samples were dried and then observed at 7.5 kV of increasing voltage. Using the SEM images shown in Fig. 11.7, differences in surface structure were detected. Hydrogel surface structure was shown to be dependent on TOCNF concentration by SEM micrographs. Without TOCNF, the CS hydrogel surface was compact and rough. At increasing TOCNF concentrations, these surfaces turned more porous and less compact [67]. (iv) Fourier Transform-Infrared (FT-IR) Spectroscopy Analysis: A Nicolet iS10–Smart FTIR spectrophotometer was used to acquire the spectra of the materials in the 4000–600 cm-1 range as shown in Fig. 6. The OH stretch and OH bend responsible for the TOCNF bands were observed at 3361 and 1635 cm−1 , respectively. Given that CS is a byproduct of partly deacetylated chitin, the CS spectrum displayed the frequencies corresponding to NH stretches in the range of 3200–3500 cm−1 ; C–H vibrational frequencies at around 2880 cm−1 ;

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Fig. 6 a FT-IR spectra of TOCNF and CS raw materials, CS and CS/TOCNF hydrogels containing different concentrations of TOCNF. b XRD spectra of TOCNF, CS, and CS/TOCNF hydrogels. Reprinted with permission from Ref. [67] Copyright 2017 Elsevier B.V.

C=O stretch of amide bonds at ~1650 cm−1 ; NH bending frequencies at around 1645 cm−1 . The C–H bending occurred at around 1420 cm−1 , symmetrical CH3 deformation, and amide at 1380 cm−1 and 1320 cm−1 respectively, supporting the CS structure. The presence of these bands in the spectra of these hydrogels confirmed the composition of the hydrogels. On the other hand, the fingerprint region of the CS/TOCNF hydrogels did not reveal any additional bands. This suggested that there was no covalent link between CS, glycerophosphate, and TOCNF. There may have been electrostatic contact [67]. (v) X-ray Diffraction (XRD): The XRD of CS and hydrogels of CS/TOCNF were recorded on a MiniFlexII instrument using CuK radiation to investigate variations in the crystalline nature of the material. TOCNF suspension, solid CS, and their hydrogels with TOCNF were among the materials that were measured. The measured dispersion angle ranged from 10° to 60°. The XRD data unambiguously show that TOCNF addition alters the crystallite of the hydrogel. The difference in crystallinity between CS/TOCNF 0.6 and CS/TOCNF 0.8 was observed, which in turn affects the stiffness of the hydrogel as shown in Fig. 7. The information suggested that the CS-TOCNF connection was controlled by electrostatic interaction [67].

5 Applications of Sponge-Like Functional Materials from TOCNFs in Environmental Remediation Sponge-like materials have various important applications from environmental remediation as well as from a biomedical point of view. Natural Sponges have shown various environmental applications. The use of natural sponges has a variety of

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Fig. 7 a TEM image of TOCNF. SEM micrographs with magnification 35,000× of CS: b CS and CS/TOCNF set hydrogels containing different concentrations of TOCNF: c CS/TOCNF 0.2; d CS/ TOCNF 0.4; e CS/TOCNF 0.6; and f CS/TOCNF 0.8. Arrows indicate TEMPO-oxidized CNF structures. Reprinted with permission from Ref. [67] Copyright 2017 Elsevier B.V.

advantages, such as reusability, affordability, environmental kindness, etc. The use of native sponges is therefore very hopeful.

5.1 Overview of Various Applications of Sponge-Like Materials in Environmental Remediation It is well known that marine sponges are used as indicators to track the presence of heavy metals in water. Consequently, they are appropriate for metal preconcentration. When nanomaterials and sponges were combined, sorbents with more sophisticated properties and broader sorption capacities were created, making sponges ideal for the sorption of tiny organic compounds like antibiotics and pesticides; which makes sponges suitable for environmental remediation [51]. Sponge particles have great potential in biomedical applications, such as drug administration through oral, parenteral, transdermal, ophthalmic, and targeted delivery. This potential is ascribed to the notable efficiency in loading drugs, outstanding biocompatibility, and unique ability to adjust the release patterns of drugs. Despite the fact that the sponge particulates study is still in its inception, the technology is gaining popularity due to its distinctive benefits. The novel drug delivery methods are developed for the treatment of local illnesses using sponge particulates technology has a huge market and therapeutic application potential. High porosity and good flexibility are characteristics of sponge particles [9]. Sponge particulates are particularly hopeful as a platform for water decontamination, drug delivery and wound curing due to the small porous structures that provide a plentiful supply of wide surfaces and interior networks for absorption as well as adsorption. The powerful capillary action of sponge particulates, which is extensively researched and used in the formulation of liquid-based ingredients, has also been documented to absorb liquid. These particulates have

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also been investigated as receptacles for adsorbing excess oils on the surface of skin or extravasating from wounds, in addition to serving as medication transporters for liquid substances. Sponge particulates have become widely used in topical skin treatments due to their cleaning usefulness. For acne treatment and wound healing, a variety of oil-controlling products made from microsponges have been sold to calm and cleanse the skin. So, overall nanosponge-like materials have a variety of uses in different environmental fields such as for water remediation, removal of heavy metals, removal of organic pollutants from water, drug delivery, formulations, and many more which are very necessary for a pollution-free environment and healthy flora and fauna.

5.2 Specific Examples of Applications Using TOCNFs as the Starting Material for Environmental Remediation As discussed above, there are various important implementations and utilizations of these TOCNF materials in the domains of environmental remediation, biomedicine, drug delivery, and many more. Some particular examples of the applications in environmental remediation are as follows: (i) Removal of Heavy Metals: Due to high toxicity and lack of biodegradability, heavy metal ions from a variety of sources, represent a danger to both the environment and public health. Adsorption is the main method used to treat wastewater. It has several advantages over other methods, including high dependability, design flexibility, technological maturity, and the ability to regenerate used adsorbent. A preferred adsorbent for the efficient adsorption of heavy metals may be MOFs@NC with excellent adsorption capacity and flexibility. With a steady state adsorption period of under 30 min, it demonstrated the removal of 558.66 mg/g Pb (II) from polluted water. Additionally, it should be mentioned that due to their low weight and substantial surface area for loading pollutants and recycling, 3D aerogels were the type that saw the greatest use. So, the strong porous network architecture allows for a total exposure of desirable active sites, quick mass transfer, and efficient interfacial transfer of contaminant ions or molecules at MOFs@NC for improved decontamination. bPEI–TOCNF sponges are also used for water remediation purposes. The strong affinity of sponges for amines allows for the simple collection of heavy-metal cations. The concentration of metal ions in solution is considerably reduced by dipping a sponge into an aqueous solution of Cu2+ , Co2+ , or Cd2+ salts. The metal ions immediately adsorb onto the material surface leading to purification as shown in Fig. 8 [35]. (ii) Removal of Organic Pollutants: The presence of various organic pollutants in water bodies is a serious matter of concern as it is quite harmful to the whole aquatic ecosystem. Dye-derived wastewater may be toxic to plants, animals, and people because it contains

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Fig. 8 Heavy metal removal mechanism from water system using nanocelluloses: a Ion exchange mechanism which involves the adsorption of hazardous metal ions (Mn+ ) takes the place of other ions (K+ , Na+ , H+ ) already associated with the nanocellulose surface; b chemical complexation mechanism in which the carboxyl (–COO− ) and hydroxyl (–OH− ) groups of the nanocelluloses have specific site interactions with particular hazardous metal ions [61] (CC-BY-4.0)

synthetic aromatic organic compounds which are very toxic. MOFs may adsorb organic contaminants by using hydrogen bonds, p–p interactions, electrostatic forces, hydrophobic, and acid/base interactions. Positively charged organic contaminants can adhere to surface-modified NC that has had –COOH and –OH introduced. Additionally, a lot of external surface sites for adsorption are offered by the pores created by the aerogel moulding process. As a result, MOFs@NC can effectively remove organic pollutants from water. The capacity of composites to be formed and recycled makes it easier to recover organic pollutants. The bPEI-TOCNF composite displays high efficiency in removing organic pollutants from water [35]. The electrostatic interactions between NC and cationic dyes are primarily responsible for the good adsorption performance of MOFs@NC on cationic dyes like rhodamine B (RhB) and methylene blue (MB), while MOF supplied porous structure for the increased external surface to accelerate the mass transfer for quick adsorption [52]. (iii) Removal of Micropollutants: Anionic micropollutants such as Congo red, methylene orange, and diclofenac were also effectively removed by MOFs@NC. The positively charged surface of MOFs encourages the electrostatic adsorption of anions and NC can provide varied interfacial interactions with MOFs. Their tight hydrogen bonds

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assist shaping, which makes recovery easier. High surface area and hierarchical porous structure in the ZIF-67/HKUST-1/CNF aerobeads showed higher adsorption capability for both DIC (121.20 mg/g) and MO (49.21 mg/g). (iv) Removal of Pesticides: The absorption of organic molecules like insecticides, herbicides, phenolic compounds, and organic solvents can be accomplished using MOFs@NC. Through electrostatic interactions, hydrogen bonds, and chemical interactions between the oxygen-containing moieties of the CS-CNC@UiO-66-NH2 aerogel and the phosphate groups of Glyphosate molecules, the aerogel had a maximum Glyphosate (a herbicide) adsorption capacity of 133.7 mg/g. (v) Removal of Gas & PM from Polluted Air: Carbon dioxide (CO2 ) and PM, notably PM2.5, may be effectively separated from contaminated air via air filtration. For better sieving of gas, a large aspect ratio of the selective material should be employed for air purification. MOF hybrids have been found as interesting microporous materials for gas separation because of their high specific surface area, pore tunability, and affinity of their organic ligands with the polymeric matrix. MOFs are incorporated into the NC matrix to give membranes more selectivity, filler content, and mechanical stability. For the separation of gas and particulate matter, notably CO2 , MOFs@NC demonstrates remarkable adsorption capability. Besides CO2 , VOCs and PM are two of the most significant indoor air contaminants. The human central nervous system and kidney will be damaged by prolonged exposure to VOCs and PM2.5, resulting in irritation, malfunction, and cancer. Potential VOC adsorbents include MOFs with large surfaces and well-calibrated pores. But, Ag-MOFs@CNF@ZIF-8 showed improved PM2.5 and PM10 removal efficiencies in comparison to bare cellulose filter, with PM2.5 removal efficiency reaching 94.3%. So, from a bigger perspective, TOCNFs have an enormous spectrum of applications in various areas of environmental remediation and many of these are still being researched.

5.3 Potential Future Applications of Sponge-Like Materials from TOCNFs in Environmental Remediation Now, as we know that there is a broad spectrum of applications for these sponge-like materials and these materials are quite useful for human health and environment. But, despite the current breakthroughs in the fabrication and environmental applications of TOCNFs, there are still many challenges and opportunities in the field of environmental remediation. There is a plethora of research that could be put forward in order to find new pathways and useful applications of these nanofibers that can benefit humans, animals and the overall environment. So, some of the potential applications and upgradations in the field of sponge-like materials are as follows [52]:

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(i) By using simulation and molecular modelling tools, the anchoring surface and adhesion of MOF to NC can be displayed, offering recommendations and design implications to fine-tune MOFs@NC by surface alteration towards various contaminants for environmental applications. (ii) Most recent investigations have been on using MOFs@NC in lab settings to remove specific contaminants. However, wastewater has a variety of contaminants. The clarification of potential interactions between concurrent contaminants and the use of clarified processes for the creation of innovative nanocomposite materials with balanced broad-spectrum and selectivity require further focus. When used as adsorbents, MOFs@NC require lengthy equilibration times to reach high adsorption capacities and are unable to entirely breakdown contaminants on their own. Redox nanoparticles may boost the remediation processes for improved pollutants removal from wastewater by being added to MOFs@NC. (iii) A series of metal cation-incorporated MOFs in the NC matrix may therefore be used to possibly cleanse drinking water/groundwater networks and create functioning air filters. For instance, the addition of Ag and Zn in AgMOFs@CNF@ZIF-8 membranes resulted in synergistic inhibition (18.1 mm inhibition zone) against E. coli. (iv) Large-scale applications like advanced wastewater treatment of trace contaminants compliant with the upgraded standard and urgent recovery of leaking oil or nuclear wastes can be made possible by the composites’ improved moldability, a wide range of sources, and broad-spectrum adsorption properties for pollutants. So, at last in a nutshell, we can say that there can be plenty of opportunity and plenty of research that can be done in this field and could benefit not only humans but could holistically benefit the whole environment.

6 Conclusion Sponge-like functional materials derived from TOCNFs have shown great potential in environmental remediation. The need for environmental remediation arises from various activities and sources, including industrial and commercial operations, agricultural practices, oil and gas exploration and production, waste disposal sites and natural disasters. Cellulose is derived from the plant material and is therefore a renewable resource. This means that TOCNFs can be produced in large quantities without depleting finite resources, making them a sustainable choice for environmental remediation. TOCNFs are produced by oxidizing CNFs with TEMPO and sodium hypochlorite, resulting in carboxylate groups on the surface of the nanofibers. The carboxylate groups provide sites for functionalization, and the resulting TOCNs have a high surface area, high adsorption capacity and a sponge-like structure with interconnected pores. These unique properties make them effective adsorbents for

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heavy metals, organic compounds, and dyes. Additionally, TOCNFs can be functionalized with various groups to enhance their adsorption capacity and selectivity for specific pollutants. They can also be used as a substrate for the immobilization of enzymes and catalysts, which can be used for the degradation of pollutants in aqueous solutions. Therefore, sponge-like functional materials derived from TOCNFs could be used across a variety of applications, from water treatment to air purification, and offer a promising solution for addressing environmental pollution and improving environmental quality. Acknowledgements The authors are thankful to the Principal, Sri Venkateswara College, University of Delhi for her valuable cooperation and guidance. The authors would also like to acknowledge HOD, Department of Chemistry, University of Delhi, and IIT Gandhinagar, for his continuous encouragement. Declaration of Competing Interest The authors declare no competing financial interests that could have appeared to influence the work reported in this paper.

Abbreviations TOCNF CNC PM WHO VOC 3D CNT TEMPO CNF MOF CS TEM SEM ECH HMDS FT-IR XRD RhB CO2

TEMPO-oxidized Cellulose Nanofiber Cellulose Nanocrystal Particulate Matter World Health Organization Volatile Organic Compound Three-Dimensional Carbon Nanotubes 2,2,6,6 Tetramethylpiperidine-1-oxyl Cellulose Nanofiber Metal-Organic Framework Chitosan Transmission Electron Microscopy Scanning Electron Microscopy Epichlorohydrin Hexamethyldisilazane Fourier Transform-Infrared X-ray Diffraction Rhodamine B Carbon Dioxide

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Synthesis and Application of Types of Metal Oxide Nanosponges in Water Treatment Archa Gulati and Ajeet Kumar

1 Introduction Water is essential for all life forms on Earth. Without water, the sensitive geochemical cycles of the earth shall collapse, leading to dire consequences for mankind. Therefore, water is very vital for the mere sustenance of living beings. However, due to growing urbanisation and industrialisation, the global requirement for water has increased manifolds causing a huge discharge of wastewater. The wastewater from agriculture, industries, bio-medical sites, and other anthropogenic activities contains numerous obnoxious pollutants like dyes, pesticides, aromatic compounds, and heavy metals which pose a serious threat to the health and well-being of humans. Moreover, these pollutants can disrupt the natural cycles of the environment as per US Environmental Protection Agency reports [1]. For instance, it has been found that the approximately 10% of the total dyes that are highly toxic and carcinogenic in nature are discharged directly into the water bodies without any prior treatment [2]. Hence, the removal of these dyes is quintessential for the mitigation of environmental damage and damage to human health. The ever-growing population has put a tremendous pressure on the agriculture sector. Consequently, there is an expansive use of chemical pesticides, insecticides, and fertilizers to increase and protect the crop yield. Unfortunately, most of these compounds are toxic even at very low concentrations [3]. Their mindless use and presence in potable water makes it necessary to take immediate action for their removal. Similarly, the presence of heavy metals in underground water or other water resources can create havoc for humans and dangerously affect the flora and fauna [4]. Hence, immediate efforts are required to facilitate their removal from water. A. Gulati (B) Department of Chemistry, Ramjas College, University of Delhi, New Delhi, India e-mail: [email protected] A. Kumar Department of Chemistry, University of Florida, Gainesville, FL, USA © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Gulati (ed.), Nanosponges for Environmental Remediation, https://doi.org/10.1007/978-3-031-41077-2_13

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The traditional methods for removal of water pollutants like biofiltration, distillation, reverse osmosis, membrane separation and coagulation-flocculation are cumbersome, tedious, and less efficient. In this aspect, removal of the water pollutants by photocatalytic degradation and adsorption are the most suitable methods due to their easy operational process, low cost, and high efficacy [5, 6]. The materials employed for targeting the removal of water pollutants must have a large surface area, high mechanical strength, and robustness, should be environmentally benign and exhibit remarkable performance. Metal oxide-based nanosponges excellently fulfil all these criteria. Further, metal oxide nanosponges have attracted huge scientific interest in recent times, because of their ability to combine with other atoms, form porous interconnected architectures, semiconductor bandgaps and tailor-made physiochemical properties [7]. Many scientific groups have employed nanosponges for several applications such as drug delivery [8], desulfurization of fuels [9], sensors [10], energy storage [11], thermal decomposition [12], etc. owing to its various astounding qualities as shown in Scheme 1. In this review article, we have focused on the removal of hazardous pollutants from water by employing metal oxide-based nanosponges by the process of adsorption or photocatalysis. The surface modifications, the effect of reaction conditions and reaction mechanism have also been outlined. Further, we have remarked on the future scope and challenges in utilizing metal oxide nanosponges for the purpose of water remediation.

Scheme 1 Astounding properties of nanosponges

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2 Types of Metal Oxide Nanosponges Metal oxide-based nanosponges can be classified as mono-metallic or bi-metallic nanosponges depending upon the number of metals present in the nanosponge. These are discussed in detail in the subsequent sections.

2.1 Mono-metallic Nanosponge Mono-metallic nanosponge is the most primitive type of metal oxide-based nanosponge which is synthesised by employing a single metal precursor. The synthesis of the first mono-metallic nanoporous nanosponge was reported by Antonelli and Ying in 1995 [13]. The authors synthesized nanoporous TiO2 nanosponge by using surfactant alkyl(trimethyl) ammonium bromide which played the role of soft template during the synthesis. Later, a template-free synthesis of mono-metallic nanosponge was reported for various metals like gold, copper, platinum, silver, and palladium by Muthusamy and Katla [14]. The authors elaborated on the effect of the concentration of NaBH4 on the properties of various metaloxide nanosponges. Further, Mn2 O3 nanosponge was prepared by the glycine-nitrate combustion process [15]. The electron micrographs of the as-synthesized Mn2 O3 nanosponge revealed mesoporous architecture with interconnected pores (Fig. 1). Ayesha Zafar et al., in 2021 reported the synthesis of Co3 O4 nanosponges by green reduction method [16].

Fig. 1 Electron micrographs images of Mn2 O3 nanosponge a low and b high magnification synthesized by the combustion process. Replicated from Ref. [15] with the permission of Elsevier Ltd.

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2.2 Bi-metallic Nanosponge Bi-metallic nanosponges offer various superior qualities over mono-metallic nanosponges like higher surface area, more porosity, higher robustness and mechanical strength, better catalytic activity, etc. Bi-metallic nanosponges consisting of Pt and Pd metal along with other less expensive metal precursors are the most common. Zhu et al., in 2015 reported the synthesis of novel Ni-Co oxide-based hollow nanosponge (Ni–Co2 –O HNSs) by reduction method using NaBH4 (Fig. 2) [17]. Hu et al., in 2015 reported the synthesis of V2 O5 /TiO2 nanosponge by using electrostatic spray deposition [18]. Raja et al., fabricated a novel Co3 O4 /NiO nanosponge by employing the precipitation method [19]. Zhao et al., synthesized a Pd-decorated ZnO nanosponge film deposition of ZnO nanoparticles on Pd film [20]. Furthermore, there are numerous reports where TiO2 , ZnO, Al2 O3 , NiO, and Co3 O4 are used in combination with other metals to form bi-metallic nanosponges [21–25].

3 Synthesis Methods Various metal oxide-based nanosponges have been synthesized using different synthesis routes as discussed below. The different advantageous and disadvantageous aspects associated with each synthesis method are mentioned in Table 1.

3.1 Solvothermal Method In the solvothermal method, porous metal oxide structure is achieved at high conditions of temperature and pressure by employing an organic solvent like ethylene glycol [26, 27], ethanol [28, 29], methanol [30], etc. When water is used as a solvent, then that process is known as hydrothermal synthesis and it is one of the most widely used processes for synthesizing porous metal oxide structures [31, 32]. The temperature plays a vital role in controlling the morphology and size of particles. In a typical synthesis process, one or two metal precursors are dissolved in a suitable organic solvent or water. The synthesis takes place under high temperature and high-pressure conditions that are generated in the reaction vessel or autoclave depending on the solvent of choice [33]. The as-obtained product is then washed, dried, and calcined to obtain the final product. This synthesis route offers various advantages like cost effectiveness, facile synthesis approach, environmentally benign and yields contamination-free metal oxide nanosponge. For instance, Patel et al., synthesized Ni–Co layered double hydroxide nanosponges by using methanol as a solvent [34]. In another study by Suárez et al. a water–methanol mixture was used to synthesise ZnO nanosponges [35]. In another report a template-free, solvothermal synthesis route for the fabrication

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Fig. 2 SEM (a, b), TEM (c, d), HRTEM (inset: SAED pattern) (e), EDX pattern of Ni–Co2 –O HNSs (f). Reproduced from Ref. [17] with permission from The Royal Society of Chemistry

of ZnO nanosponge by employing methanol as a solvent was reported [36]. The same group reported the synthesis of ZnO-EuO1.5 nanosponge using methanol as a precursor [37]. Another research group reported hydrothermal synthesis of NiO nanosponge [38]. In 2021, Golubeva et al., reported a hydrothermal synthesis of aluminosilicate nanosponge [Al2 Si2 O5 (OH)4 ] [39].

3.2 Sol–Gel Method ‘Sol’ refers to the colloidal dispersion of metal precursor in a solvent which converts into ‘gel’ after condensation and polymerisation. Basically, the metal precursor

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Table 1 Various advantages and disadvantages of the different synthesis methods for metal oxide nanosponge Method

Advantages

Disadvantages

Solvothermal

High-quality crystallised nanostructures are obtained

High conditions of temperature and pressure are required

No harmful solvents are used in the hydrothermal route

The as-obtained product needs to be annealed for acquiring metal oxide

Facile approach

Agglomeration can occur

Sol–Gel

Particle size and morphology can be controlled Precipitation method

Simplest method

Limitations associated with reducing agents like toxicity, impurity, and high cost

Electrochemical deposition

Fast and cost-effective method

High maintenance of the electrochemical apparatus

Size and morphology can be controlled

undergoes hydrolysis and network bridges of metal oxides are formed due to condensation and polymerisation. In the synthesis process, the metal precursor is first dissolved in water or an organic solvent or a miscible mixture of water-organic solvent. Agglomeration can be avoided by adjusting the pH with the help of an acid or a base. Further, to obtain gel, it is subjected to ageing and thermal treatment. The gel can be dried by evaporation or super-critical drying. The removal of organic residue is obtained by calcination and a porous metal oxide nanosponge is obtained. In 2012, Yu et al., synthesised Ag-doped TiO2 nanosponge by using cellulose as a template with the sol–gel method [40]. Another research group reported the fabrication of TiO2 nanosponge by an inspired biotemplate filter paper using the sol–gel method [41]. Gold and Palladium nanoparticles of controllable size were immobilised on TiO2 nanosponge to obtain Au/TiO2 and Pd/TiO2 nanosponge respectively (Fig. 3). In another study, phosphorylated multiwalled carbon nanotube- βcyclodextrin nanosponge decorated with TiO2 and Ag nanoparticles (pMWCNTβCD/TiO2 -Ag) nanosponge was synthesised by sol–gel method [42]. Recently in 2022, Yadav et al., reported a sol–gel method synthesis for incorporation of CuO/ZnO nanoparticles into β-cyclodextrin nanosponge to yield β-CD-CuO/ZnO nanosponge composite (Fig. 4) [43].

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Fig. 3 Sol–gel method for synthesis of Au/TiO2 and Pd/TiO2 nanosponge

Fig. 4 Synthesis of β-CD-CuO/ZnO nanosponge using sol–gel method. Replicated from Ref. [43] with the permission from Elsevier Ltd.

3.3 Precipitation Method It is one of the most used methods for the synthesis of porous metal oxide nanosponge. In a typical process, to an aqueous solution of metal precursor, a reducing agent like NaBH4 is added followed by calcination in a muffle furnace to yield porous metal oxide nanosponge [44]. The presence of a reducing agent is crucial for the creation of a porous network of metal oxide precursors. However, the precipitation method

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Fig. 5 Fe3 O4 /CDNS-FA nanosponge. Reproduced from [45] with the permission of Elsevier Ltd.

has a few disadvantages associated with it like the variable pore size of the metal oxide because of uncontrolled precipitation of the metal salt. Three-dimensional hollow Ni–Co oxide hollow nanosponge was synthesised by facile precipitation method followed by reduction with NaBH4 and annealing in the air [17]. In another study by Raja et al., Co3 O4 /NiO nanosponge was fabricated by using the precipitation method [19]. Gholibegloo et al., reported the synthesis of folic acid decorated magnetic nanosponge [45]. In this study, cyclodextrin-based nanosponge (CDNS) was anchored with magnetic Fe3 O4 particles synthesized by the co-precipitation method. The nanosponge was further decorated with folic acid (FA) to finally yield Fe3 O4 /CDNS-FA (Fig. 5). Lu et al., synthesized Cux O/carbonised nanosponge by precipitation method followed by calcination [12].

3.4 Electrochemical Deposition In the electrochemical deposition, the reaction occurs at the electrode/electrolyte interface initiated by a current flow [46]. This method is generally used for synthesizing porous thin films deposited on carbon1aceous or metallic templates. The properties of the film can be tailored by a variation in temperature, H+ ion concentration, and current density. A series of TiO2 nanofoam-nanotube arrays has been reported by tunning the anodic voltage and electrochemical reaction time via a two-step anodic oxidation reaction as shown in Fig. 6 [47]. Batista-Grau et al., reported the influence of bicarbonate electrolytes and anodising under hydrodynamic and stagnant conditions were

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Fig. 6 Anodic oxidation reaction to yield TiO2 nanofoam-nanotube. Reproduced with the permission of the publisher [47]. Copyright 2018 American Chemical Society

studied [48]. The FE-SEM images revealed that the addition of ethanol or glycerol affected the morphology and size of the structures. The authors reported that under stagnant conditions 10% v/v glycerol leads to the formation of nanotubes with flower-like morphology, while, under hydrodynamic conditions, nanowires are obtained. When 25% v/v glycerol containing electrolyte yields nanowires at stagnant conditions and ill-defined nanosponges at hydrodynamic conditions. Navarro-Gázquez et al., synthesized bimetallic TiO2 /ZnO nanosponge by electrochemical anodization of TiO2 followed by electrodeposition of ZnO [49]. The structural and morphological characterization of synthesized nanosponge was performed using a variety of microscopic and spectroscopic methods. The FE-SEM images exhibited two distinct morphologies at different concentration ranges (Fig. 7). Hexagonal nanorods of zinc oxide with a height of 120–190 nm and diameters of 17–19 nm oriented transversely were observed at 40 mM Zn(NO3 )2 concentration (Fig. 7d). However, 50 mm concentration onwards growth proceeded both longitudinally and transversely.

4 Applications of Metal Oxide-Based Nanosponges in Water Treatment Since the past decade, metal oxide nanosponges have been employed for the purpose of water remediation by the elimination of obnoxious organic and inorganic pollutants. Metal oxide-based nanosponges can efficiently remove harmful pollutants from water by adsorption as well as by photocatalysis. For instance, TiO2 /Ag nanosponge was fabricated by sol–gel method and used for photocatalytic degradation of Rhodamine B dye and salicylic acid [40]. The experimental results show that

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Fig. 7 FE-SEM images of anodized TiO2 /ZnO hybrid samples. Images captured at two different magnifications (1 μm and 200 nm) over the shown concentration range (a–f). Reproduced from Ref. [49] with the permission of publisher Elsevier Ltd.

the bimetallic TiO2 /Ag nanosponge exhibits better photocatalytic activity than pure TiO2 nanosponge and TiO2 nanoparticles (Fig. 8). The superior performance of TiO2 / Ag nanosponge has been accredited to the unique nanosponge morphology of the composite, uniform dispersion of Ag nanoparticles and strong coherent interaction between Ag and TiO2 . In another study, Au/TiO2 nanosponge was used for the reduction of paraNitrophenol into para-Aminophenol in the presence of NaBH4 as a reducing agent [41]. It was found that TiO2 nanosponge alone is not able to catalyse the reaction of reduction of para-Nitrophenol to para-Aminophenol even after 30 min. On the contrary, Au/TiO2 nanosponge was able to catalyse the reaction in 420 s. Moreover,

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Fig. 8 Photocatalytic degradation with as-prepared TiO2 /Ag (1.1, 3.2, 5.1, and 8.3%), pure TiO2 nanosponge, P25 and TiO2 powder of a the Rhodamine B dye (25 mg/L, 90 mL) and b the aqueous salicylic acid. Reprinted with permission from Ref. [40]. Copyright 2012 American Chemical Society

the Au/TiO2 nanosponge exhibited high reusability and recyclability up to 5 cycles without any significant decrease in performance. A novel Co3 O4 /NiO nanosponge was fabricated by another research group to study the photocatalytic degradation of organic pollutants [19]. Rhodamine B and Congo Red dyes were selected as targetted pollutants to gauge the photocatalytic degradation capability of the as-synthesized nanosponge. It was found that Co3 O4 / NiO nanosponge exhibits higher catalytic activity than individual Co3 O4 and NiO nanoparticles. The enhanced catalytic activity was ascribed to the nanosponge-like morphology which offered a higher surface area for the adsorption of a large number of photons and consequently the formation of a greater number of electron–hole pairs. Around 97% degradation of the dyes was achieved in 80 min and the main radical species responsible for causing degradation was found to be OH· Taka et al. synthesized pMWCNT-βCD/TiO2 -Ag nanosponge for the adsorptive removal of metal ions from the aqueous solution [42]. The authors executed the removal of lead and cobalt metals to assess the performance of the as-synthesized nanosponge. It was found that pMWCNT-βCD/TiO2 -Ag nanosponge exhibited higher adsorption capacity than pMWCNT-βCD and CD alone because of a larger number of adsorption sites available on the surface. About 99.67 and 99.18% of lead and cobalt were removed respectively. Later, Taka et al., employed pMWCNT-βCD/TiO2 -Ag nanosponge for the adsorptive removal of trichloroethylene and Congo Red dye [50]. The adsorption process was found to be endothermic in nature and the adsorption increased with increasing temperature. This was due to a higher number of adsorption sites and enhanced mobility of the adsorbate species due to an increase in the number of pores on pMWCNT-βCD/TiO2 -Ag nanosponge. Furthermore, the investigation of kinetics data revealed that the adsorption process follows pseudo-second-order kinetics. Adsorption studies at various pH revealed that for Congo Red dye, the adsorption decreases with an increase in pH. This is because of electrostatic repulsion at higher

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pH. Whereas, for trichloroethylene, adsorption increases with an increase in pH. This can be attributed to Van der Waals forces between adsorbate and adsorbent and the non-ionic nature of trichloroethylene. Hickman et al., incorporated polydimethylsiloxane (PDMS) structure and TiO2 nanocatalyst to fabricate TiO2 -PDMS composite nanosponge [51]. TiO2 -PDMS nanosponge was used effectively for the removal of Rhodamine B dye from water. Further investigations revealed that the Rhodamine B dye is removed due to the synergistic effect of adsorption and photocatalytic degradation. TiO2 -PDMS nanosponge can remove 50% of the dye even in the absence of light by adsorption. After exposure to sunlight, a total of 80% dye is removed by the TiO2 -PDMS composite nanosponge. The results are comparable to that of TiO2 , but TiO2 -PDMS composite nanosponge offers various advantages like it is easily separable from the solution, unlike the powder TiO2 and, the nanosponge can be easily regenerated by exposing it to the sunlight. Marin et al., synthesized Eu3+ -doped ZnO nanosponge for photocatalytic degradation of Rhodamine B dye in aqueous solution [52]. The authors studied the effect of different annealing temperatures and concentrations of dopant Eu3+ on the photocatalytic activity of the Eu3+ -doped ZnO nanosponge. It was established that the undoped ZnO nanosponge annealed at 800 °C exhibited the highest catalytic activity whereas, Eu3+ doped ZnO nanosponge showcased enhanced catalytic 1activity at annealing temperatures 400 and 1000 °C (Table 2). The photocatalytic degradation followed first-order kinetics model. In another study by Naik et al., 2-D Ni1−x O nanoflakes and 3-D Ni1−x O nanosponge were fabricated by direct calcination of Ni(II) coordination compound of 4-Nitrobenzoate [53]. Amaranth dye was chosen as the targeted water pollutant to study the removal of organic compounds by adsorption using the as-synthesized nanosponge. It was found that 3-D Ni1−x O nanosponge exhibited the best results and was able to adsorb Amaranth dye with 82.81% efficiency. The better performance of 3-D nanosponge over 2-D nanoflakes can be attributed to its high surface area, more Table 2 Rate constant of Rhodamine B degradation with the doping percentage of Eu3+ doped ZnO nanosponges at various annealing temperaturesa Eu3+ doping Annealing T (°C)

0% (min−1 )

0.5% (min−1 )

1% (min−1 )

2% (min−1 )

5% (min−1 )

200

0.019(1)

400

0.036(1)

0.052(2)

0.088(2)

0.075(2)

0.063(2)

600

0.148(5)

0.085(2)

0.110(6)

0.066(2)

0.036(4)

800

0.219(11)

0.141(4)

0.116(10)

0.135(4)

1000

0.091(5) 0.091(6)

900 0.115(10)

0.076(3)

0.151(11)

Reproduced from Ref. [52] with permission of The Royal Society of Chemistry a Standard errors of the linear fits are given in parentheses

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active surface sites and interconnected mesoporous spongy nature which allows easy diffusion of the dye molecules. The adsorption mechanism was studied by applying various adsorption models and it was found that the Sips isotherm model is followed. The adsorption kinetics followed pseudo-first order kinetics model indicating physical interaction between adsorption sites at the surface of the nanosponge and the dye molecules. However, the R2 values of pseudo-second order kinetics were also reasonably high indicating chemical interaction between dye molecules and Ni1−x O nanosponge. This indicates chemisorption as well as the physisorption nature of the adsorption process. Salazar et al., synthesized β-cyclodextrin based carbonate nanosponges decorated with Fe3 O4 nanoparticles and studied the removal of dinotefuran (DTF) insecticide by adsorption [54]. The results showed that the as-synthesized nanosponge was able to remove 100% of the insecticide up to the eighth reusability cycle. In another report, [Al2 Si2 O5 (OH)4 ] nanosponge were used for adsorptive removal of cationic (Methylene Blue) and anionic dye (Azorubine) from wastewater [39]. The authors compared the adsorption performance of [Al2 Si2 O5 (OH)4 ] possessing various morphologies and established that the nanosponge morphology exhibited the highest adsorption performance. This can be ascribed to the higher surface area of the nanosponge morphology than that of spherical morphology and raw kaolinite. Moreover, [Al2 Si2 O5 (OH)4 ] nanosponge was able to successfully adsorb both cationic and anionic dye from the water. Periyayya et al., reported an in-situ conversion of Cu/CuO nanodisk into Cu/CuO/ Cu2 O nanosponge upon irradiation with visible light [55]. The synthesized material was employed for the photocatalytic decomposition of Rhodamine B and Methylene Blue dyes in an aqueous solution. The authors found that the in-situ transformation of CuO nanodisk into Cu2 O nanosponge accelerates the transfer of electrons between the conduction band of Cu2 O and CuO phases. This further increases the breakdown of dyes due to more favourable electron–hole separation. Approximately 87% of Rhodamine B dye and 94% of Methylene Blue dye were degraded after 80 min exposure to daylight. Yadav et al., employed β-CD-CuO/ZnO nanosponge composite for photocatalytic degradation of Malachite Green and Methylene Blue dyes [43]. It was observed that the photocatalytic activity of β-CD-CuO/ZnO nanosponge is higher than compared to that of the CuO/ZnO composite. It was found that the nanosponge composite can degrade up to 89.15% of Methylene Blue dye and 79.90% of Malachite Green dye which was higher than that of the CuO/ZnO composite. First order kinetics model was followed by the degradation process. The greater performance can be attributed to the improvisation of electron–hole pairs due to the addition of β-CD and also to the hydrophilic exterior and hydrophobic interior cavity of the β-CD which can encapsulate pollutant molecules (Fig. 9). Oksuz et al., fabricated ZnO nanoflower, nanowires and nanosponge to study the photocatalytic activity by degradation of Methylene Blue dye [56]. The best degradation results were shown by ZnO nanowires due to their dense and homogeneously dispersed structure.

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Fig. 9 Structural representation of β-CD. Reprinted from Ref. [43] with the permission of publisher Elsevier Ltd.

5 Conclusions and Future Challenges In this review, the synthesis, and applications of metal oxide nanosponges have been summarized. The metal oxide nanosponges exhibit various superior qualities like high porosity, robustness, facile synthesis, easy functionalisation, and recyclability, thus making them worthy candidates for the removal of hazardous pollutants from water. Notably, metal oxide-based nanosponges, with characteristic physical as well as chemical properties and unique architecture, entailing vast surface area, low toxicity and facile surface modifications can be effectively utilised for the effective removal of organic as well as inorganic water pollutants from the wastewater by photocatalytic degradation and/or adsorption. Moreover, high reusability and recyclability owing to the mechanical strength of the metal oxide nanosponges make them an attractive choice for the purpose of water remediation. Further, bimetallic, or doped metal oxide nanosponges exhibit better performance as compared to monometallic nanosponges. However, to address the various challenges like adsorption mechanism, electrostatic and hydrophobic interactions with the targeted pollutant, nature of degraded products and toxicity issues, more elaborative studies are required. Further, factors which directly affect the performance of the metal oxide nanosponge like dosage, the effect of contact time, competing ions, solution pH, and isotherm models need to be addressed carefully for optimisation of the performance of the material. Most of the studies have targeted conventional water pollutants like textile dyes, para-nitrophenol, etc. Therefore, more studies focussing on the removal of emerging water pollutants like oxygen-demanding waste, oil, grease, pharmaceutical by-products, and octanoic acid derivatives are warranted. The other environmental remediation applications of metal oxide nanosponges like air purification, and soil purification should also be explored and not just limited to water purification. Despite proving the worthiness and remarkable properties of metal oxide nanosponges, their large-scale application for water treatment is still not acceptable. This might be due to the unattended concern of the toxicity and metal ion leaching associated with the use of metal oxide nanosponges. Though metal oxide nanosponges have proved to be an excellent choice of materials for water treatment, further studies require deeper

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investigations and are in the juvenile stage yet. Future research should focus on synthesizing tailored metal oxide nanosponges with alterations in the manufacturing of the nanomaterials. This review can be of assistance for comprehending various aspects of metal oxide nanosponges employed for water treatment and can enable the researchers for maximising the potential of metal oxide nanosponges.

List of Abbreviations CDNS EDX FA FE-SEM HRTEM Ni–Co2 –O HNSs pMWCNT-βCD SAED SEM TEM β-CD

Cyclodextrin-Based Nanosponge Energy Dispersive X-Ray Folic Acid Field Emission Scanning Electron Microscopy High-resolution transmission electron microscopy Nickel-Cobalt Oxide-Based Hollow Nanosponges Phosphorylated Multiwalled Carbon Nanotube- β Cyclodextrin Selected Area Electron Diffraction Scanning Electron Microscopy Transmission Electron Microscopy β-Cyclodextrin

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Synthesis and Application of Metal and Metal Oxide-Based Nanosponges as Sensors Vijay Beniwal, Naveen Sharma, and Jyoti Jain

1 Introduction Nanotechnology offers the vital prospects required in the fields of science and industry. Applications of nanotechnology are well established in diverse fields such as energy, electronics, space exploration, aeronautics, environmental remediation, and medicine among many others [1]. Synergistic results for the advancement of nanotechnology have been achieved with the help of collaborative research among different technologies. The classic products of this collaborative research can be best exemplified in the form of nanosensing and nanobiocatalysis [2]. Materials with at least one dimension between 1 and 100 nm are referred to as nanomaterials. Based on a few dimensions that are not in the nanoscale range, these materials are divided into 0-D, 1-D, and 2-D nanomaterials, known as nanospheres, nanorods, and nanosheets, respectively [3]. Here, it is crucial to emphasise the fact that several applications in the domains of electronics and energy storage, construction materials, food packaging and processing, biomedical and healthcare, and nanosensors have been developed with the help of nanomaterials using a variety of classes of functional materials like metal, metal oxide, carbon, polymers, and semiconductors [4]. Nanosensors represent one of the most important applications of nanomaterials, which, as explained in the next sections, are employed for the detection of a range of analytes.

V. Beniwal · N. Sharma · J. Jain (B) University of Rajasthan, Jaipur, Rajasthan 302004, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Gulati (ed.), Nanosponges for Environmental Remediation, https://doi.org/10.1007/978-3-031-41077-2_14

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2 Nanosensors Nanosensors represent a class of devices that either use nanotechnology or nanomaterials for the sensing and measurement of changes in the chemical, physical, and biological environment at the nanoscale range. Nanosensors are equipped with remarkable technology that offers unique detection capabilities for the identification of a variety of analytes, including pathogens, environmental contaminants, biomolecules, and other specific and high-sensitivity cells and tissues [5–9]. These devices can be engineered as per the requirements of the analyte for their detection and sensing purposes, for single analyte as well as for multiple analyte, which is known as multiplex detection. This is the main application of nanosensors in the area of medicines, safety of food, monitoring of environment, and security, etc. Specific interest in the field of nanosensors has been developed recently, owing to the proficiency of these devices for direct and on-site sensing of contaminants in various types of samples, without the requirement of costly laboratory equipment [10]. Nanosensors mainly consist of three components, a nanomaterial or a combination of nanomaterials for the amplification of the signal, a recognition element with very high specificity, and a signal transducer that has a mechanism for the detection of the presence of an analyte and signalling. The transducer works with detection principles based on several signal kinds, including optical, thermal, electrochemical, or piezoelectric. Different components of the nanosensor are represented diagrammatically in Fig. 1. These components of the nanosensor device may not necessarily be separate entities; however, the classification of nanosensing devices is generally done on the basis of these three components. In addition to producing a signal, some of the sensors, can also operate on the ‘turn-on’ or ‘off/on’ principle. In such devices, the reduction of the signal indicates the presence of the analyte. Advancements in sensor technology have been assisted by the development of nanomaterials. The development of portable sensors with reduced sizes and quick signal responses is made possible by the use of nanomaterials. The large surface area per volume of the nanomaterials facilitates the surface functionalization of these materials, and due to this, nanomaterials are especially responsive to variations in the surface chemistry. This allows the exceedingly low detection limit of the nanosensors. The greater sensitivity of nanosensors in some cases is also attributed due to the fact that the nanomaterials are similar to the size of the target analytes, like, biomolecules, antibodies, pathogens,

Fig. 1 Principal components of the sensor, a recognition element, b transducer and c signal amplifier

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metal ions, or DNA. As a result, nanosensors may now be utilised to detect incredibly tiny analytes that were previously inaccessible by conventional sensing technologies [8, 10].

3 Advantages of Nanosensors Advancements in the field of nanotechnology have led to the production of nanosensors that are sensitive, flexible, specific, and versatile. The main goal of the nanosensors is the screening and measurement of physical, chemical, and mechanical changes that are associated with an interest marker. Various types of sensing technologies can be incorporated into many systems, such as labs-on-a-chip, for the simplification of detection methods. Nanotechnology, by way of its different properties, which include compact instrumentation, high sensitivity, and speed, can help the expansion of the existing range of analytical detection limits. Nanoscale materials allow multiplexing in addition to being selective as well as being cost-effective [11]. Integration of ultrasensitive nanosensors with detection phenomena and other instruments can enlarge the proficiency of new nanotechnology to address prevalent detection systems like point-of-care type systems [12, 13]. There are many ways to name the nanosensors, e.g., optical, electrical, and, mechanical which are manufactured by different pathways. Nanosensing is a fascinating and active area of research, as it is in a nascent stage as of yet, is cross disciplinary and has a wide-range of applications [14]. Nanosensors are devices that are based on the nanoparticle and sense a variety of signals in the form of force, electrochemical, or biological substances. The specific nature of these nanosensors is imparted in the form of targeting ligands, which are conjugated directly to the nanoparticles in use. These ligands attract a particular marker of interest, also known as the analyte, which depends on the functionality of the ligand. Nanoparticles, on the other hand, transform signals from one form to another, function as a detector for the signals produced, and also increase sensitivity [15, 16]. In this chapter, we are predominantly concerned with the nanosensors that are utilised for environmental monitoring and remediation. So, we will discuss the recent advancements made in the development of environmental monitoring sensors and equipment, for example, the detection of heavy metal ions, toxic gases, pesticides, insecticides, and industrial wastewater, using different classes of nanomaterials, such as, silicon-based nanoparticles, carbon-based nanoparticles, metal/ metal-oxide nanoparticles, semiconductors, and other nanomaterials.

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4 Nanosensors for Environmental Monitoring The agricultural and industrial activities of the humans are responsible for many forms of environmental contamination, including soil, water, and air pollution, and pose a significant danger to the ecosystem as well as to human health. The pollutants responsible for the environmental imbalance include toxic gases (for example, the oxides of nitrogen (NOx), sulphur dioxide (SO2 ), hydrochloric acid (HCl) etc.), organophosphorus compounds (such as pesticides and insecticides), hazardous metal ions (like As3+ , Pb2+ , Hg2+ , Cd2+ ) and industrial and domestic wastewater (such as H2 O2 and phenol). Exposure of the environment to these pollutants directly or indirectly can cause substantial damage to our ecosystem and the environment [17, 18]. This causes danger to human health and environmental security. For instance, heavy metal contamination has become a progressively severe environmental problem in today’s world. This contamination is responsible for different ecological disorders and accumulations in the food chain. In addition to this, the consumption of food and water contaminated with heavy metals in the long lasting results in the accumulation of the metals in the bodies of living organisms and causes chronic diseases. In light of this, it has become extremely crucial to develop reliable, selective, sensitive and inexpensive methods for the identifying and keeping track of environmental toxins to mitigate the harmful effects of these pollutants [17]. Various technologies like high-performance liquid chromatography (HPLC), inductively coupled plasma mass spectrometry (ICP-MS), surface plasmon resonance (SPR) and gas chromatography-mass spectrometry (GC-MS) are already established in the literature for the monitoring of environmental pollutions [19– 22]. These traditional diagnostic methods are regarded as “gold standards” for the monitoring of environmental pollution and are capable enough of providing good output efficacy; however they are very costly and time-tacking because of the big machinery size and sophisticated sample preparation techniques, making them not appropriate for regular detection work [23–27]. Since a few years ago, sensors have appeared as a promising alternative for environmental monitoring because they are inexpensive, simple to prepare, and very sensitive and selective. Nanomaterials with various dimensions, forms, and compositions display distinctive catalytic, physical, optical, chemical, and electronic properties, which makes them ideal for sensor applications. Due to this, one of the most active areas of research in the analytical chemistry sector is the development of sensors based on nanomaterials. Moreover, to attain successful point of- care devices in developing countries, it is needed that the sensors used for environmental monitoring are sensitive, cheap, easy to be operated in different environmental conditions and able to detect multiple analytes [11]. Many of these issues can be solved with the help of nanotechnology which is already playing a central role in the construction of sensors with outstanding properties [16].

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4.1 Application of Nanomaterials for the Detection of Analytes Related to the Environment Nanomaterials have displayed significant potential for environmental monitoring’s field of trace pollutant detection, because of their highly reactive surface, large surface area per volume, great catalytic efficiency, and robust adsorption capability. Nanomaterials assist a variety of functions in sensor design. For example, the basic principle of colorimetric sensors is the changes in color of the nanomaterials i.e., silver nanoparticles, and gold nanoparticles. Nanomaterials with enhance surface area along with very good adsorption capacity, like CNTs, and grapheme, may be utilized to effectively capture the target markers or to enhance the signals used for detection. In addition to this, some of the nanomaterials, such as silicon nanowires and CNTs have the ability to be functionalized readily, which are utilised to create nanosensors that can detect very sensitive pollutants [28]. So, it can be summarized that sensors made from nanomaterials (nanosensors) offer significant benefits over traditional sensors which is attributed to their small size, high sensitivity and selectivity, and quick response time. The scope of nanomaterialbased techniques for environmental monitoring has been extended significantly with the help of a combination of these techniques with advanced analytical techniques, like electrochemistry, surface-enhanced Raman scattering (SERS), field-effect transistor (FET), fluorescence and colorimetric methods. Nanomaterial-based sensing apparatus have been developed for the monitoring of environmental contaminants present in air, water, and soil. These instruments use a variety of nanomaterials with high sensitivity and selectivity for the detection and measurement of toxic gases, toxic metal ions, pesticides, insecticides, and harmful industrial waste chemicals, including carbon nanotubes, silicon nanowires, gold nanoparticles, and quantum dots. The difficulties associated with the environmental pollution detection and prevention technologies have been generated, and these novel sensing technologies provide an exciting answer and in-depth research to get accurate and dependable outcomes.

4.2 Nanostructures and Associated Characteristics The often utilized nanostructures for the creation of nanosensors comprise nanoparticles, quantum dots, nanowires, nanotubes, nanorods, nanopores, nanowires, nanosponges and graphene [29]. Different applications of the nanomaterials according to here are measurements and forms given in Table 1. Though the list provided in Table 1 is not complete as there are so many nanoparticles which have already been developed in the literature and new ones are being explored, however it provides a brief introduction of different types of shapes and sizes. For the advancements of the nanotechnology, it is extremely important to thoroughly investigate the nanoparticles having novel and unique functionalities. Technological improvements in the sensor devices can be made with the help of

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Table 1 Different shapes of nanomaterials and their applications Nanostructure Description

Applications

Nanoparticles

0-D nanomaterials with extremely high surface to volume Chemical, ratio in addition to quantum dot effects biological and environmental sensing

Nanotubes

1-D nanomaterials with cylindrical shapes made of carbon or other materials possess exceptional electrical and mechanical properties

Nanopores

Materials with tiny openings or pores of nanoscale range. DNA sequencing, Used for the detection of changes in ionic currents or drug screening molecular interactions

Nanorods

Elongated structures with an aspect ratio greater than 1, possess unique optical and electronic properties

Photo detection, chemical and biological sensing

Nanosponges

Porous materials with intersected voids or cavities of nanoscale range capable to absorb or encapsulate various substances. High surface area and selective absorption properties

Drug delivery systems, environmental and gas sensing

Nanowires

Thin and elongated structures with nanoscale diameters. Offer extremely high surface area as well as high sensitivity

Chemical, biological, and gas sensing

Graphene

Carbon atoms arranged in the form of a layer in a 2-Dhoneycomb like structure which exhibits remarkable electrical, thermal and mechanical properties

Biological, gas, strain and environmental sensing

Quantum dots Semiconductor materials with quantum confinement characteristic. Emit light of specific wavelengths corresponds to their size. Widely used in optical devices

Chemical, biological, gas, strain sensing

Fluorescence-based sensing, photo detection, and biological imaging

nanomaterials as these materials offer intriguing and unique properties for sensor devices viz., enhanced electrical conductivity, highly reactive nature, biocompatibility, quantum effects, optical properties, excellent magnetic properties, electronic properties, strength and considerably high surface area to volume ratio. Nanomaterials provide a larger surface that can be functionalized for example, the functionalization of silica or gold nanoparticles [30]. Certain functional molecules can be stabilized with the help of immobilization on nanoparticles as demonstrated with enzymes [31]. With the help of immobilization, the rates of directelectron-transfer reactions in the electro-active species have been accelerated in a suitable matrix [32]. For the case in point, nanoparticles own a property to sense the presence of analyte in a sample with extremely low quantity owing to the high surface area of these particles. The importance of the 2-D nanomaterials is also increasing in recent years, which is attributable to the electrical conductivity and quantum effects associated with these 2-D nanostructures. Among 2-D nanomaterials, graphene and its derivatives are particularly interesting [29].

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Unique properties of nanomaterials along with their functionalities permit the fabrication of advanced sensing devices. Besides this, the nanomaterial properties are tunable, as we can change the morphology (in the form of shape and size) as well as structural and chemical functionalities of the nanomaterials as per the requirement. For instance, nanotubes, thin platted films, nanowires, nanocantilevers, and nanorods impart high sensitivity, versatility and selectivity in nanosensors. Nanotechnology provides in comparable methods to engineer the sensing platforms which are specific and sensitive to various types of analytes. Nanosensors often involve smaller volumes in comparison to the conventional analytical tools and are robust [33]. Though the nanosensors offer unconventional and inimitable diagnostic approaches, they are not fully optimized in most of the cases for scaling up the fabrication process for commercial applications.

4.3 Types of Nano Sensors for Environmental Monitoring The nanosensors used for the environmental monitoring have been categorized into three different classes as per the target area where they are used such as, gas sensors, bio-sensors and sensors for other environmental pollutants.

Nanogassensors For the monitoring of environment it is extremely important to have all the information about different environmental parameters, presence of hazards chemicals and pollution levels that can affect our ecosystem [34]. To address this, the development of a gas sensor is particularly essential which consumes less power and has high sensitivity. The importance of gas sensors lies in the ability of these sensors to sense the harmful gases that can be injurious to the environment. Fundamental principle on which the gas sensors work is related to the variations in the electrical resistance, which arises by the change in concentration of the analyte molecules in the sensor devices [35–37]. Applications of the gas sensors are already established in the fields like environmental monitoring, medical diagnosis as well as in industrial and automotive waste detection processes, though, many of these chemi resistive sensors require high operational temperatures and high power to maintain the performance of gas sensors [38]. The performance of the gas sensors is evaluated on the basis of a variety of factors, which includes, detection limit that is the lowest possible concentration of analyte molecules the sensor can detect, the ability of the sensor to differentiate a specific gas from a mixture of gases that is known as selectivity, and the reversible nature of gas sensor which is demonstrated by the ability of the gas sensor to return back to its original position once the exposer to the target gas molecules is removed. In modern world, there is a much need for the precise monitoring of hazardous materials and pollution levels caused by them. The seepage of poisonous and explosive gases might cause the instant loss of human lives [39]. So, the advancements of gas

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Fig. 2 Proposed mechanism for the detection of NO2 gas by ZnO pellet gas sensor

sensors are extremely crucial for the prevention of accidental explosions in addition to trace the spread of poisonous and harmful gases to the environment. The devices which are used to sense the presence and the concentration of different types of gaseous substances are known as nano gas sensors. The morphology of the nanoparticles is the most important parameter for the fabrication of gas sensors. Nano gas sensors are widely investigated sensors due to their compactness, fast response time, low recovery times, cost effectiveness, compatibility with integrated circuits, and extremely low detection limits. Reduction in the size of particles in the range of nanometer scale escalates the ratio of surface area to volume, thus the surface sites responsible for gas adsorption gets exposed [40]. It is because of this, the gas sensing devices based on metal/metal-oxide nanospongematerials are adept for improved sensing properties.

Nanosponge Metal Oxide Based Gas Sensors Semiconductor materials based on metal oxides nano sponges possess unique physical properties, such as well-defined geometry, fine pores, large surface areas and remarkable stability. For instance, the process of gas sensing consists of an exchange of electrons between the target gas molecules and the interface of the material used for sensing of the gas. The transduction function arising from this exchange is closely associated with the microstructure of the semiconductor oxides [34, 41]. For example, the mechanism proposed for the detection of NO2 gas by the ZnO gas sensor can be represented as Fig. 2 [42]. Highly oxidizing nature of the gases like NO2 make them sensitive towards reduction and when these gases are exposed to the materials like ZnO, they tend to capture an electron from these materials. This process results in the reduction in the density of carrier, which in turn is responsible for the decrease in the electrical resistance

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of ZnO unless an equilibrium is reached. NO2 molecules combine with O− ions to form NO3 − ions. The microstructural features (like morphology and particle size) and the structure of the sensitive layer of the nano sponges are significantly important for the identification of the gas molecules. These materials are produced by the dealloying technique. This technique is based on the dissolution of an active component, selectively from a metal alloy comprising metals with varying chemical activities. This procedure produces a residue with a porous microstructure comprising struts and ligaments of the nanoscale range. Such network architectures improve the mechanical performance of these materials to a remarkable extent, making them appropriate for various applications as a functional, lightweight and good strength materials for sensing devices. Different types of gas sensors have been constructed with the help of metal oxide nano sponges, including SnO2 , TiO2 and ZnO for the detection of gaseous substances. Nano Sponge TiO2 based Gas Sensors 3-D morphology of TiO2 nanosponges allows the fabrication of sensors with very high sensitivity, leading to the construction of miniaturized and economical sensors. Because of this, 3-D TiO2 nano sponges have been investigated extensively in the various sensing devices. For example, TiO2 based nanosponges have been utilized for the detection of H2 gas as an ultra-sensitive sensing component, finding the applications in the field of fuel cells. Nano sponge TiO2 pads, consisting higher concentration of Ti ion have smaller pore size, narrower diameter for nanowires, thinner nanowalls and denser gel layer. This dependence of different feature of these materials on the concentration of Ti ion can be explored to modify the properties of nano sponges TiO2 such as morphology [43]. These nanosensors are extremely sensitive towards the H2 gas as suggested by their capability to detect the presence of H2 gas at extremely low concentrations, as low as 1 ppm. The sensitivity of these sensors can be further enhanced by increasing the size of TiO2 pads and connecting them in a series [43]. Gas Sensors Based on ZnO Nano Sponges Zinc oxide is a semiconductor material based on n-type metal oxides which has remarkable potential for the detection of various gases. It shows sensitivity to several gases like acetone, NH3 , ethanol and CO under different conditions. Furthermore, it has variety of desirable properties such as, high luminescence, larger electrochemical coupling coefficient, low dielectric constant and high stability in addition to being cost effective. However, the performance of these materials for sensing applications is limited owing to poor sensitivity, less selectivity and high functional temperature. Different technologies have been tried to overcome these limitations so that the sensor applications of the nanosponge ZnO materials can be developed which includes, element doping, organic–inorganic hybridization and tuning of size

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and morphology [44–46]. Several morphologies of ZnO materials have been synthesized for gas sensing applications, including nanoparticles, nanorods, nanowires, nanospheres and nanosheets. These morphologies possess different active sites and unique surface effect that can affect their sensing abilities significantly. Zinc oxide (ZnO) tetrapods have unique 3-D morphology and display remarkable properties that make them suitable in different device engineering applications. In a study of significance, ZnO tetrapods were synthesized with variable arm sizes with the help of flame transport method at different growth circumstances. To understand the synthesis of these materials, their physicochemical and structural properties were also examined. The results has shown that the gas sensors based on nanosponge ZnO materials are able to detect the NO2 gas selectively, in the concentrations as low as 8.56 ppb [42]. Gas Sensors Based on Au Doped ZnO nano Sponges Mesoporous ZnO nanospheres with a doping of gold (Au) particles have been synthesized with the help of a formaldehyde supported metal–ligand cross linking method through simple photo reduction method and have been used for the detection of ethanol. The mesoporous ZnO–Au nanospheres thus formed, have displayed exceptional sensitivity and selectivity towards ethanol with reduced functional temperature and quick response in comparison to pure ZnO microstructures. Especially, mesoporous ZnO loaded with 1.0%wt of Au has shown the optimal ethanol response at 200 °C, approximately 159 for 50 ppm. Likewise, the integration of Au-NPs on the surface of ZnO microstructures have shown enhanced response towards gaseous ether in comparison to pure mesoporous ZnO [47]. For these gas sensors the increasing temperature was found to be followed by the increasing gas response ability. Addition of impurities or doping to the metal oxides effectively increases the gas sensing properties in these materials. These hybrid materials generally show superior performance in comparison to the pure metal oxides. Incorporation of noble metals like Pt, Pd, Au and Ag can improve the selectivity and sensitivity of the sensors to a great extent. The improved sensing performance of the hybrid materials is attributed to their rough surface (responsible for high surface area) and open mesopores that facilitates the adsorption and desorption of ethanol on sensing layers. Besides this, the strong spillover effect of gold nanoparticles speeds up the surface catalytic oxidation and electron transfer [47]. Hence, an effective methodology to improve the gas sensing performance of metal oxide materials is the doping of noble metal in to them. Nano Sponge SnO2 Based Gas Sensors Nanomaterials based on SnO2 microstructures are extremely important for the devices used for gas sensing and electrochemical applications due to their exceptional properties [48]. SnO2 has a very high sensitivity for the target gases that makes it a popular choice for the applications related to gas sensing [49, 50]. A report has suggested that, out of four different metal oxide, viz., tungsten trioxide (WO3 ), tungsten dioxide (WO2 ), tin oxide (SnO2 ), and tungsten trioxide doped with tin (Sn-doped

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WO3 ), SnO2 thin films were observed to be the most effective material for the detection of gaseous acetone [51]. SnO2 can also detect the gases like CO2 , NOx , and HC at lower temperatures. Furthermore, SnO2 is adept for the detection of high concentrations of CO even at high temperatures, making it appropriate material for the sensing applications [52]. To enhance the performance of SnO2 based nanomaterials, special attention has been paid to produce well-ordered mesoporous structures [53]. These materials have uniform and well-connected pores, which facilitate the diffusion of molecules and offer ample active sites ascribed to the increased surface area. Gas sensor devices based on mesoporous SnO2 demonstrate remarkable detection capabilities for H2 S gas, with very high sensitivity and stability. This is accredited to the larger surface area, high density of active sites and well-connected mesopores in crystalline pore walls. Different methodologies have been applied for the synthesis of SnO2 nanomaterials, which includes spray drying, hydrothermal reaction, chemical vapor deposition, sol–gel procedures and the soft-templating method [54]. The soft-templating method involves the gathering of amphiphilic surfactants or block copolymers with organic or inorganic precursors. With the help of this method, a nanostructure with continuous porosity can be obtained with modifiable pore sizes. Noble Metal Doped SnO2 Nano Sponge Gas Sensors Addition of the noble metals to the SnO2 increases the sensitive nature of these materials. For example, mesostructured gold-doped SnO2 display high selectivity towards CO [52], and nickel-doped SnO2 sensors exhibit improved gas response in comparison to pure SnO2 based sensors [53]. In addition to this, highly-uniform SnO2 nanowires are used in the gas sensors that are fast, sensitive, stable, and reproducible and can also be incorporated into a multi-component array. In the case of cobalt-doped multilayer SnO2 , it displays a high response of about 41% towards CO, where as undoped SnO2 exhibits a weak response [55]. Likewise, copper-doped SnO2 films show improved response towards CO gas [56]. In a similar manner, Pd/Ptdoped SnO2 display improved response of the sensor towards methane (CH4 ) gas and also improves its stability through continuous cycles [57]. The Pd-SnO2 composite nanoporous materials also display significantly high methane gas sensing ability in comparison to the pure nanoparticles, showing long term stability and high repeatability [58]. SnO2 based nanomaterials with a doping of palladium (Pd) metal have been used for the sensing applications of carbon monoxide (CO) gas. For the synthesis of SnO2 NPs, chemical precipitation approach was employed whereas stannous chloride dihydrate was used as the starting material. In these samples palladium metal was added in the concentration range of about 0.1, 0.2 and 0.3% (by weight) into the SiO2 and the samples synthesized by this method were identified with the help of XRD and FESEM. The structure of SnO2 nanoparticles was found to be tetragonal as confirmed by XRD and the size of these particles was observed to be ranging from 7–20 nm as revealed by FESEM. SnO2 sensor doped with 0.2% Pd metal displayed improved sensitivity and efficiency for the detection of CO gas as shown by real-time sensor testing [59].

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Metal Nano Sponge Based Gas Sensors Nano sponge materials based on gold and palladium nanoparticles have been used for the production of gas sensors to detect the gases like CO, H2 and nitrogen oxides. Few recent reports have indicated the potential application of mesoporous silicon layers; incorporated with gold nanoparticles, for CO2 gas sensors [60]. Nano sponge sensors for the hydrogen gas have been synthesized by the stretching of Pd–Ni films equipped with nanogaps on an elastomeric substrate. These sponges have displayed better performance in regard to the sensitivity at high H2 concentrations, response time and H2 detection limits than that of pure palladium thin films. Specifically, the hydrogen sensing sponges based on Pd87.5 Ni12.5 , have displayed reversible on–off behaviors, ultra-high sensitivity and a low detection limit. These improvements can be ascribed to the reduction in width of nanogap and expanded volume of the Pd–Ni lattice. The study has observed the effects of addition of Ni to Pd and shown the optimal Ni concentration required for the best results [61]. Given the renewed interest in the field of H2 economy in recent times, the demand for highly sensitive H2 gas sensorsis increasing significantly. Hydrogen is highly flammable and explosive gas, and because of this the development of the gas sensors that can detect and respond towards a gas leakage very quickly and accurately is extremely crucial for safety purpose. Nanomaterials based on the Pd metal are widely used in the development of H2 gas sensors owing to the specifically high affinity of this metal towards H2 gas [62]. Among the Pd based nanosensors, the sensors related to the nanogap technique have been investigated more significantly, they can operate in an on–off manner, causing improved detection abilities, such as rapid response, high sensitivity, good reliability and short recovery time. Interestingly, Pd nanogaps prepared by the use of elastomeric substrate method leads to a substantial improvement in the performance of H2 gas sensor [63]. Now, it has become extremely important to emphasize the approaches to reduce the detection limits and to increase the sensitivity, stability and reliability of elastomeric substrates supported Pd nanogap-based H2 gas sensors. A sensor using the platinum bridges has been reported to detect the ammonia (NH3 ) in the concentration range as low as 100–1400 ppm. The device has shown approximately 4% response at 100 ppm level of ammonia gas at ambient temperature, with a response rate of 70%and response time of about 8 min [64]. For the advancement of the sensing capabilities, nanoporous gold can be utilized in place of solid gold having increased surface area. Nanoporous materials based on gold have abundant usage in the catalysis. These materials can be synthesized via alloying of gold with less noble metals which are removed subsequently with the help of etchant, resulting in the formation of porous gold structure known as gold nanosponges [65]. Nanosponge Gas Sensors Based on Platinum Metal Platinum (Pt), is a noble metal and has widespread applications in the chemical, pharmaceutical, petrochemical, automotive and electronic industries. This metal is exceedingly less sensitive towards the corrosion and possess excellent electrical and

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catalytic activity. Pt, in association with palladium and their alloys with other metals, has found tremendous applications as a detection layer in resistive metallic hydrogen sensors. For example, these materials have been used for the fabrication of optical, magnetic and mechanical H2 gas sensors to form the detection layers. Additionally these materials have also been used in the H2 gas sensors based on the work function and semiconductor resistance [66]. A successful construction of a nanoporous film of the Pt has been attained by the dealloying of thin films of Pt-Cu alloy. These films were coated on the microscopic glass substrates using the magnetron co-sputtering method. The gas detection abilities of these films have been investigated with a particular attention paid towards the temperature dependence and the concentration of H2 gas. The nanoporous Pt films used in this study, were able to sense the very low concentrations of H2 gas, making it appropriate for applications related to safety, detection of leakage of H2 gas and for diagnostic purpose by the detection of molecular H2 in the air coming out through the exhalation in the respiration process [67].

Nanobiosensors The traditional methods involving the bio sensing applications uses the detection of immunological attraction or the assessment of metabolic changes. Some of the molecular techniques like cell culture, spectroscopy, Blotting or sequencing results from the polymerase chain reaction generally require enough response time (which ranges from few hours to several days). Due to this reason, these techniques may not be able to deliver swift and clear results, in the cases where very high accuracy is needed to detect the specific pathogen and the toxins related to this pathogen. The techniques of the microbiology such as colony counting and cell culture are more time consuming in comparison to other advanced technologies, although these two techniques also possess some distinct benefits such as precise and unambiguous results. On the contrary to this, advances in PCR technology, also known as real time PCR, facilitates the reaction to be completed within a few hours. Owing to its sensitivity and selectivity, ELISA is a well-established and accepted method, however it is very costly and time consuming, because of the tedious reactions involved. Specific nature of the biosensors is determined by the presence of analytes such as short DNAs with complimentary strand and antibodies. Many of the techniques used for the sensing applications involve tedious methods for sample preparation for the handling of biological samples i.e., urine, blood and tissues [68–70]. Biosensors based on the nanotechnology, also known as nanobiosensors, have the potential to monitor various soil parameters, like temperature, humidity, and other components, thus facilitates the targeted usage of chemicals and water. Applications of the nano bio sensors include monitoring of metabolites in the body fluids, detection of micro-organisms in the samples and discovering the pathology of tissues like tumors [2]. To be able to detect the important molecules i.e., metabolites related to various diseases, pathogens, DNAs, RNAs, proteins and tumor cells, is extremely

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important for the analysis of the disease in the clinical setting as well as for environmental, agricultural and industrial research. Nanobiosensors can also be used to detect the agricultural contaminants for example, acetamiprid, phenols, dinosulfon, and organophosphate pesticides, with high degree of accuracy and sensitivity. These sensitive detection systems are extremely useful for discovery of biomarkers related to unusual disorders [33]. An overview of different nanomaterials used for biosensing is provided in Table 2. Application of the biocompatible and chemically inert bio nanomaterials significantly improves the specificity of detection measurements because of the fact that the dimensions of many of the microorganisms such as, bacteria, viruses and pathogens, are similar to that of bionanosensors. The biosensors based on the nanomaterials have proven to be efficient for measuring and detecting harmful chemicals in the environment and in food [70]. Traditional biosensors consist of three main parts: a receptor for biological recognition, such as an antibody, nucleic acid, enzyme, or cell; a transducer that converts the biological binding incident into a measureable signal; and a signal display or readout that shows the presence as well as the concentration of analyte molecules [71, 72]. Based on the mechanism of recognition, biosensors may be divided into two categories: biosensors based on bio catalysis and biosensors based on bio affinity. In the sensors based on bio catalytic activity, bio receptor (which can be an enzyme, a cell or a tissue) identifies the analyte and catalyses a process that results in analyte consumption. In contrast, the bio receptor (such as an antibody or aptamer) precisely binds to the analyte in biosensors based on bio affinity, where an equilibrium state is obtained. Metal and metal oxides nanosponges can serve as excellent materials for Table 2 Different types of nanoparticles which are used for the sensing of biological analytes with the help of various sensing methods (reproduced from Ref. [29]) Analytes

Nanoparticles

Element used for recognition

Sensing technology

Bacteria

Au nanoparticles

Oligonucleotide

Colorimetry

Bacteria

Nanoparticles with magnetic properties

Antibody

Magnetic susceptibility

Microorganism

Ag nanorods

Electrostatic attraction forces

Raman spectroscopy

Toxins

Semiconductor nanocrystals

Chain of nucleotides

Fluorescence

Spore

Nanoparticles doped with Lanthanides

EDTA

PL (photoluminescence)

Deoxyribonucleic acid

Nanoparticles with magnetic properties

Electrostatic attraction forces

Chain reaction of polymerase

Pathogenic analytes Heterogeneous nanowires

Antibody

PL (photoluminescence)

Microbacterium tuberculosis

Oligonucleotide

Electrical impedance

CNTs

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the fabrication of transducers in different biosensors, which allows the amplification of the signal produced by biological molecules and leads to the superior sensitivity of the devices [73, 74].

Noble Metal Based Nano Biosensors There are two principal noble metals which have been used extensively for the applications related to nano bio sensors viz., gold and platinum. Nano Bio Sensors Based on Nanosponge Gold Nanoporous gold materials exhibit interesting properties and offer a variety of uses. These materials possess remarkable surface area to volume ratio which makes them appropriate for effective catalysis. In addition to this, specific geometrical shapes, like nanorods, can improve the existing systems when assimilated into nanoporous gold materials, termed as nanosponges [75]. Gold nanosponges produced by the dealloying of thin Au/Ag alloy films are promising materials to ensure effective medication delivery. Also, the optical properties of gold nanosponges, that include highly improved fluorescence or massive Purcell factors, are particularly important for the sensor applications. Despite the technological potential of these materials, the optical properties of these materials are yet to be utilized fully. The gold nanosponges are composed of two phases, a gold phase, and an air phase. A study was performed to establish a connection between the morphology of gold nanoparticles, studied with the help of electron microscopy, and their optical scattering spectra, examined through dark field microscopy. From the perspective of biosensing applications, gold nanoparticles are equipped with a large surface area and have several plasmon resonances in the near-IR or visible spectrum. These properties of gold-based nanobiosensors enable them to be used in a variety of applications, including surface-enhanced IR spectroscopy and surface-enhanced Raman scattering [76]. Gold nanoparticles have emerged as a desirable platform for advanced sensing devices. These particles have distinct physical and chemical characteristics that set them apart from both bulk and atomic particles. Gold nanoparticles have great biocompatibility, enabling the addition of a variety of organic and biological ligands to them, and they are simple to synthesize using various chemical or physical processes. To meet the requirements, biosensor technologies employ a number of detecting techniques, including optical, magnetic, electrochemical, and mechanical. The detection system aims to be highly sensitive, accurate, and fast. Furthermore, another important parameter to be considered here-, is the integration of these platforms with electronics for multiplex biomarker detection and scalability. Out of these techniques, electrochemical biosensors are of special interest as they have been employed as powerful tools for bioanalysis, particularly for the detection of nucleic acids. Conventionally, planar gold electrodes are utilized in electrochemical DNA

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sensors; however, they have limitations in context of selectivity, sensitivity and detection limit. These limitations are a result of crowded arrangement of capture probes on the surface of electrodes, which hampers the availability of target molecules. On the contrary, nanoporous gold electrodes have emerged as a likely alternative [65]. Nanoporous gold electrodes assist the detection limits in the range of 10–100 nm, which can be further shifted by one order of magnitude using the nanoparticulate gold with coarsened morphology. The coarse nature of these particles is responsible for the enhancement of target penetration inside the porous network. These findings offer valuable insights into the effects of the structural features of nanoparticles on the transport mechanism of nucleic acids and small molecules. This also helps in the understanding of the performance of sensors in the form of hybridization efficiency, dynamic range, DNA grafting density and sensitivity [77]. Nano Bio Sensors Based on Platinum A simple, rapid, reliable and cost-effective glucose sensing technology is required for diverse applications, such as clinical diagnosis, the food sector, bioprocessing and the creation of environment friendly and renewable fuel cells. It has become clear that nanoporous Pt makes an excellent component for non-enzymatic glucose sensors. A variety of methods have been employed for the fabrication of porous Pt films, for instance, sono electrochemical synthesis, template synthesis and electrochemical dealloying. Out of these methods, template synthesis method has been able to gain a widespread acceptance. By selectively removing Cu from a Pt-Cu alloy, robust nanoporous platinum with a sizably large surface area has been created at ambient temperature. This fabrication method has been proven to be simple, efficient, adaptable, and reproducible [78]. This technique yields nanoporous platinum that exhibits remarkable stability and considerable glucose oxidation catalytic activity. It shows a acceptable performance in the terms of sensitivity, selectivity, linear range consistency and repeatability when used in the glucose sensors. The sensor can recognise the glucose concentrations effectively in standard serum samples, providing satisfactory results. Some of the applications such as food quality, clinical, and bioprocess, require the determination of glucose concentration at neutral pH and the Pt nanoporous materials comply well with these requirements. Moreover, the porous platinum material functions as a catalyst in the fuel cells. By using chloroplatinic acid/copper sulphate solutions to repeatedly electro deposit nanoporous Pt layer on top of screen-printed carbon electrodes, a glucose sensor has been created. While performing successive cycles of electrodeposition that range from 1.4 to − 0.6 V versus Ag/AgCl electrode, the electrode’s electrochemical surface area was shown to rise. With the aid of SEM, the surface morphology of the modified electrode was characterized. For the evaluation of electrocatalytic performance of these modified electrodes towards glucose, amperometry technique was employed in buffer solutions of phosphate at pH 7.4, having potential of 0.4 V versus Ag/AgCl. This sensor has displayed exceptional stability, quick response period of under 5 s and a linear range of indication ~ 13 mm [79]. The expanded electrochemical surface area leads to the improved

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selectivity for glucose, while the signals from other interferons get lowered. The results closely matched those of the commercial glucometer when the glucose sensor was used to quantify the level of glucose in samples of blood plasma, showing the reliability of this sensor for real-world applications. Application of nanoporous materials holds significant importance in the improvement of non-enzymatic glucose sensors. Porous materials are equipped with high ratio of surface to volume, which is particularly beneficial while working with platinum electrodes. The electrochemical activity of these electrodes is generally measured by the roughness factor, greater roughness corresponds to better electrochemical activity. This is in accordance to the Pletcher’s theory that highlights the significance of active sites for the adsorption of analytes [80]. Compared to its geometric surface area, nanoscale electrode materials have a remarkable large surface area. This feature is suitable for the surface bound reactions i.e., glucose oxidation which are kinetically controlled [79]. Platinum displays very high catalytic activity towards substances like glucose and hydrogen peroxide, however the use of a smooth platinum electrode for the detection of glucose also offers several disadvantages. The smooth platinum electrode’s unsuitability for detecting glucose has been attributed to a variety of issues. Firstly, the number of the attachment’s active sites of glucose is very limited in the case of smooth platinum electrode, which results in the poor sensitivity. Secondly, the surface of electrodes is prone to interfering species, like, amino acids, ascorbic acid, and chloride ions present in physiological solutions. These species get absorbed onto the platinum surface and are responsible for the limited access of the analytes to the platinum surface. In past few years, numerous resources have been used to the research on the application of nanoporous platinum for glucose sensing. Porous platinum has displayed a high degree of sensitivity and very good selectivity for glucose. In addition to this, the rough surface of these electrodes also resists the attachment of interfering species. Fabrication of the porous platinum electrodes can be done with the help of different methods, such as from nanoparticles, by electrodeposition or dealloying [78]. Of these techniques, dealloying, which entails the selective dissolution of one or more elements from an alloy, has proved to be the most practical way for the manufacture of nanoporous metals with high surface area. A nanoporous material with beneficial qualities is obtained by the selective removal of less noble component like copper, cobalt or silver from their alloys with platinum either through the chemical or electrochemical means. This results in the characteristics like self-supporting structure, high electrochemically active surface area, particle distribution of nanoscale range and increased roughness [81].

Metal Oxide Based Nanobiosensors Owing to the stability and biocompatibility of the metal oxide nanosponges such as TiO2 , ZnO, and FeO, these materials have been used for the development of the bio sensors and have found the applications in the form of DNA, glucose and enzyme sensors [82]. A recent study found that nanoporous ZnO is an excellent

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immobilisation medium for enzymes and proteins with acceptable biocompatibility. It was utilised to make direct electrochemical biosensors. ZnO based nanosheets have also proved to be a promising immobilization matrix for the construction of electrochemical biosensors and have found applications in the environmental analysis and biomedical detection methods. Organic–inorganic based hybrid nanomaterials support direct electron transfer processes, and by trapping metalloenzymes in the ZnO composite, it is possible to increase the bioactivity of these materials. Comparative studies have discovered that the porous ZnO microspheres based on nanosheet can offer major advantages for direct electron transfer over solid ZnO microspheres. Third-generation biosensors without mediators show good sensitivity and great repeatability along with low detection threshold, wide linear range, high long-term stability and quick detecting response of H2 O2 and NaNO2 [83].

Bio Nano Sensors Based on Metal/Metal-Oxide Nano Sponges The nano sponge materials prepared by copper, silver and gold have been employed for the bio sensors, owing to their unique electrical and optical characteristics. Application of the metal and metal oxide nano sponges has the enormous potential in the biosensors because of enhanced sensitivity and specificity, which leads to the accurate and reliable detection of biological molecules. Functionalization of the metal and metal oxidesnano sponges can be done with different types of biomolecules like antibodies, enzymes or DNA to create the biosensors which are capable for the detection of a particular pathogen or biomarkers with high sensitivity and selectivity [72, 84, 85].

Sensors for Other Environmental Pollutants Along with gaseous and biological pollutants, various types of other pollutants are also present in the surroundings, including heavy metal ions, Organic pollutants, pesticides, and insecticides. Likewise, for the effective monitoring of the environment we need the data of all the environmental parameters such as temperature, pressure, humidity etc. So, the need of environmental sensors other than gaseous and biological sensors is quite obvious for the effective environmental monitoring. Nanosensing devices have recently been used for the monitoring of physical parameters like temperature as well as for the detection of the pollutants and the concentrations of chemical species. Nano sponges have the ability to efficiently adsorb the target molecules and to interact with them, which is attributed to their large surface area—to-volume ratio, causing much sensitivity, and selectivity of nano sponges. Moreover, these types of nano sponges offer some distinct advantages, such as high sensitivity, low cost, compact size, ease of production, simple measurement procedure and their capacity to find a variety of analytes [86, 87]. The examples of sensor devices that uses metal/metal oxide nano sponges for the sensing of environmental impurity present in water and soil samples are discussed in the following section.

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Sensors for the Detection of Hazardous Metals Application of the metal nano sponge using gold (Au) nanoparticle-based sensors have been studied for the detection of heavy (hazardous) metal ions like lead, cadmium and mercury in water and soil samples. Exceptionally large surface area and exclusive active surface of these materials make them efficient materials for the adsorption of heavy metal ions, responsible for extremely high sensitive and selective nature of these metal sensors [88–91].

Sensors for the Detection of Organic Pollutants Metal nano sponges based on the Pd and Pt nanoparticles have been used to identify organic contaminants in water and soil samples, such as polycyclic aromatic hydrocarbons. Functionalization of these materials can be done with specific receptors or antibodies that bind to target molecules, selectively. This enables thefinding organic contaminants in a specific manner [92, 93].

Sensors for the Detection of Water Quality Parameters Copper nanoparticles have been employed as the sensors for the measurement of water quality indicators including pH, dissolved oxygen, and temperature, in the form of metal nano sponges. Functionalization of these materials can be done with specific receptors or enzymes that are responsible for the catalysis of chemical reactions that help in the detection of water quality parameters [10, 94].

5 Conclusion It can be summarized that, metal nano sponge-based sensors that are used to detect pollutants and toxins show significant promise for environmental monitoring. They offer high selectivity and sensitivity and can be tailored for explicit environmental applications. These sensors have the ability to deliver reliable and accurate data for environmental monitoring. Addition of impurities or doping to the metal oxides effectively increases the ability of certain metal oxides to detect gases. Hybrid materials comprising mesoporous oxides generally show better results compared to the pure metal oxides. In addition to this, the incorporation of noble metals like Pt, Pd, Au and Ag can improve sensor selectivity and sensitivity to a great extent attributed to their sensitizing effects and high surface area. Hence, an effective methodology to improve the gas sensing performance of metal oxide materials is the doping of noble metal in them. Though, the nano sensors based on different metal/metal oxides and noble metal display a wide range of technological advancements over the conventional sensor devices, the full potential of these materials has not been utilized yet, as

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most of these nanosensors have not been scaled up at the commercial levels. In this regard, it becomes extremely important to perform the research that is focused on the commercial viability of these nanosensor equipments for environmental monitoring, so that low cost environmental sensors can be fabricated at the commercial levels. Acknowledgements V. Beniwal and J. Jain* , both acknowledge financial support by the University Grant Commission, MHRD, New Delhi, through the Strat Up grant (Award No. 30-572/2021 BSR).

Abbreviation HPLC ICP-MS SPR GC-MS SERS FET XRD FESEM PCR ELISA EDTA NPs CNTs SEM

High-performance liquid chromatography Inductive coupled plasma-mass spectrometry Surface plasmon resonance Gas chromatography-mass spectrometry Surface-enhanced Raman scattering Field-effect transistor X-ray diffraction Field emission scanning electron microscopy. Polymerase chain reaction Enzyme-linked immunosorbent assay Ethylenediaminetetraacetic acid Nanoparticles Carbon nano tubes Scanning electron microscopy

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Polymer-Based Nanobiocomposite as a Filter Nanosponge for Wastewater Remediation Shikha Gulati, Aashleshaa Mishra, Manan Rana, and Nabeela Ansari

1 Introduction Water pollution is a major environmental issue that poses significant threats to human health, aquatic life, and ecosystems. One of the primary sources of water pollution is untreated or poorly treated wastewater, which can contain various contaminants such as heavy metals, organic pollutants, and pathogens. The studies show that over the last sanctuary industrial chemical production has rose from 1 to 400 million tons [1]. Along with it, global industrial production has also increased from 1.2 billion tons to 3.2 billion tons over the last decade [1]. These growths and advances in the technological area have also increased the amount of waste produced by industries, thereby increasing the contaminants and pollutants in the environment. To address this problem, there is a growing need for efficient, cost-effective, and sustainable technologies for wastewater treatment and remediation. Wastewater remediation refers to the process of treating contaminated water to remove pollutants and make it safe for disposal or reuse. This process is essential for protecting public health and the environment, as untreated wastewater can contain harmful chemicals, pathogens, and other contaminants that can cause illness or damage ecosystems. There are various methods for wastewater remediation, including physical, chemical, and biological processes. Physical methods involve removing contaminants through filtration, sedimentation, or other physical means. Chemical methods involve S. Gulati (B) Department of Chemistry, Sri Venkateswara College, University of Delhi, Delhi 110021, India e-mail: [email protected] A. Mishra Department of Biological Sciences, Sri Venkateswara College, University of Delhi, Delhi 110021, India M. Rana · N. Ansari Department of Biochemistry, Sri Venkateswara College, University of Delhi, Delhi 110021, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Gulati (ed.), Nanosponges for Environmental Remediation, https://doi.org/10.1007/978-3-031-41077-2_15

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using chemicals to treat wastewater and remove contaminants, while biological methods use microorganisms to break down pollutants. Nowadays, advanced technologies, such as nanotechnology and membrane filtration, have also been developed to improve the efficiency of wastewater remediation. These technologies can provide higher levels of contaminant removal, reduce the cost of treatment, and enable the reuse of treated wastewater. This chapter discusses one of the promising approaches to wastewater treatment by using nanotechnology-based materials, specifically polymer-based nanobiocomposites (PNBs). To produce novel nanotechnology-based products to be used as wastewater purifiers, the materials should obey some fundamental properties like excellent filtration efficiency, affordability, porous structure, and ability to be reused [17] along with biocompatibility. A bionanocomposite is a composite material consisting of a biopolymer matrix, such as chitosan or cellulose, and nanoscale particles or fillers, such as nanoparticles, nanotubes, or graphene oxide. These nanofillers are incorporated into the biopolymer matrix to create a composite material with unique properties that can be tailored for specific applications. Bionanocomposites have gained significant attention recently due to their biocompatibility, biodegradability, and potential for various applications, including drug delivery, tissue engineering, and water treatment. PNBs as a filter nanosponge refer to a type of composite material that is designed to filter and remove contaminants from wastewater. They are designed like sponge-like materials with high porosity and surface area-to-volume ratio. This unique structure enables the nanobiocomposite to effectively adsorb and remove various types of contaminants, such as heavy metals, dyes, and organic pollutants, from wastewater. The term “filter nanosponge” refers to the ability of the material to act as a filter, similar to a sponge, by capturing and removing contaminants from wastewater. Despite their significant potential, the practical implementation of polymer-based nanobiocomposites for wastewater treatment and remediation faces several challenges. These include the need for cost-effective and scalable fabrication techniques, the potential for environmental impacts of nanomaterials, and the need for long-term sustainability.

2 Types of Nanobiocomposites On the basis of the shape of particle reinforcements, nano biocomposites can be categorized into three different classes: particulate (iso dimensional particles are used as reinforcements), elongated particle (carbon nanotubes (CNTs) and cellulose nanofibrils as reinforcement), and layered structure biocomposites [18] (Fig. 1). 1. Particulate nano biocomposite: These nano biocomposites are reinforced with dimensional particles, which have a low reinforcing impact. The low aspect ratio

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Fig. 1 Types of nanobiocomposites

is the reason for this. Additionally, the major goals of adopting these reinforcements are to bring down the price of composites, reduce permeability, and make them inflammable. Here are some examples of particulate bio-nanocomposites: (a) Cellulose nanocrystals (CNC) reinforced polymer composites: CNCs are rodlike nanoparticles extracted from cellulose fibers. They have high stiffness, strength, and aspect ratio, making them ideal for reinforcing polymers. CNCreinforced composites have potential applications in packaging, automotive, and biomedical industries. (b) Chitin nanofibers reinforced polymer composites: CNFs are nanofibers extracted from chitin, a natural polymer found in crustacean shells. They have high strength, modulus, and biocompatibility, making them ideal for reinforcing polymers. CNF-reinforced composites have potential applications in wound healing, tissue engineering, and drug delivery. (c) Silver nanoparticle-reinforced polymer composites: Silver nanoparticles have antimicrobial properties and can be used to enhance the antibacterial properties of polymer composites. Silver nanoparticle-reinforced composites have

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potential applications in biomedical devices, food packaging, and water treatment. (d) Graphene nanoplatelet-reinforced polymer composites: Graphene is a twodimensional material with high mechanical strength and electrical conductivity. Graphene nanoplatelets can be used to reinforce polymer composites, improving their mechanical and electrical properties. Graphene-reinforced composites have potential applications in the electronics, aerospace, and automotive industries. (e) Clay nanoparticle-reinforced polymer composites: Clay nanoparticles, such as montmorillonite or kaolinite, can be used to reinforce polymer composites. These composites have improved mechanical, barrier, and thermal properties, making them suitable for food packaging, automotive, and aerospace industries. 2. Elongated particle nano biocomposite: Stretched or elongated particle nano biocomposites are those that utilize elongated particles as reinforcement, like carbon nanotubes and cellulose nanofibrils. These show the reinforcement’s high aspect ratio and provide enhanced biomechanical behavior as a result. Here are some examples of elongated particle bio-nanocomposites: (a) Carbon nanotube (CNT) reinforced polymer composites: CNTs are elongated nanoparticles with high strength, stiffness, and electrical conductivity. They can be used to reinforce polymers, improving their mechanical and electrical properties. CNT-reinforced composites have potential applications in the electronics, aerospace, and automotive industries. (b) Cellulose nanofibers (CNF) reinforced polymer composites: CNFs are elongated nanoparticles extracted from cellulose fibers. They have a high aspect ratio, stiffness, and strength, making them ideal for reinforcing polymers. CNFreinforced composites have potential applications in packaging, automotive, and biomedical industries. (c) Silk fibroin nanofibers (SFNF) reinforced polymer composites: SFNFs are elongated nanoparticles extracted from silk fibers. They have high strength, biocompatibility, and biodegradability, making them suitable for biomedical applications. SFNF-reinforced composites have potential applications in tissue engineering and drug delivery. (d) Polymer nanofiber (PNF) reinforced polymer composites: PNFs are elongated nanoparticles made by electrospinning polymer solutions. They have a high aspect ratio and can be used to reinforce polymers, improving their mechanical properties. PNF-reinforced composites have potential applications in tissue engineering, drug delivery, and filtration. (e) Nanocellulose whiskers (NCW) reinforced polymer composites: NCWs are elongated nanoparticles extracted from cellulose fibers. They have a high aspect ratio, stiffness, and strength, making them ideal for reinforcing polymers. NCWreinforced composites have potential applications in packaging, automotive, and biomedical industries.

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3. Layered particle-reinforced nano biocomposites: Layered polymer nanocomposite is the other name for this. According to the rate of dispersion in the matrix, other subclasses (such as flocculated/phase-separated nanocomposites, micro composites, intercalated, and exfoliated nanocomposites) could also exist. Particle–particle interactions produce flocculated or phase-separated nanocomposites when there is no separation between the layers as a result. The polymeric matrix acts as a dispersion medium for the micro composites. Furthermore, intercalated nanocomposites are made of polymer chains inserted between the sheets of layered nanoparticles, whereas exfoliated nanocomposites are made when individual layers are split. Regardless of the polymer-to-polymer or polymer-to-inorganic material ratio, intercalated nano-composites are composites in which the insertion of inorganic materials or polymers (extended polymers or inorganic materials) into another polymer (host polymer) occurs on a regular basis. Regular interlayering of the composites gives them ceramic-like characteristics. The only difference between flocculated and intercalated nanocomposites is how the extended polymer is arranged within the host polymer. In Exfoliated nano-composites, extended polymer layers should be separated in a continuous polymer matrix with a mean space that relies on the loading of the filler. As a rule, less filler has been seen in comparison to other forms of nano-biocomposites. Here are some examples of layered particle-reinforced nano biocomposites: (a) Montmorillonite clay-reinforced polymer composites: Montmorillonite is a layered clay mineral with a high aspect ratio and large surface area. It can be used to reinforce polymers, improving their mechanical, thermal, and barrier properties. Montmorillonite clay-reinforced composites have potential applications in food packaging, automotive, and biomedical industries. (b) Halloysite clay-reinforced polymer composites: Halloysite is another type of layered clay mineral with a high aspect ratio and large surface area. It can be used to reinforce polymers, improving their mechanical, thermal, and barrier properties. Halloysite clay-reinforced composites have potential applications in food packaging, automotive, and biomedical industries. (c) Graphene oxide (GO)-reinforced polymer composites: GO is a layered material with a high aspect ratio and large surface area. It can be used to reinforce polymers, improving their mechanical, thermal, and electrical properties. GOreinforced composites have potential applications in electronics, energy, and biomedical industries. (d) Layered double hydroxide (LDH)-reinforced polymer composites: LDH is a type of layered material with a high aspect ratio and large surface area. It can be used to reinforce polymers, improving their mechanical, thermal, and barrier properties. LDH-reinforced composites have potential applications in food packaging, automotive, and biomedical industries. (e) Mica-reinforced polymer composites: Mica is a layered mineral with a high aspect ratio and large surface area. It can be used to reinforce polymers, improving their mechanical, thermal, and barrier properties. Mica-reinforced

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composites have potential applications in cosmetics, coatings, and biomedical industries [3].

3 Synthesis of Polymer Nanobiocomposites To initiate the synthesis process of polymer nanobiocomposites (PNBs), it is necessary to determine their intended purpose and to carefully choose the appropriate fillers, polymers, and other composite materials based on the desired properties of the final material. The selection of fillers is crucial, as they can significantly impact the performance of the composite material, and should be chosen based on their chemical and physical properties, as well as their compatibility with the polymer matrix. Similarly, the choice of polymers should be based on their intended application and compatibility with the fillers. Natural polymers, such as chitosan or cellulose, are often preferred due to their biocompatibility and biodegradability, making them suitable for applications in areas such as biomedical engineering and environmental remediation. Once the choice of fillers and polymers has been made, the synthesis process can begin. (i) Solution Intercalation A solution intercalating method is a common approach for synthesizing polymer nanobiocomposites (PNBs). In this method, the polymer matrix like starch is dissolved in a solvent, and the nanofillers are added to the solution. The mixture is then stirred, and the nanofillers are intercalated within the polymer matrix through a process of solvent evaporation or precipitation. The solvent is then removed and the remaining structure is the biopolymer [16]. The resulting PNBs exhibit improved mechanical, thermal, and barrier properties compared to the individual components. (ii) Melt Mixing Solution melt mixing is another common approach for synthesizing polymer nanobiocomposites (PNBs). In this method, the polymer is heated at very high temperatures to convert them to its molten state, upon which the nanoparticles are added and evenly distributed. The most preferable biopolymers to be used for this type of synthesis involves proteins, wheat gluten, etc. [21]. (iii) Template Synthesis The synthesis of polymer-based nanobiocomposites (PNBs) can utilize biomolecules, whole cells, and microorganisms as templates for inorganics generated from a precursor. The resulting templates are of the nanosized order which are entrapped in the mesoporous matrix [16]. One advantage of this technique is that it has synergistic effects [17]. It refers to the combined effect of the template and the inorganic constituent, which results in a material with properties superior to those of its individual components.

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4 Properties of Polymer-Based Nanobiocomposites Nanobiocomposites are created with different materials and designed for specific purposes. However, their characteristics are influenced by several variables, such as the types of nanomaterials and biofibers utilized, matrix properties, and manufacturing methods employed [7] (Fig. 2). Nanobiocomposites or biopolymeric-based nanocomposites show sustainable characteristics which have gathered much interest from the scientific community. Nanobiocomposites (NBCs) are classified into various categories depending on factors like their origin, fortification shape, size, and the type of matrix used (a few types based on the shape of particles have been discussed in Sect. 2). Biomedical research has advanced considerably in the last few decades, with scientists striving to replace damaged tissues and organs with natural or artificial biocomposites having unique properties. These biocomposites can interact directly with living tissues and are typically delivered through procedures that mimic natural conditions demonstrating the biocompatible nature of NBCs in living systems. Therefore, NBCs have been widely used in various biomedical applications as biomaterials. The mechanical properties of NBCs are crucial in pharmaceutical applications. Advanced drug delivery systems are now based on novel approaches and targeted drug delivery systems [19].

Fig. 2 The properties of Nanobiocomposites

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The use of polymer-based NBCs comes with multiple advantages, such as improved industrial productivity, simplified processing technology, and reduced manufacturing expenses. Combining natural fibers and organic and inorganic polymers with nanoparticles such as seen in the case of NBCs, improves its mechanical strength and also displays biodegradability due to the presence of biofibers. Cellulose fiber-based composites are also an eco-friendly choice since no or minimal greenhouse gases are generated in their synthesis as opposed to the composites synthesized from petroleum-based fibers. Natural fibers involved in the making of NBCs also possess excellent insulation characteristics which make them a suitable choice for applications in the automotive and construction industries. The properties of an NBC also depend on the specific surface area of the nanofiller used, which could be either in organic or inorganic forms. The most convincing aspect of an NBC is that it displays great strength, despite lacking inherent stiffness. The exceptional physical, functional, and mechanical properties of biofiber-reinforced biocomposites (BCs) have made them useful in the construction industries where they are employed in imparting flexibility to the building material. With exceptional renewability, biodegradability, stiffness, decomposability, cost-effectiveness, and high length-to-weight ratio, natural fiber-based composites are rapidly gaining popularity and gradually replacing synthetic fiber-oriented composites. Automotive panels made from hemp fiber composites have been in use for a significant period, owing to their environmentally friendly, cost-effective, and sustainable nature. NBCs reinforced with flax fibers show great strength despite being lightweight as compared to those made from synthetic components. NBCs synthesized by incorporating graphene nanoparticles in flax fibers are found to be cost-effective. BCs made from PLA and cotton show great mechanical properties with considerable upgrades in tensile strength. The addition of nanocellulose to NBCs has the potential to increase their tensile strength, viscosity, and elastic modulus, while simultaneously reducing their weight. BCs show improved mechanical strength when maleic acid is utilized in grafting the polymers which facilitate its binding with the biofibers in the matrix. However, when exposed to sunlight, humidity, heat, etc. the properties of BCs are deteriorated. Due to their excellent thermal stability, nanofillers do not undergo any changes in their physical characteristics during processing, making the recycling of NBCs a straightforward process [7]. Khutsishvili et al. synthesized a nanobiocomposite from natural polysaccharides, namely, arabinogalactan (AG), arabinogalactan sulfate (AGS), and k-carrageenan (k-CG), infused with manganese. These prepared NBCs exhibited antibacterial properties against Clavibacter sepedonicus Phytopathogen. A few experiments were performed to determine the outcomes of NBCs on the viability of Cms, the pathogen that causes ring rot in potatoes. It was observed that the NBCs decreased bacterial growth during the 28 h time the experiment was performed for. This effect was most noticeable in AGS-Mn with a 0.00625% manganese concentration. Biofilm formation by the bacteria is necessary for its survival as it provides resistance to the bacteria against its external environment. AG-Mn as well as AGS-Mn were found to inhibit

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this biofilm formation further proving the antibacterial nature of the NBCs. The number of dead bacterial cells displayed a linear relationship with the concentration of the nanocomposites. Additionally, these Mn-containing NBCs did not show any bactericidal effects against the soil microbiome and were thus found to be safe [11]. Gálvez-Iriqui et al. demonstrated the antifungal properties of chitosan-pyrrole-2carboxylic acid nanobiocomposites as it reduced the growth of Aspergillus niger in tomatoes which suggests that their treatment with the mentioned nanobiocomposite can prove useful in storage applications [6]. A combination of poly(3-hydroxybutyrate) matrix, organic nano clay (Cloisite 30B), and linear polyurethane were utilized to create hybrid nanobiocomposites. These composites have an intercalated structure and exhibit greater thermal stability and a broader range of processing temperatures in comparison to the unfilled matrix [27]. The microbial exopolysaccharide known as Pullulan (PULL) is produced under aerobic conditions by certain strains of Aureobasidium pullulans, a yeast species that exhibits polymorphism. Yeasmin et al. prepared a nanobiocomposite with a combination of PULL, TOCNs, and CNT. The composite film was found to be easily degradable since it degraded within 64 days. Also, the addition of CNT to the hybrid nanocomposite film made it electrically conducting which otherwise is an insulator that can be further used in fields such as biosensing, electronics, and biomedicine [26].

5 Why is There a Need for New Wastewater Management Methods? Water constitutes one of the most important natural resources on earth without which life is clearly impossible. As the population continues to increase the burden of sustenance of a billion people lies on water resources worldwide. Therefore, it becomes important to come up with new and improved technologies for wastewater treatment. As the identification of more contaminants, rapid population growth, expanding industrial activities, and dwindling freshwater sources become more prevalent, the traditional methods of wastewater treatment face mounting challenges. While conventional processes have successfully removed many chemical and microbial pollutants from wastewater, their effectiveness has become restricted in the past few decades due to the emergence of new obstacles. The growing understanding of the consequences of water pollution and public demand for better water quality has led to the implementation of more stringent regulations that broaden the scope of regulated contaminants and lower the maximum allowable levels for wastewater discharge [20]. Wastewater treatment and management refers to the overall process of enhancing water quality between the points of production and discharge. The objective is to

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improve the physical, biological, and chemical attributes of wastewater, thereby removing both known and emerging contaminants from the water that is ultimately released into the environment. Biological treatment has become a crucial aspect of wastewater treatment plants that process wastewater from various sources containing soluble organic impurities or a blend of different wastewater sources. For over a century, the aerobic-activated sludge process has been utilized for biological treatment and has undergone numerous enhancements and modifications. Recent years have witnessed the implementation of various advanced biological treatment processes due to increased pressure to comply with more stringent discharge regulations enforced by different agencies. As Donde concludes in his paper, to achieve sustainability in managing wastewater, it is essential to create novel concepts that can comprehensively solve problems related to known and emerging pollutants. Biological wastewater treatment systems, which have been previously known, may not always be appropriate. Therefore, it is recommended to first consider simpler, more cost-effective, and sustainable treatment technologies. New wastewater treatment ideas should fulfill the requirements of adapted wastewater treatment solutions. One approach is to integrate biological wastewater treatment into the mechanical treatment system and introduce hybrid pathways to enhance purification efficiency [15]. Nanotechnology is proving to be a great alternative for wastewater treatment as compared to the conventional treatment methods which involve the use of chemicals, are energetically intensive, and are hard to operate requiring suitable infrastructure and engineering expertise. Nanostructured materials exhibit distinct characteristics, such as a high ratio of surface-to-volume, exceptional sensitivity and reactivity, superior adsorption capacity, and are easy to functionalize. These properties make them promising candidates to overcome the limitations associated with conventional techniques [10]. Nanotechnology’s greatest strength lies in its capacity to seamlessly combine with other fields of study, modify and enhance established concepts, and create novel pathways for development and exploitation of processes. This provides a new and innovative perspective on utilizing these technologies to achieve unprecedented results [20].

6 Polymer-Based Nanobiocomposite as a Filter Nanosponge for Wastewater Remediation Adsorbent nanomaterials, also known as nanosorbents, are nanostructured materials with pores that range in size from 1 to 100 nm and can be used to adsorb pollutant molecules. They are additionally categorized as nanoporous substances. Nanosponges with cavities and mesh-like/colloidal structures made of solid nanomaterials are useful for encapsulating a variety of chemicals and molecules, including

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volatile oils, proteins/peptides, medications, genetic materials, and antineoplastic medicines [13]. The nanosponge cyclodextrins, carbon nanotubes, and metal nanoparticles are among the nanosorbents. Zeolites, clays, chitosan, activated carbon, silica beads, agricultural solid wastes (like rice husk), and industrial byproducts are some typical forms of conventional adsorbent materials that have already been employed for water purification. Insoluble nanoporous polymers known as nanosponge cyclodextrin polyurethanes have been shown to be effective adsorbents for the elimination of organic pollutants such as dyes, fertilizers, and pesticides. According to earlier research, cyclodextrin polymers can remove contaminants down to parts per trillion while zeolites and activated carbon can only do so down to parts per million. The removal of organic and inorganic pollutants from water is made possible by the excellent potential of sorbents based on cyclodextrin for removing heavy metal ions from aqueous solutions. Because of the cyclodextrin cavities that create a hydrophobic environment, cyclodextrin-based nanosponges (0.7–1.2 nm) are created with a great affinity for absorbing hazardous organic pollutants in wastewater. Nanosponges made of cyclodextrin can also be used to quickly remove contaminants from water (up to 90%), with a maximum adsorption capacity of 2 mg g−1 . The removal of certain pharmaceutical pollutants from water, such as carbendazim, diclofenac, sulfamethoxazole, and furosemide, is greatly enhanced by these nanostructures. A cross-linking oligomerization of cyclodextrin using phosphorylated multiwalled carbon nanotubes (MWCNTs) is also used to create polymeric nanobiocomposites with multifunctionality, followed by a sol–gel step to integrate silver (Ag) and titanium dioxide (TiO2 ) nanoparticles. These composites can be used as possible filter nanosponges to remove impurities from water and wastewater, such as harmful bacteria and organic/inorganic compounds. Due to the polymerization events, carbamate-bearing distinctive peaks (at 1645 cm−1 ) and oxygen-containing groups are maintained on these composites, according to Fourier-transform infrared (FTIR) research. There are three types of natural cyclodextrins (α-CD, β-CD, and γ-CD), each of which has six, seven, or eight glucose units and is connected to the others by α-(1, 4) glycosidic linkage, forming a ring. Given their high reactivity and reduced price, β-CDs are the most popular among these three natural CDs, which also range in ring size and solubility. Furthermore, cyclodextrins are valuable molecular chelating agents that are employed in a variety of industries, such as polymer synthesis, pharmaceuticals, cosmetics, food processing, textiles, fermentation, catalysis, and environmental protection. DeQuan Li and Min Ma were the first to use the name “cyclodextrin nanosponge” and to demonstrate its applicability in water filtration in 1998. Then later on, further research on the synthesis and other potential uses of new types of nanosponge cyclodextrins was proposed by different people from all over the world. These nanoporous cyclodextrin-insoluble polymers were made using a number of processes, including condensation (the best way to make nanosponge CD polyurethanes), dehydration (the reaction of CDs with a diacid (dicarboxylic acid) or diol), and protonation (where CDs are polymerized using an oxyanion) [12].

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When β-cyclodextrin covalently-cross-linked tannic acid produces polymer nanosponge through a condensation reaction, it can be used for holding back Pb2+ from wastewater with high selectivity. The production of phosphorylated multiwalled carbon nanotube-cyclodextrin/Ag-doped TiO2 nanosponges for eliminating Pb2+ and Co2+ metal ions from wastewater uses cross-linking polymerization, sol– gel, and amidation reaction methods. The pH solution, which can alter speciation, the degree of ionization of pollutants, and the surface charge of adsorbents, is one of the critical parameters with substantial effects on the adsorption capabilities of nanosponges. The elimination of Pb2+ and Co2+ is more effective when a solution’s pH is raised. Cyclodextrin-based nanosponges made by reacting β-cyclodextrin with the crosslinking agent hexamethylene diisocyanate, are intended to remove p-nitrophenol from aqueous streams through the adsorption process. Additionally, mesoporous nanosponges made of surface-functionalized cis-diol were tested for their ability to quickly remove boric acid and organic micropollutants from water. In addition to being faster (up to 60 times) at adsorbing boron, they also have an effective capacity for doing so. Furthermore, these nanosponges are capable of absorbing bisphenol A, with equilibrium adsorption occurring in under 2 min. The host–guest interactions of the nanosponge cyclodextrin insoluble polymers, primarily as polyurethanes, have shown their ability to absorb a group of organic adulterants from wastewater at very low concentrations. The host–guest complex, also known as an inclusion complex, is formed when the hydrophobic organic guest molecules (apolar contaminants) are absorbed into the hydrophobic cavities of the CDs (host) during the absorption process. Through the amidation reaction, crosspolymerization using diisocyanate (as a linker), and the sol–gel process, phosphorylated MWCNTs, Ag nanoparticles, and TiO2 are also added to nanosponges made from cyclodextrin polyurethane that possess great surface area features, insolubility, and recyclability. The created nanosponges had the highest capacity for removing trichloroethylene and Congo red dye from wastewater. Cyclodextrin nanosponges with reusability are designed via cross-linking of 1,2,3,4-butane tetracarboxylic acid with β-cyclodextrin in the company of poly(vinyl alcohol) for the adsorptive elimination of cationic contaminants from water. The maximum adsorption was reported for the removal of paraquat safranin and malachite green. For the purpose of eliminating dyestuffs from the waste stream, biodegradable cyclodextrin-based nanosponges with high biosafety properties are created utilizing a one-step solvothermal approach combining β-cyclodextrin and diphenyl carbonate. For the removal of dinotefuran from water, superparamagnetic Fe3 O4 nanoparticles are present on the surface of carbonate nanosponges generated from βcyclodextrin, with good reusability. These magnetic nanosponges seem to be viable options for removing neonicotinoids from aqueous solutions because of their great effectiveness, affordability, non-toxicity, and reusability. In the presence of poly(vinyl alcohol), cyclodextrin is used to create poly(vinyl alcohol)-cyclodextrin nanosponges via citric acid. These nanosponges use the adsorption procedure to remove paraquat from water [9].

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In comparison to other materials, nanosponges have great porosity, and simple functionalization, and added to that their cost-effectiveness, makes them an excellent alternative for removing contaminants like pharmaceutical waste, dyes, and heavy metals from water. A variety of adulterants can be removed from water via adsorption or inclusion using cyclodextrin-based nanosponges with special physicochemical properties and topologies, such as strong biocompatibility, low/non-toxicity, convenient surface functionalization, and bio-absorbent qualities. However, there are some areas that still need to be improved in order to make the process of waste removal more efficient like adsorption mechanisms, removal efficiency, biosafety issues, hydrophobic interactions, complexation agents, and certain others. Other Bio Nanocomposites in wastewater remediation Nanomaterials, because of their high adsorption efficiency, larger surface area, and a greater number of active sites for contact with contaminants in water, have the potential to significantly reduce the number of the current issues relating to water quality. Nanocellulose is the most abundant and renewable polymer in the world. It is made up of repeated -d-glucopyranose units that are chirally and chemically reactive and are covalently bonded by acetal functionalities between the hydroxyl groups of the C4 and C1 carbon atoms. Nanocellulose has significant potential as a component in water filtration membranes due to its intrinsic fibrous nature, excellent mechanical capabilities, low maintenance cost, and biocompatibility. Due to their high surface-area-to-volume ratio, low cost, high natural abundance, and intrinsic environmental inertness, nano celluloses are particularly promising adsorbents for removing heavy metals, viruses, and dyes, among other things. Additionally, the nanocellulose surface possesses highly functionalizable OH groups, which makes it easier to incorporate chemical elements that could improve the efficiency of binding pollutants to the nano cellulosic materials. One variety of cellulose nanomaterial is cellulose nanofiber (CNF). Due to the same reasons as stated above, CNFs are a viable alternative adsorbent. A few scientists from around the globe intended to produce a unique adsorbent material from cellulose nanofibers (CNFs) via a nonsolvent assisted method using Meldrum’s acid as an esterification agent which can remove the dyes from water. This is to fix the problem of toxic reagents and solvents being used with cellulose nanofibrils to adsorb various kinds of dyes. According to them, preparation of a new green-based adsorbent to adsorbing dyes from CNFs via a nonsolvent assisted procedure using Meldrum’s acid as an esterification agent has not been addressed to date and this finding offers a new platform for the surface treatment of cellulose nanofibers using solvent free green technology [4]. The contamination of dyes from many chemical and pharmaceutical industries, textile, printing, plastics, and paper, has emerged as a global issue. The removal of various colors from contaminated water has since been accomplished using a variety of treatment techniques, including adsorption, coagulation, flocculation, oxidation, ion exchange, membrane separation, and catalytic degradation. Adsorption is strikingly superior to other treatment techniques in terms of its ease of use and simplicity of design among these procedures. Due to their renewability, biodegradability, and biocompatibility, natural polymeric materials, notably polysaccharides, have drawn

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attention for use as adsorbents in wastewater sanitization. Because of its special features, increasing the use of polysaccharide material in wastewater treatment, biomedical engineering, and agriculture has attracted a lot of interest. Salecan, a water-soluble, environmentally friendly, and reasonably priced bacterial polysaccharide, was used as a matrix and poly(acrylamide-co-itaconic acid), or PAI, as the synthetic component to create salecang-PAI hydrogels using a simple chemical cross-linking process. Salecan not only served as the point of contact for regulating the water content of the hydrogels as they developed but also provided them with a customizable shape. The created salecang-PAI hydrogels showed exceptional adsorption capabilities for the dye methylene blue (MB). This work has expanded the use of salecan polysaccharides and introduced a new dye decontamination mechanism. Water contamination due to toxic heavy metals such as Pb(II), Cd(II), Hg(II), Cu(II), and Ni(II) is hazardous due to its selective toxicity even at low concentrations. Chronic lead exposure can result in anemia, encephalopathy, nephropathy, palsy, and other conditions. Acute lead exposure can cause brain dysfunction, nausea, and vomiting and that of Cd(II) can cause lung cancer, osteomalacia, and proteinuria. Precipitation, flotation, oxidation, evaporation, electrochemical removal, reduction, ion exchange, filtering, reverse osmosis, and adsorption are a few of the techniques/ methods that could be used to treat heavy metal ions. Adsorption, however, is typically favored for the removal of low concentrations of heavy metal ions due to its high efficiency, ease of handling, accessibility to raw materials, and low energy consumption. They were removed using a methionine-modified bentonite/alginate nanocomposite (meth-bent/alg). For heavy metal cleanup, this was reported to be an efficient adsorbent up to around 98 and 82%. Bentonites are a better foundation for composite materials since they offer several benefits including being nontoxic, chemically inert, and hydrophilic. A straightforward sorption process was used to modify bentonite by the biobased ligand (l-methionine). Because of its side chains with amino, carboxylic, and thiol ligands, l-methionine is an intriguing benign biomolecule in the field of heavy metal trapping. The removal of heavy metal ions has recently been reported using mentha piperita carbon, Fe3 O4 /CD polymer nanocomposites, cysteine-modified bentonite, alginate-alumina collagen fibers, natural phosphate as an adsorbent, aminated polyacrylonitrile nanofiber mats, silica aerogel activated carbon nanocomposite, and carbon-iron oxide nanocomposite [2]. When textile dyes (acid red 88-AR88) were removed from aqueous solutions using a biosilica/chitosan nanocomposite, it was discovered that the amount of adsorbed AR88 (mg/g) increased with longer reaction times, higher adsorbate concentrations, and lower temperatures as well as initial pH. When the adsorption dosage was increased from 1 to 3 g/L, the adsorption increased quickly, but further increasing the dosage only marginally increased the adsorption (1.66 mg/g). Nearly 30% of synthetic dyes are estimated to escape during production and end up in the environment as effluent from these companies and it has been reported that exposure to MB (methylene blue) is mutagenic and may lead to reproductive problems. For the purpose of improving MB-contaminated wastewaters, the viability

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of a hydrogel composite made of starch and cellulose nanowhiskers (CNWs) was examined. Fast adsorption kinetics based on the pseudo-second-order model, which is based on the chemisorption phenomenon, distinguish this system from other known adsorbents (such as starch and cellulose). Additionally, cellulose and starch, two biodegradable polymers, are used to create the hydrogel composite. Thus, this composite of starch and CNWs hydrogel demonstrated an exceptional ability to be used in the treatment of MB-contaminated wastewater. A cellulose clay hydrogel with superabsorbent qualities, exceptional mechanical capability, and excessive dye removal effectiveness has been described. Chemical cross-linking of cellulose, carboxymethyl cellulose (CMC), and the intercalated clay in the NaOH/urea aqueous solution was the primary method for creating superabsorbent hydrogels. These hydrogels showed excellent ability to remove MB from wastewater, superior mechanical performance compared to the hydrogel containing unmodified clays, and superabsorbent characteristics in distilled water. Due to their biodegradable and non-toxic nature, polysaccharide materials are garnering interest for use as adsorbents in wastewater treatment. The second most common natural biopolymer is chitosan (CTS), an N-deacetylated derivative of chitin. CTS is a popular sorbent that is frequently used to remove heavy, transitional metals and pigments. However, it adsorbs very small amounts of cationic dyes. Hence, MB cationic dye was removed from its aqueous solution utilizing batch adsorption techniques with chitosan-g-poly(acrylic acid)/montmorillonite (CTS-gPAA/MMT) nanocomposites as the adsorbent. The research discovered that when pH rose, so did the nanocomposite’s ability to adsorb MB. Thus, CTS-g-PAA/MMT nanocomposite is found to be an effective adsorbent for the removal of MB from an aqueous solution [5].

7 A Revolutionary Technique? Wastewater is used water from residential, agricultural, industrial, or commercial activities as well as surface run-offs from rains, storms, and any sewer infiltration or inflow [22]. Considering the limited nature of useful water on the planet, there’s a dire need for us to have this wastewater be recycled to an extent where most of it can be released into the nearby water bodies without the concern of contamination. To achieve this, the process of wastewater treatment is carried out. Through this process, toxins are removed from wastewater and the water is transformed into an effluent that can be recycled back into the water cycle. The conventional techniques of wastewater treatment include phase separation (sedimentation) and biological processes. A type of sludge from the wastewater treatment plant is the major by-product and it is treated in either the same water treatment plant or a different one. When anaerobic (absence of oxygen) processes are involved, biogas is produced as a by-product. There are different types of water treatments but wastewater treatment is usually interchangeable with another term ‘sewage treatment’ [23].

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Water Pollution is a major concern for the world and especially India. The majority of the metropolitan cities in India are facing the problem of water scarcity and it is primarily due to the unavailability of clean and reusable water. The key cause of water pollution in India is untreated sewage [24]. The majority of health issues that drinking water might bring about are caused by chemical contaminants. There may be widespread symptoms including nausea, skin discoloration, or even worse. Chemical contaminants that are continuously absorbed can harm organs and raise the risk of developing cancer. They do not significantly change the properties of water as do physical sediments. There are both natural and man-made chemical contaminants. Nitrogen, arsenic, and toxins produced by bacteria are examples of natural chemical pollutants. They frequently result from runoff, erosion, or water-based organisms. These substances often leak into groundwater in trace amounts, but ongoing exposure to and consumption of them can have substantial negative effects on health. Bleach, insecticides and pesticides, and corrosion of copper and lead pipes are examples of anthropogenic chemical pollution. These contaminants can occasionally result from improperly discarding chemical wastes, such as copper sulfate, which is used in pest control and gardening. Hazardous contaminants are carried by streams and groundwater into reservoirs by soil erosion [25]. Rivers and streams in India, a nation whose population rose to1.35 billion in the former 40 times, have suffered the consequences of recent civic expansion. Due to unplanned growth, water bodies are now being employed as jilting grounds for sewage and artificial scrap. According to the Central Pollution Control Board of India, 63%, or around 62 billion litres, of the civic sewage that flows into rivers every day is untreated. According to a research completed last year, the extreme pollution of the Yamuna River, which provides the maturity of the megacity’s drinking water, is to blame for the rising number of typhoid, hepatitis, and diarrhoea cases in New Delhi. Large sections of the Yamuna, the Cooum River in Chennai, the Mithi and Ulhas Rivers in Mumbai, and other rivers are regarded as dead zones because the oxygen conditions are too low to support the majority of fish life. All of this is passing when rivers in India are given the status of deities! A 2019 exploration paper claims that only 7000 million liters per day (MLD) of the 33,000 MLD of waste generated each day gets collected and treated at the majority of the sewage treatment shops erected under the Ganga Action Plan and Yamuna Action Plan [14]. Pharmaceuticals in the environment are difficult to control since they are made to interact with living things and elicit a reaction at low levels, and they can still pose a threat to the environment even at low quantities. They can stay in the water for longer periods of time and have an impact on aquatic systems because they are made to be stable and difficult to deteriorate. Pharmaceuticals from wastewater cannot be removed using conventional wastewater treatment plants [8]. Considering the present situation, it becomes of prime importance for us to find effective methods that not just are cost-effective but also less time-consuming. And here is where the Nanomaterials/Nanosponges come in as saviors! It is going to come

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as a major breakthrough once they are employed on a large scale for wastewater remediation and we can bring about a Blue Revolution in the country by cleaning water at a faster rate with better results than earlier. Their peculiar qualities such as high porosity, large surface area, affordability, simple functionalization, non-toxicity, and reusability make them a strong contender for utilization in the wastewater industry. India’s water bodies such as the rivers, lakes, ponds, etc. are in pathetic condition and if this continues, we are going to be struggling to find clean water for such a huge population’s needs.

8 Conclusion As the stress on existing freshwater resources continue to increase with great magnitude, it is highly crucial that we develop effective wastewater treatment techniques to cope with the current water crisis. Though some technologies already exist for this purpose, they are certainly not enough to cope with the problem. Moreover, with the progression of science, there are new kinds of contaminants in the environment that need to be taken care of. The developing field of nanotechnology offers lucrative methods to deal with wastewater management and treatment with their exceptional and unique characteristics. Additionally, nanobiocomposites involve the use of biodegradable and environment-friendly substances to treat wastewater which relieves the burden of using synthetic substances that can potentially persist in the environment and continue polluting it. In an era that gives much importance to sustainable development and justifiably so, more sustainable water treatment methods such as this must be developed in order to managing water resources more efficiently without worrying about harming the earth in the process.

Abbreviations AG AGS BC CD CMC CNC CNF CNT CTS CTS-gPAA/MMT GO k-CG LDH

Arabinogalactan Arabinogalactan sulfate Biofiber-reinforced biocomposites Cyclodextrins Carboxy Methyl Cellulose Cellulose Nanocrystals Chitin/Cellulose Nanofibers Carbon Nanotube Chitosan Chitosan-g-poly(acrylic acid)/montmorillonite Graphene Oxide K-carrageenan Layered Double Hydroxide

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MB MLD MWCNT NBC NCW PAI PLA PNB PNF PULL SFNF TOCN

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Methylene Blue Million Litres per Day Multiwalled Carbon Nanotube Nanobiocomposite Nanocellulose Whiskers Poly(acrylamide-co-itaconic acid) Polylactic Acid Polymer Nanobiocomposites Polymer Nanofiber Pullulan Silk Fibroin Nanofibers TEMPO-Oxidised Cellulose Nanofibrils

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Cellulose-Based Nanosponges for Wastewater Remediation Laishram Saya, Ratandeep, Bikaramjeet, and Pooja

1 Introduction Water pollution is a critical global issue and accessibility to safe drinking water has become a persistent crucial problem human beings are facing currently. With the growth of the population and the increasing burden of environmental pollution caused by numerous sources such as sewage, industrial effluents, and chemical materials, the treatment of water through various techniques has become vital. Additionally, microbial contamination is a serious concern. These pollutants can prove hazardous to human health or alter the taste, odor, and color of water [1]. Water remediation comprises several techniques and ways of sequestration or reduction of contaminants from polluted water to make it safe for human consumption and environmental use. Although conventional water treatment techniques, including coagulation methods, membrane-based techniques, and direct filtration, have been widely used, they have various drawbacks and limitations. These methods are often associated with low efficiency, selectivity, and specificity, which can affect their ability to improve water quality to desired levels [2]. The choice of the remediation method relies on the nature and concentration of pollutants present in the water, as well as the quality of the treated water desired. Moreover, the selection of a remediation technique also considers factors such as cost, sustainability, and scalability of the method. Therefore, the demand for effective and sustainable water remediation methods is increasing. Adsorption-based systems are being used more widely owing to their better results.

L. Saya · Bikaramjeet · Pooja (B) Department of Chemistry, Sri Venkateswara College (University of Delhi), Dhaula Kuan, New Delhi 110021, India e-mail: [email protected] Ratandeep Department of Chemistry, University of Delhi, New Delhi 110007, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Gulati (ed.), Nanosponges for Environmental Remediation, https://doi.org/10.1007/978-3-031-41077-2_16

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In recent years, nanotechnology has been witnessed as an efficient approach for water treatment as a result of its high efficiency and selectivity in removing contaminants. Engineered nanomaterials (ENMs) have become more popular in recent years as a more convenient and cost-effective solution for remediating polluted environments. This approach, known as “nano remediation”, has shown significant progress in cleaning up polluted soils and waters. However, the potential risks due to the release of ENMs into the environment are not yet fully understood. To address this, researchers are now focusing on designing environmentally friendly ENMs that pose minimal risks to living beings or ecosystems, while remaining economically and environmentally sustainable. This has led to the development of bio-based ENMs associated with a great potential tom minimise environmental pollution and can be used onsite. One such nanomaterial that has gained significant attention toward water remediation is cellulose-based nanosponges. Cellulose has received significant research attention owing to its abundance and potential for chemical modification [3]. NC is a highly valued component in the production of innovative materials for various advanced applications. Being derived from renewable biomass, it is associated with various desirable properties inherent of cellulose including its non-toxic and high biodegradable nature, as well as additional benefits like high surface area and mechanical strength. The biorefinery of NC is pictorially represented in Fig. 1. It is easy to modify and functionalize NC for desired applications as shown in Fig. 2. This helps reduce our reliance on non-renewable natural resources and slow down the depletion rate of these resources. NC can be acquired through two forms: Cellulose Nanocrystals (CNCs) or Cellulose Nanofibers (CNFs) by breaking down the hierarchical structure of natural cellulose through chemical or mechanical methods. Aerogels based in NC have also become increasingly important for water treatment applications [5, 6]. Nanosponges have gained special attention in various domains, including environmental studies, drug therapy, and analysis, in recent years. Nanosponges consist of three-dimensional solid nano-materials and have mesh-like/colloidal structures of high surface area and porosity with cavities, which are well-suited for the encapsulation of various substances/compounds [1]. They have been identified as promising adsorbents for wastewater treatment and have attracted increasing attention for their potential use in water remediation. One of the advantages of using cellulose-based nanosponges for water remediation is their biocompatibility and low toxicity, which make them safe for use in various settings, including drinking water treatment. In addition, their renewable and biodegradable nature makes them a sustainable and environmentally friendly alternative to traditional water remediation methods. Their network structure makes them particularly useful for the eradication of various harmful materials including heavy metal ions [8], organic pollutants including dyes [9], oils [10], salts [9], pharmaceuticals [11], pesticides [7] and pathogenic microorganisms [12]. However, the use of cellulose-based nanosponges for water remediation is also associated with certain challenges and limitations, such as limited selectivity, poor stability, high cost, and limited knowledge of their long-term effects. Addressing these challenges will require further research and development efforts to optimize

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Fig. 1 Nanocellulose biorefinery. Reprinted with permission from Ref. [4], Copyright Springer Nature

the synthesis methods and improve the performance of cellulose-based nanosponges in water remediation applications. Overall, cellulose-based nanosponges have great potential for water remediation and represent a promising avenue for developing sustainable and effective solutions for water treatment and purification [1, 7, 13]. In this chapter, the structure, synthesis, properties, and applications of cellulosebased nanosponges in the filed of water remediation will be outlined. Moreover, the advantages of these nanosponges over other nanomaterials and their potential impact on the environment will also be illustrated. Moreover, a discussion on the challenges and future aspects of using cellulose-based nanosponges for the purpose of water remediation, including optimization of their properties and scalability of production processes is also included.

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Fig. 2 Schematic depiction of main routes of surface functionalization of NC [7]. (CC-BY-4.0)

2 Sources of Nanocellulose There are numerous sources from which NC can be obtained from such as grasses including bagasse and algae, bamboo, bacteria, fungi, hardwood and softwood, various aquatic species such as tunicates, various plant fibers such as flax, hemp, ramie jute, kenaf, coir and seed fibers like cotton. Wood has been the most frequently utilized source for obtaining cellulose. Chemical and mechanical treatments can be adapted to separate cellulose from non-cellulosic materials such as hemicelluloses, lignin, waxes, and pectin that are present in the wood. After wood, the second-highest source of cellulose is crop waste. Additionally, industrial wastes, like tomato peels and garlic, sugarcane bagasse, and other processed waste products from industries, have recently emerged as another potential source of cellulose [4]. NC can also be produced from various natural cellulose sources which are elaborated below: 1. Wood pulp: This is the most common source of NC. The process of producing NC from wood pulp typically involves several steps. First, the wood pulp is treated with chemicals to break down the lignin and hemicellulose components, leaving behind the cellulose fibers. These fibers are then mechanically or chemically treated to break them down into nanoscale particles. There are several types of NC that are obtained from wood pulp, including CNCs and CNFs [14]. 2. Plants: NC is mainly derived from plants, as they are the most prevalent source of cellulose. Plants are abundant, easily accessible, and offer a variety of options for sourcing NC, as various plant parts contain cellulose in their cell walls, along with hemicellulose and lignin in varying amounts (25–50%, 20–35%, and 10–25%,

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respectively). The conventional method of producing NC from plants involves a top-down approach via acid hydrolysis [5]. 3. Cotton: Cotton fibers are also a popular source of NC. The fibers are typically treated with acid or enzymes to extract the NC. The resulting NC fibers can be further processed into several forms, such as gels, films, and coatings. These materials have potential applications in diverse fields like food packaging, water filtration, biomedical applications, and electronic devices [11]. 4. Bacteria: The bottom-up approach involving bacteria is also adopted to produce bacterial Nanocellulose (BNC). Gluconacetobacter xylinus is the most common bacterium utilized for the production of BNCs. The bacteria are cultured and the NC is harvested from the culture medium. Additionally, bacterial synthesis of NC offers the advantage of achieving high levels of crystallinity and purity in the end product [15–17]. (a) Algae: Algae constitutes one of the most commonly utilized raw materials in various industries including cosmetics, food, pharmaceuticals, and many others. Certain types of algae, such as diatoms and green algae, contain cellulose in their cell walls. This cellulose can be extracted and processed to produce NC. While algae are known to contain other useful components like alginates, carrageenans, and agar, cellulose has been an overlooked component until recently. Researchers have therefore begun investigating the extraction of cellulose from algae for various applications, including the production of NC [18]. (b) Fruit waste: Fruit waste, such as banana peels, apple pomace, and citrus peels, can also be used as a source of NC [19]. (c) Agricultural waste: Agricultural waste products, such as sugarcane bagasse, corn husks, and wheat straw, can be used as a source of NC [4]. (d) Invertebrates: Researchers have identified tunicates, which are invertebrate animals found in the sea, as another potential source for NC production. Sea squirts, a type of tunicate, in particular, have been recognized as a viable alternative for NC preparation [20]. It is noteworthy that while the chemical makeup of the different types of nanostructured cellulose is similar, their physical and solid-state properties can vary significantly. This can be due to differences in factors such as the shape and size of the NC particles, the fabrication method, and the degree of crystallinity [21]. Various types, sources, extraction techniques, and dimensions of NC are discussed in Table 1. Overall, NC can be obtained from versatile natural sources, making it a sustainable and renewable material for various applications.

Type of NC materials

Cellulose nanocrystals (CNCs)

Cellulose nanofibrils (CNFs)

Bacterial nanocellulose (BNC)

S. No.

1

2

3

Glucose like low molecular weight sugars

Lignocellulosic material of plants, paper pulp, animal tunicates

Wood fibers, paper pulp, animal tunicates

Typical sources

Static; agitated; bioreactor-based fermentation

Disc mills High-pressure homogenization; Ultrasonication TEMPO (2,2,6,6-Tetramethylpiperidinyloxyl) oxidation; Quaternization with glycidyl trimethylammonium chloride

Ball milling; Steam explosion Acidic hydrolysis; Sodium periodate oxidation; Reaction with Epoxypropyltrimethylammonium chloride

Extraction methods

Table 1 Types, sources, particle size, and extraction techniques of NC

> 1000 nm in length 20–100 nm in diameter 10–50 nm in width

100–2000 nm in length 2–20 nm in diameter 1–50 nm in width

100–2000 nm in length 2–30 nm in diameter 3–50 nm in width

Dimension (approx.)

[7, 13, 21, 22]

[7, 15, 21, 22]

Refs.

Greater thermal and [21–23] mechanical strength, purity, high crystallinity, and stability

High mechanical strength, low density, and high surface area

Large surface area, low density, good mechanical properties, low coefficient of thermal expansion

Advantageous properties

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3 Structure and Properties of Cellulose-Based Nanosponges NC, which comprises the structural framework of plants and wood, has numerous advantages. Nanosponges are extremely small sponges that have a similar size to a virus, and they have the capability to contain diverse types of drugs. NC possesses a substantial specific area of 100–200 g/m2 , an adjustable aspect ratio of 100–150, and unique mechanical properties such as Young’s modulus and tensile strength of 110–220 GPa and 7.5–7.7 GPa respectively. They show high resistance to chemicals, controllable crystallinity, and ease of surface customization. NC-based films have garnered significant attention from both industry and academia, making them a highly discussed topic [14]. Cellulose exhibits increased stability due to the presence of CH2 OH and OH moieties in equatorial positions [4]. Due to these unique physicochemical attributes, NC finds great applications in various interdisciplinary fields such as wastewater treatment [12, 14], automotive, aerospace, and energy sectors, as well as for packaging, 3D printing, membranes, and flexible electronics [14]. Cellulose can be extracted into two types of NC, namely CNFs and CNCs, depending on the isolation technique used. A variety of bacterial species are capable of producing BNC, but the most productive strain is typically Acetobacter xylinum, a non-pathogenic, rod-shaped, aerobic, and Gram-positive bacterium. The characteristics of CNCs are greatly influenced by the reaction parameters of hydrolysis including duration of reaction, temperature, and acid concentration. CNCs have typically needle-like or rod-shaped structures, with dimensions of 150 nm broad and a few hundred nm long. Furthermore, CNCs with a high crystallinity have a huge surface area, as well as great thermal stability and strength. CNFs typically have a diameter that is smaller than 100 nm, but they can have a length in the range of micrometers. CNFs possess greater aspect ratio relative to CNCs owing to their elongated shape, high surface area, and plenty of hydroxyl groups on the surface that can be modified. Although BNC has the same chemical compositions as other types of NC, BNC possesses greater purity, crystallinity and waterholding capacity imparting them superior thermal and mechanical strength [19]. The process of creating crosslinked nano-structures involves the use of CNCs and CNFs, which are modified through electrostatic interaction or covalent bond formation to produce mechanically stable porous materials. These materials can exhibit a variety of enhanced properties. Researchers imagined that the potential of cellulose nanosponge (CNS) could be utilized, and the interaction mechanisms between the sorbent and organic pollutants could be better understood by taking into account the morphology of the sponge, which is known for its significant micro- and nanoporosity. However, to achieve this, the NC is typically modified chemically to introduce the necessary bridges for further crosslinking [6, 24]. Figure 3 depicts the cellulose fibrils isolation into CNCs. Thus in 2019, G. Paladini et al. reported in a review article, a novel method for crosslinking Transparent, Outstanding, Uniform, and Sustainable Cellulose Nanofibers (TOUS-CNFs) that involved the use of branched polyethyleneimine (bPE). Hence, by subjecting bPEI/TOUS-CNF hydrogels to thermal treatment

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Fig. 3 Isolation of cellulose fibrils into CNCs. Reprinted with permission from Ref. [19]. Copyright 2020 Elsevier B.V

(freeze-drying followed by mild heating at 100 °C), amide bond formation occurred and resulted in the formation of CNSs, a new class of nanostructured sorbent. This process enhanced the structural rigidity of the aerogel and improved its adsorption efficiency by introducing amino groups due to their strong chelating action. The resulting CNSs has a 2-D sheet morphology with a sponge-like porous structure, making them effective adsorbents of both organic and inorganic pollutants (including metal ions as Cd2+ , Cr3+ , Zn2+ , Pb+ , Cu2+ ), from aqueous medium with an adsorption capacity over 200 mg/g [6]. Figure 4 depicts the BNC to synthesize cellulose-based membranes.

4 Synthesis Methods of Cellulose-Based Nanosponges Cellulose-based nanosponges can be synthesized using various methods, including chemical and mechanical methods, as well as bacterial fermentation. These methods allow for the production of nanosponges with different properties and applications, depending on the desired use and environment.

4.1 Mechanical Methods Mechanical methods represent top-down approaches that can be used to synthesize cellulose-based nanosponges. These methods involve mechanical shearing or milling of cellulose fibers to break them down into nanoscale dimensions, followed

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Fig. 4 BNC growth into the membrane. Reprinted with permission from Ref. [19]. Copyright 2020 Elsevier B.V

by chemical modification or crosslinking to result in a three-dimensional network structure. Some examples of mechanical methods for synthesizing cellulose-based nanosponges include.

Homogenization Under High-Pressure This method involves passing cellulose fibers through a high-pressure homogenizer, which uses intense shear forces to break down the fibers into nanoscale dimensions. The resulting CNCs can then be crosslinked to form a three-dimensional network structure. The extracted nanosponge has a width of 10–20 nm [15, 25].

Ultrasonication Ultrasonication involves exposing the cellulose fibers to high-frequency sound waves, which create cavitation bubbles that cause the fibers to break down into nanoscale dimensions. The resulting CNCs can then be crosslinked to form a three-dimensional network structure [26].

Ball Milling In ball milling, cellulose fibers are placed in a ball mill and subjected to mechanical shearing and impact forces. This breaks down the fibers into nanoscale dimensions, which can then be crosslinked to form a three-dimensional network structure [4].

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These mechanical methods offer advantages such as scalability and simplicity and can be used to produce cellulose-based nanosponges with unique properties and applications in areas such as drug delivery, tissue engineering, and water purification.

4.2 Chemical Methods There are several chemical techniques for the synthesis of cellulose-based nanosponges some of which are as follows.

Phase Separation Phase separation is typically achieved by exploiting the variation in the solubility of polymeric compounds in two miscible solvents, with the polymer being soluble in only one of the two solvents. Four methods for carrying out phase separation are known, viz, thermally induced phase separation, vapor phase precipitation from the, air-casting of a polymer solution, and immersion precipitation [8].

Crosslinking of CNCs with a Crosslinking Agent This method involves mixing CNCs with a crosslinking agent, such as epichlorohydrin or glutaraldehyde, and an aqueous solution, followed by heating to allow initiation of the crosslinking reaction. The resulting crosslinked network structure forms a three-dimensional cellulose-based nanosponge. In 2015, a new crosslinking protocol for TEMPO-Oxidized Cellulose Nanofibers (TOCNFs) was introduced, utilizing bPEI (25 kDa) to synthesize CNS through thermal treatment. The resulting CNSs exhibited a sponge-like porous structure with amino groups, allowing for efficient adsorption of both organic and inorganic pollutants such as metal ions including Cd2+ , Zn2+ , Pb2+ , Cr3+ , Cu2+ , etc. with an adsorption potential of over 200 mg/g [25, 27].

Free Radical Polymerization This method involves addition of a monomer to a solution of CNCs and a crosslinking agent, and a free radical initiator is added to initiate the polymerization reaction. The reaction is typically carried out under mild conditions, such as at room temperature and in an inert atmosphere, to prevent unwanted side reactions. The resulting polymer network structure forms a three-dimensional cellulose-based nanosponge. This nanosponge structure contains a large number of small pores and a high surface area, thereby making it an ideal material for various applications [7, 26].

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Emulsion Polymerization In this process, the cellulose is first dispersed in water to form a stable emulsion, which is later polymerized in the presence of a surfactant and a crosslinking agent to obtain the nanosponge structure. The emulsion is then subjected to high shear forces, such as sonication or homogenization, to break down the cellulose into smaller particles and disperse it evenly throughout the water. A free radical initiator is added to initiate the polymerization reaction, that results in a 3-D cellulose-based nanosponge [28].

Solvent Casting and Freeze-Drying In this method, a solution of CNCs and a crosslinking agent is cast onto a substrate and dried. The resulting film is then freeze-dried, resulting in a porous cellulosebased nanosponge. The solvent casting and freeze-drying can be used to produce nanosponges with distinct properties depending on the concentration of cellulose and crosslinking agent, the choice of solvent, and the processing conditions. These methods offer a versatile and scalable approach to the synthesis of CNS with a broad spectrum of properties and applications [7, 25]. Overall, the synthesis of CNS can be achieved through a variety of methods, each with its advantages and disadvantages. The choice of synthesis method relies on the specific application requirements and the desired properties of the resulting nanosponge.

4.3 Bacterial Methods NC produced via the bacterial method has a chemical structure that resembles NC obtained by chemical and mechanical methods from lignocellulosic biomass. Furthermore, when grown in an appropriate culture medium, it forms an ultrafine nanofiber network with exceptional properties such as high purity, good water absorption capacity, uniform morphology and superior mechanical properties as well as flexibility [25]. Bacterial methods for the synthesis of CNS have gained much research attention owing to their potential for the large-scale production of high-quality NCFs. Cellulose-producing bacteria, such as Gluconacetobacter xylinus, are used in these methods to produce nanoscale cellulose fibers that can be further processed into nanosponges [17]. The bacterial synthesis of nanosponges involves the following steps: (a) Bacterial culture: Cellulose-producing bacteria are cultured in a nutrient-rich medium to facilitate the production of cellulose fibers [25, 29]. (b) Cellulose fiber production: The aerobic bacteria produce cellulose fibers through a process of extracellular polymerization, leading to the formation of a cellulose pellicle on the surface of the culture medium [30].

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(c) Harvesting of cellulose pellicle: The cellulose pellicle is harvested from the culture medium and undergoes mechanical treatment to break down the fibers into smaller NCFs [29]. (d) Crosslinking and formation of nanosponges: The NC fibers are then crosslinked using a suitable crosslinking agent to form a 3-D network structure, which is further processed to obtain CNS [31]. The nanosponges obtained possess excellent properties such as high porosity, high surface area, and biocompatibility, making them an attractive material for versalite applications in the field of biomedicine, including drug delivery, tissue engineering, and wound healing. It also offers many advantages, such as the ability to produce NC with high purity and mechanical strength and the potential for large-scale production using low-cost media [25]. Figure 5 shows the stepwise representation of the production of CNS. However, challenges remain in terms of optimizing the bacterial culture conditions, improving the mechanical properties of the nanosponges, and developing sustainable and environmentally friendly processes. Hence, bacterial methods for the synthesis of CNS offer a promising approach for the production of high-quality NC fibers with unique properties for various biomedical applications. Ongoing research in this area is expected to lead to further developments in the field of cellulose-based nanomaterials.

Fig. 5 Stepwise representation of the production of CNS [32]. (CC-BY-4.0)

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5 Active Sorption of Harmful Materials by Cellulose-Based Nanosponges for Water Remediation Several methods have been practiced to minimize water pollution, including physical adsorption, chemical precipitation, biological treatment, ion exchange, photocatalysis, and so on. However, physical adsorption is highly preferred due to its simplicity, cost-effectiveness, and environmentally friendly nature [9]. Adsorption is a phenomenon wherein a chemical substance is accumulated on the surface of the sorbent, being governed by the nature of intercations between adsorbate and the adsorbent. Adsorption isotherms show the relationship between the concentration of the adsorbate on the surface and the concentration of the adsorbent in bulk, while the temperature remains constant [7]. Several adsorption isotherm models are usually used to model and describe the adsorption behaviour of adsorbate-adsorbent interactions, of which the Langmuir isotherm model is the most adopted one used in the case of cellulose substrates. This model assumes monolayer adsorption with homogeneous site energies and is represented by Eq. (1). Adsorption is a process where a chemical substance is accumulated in a sorbent. The interactions between the sorbent and the sorbate govern this process, and it is often described using adsorption isotherms. Adsorption isotherms show the relationship between the concentration of the adsorbate on the surface and the concentration of the adsorbent in bulk, while the temperature remains constant [7]. qe =

Q max bCe 1 + bCe

(1)

where, Ce is the concentration of the species to be adsorbed (mg/L), qe is the number of species adsorbed at equilibrium (mg/L), and Qmax indicates the maximum amount of species that can be adsorbed per unit weight of adsorbate (mg/L), and b is the Langmuir constant related to adsorption energy (L/mg). Qmax and b can be determined experimentally and used to compare various adsorbents for a specific adsorbate. The Langmuir model is limited to low-pressure conditions and gives inference for chemical interactions between the adsorbent and the adsorbate. Freundlich isotherm model is another model that is often considered fro illustration of multiple layers of adsorption and energies corresponding to heterogeneous sites. The Eq. (2) can be written as: 1

qe = K F Cen

(2)

where KF and n are known as Freundlich constants. The Langmuir and Freundlich isotherm models can be used together to explain the adsorption process, especially in cases where different adsorbates compete [7, 33]. The kinetics studies are also crucial in the use of powdered sorbents in the field of water treatment because the rate at which solutes are removed impacts the amount

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of time the reactor needs to achieve the desired level of sorption and, consequently, the quality standard of the treated water [24]. Adsorption pertains to the binding of contaminants, onto the exterior of a solid material through chemical bonding or electrostatic interaction. NC adsorbents primarily employ complexation, electrostatic interactions, hydrogen bonds, ion exchange, van der Waals forces, and π-π interactions as adsorption mechanisms for metal ions and dyes [31]. An essential aspect of the effective utilization of adsorption is the provision of a highly specific surface area, which permits access to functional groups on the surface. This method facilitates the elimination of various pollutants from water, particularly cations such as heavy metal ions. Furthermore, through appropriate modifications such as the incorporation of ammonium groups, negatively charged components (like nitrates and phosphates) and organic pollutants (like dyes, pharmaceuticals oils, pesticides) can also be removed [15]. Adsorbent materials derived from cellulose have garnered significant interest owing to their hydrophilicity, functionalization capability, and feasibility of modifying various characteristics, including aspect ratio, surface area, and chemical accessibility. Nevertheless, traditional cellulose-based adsorbents possess limited selectivity due to a restricted number of active sites or surface area, leading to lower adsorption kinetics, and are challenging to recover from wastewater. Compared to cellulose particles that are measured in micrometers, the cellulose particles having nanometers size possess a larger surface area and greater porosity. This makes them more effective in removing impurities from wastewater, which is a significant advantage. The increased adsorbent potential of NC is attributed to the presence of several functional groups, as well as its increased surface area, good chemical resistance, mechanical properties, fine crystalline structure, and stereochemistry rules. The adsorption capacity cannot be greater than half the surface ionic site content. Therefore, functionalization of NC by increasing the surface area is essential to introduce more complex sites for enabling the adsorption of hazardous metal ions. Furthermore, NC is more thermodynamically stable than metallic nanoparticles, making it a suitable material for this particular application [12, 16, 21]. Nanosponges are novel materials with small sponges and consist of microscopic particles with cavities that are only a few nanometers wide. They possess the ability to entrap a wide range of substances in their polymeric matrix water-based solutions. NSs are proficient in adsorbing various types of molecules, whether they are organic or inorganic, by forming either inclusion or non-inclusion complexes [34]. This chapter focuses on the removal of various water contaminants using CNS which are described elaborately in the following sections.

5.1 Removal of Heavy Metal Ions Water can be contaminated with toxic heavy metal pollutants that come from natural or industrial sources. These contaminants are highly hazardous and harmful to the environment [7]. Human health is at significant risk owing to heavy metal pollution.

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According to Fu and Wang, heavy metals are defined as metals having atomic weight that ranges from 63.5 to 200.6 and a specific gravity higher than 5 [35]. According to the World Health Organization (WHO), certain heavy metals, including mercury, cadmium, arsenic, and lead are among the top ten chemicals of high concern [16]. Such hazardous metal ions involving Ni2+ , Ag+ , Cd2+ , Cu2+ , Pd2+ , Hg2+ , U6+ , etc. are frequently released into the environment due to activities such as paint manufacturing, battery manufacturing, petroleum refining, mining, and photographic products [12]. One example of this is the use of copper in various industries, which can contaminate water sources, including groundwater, through leakage. The non-biodegradable nature of heavy metals and their tendency to accumulate in living organisms make metal pollution a significant concern [36]. This contamination can result in serious implications for human health due to excessive intake of Cu2+ ions in the body [9]. Removing heavy metals from water is crucial because their high concentrations can lead to neurological diseases, cancer, organ damage, and even death in severe cases [16]. Adsorption of hazardous metal ions, both radioactive and heavy, is a suitable method for treating water owing to its good potency, low cost, and ease of operation. Many studies have suggested that nano sorbents, such as graphene oxide, carbon tubes, zeolites polymeric materials, metals, and metal oxides, are effective in removing heavy as well as radioactive metals from wastewater. When using NCbased adsorbents for heavy metal removal, two primary mechanisms are involved, i.e., ion exchange and complexation. The ion exchange mechanism occurs when hazardous metal ions (Mn+ ) are adsorbed onto the NC surface in place of other ions (K+ , Na+ , H+ ) already associated with it. In chemical complexation, specific site interactions occur between the –COO− (carboxyl) and –OH− (hydroxyl) groups of the NC and certain hazardous metal ions (Mn+ ) [12]. Figure 6 represents both the above discussed heavy metal removal mechanisms from water systems using NC. Fiorati et al. introduced an eco-friendly strategy for the development of CNS, which proved to be effective sorbents for removing heavy metals from sea-water [37]. The materials were obtained via a two-step method involving the production of TOCNFs and their subsequent crosslinking with branched polyethyleneimine. The CNSs showed high performance in removing several heavy metal ions, such as Zn2+ , Cd2+ , Cr3+ , Hg2+ , Ni2+ , and Cu2+ , from sea-water in 1–250 ppm concentration range A rapid, cost-effective, and straightforward solid-phase extraction (SPE) method was tested using dithizone-modified cellulose acetate nanosponges by Zargar and co-workers. It was found to be effective in eliminating Pb+2 and Cd+2 from aqueous solutions. The adsorption process followed Langmuir isotherm model. The adsorption process was completed in just 0.1 min, indicating a fast kinetics of the adsorption process. The dithizone-modified adsorbent was found to have a maximum adsorption capacity of 787 mg/g and 195 mg/g toward lead and cadmium respectively [8]. Guidi and coworkers analyzed a solution of artificially contaminated freshwater with CdCl2 (0.05 mg L−1 ) by treating it with CNS (1.25 g L−1 ) for 2 h. The cellular responses of Dreissena polymorpha hemocytes were analyzed both before and after the treatment with CNS. CNS turned out to be a safe and effective solution for cadmium remediation, efficiently sequestering the metal and restoring cellular damage caused by Cd+2

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Fig. 6 Mechanism of Heavy metal adsorption from water system using NCs: a Ion exchange mechanism b chemical complexation mechanism [12]. (CC-BY-4.0)

exposure without altering the cellular physiological activity [38]. An inexpensive and easily scalable method devised by Melone and colleagues for producing cellulosebased adsorbent materials involves using TOCNFs and bPEI. These materials have a sponge-like structure and good mechanical stability in water. These properties are attributed to the stable amide bonds formed between the carboxylic groups of TOCNF and the primary amines of the bPEI. The use of these cellulose-based sponges has been effective in removing heavy metals such as copper, cobalt, nickel, and cadmium [27].

5.2 Removal of Organic Pollutants A vast array of substances, such as dyes, oils, pesticides, and pharmaceuticals, make up the category of organic pollutants found in water [7]. Organic pollutants in water can cause various health issues, such as skin rashes, headaches, and respiratory problems. Ingesting or inhaling these pollutants can lead to acute toxicity, cancer, skin irritation, and allergic reactions. NCs based sponges are being studied as potential adsorbents to remove contaminants like pesticides, fertilizers, and drugs [12]. Carboxylation is the most studied method for enhancing the sorptive capacity of CNSs. Their hydrophilicity can be reduced by modifying their surface chemistry, making them

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effective at adsorbing hydrophobic compounds. Advances in nanoscience offer the potential for developing nano-based sponges for water treatment from organic pollutants such as toxic dyes, oils and solvents, salts, pharmaceuticals, pesticides, and pathogenic microorganisms [36].

Removal of Toxic Dyes Dyes are substances that can attach to surfaces or fabrics and produce vibrant and long-lasting colors. Synthetic dyes have been quickly replacing natural dyes due to their advantages, and over time, more synthetic dyes have been manufactured for use in various industries. Currently, there are over 100,000 commercial dyes available, with annual production exceeding 7 × 105 tonnes. However, with increasing production comes an increase in waste, as 450,000 tonnes of organic dyes are produced each year, and over 11% of this is lost during manufacturing and application processes [16]. Organic dye pollutants show anionic, cationic, or non-ionic properties. Most importantly, artificial dyes have been favored over natural dyes due to their superior stability to factors such as light, oxygen, heat, and pH, as well as their consistent color and lower cost [34]. Complex aromatic structures found in most dyes exhibit biological activity, rendering them challenging to be removed through conventional waste treatment methods [36]. Cationic dyes can be adsorbed by using anionic moieties that have been functionalized on NC-based materials and vice-versa for anionic dyes [9]. For instance, research by Shiralipour et al. involved the preparation of a new absorbent using CNS that was modified by incorporating methyltrioctylammonium chloride. The purpose was to apply this absorbent for pre-concentration, elimination, and quantification of tartrazine dye using UV–visible spectrophotometry. When conventional isotherm models were used to analyze the experimental data, it was discovered that the adsorption process followed Brunauer–Emmett–Teller (BET) model. The modified nanosponges had a maximum adsorption capacity of 180 mg/g for tartrazine, as calculated through the model [34]. Another study by Riva and coworkers consisted of a system which is composed of micro- and nano-porous sponge-like material formed through thermal crosslinking of TOCNFs, bPEI (25 KDa), and citric acid. Its capacity to adsorb organic dyes was examined through both thermodynamic and kinetic studies using four different dyes. Overall, cellulose-based sponges offer advantages in terms of reusability, handling, ecofriendly, and sustainability. Another added advantage is that it can be produced using waste biomass, making it a sustainable choice that goes well with circular economy principles [24].

Removal of Oils Oil pollution is mainly caused by oil spills from tankers, although there exist other sources that cumulatively contribute to this problem. The level of toxicity in oil

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is primarily associated with the aromatic component, which is comprised of polycyclic aromatic hydrocarbons (PAHs) of both low molecular weight (LMW) and high molecular weight (HMW), and the ratio of these LMW and HMW PAHs is particularly significant [12]. The key issue with oil pollution is its rapid dispersion, particularly in the case of lighter oils. The spreading can be intensified by wind and high temperatures. The coverage of water surfaces by oil has adverse effects on the aquatic ecosystem and may lead to the contamination of drinking water sources for these activities for food. Oil pollution can also have a significant impact on humans, especially on those who rely on fishing and hunting. Additionally, it can contaminate sources of drinking water, making it crucial to remove oil spills from water. To address this, researchers have studied the development of selective materials that can rapidly absorb large amounts of oil while being robust, floating, reusable, environmentally friendly, and incinerable with the absorbed oil. Remediating water contaminated with oily substances is challenging due to its various forms and diverse nature. Therefore, sorbent materials used for bioremediation should ideally be recoverable and biodegradable with minimal environmental impact [16, 36]. Nanosponges possess the desired qualities which are shown in the studies reported here. A research work carried out by Meng and co-workers describes the synthesis and properties of an NC-based carbon aerogel, which has a sponge-like structure, high porosity of 99%, ultralight density of 0.01 g/cm3 , hydrophobicity with a contact angle of 149°, fast absorption rate, BET surface area of 521 m2 /g and can be reused multiple times. The exceptional characteristics of the cellulose-based sponge, combined with the benefits of utilizing an affordable, sustainable, and renewable material, make these materials ideal for a broad spectrum of applications related to oil spill clean-up [39]. An octadecyl trichlorosilane-grafted cellulose-based sponge was successfully prepared by Meng and co-workers to exhibit super hydrophobicity causing it to repel water and attract oil, as demonstrated by its water contact angle of 153.5° and oil contact angle of 0°. The modified adsorption capacity of the sponge was quite impressive, with an uptake capacity of up to 700% of its weight in chloroform and still maintaining about 200% of its weight in vegetable oil after being extruded five times, indicating both excellent absorption performance and recyclability. Overall, the sponge was found to be highly efficient in separating water and oil, achieving a separation efficiency of 98% for chloroform and displaying excellent selectivity for different oil/water mixtures [40].

Removal of Salts With the scarcity of clean water, the treatment of contaminated water is in high demand [16]. Recent contributions have made emphasis on the use of nanomaterials to improve the detection, capture, followed by removal of harmful pollutants as they are eco-friendly and sustainable in nature [11, 41]. Water filtering was also demonstrated by polyamide-based thin-film composites (TFCs), a membrane separation method, for the removal of salts. Several ways have been used throughout the years to get optimized results of polyamide-based thin film composite. Nanofiber

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Fig. 7 a Variation of adsorption with pressures on TFN-50 NFMs; b effect of alkaline (pH = 13) and acidic (pH = 1) treatments on normalized salt rejection and water flux of the TFN-50 NFMs [9]. (CC-BY-4.0)

membranes (NFM). More frequently in 2020, Ahankara et al. reported the incorporation of nanoparticles into membranes to alter the permeability capabilities of polyamide selective layers. Utilizing thin-film nanocomposite NFMs, also known as TFN NFMs, the nanofillers were found to have improved antifouling, chlorine, and antibacterial resistance. Various organic and inorganic nanomaterials have been produced, but their synthesis techniques have been notoriously difficult. Then, the researchers reported CNCs as one of the best adsorbents because it is abundant and also easily synthesized through acid hydrolysis of cellulose. It also showed good antifouling characteristics when added to reverse osmosis membranes. Thus, an increase in water flux by 35%, is observed on the addition of CNCs leading to higher performance by making the membrane rougher and more hydrophilic than plain TFN NFMs. The normalized salt rejection and water flux after alkaline and acidic treatments demonstrated good compatibility between CNCs and polyamide matrix (as shown in Fig. 7). It was concluded that the CNCs provide ecofriendly nanomaterials with anti-chlorine properties and a stable water permeation rate, making them ideal for TFN NFMs. Figure 8 shows a graphical depiction of the variation of the copper and chromium metals removal percentage with time for various modified CNC percentages [9].

Removal of Pharmaceuticals and Pesticides Various organic pollutants such as pesticides, dyes, and pharmaceuticals are harmful even at low levels in the environment. The accumulation of pharmaceutical molecules, including antibiotics, analgesics, and anti-inflammatory drugs (ibuprofen) in wastewater is becoming an increasingly alarming issue due to their potential to build up in aquatic organisms [7, 11]. While there have been numerous reports on the effectiveness of using NC as a means of transporting drugs, there has been limited research on how drugs adhere to NC [7]. Thus in 2017, Voisin et al. reported the ability of sulfated CNCs as an effective adsorbent of ionized drugs from water. It is possible

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Fig. 8 a Variation of percentage removal of Cu+2 ions with contanct time and modified CNC concentration b concentration of chromium metals passed through membranes having PCL and CNFs in different proportions [9]. (CC-BY-4.0)

to adsorb hydrophobic drugs by attaching a surfactant, such as a cetyltrimethylammonium bromide, to CNCs to make them more hydrophobic. The highest uptake observed for docetaxel and paclitaxel was around 0.1 mg/g, and for tetracycline hydrochloride (TC) was approximately 7 mg/g at pH = 5. NC has demonstrated a similar ability to bind cationic drugs as Carboxylated CNCs acquired through TEMPO-mediated oxidation, despite limited research on the subject. Thus evidence by Voisin H. and his co-researchers indicated that NC has the potential to be an effective drug adsorbent [7]. In 2021, an insoluble cyclodextrin (CD) polymer was introduced as a costeffective adsorbent, having attractive removal capabilities and greater recyclability in comparison to traditional methods. The team of researchers created three waterinsoluble polymers based on CDs to remove three specific pharmaceutical ingredients (levofloxacin, aspirin, and acetaminophen) from an aqueous medium. The β-CD polymer had the highest sorption capacity and showed a greater affinity for aspirin in comparison to the other two polymers. Additionally, the studies related to the removal of ibuprofen and progesterone in aqueous solutions of various poly CDs crosslinked with citric acid were investigated. It was observed that (i) poly(β-CD) exhibited superior adsorption capabilities, greater reusability, and faster adsorption kinetics; (ii) the adsorption capacity did not noticeably fluctuate with the conditions and amount of adsorbent; (iii) the improved adsorption kinetics with temperature and ionic strength. Later, other methods were developed to detect and measure the drug concentration of diclofenac in wastewater samples with concentrations ranging from 0.3 to 15.9 mg/ L. This was achieved by using poly-CD with a fluorescent dye [11]. Worldwide, pesticides are widely used as organic pollutants to prevent plants and improve food quality. They primarily affect soil pollution and ultimately end up contaminating water bodies. Since pesticides are usually water-insoluble, the options for remediation are limited. Cova et al. applied the strategy of CDs for its removal as they have a significant advantage in terms of solubilizing and eliminating such harmful contaminants. The study aimed to develop effective CD polymers for

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separating and removing ten different pesticides. The researchers synthesized poly CDs using one type of CD or a mixture of two CDs with EPI as a crosslinker, resulting in porosity. The adsorption isotherms of the pesticides revealed the high adsorption capability of the adsorbents via multiple adsorption interactions. The efficiency to remove depended on the CD surface area and its content and capacity to swell up. Pesticides with a high partition coefficient for octanol–water systems, such as butenefipronil and fipronil, showed higher removal efficiencies, indicating the importance of hydrophobic interactions with CDs in the adsorption mechanism. Nano-porous carbon has proven to be highly effective in removing pesticides such as DDT, DDD, and DDE, which belong to the p, p' -substituted diphenyl class. CDs contained within the carbon have demonstrated excellent adsorption capabilities due to the ability of pesticides to fit precisely within the CD’s cavity, forming stable inclusion complexes. The researchers proposed and tested the use of a textile coated with anionic polyCD as an anchor agent for the adsorption of a toxic herbicide in aqueous solutions. The removal efficiency was found to be high, with neutral pH and low-temperature range being the optimal conditions for adsorption. The material was also found to be reusable, with the maximum adsorption capacity retained even after regeneration. Adsorption was shown to be an effective, low-cost, and straightforward method for removing paraquat from aqueous systems [11].

5.3 Removal of Pathogenic Microorganisms Disinfection is a crucial step in wastewater treatment involving pathogens removal. NC, however, has minor impacts on microbial inhibition and can even serve for microbial growth. Therefore, antimicrobial and antibacterial properties can be added to NC-based materials to inhibit microbial growth in the contaminated water. This can be achieved through modification of the surface with antimicrobial polymers, metal NPs (such as Ag and Au), and metal oxide NPs (such as TiO2 , CuO, ZnO, and MgO). Surface modification of NC using TEMPO oxidation to introduce functionalized carboxyl groups has been shown to create active sites with high affinity towards E. coli, resulting in a rejection rate of approximately 96–99%. However, the carboxyl groups on their own do not have intrinsic antibacterial properties. They can, however, be effective antibacterial agents. For an epitome, modification of cationic surface via quaternary ammonium compounds is a direct method to induce antibacterial activity. NC membranes with smaller pores can show antifouling properties and antimicrobial filtration applications. In a research project, a composite material made of aluminum oxyhydroxide-NC was used to embed antibacterial Ag NPs. The purpose was to determine the best concentration of chloride in water to achieve effective antibacterial properties against bacteria. The results showed that a higher concentration of chloride led to a great enhancement in the ability of the material to prevent the proliferation of bacterial cells, indicating its potential as an effective antibacterial agent. Recently the composites of carboxylated CNFs containing silver nanoparticles (Ag NP) and dopamine were developed which exhibited improved

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electrical conductivity, mechanical properties, and antimicrobial activity. The strong interaction between catechol moieties and Ag NPs resulted in anisotropic alignment of the composite, which imparted flexibility and antibacterial activity to the biomaterials. The authors suggested that these materials could be useful in disinfecting wastewater [12].

6 Current Challenges and Limitations NC is a potential candidate for wastewater treatment and desalination as it possesses various unique properties, such as chemical stability, mechanical strength, and resistance to harsh environmental conditions. As a result, NC is a promising solution for the treatment of wastewater [29]. There are significant challenges involved in scaling up the production of aerogels, which form nanosponges, using CNFs and CNCs. These challenges include the high energy requirements of the manufacturing processes for CNFs and CNCs, as well as the time-consuming and cost-ineffective freeze-drying process required to produce CNF- and CNC-aerogels. Another challenge is the need to regenerate aerogel-type sorbents for reuse, which can be a slow and difficult process. Additionally, these can be subjected to degradation, fouling, and cracking, which can limit their effectiveness over time [13]. Although CNS exhibit potential for use in water purification to remove organic pollutants and heavy metals, it is still difficult to tune the key parameters such as surface area, permeability, porosity, and targeted adsorption [12]. Some several challenges and limitations need to be addressed some of which are: (a) Limited selectivity: While CNS have been shown to effectively remove various contaminants from water, they lack selectivity towards specific pollutants due to their moisture sensitivity. This can limit their effectiveness for targeted water remediation [14]. (b) Poor stability: CNS can be susceptible to degradation in certain environments, which can limit their effectiveness and lifespan in water remediation applications [7]. (c) Scaling up production: Current methods for synthesizing CNS are often smallscale and may not be easily scalable for large-scale production, which can limit their commercial viability for water remediation applications [1, 7, 13]. (d) High cost: The cost of producing CNS can be relatively high compared to other water remediation methods, which can limit their practical application in certain settings [7]. (e) Limited knowledge of long-term effects: The quantum of research on the longterm effects of using CNS for water remediation has been limited, including potential environmental impacts and health risks [42]. Comparative analyses have been carried out for various nano adsorbents along with nanosponges to address the challenges and advantages of nanosponges over other materials. The analysis is shown below in Table 2.

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Table 2 Comparative analysis of challenges and advantages of nanosponges with other nano adsorbents in wastewater remediation Nanoadsorbents

Challenges

Advantages

Refs.

1

Nanosponges

• Feasibility for mass production • Potential hazards regarding health and the environment • Complexation behaviour

• Simple to use • Strong compatibility with biological systems • Simple surface functionalization capabilities • Bio-absorption characteristics • Biodegradable • Specificity • Porosity

[1, 7, 12]

2

Carbon-based nanomaterials

• Expensive • Low level of specificity • Inadequate ability to regenerate

• Excellent performance in [43] eliminating water pollutants • Strong adsorption capacity • Potential for commercial development

3

Silica-based nanomaterials

• Low resistivity • Inexpensive towards alkaline • Abundant in nature solutions • Not suitable for solutions with pH < 8

[44]

4

Nano-biosorbent materials

• Sensitive to changes in pH • Requires modifications to enhance characteristics

[45]

5

Nano-photocatalysts • Preparation • Affordable • Feasibility for mass • Stable production • Reusability • High sensitivity • Environmentally friendly • Large specific surface area • Adjustable pore sizes and shapes

S. No.

• Inexpensive • Simplicity in design and use • Effective sorption capabilities • More selectivity • Highly efficient • Biodegradability

[45, 46]

(continued)

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Table 2 (continued) Nanoadsorbents

Challenges

Advantages

6

Nano-zeolites

• Limited adsorption ability • Poor permeation • Inability to effectively eliminate organic pollutants • Complicated adsorption mechanism

• Affordable [47] • Large specific surface area • Suitable for removing a wide range of contaminants

7

Nano-clays

• Limited adsorption capacity • Sensitive to changes in pH

• Affordable [43] • Suitable for removing various types of organic and inorganic contaminants

S.

Refs.

No.

Somewhere, the use of nanosponges is limited by a few challenges and limitations that need to be addressed in future research and development efforts. Overall, CNS show promise for water remediation when compared with other nano adsorbents.

7 Conclusions and Future Outlook Water pollution is a major environmental problem that is a result of urbanization, industrialization, and agriculture. Polluted water can contain various contaminants ranging from pathogens and heavy metals to highly toxic compounds like pesticides and pharmaceuticals that can pose serious threat to human health and the environment [13]. The process of water remediation aims to eliminate or decrease pollutants from contaminated water to make it suitable for human consumption and safe for environmental utilization [42]. Nanosponges have become increasingly popular as a potential candidate for removing a wide range of pollutants incluing both organic and inorganic species from water. NC is an effective adsorbent due to its high strength and flexibility, as well as its large surface area and versatile surface chemistry. CNFs and nanocrystals can be produced from NC through various hydrolysis and catalysis processes, resulting in tailored surface properties. NC possessing various cationic or anionic surface groups has been demonstrated to effectively remove pollutants such as heavy metals from water bodies. Moreover, NC having both ionic as well as non-ionic surface groups has also been utilized to adsorb a variety of organic pollutants such as pesticides, dyes, solvents and oil for water treatments [7]. Overall, while many nanomaterials show promise for water remediation, CNS offer unique advantages in terms

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of their biodegradability, adsorption capacity, and specificity for targeting contaminants. This is due to their unique properties, such as high porosity, ease of functionalization, simplicity, and cost-effectiveness, which make them stand out amongst other nanosystems and materials [12]. One of the key advantages of CNS is their ability to be easily functionalized, which allows them to selectively target specific contaminants. Additionally, they can be easily regenerated and reused, making them a cost-effective and sustainable solution for water remediation [7]. Overall, CNS show great potential to capture and remove substances like pharmaceuticals, heavy metals, and dyes for addressing water pollution and promoting cleaner water sources [1]. However, more research is needed to optimize the synthesis and properties of CNS to improve their efficiency and selectivity in removing contaminants from water. As research in this area progresses, several future outlooks can be envisioned. One potential future direction is the development of new and innovative methods for the synthesis of the scalability of production processes and the functionalization of cellulose-based nanosponges. By optimizing the properties of these materials, such as their pore size and surface area, researchers can improve their efficiency and selectivity in removing specific contaminants from water. Another future outlook for CNS is the integration of these materials into larger-scale water treatment systems. Lastly, the impact of CNS on the environment will need to be thoroughly evaluated as their use becomes more widespread. While these materials are biodegradable, their impact on the environment, especially if released into aquatic systems, needs to be studied to ensure that their use does not cause unintended harm. In conclusion, the future outlook for CNS for water remediation is bright. With further research and development, these materials have the potential to become a key component in sustainable and efficient water treatment systems that may eventually prove helpful in addressing the perennial global problem of water pollution. Acknowledgements The authors are thankful to the Principal, Sri Venkateswara College, University of Delhi for her continuous cooperation and guidance. The authors would also like to acknowledge the HOD, Department of Chemistry, University of Delhi, for his constant encouragement and support. Declaration of Competing Interest The authors declare no competing financial interests that could have appeared to influence the work reported in this paper.

Abbreviations BET BNC bPEI CD CNCs

Brunauer-Emmett-Teller Bacterial Nanocellulose Branched polyethyleneimine Cyclodextrin Cellulose Nanocrystals

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CNFs CNSs DDD DDE DDT ENMs EPI HMW LMW NC NCFs NFM NPs NSs PAHs PCL SPE TC TEMPO TFCs TOCNFs TOUS-CNFs WHO

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Cellulose Nanofibers Cellulose-based nanosponges Dichlorodiphenyldichloroethane Dichlorodiphenyldichloroethylene Dichlorodiphenyltrichloroethane Engineered nanomaterials Epichlorohydrin High molecular weight Low molecular weight Nanocellulose Nanocellulose fibres Nanofiber membranes Nanoparticles Nanosponges Polycyclic aromatic hydrocarbons Polycaprolactone Solid-phase extraction Tetracycline hydrochloride 2,2,6,6-Tetramethylpiperidinyloxyl Thin-film composites TEMPO-Oxidized Cellulose Nanofibers Transparent, Outstanding, Uniform, and Sustainable Cellulose Nanofibers World Health Organization

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32. Riva, L., Lotito, A. D., Punta, C., & Sacchetti, A. (2022). Zinc-and copper-loaded nanosponges from cellulose nanofibers hydrogels: New heterogeneous catalysts for the synthesis of aromatic acetals. Gels, 8(1), 54. 33. Gopakumar, D. A., Arumughan, V., Pasquini, D., et al. (2019). Nanocellulose-based membranes for water purification (pp. 59–85). Elsevier. 34. Shiralipour, R., & Larki, A. (2017) Pre-concentration and determination of tartrazine dye from aqueous solutions using modified cellulose nanosponges. Ecotoxicology and Environmental Safety, 135, 123–129. 35. Fu, F., & Wang, Q. (2011). Removal of heavy metal ions from wastewaters: A review. Journal of Environmental Management, 92, 407–418. 36. Gopakumar, D. A., Manna, S., Pasquini, D., et al. (2018). Nanocellulose: Extraction and application as a sustainable material for wastewater purification (pp. 469–486). Elsevier. 37. Fiorati, A., Grassi, G., Graziano, A., et al. (2020). Eco-design of nanostructured cellulose sponges for sea-water decontamination from heavy metal ions. Journal of Cleaner Production, 246, 119009. 38. Guidi, P., Bernardeschi, M., Palumbo, M., et al. (2020). Suitability of a cellulose-based nanomaterial for the remediation of heavy metal contaminated freshwaters: A case-study showing the recovery of cadmium induced dna integrity loss, cell proliferation increase, nuclear morphology and chromosomal alterations. Nanomaterials, 10, 1–15. 39. Meng, Y., Young, T. M., Liu, P., et al. (2015). Ultralight carbon aerogel from nanocellulose as a highly selective oil absorption material. Cellulose, 22, 435–447. 40. Meng, X., Dong, Y., Zhao, Y., & Liang, L. (2020). Preparation and modification of cellulose sponge and application of oil/water separation. RSC Advances, 10(68), 41713–41719. 41. Paul, J., & Ahankari, S. S. (2023). Nanocellulose-based aerogels for water purification: A review. Carbohydrate Polymers, 309, 120677. 42. Fiorati, A., Bellingeri, A., Punta, C., & Corsi, I. (2020). Silver nanoparticles for water pollution monitoring and treatments: Ecosafety challenge and cellulose-based hybrids solution. Polymers, 12(8), 1635. 43. Nasrollahzadeh, M., Sajjadi, M., Iravani, S., & Varma, R. S. (2021). Carbon-based sustainable nanomaterials for water treatment: State-of-art and future perspectives. Chemosphere, 263, 128005. 44. Shafiei, N., Nasrollahzadeh, M., & Iravani, S. (2021). Green synthesis of silica and silicon nanoparticles and their biomedical and catalytic applications. Comments on Inorganic Chemistry, 41, 317–372. 45. Nasrollahzadeh, M., Sajjadi, M., Iravani, S., & Varma, R. S. (2021). Green-synthesized nanocatalysts and nanomaterials for water treatment: Current challenges and future perspectives. Journal of Hazardous Materials, 401, 123401. 46. Nasrollahzadeh, M., Sajjadi, M., Iravani, S., & Varma, R. S. (2020). Trimetallic nanoparticles: Greener synthesis and their applications. Nanomaterials, 10(9), 1784. 47. Awasthi, A., Jadhao, P., & Kumari, K. (2019). Clay nano-adsorbent: Structures, applications and mechanism for water treatment. SN Applied Sciences, 1, 1–21.

Application of Nanosponges for Aquifer Bioremediation Shikha Gulati, Himshweta, Manan Rana, Nabeela Ansari, and Shalu Sachdeva

1 Introduction The term Nanosponges (NS) refers to a class of insoluble materials with distinctive nanometric porosity and superior absorption/complexion properties. NSs can be synthesized using either organic or inorganic compounds [1]. Thanks to its unique toroidal, truncated cone-shaped cyclic structure, with a hydrophobic cavity—that can house different types of guest molecules, along with reactive hydroxyl groups facing outwards—due to its susceptibility to various chemical modifications such as cross-linking, a range of chemical derivatives have been created and utilized in both research and practical applications [2]. It is a modern category of materials and has narrow cavities that can be filled with various types of substances. These minute particles possess the ability to transport both hydrophilic and lipophilic substances. The nanosponges have a three-dimensional scaffold (backbone) or network polyester that is actually capable of degrading naturally. These polyesters are mixed with a cross-linker in a solution to form nanosponges. Here, the polyester is generally biodegradable [3]. One such polymer that has gained prominence in the past is known as cyclodextrin nanosponges (CD-NS). Cyclodextrins can include organic compounds inside their cavities. Inclusion is based mainly on an interaction between the guest molecules and the cavity which is particularly hydrophobic. This type of reaction between cyclodextrins and organic molecules can be used as the basis for the adsorption S. Gulati (B) Department of Chemistry, Sri Venkateswara College, University of Delhi, Delhi 110021, India e-mail: [email protected] Himshweta · M. Rana · N. Ansari Department of Biochemistry, Sri Venkateswara College, University of Delhi, Delhi 110021, India S. Sachdeva Department of Chemistry, Acharya Narendra Dev College, University of Delhi, Delhi 110019, India © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Gulati (ed.), Nanosponges for Environmental Remediation, https://doi.org/10.1007/978-3-031-41077-2_17

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or separation of several organic agents. Nevertheless, cyclodextrins are soluble in water and in some organic solvents. Given their solubility in water, cyclodextrins can’t, therefore, be used directly for retaining organic substances. To overcome these problems of solubility, cyclodextrins have been used as basic building blocks in the preparation of cross-linked insoluble polymers. The utilization of NSs offers various benefits, one of which pertains to the polymer network encompassing the cavities, impeding the movement of the diffusion of entrapped guest molecules, in addition to facilitating slower release kinetics, another significant advantage of NSs is their insolubility, which enables easy recovery and recycling from aqueous media [4]. Nowadays, human activity and the modern way of life are responsible for the increase in environmental pollution. Industrial processes generate a variety of molecules that may pollute air, water, and soils due to negative impacts on ecosystems and humans. The emergence of effective environmental remediation technologies has become a crucial priority, and supramolecular chemistry, a relatively new discipline, holds promise in offering solutions. Here, we present the use of nanosponges derivatives in remediation technologies for aquifers [5].

2 Types of Nanosponges Nanosponges have been categorized as follows (Fig. 1):

Fig. 1 The classification of nanosponges [1, 6]

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I Cyclodextrin-Based Nanosponges Cyclodextrin NSs usually have an average diameter within 1 μm, with a very low polydispersity index, showing mono-dispersed particles. The NSs exhibit remarkable stability due to their high zeta potential values, typically negative, and their swelling characteristics are determined by the type of cross-linker used. The branching of cross-linker, and attachment of basic and acidic groups, also influence the surface area and porosity. The NSs made up of cyclodextrin are thermally stable up to 300 °C and show different peaks in FTIR spectroscopy [7]. The chemical composition and properties of cyclodextrin-based nanosponges allow them to be classified into four successive generations. • First Generation of Cyclodextrin Nanosponges (i) Cyclodextrin Based urethane nanosponges Urethane (or carbamate) CD-NSs are primarily identified by their inflexible framework, exceptional durability against chemical breakdown, and minimal degree of expansion in both aqueous and organic solvents. Carbamate CD-NSs were first developed to use in treating wastewater. Compared with activated carbon, these NSs gave a remarkable performance in the removal of some organic molecules, like p-nitrophenol, which was absorbed even at low concentrations (10–7 –10–9 M) and reduced to ppb levels. Mamba et al. employed urethane CD-NSs to remove organic pollutants from the feed water of power generation plants. It also demonstrated a significant affinity for biologically important compounds such as amino acids and bilirubin. (ii) Cyclodextrin-Based Carbonate nanosponges Carbonate-based nanosponges using CD exhibit limited swelling capacity due to their short cross-linking bridges. However, they display excellent stability when exposed to mildly alkaline or acidic solutions. Moreover, like urethane, they also exhibit a strong affinity towards certain organic molecules [8]. In some cases, absorption performances are comparable with or even better than those of activated carbon. (iii) Cyclodextrin-Based Ester Nanosponges In contrast to carbonate and urethane nanosponges, ester-based nanosponges possess a remarkable water absorption capacity, up to 25 times their weight, and can form hydrogels. The swelling ability of their NSs is usually dependent on the degree of cross-linking: the lower the density of cross-linking, the higher the water uptake. In terms of chemical stability, ester-based nanosponges are more susceptible to hydrolysis in aqueous environments compared to carbonate and urethane nanosponges [8]. (iv) Cyclodextrin-Based Ether Nanosponges CD-based ether NSs exhibit a high chemical resistance and a tunable swelling capacity. The synthesis of CD-based ether NSs is usually performed in water in the

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presence of a base. According to the experimental conditions, either soluble or insoluble polymers can be obtained. Alsbaiee et al. synthesized an aromatic ether NS based on CD, which possesses a substantial surface area ranging from 35 to 263 m2 /g from a potassium carbonate-catalyzed reaction between beta-CD and tetrafluoroterephthalonitrile. Permanent porosity and water reagin ability were tested by operating on the reaction conditions. When tested in the removal of organic pollutants, such as endocrine disruptive agents and pharmaceuticals, the polymer outperformed a leading activated carbon with surprisingly rapid adsorption kinetics. • Second Generation of Cyclodextrin Nanosponges Introducing desired functionalities into the CD polymers described thus far has expanded their range of potential applications and paved the way for a novel generation of NSs. Specific moieties can be introduced according to three strategies: Post-cross-linking functionalization of an NS, pre-cross-linking functionalization of CDs, or, the addition of a functionalizing agent simultaneously to the cross-linking step. In the first case, mostly the particle surface will be altered, whereas in the other cases, a homogenous concentration of chemical functionalities will be distributed throughout the entire NS particle. So far, two primary modifications have been made to the chemical structure of CD-based nanosponges: 1. The labeling with fluorescence active moiety. 2. And, the introduction of either electrically charged or hydrophobic groups for tuning NS polarity. • Third Generation of Cyclodextrin Nanosponges Polymers that are sensitive to external stimuli can modify their characteristics, such as shape, permeability, or color, in response to environmental changes. The capacity to detect and respond to a stimulus is attributed to stimuli-responsive mechanisms at the molecular and supramolecular levels, as well as morphology. Stimuli-sensitive nanocarriers are known for their controlled target/release upon initiation by stimulating signals or specific transport pathways, as well as their ability to enhance therapeutic efficacy with minimal side effects. The usefulness and safety of CDs for drug formulation and gene delivery are due to the reversibility of host–guest interactions between CDs guest molecules easily achieving the chemical responsive moieties by supramolecular recognition or covalent conjugation to attain physio-chemical stimuli sensitivity. • Fourth Generation of Cyclodextrin Nanosponges Molecular imprinting is a method of inducing molecular recognition properties in three-dimensional polymers in response to the presence of a template molecule during the formation of a polymer. MIPs can function as materials for biosensors, systems for drug delivery, catalysts, and mimics of antibodies to enable molecular recognition and quantitative assays. In biosensor applications, MIP synthetic materials are used

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in proteins or enzyme recognition, cholesterol estimation, antibody isolation, etc. [1]. II Titanium-Based Nanosponges Nanosponges composed of titanium are coatings on micro-spheres of polystyrene, which were synthesized by copolymerizing polymerizable surfactants and styrene. Fabrication of TiO2 /ZnO hybrid NSs is used as photoanodes for photoelectrochemical applications. A diverse range of applications have been reported for metallic nanosponges, including TiO2 nanosponges, carbon-coated metallic nanosponges, and silicon nanosponges. These applications include serving as recyclable oil absorbents, possessing photo-catalytic properties, acting as electrodes, and reducing pollutants. III Silicon-Based Nanosponges Silicon nanosponges particles are prepared from metallurgical grade, silicon powder, and nearly 1–4 microparticles were scratched to yield silicon nanosponges particles. The highly porous silicon NS acts as a carrier material in the field of sensors, catalysts and drugs, photosensitizers, absorbents, explosives, and electrodes in fuel cells. It is also turned precursor for ceramic materials which have high surface areas like SiNa and SiC. IV Hyper Cross-Linked polystyrene-based nanosponges The isolated coils of polystyrene were immersed in sparse solvents, and copious amounts of rigid intramolecular bridges were added, causing the coils to contract significantly and transform into spherical NSs. The NSs showed high diffusion, low viscosity, and high rates of sedimentation. These NSs exhibited enchanted inner surface area and the swelling was very strong in the presence of linear polystyrene nonsolvent. The hyper cross-linked NSs were used in the appropriate separation of inorganic electrolytes by applying the principles of size exclusion chromatography. Hyper-cross-linked polystyrene NS and cyclodextrin-based NS have been used in tissue scaffolds [7].

3 Synthesis and Manufacture of Nanosponges Generally, the nanosponges consist of a polymer matrix often made up of biocompatible and biodegradable polymers like cyclodextrin infused with cross-linking agents to create a network of polymer chains that form the sponge-like structure [9]. There are several methods that have been introduced in the synthesis of nanosponges which are described in the later section. Crosslinkers play a key role in creating a threedimensional network of polymer chains within the nanosponge structure which is accountable for their unique porous structure. The choice of crosslinker can affect several important properties of the nanosponge, including its mechanical strength,

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stability, porosity, and drug-loading capacity. For example, when cyclodextrins (CD) are synthesized in the presence of hexamethylene diisocyanates, and toluene-2,4diisocyanate, the resulting carbamate nanosponges were used for water filtration while the nanosponges synthesized from CD by using 1,1' -Carbonyldiimidazole, triphosgene, and diphenyl carbonate as crosslinkers are better suitable and betterexplored nanosponges for drug delivery systems along with water purification [2]. The crosslinker must be carefully selected to form stable and durable bonds with the polymer matrix, without causing excessive crosslinking or chain entanglement. Thus it is important to carefully evaluate and compare the different crosslinkers and their effects on the nanosponge structure and properties before selecting the optimal one for a given application. Methods of Nanosponges Synthesis 1. Solvent Method: The solvent method is a commonly used technique for synthesizing nanosponges (NS). This method involves the dissolution of a polymer in a suitable solvent followed by the addition of a crosslinker, which reacts with the polymer chains to form a threedimensional network structure. The solvents commonly used are aprotic solvents (these are the solvents that cannot donate a proton in a solution) such as dimethyl form amide (DMF) and dimethyl sulfoxide (DMSO) [7]. The general procedure for the solvent method synthesis of NS involves the following steps: Selection of polymer and crosslinker: The polymer and crosslinker must be carefully selected based on the desired properties and performance of the NS. The polymer should be soluble in a suitable solvent, and the crosslinker should be capable of reacting with the polymer chains to form a stable and crosslinked network structure. Preparation of the polymer solution: The polymer is typically dissolved in a suitable solvent, such as DMSO, or DMF to form a clear solution. The concentration of the polymer in the solvent can vary depending on the desired size and porosity of the NS. Addition of crosslinker: The crosslinker is added to the polymer solution in the desired ratio and mixed thoroughly to ensure uniform distribution. Generally, it is added in the ratio of 8:2 [9]. Crosslinking: The crosslinker reacts with the polymer chains to form a threedimensional network structure, resulting in the formation of the NS. The incubation period is from 1 to 48 h around a temperature range of 10 °C. Purification and drying: After the reflux time, the solution is let to cool down to room temperature followed by the subsequent addition of distilled water and purification by Soxhlet extraction with ethanol. Afterward, it was left under a vacuum to get dry [7]. 2. Ultrasound-Assisted Synthesis: Ultrasound-assisted synthesis is a relatively new technique for synthesizing nanosponges (NS). This method involves the application of high-frequency ultrasound waves to a mixture of a polymer, crosslinker, and solvent, resulting in the

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formation of NS through a rapid and efficient crosslinking process. This method enables the manufacturing of uniformly sized spherical nanosponges. No solvent is required for the method, and the ultrasound waves are responsible for carrying out the cross-linking of the polymer [9]. The general procedure for the ultrasound-assisted synthesis of NS involves the following steps: Panda et al. [10] Mixing of the polymer and crosslinker: The polymer is typically homogenized with the crosslinker in a specific molar ratio and mixed thoroughly to ensure uniform distribution in the flask. Application of ultrasound waves: The flask is then subjected to high-frequency ultrasound waves using an ultrasonic homogenizer or sonicator along with the application of heat, making the temperature up to 90 °C. The ultrasound waves cause the formation of free radicals, which react with the polymer chains to form a threedimensional network structure, resulting in the formation of the NS. The process is incubated for 5 h. Purification and drying: After cooling down, the product is broken down and washed using water in order to remove any unreacted polymer. For purification, it was administered to Soxhlet extraction with ethanol. Afterward, it was left under a vacuum to get dry and stored at 25 °C. The ultrasound-assisted method is a promising technique for the synthesis of NS and has been successfully used to synthesize NS for various applications, including drug delivery and environmental remediation. However, the optimal reaction conditions, such as the power and frequency of the ultrasound waves, must be carefully controlled and optimized to ensure the desired properties and performance of the NS. 3. Melt Method: The melt method is a straightforward and efficient approach to the synthesis of nanosponges. This method involves the melting of a polymer and crosslinker mixture at a high temperature to obtain a homogenous melt, which is then cooled to form the NS. The melt method offers several advantages over other synthesis methods, including simplicity, cost-effectiveness, and scalability. The general procedure for the melt method of NS synthesis involves the following steps [9]: Preparation of the polymer and crosslinker mixture: The polymer and crosslinker are mixed in the desired ratio to form a homogeneous mixture. Melting the mixture: The mixture is heated to a high temperature until it melts and forms a clear, homogenous melt. Cooling and solidification: The melt is allowed to cool and solidify, resulting in the formation of NS. Purification: The NS are typically purified by washing with a suitable solvent to remove any residual impurities. The melt method has been used to synthesize a variety of NS, including those based on cyclodextrins, chitosan, and other polymers. The properties of the NS,

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such as size, porosity, and surface area, can be controlled by adjusting the synthesis parameters, such as the heating and cooling rates, and the crosslinker and polymer ratios. 4. Bubble Electrospinning: Bubble electrospinning is a novel and efficient method for the synthesis of NS. This method involves using an electrospinning setup equipped with a bubble generator to create NS with a highly porous structure. The general procedure for the bubble electrospinning method of NS synthesis involves the following steps: Preparation of the polymer and crosslinker solution: The polymer and crosslinker are dissolved in a suitable solvent to form a homogeneous solution. Bubble generation: The electrospinning setup is equipped with a bubble generator, which creates bubbles of gas (usually air) in the polymer solution. Electrospinning: The polymer solution containing the bubbles is then electrospun to form NS with a highly porous structure. Crosslinking: The NS are then cross-linked to stabilize their structure and prevent them from collapsing. 5. Microwave Radiation: The microwave-assisted synthesis of NS involves the use of microwave radiation to initiate and facilitate chemical reactions between the polymer and crosslinking agents. Microwave radiation has emerged as a promising method for the synthesis of nanosponges. One advantage this method has over others is the reduced reaction time and the nanosponges obtained acquire a high level of crystallinity in their structure [9]. The general procedure for the synthesis of NS using microwave radiation involves the following steps: Preparation of the polymer and crosslinker solution: The polymer and crosslinker are dissolved in a suitable solvent to form a homogeneous solution. Microwave irradiation: The polymer and crosslinker solution is then subjected to microwave irradiation under controlled conditions. Crosslinking: The NS are then cross-linked to stabilize their structure and prevent them from collapsing.

4 Properties of Nanosponges Nanosponges possess a range of dimensions, can exist in both crystalline and paracrystalline forms which depend on the conditions of processing, and also show stability within a broad pH range of 1–11 [11]. Carbonate nanosponges have high thermal stability for temperatures even up to 300 °C which means that they can also be easily sterilized by autoclaving at high

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temperatures. They also displayed stability in both acidic and basic mediums. The basic conditions did not have much effect on the stability of nanosponges but some release of cyclodextrin units was seen post 2 h because of the degradation of the nanostructure [12]. This stable nature of carbonate nanosponges is also beneficial for their long-term storage (about six months) even among high relative humidity conditions (~ 75%) and more than normal room temperature (40%) [8]. They also impart photostability to the molecules encapsulated by them and prevent them from degradation against UV light. This is especially useful in drug delivery through cyclodextrin carriers. When two drugs, namely, Resveratrol (RES) and Oxyresveratrol (OXY) were encapsulated in cyclodextrin-based nanosponges they showed resistance to degradation by UV light (wavelength 320–400 nm fixed at a distance of 10 cm). While RES and OXY alone showed degradation of 59.7 and 27.5%, the drugs incorporated in nanosponges showed two-fold and three-fold protection, for RES-NS and OXY-NS respectively, from UV light in comparison to their pure samples [13]. Photodegradation of Gamma-oryzanol, a mixture mainly employed as sunscreen in cosmetics, was found to be slower when it was incorporated into β cyclodextrin-based nanosponges proving the photostability property of nanosponges [14]. Nanosponges also show less to no cytotoxicity which makes them a favorable agent for drug delivery systems and to be used inside living systems without the risk of side effects. Toxicity studies carried out in cell lines in vitro showed no difference in cell viability or cytotoxicity upon incubation with nanosponges for 24–72 h. Similarly, no cytotoxic effects were observed in Swiss albino mice when injected with cyclodextrin-based nanosponges and were thus considered safe within the range of 500–5000 mg/Kg [12]. Incubating human erythrocytes with nanosponges for 90 min also did not show any adverse effects like hemolysis demonstrating the good biocompatibility of nanosponges with the blood [8]. Cytotoxic studies were done on fibroblast and kidney cell lines to check for any toxic activity of the nanosponges also showed promising results as the cell viability was seen to be > 70%, even with a high concentration of nanosponges, which is the accepted minimum limit for judging biocompatibility in all cell types, according to ISO-10993-5, (2009). Cyclodextrinbased nanosponges also show less systemic toxicity since these do not get absorbed in the blood through intestinal absorption and undergo degradation by the colonic microflora [15]. Cyclodextrins have lipophilic cavities within their monomers and hydrophilic CD-NS channels that are porous, which enables them to accommodate a diverse range of compounds [16]. The mesh-like structure of the cyclodextrin nanosponges enhances the solubility of lipophilic drugs which improves their bioavailability and helps in drug delivery and release of the drug at specified targets in the body. The hydrophobic cavity in the nanosponges can encapsulate drug molecules and improve their solubilization [12]. Nanosponges have been found to have the ability to suppress the melting point of certain molecules, such as meloxicam, a drug commonly used for pain relief. Meloxicam normally has a distinct melting point of 250 °C, but when encapsulated within a cyclodextrin nanosponge, this peak disappears, indicating the formation of

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a complex between the drug and the nanosponge. This suggests that the nanosponge is able to protect the meloxicam by enclosing it within its structure [17]. The 3D structure of nanosponges is useful for fractionalizing peptides for application in proteomics. CD-based nanosponges act as suitable carriers for the adsorption of proteins, enzymes, antibodies, and macromolecules [18]. Nanosponges have good porosity resulting from which they can encapsulate many different molecules like drugs, proteins, and gases [12]. Different functional groups can be easily sequestered in carbonate nanosponges and the nanosponges can also be imbibed with magnetic properties and can be additionally modified. Nanoporous polymers were synthesized from cyclodextrin and diisocyanates, resulting in granular solids, powders, and films. The polymers could bind organic molecules, and nitrophenol can be removed from water solutions, even at low concentrations. The process involved reacting CD with diisocyanate in a 1:8 molar ratio in DMF solution under a nitrogen atmosphere. The resulting polymer can absorb nitrophenol from a colorless solution, turning yellow due to the concentration of nitrophenol in the nanosponge powder [8]. CD-based carbonate nanosponges also have the property of adsorbing enzymes onto their surface for enzyme immobilization, a technique that is used in many industries for exploiting the catalytic properties of enzymes. The nanosponges stabilize the enzyme whilst enhancing its activity [8]. A yield of 29 mg of enzyme per gram of support was obtained when catechol 1,2-dioxygenase, an enzyme derived from Acinetobacter radoresistens S13, was immobilized on CD-based carbonate nanosponges support [19]. While CD-based carbonate nanosponges do not display considerable swelling, CD-based polyamidoamine nanosponges do so in water. The uptake of water is pH dependent [8]. The loading capacity of CD-based carbamate is very similar to that of activated carbon (20–40 mg/cm3 ) despite its low surface area [8]. Li and Ma revealed that nanosponges are capable of forming an inclusion compound with the molecules that enter the cavity of the nanosponges (guest molecules) by employing circular dichroism. In nanosponges, merely the adsorption of the molecules on the surface does not take place; instead, the molecules enter the cavity to form an inclusion compound with the nanosponge [8]. Nanosponges can be regenerated with ease by washing them with some organic compounds without losing their functional ability. This happens because nanosponges readily form inclusion compounds in an aqueous environment, however, the process is almost reversible in the case of organic and comparatively less polar solvents, like ethanol. Cyclodextrin-based carbonate nanosponges can be readily obtained again by washing them with acetone, ethanol, or some other organic compound similar to these [8]. A reusability study of anionic nanosponges synthesized by the cross-linking of citric acid with β cyclodextrin was performed in methanol and 90.3% efficiency was achieved in releasing paraquat from nanosponges after five cycles [20].

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A new kind of nanosponges, called pyro-nanosponges, developed by reacting pyromellitic dianhydride with cyclodextrins, can display free carboxylic groups that remain associated at acidic pH but dissociate at physiological pH with a net negative charge. These can then electrostatically associate with positively charged groups. As a result of these electrostatic interactions, pyro-nanosponges have an additional binding site that demonstrates a good ability for encapsulating drugs [21]. By attaching molecules to their surfaces, it is possible to prepare functionalized nanosponges. In the case of carboxylated nanosponges, this was achieved by reacting succinic anhydride with performed carbonated nanosponges. Nanosponges are also easy to prepare using a straightforward synthetic process, without the use of toxic solvents or sophisticated instruments, and can be scaled up for potential industrial applications. Additionally, compared to other nanocarriers like liposomes, they are cost-effective [21].

5 Introduction to Aquifers and Their Contamination Aquifers are geological formations that contain and transmit groundwater. They are layers of porous and permeable rocks or sediments that are capable of storing and transmitting water. Aquifers are important sources of freshwater for many communities around the world and are used for a variety of purposes such as drinking water, irrigation, and industrial processes. There are many parameters on which aquifers are characterized, for example, their thickness, and hydraulic conductivity. Here are some terminologies that come into the picture regards to groundwater and aquifers: Porosity: The measure of the volume of empty spaces, or pores, within a rock or sediment that can hold water. Permeability: The ability of a rock or sediment to allow water to flow through it. Recharge: The process by which water from precipitation and surface water bodies enters an aquifer. Discharge: The release of water from an aquifer to the surface through natural springs or wells. Water table: The upper boundary of the saturated zone in an unconfined aquifer. Groundwater depletion: The long-term reduction of the amount of water stored in an aquifer, which can lead to a range of problems such as land subsidence, water quality deterioration, and reduced groundwater availability. Aquifer recharge zone: An area where surface water enters the ground and recharges an aquifer. Aquitard: Geological formation or layer of rock or sediment that restricts or slows the movement of groundwater. Rocks with high porosity for water include sandstone, limestone, and fractured volcanic rocks, among others. These rocks have significant void spaces that can hold and transmit water, allowing for the formation of aquifers.

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On the other hand, rocks with low porosity for water include shale, granite, and other crystalline rocks, as well as unfractured volcanic rocks. These rocks have very few or no void spaces, making it difficult for water to flow through them and resulting in very low permeability. As a result, they are typically unsuitable for use as aquifers. Aquifers can be classified into two types: unconfined and confined aquifers (Fig. 2) [22]. • Unconfined aquifers are located near the ground surface and are in direct contact with the atmosphere. They are not confined by an overlying layer of impermeable rock or sediment, allowing water to move freely through it. Unconfined aquifers are also known as water table aquifers, as the water table is the upper surface of the zone of saturation (the area where water fills the pore spaces of soil and rock). Unconfined aquifers are typically found in areas with permeable soils or rocks, such as sand and gravel deposits or fractured rock formations. They are often recharged by surface water, such as rain and snowmelt, which infiltrates through the ground and fills the aquifer. Groundwater in unconfined aquifers is generally more vulnerable to contamination than groundwater in confined aquifers since there is no overlying layer of impermeable rock or sediment to act as a natural

Fig. 2 Image showing unconfined and confined aquifers

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filter. As a result, unconfined aquifers are often the focus of groundwater management and protection efforts. The extraction of water from unconfined aquifers can lead to the lowering of the water table, which can cause a variety of problems such as land subsidence and saltwater intrusion. In some cases, excessive pumping can even lead to the complete depletion of the aquifer. Overall, unconfined aquifers play a crucial role in the Earth’s hydrological cycle, providing a significant source of fresh water for human use, agriculture, and other activities. However, their vulnerability to contamination and overuse underscores the importance of responsible management and conservation practices. • Confined aquifers are located beneath impermeable layers of rock or clay and are under pressure from the overlying rock layers (Fig. 2). The extraction of groundwater from aquifers is a critical process that requires careful management to ensure sustainability and prevent depletion. Confined aquifers are also commonly referred to as artesian aquifers because when water is extracted from a well that taps into a confined aquifer, the water often rises above the top of the aquifer and may even flow onto the land surface due to the pressure built up within the aquifer. The water that is confined in this manner is said to be under artesian pressure, which is a result of the water being trapped between two layers of relatively impermeable rock or sediment. This pressure can force water to the surface without the need for a pump, making artesian aquifers an important source of water for many regions [22]. The layer of impermeable rock or sediment prevents water from entering or leaving the confined aquifer except through specific locations such as wells or springs. Confined aquifers are often found in areas with a layer of impermeable rock or clay below a layer of permeable rock such as sandstone or limestone. The water in a confined aquifer is under pressure, which can cause the water to rise to the surface through a well or spring without any pumping. The recharge of confined aquifers often occurs at a distance from the well or spring where water is extracted. Rain or snowmelt that infiltrates the permeable layer above the confined aquifer recharges the aquifer, which can then be accessed through a well or spring. Confined aquifers are often used as a source of drinking water and for agricultural and industrial purposes. However, over-extraction of water from a confined aquifer can cause the water table to drop, which can result in decreased water pressure and reduced water flow. This can lead to the depletion of the aquifer, which may take decades or even centuries to recover. Another concern with confined aquifers is the possibility of contamination from surface sources. Since the water is under pressure, any contaminants that enter the confined aquifer can spread rapidly, making it difficult to remove the contaminants and restore the aquifer to its original condition. Overall, confined aquifers are an important source of fresh water in many areas, but their sustainable management and protection require careful planning and monitoring. Aquifers are vulnerable to contamination by various pollutants, including chemicals, microorganisms, and radioactive materials. Surface water and human activities such as agriculture, mining, and industrial practices can contaminate aquifers with chemicals such as nitrates, pesticides, and heavy metals [23]. Microorganisms such as

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bacteria and viruses can also contaminate aquifers, especially those that are not properly protected [24]. In addition, aquifers can be affected by the disposal of radioactive waste, which can cause long-term contamination of groundwater. Contamination of aquifers poses a serious threat to human health and the environment, making it crucial to monitor and protect groundwater resources (Table 1). Most studies showing the negative effects of these contaminants have used high doses or exposure levels that are not representative of typical exposure in real-world settings and their experiments need not necessarily include human tests, some have been done on animals like rats, etc. Further research is needed to fully understand the prospective risks linked with exposure to these contaminants.

6 Treatment of Aquifers with Nanosponges Dendrimers are hyperbranched molecules with terminal functional groups that exhibit monodispersity and molecular uniformity. Cyclodextrins have a lipophilic interior cavity. Because of their unique structure and solubilization properties, these are able to entrap organic pollutants from contaminated water. Hybrid filter modules were synthesized by impregnating ceramic porous filters with these macromolecules and were subsequently tested for their effectiveness in water purification. The filters were efficient in removing PAHs (Polycyclic Aromatic Hydrocarbons) (> 95%). The filters were also established to remove typical pollutant groups of Trihalogen Methanes (THMs), pesticides (simazine), and monoaromatic hydrocarbons (BTX) (> 80%), although quick saturation of filters was observed in these cases [59]. The results found in the work of Pifferi et al. for the removal of o-toluidine from water were considerably superior as compared to other o-toluidine sorbents. Nanosponges containing high concentrations of α-, β-, and γ- cyclodextrins were created by connecting the CD moieties through either ionizable or neutral bisacrylamide-deriving arms. The synthesis process involved stepwise oxo-Michael polyaddition in an aqueous solution, which was catalyzed by a base. Regardless of the size of the CD inner cavity, all the nanosponges demonstrated the ability to remove o-toluidine from diluted aqueous solutions. These solutions were similar in composition to what is commonly found in o-toluidine-polluted aquifers. The nanosponges were able to reduce the concentration of o-toluidine to at least 0.16 parts per million (ppm) [60]. Metal ions constitute one of the most dangerous and indestructible pollutants present in groundwater and they accumulate in the food chain over time. Human beings being at the top of the food chain, therefore, become highly susceptible to their deleterious effects. High concentrations of these metal ions can cause symptoms like allergic reactions, dementia, insomnia, etc. to develop [61]. Pedrazzo et al. created a kind of nature-friendly nanosponges by combining βcyclodextrin (β-CD) and line caps (LC) (a pea starch derivative) with citric acid in the presence of sodium hypophosphite monohydrate as a catalyst. The performance of these nanosponges in water detoxification was compared with that of nanosponges

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Table 1 Summarizes the different types of contaminants in groundwater and aquifers, their sources, and their potential health effects [23] Contaminant

How do they get access to groundwater

Potential adverse effect

References

Aluminium

Natural existence in rocks

Discoloration of water, may serve as a risk factor for Alzheimer’s disease

[23, 25]

Antimony

Industrial production and discharge, municipal waste disposal, and fertilizers

Alter levels of blood glucose and cholesterol, nausea, vomiting, and diarrhea

[23, 26]

Arsenic

Industrial waste, mineral deposits, improper waste disposal

Vomiting, abdominal pain, hyperkeratosis, skin cancer, decreased hemoglobin levels, liver damage

[23, 27, 28]

Barium

Natural existence in rocks

Hypertension, hypokalemia, vomiting, abdominal cramps

[23, 29, 30]

Beryllium

Natural existence in soil and rocks, improper waste disposal, coal burning, deposition of atmospheric beryllium

Lung damage

[23, 31, 32]

Cadmium

Industrial discharge, mining waste, sewage sludge

Decreased bone density, [23, 33, 34] kidney disorders, hypertension, RBC lysis, liver damage

Chloride

Industrial discharge, weathering of soils, deposition of salt spray, domestic discharge

Above some specified levels it [23, 35] can change the taste of water

Chromium

Inadequate industrial waste disposal, fossil fuel combustion, mineral leaching, weathering of rocks, landfills

Skin problems, kidney and liver damage, GI tract disorders, cardiovascular collapse, hypovolemia

[23, 36–38]

Copper

Mineral deposition, metal platings, plumbings

(RARELY) stomach cramps, nausea, vomiting, kidney and liver damage, headaches

[23, 39, 40]

Cyanide

Metal mining discharges, industrial waste, coal processing, fertilizers, improper waste disposal

Brain malfunctioning, liver and spleen damage

[23, 41]

Fluoride

Weathering of fluoride containing rocks, naturally existing

Fluorosis, crippling bones, headaches, hypothyroidism, muscular damage

[23, 42, 43]

Lead

Industrial discharge, mining, plumbings

Anemia, hypertension, cardiovascular effects, decreased kidney activity, hearing defects in children

[23, 44]

(continued)

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Table 1 (continued) Contaminant

How do they get access to groundwater

Potential adverse effect

References

Manganese

Mineral deposition, weathering of rocks and sediments, and industrial discharge

Can change the taste of water, [23, 45] rare cases it can cause toxicity to humans

Mercury

Atmospheric deposition, industrial waste, mining waste, pesticides, fossil fuel burning, mineral deposits

Impaired nervous system, kidney dysfunction, cardiovascular collapse, abdominal pain, vomiting, GI tract damage, and diarrhea

[23, 46, 47]

Nickel

Natural existence in soil, leaching from metal alloys

Cardiac arrest, nausea, vomiting, diarrhea, giddiness, headache, nephrotoxicity, GI tract problems

[23, 48, 49]

Selenium

Weathering, surface mining, coal power plants

Nausea, vomiting, nail discoloration, hair loss, garlic breath

[23, 50, 51]

Silver

Ore mining, improper disposal

Argyria

[23]

Sulfate

Naturally existing in rocks and soil

Improper digestion, laxative effects

[23, 52]

Thallium

Erosion of rocks, smelting facilities, coal combustion

Liver and kidney damage, stomach and intestinal ulcers, insomnia, hair loss, vomiting

[23, 53–55]

Zinc

Natural existence, industrial discharge, erosion of rocks, mining

Vomiting

[23, 56, 57]

Volatile organic compounds (e.g. benzene, chloroform, etc.)

Insecticides, paint removers, inks, etc.

Liver damage, anemia, carcinogenic, nervous system damage

[23, 58]

prepared by the reaction of β-cyclodextrin and pyromellitic dianhydride (PMDA) in dimethyl sulfoxide (DMSO) along with the introduction of triethylamine and another nanosponge which was prepared using LC and PMDA in a similar manner. Adsorption tests were performed at varying concentrations of metals (Cu2+ and Zn2+ ). When the concentration was at 500 ppm, PMDA NSs exhibited a better metal retention capability compared to citrate NSs because of their higher carboxyl group content. However, at low metal concentrations (≤ 50 ppm), both citrate and pyromellitic NSs were able to retain a large amount of metal, up to 94% [61]. Cadmium is one of the primary metal contaminants present in water which threatens the environment and human health as a carcinogen and genotoxic element. Therefore, there is a need for a nanomaterial that is sustainable, renewable and does not cause any toxic effects on either the environment or the living systems.

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Guidi et al. developed a cellulose-based nanosponge that can be used in the removal of cadmium ions from contaminated water samples. Owing to its high-porosity and eco-friendly biodegradable nature, it has proved its efficiency in sequestering metal ions without adversely affecting the cellular activity of freshwater organisms. Cytome assay was performed to confirm the competence of CNS at a chromosomal level. The strong ability to bind with metal ions is attributed to the chelation process facilitated by the presence of amino groups in the inner structure which is observed in 2,2,6,6-tetramethyl-piperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibers (TOCNF) cross-linked with branched polyethyleneimine (bPEI) polymer [62]. β-cyclodextrin (β-CD) and (6-deoxy)-(6-benzylimidazolium)-β-CD nanosponges were synthesized using toluene diisocyanate. The latter demonstrated superior effectiveness in removing metal ions, attributed to the presence of imidazolium groups that facilitate electrostatic interactions and chelation [63, 64]. Four types of cyclodextrin-calixarene nanosponges were developed to effectively adsorb Pb2+ in an attempt to treat wastewater contaminated with the metal. The nanosponges underwent modification which included functionalizing them with carboxyl and amino groups. The resulting nanosponges displayed appreciable affinity as well as adsorption ability for the metal ions. The adsorption performance of the nanosponges was studied as a function of ionic strength, ionic medium, temperature, and pH. The nanosponges demonstrated the best adsorption ability at pH 5.0 [65]. Chhetri et al. synthesized magnetic nanosponges (MNSs) by combining magnetic nanoparticles (MNPs) with polymer coagulants for efficient treatment of groundwater contaminated by concentrated animal feeding operations (CAFOs). Separating non-magnetic nanomaterials from the wastewater system is a cumbersome task, however, the application of an external magnetic field subdues the shortcomings associated with the use of non-magnetic nanomaterials. Four parameters were taken to assess the efficient treatment of contaminated water samples, namely, total organic content (TOC), biological oxygen demand (BOD), total suspended solids (TSS), and turbidity. The study showed that using MNSs in combination with coagulants at a ratio of 75% resulted in a significant reduction in the parameters. However, this reduction is dependent on the type of MNPs and coagulant used in preparing MNSs. The results suggest that MNSs can enhance the coagulation process by quickly and efficiently forming flocs, which can settle completely in the sedimentation tanks [66]. Triclopyr is a frequently used auxinic type herbicide used to eradicate some annual and perennial weeds and woody plant species. It is adsorbed by leaves and roots and gets concentrated in the meristematic tissues of the plant. Baglieri et al. synthesized a nanosponge from β-cyclodextrin by reacting it with carbonyldiimidazole in dimethylformamide at a high temperature of 90 °C for a duration of 5 h. The aim of the experiment was to use nanosponges prepared to remove triclopyr from a contaminated sample through adsorption. Interestingly and unexpectedly enough, very less interaction was observed between triclopyr and the nanosponge. This might be because the structure of triclopyr prevented it from forming inclusion complexes with the nanosponge. Nevertheless, the active ingredient degraded rapidly in the presence of nanosponges which can lead us to conclude that nanosponges can be

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utilized in mitigating the dangers of water pollution since they were seen to favor degradation of the molecule, though there is no proof of nanosponges catalyzing the degradation process themselves [67]. Paraquat (1,1' -Dimethyl-4,4' -bipyridinium dichloride) is a hydrosoluble herbicide that has significant adverse effects on the health and environment. Environmentfriendly nanosponges, which were anionic in character, were synthesized by crosslinking citric acid with β-cyclodextrin in the presence of polyvinyl alcohol. These nanosponges were employed in the removal of paraquat dye from samples and were eventually successful in removing 94.5% paraquat from the solution [20]. Nanoporous polymers were synthesized by the reaction of cyclodextrins with appropriate diisocyanates in a dimethylformamide solution at a temperature of 70°C for a duration of 16–24 h in a nitrogen atmosphere. These nanoporous polymers were successful in removing the nitrophenol present even at a very low concentration from the solution by binding to it. CD-based carbamate nanosponges have shown promising results in removing various contaminants from wastewater [8]. Mamba et al. demonstrated that these nanosponges can remove up to 84% of dissolved organic carbon (DOC) from wastewater [68] The same group also found that CD polyurethane nanosponges can effectively remove unwanted odor and taste compounds like 2-methylisoborneol and geosmin [69]. In another study by Tang et al., CD-based carbamate nanosponges were used to adsorb aromatic amino acids from phosphate buffer [70]. CD-based carbonate nanosponges are able to remove organic pollutants from water such as methyl red and hexachlorobenzene [8]. Taka et al. synthesized an insoluble nanosponge biopolymer composite using a process that involved cross-linking polymerization, amidation reaction, and sol–gel method. The synthesized phosphorylated multiwalled carbon nanotube-cyclodextrin/ silver-doped titania (pMWCNT-βCD/TiO2 Ag) composite was then evaluated as a biosorbent for its effectiveness in removing Pb2+ and Co2+ from both synthetic and real wastewater samples. The results of the study showed that pMWCNT-βCD/ TiO2 Ag composite had a maximum removal capacity of 35.86 mg/g for Pb2+ and 7.812 mg/g for Co2+ in mine effluent samples [71].

7 Conclusion Nanotechnology has proven its usefulness in many industries like medicine, bioremediation, and cosmetics only to name a few. Among these, nanosponges are quite extraordinary in that they possess a very unique structure that can be exploited in various ways to accomplish tasks such as drug delivery, wastewater treatment, environmental remediation, etc. Nanosponges have a structure that is suitable for trapping molecules for their targeted delivery to certain locations. They can form inclusion complexes with compounds in their cavity and show increased adsorption ability. These fascinating properties of nanosponges are tweaked to facilitate their use in

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removing pollutants from aquifers. Aquifers are a source of freshwater present below the ground where the water seeps through between layers of rocks. Their contamination poses a great risk to the health of human beings and the environment. Here in, we discussed many nanosponges, synthesized and tested in vitro, that display the capacity of bioremediation of the aquifers. Much attention was given to the cyclodextrin-based nanosponges since they are the most preferred choice for synthesizing nanosponges. Nanosponges can also be easily regenerated by organic solvents which reduces the chemical waste produced and leads to a greener research process. All in all, nanosponges have proven to be an extremely useful alternative to aquifer bioremediation.

Abbreviations BOD bPEI CAFOs CD CD-NS CNS DMF DMSO DOC FTIR GI Tract LC MIP MNPs MNSs NS OXY PAHs PMDA Ppm RES THMs TOC TOCNF TSS β-CD

Biological Oxygen Demand Branched polyethyleneimine Concentrated Animal Feeding Operations Cyclodextrin Cyclodextrin nanosponge Cellulose-based nanosponges Dimethyl form amide Dimethyl sulfoxide Dissolved Organic Carbon Fourier transform infrared Gastrointestinal Tract Line Caps Molecular Imprinting Magnetic Nanoparticles Magnetic Nanosponges Nanosponge Oxyresveratrol Polycyclic Aromatic Hydrocarbons Pyromellitic dianhydride Parts per million Resveratrol Trihalogen Methanes Total Organic Content 2,2,6,6-Tetramethyl-piperidine-1-oxyl (TEMPO)-oxidized nanofibers Total Suspended Solids β-Cyclodextrin

cellulose

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Nanostructured Sponges for the Removal of Toxic Dyes from Wastewater Gunjan Purohit, Manish Rawat, and Diwan S. Rawat

1 Introduction Water pollution arising from continuous industrialization and population growth is a challenging issue around the globe to solve [1, 2]. Therefore, one of the severe environmental crises around the world-wide is access to safe drinking water. Water contamination not only damages the ecosystem and environment but is responsible for the serious threat to human health [4, 3]. Figure 1 illustrates some common water pollutants whose presence not only determines the water quality but also is hazardous to the environment and ecology [5, 6]. In order to rectify this problem various water purification methods and new materials have been developed and are currently used in day-to-day life [7]. The presence of water pollutants/presence of microbes lowers the water quality and generates toxic concerns regarding the odor, color, and taste of water [8]. Table 1 enlists the different water remediation methods and conventional techniques to improvise the water quality along with their advantages and setbacks. The techniques or remedies to improvise water quality are reverse osmosis, ion exchange, membrane filtration, coagulation, absorption based, etc., and have been widely used [9]. However, these methods suffer from critical setbacks of lower selectivity, efficiency, high cost, and maintenance. Therefore, there is a need to design and develop efficient strategies encompassing low-cost (nano-based) systems for the effective toxins/pollutants removal from wastewater to improvise drinking water quality.

Gunjan Purohit and Manish Rawat are authors contributed equally. G. Purohit · M. Rawat · D. S. Rawat (B) Department of Chemistry, University of Delhi, Delhi 110007, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Gulati (ed.), Nanosponges for Environmental Remediation, https://doi.org/10.1007/978-3-031-41077-2_18

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Fig. 1 Some common water pollutants

The term nanostructured sponge first came into the picture in the 1990s owing to its nano porous-like structure derived from cyclodextrins (CDs) [21]. Nanostructured sponges (NSS) are solid hyper cross-linked polymeric-based colloidal structures consisting of sub-microscopic particles with nanosized cavities. These NSS can be of various types depending on their constituents organic or inorganic materials representing a nanometric dimension [22] Metal oxide based nanosponges, silicon/ titanium-based nanosponges, cyclodextrin-based nanosponges, or hyper-crosslinked polystyrene-based nanosponges are some well-known examples that come under the arena of nanostructured sponges [23]. Cyclodextrins are derived from starch and are cyclic glucose-based i.e., α-(1,4)linked glucopyranose units-based oligomers [24, 25] The chair conformations of glucopyranose are responsible for CDs to form a truncated cone-like structure constituting a hydrophobic cavity [26]. The external hydrophilicity and internal hydrophobic surficial characteristics of CDs are responsible for showing the inclusive host–guest relationship [27]. The hydrophilic outer layer of CDs is basically because of flanked hydroxyl groups thereby making it water-soluble. One of the most common strategies to prepare CDs-based nanosponges involves a reaction between cyclodextrins and polyfunctional moiety viz., pyrometallitic anhydride or carbonyldiimidazole. The formed product is a hyper cross-linked porous polymer of nano dimensions displaying inclusive properties. CDs-based nanostructured sponges are water-insoluble, 3D-cross-linked nanoporous, hydrophobic structures adhering to greater stability over a wide range of pH and temperature ranges [28]. The nanosized cavities of nanostructured sponges make them a versatile carrier to encapsulate metal ions, organic contaminants, organic/inorganic dyes and thus show vast application in the area of water remediation [28]. NSSs can encapsulate various molecules in their nano-sized porous cavities by forming inclusion/non-inclusion complexes. Figure 2 displays the application of CDs-based nanostructured sponges in various areas including biomedical, pharmaceuticals, water remediation/treatment, catalysis,

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Table 1 Advantages and setbacks of various water remediation methods Technologies/ Strategies

Advantages

Setbacks

References

Adsorption

Easy process High affinity and binding capacity to metals Reusability

High cost of adsorbents Difficult separation Lower selectivity

[9]

Reverse osmosis technique

Seawater desalinization Removal of microbes organic/inorganic pollutants from wastewater

Energy demanding and high cost Ill-suitable for small organic pollutants

[10, 11]

Electrochemical techniques

High selectivity and separation Scalable and flexible operations

Energy demanding and high cost

[12]

Membrane filtration Requires less space and can Higher membrane, operation, [13] techniques be processed at low pressure and maintenance costs High selectivity Cumbersome processes and lower productivity The membrane regeneration process is troublesome Chemical precipitation

Simple and cheap process

Ill-suited for a lower [14, 15] concentration of heavy metal water contamination Higher sludge production and removal cost

Ion exchange method

High treatment capacity High efficiency and kinetics Efficient in removing arsenic, nitrates from water Scalable

High maintenance cost Increased secondary pollutants Plausible microbial contamination

[16]

Flotation

High overflow rate and efficiency Less detention period

Higher capital cost, membrane, operation and maintenance cost

[17, 18]

Flocculation/ coagulation

Improvements in water clarity to decrease turbidity

Increased sludge formation containing inorganic coagulants/toxins Ill-suited for heavy metal removal/contaminants

[19, 20]

etc. [29, 30]. The main focus of the present chapter is on cyclodextrin-based nanostructured sponges and their application to the removal of toxic dyes from wastewater. Finally, a concluding remark on future perspective and critical assessment of nanostructured sponges.

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Fig. 2 Application of cyclodextrin-based nanostructured sponges

2 Types of Nanostructured Sponges Nanostructured sponges can be classified into various types such as titaniumbased nanosponges, cyclodextrin-based nanosponges, silica-based nanosponges, and hypercross linked polystyrene-based nanosponges [31–34]. Figure 3 depicts the generalized overview of nanostructured sponges.

2.1 Cyclodextrin-Based Nanosponges Cyclodextrin-based nanosponges have been widely explored as pharmaceutical excipient which is capable of incorporating guest molecules within the central cavity. Cyclodextrin-based nanosponges can be obtained by reacting cyclodextrin with suitable crosslinking agents such as diisocyanates, dialdehydes, diacyl chlorides, etc. [35].

Cyclodextrin-Based Carbamate Nanosponges CD-based carbamate nanosponges can be obtained by using appropriate diisocyanates such as 1,6-diisocyanatohexane (HDI) or 2,4-diisocyanato-1methylbenzene (TDI) in a polar protic solvent such as dimethylformamide, dimethyl sulfoxide, etc. at 70 °C for at least 16 h under inert condition. These nanosponges exhibit a strong affinity for organic molecules and are employed in the process of

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Fig. 3 Types of nanostructured sponges

water purification. Their loading capacity for organic molecules is approximately 20–40 mg per cubic centimeter [36].

Cyclodextrin-Based Carbonate Nanosponges Active carbonyl compounds such as DPC, CDI, and triphosgene are the main crosslinkers for the preparation of these nanosponges. The obtained CD nanosponges contain carbonate bonds between two CD monomers. Carbonate-based CD nanosponges have various important characteristics such as tunable polarity and variable dimensions of their cavities. Usually, these nanosponges are found in different forms such as amorphous and semi-crystalline based on different reaction conditions. The degree of solubility enhancement for these CDs is directly influenced by the level of crystallinity, which imparts a distinctive characteristic to them [37].

Cyclodextrin-Based Ester Nanosponges The crosslinking agent used in this type of nansponges is dianhydride such as pyromellitic anhydride. The reaction proceeds rapidly and is conducted at ambient temperature, by simply dissolving the CD and suitable dianhydride in DMSO and organic base like pyridine or triethylamine. These NSs exhibit free polar carboxylic

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acid groups which allow apolar organic groups and cations to a guest simultaneously [38].

2.2 Polyamidoamine Nanosponges Water is the most suitable solvent used in the synthesis of these nanosponges. Polymerization of β-CD takes place in the presence of acetic acid 2, 20-bis (acrylamide) at ambient temperature after long-standing. This NS contains both acids as well as basic residues and swelling takes place in water. A translucent gel is formed whenever these polymers come in contact with water. The translucent gel is stable up to 72 h confirmed by time-dependent swelling studies. The stability of the product is verified through the application of sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) technique [39].

2.3 Modified Nanosponges Cyclodextrin-based carbonate nanosponges can be modulated by altering the reaction condition to obtain modified nanosponges. The reaction between carbonate nanosponges with fluorescein isothiocyanate dissolved in dimethyl sulfoxide at 90 °C for a few hours led to the formation of fluorescent nanosponges. Similarly, succinic or maleic anhydride which are a cyclic organic hydride, is used for the synthesis of carboxylated nanosponges. A promising specific receptor can be easily obtained by treating these carboxylated nanosponges with biologically active carriers like biotin, chitosan, etc. These nanosponges are amorphous in nature confirmed by powder XRD. These nanosponges are also non-hemolytic and non-cytotoxic in nature [40].

3 Methods of Synthesis of Nanostructured Sponges and Properties There are majorly six methods used for the synthesis of nanostructured sponges like solvent method, emulsion solvent diffusion method, ultrasound-assisted synthesis, quasi-emulsion solvent diffusion, hyper cross-linking method for β-cyclodextrin synthesis, and polymerization.

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3.1 Solvent Method Nanosponges can be synthesized using a solvent method which involves the mixing of polymer with a suitable polar aprotic solvent such as dimethyl formamide and dimethyl sulfoxide. 1:4 molar ratio of cross-linker and polymer was used in this method with refluxing up to 1–48 h. After the reaction is complete, allow the reaction mixture to gradually cool down to room temperature. Subsequently, add distilled water to the resulting product. Filter the product under and prolonged Soxhlet can be used for the purification of the final product [41].

3.2 Emulsion Solvent Diffusion Method The emulsion solvent diffusion method involves the usage of ethyl cellulose and polyvinyl alcohol at different concentrations. Drug loading can be improved by changing the ratio of the drug to a polymer. The procedure of this method involves the dissolution of the dispersed phase containing drug and polymer into dichloromethane solvent and then the addition of this mixture into another solution containing polyvinyl alcohol in the aqueous phase. Finally, stir the reaction mixture for 3– 5 h at 1000–1500 rpm. Nanosponges can be filtered and dried in an oven for 24 h [42].

3.3 Ultrasound-Assisted Synthesis Ultrasound-assisted synthesis of nanosponges involves the solvent-free reaction of the polymer with cross-linkers. Place the reaction mixture under sonication for 5 h at 90 °C. Cool the reaction mixture and proceed with a thorough washing using water to eliminate any residual unreacted starting materials. Finally, purify the product using prolonged Soxhlet extraction [43].

3.4 Quasi-Emulsion Solvent Diffusion Different amount of polymer is used in the quasi-emulsion solvent diffusion method for the synthesis of nanosponges. Inner and outer phases are prepared in this method. Eudragit RS100 was dissolved in a solvent for the preparation of the inner phase whereas PVA solution in water is an outer phase. Both phases are mixed and stirred for one hour. Finally, the reaction mixture is filtered to isolate the nanosponges [44].

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3.5 Hyper Cross-Linking Method for β-cyclodextrin Synthesis Non-porous β-cyclodextrin can be explored as a carrier for drug delivery. The reaction between cyclodextrin and cross-linker leads to the formation of acidic or neutral nanosponges. These obtained nanosponges have 3D networks which are roughly spherical in shape with a size of a protein possessing well-defined channels and pores within its internal structure. The size of these nanosponges can be readily adjusted based on the surface charge density and porosity, enabling tailored bonding to various molecules. The diameter of these nanosponges is within 1 μm but usually, a fraction of nanosponges having a diameter below 500 nm can be preferred. The solubility of badly water-soluble drugs can be increased by these nanosponges. They are usually solid particles which further transformed into crystalline form [45].

3.6 Hyper Cross-Linking Method for β-cyclodextrin Synthesis A solution containing a non-polar drug is developed in monomer and an aqueous solution containing surfactant and dispersant is mixed. The rate of polymerization can be affected, once suspension having a discrete droplet of desired size is achieved, by the activation of monomers either by catalyst or elevated temperature. A reservoir type of system is formed in this process which activates at the surface via pores [46]. Figure 4 illustrates the various synthetic protocols or methods of nanosponges known in the literature.

4 Application of Nanostructured Sponges: Removal of Toxic Dyes from Wastewater The 3-dimensional architecture of cyclodextrin-based nanosponges exhibits wide application in wastewater remediation as it shows similar pollutants removal efficiency in comparison to those when using activated carbon. When comparing the activated carbon and NSSs, the CDs and their derivatives-based nanostructured sponges are cost-wise cheaper having ease of functionalization. Therefore, the functionalized NSSs manifest the property of highly structured sorbent material. Following are the spectacular properties of cyclodextrin-based nanostructured sponges which make them an excellent candidate for the removal of toxins from wastewater [47]. 1. Hydrophobic cavities owing to host–guest complex. 2. Diffusion tendency via the porous channels of the hydrophilic network. 3. Functionalization leads to interactive active sites on nanostructured sponges.

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Fig. 4 Different methods for synthesis of nanostructured sponges

It is well-known in the literature that these physicochemical properties can be easily fine-tuned by altering the synthetic strategies or conditions including linker type (inorganic/organic), molar ratio of linked: CDs, solvents, etc. thereby affecting the removal efficacy of pollutants present in water. It is well known in literature that the nanostructured sponges have found their application in dye removal from wastewater. Dyes are particularly used in industries encompassing or associated with consumables products for example textiles, leather, paper, printing, inks, etc. [48]. The aforementioned industries consume the greatest amount of water, especially the textile industry, and hence is one of the biggest water pollutants or water pollution-causing industries. In principle, dyes are complex/simple mixtures of organic compounds (carbon-based) with pigments that could be of organic/inorganic substances. Most of the dyes have carbon-based aromatic structural characteristics/ rings, are non-biodegradable, and are water-soluble thereby making them difficult to remove from wastewater. When these dyes are discharged into water it makes it mutagenic/carcinogenic and hence unfit for the environment especially humans/ animals/living organisms via contaminated drinking water [49]. Since these dyes are non-biodegradable and water soluble therefore they are generally extracted from the aqueous medium and based on that numerous technologies have been developed to remove or eliminate them from wastewater [50]. To address these serious issues

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cyclodextrin-based nanostructured sponges have merged as a promising candidate and have shown effectiveness and efficiency in removing dyes. Table 2 illustrates the selected examples of nanostructured sponges for dye removal for water remediation. In 2017, Riela et al., reported HNT-CDs hybrids by using natural halloysite clay and cyclodextrin under solvent-free conditions. The obtained HNT-CDs have been explored as a potential nano absorbent for an efficient removal of dyes like rhodamine B, methylene blue, toluidine blue, bromocresol green, methyl orange, and Cong Rubin at different pH (Table 2, entry 1). It was observed that HNT-CDs nanosponge showed the highest absorption for cationic dyes as compared to anionic dyes. HNT-CDs hybrids showed the best result with methylene blue and toluidine blue. HNT-CDs hybrids are found to be the most effective nano absorbent for methylene blue with a notable adsorption capacity of 226 mg/g as compared to the literature [51]. Junthip et al., reported the preparation of anionic nanosponges (BP5) by using 1,2,3,4-butanetetracarboxylic acid and β-CD in the presence of poly(vinyl alcohol) (Table 2, entry 2). These prepared BP5 nanosponges showed better adsorption for the removal of paraquat (PQ), safranin (SO), and1 malachite green (MG) with maximum adsorption capacities of 120.5, 92.6, and 64.9 mg/g respectively. These nanosponges are recycled and reused after five cycles and the adsorption capacities reached 94.1% for PQ, 91.6% for SO, and 94.6% for MG adsorption [52]. Liu et al., designed cyclodextrin-based nanosponges (CDNS) which were prepared by a one-step solvothermal method using β-CD and diphenyl carbonate (DPC). The as-prepared nanosponges were systematically investigated for an efficient removal of toxic dyes basic red 46 and rhodamine B at different pH, amount of adsorbent, time, and molar ratio of β-CD and DPC (Table 2, entry 3). The maximum adsorption capacity was found to be 101.43 mg/g and 52.33 mg/g for basic red 46 and rhodamine B respectively. The adsorption capacity of basic red 46 is almost double the value of rhodamine B due to the difference in structures of both dyes which explain disparate effects [53]. β-CD polyurethane nanosponges were modified by Mbianda et al., using phosphorylated multiwalled carbon nanotubes (pMWCNTs) which were further embedded with TiO2 and Ag nanoparticles leading to the formation of bio nanosponge polyurethane composite (pMWCNT/β-CD/TiO2 -Ag) (Table 2, entry 4). Amidation reaction, cross polymerization, using diisocyanate as a linker, and sol–gel are used for the synthesis of these nanosponges. The adsorption of trichloroethylene (TCE) and Congo red (CR) dye from wastewater was investigated using these nanosponges. These nanosponges were found to be efficient nano-biosorbent for the removal of TCE and CR using Langmuir model with maximum adsorption capacities, of 27,507 mg/g and 146.96 mg/g (CR dye) respectively [54]. Swaminathan et al., reported novel Co3 O4 /NiO nanosponges which were synthesized using the facile precipitation method (Table 2, entry 5). These nanosponges were explored for the photocatalytic degradation of organic dyes rhodamine B (RhB) and congo red (CR). The degradation of RhB and CR dyes was completed within 80 min of irradiation. A trapping experiment was used to check the active involvement of ROS, and OH radical was found to be active ROS [55]. Golubeva et al., reported a simple method for the preparation of inorganic nanosponges of a kaolinite [Al2 Si2 O5 (OH)4 ] structure (Table 2, entry 6). Hydrothermal treatment of aluminosilicate gel in an acidic

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Table 2 Examples of various nanostructured sponges for toxic dye removal from wastewater S.no.

Nanosponge

Pore Size

Surface area (m2 /g)

Dye

References

1

Halloysite–Cyclodextrin (HNT-CDs)

85.2

19.9

Rhodamine B, Methylene blue, Toluidine blue, Bromocresol green, Methyl orange, Cong rubin

[51]

2

Anionic nanosponges of β-CD and 1,2,3,4-butanetetracarboxylic acid (BP5)

n.a

n.a

Paraquat (PQ), Safranin (SO), and Malachite green (MG)

[52]

3

β-CD and diphenylcarbonate (DPC) based nanosponges

n.a

n.a

Basic red 46 and [53] Rhodamine B

4

pMWCNT/β-CD/TiO2 -Ag

n.a

352.5

Congo Red

[54]

5

Co3 O4 /NiO

n.a

n.a

Rhodamine B and Congo red

[55]

6

Aluminosilicate Nanosponges n.a

n.a

Methylene blue and azorubin

[56]

7

EDTA/β-CD

n.a

n.a

Methylene blue (MB), Safranin O (SF) and Crystal violet (CV)

[57]

8

β-CD based polymeric (β-MCD VI)

0.02

28

Rhodamine B [58] (RB) and Congo red (CR)

9

β-CD /pillar[5]arene (β-CD-P5)

4

479

Methylene blue

[59]

10

β-CD:PD

Non-porous

22

Methyl orange, Congo red, Rhodamine B and Methylene blue

[60]

11

βCD:EPI (1:135)

n.a

n.a

DirectBlue 78/ 24

[61]

12

α-CD:EPI (1:115)

n.a

n.a

DirectRed83:1

[62]

13

βCD:EPI (1:4, 1:6, 1:8)

n.a

n.a

Basic red 46 and [63] Rhodamine B

14

βCD:FPS (1:2)

5–6

na

2–naphthol

[64]

15

βCD:CMP(H)

1–10

1099

Nitrophenol, nitroaniline, nitrobenzene

[65]

418

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medium without the use of any organic cross-linking agents leads to the formation of these nanosponges. These nanosponges were explored for the adsorption of cationic (methylene blue) and anionic (azorubine) dyes in water [56]. Zhao et al., reported EDTA- β-CD nanosponges which were synthesized by a green approach via polycondensation reaction between β-cyclodextrin and EDTA as a cross-linker (Table 2, entry 7). The adsorption capability of these nanosponges was explored for an efficient removal of methylene blue (MB), safranin O (SF), and crystal violet (CV) dyes from wastewater. EDTA provides an adsorption site whereas β-cyclodextrin forms an inclusion complex. The adsorption capacity of methylene blue (MB), safranin O (SF), and crystal violet (CV) were found to be 0.262, 0.169, and 0.280 mmol/g [57]. Qin et al., reported a β-cyclodextrin-based polymeric nanosponges (MCD:VI) through free radical polymerization (Table 2, entry 8). Further, MCD:VI was used for the adsorption of rhodamine and congo red dye, and maximum adsorption capacity was found to be 1.12 mg/g and 336 mg/g for CR and RB respectively. The electrostatic and π–π interactions between NSs and dye molecules are easily affected by pH [58]. Lu et al., developed a porous β-cyclodextrin/pillar[5]arene (β-CD-P5) using tetrafluoroterephthalonitrile as crosslinking agent (Table 2, entry 9). β-CD-P5 NSs were utilized for the removal of methylene blue. It was found that the hydrophobicity of β-CD-P5 NSs increases by increasing P5 molar fraction and 78% of methylene blue was removed [59]. Jia et al., reported viologen-based β-cyclodextrin polymer through the Menshutkin reaction (Table 2, entry 10). The adsorption of two anionic (CR and MO) and two cationic (RB and MB) dyes were investigated using β-CD-PD NSs. Results revealed that only anionic dyes can be removed using these NSs, 80% for CR and 77% for MO [60] Nanosponges synthesized from β-cyclodextrins having greater surface area proved to be effective in removing dyes from wastewater. Murcia-Salvador et al., in 2019 synthesized βCD:EPI macroparticles for the spontaneous sorption of DirectBlue-78 dye [61] (Table 2, entry 11). In 2018 Pellicer et al., demonstrated the synthesis of nanostructures sponges using α-cyclodextrins (α-CD) and hydroxylpropyl-α-CD and its application for the successful removal of DirectRed83:1 dye (Table 2, entry 12). The as-synthesized NSSs using α-CD:EPI displays the removal efficiency (RE) and maximum sorbed amount (qm) as 92.8% and 31.5 mg/g respectively [62]. Li et al., in 2020 successfully displayed that the c1yclodextrin-based nanosponges are not only biodegradable but also possess high biosafety and could be synthesized by employing a single-step solvothermal technique using diphenyl carbonate and β-cyclodextrins (Table 2, entry 13). The synthesized NSSs showed the highest adsorption capacity of 101.3 and 52.33 mg/g for Basic Red46 and Rhodamine B dye/dyestuff from wastewater [63]. Wang et al., in 2017 studied various dye intermediates derived from benzene moiety namely, 4,4' -bis(chloromethyl)-1,1' -biphenyl (CMP) and 4,4' difluorodiphenylsulfone (FPS) for β-cyclodextrines derived nanostructured sponges (Table 2, entry 14). The removal efficiency was found to be > 99% for βCD:FPS in batch sorption experiments respectively [64]. Conclusively, properly functionalized nanostructured sponges prove to be a promising candidate to eliminate dye (organic/ inorganic) toxins from wastewater with greater sorbent efficiency and capabilities [65, 66].

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5 Conclusions: Future Perspectives In light of new findings, it can be concluded that the nanostructured sponges possess high porosity, ease of functionalization, involves simple synthesis steps, and are costeffective in comparison to other nanomaterials/systems. Moreover, cyclodextrinbased nanostructured sponges are not only biocompatible but also display the versatile characteristics of hyper-cross-linked polymeric specifications which in turn enhances the efficiency and performances of their parent CDs. While synthesizing these nanosponges one can control the physico-chemical properties and hence control on porosity or pore size could be achieved effectively. Such properties of nanostructured sponges make them an ideal and attractive candidate to capture and remove organic/inorganic pollutants/contaminants especially dyes from wastewater. However, it is interesting to mention that advanced elaborative studies are required to critically evaluate to overcome the shortcomings involving enhanced absorption capacities/toxin removal efficacy and removal efficiency to improvise the targeting pollutants/dyes from wastewater. It would be wonderful if this technology could come into the stage of commercialization and are cost-effective techniques for the betterment of mankind and the environment.

Abbreviations BP5 CDNS CDs CMP CR CV DMF DMSO DPC EDTA EPI FPS HDI HNT MG MWCNTs nm NS NSSs PQ PVA

1,2,3,4-Butanetetracarboxylic acid Cyclodextrin-based nanosponges. Cyclodextrins 4,4' - Bis(chloromethyl)-1,1' -biphenyl Congo red Crystal violet Dimethylformamide Dimethylsulfoxide Diphenylcarbonate Ethylenediaminetetraacetic acid Epichlorohydrin 4,4' –Difluorodiphenylsulfone Hexamethylene diisicyanate Halloysite Nanotubes Malachite green Multi walled carbon nanotubes Nanometer Nanosponge Nanostructured sponges Paraquat Polyvinylalcohol

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Maximum sorbed amount Removal efficiency Rhodamine B Rounds per minute Sodium dodecyl sulphate Safranin O Safranin Trichloroethylene Toluene-2,4-diisocyanate X-ray diffraction β-Cyclodextrin/pillar[5]arene

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Environmental Applications of Nanosponges (NSPs) to Clean up Oil Spills Yamini, Vikrant Singh Rao, Neeraj Mishra, and Sanjay Kumar

1 Introduction Oil Spills are when HC extracted by petroleum are released into the coastal area or ocean because of human activities. It can occur at various steps of exploration, areas of extraction like crude oil from offshore platforms, digging rigs and wells, or during transportation spillage by a tanker of refined petroleum derivatives like gasoline or diesel and other by-products; bunker fuel used by bigger ships as their own supply. Small spills are quite prevalent and mostly go unreported because of poorly regulated and minimally enforced rules in the area [49]. These oil spills are a critical threat [7] to our delicate ecosystem. When any derivative of oil is released into ocean water, it pollutes the water bodies, suffocates the marine organisms [21], and disturbs the interspecific balance. It is also responsible for species extinction due to their ecosystem destruction [47]. It further impacts the livelihood of coastal communities which rely on these ecosystems for their sustenance. The extent of these disasters can be enormous with several metric tons of oil making a film over the ocean surface. These incidents not only impact the immediate marine system but also have an everlasting socioeconomic crisis in the long term, which can leave its mark for years [26]. Therefore, it is near the hour to come up with strategies that are not only efficient but also a long-term solutions. The number of efforts that have been made over time to tackle the situation with developing technologies [15]. Methods like booms, skimmers, and sorbents Yamini · V. S. Rao Department of Life Sciences, Sri Venkateswara College, University of Delhi, Delhi 110021, India N. Mishra Department of Chemistry, ARSD College, University of Delhi, Delhi 110021, India S. Kumar (B) Department of Chemistry, Sri Venkateswara College, University of Delhi, Delhi 110021, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Gulati (ed.), Nanosponges for Environmental Remediation, https://doi.org/10.1007/978-3-031-41077-2_19

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have been employed to contain the spread and removal from the areas affected. Nevertheless, conventional techniques experience challenges with harsh weather, isolated areas, or fragile habitats. With the recently developing field of environmental NTch, studies show promising results in sustainable ways to clean oil spills. NPs s have a high surface-to-volume ratio and distinct physiochemical properties which proposes extraordinary opportunities in the respective field. In particular, NSPs demonstrate astounding properties like adsorbing, absorbing, or fragmenting oil particles, which gives them an upper edge as a tool for the mission. In this chapter, the potential application of NSPs and its impact in mitigating and contributing to the ongoing efforts to evolve a sustainable and efficient solution for oil spills. Through this chapter, we go over the importance of employing NSPs to clean up oil spills. We also look at new advances in the sector as well as the many approaches and methods used to extract oil from water. We will concentrate on its distinctive mechanical and physical characteristics, such as increased surface morphology and voids that permit the oil particles to rest therein as well as porosity, diameter, and surface tension of the fibres that make them appropriate for the task.

2 Environmental Impact of Oil Spills The impact of Oil release can be far greater in extent [17] than one expects and varied depending on the factors [3] like the volume and nature of released hydrocarbon, oil toxicity (linked to its density), energy in the physical environment to react with it and mixing with water and breaking down [28]. The results are degrading in nature no matter the geographical location, though they are more dramatic at excavating locations.

2.1 Ecological Consequences Marine Life Out of all, marine wildlife which is concentrated in intertidal zones (where a large amount of oil is present) experiences the most degrading effects as it is more innately exposed than terrestrial lifeforms. Static organisms [12] have it worst of all, though species that require traveling to the beach to complete their life cycle like turtles [9] for nesting, as also heavily impacted. Long exposure to the oil has different levels of results in different species populations [13]. It causes habitat redistribution due to the loss of existing habitats and disturbs the interspecific interaction [31], down to a decline in the number of species. It has a deleterious effect on MOGs like phytoplankton [14], it forms a film over the surface that doesn’t allow diffusion of gases and doesn’t let light penetrate

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through decreasing photosynthesis [39]. The structure of crude oil is made of simple methane mole forming a long chain of HC which can cause toxicity in the MOG. Phytoplankton are vital to multiple food webs and disruption in the same can mutilate multiple interlinked relationships. Oil Spills also affect marine birds in a multitude of ways [37]. They can be exposed to it by direct consumption when feeding on marine life or indirectly through contaminated species. Ingestion of the oil can cause physical repercussions. If they physically come in contact with the petroleum, it can cling onto their feathers and might affect their flight [8]. Studies show, even inhaled volatile petroleum causes toxic effects. Heavy metals like Copper or Cadmium alter the internal biochemistry of the organisms on a tissue level [25].

Effect on Mangroves Mangroves are a plant community that lines coastal areas in sub-tropical, tropical, and warm temperate regions, and have a characteristic resistance to water salinity. They are susceptible to change in their environment, especially endangered by oil spills, as the sensitive plant surfaces are seen getting deposits of oil, which further affects soils and the surrounding life forms. The oil deposits can suffocate the aerial roots [6], it also reduced transpiration by blocking the stomata [35] and hence causes mortality as oil particles can adhere to the surface for more than 6 months at a time. The volume of oil released directly influences the extent of damage caused, less oil spillage showed minimal damage to the leaves and quick recovery whereas major oil spills caused irreversible damage to the plants. Even the most established Mangroves can’t survive prolonged exposure to oil toxicity, as it asphyxiates and intoxicates the plants until it is starved.

2.2 Socioeconomic Impact The spillage causes enormous economic losses, aside, from direct spillage losses, there is an additional loss in cleaning the floating pollutant. The removal of the oil is a tedious process that depends on various environmental factors in addition to the properties of the material and how it reacts with its surroundings. It has been observed, the longer it is left untreated arduous it gets. The release of HC directly influences the life of people working on the site [27] and has a significant effect on livelihood along with visible health deterioration of coastal residents [1]. Directly hazardous toxicological impact on the employees engaging in the cleaning process is recorded. Furthermore, the locals who work in fish production face significant losses due to low production in hatcheries and contamination of the fish [36]. This makes a complex cycle [44], where human health is affected adversely, both physically and mentally [4]. Adding on to those contaminated beaches and food contributes to physical ailments which in turn cause mental distress. Income and productivity have

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been studied to be liked with mental health, which largely reflects on a community’s well-being.

3 NMs in Environmental Application NTch is an evolving branch of science that has founded its roots in various fields of environmental remediation [11] being one of them. It offers unique properties and a concise alternative as compared to its voluminous equivalents. It involves the synthesis of nanosized materials with added properties for a particular function. It uses the basics of surface chemistry and when synthesized in combination with other materials, it adds to their properties providing an advanced alternative that can target specific groups of interests. With increasing anthropogenic activities, organic and inorganic waste is released in water, polluting it. NTch provides an aid as nano filters, which removes a wide number of contaminants such as ions, pathogens, or even heavy metals. Their high surface area also makes them a good catalyst used to adsorb heavy metals and aid in other processes [45]. Similarly, membranes made up of NMs are used to trap large particles while gases pass through the porous structure. These are more efficient due to their small size which can trap minute particles as well. The same are also used as sensors, which can sense the nano amount of a substance leaks. Oil spillage is a massive contributor to marine water pollution, current techniques like skimming or igniting the surface oil contribute to pollution hence other alternatives are evolved for an efficient and sustainable way for clean-ups. NTch helps the synthesis of substance with properties that allows it to encapsulate the oil from water and has selectivity unlike traditionally used sorbent materials like raw cotton, Silicon coated glass etc. [22] along with other properties and can often also be reused. Types of NMs used for clean-up are in Fig. 1.

4 NSPs 4.1 Overview A contemporary type of material known as a “NSP” [5] is composed of minute particles with a small, nanometer-wide hollow. Different kinds of materials can be used to fill these little cavities. These small particles have the potential to hold [34] both hydrophilic and lipophilic [23] therapeutic substances, which increases the stability of molecules or medication substances that are weakly water-soluble. The NSPs constitute a network or multifaceted framework made of polyester that can break down organically. To create NSPs, these polyesters are dissolved in a

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Fig. 1 Types of NMs used in the cleanup of oil spills adapted from ref. [33] copyrights Elsevier Fig. 2 Structure of NSPs [46]

solution along with a crosslinker. Here, the polyester degrades gradually in the body because it tends to be biodegradable [16]. The organic molecules [19] that are loaded are released once the NSPs’ framework degrades (Fig. 2).

4.2 Properties of NSPs NSPs offer a distinct type of properties (Fig. 3), which can also be modified when combined with other materials and forming nanocomposites. These properties give it an edge over the existing techniques and hence increases the efficiency [40].

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Fig. 3 Properties of NSPs [24]

Surface Texture: The surface tends to be either of a single type or a mix of the aforementioned textures. The material’s rough surface makes it easier for oil to stick to the material’s surface, boosting adsorption. Voids [30]: To achieve high sorption capacity, it is also important to consider the number of voids present among the fibres. If there are interconnected gaps with a suitable size structure between these polymer fibres, which can hold oil via capillary motion through the porous inside of the NFs, a fast sorption rate can be attained. Width of Fibre: Because there are so many linked spaces in small-diameter, highporosity fibre, they have a great capacity for sorbing oil. A fibre’s tiny width also helps high-viscosity oil adhere to it. Larger-diameter fibres have more void space between them and less capacity to absorb oil than smaller-diameter fibres do. Porosity [38]: Pore size is classified on the basis of its diameter. The increased specific surface area of fibres with porous sectioning also improves their capacity to absorb oil. Surface Tension: Surface tension is a liquid surface’s tendency to reduce, which enables it to withstand an external force. A surface having a superhydrophobic property may change to an oleophilic or superoleophilic surface due to variations in the surface tension between water and oil. It was discovered that the surface tension of water and oil were 72.8 mN and less than 30 mN m1, respectively [29]. Therefore, hydrophobicity and oleophilic properties [18] could be present on any solid surface that has surface tension between water and oil. On the other hand, there are surfaces that exhibit super oleophilicity due to high surface roughness and surface energies that are comparable to those of oil. Oleophilicity and hydrophobic properties of a fibrous film’s high surface roughness and the natural qualities of the raw material, polystyrene, are what primarily generate a fiber’s superhydrophobicity and superoleophilicity behaviours. Furthermore, if the porous nanofibre structure is absent, superhydrophobicity and superoleophilicity are reduced. Additionally, because they may be properly disposed of

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during oil spill clean-up operations, it is simpler to collect sorbent with oleophilic and hydrophobic qualities, preventing additional environmental pollution.

5 Synthesis and Characterization of NSPs Because of their distinct characteristics and prospective uses in a variety of industries, such as medication delivery, sensing, catalysis, and environmental remediation, NSPs, a class of NMs, have drawn a lot of attention. I’ll list the various stages that go into the synthesis and characterization of NSPs below.

5.1 Synthesis of NSPs NSP Fabrication Techniques 1. Selection of Polymer: Selecting a suitable polymer for the manufacture of NSPs is the first step. Polymeric micelles, dendrimers, and cross-linked polymers are frequently utilized polymers. The preferred characteristics of the NSP determine the polymer to use. 2. Cross-linking Agent: To encourage cross-linking and create a three-dimensional network, a cross-linking agent is added to the polymer solution. A bifunctional or multifunctional substance that can create covalent connections with the polymer chains serves as the cross-linking agent. Examples include divinylbenzene and ethylene glycol dimethacrylate (EGDMA) (DVB). 3. Polymerization or Cross-linking Reaction: A polymerization or cross-linking reaction is carried out on the polymer solution and the cross-linking agent. Depending on the particular polymer and cross-linking agent utilised, this reaction can be started using a variety of techniques, including thermal, photochemical, or chemical initiators. 4. Templating: A template is utilized to build a porous structure inside the NSP. The porous structure can be left behind after the template, which could be a sacrificed material, is later removed. Another option for the template is an emulsion or surfactant that is spread throughout the polymer solution and stabilized throughout the cross-linking reaction. 5. Removal of Template (if applicable): If a sacrificial template was used, it is removed through methods such as dissolution, calcination, or thermal degradation. This step results in the formation of pores within the NSP.

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5.2 Characterization Techniques of NSPs Structural and Morphological Analysis NSPs must be described after synthesis in order to comprehend their structure, morphology, and characteristics. Here are some typical methods for characterizing things: 1. Scanning Electron Microscopy (SEM): The surface appearance and structure of the NSPs are revealed by SEM. In order to produce high-resolution images, it directs an electron beam. 2. Transmission Electron Microscopy (TEM): The internal structure and size distribution of NSPs can be characterized in great detail using TEM. It entails sending an electron beam across the sample and examining the image that is produced. 3. Fourier Transform Infrared Spectroscopy (FTIR): The chemical makeup and functional groups found in NSPs are investigated using FTIR. It examines how well the sample transmits and absorbs infrared light. 4. X-ray Diffraction (XRD): The crystallinity and crystal structure of NSPs are assessed using XRD. It entails exposing the material to X-rays and examining the diffraction pattern that appears. 5. Thermogravimetric Analysis (TGA): TGA calculates the weight variation of NSPs in relation to temperature. It offers details on their thermal stability and behaviour during disintegration. 6. Dynamic Light Scattering (DLS): DLS analyses the zeta potential and size distribution of NSPs in solution. It is based on an examination of the light reflected by suspended NPs s. 7. Brunauer–Emmett–Teller (BET) Analysis: NSPs’ precise surface area and porosity are measured via BET analysis. The adsorption of gas molecules onto the sample surface serves as its foundation. These are only a few illustrations of the methods applied to characterize NSPs. The particular characteristics and intended uses of the NSPs under investigation determine the characterization procedures to be used.

5.3 Surface Modification for Enhanced Oil Absorption Techniques for surface modification can be applied to improve a material’s capacity to absorb oil. It is possible to improve a material’s ability to absorb oil by altering its surface qualities, such as surface chemistry and roughness. A few methods for surface alteration are listed below: 1. Chemical modification: To improve a substance’s ability to absorb oil, chemicals can be used to change the surface chemistry of the material. For instance,

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hydrophobic groups or surfactants with a high affinity for oil can be used to functionalize materials. This alteration raises the material’s surface energy, which improves its ability to draw in and absorb oil. Plasma treatment: The surface of the material is exposed to a low-temperature plasma during the plasma treatment process, which might introduce desired chemical functionality. Surface roughness can be increased by plasma treatment, and reactive sites can be formed on the material, increasing its capacity for wettability and oil absorption. Coating deposition: The surface of the material can be made to absorb oil more effectively by coating it with a particular substance. It is possible to create coatings with a strong affinity for oil or with porous features that can catch oil droplets. The required coating can be applied to the material using a variety of coating processes, including chemical vapour deposition, dip coating, and spray coating. NM incorporation: The ability of a material to absorb oil can be considerably improved by adding NPs to its structure. For instance, increasing the surface roughness and hydrophobicity with hydrophobic NPs like carbon nanotubes or graphene can improve the surface’s ability to absorb oil. Electrospinning: A process called electrospinning can result in fibres with a high surface area-to-volume ratio. The material can absorb more oil by electrospinning hydrophobic polymers into fibrous structures. Increased oil adsorption contact points due to the fibrous structure result in increased absorption effectiveness. Surface roughening: By offering additional surface area for oil adsorption, increasing a material’s surface roughness can improve its ability to absorb oil. A rough surface texture can be produced using methods like sandblasting or etching, which improves the material’s capacity to bind oil.

It is significant to remember that the unique material and application requirements determine which surface modification technique is best. The type of oil to be absorbed, the material’s structural integrity, and the intended capacity for absorption should all be taken into account when choosing the best surface modification strategy.

5.4 Application of NSPs to Clean-Up Oil Spills A viable technology for a variety of uses, such as environmental cleanup, is NSPs. They may be helpful for cleaning up oil spills due to their special features. Here is how using NSPs to clean up oil spills works: 1. Absorption of oil: Composites interfacial area that are porous are used to make NSPs. These substances can be created at the nano size to have hydrophobic characteristics, which means they draw in and absorb oil but reject water. When applied to an oil spill, NSPs can effectively remove the oil from the water by absorbing it. 2. Floating capability: It is possible to create floating NSPs that wander on the water’s surface. This property makes the deployment and retrieval of the NSPs

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extremely convenient in cases of oil spills. The spread of the oil and its effects on the environment can be reduced by dispersing the NSPs across the afflicted area. 3. Reusability: Reusable anosponges may be created, which is a big benefit for cleaning up oil spills. The NSPs may be cleaned and reused numerous times after absorbing the oil since they can be heated or squeezed to liberate the trapped oil. This reuse lessens the expense and negative effects on the environment of the cleanup procedure. 4. Selective absorption: NSPs are extremely adaptable in various cleanup settings because they may be designed to specifically absorb particular types of oil or HC. The NSPs can be customised to target specific contaminants while disregarding other impurities in the water by adjusting their surface chemistry or structure. The clean-up process is more effective and efficient thanks to this selectivity. 5. Biodegradability: However, many NSP products can be obtained to be biodegradable, which means that they will eventually decompose naturally into nontoxic constituents. This feature helps the environment by removing the need for additional clean-up procedures after the oil has been absorbed. NSPs that are biodegradable can safely dissolve, leaving behind few remnants and minimising their long-term ecological impact. Although NSPs have potential for cleaning up oil spills, it’s vital to remember that their practical application is still being developed. Before they can be extensively used as a standard method for oil spill remediation, more research is required to enhance their design, scalability, and environmental impact.

6 Oil Extraction Mechanism of NSPs 6.1 Precipitation and Degradation Degradation, or alterations [43] to the toughness and chemical makeup of polymers, is what causes them to lose some of their physical qualities. The process of precipitation is used to separate and purify polymers. At particular temperature, salinity, and pH exists in an oil reservoir. When high some injected substances degrade and become unstable due to extreme temperatures, high levels of salt, and low pH [32]. The presence of polymer has a big impact on the viscosity of the polymer solution. The reservoir brine cations screen the polymer molecules under salinity. Thus, it causes macromolecule shrinkage under saline circumstances by lowering the interaction of polymer chains and hydrodynamic radius [41]. As a result, the polymer solution’s viscosity is decreased. High temperatures, meanwhile, encourage hydrolysis of polymer and precipitate at divalent ions (Fig. 4). The injection of chemicals could become acidic because of low pH conditions in the reservoir [2].

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Fig. 4 Degradation of pollutants with the help of Nanostructure [42]

6.2 Adsorption Chemicals are absorbed and retained when they pass through porous NM (Fig. 5). The kind of chemicals poured into the reservoir affects these variables. Chemicals can affect the reservoir rock in a number of ways, including by lowering the concentration of chemical solutions injected, electrostatic attraction, steric interaction, van der Waal forces etc. [20]. The adherence of polymer molecules to the rock surface is what causes polymer adsorption. A chemical molecule travels from the bulk solution stage, where adsorption at the contact takes place. Alkali chemicals and surfactants are adsorbents.

6.3 Factors Affecting Oil Uptake There are numerous factors that might affect the uptake of the oil, it could be the factors related to the NPss or something related to the surroundings. The characteristics related to the surrounding is pH and viscosity and the ones related to the material are as follows: Surface Tension of Adsorbing material: The adsorbing NPs have pores, which means lower will be the surface tension easier it will be for the oil to get attached to the surface of the wall [48]. Sorption between Water and Oil: A sorbent can absorb and remove nearly every bit of the oil that has leaked into the water. Furthermore, oil thickness has an impact on oil selectivity and subsequently oil sorption capability. Given that the sorbent remains overloaded with oil, a rise in the oil thickness increases the sorption capacity. In addition, differential wettability and an elevated fibre porosity work together to

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Fig. 5 Oil getting adsorbed on the surface of sorbent cleaning the water surface [10]

effectively separate oil–water combinations. This superior oil–water selectivity will lessen the sorbent’s bulk and decrease the amount of water collected from the spill site [50]. Surface Area of the material: A fibre that has a significant amount of specific surface area and porous structures that boost its efficiency and capacity for choosing oil for sorption in an oil–water combination. Additionally, a fiber’s capacity to absorb oil increases with its diameter, both at a faster rate and with a greater capacity [48]. Buoyancy: Low-density fibres often have more buoyancy in water. The short widths and elevated porosity are responsible for the low density. These properties make it easier for oil to bind to the walls of the fibres to fill in any gaps between them, leading to a higher capacity for sorption [50].

7 Performance Evaluation of NSPs Owing to their unique characteristics and prospective uses in numerous industries, such as medicine delivery, sensing, and environmental cleanup, NSPs, a particular class of NM, have drawn a lot of attention recently. NSPs’ functionality in particular applications, as well as their physical, chemical, and biological features, are all taken into account when evaluating their performance. Here are some important factors to take into account when assessing the effectiveness of NSPs: 1. Physical characterization: It is important to describe the stability, size, form, and surface morphology of NSPs. The physical characteristics of NSPs can be observed and measured using methods including atomic force microscopy

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(AFM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Chemical composition: The behaviour and effectiveness of NSPs are greatly influenced by their chemical makeup. The elemental and molecular makeup of NSPs can be determined using methods such as X-ray diffraction (XRD), Fouriertransform infrared spectroscopy (FTIR), and elemental analysis (such as energydispersive X-ray spectroscopy). Porosity and absorption capacity: High porosity is a characteristic of NSPs that enables them to absorb and hold a variety of substances. Measuring variables including specific surface area, pore size distribution, and absorption kinetics is necessary to assess the porosity and absorption capability of NSPs. These properties can be studied using methods like gas or liquid adsorption tests and Brunauer–Emmett–Teller (BET) analysis. Drug delivery efficiency: In medication delivery systems, NSPs are used extensively. Analyzing factors including drug loading capacity, release kinetics, and the stability of the loaded pharmaceuticals is necessary to evaluate how well NSPs carry drugs. Studies can be carried out in vitro and in vivo to assess the therapeutic efficacy and release profile of NSP-based drug delivery systems. Biocompatibility and toxicity: For biomedical uses, NSPs should be examined for potential toxicity and biocompatibility. To evaluate the cytotoxicity of NSPs, cell viability experiments, such as MTT or cell counting assays, can be carried out. Histopathological examination and hemocompatibility investigations can also shed light on their compatibility with blood and potential negative effects on organs. Application-specific performance: For specialized uses like sensing or environmental cleanup, NSPs can be customized. In these circumstances, the effectiveness of their performance in eliminating contaminants or detecting target analytes should be assessed. Experimental setups created for particular applications can be used to assess parameters like removal efficiency, selectivity, and sensitivity. Stability and reusability: For practical use and cost-effectiveness, NSPs’ stability and reusability must be evaluated. Evaluation of variables including shelf life, storage conditions, and the stability of functional groups on the surface of NSPs are all examples of stability studies. By running a particular application through several cycles and assessing the performance after each cycle, reusability may be determined.

Analyzing the physical, chemical, and biological characteristics of NSPs as well as how well they work in particular applications constitutes a thorough performance evaluation of NSPs. To gain a thorough grasp of their performance capabilities, it frequently takes a mix of characterisation methods, in vitro and in vivo research, and application-specific testing.

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7.1 Oil Absorption Capacity and Efficiency of NSPs A sort of NM with a high capacity and effectiveness for absorbing oil is called a NSP. They are made to absorb and remove hydrocarbon-based materials like oil from a variety of surfaces or situations. NSPs are useful in oil spill cleanup, wastewater treatment, and other oil absorption applications due to their special characteristics. The amount of oil that NSPs can absorb in relation to their own weight is referred to as their “oil absorption capacity”. NSPs can adsorb and encapsulate oil molecules within their network of nanopores due to their highly porous structure and huge surface area. They can absorb a lot of oil thanks to this structure, frequently more than their own weight. Depending on the composition and design of the NSP, the specific absorption capacity may vary, but it is often rather high. The advantages of NSPs in terms of effectiveness are numerous. First off, the high porosity and surface area of these materials improve the interface between the oil and the NSP, promoting quick absorption. This makes it possible for them to remove oil from the environment fast, so limiting its spread and potential for environmental harm. Furthermore, NSPs frequently have reusability in their design. They can be used again without significantly losing their ability to absorb oil because after collecting them, they may be pressed or treated to release the gathered oil. Chemical characteristics and surface changes of NSPs also affect how effective they are. NSPs can have their surface chemistry modified to increase their affinity for oil molecules, enhancing their capacity to absorb a given class of oils or HC. NSPs can maintain their porosity and absorption capacity over time by having surface changes that help stop the aggregation of the NMs. Due to their porous structure, substantial surface area, and surface changes, NSPs exhibit excellent oil absorption capacity and efficiency overall. These qualities make them viable materials for a range of oil cleanup and remediation applications. It is crucial to remember that the design, composition, and environmental circumstances in which NSPs are employed can all affect how well they operate.

7.2 Reusability and Regeneration One kind of NM called a NSP is intended to absorb or collect chemicals, poisons, or other things. They are made up of microscopic particles having a porous structure that makes it possible for them to effectively absorb and trap different chemicals. The reusability of NSPs is dependent on the unique structure and make-up of the NSP material. While some NSPs are single-use or have limited reusability, others can be reused several times. The capacity to regenerate NSPs easily determines how many times they may be employed to absorb or eliminate pollutants or poisons. Regeneration is the process of releasing the compounds that were caught from the NSPs, regaining their original characteristics, and making them usable again. Depending on the type of substances

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being caught and the material of the NSPs, several regeneration techniques may be used. Typical methods for regeneration include: 1. Desorption: In this technique, the NSPs’ trapped materials are released using an appropriate solvent or chemical agent. The NSPs can then be cleaned or given a special treatment to get rid of any remaining impurities so they can be used again. 2. Thermal treatment: The desorption of molecules collected by NSPs can be induced by heating them up. The confined molecules may evaporate or dissolve when heated, releasing the NSPs for further use. But not all kinds of NSPs may be acceptable for this technique, as some may lose their structural integrity or deteriorate at high temperatures. 3. Chemical treatment: It is possible to utilize certain chemical agents to interact with the compounds that have been caught and transform them into less dangerous or removable forms. This enables the regeneration and reuse of the NSPs. 4. Physical methods: There are a variety of physical procedures that can be performed, such as filtering, centrifugation, and ultrasonication, to remove the trapped components from the NSPs. These methods rely on mechanical forces or separation mechanisms to separate the contaminants and allow for the regeneration of the NSPs. It is important to remember that research into the reuse and rejuvenation of NSPs is continuous, and new methods and materials are frequently developed to increase their potency and effectiveness. As a result, the specific facts and limitations on renewability and rejuvenation may change depending on the most recent advances in the area.

7.3 Compatibility with Marine Ecosystems One of its future applications for NSPs, which are primarily made of porous materials with nanoscale characteristics, is environmental cleanup. Their suitability for maritime environments is a complex subject that requires careful consideration. 1. Material Composition: The materials used to make NSPs affect whether they are compatible with marine ecosystems. While some NSP materials might be harmful or have negative impacts, others might be biocompatible and have little to no influence on marine life. It’s important to select substances that are non-toxic and safe for marine life. 2. Particle Size: Varied NSPs can have different particle sizes, and interactions with marine life may be more likely with smaller particles. Marine species may consume NPss, which could result in bioaccumulation and biomagnification within the food chain. Therefore, it’s crucial to evaluate the particle size and marine organisms’ capacity to consume or absorb it.

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3. Ecotoxicological Effects: Ecotoxicological research should be carried out indepth to assess the potential impacts of NSPs on marine species before they are introduced into marine environments. These studies ought to evaluate variables like acute and chronic toxicity, bioaccumulation, and any potential effects on marine organisms’ ability to reproduce, grow, or behave. 4. Disposal and Environmental Fate: After use, NSPs must be disposed of properly to avoid building up in marine habitats. The fate of NSPs and any potential longterm impacts must be taken into account if they are released into the ocean. It is essential for determining if NSPs are compatible with marine ecosystems to comprehend how they degrade, persist, and move through them. 5. Environmental Monitoring: Any potential ecological effects can be discovered with the use of ongoing monitoring of NSP deployment regions. Monitoring programmes can identify any negative consequences and, if necessary, take appropriate action by assessing changes in water quality, species abundance, and the overall health of ecosystems. A cautious and thorough approach is required to verify that NSPs are compatible with marine environments. To reduce potential dangers and make sure marine ecosystems are sustained over the long term, rigorous testing, regulation, and monitoring are necessary.

8 Challenges and Limitations In compact size and porous nature, NSPs are a viable solution for reducing water pollution since they may absorb toxins from water. But when putting them into practice, there are a number of difficulties and constraints to take into account. Some of them are as follows: 1. Scalability: Scaling up the manufacture of NSPs for extensive water treatment applications is one of the main challenges. To generate NSPs in large quantities without sacrificing their effectiveness and quality, manufacturing procedures must be established. A big challenge can be achieving mass production at an affordable price. 2. Contaminant selectivity: Different pollutants may be more or less selectively removed by NSPs. They may not be as effective for other contaminants even though they can eliminate some of them. The diverse and complicated mixture of pollutants prevalent in water sources must be addressed by designing NSPs with a wider range of contaminant selectivity. 3. Stability and reusability: Under various environmental circumstances, NSPs ought to remain stable and not deteriorate over time. During repeated use, they ought to maintain their structural stability and capability for absorption. In order to maximize their effectiveness and reduce waste, NSPs must be stable and reusable.

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4. Fouling and fouling mitigation: The term “fouling” describes the buildup of contaminants or particles on the surface of NSPs, which lowers their efficiency. NSPs may become clogged with fouling, which calls for frequent cleaning or replacement. For long-term and effective operation, fouling must be reduced using procedures such as surface changes or backwashing. 5. Environmental impact: The possible environmental impact of NSPs must be evaluated throughout their life cycle. This entails assessing the technique of manufacture, the means of disposal, and any potential environmental discharge of NPs. NSP-based water treatment systems must be made safe and sustainable overall by being aware of potential dangers and putting in place the necessary precautions. 6. Regulatory considerations: Through the use of NSPs for water treatment may be governed by legal specifications and approvals. These could involve assessing their effectiveness, safety, and conformance to water quality regulations. It might be difficult and time-consuming to comply with applicable laws and gain required clearances. 7. Cost-effectiveness: Cost is a crucial factor when creating NSPs and integrating them into water treatment systems. For NSPs to be widely used, it is crucial to develop ways for producing them that is both affordable and efficient, as well as techniques to extend their lifespan and capacity for reuse. 8. Integration with existing infrastructure: It can be difficult to integrate NSP-based water treatment devices with currently in-place infrastructure and water treatment procedures. To make the implementation of NSP technology easier, compatibility with traditional treatment techniques and the capacity to adapt or update current systems should be taken into account. It will involve ongoing study, technological development, and cooperation between scientists, engineers, policymakers, and stakeholders to address these problems and constraints. Despite these challenges, NSPs have the potential to improve water treatment processes and increase access to clean water.

8.1 Environmental Impacts and Disposal of Used NSPs A subgroup of NMs called NSPs has been developed for a number of applications, such as water filtration, drug delivery, and environmental remediation. These NSPs can absorb and remove contaminants from their environment since they are typically made of porous materials with wide surfaces. The environmental impact of NSPs must be considered both during use and after disposal. Here are some ideas to consider: 1. Production: Energy, chemicals, and other materials may be used in the manufacturing of NSPs. The manufacturing techniques and materials utilised determine the production’s environmental impact. Adopting environmentally friendly and sustainable production practices is crucial for manufacturers.

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2. Applications: MOGs, chemical compounds, and heavy metals can all be removed from water using NSPs. NSPs can contribute to environmental and human health protection by successfully lowering pollution. However, it is essential to make sure that when employed in applications, NSPs do not raise new environmental issues. 3. Fate in the environment: The fate and behaviour of NSPs should be taken into account when they are employed and released into the environment. According to studies, NPss can interact with various environmental components like soil, water, and living things. The persistence, motility, and possible bioaccumulation of NSPs must all be considered when evaluating their potential ecological effects. 4. Disposal: When a NSP reaches the end of its functional life, it may eventually need to be discarded. The disposal procedures must to be designed to reduce any possible threats to both the environment and public health. Depending on the exact material composition, NSPs should ideally be disposed of using sound waste management techniques, such as recycling or cremation. It is crucial to adhere to local laws and regulations regarding the disposal of NMs. 5. Risk assessment: Comprehensive risk assessments must be carried out in order to determine the overall effects of NSPs on the environment. These evaluations ought to take into account things like exposure routes, organism toxicity, and potential long-term impacts. Researchers and governments can decide how to use and discard NSPs in an informed manner by identifying and resolving potential dangers. It’s important to remember that research into the topic of NTch and its effects on the environment is currently ongoing. To ensure the safe and sustainable application of NSPs, more specific standards and best practices for their usage and disposal are anticipated to develop as scientific knowledge increases.

9 Emerging Trends and Future Directions A promising technique for water cleanup, NSPs have a variety of possible uses and special characteristics. In the area of NSPs for water cleanup, the following are some recent developments and future directions: 1. Enhanced adsorption capacity: The creation of better adsorption-capable NSPs is a current area of research focus. Researchers are working to improve the adsorption and removal of several contaminants from water, including heavy metals, organic pollutants, and microbes, by altering the surface chemistry and structure of NSPs. 2. Selective adsorption: Designing NSPs with characteristics for selective adsorption is the subject of current research. This entails modifying the surface characteristics of NSPs to target particular contaminants, enabling them to remove specific pollutants from water with specificity. Selective adsorption can reduce

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the need for additional chemicals while increasing the effectiveness of water treatment procedures. Regeneration and reuse: The creation of NSPs that are simple to regenerate and reuse is a crucial future direction. This would lessen the overall cost and negative effects of the water treatment procedures on the environment. In order to ensure the long-term and sustainable usage of NSPs in water remediation applications, researchers are investigating strategies to efficiently regenerate them. NSP composites: The incorporation of NSPs into composite materials is another recent development. Researchers want to develop better water treatment systems with improved performance and scalability by integrating NSPs with other materials, including membranes or porous matrices. In practical applications, these composite materials can increase the filtering effectiveness, robustness, and stability of NSPs. Smart NSPs: One promising area for future research is the creation of intelligent NSPs. These NSPs have the ability to react to particular stimuli or circumstances, such as pH changes, temperature changes, or the presence of particular pollutants. Water remediation procedures can be made more effective and regulated by using smart NSPs, which can be created to release collected contaminants when needed or to give real-time feedback on the quality of the water. Scale-up and commercialization: Scaling up manufacturing and looking at commercialization options are being done as the field of NSPs for water cleanup develops. The synthesis techniques, manufacturing procedures, and costeffectiveness of NSPs are being improved by researchers and business partners, laying the groundwork for their broad use in water treatment applications. Environmental impact assessment: The assessment of NSPs’ environmental effects at every stage of their life cycle, from manufacturing to disposal, will be a key component of future research. Understanding any potential concerns related to the use of NSPs for water cleanup is essential, as is coming up with plans to reduce their environmental impact.

Ultimately, NPs show great promise for water cleanup, and further development of their capability, effectiveness, and sustainable development is anticipated as a result of current research.

10 Conclusion In outcome, using NSPs to clean up oil spills is a creative and novel way to deal with environmental problems. Highly porous materials with nanoscale structures, known as NSPs, have shown exceptional promise in the adsorption and removal of oil-based pollutants from water. Compared to traditional oil spill cleanup techniques, this approach has a number of significant advantages. First of all, NSPs have a very high surface area-to-volume ratio, which enables them to absorb a sizable amount of oil. They effectively trap oil molecules due

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to their porous nature, inhibiting their dispersal and further contaminating aquatic habitats. The great adsorption capacity of NSPs makes them an efficient instrument for cleaning up oil spills, perhaps requiring less time and money. Second, hydrophobic NSPs can be created, which means they attract and absorb oil while repelling water. This characteristic permits oil removal that is selective, minimizing First off, NSPs have an extremely high surface area to volume ratio, which makes it possible for them to absorb a significant amount of oil. Due to their porous nature, they effectively trap oil molecules, preventing their diffusion and further damaging aquatic ecosystems. NSPs are an effective tool for cleaning up oil spills due to their high adsorption capacity, possibly needing less time and money. The volume of water gathered throughout the cleanup operation. As a result, there might be a major reduction in the environmental impact on marine species and the entire ecosystem. The versatility of HC that NSPs can adsorb, including both light and heavy oils, is one of their main benefits. Because of their adaptability, they may be used to clean up a variety of oil spills, independent of the viscosity or makeup of the oil. Furthermore, NSPs can be designed to have particular characteristics, such as hydrophobic or oleophilic surfaces, which improve their capacity to absorb oil. The ability to reuse NSPs is an important advantage. After absorbing the oil, they are simple to collect and regenerate for use in the future, limiting waste and harm to the environment. Additionally, NSPs can be functionalized with components that aid in the biodegradation of the oil they have absorbed, further enhancing their environmental friendliness. Moreover, biodegradable NSPs can be generated. They can go through regeneration procedures after collecting oil, such as solvent or heating treatment, to flush out the absorbed oil and regain their ability to adsorb. Due to their capacity to be reused, NSPs are a cost-effective way to clean up oil spills because they require less frequent replacement and produce less trash. It is significant to mention that innovations are still ongoing for the use of NSPs to clean up oil spills. Additional research is required to improve their performance, analyze their long-term environmental consequences, and optimize their design. The expansion of NSP production and their practical application in actual oil spill scenarios must also be taken into account. Nevertheless, despite these obstacles, the use of NSPs in environmental remediation holds considerable potential for dealing with oil spills and reducing their ecological impact. To fully utilize the capabilities of this ground-breaking technology and guarantee its secure and efficient application in the real world, it is imperative that scientists, engineers, and policymakers continue their studies and work together. We may work toward a cleaner, more sustainable future for our planet by utilizing NSPs.

Abbreviations NPs NSP

Nanoparticles Nanosponge

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NM NTch NFs MOG HC

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Concluding Remarks and Future Perspectives of Nanosponges in Environmental Remediation Shefali Shukla, Ankita Sangwan, Nandini Pabreja, and Shikha Gulati

1 Introduction Nanosponges are cross-linked, thermally stable, polymeric nano-sized structures having large surface areas with a well-defined pore size that are suitable for enclosing various substances [76]. These are the class of nanomaterials that have gained significant attention in recent years due to their unique properties and wide range of applications. They are essentially nanoscale structures composed of porous networks that can absorb and carry various substances, making them versatile and multifunctional materials [18]. The term “nanosponges” is derived from their resemblance to traditional sponges, which are highly porous and capable of absorbing liquids. However, nanosponges operate at a much smaller scale, typically ranging from a few nanometers to hundreds of nanometers in size. Nanoparticles have a wide variety of applications such as biocompatible material, textile functionalization, and coatings against UV-radiation or allowing microbial degradation, dug delivery, DNA delivery, enzyme immobilization, removal of water contaminants like heavy metals, dyes, organic compounds, etc. Nanotechnology is potentially the most important engineering revolution since the industrial age. So far nanotechnology resulted in variants of formulation like nanoparticles, nanocapsules/nanospheres, nanosuspensions, nanocrystals, nano-erythosomes, etc. In recent years, nanomaterials are gaining a lot of attention. In 1955 Richard R. Feynman, a physicist, at Cal Teach, forecasted about nanomaterials. He said “There is plenty of room at the bottom”, and suggested that scaling down to the nano level and starting from the bottom was the key to future advancement in nanotechnology. Nanosponges are sorbent materials based on sorption properties that is absorption and adsorption together. Nanosponges have a great potential for the systematic removal of organic/ inorganic pollutants from wastewater based on disinfection and sorption processes. S. Shukla (B) · A. Sangwan · N. Pabreja · S. Gulati Department of Chemistry, Sri Venkateswara College, University of Delhi, Delhi 110021, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2023 S. Gulati (ed.), Nanosponges for Environmental Remediation, https://doi.org/10.1007/978-3-031-41077-2_20

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Nanosponges are adsorbent structures with cavities or voids (sized between 1 and 100 nm) which are capable of encapsulating small molecular weight substances through host–guest interactions. They are classified as nanoporous materials which are suitable for removing the contaminants which are the main focus of environmental research. The Nanosponges can address the problem of purifying the water by separating the metal ions or small organic molecules from wastewater samples. Conventional water purification approaches involve both filtration through activated carbons and reverse osmosis. But these methods have limited applicability as activated carbon has a poor affinity to natural compounds and it can take away limited type of natural materials from water and is not suited for removing most of them. The advantage of using nanosponges is that they effectively remove organic contaminants at concentration levels well below that of the activated carbon.

Cyclodextrin

Cross-linking Agent

Nanosponges

Nanotechnology is the branch of science that deals with the development of materials at a nanoscale level that exhibits novel properties. These nanomaterials have been explored for applications in various fields such as pharmaceuticals, polymer industries, environmental research, and many more. These Nanosponges could be developed using different strategies where organic compounds with organic linkers may be used or it could be a hybrid of organic linkers and metal ions. Nanosponges are supramolecular structures and their specific morphology provides a well-defined space or void which can accommodate a wide variety of molecules depending on their size and polar characteristics. Their high porosity with specific pore size, easy functionalization, and cost-effectiveness make them a suitable choice for capturing and removing organic/inorganic contaminants (e.g. Dyes, pharmaceuticals, and heavy metals) from wastewater.

2 Classification of Nanosponges Various classes of nanosponges have shown potential in environmental remediation, offering innovative solutions for addressing pollution and contaminants. These nanosponges may be classified on the basis of the material used in their synthesis. A few popularly recognized nanosponges include (Fig. 1).

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Fig. 1 Types of nanosponges

2.1 Cyclodextrin Based Nanosponges Cyclodextrin (CD) is dynamic molecule having peculiar and amphiphilic structures. They are cyclic oligomers formed by six, seven, and eight α-(1,4) linked glucopyranose units. These oligosaccharides have a truncated cone or torus-like structures because of chair conformation of the glucopyranose units [19, 43]. These molecules have hydrophobic interior and their outer surface is hydrophilic. The combination of external hydrophilicity and internal hydrophobic surface comprises a unique “microenvironment”, the ability of cyclodextrins to form inclusion host– guest complexes with many hydrophobic substances. The hydrophobic pocket of CDs forms stable host–guest inclusion complexes with small molecules but not with hydrophilic and high molecular weight molecules [28]. To overcome these limitations of cyclodextrins, the outer surface of CDs which show high reactivity as a consequence of their hydroxyl groups is generally modified. These functional groups undergo easy substitution and elimination. To accomplish these goals, a monomer is often added to polymerize by using a crosslinker to form ‘Cyclodextrin Nanosponges’. Theoretically, these Cyclodextrin nanosponges could be 3D crosslinked polymer networks of α-, β-, or γ-cyclodextrins but these are generally prepared from β-cyclodextrins as they have the highest complexity and stability owing to their suitable cavity size with cross-linkable polymers [6, 42]. These nanostructures have wide applications in the delivery system, bioremediation, chemical sensors, and catalysis [1, 56]. These complexes may impart beneficial modifications of the properties of guest molecules such as solubility enhancement and stabilization of labile guests [64]. The versatility of these sponges crosslinking to a high polymeric surface makes them a suitable candidate for a wide range of applications like energy storage and

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environmental remediation. Cross-linking agents can also introduce various functional groups into β-cyclodextrin (β-CD) polymers, expanding their applications and properties [29, 36]. Some examples of functional groups that can be induced in βCD based nanosponges include Urethane-based nanosponges, Ester or Acid-based nanosponges, Carbonate-based nanosponges, and Ether-based nanosponges. In the last 2 decades, cyclodextrin-based nanosponges have been developed and synthesized by various methods for pharmaceutical and biomedical applications, taking advantage of the nontoxicity of cyclodextrins towards humans [80]. These crosslinkers may influence the physical characteristics (e.g. water solubility), stability (e.g. biodegradability), and the applicability (removal of heavy metals, dyes, organic pollutants, etc.) of the resultant polymeric nanosponge [68, 82]. For example- the cyclodextrin-based polyester nanosponges are capable of degrading naturally. These polyesters are mixed with a crosslinker in a solution to form Nanosponges [70]. For such modified nanosponges—the type of applied material (e.g. Native/Modified polymers) adsorbent doses, contact time, competing ions, experimental conditions (e.g. Molar ratios, temperature and, pH solution) are the important factors that affect the adsorption and removal of pollutants from wastewater [55, 76].

2.2 Metal–Organic Framework-Based Nanosponges MOFs are a form of metal–organic framework crystal materials formed by selfassembly of organic ligands and metal ions or clusters of ions on periodic network structure. The metal which isutilized can be divalent (Cu, Zn, Mg), trivalent (Al, Cr, Ga, Fe, In), and tetravalent (V, Zr, Ti, Hf). In this regard, metal–organic frameworks (MOFs) are amongst the most researched group of materials that are porous, crystalline, and have structural regularity and easily tunable functionality. All these characteristics have distinguished the MOFs from the rest of the porous materials under research. The crystallinity of MOFs is ensured at the synthesis stage where the organic building blocks are used which are coupled with organic linkers and the metal ions/clusters serve as nodes during the molecular designing (Fig. 2). The MOFs are hybrid structures that have been explored extensively in the area of adsorption of metal ions, gases, pollutants, catalysis, optoelectronics, drug delivery, etc. [1]. These tailored structures have well-defined nanopores (0.5–1 nm shows good adsorption capacity) which provide a specific space for various industrial, bio-remedial, and pharmaceutical applications [57]. Since the characteristics of these MOFs can be modified by judicious use of organic building blocks, linkers, and metal ions these supramolecules are called ‘Designer Molecules’ which can be easily tuned as per the application [23].

Fig. 2 Factors affecting the crystallinity and pore size of metal–organic frameworks

Factors affecting the Crystallinity and pore size

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Organic Building Block Metal ion/cluster Organic linker

2.3 Cellulose-Based Nanosponges Cellulose-based nanosponges (CNS) are a type of micro and nanoporous materials derived from cellulose nanofibers. They are obtained by thermal cross-linking between cellulose fibers. The synthesis of cellulose-based nanosponges typically involves the conversion of cellulose into a gel-like state, followed by the formation of a three-dimensional network through a crosslinking process [47]. Crosslinking can be achieved using various methods, such as chemical crosslinking, physical crosslinking, or a combination of both. The resulting nanosponges exhibit interconnected pores within the cellulose matrix, which contribute to their high surface area and porosity. CNS is formed by a process wherein the cellulose is oxidized by TEMPO/NaBr/NaClO to obtain carbo-oxylated cellulose which further undergoes ultrasonification and a series of thermal treatments and washing, the inner nanostructured network is formed referred to CNS. These are a type of bio-nanomaterials that are derived from natural and renewable sources such as plants and bacteria. They have unique structural, mechanical, and optical properties that make them suitable for water and wastewater treatments [63]. Some of the cellulose-based nanomaterials that have been used for water treatment are cellulose nanocrystals (CNCs), cellulose nanofibers (CNFs), cellulose nanoparticles (CNPs), and bacterial nanocellulose (BNC). These nanomaterials can be used for various water treatment techniques, such as membrane filtration, flocculation, sorption, catalytic degradation, and disinfection. Cellulose-based nanomaterials have many advantages they are environmentally friendly (Renewable and biodegradable, bioresorbable), have excellent absorbent and adsorbent properties, biocompatible and sustainable. They can be produced at a low cost and with less energy consumption than other nanomaterials such as carbon nanotubes. They also have potential applications in other fields such as biomedical engineering, electronics, packaging, and textiles [78].

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2.4 Hyper-Cross Linked Polystyrene Nanosponges Hyper-crosslinked polystyrene nanosponges (HCPSNs) are polystyrene-derived porous nanomaterials having highly crosslinked three-dimensional network structures. The HCPSNs are synthesized from a polymerization process where polystyrene is crosslinked using a crosslinking agent, typically a divinyl benzene (DVB) or a similar compound. This crosslinking reaction results in the formation of a sponge-like structure with interconnected pores. Hyper cross-linked polymers can form crosslinked networks that are effective in breaking down and decomposing into individual elements. These materials are biodegradable material and easily recyclable hence have improved performance as compared to traditional polystyrene in terms of applications [45]. Hyper cross-linked nanosponges were synthesized by Davank et al. The modification of hyper cross-linked nanosponges is the formation of spherical nanosponges which takes place when rigid intermolecular bridges are introduced into polystyrene coils in a dilute solution. Due to this formation, spherical nanosponges exhibit high inner surface area and this is increased by the cross-linking process which allows the polymer to have a higher degree of linkage between its carbon chains. This in turn increases the number of turns per inch (N/A) and also the degree of cross-linking, which ultimately leads to a higher degree and high rate of polymerization. The high surface area enhances their adsorption capacity and allows for increased contact with target molecules or contaminants. HCPSNs are also known to be more efficient at and absorbing water and other molecules. HCPSNs are chemically stable structures that ensure the longevity and reliability of HCPSNs in different settings. The applications of HCPSNs span various fields, including water treatment, environmental remediation, drug delivery, catalysis, sensing and detection, and gas separation [74]. Ongoing research aims to further explore the potential of HCPSNs by optimizing their synthesis techniques, tailoring their properties, and expanding their range of applications.

2.5 Carbon-Coated Metallic Nanosponges The process begins by adding a small amount of activated carbon to the water or gas stream and stirring it around to ensure that it adsorbs against the carbon. After adducts have adsorbed, the activated carbon is removed from the water or gas by a simple adsorption process to develop carbon-coated metallic nanosponges that have a three-dimensional structure and can be easily grown on metal surfaces by using a thin film of activated carbon. The fibres are then chopped into small pieces and heated up on a stove. The chopped fibres are then mixed with water and the resulting mixture is heated up until the metal is vaporized and the fibres are connected to another metal [65]. This leaves the carbon and the metal sintered together. The surface area of a nanosponges tube is higher than that of a traditional fibre because the surface of a nanosponges tube is made up of a higher number of carbonized fibre cells. This

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makes it easier to produce a high-quality nanosponges product. Nanosponges are a great choice for applications where high surface-to-volume ratios are important, such as in dental products and dental implants. The adsorption tendency of pollutants is improved by the use of nanosponges. This is because nanosponges have a much larger surface area which allows the pollutants to adsorb onto the nanosponges to a greater degree. Additionally, the use of nanosponges has a greater ability to strongly adsorbed pollutants. This is because nanosponges have a much larger surface area which allows the pollutants to adsorb onto the nanosponges to a greater degree [30].

2.6 Metal Oxide Based Nanosponges Those nanosponges that primarily use metal oxides (MOs) in the synthesis of nanosponges are characterized as Metal oxide-based nanosponges. These metal oxides form an integral part of the structural components as well as these MOs are the functional additives in the nanosponge formation process. Some common applications of MOs in designing the MO-based nanosponges include Functionalizing and surface modifiers, as cross-linking agents, porosity generators, and in catalytic activity especially as photocatalysts against organic contaminants [62]. When the specific metal oxides are incorporated into the nanosponge matrix during the synthetic procedure, the designing and tailoring of the structure is made possible. Metal oxide-based and derived nanosponges offer unique properties and have applications in various fields, including catalysis, energy storage, sensing, and environmental remediation [48, 49]. These nanostructures are planned and fabricated to meet the specific application. The choice of metal oxide in the structural matrix depends on the desired specific functionalization and application. For Example: (i) TiO2 nanosponges exhibit excellent photocatalytic activity and have been used in applications such as water purification, air filtration, self-cleaning surfaces, and sensors [16, 83]. (ii) CuO-based nanosponges are used in catalytic reactions, gas sensing, and energy storage devices [75]. (iii) Fe2 O3 -based nanosponges exhibit magnetic properties and have applications in areas such as magnetic resonance imaging (MRI), drug delivery, and environmental remediation [73]. These MO-based nanosponges have a porous structure with interconnected voids or pores, similar to other nanosponge materials.

2.7 Silicon-Based Nanosponges The porous nature of silicon-based nanosponges is one of their defining characteristics. Just like other classes of nanosponges, these materials also have a high surface

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area-to-volume ratio, providing a large active surface area for interactions with other external molecules. Some of the salient features of these materials include tunable porosity that ensures their versatile application in various fields. These materials find usage in those applications where high chemical stability is required as these nanosponges have the ability to withstand harsh chemical environments and resist degradation. Even in high-temperature applications, these nanostructures exhibit good potential as these nanosponges can maintain their structural and functional integrity in these conditions. Silicon-based nanosponges, characterized by their ordered porous structure and high activity, have emerged as a promising class of nanosponges. These nanosponges have been found to have uniform pore distribution between the nanocrystals. In a report, Chadwick et al. employed an electrochemical etching technique in a hydrofluoric acid solution to create silicon nanosponges. During the etching process, the impurities present on the surface of silicon particles played a crucial role in the formation of a porous structure. The mechanism involved in pore nucleation and subsequent growth determined the composition and morphology of the resulting porous silicon nanosponges [22]. In the development of electrochemical sensors and biosensors, materials with silica chemistry, specifically silicon-based nanosponges, are utilized as potential electrode modifiers. The fabrication of these nanosponges involves using metallurgical-grade silicon powder, which undergoes an etching process to generate particles with silicon particle sizes ranging from 1 to 4 μm [32].

3 Characterization Techniques of Nanosponges Characterization techniques play a crucial role in understanding the structural, chemical, and physical properties of nanosponges. Here are some common characterization techniques used for nanosponges Fig. 3. Transmission Electron Microscopy (TEM): TEM is a high-resolution imaging technique that uses a beam of electrons to visualize the nanosponges at an atomic scale. It provides information about the size, shape, and morphology of nanosponges [66, 83]. Scanning Electron Microscopy (SEM): SEM is another imaging technique that uses a focused beam of electrons to obtain high-resolution images of the nanosponges. It provides information about the surface morphology and topography of the nanosponges [66, 83]. X-ray Diffraction (XRD): XRD is used to determine the crystalline structure of nanosponges. By directing X-rays onto the sample, the resulting diffraction pattern can be analyzed to identify the crystal phases, crystallinity, and lattice parameters of the nanosponges [11, 67]. Fourier Transform Infrared Spectroscopy (FTIR): FTIR is a technique that analyzes the interaction between infrared radiation and the nanosponge material. It provides information about the functional groups present in the nanosponges, which can help in identifying the chemical composition and structure [11].

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Fig. 3 Characterization techniques of nanosponges

Raman Spectroscopy: Raman spectroscopy involves the scattering of laser light by the nanosponges. It provides information about the vibrational modes and molecular structure of the nanosponge material, allowing for chemical identification and analysis [54]. Dynamic Light Scattering (DLS): DLS is a technique used to measure the size distribution and hydrodynamic properties of nanosponges in a liquid suspension. It provides information about the particle size, size distribution, and stability of the nanosponges [9]. Thermogravimetric Analysis (TGA): TGA is used to determine the thermal stability and decomposition temperature of nanosponges. It measures the weight loss of the sample as a function of temperature, providing information about the thermal behavior and composition [58]. Surface Area and Porosity Analysis: Techniques such as Brunauer–Emmett–Teller (BET) analysis and pore size distribution measurements are used to determine the surface area and porosity of nanosponges. These parameters are essential for understanding the adsorption and release properties of nanosponges [9]. Zeta Potential Measurement: Zeta potential analysis determines the surface charge of nanosponges in a liquid medium. It provides information about the stability and

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colloidal behavior of the nanosponges by assessing their electrostatic interactions [35].

4 Applications of Nanosponges in Environmental Remediation: A Futuristic Approach Nanosponges are the type of nanomaterials that have gained significant attention in the scientific fraternity because of their numerous applications in various fields owing to their unique properties and applications. One of the defining features of nanosponges is their high surface area-to-volume ratio. The presence of numerous interconnected pores within their structure contributes to this property. The porosity allows nanosponges to accommodate and store a wide range of substances like organic molecules, gases, liquids, and metals. The unique properties of nanosponges make them valuable in various fields. Some of the key applications include.

4.1 Wastewater Remediation Water is immensely contaminated by sewage, effluent, and agricultural pollutants. The effluent contains heavy metal ions dyes and organic compounds and agricultural pollutants include chemical fertilizers and pesticides [25, 26]. Heavy metals like lead, chromium, cadmium, mercury, arsenic, nickel, copper, and zinc are considered one of the major classes of pollutant worldwide because of their easy absorption by living organisms that lead to bioaugmentation and bioaccumulation. The industrial effluent, especially from metal plating facilities, battery manufacturing, fertilizer industry, mining activity, pesticides, metallurgical, fossil fuel, and tannery, are rich in heavy metals creates an imbalance in the aquatic ecosystem and increases biological oxygen demand [10]. In recent years, rapid industrialization has immensely contributed to the release of heavy metals, pharmaceutical, biological, and other organic contaminants directly into the water bodies. Due to the prevailing water crisis, adsorbenano adsorbents are widely looked over for the treatment of metal-contaminated waters because of their unique adsorption properties [24, 31]. Cyclodextrin as are capable of including various small organic compounds that have geometry suitable to their tubular cavities. The inclusion relies mostly on hydrophobic interaction between the host cyclodextrin and the guest organic molecules [33]. Indeed, long alkyl chains and aromatic compounds such as substituted benzenes were found to form inclusion complexes with cyclodextrin. The interaction between cyclodextrin and the organic molecule can be used as a basis for the absorption or separation of various organic agents. However, cyclodextrin is soluble in water and certain organic solvents, in addition their inclusion formation constant (K) is only in the region of 10–1000 [72]. Because of solubility in water, cyclodextrin

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cannot be employed directly in the separation of organic pollutants from water. To overcome the solubility issues, cyclodextrin is often immobilized on solid particles as a stationary phase or is linked together using a suitable crosslinker. Such cyclodextrinbased nanosponges have unique physicochemical properties and architectures that include good biocompatibility, non-toxicity, easy surface functionalization, and bioabsorbent features. Therefore these nanostructures can be employed for eliminating a wide variety of pollutants from water by adsorption and absorption [53]. Nanosponges have shown great potential for the removal of contaminants from wastewater. Their unique properties, such as high surface area, porosity, and adsorption capacity, make them effective in heavy metal remediation. • The high porosity of nanosponges allows efficient and effective trapping and binding of contaminants and thus removing them from wastewater. • These nanosponges are regenerable through desorption processes, either through changes in pH, temperature, or chemical treatments. This ability to release and recover the adsorbed heavy metals allows for the regeneration of nanosponges, making them more cost-effective and environmentally friendly than one-time-use materials. • The controlled release of the captured materials allows for the recovery of contaminants facilitating their proper disposal or potential reuse [50, 51].

4.2 Removal of Heavy Metals from Wastewater Heavy metal contamination in water and wastewater is a significant environmental concern due to its detrimental effects on human health and ecosystems [41]. To address this issue, nanosponges have emerged as promising materials for the removal of heavy metal ions from aqueous solutions. Nanosponges are highly porous materials with a large surface area, making them efficient adsorbents for heavy metal removal. The unique structure of nanosponges enables them to effectively trap heavy metal ions through adsorption mechanisms such as ion exchange, electrostatic attraction, and surface complexation [2]. The large surface area-to-volume ratio of nanosponges provides numerous active sites for the interaction and binding of heavy metal ions, leading to high adsorption capacities. Nanosponges can be synthesized using various materials, including polymers, cyclodextrins, or inorganic compounds, depending on the specific application requirements [7]. These materials can be modified or functionalized to enhance their adsorption properties, selectivity, and stability. The use of nanosponges offers several advantages for heavy metal removal. Firstly, their porous structure allows for rapid adsorption kinetics, resulting in the efficient removal of heavy metal ions from water. Secondly, the tunable surface chemistry of nanosponges enables specific targeting of different heavy metal contaminants. Functionalization with specific ligands or functional groups enhances the affinity and selectivity towards certain heavy metal ions. Nanosponges have shown promising capabilities for the efficient removal of organic/inorganic pollutants from water based on absorption/ adsorption and disinfection processes. In general, nanotechnology for wastewater

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treatment has proven to be cost-effective, less energy, and time-consuming compared to the conventional water remediation processes which use bulk materials [39, 60]. The unique properties of nanomaterials and their convergence with current treatment technologies present great opportunities to revolutionize water and wastewater treatment [61]. Polymer Nanosponges constructed from beta-cyclodextrin covalently cross-linked tannic acid we can capture Pb2+ from wastewater with through a condensation reaction. Within a short duration of 3 min, Tannic acid linked nanosponges were able to remove around 81% of Pb2+ ions with an adsorption saturation capacity of 136.8 mg g−1 at an initial concentration of 200 mg/L [77]. Novel nanosponge biopolymers were synthesized by Taka et al. and phosphorylated with carbon nanotube-cyclodextrin and silver-doped Titania. These novel nanosponges biopolymers were able to remove efficiently remove Pb2+ and Co2+ metal ions from synthetic and mine effluent samples. Various factors affected the adsorption of both the ions, Pb2+ or Co2+ , like pH of the solution, adsorbent dosage, initial concentration, contact time, and temperature. Primarily the adsorption took place through Ion exchange-electrostatic interactions and some other multilayer heterogeneous adsorption mechanisms were found to be responsible for adsorption for both Pb2+ and Co2+ . These nanosponge biopolymer composites could effectively remove up to 99.30% of Pb2+ and 95.05% of Co2+ from wastewater samples [20, 71].

4.3 Removal of Organic Contaminants and Dyes from Wastewater The Nanoporous materials/nanosponges can address the need for the removal of a large variety of organic contaminants from water [46]. The advantage of nanosponges is that they effectively remove organic contaminants at a concentration level well below that of activated carbon. Nanosponges remove modal compounds at a concentration as low as 1–50 ppb, where the activated carbon essentially has no capabilities [2]. This means that the Nanosponges are suitable for removing the highconcentration priority, highly toxic contaminants that are the focus of current water treatment and environmental research. Specifically, one potential application for current Nanosponges is that they may be used in the treatment processes for ultrapure water, which is required in semiconductor fabrication/ manufacturing facilities [3]. These Nanosponges exhibit great potential for removing pharmaceutical contaminants from water namely carbendazim, diclofenac, etc. In a recent study, mesoporous nanosponges were designed that were constructed using magnetic nanoparticles with porous β-CD polymer (MN-PCDP). These nanosponges revealed 90% removal efficiency of various organic micropollutants within a short period of 1 min. The literature data also documented numerous advantages of these nanosponges in selective adsorption with an enrichment factor of up to ~ 1000. Additionally, MN-PCDPs were also found applicable in a wider range of sensing devices, including fluorescence, Raman, and infrared spectroscopes [81].

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The removal of dyes from wastewater is a crucial aspect of water treatment due to the environmental and health hazards associated with dye pollution. Industrial activities such as textile dyeing, printing, and manufacturing release large quantities of colored effluents into water bodies, leading to water pollution [4]. To mitigate these issues, various techniques and technologies are employed for the efficient removal of dyes from wastewater [15]. Water pollution from synthetic dyes is a growing environmental concern because many dyes have carcinogenic effects on humans and aquatic life [5]. Adsorption is a widely used technology for the separation and removal of dyes from wastewater. In a report, novel nanosponges were synthesized by crosslinking 1,2,3,4-butane tetracarboxylic acid and β-CD using poly (vinyl alcohol) to eliminate some common organic dyes like paraquat, safranin, and malachite green. The results documented the maximum adsorption of 120.5, 92.6, and 64.9 mg g−1 with their reusability performance of 94.1%, 91.6%, and 94.6% for the dyes under study [52]. In another report, β-CDs were functionalized with magnetic graphene, and the synthesized nanosponge was used in the removal of Rhodamine 6G dye with 90% efficiency even after several cycles [48]. MOF has shown huge progress in the removal and degradation of contaminants due to its multifunctional properties like water stability, large surface area, recyclability, and pore size. MOF’s have drawn attention because of their capability of removal of dyes in wastewater treatment [8].

4.4 Metal Organic Frameworks (MOF)-Based Materials for Capture of Greenhouse Gases (CO2 and CH4 ) Greenhouse gas emissions is increasing for the last 50–100 years. The two major gases such as CO2 and CH4 contribute to global warming. The excessive emissions of CO2 have led to a series of various environmental issues. Emerging technologies are focusing on chemical absorption and are most suitable for post-combustion capture in power plants. However, traditional adsorbents and catalysis still have many defects in CO2 capture and transformation. In recent years, nanomaterials (nanosponges) have been considered promising adsorbents for CO2 capture due to their unique properties such as high adsorption capacity, low cost, and wide availability [44]. Nanomaterials have unique physical and chemical properties by which purification and capturing of gas can be done. They can be functionalities with different groups of compounds such as surfactants thereby increasing affinity for a target molecule. Their nanoscale size makes their surfaces active and additional stability is provided and robustness for multiple uses. The development of MOF-based materials has opened a door leading to a new set of promising adsorbents and catalysts for CO2 capture and conversion. MOFs incorporate the elegance of chemical structures and the relevance of a combination of inorganic and organic components. As compared to traditional adsorbent materials, MOFs show a broad application in the field of carbon dioxide capture and storage due to their ultra-high specific surface area, adjustable pore size, ultra-high porosity (up to 90%), high crystallinity,

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adjustable internal surface performance, and controllable structure. The interaction between MOFs and carbon dioxide adsorbate molecules plays an important role in the carbon dioxide capture process as increasing the strength of interaction augments the material’s carbon dioxide uptake capacity, especially at low loading pressure. Hence the chemical adjustability and compatibility of MOFs can be used to regulate the affinity of the carbon dioxide molecules framework. In a report, a carbon-based MOF was synthesized by Farha et al. [Zn3 (OH)(p-CDC)2.5 (DEF)4 ]n [p-CDC2− = deprotonated form of 1,12-dihydroxydicarbonyl-1,12-dicarba-closo-dodecaborane; DEF = diethylformamide]. The MOF where DMF (dimethylformamide) was used in place of DEF was post-modified by Bae et al. by heating it under vacuum for 24 h at 300 °C to yield a DMF-free version of MOF. The modified MOF was shown as a promising material for the absorption of CO2 from a mixture of CO2 and CH4 [12]. In an another report, Cu2+ functionalized, highly porousmetal–organic framework Cu2 (BBCDC) (BBCDC = 9,9' -([1,1' -biphenyl]-4,4' -diyl)bis(9H-carbazole3,6-dicarboxylate) (DUT-49) were designed having a large surface area and pore volume of 5476 m2 g−1 and 2.91 cm3 g−1 respectively. With a slight modification, the supramolecule demonstrated an excellent uptake of 2.01 g g−1 (298 K, 50 bar) of CO2 and a great storage capacity of 308 mg g− 1 (298 K, 110 bar) for CH4 [69]. Besides the capture of greenhouse gases nowadays these nanostructures are also explored for the uptake of many other gases. In a recent article by Li et al. various Metal–Organic Frameworks including post-modified structures are discussed for storage and separation of different types of gases like CO2 , a greenhouse gas; H2 and CH4 (Fuel gases) and; CO and NH3 (toxic gases) [44].

4.5 Cleaning of Oil Spillage Oil spills pose a significant threat to the environment, ecosystems, and human health. The devastating consequences of these spills necessitate the development of innovative technologies to mitigate their impact. One such groundbreaking solution is the use of nanosponges. These nanoscale materials exhibit unique properties that make them highly effective in tackling oil spillage and facilitating efficient cleanup operations. Nanosponges are porous materials composed of nanoscale particles or structures [27]. They possess an extraordinary surface-to-volume ratio, allowing for increased adsorption capabilities. Their inherent porosity provides a large number of sites for capturing and retaining oil molecules, making them ideal for oil spill remediation. Through various approaches, nanosponges can be deployed to absorb, separate, recover, and even recycle spilled oil, offering a versatile and sustainable solution to this pressing environmental challenge. Nanosponges can play a significant role in mitigating the environmental impact of oil spillage [34]. These innovative materials, typically composed of nanoscale particles or structures, possess unique properties that make them effective in oil spill cleanup. Here’s how nanosponges can be utilized.

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Absorption of oil: Nanosponges have high surface areas with abundant nanopores, allowing them to absorb oil and hydrocarbons efficiently. When deployed in oilcontaminated water, nanosponges can attract and adsorb oil molecules, effectively removing them from the environment. This property makes nanosponges ideal for oil spill cleanup operations. Remediation of contaminated water: Nanosponges can be used in filtration systems to treat water contaminated with oil. Bypassing the water through a filter containing nanosponges, the oil molecules selectively adhere to the nanosponge surfaces while the purified water passes through. This process helps separate the oil from the water, facilitating the cleanup process. Facilitation of oil recovery: Nanosponges can aid in oil recovery efforts by enhancing the efficiency of traditional methods such as skimming. By dispersing nanosponges on the surface of the water, they can attract and capture oil, forming larger oil-nanosponge aggregates that are easier to skim off. This approach increases the effectiveness of oil recovery and minimizes the environmental impact. Controlled release of captured oil: Nanosponges can be designed to release the absorbed oil selectively under specific conditions. This capability allows for the controlled recovery of the captured oil and the potential reuse or proper disposal of the nanosponge material. It provides a versatile approach to managing the recovered oil effectively. Reusability and recyclability: Nanosponges can be engineered to be reusable or recyclable, making them cost-effective and environmentally friendly options for oil spill cleanup. By developing nanosponge materials that can be easily regenerated or processed to extract the captured oil, the same nanosponges can be used multiple times, reducing waste generation.

4.6 Use of Nanobioremediation in Agriculture Nano-bioremediation is the process of employing nanoparticle/nanomaterial generated by plants, fungi, and bacteria to remove environmental toxins (such as organic and inorganic pollutants) from polluted locations [14, 79]. Apart from the many methods available for the remediation of contaminated sites (such as chemical and physical remediation), bioremediation provides an ecologically acceptable and costeffective option for removing toxins from the environment (Fig. 4). Bioremediation encompasses bioaccumulation, biotransformation, biosorption, and biological stabilization, through microbes, plants, and enzymes or a combination of them in the technique [17]. The utilization of microorganisms as catalysts, due to the presence of specific enzymes, represents a novel and coherent biosynthesis approach for the production of various nanoparticles. This approach involves the application of various components such as polysaccharides, biodegradable polymers, microorganisms, enzymes, microbial enzymes, vitamins, and organic structures. Nanobioremediation has provided a tool to merge nanotechnology and biotechnology, in which nano-encapsulated enzymes transform complex organic molecules into simpler ones,

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Fig. 4 Application of nanobioremediation in agriculture. Reused with permission from [38]. Copyright Springer Nature 2022

which are subsequently quickly eliminated by bacteria and plants working together [13, 40]. Nanosponges can serve as carriers for enzymes, bacteria, or other bioremediation agents. These agents can be loaded onto the nanosponge surface or incorporated within their porous structure. The nanosponges protect and deliver the bioremediation agents to the contaminated sites, enhancing their stability and activity. In agriculture, nanosponges help by remediating soil through the removal of organic/inorganic and other emerging pollutants (including metal ions, dyes, organic pollutants, pesticides, etc.), improving soil health, and plant growth leading to improvement of the degraded land [38]. Here are some key applications of nanosponges in bioremediation. Enhanced Bioavailability of Contaminants: Nanosponges can act as carriers or delivery systems for biological agents used in bioremediation. By encapsulating microorganisms, enzymes, or other biocatalysts within their porous structures, nanosponges protect these agents and enhance their stability and bioavailability. This allows for targeted delivery and controlled release of the biological agents, optimizing their performance in degrading or transforming contaminants. Protection against Toxic Substances: Some contaminants present in environmental pollution can be toxic to microorganisms used in bioremediation. Nanosponges can provide a protective barrier around the encapsulated biological agents, shielding them from harmful substances. This protection ensures the survival and activity of the microorganisms in hostile environments, thereby increasing the efficiency of bioremediation processes. Controlled Release of Nutrients: Nanosponges can be designed to release nutrients, such as carbon sources or essential elements, in a controlled manner. These nutrients serve as growth factors for microorganisms involved in biodegradation processes. By gradually releasing the required nutrients, nanosponges support the

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sustained growth and metabolic activity of the microorganisms, optimizing their efficiency in breaking down contaminants [21]. Adsorption of Contaminants: Nanosponges possess high surface areas and abundant nanopores, making them effective adsorbents for a wide range of pollutants. They can capture and concentrate contaminants from the environment, reducing their concentration and potential toxicity. Nanosponges can be functionalized with specific surface chemistries to enhance their selectivity for particular pollutants, allowing for the targeted removal of specific contaminants from contaminated sites [59]. In-situ Remediation Applications: Nanosponges offer the potential for in-situ remediation, meaning they can be directly applied to contaminated sites. They can be dispersed or injected into soil, sediment, or groundwater to facilitate the removal or degradation of pollutants. Nanosponges can create a favorable microenvironment for microbial growth and activity, improving the efficiency and effectiveness of bioremediation processes at the site. Monitoring and Sensing: Nanosponges can be engineered to incorporate sensing elements or indicators, enabling real-time monitoring of the bioremediation process. By integrating nanosponges with specific sensors, changes in environmental parameters, such as contaminant concentration, pH, or microbial activity, can be detected. This monitoring capability allows for better control and optimization of bioremediation strategies.

5 Advantages and Limitations of Nanosponges The various advantages associated with various types of nanosponges have made these nanomaterials a very promising tool for addressing various environmental challenges. Some key features of nanosponges that have made the materials popular among the various groups of researchers include. • Versatile and Easily tunable structures. – Functionality can be easily modified by the appropriate use of crosslinker. – The extent of crosslinking controls the size of nanosponges and hence the applicability. • Have a definite and designed 3-D structure that allows specific capture, transportation, and release of a variety of substances in a controlled manner. • Ability to uptake a wide variety of pollutants viz. heavy metals, organic molecules, dyes, pharmaceutical waste, and other contaminants present in water. • Compatibility with Other Techniques: Nanosponges can be combined with other remediation techniques to enhance their efficiency and effectiveness. They can complement processes such as bioremediation, chemical oxidation, or filtration systems, providing a synergistic effect for pollutant removal and cleanup.

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• Minimized Secondary Pollution: The use of nanosponges can help minimize the generation of secondary pollutants. By efficiently adsorbing and immobilizing contaminants, nanosponges prevent their dispersion or transformation into more harmful forms, reducing the risk of secondary pollution during remediation processes. • Contaminant Stabilization: Nanosponges can immobilize and stabilize contaminants, preventing their release back into the environment. This is particularly important for certain pollutants that may be volatile, mobile, or subject to leaching. By encapsulating and sequestering contaminants, nanosponges contribute to long-term environmental protection. • Ability to mask unpleasant odor and flavours in water. • Magnetic properties may also be introduced in nanosponges by adding magnetic particles during the design and synthesis of nanostructures. • In drug delivery, the controlled functionality can control the drug release and make it more predictable. • May act as ‘self-sterilizers’ as these nanosponges have very small pore sizes and bacteria is not able to penetrate through them. • Thermally stable, Nontoxic, and Biodegradable: The thermal stability of nanosponges is an important aspect to consider when assessing their suitability for various applications. The thermal stability of nanosponges refers to their ability to withstand high temperatures without significant degradation or structural changes. The stability, toxicity, and biodegradability of nanosponges depend on material composition and the conditions of application. While nanosponges offer several advantages in environmental applications, there are also some potential disadvantages and challenges associated with their use. Here are a few considerations: • Environmental Impact: The potential environmental impact of nanosponges is a concern. The release of nanosponges into the environment, either during production, use, or disposal, raises questions about their long-term fate and potential accumulation in ecosystems. It is crucial to assess the toxicity and potential ecological effects of nanosponges to ensure their safe implementation. • Cost and Scalability: The cost of producing nanosponges can be relatively high, particularly if they require specialized fabrication techniques or the use of expensive materials. Additionally, scaling up the production of nanosponges to meet large-scale environmental remediation demands may present challenges in terms of cost-effectiveness and feasibility. • Stability and Durability: The stability and durability of nanosponges are important considerations, especially in harsh environmental conditions. Factors such as pH, temperature, and the presence of other chemicals can potentially impact the structural integrity and performance of nanosponges over time. Ensuring the long-term stability of nanosponges is crucial to maintain their effectiveness in environmental remediation applications.

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• Selectivity and Specificity: Nanosponges may exhibit varying degrees of selectivity and specificity in their adsorption capabilities. While this can be advantageous in some cases, it may also limit their ability to target specific pollutants or contaminants in complex environmental matrices. The challenge lies in designing nanosponges with high selectivity and affinity towards the desired pollutants while minimizing interference from other substances. • Regulation and Standardization: As nanosponges are relatively new materials, the regulatory framework surrounding their use in environmental applications is still developing. Standardization of production methods, characterization techniques, and guidelines for safe use and disposal is necessary to ensure consistent and reliable performance while minimizing potential risks to human health and the environment. • Reusability and Disposal: The reusability and disposal of nanosponges are important considerations for sustainable environmental remediation. Some nanosponges may have limitations in terms of their ability to be easily regenerated or recovered for multiple cycles of use. Additionally, the proper disposal of spent or contaminated nanosponges should be carefully managed to prevent their unintended release into the environment.

6 Conclusion and Future Prospects of Nanosponges Nanosponges offer the advantage of high surface area, controllable porosity, and tunable properties, making them effective in capturing and removing pollutants from various environmental matrices. Nanosponges have shown great potential in various fields, and their future prospects are promising. However, the nanosponges are still at the budding stage and need thorough investigation in the direction of potential risks associated with the release of nanosponges into the environment. Thus a lot more needs to be done in the direction of the development of biodegradable nanosponges to prevent unintended ecological impacts. In the future nanosponges may be used for the removal of radioactive contaminants and microplastics also from soil or water which are the most serious future problems. They may be used for the absorption of radioactive ions, such as uranium or cesium, reducing their concentration and facilitating the safe disposal/recovery/storage [37]. Their unique properties, including high adsorption capacity, filtration capabilities, oil recovery enhancement, controlled release, and potential reusability, make them invaluable tools in mitigating the environmental impact of various pollutants. While further research and development are necessary to optimize their performance and ensure safety, nanosponges offer a powerful solution for addressing one of the most challenging environmental issues of our time. By harnessing their potential, we can move towards a more effective, efficient, and sustainable approach to cleanup and environmental stewardship.

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Abbreviations BET BNC CD CNC CNF CNP CNS DLS DNA DVB FTIR HCPSN MO MOF MRI SEM TEM TEMPO TGA UV XRD ZVI

Brunauer–Emmett–Teller Bacterial Nanocellulose Cyclodextrin Cellulose Nanocrystals Cellulose Nanofibres Cellulose Nanoparticles Cellulose Based Nanosponges Dynamic Light Scattering Deoxyribo Nucleic Acid Divinyl benzene Fourier Transform Infrared Spectroscopy Hyper-Cross Linked Polystyrene Nanosponges Metal Oxide Based Nanosponges Metal Organic Framework Magnetic Resonance Imaging Scanning Electron Microscopy Transmission Electron Microscopy 2,2,6,6-Tetramethyl-1-piperidinyloxy Thermograwmetric Analysis Ultra Violet X-Ray Diffraction Zero Valent Iron

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