Graphene Quantum Dots: Biomedical and Environmental Sustainability Applications 0323857213, 9780323857215

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Woodhead Publishing Series in Electronic and Optical Materials

Graphene Quantum Dots Biomedical and Environmental Sustainability Applications

Edited by Mohammad Oves Khalid Umar Iqbal M. I. Ismail Mohamad Nasir Mohamad Ibrahim

Woodhead Publishing is an imprint of Elsevier 50 Hampshire Street, 5th Floor, Cambridge, MA 02139, United States The Boulevard, Langford Lane, Kidlington, OX5 1GB, United Kingdom Copyright Ó 2023 Elsevier Ltd. All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Details on how to seek permission, further information about the Publisher’s permissions policies and our arrangements with organizations such as the Copyright Clearance Center and the Copyright Licensing Agency, can be found at our website: www.elsevier.com/permissions. This book and the individual contributions contained in it are protected under copyright by the Publisher (other than as may be noted herein).

Notices

Knowledge and best practice in this field are constantly changing. As new research and experience broaden our understanding, changes in research methods, professional practices, or medical treatment may become necessary. Practitioners and researchers must always rely on their own experience and knowledge in evaluating and using any information, methods, compounds, or experiments described herein. In using such information or methods they should be mindful of their own safety and the safety of others, including parties for whom they have a professional responsibility. To the fullest extent of the law, neither the Publisher nor the authors, contributors, or editors, assume any liability for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions, or ideas contained in the material herein. ISBN: 978-0-323-85721-5 For information on all Woodhead Publishing publications visit our website at https://www.elsevier.com/books-and-journals

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List of contributors Rohana Adnan, School of Chemical Sciences, Universiti Sains Malaysia, Penang, Malaysia Rameez Ahmad Aftab, Department of Chemical Engineering, Zakir Hussain College of Engineering and Technology, Aligarh Muslim University, Aligarh, Uttar Pradesh, India Shaikh Ziauddin Ahammad, Department of Biochemical Engineering and Biotechnology, Indian Institute of Technology IIT Delhi, New Delhi, India Hilal Ahmad, Centre for Nanoscience and Nanotechnology, Jamia Millia Islamia, New Delhi, India Mohd Ashraf Alam, Department of Pharmacology, IIMS & R, Integral University, Lucknow, Uttar Pradesh, India Syed Wazed Ali, Department of Textile and Fibre Engineering, Indian Institute of Technology Delhi, New Delhi, India Marai Almari, Department of Surgery, Faculty of Medicine, University of Tabuk, Tabuk, Saudi Arabia Mohammad Azam Ansari, Department of Epidemic Disease Research, Institute for Research & Medical Consultations (IRMC), Imam Abdulrahman Bin Faisal University, Dammam, Saudi Arabia Mohammad Omaish Ansari, Centre of Nanotechnology, King Abdulaziz University, Jeddah, Saudi Arabia Abdul Hakeem Anwer, School of Mechanical Engineering, Yeungnam University, Gyeongsan, Gyeongaan, Republic of Korea Mohd Arshad, Department of Physics, Aligarh Muslim University, Aligarh, Uttar Pradesh, India Abdullah M. Asiri, Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia; Centre of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, Saudi Arabia Mohd Ayaz, Ministry of Higher Education, Applied Biotechnology Department, Sur College of Applied Sciences, Sur, Oman Ogechukwu Bose Chukwuma, School of Industrial Technology, Universiti Sains Malaysia, Penang, Malaysia Farha Fatima, Department of Zoology, Aligarh Muslim University, Aligarh, Uttar Pradesh, India

xi

xii List of contributors Sufia ul Haque, Advance Functional Material Laboratory, Department of Applied Chemistry, Zakir Husain College of Engineering and Technology, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh, Uttar Pradesh, India Uzma Haseen, Department of Chemistry, Aligarh Muslim University, Aligarh, Uttar Pradesh, India Bahaa A. Hemdan, Environmental Microbiology, Laboratory, Water Pollution Research Department, National Research Centre, Dokki, Giza, Egypt Fahad Mabood Husain, Department of Food Science and Nutrition, Faculty of Food and Agricultural Sciences, King Saud University, Riyadh, Saudi Arabia Iqbal M. I. Ismail, Centre of Excellence in Environmental Studies, King Abdulaziz University, Jeddah, Saudi Arabia; Department of Chemistry, King Abdulaziz University, Jeddah, Saudi Arabia Mohd Jameel, Department of Zoology, Aligarh Muslim University, Aligarh, Uttar Pradesh, India Kiran Jeet, Electron Microscopy and Nanoscience Laboratory, Punjab Agricultural University, Ludhiana, Punjab, India Mangala Joshi, Department of Textile and Fibre Engineering, Indian Institute of Technology Delhi, New Delhi, India Sakshi Kapoor, Centre for Nanoscience and Nanotechnology, Jamia Millia Islamia, New Delhi, India Mohammad Zain Khan, Industrial Chemistry Research Laboratory, Department of Chemistry, Faculty of Science, Aligarh Muslim University, Aligarh, Uttar Pradesh, India Mohamad Nasir Mohamad Ibrahim, School of Chemical Sciences, Universiti Sains Malaysia, Penang, Malaysia Mohammad Nazim, Department of Chemical Engineering, Kumoh National Institute of Technology, Gumi-si, Gyeongbuk-do, Republic of Korea Mohammad Oves, Centre of Excellence in Environmental Studies, King Abdulaziz University, Jeddah, Saudi Arabia Tabassum Parveen, Department of Botany, Aligarh Muslim University, Aligarh, Uttar Pradesh, India Aftab Aslam Parwaz Khan, Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia; Centre of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, Saudi Arabia Huda A. Qari, Centre of Excellence in Environmental Studies, King Abdulaziz University, Jeddah, Saudi Arabia; Department of Biological Science, King Abdulaziz University, Jeddah, Saudi Arabia Mohammad Amir Qureshi, Department of Chemistry, Faculty of Natural Science, Jamia Millia Islamia, New Delhi, India

List of contributors

xiii

Mohd Rafatullah, School of Industrial Technology, Universiti Sains Malaysia, Penang, Malaysia Rani Rahat, College of Dentistry, University of Illinois, Chicago, IL, United States Mohd Ahmar Rauf, Use-Inspired Biomaterials & Integrated Nano Delivery (U-Bind) Systems Laboratory, Department of Pharmaceutical Sciences, Eugene Applebaum College of Pharmacy and Health Sciences, Wayne State University, Detroit, MI, United States Asif Saud, School of Chemical Sciences, Universiti Sains Malaysia, Penang, Malaysia; Department of Chemistry, Aligarh Muslim University, Aligarh, Uttar Pradesh, India Mohammad Shahadat, School of Chemical Sciences, Universiti Sains Malaysia, Penang, Malaysia Muhammad Taqi-uddeen bin Safian, School of Chemical Sciences, Universiti Sains Malaysia, Penang, Malaysia Shuchita Tomar, Department of Textile and Fibre Engineering, Indian Institute of Technology Delhi, New Delhi, India Khalid Umar, School of Chemical Sciences, Universiti Sains Malaysia, Penang, Malaysia Mohammad Faisal Umar, School of Industrial Technology, Universiti Sains Malaysia, Penang, Malaysia Sadiq Umar, College of Dentistry, University of Illinois, Chicago, IL, United States Waris, Industrial Chemistry Research Laboratory, Department of Chemistry, Faculty of Science, Aligarh Muslim University, Aligarh, Uttar Pradesh, India Mohammad Zubair, Department of Medical Microbiology, Faculty of Medicine, University of Tabuk, Tabuk, Saudi Arabia

Preface The book provides an overview of the fundamentals and advances in applications of graphene quantum dots. Concepts covered include a brief introduction, an overview of the structure and chemistry, fundamental properties of different characterization techniques, and methods of preparation of graphene quantum dots. The book has also included graphene and quantum dots recent and emerging applications in various fields: antimicrobial therapy and medical device development, bioimaging, biomedical tools development and clean energy for environmental sustainability. It has a helpful, informative, and valuable resource for researchers, scientists, and postgraduate and undergraduate students from various sectors. Thus, there is an immediate urge to educate the young masses and professionals about the newer materials capable of biomedical and environmental sustainability applications as well as various advanced imaging, disinfectant, and environmental remediation technologies. Graphene quantum dots, light yet mechanically strong materials, has the potential of being among the pioneer materials for the aforementioned field due to wide possibilities such as large surface area, ease of functionality, fabrication from natural and synthetic materials, and so forth. In spite of these qualities, the graphene quantum dots have yet to gain attention and wide popularity, and this is the reason for this book. The intended potential audience of this book is materials scientists and engineers working on biomedical tool development and environmental remediation, scientists, practising engineers and students working in the field of biomedical, energy, and environmental science. The book content covers the basic properties of graphene quantum dots, their special properties and fabrication techniques. The book has discussed most of the major biomedical and environmental remediation applications of graphene-based quantum dots. The advantages will be readers of graphene quantum dots in biomedical tool development and clean energy, and environmental sustainability to aid in materials selection. Apart from this, prospects about utilizing quantum dots of graphene in modern day-to-day life and the fabrication of tangible products have also been deliberated.

xv

xvi Preface

The information presented about the graphene quantum dots in simple and lucid form and much attention has been paid to covering all aspects of the title through to the present era. Dr. Mohammad Oves Dr. Khalid Umar Prof. Iqbal Mohammad Ibrahim Ismail Prof. Mohamad Nasir Mohamad Ibrahim

Chapter 1

Graphene and its quantum dots: fabrication and properties Sakshi Kapoor1, Uzma Haseen2 and Hilal Ahmad1 1 2

Centre for Nanoscience and Nanotechnology, Jamia Millia Islamia, New Delhi, India; Department of Chemistry, Aligarh Muslim University, Aligarh, Uttar Pradesh, India

1.1 Introduction “Nano” is a Latin word meaning “dwarf.” Scientifically materials that have at least one dimension between 1 and 100 nm are considered a nanomaterial. The name nanotechnology was proposed by Norio Taniguchi in 1974 and the theory was explained by physicist Richard P. Feynman in 1959 in his lecture “There’s Plenty of Room at the Bottom” [1]. Nanotechnology has entered fields such as electronics, medicines, agriculture, aerospace, and many more, highlighting its importance in every sector [2]. The physicochemical properties are enhanced when reducing from bulk to nanodimensional particles [3]. This implies particles smaller than the Bohr exciton radius, where the movement of excitons is confined, energy band splits up into discrete energy levels due to quantum confinement and size effect [4]. Therefore, it is significant to understand the properties of nanostructured materials to integrate them into nanoscale devices for higher efficiency [5]. The carbon nanomaterials possess exemplary properties and diversified applications, where graphene is one of the most popular allotropes of this family due to its remarkable performance [6] in every stratum vis-a`-vis other classic nanomaterials [7]. The name “graphene” was designated by Mouras et al. in 1987 [8], while the material was extensively explored after A. K Geim and K. S Novoselov in 2007 isolated graphene from highly oriented pyrolytic graphite (HOPG) [9]. Two-dimensional (2D) graphene nanomaterial is formed of a single layer of sp2 hybridized carbon atoms organized in a honeycomb matrix and possesses excellent properties at room temperature (RT) [10e12] as shown in Table 1.1.

Graphene Quantum Dots. https://doi.org/10.1016/B978-0-323-85721-5.00006-6 Copyright © 2023 Elsevier Ltd. All rights reserved.

1

2 Graphene Quantum Dots

TABLE 1.1 List of graphene properties at room temperature. S. No.

Property

Approximate value

1.

Large surface area

2630 m2 g
1

2.

Young’s modulus

2.0 TPa

3.

Superior tensile strength

130 GPa

4.

High electron mobility

15,000 cm2 V
1 s
1

5.

Enhanced electrical conductivity

Due to zero-bandgap

6.

Great thermal conductivity

3500 W mK
1e5300 W mK
1

7.

Chemically stable with high optical transparency and flexibility

88%e97.7% and (20% elongation)

Monolayer, transferable, device quality graphene sheets (GSs) are obtained by mechanical exfoliation [9] and epitaxial chemical vapor deposition [10,13]. These methods suffer a major drawback of large-scale production plus low throughput due to time and cost constraint [14]. Therefore, this led to the development of structurally similar compounds; graphene oxide (GO) and reduced graphene oxide (rGO) that has great scientific benefits [15,16]. In the case of GO, the aromatic crystal lattice of graphene is obstructed by epoxides, alcohols, ketones, and carboxylic groups, marked by a rise in interlayer spacing from 0.335 nm for graphite to more than 0.625 nm for GO and hydrophilicity [17,18]. The structure of GO and rGO possess a conjugated system of sp2 carbon atoms, few hundreds or thousands of nanometres wide and domains of conjugation are up to few centimeters in length as in the case of carbon nanotubes (CNTs) [11,19]. The unprecedented features are attained in case of either bilayer or few-layer graphene. The reason is that functional groups and defects attached on the aromatic carbon ring during various synthesis methods dramatically alter the structure of the carbon plane, thereby making it inappropriate to use for device assembly [20]. Therefore, numerous attempts have been made to achieve mono-to-bilayer graphene structure at mass scale and reasonable cost through ecofriendly synthesis technique [21], but the final target is still a dream. But it is hoped that soon efficient growth methods will be developed, as happened for CNTs [22]. GS has extraordinary physical, chemical, and electronic properties as listed earlier. Because of the hydrophobic nature of graphene, the suspension in almost all solvents gets agglomerated in short period of time, inhibiting its application in different areas. Many attempts had been made in the past to enhance its dispersion by attaching more oxygen functionalities, cutting it into one-dimensional (1D) graphene nanoribbons (GNRs) or zero-dimensional (0D)

Graphene and its quantum dots: fabrication and properties Chapter | 1

3

graphene quantum dots (GQDs) [23]. Therefore, more emphasis is laid down in converting micron sized graphene sheets into GQD and graphene nanoribbons [24]. A quantum dot (QD) is a crystal possessing dimension 1 min.

[13]

2.

Microwave heating irradiation

Graphite flakes:(2,2,6,6tetramethylpiperidine 1-oxyl) TEMPO:H2O2::1:0.1:2 heated at 1200 W for 100 s

[12]

3.

Mechanical cleavage

Tapping process is continuous attaching and detaching of exfoliated graphite (HOPG) using scotch tape.

[9]

4.

Solvothermal synthesis

Pentachloropyridine and metallic potassium processed at 160 C for 10 h in Teflon-lined autoclave.

[28]

5.

Liquid-phase exfoliation

(a) Graphite powder dispersed in NMP/PVA/surfactant NaC in a mixer with variable rotor diameter and durations,

[29]

(b)H2SO4 and HNO3 refluxed graphene sheets dispersed in DMF, followed by multi-frequency ultrasonication and centrifugation,

[30]

(c)Graphite flakes dispersed in NMP and o-DCB were sonicated at 40  2 C (600 W) for 6 h at RT,

[31,32]

(d)Graphite dispersed in NMP and Cl was produced in the mixture by adding HCl and KMnO4.

[33e35]

(a) Graphite rod in H2SO4:HNO3::3:1 under 2e10 V at RT

[6]

(b)Graphite sheet in H2SO4 under 0e8 V at RT,

[36]

(c)Graphite foil in NaOH under 3 V, H2SO4, Na2SO4, and LiClO4 under 2e10 V at RT and

[14,37,38]

6.

Electrochemical exfoliation

Continued

6 Graphene Quantum Dots

TABLE 1.2 List of major techniques followed to synthesize graphene and its derivatives.dcont’d S.No.

Approach

Protocol followed

References

H2SO4 þ KOH þ DW under 10 V at 80 C,

7.

Chemical synthesis

(d)Graphite powder in sodium chloride, sodium hydroxide, PVP, SDBS and DTAB,

[39]

(e)HOPG in Na2SO4, K2SO4, (NH4)2SO4, H2SO4 under 10 V at RT,

[20,21]

(f)Graphite foil in TBA/HSO4 under 10 V, 0.1 Hz in an ice bath.

[40]

GO synthesis (oxidant þ solvent þ additiveheating at 90 C): (a) Brodie: KClO3 þ HNO3, (b) Staudenmaier: KClO3 þ Fuming HNO3, (c) Hummers: KMnO4 þ H2SO4 þ NaNO3, (d) Tour: KMnO4 þ H2SO4 þ H3PO4 and many more methods.

[15,17,18]

rGO synthesis: (a) Thermal annealing at 900 e1100 C in ultra-high vacuum (UHV) and Ar/H2, (b) Microwave and photoreduction, (c) Chemical reduction with hydrazine hydrate and NaBH4, (d) Photocatalytic reduction, (e) Solvothermal reduction at supercritical temperature of solvent and many more methods.

[16,19]

After the process, the obtained graphene or its derivatives were sonicated, centrifuged, vacuum filtrated, heated or freeze dried.

1.3.2 Surface-enhanced Raman scattering (SERS) The GQDs were synthesized by electrochemical oxidation of graphene and GQD-NTs by electrodeposition of GQDs into 200 nm anodic aluminum oxide (AAO) membrane as shown in Fig. 1.4. GQD microbowls (Fig. 1.5d, inset) were fabricated by utilizing the microspheres as electrodeposition template

TABLE 1.3 List of synthesis techniques and features of GQD comparative to its source and derivatives. Products

Method

Yield (%)

C/O or O/C atomic ratio

Functional groups attached

Absorption shoulder (nm)

Fluorescence Ex/Em (nm)

References

11.43

e

eOH, NeH, C] O, CH2

420

Green

[23]

I. GQD versus CQD Microwave- CA and urea heated at 750 W for 4 e5 min

CQD

Electrochemical- ablation of graphite rods under 50 V at RT

0.81

e

eOH, NeH, C] O, CH2

420

Green

GQD

Solvothermal-GO dispersed in DMF heated at 200 C for 8h

9.81

e

eOH, NeH, C] O, CH2, CeH

420

Green

II. GQD versus graphene Electrochemical- stripping of GEs in H2SO4, NaCl, NaOH under 10V at RT

50

4.03

eOH, C]O, C] C, CeO, CeH

208, 286

e

PGS

Refluxing-HNO3 acid refluxing of GS at 100 C for 24 h

e

4.02

eOH, C]O, C] C, CeO

236, 275

Blue 300/397

GQD5

Electrochemical- ablation of GE in NaOH under 10 V at RT

e

11.64

eOH, C]O, C] C, CeO

251, 290

Blue 300/375, 394, and 414

GQD4

Hydrothermal-PGS and DI water heated at 200 C for 8h

e

6.59

eOH, C]O, C] C, CeO

214, 263

Blue 300/405

GQD2

Solvothermal- PGS, H3PO4 and ethylenediamine heated at 200 C for 8 h

e

2.30

eOH, C]O, C] C, CeO

220, 265

Blue 300/400

[5]

7

GS

Graphene and its quantum dots: fabrication and properties Chapter | 1

CQD

Continued

Products

Method

Yield (%)

C/O or O/C atomic ratio

Functional groups attached

Absorption shoulder (nm)

Fluorescence Ex/Em (nm)

References

III. GQD versus GO GO

Pyrolysis- thermal heating of citric acid at 200 C for 2 h

2.2

1.16

eCOOH, CeO eC, C]O, eOH

Broad peak below 600

Blue 330e480/450 e542

GQD

Pyrolysis- thermal heating of citric acid at 200 C for 30 min

9

0.92

eCOOH, CeH, C]O, eOH

362

Blue 300e440/362 and 460

[48]

IV. GQD versus surface-passivated GQD GQD

Hydrothermal- HNO3 acid refluxed GO heated at 200 C for 24 h

13.1

e

No polymer chains

320

Blue 300e400/340 e320

GQDPEG

Hydrothermal- HNO3 acid refluxed GO and PEG heated at 200 C for 24 h.

28

e

eC]C, eOH, C eH

No clear absorption peak

Blue 300e400/377 e493

[50]

After the process, the obtained GQDs were isolated by either column chromatography, decantation of supernatant, dialysis, centrifugation, vacuum filtration, evaporation, or lyophilization.

8 Graphene Quantum Dots

TABLE 1.3 List of synthesis techniques and features of GQD comparative to its source and derivatives.dcont’d

Graphene and its quantum dots: fabrication and properties Chapter | 1

9

FIGURE 1.2 FESEM images: (a) electrochemically exfoliated thin graphene sheets and (b) unseparated GQDs scattered densely on the surface of graphene sheets.

FIGURE 1.3 (a) XRD spectra of GS versus GQD, (b) Lorentzian peak fitted 2D band of GS versus GE and (c) Complete Raman Spectra of PGS versus GQD with D, G, 2D, D þ G and 2G as resonant modes. Reproduced with authorization from Kapoor S, Jha A, Ahmad H, Islam SS. Avenue to large-scale production of graphene quantum dots from high-purity graphene sheets using laboratory-grade graphite electrodes. ACS Omega 2020;5(30):18831e41.

FIGURE 1.4 Fabrication of GQDs and GQD-NTs for SERS analysis. Imprinted with authorization from Cheng H, Zhao Y, Fan Y, Xie X, Qu L, Shi G. Graphene-quantum-dot assembled nanotubes: a new platform for efficient Raman enhancement. ACS Nano 2012;6(3):2237e44.

10 Graphene Quantum Dots

FIGURE 1.5 632.8 nm stimulated Raman plot of (a) 4  10
6 mol L
1 R6G and (c) 2  10
2 mol L
1 2,4-DNT dripped on: bare Si substrate, GQDs film (GQDs/Si) and GQD-NTs (GQD-NTs/Si) adhered on Si substrate, respectively, (d) 4  10
6 mol L
1 R6G adsorbed on (i) GQD-NTs/Si, and (ii) GQD microbowls/Si, respectively inset: FESEM image depicting synthesized GQD microbowls with a width of 500 nm and a height of ca. 250 nm distinct from that of GQD-NTs (ca. 25). and (b) 623.8 nm stimulated Raman plot of different concentration R6G 8  10
11, 8  10
9, 8  10
7, 4  10
6, and 4  10
5 mol L
1 (bottom to up) adsorbed on GQD-NTs/Si, respectively. (e) 532 nm excited Raman spectra of GSs/glass (magenta) and GSs/ AAO (green), i.e., graphene sheets spin coated on glass and AAO membrane (90 nm), respectively. Imprinted with authorization from Cheng H, Zhao Y, Fan Y, Xie X, Qu L, Shi G. Graphenequantum-dot assembled nanotubes: a new platform for efficient Raman enhancement. ACS Nano 2012;6(3):2237e44.

Graphene and its quantum dots: fabrication and properties Chapter | 1

11

under similar conditions as that for GQD-NTs, with a greatly lower aspect ratio of ca. 0.5 unlike GQD-NTs (ca. 25). Rhodamine 6G (R6G) and 2,4dinitrotoluene (2,4- DNT) were used as the model molecules, to investigate GQDs, GQD-NTs, GQD microbowls for SERS applications [43]. The adsorption of molecules on different SERS substrates are discussed below: (a) GQD-NTs: Intense peaks with high signal-to-noise (S/N) ratio were recorded for R6G adsorbed on GQD-NTs, while GQD film depicted only D and G bands and no peaks were observed on Si wafer under similar environment (Fig. 1.5a). The Raman signal was 40- to 74- fold amplified on GQD-NTs as compared to only 2- to 17-fold increase on graphene film, with Si substrate as normalization reference. The limit of detection measured for R6G concentration was found to be 10
9 M at the level of (S/N) ¼ 3 (Fig. 1.5b). Hence, these results inferred that arrangement of GQDs into geometrically well-defined nanotubes (NTs) proved to be a promising substrate for SERS. The Raman peaks for 2,4-DNT on GQD film were also found seven times less intense via-a-vis GQD-NTs as shown in Fig. 1.5c, with no relevant peak detected on Si surface [43]. (b) GQD Microbowls: The much effective trap of R6G molecules on GQDNTs than GQD microbowls (Fig. 1.5d), played a vital part in strengthening the charge transmission among target molecules and graphene dots, which is accountable for attaining intensified Raman signals [43]. Although, all the three assemblies are composed of GQDs (GQD film, NTs, microbowls) but recorded measurements concludes that Raman signal strongly depend on morphology, surface and interface chemistry. This is due to the fact [43]: (a) The porous walls and hollow nanostructure of GQD-NTs provided large surface area for adsorption of molecules, effective charge transfer, and its rough surface captured incident light more constructively. In addition, GQD-NTs electrodeposited into porous AAO membrane exhibited a uniform array structure with GQDs (ca. 5 nm) systematically confined in the NTs; meeting the requirement of ordered and regular display of the nanoparticles for SERS. (b) Contrarily, drop-casted GQD film had an uneven surface comprising of agglomerated GQDs with a comparatively big size of ca. 100e200 nm caused due to wet-to-dry process. The adsorbed molecules struggle to establish a homogeneous molecular layer, coating the surfaces of the loosely packed GQD group in this scenario. Further, we inspected recently SERS study by spin-coating synthesized GSs (Table 1.3(II)) on the surface of lab-made porous alumina membrane [44]. The Raman modes of graphene sheets (D, G, and 2D) on AAO membrane (90 nm) were greatly enhanced vis-a`-vis GSs coated on glass substrate as seen

12 Graphene Quantum Dots

in Fig. 1.5e. The Raman intensity increased 10 times due to the presence of nanohole matrix of AAO membrane below the freestanding film of graphene sheets [45].

1.3.3 Chemical study of nitrogen (N)- doping The type of functional elements or molecules attached to the structure of GSs and GQDs depends on the fabrication method, raw materials and chemicals used for its synthesis. This can be evaluated via Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and energydispersive X-ray (EDX) as described below and listed in Table 1.3. The heteroatoms, functional groups, and ligands on the basal plane and edges of the polyaromatic structure of GQDs and GSs helps them to gain hydrophilicity in solvents [5]. The degree of oxidation and N-doping after following complete synthesis mechanism as depicted in Fig. 1.6 was analyzed through XPS. The magnified view of the survey range for C 1s (Fig. 1.7a,b) depicted rise in the quantity of

FIGURE 1.6 Mechanism followed for the preparation of N-doped GO and GQDs.

FIGURE 1.7 (A) Complete XPS survey constituting carbon, oxygen, hydrogen, nitrogen and magnified deconvoluted peaks of C 1s for (a) GO, (b) OeGO, (c) ReOeGO, (d) GQD1 and (e) GQD2. Imprinted with authorization from Zhao M. Direct synthesis of GQD with different fluorescence properties by oxidation of GO using nitric acid. Appl Sci 2018;8(8):1303.

Graphene and its quantum dots: fabrication and properties Chapter | 1

13

CeO and decrease in CeC and C]C, after oxidation of GO by HNO3 (i.e., OeGO). These results confirmed that oxidation causes cleavage of the CeC and C]C bonds and attachment of oxygen functional batch. Further, hydrothermal procedure caused loss of oxygen groups in the form of CO2, CO, and H2O in ReOeGO and GQD1 (Fig. 1.7c,d) due to reduction process [46]. The complete survey in Fig. 1.7a, revealed introduction of N atoms into GO after oxidation by HNO3 in the form of CeNeC (w399.5 eV) and Ne(C)3 (w401.5 eV) as seen in Fig. 1.8aed. Further, oxidation of R-O-GO disclosed increase in oxygen and additional nitrogen molecules (eNO2 w406.4 eV) in GQD2 as seen in Figs. 1.7e and 1.8d due to refluxing in HNO3. It was observed that nitrogen content was small in case of O-GO obtained after 8M HNO3 acid reflux treatment [46] and negligible in 5 M HNO3 refluxed GSs [5], which confirms the fact that degree of N-doping also depends upon the molarity of the source dopant.

1.3.4 Optical analysis 1.3.4.1 pH-dependent properties GQDs were obtained by heating refluxed graphene sheets (H2SO4 and HNO3) in DI water at 200 C for 10 h [4]. The absorption peak around 230 and 320 nm was observed for oxidized GSs, that is attributed to pep* jump of aromatic sp2 zones as shown in Fig. 1.9a. The photoluminescence (PL) peak red shifts on changing the excitation wavelength from 320 to 400 nm, with sudden decrease in intensity as shown in Fig. 1.9b. The intense PL emission peak (430 nm) was recorded from the absorption bands of 320 and 257 nm excitation as depicted in Fig. 1.9c. The PL quenching phenomenon in GQDs is observed in pH variant solutions [47], with strong emission in alkaline medium. The PL is quenched in acidic medium and depicts reversible phenomenon on changing the pH between 13 and 1 (Fig. 1.9d). The strong PL is originated from the unbound zigzag sites on GQDs with a carbene-like triplet ground state as explained by Pan et al. (Fig. 1.10a,b). The zigzag sites are protonated under acidic environment due to reversible compound formed

FIGURE 1.8 Magnified deconvoluted XPS spectra of N 1s for (a) OeGO, (b) ReOeGO, (c) GQD1 and (d) GQD2. Imprinted with authorization from Zhao M. Direct synthesis of GQD with different fluorescence properties by oxidation of GO using nitric acid. Appl Sci 2018;8(8):1303.

14 Graphene Quantum Dots

FIGURE 1.9 (a) Absorption and PL combined plot of GQDs and oxidized GSs (Inset: digital image of dispersed GQDs in water beneath visible light), (b) PL plot of the GQDs excited at variable wavelengths, (c) PLE and PL plot with sensing wavelength of 430 nm and excited at 257 nm, respectively (Inset: digital image of dispersed GQDs in water under UV light) and (d) pHdependent PL plot where pH is switched between 13 and 1. Imprinted with authorization from Pan D, Zhang J, Li Z, Wu M. Hydrothermal route for cutting graphene sheets into blue-luminescent graphene quantum dots. Adv Mater 2010;22(6):734e8.

between the zigzag sites and H⁺ and restored in alkaline conditions producing enhanced PL. The ground state of carbene possess two electronic configurations: singlet and triplet. The s1 and p1 orbitals are singly occupied in the triplet condition, while two electrons that aren’t bound together are coupled in s2 orbital with p2 orbital empty in singlet state. The valence band spin orbital energy difference in the ground state of the carbene is dE b-GQDs > g-GQDs (b) Bacterial cells survival rate: g-GQDs < b-GQDs < y-GQDs

1.3.6.4 GQDs versus surface-passivated GQDs The interaction of pristine GQDs (P-GQDs) and functionalized GQDs marked the cytoplasm of Human HeLa cells smoothly (Table 1.4(4)). P-GQDs were developed by heating graphite powder in H2SO4 and HNO3 at 120 C for 24 h. 1,2-ethylenediamine functionalized GQDs (EDA-GQDs) were prepared by mixing P-GQDs and SOCl2 at 80 C for 2 h and heating again after adding 1,2ethylenediamine at 100 C for 4 h [49]. The fluorescence cell imaging performance obtained from P-GQDs were exactly obtained from relatively smaller quantity of EDA-GQDs because of its improved emission efficiency (17.6%). The impact of diamine functionalized GQDs was comparatively less harmful with superior bioimaging execution than P-GQDs, as inferred from toxicity practicals. This concluded that organic molecule functionalization on GQDs effects little on the cell toxicity [49].

1.4 Conclusion and future prospects This chapter described physical phenomena observed in graphene and its QDs in static form. The traits of graphene quantum dots were studied comparatively with graphene and its derivatives, semiconductor (Cd X¼ S, Te) QDs, and surface-passivated GQDs. The fundamental properties are vital to understand and utilize the material extensively in every field of application. The defects originated and functional groups attached while synthesis procedure are evaluated through structural and chemical characterization tools. The mannerism in which PL is dependent on temperature, pH and passivation was thoroughly studied with experimental and theoretical aspects and profound results of SERS were obtained from uniformly arranged GQD assemblies plus AAO membrane. The remarkable qualities of robust and nontoxic GQDs as discussed in this chapter are compelling to explore this wonder nanostructure material extensively in every field. The future scope of GQDs can be foreseen in all fields of electronics, medicines, agriculture, and biological applications.

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24 Graphene Quantum Dots [22] Prasek J, Drbohlavova J, Chomoucka J, Hubalek J, Jasek O, Adam V, Kizek R. Methods for carbon nanotubes synthesis. J Mater Chem 2011;21(40):15872e84. [23] Wang L, Zhu SJ, Wang HY, Qu SN, Zhang YL, Zhang JH, Chen QD, Xu HL, Han W, Yang B, Sun HB. Common origin of green luminescence in carbon nanodots and graphene quantum dots. ACS Nano 2014;8(3):2541e7. [24] Nguyen HY, Le XH, Dao NT, Pham NT, Vu TH, Nguyen NH, Pham TN. Microwaveassisted synthesis of graphene quantum dots and nitrogen-doped graphene quantum dots: Raman characterization and their optical properties. Adv Nat Sci Nanosci Nanotechnol 2019;10(2):025005. [25] Tian R, Zhong S, Wu J, Jiang W, Shen Y, Wang T. Solvothermal method to prepare graphene quantum dots by hydrogen peroxide. Opt Mater 2016;60:204e8. [26] Ahirwar S, Mallick S, Bahadur D. Electrochemical method to prepare graphene quantum dots and graphene oxide quantum dots. ACS Omega 2017;2(11):8343e53. [27] Joshi PN, Sunil S, Sanghi SK, Sarkar D. Graphene quantum dotsdfrom emergence to nanotheranostic applications. Smart Drug Deliv Syst 2016;7:159e95. [28] Geng D, Hu Y, Li Y, Li R, Sun X. One-pot solvothermal synthesis of doped graphene with the designed nitrogen type used as a Pt support for fuel cells. Electrochem Commun 2012;22:65e8. [29] Paton KR, Varrla E, Backes C, Smith RJ, Khan U, O’Neill A, Boland C, Lotya M, Istrate OM, King P, Higgins T. Scalable production of large quantities of defect-free fewlayer graphene by shear exfoliation in liquids. Nat Mater 2014;13(6):624e30. ¨ , Gu¨ler SH, Selen V, Albayrak MG, Evin E. Production of graphene layer by liquid[30] Gu¨ler O phase exfoliation with low sonication power and sonication time from synthesized expanded graphite. Fullerenes, Nanotub Carbon Nanostruct 2016;24(2):123e7. [31] Haar S, El Gemayel M, Shin Y, Melinte G, Squillaci MA, Ersen O, Casiraghi C, Ciesielski A, Samorı` P. Enhancing the liquid-phase exfoliation of graphene in organic solvents upon addition of n-octylbenzene. Sci Rep 2015;5(1):1e9. [32] Coleman JN. Liquid exfoliation of defect-free graphene. Acc Chem Res 2013;46(1):14e22. [33] Tang XF, Yang ZG, Liang JH. Efficient strategy of chlorine-assisted liquid-phase exfoliation of graphite. J Mater Sci 2017;52(7):3786e93. [34] Backes C, Higgins TM, Kelly A, Boland C, Harvey A, Hanlon D, Coleman JN. Guidelines for exfoliation, characterization and processing of layered materials produced by liquid exfoliation. Chem Mater 2017;29(1):243e55. [35] Pavlova AS, Obraztsova EA, Belkin AV, Monat C, Rojo-Romeo P, Obraztsova ED. Liquidphase exfoliation of flaky graphite. J Nanophotonics 2016;10(1):012525. [36] Sahoo SK, Behera AK, Chandran R, Mallik A. Industrial scale synthesis of few-layer graphene nanosheets (FLGNSs): an exploration of electrochemical exfoliation approach. J Appl Electrochem 2020;50(6):673e88. [37] Wang H, Wei C, Zhu K, Zhang Y, Gong C, Guo J, Zhang J, Yu L, Zhang J. Preparation of graphene sheets by electrochemical exfoliation of graphite in confined space and their application in transparent conductive films. ACS Appl Mater Interfaces 2017;9 (39):34456e66. [38] Tripathi P, Patel CR, Dixit A, Singh AP, Kumar P, Shaz MA, Srivastava R, Gupta G, Dhawan SK, Gupta BK, Srivastava ON. High yield synthesis of electrolyte heating assisted electrochemically exfoliated graphene for electromagnetic interference shielding applications. RSC Adv 2015;5(25):19074e81.

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Chapter 2

Graphene quantum dots characterization and surface modification Muhammad Taqi-uddeen bin Safian1, Khalid Umar1, Tabassum Parveen2, Iqbal M. I. Ismail3, 4, Huda A. Qari3, 5 and Mohamad Nasir Mohamad Ibrahim1 1

School of Chemical Sciences, Universiti Sains Malaysia, Penang, Malaysia; 2Department of Botany, Aligarh Muslim University, Aligarh, Uttar Pradesh, India; 3Centre of Excellence in Environmental Studies, King Abdulaziz University, Jeddah, Saudi Arabia; 4Department of Chemistry, King Abdulaziz University, Jeddah, Saudi Arabia; 5Department of Biological Science, King Abdulaziz University, Jeddah, Saudi Arabia

2.1 Introduction Graphene quantum dots (GQDs) are the tiny version of graphene represented by zero-dimensional (0D) nanomaterial structures with its length only reaches less than 100 nm [1e3]. Given that it’s graphene structurewise, it has comparable physical and chemical properties of graphene, which include the likes of high surface area, chemically stable, and high electronic mobility [4]. The quantum size of GQDs enables quantum confinement characteristics of the Broglie wavelength nanosystem. This phenomenon can be attributed to the Bohr radius of the bulk exciton. When the size of quantum dot structures reducing its dimension, the exciton is free to move in any direction due to less restriction [5]. This leads to the unique optical and electrical properties of quantum dots structures. Depending on the size of the GQDs, it might illuminate unique photoluminescence (PL) spectra that fit that particular size [6]. In this case, it was possible to customize a GQD based on its PL spectra to get a specific size of the GQDs. Apart from the typical graphene properties, GQDs also possessed NIR light absorption, photothermal properties, fluorescence emission, and many more. Moreover, due to the tiny structure of GQDs, the surface-to-volume ratio indicates there are many sp2 networks that available which allow functional group attached from p-p stacking interactions [7]. Therefore, GQDs can be dispersed easily in water due to the functional group available around the edges. In the characterization of GQDs, a related Graphene Quantum Dots. https://doi.org/10.1016/B978-0-323-85721-5.00001-7 Copyright © 2023 Elsevier Ltd. All rights reserved.

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28 Graphene Quantum Dots

approach to graphene characterization can be used due to similarity in the structure. In addition to that, optical studies such as photoluminescence are useful to determine the particle size. Surface modification of GQDs is essential to open up the restriction in applications. Even size modification can also be classified as a surface modification. Surface modification is also known as changing the functionalization of the GDPs. These functionalized methods are done to tailor-fit GDPs to various applications. The basic properties of GDPs can be changed via doping with heteroatoms, composites with polymers or inorganic materials, and changing the shape and size of the nanomaterial structures [8]. In this chapter, we would touch upon the modification toward GQDs, which potentially address some challenges of pure GQDs limitation.

2.2 GQDs characterization The characterization of GQDs is similar to graphene. The properties of GQDs can be measured by using optical, microscopy, and surface state characterization. The optical characterizations are used to monitor the fluorescence properties and vibration patterns as provided by UV-Vis spectroscopy, Raman spectroscopy, and photoluminescence. The microscopy characterizations are used to study its surface morphology can be provided by transmission electron microscopy (TEM) and atomic force microscopy (AFM). While the surface state characterizations to study its functional group composition are provided by Fourier transform infrared spectrometer (FTIR) and X-ray photoelectron spectroscopy (XPS).

2.2.1 Optical characterizations Optical characterization techniques investigate the change in the intensity, polarization, energy, or phase of the light wave when interacting with the studied sample. The advantages of this characterization are that it required little sample preparation while being nondestructive, fast, and can be performed at room temperature. Optical absorption of graphene has been used to monitor the feedback of the graphene extraction since the size, defects, and concentration plays an essential role in defining the optical properties [9]. In addition to that, fluorescence properties added to the quantum structure of GQDs can be studied by using the PL spectrum. Combining all the optical characterizations data, we can get a meaningful analysis to paint a picture of GQDs from such a brief and short analysis.

2.2.1.1 UV-Vis spectroscopy The study of UV spectra is based on the BeereLambert law, which obeys a linear relationship between the absorbance and the concentration of one solution [10]. This means that the intensity of the beam that passed through a

Graphene quantum dots characterization Chapter | 2

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solution is influenced by the concentration of the solution. The molecule inside that solution absorbing certain wavelength energy from the beam, which transitions electrons into higher energy orbital. Thus, by observing a sample at a different wavelength, one can get the absorbance of the sample. UV-Vis comprises the spectrum of the ultraviolet, visible, and IR regions. For graphene, it produces multiple responses depending on the energy. At lower energies, classical electrodynamics responses from the electric field interaction of graphene structure responsible for the absorption. While at higher energies, the electronic excitation due to photon absorption resulting in interference [11]. Graphene reflects very little given its constant absorbance value of 2.3%. The difference in incident light reflection between a monolayer and multilayer graphene is 0.1%e2% for 10 layers [12]. Hence, the absorption data can be used as a good measurement to determine the number of layers for graphene derivatives. Graphene feature a prominent peak in the range of 250e300 nm due to the excitation of the p-electrons [13]. The same peak can be observed for GQDs as the pep* transition of graphitic sp2 is the domain structure for graphene relative [14]. Another peak that can be observed mainly for GQDs is at around 300e390 nm, attributed to the oxygenated functional group present on the GQDs [15].

2.2.1.2 Raman spectroscopy Raman spectroscopy is based on the interaction of the visible light with the chemical bonds of the sample. It is a proven tool to provide details such as chemical structure, crystallinity, and molecular interaction based on the band structure. Raman has been widely used for graphene characterization as its responsiveness toward carbon-based materials [16]. The mechanic of Raman lies in the resonance effect that increases when the laser source excitation equal to the electronic band structure translation to the Raman intensity [17]. The Raman intensity reveals the functional groups attached, modification of the matrix, the edge types, presence and type of defects, and the number of layers. The signals formulated for the Raman intensity came from the vibration of the atoms in the structure during the scattering of two phonon modes. For any graphitic materials, the specific position and intensity of two bands known as D band and G band are used to determine the graphitic structure. The same applies to GQDs. From Fig. 2.1, the G band located around 1581 cm
1 is generated from the vibration of the carbon atoms among each other in the lattice plane [18]. It also represents an aromatic structure in a 2D plane. At the same time, the D band located around 1350 cm
1 represents the disorder of graphene matrix or defects [18]. Another important frequency often used in graphitic materials is D0 band around 1620 cm
1 is a weaker frequency which generates from the carbon on the same plane as the G band originated from, but relative toward the longitude of the neighbor carbon atoms. The other band that predominant in graphene is called 2D band which does not represent

30 Graphene Quantum Dots FIGURE 2.1 Raman spectroscopy of GQDs. Reproduced with permission from Liu D, Chen X, Hu Y, Sun T, Song Z, Zheng Y, et al. Raman enhancement on ultra-clean graphene quantum dots produced by quasi-equilibrium plasmaenhanced chemical vapor deposition. Nat Commun 2018;9:1e10. https://doi. org/10.1038/s41467-017-02627-5.

defects in the graphene lattice but due to the wavelength excitation dispersion located at 2700 cm
1. For GQDs, most functional groups attached are tallied in the D band intensity [19]. Hence, the ID/IG ratios can be used to determine the oxidation level proportion to the carbon atoms. The 2D band also signifies the number of layers in the graphene. Increasing layers will shift the 2D band while it also becoming broader and shorter. Fig. 2.2 showed the change in the 2D band with the increasing of layers up to graphite. At the same time, noticeable G band intensity was also observed due to the interaction between carbon atoms to its neighboring carbon atoms perpendicular to the plane. The horizontal displacement produces phonons with an elevated energy level resulting in the intensity of the G band [18]. In the case of doping, the assimilation of heteroatoms into the GQDs lattice will prompt more defective sites. This can be monitor through the ID/IG ratio, where more than one ratio equal to highly functional GQDs. Raman spectroscopy is an essential tool in graphene derivative characterization, including GQDs. Furthermore, a deep understanding of the Raman spectra can generate lots of information regarding the GQDs characteristics, including its properties.

2.2.1.3 Photoluminescence Photoluminescence (PL) relies on the unyielding scattering of the lightning source. The process of the light source illuminating the sample and reemission causes a delay which may vary depending on the structure of the sample. The mechanism is quite familiar to that of Raman spectroscopy. However, the distinctive difference is that while Raman causes excitation to the molecules, PL is determined by the emission energy by the electronic structure of the sample [22]. GQDs accommodate quantum confinement effect due to link up p-domains, especially around the edge of the lattice [23]. This behavior is

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31

FIGURE 2.2 2D bands intensity in response to the number of layers. Reproduced with permission Wu JB., Lin ML, Cong X, Liu HN, Tan PH. Raman spectroscopy of graphene-based materials and its applications in related devices. Chem Soc Rev 2018;47:1822e1873. https:// doi.org/10.1039/c6cs00915h. Published by The Royal Society of Chemistry.

quite the opposite of the graphene due to its naturally high electron mobility with zero band-gap. The photoexcited carriers entering the graphene structure would typically slow down and recombined, which reduced the emission process [24,25]. However, this scenario can be overcome by introducing bandgap toward graphene structure with defects. Defects do play an essential role in affecting the electronic transitions and optical properties of the graphene structure. GQDs are considered defected graphene derivatives as it cuts graphene sheets into smaller pieces. Any domains other than the perfect sp2 networks are regarded as defects in graphene and its derivative structures [26]. Therefore, PL can be confirmation tools for GQDs or any other functionalized graphene traceable by the PL measurement. It should be noted that GQDs are characterized differently than other functionalized graphene due to their dissociation with isolated sp2 islands [26]. In many reports, color variations were used to determine the variance of GQDs [1,6,27]. The GQDs are built with light atoms resulting in great carrier-carrier interactions and a defining spin multiplicity due to its weak spin-orbit coupling [23]. Therefore, it possessed a much larger energy band than other compounds with a comparable build. The range of spectrum it possessed is ranging from blue to green. In addition to that, the PL is also strongly affected by pH. In acidic conditions, the PL intensity was found to diminished, which probably due to the

32 Graphene Quantum Dots

protonation of Hþ ions at the edge of the GQDs. Vice versa in the alkaline condition as the edge sites remain in this condition [28]. The size of the flakes also is a big part of the energy gap. As such, PL can be used to determine the size of GQDs fragments. It was found that the energy gap of GQDs is disproportion to the diameter of the GQDs, as shown in Fig. 2.3 [29].

FIGURE 2.3 (a) TEM images of the GQDs and their shape and size. (b) PL intensity-dependent to the size of the GQDs. Reprinted (adapted) with permission from Kim S, Hwang SW, Kim MK, Shin DY, Shin DH, Kim CO, et al. Anomalous behaviors of visible luminescence from graphene quantum dots: Interplay between size and shape. ACS Nano 2012;6:8203e8208. https://doi.org/ 10.1021/nn302878r. Copyright (2012) American Chemical Society.

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2.2.2 Microscopy characterization Microscopy characterization analyzes and draws the surface of the sample using photons, electrons, ions, or physical probes. In graphene characterization, TEM and AFM are often used as the tools to study the surface, which also can be used to study any graphene derivatives such as GQDs.

2.2.2.1 Transmission electron microscopy (TEM) TEM has been used to study the surface level of a sample down to the atomic scale by using imaging techniques. In short, the electron-optical beam is focused onto the sample attaining a magnified picture projected to the screen or film. The picture is captured by converting the electron’s intensity on the detector. The high-resolution picture taken can be used to identify ultrastructure as minuscule as 0.1e0.2 nm. TEM has been used in graphene and its derivative to study the structure size and morphology [3,30e32]. A thin and light structure such as graphene or its derivatives would require a free-standing and stronger contrast for a better image. Layers can be analyzed by firing the primary electron beam toward the graphene structure a bit tilted; just exterior to the intention aperture region causing a small angle of electron scattered highlighting the thickness of graphene film with dark field images. A better sample preparation that includes transferring the graphene onto the TEM grids needed to be addressed by either using a simple mechanical transfer of direct synthesis transfer via chemical vapor deposition (CVD) [33,34]. For GQDs, the lattice spacing around 0.24 nm can be used as a successful confirmation of synthesis depending on the layers. The lattice spacing of GQDs should be the same as the graphene it’s derived from [35]. Characterization of GQDs should be straightforward as graphene with some addition of high degree in crystallinity. 2.2.2.2 Atomic force microscopy (AFM) AFM utilized a sharp tip of a cantilever to analyze the surface of a sample. The sensitivity of the cantilever will react toward any surface-level changes causing a better view of the nanoscale topography of a solid surface. A rise in a surface will bend the cantilever, which in turn changing the direction of the reflection beam tracking the cantilever movement. At the same time, the cantilever is pulled down when there is a decline in level by attractive forces by the surface [36]. The lever moves from point A to B, and the back and forth movement of the cantilever was recorded describing the morphology of the solid surface. In graphene characterization, the sensitivity of the cantilever can detect the differences in height between functionalized-riddled graphene oxide and graphene [37]. This is based on the principle that functionalized graphene is slightly thicker than flat pure graphene due to the oxygenated functional groups present across the surface. Other than that, AFM can be used as

34 Graphene Quantum Dots

indentation by microbending to test the mechanical properties of a solid structure [38]. By using an indenter tip, force is introduced by pushing hard the tip onto the surface. It is the only way to measure the physical properties of a nanoscale structure for graphene. Graphene also responds to electrical interaction on its surface. Applying various voltages onto the AFM allows the study of the potential barrier and other electrical properties of graphene [39]. For GQDs, AFM can be used to determine the average diameter of the GQDs presented between point A to B.

2.2.3 Surface state characterization Surface state characterization study the electronic states or elements decorated the surface of graphene and its derivatives. Two surface state characterization normally used are FTIR and XPS. Both utilized the changing of the electronic band structure on the surface due to the mass material forming an electronic state called surface state.

2.2.3.1 Fourier transform infrared spectrometer (FTIR) The mechanism of FTIR lies in the ability of molecules in absorbing certain infrared light regions due to the bonds present in the molecule. Therefore, functional groups attached to the surface of graphene sheets vibrate at a different wavelength. The FTIR is normally used as a confirmation for GO and RGO synthesis by monitoring the functional groups present in each structure. In Fig. 2.4, the typical oxygenated functional groups discovered on GO are the same can be found on GQDs; such as OeH, eCeO, C]O, C]OH, etc. [1,40]. However, the variety of GQDs sizes may result in a variant of oxidation exposure [41]. Table 2.1 showed the functional groups and their representative absorption frequency. The hydroxyl group from a phenolic OH or OH from a carboxylic group presents an intense peak around 3700e3000 cm
1. There is also a tendency for vicinity hydrogen being bonded together with other OH groups, which creates the intensity [42]. Another peak around 1300 to 1000 cm
1 is responsible for the existence of CeO with toward 1300 cm
1 attributed by the bending of CeOeC [43]. A multilayer GQDs will also see the existence of eCH2 and eCH peaks around 2900 to 2800 cm
1. 2.2.3.2 X-ray photoelectron spectroscopy (XPS) XPS is a surface quantitative spectroscopic that can identify the elements within the material surface based on the photoelectric effect. The average depth of XPS analysis is approximately 5 nm. The instrument measured spatial distribution information within a resolution of w7 mm by scanning the microfocused X-ray beam across the sample surface. The X-rays excite the sample surface causing photoelectrons to be emitted that will be analyzed by the binding energy and the intensity of a photoelectron peak. The

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FIGURE 2.4 FTIR spectra of various carboxyl functionalized GQDs and GO [41]. Published by The Royal Society of Chemistry.

TABLE 2.1 Major functional groups in GQDs and their absorption frequency. Functional group

Absorption frequency

OeH stretching

3400 cm
1

CeH stretching

2910 cm
1

CeH system stretching

2875 cm
1

C¼O stretching

1687, 1710 cm
1

C¼C stretching

1542, 1568 cm
1

CeOeH bending

1409 cm
1

CeO stretching

1208 cm
1

CeOH stretching

1113 cm
1

functionalized graphene across its surface can be studied using this method by measuring its elemental composition, empirical formula, and electronic state of the elements present. In most cases, XPS was used to study the morphology of the graphene composite, especially with metal [44]. GQDs posed a typical carbon structure signal such as CeC, C]C, and CeO. Besides the oxygenated functionalized group provide the intensity of the peaks. Hence, two

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intense peaks of C1s and O1s were typically used to characterized GQDs [45]. The measurement of C1s usually are build-up from corresponding to sp2 and sp3 C of C]C, CeC, CeOH, CeOeC, etc., as shown in Fig. 2.5. Moreover, the presence of sp3 suggested that GQDs are filled with defects. The C/O ratio can be calculated from this result to analyze the richness of oxygen in GQDs.

2.3 Surface modifications Pure GQDs have many limitations that constrain their ability to be used in many applications. Surface modification has been used to alter the GQDs morphology in a way that enhanced its chemical, electronic, and optical properties. This can be done by either controlling the size and shape of the GQDs or merging GQDs with other substances such as heteroatoms, or polymers. In other terms, changing the functionalization of GQDs will improve their properties.

2.3.1 Tunable through size Previously, it was mentioned that size played an essential role in synthesizing GQDs. Some applications can only use a specific size of the material. For example, a larger size GQDs can be used to reduce immune-mediated liver damage which comparatively smaller sized GQDs would not work [47]. This is due to large nanoparticles accumulated more in the liver after intravenous injection for the intended procedure to work. In the other hand, smaller size GQDs promote more edge states that can serve as a position to store charge carriers suitable for energy storage applications [7]. Moreover, the color changes can be observed depending on the structure size of GQDs due to functional groups surrounding the GQDs [6]. More functional groups such as OH, C]O, and COOH attaching themselves at the edge of the GQDs can shift the tunes emissions from green to red. This also affects their chemical moieties which will add to the potential in various applications. The size difference is also emitting light differently due to the band-gap reduction, as shown in

FIGURE 2.5 The XPS spectra of C 1s for GQDs [46].

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Fig. 2.6. GQDs are capable of emitting light ranging from blue-green (2.9 eV) to orange-red (2.05 eV) [48]. The band-gap differences by the size and morphology of GQDs have been studied intensively using density-functional theory (DFT). This characteristic is attributed to the band-gap difference of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). A large GQD mainly consists of more functional groups, defects, and zig-zag sites that generate a different energy level than smaller GQDs. A smaller GQD will possess a large pep* energy gap, thus emitted higher energy blueish light [49]. The HOMO-LUMO gaps are also being affected by the presence of functional groups, typically on the surface of the GQDs. Edge-functionalization tends to boost only a little on the optical and electronic properties, whereas a much more massive boost can be examined in surface-functionalized GQDs [50]. This is due to the great shift in the electron density distribution for surface-functionalized GQDs compared to pure GQDs and edge-functionalized GQDs. Bigger GQDs also can host more functional groups on its surface, hence more favorable for more significant electron density.

2.3.2 Doping of GQDs with heteroatoms Doping is an essential process in the graphene family, especially in the semiconductor industry. Modifying the structure with heteroatoms means to enhance the chemical, electronic, and optical properties. The insertion of

FIGURE 2.6 Illustration of the reduction in band-gap with increasing size of GQDs.

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heteroatoms such as nitrogen, phosphorus, oxygen, sulfur, etc. into the carbon network structure can shift the electron balance and the Fermi level [51]. This typically will improve the material toward a specific reaction which can be utilized as a catalyst [52]. Similarly, the assimilation of heteroatoms onto a GQD surface will modify its properties. This doping can be categorized into three groups based on the number of type heteroatoms in GQDs lattice, single heteroatom, and double heteroatoms.

2.3.2.1 Single heteroatom Single heteroatom doping has been used to produce materials in energy-related applications. The doping is done by following either substitution of the C atoms from the lattice with the heteroatoms or attachment on the surface through adsorption. The substitution method is more stable due to the stronger covalent bonds created compare to the attachment on the lattice. It should also be noted that the substitution also includes filing the empty carbon holes of the defective sites in the GQDs lattice. Depending on the atom size and concentration, it might damage the honeycomb structure. This concern can be address by properly monitor the balance of the doping concentration. Nitrogen in GQDs is quite popular and widely used due to its reliable performance in many fields. N has high electronegativity and will create polarization in the GQDs structure [53]. Hence, a high concentration of N will heavily influence the band-gap. However, in the acidic condition, the band-gap of N-GQDs will reduce due to the N protonate. This phenomenon created a gap between the protonated N atoms and the unprotonated one, which will result in a high p
electron density, thus reducing the band-gap [54]. This means that the bandgap of N-GQDs can be fine-tuned through the pH level. PL is an essential characteristic possessed by GQDs. This property enables GQDs to be utilized as photoelectrochemical (PEC) cells materials. Although the performance for GQDs in producing hydrogen and oxygen via solar energy is less efficient from the practical operation, N-GQDs proved to be effective due to the N dopants acting as active sites to promote kinetics of PEC mechanics [55]. Further improvement of longer carrier life-time was also noticeable with the increasing of N heteroatom concentration. This was primarily due to the intrinsic sp2-bonded carbons was further strengthened by the existence of quaternary N in the network. Fig. 2.7 showed the PL spectra of the pure GQDs and various concentrations of N-GQDs. A longer life-time can be observed as the N dopant significantly participate in redox reactions, therefore extended the carrier relaxation of the PEC cells. A boron atom can be easily introduced into the GQDs lattice as the size is almost similar to carbon. Furthermore, B is situated near carbon in the periodic table while having fewer valence electrons. This means that carbon is more electronegative than the B atom. As such, a B-doped GQDs will result in a ptype extrinsic semiconductor as B atoms are more likely to become an electron

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FIGURE 2.7 PL spectra of pristine GQDs and various concentrations of N-GQDs at (a) 430 nm and (b) 525 nm. Reprinted with permission from Tsai KA, Hsieh PY, Lai TH, Tsao CW, Pan H, Lin YG, et al. Nitrogen-doped graphene quantum dots for remarkable solar hydrogen production. ACS Appl Energy Mater 2020;3:5322e5332. https://doi.org/10.1021/acsaem.0c00335. Copyright 2020 American Chemical Society.

acceptor [51]. These electrons shift will create voids disrupting other selectrons which will further create more electron holes. More holes equal to more shortage of electrons which in turn leads to better electrocatalytic performance [56]. However, there is a certain amount of B concentration allowed before oversaturation will break the honeycomb structure. Optoelectronic is one of the applications suitable for GQDs due to its PL properties. Doping with heteroatoms enhanced said properties by the strong electron-withdrawing effect. This phenomenon different from B heteroatoms as B-doped GQDs would offer more active sites. As such, B-GQDs can be used as sensor providing its intramolecular chain structure. This has been supported by a study that showed the PL observed increased upon the interaction of glucose with B-GQDs [57]. The boronic acid on the B-GQDs surface acted as a glucose sensor where it aggregates with glucose interaction, thus increasing the PL intensity. Apart from that, sulfur also has been demonstrated to be useful once doped into GQDs lattice [58,59]. Although, the size of the S atom is bigger compare to the C atom makes it difficult for the assimilation process. Furthermore, the electronegativity of the S atom is almost identical to that of the C atom; 258 and 2.55 respectively, which offer less significant electron transfer compare to the other heteroatoms [59]. However, it was reported that a significant fluorescent characteristic was observed from S-GQDs [58]. The SGQDs solution is on the softer side of yellow color but shown strong blue fluorescence around 365 nm UV. S-GQDs was shown to favor selectivity toward Ag ions owing to the synergistic effect of the S and O functional groups on S-GQDs [59].

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2.3.2.2 Double heteroatoms Double heteroatoms are also known as codoped, where two heteroatoms were introduced into the GQDs network structure. The ability to utilized multiple enhance abilities provided by two different heteroatoms has been the staple in graphene doping scenes. Similar to the single-doped GQDs, codoped affected the UV-VIS spectra of the structure. For example, a codoped N, P-GQDs efficiently absorb shorter wavelengths due to the pep* shifts of the sp2 networks in the structures, which appear as brown under visible light. At the same time, the PL spectrum exhibited a noticeable peak and a shoulder peak around 460 and 360 nm, respectively [60]. The same scenario occurred with S, N-GQDs UV-VIS spectra, where absorption peak due to the pep* transition was observed at 237 and 369 nm [40]. In both cases, the codoped GQDs showed an excellent ability as sensor materials. This enhancement has to do with the PL quenching effect, which can increase efficiency in detection and selectivity; both are good attributes for a sensor. The PL quenching effect came from the transition of the localized bonds due to the dopants insertion. Hence, codoped has the ability to increase the PL quenching factors by creating more localized energy levels [61]. This phenomenon has been proven based on the selectivity of Hg2þ ions sensing. The experiment showed that the sensitivity of codoped S, N-GQDs is four times higher than only one heteroatom-doped of N-GQDs [62]. The S, N-GQDs and N-GQDs exhibit blue and bright green color under 360 nm UV irradiation before it became darker after exposure with increasing of Hg2þ ions due to the PL quenching. However, the S, N-GQDs showed better sensitivity as it took only 30 ppm of the Hg2þ ions for the quenching process to start while 75 ppm for N-GQDs, as shown in Fig. 2.8. The different efficiency attributed to the S, N-GQDs are having S and N atom more electron-rich than the C network. The doping process increased the electron density of the structure resulting in strong PL quenching with additional levels possibly were generated during the transition phase [61]. Meanwhile, Yang et al. [63] utilized B, N-GQDs as a sensor material for Hg2þ detection. The multi heteroatoms doping factor improves the detection limit for Hg2þ ions as low as 0.16 mM. Furthermore, it was found that the selectivity of the sensor also improves as the PL quenching efficiency does not react toward other metal ions. This indicates that the material suitable in most environments due to its selectivity but also highlighted the one-dimensional (1D) of its usage. Apart from its fluorescence capability, GQDs also has been developed heavily in the oxygen reduction reaction (ORR). This is due to its surface structure, enabling quantum confinement and edge effects, resulting in better electrocatalyst reactions [7]. However, it was found that is electrocatalytic activity is limited due to the nature of GQDs, which is too small as the base material in the electrode as it posed significant percolation threshold values with low electrical conductivity [64]. Fan et al. [65] compared codoped

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FIGURE 2.8 The quenching effect shown by (a) N-GQDs and (b) S, N-GQDs with the increasing of Hg2þ ions. Reprinted with permission from Gu S, Hsieh C T., Tsai YY, Ashraf Gandomi Y, Yeom S, Kihm KD, et al. Sulfur and nitrogen co-doped graphene quantum dots as a fluorescent quenching probe for highly sensitive detection toward mercury ions. ACS Appl Nano Mater 2019;2:790e798. https://doi.org/10.1021/acsanm.8b02010. Copyright 2020 American Chemical Society.

GQDs against multiple single-doped GQDs and commercially used ORR material. The electrocatalytic performance of N, S-GQDs worked significantly better than N-GQDs, S-GQDs and undoped GQDs with 3.82 compared to 3.19, 3.41 and 3.19, respectively. It was equally performed Pt/C, which is the commercial ORR electrode. Codoping creates more active sites available for catalytic reaction than a single-doped material forcing more synergistic coupling effects between heteroatoms. Moreover, a few recent articles also demonstrate the application of graphene derivative for various applications toward sustainability [66e71].

2.4 Conclusions GQDs are still in the early stages of their development. However, GQDs possess unique abilities due to their optical, physical, and chemical properties. Given the development of this material, it is essential to know how to characterize and analyze its structure and morphology. Raman spectroscopy is the best way to know the layers and defect of GQDs. At the same time, TEM and AFM reveal the surface morphology, which is essential in GQDs utilization. The many functional groups attached on the GQDs can be identified by using FTIR and together with XPS, giving a better understanding of the oxygenated functional groups. The PL phenomena possessed by GQDs is one of the main attributes that open up this material toward all types of applications. The PL spectra also help in interpreting the size of the GQDs sheets as it is one of the

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important features of the material. No materials are perfect. However, GQDs have the ability to be manipulated in such a way to fit certain applications. Surface manipulation can be implemented to alter GQDs properties. The ability to be tuned by controlling the size of GQDs showed how flexible GQDs are as prospect materials. Even then, there still some concerns that needed to be answered in GQDs’ development. Understanding the limit and finding a way to overcome it is a positive step in moving forward.

Acknowledgments This article was financially supported by Universiti Sains Malaysia, (Malaysia) under short term grant; 304/PKIMIA/6315580.

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ions. Anal Methods 2019;11:1879e83. https://doi.org/ 10.1039/c9ay00249a. [64] Fei H, Ye R, Ye G, Gong Y, Peng Z, Xiujun F, et al. Boron- and nitrogen-doped graphene quantum dots/graphene hybrid nanoplatelets as efficient electrocatalysts for oxygen reduction. ACS Nano 2014;8:10837e43. [65] Fan T, Zhang G, Jian L, Murtaza I, Meng H, Liu Y, et al. Facile synthesis of defect-rich nitrogen and sulfur Co-doped graphene quantum dots as metal-free electrocatalyst for the oxygen reduction reaction. J Alloys Compd 2019;792:844e50. https://doi.org/10.1016/ j.jallcom.2019.04.097. [66] Asim AY, Albert S, Showkat AB, Mohamad MNI, et al. Utilizing biomass-based graphene oxideepolyanilineeAg electrodes in microbial fuel cells to boost energy generation and heavy metal removal. Polymers 2022:845. [67] Safian MT, Umar K, Ibrahim MNM. Synthesis and scalability of graphene and its derivatives: a journey towards sustainable and commercial material. J Clean Prod 2021;318:128603. [68] Yaqoob AA, Ibrahim MNM, Yaakop AS, Umar K, Ahmad A. Modified graphene oxide anode: a bioinspired waste material for bioremediation of Pb2þ with energy generation through microbial fuel cells. Chem Eng J 2021;417:128052. [69] Yaqoob AA, Umar K, Adnan R, Ibrahim MNM, Rashid M. Graphene oxideeZnO nanocomposite: an efficient visible light photocatalyst for degradation of rhodamine B. Appl Nanosci 2021;11(4):1291e302. [70] Yaqoob AA, Ibrahim MNM, Umar K, et al. Cellulose derived graphene/polyaniline nanocomposite anode for energy generation and bioremediation of toxic metals via benthic microbial fuel cells. Polymers 2021;13(1):135. [71] AlShammari A, Halim MM, Yam FK, Al-Hardan NHM, et al. The effect of substrate temperatures on the structural and conversion of thin films of reduced graphene oxide. Phys B Condens Matter 2019;572:296e301.

Chapter 3

Graphene quantum dots application in bacterial and viral pathogen disinfection Shuchita Tomar3, Mohammad Shahadat1, Rohana Adnan1, Syed Wazed Ali3, Shaikh Ziauddin Ahammad2 and Mangala Joshi3 1

School of Chemical Sciences, Universiti Sains Malaysia, Penang, Malaysia; Department of Biochemical Engineering and Biotechnology, Indian Institute of Technology IIT Delhi, New Delhi, India; 3Department of Textile and Fibre Engineering, Indian Institute of Technology Delhi, New Delhi, India 2

3.1 Introduction Water is one of the most vital substances for all living beings on earth and the utmost valuable resource for human civilization. With the rapid increase in global population and industrialization, people are experiencing various climate change and environment-related issues [1]. Among these environmental issues, water pollution or contaminated water has become the most serious issue all over the world. Several anthropogenic activities like mining, pharmaceuticals, textile industries, chemical industries, etc., are indirectly producing wastewater in the environment [2]. Such wastewater includes various types of organic and inorganic compounds, and microbial species. Contaminated water generally contains different types of pathogens such as viruses, microalgae, helminths, bacteria, protozoa and fungi that may lead to severe waterborne diseases like diarrhea, typhoid, cholera, etc. According to a recent survey of the World Health Organization (WHO), around 2 billion people (less than 25% of the worldwide population) are taking polluted water [3,4]. Approximately, 155 million people are still dependent on unprocessed surface water. However, the water supply at worldwide level, particularly in developing countries is rigorously stressed due to bad weather conditions and contaminated water sources [5,6]. Also, the United Nations World Water Development Report (2020), reveals that the water consumption demand is getting increased by 1.0% every year [7]. The water consumption demand in industrial field as well as in domestic field are rising more quickly than the agricultural field (UN-Water, 2020) and this will undoubtedly continue for the Graphene Quantum Dots. https://doi.org/10.1016/B978-0-323-85721-5.00009-1 Copyright © 2023 Elsevier Ltd. All rights reserved.

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48 Graphene Quantum Dots

next two to 3 decades [8]. Therefore, the requirement for clean and reliable treated drinking water increases constantly over the years. Unfortunately, those people who are living in rustic and remote areas generally have less incomes and are not enough capable to access better-quality drinking water sources, thus, higher chances of getting infections are there through pathogenic microorganisms. Hence, there is a persuasive requirement to produce an effective and low-priced technique for bacterial control and water decontamination [4]. Conventional techniques used for water treatment mostly involves adsorption (ion-exchange), microbial fuel cell (MFC), photocatalytic degradation, chemical oxidation (for example, chloramines, chlorine/dioxide, and ozone), that can efficiently control the infection of waterborne microorganisms [2,9e13]. Moreover, some of the corrosive substances raise a concern during operational challenges in the generation, storage and transport processes. Many consuming water efficacies are now progressing toward the use of UV disinfection to overwhelm the limitations. Though, uses of UV radiation for disinfection purpose can be more expensive and energy-intensive rather than oxidation (chemical), and also this method could be less effective to UVresistant pathogens. In addition to this, no remaining disinfectant could be maintained afterward UV disinfection [14]. Thus, the world needs a green, superficial, steady, and sustainable water disinfection technique having less cost and high efficacy (chi zang et al., 2018). This chapter mainly focuses on graphene quantum dots (GQDs) and their applications. GQDs carbon-based materials recently grabbed more attention of researchers because of having special advantages for example robust chemical inertness, low noxiousness, high fluorescent activity, and excellent photostability.

3.2 What are quantum dots? The quantum dot (QD) is one of the beneficial discoveries by researchers in nanotechnology, which has emerged as a semiconductor inorganic crystal that contain several numbers of electrons well-defined and discrete quantum state. QDs are basically the arrangement of atoms as in the bulk materials, but more of the atoms are present on their surfaces due to the 3-dimensional (3D) truncation. Moreover, QDs are small-sized particles that offer a wide range of variable element ratios, which may result in fluorescent properties. QDs are generally semiconductor nanoparticles that contain various unique properties such as size-dependent emission wavelength, broad excitation range, and also capable to produce glowing light when they are stimulated by UV light to create interesting phenomena. Moreover, the structure of QDs can be controlled simply while obeying the principle of quantum confinement. The emission and absorption spectra conforming to the energy bandgap of the QDs are basically controlled by quantum confinement principles, which is the energy essential to excite the electrons from the electronic band to higher energy levels. The excited electrons instinctively form an electron-hole pair in

GQD application in bacterial and viral pathogen disinfection Chapter | 3

49

which this excitation can emit energy in the form of a fluorescent photon. QDs can also be considered as artificial atoms that are able to produce distinct energy levels, and their bandgap can be moderated exactly via changing their sizes. Bandgap can be related to the nanocrystalline size because it depends on the number of atoms that make up the band [15]. Thus, QDs show optical properties which are dependent on size, where smaller nanocrystals can have larger band gaps. Generally, the energy bandgap increases with a decrease in QD particle size and corresponding wavelength [16]. QDs mostly contain distinctive luminescent characteristics and electric properties for example, narrow emission, wide and continuous absorption spectra, and high light stability. Their ability to captivate white light and further reemission of particular colors in a few nanoseconds later, generally depends on band gap of the materials. Moreover, QDs are potentially used in several devices such as telecommunication lasers, light-emitting diodes, and also in biomedical applications (which can be used as tools for monitoring cancerous cells, tumor imaging, and therapy). Though, confinement of the QDs can be understood by fabricating the semiconductors in extremely small sizes (within hundreds to thousands of atoms per particle). Due to this confinement effect, QDs have the potential to show controllable discrete energy levels as well as have a tendency to emit different colors of light at various wavelengths. QDs also exhibit stable and tunable wavelengths [17]. Thus, the potential properties of QDs grab the attention of the researchers toward its applications in wastewater treatment, adsorption treatment of environmental pollutants, detection of heavy metals, etc. Here, we focus more on the application of GQD.

3.3 Graphene quantum dots (GQDs): structure, synthesis, and Characteristics With rise in nanotechnology, GQDs have become a novel associate with the nanocarbon category. Usually, GQDs could be observed as the smaller particles of graphene. Graphene is basically a, 2D single-layer nanostructure with sp2-hybridized conjugated carbons, become an exciting material to inspect because of its unusual thermal, electrical and mechanical properties. In addition to this, its surface area and chemical stability are very high/and has low fabrication cost. Also, graphene is suitable and good alternative component to silicon for nanoelectronic applications as it shows high carrier agility. However, to implement graphene sheets always challenging because of some unresolved restrictions: firstly, the surface reactivity becomes reason for easy aggregation; secondly, it is quite difficult to get them disperse in eminent solvents; thirdly, during etching method, graphene rearranges into 1D nanoribbons. Moreover, it is a zero-bandgap component but still the optical properties of graphene made it inappropriate for optoelectronics application. Though, by changing the sheet size it is possible to tune the bandgap of graphene from 0eV to that of benzene [18].

50 Graphene Quantum Dots

Recently, many researchers have focused on converting 2D to 0D GQDs and further observing the edge effects along with quantum confinement impact on the characteristic of this innovative substance. GQDs can be generally defined as 0D and sp2 bonded carbon atoms that are organized in a flat and honeycomb assembly like graphene having adjacent dimensions less than 10 nm. Similar to graphene, GQDs can also be a single or few-layered structures. Generally, GQDs shows characteristic which is consequent to both graphene and GQDs. However, sometimes there is misperception among (CDs) and GQDs, that occurs because they both belongs to carbon-based nanomaterials having lateral dimensions 400 nm) /Ag*

(3.1)

AgBr þ hv (ʎ > 400 nm) /e þ hþ

(3.2)

-

The excited e s have potential to transfer from conduction band of AgBr toward the nanoparticle of Ag owing to the high conductivity of Ag nanoparticle. Additionally, the electrons obtained in the conduction band (AgBr) also transfer to the rGO surface and facilitate the fast charge transfer along the p-p graphitic carbon network. Moreover, the e generated from the nanoparticles of Ag could also flow toward rGO [43]. Consequently, the nanoparticle of silver and rGO act as a medium to transport e-s from AgBr/RGO and/or AgBr/Ag/RGO, which reduced the recombination chance with holes. Therefore, high density (high concentration) of e-s having high reduction potential (CBAgBr1/4 1.04 V vs. Normal Hydrogen Electrode (NHE)) amass onto the surface of RGO, which can readily stimulate the generation of 1O2 with the help of energy transfer to O2, or it also reduce chemisorbed (surface) O2/H2O to produce .O2 (E0 (O2/.O-2) ¼ 0.33 V vs. NHE) [44]. Consequently, . O2 go through simplistic disproportionation reaction to generate .OH, H2O2 and 1O2 as shown in Eqs. (3.3) to (3.6) [45,46]. Furthermore, this kind of transportation of photogenerated electrons stabilize the photogenerated hþ onto the surface of AgBr and Ag nanoparticles [47]. Thus, the generation of reactive species namely e, hþ, .O-2, .OH, 1O2, and H2O2, play an important role to inactive E. coli K-12; and scavengers test exposed that the plasmoninduced H2O2 is the main reactive species in this photocatalytic inactivation process. O2 þ e_

1

O2/.O-2

(3.3)

O-2 þ H2O2 þ 2OH

(3.4)

OH þ OH

(3.5)

OH þ O2 þ OH

(3.6)

hþ, e þ O2, OH, O2, H2O2 þ E. coli Bacterial cells (Organic debris)

(3.7)

2.O2þ 2H2O H2O2 þ e

.

H2O2 þ .O-2 

1

. .

.

-

62 Graphene Quantum Dots

3.7 Conclusions Water pollution caused by the presence of heavy metal ions, microbial species, and other organic and inorganic pollutants have become a serious threat to the clean and safe drinking water supply. Nanostructures derived from GQDs appeared like one of the efficient potential solutions toward water pollution extenuation. GQDs and their nanocomposites have been magnificently developed and assessed for catalytic elimination of various organic pollutants like dyes and other evolving contaminants, pollutants adsorption, filtration, and decontamination. Fabricating GQDs within several nanocomposites lead to nanocomposite properties modification and enhanced the elimination efficacies of various contaminants. Here, we discussed different methods of synthesizing GQDs such as hydrothermal method, solvothermal method, and so forth that could be utilized for treating wastewater. The amount of the incorporation of GQDs should be optimized carefully to confirm the optimistic effect in pollutant elimination efficacies of various nanocomposites. Although there are many developments on nanostructures of GQDs, but still many works left that is to be done to confirm the pattern and use of these materials at extensive level. However, nanostructures derived from GQD has capability as pollutant remedy tool, because of its nontoxicity, decomposable, and ample functional groups. Still, it is essential to make enhanced synthesis situations having ability to produce GQDs that are even in relation to size and surface functionalities as well as to make better synthesis methods which confirm suitable spreading of GQDs in nanocomposite matrix. Further, this can come up with consistency regarding the stated performances of several nanostructures derived from GQD for water effluence reduction.

Acknowledgments The authors would like to express their appreciations to Science and Engineering Research Board (DST) fast tract young scientist scheme (SB/FT/CS-122/2014) for providing Postdoctoral Fellowship to Mohammad Shahadat.

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Chapter 4

Microbial sensing and antimicrobial properties of graphene quantum dots Mohammad Oves1, Mohammad Azam Ansari2, Mohd Ahmar Rauf3, Bahaa A. Hemdan4 and Iqbal M. I. Ismail5 1

Centre of Excellence in Environmental Studies, King Abdulaziz Univesity, Jeddah, Saudi Arabia; Department of Epidemic Disease Research, Institute for Research & Medical Consultations (IRMC), Imam Abdulrahman Bin Faisal University, Dammam, Saudi Arabia; 3Use-Inspired Biomaterials & Integrated Nano Delivery (U-Bind) Systems Laboratory, Department of Pharmaceutical Sciences, Eugene Applebaum College of Pharmacy and Health Sciences, Wayne State University, Detroit, MI, United States; 4Environmental Microbiology, Laboratory, Water Pollution Research Department, National Research Centre, Dokki, Giza, Egypt; 5Department of Chemistry, King Abdulaziz Univesity, Jeddah, Saudi Arabia 2

4.1 Introduction Most communicable diseases are caused by viruses, bacteria, fungi, or parasites, and these are the prominent leading source of mortality around the world [1]. If infections become multidrug resistant (MDR), many of these diseases become more difficult to treat, resulting in increased mortality rates hospital bills [2]. The number of Gram-negative bacteria caused MDR bacterial infections; they are progressing at an alarming rate, all available antibacterial treatments become useless due to developing resistance against antibiotics. The Centers for Disease Control and Prevention (CDC) has characterized multidrug resistance pathogen as methicillin-resistant Staphylococcus aureus (MRSA), Enterococcus faecium, or Vancomycin-resistant Enterococci (VRE), and other antibiotics-resistant of Streptococcus pneumonia [3]. Pathogen-caused MDR has been accompanied by a steady drop in discovering and developing novel medicines, providing significant worldwide difficulties. According to some estimates, antibiotic resistance might be responsible for 300 million deaths and an additional cost of trillion 100 dollars by 2050, necessitating immediate and comprehensive action to solve the MDR problem [4]. The skin is the biggest organ in the human body and defends and touches harmful bacteria. Bacteria can enter a wound through a cut or abrasion, Graphene Quantum Dots. https://doi.org/10.1016/B978-0-323-85721-5.00003-0 Copyright © 2023 Elsevier Ltd. All rights reserved.

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producing local infection and even systemic sepsis. Antibiotics can treat superficial wound infections on a local level [5]. Because of abuse and quick activation of resistance genes, toxic bacteria have developed resistance to conventional antibiotics. A novel drug class with broad antibacterial action and adequate biocompatibility is required. In clinics and other healthcare institutions, various antiseptics and disinfectants are widely employed to inactivate harmful bacteria and prevent infections. On the other hand, most disinfectants are poisonous and irritating, causing health concerns and dermatitis [6]. Some are losing effectiveness as germs evolve and develop resistance to them. As a result, there is a sense of urgency. There is a growing demand for alternate antimicrobial methods with improved characteristics and reduced harmfulness for infection treatment [7]. The antimicrobial photodynamic inactivation (PDI) has revealed considerable promise in the decontamination of several microbe types, with essential compensations such as little intrusiveness, low side effect incidence, and adaptability for quick and recurrent presentation [8]. Because it caused nonspecific oxidative damage of proteins, lipids, enzymes, and nucleic acids within the cells and in the cellular membrane, the PDI therapy is less likely to elicit resistance by the targeted microorganisms [9]. When photosensitizers are stimulated by mild light of the proper wavelength, then reactive oxygen species (ROS) are formed in PDI [10]. The ROS can comprise free radical ions of ,O2 (superoxide), ,OH (hydroxyl radical), resultant lipid ions, and singlet oxygen (1O2), and the generation is related with type I and/or type II photodynamic special effects [11]. The dye moleculesdporphyrins, phthalocyanines, bacteriochlorins, phenothiazines, and derivativesdhave all been used as photosensitizers [12]. The alternative, emerging, and effective antimicrobial medicine are developed from nanoscale materials because it is a carrier for selective transport and diffuse photosensitizers in targeted cells [13]. While semiconductors and metal nanoparticles have been extensively researched for the conveyance of medicine at a particular site, carbon nanomaterials have lately received much attention in PDI-related applications due to their wide-ranging optical spectrum exposure and other favorable aspects material features [14]. Beyond the well-known fullerenes, nanotubes, and graphenes of carbon allotropes, the current identification of C-nanoparticles as a distinct zero-dimensional Callotrope, as opposed to fullerenes as “zero-dimensional C molecules.” The well-defined chemical structures and stoichiometry have opened new avenues for contesting antimicrobial agents [15]. CDs are tiny carbon nanoforms with varying surface passivation. They have emerged as a potential raised area for natural light-activated antimicrobial drugs by utilizing and increasing their inherent optical features and photoinduced redox characteristics [16e18]. In this sense, GQDs are exceptionally visible photosensitizers for operative PDI, with extra benefits owing to nontoxicity, photostability, plasticity in surface functioning for preferred contagious linkage and interactions, and so on [19].

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4.2 GQDs for bacterial sensing GQDs are versatile revolutionary bionanomaterials with many functional groups and the ability to generate multidonor ligands with macromolecules, which brings a lot of opportunities for biomedical purposes [20]. Another fascinating and prospective implementation of GQDs mentioned here seems to be photothermal/photodynamic therapy (PTT/PDT). PDT highly depends on molecular oxygen, which can kill malignant cells immediately. Ge et al. described a GQD-based PDT molecule exhibiting widespread uptake, significant emission of deep-red, and a quantum value of almost 1.3 [21]. Radiation exposure produced cell shrinkage, the development of numerous benign growths, and the mortality of HeLa cells in the experiments. With increasing GQDs dosage, cell viability diminished, although GQDs had no impact on the rate fluctuations of cells (HeLa) in the dark. In addition, in vivo experiments on cytotoxicity, photothermal activity, imaging, and PDT have been undertaken. In vivo and in vitro studies disclosed that simultaneously, GQDs might be deployed as a PDT and image analysis agent. Moreover, heteroatom doped GQDs have a greater chance of succeeding PDT effectiveness than free GQDs. Kuo and his colleagues investigated Ndoped GQDs with a 3-min photoexcitation period [22]. The findings revealed that higher N content in N-GQDs extra effectively implemented PDT actions than lesser N content N-GQDs treated similarly. The amalgamation of chemotherapy with PTT has developed an immediate research priority to achieve a better therapeutic impact with more secondary detrimental effects. Synergistic chemophotothermal chemotherapy has been successfully performed using silica nanoparticles capped with GQDs (GQDMSNs) [23]. DOXloaded GQDMSNs (DOXGQDMSNs) had a perfect temperature and pH, sensitivity, regulated drug target release, and exceptional near IR-absorption, indicating that GQDs could be useful in cancer treatment. DOX extraction was quicker in the solution at pH 7.4 than in the media at a pH of 5.0. The DOX discharge rate was significantly larger at 50
C than at 37
C, demonstrating that DOXGQDMSNs had an excellent pH and temperature responsiveness. With near-infrared (NIR) irradiation, the surface temperature of the GQDMSNs increased and was substantially more significant than that of the suspension, showing that the GQDMSNs had a remarkable photothermal outcome. As a result of the successful chemo-photothermal synergetic therapy, DOXGQDMSNs may result in increased cancer cell death. Following the chemophotothermal treatment study, the PDT/PTT synergistic cancer treatment has become a challenging topic in cancer cure research. Cao et al. recently published a report using aptamer-conjugated GQD/porphyrin hybrid drug delivery applications [24]. These GQD-PEG-P had several unique, essential materials, including high photothermal transformation performance, high 1O2 formation capacity, decent luminescence characteristics, and a large specific surface area, all of which make them suitable for intracellular miRNA revealing and photodynamic treatment.

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4.2.1 Antimicrobial property of carbon dots Carbon dots (CDs) have emerged as a prominent, interesting revolutionary framework for microbicidal remedies triggered by visible/natural light. CDs have recently gained considerable attention for their antimicrobial properties due to their preferred visual qualities, minimal toxic effects to cell cultures, and bifunctional contact capabilities with bacteria. The formulations, architectures, and features of CDs are considered in this section, and comparative investigations on their antibacterial, antifungal, and antiviral activity and related mechanistic considerations [13].

4.2.2 Potential of CDs for combating bacteria CDs with favorable surface qualities might react with microbes with a negative surface charge, causing superficial damage, intracellular permeability, and, eventually, bacterial cell loss. To make complex þ ve charged CDs, bioactive polyamines were used as precursors or functionalization agents right away [25]. Polyamines, such as cadaverine, spermine, putrescine, and spermidine, are tiny fragments with two or more amine groups created at the most excellent density of millimoles per liter in existing cells. They can be used in nanoparticle surface functionalization because of their strong positive charge and high-grade biocompatibility [26]. When SC-dots were combined with microorganisms, internal ROS increased, contributing to their antibacterial characteristics. The CDs made with identical components, including spermine, appeared to have no antibacterial properties. Straight pyrolysis of spermidine powder followed by dry annealing produced an alternative category of supercationic CDs (CQDSpds) [27]. The non-MDR bacterium E. coli, S. aureus, Salmonella enterica, P. aeruginosa, and the multidrug-resistant MRSA exhibit outstanding antibacterial activity [28]. The inhibitory activity (MICs) of CQDSpds against these microbes is around 2e4 g mL1, which is about 2500-fold lower than free spermidine. Due to their solid biological properties in both in vitro and in vivo systems, such CDs were effectively applied for keratitis treatment, indicating a novel nanoantibiotic agent for the topical application of eye-related infectious disorders [29]. The CDs were discovered to break bacterial membranes and connect to bacteria’s genome, resulting in cell death. The CDs’ antibacterial properties were reportedly aided by their positive ions. On the other hand, spermidine was used to enhance the characteristics of CDs and give them highly positive charges [30]. The spermidine-capped CQDs have produced and exhibit remarkable antibacterial effectiveness against E. coli, B. subtilis, S. aureus, MRSA, and P. aeruginosa, with a zeta potential of þ60.6 mV [31]. Photoexcited CDs might produce ROS designed to wipe out or inhibit microbes. According to research, the underlying processes implicated in CDs’ antibacterial effects are commonly linked to ROS formation. The action and

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mechanism of CDs to the bacterial cell surface, photoexcitation of ROS, interruption, and entry of the microbial cell wall/membrane, induction of oxidative stress with DNA/RNA losses, resulting in changes or restrictions of essential transcriptional activation, and initiation of oxidative damages to intracellular biomolecules like proteins and other, as shown in Fig. 4.1 [32]. The EDA-CDs could significantly reduce E. coli cells in liquids and on the agar surface, demonstrating the CDs’ visible/natural light-activated antibacterial capabilities. When E. coli cells are exposed to light for 30 min, the number of viable cells drops by four logs, whereas dark therapy only reduces possible cell concentrations by one record [33]. In approximately the same study, treatment with EDA-CDs with photoexcitation significantly suppressed the proliferation of Escherichia coli in solution and decreased population density on agar medium. Photosensitizer reactions, analogous to those experimentally demonstrated in the destruction of tumor cancer cells by CDs in photoirradiation, are attributable to these antibiotic capabilities [34]. Subsequently, numerous analyzes of CDs using different combinations revealed results similar to those described above. Lee and colleagues used a single-step electrochemical technique. Vitamin C was used as an originator to CDs and discovered that they had broad antibacterial action against B. subtilis, S. aureus, Bacillus sp., and E. coli [35]. For example, B. subtilis and Bacillus sp. cells can be inhibited by CDs at a dose of

FIGURE 4.1 illustrates the mechanism of action for CDs with photoactivated antimicrobial properties. (a) CD adsorption to bacterial surfaces and ROS production induced by natural light, (b) ROS generated resulting in bacterial cell rupture.

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50 g mL1. The MIC rate significance in the point sample was lesser for E.coli than for S. aureus, indicating that the model had a more negligible antibacterial effect on Gram-positive than Gram-negative bacteria. In a similar hydrothermal carbonization process, penicillin was exploited as a precursor to CDs, but at a temperature well below only 120
C [33]. CDs were also synthesized using a separate nonpenicillin-containing precursor array, followed by dot surface-binding of penicillin, for comparison. The antibacterial activity of two CDs, which should include penicillin but indifferent skeletal configurations, was tested against S. aureus, MDR E. coli, and MRSA. Despite this, metronidazole CDs have not been able to prevent the development of S. aureus. The antimicrobial action of CDs is influenced by a variety of factors, including their optical properties, photosynthetic properties, and surface functions, as expected, and these requirements provide additional possibilities to deceive and enhance their bactericidal capabilities in light [36].

4.2.3 Combination with other antimicrobial reagents The antimicrobial activities of nanostructures, especially carbon hybrid dots with metal oxides in the dot formation, were characterized. Nanoscale carbon was effectively connected with a multitude of nanometer semiconductors, such ZnO, TiO2, Cu2O, Na2W4O13/WO3, and others, for hybrid CDs. Under UV/ vis photoirradiation, a few mixed dots exhibited markedly higher activity versus bacterial infections, which was ascribed to much more effective charge transference and a suppressive influence on electron-hole pair combination for enhanced ROS generation [13]. Spotless TiO2 nanoparticles, in particular, are recognized for their photovoltaic performance. They’ve been used in antimicrobial and overall disinfection applications because of their chemical inertness, sizable specific surface extent, less toxicity, and capacity to make an electric charge when exposed to UV light [37]. Because of the expansive bandgap energy and the comparatively fast recombination of electron-hole pairs, colloidal TiO2 also requires UV stimulation. The former is a significant constraint for the more ideal natural light activation application conditions, while the latter links to less operative ROS generation and antimicrobial effectiveness. Because carbon in CDs effectively produces visible photons, other carbon/TiO2 hybrid dots extend the light stimulation into the visible spectrum to initiate nanoscale C and TiO2 photocatalytic properties in the dot architecture. The nanoscale carbon and TiO2 domains have a structural configuration in the core of each carbon/TiO2 hybrid dot which played an essential part in the practical optical and photoinduced redox characteristics [38]. On the entire carbon/TiO2 hybrid dots, properties were comparable to TiO2 systems with dye-sensitized, here C-domains serving as photonharvesting by the dye function in the visible spectral region, and while TiO2 has no absorptions, and the collected photon energies sensitizing the TiO2 in the hybrid dots [38]. Yan et al. has developed CDs furnished with TiO2 via

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hydrothermal process and explained the antibacterial potentials toward S. aureus and E. coli. The pure TiO2 antimicrobial results were rationalized compared to the combination’s CeTiO2 because the mixture has high visible light uptake, enhanced dispersibility, enhanced ROS generation [39]. Moreover, Zhang et al. proved that the designed CDs with Na2W4O13/WO3 layers to be utilized as an ecofriendly photo-disinfection substance and revealed that the material might decrease E. coli cells by seven logs in 100 min under solar light illumination, connected with one log and two log reductions by WO3/ Na2W4O13 and WO3, respectively (Fig. 4.2). The photocatalyst’s developments for the creation of ROS species can be observed in reactive species scavenging studies and electron spin resonance spectroscopy, which have a significant role in the enhanced photocatalytic disinfection efficiency [40].

4.2.4 Potential of CDs for combating the virus The application of CDs to neutralize viruses and diminish contagion rates has garnered limited investigation. The interferons (IFNs) are the most distinguished antiviral cellular defense components inside the human body, exhibiting a potent antiviral response to viral illness [41]. EDA-CDs had more powerful suppressive activities on VLPs’ adsorption to HBGA and their corresponding antibodies than EPA-CDs, displaying a similar charge transfer impact. Huang et al. recently discovered CDs could be made from the monomer of benzoxazine, which may block the infectious disease caused by

FIGURE 4.2 Schematic depiction of tungsten oxide-coated CDs for photocatalytic disinfection purposes.

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flaviviruses (Dengue, Zika, and Japanese encephalitis virus) and withoutenveloped viruses (parvovirus, porcine, and adenovirus-associated virus) in vitro, most likely by straight coupling to the virion’s exterior and ultimately obstructing the virus-cell communication [42]. By changing the cell surface membrane and binding protein, invasion and viral entrance can be blocked. According to the plaque reduction assay, CDs from curcumin had numerous concentration-dependent suppressive activities on the swine endemic diarrhea virus. The curcumin-derived C-dots can inhibit infectious disease via viruses at a preliminary phase. According to the Raman spectroscopy and fluorescence investigation, the electrostatic interaction of þve charged C-dots causes viral accumulation and deactivation [43]. The Garg et al. observed suppressive strategy of coronaviruses using heterogeneous CDs in a recent publication. Using a succession of unique bioactive, the authors recommended developing triazole-based CDs to fight SARS CoV-2 illness. CDa are suitable for a wide range of biomedical applications in line with the significant number of hydrophilic functional groups on their edges. Besides, the surface functioning of these mysterious nanomaterials is effective for fine-tuning the level of virus contact [44]. Coronavirus infection can be effectively inhibited by curcumin þ ve C-dots (CCM-CDs). CCMCDs were made by hydrothermally reacting citric acid and curcumin in a Teflon-coated autoclave, then centrifuging and dialysis to purify the product. It was also determined to prevent virus entrance synthesis of the ve strand of RNA. ROS accumulation and stimulation of exciting interferon genes and proinflammatory cytokines suppressed viral replication. This was found to be a multisite enteric coronavirus inhibitor (Fig. 4.2). CDs can successfully stop the replicating RNA viruses, for example, respiratory syndrome virus and porcine reproductive virus. In a Teflon-coated autoclave chamber, the PEG-diamine and ascorbic acid require hydrothermal reaction to produce CDs. The antiviral activity was assessed on monkey kidney cells infected in vitro and WUH3 virus strain of the pig cardiovascular and respiratory illness. Higher interferon combination and increased expression of interferon-stimulating genes stop viral replication [45]. The polyamine-modified CQDs could inhibit the white spot syndrome virus infection by adhering to the viral particle in a dosages manner [46]. The C-dots for the antiviral property has summarized in Table 4.1.

4.2.5 GQDs application in wound pathogen disinfection Recently, drug delivery via GQDs has sparked renewed attention due to their enormous precise surface area and constant contact with various molecules via electrostatic interactions and hydrophobic stacking. The acidic or hydrophobic environment may weaken the drug placed on GQDs. As expected, unmodified GQDs could efficiently deliver the anticancer medication DOX to the nucleus via GQDs/DOX conjugates. The augmentation of DOX nuclear accumulation

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TABLE 4.1 Summarizes of antiviral mechanisms of CDs and their derivatives. CD

Antiviral mechanism

Antiviral against

References

CCM-CDs (1.5 nm)

Penetration, multiplication, and budding

Porcine epidemic diarrhea virus (coronavirus model)

[43]

DCs (4.7 nm)

Multiplication

Porcine respiratory and reproductive syndrome virus

[45]

Functionalized CQD

Penetration and multiplication

Human coronavirus

[47]

Boronic acid/ aminefunctionalized CD

Penetration

Herpes simplex virus type 1

[48]

Benzoxamine CD (4.4 nm)

Virus adhesion

Porcine parvovirus, adenovirusassociated virus, Zika virus, and Dengue virus

[46]

CCM-CDs (4.2e5.2 nm)

Penetration, and multiplication

Enterovirus

[49]

CDs

Blocking of binding

Human norovirus virus-likeparticles

[41]

Glycyrrhizic acid CD (11.4 nm)

Invasion and replication

Porcine reproductive and respiratory syndrome virus, coronavirus, and herpes viridae

[50]

Bluefluorescent CQD (1.9 nm) Cyan fluorescent C-QD (2.7 nm)

mRNA expression level of IFN-a, IFN-b,

Pseudorabies virus

[51]

Polyaminemodified CQD

Penetration

White spot syndrome virus

[46]

by GQDs results in increased cytotoxicity against cancer cells. In the presence of GQDs, the luminous properties of DOX were quenched [52]. Additionally, GQDs passivated with PEG exhibited a remarkable capacity (2.5 mg mg1) to deliver the anticancer medication DOX. The PEG-passivated surface increased the solubility of the GQDs and drug load via hydrogen bonding. Nahain et al. described a method for efficiently and precisely

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delivering GQDs using hyaluronic acid (HA) as a targeting agent [53]. DOX has a quenching effect, and quantitative investigation revealed that 75% of DOX was loaded onto the surface of GQDs. DOX release increased to 42% during 6 h of mildly acidic accommodation (pH 5.0), and the GQDs-HA matrix released all DOX within 48 h. At the same time, DOX release was significantly slower (only 20%) at pH 7.4 after 48 h. This pH-dependent behavior proved favorable due to the tumor’s somewhat acidic environment. Additionally, in vitro cellular imaging and in vivo distribution can verify the target specificity in tumor tissue and cancer cells. Later in the study, Wang and colleagues produced folic acid (FA)-conjugated GQDs for loading DOX [54]. Even after DOX loading, the GQDs-FA could reliably identify HeLa cancer cells from normal cells and deliver DOX to targeted cells. DOX had loading effectiveness of 689 wt percent on the GQD-FA surface. Additionally, because GQDs have an intrinsic steady fluorescence, the delivery complex should be observed in real-time via two channels comparable to DOX and GQDs. Within 30 min of internalization of the DOX-GQD-FA nanomaterials by HeLa cells, the fluorescence of DOX was lost because its adsorption on the GQDs superficial, but the GQDs green fluorescence was perceived in the cell cytoplasm and then incubated for 8 h, free DOX fluorescence was visible in both the cytoplasm and nucleus.

4.3 The live cells real-time molecular tracking by GQD The GQDs have enormous potential as emerging bioimaging nanomaterials for real-time molecular tracking because of their excellent conjugation properties with protein and robust photostability in light. Zheng et al. used insulin-GQD conjugates as fluorotags to monitor the real-time dynamics of insulin receptors in alive adipocytes tissue [55]. They also investigate the distribution, internalization, and recycling of insulin receptors in adipocytes tissue. The graphene materials have been used to reveal regulated insulin receptor trafficking in adipocytes and demonstrate that apelin facilitates insulin receptor dynamics in adipocytes. At the same time, TNF inhibits it indicates the first time and opens the gate for more investigation of insulin cellular interaction. Ananthanarayanan et al. revealed that GQD transferrin-conjugated (Tr-GQD) might be used to detect transferrin receptors in human cervical carcinoma (HeLa) cells in real-time [56]. Since HeLa cells can overexpress transferrin receptors, they were used to investigate the reutilizing of iron-bound transferrin molecules and internalization. Confocal microscopy revealed that selective binding of Tr-GQDs to transferrin receptors induced their endocytosis. Moreover, GQDs might be applied for specific drug carriers and bioimaging nanomaterials to monitor the real-time release kinetics of drug doxorubicin (DOX). The stocking and release characteristics of DOX from GQDs have been demonstrated using GQDs coupled hyaluronic acid (GQDHA). Due to the extraordinary attraction between HA and the CD44

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receptor, HA acts as a targeting molecule and boosts the bright fluorescence. As a result, GQD-HA penetrated the cells more than GQD. According to the findings, pharmaceuticals agents were released from the QGD-HA, indicating that GQDs could be used as drug transporters and fluorescent probes in the future. Kim et al. demonstrated the utility of GQDs for in vivo tracking of human adipose-derived stem cells [57] and revealed that GQDs were noncytotoxic, had no substantial effect on their survival or functionality, were primarily dispersed in the cytoplasm via endocytosis, and retained their fluorescent signal for 24 h. Most of the characteristics mentioned earlier meet the primary criterion for stem cell tracking molecules. Hollow-structured nanospheres created on GQDs with Pd nanoparticles were utilized to monitor trace levels of H2O2 in real-time, a viable stage for cancer diagnostics due to the increased H2O2 generated by active tumor cells. Due to the high electrocatalytic activity of Pd and the innate peroxidase mimicking the action of GQDs, nanospheres can be used as H2O2 tracking agents. Imaging in vivo, numerous photoluminescent nanomaterials have been described for labeling cells in vitro. However, in vivo imaging and biomaterial application are well established because their excitation-dependent photoluminescence makes multi-color fluorescent GQDs possible. Autofluorescence of GQD significantly affects imaging superiority because it is critical to validate the ideal excitation and emission wavelengths before imaging. The Nurunnabi et al. initially showed that carboxylated GQDs collected mainly in the liver, spleen, lung, and tumor simultaneously and did not produce acute toxicity [58] as the fluorescence intensity increased and then decreased with increasing time, which was primarily due to the GQDs’ time dependence and excretion. Analyses of the organs from GQDs injected mice revealed that the GQDs were disseminated throughout the body and gathered in various probable locations. Biomaterials that integrate imaging and photodynamic treatment (PDT) have increased interest. For instance, a study showed that due to their extensive absorption, high singlet oxygen (1 O2) generation yield, strong deep-red emission, GQDs could be used as PDT biomaterials for real-time imaging and highly effective cancer treatment. Where GQD was injected in the mice, a spot of high red fluorescence intensity was reported, and the power remained constant for one week, establishing the parameters for PDT. This study examined the photothermal outcome and 1O2 quantum yields of GQDs in tumor cell death. The abovementioned findings showed that GQDs should be used in imaging and cancer therapy. The nanocomposite of GQDs and annexin V antibody (AbA5-GQDs) has been used to sense apoptotic cells required for homeostasis. Due to the transparency of the zebrafish, it was chosen as a unique model for studying apoptotic cell death start and progression. GQDs have emerged as a special material for building biosensors due to their intrinsic flaw structure, ease of modification, tunable PL property, and superior electrochemical property. They can be used as fluorescence probes to detect various substances with excellent selectivity and

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sensitivity, including metal ions, hydrogen peroxide (H2O2), glucose, cholesterol, protein, and nucleotides.

4.4 Conclusion Nanoscience breakthroughs are now being studied for their potential to improve and create detection methods and adapt therapy for a variety of complex ailments. In particular, QDs have been designed as cutting-edge raised areas aimed at high-throughput computable investigations of various indicators in clinical tissue samples and biomarkers in cells, in vivo assessments of cells with illnesses, and possibly tailored and perceptible medicine administration. Quantum dots indeed offer great potential in pharmacy, bioimaging, medical, and photoluminescent uses. They can be used as capable fluorescent probes for imaging with little toxicity in cells other applications, such as bioanalysis and others. Due to its good chemical inertness, outstanding biocompatibility, and resistance to photobleaching that graphene and carbon quantum dots have become more popular. Their optical characteristics may also be modified for specialized and specific applications through size control, chemical doping, and functionalization, among other techniques.

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80 Graphene Quantum Dots [28] El Hotaby W, Sherif HH, Hemdan BA, Khalil WA, Khalil SK. Assessment of in situPrepared polyvinylpyrrolidone-silver nanocomposite for antimicrobial applications. Acta Phys Pol June 1, 2017;131(6). [29] Lin F, Bao YW, Wu FG. Review: Carbon dots for sensing and killing microorganisms. C: J. Carbon Res. 2019;5:33. [30] Li C, Lin F, Sun W, Wu FG, Yang H, Lv R, Zhu YX, Jia HR, Wang C, Gao G, Chen Z. Selfassembled rose bengal-exopolysaccharide nanoparticles for improved photodynamic inactivation of bacteria by enhancing singlet oxygen generation directly in the solution. ACS Appl Mater Interfaces April 11, 2018;10(19):16715e22. [31] Khalil WA, Sherif HH, Hemdan BA, Khalil SK, El Hotaby W. Biocompatibility enhancement of graphene oxide-silver nanocomposite by functionalisation with polyvinylpyrrolidone. IET Nanobiotechnol October 17, 2019;13(8):816e23. [32] Han S, Zhang H, Xie Y, Liu L, Shan C, Li X, Liu W, Tang Y. Application of cow milkderived carbon dots/Ag NPs composite as the antibacterial agent. Appl Surf Sci February 15, 2015;328:368e73. [33] Moradlou O, Rabiei Z, Delavari N. Antibacterial effects of carbon quantum dots@ hematite nanostructures deposited on titanium against Gram-positive and Gram-negative bacteria. J Photochem Photobiol Chem June 15, 2019;379:144e9. [34] Jhonsi MA, Ananth DA, Nambirajan G, Sivasudha T, Yamini R, Bera S, Kathiravan A. Antimicrobial activity, cytotoxicity and DNA binding studies of carbon dots. Spectrochim Acta Mol Biomol Spectrosc May 5, 2018;196:295e302. [35] Sattarahmady N, Rezaie-Yazdi M, Tondro GH, Akbari N. Bactericidal laser ablation of carbon dots: an in vitro study on wild-type and antibiotic-resistant Staphylococcus aureus. J Photochem Photobiol B Biol January 1, 2017;166:323e32. [36] Kovacova M, Markovic ZM, Humpolicek P, Micusik M, Svajdlenkova H, Kleinova A, Danko M, Kubat P, Vajdak J, Capakova Z, Lehocky M. Carbon quantum dots modified polyurethane nanocomposite as effective photocatalytic and antibacterial agents. ACS Biomater Sci Eng September 27, 2018;4(12):3983e93. [37] Priyadarshini E, Rawat K, Prasad T, Bohidar HB. Antifungal efficacy of Au@ carbon dots nanoconjugates against opportunistic fungal pathogen, Candida albicans. Colloids Surf B Biointerfaces March 1, 2018;163:355e61. [38] Liu Y, Liu Y, Qian H, Wang P, LeCroy GE, Bunker CE, Fernando KS, Yang L, Reibold M, Sun YP. CarboneTiO 2 hybrid dots in different configurationseoptical properties, redox characteristics, and mechanistic implications. New J Chem 2018;42(13):10798e806. [39] Yan Y, Kuang W, Shi L, Ye X, Yang Y, Xie X, Shi Q, Tan S. Carbon quantum dot-decorated TiO2 for fast and sustainable antibacterial properties under visible-light. J Alloys Compd March 10, 2019;777:234e43. [40] Zhang J, Liu X, Wang X, Mu L, Yuan M, Liu B, Shi H. Carbon dots-decorated Na2W4O13 composite with WO3 for highly efficient photocatalytic antibacterial activity. J Hazard Mater October 5, 2018;359:1e8. [41] Dong X, Moyer MM, Yang F, Sun YP, Yang L. Carbon dots’ antiviral functions against noroviruses. Sci Rep March 31, 2017;7(1). 1e0. [42] Huang S, Gu J, Ye J, Fang B, Wan S, Wang C, Ashraf U, Li Q, Wang X, Shao L, Song Y. Benzoxazine monomer derived carbon dots as a broad-spectrum agent to block viral infectivity. J Colloid Interface Sci April 15, 2019;542:198e206. [43] Ting D, Dong N, Fang L, Lu J, Bi J, Xiao S, Han H. Correction to multisite inhibitors for enteric coronavirus: antiviral cationic carbon dots based on curcumin. ACS Appl Nano Mater April 24, 2020;3(5):4913.

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Chapter 5

Graphene quantum dots for drug biodistribution and pharmacokinetics Mohammad Zubair1, Fahad Mabood Husain2, Farha Fatima3, Mohammad Oves4, Mohammad Azam Ansari5 and Marai Almari6 1

Department of Medical Microbiology, Faculty of Medicine, University of Tabuk, Tabuk, Saudi Arabia; 2Department of Food Science and Nutrition, Faculty of Food and Agricultural Sciences, King Saud University, Riyadh, Saudi Arabia; 3Department of Zoology, Aligarh Muslim University, Aligarh, Uttar Pradesh, India; 4Centre of Excellence in Environmental Studies, King Abdulaziz University, Jeddah, Saudi Arabia; 5Department of Epidemic Disease Research, Institute for Research & Medical Consultations (IRMC), Imam Abdulrahman Bin Faisal University, Dammam, Saudi Arabia; 6Department of Surgery, Faculty of Medicine, University of Tabuk, Tabuk, Saudi Arabia

5.1 Introduction Quantum dots (QDs) are semiconducting nanoparticles gaining ground in numerous applications, counting the biomedical field, because of their exclusive optical characteristics. Recently, graphene quantum dots (GQDs) have earned some attention in biomedicine and nanomedicine because of their higher biocompatibility and low cytotoxicity compared to other QDs. GQDs share the optical characteristics of QD and have the established capability to cross the bloodebrain barrier (BBB). For this reason, GQDs are now being employed to develop our understanding of neuroscience diagnostics and therapeutics. Their size and surface chemistry that ease the loading of chemotherapeutic drugs makes them the perfect drug delivery systems through the bloodstream, across the BBB, and up to the brain. GQDs-based neuroimaging techniques and theragnostic applications, such as photothermal and photodynamic therapy alone or in combination with chemotherapy, have been designed [1]. Ever since the first synthesis that occurred in the 1980s QDs have been studied in-depth and applied in numerous devices such as optical devices and solar cells. However, more recently, the interest in QDs has deepened further and spread in various branches of medicine and biology primarily because of Graphene Quantum Dots. https://doi.org/10.1016/B978-0-323-85721-5.00010-8 Copyright © 2023 Elsevier Ltd. All rights reserved.

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their photophysical properties. Due to this property, QDs are best suited to be used in drug delivery, bioimaging, and much more important because they can be used in theragnostic applications such as photothermal therapy [2]. Quantum dots are semiconducting nanomaterials with dimensions below 100 nm. The chemical and physical characteristics of QDs are dependent on their size, this is because of the quantum confinement effect. When an electron gets promoted to conduction band from valence band what happens is that an empty electron state is left which is known as a “hole.” Electrons and holes are attracted to each other by electrostatic Coulomb force, resulting in a bound state, called excitons, which are neutral quasiparticles. QDs can also be treated as particles-in-a-box and the reason is the dimensions of QDs are comparable to the exciton diameter and this aspect can be expressed through the following formula [3]: In the aforementioned formula, h is reduced Planck constant, m is the mass h2 p2 leads to a of the particle and L is the length of the box. In QDs, this E ¼ 2mL 2 dependence of the bandgap energy on the size: the smaller the size, the bigger the bandgap energy [3]. This generally becomes evident by quantifying the variations in absorption and emission as a function of the decreasing size and this is called “blue-shifts” and these shifts indicate an increase in bandgap energy. Similar to particle-in-a-box, electrons can occupy only specific discrete levels of energy and because of this they are also known as “artificial atoms.” As compared to organic fluorophores, QDs have unique features and one such feature is the broader absorption of spectra, this enables excitation by a wide range of wavelengths and narrower emission spectra and this helps to reduce the signal overlap. The methods involving synthesis primarily involve surface passivation of an inner layer and this is done by deposing a capping layer, which generally is an inorganic semiconductor material [4]. The core layer has a slender bandgap as compared to the shell. It has been found that the synthesis of QDs generally occurs in a coordinating solvent and the presence of core material from the 16th group and of an organometallic shell precursor at great temperatures. QDs are also less vulnerable to photobleaching as compared to other molecules and the reason is that they have a very stable light emission. This has been showcased in various biological labeling experimentations where the photostability of QDs was likened with generally used fluorophores [5]. As the popularity of QDs is increasing day by day, concerns have also risen because of their cytotoxicity. The primary concern is regarding the inherent toxicity of the core elements of QDs and they include selenium and cadmium, both of these elements can affect the cell cultures and as well as live animals. In the last couple of years, QDs have displayed tremendous photophysical properties as well as good biocompatibility. This suggests that their character is more similar to the molecule as compared to other QDs, and this is the reason that their popularity has increased drastically in life sciences [6].

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Graphene is a bidimensional material, a single-atom-thick sheet of honeycomb-arranged, sp2 -bonded carbon atoms. Graphene and graphene oxide (GO) has been employed in several biomedical applications, as antibacterial agents, scaffolds for bone regeneration and diagnostic tools. Despite the wide range of applications of graphene and GO in the biomedical field and their biocompatibility, the use of GQDs comes with further advantages. GQDs, as QDs, have unique optical properties which strictly depend on their size, shape and surface chemistry, making them highly suitable for bioimaging compared to other organic dyes. Furthermore, these nanoparticles’ small dimensions easily allow them to cross biological barriers and target specific anatomical regions inaccessible for graphene and GO, making them good candidates for drug delivery [7].

5.2 Graphene quantum dots Graphene quantum dots (GQDs) are small flakes of graphene in which quantum confinement of excitons becomes prevailing, and this causes a casual spacing Coulomb blockade peaks as compared to a periodic distribution. The interesting fact is in GQDs quantum confinement is not just given by the size, the reason is that different boundaries lead to a diverse spectrum of energy and as well as the polarization of spin. Furthermore, have absorption peaks that are similar to those of other graphene-based materials. These two separate summits depend on two precise electronic transitions: the first is determined by pp* alterations within the aromatic rings and, in general, by the sp2 -hybridized portions. The second, less concentrated, the peak is due to pp* evolutions, and it is given by the existence of lone pairs contained by oxygens. As evidenced, pristine graphene exhibitions only a pp* transition peak at 270 nm, and no np* [8]. The first absorption peak of GQDs is generally in the range of 200 and 270 nm. One occurs at the wavelengths of more than 280 nm which occur within pp* and np* transitions, respectively. The primary characteristics that determine the photoluminescence and absorption features of graphenebased materials comprise its sp2-hybridized fraction, the existence of efficient groups comprising, specifically, oxygen or nitrogen, and the comparative synthesis process and the dimension of the molecule. Since the dimensions of GQDs are similar to the exciton diameter, they can be considered as particlesin-a-box for which the bandgap energy is contrariwise proportional to the squared size. It is also shown that higher bandgap energies match up to minor emissions wavelengths. Correspondingly, it has been testified that the peak of absorption of GQDs with lateral proportions ranging from 1 to 4 nm is situated at 270 nm, while 7e11 nm GQDs displayed a maximum located at 330 nm [9]. This shift could simply be elucidated by taking into account the quantity of sp2 constructions inside the molecule. The carbon materials that contain a mixture of sp3 and sp2 bonding, the optoelectronic characteristics are

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determined by the p conditions of the sp2 sites, which are engrained in the unvaryingly advanced bandgap of s and s* orbitals and this is the reason that repeat combination of electron-hole couples in sp2 clusters results in PL. To tune the emission of PL, nature and as well the quantity of sp2 sites can be manipulated, this reason is that bandgap depends not only on size and shapes but also on the portion of sp2 domains [10]. Carbon materials that have disordered sp2 clusters generally act as unstructured semiconductors. In these semiconductors the density of states of aromatic chains falls in contained states within the bandgap, plummeting the energy gap among valence and transmission band. The quantity of functional clusters comprising nitrogen gives as well in GQDs optical features: their lone pairs upsurge the second absorbance peak connected to n  p* conversions. Other agents of bioimaging, such as QDs or other organic compounds lack specific to surface functionalization that can be tuned. Because the chemistry of carbon is the most characterized and studied this allows a more sophisticated surface engineering [11]. It has also been discovered that the method of synthesis can upset the optical characteristics as well. The methods of synthesis that involve hydrothermal approaches can reduce the mean number of oxygens, therefore, the quantity of aromatic rings also gets increased and the result is that bandgap energy is reduced [12].

5.3 Synthesis of GQDs GQDs are blocks of graphene with a two-dimensional (2D) cross-sectional size and excellent chemical, physical and biological characteristics. An ideal form of GQD consists of a single atomic shell of carbon atoms. Though, most of the synthesized GQDs also contain functional groups such as oxygen and hydrogen and generally have multiple layers of atoms with sizes less than 10 nm. Because of its small size, GQD has better prospects than graphene, graphene oxide, or graphene in applications related to biomedicine. Though, before scheming GQD for applied applications, its biocompatibility and toxicity continue to remain foremost trepidations. Studies have revealed that GQDs have decent low and biotoxicity biocompatibility [13]. Currently, the methods that are used for GQD synthesis can usually be categorized into two processes which names top-down or bottom-up. The bottom-up method requires reaction steps that are more complicated and it also requires precise organic materials and because of these reasons the top-down approach is preferred as compared to the bottom-up. The top-down approach primarily involves cutting big blocks of carbon materials into lesser fragments. Furthermore, there are other numerous methods for top-down processes as well such as hydrothermal method, chemical oxidation method, chemical vapor deposition, electrochemical oxidation method and pulsed laser ablation, even a combination of these methods could be used as well [13]. The details of these methods are as follows.

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5.3.1 Chemical oxidation method This method is also identified as an oxidation cutting method. In this method, the carbon bonds of graphene or sometimes carbon nanotubes are generally destroyed by oxidants. An experimental system was developed by Liu et al. [14]. This method uses Vulcan XC-72 carbon black as a carbon source and robust oxidant concerted nitric acid reflux to formulate high concentration GQDs. The yield and the purity of this method is generally 75 wt% and 99.96 wt% correspondingly. As compared to other methods, this method of synthesis is faster and as well as eco-friendlier. However, since this method uses strong oxidants such as HNO3 and H2SO4 it is not considered that safe and also, the chemical waste generated is also considered to be hazardous [15].

5.3.2 Hydrothermal method This method is quicker and simpler to prepare GQDs. In this method, GQDs can be attained using a diversity of macromolecular or small molecular materials. Very pure GQD can be obtained by evaporation or dissolution and filtration without dialysis. The results showed that the diameter and thickness of the GQDs were mainly distributed in the range of 20e40 nm and 1e1.5 nm, respectively. The PL signal has shown good stability under various pH conditions, indicating that it has broad application prospects in various environments. This method offers many advantages, such as low cost, high quantum efficiency, no need for dialysis and cleaning, simple experimental setup, etc. The GQDs produced were environmentally friendly and demonstrated solubility in solid water to illustrate their promising applications in biomedical and bioelectronics devices. The hydrothermal method can be used to dot many elements or groups, and the raw materials come from a wide variety of compounds. Furthermore, the hydrothermal process can be combined with the chemical oxidation process to produce various GQDs. However, it suffers from the high temperature and high-pressure safety problem, and it usually takes a long time, typically at least 5 h [15].

5.3.3 Ultrasound assisted method This is an innovative technology that uses ultrasound to extract many compounds from a variety of matrices. Ultrasound propagation causes bubbles to burst, known as the cavitation phenomenon, large turbulence, high-velocity collisions between particles and perturbations in the arrays induce small, porous particles in the sample. As a result, the solute rapidly expands from the solid phase to the solvent. This method offers a clean, ecological extraction with several benefits. This technique is simple, effective and has a lower cost as compared to other methods. Its main advantages are related to increased extraction performance and speeded up mobility compared to conventional

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extraction. However, the tool has a significant disadvantage with age, since the performance gradually decreases with decreasing intensity, thus reducing the possibility of repeat experiments [15].

5.4 Applications of GQDs The exceptional properties of GQDs facilitated their promising application in biosensors. Though, the electrochemical properties of the biosensor have shown an excellent benefit for GQD in electrochemical biosensors. The high stability and accuracy of the biosensor upon glucose assay and the low toxicity of GQD for enzyme control are major advantages of its practical application. The glucose sensing system was developed based on the electrostatic attraction between anionic fluorescent GQDs and a cationic boronic acid dipyridinium salt [16]. This showed that the electrostatic attraction between GQD and BBV occurred in transferring electrons in the excitation state from GQD to BBV and also reduced the fluorescence intensity of GQD. A glucose biosensor has also been developed that uses GQD as a modified carbon-ceramic electrode with an enzyme fixation substrate. Razmi et al. confirmed the excellent accuracy of the glucose study and noted the practicality of the glucose biosensor in clinical studies. It has also been used to determine glycemic reflection in human plasma models. Trypsin is the main gastric enzyme produced by pancreatic acinar cells and breaks the peptide bonds at the C-end. However, these approaches have certain drawbacks such as complications of conjugate electrolyte synthesis, fluorophore classification, photobleaching, and as well as requirements for dissimilar kinds of substances and cytotoxicity for GQDs [16]. In modern times, GQD applications have increased in drug delivery, sensor imaging, magnetic hyperthermia, phototherapy, antibacterial activity, catalyst, environmental protection, and energy. To better apply GQDs to drug delivery, some researchers have used functional density theory calculations, molecular dynamics simulations, or other methods to study the properties of GQDs in theory. There are many ways to administer medications, but focusing on administering and ignoring medications cannot enhance the therapeutic effect of medications. Therefore, more and more researchers paid attention to the close relationship between drug delivery and drug release and tried to develop a variety of drug delivery methods to enhance the effect of therapeutic drugs by enhancing drug delivery and discharge efficiency [14]. GQDs are innovative and effective nanomaterials for biological treatment. There have been reports of graphene or graphene-based nanomaterials for drug delivery and drug delivery to improve conduction efficiency and enhance therapeutic effects. Compared to graphene, they have better water solubility, lower cytotoxicity, and higher specific surface area, making them more efficient cores in the molecular loading of the drug. As a member of the graphene and carbon family of nanomaterials, it shows several advantages over other

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nanoparticles in drug delivery applications due to their low toxicity, high surface ratio or size, and ability to operate a massive surface. Compared with other traditional nano plants such as polyethylene glycol, it can offer more binding sites for chemical coupling and improve cell absorption capacity. Compared to quantum dots of similar size, it has superior properties such as quantum confinement effects and simultaneous tracking due to its modifiable optical luminescence, but relatively lower toxicity due to lack of heavy metal components. Therefore, they have great potential for biomedical application. Besides, due to its flat structure, it has a large surface area-to-volume proportion, which allows for more efficiency in loading and administering drugs [17].

5.5 Drug delivery methods One of the major issues in medical treatment to lower the side effects of chemotherapy is localized drug delivery. There are numerous ways in which targeted drug delivery could be used such as through temperature control, pH control, or the use of ultrasonic and optical waves [18]. Numerous nanoparticles have been used for this purpose as shown in Fig. 5.1 below. Sample formulations of treatment agents require membrane cells that do not fit into the hydrophilic type. Some proteins and peptides can penetrate membrane cells and carry bio-conjugated nanocarriers including total cells. Cells embedded in peptides are the most popular biomolecules used for the synthesis of nanoorganisms including agents. A large amount of protein, nucleic acid, synthetic drugs, and liposomes are compatible with CPPs, so they can be used for a wide range of loads. However, some studies have suggested that certain factors include size, charge and the properties of the final dots in

FIGURE 5.1 Different nanoparticles used for localized drug. Adopted from Itani R, Al Faraj A. siRNA conjugated nanoparticlesda next generation strategy to treat lung cancer. Int J Mol Sci 2019;20(23):6088.

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the cell without the need for bio-fictionalization [20]. They showed that a good total number of tumors fit into simple cells and that a large number of spots are stored in the cytoplasm while small particles pass through the vital component. Depending on the target, the bio-molecule attached should be selected by the number of points. For the establishment of specific antigens, the dots should be set with the appropriate drug. As well as detecting a unique immune system in any type of cell requires total bio-contact points in molecular molecules, peptides or aptamers [21]. Foliate is a useful quantitative compound of the best bioconjugation studies for the synthesis of folate receptor, overexpressed in many types of tumor cells. Similarly, total molecules can act as biomolecular sensors, such as maltose sugar, by binding the sum of points to a protein that has a close affinity for maltose. Recent studies have demonstrated the potential of dots concerning the delivery of chemicals to specific cells while monitoring the administration of drugs [22]. Chakravarthy et al. [23] proved to deliver a beneficial total score with doxorubicin in alveolar macrophages, severe cells in diseased lungs. Savla et al. use the mucin one quantum dot aptamer to target ovarian cancer cells and administer the anticancer drug doxorubicin. Quantum dots encapsulated in biodegradable polymers such as chitosan are candidates for regulating the delivery of cancer drugs. The use of chitosan dots in chitosan prevents the drug from being released before the body part of the body is implanted. The effect of a large number of points on its television transmission is very useful for many different images of different parts of the body.

5.5.1 Fluorescent graphene quantum dots application Detection of heavy metal ions is significant within the environmental setting. Various graphene quantum dots (GQD) based fluorescence sensors are made for determining sensitivity of different metal ions. The mechanics to detect Fe3þ considering GQD fluorescence sensor feature the special relationship between phenolic hydroxyl groups and Fe3þ [24]. The efficient use of functionalized GQDs along with amine and heteroatom doping groups is observed to detect Fe3þ. A wide linear range of sulfur and nitrogen codoped GQDs are used ranging between 0 and 130 mM for sensing fluorescent material for the presence of Fe3þ. It possess low detection limit of 0.07 mM. A sensitive response toward Fe3þ is exhibited by amino acid-functionalized and nitrogen GQDs [25]. The anomaly is caused not only by the high thermodynamic attraction between Fe3þ and nitrogen atoms in GQDs, but also by Fe3þ paramagnetism. This suggests a related detection mechanism. The block copolymer-integrated GQDs can also be used for multifunctionally colorimetric temperature, metal ion sensing, and pH. The bcp-GQDs obtained exhibit rapid fluorescence quenching when 100 M Fe3þ is added. A turn-on orange-red fluorescence nanosensor based on rhodamine B derivative-

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functionalized GQDs (RBD-GQDs) has been developed for the detection of Fe3þ in cancer stem cells, unlike the previously described turnoff fluorescent nanosensor [26]. With the addition of Fe3þ, a new solid peak of orange-red emission oriented at 580 nm occurs, owing to the on-switch of the spirocyclic moiety in the RBD. There is production of different GQDs, which are sensitive toward detecting Cu2þ, besides Fe3þ. Cu2þ has been detected using a sensing technique based on amino-functionalized GQDs (af-GQDs). The facilitation of nonradiative electron/hole recombination annihilation by an efficient electron transfer mechanism, which is due to static and dynamic phenomena, causes fluorescence quenching. GQDs obtained by a hydrothermal system as a result of the forming of a complex between Cu2þ ions and the GQDs have been formed as an efficient Cu2þ sensing approach. The static (rather than dynamic) kind of fluorescence quenching mechanism tends to be the most common. Furthermore, pitch graphite fibers could be chemically oxidized to produce pristine GQDs. These GQDs may be used to determine Cu2þ in water samples in a simple and environmentally safe manner [27]. A new “off-on” fluorescence probe using Eu-GQDs was recently published for the label-free determination of Cu2þ with high sensitivity and selectivity. The use of coffee grounds as a precursor to prepare fluorescent GQDs functionalized by poly(ethylene imine) (PEI) has been suggested as an environmentally safe and simple method. As a fluorescent probe, PEI-GQDs are extremely vulnerable to Cu2þ ions. The methods of doping and functionalization of GQDs are widely used to increase selectivity. For instance, QDs modified with thymine-rich DNA (DNA-GQDs) increase selectivity; however, their fluorescence is quenched in the presence of Hg2þ. The binding of Hg2þ to the thymine bases of the DNA inhibits transfer of electrons to DNA-GQDs. A related approach can be used to make cysteine-doped GQDs (Cys-GQDs). A highly selective “on-off” fluorescence sensing technique for the detection of Hg2þ was designed on the premise that the aggregation of GQDs can be validated using dynamic light scattering in the presence of Hg2þ and that the aggregation induces the fluorescence quenching of GQDs [28]. It is possible to constrict a ratiometric fluorescence sensor using CdTe as the internal standard, along with GQDs and a CdTe-based core-satellite hybrid to detect Hg2þ as it is easy to observe ratiometric fluorescence sensing with the naked eye. Fig. 5.2 shows the steady decline in fluorescence intensity of GQDs as the concentration of Hg2þ rises. The test solution undergoes a vibrant visual color transition from blue to red with high sensitivity and selectivity (limited to 3.3 nM) [30]. Synthesis of 3,9-dithia-6-monoazaundecane (DMA)-functionalized GQDs is carried on with the help of hydrothermal method via electrostatic interaction on its surface. A microfluidic sample pretreatment system for Pb2þ preconcentration has recently been developed, which includes a large-volume SPE chamber and a peristaltic pneumatic micropump for automated sample

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FIGURE 5.2 (a) Structure of the ratiometric fluorescence probe and its working principle; (b) CdTe@SiO2@GQD ratiometric probes; (c) GQDs probes. Adopted from Hua M, Wang C, Qian J, Wang K, Yang Z, Liu Q, Wang K. Preparation of graphene quantum dots based core-satellite hybrid spheres and their use as the ratiometric fluorescence probe for visual determination of mercury (II) ions. Anal Chim Acta 2015;888:173e81 with License no 5096510894333 (for a) & 5098180378004 (for b & c).

loading and recovery. Fig. 5.3 shows the process in which DNA aptamer functionalized GQDs serve as “on-off” sensing units to detect transfer of electrons from GQDs to Pb2þ. In this process, high sensitivity is exhibited toward Pb2þ with the sensor with a linearity ranging between 1 and 1000 nM, and detection limit of 0.64 nM [30]. An innovative platform for on-site water emission screening is provided by on-chip preconcentration of trace metal ions from a large-volume sample accompanied by metal ion detection using the fluorescence GQD sensor. AgNPs developed on GQDs quench the FL of GQDs by charge-transfer processes, resulting in a novel GQD-based probe for the detection of Agþ. The protocol may also be used to detect Au on a limited basis (III). Based on Pearson’s HSAB theory [32], amine-functionalized GQDs (Am-GQDs) have been synthesized as a “on-off-on” probe for the selective detection of Agþ. Soft acids react faster and form stronger bonds with soft bases, according to Pearson’s HSAB principle, while hard acids react faster and form stronger bonds with hard bases. According to the HSAB principle, S and Ag form stronger bonds than the other metal ions, since they are soft base and soft acid, resulting in recovery of Agþ ions selectively.

5.5.2 Long-term biodistribution Fig. 5.4 shows the rate and degree of substance absorption, distribution, metabolism, and elimination (ADME) determine the behavior of any nanomaterial at the tissue or cellular level. These processes are known as biokinetics in nanoscience, and they include uptake, biodistribution, and removal [34]. Nanomaterials have a dramatically different volume-to-surface ratio than bulk materials, so their physicochemical properties can vary significantly. These variations have an effect on not only biodistribution but also

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FIGURE 5.3 (a) Schematic illustration of a five-layered sample pretreatment microdevice; (b) Digital image of the cation exchange resin; (c) Experimental scheme for Pb2þ detection. Adopted from Park M, Ha HD, Kim YT, Jung JH, Kim SH, Kim DH, Seo T. Combination of a sample pretreatment microfluidic device with a photoluminescent graphene oxide quantum dot sensor for trace lead detection. Anal Chem 2015;87:10969e75.

FIGURE 5.4 Biokinetics of nanomaterials. Adopted from Zhao FD, Yao, Guo R, Deng L, Dong A, Zhang J. Composites of polymer hydrogels and nanoparticulate systems for biomedical and pharmaceutical applications. Nanomaterials 2015;5(4):2054e130.

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biodegradation rates in vivo. The oxidation products of soluble nanomaterials, mainly ions, follow the same path as their organic solutes. This results in a global dissemination, but it can also result in localized preservation of tissues or cells. In summary, several factors influence nanomaterial biodistribution, including the nanomaterial’s size, surface properties, and dissolution rate, as well as tissue- or organ-dependent factors including barrier tightness or permeability [35]. Generally, nanomaterials are found in spleen, bone marrow, lymph nodes, liver, kidneys, and central nervous system after being exposed orally, through inhalation, of intravenous injections. The method of exposure is also important. There is variation in biodistribution and biokinetics of nanomaterials from the administered ones and the respiratory tract after intravenous administration. Furthermore, it is demonstrated that a surrogate biokinetic approach for oral or pulmonary routes of exposure is not presented through biokinetics of intravenously injected nanomaterials [36]. After 1 h, the liver, spleen, carcass, skeleton, and blood had the greatest titanium accumulation, followed by the spleen, spleen, carcass, skeleton, and blood, after which the blood content quickly declined, while the distribution in the other organs and tissues remained steady until day 28. Following oral administration, the majority of the dose was excreted in the feces, but 0.6% of the dose translocated through the GI tract and was eventually contained in the lung, brain, spleen, skeleton, liver, kidney, and uterus. After 1 h, a 4% translocation rate of the original administered dose occurred, with the majority of the dose remaining in the carcass, and this level decreased to 0.3% after 28 days. The liver and kidney organ fractions remained unchanged. For all translocated/absorbed particles, the clearance from the lungs through the larynx increased from 5% to 20%. The development of physiologically based pharmacokinetic (PBPK) models can reliably predict such relationships have been established. Most of these models characterize the fate of nondegradable nanomaterials injected intravenously [37]. The most significant considerations have been determined to be the nanomaterials’ physicochemical properties (such as shape and size), blood/tissue permeability coefficients, and macrophage phagocytosis. A novel two-step method for assessing the biokinetics of inhaled nanomaterials was recently proposed for the lung. The translocation kinetics of aerosolized gold nanoparticles through the epithelial tissue barrier were determined in vitro, and the distribution to secondary organs was predicted using a PBPK model in a second phase [37].

5.5.3 Biodistribution and toxicology of carboxylated graphene quantum dots GQDs have gathered a lot of attention as a new kind of quantum dot because of their chemical stability, electrical properties, and photoluminescence. Furthermore, because of the high quantum confinement and edge effects,

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transforming two-dimensional graphene sheets into 0-dimensional GQDs will expand the electronic and optoelectronic applications. The majority of GQD research has been based on biomedical studies, and experimental synthesis is a relatively new endeavor. There have been various studies of the production of graphene-based biosensors aimed at detecting biomolecules with high sensitivity using GQDs in biomedicine [38]. GQDs with sufficient surface functionalization can be used for drug and gene distribution due to their ultrahigh sensitive surface region. A substance that incorporates the modalities of medical imaging and therapy is referred to as theranostic. It concurrently releases medicinal drugs and medical imaging agents in a single dosage. Theranostic methods have the ability to solve the unfavorable biodistribution and selectivity gaps that currently occur between different imaging and therapeutic agents. Graphene is reports as a carrier for a drug delivery system; for instance, the hybrid SiOe 2 coated quantum dots (HQDs)-conjugated graphene is used to monitor drug delivery ad cancer therapy [39]. The most exciting feature of using GQDs as a theranostic nanoparticle is that they have large surface area-to-volume ratios, which means they can hold a lot of information. Since a variety of drug molecules could be found on both surfaces, the large planar surface of GQDs is called an additional window for drug distribution. Chemical conjugation may also be used to conjugate the edge of drug molecules, as previously mentioned. Furthermore, based on their surface functionalization, nondegradable nanoparticles prevent them from being easily cleared by the kidneys without producing harmful moieties. As a result, it is one of the most appealing materials for advancing precision medicine and imaging in the biomedical field. To make photoluminescent GQDs, exfoliation of carbon fiber first occurred during ultrasonication in acidic media, as reported previously. Synthesis process of GQDs from carbon fiber is described in our previous report. However, in brief, due to sonication in acid, the carbon fiber partially exfoliated and formed multilayered and/or monolayered graphene. The development of a zigzag-shaped graphene was aided by these conditions, which resulted in a nearly transparent gray-black color in solution. Fig. 5.5 shows the result of long-term exposure to strong acid and intense stirring that produces hexagonal nanosized graphene shaped particles [40]. Some of the synthesis methods and its applications were sum up in Tables 5.1 and 5.2.

5.6 Critical issues Nanomaterials such as GQDs alone or in combination with many other nanomaterials to form hybrid nanocomposites have remarkable properties due to their unique properties and/or synergies required in the fabrication of biosensors. Over the past decade, these biosensor platforms have offered many options for diagnosing a wide range of diseases, from autoimmune diseases to

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FIGURE 5.5 (a) Synthesis of PL GQDs from carbon fiber; (b) PL intensities of the carboxylated GQDs; (c) TEM images of the carboxylated GQDs; (d) HR-TEM image; (e) Size distribution of the carboxylated GQDs; (f) confocal laser scanning microscopic (CLSM) images. Adopted from Zeng Z, Chen S, Tan TTY, Xiao FX. Graphene quantum dots (GQDs) and its derivatives for multifarious photocatalysis and photoelectrocatalysis. Catal Today 2018;315:171e83 with Licence no 5098190151952.

TABLE 5.1 Biomedical applications of Bottom-Up synthesis methods GQDs [41]. Synthesis methods

Applications

Size obtained

GQD-RhB-silka

Diagnosis

3e20 nm

Mango leaf extract mGQDs

NIR responsive fluorescence bioimaging

2e8 nm

PEGylated GQD

Fluorescence imaging of tumors

2.75 nm

GQD-PEI

Gene transfection

3e4 nm

GQDs

Drug delivery and bioimaging

w12 nm

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TABLE 5.2 Biomedical applications of Top-Down synthesis methods GQDs [41]. Synthesis methods

Applications

Size obtained

GQDs

Diagnosis

5 nm

Durian extract GQDs

Bioimaging

2e6 nm

NP-GQD

Cysteine detection

10e30 nm

GQD-PEG-AG

Radiotherapy

3e4 nm

Lignin-GQDs

Bioimaging

2e6 nm

neurodegenerative diseases, cardiovascular disease, infectious diseases and even cancer diagnostics. Regarding the degree of specificity of this pathophysiology, early detection of it through the use of biomarkers of diseases, toxins, and pathogens in biological, environmental, and nutritional specimens shows important and concrete ideas about their severity. The sensitivity, selectivity, accuracy, and reliability of disease-related molecules always seem difficult due to their extremely low concentrations. However, our review article demonstrates that GQD-based sensors have now clearly reached the threshold for detecting some target biomolecules. These biosensors for the early detection of diseases have achieved tremendous popularity in many areas, including clinical treatment, disease surveillance, the discovery of preventive treatments, and the development of treatment-based drugs [42]. GQDs are in a position to operate multiple electrodes with ease and enable fast, simple, stable, reproducible, and cost-effective sensor systems for clinical and practical applications. Moreover, these GQD sensors are known for excellent specificity, super selectivity and sensitivity in biological matrices such as human blood, urine, sputum, saliva, milk, hard water, soil, etc. Which can be attributed to both the material and the material content. GQD chemical properties. On the other hand, the sharing of more expensive nanomaterials with GQD, complex test methods, insufficient storage stability, and a host of unpleasant factors at the nanoscale are obstacles that are obstacles to their production. Moreover, most of these recently reported sensors have yet to be verified for their clinical application. Therefore, strategies must be developed to create large groups of sensors and to have mass production and approval in the real world [19].

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Chapter 6

Graphene quantum dots: application in biomedical science Rani Rahat1, Khalid Umar2, Sadiq Umar1, Mohd Jameel3, Mohd Ashraf Alam4, Tabassum Parveen5 and Rohana Adnan2 1

College of Dentistry, University of Illinois, Chicago, IL, United States; 2School of Chemical Sciences, Universiti Sains Malaysia, Penang, Malaysia; 3Department of Zoology, Aligarh Muslim University, Aligarh, Uttar Pradesh, India; 4Department of Pharmacology, IIMS & R, Integral University, Lucknow, Uttar Pradesh, India; 5Department of Botany, Aligarh Muslim University, Aligarh, Uttar Pradesh, India

6.1 Introduction Carbon is one of the most prevalent elements on the planet and found in a variety of allotropes. The discovery of football-shaped fullerenes by Kroto in 1985, and needle-like carbon nanotubes (CNTs) by Iijima in 1991, sparked a huge and escalate the interest of using this material in carbon material field. Geim and Novoselov got the Nobel Prize in Physics in 2010 for their discovery of graphene. They spun a graphite flake into graphene using a particular type of tape with a single layer of carbon atoms. In comparison to zero-dimensional (0D) fullerenes and one-dimensional (1D) carbon nanotubes, two-dimensional (2D) graphene has unlocked new possibilities for studying several fundamental quantum relativistic phenomena that were previously thought to be highly unusual. Two-dimensional graphene has a high inherent carrier mobility, a huge surface area, outstanding mechanical characteristics, and exceptional flexibility. It has some drawbacks such as easy agglomeration and poor dispersion. Graphene quantum dots (GQDs) belong to the graphene family, recognized as a unique form of zero dimensional luminous nanomaterials produces a quantum-size effect excitation in 3e20 nm particle’s range [1]. They also have remarkable optoelectronic characteristics, as well as good biocompatibility and a low-cost production technique, and so have the potential to replace the well-known metal chalcogenides-based quantum dots [2]. Furthermore, the pep bonds below and above the atomic plane provide graphene excellent thermal and electrical conductivity when compared to Graphene Quantum Dots. https://doi.org/10.1016/B978-0-323-85721-5.00002-9 Copyright © 2023 Elsevier Ltd. All rights reserved.

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traditional semiconductor quantum dots, allowing GQDs to have their advantageous properties without the weight of inherent toxicity [3,4]. The quantum confinement effect, as well as changes in the density and type of the sp2 sites accessible in GQDs, cause their optical characteristics to be highly dependent on their size, allowing GQDs’ energy band gaps to be adjusted by varying their size [5]. Over the last few decades, quantum dots have a distinctive place in nanomaterial areas, with continuously expanding research, and have achieved significant progress [6]. GQDs’ remarkable chemical, physical, and biological characteristics permit them to flourish in a broad spectrum application in the field of nanomedicine. The exclusive electronic structure of GQDs deliberates functional qualities onto these nanoparticles like a robust and tunable photoluminescence for its usage in fluorescence bioimaging and biosensing processes. In this chapter, importance and various application of GQD have been discussed.

6.2 Applications of GQDs in biomedical sciences 6.2.1 Immunological assay based on GQDs Immunosensors are quick and easy analytical techniques for determining numerous clinical illnesses and different biochemical compositions that rely on traditional method of interaction of antibody-antigen (Ab-Ag) and provide a promising clinical diagnostic as their precise and delicate characteristics. Generally, immunosensors work by detecting the antigen’s complexity with a definite antibody pairing, one of which might be mounted on a solid surface and Ab-Ag complex formation change the signal. Enzymatic processes involve fixing enzyme-labeled antigens and can therefore be used to create very sensitive immunosensors [7]. Because of graphene’s advantageous structural and compositional synergy, GQDs are suitable materials for constructing different immunosensing systems. According to the kind of transduction, immunosensors can be categorized as amperometric immunosensors, electrochemical immunosensors [8], piezoelectric immunosensors [9], thermometric immunosensors, or magnetic immunosensors.

6.2.1.1 Electrochemical immunosensors Electrochemical immunosensors have sparked a lot of interest in the research community as they have the benefits of label-free and interaction of antigenantibody at the detection surface of device, permitting change in potential that reveals the presence of a particular protein or peptide that is to be measured. Yang and colleagues developed a highly sensitive electrochemical immunosensor stranded on nitrogen-doped graphene quantum dots (N-GQDs) sustained PtPd nanoparticles (PtPd/N-GQDs) to the detect the carcinoembryonic antigen, showing a wide range ranging from 5 fge50 ng mL 1 [10].

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Moreover, GQD-based immunosensors for detecting biomarker in human heart attacks have piqued the curiosity of researchers. When compared to existing detection methods, the results of a GQDs fluorescence resonance emergency transference (FRET) based biosensor for detecting the cardiac Troponin I (cTnI; Fig. 6.1), exhibited greater specificity, and low limit of detection (0.192 pg mL 1), with a shorter detection time of 10 min [11]. In other research, for the detection cTnI by means of a GQD-PAMAM nanohybrid a kind of improved gold (Au) screen printed electrode (Fig. 6.2) generated similar results [13]. GQD-based electrochemical immunosensors have become the most popular research topic in recent years, and interest in this field is not going to diminish.

6.2.1.2 Amperometric immunosensors Amperometric immunosensors are considerably more widely researched than other types of immunosensors, owing to their ease of manufacture, contraction, toughness, and economy [14]. Huang et al. made an amperometric immunosensor for the detection of protein (Fig. 6.3.). The immunosensor has been developed on chitosan, TiO2-graphene, and the composite of gold nanoparticles (AuNPs) film-modified glassy carbon electrode (GCE). Electrostatic adsorption allows negative charged AuNPs to get adsorbed on the charged chitosan/TiO2-graphene nanocomposite film, which may subsequently be utilized to restrain a-fetoprotein antibodies for a-fetoprotein testing (AFP).

FIGURE 6.1 Showing the immobilization of the cardiac troponin I antibody (anti-cTnI) probe on the Au/GQD/PAMAM nanohybrid electrode and electrochemical detection of cTnI. SPE: Screen printed electrode (CE: counter electrode, WE: working electrode, RE: reference electrode) [12].

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FIGURE 6.2 Showing the schematic mechanism of immunosensing based on specific interaction of anti-cTnI/afGQDs with graphene [12].

This approach yields a broad detecting range (0.1e300 ng mL 1) for the model to target AFP [15]. It has been also reported that the study on immunosensors of amperometric nature has similarity to immunosensors of electrochemical nature.

6.2.1.3 Other types of immunosensors The concepts of optical immunosensors are based on linking immunoassay with surface plasmon resonance (SPR) technology. An alteration in the

FIGURE 6.3 Schematic showing the preparation of immunosensor [12].

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medium’s refractive index may be detected when Ag (tumor marker) interacts with particular antibodies mounted on the sensor surface. Instead, antibodies might be fixed to the surface of an optical fiber, allowing some modification in refractive index, luminescence, or fluorescence associated with a various antigen and their interaction to the antibodies to be detected [16]. The concept of piezoelectric immunosensors is used by evaluating the variation in oscillation frequency due to variation in mass and antigens interact with antibodies mounted on quartz crystal [17e21]. Immunosensors of this category are not extensively utilized in biomedical applications, owing to their excessive cost, difficulties in bulk manufacture, and the typical issues related to electromagnetic and mechanical interference.

6.3 GQD-based platforms for drug delivery It has been well noted that various nanocarriers have been developed to increase drug solubility and its precise targeting. Drug carriers and targeted cellular imaging are generally combined in multifunctional GQDs, which can be utilized in the cancer. To comprehend cellular uptake, drug delivery systems might be observed by utilizing semiconductor quantum dots, and organic fluorophores, by using intrinsic fluorescence of GQDs. We could be able to readily track movement inside the cells in full detail deprived of using any external dyes [22]. In a study, synthetic folic acid (FA)-conjugated GQDs were used to load doxorubicin (DOX), an anticancer drug. The prepared nanoassembly can differentiate cancerous from healthy cells and proficiently deliver the drugs to target tissues. HeLa cells immediately import the nanoassembly with the help of receptor-mediated endocytosis, however release and accumulation of DOX take longer time. The results of in vitro toxicity show that the nanoassembly DOXeGQDeFA may selectively target the HeLa cells with minimum toxicity in nontarget cells (Fig. 6.4a and b) [23].

FIGURE 6.4 GQDs TEM image (a). (b) GQD and GQDeFA FTIR spectra [12].

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A novel hybrid nanosystem is multimodal device for treating and obtaining image of cancerous cell. The GQDs were employed as a carrier to load lutetium texaphyrin and gadolinium texaphyrin for biological redox therapy as well as increased photothermal and photodynamic therapy [24]. Khodadade et al. has developed a drug delivery system by making 10 nm nitrogen-doped GQDs (N-GQDs) having 10 graphitic layers also loaded with methotrexate (MTX). The findings indicate that utilizing GQDs as a nanocarriers had better antitumor activity as they can extend the cytotoxic effects of loaded drug (Fig. 6.5a and b) [25]. GQD receptor-mediated endocytosis offered a more precise and selective cancer diagnosis method.

6.4 Bioimaging applications of GQDs Bioimaging is considered to be one of the vital field of science where GQDs have various advantage [26,27]. Because bioimaging is a technique of seeing, observing, and detecting targeted tissues, cells, as well as molecules in the body devoid of using any intrusive techniques [28], which provide us broad knowledge of the biochemical processes within the body of organism [29]. In 1896, Wilhelm Roentgen was able to record the first X-ray image [30], a new door for future applications of bioimaging has opened to identify and observe different type of diseases and its symptoms such as bone fractures [31], cancer [32], Parkinson’s disease [33], tumor imaging [34], and so on. GQDs are useful and desired material in bioimaging process because of their outstanding adjustable PL properties, strong photostability, chemical inertness, and great biocompatibility [35]. Moreover, GQDs have superior characteristics when compared to other carbon-derived materials [36], such as notable resistance to photobleaching, which allows for effective bioimaging applications [37]. Inorganic semiconductor QD-based fluorophores and organic dyes have traditionally been used for cell visualization and bioimaging system. While there low extinction coefficient and photobleaching and inherent toxicity, are the primary barriers to their use in bioimaging [38].

FIGURE 6.5 N-GQDs TEM image (c). (d) FTIR spectra of N-GQDs and MTX-GQDs (N-GQDs) [12].

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6.4.1 Fluorescence imaging GQDs have been designed and introduced as a probe of fluorescent for observing cellular activities and taking images for cell based and in vivo tumor cells [39]. Researcher develop a redox-sensitive fluorescence probe which can record the dynamic fluctuations in the intracellular redox status caused by reductive or oxidative stress in real time. Chen and coworkers later determined the GQDs functionalization by sugar monosaccharide in order to regulate trafficking as well as general dispersal of carbohydrate receptors of cell surface [26]. The same group recently completed a real-time estimation of intracellular hydrogen sulfide (H2S) level changes inside the living cells [40]. The suggested fluorescence turn-on approach allows the specific evaluation of H2S as compared to earlier produced GQD-based fluorescent probes. For in vitro tumor cell imaging, Gao et al. reported GQDs coated with polyethyleneimine (PEI) instead of monitoring cellular dynamics [41]. The produced GQDs emitted red, yellow, or blue light depending on the molecular weight (MW) of the PEI, permitting in vitro multicolor imaging of U87 tumor cells [6]. GQDs are water soluble, have high biocompatibility, and are nontoxic. Ding et al. synthesized a doxorubicin (DOX)-loaded GQD-based theranostic nanoagent [42]. The blue fluorescence generated by the GQDs tracked the nanoagent’s internalization, but the DOX fluorescence was considerably muted by GQDs owing to their near proximity. However, after internalization, DOX emits a strong green fluorescence, indicating that DOX was effectively released from the nanoagent, bring about significant chemotherapeutic death noteworthy inhibition of cancerous cell [43]. Campbell et al. demonstrated in vitro multicolor emissive N, S-doped, as well as B, N-co-doped GQDs mutually for NIR-I and visible imaging [44]. These doped GQDs produced red, green, and blue, light at various excitation wavelengths. These emissions of multicolor is ascribed to the GQDs of quantum size as well as their electronic state or surface defect configurations. Furthermore, the ratiometric identification of healthy (HEK-293 cell) and malignant cells was possible because to the pH-dependent fluorescence emission of these GQDs (HeLa and MCF-7 cell). Different research groups have also shown heteroatom codoped GQDs for cellular imaging, like Fe, N and P, N codoped GQDs [45e48].

6.5 Toxicity of GQD materials The major challenges of nanomaterials used is their toxicity. As illustrated in Fig. 6.6, graphene influences the biological system at the cellular, protein as well as genetic levels. The toxicity of graphene is based on its uptake in various particular organs as well as on its physical and chemical interference. Accumulation of graphene in such organs affects cellular performance [49]. Their impeachment, dispensation, and excretion after entering in a cellular environment retrieve the information about their cytotoxicity.

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FIGURE 6.6 Schematic representation of the effective mechanisms by which ROS are linked with the toxicity of graphene at a cellular level.

6.6 Conclusion GQDs are recognized as a novel type of nanomaterials at 0D and are gaining attention in the field of biomedical science due to their unique physiochemical and biocompatibility properties. They are used for in vitro biosensing as well as in vivo imaging applications such as magnetic resonance imaging, dualmodel imaging fluorescence imaging, and two photon imaging. Moreover, GQDs were also helpful directly or indirectly to human life. However, regarding safety as well as toxicity of these nanomaterials is one of the biggest challenge arising in using these in the field of biotechnology. Therefore, the future research must be oriented based on the outcomes arising in this field and whether or not these will be beneficial to human beings.

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Chapter 7

Graphene quantum dot application in water purification Mohammad Oves1, Mohammad Omaish Ansari2 and Iqbal M. I. Ismail3 1

Centre of Excellence in Environmental Studies, King Abdulaziz University, Jeddah, Saudi Arabia; Centre of Nanotechnology, King Abdulaziz University, Jeddah, Saudi Arabia; 3Department of Chemistry, King Abdulaziz University, Jeddah, Saudi Arabia 2

7.1 Introduction Providing safe drinking water and enough freshwater resources is becoming increasingly problematic in many parts of the world’s population [1,26]. Despite several substantial challenges and constraints, using environmentally friendly nanomaterials with unique features including high precision and specificity, earth-abundance, reusability, and low-cost fabrication routes [2]. The first carbon nanomaterials used in wastewater treatment and purification were activated carbon, multiwalled carbon nanotubes, and single-walled carbon nanotubes [3]. Now, more advanced graphene and graphene oxide-based nanomaterials and graphene quantum dots-based nanomaterials have shown significant promise for wastewater treatment [4]. The graphene quantum dots (GQDs) have aroused much interest in science since they are zero-dimensional graphene derivatives. It has excellent optical and electrical properties because of its edge effects and strong quantum confinement. The GQDs have considerable benefits over typical semiconductor quantum dots (QDs) in terms of inexpensive, least toxic, high water solubility, persistent fluorescence, adjustable band-gap, and good biocompatibility, making them a genuine competitor. GQDs have a lot of potential in various fields, including medical diagnosis, sensors, catalysis, bioimaging, energy storage, and optoelectronics [5]. Scientists have synthesized GQDs using various carbon-based resources, including graphite flakes and other nanotubes, graphene and carbon fiber, and coal and other materials. Different synthesis techniques like hydrothermal, electrochemical, sonochemical, solvothermal, laser ablation, microwave cutting, etc., have been widely adopted Graphene Quantum Dots. https://doi.org/10.1016/B978-0-323-85721-5.00012-1 Copyright © 2023 Elsevier Ltd. All rights reserved.

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in recent years [6]. Microwave treatment is the most advantageous of these methods because it heats uniformly and quickly, resulting in a terse reaction time. Therefore, an approach to synthesizing high yields of GQDs from lowcost precursors that are simple, fast, appropriate, and ecofriendly. Wang et al. synthesized white light-emitting GQDs using a two-step microwave-assisted hydrothermal approach from corrosive acids within a 14-h reaction time [7]. Toxic and corrosive chemicals (H2SO4, KMnO4) were used in large quantities in the process, but the yield was good. In the main principle of the photocatalytic operation, electron-hole pair formation is driven by light with a particular frequency and energy more significant than the band-gap of semiconductors [8]. Recombining or assorting the produced charge carriers can lead to hydroxyl and superoxide radicals, respectively, in the valence and conduction bands. The electron has a higher þ0.5e1.5 V reductive potential than a hydrogen electrode. A typical hydrogen electrode has an oxidative potential of about þ1.0 to þ3 V, while holes have a much higher value. In the presence of reactive oxidation species, pollutants can be converted to harmless byproducts [9]. Electrons reduce oxygen in the conduction band to generate a superoxide anion, whereas holes oxidize water in the valence band to create hydroxyl radicals [10]. Many contaminants and biomolecules can be oxidized by either the superoxide anion or the hydroxyl radical, two oxidation species that are incredibly reactive themselves. Disinfectants and deodorants can be used to clean the air and water. Contaminants deteriorate over time as reactive oxidation species continually attack them. During photocatalysis, most of the organic water contaminants were oxidized by free radicals or reactive oxidation species (OH,, O2,, and H2O2). Cadmium is the primary component of conventional QD, and this contributes to cytotoxicity by allowing cadmium ions to seep out [11]. As a result, scientists create QD derived from cadmium, such as carbon QD, GQDs, and silicon QD [12e14]. Carbon-coated nanostructures have the advantage of improving charge carrier mobility because the carbonaceous material acts as an electron sink/acceptor [4]. Carbon framework codoping through N & S also persuades surface imperfections that intensify electron delocalization [15]. For example, a metal-free carbon-based photocatalytic system is famous for water treatments. Carbohydrates are primarily carbon, ranking second in the periodic table only to oxygen. Carbon QD are getting a lot of attention because they are used as photocatalytic nanomaterials, which use the most abundant carbon.

7.2 The worldwide water crisis The World Water Development Report (WWDR) from the United Nations (UN) provides insight into current safe water supply developments and plans [16]. Water protection, described as a population’s ability to maintain longterm access to sufficient quantities of water of appropriate quality, is already

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in jeopardy for many, and the problem is only going to get worse in the coming decades [16]. In today’s world of 7.7 billion inhabitants, clean water shortage is a big concern. By 2050, the global population will have risen by 22%e34% to 9.4e10.2 billion people, burdening the water supply [17]. Uneven population increase in various regions, which is unrelated to local capital, would exacerbate the burden. The majority of this population boom will occur in developing countries, first in Africa and then in Asia, where clean water shortage is already a significant problem. As the world’s population and economy accelerate, safe, adequate freshwater demand increases. Nevertheless, the global water system has been seriously strained, particularly in developing countries, due to extreme climate situations (high temperatures and droughts) and contamination of pure water supplies. Water demand has increased by 1% annually since 2000, conferring to the UN World Water Development unit [18]. This could be due to economic growth, population growth, and changing water use habits. This upward industrial and residential water use trend is predictable to duplicate for the next 2 decades, outpacing agricultural needs [18]. Up to 26 African countries, home to half of the continent’s population, are expected to experience water stress or shortage by 2025. Water shortages will affect nearly 3.6 billion people annually, and that number is expected to rise to 5.7 billion by 2050 [16]. The UN Water Agency (2018) Most water still goes to agricultural operations, which is vital if we have enough food to feed the world. Because of this, a steady source of freshwater is needed to ensure food safety [19]. Currently, approx. 3.6 billion people, somewhat less than half of the world’s population (47%), live in places where water shortage occurs at least once a year. According to the percentage, up to 52% of the world’s population, or w4.0 billion inhabitants, face water crisis problems [20]. By 2050, more than half of the world’s population (57%) will live in areas where water shortage occurs at least 1 month a year. Many geopolitical variables are challenging to forecast regarding water production, water supplies, and water quality. The deterioration of water supply and water quality was only briefly explored, maybe even more challenging to regulate [21].

7.2.1 Source of water pollution and impact on life The global population is increasing significantly, and parallel the demand for quality life leads to industrialization and other anthropogenic activities, and this development generates high pollution and climate changes [22]. The heavy industrialization and megacities municipality activities release a tremendous amount of wastewater that directly and indirectly pollute natural resources because wastewater from the pharmaceutical, pesticide, and fertilizer industries contains a high amount of potentially toxic compounds responsible for freshwater eutrophication [23]. The mining industries also release wastewater with a high load of heavy metals, textiles, and tanning

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industries known for the colored effluent discharge, which is harmful to plant and animal life because most of the dyes are used in the coloring of fabric and leather are carcinogenic. Mining and metal purification processing released a considerable amount of heavy metal-rich wastewater; most heavy metals are harmful to life. Pharmaceutical industrialization is also booming because drug abuse increased in middle-income countries due to pandemic coronavirus [24]. Due to the high consumption of drugs, most of them are released through the excreta into the wastewater. Most antibiotics and medicines belong to a specific group of pollutants because they disrupt the natural function of an aqueous ecosystem and sustain a long time in the water. Some lifestyle-based contaminants are emerging in wastewater which quickly skips from the existing water treatment plants and reaches our tap water, where they cause severe health problems to human health [25]. Nowadays, most agricultural lands use high pesticides and artificial fertilizers during irrigation or high rain. These pollutants leach out and contaminate water bodies like lakes and rivers. Instead, these pollutants accumulated in crop plant parts and directly reached the food web. When these organic and inorganic pollutants are in different cellular environment becomes transformed into various forms, they may be more toxic or less toxic to the surrounding environment. So, urgent need to develop a specific wastewater treatment protocol and management to ensure decontamination of pure water and maintain wastewater discharge quality with a limited amount of pollutants. Recently, a developed nanomaterials-based water purification system has been more efficient in treating and purifying wastewater. This chapter provides extended details about the graphene and GQD application and photocatalytic activity in wastewater treatment.

7.3 Graphene quantum dot (GQD) The GQD is the small block of nanocarbon and emerging as a primary structure of graphene [26]. Carbon chain arranged honeycomb-like construction, and each carbon attached with sp2 bond with zero-dimensional (0D) and 10 nm in lateral dimensions [27]. GQD is a single and less than 10 layers honeycomb structure is typically known as GQD [28]. The optical properties of graphene QD are derived from both quantum dots and graphene. Still, carbon dots (CD) are different from the GQD in lateral dimensions, always less than 10 nm and CD amorphous in nature, while GQD has crystalline properties [29]. For the surface, modification GQD contains oxygen as a functional group, making it more feasible for diverse applications because it possesses chemical, physical, thermal, and electronic properties [30]. However, the band-gap of graphene is 0, but the band-gap of GQDs is nonzero and may be altered by altering the size of the dots [31,32]. The electrical, catalytic, and photoluminescence characteristics of GQD in heteroatoms may be tweaked [33]. Bottom-up or top-down approaches can be used to make graphene quantum dots. Larger graphene sheets can be

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fragmented using the top-down approach to create GQDs, while chemical precursors may be used to fuse the smaller ones using the bottom-up method. A distinctive top-down synthesis method includes electrochemical exfoliation, liquid exfoliation, e-beam lithography, microwave cutting, ultrasonic shearing, and hydrothermal/solvothermal cutting. Intermolecular interaction, precursor pyrolysis, and opening fullerene cages are examples of bottom-up techniques. Bioimaging, photocatalysis, sensors, solar cells, tissue engineering, anticorrosion materials, and pollutant sorption are just a few examples of the many uses for these materials [34,35]. This chapter discusses the use of GQDderived nanomaterials in water management to assure the accessibility of clean and sufficient water for agricultural production and other critical industries. The capacity of nanostructures to remove contaminants will be demonstrated to be significantly impacted by GQDs.

7.3.1 GQDs application The GQD nanostructures have been widely used to eliminate different contaminants from polluted water [36]. For the past few years, numerous researchers have investigated the use of doped-green quantum dots and GQD-based nanocomposites in pollutant removal via various methods, including photocatalysis and photoelectrocatalysis [37,38]. All of the experiments found that adding GQDs to the nanocomposites significantly improved their capacity to remove pollution. For removing colors, emerging pollutants, and heavy metals, several GQD-derived nanocomposites have been employed. Inorganic, organic, and microbiological impurities may be removed from water via photocatalytic water treatment [39]. However, significant obstacles are charge carrier recombination, a small surface area, poorly visible light consumption, and water-based nano photocatalyst aggregation. Carbon nanomaterials such as graphene and graphene oxide may be combined with a wide range of semiconductors to increase the overall activity of the semiconductors. This strategy has been used to advance the semiconductor potential. The GQD seems to be the highly fascinating and valuable of all carbon nanomaterials because of their extraordinary adsorption and charge separation abilities as well as their ability to absorb visible light [40].

7.3.2 GQD for organic pollutants degradation The researcher uses GQD-derived nanostructures in photocatalytic dye degradation is one of the most intended uses widely [41]. Dye contamination is a major environmental problem in areas where dye pollution colors freshwater and poses a toxic threat to aquatic life. Studies have also shown that certain dyes can cause cancer and mutations in the human body, kidney, and skin irritation, and a wide range of allergies. The organic day such as rhodamine B can be degraded photocatalytically by the applied hydrogel immobilized with

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the GQD. In another study, Methylene blue dye was degraded by the N-doped GQDs (NGQDs) in visible light. Similarly, NGQDs combined with bismuth oxyhalides (BiOX, X ¼ Br, Cl) can be used for the RhB degradation. The RhB can be eliminated from the wastewater by adding GQD-TiO2 material synthesized by ultrasonic and hydrothermal synthesis methods [41]. Both materials were shown to be more active against RhB than pure TiO2 in general. Additionally, the hydrothermalprepared binary nanostructures had somewhat more significant activity than the materials generated by ultrasonic. Many of the flaws caused by ultrasound might serve as recombination centers for the charge carriers, explaining this observation. Using the binary nanostructures, this was accomplished in 135 min, which reduced RhB to almost zero in 30 min. The inclusion of GQDs boosted RhB adsorption via the interactions and changed the band-gap of TiO2, that enhanced visible light absorption and benefited by charge split-up [42]. Nonmetal (P, N, S, B) doped GQDs/g-C3N4 nanostructures were tailored using an ultrasonic exfoliation to the breakdown of RhB in visible light-assisted reaction. Oddly enough, the activity of g-C3N4 was not enhanced when combined with pure Gqds, Sqds, or Nqds, with B-Gqds having even less activity than bulk g-C3N4. Insufficient visible light absorption and poorer charge separation were blamed for this. However, combining g-C3N4 with PGQDs improved RhB degradation significantly, with a degradation rate 17 times faster than bulk g-C3N4 in just 40 min [44]. Under visible light, RhB showed improved photocatalytic activity over PGQDs/g-C3N4 similarly. When compared to pristine g-C3N4, the composite had more significant action due to better visible light consumption and the development of close p-n heterojunction that facilitated effective charge splitup [44]. Other studies have shown enhanced photocatalytic degradation of MO and MB and RhB GQDs, which improved optical characteristics and charge separation efficiency, may be to blame for the increasing inactivity [46]. Recent studies have shown that nonmetal doped and pure GQDs exhibit photocatalytic capabilities, pointing to possible uses for metal-free photocatalysts in dye degradation [47]. The basic fuchsin (BF) >80% was removed in 120 min over S-GQDs, considerably >18% higher elimination reported over only GQDs. That’s because S-GQDs and BF both use more visible light and have similar energy levels [47]. Another study found that after 100 min of sunlight exposure, clean GQDs could only remove 45% of the MB. The photocatalyst remained stable after three cycles despite the poor removal efficiency and showed a slight activity decline [48]. Even if activity levels are down, the sun’s fantastic stability and capacity to harness its energy may make up for it. An in-depth examination of the photodegradation findings, in conjunction with the response of optical analysis, most prevalent reactive species identification, and the band structure of the catalyst, offers crucial information for understanding the charge transfer process. As the photocatalyst complexity increases, so does the charge transfer route. Because of

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this, ternary semiconductor nanocomposites degrade more slowly than binary nanostructures. While in the conduction band of BiVO4, for example, electrons inhabited the region upon visible light stimulation, parting holes in the adjacent valence region. Subsequently, the electrons were transported to NGQDs, someplace O2 trapped them to produce superoxide radicals, although the holes interacted by H2O and generated OH radicals. Dye molecules degraded due to the presence of both radical species. The RhB breakdown through the BiOBr/ NGQDs and BiOCl/NGQDs has also been proposed to be similar to the dye sensitization method. By the dye sensitization approaches, in visible light, RhB molecules could absorb and generate an excited state that might introduce einto the conduction band of BiOX’s. Conspicuously, even though BiOCl is not light-sensitive, this methodology was postulated as the sole cause of its activity [49]. Instead, the conduction band of GQDs/ZnO, in this case, consisted of GQD and ZnO conduction bands with energies of 3 and 4.09 eV in opposition to vacuum energy, respectively, and valence bands with powers of 6.62 and 7.39 eV in opposition to vacuum energy. The charge transfer route of the photocatalyst must be thoroughly understood when constructing nanomaterials with extraordinary charge separation efficiency, suitable optical characteristics, and corresponding band potentials. Improved photodegradation achievement may be the result of this development. Instead of dye pollution, micropollutants (MPs), and emerging pollutants (EPs) are the main classes of organic pollutants that have gotten considerable interest due to the absence of regulatory oversight and the unknown fate of these new and emerging pollutants in the environment (MPs). There are both recently discovered and long-established pollutants in this category and the chemicals’ metabolites. The personal care and detergents EPs, hormonal, antiviral, fire retardant, and narcotics are also included in the list of possible EPs. Drug-resistant bacteria have emerged due to antibiotics in the environment, posing a significant danger to health care. These microorganisms have been related to cancer- and mutation-causing properties. GQD-derived nanostructures have been utilized to study the photocatalytic breakdown of EPs in water. The solar light promoted the research of 4-nitrophenol (4-NP), ciprofloxacin (CIP), and diethyl phthalate (DEP) because the researcher found that all emerging pollutants degraded simultaneously. At the same time, hydrogen was generated with the treatment of GQDs/MneN TiO2/g-C3N4 using a cocatalyzer of Pt [50]. A fascinating side note to this study is the possibility of solving pollution prevention and fuel creation simultaneously. g-C3N4 (S/GQDs/TCN-0.4) was shown to be the most effective photocatalyst for pollution elimination and H2 evolution. To our surprise, the pollutant solution exhibited a greater rate of H2 generation than clean water, showing that the two phases are interconnected. According to DFT calculations and GCeMS analyses, electrons were implicated in the breakdown of 4-NP, but not CIP or DEP. As a result, decreased H2 evolution in the 4-NP solution was observed [50]. According to the researchers

120 Graphene Quantum Dots

working with Ag/NGQDs/g-C3N4 ternary nanostructures, the light source affected the antibiotic tetracycline (TC) breakdown. For the same circumstances, ternary nanomaterials eliminated five times the TC amount as g-C3N4 did. The TC removal productivities were 92.8% under full-spectrum illumination and 32.3% under NIR illumination. NGQDs and Ag cannot aggregate due to g-C3N4 acting as a scaffold for their anchoring, which prevents them from doing so. Another work combined hydrothermal and physical mixing to produce an NGQD and nano cubic TiO2 sunlight-responsive photocatalyst. Bisphenol A (BPA) was degraded entirely in 30 min using a photocatalyst comprising 0.5 weight percent NGQDs. When exposed to visible light, BPA degradation was also detected in ternary nanocomposites composed of BiOCl/BiVO4/NGQDs. Charge separation in the ternary nanostructure was enhanced above that in the binary and sole semiconductors due to the many heterojunctions generated therein. When it comes to radical scavenging, both holes and hydroxyl radicals were found to be active in the GQDs/MneNeTiO2- g-C3N4 investigations. This information, combined with the band structure and optical response of the photocatalyst, led to the hypothesis that, when exposed to solar light, electrons in both g-C3N4 and MneNeTiO2 were advanced to their conduction bands while holes remained in their valence bands. The GQDs could also absorb and emit illumination with different wavelengths, such as light with an 800-nm absorption wavelength created by MnNeTiO2 and light with a 500-nm emission wavelength generated by g-C3N4. g-C3N4 will then move its electrons to the MneNeTiO2 conduction band, although the holes will go in reverse. The Pt nanoparticles deposited on the surface that caught the electrons reduced 4-NP and generated H2. Hydroxide ions formed hydrogen peroxide in the holes of g-C3N4 and MneNeTiO2, whereas hydrogen peroxide itself was oxidized in the pits of MneNeTiO2. Degradation byproducts might potentially develop in the perforations, targeting organic contaminants directly [50]. The internal Z-scheme mechanism was proposed to explain the charge transfer pathway in BiOCl/ BiVO4/NGQDs. The conduction bands of NGQDs and BiVO4 were filled with electrons when the visible light irradiated numerous heterojunctions between the three semiconductors. However, electrons from BiOCl’s valence band could transfer to BiVO4 and recombine with holes from BiVO4’s valence band, while electrons from BiVO4 can transferal to the NGQDs’ valence band. When using the Z-scheme approach to create these radicals, the electrons and holes on NGQDs were preserved in their highly reductive conduction band and their highly oxidative state on BiOCl. Due to the invasiveness of these radical species, the emerging pollutant degraded. Materials that can remediate a wide range of contaminants are required because wastewater is complex [51]. The development of materials with multiple functions may also compensate for the expense of synthesis/precursors. Many materials may be used to remove dye molecules and other impurities throughout the development process. RhB, TC, and BPA were all degraded

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photocatalytically using NS-GQDs, and (BiO)2CO3 (NS-GQDs/(BiO)2CO3), with elimination efficiencies that were greater than those of pure (BiO)2CO3. The fact that the photocatalyst was activated by both light and shade was an important finding. The biomimetic action of N,S-GQDs, which might break down H2O2 into hydroxyl radicals in a way similar to peroxidase, was attributed to dark activity [45,52]. To show that black TiO2/N,S-GQDs are adaptable, many dyes were degraded using sunlight aided sunlight and EPs, such as phenol, aniline, nitrobenzene, and dimethyl ophthalate these dyes included RhB, MO, Flu, MB, and Bpb. Researchers also looked at using the nanocomposite in sewage treatment. The photocatalyst exhibited excellent durability, retaining 95% of its original effectiveness after 30 cycles of light exposure in addition to a more significant removal percentage (k > 0.68 min1) for all contaminants. Photocatalysts show great promise for wastewater treatment because they can remove nearly 85% of the total organic compounds (TOC) in just 30 min [53]. Because dyes (colored substances) and EPs coexist in natural water, the degradation experiments were conducted separately, despite the impressive results of both pollutants’ degradation. As a result, studying the degradation process in solutions containing multiple pollutants will provide a more accurate picture of how each pollutant interacts with the others. However, the future seems bright in utilizing GQD-based structures to eliminate organic contaminants from water photocatalytic. Table 7.1 summarizes various studies on photocatalytic degradation of organic pollutants by applying GQD-based nanocomposites.

7.3.3 Microbial and heavy metal load reduction by graphene quantum dot Disinfection is critical in water treatment since it ensures that the water is free of microorganisms. Bacteria, fungi, algae, and viruses might enter the drinking water supply system if it is not disinfected, resulting in waterborne diseases and epidemics [54]. Drug-resistant bacteria have emerged due to the widespread use and disposal of antibiotics in the environment, creating a severe threat to public health. To prevent the spread of such superbugs in water needs to specific disinfectants [55,56]. As an alternative or supplement to chlorine disinfection, photocatalytic water disinfection presents the possibility of creating potentially harmful disinfection byproducts. Photocatalytic disinfection can be used in addition to chlorination to destroy bacteria and disinfection byproducts [57]. According to recent research, the antibacterial properties of the ternary nanostructure made of TiO2/Sb2S3/GQDs against Escherichia coli and staphylococcus aureus using a solvothermal synthesizing technique in a visible light environment [58,59]. When GQDs were combined with TiO2 and Sb2S3, the inhibitory concentration for S. aureus and E. coli was significantly reduced (GQDs/TiO2 and GQDs/Sb2S3) [59]. Charge separation efficiency and greater use of visible light were cited as reasons for the rise. In addition, the

GQDs or GQDsnanocomposite

Synthesis rout

Precursor material

Rate of dye degradation

GQDs

Hydrothermal

Graphene oxide

GQDs

Hydrothermal

GQDs

Dye degradation

References

In sunlight driven photocatalysts degrade upto 45% of MB

Photodegradation of MB dye

[74]

Graphite powder

827.5 mg g1 highest adsorption capacity

Adsorption of MB

[70]

Microwaveassisted hydrothermal route

Citric acid

Upto 150 mg

Adsorption of pesticide compound oxamyl

[75]

S-doped GQDs

Hydrothermal

1,3,6Trinitropyrene and Na2S

81% after 2 h

Photodegradation of fuschin dye

[47]

GQDs/AgVO3

Hydrothermal

1,3,6Trinitropyrene

90% after 120 min

Degradation of Ibuprofen

[76]

QDs@ZnO-NRs and GQDs@ZnO-NFs

Pyrolysis

Citric acid

80% after 180 min

Photodegradation of BB dye A

[77,78]

NGQDs/BiOCl and

Hydrothermal

Citric acid

Nearly 100% in 75 min

Photodegradation of RhB

[79]

NGQDs/BiOBr

Hydrothermal

Citric acid

Nearly 100% in 60 min

Photodegradation of RhB

[80]

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TABLE 7.1 GQDs-nanocomposites synthesis methods and use in dye and pollutant degradation.

Citric acid

365 nm for 92.8%; 420 nm for 90.1%; and760 nm for 31.3%; removal under irradiation light

Tetracycline (TC)

[81]

NGQDs-BiVO4

Hydrothermal

Citric acid

90% after 200 min irradiation

Degradation of MB

[82]

Bi2S3-GQDs/TiO2

Pyrolysis

Citric acid

Under irradiation 52%e92%

Removal of Cr (VI) and methyl orange

[83]

S,N:GQDs-(BiO)2CO3

Hydrothermal

Citric acid

GQDs create ROS production and degrade pollutant in light

Degradation of Rhb, tetracycline and BPA

[84]

GQDs-mpg-C3N4

Hydrothermal

Pyrene

97% removed

Degradation of RhB and tetracycline

[85]

GQDs þ SDS-(dodecyl sulfates)-LDHs (layered double hydroxides)

Hydrothermal

Citric acid

Adsorption capacity of 80%

Adsorption of 2,4, 6- trichlorophenol

[68]

GQDs/TiO2

Pyrolysis

Citric acid

100% of the RhB within 30 min

Photodegradation of Rhb

[43]

Ti3þ-TiO2/GQDs NSs

Hydrothermal

Citric acid

Enhanced photocatalytic efficiency

RhB and MB dye

[86]

ZnO-GQD

Hydrothermal

1,3,6Trinitropyrene

95% of both MB and CZ within 70 min

MB and a colourless pollutant; carbendazim (CZ)

[87]

GQDs/MneNeTiO2/g- C3N4

Pyrolysis

Citric acid

The photodegradation rate of p- nitrophenol was the highest; the H2 evolution rates in solutions system

f p- nitrophenol, diethyl phthalate and ciprofloxacin and fabrication of H2

[88]

123

Pyrolysis

Graphene quantum dot application in water purification Chapter | 7

Ag/N-GQDs/g-C3N4

124 Graphene Quantum Dots

ternary nanostructure of TiO2/Sb2S3/GQDs recorded the minor minimum inhibitory concentration for both bacterial strains when equated to the standard TiO2, GQDs/TiO2, and GQDs/Sb2S3. E. coli had a minimum inhibitory concentration of 0.03, whereas S. aureus had a minimum inhibitory concentration of 0.1. A possible explanation is that GQD and Sb2S3 on TiO2 operate in concert to promote split-up charge and the generation of free radicals or reactive species that inhibit bacterial development. As the irradiation duration increased, the amount of bacterial growth decreased until there was almost no growth left after 24 h [59]. The antibacterial effectiveness of GQDs in combination with methylene blue (MB) was studied by Kholikov et al. during light irradiation [60]. The greatest singlet oxygen production was when the GQDs:MB ratio was 1:1 when combined GQDs and MB. The inclusion of GQDs did not affect cell viability in low-light conditions, suggesting that MBGQDs might be used in photodynamic therapy. Both M. luteus and E. coli were almost disinfected in about 5 min when exposed to 660 nm light, as well. However, the disinfection time for M. luteus was substantially less than that for E. coli. In contrast to the M. luteus as a Gram-positive, E. coli as Gramnegative strains have an additional complicated and intense cell wall, which offers more defense. An antibacterial method developed from a combination of GQDs could disrupt bacteria’s cell membrane, allowing up passageways for MB to enter and create free radicals of oxygen that transformed protein synthesis and Gene [60]. The variation in bacterial cells deactivation rates between Gramþve and Grameve bacteria appear to be more complex than just changes in the cell envelope structure. The photocatalyst’s surface charge concerning the bacterial cell may be crucial in the interactions between bacteria and photocatalyst. In comparison to Gramþve bacteria, the repulsive interface with Grameve bacteria on a negatively charged photocatalyst surface might lead to reduced inactivation kinetics. Heavy metal pollution is a significant obstacle to safe drinking water and fertile agricultural land. Heavy metals, including Cr(VI), Pb(II), and Hg(II), among several others, have been found in variable amounts in the milieu; most of the metals are frequently the result of industrial activity [61]. The heavy metals have been identified as carcinogenic, mutagenic, embryotoxic, teratogenic, and reproductive system toxicity; some metals are linked to heart disease, liver impairment, and various health complications [61e63]. Furthermore, heavy metals can be delivered to crops by contaminated water irrigation and then reach people by consuming such goods. Because of the process’s nonselective nature and capacity to convert dangerous metal ions to harmless metal species, catalytic heavy metal removal is a viable approach. Under visible light illumination, Cr(VI) photocatalytic reduction was investigated over Bi2S3/GQDs/ TiO2 nano assembly as nanowires with metal oxide. Intriguingly, there was a more significant drop in Cr(VI) in the presence of MO (97%) than in the absence of MO (92%) percent.

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Meanwhile, when Cr(VI) was present, MO removal was about 52%, compared to 43% when Cr(VI) was absent (VI). According to this idea, photoinduced electrons were efficiently captured by Cr(VI), which was then reduced to Cr(III), freeing up the holes for oxidation processes that resulted in MO’s MO degradation. An electron collector, such as Cr, made it much easier just to get rid of MO (VI). GQDs to the nanocomposite improved the visible light sensitivity and charge split-up potential [64]. The photocatalytic (PC), electrocatalytic (EC), and photoelectrocatalytic (PEC) ability of Fe2O3-GQDs/ NFeTiO2 nanocomposite for Cr(VI) reducing and EDTA disintegration in aqueous solutions. PEC activity was substantially higher in the ternary nanostructure than in the binary. Furthermore, the PEC process surpassed the EC and PC procedures in terms of efficiency. PEC activity had an apparent rate constant of 7.67 times greater than PC and EC activity. Cr(VI) and EDTA were eliminated instantaneously during the PEC experiment, with Cr(IV) reduction enhanced from 40% to 91% in 80 min. FeeO3-GQDs/NFeTiO2 are adequate for the PEC decomposition of pollutants with heavy metals in wastewater. The GQDs work primarily as electron accelerators between Fe2O3 and NFeTiO2 [65]. In industrial wastewater containing these pollutant species, the power of collaboration offers an attractive possibility to treat organic pollutants and heavy metals simultaneously time.

7.3.4 Membrane filter based on graphene quantum dot The adsorption phenomenon of materials can remove a wide range of organicinorganic and microbiological contaminants [66]. Activated charcoal has long been used in industry as an adsorbent during water treatment. Different adsorbents based on graphene-related materials demonstrating outstanding absorption capabilities have also been investigated. GQD-based adsorbents in recent times appeared as possible adsorbents among these graphene materials due to their high surface area, cost-effectiveness, biocompatibility, and abundance of functional groups attachment sites [67]. Because GQDs are greatly soluble in aqueous solutions, more research into their ability to adsorb contaminants in the aqueous environment is crucial before using them. Yao and coworkers exploited the coinciding intercalation of citrate and sodium dodecyl sulfate (SDS) in the number of layers of the layer with double hydroxides to create GQDs in two-dimensional (2D) hydrophobic space. The adsorbent (GQDs þ SDS)-LDH was used to extract 2,4,6-trichlorophenol (2,4,6-TCP) [68]. To put this in perspective, the nanocomposite removed 80% of the 2,4,5-TCP, which was 65% greater than elimination using GQDsLDHs and 40% higher than removal using SDS-LDH. Pseudosecond-order dynamics were employed to model adsorption. The results matched the Langmuir model’s equilibrium adsorption capacity of 119.00 mg g1. The intercalation technique addresses both GQD aggregation and water solubility [67]. Furthermore, the best extraction results were obtained with 150 mg of

126 Graphene Quantum Dots

nano sorbent. The extraction of PAHs from real water samples spiked with known analyte quantities using GQDs/eggshell nano sorbent was investigated further to ensure the extraction procedure’s efficiency. The procedure’s extraction efficiency ranged from 92.4% to 112.84%. This demonstrated that the adsorbent might be employed in the solid-phase extraction of PAHs. Because of their adsorption properties, GQDs and eggshells functioned well together, resulting in high extraction yields [69]. In previous research, GQDs have been used as adsorbents for MB, RhB, and oxamyl, their water solubility makes it difficult to separate and recover the GQD in its pristine condition [70]. Centrifugation was often employed to remove the sorbent from the treated solution. Pure GQD, on the other hand, does not provide an appealing alternative as an adsorbent when compared to GQD-based nanocomposites because it is difficult to recover and recycle. Recent research on modified membranes of GQD has revealed that these nanocarbons play an essential role in the performance of the membrane. Interfacial polymerization was used to make polyethyleneimine augmented with GQDs in the presence of trimethyl chloride linked to polyacrylonitrile ultrafiltration backings. PEI chains were covalently bonded to GQDs with an average diameter of 2.19 nm and a thickness of one to three layers [71]. When tested under ideal conditions, the membrane of mixed matrix possesses a neutral surface, strong hydrophilicity, high water flow, and suitable antifouling (0.05 wt% GQDs). In addition, the membrane rejected salt (MgCl2) at a rate of 96.80.4%, showing that it can desalinate. In addition, the membrane could be used in wastewater treatment [72]. TMC and GQDs were polymerized in situ in the pores of ultrafiltration (UF) membranes, then thermally processed to form nanofiltration membranes in other research locations. The membrane chemical stability, porosity, and water flow were all influenced by the addition of GQDs [73]. Compared to the nanofilter membrane, Na2SO4 demonstrated improved water flow and rejection properties toward OGII, MBII, AB, and CR.

7.4 Conclusion Threats to water and food security come from organic, inorganic, and microbiological species polluting the water. Water pollution prevention may be made more accessible with graphene quantum dot-derived nanostructures. Organic pollutants like dyes and emerging contaminants may be removed catalytically using these materials, and they can be adsorbed, filtered, and disinfected with them. Nanocomposites modified by the addition of GQDs have increased removal efficiency for various contaminants. GQD incorporation levels must be carefully monitored and optimized to positively impact the pollutant-removal efficiency of multiple nanocomposites. Yet more effort must be made to ensure the design and use of GQD-based nanostructures for large-scale applications. The numerous functional groups, nontoxicity, and

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biodegradability of GQD-derived nanostructures make them promising as pollution remediation agents. To achieve uniform GQDs in terms of chemical stability, size, and surface functions, it is necessary to establish optimal synthesis conditions and develop synthesis methods that enable the appropriate scattering of the GQDs inside the nanocomposite matrix. As a result, there may be more uniformity in the claimed water pollution abatement effectiveness of diverse GQD-derived nanostructures.

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Chapter 8

Graphene-based organicinorganic hybrid quantum dots for organic pollutants treatment Asif Saud1, 3, Mohammad Oves2, Mohammad Shahadat1, Mohd Arshad4, Rohana Adnan1 and Mohammad Amir Qureshi5 1

School of Chemical Sciences, Universiti Sains Malaysia, Penang, Malaysia; 2Centre of Excellence in Environmental Studies, King Abdulaziz University, Jeddah, Saudi Arabia; 3 Department of Chemistry, Aligarh Muslim University, Aligarh, Uttar Pradesh, India; 4 Department of Physics, Aligarh Muslim University, Aligarh, Uttar Pradesh, India; 5Department of Chemistry, Faculty of Natural Science, Jamia Millia Islamia, New Delhi, India

8.1 Introduction Water pollution caused by pollutants, including dyes, polyaromatic hydrocarbons, hormones, major organic waste, and pesticides. Most of the pollutants are highly nonbiodegradable having toxic nature that can potentially be transformed into carcinogenic, teratogenic, and become hazardous, and also can increase the risk of chronic diseases [1e4]. Thus, releasing pollutants containing water into natural water resources is severely affected the life of aquatic animals. On the other hand, the consumption of contaminated water and aquatic creatures by human beings has become another serious cause of diseases and ecosystem disturbance. Therefore, attention has been paid to treating wastewater containing organic and inorganic waste before releasing it into water systems. Nanotechnology’s advent and application in wastewater treatment has been a benefit, and many metal nanoparticles (NPs) have been employed to decolorize harmful effluent dyes [5,6]. Alternatively, semiconducting metal oxides have a large enough bandgap to accelerate photochemical breakdown, the majority of them are carcinogenic [7]. Nano-metal oxides are widely known for their inability to dissolve in water or disperse properly in any solvent. As a result, after multiple photocatalytic cycles, a considerable number of these metal nanoparticles are called exhausted leftovers. Although these metal NPs are efficient photocatalysts, their fatal effect on living beings should not be Graphene Quantum Dots. https://doi.org/10.1016/B978-0-323-85721-5.00005-4 Copyright © 2023 Elsevier Ltd. All rights reserved.

133

134 Graphene Quantum Dots

overlooked [8]. These metal nanoparticles are either created with a lot of heat, or a more precise method that deposit chemical with the help of vapor deposition technique, or their starting materials are quite expensive [9]. GQDs as a supplement emerges as low-cost, easily manufactured materials that reduce organic pollutants effectively. Some organic pollutants namely methyl blue, bisphenol-A methyl orange, parachlorophenol, tetracycline, 4-nitrophenol, triclocarban [10], and carbendazim (CZ) [11] have been degraded using GQDs. GQDs are a new type of nanocarbon that has appeared in the field of nanomaterials. GQDs are graphene fragments that are smaller in size. A GQD is defined as a zero dimension, sp2 bonded carbon atom array, well-arranged hexagonal structure with sideward dimensions less than 10 nm, similar to graphene [12e14]. GQDs exhibit diverse, unique physical, chemical, and electrical properties. Chemical Stability and heat resistance, a great specific surface area, solubility, minimal cell damage, instinctive functional groups that allow for facile modifications are just a few of the advantages. Moreover, GQD’s bandgap can be altered by varying the dimension of the dots, among other variables as it has a nonzero band gap [13,15e18]. GQDs have been prepared by using different types of materials such as graphite flakes [19], carbon nanotubes [20], graphene [21], carbon fiber [22], coal [23], and others [24,25] have all been employed to synthesize GQDs to far [26]. A bottom-up and a top-down technique are commonly used to create graphene quantum dots [27]. GQDs are created by fragmenting bulk graphene material and respectively. Lithographic patterning techniques, electrochemical exfoliation, ultrasonic shearing, microwave-influenced cutting, Liquid exfoliation, hydrothermal cutting are all commonly used top-down synthesis techniques [16,28,29]. Intermolecular coupling, revere-micelle route, precursor pyrolysis, sol-gel synthesis, cage opening of fullerenes, colloidal precipitation are examples of bottom-up methods [28,29]. GQDs show various applications due to their exceptional features including photocatalysis, sensors, solar cells, adsorption of pollutants, lithium-ion batteries, and organic synthesis [30]. An outline of the top-down and bottom-up approaches is shown in Fig. 8.1 [31]. The present chapter consists of the synthesis and application of GQD-derived nanostructures for dye and organic waste treatment in wastewater.

8.2 Synthesis of quantum dots (GQDs) 8.2.1 Synthesis of (GQDs) using pyrocatechol The GQD were prepared by using precure; pyrocatechol, or 1,2dihydroxybenzene, having a molecular formula C₆H₄(OH)₂ of the three benzenediol isomers (it is the ortho isomer). For the preparation of GQDs, the suspension of pyrocatechol was heated in the temperature range 100e105
C for 45 min. Heat treatment turned the ash-gray powder into a pale dark syrupy liquid. The molten liquid was held for an hour at room temperature to the

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135

FIGURE 8.1 An outline of the top-down and bottom-up approaches.

decreased temperature of up to 60
C to continue heating. Before moving on to the next step, the syrupy liquid had to be brought to room temperature. To modify and get multiple independent pH values of prepared GQD solution, prepared a 0.5 M standard sodium hydroxide (NaOH) solution was prepared and introduce dropwise over the molten brown color solution [32].

8.2.2 Graphene quantum dot using citric acid coated with iron codoped TiO2 The synthesis of GQDs decorated with iron codoped TiO2 was carried out using citric acid; 2 g citric acid (as a precursor) was pyrolyzed on a mantle heating at 200
C for 10 min in a typical synthetic manner. Dong et al. established [33], that the appearance of the solution was pale yellow and product formation was confirmed by observing the color change which appears to be orange, indicating desired GQDs synthesis. Under vigorous stirring, the liquid state GQDs were neutralized by CH3CH2OH [34]. The prepared GQD solution was modified using a modified sol-gel technique. For the preparation of GQD-Fe-TiO2 composite photocatalysts, the solution of titanium tetraisopropoxide (37 mL) was mixed with 60 mL pure ethanol. Moreover, solution 2 was prepared by mixing a fixed quantity of GQD solution, Fe (iron) precursor, acetic acid (15 mL), DI (deionized) water, and ethanol (20 mL). Then, solution 2 was added to solution 1 with vigorous stirring until gel formation at room temperature (25  2
C). Before being dried in an oven, the gel was aged for 24 h between 25 and 28
C. The obtained material was then calcined at 300
C [35,36]. The dried material was kept in a desiccator for future use.

136 Graphene Quantum Dots

8.2.3 Preparation of graphene quantum dots (GQDs) using spent tea Pyrolysis accompanied by microwave thermochemical cutting produces lowordered array of carbon from spent tea [26]. Waste tea is rinsed with distilled water (DW), dried for 12e13 h at 80.5
C, and then compressed into a fine powder (more than 89 mm), by the controlled rate of heating 10
C min1, the powder then pyrolyzed at 500
C in a passive environment. The pyrolysis reaction was carried out for 3 h, yielding a rich carbon biochar precursor. After the pyrolysis reaction of precursor, the prepared solution gets mixed with 0.1 M HCl after rinsing with DI water and the solution gets exposed to boiling for the reduction of contaminants. The sample was rinsed again with deionized water before being dried in a 60
C to yield the final product, following that, around 20 mg of the C-rich sample as prepared was kept in a reactor containing 11 mL DI water, for oxidative cutting of carbon spheres around 1e2.5 mL of nitric acid was supplemented for lowering the pH. The reactor was then microwave-assisted for 15e180 min under reflux at a power of 100e900 W, to discrete the bigger unreacted particles, the resultant browncolored solution was thinned with 100 mL DI water and filtered through 0.1 um PVDF filtering films after the preadjusted duration was completed. The pale-yellow appearance confirmed the formation of GQDs in the filtrate. The acidic character of the solution necessitated the purification of the synthesized GQDs. To make the pH of GQDs solution (w7), NaOH solution was used and the neutral solution get dialyzed for 24 h, then freeze-dried to give dry GQDs in order to achieve the pure GQDs solution.

8.2.4 Synthesis of graphene quantum dots by using ground coffee Using a hydrothermal method, GQDs were prepared by using spent coffee grounds of coffee beans [37]. For the GQDs preparation, the powdered form of beans were obtained by crushing them in the grinder and then converted to hot coffee using a coffee machine. Used coffee grinds were cleaned and dried in an oven at 80
C. After that 0.1 g coffee was ground, and 1 mL H6N2O was dissolved in 10 mL water in an ultrasonic bath for 30 min in a traditional process. After that, the solution was transferred to a stainless autoclave lined with Teflon. The sealed autoclave was heated to 150e200
C in an electric oven and then let to cool for 6e10 h. The water-soluble GQD-containing product was filtered through a micro dimension membrane to remove insoluble carbon product and dialyzed for 2 days to remove unfused small molecules after cooling to ambient temperature. GQDs were purified and dried at 80
C.

8.2.5 Synthesis of rice husk derived GQDs The fixed content of rice husks (5.0 g) was washed and cleaned in DI water before being ground to powders [38]. After that, the powders were processed for 2 h in a tube furnace at 700
C in a N2 environment. Then, 1.95 g of rise

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husk ash was obtained, which contains both carbon and silica. The rice husk ash was then treated with a high concentrated NaOH solution for 2 hat 900
C in a protected environment. Rice husk ash was transformed to rice husk carbon and sodium silicate throughout this process. The residual dark black sample, after being properly washed with DI water and vacuum filtered, it was dried at 80
C for 12 h. Moving on, a 50 mg rice husk carbon sample was combined with 10 mL conc. H2SO4 and 3 mL DI water, for the next 5 h the dark colored dispersion was ultrasonically cleaned. After that, nitric acid (69%e70%) was gradually added (measured volume 20 mL), and the dispersed mixture was filtered by applying vacuum through a filtrate and thoroughly cleaned after another 10 h of ultrasonication. To make the pH (w8) 1 M aqueous NaOH solution along with prepared black colored sample and 30 mL of DI water were mixed. The prepared mixture was then ready to move into Teflon-lined autoclave and the temperature should be maintained at (200
C) for about 10 h. The resultant dispersion was filtered using the microporous membrane after cooling to room temperature, yielding the desired GQD in the filtrate.

8.2.6 Synthesis of lignin-based graphene quantum dots [39] GQDs have also been prepared using lignin by using 2 g of o-aminobenzenesulfonic acid and to achieve a consistent solution the temperature was raised to 80
C by mixing it with 38 g of DI water. Then, while stirring at 80
C for 20 min, 2 g of alkali lignin was manually added to the mixture. The dispersion was vacuum filtered through the filter membrane as soon as possible. After air cooling to ambient temperature, the filtrate was vacuum filtered to form a new filtrate with lignin nanoparticles. To neutralize the residual o-aminobenzenesulfonic acid, sodium hydroxide (0.5 g) was ultrasonically added to the obtained filtrate residue for next the 2 h. The mixture was placed in a Teflon-lined autoclaved for 12 h at 200
C. The generated graphene quantum dots were put into the process of dialyzation with the help of DI water for continuous 72 h, before this the filtering process was performed with the appropriate pore size filtrate and lowering the temperature to 25e27
C. GQDs based on lignin were made in solid form by lowering the temperature to the extent of freezing (16
C) and this step requires about 12 h, proceeding with the step the obtained samples were then freeze-drying by keeping the temperature at 50
C and pressure about 20 Pa.

8.2.7 Synthesis of N, S codoped commercial TiO2/GQDs [40] In this method, carbon assembly and thiourea were dissolved in 8 mL dimethylformamide at varying molar ratios and agitated to make a clear solution [41]. The solution was then added to and agitated to obtain a homogeneous suspension of 100 mg P25 (20% rutile and 80% anatase). The suspension was then placed in a stainless-steel autoclave with Teflon lining for additional

138 Graphene Quantum Dots

processing. At 180
C, the solvothermal treatment lasted for 8 h. After allowing the products to cool to ambient temperature, they were separated using ethanol and centrifugation at 9000 rpm for 15 min.

8.2.8 Development of CdS/GQDs using g-C3N4 nanosheet To prepare CdS/GQDs nanocomposite, fluctuating quantity of g-C3N4 (GCN) nanosheet and an established/measured quantity (133 mg) of Cd(Ac)2H2O were mixed with known volume (50 mL) of Dimethyl Sulfoxide solvent under the continuous dynamic condition [10]. The generated suspension was then put into the Teflon-lined stainless-steel autoclave and kept on heating for 12 h at a constant temperature of about 180
C. The obtained sample was then treated several times with water and CH3CH2OH solutions to make it contaminantfree, after cleaning up the complete sample it gets dried with the help of vacuum ovum, by making changes in the weight proportion of CdS/GQD, photocatalyst samples with varying molar percentages can be created.

8.2.9 Synthesis of metal free N dopped carbon quantum dots The nitrogen doped CQDs were obtained by putting a 25% ammonia hydroxide with a 10% glucose water solution. Aqueous glucose/NH4OH (5:1) solutions were exposed to high a temperature environment (heating) for 60 s by keeping the temperature constant at 100
C [42]. The reaction was microwave-assisted with the two variable values of power (200 and 100 W). The obtained solution was colourless and transparent in nature, but after the MW heating was performed the color of the solution will appear as light brown indicating the formation of NGQDs. The prepared solution was in an aqueous form, to get the desired QD (N-GQD) in the dry form the solution gets cooled under the controlled temperature to make it around 27
C, proceeding with the dialyzation step (300 Da) which was performed for the next 120 h, at last, the sample was obtained by filtering it through the manual filtration process keeping the pore size in the range of (450e10 nm).

8.2.10 Synthesis of GQDs using graphene oxide (GO) A straightforward method was employed used to prepare GQDs by using the modified Hummers’ method. To obtain a brown-colored graphene oxide (GO) solution, GO was dispersed in DI water and then sonicated [43]. The assorted solution was then preserved at 60
C for 24 h after H5IO6 was introduced to the GO dispersion. After centrifugation the precipitate out, it was rinsed with deionized water until to get a clear solution. After that, sodium polystyrene sulfonate was used to sonicate the graphene oxide nanosheet solution for 2 h. Finally, the temperature was raised to 50
C for the agitating purpose with the addition of L-ascorbic acid to the prepared solution. As soon as the appearance

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139

of the solution will change to dark black after the reaction has been completed, suggesting that the Graphene oxide nanosheets had been efficiently reduced into GQDs of diverse sizes (5e15 nm) (Table 8.1).

8.3 Application for the removal of organic pollutants GQDs can be used to degrade a wide range of organic pollutants and dyes. such as Methyl Orange, Methylene blue, 4-NP, p-chlorophenol, bisphenol-A, tetracycline, triclocarban [10], CZ [11], and reactive black 5 (RB5) dye, with the help of different types of GDQs which can be monitored by the change in color/absorbance of the solution. UV irradiation was used to test organic dyes degradation such as MO and MB with GQDseZnO nanocomposites [55]. The nanostructures are thought to attain impressive photocatalytic activity because of the high surface-to-volume ratio [57]. GQDseZnO nanocomposites show variable morphology for different GQD concentrations, and the nanostructure production of nanocomposites was established by the dominant count frequency of unit size in the 80e100 nm range. The (unit) particle size distribution of these ZnO-GQDs nanocomposites with different concentrations of GQDs are shown in Fig. 8.2 [55]. The absorbance of MO and MB at different time intervals of irradiation was observed After UV irradiation, both organic dyes show a drop in absorbance, indicating that dye molecules are degrading. The lower absorbance could indicate that GQDseZnO nanocomposites could be used as photocatalytic materials. The purpose of the study was to look into the degradation efficiency (DE) of organic waste [55] Another observation for the degradation was, after the process of decomposition had been going on for a while, in 2 h absorbance spectra for MB and MO dyes showed enhancement in dye degradation to 52% and 79.4%, respectively. The rate constant (k) for MO and MB was found to be 0.00519 min1 as well as 0.01133 min1 and so forth [32,58]. The degradation of MO by TiO2 NTAs can be estimated by observing the absorption and degradation of MO under UV-vis radiation. The high photocatalytic activity of composite is due to the broad absorption in the visible wavelength region, significant photoinduced charge separation through the transfer of photogenerated electrons from TiO2 NTAs to GQDs, and the high adsorption capacity of GQDs toward MB molecules. At varied UV exposure times, optical absorption profiles were obtained (365 nm, 6 W). GQDs infilled TiO2 annealed samples disassembled 99.8% of MB in 180 min, which could be owing to the tight bandgap, low charge carrier recombination rate, stability, and quick electron transport (Fig. 8.3) Another type of dye is Reactive Black 5 (RB5) dye. RB5 is a synthetic reactive dye that is widely used in the dyeing business. It is water-soluble and has reactive groups that can establish a covalent bond between the dye and fiber. RB5 is discharged into the environment in a variety of ways, causing severe ecological issues [59]. A considerable reduction of RB5 can be noticed

TABLE 8.1 Preparation of QDS using different chemical routes.

N-CQDs GQD

Raw materials Citric acid Citric acid

GQD GQD-DMA

Graphite powder

Reaction condition

Methods

References




Hydrothermal

[44]




160 C for 4 h

Hydrothermal

[45]

15e180 min

Oxidative cutting & microwave heating

[3,26]

120
C for 24 h

Microwave irradiation.

[30,46]

200 C for 5 h




GQD/CdSe

Citric acid

180 C for 24 h

Hydrothermal

[29,47]

GQD

Natural graphene powder

600 W for 1 h

Microwave irradiation

[48]

GQD

Graphene

12 h

Ultrasonic

[49]

GQD

Glucose

MW heating at 700 W for 11 min

Precursor pyrolysis

[50]

Amino functionalized GQDs

GO, ammonia

150 C for 5 h

Acidic oxidation

[50]

NGQDs/TiO2

Citric acid, urea,

180
C for 8 h

Hydrothermal and ultrasonic dispersion

[51]

GQD

Citric acid and alkali hydroxide

Electrochemical exfoliation

[52]

CQDs/Bi2O2CO3

Bi(NO3)3$5H2O, HNO3, citric acid

Centrifugation with CH3CH2OH and distilled wate subsequently dries at 80
C for 8 h

Dynamic-adsorption precipitation

[53]

Multi layered graphene quantum dot

Graphene oxide

200 ⁰C for 4 h

Hydrothermal method

[54]

GQDseZnO

Citric acid, Zn(NO3)2.6H2O

180 for 6 h

Hydrothermal

[55]

NGQDs

Citric acid

Hydrothermal

[56]

140 Graphene Quantum Dots

Type of QDs

Graphene-based organic-inorganic hybrid quantum dots Chapter | 8

141

FIGURE 8.2 Particle size distribution of GQDseZnO nanocomposites for different GQD concentrations of: (a) 0.0 M, (b) 0.1 M, (c) 0.2 M, (d) 0.5 M, and (e) 1.0 M.

142 Graphene Quantum Dots

FIGURE 8.3 Photodegradation of MB using GQDs. Adopted with permission from Kalkan E, Nadaro glu H, Celebi N, Tozsin G. Removal of textile dye Reactive Black 5 from aqueous solution by adsorption on laccase-modified silica fume. Desalin Water Treat 2014;52:6122e34. https://doi. org/10.1080/19443994.2013.811114.

using GQDs and Fe-codoped TiO2. The degradation of the RB5 dye was aided by the holes and OH radicals. Additionally, both GQDs and Fe enhanced TiO2’s photon energy harvesting capability. GQD-0.1FeeTiO2-300 was shown to be more robust and energy-efficient than undoped photocatalysts after four cycles. Photolysis of a 4-chlorophenol solution in the presence of ZnPc-(NH2) GQDs-PS-membrane (adjourned in solution) can be detected [60] during various irradiation times. 4-chlorophenol is characterized by the presence of two absorption peaks [61]. Within 1 min, noticeable degradation was detected, which is a desired attribute to the material because the window for catalytic degradation in actual membrane applications (continuous process system) will be partial. The kinetic restrictions of the experiment caused by membrane confinement in the catalytic system should be minimized by irradiating for short periods compared to reported intervals. On the other hand, when compared to pure TiO2, synthetic composite dots (N-GQDs) achieve full elimination of bisphenol-A (BPA) after 30 min of sunshine irradiation [56]. Degradation of glyphosate using GQDseZnO nanoparticles [55] was observed, they found that when compared to pure ZnO, GQDs degrade glyphosate very well. Photocatalytic reaction is responsible for the absorbance of glyphosate sample in the presence of sunlight, and it can evaluated on that basis of the photocatalytic reaction. Both pure ZnO and composite of GQDseZnO showed a slight decrease in absorbance peak around 435 nm, representing the photo degradation property of GQDs. To assess photocatalytic activity, the DE (Degradation efficiency) was considered from the peak shown during the absorption. After 179 min of photocatalytic processing, DEs for

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143

ZnO and GQDseZnO nanocomposites were determined to be 12.07% and 14.05%, respectively [62]. Degradation of New Fuchsin (NF) [(H2N(CH3)C6H3)3C]Cl with GQD are as follows [63]; NF dye solution at a fixed amount was formed and treated ultrasonically for 10 min. The pH of the solution was changed to the desired value by adding NaOH or HCl solution. By taking 10 mL of GQDs which is supplemented to a quartz cell holding solution of NF dye, further the amount of NF degradation was examined. UV-Vis measurements at a predetermined time interval (ʎmax ¼ 553 nm) were used to track the change in NF concentration. The Co  Ct/Co  100 was used to compute the percentage of degradation (D %), where Co and Ct were the dye concentrations at 0 and t, respectively. In the presence of visible light, GQDs shows the photocatalytic activity and this step results into the degradation of pollutant as shown in Fig. 8.4. Data was collected for NF dye degradation in the presence of 10 mL photocatalyst. These observations were taken at various concentrations of the dye solution (2.0e40.0 mg L1). In the presence of GQD, NF (20.0 mg L1) decolorizes as seen in Fig. 8.4a at various contact time intervals. Ibrahim et al., [64] shows the different degradation data model of dye rhodamine B (RhB) (for 6 h) with

FIGURE 8.4 (a) Show the decrease in the dye’s corresponding absorption spectra in the presence of photocatalyst, (b) Change in appearance before and after the introduction of GQDs [63].

144 Graphene Quantum Dots

three different types of graphene quantum dots GQD-1 (oxygen content 37.0%), GDQ-2 (nitrogen content 15.0%), GDQ-3 (oxygen 25.4% and nitrogen 10.8%) for the three forms of synthesized GQD in the dark and under CFL irradiation, indicating considerable activity variations under equivalent experimental conditions (Table 8.2). GQD-2 (55% degradation in 6 h), GQD-1 (25% degradation in 6 h), and GQD-3 (No degradation observed). The degradation of RhB can be investigated with the decrement in the concentration and the reason behind this was either absorption of RhB dye molecules on the surface of photocatalyst (GQD3) or the photocatalysis of the dye molecule. A small amount of dye (8% avg.) were absorbed on the surface of three type photocatalyst (GQD-1,2,3) under the no photonic condition and this was also observed in other similarly charged dyeeGQD systems due to the interaction (electrostatic) between the positively charged dye surface and negatively charged GQDs surface. Indeed, there were no significant variations in RhB degraded under dark and light irradiation conditions for the GQD-3 [65,66]. The large negative charge of GQD-2 (56.0 mV, about double that of the other two GQDs) is responsible for its increased photocatalytic activity, the photodegradation of RhB with GQD-2 during 6 h is shown in Fig. 8.5 [24,64]. Nitrogen doped graphene quantum dots NGDQ/TiO2 composites degrade RhB [51] solutions and the degradation was compared with different NGQDs indicated as green and yellow (gNGQD/TiO2 and yNGQD/TiO2) and also with TiO2. After 2 h of irradiation (>420 nm), the degradation capability of the composites was examined, and it was found that the max % of degradation was observed with y-NGQDs/TiO2 i.e., 94% while on the other hand the minimum degradation was shown by pure TiO2 is 10% only [51]. 17% and 37% of degradation of RhB molecules was shown by GQDs/TiO2 and g-NGQDs/TiO2 photocatalysts respectively (Fig. 8.6).

8.4 Proposed mechanisms Different mechanisms of the interaction of dye pollutants on the surface of the GQDs have been proposed as described below:

8.4.1 Photocatalytic activity of ZnO-GQD In the presence of sunlight, both ZnO & GQD by UV radiation and visible incoming photons-are photoexcited respectively, resulting in the formation of electronehole pairs [11]. Photoexcited electrons pass from the more negative conduction band of GQD to the less negative conduction band of ZnO due to the high interfacial contact between the two materials. Meanwhile, the generated holes accelerated from ZnO to GQD, triggering induced electronehole separation across the heterojunction effectively [72]. The highly oxidizing species (*OH radicals) were produced by the breakdown of H2O2

Percentage of degradation (%)

Time of degradation

References

100

12 min

[53]

89

1h

[54]

10

99

4h

[67]

5

81

2h

[68]

e

73

30 min

[69]

100

80

80 min

[70]

95

1h

[71]

4

1h

[71]

14.05

3h

[55]

25

6h

[64]

Concentration (mg L1)

GQDs types

Pollutant

O, N, S-GQD

Rhodamine B

O-GQD

Methyl blue

917

O, N-GQD

Methyl blue

O, S-GQD

Basic fuchsin

GQDs/Bi2WO6

Nitric oxide

GQD- maltose

Imipramine

GQDs/ZnPor

Methyl blue

GQDs

Methyl blue

GQDseZnO

Glyphosate

17.8

GQDs with 37% oxygen

Rhodamine B

10

4.8

17.8 e

Graphene-based organic-inorganic hybrid quantum dots Chapter | 8

TABLE 8.2 Graphene quantum dot photocatalytic efficiency with different organic pollutants.

145

146 Graphene Quantum Dots

FIGURE 8.5 Photodegradation of RhB (C0 ¼ 10 mg L1) with GQD-2 (C ¼ 100 mg L1) during 6 h.

FIGURE 8.6 Photocatalytic degradation rate of Rhodamine B-Hyaluronate solution [51].

molecules in the presence of light energy and electrons generated in the process [55,58]. Breaking of pollutants (MB/carbendazim) into CO2 and H2O is caused by these hydroxide radicals. The hþ generates from Valance Band of GQD degraded pollutants directly. As a result, the generation of *OH and hþ as active species on the photocatalyst surface oxidizes organic contaminants, resulting in improved photocatalytic performance. Furthermore, by harvesting solar energy, higher solar spectrum utilization from UV to visible, quick

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charge transfer across heterojunction, and high specific surface area of GQD lead to strong photocatalytic activity [11]. The proposed mechanism of the charge carrier transitions in ZnO-GQD toward photocatalytic pollutant degradation under natural sunlight irradiation is shown in Fig. 8.7.

8.4.2 Degradation of MO and MB Fig. 8.8 shows the photocatalytic activity of GQDs in the presence of MO and MB. The following processes could have occurred during the photocatalytic degradation of organic pigments: The e jumped into the conductive band (CB) from the excited state of GQD, and as a result, radicals from water and O2 were produced, and dyes were oxidized through the formed active radicals.

FIGURE 8.7 Photocatalytic activity of ZnO-GQD.

FIGURE 8.8 Mechanism of the photocatalysis undergoing on the surface of the GQDs [32].

148 Graphene Quantum Dots

8.4.3 Degradation of New Fuchsin dye [63] After irradiation of visible light, generation of e/hþ pair takes place. The electron that generates due to the irradiation of visible light can then rapidly reduce (O2) near the edge of photocatalyst and the formation of superoxide radicals (O2 ) and hydrogen peroxide radicals (lOOH) takes place, whereas the hþ can either directly degrade the dye pollutants which were present on the surface of the photocatalyst or circuitously degrade through formed hydroxyl radicals (lOH) during the process. GQDs, for starters, have a large surface area that allows them to effectively adsorb NF molecules. The movement of an electron from an excited state to conducting band of NF absorbed dye was assisted by the presence of visible light during the process of degradation. As a result, photoinduced active species (free radicals) would have an easier time oxidizing or decomposing the organic pollutants absorbed on the surface. The oxygen radicals are then dissociated as NFþl reacts quickly with them (Fig. 8.9). l

8.4.4 Photodegradation of dye rhodamine-B RhB catalyzed by GQD Irradiation of light will direct the electron to get jumped into the accommodative VB (Valance ban), this particular step will generate e and hþ pair and in this scenario electron and hole accelerated toward the active site present on GQD. GQDs show excellent donor and acceptor capability of electron and hole respectively and this character of GQDs will definitely open up the way for some of the charge carriers to move toward the RhB molecule and gives way to the process of degradation. The carriers, on the other hand, might react with H2O and dissolved O2 to form the radicals lOH and lO2, respectively (Fig. 8.10).

FIGURE 8.9 The hypothesized method for NF degradation under visible light irradiation by (a) photocatalytic and (b) photosensitization routes is depicted schematically. Adopted with permission from Roushani M, Mavaei M, Rajabi HR. Graphene quantum dots as novel and green nanomaterials for the visible-light-driven photocatalytic degradation of cationic dye. J Mol Catal Chem 2015;409:102e9. https://doi.org/10.1016/j.molcata.2015.08.011.

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FIGURE 8.10 Proposed photocatalytic degradation of RhB using GQD. Adapted with permission from Ibarbia A, Grande HJ, Ruiz V. On the factors behind the photocatalytic activity of graphene quantum dots for organic dye degradation. Part Part Syst Char 2020;37:1e9. https://doi.org/10. 1002/ppsc.202000061.

8.4.5 Pathway proposed for catalytic oxidative degradation of amines on dimethylamino functionalized graphene dot (GQDDMA) The mechanism behind the degradation is shown in Fig. 8.11 [46], based on the observations and earlier reports [73e76]. GQD-DMA has sufficient oxidation potential and can easily oxidize benzylamine in the given condition.

FIGURE 8.11 Pathway representation followed by GQD-DMA photocatalyst for coupling reaction. “s” and “t” represents singlet and triplet state respectively.

150 Graphene Quantum Dots

The electrons get separated from the hole and the mixing of these pairs (e/hþ) pair occurs with the benzylamine and surrounding oxygen as the sample gets exposed to the light. As soon as the formed radical cation reacts with oxygen species, the formation of benzyliminium and H2O2 takes place. After removing the ammonia, benzyliminium can be added to another benzylamine to produce N-benzyl-1-phenylmethanediamine, which is the final linked product.

8.5 Conclusions and prospects GQDs due to the unique properties gained attention in the field of biosensing, supercapacitors, sensors, light-emitting diodes, bioimaging, nanocarriers for gene transport, optoelectronic devices, electrochemical application, medical diagnosis, and energy storage devices but the excellent property of GQDs, i.e., photocatalytic activity have been somehow ignored, we have tried to enlighten the emergence of GQDs as an effective organic waste degradant as they have unique photocatalytic properties, in this article the different methods are shown to synthesize GQDs and explain the mechanism behind stimulation of e to the conduction band, resulting in electronehole pairs and proposed many hypothesized route for showing photocatalytic activity of GQDs. Because of their simple manufacturing method and distinctive photocatalytic properties, GQDs are a promising contender in the field of waste treatment.

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154 Graphene Quantum Dots [56] Lim PF, Leong KH, Sim LC, Abd Aziz A, Saravanan P. Amalgamation of N-graphene quantum dots with nanocubic like TiO2: an insight study of sunlight sensitive photocatalysis. Environ Sci Pollut Res 2019;26:3455e64. https://doi.org/10.1007/s11356-018-3821-1. [57] Wongrat E, Ponhan W, Choopun S. Room temperature ethanol sensing properties of FET sensors based on ZnO nanostructures. Ceram Int 2017;43:S520e4. https://doi.org/10.1016/ j.ceramint.2017.05.268. [58] Gupta BK, Kedawat G, Agrawal Y, Kumar P, Dwivedi J, Dhawan SK. A novel strategy to enhance ultraviolet light driven photocatalysis from graphene quantum dots infilled TiO2 nanotube arrays. RSC Adv 2015;5:10623e31. https://doi.org/10.1039/c4ra14039g. [59] Kalkan E, Nadaroglu H, Celebi N, Tozsin G. Removal of textile dye Reactive Black 5 from aqueous solution by adsorption on laccase-modified silica fume. Desalination Water Treat 2014;52:6122e34. https://doi.org/10.1080/19443994.2013.811114. [60] Mafukidze DM, Nyokong T. Graphene quantum dot-phthalocyanine polystyrene conjugate embedded in asymmetric polymer membranes for photocatalytic oxidation of 4-chlorophenol. J Coord Chem 2017;70:3598e618. https://doi.org/10.1080/00958972.2017.1400664. [61] Zugle R, Nyokong T. Comparative phototransformation of environmental pollutants using metallophthalocyanines supported on electrospun polymer fibers. J Appl Polym Sci 2013;128:1131e42. https://doi.org/10.1002/app.38381. [62] Sarkar S, Das R. PVP capped silver nanocubes assisted removal of glyphosate from water - a photoluminescence study. J Hazard Mater 2017. https://doi.org/10.1016/j.jhazmat.2017.06.014. [63] Roushani M, Mavaei M, Rajabi HR. Graphene quantum dots as novel and green nanomaterials for the visible-light-driven photocatalytic degradation of cationic dye. J Mol Catal Chem 2015;409:102e9. https://doi.org/10.1016/j.molcata.2015.08.011. [64] Ibarbia A, Grande HJ, Ruiz V. On the factors behind the photocatalytic activity of graphene quantum dots for organic dye degradation. Part Part Syst Char 2020;37:1e9. https://doi.org/ 10.1002/ppsc.202000061. [65] Bijimol D, Punnoose MS, Korah BK, Mathew B. Optimization of the properties of Mn doped ZnS quantum dots capped with thiourea. AIP Conf Proc 2020;2269. https://doi.org/ 10.1063/5.0019540. [66] Krawczyk P, Gurzeda B, Bachar A. Thermal exfoliation of electrochemically obtained graphitic materials. Appl Surf Sci 2019;481:466e72. https://doi.org/10.1016/j.apsusc.2019.03.154. [67] Mashali F, Languri EM, Davidson J, Kerns D, Johnson W, Nawaz K, Cunningham G. Thermo-physical properties of diamond nanofluids: a review. Int J Heat Mass Tran 2019;129:1123e35. https://doi.org/10.1016/j.ijheatmasstransfer.2018.10.033. [68] Kang S, Jeong YK, Jung KH, Son Y, Kim WR, Ryu JH, Kim KM. One-step synthesis of sulfur-incorporated graphene quantum dots using pulsed laser ablation for enhancing optical properties. Opt Express 2020;28:21659. https://doi.org/10.1364/oe.398124. [69] Cui Y, Wang T, Liu J, Hu L, Nie Q, Tan Z, Yu H. Enhanced solar photocatalytic degradation of nitric oxide using graphene quantum dots/bismuth tungstate composite catalysts. Chem Eng J 2021;420:129595. https://doi.org/10.1016/j.cej.2021.129595. [70] Hatefi R, Mashinchian-Moradi A, Younesi H, Nojavan S. Graphene quantum dots based on maltose as a high yield photocatalyst for efficient photodegradation of imipramine in wastewater samples. J Environ Heal Sci Eng 2020;18:1531e40. https://doi.org/10.1007/ s40201-020-00569-7. [71] Lu Q, Zhang Y, Liu S. Graphene quantum dots enhanced photocatalytic activity of zinc porphyrin toward the degradation of methylene blue under visible-light irradiation. J Mater Chem A 2015;3:8552e8. https://doi.org/10.1039/c5ta00525f.

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Chapter 9

Graphene quantum dots for heavy metal detection and removal Sufia ul Haque1, Mohammad Faisal Umar2, Ogechukwu Bose Chukwuma2 and Mohd Rafatullah2 1

Advance Functional Material Laboratory, Department of Applied Chemistry, Zakir Husain College of Engineering and Technology, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh, Uttar Pradesh, India; 2School of Industrial Technology, Universiti Sains Malaysia, Penang, Malaysia

9.1 Introduction 9.1.1 Background In 1974, Norio Taniguchi coined the term nanotechnology, which has drawn the attention of researchers to a new area of knowledge and application [1,2]. The plethora of materials can be synthesized and designed with optimized properties for various applications by playing with the dimensions of the material at atomic scale [3,4]. The most suitable example is nanomaterials of carbon. The discovery of nanomaterials of carbon like graphene [5], carbon nanotubes (CNTs) [6], and fullerene [7] had a massive influence in science and technology development, which resulted in Nobel Prize. That unquestionably shows the interest of scientists in nanostructures of carbon and their applications in many other fields of application [8]. In 2004, another example of nanomaterial of carbon i.e., luminescent carbon nanoparticle with diameter ( GQDs@3N > GQDs@1N > GQDs@2N > GQDs). This result was validated by the noncovalent interaction plots and quantum theory of atoms in molecules analysis. However, several research groups were investigated the interaction of graphene oxide quantum dots (GOQDs) with heavy metals form a specific point of view (fluorescence quenching effect) [79e83]. It was observed that the intensity of intrinsic blue/green fluorescent emission of GOQDs gradually decreased with the increase in heavy metal ions (Hg2þ, Fe3þ, Cu2þ, Pb2þ) concentration. Chelation of metal ions by various functional moieties of GOQDs might be the reason for this effect, as it causes reduction in fluorescence emission rate owing to the appearance of nonradiative processes. Further, research based on the coating of GQDs on quartz sand was carried out for removing Hg2þ and Pb2þ in an aqueous solution [84]. The maximum adsorption capacity reported was 24.65 and 24.92 mg g1 for Hg2þ and Pb2þ, respectively. That showed better adsorption capacity of the GQDs coated on quartz sand compared to the quartz sand alone. The experimental outcomes showed the adsorption of Hg2þand Pb2þ on the GQDs coated on quartz sand pursued pseudo-second order kinetic model whereas the experimental data at equilibrium followed Langmuir isotherm (R2 > 0.99). It was also observed that the heavy metal removal performance of the GQDs coated on quartz sand relied on the GQDs particle size. This adsorbent can be used to treat contaminated water at industrial scale due to its low cost, effectiveness, and stability. Research work focused on the interaction between heavy metals (Cd, Pb, and Hg) and GQDs was carried out. The binding energy and height of heavy metal (neutral and charged) ions on GQDs was determined and the findings

168 Graphene Quantum Dots

showed adsorption energy of donors like physisorbed neutral Pb atoms was greater than that of Cd and Hg. Nevertheless, the charged heavy metal species acted as typical acceptors with the study, revealing how replacing a carbon atom with a heavy metal adatom modifies the geometric structure of GQDs and the changes in their vibrational and electronic properties. This research work suggested the route toward optically detecting toxic heavy metals using GQD-based sensing. Ivan et al. [85] reported the artificial vacancy-type defects created in GQDs which were utilized for the reaction of the active sites with Cd, Hg, and Pb. The researchers employed density functional theory and time-dependent density functional theory methods to predict the effect of the vacancy-based complexes to elaborate the ability of GQDs to bind to heavy metals. Monovacancy and trivacancy defects were very active in reacting with heavy metals in contrast to the even-numbered vacancy and defect-free GQDs. Meanwhile the interaction between GQDs and Pb is controlled by the charge transfer, which showed that the Pb atoms could bind more strongly close to the vacancy defects than the other heavy metals (Cd and Hg). In 2013, the detection of Hg2þ.for the first time by employing GQDs was reported. They synthesized the GQDs by carbonizing the citric acid and the solution was made in the alkaline solution. Various metal ions were tested with this solution, and Hg2þ.entirely quenched the emission. This quenching fluorescence property aroused because of the Hg2þ.adsorption on GQDs surface. The detection limit of Hg2þ.was detected to be 3360 nM [86]. Later, GQDs was prepared through ultrasonic method utilized by Hg2þ.to quench the fluorescence of GQDs [87]. As the transfer of electron occurred by the adding of Hg2þ, it caused nonradiative electron-hole annihilation. The synthesized GQDs augmented with carboxylate groups revealed high affinity for Hg2þ. The detection limit of Hg2þ was detected to be 100 nM under a linear range between 0.8 and 9 mM. In 2015, the investigation on the optical detection was further advanced by Li et al. [88]. The research group prepared the GQDs through the citric acid pyrolysis and utilized them to detect Hg2þ. The GQDs fluorescence was quenched by the Hg2þ through the mechanism of charge transfer. The limit of detection of Hg2þ.was detected to be 0.439 nM under a linear range (1e50 nM) concentration of Hg2þ. A recent research work using the electrochemiluminescence sensor effectively attained the lowest detection limit (¼0.00.248 nM) for Hg2þ. First, citric acid underwent pyrolysis to produce GQDs as a by-product. Then, an integration of DNA (single stranded) through the sulfhydryl and amino groups at each end of the gold nanoparticle and GQDs, respectively was carried out. The significant role of gold nanoparticle was to enhance the signal amplification by increasing the GQDs load. The sensor showed a wide linear range of 0.01e100 nM [89].The various research work done upon the Hg2þ detection by using different GQDs-based optical sensors is given in Table 9.1.

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169

As discussed earlier, exposure to Pb2þ albeit at minute concentration has harmful effects on the central nervous system, reproductive, and human health. One study reported the effects of Pb2þ.exposure to guinea pig, which showed the increment in heart beat frequency [106]. Qi et al. used GQDs for the first time for the detection of Pb2þ in 2013. This study used 3,9-dithia-6monoazaundecane functionalized GQDs and tryptophan for the fluorescent detection of Pb2þ. Electrostatic interaction of the Pb2þ produced a rigid structure. The carboxylate group of tryptophan and S atom on 3,9-dithia-6monoazaundecane functionalized GQDs surface, which resulted in the improvement of the fluorescent. This occurred by the energy-transfer interactions between 3,9-dithia-6-monoazaundecane functionalized GQDs and tryptophan. However, this investigation reported the lowest detection limit (0.009 nM) so far, with a linear range of 0.01e1 nM [107]. Dong et al. [108] developed a coreactant (GQDs and L-cysteine) electroluminescence system for the Pb2þ.sensing. According to this investigation, the electroluminescence signals were developed by dissolved oxygen, oxidation of L-cysteine, and the reduction of GQDs. The L-cysteine oxidation checked free radicals RSO , RSO2 , and RSO3 and a quenched the electroluminescence signal confirmed their formation. A linear dependence between the Pb2þ.concentration and the quenched ratio was attained in the range of o 0.10e10 mM with a limit of detection of 70 nM [108]. Later, graphene oxide and the GQDs-aptamer conjugate were utilized to detect Pb2þ. Graphene oxide acted as a quenching agent and electron acceptor, while GQDs acted as a fluorophore. The graphite powder was used to synthesize both graphene oxide and GQDs. Further, GQDs was reduced by using sodium borohydride to produce rGQDs. The GQDs was accumulated on the graphene oxide surface via electrostatic force and p-p stacking, that quenched the fluorescence. The complexation of rGQDs and Pb2þ caused the fluorescence with a linear relation between Pb2þconcentration and fluorescence intensity. The detection limit was evaluated to be 0.6 nM in a wide range of 9.9e435 nM [109]. Another research work utilized the combination of gold nanoparticles and GQDs via the pairing reaction between GQDs and gold nanoparticles both altered with DNAs. Thus, the fluorescence resonance energy transfer between gold nanoparticles and GQDs caused fluorescence quenched, which can be recovered by adding Pb2þ.The presence of Pb2þstimulates the catalytic activity to cut the DNAs linker in order to detach GQDs and gold nanoparticles, which detect the Pb2þdue to the fluorescence recovery. This fabricated sensor exhibited a detection limit (¼16.7 nM) with a wide linear range of Pb2þ (0.05e4 mM) [110]. Another investigation employed transferring fluorescence resonance energy in self-assembled multilayers to detect Pb2þ. The authors built the sensor by employing glutathione-functionalized GQDs as the energy donor, graphene oxide as energy acceptor, and poly(diallydimethylammonium) chloride and G5

170 Graphene Quantum Dots

(G-rich DNA) strand as the linker. Pb2þ being present in specific amounts, causes G5 DNA to fold into quadruplex, thus shortening the distance between graphene oxide and glutathione-functionalized GQDs, hence causing the enhancement in the energy transfer between graphene oxide and glutathionefunctionalized GQDs with the quenching of fluorescence to recognize Pb2þ.The value of detection limit was found to be 2.2 nM within broad range of concentration (2.4e11.5 nM) [111]. The GQDs doped by diethyl dithiocarbamate was synthesized by citric acid and diethyl dithiocarbamate pyrolysis. For the detection of Pb2þ., the doped GQDs was used as resonance light scattering probe. The intensity of resonance light scattering linearly increased, confirming the presence of Pb2þ with a concentration range of 4.83e48.3 nM. The limit of detection was found to be 3.86 nM [112].The selectivity of this sensor was enhanced by the addition of tartaric acid solution used to mask the Pb2þ. Table 9.2 mentioned below showed different combination of GQDs for the Pb2þ. ion detection. A hydrogel adsorbent was prepared by employing in situ polymerization of N-vinyl imidazole (VI), two different cross-linkers and appropriate quantity of nitrogen-doped GQDs [115]. This nitrogen-doped GQDs hydrogel was used to remove Cd2þ, Ni2þand Cr6þ ions form water. The adsorption of these heavy metal ions was analyzed at pH 1.0, 7.0, and 9.0 and optimum removal efficiencies of Cd2þ, Ni2þ, and Cr6þ ions were found to be 75%, 94.6%, and 70.9%, respectively at pH 7.0. The kinetics and adsorption isotherm were also analyzed to calculate the adsorption behavior of the nitrogen-doped GQDsbased hydrogel. The maximum adsorption capacity of nitrogen-doped GQDs hydrogel was found to be 5000, 5000, and 370 mg g1 for Cd2þ, Ni2þ, and Cr6þ ions, respectively. Another research work has reported a nanoadsorbent (magnetic) made of NiFe2O4/hydroxyapatite (HAP)/GQDs to remove Cd2þ ions from aqueous solutions [116]. The outcome of the study revealed that equilibrium time was 10 min for the adsorption of the Cd2þ ions onto the NiFe2O4/HAP/GQDs adsorbent. The removal of Cd2þ ions was studied at pH 6.0 with maximum absorption capacity of 344.83 mg g1 at 25 C. This study showed that the NiFe2O4/HAP/GQDs nanoadsorbent was effective for the removal of Cd2þ ions from aqueous solutions. The possible adsorption mechanism of Cd2þ ions on NiFe2O4/HAP/GQDs nanoadsorbent is shown in Fig. 9.5. For the removal of Hg2þ ions the synthesis of multifunctional nanocomposite (Fe3O4@SiO2@GQDs) was carried out [100]. Here, the GQDs were covalently loaded on the silica-coated magnetite nanospheres surface. This nanocomposite exhibited a strong fluorescence, which was quenched by Hg2þ ions. The GQDs in the nanocomposite have high specific surface area and enough binding sites, which resulted in a good adsorption capacity of 68 mg g1 for Hg2þ ions. This nanocomposite material was recycled with EDTA and repeatedly utilized for Hg2þ ions detection and removal from wastewater.

Optical method

Linear range

Detection limit (nM)

References

Pyrolysis

Resonance light scattering

4.83e48.3 nM

3.86

[112]

Anthracite coal

Electrochemical oxidation

FP

1e20 mM

750

[113]

Glutathionefunctionalized GQDs

Citric acid/ Glutathione

Pyrolysis

Fluorescence resonance energy transfer

2.4e11.5 nM

2.2

[111]

GQDs and gold nanoparticles

Purchased

e

Fluorescence resonance energy transfer

0.05e4 mM

16.7

[110]

S-doped GQDs

Pyrene/1,3,6trinitropyrene

Hydrothermal

FP

0.1e140.0 mM

30

[114]

rGQDs

Graphite powder

Oxidation/ reduction

Fluorescence

9.9e435 nM

0.6

[109]

GQDs/L-Cysteine

Carbon black

Chemical oxidation

Electroluminisence

100e1000 nM

70

[108]

GQDs- 3,9-dithia-6monoazaundecane

GO/3,9-dithia-6monoazaundecane

Hydrothermal

Fluorescent probe

0.01e1 nM

0.009

[107]

Variety of GQDs

Starting materials

Diethyl dithiocarbamatedoped GQDs

Citric acid/Diethyl ddithiocarbamate

N, P, S co-doped GQDs

Synthesis method

Graphene quantum dots for heavy metal detection and removal Chapter | 9

TABLE 9.2 The various GQDs-based sensors for the detection of Pb2þ.

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FIGURE 9.5 Cd2þ adsorption mechanism on NiFe2O4/hydroxyapatite/GQDs nanoadsorbent.

In a study, a simple and commercial nanosorbent made of GQDs coated on quartz sand were prepared for the removal of Hg2þ and Pb2þ [84]. This study showed the improved adsorption performance of the GQDs coated on quartz sand in comparison to the quartz sand, owing to the large surface area of the GQDs. The study revealed the influence of particle size of GQDs on the heavy metal removal efficiency. GQDs obtained from carbonization process for 30 min showed the highest removal percentage. The maximum adsorption capacity of this nanosorbent for Hg2þ and Pb2þ were evaluated to be 24.65 and 24.92 mg g1, respectively. This study provided a cheap, stable, and effective nanosorbent (GQDs coated on quartz sand), which could be utilized for industrial wastewater treatment. This study on the development of a Fo¨rster resonance energy transfere based optical sensing system for the detection of heavy metals. The sensing system is made of GQDs as donor and carbon nanodots as an acceptor. These optical sensor works when fluorescent carbon nanodots are in the range of Fo¨rster resonance energy-transfer distance, the donor GQDs is quenched by the transfer of energy to acceptor carbon dots. The Fo¨rster resonance energytransfer signals were reduced upon the addition of heavy metals like As5þ and Hg2þ. The mechanism of Fo¨rster resonance energy transfer on the molecular interactions between GQDs and C-dots are shown in Fig. 9.6. At present, heavy metal contamination has still been a critical issue. Beyond a certain limit, the presence of heavy metals adversely affects the

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FIGURE 9.6 Schematic diagram of (a) Fo¨rster resonance energy-transfer mechanism and (b) spectral overlap of Fo¨rster resonance energy-transfer pair. Adapted with permission Mohammad-Rezaei R, Jaymand M. Graphene quantum dots coated on quartz sand as efficient and low-cost adsorbent for removal of Hg2þ and Pb2þ from aqueous solutions. Environ Prog Sustain Energy 2019;38:S24eS31. https://doi.org/10.1002/ep.12911. Elsevier.

human health and environment. For this reason, an immediate solution is required to sort out this problem. An immediate detection of these heavy metals below their hazardous limit is required. Now, it is the immense need to develop a sensor with high selectivity and sensitivity to detect these heavy metals at very low concentration. That’s why GQDs have been used commonly to detect the heavy metal ions. The combination of surface plasmon resonance and GQDs and their hybrid materials has the potential to detect heavy metal ions frequently. The portable heavy metal ion detection can be developed by using various GQDs fluorescence technique allowing high specificity and sensitivity. However, quantum dots fluorescence based heavy metal detection is still in progress.

9.4 Conclusions This chapter explained the background, development of the synthesis methods, and various applications of the GQDs. Several methods for the fabrication of GQDs have been discussed with their various unique properties which can be utilized for many applications. Since graphene been discovered, finds limited application due to zero band gap. However, the development of GQDs has turned out to be an excellent way to utilize various applications of graphene. Unquestionably, vital research efforts have been carried out for the synthesis and employability of GQDs. As discussed in this chapter, the GQDs materials gain recognition as essential materials whose functions can be harnessed in

174 Graphene Quantum Dots

different areas such as the optical, medical, and energy fields. Though, the further investigations are mandatory for some of the applications of GQDs where the working mechanism is not precise. However, the utilization of GQDs in various applications grabbed colossal attention among researchers owing to its several outstanding benefits. This chapter has provided an outlook of synthesis and functionalization routes with various ways of the GQDs. Further, the identification and treatment of toxic transition metals (Pb2þand Hg2þ) by employing GQDs are explained with the reporting of detection limit with certain linear range of concentration.

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Chapter 10

Graphene quantum dots for clean energy solutions Waris1, Abdul Hakeem Anwer2 and Mohammad Zain Khan1 1 Industrial Chemistry Research Laboratory, Department of Chemistry, Faculty of Science, Aligarh Muslim University, Aligarh, Uttar Pradesh, India; 2School of Mechanical Engineering, Yeungnam University, Gyeongsan, Gyeongaan, Republic of Korea

10.1 Introduction One of the world’s biggest challenges is to meet the rising demand for renewable energy in an environmentally friendly and sustainable way, especially in developing countries with rapidly increasing populations and living standards. The main requirement is to include clean energy solutions in this respect [1]. Energy is a fundamental part of a developing country’s economy. The utilization of resources worldwide was approximately 520 quintillion BTUs (British Thermal Units) in 2010 and is expected to shoot up by 56% (820 quintillion BTU) in 2040. 78% of the world’s energy consumption is derived from natural resources such as coal, oil, and natural gas, while only 19% of existing energy comes from renewable sources [2,3]. As natural (nonrenewable) energy sources are being used up continuously, it is also harming our environment by petroleum products. Decreasing renewable energy sources and growing energy demand suggest that we should act honestly and decisively to establish completely clean, sustainable, progressive, and feasible sources of energy. A large number of scientific experts in the world are looking for a source of sustainable substitutions to fulfill the energy demand of the world. Renewable energy can be generated by different resources such as wind, solar, geothermal, waste energy, etc. The biggest worry may not be the exhausting petroleum products but the rising global pollution that deviates from the atmospheric balance which could affect humans and other living beings [4]. In this sense, the growth of renewable, sustainable and clean energy sources has become a subject of urgency [5]. According to the studies, energy consumption around the world has increased by 92% from 1975 to 2020. Unfortunately, only 10% of this consumption energy is being derived from renewable energy sources [6]. Graphene Quantum Dots. https://doi.org/10.1016/B978-0-323-85721-5.00004-2 Copyright © 2023 Elsevier Ltd. All rights reserved.

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Energy and the environment are now critical subjects around the world. The use of clean energies, including solar and wind energy, attracts growing attention. However, owing to the wind and solar energy fluctuations, energy storage, and conversion is of particular significance in the effective use of clean energy. Supercapacitors are considered promising ways to store renewable energy [7]. The solar cell directly converting a solar photon into electricity through its photoelectric effect or photochemical effect [5]. However, the performance of the available solar cells, supercapacitors, and batteries is unlikely to meet the demands for enormous storing energy. To address this problem, it is urgent to pursue new carbon-based materials with improved efficiency for energy storage and conversion devices [7]. Research toward sustainable, clean and environmentally friendly renewable resources to develop carbon-based materials used in energy storage and conversion devices has been prompted by the challenges of environmental pollution degradation [8]. The excellence of carbon materials such as quantum dots (QDs), carbon quantum dots (CQDs), and graphene quantum dots (GQDs) have gained significant interest in several potential fields, like broad surface area, controllable structure, conductivity, high durability and low toxicity, etc. [9e11]. These materials have a very promising and remarkable potential in the area of energy storage and conversion systems, and electrode materials [12]. The development of carbon-based nanomaterials with a new generation and forming a more sustainable energy materials industry can be seen as a potential precursor. Recently, biowaste-derived carbon materials have revealed potential applications in the area of energy storage and conversion. However, further work is needed to commercialize carbon materials derived from biomass for adequate performance, operation, and productivity [13]. Due to their chemical inertness and lower photobleaching and low toxicity, carbon-based QDs (such as GQDs) have several favorable conditions over noncarbon QDs. They can be formed from biomass for instance. Carbon-based QDs have been studied as supercapacitors, batteries, water splitting devices, LEDs, solar cells, biosensors, and catalysts in recent years and have also been mixed in optoelectronic devices with noncarbon dots [14]. The latest research progresses on the preparation and applications of GQDs are covered in this chapter. Acid etching [15], hydrothermal methods [16], ultrasonication [15], electrochemical exfoliation [17], microwave-assisted hydrothermal [18], and carbonization [15] were discussed. Furthermore, the potential applications of GQDs, such as energy storage devices (supercapacitors, batteries) and conversion devices (solar cells), are thoroughly discussed [19]. The analysis demonstrates that GQDs outperform conventional semiconductors and will be employed in a variety of innovative materials and clean energy storage and conversion devices Scheme 10.1.

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SCHEME 10.1 GQDs in electrochemical clean energy devices. All the devices shown in the image can benefit from the improved performance afforded by GQDs electrodes. Reproduced with the permission of Kumar R, Sahoo S, Joanni E, Singh RK, Maegawa K, Tan WK, et al. Heteroatom doped graphene engineering for energy storage and conversion. Mater Today 2020;39:47e65.

10.1.1 Challenges of clean energy Environmental changes, energy protection, and monetary solidity are inseparable related [21]. Without understanding and moving against this fact, the eager targets that were set to reach all of the difficulties can never be fully understood. The world will be facing an overwhelming test in the coming century to monitor the world’s financial growth flexibly with constant and moderate resources, without creating insupportable disruption to the world’s atmosphere due to fossil fuel use. The challenge involves two key proportions [22,23]: l

In the coming decades, global energy will remain dominated by fossil fuels, mainly with the increase in energy demand in developing countries: The International Energy Agency’s (IEA) latest policy scenario, which anticipates that recent government promises will be thoroughly implemented, will allow global demand for primary energy to increase by one-third between 2010 and 2035 with 90% of the non-OECD growth. The percentage of fossil fuels in global primary energy consumption has fallen

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l

around 81%e75% in 2035. Renewables energy increased from 13% of the mix today to 18% in 2035. Energy destitution in nonindustrial nations stays a worldwide challenge in near future: Indeed, even with quickly developing economies in some nonindustrial countries, the insights are still stunning. Today, more than 20% of the world’s population need access to electricity, and 40% of the world’s population depends on traditional biomass cooking methods. It’s extended that the difficulty will endure and even develop in the more drawn-out term: 1.2 billion individuals need admittance to power in 2030, and the quantity of individuals depending on the conventional utilization of biomass for cooking ascends to 2.8 billion out of 2030. The far and wide utilization of biomass prompts deforestation and the natural atmospheric effect of dark carbon, which is a significant part of a worldwide temperature alteration. Much more terrible, the family air contamination from the biomass utilization in inefficient ovens would prompt over 1.5 million unexpected losses every year in 2030 more regrettable even than unexpected losses from jungle fever, tuberculosis. Many attentions have been drawn to the environmental concerns of renewable and sustainable energy research and commercialization, and high-energy and high-power storage technologies are imperative for the rapidly growing energy demand.

10.1.2 Clean energy solution The energy generated by employing renewable energy implies that the environment is not polluted. Sustainable energy sources that do not have a natural duty may also be applied to spending money that cannot be supplanted or seriously damage the environment so that citizens in the future have to take care of today’s problems. Who doesn’t need clean or green energy? Yet, conjure up a scenario in which this cost somewhat more. We live in a period where all that we use and all we collaborate with inside our everyday lives must be clean. Therefore, it is worthy to state that perfect energy arrangements, including supercapacitors, batteries, photovoltaic devices, and lighttransmitting diodes, must be clean too. As to the clean energy portfolio, we have to make it under these clean energy arrangements [24]. With the created front line innovations and computerized reasoning applications, we have to change the game-plan in managing energy matters, covering the whole energy range under these classifications, energy basics and ideas, energy materials, energy transformation, and energy the board. With the worldwide energy emergency and environmental change concerns, it is turning out to be increasingly clearer that we have to change the course of action and change from traditional techniques, approaches, frameworks, answers for a spotless energy portfolio where the practicality arrangements are focused on. Changing to clean energy arrangements doesn’t imply that we can disregard the ideas

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and basics which should be treated as a structure that can’t remain without columns. Energy arrangements can’t make do without ideas and essentials [24]. Through a source-system-service approach, renewable energy systems provide promising solutions that require explicit tasks to be refined to provide a complete and sustainable base, as shown in Fig. 10.1. It is crucial to select a perfect wellspring of energy, to begin with. There are surely a few standards to hold up under as a top priority, for instance, wealth, attainable neighborhood quality, moderateness, trustworthiness, security, natural effects, and so on. Most of the promising options appear to be renewables. Next, it is basic to look at framework misfortunes and proficiencies on top of choosing a perfect clean energy source. By and large, a system can be explored through the following important steps: l

l l

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For minimum degradation and the maximum number of usable outputs, process upgrade. Device integration with enhanced outcomes for extrareliable service. Multigeneration by using the same power input to maximize the number of valuable items. Growing performance by defining and modifying loss causes.

When it comes to the service stage, which can be called an application step, it is equally necessary to mitigate losses, irreversibility, waste, and so on, and to recover valuable resources such as heat from carbon-based materials (such as CDs, CQDs, GQDs, etc.) [1]. Clean energy systems are needed to resolve global energy concerns without adversely affecting the climate, the economy, and human capital in the future as well as sustainability. Clean energy strategies are intended to achieve the accompanying basic objectives for better sustainability [24].

FIGURE 10.1 The source-System-Service path to sustainability. Reproduced with the permission of Dincer I, Acar C. Smart energy solutions with hydrogen options. Int J Hydrogen Energy 2018;43:8579e8599, copyright 2018, Elsevier.

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10.2 Theoretical background 10.2.1 Quantum dots background and creation of graphene QDs Since the 19th century, carbon has been widely explored and has continuously expanded in studies as one of the most abundant elements. Fig. 10.2 shows the complete development of carbon content. In 1859 Brodie synthesized graphite oxide in several risky experiments to determine the molecular weight of graphite and coined the name “Graphon.” Kohlschutter and Haenni did not define the properties of graphite oxide at that time until 1918. Bernal continued work in 1924 with single-crystal division measurements on the structural properties of graphite oxide. As for advances in characterization technology, Wallace first studied graphite electronic properties and suggested the graphene principle in 1947 [25]. One year later, a few layers of graphite were successfully identified by transmission electron microscopy (TEM) by Ruess and Vogt. In 1957, a new process for preparing graphite oxide was developed by Hummers and Offeman, which was more productive and environmentally friendly as compared to Brodie’s method. Boehm introduced graphene in 1962 and after that, the theoretical study was done by David DiVincenzo and Mele in 1984 using a massless Dirac equation, which was a very unconvincing thing during that time. Smalley discovered a soccer-shaped

FIGURE 10.2 Development of carbon materials [25].

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fullerene during the production of carbon materials in 1985. It is the first carbon allotropy to be used by scientists around the world. In 1987 Mouras introduced ’graphene’ as a single graphite sheet, although theoretical calculations left the stable graphite sheets inexistent. The other carbon allotropy was discovered by Lijima 1991, identified as carbon nanotubes (CNTs). More than a decade later, A. K. Geim and K. S. Novoslov performed a very outstanding graphene experiment using scotch tape technology in 2004 at Manchester University, leading to monolayer graphene. This extraordinary work has been open a wide range of 2D material science and offers a new technology for different applications. Geim and Novoslov were awarded the Nobel Prize in 2010 for this remarkable experimental work on graphene. The second time Novel prized was awarded to researchers for working on the carbon materials. The various form of the carbon materials, such as 3D (graphite), 2D (graphene), 1D (nanotubes), and 0D (quantum dots), have been successfully explored till now. After the discovery of graphene, the scientific community has explored the properties and the application of graphene. However, during the study of graphene, scientists have observed many problems of graphene, like no banding in the band and less absorptivity, etc. To reduce this drawback, the modification in structural properties of the graphene was subsequently investigated. Ponomarenko was successfully developed the GQDs in 2008 inspired by Xu et al. experiment work on carbon dots (CDs) in 2004. The GQDs consist of graphene lattice in the dot, which has a size of 100 nm and 10 layers of graphene, due to which GQDs and CDs have a different structure [26]. The structure of CDs is typically quasi-spherical carbon nanoparticles (NPs) with a size of approximately 10 nm [26]. Due to the quantum confinement effect in the GQDs, Pan et al. observed fluorescent properties of the GQDs in 2010 [27]. The doping in the GQDs was first introduced by Zhao and their colleagues in 2012 by using it as a dopant in GQDs to change their physical and chemical properties [28]. Moreover, GQDs have larger solubility in comparison to CNTs due to the wide edge effect, this can be changed by using different functional groups, on the other hand, CNTs have a limitation due to 1D characteristics. The 0D GQDs have been developed by changing 2D materials but have a very large Bohr radius of excitement [29]. These results indicated quantum confinement and wide edge effects in the GQDs, and the crystal edge changed the electron distribution by reducing the structural size at the nanometer scale. Additionally, GQDs possessed a bandgap but graphene has zero bandgaps. The band gaps in GQDs have been demonstrated by theoretical studies and also experimentally confirmed by optical and electrical measurements. The semiconductor or insulator GQDs can be transformed by semiconducting graphene. The expansion of graphene optical absorption improved energy range since the bandgap is opened in GQDs. GQDs are often chemically and physically distinguished by their special edge and quantum captivity effects compared to

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other carbon materials, such as graphene, CDs, and CNTs, etc. In the field of nanomaterials, the manufacturing methods typically affect numerous physicochemical properties of nanomaterials. In recent years, there are many techniques have been developed to synthesized GQDs, based on the precursor material, these techniques are classified into two major sections, (1) top-down and (2) bottom-up [25]. The edge and quantum confinement effect are the fundamental properties of the GQDs. These fundamental properties of the GQDs provided remarkable physical and chemical properties, such as solubility, surface grafting, nontoxicity, and biocompatibility [25]. This chapter covers the different uses of these fascinating materials. We will focus on optoelectronics and energy application and introduce us briefly to other applications. Also, we provide a perspective for GQDs, with possible applications and trends in growth. With numerous outstanding reviews focusing on various aspects of GQDs including their synthesis and environmental applications, we expect this segment will be a valuable insight into and inspire new thinking and more study on the current state of research on optoelectronic devices clean energy systems in GODs.

10.2.2 The outlooks of graphene quantum dots The study of GQDs is still in its early stages and the researcher must focus on a variety of other issues, as shown in Fig. 10.3. However, there are numerous

FIGURE 10.3

In line with the latest study, the challenges of GQDs [25].

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significant favorable circumstances and potential uses of GQDs which is deciding further analysis to explore the properties of the GQDs material and achieved the requirements of the application. As a result, GQDs studies have been ongoing to resolve the five crucial issues as mentioned in Fig. 10.3. It is necessary to produced GQDs at a large scale with minimal cost to meet the business prerequisites. However, the production yield of GQDs by using present techniques is significantly poor (