Electrochemistry on the edge

Graphene edges can be used as nanoelectrodes for electrochemistry, new research shows. Researchers from five countries have joined hands to produce protruding graphene edges in a nanostructure, and to perform cyclic voltammetry with reactions on the edge. This method allowed for detection of redox species down to micromolar concentrations with sub-second time resolution. The research opens a path towards electrochemistry on a chip with a number of electrodes in parallel, as well as the use of low volumes of sample solution, and new effects such as studying electron transfer without an electrolyte.

Figure 1: (Left) Geometry of the device. Gold contacts attach to graphene sandwiched between two layers of hBN, with the graphene edge protruding. (Right) False-color SEM image of graphene edge between two edges of hBN, shown in yellow. Images taken from Plačkić et al, Small 2023, 2306361, under the license CC BY 4.0 DEED.

To make the device, the researchers sandwiched a layer of graphene between two layers of hexagonal boron nitride (hBN). The hBN serves as an insulator that prevents the graphene from reacting to chemicals in the environment. The edge of graphene is accessible, however, on the edge of the device, as depicted in Figure 1. The scientists then use the exposed edge to study electron transfer using several redox probes, including ferrocene(di)methanol, hexaammineruthenium, methylene blue, dopamine and ferrocyanide.

Figure 2: CV curves of reduction of methylene blue and hexaammineruthenium, at concentrations of 0.1 millimoles.

There are distinct benefits to studying electrochemistry at pristine single graphene edges. From a geometric standpoint, the small size of the electrode allows for improved mass transport capabilities. This is crucial for investigating rapid electron transfer kinetics (typically > 1 cm/s), as demonstrated earlier with single carbon nanotube electrodes. Also, the edge exhibits a unique electronic structure compared to the basal plane, leading to anticipated enhancements in electrochemical activity and catalytic effects. A graphene edge acts as a one-dimensional system, functioning akin to a nanoband electrode. While micro-/nano-band electrodes are commonly fabricated using metals like Au or Pt, utilizing carbon as a nanoelectrode provides an opportunity to explore specific redox active species, such as nicotinamide adenine dinucleotide (NADH), known to exhibit higher electron transfer rates on carbon surfaces than on metals. The basal plane of graphene can serve as a significant current collector for monitoring the response of the edge. In an ideal scenario, the redox current at each atom along the edge can be directly collected by the basal plane simultaneously, independent of the rest of the structure.

The achievement of one-dimensional electrochemistry on a graphene edge is a novel scientific phenomenon, with applications in fundamental studies of electrochemical reactions and practical use of reactions in small volumes on microchips.