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pH-Responsive Chitosan-Mediated Graphene Dispersions Ming Fang, Jiang Long, Weifeng Zhao, Liwei Wang, and Guohua Chen* Institute of Polymer & Nanomaterials, Huaqiao University, 269 Chenghuabei Road, Quanzhou City, Fujian Province, 362021, P.R.China Received July 6, 2010. Revised Manuscript Received September 22, 2010 Homogeneous aqueous suspensions of graphene have been prepared by chemical reduction of graphene oxide in the presence of chitosan. The graphene sheets in thus prepared suspensions can be switched reversibly between a well dispersed and a more aggregated state with pH as a stimulus.
Introduction 2
Graphene, a single layer of sp -bonded carbon atoms, has attracted enormous attention owing to its unique structure and its exceptional electronic, mechanical, and thermal properties.1-4 These properties make graphene a highly promising material for many potential applications such as nanoelectronics for next generation high-speed logic devices,5,6 reinforced composites for electrical and thermal conduction,7,8 transparent electrodes for displays and solar cells,9-11 and sensors for chemical and biological diagnosing.12,13 So far, graphene has been prepared by micromechanical cleavage,14 chemical vapor deposition,15,16 epitaxial growth,17 liquid-phase exfoliation,18 and chemical reduction of graphene oxide (GO).19 Unfortunately, as-produced graphene generally shows limited solubility in conventional solvents, which has severely hindered its practical application. *Corresponding author. E-mail:
[email protected].
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Langmuir 2010, 26(22), 16771–16774
Noncovalent functionalization using dispersants (or mediating agents) has provided a simple but effective route to solve this problem. Up to now, various polymers and surfactants such as poly(sodium 4-styrenesulfonate),20 PmPV,21 TWEEN,22 sodium cholate,23 and DNA24 have been used to facilitate the dispersion of graphene. However, little attention has been paid to the possibility of controlling the dispersion/aggregation of graphene in solutions using stimuli-responsive polymers. Chitosan is a polysaccharide derived from incomplete deacetylation of chitin. Collected with excellent biocompatibility, biodegradability, antibacterial property, and metal binding ability, chitosan has found widespread applications in sensors, medicine, metal chelating agents, adhesives, and textiles.25 In addition, chitosan is a stimulus-responsive polymer with a solubility that can be reversibly alerted by pH changes.26 Using chitosan as a mediating agent for graphene dispersing may bring about an interesting system that is possibly integrated with both the merits of graphene and chitosan. Few polymers like chitosan hold the potential of endowing graphene with both biocompatibility and stimuli responsibility. Here, we demonstrate the fabrication of such graphene/chitosan system and discuss the pH-responsiveness of the system. Homogeneous aqueous suspension of graphene has been prepared by chemical reduction of GO in the presence of chitosan, and the graphene in the as-prepared suspension can be switched reversibly between a well dispersed and a more aggregated state with pH as a stimulus.
Experimental Section Materials. Chitosan with low molecular weight (Mw=5000 Da) and medium molecular weight (Mw = 300 000 Da, with a dynamic viscosity of 28 mPa s in 1% acetic acid, 20 °C) were obtained from Zhejiang Aoxing Biotechnology Co., Ltd. Chitosans with higher molecular weights (with dynamic viscosities between 200-400 and >400 mPa s) were purchased from Aladdin Chemistry Co. Ltd. Graphite powder (
