Surface Functionalization Reactions of Graphene-Based Nanostructure and Their Practical Application

Abstract

Graphene (G) has captured the attention of scientists and researchers due to its remarkable electronic, structural, optical, and mechanical properties. While pristine G has been used for various desirable applications requiring high electrical conductivity, there is also a demand for altered or functionalized versions of G, such as G oxide, reduced G, and other functionalized variants, in numerous other applications. The structural alteration of G through chemical functionalization unveils a multitude of possibilities for adjusting its configuration, and various chemical and physical functionalization techniques have been explored to enhance G’s stability and adaptability. Functionalization allows the customization of graphene’s properties, such as electronic, chemical, and mechanical characteristics, to suit specific applications. This chapter highlights the functionalization of graphene-based nanostructure, encompassing both covalent and non-covalent approaches, for a wide range of applications as well as for addressing current challenges and for outlining potential future research directions concerning surface functional modification for G and graphene oxide (GO).

1. Introduction

The well-known materials of the carbon family prior to the 1980s were graphite and diamond. The discovery of molecular carbon allotropes, including fullerenes, carbon nanotubes (CNT), and most recently, 2-D graphene (G), has completely altered the landscape of the inorganic chemistry of carbon [1]. Graphene possesses a 2D honeycomb-like lattice structure, made up of single-layer sp²-bonded carbon atoms (Figure 1) [2].

Figure 1.

Structure of graphene.

Single-layer G exhibits a unique characteristic of having a zero-band gap, leading to its exceptional optical transparency of 97.7%. Furthermore, G has an impressive specific surface area of 2600 m²/g [3], which is greater than that of CNTs yet is less reactive because it lacks the binding stress that CNTs’ curvature produces. Therefore, G is used by researchers and scientists in a large-scale application in different fields including drug delivery systems, biosensing, polymer compositing, and fabrication of liquid crystal devices. Other exceptional qualities displayed by G are its electrical, mechanical, optical, and transport nature and the G bipolar field effect [4, 5, 6]. These unique properties of G have opened up extensive opportunities for surface chemical applications and sparked intense interest among researchers and technologies [7].

Despite its advantageous properties, G sheets tend to restack and aggregate because of van der Waals interactions between the layers. This propensity poses significant challenges in applications linked to nanomaterials and biomaterials-based technologies, where maintaining the desired structural arrangement is crucial. The attractive van der Walls forces found between G sheets prevent the G dispersion in various solvents. Moreover, due to these interactions, the monolayer G tends to re-aggregate after exfoliation and dispersion. These additional limitations of the single-component materials, including challenging processing, impose significant constraints on its practical applications.

However, as compared to G, graphene oxide (GO), a derivative material obtained by the partial oxidation of G is gaining more attention as it can be synthesized on a large scale with more ease and cost-effectiveness. Thus, GO has experienced a remarkable increase in attention in recent times. The surfaces of GO sheets are rich in oxygen-containing functional groups that may include hydroxyl, epoxide, diol, ketone, and carboxyl sites (Figure 2). These groups play a crucial role in altering the van der Waals interactions, leading to a diverse range of solubilities in both water and organic solvents [8].

Figure 2.

G oxide containing various functional groups like hydroxyl, carboxyl, and epoxy.

The functional modifications are essential to expanding the potential use of both G and its derivatives including (GO). Thus, the fundamental basis for achieving functionalization lies in effecting changes to the intrinsic structure of G and its oxide. The two faces, edges, and defect sides of G can all be functionalized through surface or substitutional doping [9]. The existence of structural imperfections and the number of layers have an impact on G’s properties [10]. For example, G’s electrical conductivity tends to decrease with the introduction of defects during covalent functionalization though the sp² structure is preserved under non-covalent functionalization [11].

With improved synthetic techniques, G is made more usable in electronics and other fields that rely on quick electron transfer mechanisms including photocatalytic applications for renewable energy production. Another significant problem that needs to be addressed is the need for conversion of the 2-D G material into 3-D, as the creation of higher-order nanostructures from G continues to show promise in applications like supercapacitors, fuel cells, water purification, drug delivery, photovoltaics, catalysis, gas adsorption, sensing [12], touch screens, spintronic devices, high-frequency circuits, toxic material removal, and flexible electronics [13, 14, 15].

The ability of the G surface to be altered and functionalized has opened a plethora of possibilities to develop specialized practical compounds [10]. The bandgap of single-layer G is altered for microelectronic devices [11]. Moreover, highly porous 3D structures can be engineered from the naturally non-porous 2D G and used in gas sorption, storage, separation, sensing, and electrochemical devices like batteries, fuel cells, and supercapacitors. Nevertheless, the vast array of potential applications can be expanded through numerous functionalization techniques for G and its derivatives. These diverse functionalization techniques provide numerous opportunities to enhance the present applications of G in other areas, such as bioimaging or increasing band gaps for electronic use. In this chapter, methods for functional modification of G and GO have been discussed focusing on the essential chemical bonds and functional groups that affect their structural integrity. The chapter is divided into two sections. Firstly, the synthesis and structure of G-based materials are discussed. Then, G functionalization and applications of the derived materials are explored. The methods of G modification discussed include reactions with organic and inorganic molecules, as well as chemical modifications through covalent and non-covalent interactions with G [16].

2. Synthesis of graphene-based nanostructure

2.1 Pristine graphene

The basic structure of all graphitic materials including charcoal, CNT, and graphite is based on G which is an indeterminately large atomic molecule. There are several approaches that range from mechanical exfoliation of high-quality graphite to direct growth on carbides by bottom-up and top-down techniques which are used to synthesize G [17].

G sheets of different thicknesses can be synthesized through mechanical exfoliation using a simple peeling process. The natural graphite or single-crystal graphite is engraved in oxygen plasma to create deep mesas followed by wedging and peeling off layers. The collected flakes of G are washed off and transferred to a substrate. Another approach to synthesizing defect-free monolayer G is physical exfoliation through ultrasonication using a high-boiling-point solvent [18].

One more promising method for synthesizing mono- or few-layer G is the chemical vapor deposition method (CVD), which also allows for the thickness and crystallinity of the G layer to be controlled using this method [19].

An alternative approach for G synthesis is plasma-induced CVD where the synthesis process takes place at low temperature. This method is more commonly used to synthesize G as compared to CVD as during synthesis, low temperature and less deposition time are required [20].

The chemical exfoliation of graphite is one of the well-established variant methods to produce G. In this method, the increment of interlayer spacing of graphite is achieved by using active functional moieties, which results in the weakening of the van der Wall forces. In this technique, the intercalated G compounds undergo exfoliation either through rapid heating, reduction process, or ultrasonication [21].

2.2 Graphene oxide

GO is highly hydrophilic and can easily swell after dispersion in water due to extra carbonyl and carboxyl groups that are present at the edges of the GO sheets. Due to this hydrophilicity, ultrasonication treatment is done to introduce a single or few layers of G which are exceedingly steady in deionized water and other solvents. This is necessary to understand that graphite oxide and GO are different [22].

Graphite oxide is a multi-layered system while GO is a few or single-layer system. On the basis of oxygen functionalities, numerous models related to GO structure have been studied. GO can be synthesized through the oxidation of graphite using an oxidizing agent in the presence of concentrated inorganic acids. Thus, GO is widely synthesized through various methods like Hummer’s method where potassium permanganate, as an oxidizing agent, concentrated sulfuric (VI), and nitric (V) acids are used to oxidize the G-to-GO [23].

Another method used to synthesize GO is the Staudenmaier method which involves the use of fuming nitric acid and KClO₃ as oxidants [24], and the Tour method using concentrated phosphoric acid with potassium permanganate oxidant [25]. All these methods are known as chemical oxidation methods where various oxygen-containing functional groups are created on the edges and surface of G (Figure 3). These functional groups are responsible for the hydrophilic nature of GO as they break van der Wall forces. The characteristics of GO can be modified through functionalization to tailor the materials for specific applications.

Figure 3.

Reduction of GO through different methods.

2.3 Reduced graphene

When GO undergoes reduction, G is produced due to the removal of oxygen-containing functional groups. Various chemical, thermal, and electrochemical methods are used for the reduction of GO. The chemical reduction has been done using various reducing agents like hydrazine [26], sodium borohydride [27], hydroquinone [28], alkaline solution [29], ascorbic acid [30], and glucose [31].

During the reduction method, a brown precipitate of G, which turns black during dispersion, results in aggregation and precipitation of reduced sheets of G oxide. During the thermal reduction process, G oxide is heated to remove various oxide groups resulting in exfoliations with the evolution of CO₂. The electrochemical reduction method can also be initiated at −0.8 V and completed at −1.5 V with the formation of black precipitates. There are some other methods like photochemical and photothermal methods which are used for the formation of reduced G oxide [32].

3. Surface functionalization

Surface functionalization is one of the most important processes that enable the use of G-based materials for various applications. The functionalization affects the hydrophobicity and surface charge of G by changing the ionization of G. G’s surface can be modified via doping with chemical agents that include elements, compounds, polymers, and nanoparticles which enhance its dispersion, stability, and tribological properties. Through surface modification, stable G-based hybrid materials can be formed. When GO and its composites are used in a water-borne coating system, the main problem is to achieve homogenous dispersion of G. Therefore, to overcome this problem, the surface functionalization of GO is done by covalent/non-covalent reactions involving hydroxyl, carboxyl, and epoxy group using chemical moieties such as organic and inorganic, polymer, and nanocomposites (Figure 4). Among the groups, the epoxy sites react with the edges and defect sites of G to enhance the dispersion ability of G by forming a stable crosslinker between the epoxy binders and functional G. To enhance practical nano-electronic devices and sensors, band gap widening of G can be achieved through doping, intercalation, and striping processes [15, 33].

Figure 4.

Covalent and non-covalent functionalization of G.

The appropriate functionalization of G and GO allows to protect their natural properties by preventing their aggregation during reduction steps [32] and it forms further functional groups to impart additional properties to the materials. As a result, the functionalization processes of G materials have been divided into four categories: that include: (1) covalent, (2) non-covalent functionalization, (3) substitutional doping, and (4) hybridization. With organic groups such as epoxide, carboxylic groups, amino, and hydroxyl groups, G can be functionalized directly through covalent bonding onto the surface sites. To increase dispersibility, for instance, carboxylic acid groups at the GO edges are then swapped out for amine groups such as ethylenediamine and ethanolamine.

3.1 Covalent functionalization

For the purpose of enhancing G’s performance, covalent bonds are used to join G with newly added groups. When an appropriate functional group establishes a covalent bond with the sp² carbon structure of the G surfaces, at the edges or basal planes, covalent functionalization takes place. The aromatic nature of G can be improved by adding functional groups via the covalent method, which alters the electrical properties of materials while boosting its solubility and stability and widening the bandgap [34].

Covalent organic functionalization offers several advantages, allowing the combination of G’s properties with other functional materials like chromophores or polymers, and it enhances the dispersibility of the material in organic solvents and water [35]. Surface functionalization at the edges and flaw functionalization are all possible. The resulting structural defects, which include structural flaws, atoms, and solvent molecules randomly adsorbed onto the materials, are brought on by the chemical processes used to produce G [5]. These defects also include damage to the carbon lattice and structural flaws. Covalent modification can, however, be categorized into different types, such as free radical addition, atomic radical addition, nucleophilic addition, cycloaddition, electrophilic substitution reactions, carboxyl, carbon-carbon skeleton functionalization, and hydroxyl functionalization.

3.1.1 Free radical addition

Functionalization of G based on the free radical addition involves the formation of covalent bonds through the interaction of free radicals with the G surfaces through thermal treatment, photochemical, or chemical treatments (Figure 5) [36]. One of the common radical reactions is carried out with aryl diazonium salts. Functionalization of G through free radical addition was performed by Tour and co-workers where aryl radical forms were formed after the removal of nitrogen from aryl diazonium ion. It was found that the addition of free radical moiety took place on the sp² surface of G by donating an electron [37]. The organic free radicals generated during the reaction then reacted with G through the covalent formation and were found to be accountable for self-polymerization of the materials. The free radical addition reaction is controlled by the G layers, and it is observed that G with a single layer is 10 times more reactive than bi or multi-layered G [38].

Figure 5.

Free radical addition with aryl diazonium salt.

This type of reaction is employed to study the antimicrobial characteristics of synthesized G and its composites using chlorophenyl groups. On functionalization with chlorophenyl groups, G has been found to be more effective as an antimicrobial agent. Numerous studies demonstrated the generation of free radicals used to induce the denaturation of DNA/RNA, leading to the inactivation of microbial cells.

An alternative method for introducing free radicals involves the reaction of benzoyl peroxide with graphene sheets, which is initiated through photochemical means. This process entails focusing an Ar-ion laser beam onto the graphene sheets immersed in a solution of benzoyl peroxide and toluene [39].

3.1.2 Nucleophilic addition reactions

In these reactions, G always behaves as an electron acceptor. For instance, when poly-9,9′-dihexyfluorene carbazole reacts with G, a base is used to initially form an anionic moiety, leading to the generation of nitrogen anions on carbazole (Figure 6). Subsequently, this anion reacts with the G surface, resulting in the formation of a covalent bond [40].

Figure 6.

Nucleophilic addition reaction with carbazole.

The primary sites of reactivity in the nucleophilic addition reaction are the epoxy groups found in GO. The amine (-NH₂) functionality present in the organic modifiers, possessing a lone pair of electrons, initiates an attack on the epoxy groups of GO. Notably, nucleophilic addition takes place with remarkable ease, even at room temperature and in an aqueous environment. Consequently, this method has garnered significant attention as a promising approach for the large-scale production of functionalized graphene when compared to other techniques [41].

3.1.3 Cycloaddition reactions

There are different types of cycloaddition reactions like [2 + 1], [2 + 2], [3 + 2], and [4 + 2]. The most prominent reaction is [3 + 2] cycloaddition reaction including 1,3-dipolar cycloaddition which was performed by Trapalis and his co-workers (Figure 7). G can be used on a large scale in the Diels-Alder reaction as it behaves both as a diene and a dienophile [42].

Figure 7.

1,3-Dipolar cycloaddition reaction.

Due to its gentle reaction condition, the Bingel reaction is one of the exceptionally valuable reactions for functionalizing carbon nanomaterials, such as graphene. This reaction entails the use of a halide derivative of a malonate ester group in the presence of a base. The base extracts a proton from this derivative, leading to the formation of an enolate that subsequently attacks the C=C bonds within the graphene structure. The resulting carbanion undergoes a nucleophilic substitution, displacing the halide and resulting in the formation of a cyclopropane ring [43].

Arynes are well-known reactive intermediates in nucleophilic aromatic substitution reactions, and they are used on a large scale in various reactions. In recent times, there have been several attempts to employ aryne cycloaddition reactions for carbon nanomaterials, demonstrating its successful application in functionalizing fullerenes and their derivatives. Notably, various research groups have reported the chemical modification of graphene through aryne cycloaddition. Based on weight loss analysis, it is estimated that the degree of functionalization is approximately more than one functional group per 17 carbon atoms. The resulting graphene exhibits exceptional solubility and thermal stability, remaining stable even at temperatures as high as 500°C. Additionally, similar reactions using 1,3-dipolar cycloaddition of azomethine ylides and cyclopropanated malonate have also yielded successful chemical functionalization of G and its derivatives [44, 45].

3.1.4 Reaction with atomic radicals

When the reaction with organic free radicals and atomic species like hydrogen, and fluorine is compared, the possibility of side chain reaction is less in case of reaction with atomic radicals due to which uniform and homogenous functionalization of G occurs (Figure 8) [46]. During hydrogenation, deformation of lattice occurs on attaching of atomic hydrogen resulting in easy formation of a second C-H bond. G in its hydrogenated form is known as graphane [47]. The fluorination reaction of G is found to be comparable with hydrogenation as fluorine attached to the carbon atoms of G through a single bond with enhanced binding strength results in high functionalization. Generally, there are three methods to synthesize fluoroG i.e. through: (1) exposition to XeF, (2) etching fluorinated complexes, and (3) graphite fluoride exfoliation [48].

Figure 8.

Reaction with gas phase atomic radicals.

Oxygenated G produced by applying Hummer’s method, exhibits significant heterogeneity. In this reaction, it is assumed that oxygen atoms will attach to graphene, resulting in the formation of epoxide groups. Researchers have successfully generated these epoxide groups by exposing graphene to oxygen plasmas and atomic oxygen beams [49].

3.1.5 Electrophilic substitution reactions

The functionalization of graphene (G) is made possible through electrophilic substitution reactions, taking into account its electron-rich structure. Various reactions like Friedel-Craft acylation and hydrogen-lithium exchange can be included in electrophilic substitution reactions. In the Friedel-Craft reaction, acyl cation is formed as a reactive intermediate after introducing ketone moieties. The reactive intermediate is an acyl cation generated in the presence of a Lewis acid. This reaction was employed to produce brominated flame-retardant high-density polyethylene composites containing graphene nanoplatelets. In the hydrogen-lithium exchange reaction, the first deprotonation or carbometallation of G takes place using butyl lithium (Figure 9). The derivative of lithium-G reacts with an electrophile resulting in the formation of a covalent bond [50].

Figure 9.

Mechanism of electrophilic reaction with BuLi.

Covalent bond functionalization is simpler on GO as compared to G due to oxygen-containing groups on its surface. Common chemical processes involving these groups include addition reactions, carboxylic acylation, epoxy ring opening, isocyanation, and diazotization [7].

By employing polystyrene particles that have been “armored” using nanoscale GO sheets made via aqueous mini-emulsion polymerization of styrene, Wang et al. [51] took advantage of GO’s amphiphilic characteristics without the use of conventional surfactants. From graphite nanofibers, a unique technique was used to manufacture the nanoscale GO sheets of 100 nm diameter. Increased processability and new functions are brought about via covalent bond modification [51].

It is possible to covalently functionalize GO using a variety of other methods. GO nanosheets were chemically modified to include a sulfanilic acid group, which enhanced water dispersibility due to ionic repulsion [52].

Covalent bond modification facilitated increased processability and introduced new functionalities. It has been reported that chemical modification of GO nanosheets with a sulfanilic acid group improved water dispersibility through ionic repulsion. Moreover, covalent functionalization with polyaniline (PANI) not only increased the surface area of reduced GO aerogel (rGOA) but also enhanced electrical conductivity and prevented G nanosheet aggregation. The functionalization started with a free radical reaction where GO donated an electron to aryl diazonium ion resulting in the formation of aryl radical. GO attached to aryl group through covalent bonding after cleavage from the N of PANI-grafted rGOA as high-performance supercapacitor electrodes [53].

The reaction was further preceded by amidation. Lastly, amino group act as active sites for polymerization of aniline on rGOA in an acidic medium using ammonium persulfate as oxidant resulting in the formation of PANI-grafted rGOA as shown in Figure 10. Consequently, the PANI-grafted rGOA exhibited superior capacitance performance (396 F/g at 10 A/g) compared to rGOA (183 F/g at 10 A/g).

Figure 10.

Covalent functionalization of rGOA with PANI.

Shin et al. [54] prepared GO-LMB 21 (locked nucleic acid molecular beacon) using amine functionalized DNA, LMB, and carboxylate GO. The synthesized GO-LMB enhanced the detection limit of miRNA sensing to the picomolar level by covalently coupling fluorescence-labeled dsDNA probes onto the GO. This approach was adopted because interactions between GO and DNA molecules were obstructed by the presence of small molecules like lipids, proteins, and nucleic acids, which caused nonspecific probe desorption [54].

A porphyrin-G nanohybrid was created by Choi et al. [55]. By adding polymers such as polyethylene glycol, dextran, and chitosan, the amidation process has frequently been employed to produce biocompatible GO [55]. The synthesis of chitosan (CS) modified graphene nanosheets under microwave irradiation in N,N-dimethylformamide medium is the best example of amidation reaction which involved the reaction between the carboxyl groups of graphene oxide nanosheets (GONS) and the amido groups of chitosan followed by the reduction of graphene oxide nanosheets into graphene nanosheets using hydrazine hydrate. The results showed that chitosan was covalently grafted onto the surface of graphene nanosheets via amido bonds. Solubility measurements indicated that the resultant nanocomposites dispersed well in aqueous acetic acid.

Four esterification methods, namely direct, carbodiimide activated, oxalyl chloride acylation, and via an acid-functionalized GO intermediate, were investigated for the preparation of surface-functionalized GO nanosheets using tannic acid (TA). In the first approach, direct esterification of GO was done with TA in an acidic medium. In the second approach, acid-functionalized GO (GO-COOH) was prepared through the activation of GO with C₂H₃ClO₂ in a basic medium. In the third experiment, the aqueous solution of GO-COOH was ultrasonicated followed by the addition of TA under ambient conditions to form the final product GO-COOH-g-TA-2. In the last stage, the formation of acyl chloride derivative (GO-CO-Cl) takes place through the conversion of carboxylic acid into acyl chloride. The covalent grafting of TA onto the GO surface renders it more hydrophobic, leading to enhanced organic solvent dispersion. Additionally, TA acts as a crosslinker between the GO nanosheets, thereby increasing its thermal resistance. Furthermore, the combined effect of GO and TA results in the suppression of bacterial growth. Among the examined methods, esterification using carbodiimide showed the highest degree of grafting, maximum thermal stability, and strongest antibacterial activity [56].

3.1.6 Carboxyl functionalization

The functionalization of GO has been extensively explored because of the abundance of carboxyl groups near the edge of the material which are highly reactive groups [57]. The reactions are often started by the carboxyl functionalization step, after which dehydration of an amino group and a hydroxyl group takes place to create an ester/amide bond. The chemicals usually employed for carboxyl activation include thionyl chloride (SOCl₂), 2-(7-aza-1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate, N,N-dicyclohexyl carbodiimide (DCC), and 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) [58].

Functionalization of semiconducting G nanoribbons (GNRs) with Stone-Wales (SW) imperfections was demonstrated using carboxyl (COOH) group. A significant change in the properties of GNR was observed due to the SW defect as the electrons of unsaturated carbon atoms in COOH interact directly with donor state of GNR nanoribbons. The significant rehybridization interaction results in noticeable downward shifts of the states originating from the two bonding carbon atoms to a lower energy range. This phenomenon helps to clarify why the defect band shifts downward after the adsorption of the COOH group. Essentially, the alteration in electrical conductivity with the density of single-wall defective carbon nanotubes (SWDCPs) can be attributed to the redistribution of electronic states. When SWDCPs axial concentration increases, the behaviors of the system shift from semiconducting to p-type metallic. Consequently, G nanoribbons (GNRs) offer promising prospects for applications in nano-electronics and chemical sensors [59]. Following the addition of CRGO-CN (chemically reduced G oxide having CN group) to a solution of sodium hydroxide and methanol, a hydrolysis reaction produced CRGO that was more abundant in carboxyl groups (CRGO-COOH) [60]. A poly(3-hexylthiophene) molecule was grafted onto the carboxylic acids to activate them in order to create heterojunction photovoltaic devices [61].

3.1.7 Carbon-carbon skeleton functionalization

The aromatic basal plane of GO can be directly functionalized using highly reactive intermediates including nitriles, carbenes, and aryl diazonium salts. This process could change the basal plane’s sp² hybridization to sp³ after functionalization. In comparison to non-functionalized GO, GO functional groups considerably increased dispersion stability in water, dimethyl sulfoxide, and N,N-dimethylformamide (DMF). In the aromatic ring of G or GO, the C=C bond is mostly used for the functional alteration of the carbon skeleton. Both the Diels-Alder reaction and the GO diazotization reaction have been documented [62]. The functionalization of G sheets was done through the one-pot process using anthraquinone molecules. In this process, the G electrode undergoes oxidative electrochemical exfoliation in the presence of 0.1 M H₂SO₄ solution containing anthraquinone diazonium ions. Functionalization takes place through the spontaneous reaction of newly formed graphene sheets with diazonium ions (Figure 11) [63].

Figure 11.

The one-pot electrochemical exfoliation of graphite leads to the spontaneous functionalization of graphene sheets (EG) with anthraquinone.

Xiong et al. [62] produced 4-propargyloxyphenyl G (GCCH) through a two-step process. They mixed solution-phase G with 4-propargyloxydiazobenzenetetrafluoroborate at 45°C for 8 hours [62]. Then, the G carbon skeleton was treated and further functionalized using a click chemical reaction with azido polyethylene glycol carboxylic acid [64]. This approach proved to be versatile and practical, allowing for the creation of biosensors and composite materials with G by modifying the functional groups connecting the layers.

The underlying mechanism involves the formation of a diazonium salt or a diazo compound through the diazotization of an aromatic amine-containing material with a reactive functional group, leading to the generation of a free radical during deaeration [65]. The benzene derivative linked through sigma bond with the reactive functional group undergoes an addition reaction with a carbon double bond (C=C) to form a new single C-C bond. Eventually, G with the reactive functional groups undergoes functional modification with G oxide.

3.1.8 Hydroxy functionalization

Another significant method of modifying G is through hydroxy functionalization, in which the occurrence of several hydroxyl groups present on the GO surface enables the functionalization of hydroxyl-based via amidation reaction or esterification. The ester produced by GO is then further modified using other functional groups [66]. The hydroxyl groups were substituted after esterification in order to create aziolated GO. Stirring GO with 2-bromoisobutyryl bromide for 2 days proceeded by dispersion of the esterified GO in dimethylformamide and sodium azide for 24 hours produced azido functionalized GO resulting in improvement in the solubility of the functionalized GO in polar solvents like chloroform and tetrahydrofuran [67].

The researchers utilized tris buffer (TB) and diethanolamine (DEA) which are water-soluble, non-toxic, and biocompatible to easily produce alcohol-functionalized GO material. This alcohol functionalization on GO resulted in the expansion of interlayer space and specific surface areas, allowing for the molecular grafting of amines to G. In a 60% KOH solution with a density of 1 A g⁻¹, in a 6% KOH solution with a density of 1 A g⁻¹, the synthesized materials exhibited high specific capacitance values of 372 and 252 F g⁻¹ [68].

Erythritol, a promising medium-temperature candidate, was combined with GO nanosheets as an additive to increase the dispersion stability of the composites. GO nanosheets treated with hydroxyl groups were used for this purpose. Moreover, the application of functionalized GO nanosheets as an additive has proven effective in enhancing erythritol performance, which is a promising medium-temperature candidate. To optimize overall performance, a trade-off evaluation on the loading of the additive is necessary. By combining GO nanosheets with erythritol, the dispersion stability improved significantly while using GO nanosheets after treatment with hydroxyl-containing compound, the highest loading of 1.0 wt% GO nanosheets led to a twofold increase in thermal conductivity and a reduction in supercooling from 64°C to 48°C. It has been demonstrated that functionalized GO nanosheets are effective in enhancing the performance of erythritol, however, to obtain the greatest overall performance, a trade-off analysis on the loading would be necessary [69].

In other words, the hydroxyl groups were selectively functionalized using a variety of chemical strategies. The Williamson reaction with an amino-terminated linker derivatized the hydroxyl groups, resulting in the production of ether bonds. In contrast to most derivatization methods for hydroxyl groups, this reaction occurred under benign conditions at ambient temperature, preventing the reduction of GO. Then, effective esterification of the hydroxyls using aminocaproic acid at room temperature has been carried out. Overall, the hydroxyls’ reactivity in the Williamson reaction and esterification was quite similar [70].

When dispersing gold nanoparticles (GNPs) in pyridine, Georgakilas et al. [39] employed 1,3-dipolar cycloaddition of azomethine ylide to add the hydroxyl group to G surfaces. For 30 days, the functionalized GNPs could persistently be disseminated in ethanol [39]. Using a ball mill to exfoliate graphite and potassium hydroxide, the authors were able to produce hydroxyl-functionalized G. The G with the hydroxyl functional is very electroactive, hydrophilic, and water dispersible. The functionalized G is made up of single- to few-layer GNPs. Esterification has been reported by the addition of poly(vinyl) alcohol to GNPs [71]. The carboxylic group was added to the pre-functionalized GNPs by oxidizing them in a cold nitric/sulfuric acid solution while subjecting them to extended sonication. Amiri et al. [70] functionalized GNPs with ethylene glycol via a microwave-aided electrophilic process. In water/ethylene glycol conditions, the functionalized GNPs exhibit good dispersibility [70].

3.2 Non-covalent functionalization

The stable dispersion of G materials in organic solvents can be achieved through covalent functionalization using aromatic compounds [72, 73] isocyanates, and aliphatic amines [74]. However, this covalent approach comes at the cost of reducing many of the advantageous characteristics of G-based materials (GBMs), such as their barrier, electrical, and mechanical properties. In contrast, the non-covalent approach focuses on achieving stable dispersion by relying on physical adsorption and/or enfolding of molecules or polymers through weak interactions like hydrogen bonding, van der Waals forces, and H-π, π-π, cation-π, and anion-π interactions. This non-covalent method allows for the preservation of the electrical properties of GO [32].

Non-covalent functionalization procedures have a significant benefit over covalent functionalization approaches since they lessen the impact on the G structure and its inherent material qualities [74]. GBMs can be non-covalently functionalized using a variety of techniques, including polymeric grafting, interaction of small molecules with aromatic rings, or use of surfactants [75].

The G structure suffers little harm from the non-covalent change, and maximum properties can be preserved [13, 76]. Previously, conjugated molecules such as 1-pyrenebutyrate, sulfonated polyaniline, dendronized perylene bisimides, polyacetylenes, and carboxylated oligoanilines have been used to disperse G [39, 77]. Through dispersant, van der Waals interactions or electrostatic repulsion prevails which may aid in G stability in solution.

3.2.1 π-π interactions

G interaction with other compounds or nanomaterials is influenced by two different forms of π-π interactions that take place between the electron-rich and electron-poor areas. This is frequently observed in C₆R₆ (R is substituents like hydrogen or other groups altering the ring’s polarity) where C and R arrange through face and edges interaction. Beyond benzene, these interactions also occur with biologically significant compounds such as DNA and porphyrins. These interactions, which are also present in small molecules, can be used to functionalize GO and G systems for processing and property modification.

A novel approach to non-covalently functionalized GO involved using hyperbranched polyesters with terminal carboxyl (HBP) to produce HBP-GO through strong π-π coupling between hyperbranched polyesters and GO nanosheets. This method was aimed at enhancing the interfacial characteristics between GO and epoxy resin (EP). The presence of hyperbranched polyesters embedded within the GO layer created a steric hindrance effect, effectively preventing the aggregation of GO nanosheets, and greatly improving their dispersibility. Additionally, this non-covalent functionalization increased surface energy, interfacial energy, and adhesion work, reduced the contact angle of HBP-GO with EP, and significantly enhanced the wetting property of HBP-GO [78].

The G compounds with non-covalent functionalization spread easily in polar aprotic liquids. According to Marcia et al., non-covalent pyrene derivatives were applied to GBMs to prevent agglomeration and improve compatibility with the polymer matrix [79]. Layek et al. [80] found that functional groups on the pyrene basal plane edge also improved the interface between G and the polymeric matrix [79]. Similar to this, non-covalently functionalized G composites with enhanced barrier characteristics and G dispersion utilizing pyrene derivates and polyketones [80]. The fluorescence qualities of the dispersant with conjugated structure are typically present; however, once the G is combined, the fluorescence capabilities will be significantly quenched [81].

The reduction of GO through the wet chemical method produced a G-based material having aggregation-induced emission (AIE) properties. During the GO reduction process, TPEP, a conjugated molecule containing tetraphenylethylene (TPE) and pyrene, was used as a stabilizer. The resulting rGO-TPEP exhibited AIE property and high dispersion in solution compared to TPEP alone. The fluorescence intensity of rGO-TPEP was 2.23 times stronger. rGO-TPEP proved to be a sensitive chemical sensor for explosive detection in both aggregated and solid states, owing to its unique optical features and AIE effect. In the aggregated state, rGO-TPEP demonstrated the ability to detect even small concentrations of 2,4-dinitrotoluene (DNT) at 0.91 ppm, with a high quenching constant of 2.47 × 104 M⁻¹ [82].

3.2.2 Functionalization with polymers, drugs, and biomolecules

The π-π interactions between G derivatives and polymers containing aromatic rings provide an excellent example of how tight binding can result in highly homogeneous polymer composites with improved mechanical, electrical, and thermal properties [81, 82]. For instance, Kevlar, an aromatic ring-containing polymer, can strongly interact with G through stacking, and when incorporated into a G nanoribbon (GNR) composite, it enhances the mechanical properties significantly. For example, the addition of 1 wt% GNR to Kevlar/PVC composite increased the Young’s modulus and yield strength by approximately 72.3% and 106%, respectively.

Non-covalent functionalization of G has been explored in other applications. One method involves non-covalently functionalizing G with amine-terminated polystyrene, which enhances its dispersibility in organic environments [71]. Another approach uses self-assembled monolayers of 1,4-benzenedimethanethiol (BDMT) to anchor gold nanoparticles on the G surface, enabling highly sensitive electrochemical detection of H₂O₂ [83].

A novel method for modifying G in field-effect transistors (FETs) application was developed, involving positive and negative doping effects induced by sequentially treating G with gold nanoparticles (AuNPs) and thiol-SAM molecules [82]. This technique demonstrated a Dirac voltage switcher on a G FET using heavy metal ions.

Similar to this, a G structure that self-assembled and adorned with glucose oxidase was employed to support the sensing of glucose within the 20 nm detection limit [84, 85, 86]. To detect nitrate, copper nanoparticles and adorned self-accumulated G was synthesized by Wang, Kim, and Cui [87]. Non-covalently functionalized G with 60 nm thickness and 300 nm diameter was obtained with dextran and chitosan via layer assembly on the GO surface for an anticancer function [88, 89].

Jung et al. [90] introduced a novel G probe functionalized with single-stranded DNA (ssDNA) for the accurate detection of H₂S and NH₃ in exhaled breathing. The functionalization of G with ssDNA creates an ion-conducting channel, enabling efficient proton hopping at humidity levels above 80%. This process results in excellent carrier density modulation, which has the potential to detect biomarkers for illnesses such as kidney diseases and halitosis. The chemiresistive probe was fabricated using a hybrid configuration, where non-covalent π-stacking interactions between ssDNA and G play a crucial role in facilitating the detection process [90].

Concha et al. [91] utilized self-limiting monolayers of ammonium-substituted pyrenes to impart a general positive charge to the G surface and sulfonate-substituted pyrenes to impart a general negative charge. Both types of pyrenes resulted in a stable hydrophilic surface, which allowed for the specific immobilization of macromolecules that carried either negative or positive charges. This straightforward and versatile non-covalent approach is applicable to G on various substrates, including Cu, SiO₂, suspended G, and graphite. By transforming G from hydrophobic to hydrophilic, this method facilitates the use of electrostatic interactions to control the adsorption of macromolecules [91].

Gan et al. [92] developed a nanocomposite to functionalize G with D-glucose using poly (vinyl alcohol) (PVA) and poly (methyl methacrylate) (PMMA) as matrices. The interaction between the polymer blend and D-glucose moieties through hydrogen bonds connected to the fillers led to the uniform diffusion of functionalized G within the matrices. This resulted in a significant improvement in the thermomechanical properties of the nanocomposite [92].

Chhetri et al. [89] used a catalyst based on 3-amino-1,2,4-triazole (TZ) in KOH to functionalize GO nanoparticles and then incorporate them into epoxy resin resulting in better thermal and mechanical resistance. More specifically, compared to composites containing pure GO, both the tensile strength and elastic modulus increased by roughly 30%, while fracture toughness increased more than a magnitude [89].

Functionalization of high-density polyethylene (HDPE) with maleic anhydride (GO-g-MA), GO with ethylenediamine (GO-EDA), and oxidized CNTs (MWCNTs-COOH) were used in a complicated system that Bian et al. [93] developed. To connect GO-EDA and MWCNTs-COOH, L-aspartic acid was employed, creating a hybrid network. This hybrid network was then melted into HDPE-g-MA [93].

The packaging business is a significant additional area where the utilization of G and its derivatives may be of tremendous relevance. Given the growing environmental concerns around waste disposal, two significant areas of application are food and electronic packaging, both requiring excellent barrier properties against gases, particularly water vapor. Functionalization of G has also been employed in packaging materials, where it can enhance mechanical qualities, chemical endurance, and barrier properties [86]. Examples include G functionalized with D-glucose to improve thermomechanical properties in polyvinyl alcohol and poly(methyl methacrylate) matrices [93], as well as GO functionalized with 3-amino-1,2,4-triazole to enhance thermal and mechanical resistance in epoxy resin [94].

G’s utilization in electronic packaging holds particular promise due to its ability to enhance mechanical properties, chemical resistance, and barrier performance [79].

Nanocomposites were formed using vinyl silicone resin prepolymer. The addition of 1% of functionalized G nanoplatelets (GNP) resulted in a substantial improvement in mechanical properties, increasing the tensile strength by approximately 500% and the elastic modulus by up to 1000%. Moreover, at higher percentages of functionalized GNP (10–15%), there was a significant enhancement in thermal conductivity, reaching 16 to 38 times the initial conductivity of the resin [94].

Hierarchical structures were created by growing in situ SiO₂ nanoparticles on reduced G oxide (rGO) nanoplatelets that had been non-covalently functionalized. These were discovered to serve as a reinforcing agent for the matrix made of hydrogenated nitrile butadiene rubber, significantly resulting in a significant improvement in both the static and dynamic mechanical properties [95, 96].

Ou et al. [97] developed an eco-friendly polyurethane comprising of poly (L-lactic acid) as flexible fragments and G. The process involved initiating the ring-opening polymerization of L-lactic acid with phenol-derivatized G, resulting in a polymer grafted with G. Subsequently, the polymer endured condensation polymerization with diphenylmethane-diisocyanate to produce the polyurethane. This polyurethane exhibited superior hydrolysis and antifouling behavior compared to the clean polyurethane, thus making it a suitable coating for flat surfaces [97].

Layer-by-layer assembly propelled by electrostatic contact was used to alternately deposit polyethyleneimine and functionalized GO (FGO) on poly (vinyl alcohol) (PVA) film surface, giving the coated PVA film remarkable flame retardancy. The PVA matrix was enclosed in a shield of defense created by the multilayer FGO-based coating, which successfully stopped the mass and heat transfer that occurs during combustion. Compared to plain PVA, coated PVA shows an initial decomposition temperature of 260°C and a nearly 60% lower total heat output [98].

3.3 Nanoparticle functionalization

Another common type of functionalization is the addition of various nanoobjects, such as nanoparticles (NPs), nanowires, nanorods, and nanospheres (NSs), to G heterostructures and nanocomposites. In contrast to pure G, nanoobjects are equipped with a variety of capabilities based on the inherent features of materials. To expand G’s potential in a variety of electronic and optoelectronic processes, the nanoobject-based G nanocomposites in particular combine specialized optical and electrical capabilities. The semiconductor nanoobjects that can outperform the low-absorption behavior of pure G have certain exceptional optical properties. For instance, when coupled with G to create nanocomposites, the semiconductor cadmium sulfide quantum dots/nanoparticles (CdS QDs/NPs) have significantly enhanced the photo-absorption as well as photoelectrical reactions [99].

When paired with G/ZnO heterostructures, ZnO nanowires/nanorod semiconductors with UV activity have a wider bandgap, which can enhance G’s UV responsiveness. Likewise, when mixed with G to create nanocomposites, the TiO₂ NPs demonstrated better photocatalytic and photoelectric activity [100, 101]. The G/nanoobjects also demonstrate exceptional activity in electrical and electrochemical fields; according to one study, in a hydrogen evolution reaction, MoS₂ NSs/G nanocomposites displayed superior electrocatalytic properties to pure MoS₂ [102]. Additionally, Ni (OH)₂, the NSs/G nanocomposites have shown improved performance in investigations pertaining to electrochemical capacitors [103].

Pure Co₃O₄ NPs and Co₃O₄ NPs/G nanocomposites demonstrated significantly higher oxygen reduction than a C/Pt catalyst. During the oxidation of methanol, the metal NPs, which are only Pt-functionalized with G, demonstrate excellent electrocatalytic activity [104]. Shahid et al. demonstrated the functionalization of G with nanoscale particles which show selectivity for hydrazine [105].

G-gold nanostructures (G-AuNS) have emerged as highly effective sensing substrates, enabling the development of electrochemical biosensors that are affordable, dependable, rapid, and sensitive. These electrochemical devices have found significant applications in the biomedical domain, for detecting glucose through enzymatic or catalytic approaches, as well as H₂O₂, biomolecules (DNA, protein), small molecules (dopamine), microorganisms, foodborne pathogens, environmental pollutants, and a wide range of other analytes. In general, electrochemical biosensors present remarkable advantages, including customization, miniaturization, and rapid analysis capabilities. Nevertheless, they also come with certain common analytical limitations, such as susceptibility to interferences from complex biological sample matrices and the inability to simultaneously detect multiple analytes [106].

3.4 Plasma hydrogenation

G is an example of a 2D material with exceptional qualities that can be used for huge on/off ratio devices and hydrogen storage. G’s outstanding electrical, optical, mechanical, and thermal capabilities have propelled it to the forefront of cutting-edge research and industry. However, a significant barrier to its use has been G’s lack of a sizable band gap. Various methods have therefore been developed to open and control a band gap in functionalized G.

The most straightforward chemical modification of G involves hydrogenation. In this process, each lattice carbon can bond with only one hydrogen atom, assuming no carbon-carbon bonds are broken. Surprisingly, due to the similar electronegativities of carbon and hydrogen, the C-H bond is expected to be nonpolar, not significantly impacting the G’s doping. Based on our knowledge of organic chemistry, the C-H bond in organic molecules is relatively unreactive, leading to the anticipation that hydrogenated G would be chemically stable and inert. This hydrogenated form of G was envisaged as a chemically stable, nonpolar, two-dimensional material with potential applications in electronics, and possibly hydrogen storage (Figure 12) [106].

Figure 12.

Plasma hydrogenation of G.

On the other hand, hydrogen plasma treatment for surface modification of single-layered G has garnered significant scholarly interest due to its potential for conventional wafer-scale production. Researchers have successfully created a monolayer chemical-vapor-deposited G, hydrogenated by an indirect hydrogen plasma, with no structural defects. They observed that adjusting the hydrogen coverage allows for precise control of the band gap, achieving values of up to 3.9 eV [107].

Plasma containing H₃⁺ ions having 3.45, 5.35, and 7.45 eV energies was used to expose G sheets. Only the specimen subjected to the lowest energy plasma could be thermally annealed back into G; the other specimens had irreversible features as a result of vacancy defects caused by ions having high energies. By employing plasma with the proper ion energy and Joule heating, the alterable property in G FETs has been demonstrated, proving that the damage caused by plasma was minimal [108].

The photoluminescence of hydrogenated G exhibits an intriguing optical characteristic. This phenomenon arises from the creation of electronically disconnected conjugated polycyclic regions, exhibiting a diverse range of absorption outlines and remarkably fluorescent emission features as atoms randomly populate the G lattice [109]. These luminous regions possess functionality comparable to carbon nanodots. Consequently, highly hydrogenated G has garnered interest for applications involving white light fluorescence, optoelectronic characteristics, and imaging capabilities, akin to quantum dots [110]. Elsewhere, Elias et al. discovered that the hydrogenation had a strong p-doping behaviour. However, it was discovered that once the sample was dried, hydrogen showed the n-doped behaviour in the G and p-doping effect was initiated by adsorbed water molecules [111].

These studies highlight the importance of surface modification of GO in tuning its properties and enabling novel applications in analytical chemistry. Fortunately, GO possesses numerous active surface functional groups, which allow for surface modification through various interactions, including covalent and non-covalent interactions. Additionally, successful surface modification of GO has been achieved through the doping of heteroatoms or nanocomposites.

3.5 Substitutional doping of graphene

The most realistic and appropriate approach to amend the band structure of G is doping where G changes from semi-metal to n or p-type semiconductor. This is accomplished by substituting or replacing carbon atoms from G lattice with foreign elements, a process known as substitutional doping of G. Substitutional doping is going to be interesting as it introduces the charge in the structure of G.

In this type of doping, G’s carbon atom is replaced by another atom like nitrogen, phosphorous, boron, sulfur, etc., near the opening (Figure 13). In a pristine G, the unpaired electrons are tightly bonded and passivated within its delocalized structure, rendering it chemically unreactive and limiting its energy absorption capacity [112]. However, G can be easily p-type doped through surface absorption. When pristine G is exposed to molecules with electron-withdrawing groups (such as H₂O, O₂, N₂, NO₂, PMMA, etc.), noticeable p-type doping occurs, but it can quickly return to its original state once the doping molecules are removed.

Figure 13.

Doping mechanism in G layer.

In contrast, achieving stable n-type doping in G presents more challenges. Although some electron-donating molecules like ammonia, potassium, phosphorus, hydrogen, and poly(ethyleneimine) (PEI) can induce n-type doping through surface electron transfer, these doping effects often prove to be unstable. As an alternative approach, introducing nitrogen-containing precursors during the growth process can partially replace lattice carbon atoms with nitrogen atoms, leading to effective n-doping.

Combining both p-type and n-type doping methods enables the creation of p-n junctions in mono- or bi-layer G. These hetero-doped G p-n junctions have paved the way for novel functional devices like photothermoelectric devices [113].

Heteroatom insertion can produce extraordinarily abundant active sites in G. The addition of a heteroatom with a different electronegativity from the carbon atom could break the electroneutrality of G resulting in the generation of unstable charged zones and such zones might act as active sites. These active sites may exist as structural defects (arising from the lattice strain). The lattice strain is mainly due to the difference in size of dopant and carbon atom. The base of these active sites introduces the band gap and semiconducting properties of G by improving their chemical properties. The heteroatom can be substituted into G through various methods like solid-phase synthesis, liquid-phase synthesis, and direct synthesis [114].

CVD and segregation-growth approach are the direct synthesis methods. The most prominent method to introduce the heteroatom in G is the CVD method where heteroatoms are directly incorporated into lattice of G [114, 115, 116]. This method is used to synthesize n-type semiconductors using G and nitrogen element as dopant accompanied by alteration of charge mobilization and transference of electron results in the transition of G from metal to semiconductor. As a result, nitrogen-doped graphene finds applications in both the electronic and optoelectronic realms [117].

Another method used to synthesize doped G is a segregation-growth method which is also a direct synthesis method. In this method, some selective doping is possible as a heteroatom incorporated into some selective surface of G. Recently, Wang et al. confirmed the fabrication of nitrogen-doped G via this method exhibited a bandgap of 0.16 eV. This method is used to control the concentration and position of doping materials hence the synthesized G used for FET application [118].

4. Conclusion, challenges, and future prospectives

The chapter delves into the ability of G materials for various applications such as energy storage, conversion devices, and many more. To leverage the unique properties of G, it can be combined with other nanomaterials like metal, metal oxide, magnetic nanoparticles, quantum dots, etc., using surface functionalization and nanoparticle functionalization, along with plasma dehydrogenation and substitutional doping. The appropriate modification of the surface through doping and/or functionalization creates promising opportunities for the use of these materials in device applications.

Surface functionalization can be achieved through covalent and non-covalent functionalization. Non-covalent functionalization, while easy and rapid, relies on hydrophobic, van der Waals, and electrostatic interactions, making it susceptible to leaching out functions from the G sheets during application. On the other hand, covalent functionalization capitalizes on oxygen functional groups present on G surfaces, such as carboxylic acid groups at the edges and epoxy/hydroxyl groups on the basal plane, to modify the surface functionality of G. Covalent functionalization addresses the drawbacks of non-covalent functionalization and allows for the precise decoration of desired functions on suitable platforms.

Regarding surface transfer doping, its long-term stability is lacking as adsorbed species can desorb from the G surface and react with reactive molecules. Substitutional doped G, where metal or heteroatoms are attached to the carbon linkage of G, offers improved stability. However, it faces critical challenges in terms of large-scale production, doping controllability, and mechanisms. The production of G indeed faces several challenges, including the need for specific conditions and high temperatures in some methods such as the CVD method where high temperatures are required (typically above 1000°C) to create the necessary conditions for graphene growth. These high temperatures can be energy-intensive and may limit the scalability and cost-effectiveness of production. Developing a controlled synthesis of doped G could provide a desirable solution. As the field of novel materials and innovative applications continuously advances, understanding the mechanisms for reactions in many electrochemical systems remains complex. Further investigations into doped G materials hold promise for contributing to energy systems. Controlled reduction offers a viable pathway for mass-producing semiconducting G.

GO has proven its versatility in various applications, including optoelectronics, drug delivery materials, biodevices, and polymer composites. Consequently, G emerges as a promising candidate for immobilizing various substances, such as metals, biomolecules, fluorescent molecules, drugs, and inorganic nanoparticles.

Acknowledgments

The authors are thankful to K. R. Mangalam University for providing technical support.

Conflict of interest

The authors declare no conflict of interest.

Abbreviations

AIE	aggregation-induced emission
BDMT	1,4-benzenedimethanethiol
CdS QDs/NPs	cadmium sulfide quantum dots/nanoparticles
CVD	chemical vapor deposition
DA	dodecyl amine
DCC	N,N-dicyclohexyl carbodiimide
DEA	diethanolamine
DMF	N,N-dimethylformamide
DNT	2,4-dinitrotoluene
EDA	ethylenediamine
EDC	1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide
FET	field-effect transistor
FGO	functionalized GO
G	graphene
GBMs	G-based materials
GO	graphene oxide
G-AuNS	G-gold nanostructures
GNPs	gold nanoparticles
GNS	graphene nanosheets
GNRs	G nanoribbons
HBP	hyperbranched polyesters with terminal carboxyl
HDPE	high-density polyethylene
LMB	locked nucleic acid molecular beacon
MA	maleic anhydride
MWCNT	multi-walled carbon nanotube
NPs	nanoparticles
NSs	nanospheres
ODA	octadecylamine
PANI	polyaniline
Py-PGMA	poly (glycidyl methacrylate) with localized pyrene groups
PMMA	poly (methyl methacrylate)
PNVP	poly-N-vinyl-2-pyrrolidone
PVA	poly (vinyl alcohol)
rGO	reduced graphene oxide
rGOA	reduced GO aerogel
ssDNA	single-stranded DNA
SW	Stone-Wales
SWDCPs	SW defect COOH pairs
TA	tannic acid
TB	tris buffer
TPE	tetraphenylethylene
thiol-SAM	thiol self-assembled monolayer