Click chemistry approach to functionalize two-dimensional macromolecules of graphene oxide nanosheets

A facile “click chemistry” approach to functionalize 2D macromolecules of graphene oxide nanosheets with poly (ethylene glycol) of different molecular weights, polystyrene, palmitic acid and various amino acids was presented. FTIR, TGA, Raman spectroscopy, XPS, XRD, TEM, AFM and SEM were utilized to characterize the products. High degree of functionalization was achieved on the flat surfaces of graphene oxide, affording polymer-grafted 2D brushes and amino acids-immobilized nanosheets, which show improved solubility in organic solvents. The click chemistry strategy reported herein provides a facile and general method for functionalization of graphene oxide with macromolecules and desired biomolecules.

graphite, possesses abundant oxygen-containing functional groups, which not only render GO moderate water-dispersibility but also offer reactive sites for the further modification. Therefore, various reactions utilizing these oxygen-containing groups have been developed [4][5][6][7][8]. Haddon et al. have modified GO with long-chain alkylamine, making GO dispersed well in organic solvent [9]. Stankovich and coworkers functionalized GO with isocyanate derivatives and the resulting products show good solubility in polar aprotic organic solvents [10]. Chen et al. demonstrated that porphyrin and fullerene modified-graphene afforded the useful nonlinear optical properties [11]. In addition to small molecules, polymers were also employed to functionalize GO in order to improve the performance of GO in distinct domains. Yang et al. synthesized poly (2-(dimethylamino) ethyl methacrylate) (PDMAEMA)-grafted GO by ATRP [12], while Salavagione et al. functionalized GO with poly(vinyl alcohol) by esterification of carboxylic groups [13]. However, to functionalize GO with a facile and general strategy is still a challenge.
In addition, click chemistry, characterized by its modular nature, high selectivity and yields, has attained great development in recent years. Among various types of click reactions, the Cu-catalyzed Huisgen 1, 3-dipolar cycloaddition of azides and alkynes plays a particularly important role in organic synthesis for its excellent performance such as stability against dimerization or hydrolysis [14]. Since the azide-alkyne click chemistry has been demonstrated as a useful tool to modify carbon nanotubes (CNTs) [15][16][17][18][19][20][21], it would be quite efficient for the functionalization of GO, the analogue of CNTs. Most recently, the click chemistry has been tried to functionalize GO [22,23]. However, the reaction conditions and related influential and alkyne-terminated PEG, PS, C16, Gly and Phe were synthesized in our lab [18,20,24]. CuBr (Aldrich, 98%) was obtained from Aldrich and purified according to the published procedures [25]. GO was synthesized from natural graphite powder and the specific process is presented in our published paper [26,27]. The water solution of GO (100 ml, 7.5 mg/ml) reacted with 3-azidopropan-1-amine (NH 2 (CH 2 ) 3 N 3 ) (7.

Measurements
Thermogravimetric analysis (TGA) was carried out on a

Results and Discussion
Our functionalization protocol is shown in Scheme 1.
Carboxylic groups of GO react with 3-azidopropan-1-amine XPS was used to study surface elemental composition of different specimens. Figure 3C shows the XPS spectra of GO, GO-N 3 and GO-PEG. The nitrogen content increased from 1.2% for GO (nitrogen absorption from atmosphere) to 4.9% for GO-N 3 and then decreased to 3.8% for GO-PEG, confirming the successful attachment of azide groups and PEG. In addition, the carbon content of GO-C16 is as high as 80.9% compared with the content of 65.8% for GO-N 3 , which also proves the accomplishment of the click coupling reaction (Table S1 in Supporting Information).
XRD measurement is also performed to further study the changes in structure. As shown in Fig. 3D, the initial graphite In addition, the final products including GO-PEG show improved solubility in a variety of solvents such as chloroform, dichloromethane and so on (see Fig. 5), which will make considerable contribution to the application of graphene in the area of composites and other aspects.

Conclusions
In summary, an efficient click chemistry approach to functionalize 2D macromolecules of graphene oxide nanosheets has been presented and demonstrated with linear polymers and various amino acids. High degree of functionalization was achieved on the flat surfaces of GO, affording polymer-grafted 2D brushes and amino acids-immobilized nanosheets. The click chemistry strategy opens the door for facile functionalization of GO with macromolecules and desired biomolecules. The relevant work is in progress and will be reported later.