Permselectivity, Sensitivity, and Amperometric pH Sensing at Thioctic Acid Monolayer Microelectrodes.

Gold fiber thioctic acid (TA) monolayer (thickness of ∼8 Å) ultramicroelectrodes (UMEs) have been fabricated and characterized. Gold fibers were acid etched in order to produce monolayers that can act as high-quality permselective membranes. The selectivity and sensitivity of the TA monolayer UMEs compares favorably to that of TA monolayers prepared on Au vacuum deposited on large (∼1 cm(2) area) single-crystal silicon wafers. The results obtained here illustrate selectivity of the monolayer electrodes to electroinactive electrolyte anions, which participate indirectly in the electrode reaction, in addition to the previously established selectivity to electroactive cations and anions. The permselectivity of the monolayer to electrolyte anions can result in lower than expected steady-state currents at the monolayer UMEs, at the reaction potentials of fast-reacting analytes, and at high overpotentials for kinetically slow probes. Application of TA UMEs to amperometric pH sensing is also described in the determination of the pK(a) of TA at the monolayer surface. The method is compared with other methods of monolayer pK(a) measurements.