Electrochemistry of phenothiazine and methylviologen biosensor electron-transfer mediators at nanoelectrode ensembles

Gold nanoelectrode ensembles (NEEs) have been prepared by using an electrol ess plating method to deposit disk-shaped Au electrode elements (diameter = 38 nm) within the pores of a microporous polycarbonate template membrane. The electrochemistry of three electron-transfer mediators used for biosenso rs based on reductase enzymes - two phenothiazines (Azure A and B) and meth ylviologen - were investigated at these NEEs. As has been observed previous ly, detection limits obtained at the NEE are lower than corresponding detec tion limits for a Au disk electrode of conventional dimensions (diameter = 3.2 mm, called a macro electrode here). However, the enhancement in the det ection limit at the NEE depends on the E-1/2 value of the mediator used. Al l three of these mediators have more negative E-1/2 values than the redox c ouples investigated previously at such NEEs. As such, their voltammetric wa ves are close to the negative limit for Au in the pH 7.4 buffer used as the electrolyte. The effects of background currents, associated with proton re duction, on the detection limits were investigated. At the macro electrode, the voltammograms for the phenothiazines are distorted by adsorption of th e reduced forms to the electrode surface. This adsorption process is concen tration dependent. The lower detection limits obtained at the NEE anew for the use of lower mediator concentrations, and this unwanted adsorption proc ess can be eliminated at the NEE. Finally, we report here the first use of the NEEs for the determination of standard heterogeneous rates constants. ( C) 2000 Elsevier Science B.V. All rights reserved.