Systematic study of the correlation between surface chemistry, conductivity and electrocatalytic properties of graphene oxide nanosheets

Abstract

A main advantage of graphene oxide (GO) over other materials is the high tunability of its surface functional groups and of its electric conductivity. However, the complex chemical composition of GO renders difficult to unravel the correlation between structural and electric properties. Here, we use a combination of electron spectroscopy and electrochemistry to correlate the surface chemistry of GO to its electrical conductivity and electrocatalytic properties with respect to two molecules of high biological interest: β-nicotinamide adenine dinucleotide (NADH) and vitamin C. We demonstrate that the electrocatalytic properties of the material are due to hydroxyl, carbonyl and carboxyl groups residues that, even if already present on pristine GO, become electroactive only upon GO reduction. The results of this study demonstrate the advantages in the use of GO in amperometric biosensing and in enzymatic biofuel cells: it allows the oxidation of the target molecules at low potential values, with a sensitivity >15 times higher with respect to standard, carbon-based electrode materials. Finally, we demonstrate that the right amount of chemical groups to achieve such high performance can be obtained also by direct electrochemical exfoliation of bulk graphite, without passing through GO production, thus rendering this approach suitable for cheap, large-scale applications.

Electrochemical sensing of glucose by chitosan modified graphene oxide

“…Due to the poor conductivity of pristine GO, SPEs modified by such a nanomaterial were electrochemically treated at −1.25 V for 300 s in 0.1 M PBS [10] before the deposition of both glutaraldehyde and Enz, obtaining rGO coatings.…”

“…A similar peak-to-peak separation was also obtained for the GO+Ch blend; this result let us conclude that the simple mixing of GO with Ch molecules induces ring opening of the epoxy groups on GO, sufficiently improving the conductivity of this material for the electrochemical purposes. Suitable conductivity of simple GO coatings was only obtained after electrochemical pre-treatment at −1.25 V [10]; this notwithstanding, the charge transfer resistance of the resulting rGO coating was slightly worse, as evident from a peak-to-peak separation of about 80 mV. CV responses of Fc oxidation recorded at GO−Ch at different scan rates show a linear relationship (R 2 =0.998) between the anodic peak current and the square root of the potential scan rate (figure S6 in ESI), meaning that the charge transfer process is controlled by diffusion, as expected for a working electrode possessing performance suitable for the electroanalytical applications.…”

“…This material, in fact, possesses a nanosized dimension, allowing a high number of oxidized residues to be well exposed to the surrounding environment. These moieties are responsible for the activation of effective electrocatalytic processes in charge of several species acting as the analyte [8][9][10]. These same can be also used as the binding sites of (bio)molecules, e.g.…”

“…Except for these purposes specifically dealing with sensing, GO is often preferred to pristine graphene since it possesses greater solubility in many polar solvents, water included, and its surface chemistry can be easily tuned from highly oxidized, i.e. rich of epoxide, hydroxyl, carbonyl and carboxyl groups, to a structure possessing mainly sp 2 -hybridized carbon atoms, thanks to a wellcontrolled electrochemical reduction [10,14].…”

Do biomedical engineers dream of graphene sheets?

“…However, rGO is conceptually a different nanomaterial relatively to graphene since both the removal of 6 oxygen functional groups and the restoration of the π-π conjugation that occur during the reduction process are not able to induce a uniform sp 3 -sp 2 hybridization throughout all the extension of the carbon network 27 . In this way, by controlling the degree of reduction, it is possible to modulate the quantity of topological defects and residual oxygen moieties that are capable of influencing the biological, chemical, electrical, mechanical, optical and thermal properties of the final nanomaterial [28][29][30] . For example, Chen et al 31 have studied the capacity of three different chitosan derivatives to work simultaneously as reducing and stabilizing agents for GO with the final purpose of selecting the composite with less oxygen content and therefore with a more integrated conjugated carbon network for further drug delivery testing.…”