Supramolecular Gels: Materials and Emerging Applications [1 ed.] 3527345116, 9783527345113

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Supramolecular Gels

Supramolecular Gels Materials and Emerging Applications

Edited by Tifeng Jiao

Editor Prof. Tifeng Jiao

Yanshan University School of Environmental and Chemical Engineering No. 438 West Hebei Street Qinhuangdao 066004 Hebei Province China Cover Cover Image: © Fotaro1965/Shutterstock

All books published by WILEY-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details, or other items may inadvertently be inaccurate. Library of Congress Card No.:

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The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at . © 2021 WILEY-VCH GmbH, Boschstr. 12, 69469 Weinheim, Germany All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Print ISBN: 978-3-527-34511-3 ePDF ISBN: 978-3-527-81703-0 ePub ISBN: 978-3-527-81701-6 oBook ISBN: 978-3-527-81700-9 Typesetting Straive, Chennai, India Printing and Binding

Printed on acid-free paper 10 9 8 7 6 5 4 3 2 1

v

Contents Preface ix 1

1.1 1.2 1.2.1 1.2.2 1.2.3 1.3 1.3.1 1.3.2 1.4

2

2.1 2.2 2.2.1 2.2.1.1 2.2.1.2 2.2.1.3 2.2.1.4 2.2.2 2.2.2.1 2.2.2.2 2.2.2.3 2.2.2.4

Molecular Gel as Medium or Intermediate in Functional Materials Synthesis 1 Rong Miao and Junxia Peng Introduction 1 Molecular Gel as Intermediate in Synthesizing Fluorescent Sensing Films with High Performance 2 Molecular Design 3 Molecular Gel Strategy-Based Sensing Film for VOC Vapor Detection 4 Molecular Gel Strategy-Based Film for Chemicals Sensing in Liquid Phase 9 Molecular Gel as Intermediate in Synthesizing Porous Materials 9 Porous Materials for Removal of Oil on Water Surface 11 Porous Materials for VOCs Adsorption 13 Summary and Perspectives 14 References 16 Preparation and Sensing Application of Fluorescent Organogels and Hydrogels 21 Xudong Yu, Lijun Geng, and Jiangbo Guo Introduction 21 Types of Gels that Respond to Different Stimuli 22 Fluorescent Gels that Respond to Physical Stimuli 22 Fluorescent Gels that Respond to Heat 22 Fluorescent Gels that Respond to Light 23 Fluorescent Gels that Respond to Ultrasound 25 Fluorescent Xerogels that Respond to Grinding or Pressure 28 Fluorescent Gels for Visual Chemical Stimulus Sensing 30 Fluorescent Gels for Cation Sensing 30 Fluorescent Gels for Anion Sensing 35 Fluorescent Gels for CO2 Sensing 37 Fluorescent Gels for Solvent and Humidity Sensing 38

vi

Contents

2.2.2.5 2.2.2.6 2.3

Fluorescent Gels for Nitroaromatic Derivative Sensing Fluorescent Gels for Amine Sensing 42 Summary and Perspectives 45 References 46

3

Preparation of Self-Assembled Composite Hydrogels and Their Application in Biomedicine and Wastewater Treatment 51 Ran Wang, Jingxin Zhou, Lexin Zhang, and Tifeng Jiao Introduction 51 Prepared Composite Hydrogels Used in Biomedicine 52 Self-Assembly and Drug Release Capacities of Organogels via Some Amide Compounds with Aromatic Substituent Headgroups 52 Prepared Composite Hydrogels Used in Wastewater Treatment 55 Preparation and Self-assembly of Some Functionalized Supramolecular Hydrogels 55 Preparation and Self-assembly of Some Graphene Oxide-Based Composite Hydrogels 57 Conclusion and Perspectives 64 Acknowledgments 65 References 65

3.1 3.2 3.2.1 3.3 3.3.1 3.3.2 3.4

4 4.1 4.2 4.2.1 4.2.2 4.3 4.3.1 4.3.2 4.4

5

5.1 5.2 5.2.1 5.2.2 5.3

42

Conductive Hydrogels for Flexible Mechanical Sensors 71 Zhihui Qin and Tifeng Jiao Introduction 71 Fabrication of Conductive Hydrogels 73 Electronically Conductive Hydrogel 74 Ionically Conductive Hydrogels 78 Conductive Hydrogel-Based Mechanical Sensors 80 Strain Sensors 81 Pressure Sensors 85 Conclusion and Outlook 89 Acknowledgments 90 References 90 Recent Progress on Heat-Set Molecular Gels 99 Yuangang Li, Zonglin Yang, Yong Chen, Huajing Li, Rong Yang, and Chenyu Huang Introduction 99 Heat-Set Molecular Gels 101 Heat-Set Molecular Hydrogel 101 Heat-Set Organic Gel 110 Conclusion and Perspectives 118 Acknowledgements 120 References 120

Contents

6

6.1 6.2 6.2.1 6.2.2 6.2.3 6.2.3.1 6.2.3.2 6.3 6.4 6.4.1 6.4.2 6.5

7

7.1 7.2 7.2.1 7.2.2 7.2.2.1 7.2.2.2 7.2.3 7.3 7.3.1 7.3.2 7.3.3 7.4

8 8.1 8.2 8.3 8.4 8.5 8.5.1 8.5.2

Supramolecular Gels from Carbohydrate Biopolymers for Water Remediation 127 Xuefeng Zhang and Weiqi Leng Introduction 127 Hydrogels from Carbohydrate Biopolymers 128 Hydrogels from Native Cellulose or Chitin 129 Hydrogels from Cellulose or Chitin Derivatives 130 Hydrogels from Biopolymer Nanomaterials 132 Hydrogels from Physically Cross-Linked NC or NCh 134 Hydrogels from Chemically Cross-Linked NC or NCh 137 Aerogels from Carbohydrate Biopolymers 138 Biopolymer-Derived Gels for Water Remediation 140 Heavy Metal Removal 141 Organic Pollutants Removal 146 Conclusions and Perspectives 156 References 156 Biobased Aerogels for Oil Spill Remediation 169 Weiqi Leng, Sheng He, Xuefeng Zhang, Xiang Wang, and Chanaka M. Navarathna Introduction 169 Aerogels: Classification, Fabrication, and Properties 172 Classification of Aerogels 172 Fabrication of Biobased Aerogels 173 Supercritical Drying 177 Freeze-drying 179 Functionalization of Biobased Aerogels 186 Biobased Aerogels for Oil Spill Remediation 193 Parameters That Affect the Oil Absorption Performance 193 Mechanisms of Oil Absorption 197 Post-processing of Aerogel Absorbent After Oil Spill Remediation 200 Conclusion and Future Scope 201 References 201 Luminescent Supramolecular Gels 215 Xue Jin and Pengfei Duan Introduction 215 Fluorescence in Supramolecular Gels 216 Phosphorescence in Supramolecular Gels 219 Upconverted Luminescence in Supramolecular Gels 224 Circularly Polarized Luminescence in Supramolecular Gels 230 CPL-Active Gel Based on Chiral Luminescent Gelators 234 CPL-Active Supramolecular Gel Based on Achiral Luminescent Gelators 238

vii

viii

Contents

8.5.3 8.5.4 8.6

CPL-Active Supramolecular Gel by Using Organic Luminophores as Guests 239 CPL-Active Supramolecular Gel Based on Inorganic Luminescence Guest 245 Conclusion and Perspectives 249 Acknowledgments 250 References 250 Index 257

ix

Preface Gels are soft materials composed of three-dimensional cross-linked networks filled with a second dispersion medium. Virtually any fluid can be used as dispersion medium, including water (hydrogels), an organic solvent (organogels), and even air (aerogels). Gels can be classified in different ways depending on their constitution, the types of cross-linking, and the media they encompass. Supramolecular gels, relying on non-covalent interactions for self-organization into hierarchical structures, are becoming one of the most attractive research subjects in the field of supramolecular chemistry and material sciences. The dynamic and reversible nature of supramolecular gels endows them with unique properties and makes their characterization diversified. Meanwhile, several functionalities such as optical, electrical, magnetic, thermal, and other properties can be also introduced into supramolecular gels by designing network structure and incorporating an active group or a receptor unit. This class of materials not only can be utilized as medium or intermediate templates to synthesize functional materials for possible applications in separation and information storage, but also serve as media for a range of applications, such as biomaterials, industry, electronic materials, and personal care products. As we all know, various efforts have been and are being made to develop novel supramolecular gels and explore their potential applications, and there are plenty of articles and reviews published. However, there are still few reports on the detailed description of supramolecular gel preparation and emerging applications. Thus, this book aims to provide such a review about the preparation and emerging application of different types and functions of supramolecular gels. In the following chapters, several examples and achievements of recent and current researches are given. Rong Miao and Yu Fang review molecular gel as medium or intermediate in functional materials synthesis. Yu et al. adds a perspective on design of supramolecular low-molecular mass organogelators with fluorescence and sensing abilities toward both physical and chemical stimuli. Preparation of self-assembled composite hydrogels and their application in biomedicine and wastewater treatment are discussed by Jiao et al. In addition, Zhihui Qin and Tifeng Jiao highlight recent progress in the field of conductive hydrogels as flexible mechanical sensors. Li et al. provide an overview of both heat-set hydrogels and heat-set organic gels and a brief description of their possible potential applications. Xuefeng Zhang and Weiqi

x

Preface

Leng review the recent advances for the synthesis of hydrogels and aerogels from cellulose and chitin and their application for water remediation. Fabrication and functionalization of biobased aerogels and their performance for oil remediation are summarized by He et al. Finally, Xue Jin and Pengfei Duan highlight and put into perspective a few recent advances in the field of luminescent gels. I would like to greatly thank all these colleagues for providing these deep and illustrative insights into their work and that of their peers. I believe that the content of supramolecular gels presented in this book can provide readers with guidance on fundamental understanding of supramolecular gels and promote the development of supramolecular gels.

1

1 Molecular Gel as Medium or Intermediate in Functional Materials Synthesis Rong Miao and Junxia Peng Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an, P. R. China

1.1 Introduction Since the 1990s, substantive development has been achieved in research related to molecular gels [1, 2]. Thousands of molecular gelators have been reported [2]. Meanwhile, with rapid development in supramolecular chemistry and nanoscience, in-depth understanding has been gained and many new research branches have emerged [3, 4]. By combining research results and theoretical calculations, theories in molecular gel study have been built. Preparation of molecular gel has also been gradually changed from the accidental discovery stage to the aimed designed stage. A typical characteristic of research on molecular gels is the interdisciplinary feature. Generally, research on molecular gels includes knowledge related to soft matter, self-assembly, rheology, thermodynamics, chemical synthesis, theoretical calculation, simulation, etc. Macroscopically, molecular gels are solid-like. Microscopically, molecular gels are made up of non-covalent interaction-based network structures consisting of a small quantity of molecular gelators and abundance of solvents [5]. The solvent molecules are trapped in the network structure via capillarity, surface tension, and other immobilization forces. Thus, molecular gels possess typical microheterogeneous structure. Different from chemical gels and polymer gels, non-covalent bonding-based network endows molecular gels prominent thermal reversibility, shear thixotropy, and stimulus response [6, 7]. These properties lay solid foundation for further application of the molecular gels. However, pronounced application of molecular gel has not been realized. Stability is an important factor that affects the practical applications of molecular gels [8]. This is because, the formation and existence of a molecular gel is based on an equilibrium of dissolve-aggregation, which is based on a variety of non-covalent interactions [7]. The equilibrium may be broken by any tiny external disturbance, causing melting of phase transition in the molecular gel. In order to get stable molecular gels, it is necessary to strengthen the factors that are helpful to maintain the dissolve-aggregation balance. For example, high-boiling point liquids are used to Supramolecular Gels: Materials and Emerging Applications, First Edition. Edited by Tifeng Jiao. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

2

1 Molecular Gel as Medium or Intermediate in Functional Materials Synthesis

prevent gel-sol phase transitions caused by solvent volatilization [9]. Thermal insulation and vibration reduction are realized through packaging, so as to overcome the gel damage caused by vibration and heat. In fact, it is in this way that the intelligent propellant based on molecular gel is prepared. Another way to promote the practical application of molecular gels is to use them as an intermediate to construct special materials with special structures and excellent properties, where the application of molecular gels will be highlighted [10, 11]. Based on this consideration, efforts have been devoted to the study of molecular gels as intermediates in functional materials fabrication (Scheme 1.1), such as in fluorescent sensing film, gel emulsions, and low-density porous materials. With molecular gel strategy, a series of materials with favorable performance have been obtained. In this chapter, the application of molecular gel strategy in the preparation of fluorescence sensing thin film materials and low-density porous materials is described in combination with laboratory practice. On this basis, the main challenges of related research are proposed, and the prospect and development trend of molecular gel expansion research are prospected. Molecular gel strategy Fluorescent sensing film Typically designed molecular gelator

Molecular gel (intermediate)

Gel emulsion and porous materials

Scheme 1.1 Molecular gel strategy in fabrication of fluorescent sensing film and gel emulsion as well as porous materials.

1.2 Molecular Gel as Intermediate in Synthesizing Fluorescent Sensing Films with High Performance Compared with homogenous chemical sensing, film-based chemical sensing owns advantages in little influence on the detection system, favorable reusability, and easy to be integrated into device. [12, 13] Among chemical sensors, fluorescent sensors have received considerable attention, because of their merits in regard to high sensitivity, multiple parameters, little consumption of reagent, as well as no reference, or even remote monitoring [14, 15]. For a film-based sensor device, fluorescent film with desired sensing performance plays a pivotal role. Thus, exploring fluorescent film with desired sensing ability will lay a solid foundation in the development of high-performance sensors. Generally, sensing performance of a fluorescent film can be evaluated by sensitivity, selectivity, response time, reversibility, reusability, and stability. Sensing behavior of a fluorescent film is not only dependent on constitution of the film, but also on microstructure of the film [12, 16]. Accordingly, rationally designed fluorescent sensing unit and suitable film fabrication strategy should be two key issues in fluorescent sensing film fabrication. A desired sensing film would show characteristics in abundance of effective sensing units, which reacts to the analyte to cause a change in fluorescence signal [17].

1.2 Molecular Gel as Intermediate in Synthesizing Fluorescent Sensing Films with High Performance

Gel

Stimuli

Figure 1.1

Sol

Casting

Drying

Gel network-based fluorescent film

Illustration of the molecular gel for fluorescent film fabrication.

As known, solvent is the main component (more than 90%) in a molecular gel and the self-assembled supramolecular gel network accounts for only ∼2%. Thus, the corresponding xerogel obtained from removing the solvent possesses features of porous network structure, which is beneficial for fluorescent sensing because of the permeability at the molecular level. Preferable permeability will lay a foundation for fast and effective interaction between the analyte and the sensing units and, therefore, result in improved sensing performance, such as fast response and high reversibility and sensitivity. In addition, shear thixotropy of some molecular gels would facilitate sensing film preparation, as they can be easily sprayed or spin coated onto the solid substrate to form porous films. Accordingly, molecular gel strategy has been developed to fabricate different types of fluorescent sensing films [17–19]. Details of the strategy are illustrated in Figure 1.1. In the strategy, fluorescent unit is introduced to the molecular design of a gelator and the achieved fluorescent molecular gels are used to construct sensing film with porous network microstructure. With the method, fluorescent films with favorable sensing performance for volatile organic compounds (VOCs), narcotics, and chemical agent have been realized and some of the sensing films have been made into devices for real-life applications.

1.2.1

Molecular Design

To make fluorescent sensing film with molecular gel strategy, structure of the molecular gelator is the basis; molecular design and synthesis play an important role. In general, there are two requirements that a molecular gelator needs to meet when it is to be used to prepare fluorescent sensing film via molecular gel strategy: (i) fluorescent unit that can respond to some typical analyte should be included and (ii) units with supramolecular binding sites should also be included to endow the molecule self-assembly ability (Scheme 1.2). Up to now, various types of fluorophores have been reported [20–22]. Based on their characteristics in molecular structures, different types of fluorophores have varied response to the analytes. For example, most of the perylene bisimide (PBI) derivatives are electron-deficient compounds and their fluorescence is easily quenched by some electron-rich compounds, such as amines and phenols. [23, 24] Fluorescence of some polycyclic aromatic hydrocarbon (PAH) derivatives shows typical difference between their monomer state and aggregated

3

4

1 Molecular Gel as Medium or Intermediate in Functional Materials Synthesis

state [23]. After more than 20 years of development, abundance of gelator units have been reported, including cholesterol and its derivatives, alkanes, amino acids, and aromatic compounds. [1]. Rational combination of a fluorophore and the gelator units will enable the access of favorable fluorescent molecular gelator, which lay solid foundation for fluorescent sensing film fabrication. Fluorescent unit

Scheme 1.2

Linker

Self-assembly unit

Structure design for gelator molecules used for fluorescent film fabrication.

In Sections 1.2.2 and 1.2.3, fluorescent gelators with different molecular structures will be introduced. Based on systematic study of the self-assembly feature, gelation behavior as well as rheological properties of the gelators, fluorescent films will be obtained via techniques of drop casting or spray coating. When gelating solvent is removed, porous sensing films can be achieved. Meanwhile, porosity and microstructure of the film can be adjusted by varying the concentration of the gelator, gelating solvent, or the drying process. On that basis, the photophysical behavior and sensing performance of the films will evaluated. Furthermore, films with outstanding sensing performance will be selected to make sensor device. The molecular gel strategy will not only broaden the application of molecular gel, but also provide convenience for sensor development.

1.2.2 Molecular Gel Strategy-Based Sensing Film for VOC Vapor Detection VOCs include a variety of chemicals, which are emitted as gases from certain solids or liquids [25, 26]. Most VOCs are toxic and some of them have short- or long-term adverse health effects, including irritation of eyes, nose, and throat, and damage to the liver, kidneys, and central nervous system. Sensors for VOCs detection would be of great help in reducing exposure risk. Amines, especially organic amines, are recognized as important environmental pollutants and they pose a direct threat to humans [27, 28]. Meanwhile, active ingredients of some illegal chemicals are derivatives of organic amines [12]. Many PBI derivatives have shown excellent sensing performance, especially for organic amines, due to their electron-deficient nature. In addition, energies of highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) can be adjusted by varying the substituents (both the substitution type and position) [29–32]. In this way, charge transfer process between some PBI derivatives and typical amines can be tuned and sensing performance of the system can be optimized. Nevertheless, strong 𝜋–𝜋 interaction between the PBI molecules results in poor solubility of the PBI derivatives and may bring difficulties for their use through molecular gel strategy. For this reason, cholesterol groups have been involved to improve the solubility of the PBI molecules and also endow them supramolecular assembly ability.

1.2 Molecular Gel as Intermediate in Synthesizing Fluorescent Sensing Films with High Performance O O

O n

N H

O N

O

O O

N

O

n

O

H N

O

Compound 1: n=2 Compound 2: n=11

O O

O O

N N H O S O

O O

N

N

O

O

Compound 3

O S O H N N

O

O

O O

N O NH

O

HN N O

Scheme 1.3

O

Compound 4

Molecular structures of compound 1, 2, 3, and 4.

A series of molecular systems (compounds 1∼4) for organic amine sensing has been achieved by the combination of the two units (Scheme 1.3) [29–32]. The four compounds show varied assembly behavior in different solvents and it was found that sensing performance of the system was highly dependent on the assembled structure of the molecules. The molecular gel-based film was obviously superior to the film obtained by simple casting of the compounds. Sensing films obtained from molecular gel strategy show features of porous network structure: film 1 is made up of nanofibers of ∼80 nm [29]; film 2 is characterized by stacking of nanoparticles of ∼150 nm (Figure 1.2a) [30]; film 3 contains abundance of nanofibers of ∼100 nm (Figure 1.2b) [31]; morphology of film 4 is similar to that of film 3 except for the smaller size of nanofibers [32]. The four films exhibited different sensing

2 μm (a)

5 μm (b)

Figure 1.2 Self-assembled structures of compound 2(a) and 3(b). (a and b) SEM images of film 2 and 3 through the assembly of compound 2 and 3, respectively. Source: Adapted with permission from He et al. [31] and Shang et al. [32]. © 2016 Elsevier.

5

1. MPEA 2. Xylidine 3. o-toluidine 4. 1,4-diaminobenzene 5. Aniline 6. Propylamine 7. Methyiamine 8. Hydrazine hydrate

0.6

9. Triethylamine 10. Diethylenetriamine 11. Triethylenetetramine

0.5

12. Ammonium hydroxide 13. Ethanediamine 14. Diethylamine

0.4

15. Toluene 16. Chloroform 17. Acetone 18. Dichloromethane 19. n-hexane 20. Cyclohexane 21. Methanol

0.3

22. Apple pomace 23. Ethanol 24. THF 25. Phenol

0.2

26. Banana juice 27. Benzene 28. Water 29. Acetonitrile.

0.1 0.0

1 2 3 4 5 6 7 8 9 10 111213141516 1718192021 2223242526272829

–0.1

Analytes

Fluorescence intensity (a.u.)

(a)

8 6 4 2

10 8

Blank After 50 s

6

MPEA vapor

91.6%

2 0 650 700 750 Wavelength (nm)

800

550

1.2

0

10

20 30 Time (s)

40

50

600 650 700 Wavelength (nm)

750

800

Film treated with cold air for five minutes

1.0 0.8 0.6 0.4 0.2

Film treated with MPEA saturated vapor

0

0 (c)

0 6.6 ppb 13.2 ppb 19.8 ppb 26.4 ppb 33.0 ppb 39.6 ppb 46.2 ppb 52.8 ppb 59.4 ppb 99 ppb 330 ppb

500

(b)

4

600

8 7 6 5 4 3 2 1 0

Fluorescence Intensity (a.u.)

Quenching efficiency 1-l/l0

0.7

Fluorescence intensity (a.u.)

1 Molecular Gel as Medium or Intermediate in Functional Materials Synthesis

Fluorescence intensity (a.u.)

6

2

4 6 8 Cycle number

10

12

(d)

Figure 1.3 Sensing performance of some typical fluorescent films based on molecular gel strategy. (a) Fluorescence response of film 2 to a series of volatile compounds; (b) fluorescence response of film 3 to aniline; (c) fluorescence response of film 3 to N-methyl-phenethylamine (MEPA); (c) reversible sensing response of film 2 to MEPA vapor; inset shows the fluorescence images of the film before (up) and after (down) exposure to MEPA vapor; (d) reversible fluorescence response of film 2 to MEPA vapor. Source: He et al. [31] /Elsevier.

performance and showed favorable sensing performance (sensitivity, response time, and reversibility) in organic amine and its analogues, such as methylamphetamine detection (Figure 1.3). Fluorescence of film can be quenched by most commonly used amines tested with quenching efficiencies in the range of 60∼80%. Sensing performance of film 2 is similar to that of film 1, but the quenching efficiencies are different (40∼85%). Film 3 showed remarkable fluorescence response (∼90% quenching efficiency) to methylamphetamine, an analogue of narcotics. Detection limit of film 3 to methylamphetamine is 5.5 ppb, which offers great opportunity for narcotics detection. Detection limit of film 4 to aniline vapor is 15 ppb and a film 4-based sensor device can be used to detect simulated exhaled (with aniline vapor) sample for lung cancer patients. The different sensing behaviors of the three films should be attributed to both varied molecular structure and self-assembled microstructure of the films. Similar to PBI, naphthalimide (NID) is also an electron-deficient fluorophore. A long-chain alkane-modified NID derivative (compound 5, Scheme 1.4) has shown strong gelation ability, and several solvents (toluene, hexane, etc.) can be gelled using the NID gelator [33]. Xerogels from toluene and hexane have different morphologies

1.2 Molecular Gel as Intermediate in Synthesizing Fluorescent Sensing Films with High Performance

and the corresponding films show different sensing performance. The film made up of uniform nanofibers was proved to be sufficient in aniline sensing with response less than 1 s and a detection limit of 12.7 mg m−3 ppm.

O N

O

O

H N

O

O O

N H

O N O

Scheme 1.4

O O O O

Compound 5

Molecular structure of compound 5.

7-Nitrobenzofuranzan is a widely used bioimaging and bioanalysis agent, whose fluorescence is sensitive to environmental change. Using phenylalanine as the linker, a derivative of 7-nitrobenzofuranzan was connected with cholesterol and compound 6 was prepared (Scheme 1.5) [34]. Owing to the chirality of the phenylalanine (D/L) group, the isomers of compound 6 showed different gelation behaviors (Table 1.1). Systematic gelling behavior study revealed that the compound was an excellent molecular gelator and a fluorescent DMSO gel was got. The gel was sensitive to chemical stimulus: introduction of ammonia could break down the gel and further removal of the ammonia by bubbling of N2 or air could result in the recovery of the gel. Meanwhile, the reversible sol-gel transition is accompanied by obvious fluorescence change: the DMSO gel is highly fluorescent, but the fluorescence is quenched when ammonia is introduced; the fluorescence can be recovered when the ammonia is removed. Therefore, sensing film for ammonia can be easily obtained by spray coating or high-speed coating. Microstructure as well sensing performance of the film can be optimized by changing the concentration of the compound. Under low gelator concentration, the film is characterized with loosely and randomly packed nanofibers, while the fibers become dense and uniform when concentration of the gelator is increased. With the sensing film as the key part, fluorescent sensor device for monitoring ammonia leakage was realized.

*O NH

O2N N

O

O

N H2C

Scheme 1.5 Structure of the typically designed molecular gelator, compound 6. Source: Based on Yu et al. [34].

7

8

1 Molecular Gel as Medium or Intermediate in Functional Materials Synthesis

Table 1.1

Gelation behavior of compound 6 (2.5 wt%).

Solvent

Methanol

C6-L

C6-D

Solvent

I

S

n-Hexane

C6-L

C6-D

I

I

Ethanol

I

G

n-Heptane

I

I

n-Propanol

I

G

n-Octane

I

I

n-Butanol

I

G

n-Nonane

I

I

n-Pentanol

I

G

n-Decane

I

I

n-Hexanol

I

P

Cyclohexane

I

I

n-Heptanol

I

P

Acetonitrile

I

G

n-Octanol

I

P

Water

I

I

n-Nonanol

I

I

DMF

S

P

n-Decanol

I

I

DMSO

I

G

Acetone

S

P

Acetic acid

I

I

Benzene

S

S

Ethyl ether

I

I

Toluene

S

S

Ethyl acetate

S

S

THF

S

S

Dichloromethane

S

S

Pyridine

S

S

Trichloromethane

S

S

Note: I represents insoluble; S represents solution; P represents precipitate; G represents gel.

Hydrogen chloride is an extremely harmful corrosive gas, which easily causes irritation to eyes, skin, and respiratory system. Moreover, the existence of hydrogen chloride in near-earth space will lead to abnormal atmospheric processes or even affect the global environment. Thus, the detection of hydrogen chloride in gas state is of great importance. By the combination of 1,4-bis(substituted phenylacetylenyl) benzene, glucose, and cholesterol, a fluorescent molecule with typical assembly ability is constructed (compound 7, Scheme 1.6) [35]. The molecules exist in the aggregated form in chloroform, and fluorescent films consisting of numerous spherical particles can be obtained by transferring the aggregates onto a solid surface. Hydrophilicity as well morphologies of the film could be tuned by controlling the drying process. Quick drying produces hydrophobic film, but slow drying in humid air leads to hydrophilic film. The hydrophobic film is made up of particles of size 1∼3 μm; the hydrophilic film is characterized by vehicles of size ∼10 μm. Difference in film hydrophilicity and morphology causes remarkable difference in sensing performance (to hydrogen chloride vapor), which should be attributed to the interaction between the analyte and the film. Fluorescence of the hydrophilic film is sensitive to the presence of hydrogen chloride vapor with a detection limit of ∼0.4 ppb. But fluorescence of the hydrophobic film showed little change upon hydrogen chloride vapor exposure: high concentration of hydrogen chloride vapor (88 ppm or even 2860 ppm) could only cause 25% of fluorescence quenching.

1.3 Molecular Gel as Intermediate in Synthesizing Porous Materials

O

HO O O HO

OH OH

NH

H H

O

O H

8

H

8O

O

HN

HO HO

Scheme 1.6

OH O O OH

Molecular structure of compound 7. Source: Based on Sun et al. [35].

1.2.3 Molecular Gel Strategy-Based Film for Chemicals Sensing in Liquid Phase Water pollution is one of the most challenging problems faced by humans. Methods that can be used for reliable, sensitive, and user-friendly detection of toxic chemicals in water are urgently demanded. With the advantages of molecular gel strategy in fluorescent sensing film fabrication, a series of fluorescent molecules with favorite self-assembly behavior have been synthesized [36–38], and corresponding fluorescent sensing film with typical self-assembled structure are prepared for the detection of toxic chemicals in water system. Some of the films can be used to detect toxic substances, such as nerve poison, pesticide, formaldehyde, in water. The detection is sensitive, selective, and the process can be realized either by spectroscopy, or by visualization.

1.3 Molecular Gel as Intermediate in Synthesizing Porous Materials Because of their advantages at low density, high porosity, large surface area, and tunable pore size, porous materials are playing increasingly important roles in many fields, such as separation, catalysis, molecular recognition, bioengineering, and environmental science [39, 40]. A key point in porous materials preparation is to control the pores, including pore size, pore shape, and pore distribution. Commonly used methods for porous materials preparation includes gas foaming,

9

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1 Molecular Gel as Medium or Intermediate in Functional Materials Synthesis

Water

Oil and gelator

Polymerization drying

Gel emulsion

Porous materials

Figure 1.4 Illustration of a typical process for the preparation of molecular gelator-stabilized gel emulsions and the corresponding porous materials. Source: Based on Chen et al. [42].

porogen adding, or templating [41]. Among these methods, templating is preferred owing to its advantage in regard to facile control of pore size and distribution. Gel emulsion, also known as highly concentrated emulsion or high-internal phase ratio emulsion, is a typical template in porous materials preparation. Similar to a gel, gel emulsion is solid like, apparently. But in the microphase, a gel emulsion is totally different from a gel. There is more than one phase in a gel emulsion, usually called continuous phase and dispersed phase. Different from traditional emulsions, gel emulsions have several advantages when used as reaction intermediate. Coexistence of hydrophobic and hydrophilic domains enables good solubility of the gel emulsion system in both nonpolar and polar reactants. High viscosity of the system helps to avoid the precipitation of the product and provide basis for the formation of network structure. Water accounts for a large portion in W/O (water in oil) gel emulsion, which is benefit for the reduction of organic liquid for a consideration of both environmental protection and cost saving. The size and shape of the produced materials can be easily controlled by using a mold. Internal structure of the porous materials can be easily tuned by variation of the constitution of the gel emulsions (Figure 1.4) [42]. When used as reaction intermediate, gel emulsions can serve as both template and solvent, and they are widely used in the fabrication of low-density porous materials. Formation of a gel emulsion is based on the stabilizer, which reduces the interfacial energy between the oil phase and the water phase [43]. Up to now, the widely used stabilizers include surfactants, micro- or nanoparticles (organic and inorganic), and molecular gelators. Usually, the stabilizer accounts for less than 3% (w/v) in a gel emulsion stabilized by molecular gelator, which is much lower than when surfactant is used as stabilizer. Compared with gel emulsion stabilized by micro- or nanoparticles, gel emulsions stabilized by molecular gelators possess better stability. In addition, undesired changes such as phase inversion and phase separation in particle-stabilized gel emulsions can be avoided in the molecular gelator-stabilized system. Moreover, content of the dispersed phase is not limited to a maximum of 74% in molecular gel-stabilized gel emulsion. Meanwhile, the ratio of the two phases (oil phase and water phase) in molecular gel-stabilized gel emulsion can be tuned in a wide range, which provides the basis for adjusting the internal structure of the porous materials. Low content of stabilizer is beneficial for cost saving and also for further purification of the obtained porous materials.

1.3 Molecular Gel as Intermediate in Synthesizing Porous Materials

Based on the advantages of molecular gelator in gel emulsion stabilization, plenty of low-density porous materials have been synthesized. Details of the porous materials as well as their preliminary applications in adsorption, absorption, and environment treatment will be introduced in Sections 1.3.1 and 1.3.2.

1.3.1

Porous Materials for Removal of Oil on Water Surface

With rapid development in chemical engineering, there is increasing demand for petroleum extraction, refining, and applications. As a consequence, leakage of petroleum or the related products has become an emerging problem, which poses huge threats to ecology and human health [44, 45]. Sorption is an efficient way to reduce the risk of petroleum leakage, where sorbents with high performance are desired. It is of great significance to explore sorbents with high sorption capacity. Porous materials are widely used as sorbents. Sorption ability of porous materials is highly dependent on the constitution, internal structure, hydrophilicity, mechanical strength, as well as stability of the materials. Meanwhile, cost and the complexity of the fabrication process are important issues that need to be considered when the materials are put into application. With typically designed molecular gelator as stabilizer, a series of gel emulsions have been prepared. Polymerizable monomers, such as styrene and acrylic acid, are used as the continuous phase and water serves as the dispersed phase. Then, the gel emulsions undergo a polymerization and a subsequent drying process and thus porous materials can be achieved. In 2009, Professor Fang’s group reported the first molecular gelator-based gel emulsion stabilizer (compound 8, Scheme 1.7) [46]. The gelator contains two cholesterol groups and has a strong gelling ability. With the gelator, different types of oils can be gelated, including n-heptane, n-octane, n-nonane, n-decane, kerosene, diesel, and gasoline. Based on the gels, a series of gel emulsions (W/O type) were made by introducing water into the gelling system. Microscopic studies and rheological measurement suggested that the gel emulsions are stable and have typical foaming structure. Gelators based on the combination of cholesterol and ferrocene have also been proven to be capable in the stabilization of gel emulsion. Some cholesterol-ferrocene gelated gels are responsive to multiple stimuli.

O O

H N CH3

4

O

O

O

H N

O CH3

Compound 8

Scheme 1.7

Molecular structure of compound 8.

Molecular structures of some typical molecular gelators that can serve as stabilizers in gel emulsions formation are shown in Scheme 1.8. As an example, a series of gel emulsions of W/O type can be obtained when either of the gelators is used as a

11

12

1 Molecular Gel as Medium or Intermediate in Functional Materials Synthesis

stabilizer and tert-butyl methacrylate is used as the continuous phase [47]. Polymeric porous materials with varied internal structure as well as mechanical properties can be achieved by polymerization of the different gel emulsions (Figure 1.5) [48]. The porous materials show high performance in absorption of organic liquids. Meanwhile, the absorbed solvents can be easily recovered by a squeezing process. In this way, the porous materials can be recycled and the absorption process can be repeated for more than 10 times. Except for optimization on the water content in the gel emulsion, properties as well absorption behavior of the porous material can also be enhanced by the introduction of some typical additives. For example, introducing some reactive silyl reagents is helpful to improve the mechanical strength and absorption capacity of the porous materials.

O O

H N

H N

N O

O O

O

Compound 9

NH Fe

O

O OH O O Compound 10

N OH

HO

H N

O

O O CH2 Compound 11

Scheme 1.8 Molecular structure of the gelators (compound 9–11) stabilizer used for gel emulsion and porous materials preparation.

Compound 10, which is a typical cholesterol gelator with two hydroxyl groups in the structure, can also serve as a stabilizer for W/O gel emulsions [49]. Using styrene as the continuous phase and water as the dispersed phase, a series of gel emulsions with outstanding stability can be prepared. After polymerization and drying, corresponding porous polystyrene materials were synthesized. Density of the materials can be adjusted by the variation of water content in the gel emulsions and polystyrene materials with ultralow density was presented. The preparation can be processed under mild conditions, where no complex or high energy cost drying process is needed. No high-cost post-treatment is needed as the water in the polymerized materials can be removed by squeezing and simple drying at 50 ∘ C. The porous polystyrene materials can be used to remove some organic liquids from water. The materials can be recycled by subsequent squeeze, washing with ethanol, and drying steps (Figure 1.6) [50]. Another representative molecular gelator stabilizer contains a phenylalanine group as a linker, and a carboxyl group in a cholesterol derivative (compound 11) [51]. With the stabilizer, gel emulsion with tunable rheological properties can be

1.3 Molecular Gel as Intermediate in Synthesizing Porous Materials

(a)

(b) 105 10

(c)

4

102 101 100

50 500 1000 2000 3000 4000

0.18

tgδ =G″/G′

G′ (Pa)

103

0.16

200 μm

0.14

50 500 1000 2000 3000 4000

0.12 0.10

10–1

0.08 0

10–2 10–1

1000 2000 3000 Water content (μl)

100

101

4000

102

Stress (Pa)

(d)

103 200 μm

Figure 1.5 Typical gel emulsions and porous materials prepared by using molecular gelator as stabilizer. (a) Photos of the styrene gel. Photos of gel emulsion with different amounts of stabilizer. (b) Rheological behaviors of gel emulsions with different water contents. (c and d) SEM images of porous materials prepared from gel emulsions with different amounts of n-heptane as additives.

prepared. In addition, low-density porous materials with favorable mechanical strength and flexibility are obtained. The synthesized materials with a density of 0.18 g m−3 have good compressibility; the materials can be fully recovered with 70% compression and there is little change after compression (at 70% compression) for 25 times. The favorable mechanical strength lays a solid foundation for further applications of the materials.

1.3.2

Porous Materials for VOCs Adsorption

Efficient method for polluted air treatment is a great challenge, especially the removal of VOCs. Organic chemicals are not only widely used in chemical production, but also used as ingredients in household products, such as paints, detergents, disinfectors, and cosmetics. Removal of VOCs from polluted air has attracted tremendous attention around the world [52, 53]. Usually, concentrations of some VOCs are much higher indoors than outdoors, which may cause shortand long-term health effects. Among the methods for VOCs removal, adsorption is preferred because of its advantages in terms of low cost and high efficiency.

13

1 Molecular Gel as Medium or Intermediate in Functional Materials Synthesis

35 Adsorption capacity (g/g)

14

Toluene Phenol Dichloromethane Trichloromethane Nitrobenzene Methanol n-Heptane Triethylamine

30 25 20 15 10 5 0

(a)

Solvents

1. Wash out

Adsorption 23 mm 24 mm

24 mm

2. Dry up Step 1

(b)

Figure 1.6 Typical application of gel emulsion-templated porous materials in organic solvent removal from water. (a) Maximum oil adsorption capacity of the porous materials. (b) shows the reusability of the materials. Source: Liu et al. [48]/Wiley.

To avoid the effect of unreacted stabilizer (molecular gelator) to the polymerized porous materials, several cholesterol derivatives with polymerizable bonds have been designed and synthesized (Scheme 1.9) [54, 55]. Using the gelators as stabilizers, a series of W/O gel emulsions (continuous phase: styrene; dispersed phase: water) have been prepared. Then, the gel emulsions were forced to undergo polymerization and drying. Accordingly, porous materials with different constitution and varied internal structures can be synthesized. Owing to the hydrophobic nature of the porous materials, they have the ability to adsorb VOCs (including benzene, toluene, ethyl benzene, and ethylene) and show higher adsorption capacity compared to the commonly used adsorbents, active carbon.

1.4 Summary and Perspectives Physical gels, especially molecular gels made from low-molecular weight gelators and solvents are a combination of synthetic technology and supramolecular self-assembly. Different from chemical gels, gel networks of molecular gels rely on supramolecular interactions (including hydrogen bonding, 𝜋–𝜋 stacking, hydrophobic interactions, electrostatic interactions, dipole interactions) between gelator molecules, solvent molecules, and gelator-solvent molecules. The remarkable difference in structures makes the two type of gels (chemical gels and physical

1.4 Summary and Perspectives

Compound 12 H N

O O

O 4 N H

Compound 13 H N

O

O O

Scheme 1.9

O

O N H

Structures of stabilizers with polymerizable bonds.

gels) show different response to external stimulus. For example, networks in both types of the gels can be broken under vigorous shearing, such as stirring, pumping, or shaking, which may result in phase separation or phase transition of the gel. It is hardly for the chemical gels to be recovered after the disruption, as the chemically bonded network is reversibly broken. But many molecular gels can be recovered when the shearing is removed. The preferable stimulus-responsive characteristics of molecular gels endow them great potential in applications of template synthesis, controlled release, tissue culturing, separation, information storage, sensing, etc. Moreover, the concentration of a gelator is usually very low in a molecular gel, which is a big advantage for further applications in molecular recognition, template synthesis, and synthetic intermediate. Introducing the molecular gel strategy to synthesize high-performance fluorescent sensing film and low-density porous materials shows some preliminary application of molecular gel in synthetic intermediate. It is expected that molecular gels will play more important roles in the research area of synthetic medium, which may include innovation in liquid–liquid extraction, cultivation of high-quality crystals, stabilization of solid–liquid suspension system extraction technology. In addition, there are problems to be solved when molecular gel strategy is used in the synthesis of fluorescent sensing films and low-density porous materials. For example, adhesion of some fluorescent films obtained from molecular gel needs to be improved. Effect of molecular structure of the gelator stabilizer on the properties of the gel emulsions as well as the low-density porous materials needs to be given more attention. This will pave way for achieving porous materials with excellent mechanical strength and tunable internal microstructure.

15

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1 Molecular Gel as Medium or Intermediate in Functional Materials Synthesis

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33 Fan, J., Chang, X., He, M. et al. (2016). Functionality-oriented derivatization of naphthalene diimide: a molecular gel strategy-based fluorescent film for aniline vapor detection. ACS Appl. Mater. Interfaces 8: 18584–18592. 34 Yu, H., Lü, Y., Chen, X. et al. (2014). Functionality-oriented molecular gels: synthesis and properties of nitrobenzoxadiazole (NBD)-containing low-molecular mass gelators. Soft Matter 10: 9159–9166. 35 Sun, X., Qi, Y., Liu, H. et al. (2014). “Yin and Yang” tuned fluorescence sensing behavior of branched 1,4-bis(phenylethynyl)benzene. ACS Appl. Mater. Interfaces 6: 20016–20024. 36 Zhang, S., Yang, H., Ma, Y., and Fang, Y. (2016). A fluorescent bis-NBD derivative of calix[4]arene: Switchable response to Ag+ and HCHO in solution phase. Sens. Actuators, B: Chem. 227: 271–276. 37 Lü, Y., Sun, Q., Hu, B. et al. (2016). Synthesis and sensing applications of a new fluorescent derivative of cholesterol. New J. Chem. 40: 1817–1824. 38 Qi, Y., Sun, X., Chang, X. et al. (2016). A new type of 1, 4-bis(phenylethynyl)benzene derivatives: optical behavior and sensing applications. Acta Phys. -Chim. Sin. 32: 373–379. 39 Slater, A.G. and Cooper, A.I. (2015). Function-led design of new porous materials. Science 348: 8075. 40 Oh, J.Y., Rondeau-Gagné, S., Chiu, Y. et al. (2015). Intrinsically stretchable and healable semiconducting polymer for organic transistors. Nature 539: 411–415. 41 Georgakilas, V., Tiwari, J.N., Kemp, K.C. et al. (2016). Noncovalent functionalization of graphene and graphene oxide for energy materials, biosensing, catalytic, and biomedical applications. Chem. Rev. 116: 5464–5519. 42 Chen, X., Liu, K., He, P. et al. (2012). Preparation of novel W/O gel-emulsions and their application in the preparation of low-density materials. Langmuir 28: 9275–9281. 43 Ikem, V.O., Menner, A., Horozov, T.S., and Bismarck, A. (2010). Highly permeable macroporous polymers synthesized from pickering medium and high internal phase emulsion templates. Adv. Mater. 22: 3588–3592. 44 Swannell, R.P., Lee, K., and Donagh, M.M. (1996). Field evaluations of marine oil spill bioremediation. Microbiol. Rev. 60: 342–365. 45 Dowd, R.M. (1984). Leaking underground storage tanks. Environ. Sci. Technol. 18: 309A. 46 Peng, J., Xia, H., Liu, K. et al. (2009). Water-in-oil gel emulsions from a cholesterol derivative: structure and unusual properties. J. Colloid Interface Sci. 336: 780–785. 47 Chen, X., Liu, L., Liu, K. et al. (2014). Facile preparation of porous polymeric composite monoliths with superior performances in oil-water separation a low-molecular mass gelators-based gel emulsion approach. J. Mater. Chem. A 2: 10081–10089. 48 Liu, J., Wang, P., He, Y. et al. (2019). Polymerizable nonconventional gel emulsions and their utilization in the template preparation of low-density, high-strength polymeric monoliths and 3D printing. Macromolecules 52: 2456–2463.

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2 Preparation and Sensing Application of Fluorescent Organogels and Hydrogels Xudong Yu, Lijun Geng and Jiangbo Guo College of Science and Hebei Research Center of Pharmaceutical and Chemical Engineering, Hebei University of Science and Technology, Shijiazhuang, PR China

2.1 Introduction In the past decades, the synthesis and design of low-molecular mass organogelators (LMOGs) has received considerable attention owing to their potential applications in the field of drug control and release, cell culture and imaging, stimuli sensing, as well as intelligent materials [1–4]. These gelators have the ability to self-assemble to three-dimensional (3D) matrix networks driven by non-covalent interaction such as hydrogen bonding, 𝜋–𝜋 stacking, electrostatic interaction and van der Waals forces [5–8]. Especially, 𝜋–𝜋 stacking is an important non-covalent interaction during gel formation process. Many kinds of fluorophores as 𝜋 units have been utilized to construct fluorescent gelators. The most used fluorophores of gelators in literatures include naphthalimide, BODIPY (boron-dipyrromethene or 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene), OPV (oligo(p-phenylenevinylene), and perylene conjugates. The fluorescent gelators can spontaneously self-assemble in water, ionic liquid or organic solvents via cooperative non-covalent interactions to form ordered microstructures, which further inhibit liquid flowing, leading to fluorescent gels. Minor changes in the 𝜋 units result in distinct assembly properties and functions. Interestingly, these gels have stimuli-responsive properties toward physical or chemical stimuli, together with fluorescent changes besides other changes including phase, morphology, color, and rheology changes, which endows the direct and visual sensing of gel platform toward environment stimuli [9–11]. To data, 𝜋-based gelators have been found the most versatile applications compared with that of hydrogen bonding gels and other gels. In this chapter, low-molecular mass organogelators (LMOGs) containing 𝜋 units with fluorescence properties will be discussed to demonstrate the importance of fluorescent unit for the self-assembly of gels and its applications with tunable optic properties toward external stimuli sensing. The following sections include three parts according to the kinds of external stimuli: physical stimulus-responsive gels, chemical stimulus-responsive gels, and summary and perspective. Supramolecular Gels: Materials and Emerging Applications, First Edition. Edited by Tifeng Jiao. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

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2.2 Types of Gels that Respond to Different Stimuli Due to the dedicate balance of non-covalent interactions, the supramolecular gels are very sensitive to environment factors, leading to visual response toward external stimuli. According to the kinds of different stimuli, the responsive gels are classified into physical stimuli-responsive gels and chemical stimuli-responsive gels. Herein, we mainly present a brief discussion and overview about fluorescent and stimuli-responsive gels are mainly discussed as the following.

2.2.1

Fluorescent Gels that Respond to Physical Stimuli

2.2.1.1 Fluorescent Gels that Respond to Heat

Heating–cooling process is the most widely used approach for construction of gels in the previous stage. In most of the cases, by heating the mixture of gelator and solvent, the gelator could dissolve in the special solvent with disordered monomers or weak non-covalent interactions. When the mixture was cooled, a stable gel formed due to the self-assembly of gelators via cooperative and strong non-covalent interactions into 3D networks that trapped solvent molecules. In most of cases, these sol-gel or suspension-gel process treated by heating–cooling process is reversible. Therefore, reversible heat-setting is an ideal method to construct phase and fluorescent switches. For example, Kamikawa and Kato demonstrate the monomer and excimer emissions of pyrenes in the pyrene-containing oligo(glutamic acid)s 1 or 2 with different emission colors, which could be tuned by sol and gel state via heating–cooling approach (Figures 2.1 and 2.2). The study presents that the dissociation and reformation of intermolecular hydrogen bonding interaction plays an important role for such changes [12]. Notably, aggregation-induced emission (AIE) or emission enhancement (AIEE) is an efficient way to construct fluorescent gels with strong emission, which is well-established in the field of soft matter, especially gels [13]. In gel networks, the movement and torsion of molecules are inhibited, leading to less energy dissipation and fluorescence enhancement. Tang and coworkers report a series of OC11H23 OC11H23

O OC11H23 O O

OC11H23

O

O

O

O

OC11H23 O

O O

N

OC11H23 OC11 H23

H

N

O

H

O

OC11H23 OC11H23

1

N H

O H

N O

OC11 H23

O O

OC11H23 OC11H23

2

Figure 2.1

Chemical structures of 1 and 2.

Figure 2.2 (a) Fluorescence spectra of compounds 1 and 2 in cyclohexane. (𝜆ex = 345 nm. [1]) (4.0 × 10−2 M; [2]) 2.0 × 10−2 M. Thin blue line, 1 at 20 ∘ C (gel); thin green line, 1 at 60 ∘ C (sol); thick violet line, 2 at 20 ∘ C (gel); and thick green line, 2 at 60 ∘ C (sol). (b) Photograph of the fluorescent gel of 1 and schematic illustration of the self-assembly in the gel state. (c) Photograph of the sol of 1 and schematic illustration of excimer formation in the sol state. Source: Kamikawa and Kato [12]. Reproduced with permission from © 2007 American Chemical Society. DOI: doi.org/10.1021/ la061581u.

PL Intensity (a.u)

2.2 Types of Gels that Respond to Different Stimuli

400

450

(a)

(b)

500

550

600

650

Wavelength (nm)

(c)

tetraphenylethene-based gelators (3–5, Figure 2.3); their organogels show much stronger emission than that of the corresponding sol at elevated temperature with isolated species, displaying gelation-enhanced emission properties [14, 15]. The organic aggregates of both gels and xerogels with AIE characteristic have potential applications in the field of OLEDs, sensors, cell imaging, optical devices, and smart materials. However, the related literatures about AIE gels were still few. 2.2.1.2 Fluorescent Gels that Respond to Light

Light is regarded as an efficient, clean, green energy that could tune gel assembly without inducing any chemical stimulus into the system. In the past decades, there

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2 Preparation and Sensing Application of Fluorescent Organogels and Hydrogels

N N N

N N N

R

R

O R=

() O 9

O

3

N N

O N (CH2)m C O

O O C (CH2)m N

N

N

4, m = 5 5, m = 10

Figure 2.3

Chemical structure of 3, 4, and 5.

have been tremendous works in the study of light-responsive gels. However, most of the fluorescent gels that respond to light are polymeric gels, and the development of such gels with both fluorescence and light-responsive properties constructed by low-molecular weight gelators (LMWGs) is still in its infancy. The typical works on light-responsive low-molecular weight gelators involve diarylethene-based gelators, which are very sensitive to light with controllable wavelengths and can be developed to prepare light-controlled switches in restricted networks. For example, de Jong et al. reported the chiral switches controlled by light by incorporation of diarylethene photo-responsive unit [16]. Tian’s group combined cholesterol unit as gelator group, naphthalimide motif as fluorescent unit, and diarylethene group as light-responsive unit into one gelator 6 (Figure 2.4), and the gel in mixed solvent of toluene and ethanol (1: 3 v/v) with yellow color and ring-closed form changes to red upon the irradiation at around 365 nm, which and the color could be recovered by triggered by visible light (𝜆 > 510 nm) [17]. They also found that the number of fluorescence bands at about 460 nm increased with the ring-closed form after irradiation by 365 nm. By introducing the Schiff base segment into naphthalimide-based gelator 7 (NSS), the gel of 7 reported by us in benzene displays significant color change from red to black when treated with sunlight or UV light, together with obvious fluorescent quenching (Figure 2.5) [18]. The isomerization of NSS from E form to Z form is proved by 1 H NMR spectra. In addition, the gels dominated by Z forms and E forms respectively exhibit different responsive properties and binding mechanism with F− anions. In this work, we show that the anion sensing properties of NSS

2.2 Types of Gels that Respond to Different Stimuli H O N

O

O N CH2

O

S

S

O

O N CH2

O

UV

Vis

O

N H

O

O

O HN

O

CH2 N

S

S

O

CH2 N

N H

O

O

O

6

Figure 2.4

The structure and photochromic process of 6 (BTE-NA-(chol)2 ).

Gel

Gel

E form

Z form O

H N N

light N R

7

Solution

CH3OH

N R

O F–

(FHF)n– n N N–

F–

CH3OH

O N R O

Gel

O

H N N

O

O N N H F-

N R O

Gel

Figure 2.5 Illustration of the solution and organogels of NSS that respond to light and fluoride anions with different conformations, colors, and emission colors. Source: Yu et al. [18]. © 2017 Elsevier.

could be controlled by light irradiation via E/Z photoisomerization pathway for the first time. The fluorescent changes controlled by light also allow for the controllable energy transfer in gel system, leading to multicolor emission system. For example, by combining efficient AIE and electron-rich gelator 8, electron-deficient compound 9, and photo-responsive spiropyran derivative 10 into a gel system, Chen et al. showed that the hybrid gel exhibits multiple emission colors tuned by light (Figures 2.6 and 2.7) [19]. 2.2.1.3 Fluorescent Gels that Respond to Ultrasound

In the past decades, ultrasound, a kind of clean energy such as light, has been shown to be an efficient tool for gelation of small molecules in solvents. As a kind

25

26

2 Preparation and Sensing Application of Fluorescent Organogels and Hydrogels

MeOOC COOMe MeOOC

O O

MeOOC

O O

O

O

MeOOC O

O MeOOC O

COOMe MeOOC

8

C12H25 C12H25 C12H25

O

O

N

N

O

O

C12H25 C12H25 C12H25

9

Figure 2.6

The chemical structure of 6 and 7. Source: Chen et al. [19].

O 2N O 2N

UV O

N

O

OR

O

Vis

N

O

OR

10

Figure 2.7

The chemical structure of 10. Source: Chen et al. [19].

of high-frequency mechanical wave, ultrasound is believed to be favorable for the intermolecular interaction of gelator such as 𝜋–𝜋 stacking, hydrogen bonding interaction with high-energy barrier [20]. On the basis of ultrasound-induced gelation, rheology and fluorescent switches controlled by ultrasound and heating–cooling stimuli have been designed and prepared.

2.2 Types of Gels that Respond to Different Stimuli

N O Pd O N n(H2C)

N

O N Pt N O

O Pt N O

n(H2C)

(CH2)n

(CH2)n

(CH2)n n(H2C)

N O Pd O N

N O Pt O N

anti-1 (11)

R-anti-2 (12)

O

N O Pt N

S-anti-2 (13)

Figure 2.8 The chemical structures of Pd- and Pt-based complexes. Source: Based on Naota and Koori [21].

Naota et al. demonstrated the first paradigm of ultrasound-triggered gelation by using dinuclear Pd complex (Figure 2.8) [21]. The sol of 11 (n = 5) in organic solvents such as 1,4-dioxane, acetone, ethyl acetate, and could be transformed to opaque gel instantly accelerated by ultrasound, which might be rationally ascribed to the transformation of intramolecular 𝜋–𝜋 stacking into intermolecular 𝜋–𝜋 stacking at the molecular level. By heating the gel, it transforms to a sol again, resulting in both phase and rheology switches. When palladium ion is replaced by platinum ion in molecule core (12, 13), ultrasound-triggered emission enhancement phenomena of 13 is observed in the phase transition from sol to gel [22]. When treated with ultrasound waves (40 kHz, 0.45 W cm−2 , 10 s), R-(±)-anti-12a (n = 5) in organic liquids could form stable gels instantly, while S-(+)-anti-13a (n = 5, 100% ee) do not gel in any organic liquid. The process is accompanied by obvious emission changes from non-emissive solution to phosphorescent gel with yellow color irradiated by UV light. Additionally, the emission intensity values could be also precisely tuned by sonication time and linkers. The experiments suggest that the high-ordered aggregation that resulted from the homo- and heterochiral aggregations plays a vital role in emission enhancement. Yi’s group reported the design and sonication-responsive properties of organometallic terpyridyl platinum gelators containing cholesterol group (Figure 2.9) [23]. The sonicated gels also show luminescence enhancement in

N CF SO – 3 3 + N Pt N

O N O

H O N H

H

H

O

14

Figure 2.9

The chemical structure of gelator 14. Source: Modified from Liu et al. [23].

27

2 Preparation and Sensing Application of Fluorescent Organogels and Hydrogels

Cationic surface

Vesicle

Precipitate

(a) nm

H-C

3.1

28

Ultras ou

nd

Self-assemble

6.2 nm Self-assemble

(b) Hydrophobic periphery tBu3tpy

Helical ribbon

4-ynyl-1,8-naphthalimide

Gel Cholesterol

Figure 2.10 The schematic mechanism of the gelation via a self-assembly process in sonicated gel 1a. (a) Heating–cooling process and (b) sonication. Source: Liu et al. [23]. © 2013 Royal Society of Chemistry.

comparison with that of corresponding solutions or suspensions. Several experiments suggest that sonication is able to enhance the hydrophobic and ionic dipolar interaction of platinum gelators (Figure 2.10), leading to gelation process with fluorescence enhancement. 2.2.1.4 Fluorescent Xerogels that Respond to Grinding or Pressure

Fluorescent molecules are found to assemble into 3D matrix with highly ordered aggregation of molecules, and the assembly pattern of molecular assembly in xerogels can be retained by evaporation of the solvents. Therefore, the xerogels without solvent molecules are able to express stimuli-responsive properties toward external stimuli such as grind or pressure. One of their outstanding properties is called “mechanochromic luminescence.” For ordered assembly, phase changes from crystalline-like phase to amorphous state are observed when triggered with pressure such as shearing and grinding stimuli. In these processes, aggregates in molecular level such as stacking modes and intermolecular interactions are destroyed to some extent, leading to fluorescent changes in emission intensity or emission colors. In the year 2012, T. Baumgartner’s group observed the mechanically responsive FRET phenomena in xerogels derived from 𝜋-conjugated fluorophore donor–acceptor systems, and the process could be thermally annealed [24]. Xue et al. synthesized two cyanostyrylanthracene-based gelators 15 (PC2AN) and 16 (PC3AN) exhibiting aggregation-induced emission phenomena in gelation process (Figure 2.11). The resulting xerogel of 15 by evaporation showed mechanofluorochromism (MFC) with emission color changes from blue-green to yellow upon grinding. Another study indicated that methylene group has significant effects on gelation ability and MFC properties of gelators [25]. In further studies, they also demonstrated that

2.2 Types of Gels that Respond to Different Stimuli

Figure 2.11 The chemical structures of gelator 15 and 16.

O C12H25

N H (S)

H N

H N

( )

n

O

O CN

15 n = 2 16 n = 3

the aggregations of salicylaldimine difluoroboron complexes possessed reversible piezofluorochromic properties [26, 27]. With intermolecular electron donor and acceptor units into one molecule, we also presented that the assembly of naphthalimide-based organogelators exhibited the grinding chromism. The 1,8-naphthalimide derivatives 17–22 exhibit color and emission color changes when the sols or suspensions are transformed to a stable gel irradiated by ultrasound (Figure 2.12) [28]. These gels showed switchable control of luminescence tuned by ultrasound and heating. When sonication-triggered S-gels are evaporated to S-xerogels, the resulting xerogels (17–22) also display mechanochromism, the color of which changes from red to yellow and the emission color of which changes from orange to green with enhanced intensity by grinding. This mechanochromic property could be reversed through a regelation process. The mechanochromic character of the S-xerogel of 17 is further applied to semiquantitatively sense the mechanical pressure ranging from 2 to 40 MPa through fluorescence changes, representing a new type of application for gelation assembly. The results revealed that ultrasound enhances the J-type aggregations of naphthalimide fluorophores, which further tended to be damaged when the O R

O

O

H N

N H

O R O

N

O N H

O

O O

N H

O

O

H N

N O

18

17 R

R

O

O

H N

N H

N

N O

O

O

O

20

19 O R

O

N H

H N

O

H N

N H

O

O R

N N

O

N H

O

H N

N N

O

O

21

22 R= H

H H

Figure 2.12

The chemical structures of gelator 17–22. Source: Yu et al. [28].

29

2 Preparation and Sensing Application of Fluorescent Organogels and Hydrogels

H

H

H N

O O

H

O N H

N O O N

Figure 2.13

H N

O

23

H

O N H

H

O

H

The chemical structure of gelator 23. Source: Yu et al. [18].

Grind

(a)

0.9

0.9 Intensity

Intensity

30

0.6 0.3 0.0

0.3 0.0

500 (b)

0.6

550

600

650

700

Wavelength/nm

500 (c)

550

600

650

700

Wavelength/nm

Figure 2.14 (a) The color and emission color changes of the 23 (NDS) xerogel obtained from CHCl3 (25 mg ml−1 ) in light and in dark (irradiated by 365 nm); the fluorescence spectral changes of the xerogel obtained from CH2 Cl2 (b and c) CHCl3 (𝜆ex = 450 nm) by grinding. Source: Yu et al. [18]. Reproduced with permission from © 2017 Royal Society of Chemistry. DOI: https://doi.org/10.1039/C7TC01331K.

assembly was subjected to pressure and heat stimuli. Further study demonstrates that the xerogel of gelator 23 from CH2 Cl2 , CHCl3 or gel emulsions with tunable honeycomb structure also show similar grind chromism (Figures 2.13 and 2.14) [29]. More importantly, the gels display different emission color in CH2 Cl2 and CHCl3 , which might have potential for solvent discrimination with similar polarity and structure.

2.2.2

Fluorescent Gels for Visual Chemical Stimulus Sensing

2.2.2.1 Fluorescent Gels for Cation Sensing

In the past decades, coordination interaction-driven metallogels from low-molecular weight gelators (LMWGs), especially coordination polymer gels (CPGs), have

2.2 Types of Gels that Respond to Different Stimuli

received great attention. These gels have the merits of diversity, controllability, and responsiveness with redox, optical, electronic, and magnetic properties [30]. The coordination geometry, radius, valence, as well as coordination number by the metal ions or metal-based materials all have a huge impact on the gelation properties. Herein, we mainly focus on the fluorescent gels that have the ability in selectively sensing toward metal ions. Selective recognition of cations or anions has been of great interest in the past decades due to their important roles in environment and biological processes. By introducing coordination units or anions into gelators, ion-responsive gels could be obtained, together with macroscopic changes, especially fluorescent changes for visual sensing purposes. Additionally, in comparison to that of the solutions, the competitive interactions such as hydrogen bonding and coordination interactions always play a vital role in the gelation and responsive properties. Lin and Sun et al. reported that the phenyl alkyl ether-based gelator 24 (Figure 2.15) could form strong fluorescent gels with AIE properties when bonded with Ca2+ . Introduction of Cu2+ in the system resulted in Cu2+ /24 complexes and fluorescent quenching of the corresponding gel. Furthermore, the fluorescence could be recovered upon the addition of selective CN− anions through the formation of CuCN. The process endowed fluorescence on recognition for CN− [31]. Such a strategy is also applied to construct fluorescent switches via alternative addition H Hf

OC16H33

He

H Ha g

Hb N Hc

OC16H33 OC18633

O

OC16H33

O

Self-assemble

N

OC16H33

H

N N

OC16H33 H

OC16H33

O

OC16H33

H

Hh

N N

Hd 24

OC16H33

H

OC16H33

H

N N

OC16H33

O

H

H

2+

Cd

H

OC16H33

O H

2+

N N 2+

OC16H33

O

Cd

H

2+

I

–

Cd

OC16H33

O

2+ –

OC16H33

I

O

OC16H33 OC16H33

H

OC16H33

O

OC16H33

H N N

OC16H33

N N Cd

OC16H33

OC16H33

H

N N

OC16H33

Cd

Cd24

– 2 Cd + I

OC16H33

N N

OC16H33

OC16H33

H

H

N N

OC16H33

O

OC16H33 OC16H33 OC16H33

– 2 Cd + I

–

Cd24+I

Figure 2.15 Chemical structure of the 24 and the presumed self-assembly and reversible stimuli-response mechanism. Source: Lin et al. [32].

31

32

2 Preparation and Sensing Application of Fluorescent Organogels and Hydrogels

DMSO

OH N HN

O (b) 3+

Recognize Al O

NH N

HO

UV-light (c)

(a)

Figure 2.16 Gelator L (a), and the formation representation of L-gel (b) and Al@gel (c, under UV at 365 nm). Source: Ma et al. [34]. Reproduced with permission from © 2017 Royal Society of Chemistry. DOI: https://doi.org/10.1039/C7SM02141K.

of I− and Cd2+ into the Cd2+ 24 gels (Figure 2.15) [32]. Based on multi-competitive binding interactions, they also successively prepared multi-analyte sensor array toward both ions and anions in water just by using one synthesized receptor [33]. The systematic works supply a novel approach for visual ion sensing and designing stimuli-responsive gel arrays. X. Ma’s group synthesized a novel symmetric Schiff base compound L that can gel in dimethyl sulfoxide (DMSO) with weak fluorescence based on photoinduced electron transfer (PET) characteristic (Figure 2.16) [34]. In the presence of Al3+ , the fluorescence of Al3+ -L gel enhances greatly (19-fold in comparison with that of the pure gel), together with more densely organized structure. By employing a terpyridine-based organogelator 25 (Figure 2.17), we demonstrate that the sonication-triggered gel is able to selectively and visually sense Ca2+ ion through emission color changes from blue to yellow in gels, while, similar ion Mg2+ could

H H

H

H N

H N

O O

O

O O

H

H

HN

H NH

N

N

N

25

Figure 2.17

Chemical structure of gelator 25.

2.2 Types of Gels that Respond to Different Stimuli

Ca2+

OH

LiOH, MeOH

N H N

N

OH O

N H

O H H OH N

H

HO

M pla ole na cul riz ar ati on

HO

N

N

Warm

Co nfo rm cha atio nge nal

O HO H

N H O HO H

Flourescent gel

LiOH, MeOH

Non-flourescent sol

OH

Figure 2.18 Illustration of ultrasound-accelerated gelation of 25 for visual sensing of Ca2+ . Source: Geng et al. [35]. Reproduced with permission from RSC. DOI: doi.org/10.1039/C5SM01851J.

Cool

Isomer 1

Isomer 2

Figure 2.19 Structures of two isomers of L-tartaric acid-based gelators and their cartoon representation of conformational changes before and after Li+ chelation. Photographs of the gel in an inverted vial under naked eye and UV light, isomer 2 forms non-fluorescent solution under similar conditions (right side). Photograph showing the gel lifting up and down upon heating at 75 ∘ C and cooling in a reversible manner (bottom). Source: Dubey et al. [36]. Reproduced with permission from © 2014 Royal Society of Chemistry. DOI: doi.org/10.1039/C4CC02591A.

not penetrate into the gel network of molecular assembly of 25 (Figure 2.18) [35]. It is presented that the competition between 25 self-assembly and the interaction of 25 with ions played the key role in the highly selective sensing of 25 toward Ca2+ . Besides the contact mode between gels and ions during the ion sensing process, in some cases, the ions could also serve as AIE-active reagents with ion-induced gelation approaches. For example, D. S. Pandey observed that the L-tartaric acid-based gelator (isomer 1) formed fluorescent gels in methanol with the aid of LiOH, which is assigned to the chelation-enhanced fluorescence (CHEF) and AIEE effects-based acid–base interaction (Figure 2.19) [36]. In this case, both the bases of LiOH and position of –OH in benzene group play key roles in the gelation. Moreover, S. Y.

33

34

2 Preparation and Sensing Application of Fluorescent Organogels and Hydrogels CF3 N NC CF3

26

AgClO4 F3C

NC

TBAF

ClO4–

CF3

N Ag+ N F3C

NC CF3

Figure 2.20 Schematic representation of the silver ion-coordinated complex between AIEE-active ligand 26 (CN-TFMBPPE) and silver perchlorate.

Park presented Ag+ -induced emissive gels starting from nonfluorescent solution of 𝛼-cyanostilbene derivative containing pyridyl group for coordination interaction and such a process could be also reversed by the addition of tetrabutylammonium fluoride (TBAF) (Figures 2.20) [37]. Additionally, chemical reaction is also utilized to prepare selective sensors toward ions in gels. For example, the naphthalimide-based gelator 27 containing ethylamine-thiourea segment has been designed and synthesized (Figure 2.21) [38]. By using the ion (Ag+ or Hg2+ )-catalyzed ring-closed reaction, the gelator could selectively recognize Ag+ and Hg2+ in solutions among tested metal ions. On the contrary, Ag+ and Hg2+ could not be discriminated in solution. The sonogel of 27 in n-propanol displays different signal responses toward Ag+ and Hg2+ . In the presence of Hg2+ (5 equiv.), the fluorescence intensity of sonogel quenches by a factor of 8.2, which displays signal magnification effect compared with what is seen in the solution; while, Ag+ triggers the gel-to-precipitate transformation of 26 gel, together with fluorescent quenching by a factor of 4.1. In nonpolar solvent such as toluene, Ag+ is unable to penetrate into the gel networks and Hg2+ triggers the S N H

O

H N

N H

O

N

H N

N H

O

H

N H

N H

H N

H

O

27 S

H

O

O O Hg

2+

+

or Ag

N R N

N R

N

HN O

O

Figure 2.21 The chemical structure of gelator 27 and intramolecular guanylation of 27 promoted by Hg2+ or Ag+ . Source: Wang et al. [38].

2.2 Types of Gels that Respond to Different Stimuli

fluorescent quenching of the gel. Therefore, the gelator 27 could discriminate Hg2+ from Ag+ in both polar and nonpolar solvents with opposite signal outputs. It is also presented that the competition between self-assembling gelator molecules and gelator with ions is responsible for the responsive difference. 2.2.2.2 Fluorescent Gels for Anion Sensing

Over the past decades, the field of selective anion sensing has led to intense interest due to their essential roles in a vast range of chemical, biological, medical, and environmental processes [39–42]. Generally, the supramolecular interaction between host and anions includes hydrogen bonding, anion–𝜋 coordination, and anion-activated interactions. To date, most of the anion-responsive gels are based on the competitive hydrogen bonding and coordination interactions and there has been tremendous amount of work on anion-responsive gels. Herein, the gels that respond to anions by fluorescence signal outputs are highlighted. In the year 2007, Yi’s group demonstrated bisurea-functionalized organogels of 28a–28c with heat-controlled fluorescence, which could directly sense F− with reversible gel-to-sol changes (Figure 2.22) [43]. Maeda et al. found that BF2 complexes of aryl-substituted dipyrrolyldiketones 29b–29d form transparent emissive gel in hydrocarbon solvents (Figure 2.23) [44]. The addition of TBACl (10 equiv.) to the 29d gel in octane leads to the disruption of Figure 2.22 28a–28c.

The chemical structure of gelator

OR

OR

RO

OR

RO

NH O

HN

NH HN

28a: R=C6H13 28b: R=C12H25 28c: R=C16H33

F

F B

O

R3O

OR3

O H N

HN

OR3

R3O R3O

Figure 2.23

OR3 29a (R3 = CH3) 29b (R3 = C8H17) 29c (R3 = C12H25) 29d (R3 = C16H33)

The chemical structure of 29a–29d. Source: Maeda et al. [44].

OR O

35

36

2 Preparation and Sensing Application of Fluorescent Organogels and Hydrogels

F

F B

R1O

O H N

O H N

R2O

OR1 OR2

R1O

OR1

30a(R1=R2=C10*) 30b(R1=C10*, R1=C16H33) 30c(R1=C10H33, R2=C10*) C10*=

Figure 2.24

The chemical structure of 30a and 30b. Source: Maeda et al. [45].

the gel, which is accompanied with emission color changes from red to orange. It has also been shown that the collapsing time could be tuned ranging from hours to days by addition of different anions. The results suggested that the conformations of the complexes changed upon the addition of anions, where 29d bonded with anions with both hydrogen bonding and C–H anion interactions. In a consequent study, chiral complexes 30b–30c were synthesized and characterized. They also exhibit gel-to-sol transition in the presence of anions (Figure 2.24) [45]. Due to strong electrostatic interaction of ion pairs between cations and anions, a set of complex gel systems was also developed to selectively and reversibly sense both ions and anions in metallogel systems. For example, T. B. Wei displayed a supramolecular polymeric gel that assembled by host–guest interaction between N-(pyridinium-4-yl)-naphthalimide functionalized-pillar[5]arene host (P5BD) and a 1,4-bis-bromohexane functionalized-pillar[5]arene guest (DPHB) (Figure 2.25). The gel could selectively sense and remove Hg2+ with fluorescence enhancement through cation–𝜋 interaction. In addition, I− is further used to bind Hg2+ in a contact mode, causing fluorescent quenching of the gel. Such process endowed the selective and visual sensing of the gel toward I− . [46]. When hydrogen bonding interaction between host and anions is strong enough, deprotonation might happen during the sensing process. For example, Xing et al. developed a kind of uracil-based 𝜋 organogelator, which could form stable gels in organic solvents and the gel shows reversible, sensitive, and sensing selective abilities toward F− , together with the gel disruption and emission color changes (Figure 2.26) [47]. Several experiments suggest that the strong hydrogen bonding interaction between F− and uracil unit results in the deprotonation process in the recognition events. In most of the cases that gels reported by coworkers are developed for anion sensing, anions are utilized to destroy the assembly of gelators. However, recent studies show that they could also serve as building blocks during the gelation process. K. Ghosh reported that the cholesterol-based derivatives 31 and 32 could gel in DMSO and water mixture (v : v = 1 : 1) in the presence of F− , while other

2.2 Types of Gels that Respond to Different Stimuli

O c

d O O

O

O O

O

–

Br N +

O

N a

O

1 3

O

O

O O

O

O

O O

O

2

Br b

O

O

e

O Host (P5BD)

O

Br

Guest (DPHB)

Self-assemble

(a)

Cation-π interaction Selectively detect and Efficiently remove

Cation-π interaction

(b)

Figure 2.25 (a) The structure of P5BD and DPHB. (b) The proposed assembly and stimuli-response mechanisms of P5BD–DPHB–G with Hg2+ and I− . Source: Lin et al. [46]. Reproduced with permission from © 2017 Royal Society of Chemistry. DOI: doi.org/10.1039/ C7SM01447C. Figure 2.26 The chemical structure of 31. Source: Modified from Xing et al. [47].

O C12H25N C12H25O

NH O

31

anions among the tested anions are nonresponsive in bringing sol-gel transformation. Such gelation approach allows selective and visual sensing toward F− (Figure 2.27) [48]. 2.2.2.3 Fluorescent Gels for CO2 Sensing

In the past decades, the coordination of CO2 with amines has been extensively used to construct stimuli-responsive polymers or gels because CO2 is inexpensive, green, and abundant, and the bonding of CO2 with amines is usually reversible. Therefore, it is developed to be an ideal trigger for constructing stimuli-responsive materials.

37

38

2 Preparation and Sensing Application of Fluorescent Organogels and Hydrogels

Figure 2.27 The chemical structures of 32 and 33. Source: Panja et al. [48]. N + N

N + N

X– O

O

O

O

R

R

33, X–=PF6–

32, X–=PF6–

O

N

O

O O F

N

N2

+

34

O

CO2+H2O

–

F H N

O

N

O

NH O

X–

O

HCO3– N H O O

O

Figure 2.28 The chemical structure of gelator 34 and the responsive properties of 34 toward F− and CO2 .

Conversely, fluorescent gels are also shown to be good candidates for reversible CO2 sensors [49]. In view of fluorescent gel systems, Yoon reported that the resulting NAP-chol 34/F− hybrid sols with orange-red emission color could transform into orange-yellow opaque gels when CO2 is bubbled into the system assisted by gentle heating (Figure 2.28). Such changes are ascribed to the hydrogen competition interaction between NAP-chol with F− and NAP-chol with CO2 . The gel could be also reversed from the sol by bubbling N2 (Figure 2.29) [50]. In a follow-up study, they employed new aroylhydrazone derivatives to prepare anion-activated CO2 chemosensors through similar strategy. The quantitative sensing toward CO2 can be achieved via the correlation between flowing volume and the enhanced fluorescence intensity [51]. 2.2.2.4 Fluorescent Gels for Solvent and Humidity Sensing

Selective solvent sensing from their mixtures is an intriguing field in the last decade due to the important role of organic solvents in environment, chemistry, and industry. Different from the time-consuming techniques and expensive equipment such as ion chromatography, mass spectrometry, and gas chromatography, recent studies show that gelation is a good platform for selective solvent discrimination [52–54]. For example, by combining terpyridyl group as intramolecular electron donor and naphthalimide segment as electron acceptor into one gelator, we observe that the compound 35 and 36 could selectively recognize DMF among tested gel arrays with color and emission difference through room temperature gelation

2.2 Types of Gels that Respond to Different Stimuli

F–

(b) (c) Heating

(d)

Cooling CO2

O

N

N2, Δ

O

NH O

O

(e) (a)

NAP-chol 1

Figure 2.29 Multiple responses of NAP-chol 34 (4 mg ml−1 ) in DMSO under the stimulations of thermal, fluoride anions, and CO2 . (a) Sol, (b) gel, (c) partial or (d) complete collapse of the gel induced by fluoride anions, (e) upon the reaction with CO2 in the presence of fluoride anions (20 equiv.). Source: Zhang et al. [50]. Reproduced with permission from © 2015 Springer Nature. DOI: doi.org/10.1038/srep04593.

approach (Figure 2.30) [55]. It is indicated that such difference is because of the specific J aggregate formation of 35 or 36 in DMSO, which is not observed in other test organic solvents. Further study shows that the aggregates could be also utilized to sense water in DMSO through gradual changes from red to yellow. In a further study, we present that the gelator 37 containing pyridyl and naphthalimide pairs can discriminate short cycloalkanes from alkanes through different aggregation modes (H-type aggregation for cycloalkanes and J-type aggregation for alkanes, Figure 2.31) [56]. The discrimination is visually achieved by different emission colors and mechanical properties. Amazingly, the gelator 37 could selectively gel in single-phase solvent mixtures with high separate efficiency (>92%). This is the first example in which the xerogel assembly can also be developed to selectively and efficiently separate similar organic solvent species just in single-phase organic solvent mixtures. Similar to that of organic solvent sensors, the design of reliable sensors for monitoring water, especially relative humidity (RH), has been of great importance in technology and daily life, for example, in agriculture, medicine, and biotechnology

39

40

2 Preparation and Sensing Application of Fluorescent Organogels and Hydrogels

Acceptor

Donor

O

N ..

Br N

..

H N

N H

O

N

.. N

O 35

O Br

O

O

N .. N H

36

..

H N

N

N

.. N

(a)

DMSO acetonepropanol butanol

THF

2-Methoxy ethanol

Benzene Ethanol DMF

(b)

Figure 2.30 (a) Chemical structures of 35 and 36; (b) photos of typical gel arrays of 35 in different organic solvents triggered by sonication and their corresponding suspensions or solutions by heating–cooling process. Source: Feng et al. [55]. Reproduced with permission from Elsevier. DOI: doi.org/10.1016/j.snb.2016.12.089.

[57–60]. Based on the partial host–guest interactions between 𝛼-cyclodextrin (𝛼-CD) and adamantane units triggered by water molecules, we successfully designed a novel kind of fluorescent gel that could respond to water, especially humidity with visualized gel-to-suspension transition [61]. In n-propanol, 𝛼-CD has been inserted into aggregations of 38 or 39 through intermolecular hydrogen bonding interactions assisted by ultrasound, leading to the weakening of intermolecular interactions between homomolecules of 38 or 39 and formation of thixotropic organogels (Figure 2.32). In the presence of water, partial inclusion between 𝛼-CD and adamantane unit of 38 happens and the hydrogen bonding junctions disappear, resulting in gel collapsing. Additionally, the gel is very sensitive to humidity when exposed to air with different RH range of 40–70%, which allows for semiquantitative and visual humidity sensing (Figure 2.33).

2.2 Types of Gels that Respond to Different Stimuli

H H

H

H H N

O

O

N

O

H

H

O

NH

37

O

N

O

3.5 nm

4.2 nm

N

Figure 2.31 The chemical structure and assembly mode of 37 (NPS) and the gels of NPS (3 wt%) in cyclohexane and hexane. Source: Wang et al. [56]. Reproduced with permission from © 2017 American Chemical Society. DOI: doi.org/10.1021/acsami.6b15249.

O

O

H N

N H

O

N O

OH OH O HO

N OH OH H

H N

H

O

H H

O

38 O

O

N O

Figure 2.32

H N

N H

N H 39

The chemical structures of 38 and 39.

H N

O O

H H H

41

2 Preparation and Sensing Application of Fluorescent Organogels and Hydrogels

500 mm 110

Collapsing time/min

42

100 90 80 70 60 50 40 40

(a)

45

50 55 60 Humidity/RH%

65

70

(b)

(c)

Figure 2.33 (a) Linear relationship of the 38/𝛼-CD gel (with a molar ratio of 1 : 1) collapsing time as a function of RH at 25 ∘ C; inset: 2 ml of high-performance liquid chromatography (HPLC) tube used for the humidity test; (b) photograph of the hygrometer STH310 used in the test; and (c) from left to right: photographs of the 1b/𝛼-CD gel kept at 54% RH after 10, 20, 30, and 43 minutes; scale bar: 1 cm. Source: Yu et al. [61]. Reproduced with permission from © 2017 American Chemical Society. DOI: doi.org/10.1021/acs. langmuir.6b04401.

2.2.2.5 Fluorescent Gels for Nitroaromatic Derivative Sensing

Nitroaromatic compounds are highly dangerous, explosive, chemical, and toxic environmental pollutants. Hence, their detection is one of the most active areas in the field of sensors. For example, A. Ajayaghosh’s group reported the fluorescent 𝜋 OPVPF organogels that display obvious fluorescent quenching in the presence of nitroaromatic compounds at different concentrations. Further study presented that the as-prepared test trips of xerogels were able to detect TNT at a record attogram level (∼12 ag cm−2 ), and the detection limit was 0.23 ppq (Figure 2.34) [62]. The low detection limit for TNT by contact method allows a low-cost and direct protocol for the on-site detection of TNT on real samples. Cao reported a naphthalimide-based gelator that contains pyridine groups (Figure 2.35); it could gel in mixed solvent of CH3 CN/H2 O (1/1, v/v), which is further developed to absorb and sense PA (picric acid) with fluorescent quenching phenomena based on the hydrogen bonding interaction between pyridine and phenol group [63]. However, there are still limited paradigms where gel assembly is utilized to selectively and visually sense nitroaromatic compounds in pure water. 2.2.2.6 Fluorescent Gels for Amine Sensing

Organic amines are a class of important intermediates because of their wide applications in material chemistry, dyes and pigments, as well as fuel additives [64, 65]. Excess presence of amines in environment is dangerous and toxic. Therefore, design

2.2 Types of Gels that Respond to Different Stimuli

F

OR

F

OR

F

OR

F

F

F F

F

F

RO

RO

RO

F

Self-assembly

TNT

10–15 M (~12 ag/cm2)

10–7 M

Gel coated filter paper

10–3 M

Figure 2.34 The chemical structure of OPVPF and sensing properties of gel and gel-coated filter paper. Source: Kartha et al. [62]. Reproduced with permission from © 2012 American Chemical Society. DOI: doi.org/10.1021/ja210728c. Figure 2.35 The chemical structure of gelator 40.

O

N O O 40

N

of novel materials for selective amine sensing is an increasing area in the past decades. In recent years, gels are also employed to visually sense or discriminate amines. We report a donor-𝜋-acceptor (D-𝜋-A) structural compound 41 that could discriminate aliphatic and aromatic amines in both solution and gels with different fluorescent signal outputs (Figure 2.36) [66]. Several experiments show that the isomerization of 41 from trans-to-cis induced by aliphatic amines triggers the O HO O

N H

H N

O O N N H

O

H N

H

e

in

m

a yl

op

Pr

P

G

An

ilin

e

H

O

41

(a)

H

O

HOAc

S

G

HOAc

G

G

(b)

Figure 2.36 (a) Chemical structure of 41 and (b) illustration of ultrasound-accelerated gelation for visual and reversible sensing of amines. Source: Pang et al. [66]. Reproduced with permission from © 2015 American Chemical Society. DOI: doi.org/10.1021/acsami. 5b03000.

43

44

2 Preparation and Sensing Application of Fluorescent Organogels and Hydrogels Propylamine Aniline

O RN O

Sol

N H

Heating-cooling

O

H N O

O

HO OH

O

N H

O

H N

NR O

Precipitate

Trans form Assembly triggered by sonication

Propylamine

Aniline Trans form

acid

Gel, FL off

Trans form

Gel

Cis form acid

Sol, FL enhancement

Figure 2.37 Proposed molecular assembly and sensing properties of the gel of 41 toward propylamine and aniline. Source: Pang et al. [66]. © 2015 American Chemical Society.

sol-gel transformation, together with significant fluorescence enhancement. In contrast, in the presence of aromatic amines, the binding of –NH with aromatic amine inhibits the ICT process of 41, resulting in the gel-to-gel transition with fluorescent quenching (Figure 2.37). The applications of the gel and gel aggregate for capturing and detecting aromatic amines in aqueous solution have also been investigated. M. Kumar reported an intramolecular charge transfer (ICT) and aggregationinduced emission enhancement (AIEE) active donor−acceptor−donor (D-A-D) system, which is also developed for aliphatic and aromatic amines sensing in aqueous media (Figure 2.38) [67]. They showed that the aggregates of compound 42 from aqueous solution with porous spherical structure can selectively sense tested aromatic amines such as aniline via fluorescent quenching. While, in the presence of Et3 N as a kind of aliphatic amine, emission color changes are observed. Amazingly, the test trip obtained from 42 assembly could be utilized to detect aniline and triethylamine with minimal limit of ∼1.01 ng cm−2 and 9.3 pg cm−2 respectively.

CN

NC

42

Figure 2.38 The chemical structure of gelator 42. Source: Modified from Pramanik et al. [67].

2.3 Summary and Perspectives

Figure 2.39 The chemical structure of gelator 43. Source: Cao et al. [68].

O

O

HO N

OCH3

O 43

Based on the alkalinity of amines, Cao et al. also reported the naphthol-based organogelator 43 that could gel in mixed solvent of methanol and H2 O (1/1, v/v, Figure 2.39) [68]. The resulting gel is able to selectively respond to Et3 N with sol-gel transition among tested amines including phenylamine; N,N-dimethylaniline; and triethylamine. The recognition process is accompanied by fluorescent quenching due to the inhibited ICT process of naphthalimide fluorophore.

2.3 Summary and Perspectives In conclusion, the hydrogels and organogels formed by non-covalent interactions of fluorescent low-molecular weight organogelators that respond to external stimuli including both physical and chemical stimuli are summarized in this chapter. The extensive sensing applications of both organogels and hydrogels are of great importance for the design and synthesis of novel intelligent and sensing platforms. The sensing processes of the gels toward analysts are exhibited multiple signal outputs such as transmittance, color or emission color, phase, as well as morphology changes, which makes them good platforms for visual, simple, and flexible sensing. Some of them display outstanding mechanical and self-healing properties, which would be more useful for potential applications. Additionally, it is found that the function groups and aggregation properties of the gelators highly impact the sensing properties of these gels such as selectivity and sensitivity. Notably, in recent years, the xerogels evaporated from the gels with ordered assembly have been given increasing attention, especially in the use of pressure-responsive materials, but the study of xerogels is still in its infancy. In view of the sensing objects, there are tremendous and increasing amounts of works that utilize gels for cation and anion sensing based on the metal–ligand interactions. However, examples of gels that respond to amines, solvent molecules as well as physical stimuli are still limited, and many of them have been found and studied accidentally. Moreover, the sensing analysts of gel platforms could be also extended to biomolecules such as amine acid, DNA, and protein and other stimuli in industry such as bare metal layers [69, 70]. Moreover, precise design of gelators and control on the sensing ability are necessary. In view of sensing abilities, there are also many problems to be addressed in the future for the gels with fluorescent sensing properties such as sensitivity and selectivity in complex environment, fast responsive rate as well as mechanical strength. However, due to the merits of easy and facile modification, visual and instant response to stimuli, and multiple functions of these gels, we believe that they would be promising sensing platforms and have potential in

45

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the field of drug control and release, adaptive and intelligent materials, switches, biological relevant processes, and others.

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3 Preparation of Self-Assembled Composite Hydrogels and Their Application in Biomedicine and Wastewater Treatment Ran Wang, Jingxin Zhou, Lexin Zhang and Tifeng Jiao Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao, P. R. China

3.1 Introduction The self-assembly technique is a common way for functionalized small molecules into supramolecular nanostructures and provide clues for the development of new nanoscale materials and composites [1–7]. The preparation and application of porous hydrogel materials have been important topics for scientists in recent years [8–11]. Hydrogel materials exhibit excellent properties due to their unique three-dimensional porous network structure, such as high mechanical strength, large surface area, ultralow density, and good compressibility. These properties make the hydrogels suitable for use in shape memory materials, drug delivery systems, tissue engineering, soft robotics, and wastewater treatment [12–21]. In the process of self-assembly, the main forces include hydrogen bonding, 𝜋–𝜋 interaction, or electrostatic interaction [22–24]. The gelator molecules in supramolecular gels self-assembled into three-dimensional networks via various non-covalent interactions, such as hydrogen bonding, 𝜋–𝜋 stacking, van der Waals interaction, dipole–dipole interaction, coordination, solvophobic interaction, and host–guest interaction [25–29]. In addition, two-component gels are a class of gel systems that have different functionalized chemical groups and nanoparticles. Moreover, the self-assembled composite hydrogels for biomedical-controlled drug release behavior and combined antitumor photothermal/photodynamic therapy have attracted more attention due to their unique properties, such as a wide therapeutic window, low toxicity, and good drug release efficacy [30–32]. Self-assembled injectable and self-healing hydrogels based on biomolecules including peptides and proteins have good biocompatibility and biodegradability, adjustable mechanical properties and flexible environmental responsiveness, so they are widely used in biomedicine [33–36]. With the rapid development of industries, people are paying

Supramolecular Gels: Materials and Emerging Applications, First Edition. Edited by Tifeng Jiao. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

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3 Preparation of Self-Assembled Composite Hydrogels

more and more attention to water pollution problems. The sewage discharged by factories contains a large amount of dye, which is harmful to human health and the environment. Effective and convenient removal of organic dyes from water has become a challenging problem [37–41]. The special porous network structure of these gel materials gives them good properties such as large surface area, ultralow density, strong mechanical strength, and good compressibility. These new properties make hydrogels suitable for removing organic dyes from aqueous environments. In the present chapter, previous main works on the preparation of self-assembly functionalized composite hydrogels, such as two-component organogels, graphene oxide-based composite hydrogels, supramolecular hydrogels, and metal hydrogels, are summarized. In addition, the prepared hydrogels showed good application prospects in wastewater treatment and biomedicine.

3.2 Prepared Composite Hydrogels Used in Biomedicine 3.2.1 Self-Assembly and Drug Release Capacities of Organogels via Some Amide Compounds with Aromatic Substituent Headgroups In the present work, we have synthesized supramolecular gels from new amide compounds with different aromatic substituent headgroups [7]. The molecular structures and abbreviations of obtained amide derivatives are shown in Figure 3.1. In the molecular skeleton, differently sized headgroups are attached to the phenyl ring via an amide bond to form a rigid and hydrophobic substituent segment [42–46]. The results showed that all compounds could be prepared in different solvents in the currently used solvents. Morphological characterization of organogels revealed various nanostructures of aggregates in the gel. In addition, drug release behaviors at different dye concentrations were also investigated. The photographs of obtained TC16-Fl gels are displayed in Figure 3.2. The data show that the aromatic substituent head group has an important effect on the gelation behavior of currently designed compounds. These results indicate that the larger aromatic head group in the molecular structure of the functional gel

NH C

H N O

OC16H33

C16H33O

OC16H33 TC16-Ben

C

H N

O

OC16H33

C16H33O

C16H33O

O

OC16H33 OC16H33

OC16H33 TC16-Np

C

TC16-F1

Figure 3.1 Molecular structures and abbreviations of three obtained amide derivatives. Source: Zhang et al. [7]. Licensed under CC BY 4.0.

3.2 Prepared Composite Hydrogels Used in Biomedicine

Figure 3.2 Photograph of TC16-Fl organogels in present used solvents (from left to right, nitrobenzene, acetone, N,N-dimethylformamide [DMF], aniline, pyridine, petroleum ether, n-hexane, ethanol, n-propanol, isopropanol, isooctanol, n-butanol, n-butyl acrylate, cyclohexanone, n-pentanol, 1,4-dioxane, cyclopentanone, and isopentanol). Source: Reproduced with permission from Zhang et al. [7]. © 2016 Materials. DOI: https://doi.org/10.3390/ma9070541.

contributes to organized self-assembly and subsequent gelation of the organic solvent, which is in good agreement with previous reports [47, 48]. The morphology and structure in the organogels were studied by scanning electron microscope (SEM) and atomic force microscope (AFM) techniques. From Figure 3.3, it can be clear seen that the TC16-Ben xerogel mainly exhibited large wrinkle-like or lamella-like aggregates with micrometer scale. Moreover, as shown in Figure 3.4, TC16-Np gels also displayed more nanostructures, such as fiber, wrinkle, and lamella. Figure 3.5 shows the SEM images of TC16-Fl xerogels from 18 solvents, which demonstrated the self-assembled diverse micro/nano- morphologies included various nano-aggregates. Due to the 3D porous structure of the gel, it is advantageous to load various drugs and characterize their drug release ability. Considering its unique nanostructures and solvents, it is preferred to add Congo Red (CR) as a typical TC16-Fl gel for model drugs in n-pentanol and to study their drug release properties. In addition, the release capacities of TC16-Fl gels in n-pentanol with different CR concentrations were also investigated, as shown in Figure 3.6. The results showed that with the change of CR concentration, the release rate before 320 minutes increased randomly, which might be mainly attributed to the original diffusion process of composited CR molecules from the outermost layer of TC16-Fl gels. However, the final release ratios were 75.2, 70.9, 65.1, and 62.2% for the CR concentrations of 8, 6, 4, and 2 mg ml−1 , respectively. Moreover, Figure 3.7 demonstrates the release kinetics curves of as-prepared TC16-Fl organogels in n-pentanol with different CR concentrations at 298 K.

53

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3 Preparation of Self-Assembled Composite Hydrogels

(a)

(b)

(c)

(d)

(e)

(f)

(g)

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Figure 3.3 SEM images of TC16-Ben xerogels from gels in various solvents: (a) aniline; (b) petroleum ether; (c) n-hexane; (d) ethanol; (e) n-propanol; (f) isopropanol; (g) n-butanol; (h) n-pentanol; (i) isopentanol. Source: Reproduced with permission from Zhang et al. [7]. © 2016 Materials. DOI: https://doi.org/10.3390/ma9070541.

(a)

(b)

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Figure 3.4 SEM images of TC16-Np xerogels from gels in various solvents: (a) DMF; (b) aniline; (c) n-propanol; (d) n-butanol; (e) n-pentanol; (f) 1,4-dioxane; (g) isopentanol. Source: Reproduced with permission from Zhang et al. [7]. © 2016 Materials. DOI: https://doi.org/10.3390/ma9070541.

3.3 Prepared Composite Hydrogels Used in Wastewater Treatment

(a)

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Figure 3.5 SEM images of TC16-Fl xerogels from gels in various solvents: (a) nitrobenzene; (b) acetone; (c) DMF; (d) aniline; (e) pyridine; (f) petroleum ether; (g) n-hexane; (h) ethanol; (i) n-propanol; (j) isopropanol; (k) isooctanol; (l) n-butanol; (m) n-butyl acrylate; (n) cyclohexanone; (o) n-pentanol; (p) 1,4-dioxane; (q) cyclopentanone; (r) isopentanol. Pictures in (s) indicate AFM images in 2D height and 3D model of TC16-Fl xerogel from gel in n-pentanol. Source: Reproduced with permission from Zhang et al. [7]. © 2016 Materials. DOI: https://doi.org/10.3390/ma9070541.

3.3 Prepared Composite Hydrogels Used in Wastewater Treatment 3.3.1 Preparation and Self-assembly of Some Functionalized Supramolecular Hydrogels Supramolecular gels can be considered as organic compound or building components that form a three-dimensional network structure with encapsulated solvents

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3 Preparation of Self-Assembled Composite Hydrogels

100

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Figure 3.6 Release capacities of TC16-Fl organogels in n-pentanol with different CR concentrations. Source: Zhang et al. [7]. Licensed under CC BY 4.0. 350 Gel with CR of 2 mg/mL Gel with CR of 4 mg/mL Gel with CR of 6 mg/mL Gel with CR of 8 mg/mL

10 8

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Figure 3.7 Release kinetics curves of as-prepared TC16-Fl organogels in n-pentanol with different CR concentrations at 298 K: (a) pseudo-first order model; (b) pseudo-second order model. Source: Zhang et al. [7]. Licensed under CC BY 4.0.

under non-covalent bonds, for example, hydrogen bonding, van der Waals force, 𝜋–𝜋 stacking, or electrostatic force [49–56]. In terms of various nanostructures, the gel molecules at the molecular level self-assemble and arrange orderly to form hierarchical microstructures/nanostructures with different levels in three-dimensional space, which produce fibrous shapes, rods, sheets, columnar or spherical shapes, respectively. At present, the reported research systems include small organic molecule gels, such as amides, hydrocarbons, cholesterol, amino acids and ureido derivatives. In a previous work, we have designed and prepared new two-component supramolecular gels via two different kinds of organic components, i.e. 2,2-bis(4-carboxyphenyl)hexafluoropropane and N-(4-Aminobenzoyl)-L-glutamic acid diethyl ester [57]. The prepared gels exhibited good adsorption performance

3.3 Prepared Composite Hydrogels Used in Wastewater Treatment

for methylene blue (MB) and rhodamine B (RhB). We also have reported a new two-component gel system composed of N-(4-aminobenzoyl)-L-glutamic acid diethyl ester with polyacrylic acid by the self-assembly process in different solvents [58]. The polymeric acid as gel component can introduce different self-assembly modes and form special nanostructures. The morphologies of the aggregates shown in the image can be rationalized by a generally accepted view that highly directional intermolecular interactions, such as hydrogen bonds or interactions, facilitate the formation of organized bands or fiber micro- and nanostructures [59–61]. And the hydrogel exhibits a continuous adsorption process, and equilibrium times are approximately 400 minutes for MB and CR, respectively. In another work, we studied a self-assembling multifunctional two-component supramolecular hydrogel based on two small organic molecules, namely, 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid and N-(4-amino-benzoyl)-L-glutamic acid diethyl ester, which self-assemble by intermolecular non-covalent bonding to form a one-dimensional fiber or band structure [62]. In addition, the surface effects of metal nanoparticles have broad application prospects in catalysis. However, metal nanoparticles are prone to aggregation, which greatly reduces their catalytic activity. In another system, because supramolecular gels contain various non-covalent interactions, they can be reversibly linked and disconnected. We designed a novel supramolecular hydrogel using cyclodextrin polymer (P-CD) as the host polymer and ferrocene (PAA-Fc)-modified poly(acrylic acid) as the guest polymer [63]. The hydrogel 3D cross-linked network is mainly formed through host–guest interaction between 𝛽-CD and ferrocene. The good removal rate of the prepared hydrogels for dyes is mainly due to porous structures facilitating the rapid passage of dye solutions. In addition, we have also designed and prepared a self-assembled hydrogel material constructed from cyclodextrin polymer (P-CD)/adamantane-modified polyacrylic acid (PAA-Ad) through host interaction [64].

3.3.2 Preparation and Self-assembly of Some Graphene Oxide-Based Composite Hydrogels In the past few years, graphene oxide (GO) sheets have received extensive attention as potential dye adsorbents because of their unique conjugated two-dimensional (2D) structure. In addition, the negative charge in the GO sheet due to various oxygen-rich functional groups (i.e. carboxyl, carbonyl, hydroxyl) allows for additional strong electrostatic interaction with the cationic dye molecules. In previous work, GO/polyethyleneimine (GO/PEI) hydrogels by combining the GO suspension and the PEI aqueous solution have been reported [65]. GO is likely to act as a visible light photocatalyst for degrading dyes during adsorption. Therefore, the adsorption experiment of the present invention was measured and repeated under dark conditions. Absorbance of 662 nm (for MB) and 554 nm (for RhB) was used to determine the residual dye concentration of samples collected at different time intervals.

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3 Preparation of Self-Assembled Composite Hydrogels

a

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Figure 3.8 SEM and TEM images for the lyophilized RGO/PEI hydrogel (a, d) and RGO/PEI/Ag hydrogel (b, e). (c) Photographs of: a, GO aqueous solution; b, GO/PEI hydrogel; c, RGO/PEI hydrogels; d, RGO/PEI/Ag hydrogel. (f) is energy dispersive x-ray spectrometer (EDXS) taken on the RGO/PEI/Ag hydrogel shown in (e). Source: Jiao et al. [56]. Licensed under CC BY 4.0.

In another work, by reducing GO and noble metal precursors within the GO gel matrix at the same time, a reduced graphene oxide-based silver-containing composite hydrogel was successfully prepared [56]. The reduced graphene oxide (RGO)-based hydrogel was obtained by the in situ reduction of GO with vitamin C under heating (90 ∘ C), which was used as a nontoxic and environmentally friendly reducing agent. The obtained hydrogels are composed of a network structure of cross-linked nanosheets by transmission electron microscope (TEM) and SEM (Figure 3.8). It can be clearly found that silver nanoparticles are uniformly dispersed on the surface of nanosheets. In addition, some other characterizations proved the successful preparation of composite hydrogels. The catalytic performance of the RGO-based composite hydrogels containing Ag nanoparticles was also studied for the photocatalytic degradation of RhB and MB solutions. Figure 3.9 shows the relationship between dye degradation rate and time of RhB and MB solutions in the presence of RGO-based composite hydrogels containing Ag nanoparticles as a catalyst. The results show that the degradation rate of the composite hydrogel for RhB can reach nearly 100% in about 70 minutes, and that for MB in about 30 minutes, which indicates that the prepared RGO/PEI/Ag hydrogel has a very good dye catalyst performance. In contrast, without photoirradiation, the degradation performance of RGO/PE/Ag hydrogels was significantly reduced. Moreover, the degradation properties of the prepared composite hydrogels on dyes have also been studied (Figure 3.10). The results show that the good dye degradation ability of the composite hydrogels may be related to their unique nanocomposite structure, for example, large three-dimensional network-like nanostructures are cross-linked with PEI through electrostatic attraction and hydrogen bonding, and

3.3 Prepared Composite Hydrogels Used in Wastewater Treatment

1.0

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Figure 3.9 Photocatalytic properties of RGO/PEI/Ag gel on MB (a) and RhB (b) solution, respectively. The inserted photos are those of dye solutions acquired for the supernatant liquids collected at different time intervals during photocatalytic experiment. Source: Jiao et al. [56]. Licensed under CC BY 4.0.

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Figure 3.10 Degradation kinetics curves of as-prepared RGO/PEI/Ag nanocomposites on MB (a, b) and RhB (c, d) at 298 K. Source: Jiao et al. [56]. Licensed under CC BY 4.0.

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Figure 3.11 SEM and TEM images for the lyophilized GO/CS hydrogel (a, e), RGO/CS hydrogel (b, f), and RGO/CS/Ag hydrogel (c, g), respectively. (d) EDXS taken on the RGO/CS/Ag hydrogel shown in part g. (h) Photographs of the following: GO aqueous solution, GO/CS, RGO/CS, and RGO/CS/Ag composite hydrogels (from left to right). Source: Reproduced with permission from Jiao et al. [66]. © 2015 American Chemical Society. DOI: doi.org/10.1021/acssuschemeng.5b00695.

3.3 Prepared Composite Hydrogels Used in Wastewater Treatment

highly dispersed Ag nanoparticles modified on the surface of RGO nanosheets as the photocatalytic active sites. In addition, we have reported the synthesis of RGO/PEI/Ag and RGO/CS/Ag composite gel materials and evaluated their dye degradation capacity [66]. The chitosan (CS) molecule was chosen for its functional amine segments in the molecular skeleton that can form porous gel nanostructures through interactions such as hydrogen bonding (Figure 3.11). The in situ formed silver nanoparticles were uniformly anchored on the RGO surface to form a ternary nanocomposite. The data of the photocatalytic ability experiment showed that the prepared 3D GO-based hydrogels can effectively remove the dyes and exhibit good photocatalytic performance for the currently used RhB and MB single or mixed solutions according to the pseudo-secondary model (Figure. 3.12). In another work, we reported the preparation of chitosan (CS) and GO composite hydrogels through in situ reduction approach [67]. The composite hydrogels consist of both hydrogen bonding and electrostatic interactions between the CS molecules and GO sheets. CS molecules were incorporated to facilitate the gelation process of GO sheets, and the dye adsorption capacity of the hydrogel was mainly attributed to the GO sheets. Furthermore, we have investigated the preparation of organogels with various nanostructures via self-assembly of functional cationic gemini amphiphilic compound–GO nanocomposites in normal organic solvents [68]. 180

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Figure 3.12 Degradation kinetics curves of as-prepared RGO/CS/Ag nanocomposites on MB (a, b) and RhB (c, d) at 298 K. Source: Jiao et al. [66]. © 2015 American Chemical Society.

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3 Preparation of Self-Assembled Composite Hydrogels

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Figure 3.13 SEM images of as-obtained PAA-Ag/AgNPs composite hydrogels with different preparation times: (a) 10 minutes; (b) 30 minutes; (c) 2 hours; (d) 5 hours; (e) 10 hours; (f) 24 hours. (g and h) TEM images of the composite hydrogels with preparation times of 30 minutes and 24 hours. (i) Typical EDXS of composite hydrogels prepared at 24 hours. Source: Hou et al. [69]. © 2016 Royal Society of Chemistry. DOI: doi.org/10.1039/C6RA23371F.

Moreover, hierarchical porous composite hydrogels of poly(acrylic acid)-silver/ silver nanoparticles (PAA-Ag/AgNPs) were successfully prepared by an in situ coordination and self-assembly process [69]. The obtained composite PAA-Ag/AgNPs hydrogels were achieved through different coordination times between PAA and silver ions in the self-assembly process (Figure 3.13). It can be clearly observed that the sizes of holes in the nanostructures become larger with increasing reaction time. In addition, the catalytic kinetic experiments of the as-prepared PAA-Ag/AgNPs hydrogel-24 hours were performed, and the results are shown in Figure 3.14. Due to the electron transfer process, charge recombination is suppressed in the PAA-Ag/AgNPs composite obtained by the present invention, thereby greatly improving the efficiency of photocatalytic behavior. In view of the above results and the self-assembly properties of the hydrogels obtained according to the invention, a reasonable explanation for the formation of PAA-Ag/AgNPs hydrogels is proposed, as shown schematically in Figure 3.15.

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Figure 3.14 Catalytic kinetics curves of the PAA/Ag hydrogel-24 hours for degradation of different dyes: (a, c and e) pseudo-first order kinetics; (b, d and f) pseudo-second order kinetics. Source: Hou et al. [69]. © 2016 Royal Society of Chemistry.

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AgNO3, short time

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–COOH PAA solution

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PAA-Ag/AgNPs composite hydrogel (complete coordination) H2O, CO2, etc.

Figure 3.15 Schematic illustration of the formation of the present PAA-Ag/AgNPs hydrogels. Source: Hou et al. [69]. © 2016 Royal Society of Chemistry.

In another work, we reported the preparation of a new graphene oxide–polyethylene glycol (GO–PEG) composite hydrogel by photopolymerization of thiol-ene without the addition of other stable or cross-linking compounds [70]. The speculated photopolymerization reaction that appeared between thiol groups and ene groups is crucial for the preparation of hierarchical composite nanostructures. The prepared GO–PEG composite hydrogels demonstrate good removal rates for three used model dyes and fit in good accordance with pseudo-second order model. Thus present research work provides new clues for the design of GO-based soft matter as well as hydrogels for dye removal.

3.4 Conclusion and Perspectives We are working on the design, preparation, and self-assembly of supramolecular and other composite hydrogels. In this chapter, various kinds of hydrogels including organogels, two-component supramolecular gels, graphene oxide-based and other composite hydrogels have all been demonstrated and investigated. The prepared gel materials are mainly used in wastewater treatment and biomedicine. For biomedicine, an injectable and self-healing collagen-protein-based hydrogel is formed by a gold-biomineralization-triggered self-assembly, mainly associated with the electrostatic interaction between positively charged collagen chains and

References

anionic clusters ([AuCl4 ]− ions). And such biocompatible collagen-based hydrogels have been developed as a novel tool for localized delivery and sustained release of therapeutic drugs, with the advantages to reduce the drug dosage, to lower toxicity, and to improve bioavailability. In addition, the organogels were prepared by three amide derivatives with aromatic substituent headgroups. The results indicate that drug release rate of the organogels can be regulated by the release times and drug concentrations. We have also prepared a series of other composite gel materials for wastewater treatment, and the adsorption is mainly accomplished through host–guest interaction and electrostatic interaction. The obtained gels exhibit good adsorption properties for dyes. The above research work may give the potential perspective for the design and preparation of new self-assembly systems and nanomaterials. This research work has provided valuable information for further exploration of self-assembled hydrogels in physics, biosensors, catalysis, nanomaterials, and environmental treatment.

Acknowledgments We greatly appreciate the financial support of the National Natural Science Foundation of China (nos. 21872119, 22072127) and the Talent Engineering Training Funding Project of Hebei Province (no. A201905004).

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4 Conductive Hydrogels for Flexible Mechanical Sensors Zhihui Qin and Tifeng Jiao Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao, P. R. China

4.1 Introduction Skin, being the largest organ in human body, is able to sense various stimuli such as temperature, humidity, pressure, and pain via numerous sensor types, playing an important role in the interface between brain and external ambient environment [1–5]. In addition, skin also has a wide variety of unique properties such as stretchability, self-healing ability, highly mechanical toughness, and environmental suitability [6–8]. In recent years, inspired by the structure and perception functions of skin, much effort has been devoted to designing and developing various electronic devices such as wearable sensors [9], electronic skins [10], and implantable electronic devices [11], exhibiting potential applications in personalized health monitoring [12, 13], human–machine interfaces [14], and humanoid robotics [15]. Development of a variety of flexible physical sensors with high sensitivity, rapid response, and high resolution is the key to realizing the simulation of skin sensing characteristics. As a widely used physical sensor, mechanical sensors are sensitive to mechanical stimuli, which can be applied to detect both large human motions such as joint motions and tiny movements caused by blood pressure, pulse, breathing, and sound [16–20]. This type of mechanical sensors has broad application prospects in various fields. For example, in the field of robotics, such sensors can enable robots to respond to external stimuli and complete complex tasks [21]. In the field of medicine, mechanical sensors can not only help amputees recover their perception functions, but also can be used to continuously monitor physiological health [22–24]. When mechanical sensors are attached to human skin or are functioning as skin-like sensors in robotics or prosthetics, they firstly need to be well adhered to moving surfaces. For example, at the human joints, the surface will undergo lateral tension, compression, and twisting [25]. Thus, mechanical flexibility is needed to provide the necessary mechanical degrees of freedom to adapt to mechanical deformation for mechanical sensors during usage. For the majority of materials used to Supramolecular Gels: Materials and Emerging Applications, First Edition. Edited by Tifeng Jiao. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

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construct flexible mechanical sensors, stretchable conductive materials composed of elastomers (polydimethylsiloxane, silk fibroin, polyurethane, and polyethylene terephthalate) [26–29] and conductive agents including metal particles [30], semiconductors [31], carbon materials [32, 33], and conducting polymers [34–36] are commonly used. The elastomers serve as flexible matrix to endow the network of conductive fillers with deformations, converting external pressure and strain into detectable electronic signals. The properties and interactions of matrix and the network have a great influence on the performances of such sensors. Although these elastomer composites are advantageous in their stretchability and have low modulus close to human tissues, low compatibility between fillers and elastomers, non-biocompatibility and low fatigue resistance usually limit the widespread use of these materials as stretchable mechanical sensors [37]. It remains a big challenge to develop flexible conductive materials with satisfied performances as mechanical sensors. Conductive hydrogels have become one of the most promising materials for the preparation of flexible mechanical sensors due to their unique skin tissue-like structure, mechanical compatibility, good biocompatibility, and conductive activity [38–42]. Conductive hydrogels are usually fabricated by incorporating conductive fillers, or adding soluble salts to traditional hydrogel networks. In this system, hydrogel networks provide the mechanical flexibility, biocompatibility, and other functionalities, which can be tuned by changing the cross-linked polymers, network structure, and cross-linked interactions. For example, natural biopolymers are chain-like molecules that contain repetitive units, such as proteins and polysaccharides with superb biological characteristics, which can be used to prepare soft biointegrated mechanical sensors [43, 44]. The introduction of dynamic covalent or non-covalent (such as hydrogen bond, ionic bond, and host–guest interactions) interactions can efficiently enhance the mechanical toughness of hydrogel networks [45, 46]. Within the hydrogel network, conductive fillers or movable salt ions provide conductive path, endowing the hydrogel with conductive activity. For conductive hydrogels containing conductive fillers such as conductive polymers, metal nanoparticles/nanowires, and carbon nanomaterials, there are interactions between conductive fillers and hydrogel matrix. Thus, when the hydrogels are loaded by pressure or strain, the links of conductive fillers can be changed, resulting in the variation of electrical signals [47]. For salt ions-based conductive hydrogel, the changes of electrical signals are mainly attributed to geometrical changes of hydrogels under external pressure or strain. Although both conductive fillers and salt ions-based hydrogels can achieve the transformation of mechanical forces to detectable electrical signals, they exhibit different advantages as mechanical sensors. For instance, conductive fillers-based hydrogels usually show much higher sensitivity due to the contact-resistance effect and tunneling effect [48]; however, most of them are nontransparent, which will influence the visualization of sensors. Meanwhile, whereas the ion conduction is ubiquitous in biosystems [38, 49], the electron conduction of conductive fillers-based hydrogels seriously hinders the integration of wearable mechanical sensors and human biotissues. Ionic conductive hydrogels are highly transparent and have the same conductive channel

4.2 Fabrication of Conductive Hydrogels

to biosystems, which make them similar more suitable for biointegrated sensors, but they usually have relatively low sensing sensitivity. From a materials design standpoint, the excellent mechanical toughness combined with great conductivity is essential for real application as mechanical sensors. However, a high filling content is needed for high conductivity, but will result in the deterioration of mechanical performances due to the phase separation between fillers and hydrogel matrix. Therefore, in order to prepare conductive hydrogels with desired mechanical properties and conductivity, optimization of the network structure and improvement of the affinity of conductive component with hydrogel matrix are needed [50]. In addition, integrating multi-functionality such as self-healing ability, self-adhesion, antifreezing property, multi-sensing capacity, and recyclability into conductive hydrogel is greatly important for practical applications [51–55]. For example, the self-healing ability can greatly improve the service life of conductive hydrogels as wearable mechanical sensors [51]. The wide extreme-temperature tolerance will broaden their application environment and season, even at ultralow temperature [53]. Taken together, to satisfy the real applications as mechanical sensors, integration of mechanical toughness, good conductivity, and multi-functionality into conductive hydrogels should be carefully considered. In this chapter, recent advances in the preparation, property, and applications of conductive hydrogels as mechanical sensors are reviewed. The content begins with the unique properties of conductive hydrogels that are required for advanced mechanical sensors and preparation techniques of conductive hydrogel materials. Next, the recent progresses in design and fabrication strategies of conductive hydrogels for mechanical sensors including strain and pressure sensors are summarized and compared. Finally, the challenges and perspectives of conductive hydrogels as mechanical sensors are discussed and proposed.

4.2 Fabrication of Conductive Hydrogels Conductive hydrogels are usually composed of conductive components and gel networks, where the hydrogel polymer network provides the scaffold and the conductive material imparts conductivity. Due to its unique conductivity, mechanical properties, and biocompatibility, conductive hydrogels have been widely used in energy storage devices [56, 57], different types of sensors [58], and tissue engineering [59]. In order to completely imitate the mechanical sensing properties of human skin, the conductive hydrogel used to prepare the mechanical sensor needs to have the following two characteristics: (i) sufficient mechanical flexibility and (ii) sensitive sensing channels. On the one hand, in practical applications, mechanical sensors continue to withstand dynamic deformation from the outside, so the conductive hydrogel used to prepare the sensors should have greater deformability and toughness [25]. On the other hand, the construction of the sensing channel in conductive hydrogels has an important influence on the sensitivity of sensors and makes it possible to convert mechanical stimuli into detectable electrical signals. Therefore, good conductivity of the hydrogel is very important for achieving the sensing characteristics. At the

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same time, the rapid self-recovery of the hydrogel can improve the responsiveness and stability of the sensors [60]. In addition, other functions such as self-healing and stimulus responsiveness can increase the service life of the sensors and realize its intelligence. According to difference of conductive mechanisms, conductive hydrogels can generally be divided into electronically conductive hydrogels and ionically conductive hydrogels. This section mainly focuses on the preparation methods and representative examples of conductive hydrogels based on different conductive mechanisms.

4.2.1

Electronically Conductive Hydrogel

Electronically conductive hydrogels are mainly formed by complexing hydrogel with conductive polymers or fillers. Conductive polymers (CPs) are synthetic polymers that are characterized by their ability to conduct electrons, which have attracted significant interests due to their tunable electrical conductivity [61, 62]. Polythiophene, polypyrrole, and polyaniline are three main conductive polymers used to synthesize conductive hydrogels. However, the highly p-conjugated structure and inherent stiffness of conductive polymers have limited the preparation of flexible conductive hydrogels as mechanical sensors. Recently, the mechanical flexibility of conductive polymer hydrogels has been greatly improved by designing the network structure or introducing dynamic cross-linked interactions [57, 63–65]. Generally, there are two main methods to prepare conductive polymer hydrogels. One is to directly add conductive polymers into hydrophilic hydrogel networks. A typical example was reported by Wang’s group [65] as shown in Figure 4.1a. First, the conductive component of poly (3,4-ethylenedioxythiophene):sulfonated lignin (PEDOT:SL) was prepared by in situ copolymerizing from EDOT and SL with ammonium persulfate (APS) as the oxidizing agent, where the SL was used as an effective dispersant for the PEDOT:SL solution. Then, the self-wrinkled PEDOT:SL-PAA organohydrogels were fabricated by incorporating the PEDOT:SL into the poly (acrylic acid) (PAA) hydrogel backbone and subsequent solvent replacement. The PEDOT:SL-PAA organohydrogel maintained high conductivity due to the introduction of PEDOT:SL, which also improved the mechanical properties of softness and elasticity. Thus, this organohydrogel with high conductivity and excellent mechanical performances could be used as biocompatible wearable sensors. Rong’s group [63] also synthesized an antifreezing conductive organohydrogel by incorporating poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT:PSS) into polyvinyl alcohol (PVA) hydrogel. In this system, conductive polymers PEDOT: PSS and PVA polymer powders were dissolved in EG/H2 O at 95 ∘ C. Then the homogeneous solution was cooled to −20 ∘ C to induce physical cross-linking points among PVA chains by combination of the hydrogen bonds and crystalline domains, giving the high mechanical strength of the obtained organohydrogel. The existence of PEDOT: PSS network endowed the organohydrogel with good conductivity. Another method is to introduce conductive components in the form of small molecule monomers, and then realize the effective combination of conductive polymer and gel network through in situ polymerization. The

4.2 Fabrication of Conductive Hydrogels EDOT,APS Fe

Sulfonated

Ionic crosslinking

PEDOT:SL

SL

L

3+

In situ polymerization

Ultrasonic dispersion O

PAM/CS IPN hydrogel

O

O

AA,APS,MBA

N H

N H

N H NH

50 °C

N H OH

2

O

O HO

NH

n

OH O

PEDOT:SL-PAA-X’ hydrogel

Water Solvent replacement

PPy nanored formed on the CS template

3+

Fe

PEDOT

PPy-PAM/CS hydrogel PPy nanored

PAA chain Conductive polymer skeleton Wrinkle

(a)

Chitosan chain entanglement

PEDOT:SL-PAA-X organohydrogel

Glycerol PAM ebemical crosslinking

Py absorbed into the IPN hydrogel

Py fixed on CS template

(b)

Figure 4.1 Preparation of conductive polymer-based conductive hydrogels. (a) Schematic illustration of the preparation of the conductive EDOT: SL-PAA organohydrogels. Source: Wang et al. [65]. © 2019 Elsevier. (b) Schematics of PPy composited conductive and tough PPy-PAM/CS hydrogels. Source: Gan et al, [57]. © 2018 American Chemical Society.

interactions between conductive polymers and hydrogel matrix, the dispersibility and the content of conductive polymers can be largely tuned by this method, resulting in the enhancement of conductivity. The first conductive polymer hydrogel synthesized by this approach was to polymerize polypyrrole (PPy) directly on a preformed polyacrylamide (PAM) hydrogel reported by Pissis and Kyritsis [66]. In a similar mechanism, Lu’s group [57] reported a tough and conductive hydrogel by controlling conducting polypyrrole (PPy) nanorods by in situ formation inside polyacrylamide (PAM)/chitosan (CS) interpenetrating polymer network hydrogel as shown in Figure 4.1b. The PPy nanorods were uniformly distributed in the PAM/CS interpenetrating network (IPN) hydrogel, endowing the hydrogel with high conductivity and the conductive hydrogel exhibited remarkable mechanical properties due to the formation of composites with the PPy nanorods. In addition, Ma’s group [64] prepared a dynamically cross-linked PANI-PVA conductive hydrogel by in situ copolymerization of aniline (ANI) and aminophenylboronic acid in polyvinyl alcohol (PVA) solution. The intermolecular interaction between boronic acid groups on PANI and hydroxy groups on PVA served as the cross-links, endowing the hydrogel with superior mechanical properties. PANI provided fast and reversible charge storage with a high specific capacitance and good chemical stability. Therefore, this hydrogel could be used as a flexible solid state supercapacitor with good cyclic stability and mechanical durability. Except for conductive polymers, carbon-based materials such as carbon nanotubes (CNTs), graphene, and carbon quantum dots are also ideal conductive fillers for preparing conductive hydrogels due to their high electrical conductivity, excellent environmental stability, and mechanical strength [67, 68]. Among them, CNT and graphene have been extensively studied in the field of materials science and are widely used as conductive fillers to prepare conductive hydrogels. However, the poor dispersion of carbon nanotubes and graphene in aqueous solutions limits their application in the preparation of conductive hydrogels. In order to improve the dispersibility of carbon nanotubes and graphene in hydrogel network, the common method is to introduce hydrophilic functional groups on the surface or

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CNT-CNF nanohybrids

CNFs

Homogeneous

Cooling gelation borax

CNFs

PVA

Dispersion

Heating

Na2B4O7 + 7H2O → 2B(OH)3 + 2B(OH)4– + 2Na

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(a) Step 1 CH2 CH CH2 CH CH CH CH 2 2 COOH O O O O H H

Dopamine

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CH2CH CH2 CH CH CH 2 COOH O O O O H H

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pH=8.5 O

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AA

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(b)

Figure 4.2 Fabrication of carbon-based conductive hydrogels. (a) Schematic of the preparation for PVAB-based composite hydrogels doping with CNT-CNF nanohybrids. Source: Han et al. [69]. © 2019 Elsevier. (b) Schematic illustration of preparation process and structure of rGO incorporated PAA nanocomposite hydrogel inspired by mussel-inspired chemistry. Source: Jing et al. [71]. © 2018 Elsevier.

modify them with hydrophilic polymers. For example, Han et al. [69] prepared multifunctional self-healable electroconductive hydrogels (ECHs) by combining polyvinyl alcohol-borax (PVAB) hydrogel with carbon nanotube-cellulose nanofiber (CNT-CNF) nanohybrids, as depicted in Figure 4.2a. The hydrophilic CNFs acted as a biotemplate to effectively disperse CNTs, forming the well-dispersed CNT-CNF nanohybrids in aqueous suspensions and borate ions cross-linked CNT-CNF nanohybrids with PVA. The CNT-CNF nanohybrids not only enhanced the viscoelasticity and mechanical toughness of the hydrogel, but also endowed the CNT-CNF/PVAB hydrogel with high conductivity. The solid state supercapacitor assembled by PVAB-based hydrogels exhibited ideal capacitance retention under various deformations. Oxidation by using high concentration of strong acid such as H2 SO4 and HNO3 is an effective method to introduce hydrophilic group on CNT surfaces. As reported by Yao’s group [70], the CNTs were first oxidized by

4.2 Fabrication of Conductive Hydrogels

HNO3 and then dispersed evenly in aqueous solution with assistance of gelatin. A CNTs-incorporated PAM hydrogel was fabricated by polymerization of AAm monomer and cross-linker in the presence of CNT dispersion and the obtained hydrogel exhibited excellent mechanical toughness and suitable conductivity. As a derivative of graphene, graphene oxide (GO) with a variety of hydrophilic functional groups on the surface is often used to prepare conductive hydrogels. However, compared with graphene, GO-based conductive hydrogels usually exhibit poor conductivity. In order to improve the conductivity of GO-based hydrogels, the researchers used the method of in situ reducing GO in the hydrogel matrix to produce reduced GO (rGO) with considerable conductivity [53, 71–73]. Various types of chemical reducers can be used to produce rGO, among which dopamine (DA) is considered to be the most potential agent because of its biocompatibility and mild reducing conditions. For instance, Turng’s group [71] fabricated a nanocomposite hydrogel comprised of poly (acrylic acid) (PAA) and rGO by mussel-inspired chemistry, as shown in Figure 4.2b. In the first step, graphene oxide was fully reduced via dopamine self-polymerization, which provided effective electrical pathways in the hydrogel. In the second step, acrylic acid (AA) monomers were in situ polymerized in the presence of a chemical cross-linker and a physical cross-linker to form a dual-cross-linked hydrogel. The obtained nanocomposite hydrogel integrated high stretchability, high toughness, and superior sensing abilities. Gan et al. [72] also prepared the PSGO-PEDOT-PAM hydrogel by first self-assembling PEDOT on a polydopamine-reduced and sulfonated graphene oxide (PSGO) template, and then incorporating the PEDOT-PSGO into a PAM hydrogel, which could be used as an implantable bioelectrode to detect biological signals. Recently, MXene, an emerging 2D material with remarkable hydrophilic, strong mechanical, and highly conductive properties has been widely investigated to prepare conductive hydrogels [74–77]. Generally, MXenes are prepared by selective etching away of the A-group layers from the MAX phases by using LiF reagents, which endows the MXene with a number of functional groups on their surfaces and edges (such as –OH, –O, and –F) [78], making them show good dispersion in aqueous solution and form stable conductive effective pathways. In addition, the abundant hydrophilic groups on MXene can interact with hydrogel network to form cross-links, improving the mechanical properties of hydrogels. As proof of concept, Ma’s group [74] fabricated a MXene nanosheets-catalyzed self-assembled, poly (acrylic acid) (PAA) hydrogel with excellent conductivity, stretchability, and anti-aggregation performances as shown in Figure 4.3. The reductive TiO2 @MXene nanosheets catalyzed the dissociation of the ammonium to initiate the polymerization of AA monomers, and meanwhile cross-linked PAA polymer chains to obtain a hydrogel. The properties of this hydrogel such as mechanical toughness, adhesion, and conductivity could be readily adjusted by altering the TiO2 @MXene content, which could be used as self-adhesive bioelectrodes for the stable detection of human physiological signals. Moreover, Yu’s group [75] fabricated an antifreezing, self-healing, and conductive MXene nanocomposite organohydrogel (MNOH) by the incorporation of the conductive MXene nanosheet networks into hydrogel polymer networks and subsequent immersing process in ethylene glycol (EG). The

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(a)

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AA TiO2@MXene suspension (b)

AA capture H2O Redox SO4· OH·

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(500 kPa). The microstructure of the hydrogel surface plays an important role in improving the sensitivity and signal-to-noise ratio of the hydrogel-based pressure sensors. As an important example, Dong’s group [116] prepared a self-patterned ion-conducting hydrogel with wrinkle microstructure on the surface and used it as resistive pressure sensors. These surface fold structures significantly increased the contact area change during the pressure change, thereby improving the sensitivity of the pressure sensor (0–3.27 kPa, 0.05 kPa−1 ) and the accurate detection of dynamic pressure. In addition, the hydrogel pressure sensor also exhibited rapid response and good sensing stability, which could be used to monitor various human motions, including phonation, swallowing process, and limb activities, showing great potential for electronic skins. Recently, skin-inspired hydrogel-based multifunctional mechanical sensors, which can be used to detect both strain and pressure, have been also widely developed [121–123]. For example, Gao’s group [122] fabricated a highly stretchable and anti-fatigue HLPs-PAAm hydrogel by incorporating hybrid latex nanoparticles (HLPs) into hydrophobic association polyacrylamide (PAAm) networks. The non-covalent synergistic interactions in the hydrogel networks

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Figure 4.10 The HLPs-PAAm hydrogels-based strain and pressure sensors. (a) Schematic structure of the HLPs-PAAm hydrogels. (b) Resistance–strain curve of the strain sensor. (c) Resistance–pressure curve of the pressure sensor. Detection of real-time signals upon (d) finger bending, (e) pressing, and (f) pronunciation. Source: Reproduced with permission from Xia et al. [122]. © 2019 American Chemical Society. DOI: doi.org/10.1021/acs.chemmater.9b03919.

4.4 Conclusion and Outlook

endowed the hydrogel with fast self-recoverable and anti-fatigue behaviors. This hydrogel could be used as both pressure and strain sensors with durability and high sensitivity for the detection of various mechanical deformations, showing broad applications in electric skins and soft robotics (Figure 4.10).

4.4 Conclusion and Outlook Given the virtues of conductive hydrogel such as tissue-like structure, good biocompatibility, mechanical compatibility, and conductive activity, conductive hydrogels are becoming the most promising materials for wearable sensors and electronic skins. In this chapter, we reviewed recent advances in the preparation, properties, and applications of conductive hydrogels as mechanical sensors. With the introduction of conductive fillers or soluble salt ion and hydrogel network structure design, the conductive hydrogels with excellent mechanical flexibility and satisfied conductivity can be obtained to assemble strain and pressure sensors for detection of human motions and physiological signals. In addition, by introducing functional groups and constituents, other functionalities such as self-adhesion, self-healing abilities, and antifreezing have been also integrated into conductive hydrogels to ensure accurate detection, extend service life, and broaden the application environment of mechanical sensors. Although recent achievements in conductive hydrogels-based mechanical sensors, the conductive hydrogels-based strain and pressure sensors are still in their infancy and there are urgent demands to achieve real applications in wearable devices and electronic skins. Integrating high sensitivity, large sensing range, and great linearity into one hydrogel-based sensor is very important for real applications, but rarely realized. High sensitivity contributes to the precise capture of subtle movements or physiological signals. Large working range can ensure sufficient detection range, which means detection of full human movements. Great linearity makes the calibration process simple and be easily used to detect different mechanics. Though conductive hydrogels containing conductive fillers display high sensitivity due to the contact-resistance effect and tunneling effect, these hydrogels-based sensors exhibit poor linearity because of the different mechanisms in different sensing range. It seems to be conflicting between the high sensitivity and good linearity. The balance between sensitivity and linearity should be considered. In addition, upon large strain or pressure, the hydrogel cannot fully return to original state, which will result in large hysteresis in signals. Moreover, mechanical performances are also a significant parameter for conductive hydrogels-based mechanical sensors. Lately, a variety of high stretchable and tough hydrogels has been designed by combining reversible non-covalent interactions (e.g. ionic bond, hydrogen bonding, crystallization, and hydrophobic interaction) with designed network structure, such as double network hydrogels, hydrophobic association hydrogels, ionically cross-linked hydrogels, and nanocomposite hydrogels. However, it seems difficult to combine high stretchability and high toughness with fast resilience. Therefore,

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the mechanical performances of conductive hydrogels need to be further optimized to satisfy different applications. Different from metals, which have a high conductivity (106 S/m), the conductivity of hydrogels is far lower (0.1–103 S/m); some strategies such as combination of ionic and electronic conduction should be used to further improve their conductivity. Environmental adaptability and long-term stability are also vital for wearable sensors. Hydrogels will inevitably deteriorate their performances in harsh environment. For example, hydrogels will lose water when stored at room temperature for a long time and will freeze at subzero temperatures due to the freezing of water. Although some antifreezing or non-drying hydrogels have been fabricated by introducing organic solvent or hygroscopic salt, these methods have an influence on the mechanical property or conductivity or sensitivity of the obtained sensors and the performances of these hydrogels-based sensors will also decline under harsh environments. Thus, it is an urgent need to develop conductive hydrogels that can work under harsh environments without compromising their intrinsic properties. Non-recyclability is also a serious problem facing current wearable electronics. There are many electronic wastes produced worldwide. These electronic wastes have great impact on the environment and cause rational concerns on valuable natural resources. Meanwhile, the single-use electronic items will increase the manufacturing cost and relevant expenditure of consumers. Developing hydrogels-based mechanical sensors with full recyclability is significant for green sustainable development. Future studies should focus on the novel design of conductive hydrogels to fabricate biomimetic sensors with desired mechanical performances and conductivity combined with functionalities.

Acknowledgments We greatly appreciate the financial support of the National Natural Science Foundation of China (nos. 21872119, 22072127) and the Talent Engineering Training Funding Project of Hebei Province (no. A201905004).

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89 Ge, G., Zhang, Y., Shao, J. et al. (2018). Stretchable, transparent, and self-patterned hydrogel-based pressure sensor for human motions detection. Adv. Funct. Mater. 28: 1802576. 90 Chossat, J.B., Park, Y.L., Wood, R.J., and Duchaine, V. (2013). A soft strain sensor based on ionic and metal liquids. IEEE Sens. J. 13: 3405–3414. 91 Gong, S., Schwalb, W., Wang, Y. et al. (2014). A wearable and highly sensitive pressure sensor with ultrathin gold nanowires. Nat. Commun. 5: 1–8. 92 Liu, Y., Hu, Y., Zhao, J. et al. (2016). Self-powered piezoionic strain sensor toward the monitoring of human activities. Small 12: 5074–5080. 93 Lipomi, D.J., Vosgueritchian, M., Tee, B.C. et al. (2011). Skin-like pressure and strain sensors based on transparent elastic films of carbon nanotubes. Nat. Nanotechnol. 6: 788–792. 94 Zhang, X., Sheng, N., Wang, L. et al. (2019). Supramolecular nanofibrillar hydrogels as highly stretchable, elastic and sensitive ionic sensors. Mater. Horiz. 6: 326–333. 95 Zhang, Z., Gao, Z., Wang, Y. et al. (2019). Eco-friendly, self-healing hydrogels for adhesive and elastic strain sensors, circuit repairing, and flexible electronic devices. Macromolecules 52: 2531–2541. 96 Lv, R., Bei, Z., Huang, Y. et al. (2020). Mussel-inspired flexible, wearable, and self-adhesive conductive hydrogels for strain sensors. Macromol. Rapid Commun. 41: 1900450. 97 Wang, Z., Zhou, H., Lai, J. et al. (2018). Extremely stretchable and electrically conductive hydrogels with dually synergistic networks for wearable strain sensors. J. Mater. Chem. C 6: 9200–9207. 98 Zhang, Q., Liu, X., Duan, L., and Gao, G. (2020). Nucleotide-driven skin-attachable hydrogels toward visual human-machine interfaces. J. Mater. Chem. A 8: 4515–4523. 99 Xu, J., Jin, R., Ren, X., and Gao, G. (2019). Cartilage-inspired hydrogel strain sensors with ultrahigh toughness, good self-recovery and stable anti-swelling properties. J. Mater. Chem. A 7: 25441–25448. 100 Zhang, X., Liu, W., Cai, J. et al. (2019). Equip the hydrogel with armor: strong and super tough biomass reinforced hydrogel with excellent conductivity and anti-bacterial performance. J. Mater. Chem. A 7: 26917–26926. 101 Wang, Z., Chen, J., Cong, Y. et al. (2018). Ultrastretchable strain sensors and arrays with high sensitivity and linearity based on super tough conductive hydrogels. Chem. Mater. 30: 8062–8069. 102 Qin, Z., Sun, X., Yu, Q. et al. (2020). Carbon nanotubes/hydrophobically associated hydrogels as ultrastretchable, highly sensitive, stable strain, and pressure sensors. ACS Appl. Mater. Interfaces 12: 4944–4953. 103 Yang, C. and Suo, Z. (2018). Hydrogel ionotronics. Nat. Rev. Mater. 3: 125. 104 Kim, C.C., Lee, H.H., Oh, K.H., and Sun, J.Y. (2016). Highly stretchable, transparent ionic touch panel. Science 353: 682–687. 105 Jing, X., Mi, H.Y., Lin, Y.J. et al. (2018). Highly stretchable and biocompatible strain sensors based on mussel-inspired super-adhesive self-healing hydrogels for human motion monitoring. ACS Appl. Mater. Interfaces 10: 20897–20909.

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106 Liao, M., Wan, P., Wen, J. et al. (2017). Wearable, healable, and adhesive epidermal sensors assembled from mussel-inspired conductive hybrid hydrogel framework. Adv. Funct. Mater. 27: 1703852. 107 Tong, R., Chen, G., Pan, D. et al. (2019). Highly stretchable and compressible cellulose ionic hydrogels for flexible strain sensors. Biomacromolecules 20: 2096–2104. 108 Liu, H., Wang, X., Cao, Y. et al. (2020). Freezing-tolerant, highly sensitive strain and pressure sensors assembled from ionic conductive hydrogels with dynamic cross-links. ACS Appl. Mater. Interfaces 12: 25334–25344. 109 Wu, J., Wu, Z., Lu, X. et al. (2019). Ultrastretchable and stable strain sensors based on antifreezing and self-healing ionic organohydrogels for human motion monitoring. ACS Appl. Mater. Interfaces 11: 9405–9414. 110 Zhang, H., Niu, W., and Zhang, S. (2019). Extremely stretchable and self-healable electrical skin with mechanical adaptability, an ultrawide linear response range, and excellent temperature tolerance. ACS Appl. Mater. Interfaces 11: 24639–24647. 111 Qin, Z., Dong, D., Yao, M. et al. (2019). Freezing-tolerant supramolecular organohydrogel with high toughness, thermoplasticity, and healable and adhesive properties. ACS Appl. Mater. Interfaces 11: 21184–21193. 112 Qin, Z., Sun, X., Zhang, H. et al. (2020). A transparent, ultrastretchable and fully recyclable gelatin organohydrogel based electronic sensor with broad operating temperature. J. Mater. Chem. A 8: 4447–4456. 113 Yang, Y., Guan, L., Li, X. et al. (2019). Conductive organohydrogels with ultrastretchability, antifreezing, self-healing, and adhesive properties for motion detection and signal transmission. ACS Appl. Mater. Interfaces 11: 3428–3437. 114 Tai, Y., Mulle, M., Ventura, I.A., and Lubineau, G. (2015). A highly sensitive, low-cost, wearable pressure sensor based on conductive hydrogel spheres. Nanoscale 7: 14766–14773. 115 Lou, Z., Chen, S., Wang, L. et al. (2016). An ultra-sensitive and rapid response speed graphene pressure sensors for electronic skin and health monitoring. Nano Energy 23: 7–14. 116 Ge, G., Zhang, Y., Shao, J. et al. (2018). Stretchable, transparent, and self-patterned hydrogel-based pressure sensor for human motions detection. Adv. Funct. Mater. 28: 1802576. 117 Si, Y., Wang, L., Wang, X. et al. (2017). Ultrahigh-water-content, superelastic, and shape-memory nanofiber-assembled hydrogels exhibiting pressure-responsive conductivity. Adv. Mater. 29: 1700339. 118 Lei, Z. and Wu, P. (2018). Zwitterionic skins with a wide scope of customizable functionalities. ACS Nano 12: 12860–12868. 119 Duan, J., Liang, X., Guo, J. et al. (2016). Ultra-stretchable and force-sensitive hydrogels reinforced with chitosan microspheres embedded in polymer networks. Adv. Mater. 28: 8037–8044. 120 Ge, G., Yuan, W., Zhao, W. et al. (2019). Highly stretchable and autonomously healable epidermal sensor based on multi-functional hydrogel frameworks. J. Mater. Chem. A 7: 5949–5956.

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121 Zhang, Q., Liu, X., Ren, X. et al. (2019). Nucleotide-regulated tough and rapidly self-recoverable hydrogels for highly sensitive and durable pressure and strain sensors. Chem. Mater. 31: 5881–5889. 122 Xia, S., Zhang, Q., Song, S. et al. (2019). Bioinspired dynamic cross-linking hydrogel sensors with skin-like strain and pressure sensing behaviors. Chem. Mater. 31: 9522–9531. 123 Ying, B., Wu, Q., Li, J., and Liu, X. (2020). An ambient-stable and stretchable ionic skin with multimodal sensation. Mater. Horiz. 2020 (7): 477–488.

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5 Recent Progress on Heat-Set Molecular Gels Yuangang Li, Zonglin Yang, Yong Chen, Huajing Li, Rong Yang and Chenyu Huang College of Chemistry and Chemical Engineering, Xi’an University of Science and Technology, Xi’an, China

5.1 Introduction “A gel is a solid jelly-like material that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state.” [1] Gels are a common class of substances in daily life, such as jelly, hair gel, and toothpaste. Although we can cite so many examples of gels, we cannot give it a very strict and satisfactory definition. Just as the researcher Dorothy Jordon Lloyd in the early twentieth century, once described this: “Gel as a colloidal state, although it is easy to identify it, it is difficult to define it…” [2] Although there are many academic disputes on the definition of gel, its importance in academic research and its contribution to human life are unquestionable. As for gels, they can be roughly classified according to the source of raw materials, chemical composition, type of solvent, cross-linking of fiber networks, etc. [3]. Figure 5.1 shows their classifications. According to the source, they can be divided into natural gel and artificial gel. According to the continuous medium, they can be divided into organic gel, hydrogel, and aerogel. According to the chemical compositions of the gel forming factors, they can be divided into supramolecular gels (again small molecule gels) and polymer gels. Strictly speaking, some gels are composed of inorganic particles. The bonding interactions in the gels can be either physical or chemical. Physical cross-linking includes hydrogen bonding, hydrophobic interaction, interchain cross-linking interaction, and partly crystal formation. Although the physical cross-links are not permanent, they are strong enough to link long-span segments together, thereby affecting the mechanical response of the gel network to external disturbances applied. The life span of physical bonds usually depends on temperature and other thermodynamic variables, making these systems thermally reversible and self-healing [4]. The chemical gels are formed by cross-linking the chains with covalent bonds, which are not reversible, and we need to use various chemical principles to make the chemical gels reversible. Supramolecular Gels: Materials and Emerging Applications, First Edition. Edited by Tifeng Jiao. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

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5 Recent Progress on Heat-Set Molecular Gels

Gels Source

Natural

Medium

Artificial

Organo

Hydro

Figure 5.1 Classification of gels. Source: Sangeetha and Maitra [3]. © 2005, Royal Society of Chemistry.

Aero/xero

Constitution

Supramolecular

Physical

Macromolecular Crosslinking

Chemical

The molecular gels are colloidal substances produced by combining small molecules with a solvent (either water or organic solvent) that has self-curing properties. Due to the interaction between the solvent and the solute, it has a certain degree of thermal reversibility, which is the well-known phenomenon of dissolving upon heating and solidification upon cooling. The process of molecular gel formation is accompanied by the self-assembly of the low-molecular weight gelators (LMWGs). Molecular self-assembly is a ubiquitous process in nature, and also plays a vital role in the generation, which maintains the development of life [5]. Molecular self-assembly is also an important branch of supramolecular chemistry. Molecular self-assembly emphasizes the spontaneous aggregation behavior of supramolecular units. Many biological events, such as protein folding and the formation of DNA double helix structure, which are typical self-assembly behaviors with hydrogen bonding interaction, and their process, structural complexity, function, and information storage all play a crucial role in the evolution of organisms [6]. Heat-set molecular gels are a class of small molecule self-assembly systems with anomalous thermal response [7–9] and reversible phase transition behavior. Interestingly, this is a smart gel characterized by its abnormal phase behavior with a gel formation behavior at a critical high temperature through heating, which is totally contrary to the normal molecular gels with a gel formation behavior through cooling [10–12]. Due to its special thermal response phase transition characteristics, heat-set molecular gels have special uses that conventional molecular gels do not have, and have potential application values in the fields of tissue engineering [13, 14], carrier for controlled drug release [15], and detection and removal of pollutants [16]. Heat-set molecular gels are classified according to the solution medium, into organic gels and hydrogels. Hydrogels are different from organic gels in that they are less toxic and nonflammable. Because of their high water content, resilience of similar tissues, easy transportation of nutrients and waste, they have been widely used in biosensing, drug delivery, and tissue engineering [17]. According to the structure of the gel classification, in addition to heat-set molecular (small molecule) gels, there is a category of heat-set polymer (macromolecular) gels. Although most natural gels are formed by macromolecular compounds (such as starch, cellulose, and

5.2 Heat-Set Molecular Gels

chitosan), these substances cannot meet the development needs of human society. Some synthetic polymer gels have emerged, for instance, PEG [18], PNIPAAM [19], PAAm [20], PAA [21], and PVA [22]. These materials of polymer gels can be used to protect electrochemical systems [23, 24], smart wearable devices [25, 26], soft robots [27] and so on. In previous work by Prof. Jeong [28] and Prof. Loh [29], polymer-based thermo-gels have been reviewed in detail from various systems and will not be discussed in this chapter. This article will briefly summarize the research progress of heat-set molecular gels and their extended applications.

5.2 Heat-Set Molecular Gels The formation of heat-set gels is due to the heat-induced self-assembly of small molecules in a solvent to induce the formation of a three-dimensional network structure, causing the solvent to lose its own fluidity. Due to its total non-covalent nature of the heat-induced self-assembly, the molecular heat-set gel can return to solution state when the temperature decreases lower than the critical point while the polymeric heat-set gels cannot have such reversible behavior. Since the first heat-set molecular gel system was found by Professor Kimizuka’s group [9] in 2004, more and more examples of heat-set molecular gels have appeared based on various kinds of gelators and mechanisms. In the following, I will describe the progress of research on the heat-set properties of gels in terms of both hydrogels and organogels according to the solvent systems.

5.2.1

Heat-Set Molecular Hydrogel

There are two limitations in the exploration of LCST-based supramolecular hydrogels based on LMWG (low-molecular weight gelators): (i) it is difficult to design small molecules with both gelation and LCST capacity and (ii) the gelation process of LMWG is fraught with ambiguities when LCST behaviors coexist. The model was studied in depth by the Zheng and Dong research groups [30]. The design idea is to choose amphiphilic structures because their hydrophilic peripheral parts can interact with water molecules to achieve LCST behavior, and the hydrophobic core of the gelling agent remains insulated from water molecules and assembles to form gels without interference from water molecules. Selecting the appropriate heat-responsive chain segments allows the preparation of LMWG-based LCST-type supramolecular hydrogels. The monomer design is shown in Figure 5.2; the benzo-21-crown-7 (B21C7) derivatives are a new class of LCST molecules that can be used in different systems to promote LCST behavior [31]. Amphiphilic monomers containing two B21C7 units were designed and synthesized by amidation reaction using the corresponding benzodicarboxylic acid (MF) as raw material. The hydrophilic unit B21C7 in the outer layer responds to temperature and exhibits LCST phase behavior, while the hydrophobic tetrafluorobenzene in the inner part controls the aggregation of the gelling agent through F–F interactions, 𝜋–𝜋 stacking, and hydrophobic interactions, which in turn leads to supramolecular gelation.

101

102

5 Recent Progress on Heat-Set Molecular Gels

F

O

O

F

N H

F

R

R

N H

F O

R

N H

O

MF

R O

O

R MH

NH

O

O

O

R=

N H

N H

O O B21C7

O

O

O

N H

R

R TC

(a)

Self-assemble > Tcloud

MF monomers

No gelation

Aggregated MF species

< Tcloud

< Tcloud

> Tcloud

Self-assemble

Self-assemble > Tcloud

The formation of hydrogel

(b)

Macroscopic MF species = MF

= water

Figure 5.2 (a) Chemical structures of LMWG MF and model compounds (TC and MH) and (b) LCST-induced supramolecular gelation: gelation of MF is only realized when the gelation temperature is above the critical transition temperature (T cloud ) of LCST behavior. Source: Wu et al. [30]. © 2020, American Chemical Society.

5.2 Heat-Set Molecular Gels

Figure 5.2 shows the process of gel formation of MF monomers where aggregated MF clusters are formed by self-assembly when the temperature is higher than T cloud . Keeping the temperature above T cloud , the self-assembly behavior remains and eventually macroscopic MF aggregates are formed, followed by MF hydrogel formation. When the temperature is lower than T cloud , there is no formation of macroscopic MF aggregates and MF hydrogels. The highlight of this study is the observation of different types of MF aggregates during LCST-induced gelation. The LCST behavior of this supramolecular hydrogel system plays a key role in the gel formation process by forming the macroscopic backbone of the hydrogel, which is quite different from that of thermal polymer hydrogel systems. Similar to the previously mentioned crown ethers, Huang et al. [32] reported a compound with five paired benzene rod imidazole bonds laterally grafted to dendritic oligomeric ether chains. In contrast to conventional gels, this compound can form an anisotropic opaque gel that can be reversibly transformed into a transparent solution upon cooling. In their work, they have designed a supramolecular nanofiber with a vector substructure that can be formed in aqueous solution, arranged in parallel to each other to form a vector-like network (Figure 5.3). Thus, when the solution is heated to a physiological temperature, the nanofiber solution can mix with the cells at room temperature and transform into a gel, allowing the cells to be encapsulated in a 3D environment, thus creating a fine environment that mimics the growth of cells and tissues in the body [33, 34]. Hydrogel can be formed because it has a balance between hydrophilic and hydrophobic interactions in the solvent (water), reasonable regulation of this balance can promote or inhibit the formation of colloids, which can be achieved by rational molecular design. The Hamachi group has developed a bola-shaped amphiphilic compound and demonstrated its flexible molecular design. The retro-D-A reaction was used as a switch to delicately control the hydrophilic-hydrophobic balance to cause the formation of heat-set hydrogels [35]. The schematic diagram of forming a heat-set gel is shown in Figure 5.4. The bola-shaped amphiphilic compound (also known as a precursor of hydrogels) contains two hydrophilic head groups (light blue and dark blue), a long-chain hydrophobic group (white), and a cleavable site (red and green). The cleavable site is composed of furan and maleimide, and this part can be cleaved by retro-D-A reaction. This bola-shaped amphiphilic molecule can form a two-dimensional nanostructure through the self-assembly process in water. At low temperature, no gel appears, but the magic is that given a certain thermal stimulation to the system, a dense and stable gel is formed. It is because of the split of the cleavable site at high temperature, the part with furan and hydrophilic head is peeled off from precursor and released into the solution, which caused the shift of hydrophilic-hydrophobic balance of the system. Then the original two-dimensional nanostructure is cross-linked to form a dense three-dimensional network structure, thus forming the gel. The molecular design of the bola-shaped amphiphilic molecule is shown in Figure 5.5, which differs from their previous research [36] in which the molecular

103

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5 Recent Progress on Heat-Set Molecular Gels

O O

O O O

O

O

N

O

O O

N H

O

O

O

O

O

O N H

O

O

O

N

O

O O

O

O

O

O

O

O

O

O

O

O

O O

2

1 (a)

9 nm

Self-assembly

(b)

Side view

Top view

Heating Cooling

(c)

Figure 5.3 Molecular structure and self-assembly. (a) Molecular structure of laterally grafted rod amphiphiles. (b) Schematic representation of the molecular packing arrangements of the nanofibers. (c) Schematic representation of a reversible isotropic sol–nematic gel phase transition of supramolecular nanofibers. Source: Huang et al. [32]. © 2011, Springer Nature.

design introduces the biphenylalanine (BPh) and the furan-modified peptide derivative of p-phenylenediamine (FurTpa) as its hydrophobic N-terminal. In order to convert this hydrogel containing FurTpa units into water-soluble amphiphiles (hydrogel precursors), polyethylene glycol-maleimide 2 was introduced. Therefore, the furan unit is attached to the hydrogelator, and the maleimide unit is attached to the “removable” part, the structure of the “removable” part is opposite to that of glycolipid-based 3.4-endo amphiphiles. After structural modification, the gelation efficiency is greatly improved, as shown in Figure 5.6. The speed of gel formation from 1.2-endo increases with the concentration of the gelator and has

5.2 Heat-Set Molecular Gels

Hydrogelator

Cleavable site

Hydrophobic

N

Hydrophilic

𝛥 r-DA

O

Hydrophilic

O N

O

DA

O

Cycloadduct

O

+

O

Maleimide

Furan

Bolaamphiphile (hydrogelator precursor) Self-assembly Heat stimuli r-DA Sol 2D nanostructure

Gel 3D network of 1D nanofiber

Figure 5.4 Schematic representation showing heat-set supramolecular hydrogelation of bola-amphiphiles via retro-Diels–Alder (r-DA) reaction. Source: Ochi et al. [35]. © 2014, Royal Society of Chemistry.

a higher gelation efficiency than the gelator 3.4-endo. In their publication, it was also mentioned that the newly developed gelator 1.2-endo with a very low (0.06 wt%/1.0 mM) CGC and the gel forming temperature can reach as high as 92 ∘ C. This bola-amphiphilic gelator with flexible molecular design provides a novel idea for the development of heat-set gels in the future. Since the retro-D-A reaction requires a special catalyst, the limitation is that the achieved heat-set gels are not revisable, that is, the gel cannot return into solution when the temperature is decreased. Charge transfer (CT) interactions are another strategy often used to construct multifunctional supramolecular assemblies. Using CT interactions as a supramolecular tool to prepare two-component gels is an amazing task. In the research of Bhattacharjee’s group, they reported for the first time that a small molecule thermally solidified hydrogel induced by charge transfer [37]. Its molecular design is shown in Figure 5.7a. Its design inspiration was derived from the study of nine different charge transfer (CT) pairs in the report of the Wilson research group. The correlation constant between pyrene and NDI-A was the highest [38]. The CT interactions between Py-D and NDI-A and the self-assembly characteristics of the resulting CT complex were investigated. The CT interaction between Py-D and NDI-A is confirmed by the color change of the DMF solution from yellow (for Py-D) to dark purple (for CT complex) (Figure 5.8a). At room temperature, when water is added to Py-D: NDI-A = 1 : 1 DMF solution (volume ratio H2 O: DMF = 99:1), a transparent hydrogel can be formed instantly, with [Py-D] CGC = 1.82 mM. Although there are acid-sensitive acyl units in Py-D, the hydrogel still has a stable period of several weeks. UV-Vis spectroscopy studies with Py-D:NDI-A = 1 : 1 in the volume ratio H2 O: DMF = 99 : 1 showed that a

105

O

N

N H

n

FurTpa

O H

O O

O

O

O

H HN

N H

O

r-DA

O

H N

OH

O

DA

O

H N

1•2-endo

O

O

n

N H

N O

2

FurTpa-BPh-F (1) (Hydrogelator)

O OH

O N H

HO

O H

O

N 9

O

O

H N

OH r-DA

O O

H

3•4-endo

O

O

O

(b)

O

O O

OH +

HO

OH HO HO

O

O

O (a)

H N

N H

O O HO

O O H

DA

HO HO

O OH

O

O N H

O

O +

N 9

O

O

O

OH O

NH

O

O O

3 (Hydrogelator)

4

HO

Figure 5.5 Molecular structure of (a) bola-amphiphile 1.2-endo constructed from a peptide-based hydrogelator 1 bearing a furan and a water-soluble PEG-maleimide 2 through D-A reaction, and (b) bola-amphiphile 3.4-endo [36] constructed from a glycolipid-based hydrogelator bearing a maleimide 3 and a water-soluble furan 4. Source: Ochi et al. [35].

5.2 Heat-Set Molecular Gels

6 Gelation time (h)

Figure 5.6 Initial concentration dependence on gelation time for 1.2-endo and 3.4-endo (heating at 60 ∘ C). Source: Ochi et al. [35]. © 2014, Royal Society of Chemistry.

3 • 4-endo

5 4 1 • 2-endo

3 2 1 0

9

18

36

18

Concentration (mM)

O H NN

O

O O O

O O O

O O O

Py-D (Donor)

(a)

O HN N

O O

N

OO

OO N NH

N

O

NDI-A (Acceptor)

N

N

(c) O C

(b)

(d)

10 μm

Figure 5.7 (a) Structures of the Py-D (donor) and NDI-A (acceptor). (b) X-ray single-crystal structure of NDI-A along the a-axis showing the hydrogen bonding and slipped stack 𝜋–𝜋 stacking interactions. (c and d) Photograph of a 1 : 2 NDI-A: AgOTf metallogel ([NDI-A] = 25 mM) in 2 : 1 v/v DMSO: H2 O and its morphology under SEM, respectively. Source: Reproduced with permission from [37]. © 2016, RSC Publishing. DOI: doi.org/10.1039/C6NR01128D.

strong CT band appeared with maximum absorption at 555 nm (Figure 5.8b). As the concentration of the CT complex decreases, the CT band gradually disappears, indicating that the decrease in the concentration of the complex will result in the loss of the thermal curing properties of the hydrogel. In order to develop a nanocomposite gel with one-dimensional coordination polymer and nanoclusters, Wu and Cao and their colleagues [39] conducted in-depth experiments and research, then evaluated and discussed the properties of this nanocomposite gel. The design idea is to add coordination metals and ligands to the aqueous solution of [Na(L)(H2 O)]⋅2H2 O (HL = 4,6-bis(2-pyridyl)-1,3,5-triazin-2-ol) and Cu2 (OAc)4 ⋅2H2 O at a ratio of Cu/L = 4 (5.6 wt% gel concentration). Thus,

107

5 Recent Progress on Heat-Set Molecular Gels [Py-D] = 0.75 mM

0.5

Py-D

1:1

NDI-A

Py-D:NDI-A

0.4 Absorbance

108

0.3

0.1 mM

0.2 0.1 0.0 300

(b)

(a)

400

500

600

700

Wavelength (nm)

Figure 5.8 (a) Evolution of intense-violet colored solution (for the CT complex) as a result of mixing of Py-D and NDI-A solution in DMF. Source: Reproduced with permission from [37]. © 2016, RSC Publishing. DOI: doi.org/10.1039/C6NR01128D. (b) Concentration-dependent UV-Vis study of 1 : 1 Py-D: NDI-A in 99 : 1 (v/v) H2 O: DMF. Source: Bhattacharjee et al. [37]. © 2016, Royal Society of Chemistry.

a one-dimensional coordination polymer [Cu2 L(m-OAc)2 (m-OH)] (1) and coordination nanoclusters are obtained ([Cu9 L4 (OAc)7 (OH)5 ])2 +2 (3). Nanocluster 3 stabilizes the one-dimensional coordination polymer, inhibits the crystallization of 1 in the system, and forms a thermally solidified nanocomposite hydrogel based on coordination at room temperature and higher temperatures. After cooling to 4 ∘ C, the system appears as a light green transparent liquid, which is in contrast to most supramolecular gels. T gel in this system can be tuned by changing the pH of hydrogel, concentration of gelators, aging time, and coordination ratio. First, the thermal response and pH response of the gel were tested. The gel becomes a solution when cooled below 4 ∘ C (as shown in Figure 5.9), instead of the traditional heat-dissolving gel, which indicates that it is a heat-set gel. The gel-to-solution transition is reversible upon temperature changes. It is worth noting that the solution is stable at low temperatures, unlike the metastable gel [40]. This indicates that the stability of the gel largely depends on the Cu/L ratio and the concentration of the gelling agent. Secondly, the pH response of the gel is reversible.

R.T.

NH3 • H2O

(a) Heat

Cool

R.T.

HOAc (c) Cool

–4°C

(b)

Heat 60°C

(d)

Figure 5.9 Stimuli-responsive behaviors and phase transitions of the metallogel. (a) Green gel at room temperature in the alkaline state, cooling to −4∘ C results in (b) green sol; (c) conversion to blue sol at room temperature by conditioning the system to acid with acetic acid; (d) formation of a green gel by heating to 60∘ C. Source: Reproduced with permission from [39], © 2012, RSC Publishing. DOI: 10.1039/C2RA22549B.

5.2 Heat-Set Molecular Gels

35

60 50

Gel

25

Tgel /°C

Tgel /°C

30

20 15

Sol

10 0 2

4 6 8 HOAc (μL)

10

Sol 3

12

4 5 6 Gelator (wt%)

(b)

7

40

40 30

30 Gel

20

Tgel /°C

Tgel /°C

20

5 0

Gel

30

10

(a)

10

Gel

20 10

0

Sol

Sol 0

–10 0 (c)

40

1 2 3 4 Ageing time / day

5

4 (d)

6

8

10 12 Cu/L

14

16

Figure 5.10 Phase diagram plotted in terms of T gel vs. various factors. (a) The addition of HOAc (Cu/L = 4, 5.6 wt%, one-day-aged); (b) the change of gelator (Cu/L = 4, one-day-aged); (c) the aged time (Cu/L = 4, 4.2 wt%); (d) the change of the molar ratio of Cu/L (4.2 wt%, one-day-aged). Source: Wu et al. [39]. © 2012, Royal Society of Chemistry.

When HOAc was added to the gel at room temperature, it became a solution. When 25% NH3 ⋅H2 O was added, the reverse process was observed. Interestingly, the acidic solution gelled again after heating at 60 ∘ C, and the gels can be reverse fluidized after cooling to room temperature. This fact indicates that the stability of the gel also depends on the pH value. One can use the “drop ball method” to quantitatively evaluate its properties [41]. As the pH value of the experimental environment decreases, the T gel value (sol-gel transition temperature) gradually increases, indicating that the thermal stability of the gel can be adjusted by the pH value. The T gel value of the gel decreases with increase in gel concentration. This behavior illustrates that the stability of the gel network increases with gel concentration increase. The change rule of T gel value with aging time is similar to the influence rule of gel concentration on T gel value, which shows that aging seems to significantly improve the thermal stability of the gel. Interestingly, the gel is enhanced, as the Cu/L molar ratio increases from four to seven, but further increases in the Cu/L molar ratio destabilize the gel, which shows that the proper Cu/L ratio plays a vital role in the process of gel stabilization (Figure 5.10). In summary, a coordination-based nanocomposite heat-set hydrogel was demonstrated, in which nanoclusters (3) stabilized the one-dimensional coordination polymer (1) and suppressed the crystallization of 1. Hydrogels respond to a variety of stimuli including heat, pH, and concentration. It proposes a nanocomposite gel formation mechanism, explaining the rare properties of low viscosity at low

109

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temperature and gel formation at high temperature. The findings of this study provide a new idea for the development of nano functional materials.

5.2.2

Heat-Set Organic Gel

Because heat-set supramolecular gels have a good response to heat and the phenomenon of thermal curation may appear after thermal stimulation, they have attracted the interest of researchers. Since 2004, a large number of research reports on heat-set gels have been published. Keita Kuroiwa and Tomoko Shibata published the first example of a heat-set molecular gel in 2004 [9]. It was reported that heat-set organic gels were formed due to the different spatial configurations of transition metal complexes, introducing a solvent-soluble dodecyloxypropyl chain into the side chain of the bridging ligand 1,2,4-triazole. To ensure its solubility, adding chloroform at room temperature will form a blue gel-like phase, and when the temperature decreases to 0 ∘ C, a pale pink solution will be formed, as shown in Figure 5.11. The mutual conversion between the octahedral coordination configuration (Oh ) and the tetrahedral coordination configuration (Td ) illustrates the formation mechanism of network of the heat-set gels. The Gibbs free energy is temperature dependent according to the (DSC) differential scanning calorimetry analysis (Figure 5.12). It can be seen that the formation of the gel network as a whole is driven by enthalpy. The exothermic transition during heating is due to the higher thermodynamic stability of the tetrahedral complexes in solution than the octahedral complexes. The octahedral complex in the low-temperature solution on the solid line at 25 ∘ C is transformed into a tetrahedral complex with a gel-like state. This study describes the first example of a thermally reversible, heat-set gel-like network in an organic medium. The lipophilic modification of the bridging ligand provides another strategy for the electrostatic binding of lipid molecules to their oppositely charged complexes reported previously [42, 43]. The introduction of ether groups in the 4-alkylated triazole Co(II) complex is very important because it promotes the formation of a well-developed gel-like network in chloroform. This process is reversible, Figure 5.11 Pictures of Co(1)3 Cl2 in chloroform: (a) a blue gel-like phase at 25 ∘ C; (b) a pale pink solution at 0 ∘ C. Source: Reproduced with permission from [9]. © 2004, American Chemical Society. DOI:doi.org/10.1021/ja037847q. (a)

(b)

5.2 Heat-Set Molecular Gels

Heating scan

2nd scan

3rd scan 0 (a)

10

20

30

Temperature / °C

40

Gibbs free energy

Exothemic

1st scan

Solution state (Oh)

50

Metastable phase Exothermic Gel-like state (Td)

Crystal

0 (b)

25 40

Temperature / °C

Figure 5.12 (a) DSC thermograms of Co(1)3 Cl2 ; (b) schematic temperature dependence of Gibbs free energy for Co(1)3 Cl2 in chloroform. Source: Kuroiwa and Shibata [9]. © 2004, American Chemical Society.

and the heat-set transition (from the Oh complex in solution to the gel-like Td complex) is driven by enthalpy. These characteristics are clearly different from the traditional organic gel that dissolves upon heating. In addition, it also contrasts with the heat-set polymer hydrogel driven by entropy. Therefore, the amphiphilic one-dimensional coordination system provides unique self-assembly properties that are not available in traditional inorganic or polymer chemistry. With the pavement of the pioneer work, the study of metal-organic heat-set gels continues to advance. In order to further functionalize the gel and make it more responsive to stimuli (more obviously thermal stimuli and curing), Hatten’s research group [44] carried out further research on the gelation reaction of transition metal coordination. It used a special equilibrium formula to explain the mechanism of gel formation. As demonstrated in Figure 5.13, based on the ability of CuI to stabilize the imine ligands, it is inferred that the CuI template can weave linear 1,4-diaminobenzene and 4,4′ -dimethyl-3,3′ -bispyridine subcomponent to form polymer 1 [45]. And this polymer 1 happens to be the key compound to form a heat-set gel balance. The amino group of 1,4-diaminobenzene may have more nucleophilic than the terminal amino group of compound 2 (as shown in Figure 5.14). Therefore, the molecule has a more electron-rich nature, resulting in longer oligomers being more unstable than compound 2 in the presence of free 1,4-diaminobenzene. The formation of longer oligomers at the expense of compound 2 will require the release of CuI . This CuI will not be able to obtain a coordinated saturation state with two phosphines and two imine ligands (this is a particularly stable coordination configuration for CuI ). They attribute the sol-gel transition to the formation of CuI N4 cross-links because the equilibrium 2[CuI N2 P2 ] ↔ [CuI N4 ] + [CuPn ]+ + (4 − n) P shifts to the right at higher temperatures. Figure 5.15 shows that oligomer 1 appears as a solution in the NMR tube at 20 ∘ C, and changes into a solid-like gel state attached to the NMR tube when it is heated to 140 ∘ C. The explanation for the formation mechanism of this heat-set gel is that each [CuN4 ]+ cross-link must come from the reaction between two [CuN2 P2 ]+ units on different chains, and one of the [Cu(P)4 ]+ units, which is released into the

111

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5 Recent Progress on Heat-Set Molecular Gels

P

2n

2

N

2 2

Cu+ N

NH2

P(C8H17)3 P

P(C8H17)3

Cu P

n (C H ) P N NH2 n CuBF4 8 17 3 Cu+ O DMSO (C8H17)3P N

P Cu P

N

P Cu

n

N

P(C8H17)3

Cu+

N

N

P(C8H17)3

n

O

P

1

Figure 5.13 The preparation of conjugated metal-organic polymer 1 from subcomponents (left) and its cartoon representation (right). Source: de Hatten et al. [44]. © 2011, American Chemical Society. (C8H17)3P

P(C8H17)3 +

Cu N H2N

N

N

NH2

Figure 5.14 Diagram of the structure of model compound 2. Source: de Hatten et al. [44].

N Cu+

(C8H17)3P

P(C8H17)3

2

solution. According to the observation, [CuN2 P2 ]+ does not tend to undergo ligand exchange to produce [CuN4 ]+ and [CuP4 ]+ . So, it can be concluded that [CuN4 ]+ is cross-linked between the chains of oligomeric 1. It is enthalpically unfavorable, but due to the increase in entropy, the equilibrium [CuP4 ]+ ↔ [CuPn ]+ + (4 − n) P shifts to the right with increasing temperature (at higher temperatures) [46–48], forming a dense fibrous network structure, which leads to the formation of gels. In a report on the temperature- and voltage-induced weight allocation of a dynamic electroluminescent metal polymer, Friend research group [49] reported that a polymer containing dynamic covalent metals was synthesized by condensing linear diamine and dialdehyde subcomponents around the copper (I) template in the presence of a bisphosphine ligand. In the solution, the red polymer undergoes a sol-gel transition when heated, thereby forming a yellow gel. This process can be reversible or irreversible depending on the solvent used. Similar to the self-assembly process of a dynamic covalent metal polymer reported previously [50], it undergoes reversible structural rearrangement in response to temperature changes in solution and solid state electric field, changing its mechanical and photophysical

5.2 Heat-Set Molecular Gels

Figure 5.15 Schematic representation of the gelation mechanism of 1(left); photographs of inverted NMR tubes showing 1 in solution (top right) and following the sol-gel transition (bottom right). Source: Reproduced with permission from [44], © 2011, American Chemical Society. DOI: doi.org/10.1021/ja110575s.

20°C

140°C

O

O P

2n

P

P 2n CuBF4

O

N

N

C2H4Cl2 n

N

N

O NH2

N n

N Cu

P H2N

P

Cu+

+

P

1

O

n

Figure 5.16

Synthesis of polymer 1 from subcomponents. Source: Asil et al. [49].

properties. The polymer is formed by self-assembly of subcomponents, wherein the condensation of aromatic amine and pyridyl subcomponents is driven by the preferential coordination of the resulting imines with metal ions [51]. The selection of its subcomponents and the synthesis idea are shown in Figure 5.16. In solution, the polymer exhibits thermochromism [52] and heat-set gel formation [44]. Depending on the solvent used, the gel formation may be reversible or irreversible. In addition, the polymer is electroluminescent and can be made into a light-emitting electrochemical cell (LECs), the simplest type of light-emitting device formed by clamping a conjugated polymer containing ionic conductive material between two electrodes. This provides an idea for the versatility of application for soft materials. In the research of the Wei and Pan research group, photoinduced and thermo-induced gels with ultraviolet and visible light responses were designed, [53] to continue to research the response of heat-set gels to light stimuli. In this study, photochromic metal-organic gels (MOGs) were prepared by incorporating

113

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5 Recent Progress on Heat-Set Molecular Gels

photosensitive reactive molecules into organic ligands. Surprisingly, the resulting MOG exhibits the unusual characteristics of supramolecular gels, namely a rare reversible gel-sol transition. Moreover, the gel can be formed at a very low concentration of gelling agent (0.01 mol L−1 ), and it reacts significantly to weakly coordinated and uncoordinated anions. The transition time from gel to solution depends on the heating time required for gelation, thereby generating a heating memory effect [54]. In addition, the emission characteristics of the gel are easily adjusted by ultraviolet and visible light. The gelation study was carried out with a dicarboxylic acid DCBTF6 , which contains a DAE (diarylethylene) fragment that can undergo a reversible photochromic reaction under ultraviolet and visible light irradiation [55, 56]. As shown in Figure 5.17, the design molecule DCBTF6 has two structures, closed form (C-DCBTF6 ) and open form (O-DCBTF6 ). The structure can be controlled by visible light irradiation and ultraviolet [57] light irradiation as “switches.” When the molecule maintains the open state, the appropriate L/M (ligand/coordination metal) can be controlled by adding Al3+ to form an o-solution in an open state. The closed state of the sol phase (C-solution) can be formed by ultraviolet light. At this moment, the unique heat-set properties of the system appear, and by giving it an appropriate F F F F

F F F F

O

Closed form

HO

O S

S

F

F

VIS OH

F

F

O

UV

HO

C-DCBTF6

O S

S

OH

O-DCBTF6

Open form

Al3+ cluster RT

UV 80 °C C-solution

UV C-gel

VIS

O-gel

VIS

RT 80 °C

O-solution

Accelerating Non-coordinating anions (BF4–, PF6–) and water

Figure 5.17 Reversible photoisomerization between the open- and closed-ring forms of the photochromic dicarboxylic acid ligand DCBTF6 , and schematic representation of multiple transformations among gels and solutions in both open and closed forms. Source: Wei et al. [53]. © 2014, John Wiley & Sons.

5.2 Heat-Set Molecular Gels

thermal stimulation (80 ∘ C is the sol-gel transition temperature) a dense gel can be obtained. When the environment returns to room temperature and is given irradiation with visible light, the molecule will return to its open state and sol state. The development of this part of the research has opened up new ideas for the functionalization of supramolecular gels. Skirt cyclodextrins (CDs, including 𝛼-, 𝛽- and 𝛾-CDs) play a very important role in the study of host–guest supramolecular interactions as important supramolecular [58] hosts. The hydrophobic and hydrophilic surfaces can be used to complex with various organic guest fractions via supramolecular interactions [59]. Li et al. reported that heat-set organogels can be formed through a host–guest supramolecular interaction [57, 60],which is the first report of reversible heat-set organogels based on 𝛽-cyclodextrin (𝛽-CD) supramolecular interactions. The idea is to prepare gels by reacting diphenylamine (DPA) with 𝛽-cyclodextrin and lithium chloride in N,N-dimethylformamide (DMF). In this gel system, DPA can be gelatinized in DMF with increasing temperature and then dissolved again with decreasing temperature, thus achieving a reversible sol-gel transition. The introduction of the guest molecules affects the formation of the gel system (as shown in Table 5.1). In all cases, when the guest molecules were introduced into the gel system, the temperature of the heat-set gel decreased, which means that the gel was more likely to form. In particular, guest molecules such as naphthalene and 1-aminoanthraquinone were unable to form gels due to their size being close to the cavity of 𝛽-CD. The explanation for this is that if the size of the guest molecules is appropriate for the cavity of 𝛽-CD, they may prevent supramolecular interactions between 𝛽-CD and other molecules (e.g. DPA) that promote gel formation. Relatively small guest molecules, such as NaCl, KCl, CaCl2 , and MgCl2 , do not prevent supramolecular interactions between 𝛽-CD and DPA, or even complexation with 𝛽-CD, and thus do not prevent gel formation. Research on 𝛽-cyclodextrin-based heat-set gels is still ongoing. There is one such report from Hou et al. on the modification of heat-cured gels, and the article mentions how the conversion from heat-cured organic gels to room temperature-cured organic gels was achieved [58]. The system contains 𝛽-cyclodextrin (𝛽-CD), 4,4′ -isopropylbisphenol (BPA), LiCl, and N,N-dimethylacetamide (DMAc), which can be mixed in suitable proportions to form a white organogel, and can be converted to a transparent solution within five hours after cooling to ambient temperature; the process can be repeated many times, indicating that the heat-set organogel is totally reversible. However, unlike conventional heat-set gels, room temperature organic gels can be obtained by adding ethanol (EtOH) to them at room temperature, even without heating. In the formation of organic gels at room temperature, EtOH can be considered as a cosolvent that may be involved in the molecular self-assembly process [61]. The different solubility of this gel in different solvents at different temperatures leads to its heat-curing properties in the high-temperature DMAc and DMF systems. When the solvent was kept constant, the T gel of these systems followed this order due to 𝜋–𝜋 stacking and electronic effects of the guest molecules: BPA < bisphenol F < phenol < p-chlorophenol < p-nitrophenol.

115

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5 Recent Progress on Heat-Set Molecular Gels

Table 5.1

Results of adding guest molecules to the gel. T gel /∘ C

𝚫 T gel /∘ C

No guest

120

0

Benzene

99

21

Guest molecules (0.5 mmol)

Molecular formula

Toluene

CH3

108

12

Chlorobenzene

Cl

111

9

o-Nitrochlorobenzene

CH3

94

26

99

21

99

21

o-Xylene

m-Xylene

CH3

NO2

CH3

CH3

CH3

Dodecyl alcohol

CH3 (CH2 )11 OH

92

28

Tetradecyl alcohol

CH3 (CH2 )13 OH

89

31

Octadecanol

CH3 (CH2 )17 OH

95

25

n-Butylamine

CH3 (CH2 )3 NH2

111

9

Hexylamine

CH3 (CH2 )5 NH2

116

4

Decylamine

CH3 (CH2 )9 NH2

110

10

Dodecylamine

CH3 (CH2 )11 NH2

116

4

Tetradecylamine

CH3 (CH2 )13 NH2

101

19

Hexadecylamine

CH3 (CH2 )15 NH2

116

4

Octadecylamine

CH3 (CH2 )17 NH2

110

10

Sodium chloride

NaCl

105

15

Potassium chloride

KCl

110

10

Calcium chloride

CaCl2

102

18

Magnesium chloride

MgCl2

111

9

118

2

118

2

Cyclohexane

Acetic acid

CH3 COOH

(continued)

5.2 Heat-Set Molecular Gels

Table 5.1

(Continued)

Guest molecules (0.5 mmol)

Molecular formula

Anthranilic acid

COOH NH2

T gel /∘ C

𝚫 T gel /∘ C

110

10

Stearic acid

CH3 (CH2 )16 COOH

84

36

Heptylic acid

CH3 (CH2 )5 COOH

118

2

99

21

Amoxicillin

O O

HO

O

N S

1-aminoanthraquinone

N H

H

O

OH H2N

No gel

NH2

O

Naphthalene

No gel

Piroxicam

No gel

F

N

N

F OH N N

F

N

Voriconazole

OH

N O

S

No gel

O N H

N

O

Source: Li et al. [60]. © 2010, American Chemical Society.

Both metallic gel systems and host–guest gel systems have their own formation mechanisms. Many molecular and supramolecular properties are employed in the formation of heat-set gels, and molecular chirality is no exception [62–64]; Zheng’s team first reported a first method for the formation of heat-set gels by the mixing of two components of chiral calix [4] aromatics with long tertiary alkyl chains on the upper edge of the chiral calix [4] aromatics [65]. The compound with S-1-phenylethylamine group on the lower margin (shown on the left in Figure 5.18) can enantioselectivity form heat-set gels and egg vesicles with D-2,3-dibenzoyl tartaric acid in cyclohexane due to differences in the interaction between the two

117

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5 Recent Progress on Heat-Set Molecular Gels

R

R

R

R

60 °C R

O OH

OH

O

H2C

Ph

NH

CH3 Ph H

1b: -C(CH3)2(CH2)9CH3

H2C

CH2 HN

1a: -C(CH3)2(CH2)13CH3 CH2

1

H

CH3

20 °C

1c: -C(CH3)2(CH2)5CH3 1a + L-2

1a + D-2

Figure 5.18 Schematic structure of calix[4]arene 1(left). Gels changed with temperature (right). Source: Reproduced with permission from [65]. © 2007, Royal Society of Chemistry. DOI: doi.org/10.1039/B713548C.

component gels. In addition, the diameter of vesicles decreases as the alkyl length increases (Figure 5.19, right), which can be used to control the size of vesicles. The biggest advantage and characteristic of the two-component gel system over the one-component system is that the structure and properties of the gel nanomaterials and micromaterials can be adjusted by changing the molecular ratio of the two components or by changing one of the two components. Chiral cup [4] arylene diamine 1 shows a conical conformation, which may be due to the intramolecular hydrogen bond between the hydroxyl group at the lower edge and the ether oxygen atom. A solid mixture of 1a and L-2 (L-dibenzoyltartaric acid) was dissolved in cyclohexane by heating, and the resulting solution was cooled to 20 ∘ C to obtain a gel. Performing the same procedure using 1a and D-2 (D-dibenzoyltartaric acid) under the same conditions, the cyclohexane gels of 1a and L-2 turned into solution when heated to 60 ∘ C, but the cyclohexane solution of 1a and D-2 turned into a gel (as shown in Figure 5.19). When the gel formed at high temperature is cooled to about 20 ∘ C, the solution is regenerated after resting for about five minutes, and it remains a solution after resting at 10 ∘ C for a long time. The gel-sol reversible transformation process can be repeated several times with temperature changes and 1a still shows thermally induced gelation after storage at room temperature for more than one year. The thermal curing properties and the extremely long aging time allow this soft material to be used in a very wide range of applications.

5.3 Conclusion and Perspectives In the past two decades, heat-set molecular gels have undergone a progress from the first discovery to the appearance of more and more examples. Up to now, huge improvements have been achieved in this special field. According to the solvent in which the heat-set gels formed, both heat-set organic gels and hydrogels were

(a)

(b)

(c)

(d)

Figure 5.19 (a, b and c) TEM images of gel from (a) 1a and D-2, (b) 1b and D-2, (c) 1c and D-2 (scale bar for a, b and c, 1 mm). (d) AFM images of gel from 1b and D-2 on freshly broken mica (25 × 25 mm). Source: Reproduced with permission from [65]. © 2007, Royal Society of Chemistry. DOI: doi.org/10.1039/B713548C.

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5 Recent Progress on Heat-Set Molecular Gels

explored equally. As far as the structures of the involved gelators was concerned, although most of the reported gelators are metal complex consistent with inorganic metal ions and organic ligands, more and more gelators based on pure oranic content were upcoming. Despite the aforementioned advances that have been achieved, we have to admit that almost all the examples of heat-set molecular gels were from the accident discoveries in the labs. Therefore, achieving a rational design of heat-set molecular gels is undoubtedly the main future direction in this field. Clear physical mechanism of the thermal response in the scale of molecular self-assembly must be considered at the first step of rational design. Secondly, the design and synthesis of molecular gelators, which can maintain the subtle assembly balance and response to the thermal stimuli at the same time, are very important. Fortunately, the tremendous progress achieved in the past decades in the field of molecular gels [9, 66, 67] and the huge database of molecular gelator structures accumulated can provide helpful assistance for the future design of gelators. Although the rational design of heat-set gels is a great challenge for the further development of heat-set molecular gels, some pioneering work [35, 36, 53, 68] has already emerged to pave the way from accidental discovery to rational design of heat-set molecular gels, and it is believed that subsequent researchers will soon conquer this big challenge. Another important aspect of the heat-set molecular gels is their applications in the process of real industry and engineering fields. Due to their special heat-set gel formation property, response to thermal stimuli, and total reverse phase recovery into solution upon cooling, heat-set molecular gels have great potential to be applied in various fields such as tissue engineering materials, controlled drug delivery, 3D printing, and next-generation smart fire extinguisher [28, 29, 69–71]. In order to realize the application of heat-set molecular gels, it is very important to achieve rational design of such systems as a first step. However, the relationship between the molecular structures of gelators and the thermal and (or) mechanical properties also play important role in the way to real applications. The cost of preparing the material is another issue to consider when it is used on a large scale, such as in smart fire extinguishers.

Acknowledgements We greatly appreciate the financial support of the National Natural Science Foundation of China (no. 22072113).

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20 Messing, R., Frickel, N., Belkoura, L. et al. (2011). Cobalt ferrite nanoparticles as multifunctional cross-linkers in PAAm ferrohydrogels. Macromolecules 44: 2990–2999. 21 Xu, J., Wang, Z., You, J. et al. (2020). Polymerization of moldable self-healing hydrogel with liquid metal nanodroplets for flexible strain-sensing devices. Chem. Eng. J. 392: 123788. 22 Liu, M., Lu, X., Gao, L. et al. (2018). Polyvinyl alcohol-based thermogel with tunable gelation and self-healing property. Macromol. Chem. Phys. 219: 1800162. 23 Yang, H., Liu, Z., Chandran, B.K. et al. (2015). Self-protection of electrochemical storage devices via a thermal reversible sol-gel transition. Adv. Mater. 27: 5593–5598. 24 Shi, Y., Ha, H., Al-Sudani, A. et al. (2016). Thermoplastic elastomer-enabled smart electrolyte for thermoresponsive self-protection of electrochemical energy storage devices. Adv. Mater. 28: 7921–7928. 25 Ge, J., Sun, L., Zhang, F.-R. et al. (2016). A stretchable electronic fabric artificial skin with pressure-, lateral strain-, and flexion-sensitive properties. Adv. Mater. 28: 722–728. 26 Feng, J., Peng, L., Wu, C. et al. (2012). Giant moisture responsiveness of VS2 ultrathin nanosheets for novel touchless positioning interface. Adv. Mater. 24: 1969–1674. 27 Shintake, J., Cacucciolo, V., Floreano, D., and Shea, H. (2018). Soft robotic grippers. Adv. Mater.: 1707035. 28 Moon, H.J., Ko du, Y., Park, M.H. et al. (2012). Temperature-responsive compounds as in situ gelling biomedical materials. Chem. Soc. Rev. 41: 4860–4883. 29 Liow, S.S., Dou, Q., Kai, D. et al. (2016). Thermogels: in situ gelling biomaterial. ACS Biomater. Sci. Eng. 2: 295–316. 30 Wu, S., Zhang, Q., Deng, Y. et al. (2020). Assembly pattern of supramolecular hydrogel induced by lower critical solution temperature behavior of low-molecular-weight gelator. J. Am. Chem. Soc. 142: 448–455. 31 Qi, Z., Chiappisi, L., Gong, H. et al. (2018). Ion selectivity in nonpolymeric thermosensitive systems induced by water-attenuated supramolecular recognition. Chemistry 24: 3854–3861. 32 Huang, Z., Lee, H., Lee, E. et al. (2011). Responsive nematic gels from the self-assembly of aqueous nanofibres. Nat. Commun. 2: 459–463. 33 Prestwich, G.D. (2008). Evaluating drug efficacy and toxicology in three dimensions: using synthetic extracellular matrices in drug discovery. Acc. Chem. Res. 41: 139. 34 Zhang, S., Greenfield, M.A., Mata, A. et al. (2010). A self-assembly pathway to aligned monodomain gels. Nat. Mater. 9: 594–601. 35 Ochi, R., Nishida, T., Ikeda, M., and Hamachi, I. (2014). Design of peptide-based bolaamphiphiles exhibiting heat-set hydrogelation via retro-Diels-Alder reaction. J. Mater. Chem. B 2: 1464–1469. 36 Ikeda, M., Ochi, R., Kurita, Y.S. et al. (2012). Heat-induced morphological transformation of supramolecular nanostructures by retro-Diels-Alder reaction. Chem. Eur. J. 18: 13091–13096.

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37 Bhattacharjee, S., Maiti, B., and Bhattacharya, S. (2016). First report of charge-transfer induced heat-set hydrogel. Structural insights and remarkable properties. Nanoscale 8: 11224–11233. 38 Kumar, N.S., Gujrati, M.D., and Wilson, J.N. (2010). Evidence of preferential pi-stacking: a study of intermolecular and intramolecular charge transfer complexes. Chem. Commun. 46: 5464–5466. 39 Wu, J.-J., Cao, M.-L., Zhang, J.-Y., and Ye, B.-H. (2012). A nanocomposite gel based on 1D coordination polymers and nanoclusters reversibly gelate water upon heating. RSC Adv. 2: 12718–12723. 40 Wu, J.-J. and Xue, W. (2011). Temperature-dependent supramolecular isomers of a tetranuclear macrocycle and a zigzag chain based on dicopper building blocks. Cryst. Eng. Comm. 13: 5495–5510. 41 Kato, T. and Sakai, A.T.M. (1980). Melting temperature of thermally reversible gel. VI. Effect of branching on the sol–gel transition of polyethylene gels. Polym. J. 12: 335–341. 42 Kimizuka, N.O. and Kunitake, T. (1998). Supramolecular assemblies comprised of one-dimensional mixed valence platinum complex and anionic amphiphiles in organic media. Chem. Lett. 27: 695–696. 43 Lee, C. and Kimizuka, N. (2002). Solvatochromic nanowires self-assembled from cationic, chloro-bridged linear platinum complexes and anionic amphiphile. Chem. Lett.: 1252–1253. 44 de Hatten, X., Bell, N., Yufa, N. et al. (2011). A dynamic covalent, luminescent metallopolymer that undergoes sol-to-gel transition on temperature rise. J. Am. Chem. Soc. 133: 3158–3164. 45 Nitschke, J. (2004). Mutual stabilization between imine ligands and copper(i) ions in aqueous solution. Angew. Chem. Int. Ed. 116: 3135–3137. 46 Nohra, B., Réau, R., Lescop, C., and Hissler, M. (2008). Organophosphorus-conjugated materials for optoelectronic applications. Phosphorus Sulfur. 183: 253–257. 47 Oliveri, C.G. and Gianneschi, N.C. (2006). Supramolecular allosteric cofacial porphyrin complexes. J. Am. Chem. Soc. 128: 16286–16296. 48 Zhang, Q., Ding, J., Cheng, Y. et al. (2007). Novel heteroleptic CuI complexes with tunable emission color for efficient phosphorescent light-emitting diodes. Adv. Funct. Mater. 17: 2983–2990. 49 Asil, D., Foster, J.A., Patra, A. et al. (2014). Temperature- and voltage-induced ligand rearrangement of a dynamic electroluminescent metallopolymer. Angew. Chem. Int. Ed. 53: 8388–8391. 50 Fan, J., Lal Saha, M., Song, B. et al. (2012). Preparation of a poly-nanocage dynamer: correlating the growth of polymer strands using constitutional dynamic chemistry and heteroleptic aggregation. J. Am. Chem. Soc. 134: 150–153. 51 Hiroshi, D., Kadzuya, H., Seiki, Y. et al. (2009). Back to back twin bowls of D3-symmetric Tris(spiroborate)s for supramolecular chain structures. J. Am. Chem. Soc. 131: 1638–1639.

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52 Hadjoudis, E. and Mavridis, I.M. (2004). Photochromism and thermochromism of Schiff bases in the solid state: structural aspects. Chem. Soc. Rev. 33: 579–588. 53 Wei, C., Pan, M., Li, K. et al. (2014). A multistimuli-responsive photochromic metal-organic gel. Adv. Mater. 26: 2072–2077. 54 Annaka, M., Tokita, M., Tanaka, T. et al. (2000). The gel that memorizes phases. J. Chem. Phys. 112: 471–477. 55 Zhang, J., Jin, J., Zou, L., and Tian, H. (2013). Reversible photo-controllable gels based on bisthienylethene-doped lecithin micelles. Chem. Commun. 49: 9926–9928. 56 Wang, S., Shen, W., Feng, Y., and Tian, H. (2006). A multiple switching bisthienylethene and its photochromic fluorescent organogelator. Chem. Commun.: 1497–1499. 57 Li, Y., Zhao, W., Zhang, H. et al. (2010). Triple-transforming gel prepared by b-cyclodextrin, diphenylamine and lithium chloride in N, N-dimethylacetamide. Chin. Chem. Lett. 21: 1251–1254. 58 Hou, Y., Sun, T., Xin, F. et al. (2013). Transformation from a heat-set organogel to a room-temperature organogel induced by alcohols. J. Incl. Phenom. Macro. 79: 133–140. 59 Hapiot, F., Tilloy, S., and Monflier, E. (2006). Cyclodextrins as supramolecular hosts for organometallic complexes. Chem. Rev. 106: 767–781. 60 Li, Y., Liu, J., Du, G. et al. (2010). Reversible heat-set organogel based on supramolecular interactions of 𝛽-Cyclodextrin in N,N-Dimethylformamide. J. Phys. Chem. B 114: 10321–10326. 61 Zhu, P., Yan, X., Su, Y. et al. (2010). Solvent-induced structural transition of self-assembled dipeptide: from organogels to microcrystals. Chemistry 16: 3176–3183. 62 Komiya, N., Muraoka, T., Iida, M. et al. (2011). Ultrasound-induced emission enhancement based on structure-dependent homo- and heterochiral aggregations of chiral binuclear platinum complexes. J. Am. Chem. Soc. 133: 16054–16061. 63 Chen, X., Huang, Z., Chen, S.-Y. et al. (2010). Enantioselective gel collapsing: a new means of visual chiral sensing. J. Am. Chem. Soc. 132: 7297–7299. 64 Li, Y., Wang, T., and Liu, M. (2007). Gelating-induced supramolecular chirality of achiral porphyrins: chiroptical switch between achiral molecules and chiral assemblies. Soft Matter 3: 1312–1317. 65 Zhou, J., Chen, X., and Zheng, Y. (2007). Heat-set gels and egg-like vesicles using two component gel system based on chiral calix[4]arenes. Chem. Commun.: 5200–5202. 66 Du, X., Zhou, J., Shi, J., and Xu, B. (2015). Supramolecular hydrogelators and hydrogels: From soft matter to molecular biomaterials. Chem. Rev. 115: 13165–13307. 67 Li, Y., Wang, T., and Liu, M. (2007). Ultrasound induced formation of organogel from a glutamic dendron. Tetrahedron 63: 7468–7473.

References

68 Nebot, V.J., Ojeda-Flores, J.J., Smets, J. et al. (2014). Rational design of heat-set and specific-ion-responsive supramolecular hydrogels based on the Hofmeister effect. Chem. Eur. J. 20: 14465–14472. 69 Liu, D., Song, D., Guo, G. et al. (2007). The synthesis of 18beta-glycyrrhetinic acid derivatives which have increased antiproliferative and apoptotic effects in leukemia cells. Bioorg. Med. Chem. 15: 5432–5439. 70 Chang, R., Wang, X., Li, X. et al. (2016). Self-activated healable hydrogels with reversible temperature responsiveness. ACS Appl. Mater. Interfaces 8: 25544–25551. 71 Guo, J., Cai, G., Jin, Y. et al. (2020). An improved composite fly ash gel to extinguish underground coal fire in close distance coal seams: a case study. Adv. Mater. Sci. Eng. 2020: 1–11.

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6 Supramolecular Gels from Carbohydrate Biopolymers for Water Remediation Xuefeng Zhang 1 and Weiqi Leng 2 1

Department of Sustainable Bioproducts, Mississippi State University, Mississippi State, MS, USA College of Materials Science and Engineering and College of Light Industry and Food Engineering, Nanjing Forestry University, Nanjing, Jiangsu, China 2

6.1 Introduction Gels are an elastic soft matter composed of three-dimensional cross-linked polymer networks with microsized and/or nanosized pores filled up with a second medium. Gels can be classified as hydrogels, organogels, and aerogels, depending on the type of filling medium. Hydrogels can store a large amount of water as a filling medium due to their superhydrophilic polymer skeleton, while organogels consist of a superhydrophobic polymer framework that holds plenty of organic liquids. Aerogels are usually produced by solvent removal of hydrogels or organogels with their polymer networks retained without or with little shrinkage ( 1 m; Diameter 5–100 nm

HO

Su lf hy uric dro ac lys id is

OH

Acid hydrolysis HO

OH HO

Cellulose

HO

Length 50–500 nm; Diameter 3–5 nm

OSO3

–

OH

–

Cellulose Nanocrystal (CNC)

(a)

OH

–

HO

Cellulose Nanofibril (CNF)

–

COO HO

SO3-O

(b)

Mechanical homogenization –

Crystalline regions

Amorphous regions

OMP ion TE idat x o

HO –

OH NH

Length > 1 m; Diameter 5-100 nm Acid hydrolysis

HO O H 3C

Chitin

(c)

Chitin Nanocrystal (ChNC) Length 50–500 nm; Diameter 3–5 nm

OH HN

HN H3C

O

P ac ar tly ety lat o ion hy r HC dro I lys is de

OH HO HO O

(d)

OH

O

CH3

HO

Chitin Nanofibril (ChNF)

OOC

COO CH3 NH O

+

NH3 OH

HN

H3C

Figure 6.5 Preparation of NFs and NCs from cellulose and chitin through physical or chemical exfoliation processes, (a) exfoliation of cellulose to CNF and CNC, (b) distinctive surface chemistries of NC obtained by the most commonly used cellulose extraction methods. Source: Zhang et al. [73, 74]; Mendoza et al. [75]; Isogai et al. [76]; Zhu et al. [77]; Jiang and Hsieh [78]; da Silva Perez et al. [79]; Isobe et al. [80], (c) exfoliation of chitin to ChNF and ChNC, (d) distinctive surface chemistries of NCh obtained by the most commonly used chitin extraction methods.

surface area, superhydrophilicity, excellent mechanical strength, and tunable surface characteristics [10, 20, 67, 68]. The NFs and NCs differ in their preparation method, size, and rigidity (Figure 6.5). Specifically, cellulose or chitin NFs (CNFs or ChNFs) are long (0.5–10’s 𝜇m in length and 4–100 nm in diameter) and flexible filaments that are usually manufactured through high-energy mechanical homogenization with or without a combination of a pretreatment process (i.e. TEMPO-oxidation or enzymatic treatment). Conversely, cellulose or chitin NCs (CNCs or ChNCs) are rigid nanorods that are shorter than NFs (50–500 nm in length and 3–5 nm in diameter) and normally produced through a concentrated acid (e.g. sulfuric acid or hydrogen chloride acid) hydrolysis process. It is worth noting that the production of NC and NCh usually causes surface derivatization of cellulose and chitin polymeric chains (Figure 6.5b and d). For instance, sulfuric acid hydrolysis CNC usually contains anionically charged sulfate (–SO3 − ) groups [69–72], TEMPO-oxidized CNF also exhibits a negatively charged surface due to the oxidation of C6 hydroxyl (OH) groups to carboxylate (–COO− ) groups (Figure 6.5b) [73–80]. Those charged surface functional groups could not only facilitate cellulose defibrillation process by inducing the electrostatic repulsion between anionically charged cellulose microfibrils, but also increase the stability

133

134

6 Supramolecular Gels from Carbohydrate Biopolymers for Water Remediation

of CNF or CNC suspensions [76]. On the other hand, TEMPO-oxidation of chitin could also produce anionically charged ChNFs or ChNC because of the derivatization of C6 hydroxyl (OH) groups to carboxylate (–COO− ) groups [81–85]. Moreover, cationically charged nanochitins can be produced through either HCl acid hydrolysis or partly deacetylation (treated in concentrated alkaline solutions at mild temperature) techniques due to the transformation of acetylamino into –NH3 + groups [83, 86–91]. 6.2.3.1 Hydrogels from Physically Cross-Linked NC or NCh

As with the cellulose and chitin solutions, the NC and NCh can be converted to hydrogel through several types of physical gelation methods. Due to their high aspect ratio and good flexibility, CNF or ChNF hydrogels can be prepared at a very low concentration (100 [92]. Source: Pääkkö et al. [92]. Reproduced with permission from © 2007 American Chemical Society DOI-https://doi.org/10.1021/bm061215p. (b) CNC with the aspect ratio of 14.7 [94]. Source: Lewis et al. [94]. Reproduced with permission from © 2016 American Chemical Society DOI-https://doi.org/10.1021/acs.biomac.6b00906. (c) ChNC with the aspect ratio of 1.35 [95]. Source: Tzoumaki et al. [95]. Reproduced with permission from © 2010 American Chemical Society DOI-https://doi.org/10.1021/bm901046c. (d and e) Photographs of (d) switchgrass CNC and (e) cotton CNC suspensions at different CNC concentrations, switchgrass CNC has a higher aspect ratio of 39 and a lower gelation concentration of 2.5 wt%, while cotton CNC has a lower aspect ratio of 13 and a higher gelation concentration of 4.0 wt% [96]. Source: Wu et al. [96]. Reproduced with permission from © 2014 Wiley Periodicals, Inc. DOI-https://doi.org/10.1002/app.40525. (f) ChN water dispersion exhibits liquid-like behavior at low concentration (1.8 wt%) and transformed to ChN hydrogel at high concentration (5 wt%) [95]. Source: Tzoumaki et al. [95]. Reproduced with permission from © 2010 American Chemical Society DOI-https://doi.org/10.1021/bm901046c.

6.2 Hydrogels from Carbohydrate Biopolymers

Alternatively, the gelation of NC or NCh can be easily achieved by altering their surface electric charge behaviors via pH adjustment or salt addition. For example, acid addition of TEMPO-oxidized NC or NCh suspensions suppresses the electrostatic repulsion of NC or NCh polymeric chains/needles and results in influential attractive forces (e.g. hydrogen bonding and van der Waals force) and subsequent gelation (Figures 6.7a and 6.8a) [98]. On the other hand, alkaline addition to amine-modified NC (Figure 6.8b), acid-hydrolyzed NCh, or partly deacetylated NCh suspensions also leads to gelation through a similar mechanism (Figure 6.7b) [86]. Cross-linking with multivalent ions is another commonly used method for the gelation of surface charged NC or NCh (Figure 6.8c). Chau et al. tested the gelation behavior of sulfuric acid-hydrolyzed CNCs with several metal cations (Mg2+ , Ca2+ , Sr2+ , Al3+ ) and they found cations with higher charge number and/or larger ionic radii favored CNC gelation. [100] A similar phenomenon was found when investigating the gelation of TEMPO-oxidized CNF (TOCNF) suspensions with mono-, di-, and trivalent cations. Cations with higher valence-number not only facilitate the gelation process but also increase the mechanical strength of hydrogels [99, 101]. Zhu et al. used several divalent transition metal cations (i.e. Zn2+ , Cu2+ , and Co2+ ) as cross-linking agents to transform TOCNF suspensions into hydrogels; after freeze-drying for water removal, shapeable CNF aerogels decorated with transition metal cations were obtained, and the aerogels exhibited good adsorption performance for organic dyes [102]. Similarly, ion-induced gelation of partly deacetylated NCh was also reported; however, the anions (CO3 2− ) were used to bond the positively charged ChNC (due to the surface –NH3 + groups) [83]. In addition to multivalent ions, multivalent polyelectrolyte could also be used for NC and NCh gelation. Zhang et al. reported a biohybrid hydrogel fabricated through self-assembly of TOCNF (anionically charged) and partly deacetylated ChNF (PDChNF, cationically charged). The gelation was found to start immediately after mixing two nanofibril suspensions and complete within one minute (Figure 6.9a–d). They found both electrostatic attraction and hydrogen bonding interaction contributed to the cross-linking of TOCNF and PDChNF (Figure 6.9f) [73]. Dai et al. reported the preparation of NC composite hydrogels from self-assembly of TOCNF and cationic guar gum, and the electrostatic interactions between –COO− and –N+ (CH3 )3 were responsible for the gelation [103]. In recent years, a hydrothermal treatment technique has been reported to convert NC or NCh suspensions to hydrogels [94, 104, 105]. Hydrothermal treatment, normally operated at temperatures higher than 100 ∘ C, could eliminate surface charged groups on NC or NCh and suppress the electrostatic repulsion of NC or NCh polymeric chains, and leads to the gelation. For example, Lewis et al. found desulfation of the CNC could occur at 60 ∘ C within 20 hours, leading to the loss of surface repulsion among CNCs and the formation of cross-linked network structure [94]. However, high temperatures such as 120 ∘ C and long treatment time (e.g. 20 hours) are necessary to produce stable and strong hydrogels. Suenaga and Osada explored the gelation of TOCNF dispersions with the hydrothermal treatment technique

135

O

O O

O O

O M

O HO + Mx+

M

O OH

HO

Nanocellulose hydrogel

O OH

– Mx+

x+

O

O OH

HO

O

O

Ionic bond

O OH

HO O

O

O HO

O HO HO

(a)

O HO

O HO HO

Hydrogen bond O OH O OH

HO

OH

HO O

O OH

HO HO

M

x–

NH3 HN

M OH

H3C

O OH O

O

O OH

– H+

Nanocellulose hydrogel

HO

OH

O

OH

O

Electrostatic repulsion

HN

HO

HO

O OH

H3C

OH

O

OH O OH

+ Mx– CH3 NH

NH3

H3N

OH

HO

NH

NH2 OH

HO

HO

NH2

HN OH

CH3 Nanochitin suspension O

+ H+

– H+

CH3 NH

O

OH CH3 NH

H2 N

O (b) H3C

H2N

HN

H3C

O

OH

Hydrogen bond O

– Mx–

OH HO

HN H3C

O

OH

HO

Electrostatic repulsion

HO

HO OH

H3N

Ionic bond

Nanochitin hydrogel

CH3 NH

H3N

HN

OH O

x–

NH3

O

O

OH

HO

O

O

CH3 NH

H3N

NH3

H 3C

O

O

+ H+

O

O

O HO

O

Nanocellulose suspension

HO

O

x+

Nanochitin hydrogel

O

OH

HO

Figure 6.7 Schematic representation of the proposed gelation mechanisms of NC (a) and NCh. Source: Modified from Way et al. [98] (b) suspensions through pH adjustment and ions addition. Source: Based on Liu et al. [86].

6.2 Hydrogels from Carbohydrate Biopolymers

CNC-CO2H

(a)

(b)

pH 1

pH 7

CNF dispersion

pH 11

CNC-NH2

pH 1

pH 7

pH 11

(c)

Figure 6.8 Physically cross-linked hydrogels prepared from NC and NCh: (a) gelation of TEMPO-oxidized CNC by the addition of acid, (b) gelation of amine-modified CNC by the addition of alkaline solution [98]. Source: Way et al. [98]. Reproduced with permission from © 2012 American Chemical Society. DOI- https://doi.org/10.1021/mz3003006. (c) Ion-mediated gelation of 1.27 wt% TOCNF dispersion [99]. Source: Dong et al. [99]. Reproduced with permission from © 2013 American Chemical Society DOI-https://doi.org/10.1021/bm400993f.

(Figure 6.10a) [105]. Hydrothermal treatment of 1 wt% TOCNF dispersion at 160 ∘ C for 20 minutes resulted in a ∼48% reduction of carboxylic content of TOCNF from 1.44 to 0.75 mmol g−1 . Further extending the treatment time did not change the carboxylic content but increase the mechanical property of hydrogels. The gelation was attributed to the hydrolysis of glucuronate groups of TOCNF that diminished the repulsive forces among TOCNF chains and led to the formation of gel network through physical entanglements and secondary force attractions (e.g. van der Waals forces, hydrophobic interaction, and/or hydrogen bonding). The freshly obtained TOCNF hydrogels exhibit a yellowish color due to the existence of brownish TOCNF hydrolysis products (Figure 6.10b), while immersing the yellowish hydrogels into distilled water at 5 ∘ C could obtain transparent hydrogels (Figure 6.10c). Hydrogels could maintain their original cylinder shape after depigmentation only with more than 30 minutes of hydrothermal treatment, indicating that the mechanical strength of hydrogels increased with longer treatment time (Figure 6.10c). 6.2.3.2 Hydrogels from Chemically Cross-Linked NC or NCh

To obtain robust hydrogels from NC or NCh, a variety of chemical cross-linkers were used to cross-link the NC or NCh nanofilaments. Chemical cross-linkers react with –OH (for NC or NCh) or –NH2 (for amino-modified NC or NCh) groups to form covalent bonds that link NC or NCh polymeric chains. Generally, chemical cross-linkers for NC and NCh can be classified into two types according to the reaction mechanism: etherifying and esterifying cross-linkers. Etherifying cross-linkers include epoxide and organochlorine forming ether bonds (R–O–R) or secondary amine bonds (R–NH–R) with NC or NCh polymeric chains; while esterifying

137

138

6 Supramolecular Gels from Carbohydrate Biopolymers for Water Remediation

(b)

(a)

(c)

(d)

(e)

PDChNF TOCNF Suspensions (f)

Mixing

Self-assembly

TOCNF H3COCHN HO O

O –

HOH2C –

δ δ

OH

COOH

OOC HO

O O

O O

CH2OH

O CH2OH

O

δ – δ

H3N

pH Cu2+ > Cd2+ , and can be quickly regenerated by 1 mol l−1 HCl solution within 5–15 minutes with up to 98% recovery, thereby demonstrated good reusability. Later, the same group prepared the cellulose/chitin hydrogel membranes with a stronger alkaline solvent system (7 wt% NaOH/12 wt% urea aqueous solution) for biopolymer dissolution. The prepared membranes had good selectivity for Hg2+ adsorption and followed by Pb2+ and Cu2+ . In comparison with neat cellulose and neat chitin hydrogel membranes, the composite membranes exhibited the highest adsorption capacity for Pb2+ (455.4 mg g−1 ) uptake, while the neat chitin membranes showed the highest adsorption capacities for Hg2+ (481.7 mg g−1 ) and Cu2+ (149.2 mg g−1 ) [123]. CMC has been widely used to produce hydrogels for the capture of cationic heavy metal ions from water due to its high water solubility and ample surface carboxymethyl groups (–CH2 –COOH) [1–4]. The adsorption of heavy metals is pH-dependent. The increase of solution pH can facilitate the deprotonation of –CH2 –COOH to form carboxylate ions (–CH2 –COO− ), thereby enhance the adsorption of cationic heavy metal ions through electrostatic attraction [121]. The employment of a second ingredient to the biopolymer can boost the removal efficiency and capacity of the resultant composite gels for water contaminants. For instance, the addition of polyacrylamide endowed the CMC hydrogel with 1.5–3.6 times higher heavy metal (Cu, Pb, and Cd) adsorption capacity due to the active –NH2 and C=O groups in polyacrylamide [127]. In chitosan hydrogels, the introduction of polyacrylamide was found not only to double the Hg adsorption capacity but also increase the Hg sorption selectivity over Pb [125]. As the most commonly seen chitin derivatives, chitosan has been used for the synthesis of adsorbent hydrogels for the removal of both cationic (heavy metals) and anionic ions (nutrients, arsenic compounds) due to their abundant surface amido and hydroxyl groups and good processability (soluble in acidic solutions). Coagulation of chitosan solution in alkaline bath yielded chitosan hydrogel beads, which exhibited good water stability in natural and alkaline solutions, with a Cu2+ adsorption capacity of 87.7 mg g−1 [128]. After cross-linking with chemical cross-linkers, the adsorption capacity decreased significantly (Table 6.2) due to the depletion of –NH2 or –OH groups and swelling ratio. Guo et al. reported glutaraldehyde cross-linked chitosan aerogel beads, which exhibited high adsorption capacity for Cr(VI) (i.e. HCrO4 − , Cr2 O7 2− ) at low pH (i.e. 2.0) due to the positively charged surface (–NH3+ and –OH2+ ) (Figure 6.12a) [122]. While at a high pH (i.e. 5.5),

6.4 Biopolymer-Derived Gels for Water Remediation

H N OH O

O HO

O N HO

N

OH

NH

O

O

O

O

HO

HO

NH2 n O

HO

OH O

N

O

O HO

O HO

H N

O

HO

N

H

O O

NH

n

HO

N

Peristaltic pump Chitosan

Dopamine

O

O

g

kin

lin s-

Stirring Acetic acid solution

CS solution

os Cr

NaOH solution

(a) +

HCI

(b)

NH3

HCrO4– HO

Polymeric matrix

–

HCrO4

OH

Chitosan bead (CB)

+

NH3

NH2

OH

PCB

–

OH O4CrH

+

H3 N

Electrostatic attraction

Exhausted PCB

Figure 6.12 (a) Preparation of polydopamine-modified chitosan aerogel beads cross-linked with glutaraldehyde [122]. Source: Guo et al. [122]. © 2018, Elsevier. (b) Adsorptive removal of Cr(VI) using chitosan bead (CB) [120]. Source: Kousalya et al. [120]. © 2010, Elsevier.

the chitosan beads exhibited good adsorption performance for Pb2+ because the deprotonation of hydroxyl and amine groups decreased the electrostatic repulsion between hydrogel beads and Pb2+ , as well as the increase of complexation between Pb2+ and lone electron pair atoms (e.g. O and N). When polydopamine was grafted onto chitosan beads, the adsorption capacity for both Cr(VI) and Pb increased since more N (amine) and O (ketone) atoms were introduced. Another study from Kousalya et al. reported that the adsorption capacity of Cr(VI) onto chitosan hydrogel beads increased by three to four times after surface modifications, i.e. surface protonation, grafting carboxylic or amine compounds; and the electrostatic attraction between protonated chitosan amines (–NH3 + ) and anionic Cr(VI) ions (HCrO4 − ) was responsible for adsorption (Figure 6.12b) [120]. Li et al. prepared a physically cross-linked composite hydrogel from TOCNF and polyethyleneimine (PEI) via electrostatic attraction, and the composite aerogel was obtained after freeze-drying [136]. In contrast to neat TOCNF aerogel, composite aerogel showed good structural stability and shape recovery in water due to strong electrostatic interaction between anionically charged TOCNF and cationically charged PEI. The Langmuir capacities of composite aerogel were found to be 175.4

143

Table 6.2

Comparison of heavy metal adsorption among various cellulose- and/or chitin-based gel adsorbents.

Gel adsorbent

Gelation method

Initial conc. (mg l−1 )

Temp. (∘ C)

pH

Time

Cellulose/chitin

Coagulation in 5% H2 SO4

50–280

∼25

4 or 5

4–5 h

Cellulose

Coagulation in 5% Na2 SO4

∼64–1000

∼25

5

24

Capacityqe (mg g−1 )a) 2+

Pb

2+

176.0, Cu

Ref. 2+

32.4, Cd

69.7

Hg2+ 140.4, Cu2+ 17.2, Pb2+ 155.3

Chitin

Hg2+ 481.7, Cu2+ 149.2, Pb2+ 393.3

Cellulose/chitin

Hg2+ 461.38, Cu2+ 111.1, Pb2+ 455.4 25

5.8

10 h

Cu2+ 26.7, Zn2+ 19.5, Cr(VI) 13.0, Ni2+ 13.0, Pb2+ 26.9

[129]

25–500

−25

5.5

24 h

Cu2+ 99.0, Pb2+ 87.0, Cd2+ 172.4

[127]

Coagulation in water bath

CMC

Ca2+

CMC/polyacrylamide

MBA cross-linkingb)

TOCNF/cationic guar gum

Self-assembly

1000–3500

35

CMC

ECH cross-linkingc)

∼10–3100

CMC/poly(vinyl alcohol)

Freeze and thaw

100

Chitosan

Coagulation in 0.1 M NaOH

0–14

Chitosan

Cu2+ 227.3, Pb2+ 312.5, Cd2+ 256.4 NA

24 h

Cu2+ 498.5, Ni2+ 231.4

25

7

72 h

Cu2+ 412.1, Ni2+ 231.4, Pb2+ 1066.1

[121]

15

1.6

24 h

Ag+ 8.4, Ni2+ 6.0, Cu2+ 5.5, Zn2+ 5.3M

[130]

∼25

6

1h

Cu2+ 80.71

[128]

GA cross-linkingd)

Cu2+ 59.67

ECH cross-linking

Cu2+ 62.47

EDGE cross-linkinge)

Cu2+ 45.62

(1) Coagulation in 1M NaOH and (2) EGDE cross-linking

[123]

∼50–2000

Cellulose/chitosan

Chitosan/polyacrylamide

[126]

10–200

∼25

4

15 h

Hg2+ 181.8

1h

Hg2+ 322.6

[103]

[125]

Chitosan

(1) Coagulation in 10% NaOH and (2) GA cross-linking

50–600

NA

2.0

24 h

Acid-treated chitosan

(1) Coagulation in 0.5 M NaOH and (2) GA cross-linking

10

30

2.0

Cr(VI) 263.6

5.5

Pb2+ 295.4

7

[122]

Pb2+ 295.4

5.5 Chitosan/polydopamine

Cr(VI) 263.6

10 min

Cr(VI) 3.24M

[120]

Cr(VI) 3.65M

Carboxylic-modified chitosan

Cr(VI) 4.06M

Amine-grafted chitosan CMC/chitosan

Arginine cross-linking

Cellulose/acrylic acid

MBA cross-linking

MOF-modified cellulose/chitosan

Mechanical homogenization

Cellulose/TOCNF/lignin

Coagulation in 1 M HCl

50–500

∼40

6.5

1h

Pb2+ 182.5, Cd2+ 168.5

200–2000

30

5

6h

Pb2+ 825.7, Cd2+ 562.7, Ni2+ 380.1

[132]

1000

25

6

24 h

Cu2+ 200.6, Cr(VI) 152.1M

[133]

50–250

25

NA

1h

Cu2+ 540.72

[134]

[131]

TOCNF/PDChNF

Self-assembly

10–1000

25

7

2h

As(III) 217

[73]

CNF/polydopamine/polyethylenimine

Thermal treatment at 80 ∘ C

50–1200

NA

5

12 h

Cu2+ 103.5

[135]

a) b) c) d) e)

Adsorption capacities were obtained as the Langmuir capacity (if there is no mark behind) or the maximum capacity (marked withM ) reported in the article MBA: N,N ′ -methylenebis (acrylamide) ECH: epichlorohydrin GA: Glutaraldehyde EGDE: ethylene glycol diglycidyl ether.

146

6 Supramolecular Gels from Carbohydrate Biopolymers for Water Remediation

and 357.1 mg g−1 for Pb2+ and Cu2+ , respectively. Moreover, the adsorbed heavy metal ions can be easily stripped by soaking the aerogel in ethylenediaminetetraacetic acid (EDTA) solution, and the regenerated aerogel still maintained good adsorption performance. In a TOCNF/cellulose/lignin composite hydrogel, TOCNF and cellulose served as the skeleton for the deposition of lignin nanoparticles [134]. Owing to the high mechanical strength of TOCNF and abundant surface functional groups in both TOCNF (hydroxyl and carboxylic) and lignin (phenolic and carboxylic), the composite hydrogel exhibits superior mechanical strength and high Cu2+ uptake capacity of 540.7 mg g−1 . Zhang et al. reported a biohybrid hydrogel through self-organization of CNFs and ChNFs due to the electrostatic interaction between carboxylic groups on the CNFs’ surface and amino groups on the surface of ChNF. Lyophilization of biohybrid hydrogel produced aerogels that exhibited good wet stability and high arsenite [As(III): H3 AsO3 , H2 AsO3 − , HAsO3 2− , AsO3 3− ] adsorption capacity of 217.0 mg g−1 [73].

6.4.2

Organic Pollutants Removal

Industries like textile, paper, and plastics manufacturing industries consume a substantial amount of water, and discharge a considerable quantity of organic pollutants including dyes, surfactants, and pesticides to water [137, 138]. These organic pollutants have caused tremendous problems to aquatic organisms and global environment, of which, dye pollutants have negative impacts on the dissolved oxygen (DO) level in water due to their high chemical oxygen demand (COD) and are resistant to oxidizing agents. Dyes also have high toxicity and color that could inhibit sunlight penetration. Hence, there is an urgent need to remediate the aquatic environment for living organisms [139]. Common dye pollutants remediation processes can be categorized into three main types, i.e. physical separation, chemical process, and biological degradation. Methods such as oxidation, membrane separation, and adsorption have been mainly used to remove those dye pollutants, of which, adsorption has been mostly used in the actual process due to low operational cost, high effectiveness, and ease of use compared to other processes [139]. Ideal organic pollutant adsorbents need to possess merits such as easy accessibility, fast sorption rate, low cost, large surface area, massive available functional groups that are ready to react with organic pollutants, and high adsorption capacity [140]. Cross-linked hydrogels and aerogels perfectly fit these criteria. Hydrogels have high swelling capacity, adsorption capacity, physicochemical and mechanical stability and reusability, resulting in great performance in wastewater applications. Aerogels, on the other side, are a family of solid materials after the removal of solvents in hydrogels, rendering much lower density, higher porosity, and larger internal surface area [141]. Various materials have been investigated as hydrogel and aerogel adsorbents for organic pollutants removal from wastewater. Several reports have indicated that celluloseand chitin-based hydrogels and aerogels are considered to be promising candidates [20, 142]. Cellulose is the most important natural polysaccharide in the world with extraordinary physical and mechanical properties. It has a huge amount of readily available hydroxyl groups that can combine dye molecules [140]. Meanwhile, chitin

6.4 Biopolymer-Derived Gels for Water Remediation

is the second most important natural polymer in the world [143]. The most important deacetylated derivative of chitin, i.e. chitosan, also has good sorption capacity for dye pollutants due to its abundant amino acids at position C2 , acting as chelating sites [140]. In this section, the recent advances of dye pollutants removal by celluloseand chitosan-based hydrogels and aerogels are summarized. In general, dyes can be classified into two types, i.e. ionic dyes and nonionic dyes, of which ionic dyes can be further categorized into cationic and anionic dyes. Cationic dyes carry positive surface charges while anionic dyes carry negative surface charges. Table 6.3 lists some widely used dyes in staining and textile industries. As mentioned early, cellulose and chitin are usually pretreated or grafted with other chemicals to be endowed with ionic properties that are ready to react with counterion, molecules, and dyes [144]. These derivatives (Figure 6.4) are also much easier to fabricate into hydrogels and aerogels with or without the addition of a physical cross-linkers because they are water soluble. The adsorption of dye pollutants by cellulose- and chitin-based hydrogels and aerogels is a complicated process involving different kinds of synergistic interactions between cellulose and chitosan and dyes, e.g. ion exchange, chelation, hydrogen bonds, hydrophobic attractions, van der Waals force, physical adsorption, aggregation mechanisms, electrostatic interaction, and dye–dye interactions [137, 145, 146]. Researchers have not yet reached an agreement on the mechanism of dye adsorption onto cellulose- and chitin-based hydrogels and aerogels. Several removal mechanisms such as surface adsorption, chemisorption, diffusion, and adsorption-complexation were proposed. Unfortunately, those respective mechanisms were dependent on different studies and experimental conditions. Moreover, various pretreatment or preparation methods added to the uncertainty. The addition of reactive chemicals, the change of pH values, and the presence of different ligands, which chelated with cellulose and chitin, also complicated the interpretation of the dye removal mechanism. In general, the adsorption mechanisms for dye adsorption by both cellulose- and chitin-based hydrogels and aerogels are illustrated by investigating the effect of solution pH, contact time, temperature, and initial dye concentration, active functional sites, surface area from swelling, pore diameter, and total pore volume [137, 142, 147]. Four steps are believed to occur during the adsorption of dyes onto gels: (i) bulk diffusion of dye pollutants from the solution to the surface of the adsorbent; (ii) film diffusion of dye through the boundary layer to the surface of the adsorbent; (iii) pore diffusion or intraparticle diffusion for the transportation of the dye from the surface to the pores of the particle; and (iv) finally the chemical reaction: adsorption of dye at an active site on the surface of adsorbent via electrostatic interaction, ion exchange, complexation and/or chelation. The first three steps are recognized as physisorption, while the last one is chemisorption. The chemisorption step usually dominates the adsorption process. As was mentioned before, solution pH, initial dye concentration, functional groups, contact time, and adsorbent dosage play a significant effect in dye adsorption, of which, solution pH, initial dye concentration, functional groups, and adsorbent dosage are prevalent external factors that can influence the adsorption of dye pollutants and are controlled by a charge neutralization mechanism [146–148].

147

148

6 Supramolecular Gels from Carbohydrate Biopolymers for Water Remediation

Table 6.3

Classification and molecular structure of dyes.

Dye category

Anionic

Commercial name

Molecular structure

Indigo Carmine (IC)

Na+ –O

O

O

H

S

N

O

O N

S O

H

C. I. Acid Red 73 (AR73)

N

O– Na+

O

H O N N

N

NaO3S SO3Na

Methylene Orange (MO)

O H3C

N N

N

S

O–

O

Na+

H3C

Congo Red (CR)

NH2

OO S O–Na+

N N

N N

O S O O Na+

H2N

Sunset Yellow (SY)

–O

O

O

S

S

Na+ O

O– O

N

Na+

N

HO

Reactive Blue 19 (RB19)

O

O

Acid Red 112 (AR112)

NH2 O O S – O Na+

HN

O O S

O O S – O O Na+

NaO3S NaO3S N N

N N

SO3Na

NaO3S

(Continued)

6.4 Biopolymer-Derived Gels for Water Remediation

Table 6.3

(Continued)

Dye category

Commercial name

Molecular structure

Acid Blue 92 (AB92) NH

O NaO S O

N

N OH

O S ONa O

O S O ONa

Acid Red 13 (AR13)

NaO3S

N N

SO3Na

OH

Acid Blue 93 (AB93)

HO O S O N

NaO O S O

O ONa S O N H

Cationic

Methylene Blue (MB)

N H

N N

N+

S Cl–

Malachite Green (MG)

Cl– N

N

(Continued)

149

150

6 Supramolecular Gels from Carbohydrate Biopolymers for Water Remediation

Table 6.3

(Continued)

Dye category

Commercial name

Molecular structure

Rhodamine 6G (RH)

H3C

CH3

Cl– HN

O

H3C

NH

CH3

O

CH3

O

Rhodamine B (RB)

H3C H3C

CH3

Cl– O

N

N

CH3

COOH

Thioflavin T

CH3 N+ H3C

S

CH3 N CH3

Cl–

During dye adsorption, various forces are involved including adsorbent–adsorbate, adsorbent–solvent, dye–solvent, solvent–solvent interactions, and maybe other interactions, resulting in a complicated process. Despite the process complexity, two-parameter isotherm models such as Langmuir and Freundlich models are widely used to describe the adsorption of dyes and fit well to the adsorption data. Langmuir isotherm is a theoretical model comprising of monolayer adsorption and assumes a homogenous nature of the adsorbent with equal identical binding sites. On the other hand, Freundlich isotherm is an empirical model that assumes the heterogeneous surface of the adsorbate and with the possibility of multilayer adsorption. It is noteworthy that the assumptions of two-parameter isotherm models are not based on the actual adsorption process and are idealistic. Thus, it has limited applications within confined conditions. Moreover, these two models are based on the hypothesis of physical adsorptions rather than chemical adsorptions, while indeed dye removal is mainly chemical adsorptions. Thereby, three-parameter Redlich–Peterson and Sips isotherms, on the other side, are preferred. Sips isotherms is a combination of the Langmuir and Freundlich models, and is supposed to describe the more comprehensive heterogeneous surfaces. When the dye concentrations are low, it shifts to Freundlich isotherm, while it shifts to a monolayer Langmuir isotherm adsorption at high dye concentrations [149]. Table 6.4 lists the recent studies for various dye pollutant removals using cellulose-

Table 6.4

Batch studies for various dye removals using cellulose- and chitin-based gel adsorbents. Capacity, qe (mg g−1 )a)

Gelation methods

Initial conc. (mg l–1 )

Temp. (∘ C)

pH

Cellulose/acrylic acid

MBA cross-linkingb)

400–2000

30

6.5

36 h

MB 2377

[150]

Amine-modified CMC

EGDMA cross-linkingc)

10–4000

NA

3

12 h

MO 1825

[145] [151]

Gel adsorbent

Contact time

Ref.

Chitosan-Fe(III)

GA cross-linkingd)

25–200

NA

12

∼10 min

AR73 294.5

Sulfopropyl modified CMC

MBA cross-linking

25–2000

25

6

24 h

MB 1675

[148]

Chitosan/cellulose/rectorite

ECH cross-linkinge)

10–500

NA

NA

∼12 h

CR 166.10

[140]

TOCNF/PDChNF

Self-assembly

10–200

25

10

24 h

MB 530.76

[73]

Chitosan/polymer nanoparticles

Coagulation with 6% NaOH

20–1000

25

NA

NA

IC 118M , RH 75M , SY 70M

[152]

CMC/clay

ECH cross-linking

10–200

30

1–11

40 h

MB 1065

[142]

Amine-modified cellulose

PEGDEf)

10–1200

20–50

7

24 h

AR13 430, AB92 447, AR112 322

[146]

CNC/polyamide

(1) Coagulation with 0.1 M HCl NaOH (2) thermal treatment at 150 ∘ C

1–300

∼25

4.8–6.5

12 h

MB 358.4

[147]

Chitosan/poly(acrylamide)

ECH cross-linking

6–1280

25

5.5

6h

MB 750S

[149]

Chitin

ECH cross-linking

73–510

30

7

48 h

MG 33.5

[153]

UiO-66/nanocellulose

Mechanical stir and ice templating

50

NA

NA

6h

MO 71.7M , MB 51.8M

[154]

(Continued)

Table 6.4

(Continued)

Gel adsorbent

Gelation methods

Initial conc. (mg l–1 )

Temp. (∘ C)

pH

Contact time

Capacity, qe (mg g−1 )a)

Ref.

Chitosan/GO/CMC

Acid coagulation

100–600

50

6

72 h

MB 3610

[59]

CNF/polydopamine/ polyethylenimine

Thermal treatment at 80 ∘ C

20–1200

NA

4

12 h

MO 265.9

[135]

200–1280

35

NA

24 h

TT 430.2, MO 134.3M

[103]

TOChN

Coagulation with NH3 gas

30

NA

8

12 h 12 h

MG 50M

[86]

DEChN

Coagulation with HCl gas

90

CNC/alginate

Ionic gelation with Ca2+

600–2000

25

7

1h

MB 256.4

[155]

Cellulose/acrylic acid/acrylamide

MBA cross-linking

200–2500

25

7

1.5 h

MB 1814, AB93 1602

[156]

TOCNF/cationic guar gum

RB19 225M

1.5

a) Adsorption capacities were obtained as the Langmuir capacity (if there is no mark behind), Sips capacity (marked withS ), or the maximum capacity (marked withM ) reported in the article b) MBA: N,N ′ -methylenebis (acrylamide) c) EGDMA ethylene glycol dimethacrylate d) GA: Glutaraldehyde e) ECH: epichlorohydrin f) PEGDE poly(ethylene glycol) diglycidyl ethers.

6.4 Biopolymer-Derived Gels for Water Remediation

and chitin-based hydrogel and aerogel adsorbents. Types of adsorbent, dyes, specific surface area, temperature, pH value, adsorption kinetics, and adsorption capacity are summarized. Electrostatic attraction is one of the most prevalent adsorption mechanisms ever reported, and the solution pH plays an important role in the whole adsorption process. The solution pH affects the surface charge of the adsorbent, the degree of ionization in the adsorbent, the release of different functional groups on the active sites, etc., which makes the adsorption process pH-sensitive [145]. Celluloseand chitosan-based hydrogels and aerogels can be fabricated via pretreatment with NaOH and NaOH/Na2 CO3 solution to relieve the electronic repulsion of ammonium ions between polymeric chains. The electrostatic interaction between ammonium ions and anions renders the formation of pH-sensitive hydrogels and aerogels. In such pH-sensitive adsorbent, the cross-linking density was controlled by adjusting the pH value of the ionic cross-linker solution [20]. At low pH values, protonation occurs in the solution, and an excessive quantity of hydrogen ions exist, which facilitates the formation of a large number of cationic groups for hydrogels. These cationic groups readily interact with active anionic sites within the hydrogels, making adsorptions of cationic dyes such as methylene blue difficult due to electrostatic repulsion, while disfavoring anionic dyes such as methyl orange. On the other side, increasing solution pH value to the alkaline condition causes deprotonation, resulting in a negatively charged hydrogel surface, leading to higher adsorption of cationic dyes [148]. Therefore, the pH value dictates the surface charge of the adsorbent via the process of protonation and deprotonation [73, 135]. Liu et al. produced two types of ChNC-based hydrogels through a gas phase coagulation method [86]. Positively surface charged partially deacetylated ChNC (DEChN) was transformed to the DEChN hydrogel via ammonia gas coagulation, while negatively surface charged TEMPO-oxidized ChNC (TOChN) was converted to the TOChN hydrogel via hydrogen chloride gas coagulation. Owing to their different surface charge characteristics, DEChN and TOChN hydrogels were used to capture anionic dye RB-19 and cationic dye MG, respectively (Figure 6.13). The maximum adsorption capacity of RB-19 at pH of 1.5 and MG at pH of 8 was found to be 225 and 50 mg g−1 , respectively. As for the functional groups attached to the cellulose- and chitin-based hydrogels and aerogels, they are the actual active sites that mount and react with dye molecules. For example, chitin-based hydrogel combined with iron ions exhibits rapid adsorption for AR73 due to the high density of surface iron ions for active sites for dye molecule chelation. Once iron is introduced into the hydrogel network, the free dye molecules readily change places with the water molecules to chelate the iron center [140]. The initial dye concentration also plays an important role in the adsorption process. The equilibrium pH decreases with the increase of the initial dye concentration. As more dyes are adsorbed onto the adsorbent, more hydrogen ions are released, then the pH decreases, following the principles of ion exchange [137]. However, as increasing numbers of dye molecules are adsorbed onto the adsorbent, dye molecules repellant occurs, decreasing adsorption rate.

153

DEChN (b)

1.2

(c)

(d)

200 g

Absorbance

1.0 .8

(e)

(f)

1.2

(g)

RB 19-raw DEChN(0.2) DEChN(0.4) DEChN(0.6) DEChN(0.8) DEChN(1.0)

(h) 1.0

.6 200 g .4 .2 0.0 300

Absorbance

(a)

TOChN

.8

BG 4-raw TOChN(0.2) TOChN(0.4) TOChN(0.6) TOChN(0.8) TOChN(1.0)

.6 .4 .2

600 400 500 Wavelength (nm)

700

0.0 300

600 400 500 Wavelength (nm)

700

Figure 6.13 Photographs of a DEChN hydrogel (a) and a TOChN hydrogel (e) and the corresponding hydrogel bearing a 200 g weight in (c) and (g), respectively; the adsorption of RB 19 (90 mg l−1 ) into DEChN hydrogels (b, d; contact time: 12 hours, agitation speed: 150 rpm, pH 1.5) and the adsorption of BG 4 (30 mg l−1 ) into TOChN hydrogels (f, h; contact time: 12 hours, agitation speed: 150 rpm, pH 8.0). Source: Liu et al. [86]. Reproduced with permission from © 2016 American Chemical Society. DOI-https://doi.org/10.1021/acs.biomac.6b01278.

6.4 Biopolymer-Derived Gels for Water Remediation

The reaction equilibrium is achieved when the repellant and driving forces reach a balance [140]. Adsorption time does not affect the adsorption performance once reaction equilibrium has been achieved. The adsorption isotherms have been used to evaluate the performance of cellulose- and chitin-based hydrogel and aerogel adsorbents for dyes adsorption, and the isotherm shapes are adopted to illustrate the nature of the adsorption mechanisms [157]. Although the native hydrogels and aerogels from chitin, cellulose, and their derivatives have shown good adsorption performance for organic dyes, by incorporating with other inorganics and/or polymers, novel composite gels with advance mechanical properties and superior adsorption performance can be obtained. For instance, Tu et al. prepared an ECH cross-linked composite hydrogel from cellulose and chitosan, and inorganic clay (rectorite) was used as a reinforcing agent to increase the mechanical strength and adsorption capacity of CR. The composite hydrogel exhibited good elasticity, high strength, and excellent resilience (Figure 6.14). Owing to the electrostatic attraction between cationic chitosan polymeric chains and anionic CR, the hydrogel showed a high CR adsorption capacity of 166.1 mg g−1 [140]. Peng et al. also incorporated inorganic clay (montmorillonite) into an ECH cross-linked CMC hydrogel. The clay-reinforced CMC hydrogel exhibited a high MB adsorption capacity of 1065 mg g−1 , which was more (a)

(b)

(c)

1 cm

1 cm

1 cm

(d)

Figure 6.14 Photographs of hydrogels with the content of chitosan: cellulose = 3 : 1, 5% rectorite under (a) knotting, (b) twisting, and (c) compression; (d) photographs of the adsorption process of hydrogels (20 mg dry hydrogels were immersed in 15 ml CR solutions with the concentration of 100 mg l−1 for two hours). Source: Tu et al. [140]. Reproduced with permission from © 2017 Royal Society of Chemistry. DOI-https://doi.org/10.1039/C7PY00223H.

155

156

6 Supramolecular Gels from Carbohydrate Biopolymers for Water Remediation

than two times of the neat CMC hydrogel [142]. In another example, Huang et al. prepared a composite aerogel consisting of chitosan, GO, and CMC. The composite aerogel exhibited a high MB adsorption capacity of 3190 and 3610 mg g−1 at 25 and 50 ∘ C, respectively. The authors attributed the high adsorption performance to the strong interactions between MB and the aerogel building elements including the π–π conjugation interaction, hydrogen bonding interaction, and electrostatic attraction [59].

6.5 Conclusions and Perspectives In this chapter, we have summarized recent studies of two major classes of gels synthesized from carbohydrate biopolymers, cellulose and chitin, and their application for water remediation. Considering their renewability, biodegradability, natural abundance, and abundance in functional groups, gels prepared from cellulose, chitin, and their derivatives hold great potentials as absorbents for the removal of water pollutants. Although lots of studies have demonstrated that cellulose- and chitin-based gels are good absorbents for the removal of heavy metals, organic molecules, and inorganic ions from aqueous solutions, simulated contaminated water was mainly used, and less attention was given to the application of those gels for the remediation of real industrial or agricultural wastewater. On the other hand, recent studies on water purification using cellulose- and chitin-based gels were mainly conducted with a batch adsorption process, and the initial concentrations of simulated wastewater in those studies were usually very high (several tens to several thousands of ppm level), which are not practical for water remediation application. For future studies, more attention should be paid to (i) the evaluation on the adsorption performance of cellulose- and chitin-based gels using real wastewater samples; (ii) the application of cellulose- and chitin-based gels for water remediation in continuous flows; and (iii) the assessment of the adsorption performance of cellulose- and chitin-based gels using low concentration of wastewater, such as ppb or ppt level wastewater. Gels from cellulose, chitin, and their derivatives still offer abundant promising opportunities for water remediation applications, even though many challenges need to be overcome to trade off the effectiveness and economic applicability before the commercialization of carbohydrate biopolymer-based gel adsorbents; thus, fundamental research on the nature of these gels should be further continued.

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124 Zhou, D., Zhang, L., and Guo, S. (2005). Mechanisms of lead biosorption on cellulose/chitin beads. Water Res. 39: 3755–3762. https://doi.org/10.1016/j .watres.2005.06.033. 125 Li, N., Bai, R., and Liu, C. (2005). Enhanced and selective adsorption of mercury ions on chitosan beads grafted with polyacrylamide via surface-initiated atom transfer radical polymerization. Langmuir 21: 11780–11787. https://doi .org/10.1021/la051551b. 126 Zhou, D., Zhang, L., Zhou, J., and Guo, S. (2004). Cellulose/chitin beads for adsorption of heavy metals in aqueous solution. Water Res. 38: 2643–2650. https://doi.org/10.1016/j.watres.2004.03.026. 127 Godiya, C.B., Cheng, X., Li, D. et al. (2019). Carboxymethyl cellulose/polyacrylamide composite hydrogel for cascaded treatment/reuse of heavy metal ions in wastewater. J. Hazard. Mater. 364: 28–38. https://doi.org/10.1016/j .jhazmat.2018.09.076. 128 Wan Ngah, W.S., Endud, C.S., and Mayanar, R. (2002). Removal of copper(II) ions from aqueous solution onto chitosan and cross-linked chitosan beads. React. Funct. Polym. 50: 181–190. https://doi.org/10.1016/S1381-5148(01)001134. 129 Sun, X., Peng, B., Ji, Y. et al. (2009). Chitosan(chitin)/cellulose composite biosorbents prepared using ionic liquid for heavy metal ions adsorption. AIChE J. 55: 2062–2069. https://doi.org/10.1002/aic.11797. 130 Wang, L.-Y. and Wang, M.-J. (2016). Removal of heavy metal ions by poly(vinyl alcohol) and carboxymethyl cellulose composite hydrogels prepared by a freeze–thaw method. ACS Sustain. Chem. Eng. 4: 2830–2837. https://doi.org/ 10.1021/acssuschemeng.6b00336. 131 Manzoor, K., Ahmad, M., Ahmad, S., and Ikram, S. (2019). Removal of Pb(ii) and Cd(ii) from wastewater using arginine cross-linked chitosan–carboxymethyl cellulose beads as green adsorbent. RSC Adv. 9: 7890–7902. https://doi.org/10 .1039/C9RA00356H. 132 Zhou, Y., Zhang, L., Fu, S. et al. (2012). Adsorption behavior of Cd2+, Pb2+, and Ni2+ from aqueous solutions on cellulose-based hydrogels. BioResources 7: 2752–2765. 133 Li, D., Tian, X., Wang, Z. et al. (2020). Multifunctional adsorbent based on metal-organic framework modified bacterial cellulose/chitosan composite aerogel for high efficient removal of heavy metal ion and organic pollutant. Chem. Eng. J. 383: 123127. https://doi.org/10.1016/j.cej.2019.123127. 134 Zhang, L., Lu, H., Yu, J. et al. (2018). Synthesis of lignocellulose-based composite hydrogel as a novel biosorbent for Cu2+ removal. Cellulose 25: 7315–7328. https://doi.org/10.1007/s10570-018-2077-8. 135 Tang, J., Song, Y., Zhao, F. et al. (2019). Compressible cellulose nanofibril (CNF) based aerogels produced via a bio-inspired strategy for heavy metal ion and dye removal. Carbohydr. Polym. 208: 404–412. https://doi.org/10.1016/j .carbpol.2018.12.079.

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7 Biobased Aerogels for Oil Spill Remediation Weiqi Leng 1,2 , Sheng He 3 , Xuefeng Zhang 4 , Xiang Wang 5 and Chanaka M. Navarathna 6 1 College

of Materials Science and Engineering, Nanjing Forestry University, Nanjing, Jiangsu, China College of Light Industry and Food Engineering, Nanjing Forestry University, Nanjing, Jiangsu, China 3 China National Bamboo Research Center, Key Laboratory of Bamboo High Efficient Processing of Zhejiang Province, Hangzhou, Zhejiang, China 4 Department of Sustainable Bioproducts, Mississippi State University, Mississippi State, MS, USA 5 Department of Chemistry, Zhejiang University, Hangzhou, Zhejiang, China 6 Department of Chemistry, Mississippi State University, Mississippi State, MS, USA 2

7.1

Introduction

With increasing global energy demand, due to the growing population and industrialization using petroleum as a primary source of energy, global consumption of crude oil continues to rise. During the process of offshore oil production and transportation, oil and petroleum leakage from oil tankers or vessel sinking and industrial wastewater occurred quite a few times. According to statistics, over 7.3 million barrels of oil enter the natural aquatic environments from municipal and industrial sources, marine transport, natural oil seeps, and accidents [1]. Devastating oil leakages have caused significant loss of valuable energy resources. Moreover, the spilled oil can have short-term, life-threatening effects on thousands of species and the long-term, irrecoverable threat to aquatic ecosystems and even human health. [2–9]. For example, in the catastrophic explosion of the Deepwater Horizon oil rig in the Gulf of Mexico in 2010, the BP pipe had been leaking oil and gas to the ocean for 87 days about 42 miles off the coast of Louisiana, and about 3.2 million barrels of oil had leaked into the Gulf of Mexico, making it the largest accidental marine spill in history. Therefore, cleaning and removing oil spills under the ocean environment is an urgent task [6, 10–14]. However, the treatment of oily wastewater is becoming one of the most difficult and challenging problems worldwide in handling environmental pollution, especially in developing countries, due to the complexity of the sources and the stubborn affinity of oil to water and the formation of emulsions [15, 16]. Oil spills can persist for extended periods, during which the spilled oil undergoes several gradual physicochemical processes, including diffusion-driven spreading, solubilization, the formation of water–oil emulsions, evaporation, photolysis, and slow biodegradation. Various advanced materials and countermeasures Supramolecular Gels: Materials and Emerging Applications, First Edition. Edited by Tifeng Jiao. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

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have been developed for strategic and tactical oil spill cleaning, which can be classified into three categories: burning in situ or dispersion (chemical method); hydrocarbon natural degrading by dispersing the oil in water via dispersants as well as the adoption of bacteria and microorganisms to help break down the oil particles (biological method); and mechanical collection or absorption of oil from water surface (physical method) [6, 17–25], of which, physical absorption of oil from water is one of the most promising and practical cleanup approaches since it is convenient, is affordable, and does not generate secondary pollution [2, 26, 27]. Table 7.1 lists the detailed advantages and limitations of the mentioned oil spill remediation methods. Oil sorbents are insoluble materials that are able to assimilate oils on the surface of water through the mechanism of absorption or/and adsorption. Absorption is a process in/by which liquids diffuse and distribute throughout the molecular structure of the absorbents and cause the solid to swell over 50%, while adsorption refers to the liquid diffusion and pore filling of the internal structure of adsorbents without causing the significant swelling of solid materials. Generally, oil sorbents can be classified into three main categories: inorganic mineral sorbents, synthetic polymer sorbents, and biobased sorbents. Inorganic mineral sorbents consist of fly ash [29], zeolite [30], activated carbon [31], clay [32], silica [33], etc. These sorbents are inexpensive and available in large quantities. Moreover, they can adsorb up to 20 times their weight in oil from oil/water mixtures. However, owing to their high density, these sorbents tend to sink, making it difficult to recycle the adsorbents and adsorbed oil [28]. Synthetic polymer sorbents are a series of lightweight and porous plastic materials or composites such as polyurethane, polyethylene, and polystyrene-based cross-linked polymers [34], which are designed to adsorb oils onto their surfaces and/or into their solid structure. Although the oil uptake capacities of synthetic polymer sorbents are high, there is still a major concern for the large-scale utilization because of their poor biodegradability [35, 36]. Moreover, additional steps such as centrifugation, filtration, and settling are required to remove water, immensely obstructing the cyclic application of the sorbents [37]. These shortcomings have compelled the scientific communities to fabricate novel, efficient, and sustainable alternatives. Among these alternatives, biobased sorbents are generally considered as the optimal materials for oil spill remediation because of their low cost and high biodegradability. However, there are disadvantages in using natural materials such as wood chips [38], sawdust [39], or crop straws [40] directly for oil removal because those materials tend to adsorb water as well as oil, causing a low oil uptake capacity and sorbents to sink [28]. Therefore, the development of high-efficient biobased sorbents for oil removal remains a high priority. Typically, an ideal oil absorbent should have a high porosity, buoyancy, absorption capacity, oil/water selectivity, low cost, fast oil sorption rate, environmental friendliness, and recyclability [41]. Aerogels (with/without proper functionalization) are the perfect candidates to clean up oil spills. Aerogels refer to one class of lightweight, highly porous materials fabricated by replacing the liquid in a gel with air. Aerogels usually have a high specific surface area and abundant macro- to nanoscale pores. Those pores allow aerogels to adsorb a large amount of liquid, for instance, 20–100 times more than self-weight [42, 43]. Moreover, aerogels usually have tunable surface characteristics, which

Table 7.1

Comparison of various oil cleanup methods in the literature [28].

Classification

Methods

Examples

Advantages

Limitations

Environmental impact

Cost

Chemical

In situ burning

Combustion

Effectiv and quick removal of quantities of oil

Environment and safety concerns

Formation of large quantities of harmful smokes and viscous residues after combustion

Lowest

Chemical treatment

Dispersion; solidifiers

Simple operation, suitable to treat a large polluted area

Little effect on very viscous oil, ineffective in calm water, high initial and/or running costs

Harmful to aquatic organisms

High

Biological

Bioremediation

Microorganism degradation

Good oil removal efficiency, low operation cost

Ineffective in spill with large coherent mass, may affect aquatic life

Friendly

Low

Physical

Mechanical

Skimmers; booms

Efficient

Labor-intensive, time-consuming

Friendly

Highest

Adsorption

Use of oil sorbents

Good oil removal efficiency, simple operation, practically feasible, less secondary pollution

Labor-intensive

Friendly, its biodegradation depends on the used sorbents

Low

Source: Liu et al. [28]. © 2017, American Chemical Society.

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7 Biobased Aerogels for Oil Spill Remediation

can be tailored to diverse surface properties such as hydrophilic, oleophilic, or amphipathic [17, 44]. Hence, aerogels are promising candidate for the rapid removal of spilled oils from water and to float on water. To date, various types of aerogel absorbents have been developed, including inorganic type [silica, carbon nanotube (CNT), and graphene] [45–48] and organic one, especially biobased (such as those made from cellulose, and chitosan). Unmodified silica aerogels are oleophilic and can absorb oils, but it is difficult to separate the absorbent with oils from water due to their inherent brittleness and inability to tolerate the capillary force, thus cracking seriously when oils are absorbed inside the mesopores. Recently, a variety of CNT and graphene-based aerogels have been fabricated with excellent absorption capacities (100−913 g g−1 ), low density (∼0.003–0.5 g cm−3 ), large specific surface area (∼500–1200 m2 g−1 ), satisfactory reusability, and great ability to separate oils from oil/water emulsion [12, 45–48]. Nevertheless, the complicated synthesis routes as well as the high cost, non-renewability of the precursors, and inability to realize complete regeneration of aerogels make their application potentials slim [49]. Biobased aerogels, on the other side, not only possess intriguing features such as competitive absorption capacities and reusability, but also have merits including abundant sources, natural renewability, biodegradability, and tailor-made surface functionalization combined with the ability to be modified into various morphologies and sizes [50]. Therefore, biobased aerogels are one of the most fascinating natural oil absorbents with appropriate functionalization [51, 52]. In this chapter, recent advances in the preparation and property of aerogels utilizing renewable and biobased materials are reviewed. The oil absorption performances and mechanisms of those biobased aerogels are summarized and compared. The limitations and future works of utilizing biobased aerogels for oil spill remediation are discussed and proposed.

7.2

Aerogels: Classification, Fabrication, and Properties

7.2.1

Classification of Aerogels

Aerogels were first introduced by S. Kistler in the 1930s, he adopted the supercritical drying to extract the pore-filing liquid of wet gels and fill the solid materials with air, while retaining its original dimensions [53]. However, due to the intricate multistage fabrication procedures, aerogels had been quite a forgotten material for about 30 years until the advanced synthesis and drying techniques were developed. According to IUPAC (international union of pure and applied chemistry), aerogel is defined as a “gel comprised of a microporous solid in which the dispersed phase is a gas” [54]. Aerogel refers to any material derived from either organic, inorganic, or hybrid molecular precursors, and it is prepared by a sol-gel process and an appropriate drying method. Because of their high porosity and open structure filled by air, aerogels are the lightest solid materials ever known. Another widely acceptable definition given by Pierre in the “Aerogel Handbook” describes aerogels as “gels in which the liquid has been replaced by air, with very moderate shrinkage

7.2 Aerogels: Classification, Fabrication, and Properties

1930s

1970s

First aerogels invented by Kistler

SiO2 aerogel by Teichner

1989

2010s

2011

Novel aerogels Various polymer (C-) & RF aerogels i.e. CNTs, graphene aerogels by NASA by Pekala Evoluion of aerogels

Figure 7.1

Evolution of aerogels [26]. Source: Maleki [26]. © 2016, Elsevier.

of the solid network” [55]. This definition clearly demonstrated the preparation method of aerogels, namely, solvent removal from corresponding gels including hydrogels or organogels. According to their chemical and material origins, aerogels can be classified as inorganic and organic aerogels. Inorganic aerogels [45–48], including silica, carbon, metal, and ceramic aerogels have been widely used in energy storage, thermal insulation, and electronics areas due to their unique thermal and electrical properties [56–59]. Organic aerogels literally refer to those produced from organic molecules or polymers through physical or chemical cross-linking. More specifically, when aerogels are prepared from renewable resources such as cellulose, chitin, or algae, they can be categorized as biobased aerogels. Additionally, when more than one material or molecule is selected for aerogels preparation, the obtained aerogels are usually called composite aerogels or hybrid aerogels. Inorganic silica aerogels (Figure 7.1), being the first generation of aerogel materials, have been mostly studied in the research communities. However, the poor mechanical properties limited their applications. Although the second generation, synthetic polymer aerogels, could overcome the mechanical fragility demerit, their strength was constrained and they were mostly derived from petroleum-based resources. As the third generation succeeding silica and synthetic polymer-based ones, biobased aerogels not only exhibit good mechanical strength, but also have excellent biodegradability and recyclability [60]. In recent years, biobased aerogels (also designated as biobased sponges or foams) have been attracting marvelous attention from research communities due to the abundance and biodegradability of the precursors. Commonly used precursors include bacterial cellulose, starch, alginate, lignocellulose, and chitin (or chitosan) [61–63]. Cellulose and chitin, being the most and the second most abundant biopolymers, respectively, are widely used in the preparation of aerogels [20, 64, 65]. The cellulose- and chitin-based aerogels have been widely used in building thermal insulation, agriculture, sanitation and hygiene, and water purification applications due to their excellent strength-to-density and surface area-to-weight ratios, compression strength, flexibility, and tailorable pores [62, 66–72].

7.2.2

Fabrication of Biobased Aerogels

Cellulose, chitin, and their derivatives have been widely used for the preparation of aerogels due to their renewability, cost-effectiveness, and high functionality.

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7 Biobased Aerogels for Oil Spill Remediation

OCH2COO–

OCH3 O

O RO

OR

R=H or CH3

Methylcellulose (MC)

O RO

CH3 O CH2 CH OH

OCH2CH2OH

O R = H or OR CH COO– 2

Carboxymethyl cellulose (CMC)

O

O RO

R = H or OR CH2CH2OH

O

R = H or OR CH2CH(CH3)OH

O RO

Hydroxyethyl cellulose (HEC)

Hydroxypropylmethyl cellulose (HPMC)

(a) OCH2COO–

OH O HO

O NH2

Chitosan

O

O RO

NH

OCH2COO– R = H or CH2COO–

O Carboxymethyl chitin (CMCh)

O RO

O NHCH2COO–

R = H or CH2COO–

Carboxymethyl chitosan (CMCts)

(b)

Figure 7.2

Cellulose (a) and chitin (b) derivatives.

Cellulose and chitin contain abundant surface hydroxyl (-OH) and/or amino groups (–NHx ), which allow them to be processed directly using polar solvents or refined for functional materials after derivatization. Various cellulose and chitin derivatives can be obtained by either –OHs/–NH2 substitution or grafting (Figure 7.2). Two directions are mainly involved in the production of cellulose-based aerogels, namely, bottom-up and top-down protocols. The bottom-up approach consists of extracting cellulose from its original natural sources and rebuilding the block structure. While the top-down approach retains the natural hierarchical structure of wood and takes advantage of the orthotropic alignment, which enhances the mechanical properties [73]. Mechanically durable aerogel absorbents are especially favorable because they can open up a series of preceding unrealized applications, including skimmers that are able to withstand ocean towing forces, absorbents that can be used in high wind and waves conditions [1]. Cellulose nanofibrils-based aerogels are made by bottom-up approach and typically produced by an initial breakdown of the crystalline cellulose structure to dissolve cellulose [74, 75], followed by gelation, cellulose regeneration, solvent exchange, and a special drying process via supercritical CO2 or freeze-drying [76, 77]. This chapter will only briefly review the respective fabrication processes of biobased aerogels, while the main focus will be on the functionalization of the biobased aerogels for the oil remediation application. The dissolution of cellulose involves the use of ionic liquids, of which, DMAc/LiCl was reported with a satisfactory ability to extract cellulose [17]. After the ionic liquid extraction, native cellulose is successfully disrupted and regenerated in a non-ordered form. Cellulose nanofibrils, promising cellulosic units with a width in the order of nanometer range (i.e. 2–100 nm), are usually used in the fabrication of cellulose-based aerogels. The cellulose nanofibrils are generally obtained from mechanical defibrillation or acid hydrolysis. The mechanical defibrillation will generate cellulose nanofibrils with a bunch of microfibrils extending from the cellulose fiber surface (Figure 7.3). These extending hydrophilic microfibrils will significantly enhance the contact between cellulose and water, endowing better dispersion of

7.2 Aerogels: Classification, Fabrication, and Properties

Microfibrillating

Microfibrillating

Figure 7.3 Mechanical defibrillation process generates microfibrils extending from the cellulose fiber surface [78]. Source: Wang et al. [78]. © 2015, Royal Society of Chemical.

cellulose nanofibrils. Moreover, these microfibrils can produce “steric hindrance” that prevents the settling of cellulose nanofibrils, thus making the suspension stable [78]. To reduce the energy input, the mechanical defibrillation is combined with a preceding TEMPO-mediated oxidation or enzymatic pretreatment [79–81]. Chitin can be dissolved into strong polar solvents including ionic liquids, polar organic solvents, deep eutectic solvents, and strong alkali solvents [82]. Chitin consists of various acetyl amine groups, which can be converted into aqueous soluble chitosan via alkaline deacetylation. Chitosan is easier to handle because of the abundant free amino (–NH2 ) groups on the surface of its polymeric chains (Figure 7.2b). The protonation of –NH2 in dilute acid solutions (e.g. acetic acid) results in the formation of –NH3 + cations. These –NH3 + cations containing chitosan will be stably dissolved under electrostatic repulsion of polymeric chains. When the biobased precursors are successfully dissolved into the solvent, the gelation process is followed prior to the drying process, which is a key step because the 3D network is formed during the gelation process. Gel is made of a dispersed solution of colloid originated from a mixture of biobased precursors, solvents, and catalysts as a consequence of physical entanglement, hydrolysis, and polycondensation reactions. Depending on the pore-filling solvents, the gels can be named accordingly, e.g. alcogel and acetogel refer to those using alcohol and acetone as the pore solvent, respectively [26]. The gelation process is driven either by physical or chemical cross-linking, depending on the participating reactions during the gel formation. The physical cross-linking involves an intramolecular and/or intermolecular hydrogen bonding and physical entanglement between the biobased precursors, while the chemical one involves additional cross-linking agents such as resins (e.g. Kymene) to hold the 3D structure [83, 84]. As for the chemical cross-linking process, the reactions between the precursors and cross-linking agents are not completed after the gelation, and additional aging is needed [85]. During the aging process, the gelation solvent that contains reactive functional groups (e.g. –OH) continues to react with each other and strengthen the tenuous 3D network to withstand external mechanical forces. The aging process can be interpreted by a phenomenon called “Ostwald ripening” or “coarsening” [65], which originates from the fact that molecules in the solution dissolve from the upper state convex to accumulate on the lower state concave. Therefore, the 3D network gets fortified. During the “Ostwald ripening,”

175

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7 Biobased Aerogels for Oil Spill Remediation

variables such as pH value, time, and temperature are the key factors that can change the kinetics of the aging process [86]. In addition, the pore size, porosity, and specific surface areas are determined by the aging process [87, 88]. When the wet gels are stably formed, removal of solvent is required to generate enough internal space and lower the density of the 3D structure. For cellulose and chitin, owing to their intrinsic hydrophilicity, water or aqueous solution is usually used as solvent to prepare hydrogels. The main challenge in the preparation of cellulose and chitin-based aerogels is to preserve the shape and porous structure of gels with minimum shrinkage during water removal [68]. Conventional solvent removal techniques including oven-dry or vacuum evaporation fail to maintain the original shape of wet gels, since the evaporation of the liquid water from wet gels generates capillary force-induced pressure in the liquid/vapor interfaces that can cause severe pore deformation and solid structure collapse and result in significant shrinkage. Aerogels produced from conventional drying techniques usually have up to 90% volume shrinkage, thus being termed as xerogel [89]. Hence, specific drying processes are required to achieve a low surface tension at the solid–liquid–vapor interface [90], while traditional ambient pressure drying cannot fulfill this purpose and develops a capillary tension unless the inner pore walls are chemically pretreated with nonpolar agents (e.g. hydrophobization) [91]. In a study of the synthesis of cellulose-based aerogels conducted by Adelifard et al., ambient pressure drying was adopted using acetone, ethanol, and naphthalene (NPH) as the solvents and porous-making agent (Figure 7.4). Two separate temperature phases (70 and 120 ∘ C) were achieved with the first phase to remove acetone and ethanol, while the vacant space among cellulose was filled with NPH to prevent major shrinkage of the 3D structure, since NPH could forcefully resist against the surface tension applied to cellulose. At the second temperature phase, the gels were already stable, then NPH was sublimated easily at ambient pressure, leaving adequate amount of pores inside without shrinkage [92]. In most cases, supercritical CO2 drying or freeze-drying is introduced. The mechanisms behind these two drying techniques are converting gel solvents to supercritical Ethanol Naphtalene Acetone Cellulose fibers

Vigorous mechanical stirring

Molding and aging

stirrer

Second step drying (heating at 120°C) Sublimation of NPH crystals

Final synthesized cellulose aerogel

Icubator

First step drying (heating at 70°C) Evaporation of solvents Cellulose fibers NPH crystals

Icubator

Figure 7.4 A scheme of novel ambient pressure drying process [92]. Source: Ebrahimi et al. [92]. © 2020, Springer Nature.

7.2 Aerogels: Classification, Fabrication, and Properties

state or solid state, followed by direct removal of supercritical fluid or solid solvent to avoid the formation of liquid/vapor interface and prevent the capillary pressure. 7.2.2.1

Supercritical Drying

In 1931, Steven Kistler first invented several different kinds of aerogels including silica aerogel, metal aerogels, and cellulose aerogels through the supercritical drying technique [53]. The supercritical drying involves the substitution of the original gel solvent to a supercritical fluid, followed by adjusting the system temperature and pressure above its critical point, and then removing the supercritical fluid through pressure reduction. Carbon oxide (CO2 ) is the most widely used supercritical fluid for aerogel preparation due to its readily reachable critical temperature and pressure, i.e. 31.3 ∘ C and 7.39 MPa. Moreover, CO2 is an inexpensive, nontoxic product with low reactivity to the gel precursors under supercritical conditions. Figure 7.5 illustrates a typical aerogel preparation process via supercritical drying. Wet gels are firstly formed through the sol-gel transition (gelation) from a solution or suspension. Then, the gel solvent is substituted by liquid CO2 without structure collapse. After that, liquid CO2 is transformed into the supercritical phase (scCO2 ) via the increase of system temperature. In the supercritical status, the liquid and gaseous CO2 phases of the substance become indistinguishable. In the following depressurized step, the system pressure is gradually decreased to the atmospheric pressure through the evaporation of gaseous CO2 directly from scCO2 . The condensation of scCO2 to liquid CO2 is avoided during depressurization. Hence, the liquid–vapor interfaces are not formed, and the surface tension-induced shrinkage is avoided during gel drying. The solvent exchange and depressurization processes play critical roles in the properties of aerogels. Generally, the higher miscibility of gel solvent and CO2 are, the better properties (e.g. low shrinkage ratio and uniform pore structure) of the aerogels are [93]. Since CO2 is immiscible with water, directly replacing water with liquid CO2 from wet gels cannot result in satisfactory aerogels with intact porous structure. Therefore, for supercritical drying, an intermediate solvent is required to exchange water with a more CO2 miscible solvent before the final solvent exchange with CO2 . The tedious water exchange process usually takes a few days [94]. Methanol, ethanol, and acetone are widely used intermediate solvents for water exchange [93–96]. In addition, other than using liquid CO2 [94, 97], the scCO2 can be directly introduced to the system for solvent exchange [98–100]. Table 7.2 lists various biobased aerogels fabricated with supercritical drying method. Solvents,

Solvent exchange

Gelation

to liquid CO2

Solution/suspension

Gel

Heating

Removal

to form scCO2

of scCO2

Gel filled with liquid CO2

Gel filled with scCO2

Aerogel

Figure 7.5 Schematic illustration of aerogel preparation through the CO2 supercritical drying technique.

177

Table 7.2

Material

Biobased aerogel prepared from the supercritical drying process. Solvent used for water exchange

Supercritical drying process

Property

References

Solvent exchange condition

Supercritical condition

Depressurized Density condition (mg cm−3 )

Surface areaa) (m2 g−1 ) Transparency

TOCNFb)

Ethanol

1 l min−1 liquid CO2 flow at 15 ∘ C for 8 h

40 ∘ C for 30 min

1h

10–40

300–350

Transparent

[94]

TOCNF

Ethanol

Liquid CO2 at 18 ∘ C, 10 MPa for 1 h, three cycles

40 ∘ C for 1 h

0.8 MPa min−1

24.3

482

Opaque

[97]

TOCNF

Isopropanol

38 ∘ C, 8.6 MPa

N/A

69

689.5

Transparent

[101]

CNFc)

Ethanol

scCO2 at 100 ∘ C 20 MPa for 3 h 2 ml min−1 scCO2 flow at 100 ∘ C, 20 MPa for 3 h

30 min

21

31.5

Opaque

[98]

CNF

Ethanol

0.5 l min−1 scCO2 flow at 40 ∘ C, 10 MPa for 4 h

N/A

9–50

72–115

Opaque

[102]

CNF

Ethanol

12 MPa, 40 ∘ C with scCO2

N/A

4%d)

260–354

Opaque

[103]

Cellulose

Methanol

20 min

58

315

NA

[95] [104]

Cellulose

Ethanol

Stabilized with scCO2 at 40 ∘ C under 20 MPa for 1.5 h, followed by 0.42 g min−1 flow for 3 h scCO flow at 40 ∘ C, 15 MPa for 5 h

N/A

50–260

172–284

Opaque

Cellulose

Ethanol

12 MPa, 40 ∘ C

N/A

20–30

175–244

Transparent

Bacterial cellulose

Ethanol

1 g min−1 scCO2 flow at 40 ∘ C, 10 MPa for 75 min

N/A

8.25

200

Opaque

Chitosan

Methanol

20 MPa, 80 ∘ C with scCO2

N/A

39.5, 80–90%d)

NA

Transparent

[100]

Alginate

Ethanol, acetone

10 or 15 MPa; 32, 35, or 38 ∘ C; 90 or 180 min with scCO2

N/A

0.6% ethanol,d)0.3% acetoned)

NA

Opaque

[93]

CNWe)

Ethanol

2 ml min−1 scCO2 flow at 40 ∘ C, 10 MPa for 6 h

0.5 MPa min−1

78–155

300–350

Opaque

43–98

58–261

ChNWf)

a) b) c) d) e) f)

2

Surface area calculated using Brunauer–Emmett–Teller (BET) theory from N2 sorption isotherms of aerogels. TEMPO-oxidized cellulose nanofiber. Aerogel density. Aerogel volume shrinkage ratio. Cellulose nanowhisker. Chitin nanowhisker.

[105]

[99] [96]

7.2 Aerogels: Classification, Fabrication, and Properties

exchange conditions, supercritical conditions, depressurized conditions, and several corresponding aerogel properties are summarized. Although supercritical drying has the advantage of avoiding the generation of capillary pressure to collapse the aerogel, the mechanical stress could still be generated during the solvent exchange, supercritical generation, and depressurization steps due to the change of system pressure. Therefore, a slow ramp of system pressure rising and dropping during those steps is recommended. San-Moral et al. confirmed that a slow depressurization speed could eliminate the damage of aerogel by the mechanical stresses induced by CO2 rapid expansion [106]. Sakai et al. prepared aerogels with large surface area (350 m2 g−1 ), high porosity (99.7%), and transparent nanocellulose using a depressurization time of 1 hour [94]. Thielemans et al. prepared nanocellulose and nanochitin-based aerogels with high surface area (up to 350 m2 g−1 and up to 261 m2 g−1 , respectively) using a depressurization speed of 0.5 MPa min−1 [96, 99]. Della Porta et al. systematically investigated the synthesis of alginate-based aerogel beads through CO2 supercritical drying technique [93]. Both ethanol and acetone were used as water exchange solvents and that ensured the formation of alginate aerogels with uniform internal nanostructure and low shrinkage. Moreover, small amount of water, e.g. 2%, left in the wet gels after ethanol exchange was found having little effect on the preparation of aerogel through the CO2 supercritical process. Increasing the supercritical temperature from 32 to 38 ∘ C decreased the shrinkage ratio of obtained aerogel. Acetone was recommended as a better exchange solvent because it has better affinity to CO2 than that of ethanol. Thereby, low-shrinkage alginate aerogels were obtained at both 10 and 15 MPa when acetone was used, while they could only be obtained at 15 MPa when ethanol was used. Although supercritical drying is able to maintain the original solid framework and pore structure of wet gels and produces aerogels with high specific surface area and uniform pore structure, it is not broadly applied due to the complexity and high cost of the process [107]. 7.2.2.2

Freeze-drying

Freeze-drying is another commonly used solvent removal technique for the preparation of porous aerogels from wet gels. In a typical freeze-drying process, settled wet gels are firstly frozen to convert the liquid water to solid ice, followed by a sublimation step under low pressures (Figure 7.6). Although the liquid/vapor interface is avoided during sublimation that prevents the volume shrinkage of aerogels, the original porous internal structure of wet gels is altered because of the growth of ice crystal during the freezing stage [108]. Freeze-drying usually causes the aggregation of biobased precursors due to the formation of large ice crystals and pushing the precursors together, thus the specific surface areas are significantly reduced [7]. This drawback can be ameliorated by tuning the porous structure of aerogels via regulating the freezing parameters such as freeze rate and freeze direction. The freeze-drying is also widely termed as ice templating or freeze casting. Generally, the structure of aerogel networks is affected by both the freeze parameters and the properties of solutes or particles in solution, suspension, or wet gels.

179

180

7 Biobased Aerogels for Oil Spill Remediation

Gelation

Freezing

Lyophilization

Ice growth direction Solution/suspension

Gel

Cooling source

Aerogel

Directly freezing or ice templating

Figure 7.6 Schematic illustration of aerogel preparation through the freeze-drying technique.

Effect of Solute or Particle Properties In the mechanism of ice templating, the growth

of ice crystal excludes the solutes or particles from the aqueous solutions as well as suspensions and concentrates between ice crystals to form lamellar structure or 3D aperiodic nanofiber networks [109, 110]. The structure of aerogel networks is affected by the concentration of solutes or particles in solution, suspension, or wet gels. Qian and Zhang prepared chitosan-based aerogels via freeze-drying aqueous chitosan solutions [111]. They found that low-concentration (i.e. 0.02–0.1 wt%) chitosan solution produced nanofiber networks, while-high concentration chitosan solutions produced lamellar films or membranes (Figure 7.7). During the ice crystal

(a)

(b)

(c)

(d)

Figure 7.7 Scanning electron microscopy (SEM) images of chitosan-based aerogels obtained from chitosan solutions with different concentrations through freeze-drying (frozen at −196 ∘ C). (a) Aerogel obtained from 1 wt% chitosan solution exhibited oriented sheet networks, the scale bar is 50 𝜇m; (b–d) aerogel obtained from 0.1 wt%, (c) 0.05 wt%, and (d) 0.02 wt% solution exhibited nanofiber networks; the scale bars in (b–d) are 5 𝜇m, the inset scale bars are 500 nm [111]. Source: Qian and Zhang [111]. Reproduced with permission from © 2011 Royal Society of Chemical. DOI-https://doi.org/10.1039/B927125B.

7.2 Aerogels: Classification, Fabrication, and Properties

CNC I

CNF I

Ice growth direction

Particles entrapped between the ice fronts

Self-assembly

0.25 μm

100 nm

CNC II

CNF II

Cellulose suspension

High concentration

Lamellar structure

H2O molecules Cellulose fibers

Self-organization

Growing ice crystal 100 nm

Frozen Freeze-drying

100 nm

Cellulose dispersions

(a)

Low concentration

Oriented ultra-fine fibers

(b)

Figure 7.8 (a) Transmission electron microscopy (TEM) images of nanocellulose used for aerogel preparation (CNC, cellulose nanocrystals; CNF, cellulose nanofibers; I, cellulose I crystalline allomorphs; II, cellulose II crystalline allomorphs), and the inset in each image representing the corresponding dispersion state of each cellulose suspension at the concentration of 1.0 wt%. (b) Schematic of the possible formation mechanism of the lamellar geometry and the alignment of ultrafine fibers during the freeze-drying process (freeze temperature: −75 ∘ C) [112]. Source: Han et al. [112]. Reproduced with permission from © 2013 American Chemical Society DOI-https://doi.org/10.1021/bm4001734.

growth stage, the dissolved chitosan polymeric chains were gradually excluded from the solution and formed aggregates. At low concentration, the polymeric chitosan chains were aligned into packed nanofiber bundles (Figure 7.7b–d), while at high concentration, the polymeric chains are self-assembled to lamellar membranes (Figure 7.7a). Han et al. also observed a similar phenomenon when freeze-casting nanocellulose aqueous suspensions [112]. As shown in Figure 7.8, regardless of the original morphology of nanocellulose, the obtained aerogels exhibited lamellar structure at high concentration (e.g. 0.5 and 1.0 wt%), while aerogels with aperiodic nanofiber networks are produced when low-concentration nanocellulose suspensions were used. The formation of network structure during ice crystal growth is also affected by the affinity among solutes or nanoparticles. Jiang and Hsieh compared the self-assembly of cellulose nanofibers with different surface charges during freeze-drying. The nanofibers with high surface charges (i.e. containing 89.7% of deprotonated surface carboxyl groups, Figure 7.9 top row), corresponding to low affinity among nanofibers, exhibited aperiodic nanofiber networks after freeze-drying; however, the nanofibers with low surface charges (i.e. containing 0% of deprotonated surface carboxyl groups, Figure 7.9 bottom row) showed ultrathin film-like structures [113]. Because of the difficulty in producing nanosized ice crystals, the preparation of ice templated aerogels with nanopores or mesopores remains challenging. Recently, Borisova’s group applied an aqueous solvent of tertiary-butanol (TBA) to produce porous biobased aerogels with both macropores and mesopores [114]. The formation of macropores was attributed to ice templates, while the mesopores were formed due to the eutectic points of TBA-water.

181

7 Biobased Aerogels for Oil Spill Remediation

–1

–0.8

Water: 95 Toluene: 103

–1.0 –1.2

272 ºC 100

200 300 400 Temperature (ºC)

500

0.0 –0.2

COOH

–0.4 –0.6 –0.8

H

COO

COOH

Derivative weight (%. ºC–1)

COOH

–0.4 –0.6

COOH

COOH

COOH

COOH

COOH

COOH

COOH

0.0

–0.2

50 μm

HCl COOH

Thermal stability Derivative weight (%. ºC )

COO – Na +

COO – Na +

COO – Na +

COO – Na +

COOH

COO – Na +

COOH

COOH

Absorption ratio (g/g) Water: 74 Toluene: 136

Cellulose nanofibrils

– + COO Na

COOH

COOH

COOH

COO – Na +

COOH

COO – Na +

– Na+ COO

Liq. nitrogen freeze-drying

Self-assembled structure

NaOH

24.1%

100%

COO – Na +

COO – Na +

COO – Na +

COOH

10.3%

COOH

COO – Na +

COO – Na +

Protonation of surface carboxyls

COOH

182

50 μm

269 ºC 330 ºC

–1.0 –1.2 100

200 300 400 Temperature (ºC)

500

Figure 7.9 Schematic illustrates cellulose nanofibers with high surface charges (low protonation degree, top row) formed aperiodic nanofiber networks with low thermal stability after freeze-drying, while cellulose nanofibers with low surface charges (high protonation degree, bottom row) formed ultrathin film-like structures with low thermal stability after freeze-drying (freeze temperature: −75 ∘ C) [113]. Source: Jiang and Hsieh [113]. Reproduced with permission from © 2016 American Chemical Society DOIhttps://doi.org/10.1021/acssuschemeng.5b01123.

Effect of Freezing Parameters The advantage of ice templating technique for aerogel

preparation is the ability to produce aligned porous structure and this can be achieved by regulating the freeze rate and freeze direction. Directly place a polymer solution or wet gel in a cold environment such as dry ice, liquid nitrogen, or a refrigerator produces aerogel with less oriented porous structure [115, 116], and this process is also termed as homogeneous freezing [107]. Although the growth and orientation of ice crystals are not controlled in the homogeneous freezing process, the local alignment is normally observed because of the local temperature gradient [110]. On the other side, the directional freezing process could control the growth and orientation of ice crystals and produces aerogels with oriented porous structure and significantly different mechanical properties, which would influence the subsequent oil remediation performance. Directional freezing was reported to result in the formation of aerogels with unique spring-like configuration, which provided enough space for elastic deformation and the aerogels could be squeezed multiple times without obvious collapse. Another advantage of directional freezing was to provide special channels for liquid transportation within the aerogels that would reach efficient oil absorption [117]. Pan et al. obtained xylem-like aerogels through a unidirectional (one-direction) ice templating of cellulose nanofiber suspension (Figure 7.10a–e) [118]. Wang et al. produced radial and centrosymmetric structured aerogel through the bidirectional ice templating of chitosan and graphene oxide

7.2 Aerogels: Classification, Fabrication, and Properties

Ice growth direction

Unidirectional

(b)

(c)

(d)

(e)

50 μm

50 μm

Cellulose nanofibers (a) Bidirectional

ICE

Unfrozen Dispersion

ICE

Growth direction

(f) 250 μm

(g)

(h)

100 μm

(i)

Figure 7.10 Aerogels prepared from directional ice templating processes. (a–e) Xylem-like cylindrical shaped aerogels prepared from cellulose nanofibers through a unidirectional ice templating process (freeze temperature: −196 ∘ C); (a) schematic illustrates unidirectional ice templating; (b, c) photographs show the laser light passes through the cylindrical shaped aerogel only lengthways (b, the ice growth direction) and not sideways (c); (d, e) SEM micrographs show the cross-section (d) and longitudinal (e) views of cylindrical shaped aerogel [118]. Source: Pan et al. [118]. Reproduced with permission from © 2016, American Chemical Society. DOI- https://doi.org/10.1021/acsnano.6b05808.. (f–i) Chitosan/graphene oxide composite aerogel prepared from a bidirectional ice templating process using a cylindrical mold (freeze temperature: −196 ∘ C); (f) schematic illustrates bidirectional ice templating; (g) photographs show top (left) and side (right) views of cylindrical shaped aerogel (scale bar is 1 cm); (h, i) SEM micrographs show the cross-section (h) and longitudinal (i) views of cylindrical shaped aerogel [119]. Source: Wang et al. [119]. Reproduced with permission from © 2018, American Chemical Society. DOI-https://doi.org/10.1021/acsnano.8b01747.

mixture suspensions (Figure 7.10f–i) [119]. In another research by Chen’s group, various freezing methods were compared and their effects on the morphology of aerogels discussed (Figure 7.11). The bidirectional freezing method simultaneously created both vertical and horizontal temperature gradients under freezing and ice crystals formed in both directions, rendering an aligned parallel lamellar pattern in aerogels and subsequent satisfactory mechanical properties [120].

183

184

7 Biobased Aerogels for Oil Spill Remediation

tor era g g i n r i f Re freez

Unidirectional freezing

Freeze drying

Alginate/ cellulose B idi mixture r fre ectio ez na ing l

Figure 7.11 Schematic illustration of the effect of different freezing methods on the morphology of aerogels [120]. Source: Yang et al. [120]. Reproduced with permission from © 2018, Springer Science+Business Media, LLC DOI-https://doi.org/10.1007/s10570-0181801-8.

The freezing rate is another important factor that affects the porous structure of aerogels. A fast rate produces small ice crystals and subsequently small pore size, while the slow rate leads to large ice crystals and large pore size [26, 121, 122]. Martoïa et al. reported rapid freezing of the cellulose nanofiber solution in liquid ethanol (−114 ∘ C) solution resulted in aerogels with small pores (1–60 nm), while slower freezing of the solution in liquid benzonitrile (−13 ∘ C) resulted in aerogels with large pores (100–300 𝜇m) [123]. The pore size was controlled by the growth of ice crystals including ice crystal nucleation and ice crystal growth [124]. As shown in Figure 7.12, lower freezing temperatures favor the ice nucleation, thus forming relatively small and uniform crystals and producing aerogels with small pores; however, higher freezing temperatures facilitate the crystal growth, thus forming large ice crystals and producing aerogels with large pores. The freezing rate was also reported to influence the self-assembly of solutes or particles, thereby influencing the structure of aerogel networks. Wu and Meredith stated that chitin nanofibers tended to self-assemble to aperiodic nanofiber networks, random sheet-like frameworks, and directional lamellar membranes when freeze-dried at −20, −80, and −196 ∘ C, respectively (Figure 7.12b–d) [109]. The authors postulated that those structure variations occurred due to a combination of attractive interactions of nanofibers and ice crystal growth speed. At a slow freezing rate, chitin nanofibers were excluded by ice nucleates and self-assembled to interconnected nanofiber bundles. As the nanofiber bundles were tightly packed by the strong fiber–fiber interactions via van der Waals attraction, electrostatic repulsion, and hydrogen bonding and formed a strong network with high resistance to stress, the further growth of ice crystals could not push

7.2 Aerogels: Classification, Fabrication, and Properties

High

(a)

Nucleation

(b)

Imax

Rate

Crystal growth Gmax

2 μm Low

Temperature (°C)

High

(c)

(d)

100 μm

Freezing direction

30 μm

Figure 7.12 (a) Temperature influence on nucleation rate and ice crystal growth rate [124]. Source: Wang et al. [124]. © 2019, Elsevier. (b–d) SEM micrographs show the cross-section view of chitin nanofiber aerogel obtained by freeze casting at −20 ∘ C (b), −80 ∘ C (c), and −196 ∘ C (d) [109]. Source: Wu and Meredith [109]. Reproduced with permission from © 2014 American Chemical Society DOI- https://doi.org/10.1021/mz400543f.

the bundles to form sheet-like frameworks. However, at a high freezing rate, the chitin nanofibers did not have enough time to reorient and align into packed bundles and develop a strong network, whereas the quicker growth of ice expelled nanofibers and then disrupted the developed network structure under −80 and −196 ∘ C freezing to form sheet-like structures due to faster aggregation [109]. It is noteworthy that the freezing temperature also has significant effects on the morphology of aerogels [117]. It was preferred to freezing under liquid nitrogen (−196 ∘ C), otherwise, partial and discontinuous alignment of molecular chains would occur. Although there are various merits for biobased aerogels, one challenge still needs to be addressed before upscale application as ideal oil pollutant absorbents. This issue originates from the inherent hydrophilicity of the nanocellulose and chitin-based aerogels owing to the presence of abundant hydroxyl and amino groups, which leads to poor oil/water selectivity and the collapse of the porous structure of the aerogels once in contact with water. Low water-repelling ability also reduces the oil absorption efficiency of these pores. A possible method to transfer the hydrophilicity of biobased aerogels to hydrophobicity is modification of the precursor surface by hydrophobic molecules or pyrolysis treatment [4].

185

7 Biobased Aerogels for Oil Spill Remediation

CS Matrix

Polysiloxane Coating

NH3+ O

OH

CH3 O Si

Si

Si

OH

O CH3 O NH3 O O Si Si OH CH3 O CH3 O CH3 O Si

OH

OH

NH3+ CH3Si(OCH3)3 OH

Trace Water

(a)

O

+O

O

Si CH3

ν(Si-OH)MTMS

CH3

O

CH3

δ(C-H)MTMS

Si

Intensity (a.u.)

NH3+

Transmittance (%)

186

unmodified MTMS-modified

O 1800

1600

ν(Si-C)MTMS ν(Si-O-Si)MTMS

1400 1200 1000 Wavenumber(cm–1)

800

600

(b)

Figure 7.13 (a) Schematic illustration of the possible chemical reaction between MTMS and –OH groups on the surface of chitin aerogel [117]. Source: Yi et al. [117]. © 2020, Elsevier. (b) FITR spectra of unmodified and MTMS-modified aerogels [130]. Source: Zhang et al. [130]. © 2019, American Chemical Society.

7.2.3

Functionalization of Biobased Aerogels

An ideal oil absorbent should fulfill several requirements such as high hydrophobicity and oleophilicity, large oil absorption and retention capacities, decent oil/water selectivity, fast oil absorption rate, strong mechanical strength, reusability, buoyancy, low cost, resource abundance, and biodegradability [125]. The main objectives of the functionalization of biobased aerogel absorbents are to lower their surface energy, modify their morphology, and improve their mechanical strength. Generally, the hydrophobization functionalization is achieved via silanization using chemical vapor deposition (CVD), atom layer deposition (ALD), cold plasma treatment, sol-gel, esterification, fluorination, and introduction of surface roughness (i.e. micro- or nanoscale asperities) [126–129]. CVD is the most frequently applied hydrophobization method, which involves the flow of gaseous precursor into a chamber containing one or more heated solid samples to be coated on. Chemical reactions occur on and near the hot surface, resulting in the deposition of thin films on the surfaces of the samples (Figure 7.13a). Hence, CVD is deemed as a surface modification method. The CVD surface modification is proceeded through two steps: (i) hydrolysis of silane precursors and generation of –Si–OH groups; and (ii) the reaction between –Si–OH and available –OH groups. It has been reported that the hydrolyzed –Si–OH bond in silane is very reactive toward –OH protic groups, creating Si–O–C bonds [131], which is confirmed by the FTIR absorption peak at 1272 cm−1 (Figure 7.13b), ascribed to the characteristic vibrations of C–Si asymmetric stretching in C–Si–O units. Moreover, the silanol groups themselves may also condense on the substrate surface, generating rigid polysiloxanes with stable Si−O−Si bonds [130]. The polysiloxane coatings deposited inside the cell walls might also contribute to the elastic deformation and lead to repulsive interactions between the –CH3 groups on the adjacent surface of polysiloxane coating. Commonly used hydrophobizing agents include TiO2 , SiO2 , alkoxysilanes, chlorosilanes, alkyl ketene dimer, (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane,

7.2 Aerogels: Classification, Fabrication, and Properties

methyltrimethoxysilane(MTMS), methyltrichlorosilane (MTCS), methyltriethoxysilane (MTES), triethoxyl(octyl)silane (OTES), trimethylchlorosilane (TMCS), octyltrichlorosilane (OTCS), etc. [7, 132, 133]. Table 7.3 lists some frequently used hydrophobizing agents in recent research. Jiang and Hsieh treated the hydrophilic cellulose nanofibrils aerogels with triethoxyl(octyl)silane through CVD and obtained hydrophobic aerogels [17]. Similarly, Sun’s group applied CVD treatment of MTMS on the skeleton of the cellulose nanofibrils aerogels to hydrophobize the surface of the aerogels [20, 78]. It was reported that the longer the alkyl chain linked to the silane, the more hydrophobic the polymer became [122]. The degree of hydrophobization was expressed by the static and dynamic contact angles (CAs) of the aerogel surfaces. Based on the CA values, the surface is classified into hydrophilic, hydrophobic, and superhydrophobic. If the CA is less than 90∘ , the surface is categorized as hydrophilic; if the CA lies between 90∘ and 150∘ , then categorized as hydrophobic; finally, if the CA is more than 150∘ , the surface is superhydrophobic. At thermodynamic equilibrium, the relationship between apparent CA and the roughness of the aerogel surface can be expressed by Wenzel’s model: cos 𝜃w = r cos 𝜃 where 𝜃 w corresponds to the apparent CA, r represents the roughness factor, and 𝜃 refers to Young’s angle. The surface energy of the biobased precursors is decreased by hydrophobization treatment. Moreover, the biobased precursors are preserved after the hydrophobization, maintaining high surface roughness. Thus, the hydrophobized surface has a high CA [6]. Although the CVD process results in satisfactory hydrophobic property of the modified aerogels, there are three major demerits: (i) The preparation of the aerogels and their hydrophobization are in two separate steps, thus lengthy and not practical for scale-up applications; (ii) The distribution of hydrophobic groups formed by CVD is nonuniform throughout the aerogels [130, 145]; and (iii) the preparation conditions including the initial amount of reagent, temperature, pressure, and time of the CVD reactions need to be accurately controlled [6]. To address these issues, Tingaut’s group fabricated hydrophobic cellulose nanofibrils (NFC) aerogels by one-step freeze-drying of a cellulose nanofibrils suspension in the presence of acid-hydrolyzed MTMS (Figure 7.14) [20]. Compared with CVD method, the one-step freeze-drying generated uniformly dispersed hydrophobization agents that diffused into the pore. In another work conducted by Sai’s groups, instead of using traditional silane agents, water-soluble sodium silicate (Na2 SiO3 ) was used to diffuse into the wet matrix. The hydrophobization reaction occurred via the catalyzation of diluted sulfuric acid (Figure 7.15) [129]. Gao et al. applied octadecylamine (ODA) as the hydrophobization agent by simply dipping cellulose nanofibrils into a dopamine/ODA emulsion and subsequent freeze-drying. Superhydrophobic aerogels with uniform and intact structures were obtained. The ceraceous ODA molecules anchored on the cellulose nanofibrils surface decreased the surface energy and efficiently sealed off the hydrophilic –OH groups. In addition, the porous rough structure of aerogel trapped air below

187

Table 7.3

Aerogels for oil spill remediation: precursors, functionalization, basic properties, and oil absorption performance.

Precursor

Cellulose

Reusability (cycles, Si content Equilibrium retained capacity %) (atom %) time

Modifiera)

Drying Density Porosity method (mg cm−3 ) (%)

Surface areab) (m2 g−1 )

MTES

FDd)

3.41–5.08

99.7

94.8–195.5

0–6.35

≤25 s

30, ∼70%

KH570, TEOS

FD

N.A.

N.A.

139–157

4.98

30, 71.4-81%

Up to 159

DO, gasoline (E95), MO (5w40), CO, LSO; 70-150

[135]

MTMS

FD

2.4–24.2

98.42–99.84

N.A.

1.86

30 s

30, up to 61%

152–154

PO, WO, MO, SO; 178–228

[78]

MTMS

FD

N.A.

97.2–99.4

N.A.

N.A.

N.A.

N.A.

150.8–153.5

MO, MLO; up to 95

[83]

HMDC

FD

13.4–27.6

N.A.

N.A.

N.A.

N.A.

3, 41.6–54.3%

138.7

Gasoline, MO, SFO; 47.1–55.8

[125] [136]

N.A.

FD

N.A.

N.A.

399.9

N.A.

N.A.

10, 79.8%

Up to 153

DO, SBO; 95–145

TMCS

FD

≤6.77

≈99.6

≥169.1

1.13

12–20 s

10, >95%

Up to 146.5

Up to 185

[6]

MTES

FD

2.6

N.A.

N.A.

N.A.

60 s

N.A.

152

Gasoline, PTO, PO, MO; 8–14

[2]

Cellulose/ naphthalene

MTMS

AD

58

>96.5

1.5–5

N.A.

10 min

5, 30–50%

137

EO, Gasoline, WEO, HO, OO; 12–16

[92]

Microfibril cellulose

Pyrolysis

FD

10

99

Up to 521

N.A.

95%

149

CAO, DO, PRO, PO; 55–87

[84]

Bacterial cellulose

N.A.

N.A.

Oleic acid

FD

9.2

N.A.

397.5

N.A.

N.A.

N.A.

N.A.

PO; 33.2

[137]

SDS

FD

1.5

N.A.

47.5–151.2

N.A.

N.A.

N.A.

N.A.

PO; 145.2

[138]

Triethoxyl (octyl) silane

FD

1.7–8.1

99.5–99.9

10.9

0.2

2 min

6, 48–61%

N.A.

PO, SBO; 220–240

[17]

Graphite oxide

FD

N.A.

N.A.

128–219

N.A.

N.A.

4, 100%

N.A.

SO

[139]

HDTMS

FD

11–17.5

98.8–99.3

261.9–297.7

N.A.

N.A.

20, N.A.

121–139

MO, SFO; 78.8–162.4

[122]

MTMS

FD

10.2

99.4

23.4

N.A.

Within minutes

35, 84%

142

Gasoline, DO, PO, CNO, MIO, MO; 55–83

[130]

MTCS

FD

13–14.2

98.9–99.0

76–172

4.44

Within minutes

N.A.

150.3

DO, Gasoline, CDO, CNO, PO; 45.7–64.5

[44]

CNF/tannic acid

CDA

FD

15.5

N.A.

75.7

0.2

N.A.

121

DO, PO; 70–83

[140]

Carboxylate CNF

MTCS

FD

N.A.

Up to 96.5

N.A.

N.A.

Up to 16s

10, 92.4%

148.7

CAO, PO, SO; 19–24

[120]

Cellulose nanocrystal/PVA

MTCS

FD

22.5–36.1

97.7–98.7

Up to 38

N.A.

N.A.

10, 89%

Up to 144.5

MIO, PO, CAO; 21–27

[8]

Chitosan

MTMS

FD

27.1

96.8

20.6

N.A.

5s

10, 95%

152.3

CNO, VO, 40–43

[117]

Nanocellulose Cellulose nanofiber (CNF)

CNF/polyvinyl alcohol (PVA)

Agarose/chitosan

FD

DO, CVO, HCO

[141] (continued)

Table 7.3

(Continued)

Precursor

Modifiera)

Drying Density Porosity method (mg cm−3 ) (%)

Reusability Surface (cycles, b) area Contact Si content Equilibrium retained 2 −1 capacity %) pH angle (∘ ) (m g ) (atom %) time

Lettuce

PDMS

FD

N.A.

N.A.

16.02

N.A.

Up to 217 s

N.A.

144.2

DO, CDO, PNO, EO; 3–11

[142]

Wood

MTMS

FD

31.8

N.A.

N.A.

N.A.

N.A.

10, 88%

Up to 154.7

MO, OO, DO, PO, gasoline, CAO; 22.3–23.6

[143]

Oil sorption capacity, (g g−1 )c)

References

Graphene

N.A.

FD

N.A.

97.6

N.A.

N.A.

N.A.

5, 95%

150.5

MO, gasoline; 22–26

[144]

Reduced graphene oxide

HFTCS

FD

14.4–14.6

83.68–84.57

278–295

0.23

1.5s

10, 89%

144

PO; 50

[131]

a) The abbreviations of modifiers: MTES (methyltriethoxysilane), MTMS (methyltrimethoxysilane), TMCS (trimethylcholorosilane), MTCS (methyltrichlorosilane), KH570 (𝛾-methacryloxypropyltrimethoxysilane), TEOS (tetraethoxysilane), PLA [poly(lactic acid)], HDTMS (hexadecyltrimethoxysilane), HMDC (hexamethylene-diisocyanate), CDA (cardanol-derived siloxane), SDS (sodium dodecylsulfate), PDMS (polydimethylsiloxane), HFTCS [trichloro(1H,1H, 2H, 2H-heptadecafluorodecyl)silane], DHC (opamine hydrochloride). b) Surface area is tested from N2 isotherms. c) The abbreviations of oils: PO (pump oil), MIO (mineral oil), SO (silicone oil), MO (motor oil), SBO (soybean oil), DO (diesel oil), SFO (sunflower oil), MLO (machine lubricating oil), CO (castor oil), LSO (linseed oil), WO (white oil), PTO (plant oil), CAO (canola oil), CNO (corn oil), CDO (crude oil), VO (vacuum oil), EO (engine oil), WEO (waste engine oil), HO (hydraulic oil), OO (olive oil), CVO (crude vegetable oil), HCO (highly contaminated oil), PNO (peanut oil), PRO (paraffin oil). d) FD, freeze-drying.

7.2 Aerogels: Classification, Fabrication, and Properties

OMe MeO Si Me

(b)

OMe

H2O

Me O Si

Me O

Si

Me O

1) Stirring two hours, pH = 4, RT

Si O

OH

OH

OH

OH

OH

OH

(c)

2) Freeze-drying 1 cm

NFC (a)

(d) Me O

Me

Si

O Si

O

O

Me O Si O O H H O

NFC

(a) –

Si O

HO

H+

O

O

–

Si

SiO2 nanoparticle

Si O

O Si O

HO

OH

O

Si

O

Fo ge rm l s ed ke S le iO to 2 n

Figure 7.14 Schematic illustration of the synthesis of silylated NFC aerogels. (a) SEM micrograph of NFC (scale bar of 10 𝜇m). (b) Preparation of the polysiloxane sol. (c) Image of a silylated NFC sponge. (d) Possible interactions between the polysiloxane sol and the NFC surface [20]. Source: Zhang et al. [20]. Reproduced with permission from © 2014, American Chemical Society DOI-https://doi.org/10.1021/cm5004164.

O

O Si

O

OH

1 μm O

Immersed into

C H2 HO O HO H

C H2 HOO HO O OH

Na2SiO3

C H2 HO O HO O H O

C H2 HO O HO O OH

OH OH

O

–

Si O

O

–

nto

di

e ers

mm

I

Taken out

Freeze drying

H2SO4

The microcrack

(b)

1 μm

Figure 7.15 Schematic illustration of the formation mechanism of composite aerogels prepared from bacterial cellulose (BC) and silica composite. (a) SEM image and photograph of BC hydrogels 3D network (matrix). (b) SEM image and photograph of BC−silica CAs (SiO2 about 95.9% w/w). First, the Na2 SiO3 diffused into the BC hydrogel 3D network. Then, the (SiO3 )2− converted into SiO2 nanoparticles (represented by blue balls) as H+ diffused into the BC 3D network, and these nanoparticles assembled with the silica gel skeleton to form the IPN structure with the BC network. Lastly, the wet gels were dried with a freeze-drying method to obtain the CAs [129]. Source: Sai et al. [129]. Reproduced with permission from © 2014 Royal Society of Chemistry. DOI- https://doi.org/10.1039/C4RA02752C.

191

192

7 Biobased Aerogels for Oil Spill Remediation

OH OH OH OH OH OH OH MFC

CH3 Si OC2H5

OC2H5 OC2H5

CH3 Water

Si OH

OH

OH

O O O O O O O

CH3

O

Si

Si

Si

O O

CH3

CH3 O

Si

O

O

O Si Si O CH3 CH3 CH O 3 CH 3 O Si O Si O Si CH3

O

CH3

Polysiloxane particles

Figure 7.16 Reaction between silane and cellulose [4]. Source: Zhou et al. [4]. © 2016, American Chemical Society.

the water drops to form air pockets, i.e. a Cassie–Baxter wetting regime [3]. Xu’s group used a different hydrophobization treatment by directly immersion of the aerogels into ethanol/(MTES) solution, followed by vacuum drying. The MTES successfully diffused into the pores uniformly and avoided heterogeneity. In addition, polysiloxane particles with diameters around 200 nm were formed on the surface of the modified aerogels due to the special hydrophobization treatment. The formation of the polysiloxane particles was attributed to the self-polymerization of silanols (Figure 7.16) [4]. Various studies have confirmed that different types of hydrophobic coatings were formed depending on the silane reaction conditions (i.e. before or after freeze-drying) [146]. Silanes formed either self-assembled or covalently attached monolayers under dry conditions, while formed covalently attached cross-linked polymeric fibers/layers under wet conditions [44]. The selected hydrophobization agents would influence the silylation levels, determined by the degree of substitution (DS). Some agents may not react with the hydroxyl group in the interior of the cellulose nanofibrils, thus the DS is in a low range. The silylation levels can be determined by X-ray photoelectron spectroscopy (XPS) [135]. In a study by Sai et al., the hydrophobization agent with only one active group was used to modify cellulose or chitin aerogels, since the molecules would not condense with each other once they reacted with the hydroxyl group of cellulose or chitin, and made it possible to employ the contents of Si to evaluate the DS. Other than silanization, some other approaches were also demonstrated by researchers. Pyrolysis was used to enhance the hydrophobicity as well as the porosity of the biobased aerogels [83], since the pyrolysis process played an indispensable role in hydrophobic property via enhancing surface roughness and reducing surface energy [144]. Other than the pyrolysis treatment, Tang et al. fabricated novel superhydrophobicity and superhydrophilicity transformable aerogels by the introduction of carboxyl and alkyl group-modified SiO2 nanoparticles on the surface of melamine aerogel (Figure 7.17). Through manipulating pH values, protonation and deprotonation of the modified SiO2 nanoparticles occurred. The pH-responsive aerogels were superhydrophobic in acid and neutral environment, while gradually changed to superhydrophilic under basic environment. In a recent study, sodium dodecylsulfate (SDS) as a common anionic surfactant containing a 12-carbon tail attached to a sulfate group was introduced to fabricate nanocellulose aerogels. The role of SDS was

7.3 Biobased Aerogels for Oil Spill Remediation

Acid water COOH

Alkyl chain

Carboxyl chain

Si C N O H

(a)

Basic water

COO− Si

Alkyl Carboxylic ion chain chain

C N O H

(b)

Figure 7.17 Schematic diagrams of wetting state of (a) acid droplet and (b) basic droplet on the pH-responsive aerogels surface (the blue, brown, pink, and gray areas represent responsive sponge skeleton, acid droplet, basic droplet, and SiO2 nanoparticle, respectively) [49]. Source: Tang et al. [49]. © 2020 Elsevier.

in dispersing the nanocellulose and endowing hydrophobicity due to its excellent foaming ability as well as the possession of hydrophobic 12-carbon tail [138].

7.3

Biobased Aerogels for Oil Spill Remediation

Ever since the industrial revolution, the oil rig accidents, oil vessels sinking, and industrial activities have led to the discharge of oil contaminants into the marine environment. The oil spill remediation has been a continuing challenge to both research and industrial communities. In Chapter 6, fabrication and functionalization of biobased aerogels were summarized. In this section, the specially functionalized biobased aerogels for oil spill remediation will be reviewed. Basic parameters that influence oil absorption performance are listed in Table 7.3.

7.3.1

Parameters That Affect the Oil Absorption Performance

It is universally accepted that the oil sorption performance of biobased aerogels depends not only on the morphology (e.g. surface wettability, total pore volume,

193

194

7 Biobased Aerogels for Oil Spill Remediation

specific surface area, and pore structure), but also on the density and viscosity of oily liquid, the capillary effect, van der Waals forces, hydrophobic interaction between the oils and absorbents, and the mechanical properties of the aerogels [4, 17]. Promising biobased aerogels should fulfill all these requirements. Under these guidelines, quite a few researches have been conducted to fabricate appropriate biobased aerogels for satisfactory oil absorption performance. The pore morphology and porosity are two different concepts. Aerogels with the same porosity can have significantly different absorption capacity, partly due to the variation in pore morphology. The equation of porosity only takes the volume of the aerogel and matrix into account, which only provides the porosity data, while not the information concerning the status of the pores. There are different pore morphologies for aerogels, and it can be classified into closed pores, blind pores, cross-linked pores, and through pores [8]. High porosity, high specific surface areas, and large pore volume provide sufficient space to store the absorbed liquids and benefit the oil transfer [17, 136]. Aerogels usually contain diverse pores including micropore (50 nm), and micron-sized pores (up to several tens of micrometers). Those pores usually exhibit synergistic effects of hierarchical and multiscale pore structures on excellent adsorption of oils, of which, the macropores play an important role in permeating the oils into the aerogels, serving as the driving unit for oil adsorption. The pore size distribution of aerogel is usually determined by liquid N2 adsorption/desorption isotherms. However, the N2 isotherms can only estimate the volume and surface of pores with cavities less than 50 nm, thus the macropores and larger pores cannot be measured [147]. Nevertheless, N2 adsorption/desorption isotherms are still a useful technique to investigate the microstructures of aerogels, especially for the comparison of aerogels before and after surface hydrophobic modifications [3]. There are six types of isotherms in total by IUPAC classification, while three are commonly found in the adsorption on polar materials, namely type I, II, and IV (Figure 7.18a) [148]. Type I is related to micropores, and the reversible type II is related to nonporous or macropore structure (>50 nm), while type IV is related to mesopores. Quite a few studies on the hydrophobization of aerogels confirmed the existence of type IV mesopores [2, 15, 136, 137]. In addition to the types of adsorption/desorption isotherm, pore shape also influences the mechanisms of condensation and evaporation. Four types of hysteresis have been classified (Figure 7.18b). Type H1 hysteresis is related to channels with uniform sizes and shapes, type H2 is related to channels with a pore mouth smaller than the pore body, type H3 hysteresis is related to a very wide distribution of pore size, and type H4 corresponds to small amounts of mesopores limited by micropores [138]. Cao et al. introduced polydopamine (PDA) and octadecylamine (ODA) to functionalize the nanocellulose aerogels. The obtained aerogels exhibit the type H3 hysteresis loop, indicating a wide distribution of pore sizes of the aerogels. The anchoring feature of PDA improved the alignment of the pore structure and created open pores, thus making the aerogels more accessible to oils (Figure 7.19) [3].

7.3 Biobased Aerogels for Oil Spill Remediation

I

II

IV

B

B

na/ms

P/P°

(a) H1

H2

H3

H4

na/ms

(b)

P/P°

Figure 7.18 (a) The three types of adsorption isotherms usually found by nitrogen adsorption in aerogels (types I, II, and IV by IUPAC classification); (b) four typical hysteresis loops of adsorption isotherms [148]. Source: Chang et al. [148]. © 2009, Oxford University Press.

The surface wettability is important with a superhydrophobic surface resulting in great selectivity to oil. Silanization treatment is the most used approach to convert hydrophilic surfaces into hydrophobic, endowing aerogels non-wettable characteristic and excellent preferential absorption of oils over water [17]. Pyrolysis or hydrothermal treatment was also reported as feasible ways to at least partially remove hydrophilic groups of biobased aerogels and increase the non-wettability [139]. The pyrolysis treatment could significantly decrease the surface wettability from 1∘ up to 128∘ . In addition, the pyrolysis could also retain the porous structure of corresponding aerogels with slight shrinkage of 15% and a decrease of density due to the evaporation of volatile components. After oil absorption, the pyrolyzed aerogels could be easily regenerated by distillation and direct combustion in air [149]. The density of biobased aerogels also has a significant effect on the oil absorption capacity. Basically, lower density indicates greater oil absorption [78]. Hence, the incorporation of functional groups would increase the density, posing a negative impact on the absorption capacity [134]. The improvement of mechanical properties is one of the top priorities for the functionalized biobased aerogels, since it is of critical importance for cyclic application. Both inorganic and organic additives were introduced to strengthen the compressive strength. In situ growth of MnO2 nanosheets on the surface of the biobased aerogel was reported and the intercalated network endowed superior

195

196

7 Biobased Aerogels for Oil Spill Remediation

NFC NH

or pH induced self-polymerization

O

O

NH

NH

n O

NH

or O

N H 39

n

PDA

C

18

O

ODA-PDA

n N H 39 C 18

OH n

PDA interface Schiff-base reaction

NFC ODA surface layer

Figure 7.19 Schematic illustration of the fabrication mechanism of the mussel adhesive-inspired, superhydrophobic NFC-based aerogels [3]. Source: Gao et al. [3]. © 2018, American Chemical Society.

elasticity for the aerogels [15]. Xu’s groups successfully produced superhydrophobic microfibrillated cellulose aerogels (HMFCAs) with high oleophilicity, ultralow density (≤5.08 mg cm−3 ), superior porosity (≥99.68%), and high mechanical stability through an eco-friendly liquid phase silanization reaction [4]. The volume-based absorption capacities of the HMFCAs could reach up to 90%, indicating that almost all of the pores in HMFCAs were filled with oils. The aerogels also exhibited strong compressive strength and readily collected through a simple squeezing process ascribed to the cross-linked Si–O–Si bonds in the polysiloxane, guaranteeing its possibility for cyclic oil absorptions [15]. Some researchers suggested that the absence of additional components may facilitate the formation of a strong hydrogen-bonded network of cellulose, rendering improved mechanical integrity [150, 151], while others suggested that the incorporation of mechanically strong agents into the aerogel matrix be beneficial for multiple squeezing-absorption cycles (Figure 7.20) [44, 130]. Another study on the improvement of the mechanical properties of aerogels was conducted to introduce cellulose nanocrystals (CNCs) into the matrix [8]. The high compressibility of aerogels was attributed to the strong, dense, and porous microstructures. In addition, the CNCs played an important role in such high compressibility. It has been well-known that CNCs have excellent mechanical properties and cannot be easily deformed in both longitude and transverse directions. Thus, CNCs could have flexibility for shape recovery [72]. The introduction of CNCs not only increased the mechanical strength of the aerogels by acting as a strong rod in the matrix, but also helped the aerogels to bounce back to its original status under cyclic compressive mode [8]. The proposed relationship between the mechanical properties and the morphology was that the Young’s modulus and yield

7.3 Biobased Aerogels for Oil Spill Remediation

1. Compression 2. Release 70% strain

Figure 7.20 Schematic illustration of the changes in the aligned lamellar structure for modified PVA/CNF aerogels with cyclic compression in the perpendicular direction [130]. Source: Zhang et al. [130]. Reproduced with permission from © 2018, American Chemical Society DOI- https://doi.org/10.1021/acssuschemeng.8b03554.

strain increased with increased densities, smaller pores, or more closely spaced pore walls. The entanglements and high aspect ratio of cellulose nanofibrils also resulted in large specific compressive modulus of the corresponding aerogels [17]. Other than the features of aerogels, the viscosity of oils has a significant effect on the performance of oil absorption, since it has an impact on the movement velocity of the liquid molecules: higher viscosity needs more time to reach absorption equilibrium, while lower viscosity needs less time to reach the equilibrium [3, 6]. One way to lower the viscosity is to increase the temperature of the oil, while it is not feasible in some cases. To address this issue, a new type of cellulose nanotube/polydimethylsiloxane functionalized polyurethane aerogels was fabricated to in situ lower the viscosity of oils by photothermal activity [152]. The high photothermal activity (99% light absorption under solar simulator 1000 W m−2 ) efficiently reduced the viscosity of oils and made the oil absorption much easier. However, one drawback of the low viscosity of the oils is that the retainability is retarded due to the negative anchoring effect on the pore walls.

7.3.2

Mechanisms of Oil Absorption

The oil absorption process occurs in three major steps: (i) the oil molecules diffuse onto the aerogel surface; (ii) the oil stays by the capillary forces; and (iii) the oil accumulates and fills into the pores [153]. The oil adherence to the aerogel surface occurs due to the intramolecular interaction and van der Waals forces. The hydrophobic aerogel surface offers the possibility of overcoming the minimum energy needed for the oil to be absorbed [154]. In addition to that, the pore morphology and the capillary forces are considered as essential factors for a higher adsorption capacity. Moreover, the surface roughness is a key factor in the absorption process and prevents the leakage of the absorbed oil [122]. The pseudo-first and pseudo-second order models [155, 156] are commonly used for the oil absorption of biobased aerogels. If the oil absorption process fits the pseudo-first order model, then it relates to physisorption; whereas the pseudo-second order model relates to chemisorption. In most cases, the pseudo-second order model is used, and the activation energy (Ea )

197

198

7 Biobased Aerogels for Oil Spill Remediation

can be calculated from the model via the change of the absorption rate constant k, with temperature T (K), using the Arrhenius equation [38, 49]: Ea RT where A represents the pre-exponential factor and R represents the gas constant (8.314 J mol−1 K−1 ). By plotting lnk against 1/T, Ea can be determined from the slope. The activation energy values obtained from the pseudo-second order model involves higher forces than that from the pseudo-first order model [157]. To understand the first step of the oil absorption process, the intraparticle diffusion model is used to determine the controlling step of particle diffusion during the adsorption process: ln k = ln A −

qt = kid t0.5 + C where kid and C are the intraparticle diffusion rate and the intercept, respectively. It was stated that the diffusion of oils consisted of three controlling steps: (i) boundary layer diffusion; (ii) diffusion into the mesopore; and (iii) micropore diffusion. The boundary layer diffusion was believed to be the fastest step, while the micropore diffusion was the rate-controlling step [140]. To better understand the second absorption step stemmed from capillary forces, the aerogels were modeled as a bundle of capillary tubes [142]. Before applying a suction force to aerogels, the capillary pressure of the oil–air interfaces at the top of aerogels can be expressed by the Young–Laplace equation: 2𝛾OA cos 𝜃1 R where Ps represents the capillary pressure, R represents the radius of the tube, 𝛾 OA represents the oil–air interfacial tension, and 𝜃 1 represents the contact angle of oil in the tubes. Thus, after applying suction force, the capillary pressure (Ps ) at plain A increases by decreasing the contact angle from 𝜃 1 to 𝜃 3 (Figure 7.21a, b), creating a new pressure equilibrium between plains A and C. Hence, air does not break through the interfaces. The capillary pressure at the bottom of the aerogels increases with decrease in the contact angle from 𝜃 2 to 𝜃 4 (Figure 7.21a, b) after the application of a suction force. The increased capillary pressure on the interfaces and hydrophobic surface prevent water permeation into the aerogels. However, since there is no capillary pressure in plain B, the pressure equilibrium between plains B and C is broken after the application of suction force. The pressure difference drives oil from plain B to C (Figure 7.21a–c). The oil–water and oil–air interfaces act as sealed films. Only floating oil penetrates the aerogels and flows from plain B to C under a pressure difference. A similar capillary force theory was proposed by Yang et al. [120]. They stated that a capillary action was formed between the lamellar structures of the aerogels. The oil was driven by capillary force and quickly permeated through the macropores (Figure 7.21d). Once the oil was sucked clean, the suction force drove the oil to gather in the macropores. When the oil above the nozzle of the pipe was completely removed, air channels formed and left the remaining oil below the nozzle of the pipe. PS =

7.3 Biobased Aerogels for Oil Spill Remediation θ3

θ1

C

PDMS-CA

Suction Air Air

Suction P

A

Oil

Air

Water Pipe

P0

B

θ2

Oil

(b) Pump

Oil

Water

θ4

(a)

Air

(c) Pump

Pump

Oil Water

Water

Water

(d)

Figure 7.21 (a–c) Simulation schematic representations of a biobased aerogel before (a) and after (b) application of suction force. (c) Plausible mechanism of the self-controlled oil collection [142]. Source: Wang et al. [142]. © 2016, American Chemical Society. (d) Schematic illustration of absorption processes of oil by sodium alginate/cellulose aerogel [120]. Source: Yang et al. [120]. © 2018, Springer Nature.

In another study, the linear wicking of fibrous webs was determined by the Lucas–Washburn’s model [125]: ] [ rc 𝛾 cos 𝜃 M = 𝜌l A𝜑 × t 1∕2 2𝜇 where, M is the mass of liquid imbibed by the sorbent, 𝜌l is the density of wetting liquid, A is the base area of the specimen, r c is the Washburn’s ideal capillary radius, t is the time after a capillary tube touches the liquid, and 𝜑, 𝜇, 𝛾,and 𝜃 are the porosity, viscosity, surface tension of liquid, and contact angle at the liquid–solid–air interface, respectively. The porosity is expressed as: 𝜌 𝜑 = 1 − bulk 𝜌f The permeability of the aerogels was calculated as: 𝜑𝛾c 2 8 Oil accumulates and fills into the pores of aerogels during the third step. It was postulated that the nonrigid medium swelling effects hindered the oil flow through the pores. During the oil absorption, the aerogel precursors were swollen by the oil, thus the porosity and the mean capillary radius were reduced. When oil viscosity was high, the flow rate through the pores was slow, thus the biopolymers in the lower part of the sorbent had enough time to swell before the aerogels became fully saturated. Therefore, the precursor swollen and reduction of the mean capillary radius rendered a deviation from predictions made by the model. K=

199

7 Biobased Aerogels for Oil Spill Remediation

7.3.3

Post-processing of Aerogel Absorbent After Oil Spill Remediation

The convenience of post-processing of aerogels after oil absorption is essential for sustainable development and the scale-up application. Traditional post-processing methods involve extrusion [158], combustion [159], distillation, and centrifugation [46]. However, the oil removal efficiency is either not satisfactory or complicate processes are needed, resulting in subsequently reduced oil absorption capacity. In addition, some of the methods will cause permanent damage to the aerogels and compromise the cyclic usage. Therefore, traditional methods are not practical for industrial applications. Researchers have been devoted to developing novel feasible post-processing methods to realize the complete regeneration of aerogels in an eco-friendly and energy-saving way. Facile pH-sensitive aerogels were fabricated for in situ oil absorption and removal [49]. The basic idea was to graft ionizable functional groups such as carboxyl to make them protonated and deprotonated by changing the pH value (Figure 7.22). During the oil absorption process, the pH value was controlled within acidic range, while within alkaline range during the oil removal process. As a result, continuous oil/water separation could be realized. In addition to this pH-induced recycling technique, other methods were also reported such as the introduction of magnetic components into the aerogels for facile recycle of the aerogels by permanent magnets [137], as well as the fabrication of aerogels inspired with the wood hierarchical microstructure for continuous flow of oils along the freezing direction of the aerogels [117]. CH3

CH

3 CH

O

Si

N

O

Acid water

COOH

O O

O Si

Si O

Si C N O H

O N

O

N

C

C O OH

C

O CH 3

OH

O

O Si

C

O

O

O

O

SiO2

O Si

Si

CH3

O

O

N

O O

O Si

Si O

C

Si

Si

O

C O H

3

O O

C

CH3

O

H

C

O

200

Alkyl chain

O

MS

pH up

Basic water

pH down

COO−

Alkyl chain

Oil

Carboxyl chain

Carboxylic ion chain

Si C N O H

Water

Figure 7.22 pH-sensitive aerogels for oil absorption and simple removal [49]. Source: Tang et al. [49]. © 2020, Elsevier.

References

7.4

Conclusion and Future Scope

In this chapter, recent researches on the fabrication, oil absorption mechanisms, and performance of biobased aerogels were summarized. With the introduction of novel synthesis approaches and functionalization such as silanization and pyrolysis treatment, biobased aerogels with excellent oil absorption properties were obtained. Nevertheless, there are still some challenges needed to be overcome for the sake of scale-up applications. The commonly adopted bottom-up approach is time-consuming and energy intensive, thus an alternative top-down approach inspired by the structure of native biomaterials is recommended and detailed study needed. In addition, the CVD hydrophobization treatment usually ends up with nonuniform functionalization, obstructing the large-scale application. More advanced, uniform, and facile treatment is urgently needed. Increasing interests have been drawn in carbon-based materials such as carbon nanotubes and graphene owing to their extraordinary mechanical, hydrophobic, thermal properties, and strong interatomic interactions with hydrophobic agents. The incorporation of these carbon-based materials with cellulose and chitin will also introduce heteroatoms in the pore surfaces and the matrix framework, and increase the surface roughness, thus endowing the aerogels with greater oil absorption performance. For future studies, continuing attention should be focused on (i) the application of biobased aerogels in continuous oil/water flow; (ii) the evaluation of the scale-up oil adsorption performance; and (iii) the fundamental mechanisms research on the oil absorption of biobased aerogels.

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8 Luminescent Supramolecular Gels Xue Jin and Pengfei Duan CAS Center for Excellence in Nanoscience, CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, National Center for Nanoscience and Technology (NCNST), Beijing, P. R. China

8.1 Introduction Supramolecular gels, as an important class of soft material, have evolved to be one of the most attractive subjects within supramolecular chemistry and material sciences during the past few decades [1, 2]. During the process of gelation, self-assembly has proven to play an essential role by employing weak intermolecular non-covalent interactions, such as H-bonding, dipole−dipole attraction, π–π stacking, electrostatic and van der Waals force, hydrophobic and hydrophilic effect, and metal−ligand coordination [3]. Driven by such non-covalent interactions, the organic gelators self-assemble to form nanometer scale structures, which sequentially build up micrometer scale interwoven three-dimensional networks, entrapping solvent molecules. This process is spontaneous and easily controlled without using advanced techniques. By a small modification of the structure of the gelator subunit or by changing the experimental conditions, such as the polarity of solvents, the morphology and the properties of the resulting gels could be finely tuned [4]. Thus, this provides a simple yet highly efficient approach to the preparation of functional soft materials from small building blocks. In particular, the self-assembly of luminescent supramolecular gels has attracted much attention because of their potential applications in sensor materials, data storage, organic light-emitting diodes (OLEDs), anti-counterfeiting materials, etc. [5–11]. By introducing luminophores into supramolecular gel, many luminescent gels have been designed and synthesized. Generally, by virtue of inherent electronic properties such as luminescence, charge carrier mobility, and electronic conductivity, organic molecules containing π-moiety have been widely used in design of luminescence gel [12]. These luminescent gels can roughly be classified into two types: (i) directly tailor-made gelators incorporating chromophores or dye groups and (ii) doping luminescent moieties into the matrix of the gel [13, 14]. Likewise, luminescent inorganic nanomaterials are also able to be co-assembled into supramolecular gels. These hybrid luminescent gels, including quantum dot gels or perovskite nanocrystals gels, exhibit interesting chiroptical properties [15–18]. Supramolecular Gels: Materials and Emerging Applications, First Edition. Edited by Tifeng Jiao. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

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Especially, luminescent materials with circularly polarized luminescence (CPL) activity have been drawing extensive attention owing to their potential applications in the fields of photoelectric devices, security systems, chiral sensing, asymmetric catalysis, and so on [19–23]. Among various organic molecules, low-molecular weight gelator (LMWG) or simply gelator is a unique kind of molecules. Self-assembly provides a powerful solution for precisely controlling the arrangement of building blocks and enhancing certain properties [24–27]. For example, self-assembly can significantly amplify the circular polarization of some CPL-active materials [28]. In addition, not only chiral but also achiral chromophores can be endowed with CPL activity via self-assembly [29]. Based on such an efficient approach, various organic and inorganic materials can be embedded into a chiral host, which generates numerous CPL-active materials. This chapter will highlight and put into perspective a few recent advances in the field of luminescent gels. These gels will be discussed according to different types of photoluminescence: (i) fluorescence is singlet-to-singlet emission (i.e. a spin-allowed transition); (ii) phosphorescence is triplet-to-singlet emission (i.e. a spin-forbidden transition); (iii) upconverted luminescence is a higher energy emission that is excited by lower energy light; and (iv) circularly polarized luminescence is asymmetric emission of chiral emitter. A detailed look to these four kinds of luminescent gels will be remarked in the following Sections 8.2–8.5.

8.2 Fluorescence in Supramolecular Gels Organic π-conjugated molecules as fluorophores are widely exploited in soft materials as well as biological, optoelectronic, and photonic applications [30, 31]. Since most of the π-conjugated molecules have a strong tendency to aggregate and to exhibit solvatochromic behavior, self-assembly of these moieties in gel system has drawn great interest among chemists [32]. Synthetic modification of organic dyes with suitable gelators has resulted in a large number of gelators exhibiting interesting optoelectronic properties [33]. Additionally, chromophores doping in gels can markedly influence the optoelectronic properties of the constituent chromophores by tuning the assembly [34]. For instance, supramolecular gels have been devoted to avoiding ACQ of planar aromatic dyes in the condensed phase. PBI is an ACQ organic dye that exhibits strong fluorescence with high quantum yield in molecule state. Ikeda et al. synthesized an organic gelator 1, which is tris(phenylisoxazolyl)benzene possessing PBI and long alkyl side chain (Figure 8.1) [35]. The emission property of a PBI moiety can be regulated through its supramolecular assembly. Dioxane and decalin solvents facilitate the self-assembly of the tris(phenylisoxazolyl)benzene moiety without the assembly of the PBI moiety, resulting in gels with fluorescence. In contrast, benzene gel is colored in deep red without fluorescence, leading to the completely stacked J-aggregates of the PBI moieties. A difference in solvation from the PBI moiety and the phenylisoxazole moiety may drive this unique fluorescent property in the gel states.

8.2 Fluorescence in Supramolecular Gels R1O OR1

ON

1,4-

O N O

N R2

N

O

R2 = O

hv

1 R1 = n-C10H21

O

ane

diox

NO

C6H13

Ben

zen

e

C6H13

Figure 8.1 Schematic illustration of solvent-induced emission of PBI organogels. Source: Ikeda et al. [35]. © 2016, The Royal Society of Chemistry.

Stimuli-responsiveness is a vital feature of supramolecular gels. Control of gel-to-sol transition by external stimuli in supramolecular gels offers stimuli-responsive functional materials [36, 37]. For example, photochromic molecules with planar and nonplanar structures are suitable for constructing photoresponsive supramolecular gels and influencing the assembly of gels. The nonplanar structure prevents the formation of stacked structures, resulting in the transformation from gel to sol. A cyanostilbene moiety with AIE property fulfills the requirements; the planar trans-isomer can isomerize to the nonplanar twisted cis-isomer by photoirradiation [38]. The reversible control of the self-assembling process of photosensitive organogels induces a sol-to-gel transition. Park group’s has obtained an organogel assembled by gelator 2 (CN-TFMBE) with simple trifluoromethyl (CF3 ) substituents instead of the common gelaton parts, endowing the gel system with a strongly enhanced fluorescence emission by the gelation process [39]. As shown in Figure 8.2, the planar trans-isomer forms the stacked assemblies in various organic solvents, whereas the twisted cis-isomer does not assemble due to steric requirements. The remarkable 100-fold fluorescence enhancement from CN-TFMBE gels can be explained within the context of the AIE phenomenon. Isolated CN-TFMBE molecules in dilute solution are considered to be significantly twisted by the steric interactions in biphenyl units, as well as the bulky cyano groups attached into vinylene moiety, which generally suppresses the radiative decay channel. However, the more planar and conjugated conformation of CN-TFMBE gels was highly crystalline and nontransparent due to a tight and close packing of gelator, which practically hampered photoisomerization process. Another well-designed cyanostilbene gelator 3 (PyG) with bulky and flexible spacers via amide group can form transparent and fluorescent gel, which successfully demonstrates its photo-induced morphological change (gel-to-sol) upon photoisomerization, accompanying a unique fluorescence color switching, as shown in Figure 8.3 [40]. Upon the 460 nm LED light irradiation, the gel totally changed into the viscous sol within ∼15 minutes. Correspondingly, yellow gel became colorless in the sol and the emission shifted from greenish blue to blue. The main driving force for this morphological change is attributed to the trans-cis photoisomerization of the cyanostilbene moiety in the gel networks.

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2

F3C

CF3 NC

F3C

CF3

CN-TFMBE

(a)

(b) 1.0 Nanoparticle suspension

THF

0.8 0.6 0.4 0.2

Nanoparticle suspension

A N P

THF x 50

0.0 300

(c)

B C D T

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Absorbance

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500

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(d)

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Figure 8.2 (a) The chemical structure of CN-TEMBE. (b) Photo of CN-TFMBE solution at 60 ∘ C, gel at 20 ∘ C under natural light and UV light (365 nm) in 1,2-dichloroethane. (c) UV/vis absorption spectra and (d) PL spectra of CN-TFMBE in THF and its nanoparticle suspension (THF/water (1 : 4) mixture). The PL intensities were normalized by the corresponding UV absorbance. Inset photograph shows the fluorescence emission of CN-TFMBE in different solvents and solid state (B, benzene; C, chloroform; D, 1,2-dichloroethane; T, THF; A, acetonitrile; N, nanoparticle suspension; P, powder) under 365 nm UV light illumination. Source: Reproduced with permission from [39]. © 2004, American Chemical Society.

Two-component supramolecular gels provide an easy way to achieve multichannel stimuli-responsive functional materials [41]. Based on these, Kim et al. have reported a multicolor tunable and multistate switchable organogel, using the co-assembly of the abovementioned AIE organogelator 2 (CN-TFMBE) and a turn-on type photochromic diarylethene dye [42]. As shown in Figure 8.4, by a combination of orthogonal stimuli of heat and light modulation, the mixed organogel can be reversibly switched among four different states: blue-emitting gel, nonemissive sol, green-emitting gel, and green-emitting sol. It is noteworthy that this four-state switching constitutes a combinational logic circuit consisting of two stimuli inputs and three outputs. The fluorescence image patterning can be reversible written and erased on this mixture gel system. The most important aim to fabricate luminescence supramolecular gel is for the potential applications in sensor, photoelectric device, and biomedical materials [31]. Recently, Jiang’s group synthesized the ibs-cyanostilbene derivative gelator 4, showing an interesting anion binding property [43]. As shown in Figure 8.5, CO2 sensing is here achieved through the formation of a host–guest complex between the gelator molecules and the anion of carbamate ionic liquid, which is obtained in situ by reaction of CO2 with aliphatic diethylamine (DEA). While the gel aggregate system

8.3 Phosphorescence in Supramolecular Gels

O

CN N

H N C O

O O

3 (PyG)

(a)

(b)

3.5 1.0 PL intensity (normalized)

Absorbance (a.u.)

3.0 2.5 2.0 1.5 1.0 0.5 0.0

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600

650

Figure 8.3 (a) Chemical structure of the cyanostilbene-containing gelator molecule 1 (PyG). (b) Photograph of PyG transparent gel in cyclohexane in room light. The inset shows the fluorescence photograph of the gel under UV light. (c) UV-visible absorption spectra and (d) emission spectra of the gel in cyclohexane before (black) and after (red) the 465 nm light irradiation. Each inset shows the sol state after the irradiation under room light (left) and UV light (right), respectively. Source: Reproduced with permission from [40]. © 2014, The Royal Society of Chemistry.

responds to CO2 , in the presence of excess DEA, by emission quenching with moderate sensitivity (detection limit of 908 ppm). Indeed, it allows for the detection of CO2 by fluorescent quenching and modulation, reaching a very high sensitivity and a low detection limit, as low as 4.5 ppm.

8.3 Phosphorescence in Supramolecular Gels Like fluorescence, phosphorescence also is a kind of photoluminescence. The difference is that phosphorescence has a large Stokes shift and that there is less spectrum overlap of emission and excitation, thereby effectively avoiding interference of excitation light and scattered light [44]. Additionally, the lifetime of phosphorescence is longer than that of fluorescence. Although phosphorescence has a long excitation lifetime, it is easily quenched due to the motion and collision of phosphor itself. In the early days of phosphorescence study, an attempt had been made to inhibit molecular motion and collision by using low temperatures [45]. Nowadays, a number of methods to induce room temperature phosphorescence (RTP) have been

219

220

8 Luminescent Supramolecular Gels

F F

F3C

F F F F Et

CF3

(a)

CF3

F3C

Cooling Heating

NC

2

CF3

CF3

F3C

Planarization & stacking

F3C

NC

CF3

CN-TFMBE (sol)

0S

CF3

Et

CF3-BPDBTEO (closed)

twisted

F3C

F F

S S Et O O O O

CF3-BPDBTEO (open)

F3C

F F

F3C

Vis.

S S Et O O O O

F3C

UV

CF3

F F

CF3

CN-TFMBE (gel)

1S UV Vis.

Input A

Output 1

NOR

Input B Cooling

Heating

Cooling

Heating

YES

Output 2 2G

Output

1G

G

UV

Output 3

NOT

Vis.

S

YES

(b)

(c)

Figure 8.4 (a) Chemical structure and the photochromic reaction of CF3 -BPDBTEO. Green emission is switched on/off upon UV/vis irradiation. Chemical structure and thermal switching of gelator 2 (CN-TFMBE). Blue emission is turned on/off upon cooling/heating. (b) Photographs of the four states (2G, 0S, 1G, 1S) of the mixture. (c) Schematic illustration for the organogel-based integrated logic circuit. Source: Reproduced with permission from [42]. © 2017, Wiley-VCH. DOI: doi.org/10.1002/adfm.201706213.

developed [46]. However, phosphor should be entrapped in a rigid microenvironment to inhibit its motion because of fast motion of phosphor at room temperature. Therefore, RTP technology is mainly focused on the development of efficient substrates, which can locate phosphor. On the other hand, reducing the mobility (e.g. rotation) of a chromophore is one of the most explored and effective strategies to increase the phosphorescence quantum yield by diminishing the impact of quenching and non-radiative processes. Supramolecular gels represent a flexible and highly tunable matrix ideal for controlling the environment of luminophores. RTP in organic materials has also been noticed in host systems like supramolecular gel networks. As shown in Figure 8.6, Wang et al. reported thermally on–off RTP by entrapping 3-bromo-quinoline into supramolecular gels formed by the self-assembly of commercial sorbitol derivative gelator 5 (DBS) [47]. The gel state exhibits strong phosphorescence, because the chromophore is entrapped in the hydrophobic 3D network restricting its motion and avoiding the quenching of its phosphorescence. This RTP gel was found to be non-quenchable in the presence of different anion

8.3 Phosphorescence in Supramolecular Gels

δ+

OC16H33 Hb

He

δ–

O Hd

O NHc

Ha

HN

Hf Hh Anion binding site

(a)

Hg CN

NC

4

Hi 400

400

CO2

320

320

I 455 nm

Intensity (a.u.)

DEA

240

240 160 80 0 0.0

160

2.5

5.0

7.5 10.0 12.0 15.0

Equivalents of CO2

80

Gel

Gel aggregates 0

400

(b)

450 500 550 600 Wavelength (nm)

650

Equivalents of CO2

(c) 0

0.4

0.9

1.5

2.25

3

3.75

4.5

5.6

7.5

11.3

15

Figure 8.5 (a) Chemical structure and electrostatic potential map for the gelator 4 calculated at the DFT B3LYP (6-31G**) level of theory and its molecular formula. (b) Photos of the gel (10 mg in 2.9 ml toluene) and the gel aggregates (gel/DEA, 5 vs. 1 v/v) and fluorescent spectra of the gel aggregates upon bubbling increasing amounts of CO2 (𝜆ex = 365 nm). Inset: Plot of the intensity at 455 nm versus CO2 concentration (expressed as equivalents to 1). (c) Photos of the gel aggregates with different CO2 concentrations (irradiated under 365 nm light). Source: Reproduced with permission from [43]. © 2018, The Royal Society of Chemistry. DOI: doi.org/10.1039/C8TC01190G.

and heavy-metal cation. By changing the medium temperature at 10 ∘ C and 80 ∘ C, respectively, it can be switched to on–off stages. The exploration of transition metal–ligand complexes is also an important area in phosphorescent materials [48–50]. The strong propensity of d8 transition metal compounds to form non-covalent metal–metal interactions facilitates supramolecular assembly and the formation of supramolecular gels with interesting photophysical properties. For example, a trinuclear AuI pyrazolate metallacyclic gelator 6 has been successfully synthesized by Adia and coworkers (Figure 8.7) [51]. Due to the thermal response of AuI pyrazolate supramolecular gel, the red luminescence can be switched “on” (gel) and “off ” (sol) synchronously to the phase transition and repeated many times without any decay. On the other hand, Ag+ can intercalate

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8 Luminescent Supramolecular Gels

hν

No RTP

5 DBS

RTP

OH O H O

DBS self-assembly

O H O

3-BrQ

Gel

Sol

OH

N

DBS

Br

3-BrQ

(a)

(b) 600 Br

500 RTP intensity

222

N

400

3-BrQ

300 200 100 0

(c)

200

300

400 500 Wavelength (nm)

600

700

Figure 8.6 (a) Chemical structure of commercial sorbitol derivative DBS (gelator 5). (b) Schematic illustration of 3-BrQ RTP induced by DBS supramolecular gels. (c) Phosphorescence spectra of 3-BrQ in a DBS gel (black curve) and NaDC solution (red curve). Source: Reproduced with permission from [47]. © 2015, American Chemical Society. DOI: doi.org/10.1021/la5040323.

into the columnar assembly of the Au(I) complex and showed a green-blue luminescence. The emission lifetime of this gel is 6 μs, indicating that the emission is phosphorescence assignable to a triplet metal-centered excited state modified with an Au(I)-Au(I) metallophilic interaction. Due to the advantages of versatile synthesis, tunable color, and high photo- and chemical stabilities, as well as the utilization as triplet emitters and dopants in phosphorescent OLEDs, new luminescent transition metal–ligand complexes have experienced a great blossom with revival of interest, such as the triplet emitters of platinum(II) complexes [49]. Ziessel and coworkers obtained a phosphorescent supramolecular gelator 7 by the coupling of a planar PtII terpyridine unit to gallate-functionalized alkynes [52]. As shown in Figure 8.8, the gel exhibited an unusually strong emission at 830 nm, and this near-infrared emission can be tuned to red luminescence by changing solvent. Pt⋅⋅⋅Pt interactions of the complexes is the main driving force for the formation of highly colored organogels. Amide groups linked the gallate and alkyne units provide additional stabilization of the structures through hydrogen bonding. Xiao et al. designed an interesting gelation trigger of dinuclear cyclometalated platinum(II) complexes covalently connected by oligo(oxyethylene) chains; they

8.3 Phosphorescence in Supramolecular Gels OR

OR Me

Me N N

OR Me

Aul

Aul

N N N Aul N

Me

RO Me

Me

6 (a)

RO

OR

R = C18H37 (a)

(b)

Daylight

Luminescence hardly visible (λext = 254 nm)

Cool (gel)

Ag+

Cl–

Cl–

Ag+

Cool (gel)

Green (λext = 365 nm)

(b)

Red (λext = 254 nm)

Heat (sol)

Blue (λext = 365 nm)

Heat (sol)

(c)

(d)

Figure 8.7 (a) Chemical structure of Au(I) pyrazolate gelator 6. (b) Pictures and schematic self-assembling structures; (a) sol, (b) gel, (c) sol containing AgOTf (0.01 equiv), and (d) gel containing AgOTf (0.01 equiv). Source: Reproduced with permission from [51]. © 2005, American Chemical Society. DOI: doi.org/10.1021/ja0441007.

can co-assemble with phosphorescent cationic organoplatinum(II) complexes to form luminescent hydrogels in aqueous dispersions with intra- and intermolecular Pt⋅⋅⋅Pt and π–π interactions as the driving force (Figure 8.9) [53]. Their solution behavior is modulated by the length of the oligo(oxyethylene) chain connecting the two cyclometalated platinum(II) motifs. By employing dinuclear organoplatinum (II) complexes with long bridging chains as supramolecular cross-linkers, a spontaneous and reversible transition from a nematic fluid to a luminescent hydrogel has been demonstrated. This may suggest that dinuclear platinum(II) complexes bridged by a flexible linker could be used as building blocks to interconnect uniaxially aligned polyelectrolytes. This work also describes an efficient protocol for the formation of hydrogels. The excellent phosphorescent properties, water solubility, and thermo-responsiveness of these Pt(II) hydrogels may lead to new applications in “smart materials.” Strassert and his coworkers developed a straightforward one-pot synthetic method of neutral, soluble Pt(II) triplet-emitted gelator 8 bearing a dianionic tridentate terpyridine-like ligand [54]. This synthetic strategy does not require moisture- and oxygen exclusion, and the product is easily purified by repeated precipitation. Most importantly, the Pt(II) emitter is able to self-assemble into gel with an unprecedented 90% photoluminescence quantum yield. The self-assembly

223

224

8 Luminescent Supramolecular Gels +

7

BF4–

N N

N Pt

O RO

O N H

OR

N H

OR

RO OR

(a)

OR

R=

(c) 250

1.0 200 0.8 A

Em

0.6

150 100

0.4

50

0.2 0.0 350

(b)

0% 4.8 % 20.0 % 27.3 % 33.3 % 38.5 % 45.3 % 50.8 % 55.3 % 59.0 % 62.1 % 64.9 % 67.2 % 69.9 % 72.2 % 74.2 %

400

450

500 550 λ / nm

600

650

0 500

700

(d)

600

700 λ / nm

800

900

Figure 8.8 (a) The compound structures from gelator 7. (b) Absorption spectra of gelator 7 in dodecane (green line), in dichloromethane (redline), and in dichloromethane with 7% v/v methanol (yellow line). The inset shows a photograph of the corresponding solutions from left to right. (c) AFM and TEM image of a diluted gel in dodecane, showing an interconnected network of fibers. Zoom showing a fiber with the smallest observed diameter (2 nm; indicated by arrow). (d) Emission spectrum (𝜆ex = 482 nm) of gelator 7 in dodecane upon addition (% v/v) of a mixture of methanol (25% v/v) in dichloromethane. Gel photograph obtained with gelator 7 in dodecane. Source: Reproduced with permission from [52]. © 2007, Wiley-VCH. DOI: 10.1002/anie.200604012.

process can be monitored by the turn-on of the phosphorescent emission upon aggregation. In neat films and poly(methyl methacrylate) (PMMA) matrices, the photoluminescence quantum yield of Pt(II) compound reaches up to 87%, and the emission and excitation spectra do not vary with its concentration, which is suitable as a dopant in solution-processed OLEDs (Figure 8.10).

8.4 Upconverted Luminescence in Supramolecular Gels Photon upconversion (UC) is a process that converts lower energy (longer wavelength) photons to higher energy (shorter wavelength) photons. It is technologically important for a variety of applications ranging from energy to biology [55–57]. The efficiency of renewable energy production using solar energy can be enhanced by implementing the UC process. The UC mechanisms that are extensively investigated are based on two-photon absorption, multistep excitation of lanthanides, or

8.4 Upconverted Luminescence in Supramolecular Gels

2+

+ N N

Pt

Cl–

N N

R

Pt

C

C

N

N

2 X–

N O

O

nO

n = 10

Pt

N

C N

X = Cl, PF6

(a)

R = H, 1·Cl

10·Cl

(b)

Pt P t Pt

Pt Pt Pt Pt Pt Pt Pt Pt Pt Pt Pt Pt

Pt

Pt Pt Pt Pt Pt Pt Pt Pt Pt Pt Pt

Pt Pt Pt P t

Pt Pt Pt

Pt

Pt Pt Pt Pt Pt Pt Pt Pt Pt Pt Pt

Pt Pt Pt Pt Pt Pt Pt Pt Pt Pt Pt

Pt Pt Pt Pt Pt Pt Pt Pt Pt Pt Pt

(c) Viscoelastic hydrogel of cross-linked supramolecular polyelectrolytes

Viscous fluid of chromonic mesophase and supramolecular polyelectrolytes

+ 10·Cl 1·Cl

(d)

(e)

Figure 8.9 (a) and (b) The structure of cationic organoplatinum(II) complexes 1⋅Cl and 10⋅Cl. (c) Schematic presentation of organoplatinum(II) nematic hydrogels by supramolecularly cross-linking chromonic mesophases. (d) Pictures of the red hydrogel (1⋅Cl + 10⋅Cl). (e) Pictures of letters molded from hydrogel (1⋅Cl + 10⋅Cl) under daylight and under a UV lamp. Source: Reproduced with permission from [53]. © 2014, The Royal Society of Chemistry. DOI: doi.org/10.1039/C4SC00143E.

triplet–triplet annihilation (TTA). Only the TTA mechanism of organic molecules can utilize weak excitation intensity such as sunlight [58]. Therefore, combining the low-power excitation and high quantum yield, TTA-UC systems are beneficial for practical applications, ranging from sunlight-powered renewable energy production including photovoltaics and photocatalysis to bioimaging and phototherapy [59]. The most efficient TTA-based UC has been achieved for donor–acceptor pairs molecularly dissolved in organic solvents because they allow fast diffusion of the excited molecules. Supramolecular gel structures, which have macromolecular

225

226

8 Luminescent Supramolecular Gels

N N

80

104

60

103

J/ 40 mA cm–2

102

20

1

N

N N

N N

N

N

Pt N

8

0 0 C4H9

(a)

10 1

(d)

C4H9 C4H9

2

3 4 V/V

5

6

7

8

L/ cd m–2

100

10 10

1.0

I

0.0

1

ηc / cd A–1 1

0.5

300

400

500

600

700

800

λ / nm

(b)

0.01

(e)

0.1 0.1 1 J / mA cm–2

10

100

7V 8V 9V 10 V

1.0

1.0

0.5

I

ηp / Im W–1

0.5

I 0.0

(c)

0.0 300

400

500 λ / nm

600

700

800

400 (f)

500

600

λ / nm

700

800

Figure 8.10 (a) One-pot synthesis of platinum(II) gelator 8. (b) Normalized absorption, excitation (𝜆em = 580 nm), and emission (𝜆ex = 420 nm) spectra of gelator 8 in PMMA matrices (10 wt%). (c) Emission (𝜆ex = 420 nm) and excitation spectra (𝜆em = 580 nm) of the gel. Insets: Photographs of the luminescent gel. Source: Reproduced with permission from [54]. © 2010, Wiley-VCH. DOI: doi.org/10.1002/anie.201003818.(d) Current density–voltage–luminance (J–V –L) curve. (e) Luminous efficiency 𝜂 c and power efficiency 𝜂 p versus current density of the optimized device. (f) Electroluminescence spectra recorded at various applied voltages. Source: Strassert et al. [54]. © 2011, John Wiley & Sons.

solid state systems and microscopically liquid domains, were expected to yield solid state TTA-UC systems with liquid-like efficiencies [60]. Furthermore, by incorporating the sensitizer and annihilator in the gel structures, local concentrations of donor–acceptor pairs can be enhanced, resulting in increased triplet exciton migration and improved oxygen tolerance [61] (Figure 8.11). The first example of organogel-based TTA-UC was obtained by the Kimizuka’s group. A novel amphiphilic acceptor DPA-1 that possesses a 9,10-diphenylanthracene (DPA) emitter and ether-linked alkyl chains was designed [61]. The multiple hydrogen-bonding moieties of DPA-1 can spontaneously co-assemble with a triplet sensitizer Pt(II) octaethylporphyrin (PtOEP) molecules in organic solvent, showing efficient triplet sensitization and triplet energy migration among the preorganized emitters. As shown in Figure 8.12, in deaerated conditions, this supramolecular gel system shows a high UC quantum yield of 30% optimized

8.4 Upconverted Luminescence in Supramolecular Gels 1A∗

1D∗

ISC

3D∗

TTA

TTET

A∗

3

1D

Donor (D)

1A

A∗

3

1A

1A

Acceptor (A)

Figure 8.11 Scheme for the mechanism of TTA-UC. A triplet state of donor 3D*, formed by intersystem crossing (ISC) from the photo-excited (green arrow) singlet state 1D*, experiences triplet–triplet energy transfer (TTET) to an acceptor triplet 3A*. Two acceptor excited triplets annihilate to form a higher singlet energy level 1A*, which consequently produces upconverted delayed fluorescence (blue arrow).

at low excitation power. Moreover, the UC emission largely remains even in an air-saturated solution, resulting in the further investigation of UC process in gel system without deoxygenation. In the same year, they developed a very efficient TTA-UC process in a supramolecular gel nanofiber system under aerobic conditions. The gelator 9 N,N ′ -bis-(octadecyl)-L-boc-glutamic diamide (LBG) was chosen as matrix to confine sensitizer and emitter molecules, which can form a stable gel in most solvents due to hydrogen bonds and van der Waals interactions of the gelator (Figure 8.13) [62]. Excitation of the PtOEP/DPA/LBG co-assembled gel (air-saturated state) with 532 nm laser light caused an upconverted emission at 435 nm. The threshold excitation intensity of 1.48 mW cm−2 under aerobic conditions was lower than the solar irradiance of 1.6 mW cm−2 at 532 ± 5 nm. Interestingly, the intensity of upconverted emission can remain in air for 25 days. However, the value of upconversion quantum yield in-air was found to be only 3.5%. The disruption of the gel structure upon heating caused the quenching of the excited triplet species by dissolved oxygen. Therefore, UC luminescence can turn on and turn off during the gel-to-sol transition. Furthermore, the generality of air-stable TTA-UC in supramolecular gel was also observed incorporating different donor−acceptor pairs achieving upconversion of NIR-to-yellow, red-to-cyan, green-to-blue, and blue-to-UV light conversions. At the same time, Schmidt and coworkers proposed the same strategy by using the well-studied organogelator DMDBS as matrix, and PdTPP/DPA as the upconversion active material [60]. The performance of upconverter composition gelated with DMDBS is indistinguishable from the liquid sample in all measurements. Thus, these studies demonstrated the gelated photochemical upconversion material is reliable to create efficient, solid state upconverting materials for application in photovoltaics and photocatalysis.

227

8 Luminescent Supramolecular Gels

O R R

O O

O

O

H

O NH

N N H

(CH3)11

R=

(a)

N

H N 2 N H O

NH

2 O

CH3

O O

R N

UCPL intensity (arb.u.)

UCPL intensity (arb.u.)

400

(b)

N

R

PtOEP

6.90 mWcm–2 5.52 mWcm–2 4.83 mWcm–2 3.45 mWcm–2 2.76 mWcm–2 1.38 mWcm–2 0.97 mWcm–2 0.48 mWcm–2

500 600 Wavelength (nm)

400

N Pt

DPA-1

700

450 500 Wavelength (nm)

550

(c)

0.35

UCPL intensity (arb.u.)

Quantum yield

0.3 0.25 0.2 0.15 0.1 0.05

Ith = 8.9 mWcm–2 Slope = 1.16 Slope = 1.90

0 0

50

150

100

200

Power density (mWcm–2)

(e)

gel

400

(f)

450

0.1

1

10

Power density (mWcm–2)

UCPL intensity (a.u.)

(d)

UCPL intensity (a.u.)

228

500

Wavelength (nm)

550

400

(g)

100

film

450

500

550

Wavelength (nm)

Figure 8.12 (a) The chemical structure of acceptor DPA-1 and donor PtOEP. (b) Photoluminescence spectra of DPA-1 and PtOEP in deaerated chloroform ([DPA-1] = 10 mM, [PtOEP] = 10 μ M) with different incident power densities of 532 nm laser at the room temperature. (c) Photoluminescence spectrum of 9-PtOEP (red), and of DPA-PtOEP (blue, [DPA] = 10 mM, [PtOEP] = 10 μ M) in air-saturated chloroform at 77 K under 532 nm excitation (excitation power density = 982 mW cm−2 ). (d) Dependence of UC quantum yield and on the incident power density in the deaerated condition at the room temperature. Different colors represent the data of different samples in the same condition. (e) Dependence of UC emission intensity at 440 nm on the incident power density in the deaerated condition at the room temperature. (f) Photoluminescence spectra of the 1,2-dichloroethane gel of DPA-1 doped with PtOEP ([DPA-1] = 16 mM, [PtOEP] = 16 μ M) upon 532 nm laser excitation in the ambient conditions. Inset pictures show the doped gel under white light and 532 nm green laser. Source: Reproduced with permission [61]. © 2015, Springer Nature. DOI-https://doi.org/10.1038/srep10882. (g) Photoluminescence spectra of the cast film of PtOEP-doped DPA-1 (PtOEP/ DPA-1 = 0.1 mol%) obtained by 532 nm laser excitation in the ambient conditions. Source: Ogawa et al. [61] Licensed under CC BY 4.0.

570 nm

9

730 nm

LBG PdPc(OBu)8

Rubrene

PtTPBP

BPEA

PtOEP

DPA

507 nm

635 nm

(a) 440 nm

Slope = 1.1

UCPL Intensity (a.u.)

101

532 nm

100

Ith = 1.48 mW cm–2

10–1 377 nm

–2

10

Slope = 2.0

445 nm

10–3 0.1 (b)

1 Power density (mW cm–2)

10 (c)

Ir(C6)2(acac)

DBP

Figure 8.13 (a) A schematic representation of the unit structure of the upconversion gel system. (b) TTA-UC emission intensity observed for PtOEP/DPA/LBG ternary gel as a function of the 532 nm excitation power density in air-saturated DMF at room temperature. The dashed lines are fitting results with slopes of 2.0 (blue) and 1.1 (red), and Ith was determined as 1.48 mW cm−2 from the crossing point of these two lines. (c) Photographs of the UC co-gels in air-saturated DMF. Typical UC pairs were used to form ternary gels as shown. Short pass filters were used to remove the scattered excitation lights. [Donor] = 67 μM, [Acceptor] = 6.7 mM, [LBG] = 13.3 mM. Source: Reproduced with permission from [62]. © 2015, American Chemical Society. DOI: doi.org/10.1021/ja511061h.

230

8 Luminescent Supramolecular Gels

Based on the above assumption, Haring et al. reported the first application of green-to-blue photon upconversion to a chemical reaction in supramolecular gel based on the gelators 10 (G-1) and 11 (G-2) [63]. The developed method allows the photoreduction of aryl halides through cleavage of the C–halogen bond by means of low-energy visible light irradiation of PtOEP/DPA UC system embedded in a physical gel (Figure 8.14). This strategy is based on the combination of a cascade of photophysical and photochemical events involving TTA-UC, single electron transfer (SET), and H-atom transfer (HAT) as key steps. The gel network provides a suitable stabilizing microenvironment to achieve the challenging multistep process under aerobic conditions at room temperature and without additional additives such as bases or acids. Most importantly, good conversions and excellent mass balances were observed with several aryl halides. Another important research field of UC materials is biomedical application. Since visible light does not penetrate effectively through biological tissues, NIR light has more advantages than visible light as a light source for optogenetics in deep tissues. Thus, the development of biocompatible UC materials, such as hydrogels, is highly desired for various biological applications [64]. However, TTA-UC process in aqueous environments is scarce, due to the weak solubility of hydrophobic TTA chromophores and massive deactivation of excited triplets by dissolved oxygen molecules. To overcome this limitation, Bharmoria et al. have very recently developed an air-stable photon upconverting hydrogel, which was formulated based on the concept of synergistic biopolymer−surfactant interactions [65]. Further, they have used this air-stable upconverting hydrogel for optogenetic genome engineering by using a flavin-binding fungal photoreceptor Cre-recombinase (PA-Cre) [66]. As shown in Figure 8.15, under NIR-light illumination, upconverted blue light successfully activated PA-Cre and led to the morphological regulation of hippocampal neurons, which is important for learning and long-term memory. Although these O2 -tolerant UC hydrogels are fabricated by commercial polymer gelatin, the supramolecular hydrogel which consists of low-molecular weight gelators encompassing adequate mechanical properties, thixotropy, controllable gelation, self-healing, biodegradability, biocompatibility, biostability, etc. might be a preferable choice to more potential applications.

8.5 Circularly Polarized Luminescence in Supramolecular Gels Circularly polarized luminescence (CPL) is a phenomenon based on the emission of left circularly polarized light or right circularly polarized light by luminescent chiral systems, which exhibit the structural properties of excited state of chiral molecules or assemblies [67]. Recently, CPL in chiroptical materials has been drawing extensive attention owing to its potential applications in the fields of photoelectric devices, 3D displays, information security systems, chiral sensing, asymmetric catalysis, and so on [22, 68–72]. To achieve CPL, it is generally necessary to integrate the chiral part and luminescent part into one molecule [73]. In such a

t-BuO

visible light λexc = 532 nm

λem = 430 nm

@intragel

O O

HN

NHR

N

N

NHR G-1: R = n-C18H37

N

H N

Pt N

O

10

O

O R′

PtOEP (sensitizer)

R′ N H O G-2: R′ = n-C11H23

(a) @INTRAGEL in AIR ArX ArX

eV

SET

•–

(DPA)•+

1

(DPA)*

3

TTA

ISC

•

Ar + X– 3

HAT

(DPA)* + 3(DPA)*

DMF DMF

+

(PtOEP)*

volatiles

λ

2

1

TTET

BET

H2O

(b)

(PtOEP)*

3

•

DMF

ArH

1

DPA

PtOEP

430 nm

532 nm

H

λexc = 532 nm

LMW gelator, DMF 1

11 DPA (emitter)

PtOEP, DPA

Br

O 2

r.t. and aerobic conditions!

Entry

Gelator

[G]k (g L –1)

Conversionl (%)

1b 2 3c 4 5 6 7 8 9d 10 e 11f 12g 13 h 14 i 15 j 16 17 (c)

G-1 G-1 G-1 G-1 G-1 G-1 G-1 G-1 G-1 G-1 G-1 G-1 G-1 G-1 G-1 G-2

10 10 6 7 8 13 20 10 10 10 10 10 10 20 10 15

6 65 0 55 53 48 59 60 13 54 30 68 10 24 0 60 62

Mass balance (%) 96 98 93 98 99 84 88 100 99 93 93 98 97 90 89

Yieldm (%) 2 58 0 42 47 43 40 44 14 48 24 56 9 19 0 47 48

Figure 8.14 (a) Left: Illustration of green-to-blue photon UC with PtOEP/DPA. Right: Structures of the gelators 10 (G-1) and 11 (G-2). (b) Plausible mechanism for visible light photoreduction of aryl halides at room temperature in air based on the combination of TTA-UC and SET processes in a gel. (c) Intragel photoreduction of G-1 in air (ΔGET = −1.8 kcal mol−1 ). Source: Haring et al. [63]. Licensed under CC BY 3.0.

232

8 Luminescent Supramolecular Gels Gelatin

DPAS

TTA-UC λem = 442 nm

O2

Co-assembly

TEM

TTET

λex = 532 nm

(a) Triton X-100

PtOEP TTA-UC hydrogel NIR IMET

NIR excitation

TET

Os(peptpy)22+ TTA

Gelation

TTA-UC

HO

O 100

O 65

O

100H

TTBP

Blue

Pluronic F127

Blue emission

Blue light-sensitive genetically engineered neuron

STOP Activated PA-Cre Expression

(b)

Blue light stimulation

Spine formation

Figure 8.15 (a) A schematic representation of G-TX-DPAS-PtOEP co-assembly in photon upconverting hydrogel. Source: Bharmoria et al. [65]. © 2018, American Chemical Society. (b) A schematic representation of NIR optogenetics based on TTA-UC hydrogels. A UC hydrogel consisting of Pluronic F127 micelles and UC dyes was irradiated with a continuous-wave NIR laser at 724 nm. After the triplet energy transfer (TET) to acceptor TTBP, bimolecular annihilation (TTA) between TTBP triplets produced a blue upconverted emission. The upconverted blue light induced the activation of PA-Cre. Source: Sasaki et al. [66]. © 2019, John Wiley & Sons.

way, a tediously synthesis process is sometime inevitable, while the obtained chiral emitters with or without CPL -activity is unpredictable [74]. Likewise, CPL-active inorganic materials also require arduous syntheses by covalent bonding chiral reagents. But until now, only few cases of CPL-active inorganic materials have been reported [75]. Moreover, luminescence dissymmetry factor (glum ), which is used to quantify the level of CPL, is a key issue to develop CPL materials. The maximum value of |glum | is 2, which means completely left or right circularly polarized light

8.5 Circularly Polarized Luminescence in Supramolecular Gels

[76]. Thus, pursuing large glum values is one of the most urgent issues in CPL research. Supramolecular self-assembly provides a powerful tool for integrating different functional parts together and enhancing certain properties [77]. For example, supramolecular self-assembly can significantly amplify the glum value of some CPL-active materials [78]. In addition, not only chiral but also achiral chromophores can be endowed with CPL activity via self-assembly [29]. As illustrated in Scheme 8.1, there are three cases for the design of gelator molecules in order to achieve CPL, similar to the formation of the supramolecular chiral gels [79, 80].

Self-assembly

Gelator

Gel

(a) Chiral assembly

Chiral gelator (b) Chiral induction

Chiral gelator Achiral dopant (c) Symmetry breaking

Achiral gelator

Scheme 8.1 Schematics of gelation induced by self-assembly of gelator (top) and three types of gelation-induced supramolecular chirality (bottom). Chiral gels from (a) exclusive chiral gelators, (b) the co-assembly of chiral gelator and achiral dopants, (c) exclusive achiral gelators. Source: Duan et al. [79]. © 2014, The Royal Society of Chemistry.

Especially, an intriguing recent approach for fabricating CPL-active materials by the utilization of chiral supramolecular gels has been widely demonstrated. This strategy, called “chiral host–achiral luminescent guest,” can co-assemble chiral gel host with achiral luminophores, endowing the achiral component with CPL activity [81]. Based on such an efficient approach, various organic and inorganic materials can be embedded into a chiral gel host, subsequently generating numerous

233

234

8 Luminescent Supramolecular Gels

CPL-active gel materials. Herein, we focus on the recent progress of supramolecular gel with CPL activity, and summarize the CPL-active gel systems based on the different designs of gelator molecules or luminophores. The discovery of the gelator was fortuitous in the early stage of the development on supramolecular gels. However, it is now possible to design the gelator molecules on purpose. Hinted from the notion of supramolecular synthon in crystal engineering and synthon in organic synthesis, we proposed a concept of gelaton to the design of the gelator [80]. A gelator molecule can be illustrated as shown in Scheme 8.2. Linker Functional group

Gelaton

H N

O

H N

COOR

N H

HN

OR O

O

Cholesterol

Gelaton

Lysine

COOR

N HN

HN

O

H N

N H

Phe-Phe O

R

H N

R

Glutamic acid

Histidine

Gallic acid

OR O

OH HO HO

OR

RO

O

O O

OH

Glucose

O

C3 structures

O

Scheme 8.2 Three main parts of gelator and some typical gelatons. Source: Liu et al. [80]. © 2018, The Royal Society of Chemistry.

8.5.1

CPL-Active Gel Based on Chiral Luminescent Gelators

In order to achieve CPL, introducing a luminescent moiety to nonluminous chiral gelaton through covalent bond is a traditional strategy. However, in the design of CPL molecules with point chirality gelaton, considering the distance from the chiral center to the chromophore is necessary [74]. If the spacer between the chiral center and the chromophore is too long, CPL is silent. In the case of CPL from the gelators in assemblies, ACQ or AIE phenomenon will greatly influence the CPL of the system. These are very important issues that should be considered in the design of the CPL-active gelators. On account of their natural chirality, easy accessibility, and wide diversity, amino acids as excellent gelaton candidates are extensively used in the design of chiral supramolecular gelators [79]. For the CPL-active amino acid derivatives, the gelator usually does not show CPL in molecule state because the chromophore is far from the chiral center. However, upon gelation, the chirality can be transferred to the

8.5 Circularly Polarized Luminescence in Supramolecular Gels

(a)

(b)

(d)

Assembly Nanofiber Right-CPL

T-shaped stacking

(e) (c)

12 (PyHis)

Coordination & π-π stacking

(g) 0

–20

DC (V)

DC (V)

0.6 0.4 0.2 0.0

(h)

30 20 10 0 –10 –20 –30

Glum(x10–3)

L-PyHis D-PyHis

20

CPL (mdeg)

CPL (mdeg)

(f)

Nanosphere left-CPL

1.0

4

Zn2+

0

–4

0.5

EDTA 400

450 500 550 600 Wavelength (nm)

650

0.0

400

450 500 550 600 Wavelength (nm)

650

0

1

2

3

4

Cycles

Figure 8.16 (a) The structure of the chiral π-gelator 12 (PyHis). (b) The pyrenes in PyHis nanofibers adopted T-shaped stacking and showed right-handed CPL. (c) T-shaped stacking converted into π–π stacking with the aid of the formation of a distorted triangular bipyramid [Zn(PyHis)5]2+ complex. The nanofibers gradually converted into nanospheres along with the increasing of Zn2+ and showed left-handed CPL. Source: Niu et al. [82]. © 2019, John Wiley & Sons. (d) SEM image of PyHis xerogel Source: Reproduced with permission from [82]. © 2019, Wiley-VCH. DOI: doi.org/10.1002/anie.201900607 and (e) PyHis in the presence of Zn2+ (Zn2+ /PyHis = 1/5). (f) CPL spectra of L-PyHis and D-PyHis gel. (g) CPL spectra change of L-PyHis gel in the presence of different amounts of Zn2+ (𝜆ex = 330 nm). (f) Switchable CPL-inversion cycles by alternately adding certain amounts of Zn2+ and EDTA in situ (stimulus/PyHis = 1 : 16 was added for each turn). Notes: The numbers (1/128, 1/64, …, 1/5, 1/2) in the figure notes represent the molar ratios of Zn2+ to PyHis, [PyHis] = 0.02 M. Source: Niu et al. [82]. © 2019, John Wiley & Sons.

assemblies, which show intense CPL activity. For example, Niu et al. designed a histidine gelaton-linked pyrene gelator 12 (PyHis) for ion-controlled CPL switch of organogel (Figure 8.16) [82]. Interestingly, no CD and CPL signal was detected for the monomeric state. However, when PyHis formed supramolecular gel through self-assembly to nanofiber structure, the excimer of pyrene formed and emitted CPL, indicating the supramolecular chirality originated from the molecular chirality localized at the histidine moiety. When Zn2+ was added to the PyHis gel, it gradually collapsed to form dispersions, until the Zn2+ /PyHis ratio reached 1 : 5, all the nanofibers converted into nanospheres. The intensity of CD and CPL decreased gradually and became silent upon the increased adding of Zn2+ , and then reversed as it was continually increased to more than 1 : 18. Such a process is reversible and the back-switching from the PyHis/Zn2+ complex to PyHis assemblies could be realized by adding EDTA as a competing ligand. An interesting co-assembly-amplified CPL based on L-histidine derived gelator has been investigated, as shown in Figure 8.17 [83]. It was found that a

235

8 Luminescent Supramolecular Gels

π-π stacking 2.20 nm

13 (α-NapHis) Sonication Co-assembly

2.18 nm

π-π stacking Enhanced CPL

Weak CPL

CPL

20 10 0 0.4 DC(V)

236

0.2 0.0

(a)

350

400 450 500 Wavelength (nm)

550

(b)

α-NapHis Bilayer

α-NapHis/BA Hex

Figure 8.17 (a) Schematics of the achiral molecule boosted CPL in histidine-derived naphthalene organogels (up) and CPL spectra of α-NapHis and its co-assemblies with BA in different ratios (bottom). (b) Schematic illustration of the molecule packing modes of α-NapHis self-assemblies and the α-NapHis/BA co-assemblies. Source: Reproduced with permission from [83]. © 2018, The Royal Society of Chemistry DOI: doi.org/10.1039/ C7CC09049H.

sonication-induced supramolecular gel with CPL properties could be formed by gelator 13 (α-NapHis), while β-NapHis formed only a dispersion showing CPL silence. The calculated glum was ±5.0 × 10−4 at 370 nm. The gelator combined with achiral benzoic acid (BA) could form two-component co-gels. Although the BA is achiral, the co-gels exhibited unexpected enhanced CPL and one order of magnitude amplification. The largest glum value is ±3.0 × 10−3 . The possible mechanism for this CPL enhancement is illustrated in Figure 8.17b. The self-assembled of gelator α-NapHis could form a bilayer structure due to the H-bond and π–π stacking based on the imidazole moieties, urea and naphthalene. In the co-assemblies, a C3 -like binary unit firstly formed between α-NapHis and BA, then this unit further self-assembled into hexagonal structures. The tight π–π interaction in this hexagonal stacking resulted in better chirality transfer from gelator α-NapHis to the supramolecular assembles, which might further profit the glum values. Photon UC, as mentioned above, provides a view for higher energy conversion, while the integration of chiral co-assembly in photon UC provides a novel strategy of double channel to information transfer in emissive system [84]. Our group along with Liu’s group proposed an idea to modulate triplet–triplet annihilation-based photon upconversion (TTA-UC) in chiral assembly system [85]. We designed a self-assembly system based on anthracene-derived glutamic acid chiral gelator 14 (LGAn/DGAn) as acceptor and achiral PdII octaethylporphyrin derivative (PtOEP-C18) as donor/sensitizer. The co-assembly of PtOEP-C18 and chiral acceptor via co-gelation could not only generate chirality transfer from LGAn/DGAn to PtOEP-C18, but also realize triplet–triplet energy transfer (TTET) from PtOEP-C18

8.5 Circularly Polarized Luminescence in Supramolecular Gels

14

(LGAn/DGAn) O

PdOEP-C18

C18H37 NH

HN C18H37 N

HN

N

O O

Pd O O

NH C18H37

Co-assembly

N

N

HN C H 18 37

Upconverted CPL

Downconverted CPL

λex = 532 nm

λex = 532 nm

D

A

A

A

A

A

A

A

TTA

A

A

A

A

A

A

A

D

A

A

A

A

A

A

(a) Upconverted CPL

10

90

Downconverted CPL

0 –10 –20

LGAn/PdOEP-C18 DGAn/PdOEP-C18

CPL (mdeg)

CPL (mdeg)

20

Downconverted CPL

60 30 0 –30 –60 –90

0.0 400

DC (V)

DC (V)

(b)

0.2

LGAn/PdOEP-C18 DGAn/PdOEP-C18

0.4

0.4

λex = 532 nm

500 600 Wavelength (nm)

700

0.2

λex = 532 nm

0.0 550

600 650 700 Wavelength (nm)

750

Figure 8.18 (a) Schematics of energy acceptor LGAn/DGAn (gelator 14) and energy donor PdOEP-C18 in TTA-UC process. In the co-assembly system, the chirality can be transferred from gel LGAn/DGAn to PdOEP-C18. (b) Upconverted and downconverted CPL spectra of LGAn/PdOEP-C18 and DGAn/PdOEP-C18 in deaerated toluene, 𝜆ex = 532 nm laser. Source: Yang et al. [85]. © 2018, John Wiley & Sons.

to LGAn/DGAn. Thus, dual upconverted (460 nm) and downconverted CPL (550–750 nm) emission was detected in the co-gels under excitation of 532 nm laser. As shown in Figure 8.18, the co-gels in deaerated toluene showed the mirror-imaged upconverted circularly polarized luminescence (UC-CPL) signal at 460 nm. More interestingly, under high excitation laser power, strong CPL signal could be observed in the wide emission range of 550–750 nm (Figure 8.18). This strongly suggested that a downconverted CPL could be emitted from PtOEP-C18 in the co-gels at deaerated condition. Thus, a dual circularly polarized light emission involving upconverted CPL at 460 nm and downconverted CPL was realized in co-gels. Two channels of chirality and energy transfer process were successfully integrated and the interplay of energy and chirality transfer to produce a dual CPL emission was revealed simultaneously.

237

238

8 Luminescent Supramolecular Gels

8.5.2 CPL-Active Supramolecular Gel Based on Achiral Luminescent Gelators In the above description, an intrinsically chiral component is required for the generation of CPL. However, not only chiral molecules but also completely achiral molecules can form chiral supramolecular assemblies by spontaneous symmetry breaking, thus providing the possibility for the fabrication of CPL materials from exclusively achiral luminophores. In this situation, an asymmetric environment during self-assembly is essential. To date, the supramolecular chirality in achiral systems can be controlled by adding some simple chiral dopants and an external influence, such as a magnetic field, circularly polarized light, as well as the vortex motion generated by rotary evaporation or magnetic stirrers [86, 87]. An achiral C3 -symmetric molecule is usually investigated in the supramolecular gel systems for controlling the handedness. For example, it was found that when the achiral C3 -symmetric gelator 15 formed the organogels, it showed strong emission as well as the CPL [29]. But the handedness of the CPL appeared randomly, suggesting a spontaneous symmetry breaking and the glum value was ±0.8 × 10−2 , as shown in Figure 8.19. Interestingly, mechanical stirring could enhance the glum values during the supramolecular gelation process. Although the direction of the CPL signals could not be controlled by the stirring direction, the obtained gel dispersion was quite stable and the CPL remained even after stopping the stirring. More interestingly, the direction of CPL signals can be readily regulated by adding some simple chiral dopants, and the glum value was also amplified by chiral dopants (±2.3 × 10−2 ). In some cases, the assembled morphology of achiral molecules could influence CPL properties. As illustrated in Figure 8.20, twisted ribbons, nanobelts, and trumpet-like nanostructures can be formed by an achiral C3 -symmetric molecule 16 with the increased ratio of DMF/water [88]. All the nanostructures are based on the original nanobelt structure. Upon the increased amount of DMF, nanotwists and nanotrumpets were formed by such nanobelts through twisting and rolling, respectively. Intriguingly, only nanotwists showed supramolecular chirality with relatively strong CPL performance (glum = ±2.1 × 10−2 ), while the other nanostructure could not. Recently, an assembly of achiral BTAC analogue was found to not only emit CPL but also transfer its chirality to achiral emitter. The ends of compound 17 are carboxylic groups instead of ethyl cinematic groups, which are able to bind cations and cationic fluorescent dye via electrostatic interaction [89]. The stereochemical environment of the cation-binding sites was explored in the helical nanoribbon by using a cationic fluorescent dye, methylene blue (MB). Interestingly, the mirror symmetry-broken co-assemblies displayed achiral monomer that can serve as a chiral ligand for mediating enantioselective catalysis. The successful CPL observation from MB in Figure 8.21 suggests that the catalytic site having a Cu 2+ ion is chiral.

8.5 Circularly Polarized Luminescence in Supramolecular Gels

O

O

H2O HN

O H N

O

DMF solution

Gel

Solution

Suspension

O

NH

O

O

15

O

O

(a) 0

(b) CPL (mdeg)

Sample 1 Sample 2

30 CPL (mdeg)

Cooling to RT with stirring

Heat

0

|glum|: 0.8 × 10–2

–30

Sample 1 Sample 2

60 30 0

|glum|: 1.2 × 10–2

–30 –60 400

–60

(c) 150

0.4

450 500 550 Wavelength (nm)

600

(S)-1-cyclohexyl ethylamine

CPL (mdeg)

0.2

DC (V)

100 50 0

|glum|: 2.3 × 10–2

–50 –100

400

450 500 550 Wavelength (nm)

600

0.0

(R)-1-cyclohexyl ethylamine

–150 400

450 500 550 Wavelength (nm)

600

Figure 8.19 Chemical structure of 15 and photographs showing 15 assemblies in DMF/H2 O upon stirring during preparation of the gels (top). (a) CPL spectra (left axis) and fluorescence spectra (right axis) of gels in DMF/H2 O. CPL spectra of gels after 900 rpm clockwise stirring during gelation Source: Reproduced with permission from [29]. © 2015, The Royal Society of Chemistry. DOI: 10.1039/C5SC01056J. (b) and containing 900 mol% chiral 1-cyclohexyl ethylamine (c) in DMF/H2 O. Source: Shen et al. [29]. Licensed under CC BY 3.0.

8.5.3 CPL-Active Supramolecular Gel by Using Organic Luminophores as Guests As stated above, both chiral and achiral luminescent gelators could exhibit CPL through self-assembly in asymmetric environment. In addition, achiral luminophores can also be used to fabricate CPL-active materials by co-assembly. The induced chirality of achiral luminophores is attributed to chirality transfer from chiral components. Chirality transfer has potential to not only endow almost all luminophores with CPL, but also simplify the strategy to produce various CPL-active materials instead of the tedious organic synthesis [76]. For the chirality induction of the achiral luminophores, the interaction between chiral molecules

239

8 Luminescent Supramolecular Gels

O

O

DMF/H2O

60

2 6 7

40

HN

CPL (mdeg)

240

O H N

O NH

O

20 0 –20 –40

O

O O

16

O

–60 400

500

(a)

600

700

Wavelength (nm) Extend

Thin layer

Twist

Thick layer

Roll

Roll

l

ira

Ch

DMF/H2O=2

Twist

l

ira

Ch

DMF/H2O=3-4

DMF/H2O=5

DMF/H2O=6-7

(b)

Figure 8.20 (a) Molecular structure for the gelator 16 and CPL spectra of 16 at a volume ratio of DMF/H2 O = 2, 6 and 7. (b) Schematics of the formation of nanotwists and nanotrumpets from nanobelts. Bottom was the SEM images showed the nanotwists and nanotrumpets. The nanotwists showed CPL activities, while the nanotrumpets did not. Source: Reproduced with permission from [88]. © 2018, The Royal Society of Chemistry. DOI: doi.org/10.1039/C8CC02130A.

and achiral luminophores is significant. Non-covalent interactions such as electrostatic interactions, hydrophobic interactions, hydrogen bonds, and host–guest interactions can be employed to regulate the chirality induction [81]. Our group reported the first example that chiral confined spaces or environments can endow achiral components with chirality [90]. Figure 8.22 shows a general approach to fabricate CPL-active assembly nanotubes through loading achiral

8.5 Circularly Polarized Luminescence in Supramolecular Gels Cu2+ coordination CO2H

Mirror symmetry breaking (+)-PBTABA

(–)-PBTABA

H N

O

17

(a)

DMF/water (7/3, v/v)

HN

O

HO2C

1. Heating to 100 °C 2. Cooling to 25 °C Upon rotary stirring

NH

C

Methylene blue (MB)

N O N

CD (mdeg)

O

C

O

DMF/water (7/3, v/v)

O

O

C

C

N O

S

S

N

N

N

(+)-PBTABA/MB (–)-PBTABA/MB

(c)

1200 800 400 0 –400 –800 –1200

(d)

CD (mdeg)

O

M>P

(b)

(+)-PBTABA (–)-PBTABA

O

P>M

CO2H

BTABA (achiral)

1200 800 400 0 –400 –800 –1200

(e)

or 80

(+)-PBTABA CD (mdeg)

O

(–)-PBTABA

0 –40

80

(+)-PBTABA

(–)-PBTABA/MB (–)-PBTABA 300 400 500 600 700 Wavelength (nm)

(–)-PBTABA 400 500 600 Wavelength (nm)

300 400 500 600 700 Wavelength (nm)

(+)-PBTABA/MB

(+)-PBTABA

40

–80

CD (mdeg)

H-bonding

(–)-PBTABA/MB

40 0 –40 –80 600

(+)-PBTABA/MB 700 800 Wavelength (nm)

Figure 8.21 Mirror symmetry breaking of helical supramolecular nanoribbons for asymmetric catalysis. (a) and (b) molecular structure of the achiral BTABA monomer (gelator 17) and schematic representation of the preparation of helical PBTABA nanoribbons with either (P)- or (M)-dominant helical handedness by applying rotary stirring during the cooling process of a DMF/water solution of BTA BA . (c) Schematic representation of the binding of methylene blue (MB) onto PBTABA at its carboxylate units via an electrostatic interaction. (d) CD and CPL (𝜆ex = 290 nm) spectra at 25 ∘ C of DMF/water (7/3, v/v; 1 ml) suspensions of (−)-PBTABA (blue) and (+)-PBTABA (pink) ([BTABA ] = 5.3 mM). (e) CD and CPL (𝜆ex = 550 nm) spectra at 25 ∘ C of DMF/water (7/3, v/v; 1 ml) suspensions of (−)-PBTABA (blue) and (+)-PBTABA (pink) ([BTABA] = 5.3 mM) containing MB ([MB]/[BTABA] = 20%). Source: Shen et al. [89]. Licensed under CC BY 4.0.

AIE luminophores (AIEgens). As illustrated in Figure 8.22a, C3 symmetric chiral gelators 18 can construct the hexagonal nanotube structures, and the intrinsic chirality of the substituted glutamate moieties can transfer to the supramolecular nanotubes during the self-assembly process. The achiral AIE luminophores could be embedded into the confined nanotubes via co-assembly, and achiral AIE dyes aggregated during the gelation process, which showed enhanced fluorescence intensity and distinct CPL by direct excitation. As shown in Figure 8.22c, through simply altering the doped dyes, mirror-imaged CPL signals from 425 to 595 nm, covering the full-color from blue, green, yellow to orange-red color, can be tuned. For the chirality transfer in supramolecular gel, the expression of supramolecular chirality is of most importance. An example of three-component co-assembled gel

241

8 Luminescent Supramolecular Gels AIE dyes

Chiral nanotubes emitting CPL

O O O O

O HN O O

O O

(a)

HN

HN O

Self-assembly

O

O O

O

Doping with AIE dyes

O

18 2

1 CPL (mdeg)

242

0

–1

–2 400

(b)

500 600 Wavelength (nm)

700

(c)

Figure 8.22 (a) Schematic representation of chiral nanotubes formed by L-/D-17 encapsulated different AIE luminophores. (b) The photograph of AIEgens-loaded co-gels under UV light irradiation. Source: Reproduced with permission from [90]. © 2017, Wiley-VCH. DOI-https://doi.org/10.1002/adma.201606503. (c) Mirror image CPL spectra of TPE (𝜆ex = 300 nm), HPS (𝜆ex = 365 nm), 𝛽-DCS (𝜆ex = 354 nm), 𝛼-DCS (𝜆ex = 363 nm), MeCNS (𝜆ex = 376 nm), BuCNS (𝜆ex = 376 nm) in L-17 (solid lines), or D-17 (dash lines) host gels, all the co-gel samples were made in DMSO/H2 O (1/1, v/v) mixing solvents. Source: Han et al. [90]. © 2017, John Wiley & Sons.

for induced CPL is showed in Figure 8.23 [91]. The chiral molecule Fmoc-Glu (gelator 19) could form helical structure by adding the achiral nucleobase (guanine (G) or adenine (A)). Interestingly, this supramolecular chirality transferred to the achiral cationic dye ThT, only when the achiral nucleobase existed. In the absence of A or G, Fmoc-Glu will not express the chirality and self-assemble into hydrogels. With the aid of purines (G or A), Fmoc-Glu were found to form an opaque and translucent hydrogel. The co-assembly of Fmoc-Glu, adenine, and ThT exhibits helicity structure and distinct CPL, indicating that achiral nucleobases play an essential role in the expression and transfer of chirality. As stated above, the regulated chiral assembly of emitters would be one of the general and simple ways adopted for enhancing glum value. However, due to the ACQ effect of luminophores in the aggregated state, the CPL performance of most luminophores becomes even worse compared from isolate molecule to aggregated state. Fortunately, the finding of AIE effect solve this problem by combining with chiral assembly. AIE luminophore incorporated with the chiral component to fabricate novel CPL-active materials has become an efficient way to achieve both large glum values and high emission efficiency in condensed phase. Cyano-substituted stilbene (CNSB) is a well-known compound with AIE property. When CNSB conjugated with a glutamic-derived gelaton, an AIE gelator 20 was obtained [92]. As shown in Figure 8.24, the chiral donor (gelator 20) and achiral acceptor (BPEA) could form

8.5 Circularly Polarized Luminescence in Supramolecular Gels 45

Cl – OH

O O

N H

N

N S

OH O

No helices No CD No CPL

ThT NH2

O

19 (Fmoc-Glu)

N

HN N

N

N

G

N

N H

0

Fmoc-L-Glu/A/ThT Fmoc-D-Glu/A/ThT

–45 0.50

A

DC / V

H2 N

N H

CPL / mdeg

O

0.25

CPL

0.00 440

Fmoc-L-Glu/A/ThT Fmoc-D-Glu/A/ThT

480 λ / nm

520

560

Figure 8.23 Left: Schematic illustration of the achiral nucleobase-assisted helical self-assembly based on the Fmoc-Glu and supramolecular chirality transfer from Fmoc-Glu to achiral ThT. Right: CPL spectra of Fmoc-(L/D)-Glu/A/ThT. Source: Deng et al. [91]. © 2016, John Wiley & Sons.

a composite nanohelix through co-assembly, in which both energy and chirality transfer were observed simultaneously. Amazingly, not only did the chirality transfer happen in the complex, but also the dissymmetry of CPL was significantly amplified during the energy transfer. Due to the π–π stacking of CNSB and the H-bond between the amide groups, an ordered nanohelix structure could be obtained from gelator 20 through self-assembly. During the self-assembly process, not only the emission intensity of 20 remarkably increased, but also the molecular level chirality transferred to the supramolecular level, resulting in the excellent CPL properties. When gelator 20 co-assembled with achiral BPEA, the acceptor BPEA was inserted into the nanohelix through the weak π–π stacking to form the co-gel (Figure 8.24a). Interestingly, the achiral acceptors could be endowed with CPL caused by chirality transfer from the nanohelix. As shown in Figure 8.24b, the detected CPL glum value was ±1.2 × 10−3 by directly exciting the acceptor (at 400 nm), while the glum for the acceptor by exciting the donor (at 320 nm) showed a significant enhancement (up to ±3 × 10−3 ). This result exhibited that glum value of CPL was amplified more than 2.5 times through the energy transfer process. This might be resulting from the enhancement of acceptor emission via the energy transfer, which seems to further amplify the glum values. Encouraged by the result of energy transfer-amplified CPL, Ji et al. further investigated a cooperative chirality and sequential energy transfer in a supramolecular co-assembly system to explore its mechanism [93]. In Figure 8.25, a cyanostilbene-appended glutamate gelator 21 and two kinds achiral acceptors, thioflavin T (ThT) and acridine orange (AO), were employed to from a co-gel. The chiral gelator 21 could form supramolecular nanotubes with CPL activity. In addition, the supramolecular chirality could transfer to these two achiral acceptors through co-assembly. Meanwhile, the excited-state energy of 21 nanotubes could directly transfer to ThT but only be sequentially transferred to AO. More interestingly, compared with CPL from directly exciting AO, a stepwise amplified CPL

243

8 Luminescent Supramolecular Gels

(a)

20 glum = 1.2 × 103

400 nm

glum × 103

Co-assembly

BPEA (b) 6 3 0 –3

320 nm

glum = 3 × 103

DC (V)

–6 0.6 0.4 0.2 0.0 350

5.3 nm

550 450 500 Wavelength (nm)

400

600

650

O O

COOH

ChT

21

ET

ChT

AO

ET

0.4 0.2

O /A

T Th

Light-harvesting antenna

500 λ (nm) 340 nm

600 450 nm

0

0.4 0.2 0.0 400

DC (V) CPL (mdeg)

SET

20

21/ThT

–20

ChT

Helical nanotube

0

0.0 400 DC (V) CPL (mdeg)

Assembly

40

–40

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Figure 8.24 (a) Chemical structures of 19 and BPEA, and schematic illustration of energy transfer-amplified CPL in co-assembly nanohelix. Under excitation at 400 nm, the composite nanohelix showed green CPL with glum = 1.2 × 10−3 (top pattern), while energy transfer boosted CPL with a relatively large value glum = 3.0 × 10−3 under excitation at 320 nm (bottom pattern). (b) CPL dissymmetry factor glum versus wavelength, 𝜆ex = 320 nm (blue line) and 𝜆ex = 400 nm (red line). Source: Yang et al. [92]. Licensed under CC BY 4.0.

Th

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ET:

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Figure 8.25 Left: Schematics of the self-assembly of 21 nanotubes, and different chirality and energy transfer modes of the co-assembly of 21/ThT, 21/AO, and 21/ThT/AO. Inserts are SEM and TEM images of the 21 nanotubes. Right: CPL spectra of 21/ThT,21/AO, and 21/ThT/AO under different excitation wavelengths (𝜆ex = 340 nm for 21, 𝜆ex = 400 nm for ThT, and 𝜆ex = 450 nm for AO). Source: Reproduced with permission from [93]. © 2018, Wiley-VCH. DOI: doi.org/10.1002/anie.201812642.

8.5 Circularly Polarized Luminescence in Supramolecular Gels

could be observed when exciting the donor 21 or intermediate donor ThT in the 21/ThT/AO ternary system. It should be noted that energy transfer boosted CPL was feasible. And a possible analysis was proposed as following. When donor 21 nanotubes are excited by unpolarized light, a CPL is obtained due to its intrinsic chirality. However, when acceptors are added, the excited-state energy with chiral information will transfer to acceptors, resulting in a new CPL from the acceptor. Based on a theoretical calculation by Bene and coworkers, in a helicity and energy transfer process (hFRET), the helicity in fluorescence from a rotating donor dipole could be preserved. In the instance of 21/ThT/AO system, the helicity of luminescent donor 21 and ThT may be sequentially transferred to acceptor AO and amplified the glum of AO. For experimental aspect, the L-21/ThT system exhibited an amplified glum value (1.89 × 10−2 ) when excited by circularly polarized light in contrast to the glum value excited by unpolarized light (glum = 3.3 × 10−3 ). Thus, the helicity of excited state of donor could be transferred to the acceptor and the subsequent amplification of the glum was indirectly illustrated.

8.5.4 CPL-Active Supramolecular Gel Based on Inorganic Luminescence Guest Compared to CPL-active organic materials, the design and fabrication of inorganic materials with CPL has been a great challenge in developing chiroptical materials. To date, only a few CPL-active chiral inorganic nanomaterials have been developed either through tedious synthesis or with unregulated chiroptical properties. Generally, there are two feasible approaches to construct CPL-active inorganic materials. The most common approach is capping the inorganic nanostructures with chiral reagents, which has been extensively demonstrated as a common method for fabricating chiral nanomaterials. For example, CPL-active noble metal clusters, chalcogenide semiconductor quantum dots (QDs), and nanostructured ZnO films have been reported by using this method [75, 94, 95]. Another approach reported very recently is the “chiral host-luminescent guest.” Luminescent inorganic nanomaterials (guests) are achiral; however, after incorporation into a chiral host, they display induced chirality and hence become CPL active. The chiral assembly of achiral nanomaterials in confined space of chiral host was considered to be the main reason for the induced chirality and CPL behaviors. Compared with the chiral host, there are more options for the luminescent guest. So far, several inorganic nanomaterials have been reported to show CPL by incorporating into chiral guests, including QDs, perovskite nanocrystals, and lanthanide nanoparticles [96–98]. Herein, we have highlighted recent progress in CPL-active inorganic nanomaterials fabricated in supramolecular gels. The first example of CPL-active inorganic nanomaterial in supramolecular gels is QDs, which has been accomplished by our group [96]. The well-known chiral lipid gelator 22 N,N ′ -bis(octadecyl)-l-glutamic diamide (LGAm) and its enantiomer DGAm, which forms organogels with tubular nanostructures, was mixed with core–shell-type QDs modified with achiral 3-mercaptopropionic acid. Upon a

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Figure 8.26 (a) Molecular structure of the gelators 22 and the QDs used in this work. Three core–shell QDs with the same ZnS shell capped with 3-mercaptopropionic acid and different cores (CdSe, CdS, and ZnSe) were investigated. (b) Photograph of various CdSe/ZnS QD doped co-gels in EtOH/H2 O (10:1 v/v) under UV light. EtOH/H2 [LGAm] = 20 mg ml−1 , [QD] = 0.2 mg. (c) Fluorescence spectra of the corresponding co-gels (𝜆ex = 360 nm). (d) Mirror-image CPL spectra of the corresponding co-gels (𝜆ex = 360 nm). (e) Fluorescence microscopy image of the white light-emitting co-gel. (f) Commission International de I’Pclairage coordinate value of the white light-emitting co-gel. Source: Reproduced with permission from [96]. © 2017, Wiley-VCH. DOI: doi.org/10.1002/ange.201706308.

cooling process, supramolecular gel was formed. The gels consisted of nanotube structures where the QDs were arranged in the nanotubes. The commercially available 3-mercaptopropionic acid-capped QDs can be fabricated on large scale, and all emission colors can be obtained. Several kinds of core–shell-type QDs including CdSe/ZnS, CdS/ZnS, and ZnSe/ZnS (Figure 8.26a) were tested and demonstrate the generation of CPL activity by gelation. The CdSe/ZnS core–shell QDs were chosen to carefully test five kinds of colorful QDs while only blue QDs made from

8.5 Circularly Polarized Luminescence in Supramolecular Gels

CdS/ZnS and ZnSe/ZnS have been studied. The co-gel formation between the CdSe/ZnS QDs and the lipid gelator was dominated by the electrostatic interaction between carboxylate groups on the QD surface and the amine groups of the gelator. Under UV irradiation, the gels were colorful with bright emission from the blue to the red region (Figure 8.26b). The fluorescence spectra of the co-gels had the same shapes as that recorded for an aqueous solution of QDs, which indicates that the fundamental emission properties of the QDs are well preserved in the co-gels. Interestingly, these co-gels showed intense circularly polarized emission with several special features. First, all of the gel samples with various colors showed CPL although they did not show CD owing to the strong scattering of the white co-gel. Second, CPL signals with different handedness can be generated in the co-gels. For example, the red QDs in the co-gel exhibited strong mirror-image CPL relative to the molecular chirality of the gelator. This means that the sign of the CPL signal can be well regulated by the chirality of the nanotubes. The calculated value of the dissymmetry factor ∣glum ∣of the CPL signal was about 10−3 , which is comparable to that of other CPL semiconductor materials, such as cysteine-capped CdSe nanoparticles (4 × 10−3 ) and CdS nanoparticles templated in a protein nanocage (4.4 × 10−3 ). Blending five kinds of CdSe/ZnS QDs with different colors in a mass ratio of 4 : 2 : 3 : 3 : 3 (blue/cyan/green/yellow/red) led to comparatively good white CPL emission. The abovementioned method provides a general approach for fabricating CPL-active emissive nanomaterials. In the next year, with the aid of supramolecular self-assembly approach, our group successfully endowed the perovskite nanocrystals (NCs) with CPL (Figure 8.27) [97]. It is found that the chiral lipid gelators 22 can also co-assemble with the achiral all-inorganic perovskite NCs stabilized by oleic acid and oleylamine in nonpolar solvents, in which the gelator molecules modify the surface of the perovskite NCs. Through such co-gelation, the molecular chirality can transfer to the NCs resulting in CPL signals with a dissymmetric factor (glum ) up to 10−3 . Furthermore, depending on the molecular chirality of the gelator, the CPL sense can be selected and the mirror-imaged CPL is obtained. Such gels can be further embedded into the polymer film to facilitate flexible CPL devices. The above well-designed inorganic CPL gels afford a new insight into the application of the functional chiroptical materials. Soon, lanthanide-doped upconversion nanoparticles (UCNPs) showing upconverted circularly polarized luminescence were demonstrated in an organic−inorganic co-assembled system, as shown in Figure 8.28 [98]. Achiral UCNPs (NaYF4 :Yb/Er or NaYF4 :Yb/Tm) can be encapsulated into chiral helical nanotubes through the procedure of co-gelation with gelator 21. These co-gel systems display intense UC-CPL ranging from ultraviolet (UV, 300 nm) to near-infrared (NIR, 850 nm) wavelength. Importantly, the UV part of UC-CPL was used to initiate the enantioselective polymerization of diacetylene, indicating that the molecular chirality of the gelator controlled the handedness of the UC-CPL generated from the LGAm or DGAm/UCNPs-Tm co-gels and then subsequently controlled the enantioselective polymerization of diacetylene.

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Figure 8.27 (a) Photograph of colloidal dispersions of CsPbX3 NCs with different halide (X = Cl, Br, and I) compositions in hexane under UV light (𝜆ex = 365 nm). (b, b′ ) HRTEM overview images of CsPbBr3 . (c) Fluorescence spectra of the corresponding CsPbX3 NCs doped DGAm (𝜆ex = 310 nm for all but 340 nm for CsPbBrx I3−x and CsPbI3 samples); the solvents are hexane, [DGAm] = 9.2 mM, [CsPbX3 NCs] = 1.1 mg ml−1 ; CsPbCl3 NCs (blue line), CsPbClx Br3−x NCs (cyan line), CsPbBr3 NCs (green line), CsPbBrx I3−x NCs (orange line), and CsPbI3 NCs (red line). (d) Mirror-image CPL spectra of the corresponding co-assembly samples, 𝜆ex = 310 nm. (e) The photograph of the free-standing thin film of CsPbBr3 NCs/DGAm dispersed in PMMA under room light (top) and UV light (bottom,𝜆ex = 365 nm); [DGAm] = 9.2 mM, [CsPbBr3 NCs] = 2.2 mg ml−1 , PMMA = 50 mg ml−1 . (f) Mirror-image CPL spectra of the thin film, 𝜆ex = 310 nm. Source: Reproduced with permission from [97]. © 2018, Wiley-VCH. DOI: doi.org/10.1002/adma.201705011.

8.6 Conclusion and Perspectives

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Figure 8.28 (a) Schematic representation of upconverted circularly polarized luminescence (UC-CPL) based on achiral UCNPs and chiral nanotubes. Upon the excitation of 980 nm laser, blue and green UC-CPL could be obtained from the UCNPs-Tm and UCNPs-Er co-gels, respectively. (b) TEM images of LGAm gels and LGAm/UCNPs-Er co-gels. [LGAm] = 26.7 mg ml−1 and [UCNPs] = 1.7 mg ml−1 . (c) UC-CPL emission spectra of co-gels excited by 980 nm laser. The DC value in the bottom spectrum stands for fluorescence intensity. To obtain clear UC-CPL spectra, the co-gel systems were tested in several wavelength regions (300–490, 490–700, and 700–850 nm), wherein the maximum emission peaks were modulated around 0.5 V. [LGAm] = [DGAm] = 26.7 mg ml−1 and [UCNPs-Er] = [UCNPs-Tm] = 1.7 mg ml−1 . (d) Illustration of the polymerization of HA and CD spectra of PDA films after exposing to the UC-CPL generated from UCNPs-Tm doped co-gels. Source: Reproduced with permission from [98]. © 2019, American Chemical Society. DOI: doi.org/10.1021/acsnano.8b08273.

8.6 Conclusion and Perspectives In recent decades, supramolecular luminescence gels have developed rapidly because of their various advantages, including flexible molecular design, high quantum yields, tunable wavelengths, and excellent processability, indicating the potential of this area in the field of new functional materials useful for a variety of applications. It has been seen that the flexibility of self-assembly approach

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can extend the solution of material fabrication, enabling systematic design and fabrication of various luminescence materials. By introducing the design of new fluorescent materials and the concept of self-assembly, CPL-active materials have developed rapidly. From the point of view of emissive materials, the concept of ACQ and AIE has been greatly developed and researchers can more easily design materials showing CPL activity. The modulation of CPL, especially how to amplify glum values, is a key issue in the research field of chiral materials, which greatly determines the practical applications. The hybridization of energy transfer with self-assembly would provide an excellent strategy for developing highly efficient CPL-active materials. In addition, the theoretical mechanism of energy transfer-amplified circular polarization in self-assembled systems is not yet fully understood: it will require the close collaboration with theoretical scientists. On the other hand, this also opens up a new research topic: energy transfer-amplified circular polarization. We envisage that this topic is worth deep discussion. Although the applications of supramolecular luminescence gels have been partly discussed here, the development of supramolecular gel applications in biomedicines, imaging and sensing, and displays and devices is still in its infancy; further effort should be greatly devoted. We hope that the present work will help to seed and stimulate the research area of supramolecular luminescence gel materials, driving further development in supramolecular gel science.

Acknowledgments We greatly appreciate the financial support of the National Natural Science Foundation of China (21802027, 51673050, 91856115); the Ministry of Science and Technology of the People’s Republic of China (2017YFA0206600, 2016YFA0203400); and the Strategic Priority Research Program of Chinese Academy of Sciences (XDB36000000).

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257

Index a

b

absorption 1, 12, 170, 172 achiral chromphores 216, 233 acrylamide (AAm) monomers 79 active donor–acceptor–donor (D-A-D) system 44 𝛼-cyclodextrin (𝛼-CD) 40 aerogel preparation process 177 aerogels 169 classification of 172–173 fabrication of 173–177 freeze-drying 179–186 freezing parameters 182–186 functionalization 186–193 oil spill remediation mechanisms of oil absorption 197–199 parameters 193–197 post-processing of aerogel absorbent 200 solutes/particles properties 180–182 supercritical drying 177–179 surface characteristics 170 unmodified silica aerogels 172 aggregation induced emission (AIE) 22 aggregation induced emission enhancement (AIEE) 44 aging process 175, 176 amine sensing 42–45 anionic clusters 65 anion sensing 35–37 antifreezing ionic hydrogels 82, 85 aromatic amine 43–44 artificial gel 99 asymmetric catalysis 216, 230 atom layer deposition (ALD) 186

bacterial cellulose (BC) 191 𝛽-cyclodextrin-based heat-set gels 115 bidirectional freezing method 183 biobased sorbents 170 biocompatibility 51, 72, 73, 77 biocompatible collagen-based hydrogels 65 biodegradability 51, 156, 170, 172 bioimaging 7, 225 biomedical controlled drug release behavior 51 biopolymer derived gels, for water remediation heavy metal removals 141–146 organic pollutants removal 146–156 biopolymer nanomaterials 132–134 biosensing 100 biostability 230 biosystems 72, 73 biphenylalanine (BPh) 104 1,4-bis-bromohexane functionalized-pillar[5]arene guest (DPHB) 36 2,2-bis(4-carboxyphenyl)hexafluoropropane and N-(4-Aminobenzoyl)L-glutamic acid diethyl ester 56 bisphosphine ligand 112 blue-emitting gel 218 BODIPY 21

c carbohydrate biopolymer-based gel adsorbents 156 carbon materials 72

Supramolecular Gels: Materials and Emerging Applications, First Edition. Edited by Tifeng Jiao. © 2021 WILEY-VCH GmbH. Published 2021 by WILEY-VCH GmbH.

258

Index

carbon nanotubes (CNTs) 75, 81 cation sensing 30–35 cellulose base aerogels 173, 174 nanofibers 181–183 nanofibrils 174, 175, 187, 192, 197 nanofibrils-based aerogels 174 regeneration 174 cellulose nanocrystals (CNCs) 196 centrifugation 170, 200 charge transfer (CT) interactions 105 chemical gels 1, 14, 15, 99 chemically cross-linked hydrogels 141, 142 chemical oxygen demand (COD) 146 chemical vapor deposition (CVD) 186 chitin derivatives 130–132 nanofibers 184, 185 chitin-based aerogels 139, 173, 176, 179, 185 chitin-based gel adsorbents 144–145 chitin-based hydrogel 146, 147, 153, 155 chitosan (CS) 61, 75, 86, 132 circularly polarized luminescence (CPL) 216, 230–234 coassembly of the above-mentioned AIE organogelator 2 (CN-TFMBE) 218 cold plasma treatment 186 combined antitumor photothermal/photodynamic therapy 51 combustion 195, 200 composite aerogels (CAs) 143, 156, 173, 191 conducting polymers 72 conductive fillers 72, 75, 89 conductive hydrogel electronically conductive hydrogels acrylic acid (AA) monomers 77 carbon-based materials 75 conductive polymers 72 crosslinked interactions 72 GO-based conductive hydrogels 77 in-situ polymerization 74 MXene nanosheet 77–78 PANI-PVA conductive hydrogel 75 polymerize polypyrrole (PPy) 75

ionically conductive hydrogels 78–80 mechanical properties and conductivity 73 pressure sensors 85–89 strain sensors 81–85 conductive polymers 72 conjugated metal-organic polymer 112 continuous phase 10–12, 14 controllable energy transfer 25 controllable gelation 230 coordination polymer gels (CPGs) 30 CO2 sensing 37–38

d diarylethene-based gelators 24 3-dimethyl (methacryloyloxyethyl) ammonium propanesulfonate (DMAPS 86 dinuclear organoplatinum (II) complexes 223 dinuclear Pd complex 27 dinuclear platinum(II) complexes 223 9,10-diphenylanthracene (DPA) emitter 226 dipole–dipole attraction 215 dipole–dipole interaction 51, 79 directional freezing 182 directional ice templating processes 183 dispersed phase 10–12, 14, 172 distillation 195, 200 donor–acceptor pairs 225–227 donor-𝜋-acceptor (D-𝜋-A) structural compound 43 dye adsorption 61, 147, 150

e elastomer composites 72 electronic skins 71, 78, 87, 89 electron transfer process 62 electrostatic attraction 58, 128, 135, 141–143, 155 electrostatic interaction 14, 21, 36, 51, 57, 61, 64, 81, 132, 147 electrostatic repulsion 132, 133, 135 epichlorohydrin (ECH) 130 esterification 186 ether-linked alkyl chains 226 extrusion 200

Index

f

h

flavin-binding fungal photoreceptor Cre-recombinase 230 fluorescence quenching 8 fluorescent gelators 4, 21 fluorescent gels for amine sensing 42–45 for anion sensing 35–37 cation sensing 30–35 CO2 sensing 37–38 heating-cooling process 22–23 light 23–25 for nitroaromatic derivative sensing 42 solvent and humidity sensing 38–42 ultrasound 25–28 xerogels 28–30 fluorescent low molecular weight organogelators 45 fluorination 186 freeze-drying 127, 135, 139, 141, 179–186 Freundlich isotherm 150 furan-modified peptide derivative of p-phenylenediamine (FurTpa) 104

H-atom transfer (HAT) 230 H-bonding 215 heating-cooling approach 22 heating-cooling process 22–23, 28 heat-set molecular gels amphiphilic monomers 101 anisotropic opaque gel 103 biphenylalanine (BPh) 104 bola-amphiphilic gelator 105 bola-shaped amphiphilic compound 103 charge transfer (CT) interactions 105 coordination-based nanocomposite heat-set hydrogel 109 furan-modified peptide derivative of p-phenylenediamine (FurTpa) 104 hydrophilic and hydrophobic interactions 103 LCST-based supramolecular hydrogels 101 nanocomposite gel 107, 109 one-dimensional coordination polymer 107–109 self-assembly behavior 103 thermal response and pH response 108 heat-set organic gels amphiphilic one-dimensional coordination system 111 𝛽-cyclodextrin-based heat-set gels 115 𝛽-cyclodextrin (𝛽-CD) supramolecular interactions 115 1,4-diaminobenzene 111 gel nanomaterials and micromaterials 118 gel-sol reversible transformation process 118 hydrophobic and hydrophilic surfaces 115 metallic gel systems and host-guest gel systems 117 octahedral coordination configuration 110 photochromic metal-organic gels (MOGs) 113–114 tetrahedral coordination configuration 110

g gas chromatography 38 gas phase coagulation method 153 gelatin organohydrogels 85, 86 gelation 4, 6–8, 25–29, 31, 38, 52, 111 gelator 3, 4, 6, 7, 11–15, 22, 24, 25, 27, 30, 120, 215–217 gel disruption 36 gel emulsion formation 10 gel system 36, 51, 57, 101, 115 gel-to-gel transition 44 GO composite hydrogels 61 GO/polyethyleneimine (GO/PEI) hydrogels 57 graphene oxide-based composite hydrogels 52, 57–64 graphene oxide-polyethylene glycol (GO-PEG) composite hydrogel 64 green-emitting gel 218 green-emitting sol 218

259

260

Index

heavy metal adsorption 144–145 heavy metal removals 141–146 highest occupied molecular orbital (HOMO) 4 host-guest interaction 36, 40, 51, 57, 65, 72 hybrid aerogels 173 hybrid gel 25 hybrid latex nanoparticles (HLPs) 87 hydrogel-based sensors 82, 85 hydrogels biopolymer nanomaterials 132–134 cellulose and chitin derivatives 130–132 chemically cross-linked NC/NCh 137–138 native cellulose and chitin 129–130 physically cross-linked NC/NCh 134–137 hydrogels-based sensors 82, 89, 90 hydrogen bonding 14, 21, 22, 26, 31, 36, 40, 51, 61, 100, 128 hydrogen bonding interaction 22, 36, 40, 42, 100, 130, 134, 135, 156 hydrogen chloride 8, 133, 153 hydrophilic microfibrils 196 hydrophobic interaction 81, 99, 101, 103, 194 hydrophobization method 186 hydrothermal treatment 135, 137, 195

i implantable electronic devices 71 inorganic silica aerogels 173 in situ reduction approach 61 inter-chain cross-linking interaction 99 intermolecular hydrogen bonding interactions 22, 40 intermolecular non-covalent bonding 57 intramolecular charge transfer (ICT) 44 intra-particle diffusion model 198 ion chromatography 38 ionic conductive hydrogels 72, 82 ion sensing process 33 IUPAC classification 194, 195

l light-emitting electrochemical cell (LECs) 113

light-responsive low-molecular-weight gelators 24 liquid N2 adsorption/desorption isotherms 194 lowest unoccupied molecular orbital (LUMO) 4 low fatigue resistance 72 low-molecular-mass-organogelators (LMOGs) 21 low molecular weight gelators (LMWGs) 24, 30, 100 L-tartaric acid based gelator 33 luminescent supramolecular gels 215

m mass spectrometry 38 mechanical flexibility 71–74, 80, 89 mechanical sensors 71–74, 80–89 mechanofluorochromism (MFC) 28 metal hydrogels 52 metal-ligand coordination 215 methacrylic acid (MAA) 86 microfibrils 133, 174, 175 molecular gel chemicals sensing, liquid phase 9 design 3–4 synthesizing fluorescent sensing films 2–3 synthesizing porous materials 9–11 oil on water surface, removal 11–13 VOCs adsorption 13–14 VOC 4–9 molecular self-assembly 10, 14, 100, 115, 233, 247

n N-(4-amino-benzoyl)-L-glutamic acid diethyl ester 56, 57 nanocellulose 32, 179, 181, 185 nanocrystals (NCs) 132 nanofibrils (NFs) 132 naphthalimide (NID) 6, 24, 29, 34 naphthalimide-based gelator 24, 34, 42 naphthalimide-based organogelators 29 natural gel 99, 100 nitroaromatic derivative sensing 42 7-nitrobenzofuranzan 7 N,N-dimethylaniline 45

Index

N, N-methylenebisacrylamide (MBAA) crosslinkers 79 non-biocompatibility 72 nonemissive sol 218 N-(pyridinium-4-yl)-naphthalimide functionalized-pillar[5]arene host (P5BD) 36

o octadecylamine (ODA) 187, 194 oil absorption efficiency 185 oil cleanup methods 171 oil sorbents 170 oil spill remediation 72, 170, 188, 193–200 oil spills 169, 170 oligo(oxyethylene) chains 222 organic pollutants removal 146–156 organogel assembled by gelator 2 (CN-TFMBE) 217 organogels 35, 42, 45, 52–55, 65, 115, 127 organohydrogels 74, 85 organometallic terpyridyl platinum gelators 27

p PAA-Ag/AgNPs hydrogels 62, 64 𝜋-conjugated molecules 216 personalized health monitoring 71 phenylamine 45 photocatalysis 225, 227 photocatalytic behavior 62 photochromic metal-organic gels (MOGs) 113 photochromic process 25 phototherapy 225 pH responsive aerogels 192, 193 pH-sensitive hydrogels 132, 153 Physically cross-linked hydrogels 80, 128, 137, 141, 142 physical sensor 71 platinum gelators 27, 28 polyacrylamide (PAM) hydrogel 75, 82 polydopamine (PDA) 82, 194 polymer gels 1, 99, 101 𝜋−𝜋 interaction 4, 51 𝜋−𝜋 stacking 14, 16, 21, 27, 51, 56, 101, 115 pressure sensors 73, 81, 85–89

pseudo-second-order model 59, 61 pyridyl subcomponents 113 pyrolysis 185, 192, 195, 201

r reduced graphene oxide (RGO)-based hydrogel 58 relative humidity (RH) 39 room temperature phosphorescence (RTP) 219

s self-adhesion 73, 81, 82, 89 self-assembly 1, 3, 21, 51–57, 62, 100 self-assembly process 57, 62, 103, 115, 241, 243 self-assembly technique graphene oxide-based composite hydrogels 57–64 self-assembled injectable 51 self-healing hydrogels 51 supramolecular gels 55–57 self-healing ability 71, 73, 82 collagen-protein-based hydrogel 64 selfhealing ability 71, 73, 82 self-polymerization 77, 192, 196 semiconductors 72 sensing objects 45 silane 186, 187, 192 silanization 186, 192, 195, 196 silanols 192 silica composite 191 silver nanoparticles 58, 61, 62 single electron transfer (SET) 230 sodium dodecylsulfate (SDS) 192 sol-gel transformation 37, 44 solvatochromic behavior 216 solvent exchange 129, 139, 174, 177, 179 solvent exchange and depressurization processes 177 solvent molecules 1, 14, 22, 28, 215 solvent volatilization 2 solvophobic interaction 51 sonication triggered gel 32 stimuli-responsive functional materials 217 stimuli-responsive gel arrays 32 strain sensors 79, 81–85

261

262

Index

stretchability 72, 77, 79, 81, 87 stretchable conductive materials 72 supercritical drying 127, 139, 172, 177–179 supercritical drying process 178 superhydrophobic microfibrillated cellulose aerogels (HMFCAs) 196 supramolecular gels circularly polarized luminescence (CPL) 216 achiral luminescent gelators 238–239 chiral luminescent gelators 234–237 inorganic luminophores guest 245–249 organnic luminophores guest 239–245 CN-TFMBE gels 217 cyanostilbene gelator 3 (PyG) 217 fluorescence quenching and modulation 219 luminescent inorganic nanomaterials 215 luminophors 215 multi-channel stimuli-responsive functional materials 218 phosphorescence 219–224 photon upconversion 224–230 supramolecular hydrogels 55–57, 101 surface wettability 195 suspension-gel process 22 synergistic biopolymer–surfactant interactions 230 synthesized receptor 32 synthetic polymer aerogels 173 synthetic polymer gels 101 synthetic polymer sorbents 170

t tertiary-butanol (TBA) 181 1,4,7,10-tetraazacyclododecane-1,4, 7-triacetic acid 57

thermal insulation 2, 173 thixotropy 1, 3, 230 time-consuming techniques 38 tissue engineering 51, 73, 100, 120 triethylamine 44, 45 triplet–triplet annihilation (TTA) 225 triplet-triplet annihilation-based photon upconversion (TTA-UC) 236 two-component organogels 52

u ultrasound 25–29, 40 unmodified silica aerogels 172 upconversion nanoparticles (UCNPs) 247 upconverted circularly polarized luminescence (UC-CPL) 237, 247, 249

v van der Waals forces 21, 128, 135, 147, 194, 197 Van der Waals interaction 51, 227 visual ion sensing 32 volatile organic compounds (VOCs) adsorption 13–14 vapor detection 4–9

w water pollution 9, 52, 141 wearable mechanical sensors 72, 73 wearable sensors 71, 74, 82, 89, 90

x xerogel assembly 39 xerogels 6, 23, 28–30, 45, 53 X-ray photoelectron spectroscopy (XPS) 192

z zwitterionic hydrogels-based capacitive pressure sensor 87