Constant-distance mode scanning potentiometry. High resolution pH measurements in three-dimensions
by Etienne, Mathieu; Dierkes, Paul; Erichsen, Thomas; Schuhmann, Wolfgang; Fritsch, Ingrid
A very small liquid-membrane based pH probe (less than 500 nm diameter) has been implemented in scanning electrochemical microscopy (SECM) equipped with a nonoptical shear force-based distance control. This constant-distance mode pH microscopy was tested over a complex microcavity (50 mu m diameter, 8 gm deep) possessing three individually-addressable gold electrodes: a microdisk at the bottom, a microcrescent ring 4 mu m away and along the wall, and a planar macroelectrode around the rim at the top. All the experiments were performed in a buffer of pH 6.22. The pH inside the microcavity was modified locally by applying negative potential (reduction of O-2 and H2O to produce OH-) either at the ring or disk electrodes. The shear force-sensitive signal permitted the precise determination of the tip-to-sample distance. Moreover, operating in constant-distance mode, the tip-to-sample distance was kept constant (less than 200 nm) while scanning the electrode over the sample. The simultaneous collection of both topographic and potentiometric information allowed a better understanding of the complex electrochemical processes occurring within the microcavity. Since potentiometry is not disturbing diffusion profiles in front of active electrodes constant-distance scanning potentiometry can be applied for the visualization of processes occurring in close proximity to electrode surfaces. This is especially important for the elucidation of properties of redox recycling systems often applied in amplified electrochemical assays.
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