Graphene has quickly become a material under spotlight for development of new electrochemical sensors because of its unique electrochemical and structural properties. Since graphene has a large electrochemical potential window (approximately 2.5 V in 0.1 mM phosphate buffer saline solution), detection of molecules that have high oxidation or reduction potential (e.g. nucleic acids) become feasible. In addition, edges and defects on graphene provide a high electron transfer rate, suggesting that reduced graphene oxide (RGO) sheets or small flakes of pristine graphene are superior for electrochemical detection.
It has been demonstrated that the electron transfer rate of Fe3+/ 2+ on RGO is more than an order of magnitude higher than that on a glassy carbon electrode (GCE) due to the unique electronic structure of RGO, especially the high density of the electronic states over a wide energy range. Electron transfer can be enhanced also because small graphene flakes are able to provide direct electrical wiring between the electrode and the active centers of the redox enzymes. Interestingly, RGO has intrinsic catalytic activity towards some small enzymatic products such as H2O2 and NADH, making it attractive for enzyme-based sensors. Owing to its extremely high surface-to-volume ratio (theoretically, 2600 m2g-1) graphene based electrodes provide a large effective reaction area and high capacity for enzyme loading. A high surface-to-volume ratio also makes it ideal for functional composite, in which, a small percentage of graphene is able to provide percolating pathways for charge conduction.
Most graphene based electrochemical sensors use RGO because of
1) the abundant defects and chemical groups facilitate charge transfer and thus ensure high electrochemical activity
2) the populated chemical moieties on the RGO surface offer the convenience and flexibility for various fictionalizations to enhance the sensor performance
3) the chemical and electrical properties of RGO are highly tunable as compared to non-conductive graphene oxide (GO)
4) RGO can efficiently transport charges
Figure: Graphene-material based electrochemical sensor for detection of Hydrogen per Oxide. (a) Schematic of the construction of Hb-graphene–chitosan/ GCE. Hb = hemoglobin; GCE = glassy carbon electrode; graphene here is actually RGO (b) Illustration of a RGO sheet decorated with Prussian blue (PB) nanocubes.
Extracted and edited from “Biological and chemical sensors based on graphene materials by Yuxin Liu, Xiaochen Dong and Peng Chen in Chemical Society Reviews, 2012“