Quantum-sized gold nanoclusters of size <2 nm possess unique and size-dependent redox behavior. Au_(25) nanoclusters (Au_(25)NCs), in particular exhibit redox potentials closer to conventionally used mediators such as ferrocyanide and ferrocene and hence our objective was to utilize these clusters as mediators in the development of electrochemical sensors and biosensors. We have prepared modified electrodes containing Au_(25)NCs and tested their electrocatalytic activity and amperometric sensing behavior. Au_(25)NCs have behaved as an electronic conductor and redox mediator as well. Further, we have tuned the electrostatic charge on these clusters by using glutathione ligand and improved the selectivity of the modified electrode towards charged analytes. Finally, in order to examine the utility of the Au_(25)NCs in electrochemical biosensing, an ionic liquid containing Au_(25)NC has been prepared (Au_(25)IL). The composite of Au_(25)IL and glucose oxidase enzyme has exhibited to be an excellent biosensor towards the determination of glucose.
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