Hydrodynamic Modulation Voltammetry with a Variable-Height Radial Flow Microring Electrode

Abstract

A new form of hydrodynamic modulation voltammetry (HMV), employing a radial flow microring electrode (RFMRE), is described. In the RFMRE, a nozzle, bearing a thin ring electrode (thickness 0.1−0.5 μm) is positioned close to a planar inert substrate, forming a thin-layer radial flow cell. Solution flows down the nozzle, impinging on the surface of the substrate, and is then forced into the nozzle−substrate gap whereupon it flows radially across the surface of the ring electrode. At constant volume flow rate, mass transport to the RFMRE is readily modulated by periodically varying the effective height of the flow cell, on the micrometer scale. As a result of the rapid diffusional and hydrodynamic relaxation times in this device, the modulated current in phase with the oscillation of the cell height can be described quantitatively using existing models for the variation of the mass-transfer-limited current with nozzle−substrate separation. The capabilities of RFMRE-HMV, in terms of improved amperometric detection close to the solvent window and at low concentration, are illustrated through studies of the oxidation of tris(2,2‘-bipyridyl) ruthenium(II) and hexachloride iridate(III) at a Pt electrode in aqueous solution. A key feature of this modulation technique is that the in-phase currents recorded in the RFMRE-HMV configuration (which discriminate against background processes) are far in excess of the equivalent dc currents (which do not discriminate against background processes) measured at thin ring electrodes in quiescent solution.