A scanning micropipet contact method (SMCM) is described which promises wide-ranging application in imaging and quantifying electrode activity at high spatial resolution. In SMCM, a moveable micropipet probe (diameter 300 nm to 1 microm) containing an electroactive species in electrolyte solution is brought to a sample electrode surface so that the liquid meniscus makes contact. The micropipet contains a reference-counter electrode, and the sample is connected as the working electrode to make a two-electrode voltammetric measurement. SMCM thus makes possible highly localized electrochemical experiments, and furthermore, heterogeneous electrode surfaces may be investigated without the substrate being completely immersed in solution. This opens up the possibility of making measurements on a wide range of electrode materials without having to encapsulate the electrode. Furthermore, the electrode/solution contact can be made rapidly and briefly, which is useful for situations where the electrode would be unstable for longer periods (e.g., due to corrosion or surface adsorption). For heterogeneously active surfaces the technique is particularly powerful as it allows defined areas to be targeted and individual sites to be probed. To exemplify the approach, the electroactivity of basal plane highly oriented pyrolytic graphite (HOPG) and two types of aluminum alloy were investigated. SMCM measurements indicate that basal plane HOPG shows much greater activity than present consensus. Measurements of chemically heterogeneous aluminum alloy surfaces with SMCM allow variations in redox activity to be mapped with high spatial resolution.

Spatial variations in the electrical and electrochemical activity of microarray electrodes, fabricated entirely from diamond, have been investigated. The arrays contain approximately 50-mum-diameter boron-doped diamond (BDD) disks spaced 250 mum apart (center to center) in insulating intrinsic diamond supports, such that the BDD regions are coplanar with the intrinsic diamond. Atomic force microscopy (AFM) imaging of the surface reveals a roughness of no more than +/-10 nm over the array. Each BDD microdisk within the array contains polycrystalline BDD with a variety of different grains exposed. Using conducting-AFM, the conductivity of the different grains was found to vary within a BDD microdisk. Electrochemical imaging of the electroactivity of the microdisk electrodes using scanning electrochemical microscopy operating in substrate generation-tip collection mode revealed that, under apparently diffusion-limited steady-state conditions, there was a small variation in the response between electrodes. However, the majority of electrodes in the array appeared to show predominantly metallic behavior. For the electrodes that showed a lower activity, all grains within the microdisk supported electron transfer, albeit at different rates, as evidenced by studies on the electrodeposition of metallic silver, at potentials far negative of the flat band potential of oxygen-terminated polycrystalline diamond. The possibility of using these array electrodes for steady-state diffusion-limited measurements in electroanalytical applications is far-reaching. However, caution should be exercised in the kinetic analysis of voltammetric measurements, since wide variations in the electroactivity of individual grains are apparent when the potential is below the diffusion-limited value.

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