Electrochemical functionalization of a gold electrode with redox-active self-assembled monolayer for electroanalytical application

The electrochemical functionalization of a Au electrode with a redox-active monolayer and the electroanalytical applications of the functionalized electrode are described. Reaction of the electrochemically derived o-quinone on the self-assembled monolayer (SAM) of 6-mercaptopurine (MPU) on a Au electrode gives a redox-active 4-(6-mercapto-pufin-9-yl)benzene- 1,2-diol (MPBD) self-assembly under optimized conditions. Electrochemical quartz crystal microbalance technique has been employed to follow the functionalization of the electrode in real time. Electrochemically derived o-quinone reacts at the N(9) position of the self-assembled MPU in neutral pH. Raman spectral measurement confirms the reaction of o-quinone on MPU self-assembly. MPBD shows a well-defined reversible redox response, characteristic of a surface-confined redox mediator at 0.21 V in neutral pH. The anodic peak potential (E-p(a)) of MPBD shifts by -60 mV while changing the solution pH by 1 unit, indicating that the redox reaction involves two electrons and two protons. The surface coverage (F) of MPBD was 7.2 +/- 0.3 x 10(-12) mol/cm(2). The apparent heterogeneous rate constant (k(s)(app)) for MPBD was 268 +/- 6 s(-1). MPBD efficiently mediates the oxidation of nicotinamide adenine dinucleotide (NADH) and ascorbate (AA). A large decrease in the overpotential and significant increase in the peak current with respect to the unmodified electrode has been observed. Surface-confined MPBD has been successfully used for the amperometric sensing of NADH and AA in neutral pH at the nanomolar level.