Accepted Manuscript

Sweet graphene I: Toward hydrophilic graphene nanosheets via click graftingalkyne-saccharides onto azide-functionalized graphene oxide

Mina Namvari, Hassan Namazi

PII: S0008-6215(14)00250-XDOI: http://dx.doi.org/10.1016/j.carres.2014.06.012Reference: CAR 6773

To appear in: Carbohydrate Research

Received Date: 28 April 2014Revised Date: 3 June 2014Accepted Date: 11 June 2014

Please cite this article as: Namvari, M., Namazi, H., Sweet graphene I: Toward hydrophilic graphene nanosheetsvia click grafting alkyne-saccharides onto azide-functionalized graphene oxide, Carbohydrate Research (2014),doi: http://dx.doi.org/10.1016/j.carres.2014.06.012

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Elsevier Editorial System (TM) for Carbohydrate Research

Manuscript Draft

Title: Sweet graphene I: Toward hydrophilic graphene nanosheets via click grafting alkyne-

saccharides onto azide-functionalized graphene oxide

Article Type: Full Length Article

Corresponding Author: Hassan Namazi, Ph. D.

Corresponding Author's Institution: University of Tabriz.

First Author: Mina Namvari, Ph. D. student.

Order of Authors: Mina Namvaria, Ph. D. student; Hassan Namazi

a,b, Ph. D.

Affiliations: aLaboratory of Dendrimers and Nano-Biopolymers, Faculty of Chemistry,

University of Tabriz, Tabriz, Iran

bResearch Center for Pharmaceutical Nanotechnology (RCPN), Tabriz University of Medical

Science, Tabriz, Iran

Mina Namvari’s email: m.namvari@tabrizu.ac.ir

Prof. Hassan Namazi’s email: namazi@tabrizu.ac.ir

Corresponding Author P.O. Box 51666, University of Tabriz, Tabriz-Iran, Tel.: + 98 411 339

3121, Fax: +98 411 334 0191.

2

Abstract

Water-soluble graphene nanosheets (GNS) were fabricated via functionalization of graphene

oxide (GO) with mono and disaccharides on the basal plane and edges using Cu (I)-catalyzed

Huisgen 1,3-dipolar cycloaddition of azides and terminal alkynes (Click Chemistry). To graft

saccharides onto the plane of GO, it was reacted with sodium azide to introduce azide groups on

the plane. Then, it was treated with alkyne-modified glucose, mannose, galactose and maltose. In

the next approach, we attached 1, 3-diazideoprop-2-ol onto the edges of GO and it was

subsequently clicked with alkyne-glucose. The products were analyzed by Fourier-transform

infrared spectroscopy (FTIR), field-emission scanning electron microscopy, thermogravimetric

analysis (TGA) and X-ray diffraction spectrometry. FTIR and TGA results showed both sugar-

grafted GO sheets were reduced by sodium ascorbate during click-coupling reaction which is an

advantage for this reaction. Besides, glycoside-grafted GNS were easily dispersed in water and

stable for two weeks.

Keywords: Graphene oxide; Click chemistry; carbohydrate; Water-soluble

Introduction

3

In his landmark review published in 2001, Sharpless and coworkers1 defined click chemistry as a

‘set of powerful, highly reliable, and selective reactions for the rapid synthesis of useful new

compounds and combinatorial libraries.’ To qualify as “click” chemistry, the reaction itself must

be modular, wide in scope, tolerant to other functional groups, insensitive to solvent, moisture

and oxygen, giving very high yields and only inoffensive by-products that can be easily removed

by non-chromatographic methods, and be stereospecific, preferably giving only one product.1

The Cu (I)-catalyzed Huisgen 1, 3-dipolar cycloaddition of azides and terminal alkynes

(CuAAC) to form 1,2,3-triazoles2 is the model example of a click reaction resulting exclusively

in 1, 4-disubstituted product. Because of the mentioned merits, CuAAC reaction has extensively

been exploited as a novel methodology to advance drug-discovery 3,4

, functionalize monolayers,5

synthesize and functionalize different molecules,6 polymers,

7 dendrimers,

8,9 nanoparticles,

10

virus,11

posttranslationally engineer proteins12

and modify cell surfaces.13

Especially, due to the

stability of alkyne and azide during the protection and deprotection process in the carbohydrate

chemical reactions, CuAAC reaction has led to the synthesis of a large number of compounds

ranging from small molecules, such as simple glycosides, e.g., N-glycosyltriazoles, through

oligosaccharide mimetics to homogeneous and heterogeneous neoglycoconjugates.

Glycomacrocycles, glycoclusters, glycodendrimers, glycocyclodextrins, and glycopolymers can

be included in the latter group.14-18

Glycodendrimers decorated with various protected15,19-21

or

unprotected23

monosaccharaides have been widely used to study lectin-carbohydrate interaction

19,20 and also to stabilize Pd and Pt nanoparticles.

24 The new path of click chemistry has

significantly broadened the structural diversity of polysaccharides. Cellulose has been

functionalized with small molecules,25,26

dendrons27,28

and polymers29

and crosslinked with

itself30,31

and starch32

using CuAAC.

Graphene is a single layer of carbon atoms arranged in a hexagonal lattice structure. It is the first

two-dimensional crystalline material to be isolated,33

and owing to its single atom-thick nature, is

of immense scientific and applied interest. Different strategies have been introduced to prepare

graphene including metal ion intercalation, liquid phase exfoliation of graphite, chemical vapor

deposition (CVD) growth, vacuum graphitization of silicon carbide (SiC), bottom-up organic

synthesis of large polycyclic aromatic hydrocarbons (PAHs), and of course, chemical reduction

of GO.34

Each strategy has its own advantages and disadvantages; nevertheless, chemical

reduction of exfoliated GO is believed to be a promising method, mainly due to its wet chemical

4

processability and large scale availability to monolayers35

which includes non-covalent, covalent

functionalization of reduced graphene oxide and so on. The chemical reduction of GO has

usually been carried out using hydrazine/ hydrazine derivatives and sodium borohydride as the

reducing agents.36,37

Unfortunately, since hydrazine or dimethylhydrazine are highly toxic and

dangerously unstable, using them to reduce GO requires great care. Environmentally friendly

approaches for large-scale production of water-soluble graphene have been reported. Zhang et al.

has reduced GO under a mild condition using L-ascorbic acid while Dong et al. has developed a

green and facile approach to synthesize graphene nanosheets (GNS) based on reducing sugars,

such as glucose, fructose and sucrose using exfoliated GO as precursor.38

Functionalization of

graphene via different methods has led to hydrophilic39-41

and hydrophobic materials42-45

and

exfoliated GNS have also been obtained.45-47

Carbohydrate/graphene nanocomposites48-49

and

covalent attachment of polysaccharides onto graphene have also been reported.39,40,47

Furthermore, very few click reactions have been performed on GO47,50-56

and to our knowledge

neither mono nor disaccharides have been covalently attached to GO so far. Thus, we decided to

utilize the powerful click reaction to attach mono and disaccharides onto the basal plane and

edges of GO to obtain water-soluble GNS. In the first approach, treating GO with sodium azide

(GO-N3) and clicking it with alkyne-moiety-containing glucose, mannose, galactose and maltose

resulted in the attachment of glycosides on the basal plane of GO and as expected, water-soluble

graphene sheets were obtained. In the second approach, refluxing GO in thionyl chloride (SOCl2)

and subsequently treating with 1, 3-diazidopropan-2-ol yielded in azide-modified GO which was

then reacted with alkyne glucose. This way, GO was functionalized with saccharides on the

edges via click reaction and the resulted product was water-soluble as well.

2. Experimental section

2.1. Materials

Graphite (average particle size 30 µm) was commercially available and it was used without

further purification. Sodium azide (NaN3), 1, 3-dichloroprop-2-ol, D-glucose, D-mannose, D-

galactose, maltose, propargyl alcohol, sodium ascorbate, CuSO4.5H2O, triethylamine (Et3N), N,

N’-dimethylformamide (DMF), tetrahydrofuran (THF). Unless otherwise stated, the reagents

were of analytical grade and were used as received and were all purchased from Merck.

Deionized water (DI water) was used in all experiments.

5

2.2. Preparation of mono and disaccharides clicked onto GO. The approaches addressed to

clicking GO with glycosides are illustrated in Scheme 1.

Approach 1:

2.2.1. Click reaction of GO-N3 and alkyne-modified saccharides (rGO-N3-sugar)

The as-prepared GO by modified Hummers’ method,57

was reacted with NaN3 to yield GO-N3.46

Propargyl-functionalized saccharides (alkyne glycosides) were prepared through the reaction of

D-glucose, D-mannose, D-galactose, and maltose with propargyl alcohol in the presence of

H2SO4–silica as a catalyst.58

GO-N3 (10 mg) was dispersed by ultrasonication into a mixture of

H2O/DMF (2:1, 10 mL). Alkyne glucoside (1 g), sodium ascorbate (10 mg), and CuSO4.5H2O (2

mg) were added to the solution, which was then stirred at room temperature for 48 h. During the

reaction, the mixture was sonicated 4 times. Afterwards, the mixture was filtered and washed

with DI water and ethanol many times. The resulting powder was dried in vacuum overnight.

The procedure to the preparation of GO-N3-glycoside based on D-mannose, D-galactose, and

maltose was the same as mentioned above, except adding D-mannose, D-galactose, and maltose

instead of glucose.

Approach 2:

2.2.2. Preparation of 1, 3-diazidoprop-2-ol-functionalized GO (GO-diazide)

To a dispersion of acyl chloride-functionalized GO39

(100 g) in 20 mL of dry THF under argon

atmosphere, 1, 3-diazidoprop-2-ol59

in 2 mL of THF and Et3N (1 mL) were added dropwise at 0

°C. The mixture was stirred at 0 °C for 1 h, at room temperature for 6 h and reflux for 24 h. The

powder was washed with an excess amount of DI water and ethanol. After washing, the resulting

powder was dried in vacuum overnight.

2.2.3. Click reaction of GO-diazide and alkyne glucoside (rGO-diazide-Glc)

GO-diazide (10 mg) was disperesed by ultrasonication into a mixture of H2O/DMF (2:1, 10 mL).

Alkyne glucoside (1 g), sodium ascorbate (10 mg), and CuSO4.5H2O (2 mg) were added to the

solution, which was then stirred at room temperature for 24 h. During the reaction, the mixture

was sonicated 2 times. Afterwards, the mixture was filtered and washed with DI water and

ethanol many times. The resulting powder was dried in vacuum overnight.

6

Scheme 1. Grafting mono and disaccharides onto GO by CuAAC via two approaches.

2.3. Characterization

Infrared spectra were obtained on a Fourier-transform infrared (FTIR) spectrometer (Bruker

Instruments, model Aquinox 55, Germany) in the 4,000–400 cm-1

range at a resolution of 0.5

cm-1

as KBr pellets. The pattern of X-ray diffraction (XRD) of the samples was obtained by

Siemens diffractometer with Cu-ka radiation at 35 kV in the scan range of 2 h from 2 to 60° and

7

scan rate of 1°/min. Scanning electron micrographs (SEM) were obtained using a Field-Emission

Scanning Electron Microscopy (FESEM, MIRA3 FEG-SEM) operating at 10 kV. Thermo

gravimetric analysis (TGA) was performed with a TGA-PL thermal analyzer under air

atmosphere from room temperature up to 600 °C at a heating rate of 10 °C/min.

3. Results and discussion

3.1. Characterization of GO

GO, produced through exfoliation of graphite oxide created from modified Hummers’ method,

was characterized by FTIR, XRD, SEM and TGA. Four main groups of GO could be noted on

the FTIR spectrum shown in Fig. 1A: at 1074 cm-1

for C–O bonds; at 1626 cm-1

for C = C bonds;

at 1735cm-1

for C=O bonds and at 3392 cm-1

for O–H bonds. This was in good agreement with

previous reports.60

Fig. 1B showed the XRD pattern for GO. Oxidation generates oxygen-

containing groups on the originally atomically flat graphene sheets which makes individual GO

sheets thicker than individual pristine graphene sheets44

thus, GO showed a sharp (0 0 1) peak at

10.44°. Layered structure of GO was clearly observed in the related SEM image (Fig. 1C). TG

Fig. 1. (A) FTIR spectrum of GO; (B) XRD pattern of GO; (C) SEM image of GO; (D) TGA

curve of GO.

8

analysis of GO was shown in Fig. 1D. GO exhibited two steps of mass loss, first one before 120

°C which was attributed to the evaporation of adsorbed water and the main step up to 400 °C was

related to the decomposition of oxygen-containing functional groups such as carboxyl, hydroxyl

and epoxy.60

3.2. Characterization of saccharide modified-GO

3.2.1. Characterization of rGO-N3-sugar

GO has been selected very often as the starting material for the formation of graphene

derivatives through the covalent attachment of organic groups on its surface due to the rich

content of hydroxyl, carboxyl, and epoxy groups. Various approaches for the covalent

functionalization of GO has been reported so far;61

preparation of hydrophilic or organophilic

GNS was of great importance in some of them on account for specific applications. rGO-

polyvinylalcohol,62

rGO-polystyrene/polyacrylamide copolymer,63

functionalized GO with

allylamine, p-phenyl sulfonate, phenylene diamine groups64,65

and 1-(3-aminopropyl)-

imidazolium bromide,66

rGO-hydroxypropyl cellulose and rGO-chitosan39

are a few examples of

water-soluble GNS. We decided to graft mono and disaccharides onto GO using click reaction.

This method does not require a second step of reduction since sodium ascorbate is also in charge

of reducing saccharide-modified GO. This is an advantage for click reaction. GO-N3-glycoside

powders were dispersed in acidic and basic media, separately, by sonication for 10 min to

investigate in which media they were stable. It was found out that the suspensions were stable in

basic solution even after 2 weeks standing at room temperature (Fig. 2). Hydroxypropyl cellulose

and chitosan functionalized graphene nanosheets (GNS) were also stable in water for one

month.39

Fig. 2. rGO-N3-Glc dispersed in acidic and basic solutions (left), after 2 weeks (right).

FTIR spectra of GO-N3, alkyne modified-glycosides and rGO-N3-glycosides are shown in Fig 3.

In the FTIR spectra of GO-N3 the signal at 2115 cm-1

was consistent with the asymmetric

9

stretching of the azide group (Fig. 3a). Besides, the peak of O-H stretch at 3443 cm-1

became

more intense since more O-H groups existed on the sheets due to azidation (Fig 2a).46

Alkyne-

functionalized glucose, mannose, galactose and maltose had a peak at 2151 cm-1

in common

which was assigned to C≡C stretch of alkyne (Fig 3b-d). After grafting saccharides onto GO, the

disappearance of the characteristic peaks related to alkyne and azide moieties in the FTIR spectra

of rGO-N3-glycosides, indicated that the starting materials no more existed (Fig 3f-i). In

addition, a new signal around 1640 cm-1

showed up proving the formation of triazole group 46

.

As already known, ascorbic acid is able to reduce GO.67,60

As it was obvious from the FTIR

spectra of the rGO-N3-glycosides, dramatic reduction in the intensities of the peaks

Fig. 3. FTIR spectra of GO-N3 (a), azide-Glc (b), azide-Man (c), azide-Gal (d), azide-Maltose

(e), rGO-N3-Glc (f), rGO-N3-Man (g), rGO-N3-Gal (h) and rGO-N3-Maltose (i).

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corresponding to the oxygen functionalities, such as the C=O stretching vibration peak at 1735

cm-1

, the vibration peak of O–H groups at 3392 cm-1

and the C–O stretching vibration peak at

1074 cm-1

were observed, and some of them disappeared entirely. These observations proved

that most oxygen functionalities in GO were removed.

XRD patterns of GO-N3-glycosides are shown in Fig. 4A. Since the patterns are all similar, the

ones related to rGO-N3-Glc and rGO-N3-Maltose are illustrated. GO showed a sharp (0 0 1) peak

at 10.44°. In GNS, which were prepared by reducing GO with hydrazine hydrate, this peak

shifted back around the original (0 0 2) peak and was observed around 25°.41

rGO-N3-glycosides

showed an XRD pattern similar to GNS and the related peaks were seen at 2θ = 23.68° and

23.04° for rGO-N3-Glc and rGO-N3-Maltose, respectively. Besides, the (0 0 1) peak of GO was

not observed. This indicated that layered GO has been exfoliated largely and saccharides were

into interlayer spacing of GO, partially restoring its electronic conjugation.45

It can be suggested

that functionalization of GO with saccharides has led to the formation of graphene-like

platelets.46

Fig. 4. (A) XRD patterns of GO-N3-Glc and GO-N3-Maltose; (B) TGA curves of GO, rGO-N3-

Glc, rGO-N3-Man, rGO-N3-Gal and rGO-N3-Maltose.

Content of the grafted saccharides in the functionalized graphenes were estimated by TGA. The

TGA curves of GO, rGO-N3-Glc, rGO-N3-Man, rGO-N3-Gal and rGO-N3-Maltose are shown in

Fig. 4B. rGO-N3-glycosides showed higher thermal stability compared with GO due to the

higher onset temperature. Decomposition of triazole groups started around 240 oC and continued

to 430 oC and the weight losses were 30-35%. No weight loss related to the removal of oxygen-

containing groups of GO was observed and this was another proof that these sweet GNS were

11

reduced by sodium ascorbate. The mass loss over the whole temperature of 450-600 °C is low

demonstrating the high efficiency of the covalent functionalization.45

To investigate the morphology of the rGO-N3-glycosides FESEM images were taken (Fig. 5).

The objects seen on the sheets are the saccharide aggregates attached to GO on the plane.

Fig. 5. SEM images of rGO-N3-Glc (a), rGO-N3-Man (b), rGO-N3-Gal (c) and rGO-N3-Maltose

(d).

3.2.2. Characterization of rGO-diazide-Glc

After functionalizing GO with saccharides on the plane, we decided to attach them onto its edges

as well. The only difference was that the reaction was carried out with glucose only. Thus, after

chlorinating GO with SOCl2, 1, 3-diazidoprop-2-ol was attached by esterification reaction.

Subsequently, GO-diazide was clicked with alkyne glucose. The stability of rGO-diazide-Glc

was investigated in both acidic and basic media and similar to rGO-N3-Glc, rGO-diazide-Glc

was also stable in basic solution for 2 weeks.

To demonstrate the successful functionalization of GO with 1, 3-diazidoprop-2-ol and

subsequently with glucose, FTIR spectra of starting materials and products were inspected (Fig.

12

6A). After grafting diazide functionality onto the edges of GO, a sharp peak at 2088 cm-1

related

to the stretching band of azide showed up. Signals at 2923 and 1397 cm-1

were attributed to

stretching vibration of CH2-N3 and C-N, respectively. The peak assigned to O-H of carboxylic

acid groups of GO was reduced in broadness and shifted to 3438 cm-1

due to esterifaction

reaction. In addition, a sharp band at 1580 cm-1

was connected to the skeletal vibration of

reduced graphene sheets.68

McAllister et al. has reported that in nucleophilic substitution

between epoxy groups of GO and alkylamine or diaminoalkane, deoxygenation occurs which

leads to reduction of GO. In the FTIR spectra of GO-diazide the peak at 1580 cm-1

was related to

the out-of-plane A2u, increased disorder, bending graphite sheets and a change in the interplanar

bonding.69

In the reaction between GO and 1-bromooctadecane in the presence of pyridine, a

peak at 1560 cm-1

was observed and indicated that GO has been effectively reduced during the

functionalization process.43

Using excess of Et3N in this reaction and the reflux condition, it

could be speculated that Et3N could have acted like pyridin and reduction of GO has happened

associated with the nucleophilic attack of Et3N. This was in good agreement with XRD spectrum

of GO-diazide which showed a pattern similar to GNS.41

Compared to GO-diazide and alkyne-

Glc, diappearance of the signals coonected to azide and alkyne groups and the presence of the

peaks of C-O-C stretching of glucose moiety at 1089 cm-1

and triazole group at 1647 cm-1

in

rGO-diazide-Glc, verified the introduction of glucose onto GO-diazide. Moreover, similar to the

pattern seen in rGO-N3-Glc, the signals attributed to stretching vibrations of C-O and O-H of GO

were not present which indicated that Glc-functionalized GNS was reduced during click-

coupling reaction.

XRD patterns were used to further study the changes in sturucture after functionalization. Fig.

6B showed powder XRD results for GO-diazide and rGO-diazide-Glc. After grafting 1, 3-

diazidoprop-2-ol onto GO, the peak shifted back to the original (0 0 2) peak at 23o. The changes

in both d-spacing value and broadness might be attributed intercalation of dazide moiety into its

interlayer spacing as explained earlier about rGO-N3-glycosides.45

In the XRD result of rGO-

diazide-Glc, the same pattern was observed (2θ=24o) and the broadening of the peak indicates

the smaller sheet size of the reduction products as compared to GO.

The morphology of the GO-diazide and rGO-diazide-Glc was studied by SEM (Fig. 6C). Fig.

6a,b are related to GO-diazide in which layer-by-layer assembly of functionalized GO sheets

were clearly observed. White round objects on the sheets were diazide groups. After carrying out

13

the click reaction, big white objects which were known to be the glucose molecules were

observed. It should be noted that less glucose molecules were seen here and this might speculate

that there were more epoxy group on the GO plane than acid carboxylic groups at the edges.

Fig. 6. (A) FTIR spectra of GO (a), GO-diazide (b), rGO-diazide-Glc (c), (B) XRD patterns of

GO-diazide (a) and rGO-diazide-Glc (b), (C) FESEM images of GO-diazide (a,b) and rGO-

diazide-Glc (c,d).

The presence of functional groups on the graphene sheets was further analyzed using TGA (Fig.

7). GO-diazide and rGO-diazide-Glc showed higher thermal stability in comparison with GO.

GO-diazide showed very low weight loss below 125 oC which was related to low content of

adsorbed water. Next step of mass loss began at 125 oC and was 11% up to 217

oC which was

attributed to decomposition of oxygen-containing groups of GO sheets. The removal of diazide

functionality started at around 250 oC and continued to 460

oC with a weight loss of 20%. This

showed that the content of functionalization of GO with diazide group on the edges were less

than the content of azide group on the plane and this is in good agreement with SEM results. TG

analysis of rGO-diazide-Glc confirmed a two-step decomposition profile corresponding to the

removal of water (4%, <250 oC) and decomposition of glucose-containing groups (29%, 280-550

14

oC). Since no weight loss regarding the removal of oxygen-containing groups was observed, it

could be proved that Glc-modified GNS synthesized through this approach, were also reduced by

sodium ascorbate during click-coupling reaction.

Fig. 7. TGA curves of GO, GO-diazide and rGO-diazide-Glc.

4. Conclusions

In summary, two types of water-soluble graphene nanosheets were prepared via attaching mono

and disaccharides onto the plane and edges of GO which were stable in basic water for two

weeks. These glycoside-modified GO sheets were reduced by sodium ascorbate during click-

coupling reaction which is an advantage besides other significant advantages of this reaction and

was confirmed by FTIR and TGA. The content of functionalization of GO on the plane was

higher than the edges and this was proved by SEM and TGA results.

Acknowledgements

Authors are pleased to acknowledge the University of Tabriz and Research Center for

Pharmaceutical Nanotechnology (RCPN), Tabriz University of Medical Science for financial

support of this work.

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Captions for Figures

Scheme 1. Grafting mono and disaccharides onto GO by CuAAC via two approaches

Figure 1. (A) FTIR spectrum of GO; (B) XRD pattern of GO; (C) SEM image of GO; (D) TGA

curve of GO.

Figure 2. rGO-N3-Glc dispersed in acidic and basic solutions (left), after 2 weeks (right).

Figure 3. FTIR spectra of GO-N3 (a), azide-Glc (b), azide-Man (c), azide-Gal (d), azide-Maltose

(e), rGO-N3-Glc (f), rGO-N3-Man (g), rGO-N3-Gal (h) and rGO-N3-Maltose (i).

Figure 4. (A) XRD patterns of GO-N3-Glc and GO-N3-Maltose; (B) TGA curves of GO, rGO-

N3-Glc, rGO-N3-Man, rGO-N3-Gal and rGO-N3-Maltose.

Figure 5. SEM images of rGO-N3-Glc (a), rGO-N3-Man (b), rGO-N3-Gal (c) and rGO-N3-

Maltose (d).

Figure 6. (A) FTIR spectra of GO (a), GO-diazide (b), rGO-diazide-Glc (c), (B) XRD patterns

of GO-diazide (a) and rGO-diazide-Glc (b), (C) FESEM images of GO-diazide (a,b) and rGO-

diazide-Glc (c,d).

Figure 7. TGA curves of GO, GO-diazide and rGO-diazide-Glc.

19

Highlights

• Grafting mono and disaccharides onto graphene oxide (GO)

• Using click reaction to obtain hydrophilic graphene nanosheets via two approaches

• Stability of sugar-grafted GO in basic media for two weeks

• Reduction of sugar-grafted GO by sodium ascorbate during click