Reduced Graphene Oxide and Carbon Nanotubes for the development of polyphenols amperometric biosensors

Biosensors for sensitive, rapid and precise determination of phenolic compounds are attracting growing interest in environmental control and protection as well as in food industry. Laccase (Lac) and tyrosinase (Tyr) are multicopper oxidase enzymes that catalyze the oxidation of phenol derivatives to the relevant quinones with the concomitant reduction of oxygen directly to water without the formation of reactive oxygen intermediates. Here we present the development of amperometric biosensors based on Lac or Tyr physically adsorbed on glassy carbon electrodes modified with carbon-based nanomaterials. Carbon-based nanomaterials are often employed in electrochemistry for their beneficial properties and in the last few years they have been frequently used in the development of biosensors to enhance the electron transfer between the electrode and the enzyme. Graphene represents an excellent material for sensing applications due to properties like fast heterogeneous electron transfer, large surface area, high mechanical strength, ease of functionalization, high conductivity and good biocompatibility. The electron transfer rates on graphene sheets obtained by electrochemical reduction of graphene oxide (GO) are similar to those observed for carbon nanotubes and higher than those of glassy carbon electrodes since the reduction of oxygen functional groups at edge and basal planes produces defect sites. The electrochemical reduction of GO is a versatile method to obtain a graphene layer (rGO) on the electrode surface which still displays some controllable oxygen-containing functionalities, usable for covalent or physical immobilization of enzymes. The enzyme immobilization represents a rather critical issue because the methods used for this procedure significantly influence the biosensor properties, operability and long-term stability. Among the methods available in the case of tyrosinase and laccase, the most commonly employed is the physical entrapment, obtained by a cross-linking with bovine serum albumin (BSA) and glutaraldehyde (GA).