Covalently Linked, Water-Dispersible, Cyclodextrin: Reduced- Graphene Oxide Sheets Bharathi Konkena and Sukumaran Vasudevan* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India * S Supporting Information ABSTRACT: Reduced-graphene oxide (rGO) sheets have been functionalized by covalently linking β-cyclodextrin (β-CD) cavities to the sheets via an amide linkage. The functionalized β-CD:rGO sheets, in contrast to rGO, are dispersible over a wide range of pH values (2-13). Zeta potential measurements indicate that there is more than one factor responsible for the dispersibility. We show here that planar aromatic molecules adsorbed on the rGO sheet as well as nonplanar molecules included in the tethered β-CD cavities have their fluorescence effectively quenched by the β- CD:rGO sheets. The β-CD:rGO sheets combine the hydro- phobicity associated with rGO along with the hydrophobicity of the cyclodextrin cavities in a single water-dispersible material. ■ INTRODUCTION Graphene sheets that are one atom thick and consist of 2D layers of carbon atoms have gained enormous importance over the past few years because of their unique attributes - high electronic and thermal conductivities and exceptional mechan- ical strength. 1,2 Established procedures (such as mechanical exfoliation or chemical vapor deposition) are not ideal for the large-scale manufacture of processable graphene sheets. The chemical reduction of suspensions of graphene oxide (GO) has emerged as a viable route for the large scale production of graphene sheets. 3-5 GO is the material of choice for large-area electronics because it can be deposited readily and uniformly on a variety of substrates. 6,7 Subsequent chemical or thermal reduction of oxygen functionalities allows the electrical conductivity to be tuned over several orders of magnitude. 8 The reduced-graphene oxide sheets, although their conductiv- ities are comparatively lower than that of graphene obtained by physical methods such as mechanical exfoliation, are never- theless versatile materials. A major challenge is that the reduced-graphene oxide sheets (rGO), unlike graphene oxide sheets, are hydrophobic with limited aqueous dispersibility and easily form agglomerates irreversibly or even restack to form graphite in aqueous solutions. Zeta potential measurements show that the colloidal dispersions of both rGO and GO are electrostatically stabilized, but whereas GO sheets are dispersible at pH values of as low as 4.0, the rGO sheets are dispersible only above pH 8. 9,10 We had shown earlier from pK measurements that the superior dispersibility of GO as compared to that of rGO is due to the presence of ionizable groups on GO that are acidic (pK 4.3) in addition to groups with pK values of 6.6 and 9.0. In contrast, rGO sheets have ionizable groups with a single pK value of 8. 11 In both GO and rGO sheets, it is the ionization of the carboxylic groups that is primarily responsible for the built-up charge, but on GO sheets, the presence of phenolic and hydroxyl groups in close proximity to the carboxylic groups lowers the pK a value by stabilizing the carboxylate anion, resulting in superior water dispersibility. 11 There have been reports in the literature on differing strategies to enhance the aqueous dispersibility of rGO sheets. Most involve either the covalent or noncovalent attachment of suitable dispersing agents 12,13 that include octadecylamine, 14 porphyrin, 15 polystyrene, 16 poly(sodium 4-styrenesulfonate), 17 3-methylimidazolium, 18 and DNA 19 as well as large aromatic molecules. 20 Here we exploit the presence of reactive carboxylic groups at the edges of the GO sheets to link β-cyclodextrin (β- CD) cavities covalently via the formation of an amide bond. Subsequent chemical reduction removes most oxygen function- alities present on the basal plane and the edges to give rGO sheets with β-CD cavities attached to the edges of the sheet. The presence of hydrophilic -OH groups on the rim of the CD cavities renders the β-CD:rGO sheets water-dispersible. This is not the first attempt on the use of CDs to enhance the processability of graphene sheets. 21-24 A composite formed by a physical mixture of GO and CD that is subsequently reduced has recently been reported. 21 It is claimed that the CD cavities remain attached to the rGO sheets through hydrogen bonds formed between the cyclodextrin hydroxyl groups and oxygen functionalities present on the rGO sheets, although no direct evidence is provided. A composite of the oxidized GO with amino cyclodextrin cavities with the two components interacting by electrostatic forces has also been reported. 25 Received: May 21, 2012 Revised: August 8, 2012 Published: August 9, 2012 Letter pubs.acs.org/Langmuir © 2012 American Chemical Society 12432 dx.doi.org/10.1021/la3020783 | Langmuir 2012, 28, 12432-12437