Combined scanning electrochemical atomic force microscopy (SECM-AFM) is a recently introduced scanned probe microscopy technique where the probe, which consists of a tip electrode and integrated cantilever, is capable of functioning as both a force sensor, for topographical imaging, and an ultramicroelectrode for electrochemical imaging. To extend the capabilities of the technique, two strategies for noncontact amperometric imaging-in conjunction with contact mode topographical imaging-have been developed for the investigation of solid-liquid interfaces. First, SECM-AFM can be used to image an area of the surface of interest, in contact mode, to deduce the topography. The feedback loop of the AFM is then disengaged and the stepper motor employed to retract the tip a specified distance from the sample, to record a current image over the same area, but with the tip held in a fixed x-y plane above the surface. Second, Lift Mode can be employed, where a line scan of topographical AFM data is first acquired in contact mode, and the line is then rescanned to record SECM current data, with the tip maintained at a constant distance from the target interface, effectively following the contours of the surface. Both approaches are exemplified with SECM feedback and substrate generation-tip collection measurements, with a 10-microm-diameter Pt disk UME serving as a model substrate. The approaches described allow electrochemical images, acquired with the tip above the surface, to be closely correlated with the underlying topography, recorded with the tip in intimate contact with the surface.

The radial flow microring electrode (RFMRE), a new hydrodynamic ultramicroelectrode, is described. In the RFMRE, solution flows from a capillary nozzle, which is positioned very close to a planar substrate using micropositioners. The RFMRE can be operated in one of two configurations: either (a) with the ring electrode on the capillary or (b) with the ring electrode positioned in the plane of the substrate directly underneath the capillary. In both arrangements, as fluid leaves the capillary, it is forced into the nozzle/substrate gap and flows radially past the ring electrode. Under these conditions, the RFMRE is effectively analogous to a microband channel electrode. The RFMRE is shown to be characterized by well-defined, variable, and high mass-transfer rates under steady-state voltammetric conditions. Mass-transfer coefficients in excess of 2 cm s -1 have been readily achieved with the RFMRE operating at relatively low volume flow rates (1.67 × 10 -2 cm 3 s -1 ). The device thus has considerable promise for electroanalysis and the study of fast electrode kinetics.The ability to characterize both heterogeneous electrode reactions and coupled homogeneous solution reactions with increasingly faster kinetics is a major challenge in electrochemical research. 1 The attainment of this goal requires the availability and development of techniques that are able to deliver the necessary high mass-transfer rates to compete with the reaction kinetics, under defined and controllable conditions. Hydrodynamic techniques, 2,3 where the electrode moves with respect to the solution, as with rotating (or vibrating) disks, 3 rings, 4 wires, 5 and photoelectrodes, 6,7 or where solution is forced past a stationary electrode, as exemplified by conical, 8 band, 9 and S0003-2700(98)00166-8 CCC: $15.00

A new method of hydrodynamic modulation voltammetry (HMV) is introduced, based on the microjet electrode (MJE) with an oscillating nozzle position. In MJE-HMV a jet of solution is fired at high velocities from a nozzle (with a typical diameter in the range 25−50 μm) onto the surface of a disk ultramicroelectrode (UME). The mass transport rate to the electrode is modulated by oscillating the lateral position of the jet between two different coordinates:  one where the jet impinges directly on the electrode surface and the other where the flowing stream (largely) misses the electrode. The resulting modulated (transport-limited) current in phase with the moving jet is quantitative and discriminates effectively against background processes. Studies of iridium (III) hexachloride (IrCl6 3-) oxidation at a Pt MJE serve to demonstrate the general capabilities of the technique. For this system, detection limits are estimated to be ∼5 × 10-9 mol dm-3. In its present form, modulation frequencies of as much as 20 Hz can be successfully employed without serious attenuation of the current signal, and there is scope for further improvement through the use of smaller nozzles and electrodes and piezoelectric positioners with improved frequency responses.