Graphene Ring Nanoelectrodes (GRiNs): Application As an Electroanalytical Sensor

We present the fabrication and characterisation of Graphene Ring Nanoelectrodes (GRiNs) at size ranges below 10 nm. Graphene was obtained from chemically exfoliated graphite oxide (GO) prepared by a modified Hummers’ method. GO was dip coated from colloidal solution onto optical fibre and the consequential GO layer was reduced to graphene by both chemical (hydrazine) and thermal treatments. Subsequent electrical connections and isolation of the optical fibre were carried out.

In an attempt to reduce the thickness of the ring electrodes, mechanical exfoliation of the graphene layer was conducted using an adaptation of the classic Scotch tape method. This approach stripped off all the graphene layers from the optical fibre and was abandoned because it was not conducive to achieving GRiNs in the sub-5 nm domain. The lower limit for electrode thickness is 0.335 nm corresponding to a single layer of graphene.

In light of this previous trial, it was proven that the concentration of GO suspension and its viscosity governs the thickness of the graphene layer. Results show that previously used GO suspension concentrations of 10 wt% yield a thick fluid with non-newtonian rheological properties that result in GRiNs with micrometer range ring electrode thicknesses. Thinner, newtonian fluids are obtained at 1, 1.5, 2, 2.5, 4, 6, 8 and 8.5 wt%. Non-newtonian fluids are obtained at 8.6 wt%. GRINs fabricated using 4 wt% GO solution are found to be conducting and, through the application of electrochemical sizing methods, were proven to have ring electrode thicknesses in the sub-10 nm range.

The resultant nanoelectrodes are highly reliable, the nanoring design allows for efficient utilisation of electrochemically active edge sites. The behaviour of the so-formed graphene ring nanoelectrodes was studied using the well-known electrochemically reversible system of Fe(CN)₆^4/3- with GRiNs of ring thicknesses 5-75 nm. Test results revealed that at thicknesses sub-10 nm, electron transfer rate constants are one order of magnitude larger than those obtained using macroelectrodes. Further, such GRiNs exhibit specific capacitances of two orders of magnitude greater than those reported for equivalent macroelectrodes.

These nanometer dimensions result in: increased densities of charge, and thus energy storage at the electrode surface compared to macroscopic systems; and enhanced rates of reactant mass transport to, and electron transfer across, the electrode/electrolyte interface (due to the more rapid potential drop within the diffusion double layer). These effects are especially enhanced at critical electrode dimensions of below 10 nm. Thus, their study consequently provides molecular level insight into nanoelectrochemistry application areas such as energy storage systems and environmental monitoring.