Van der Waals Interaction Mediated Redox on Graphene Electrode

Abstract

Graphene, unlike (defective) graphene oxide, is generally not considered to be a highly efficient electrochemical electrode except for activity at the edges. This is because, graphene forms a highly stable hydration bilayer that screens the electric field emanating from the electrode to drive the electrochemical process. A remarkable electrochemical electrode property of graphene is discovered and explored. The property emerges from graphene’s single crystal, aromatic structure. The redox behavior of an aromatic compound, methylene blue (MB) and non-aromatic compounds, such as , potassium ferric cyanide are compared. The electrochemical behavior on monolayer of graphene is quantitatively measured by a novel opto-electrochemical instrument for both electrochemical and structural information. The instrument called, the Scanning Electrometer for electrical Double-layer (SEED) allows local measurement of redox on a 10 micron laser spot to exclusively measure redox signal away from the edges of graphene. Two distinct peaks for both reduction and oxidation during conventional cyclic voltametric (CV) waveform are observed by SEED (Figure 1). The two peaks were attributed to two distinct orientation of MB chemisorption on graphene and subsequent electron transfer. One of the peak depended on precise orientation of the MB on single crystal graphene surface indicating pi-pi interaction mediated chemisorption (Figure 2). The other peak is independent of graphene orientation. MB redox on Au showed a single peak (Figure 1) and was independent of orientation (Figure 2). The effect of the two modes of chemisorption on ionic strength was studied and compared with MB redox on pure gold performed on the same chip. The two peaks were not distinguishable by CV or differential pulse voltammetry (DPV) measured on the same sample under similar conditions. This indicated the sensitivity of SEED to obtain structural information and its ability to measure electrochemical activity on micro-spot. All the three measurements, SEED, DPV and CV were performed without taking the sample out of the solution. The redox of non-aromatic compounds showed no orientation effect. The distinct van der Waals interaction mediated chemisorption by pi-pi stacking on graphene electrode may have distinct advantage to design highly sensitive and specific sensors and biosensors in future, for example, in DNA and RNA sensing. Figure 1