Close-proximity mode experiments

As in amperometric applications, many experiments can be conducted in the close proximity mode where the tip is moved very close to the substrate surface and a perturbation is applied to the sample. This perturbation may take several forms, typically potentiostatic or galvanostatic excursions if the sample is acting as an electrode, but also optical illumination with a laser beam, change of solution, etc. The tip response is then recorded as a function of time following the application of the perturbation. In these conditions potentiometric detection offers two advantages over amperometric detection (1) the range of ions detectable is extended to nonelectroactive species such as alkali metals, and (2) the tip response is selective. There are, however, some drawbacks. Because of the high impedance of the electrometer, the response time is worse in potentiometric applications where the t90 is rarely below 30 s. This must be compared to the millisecond time scale available with amperometric responses (89). Ohmic drop may also affect the tip potential. 

Kemp et al. (92) operated in the close proximity mode to study the concentration of chloride ions generated near the surface of UV-irradiated titanium dioxide in the presence of aqueous 2,4-dichlorophenol. The Cl concentration was measured potentiometrically with a 50 pan diameter Ag/ AgCl microdisk. Experiments were performed at several tip-substrate distances, and the concentration of chloride was recorded as a function of time after turning on and turning off the UV irradiation. 

SECM amperometric methods are based on the measurement of electrode currents (tip and substrate, (p and ig, respectively) as a function of various parameters, including tip-substrate distance (d) and tip or substrate potentials (Ej or Eg). Irrespective of the system studied (e.g., an electrode, an inert surface, a catalyst, or a living organism), the probe tip is a necessary component to perform any SECM experiment. In all cases, the amperometric tip is a UME that can be positioned in close proximity to another surface. There are two amperometric operation modes feedback and generation/collection modes. The preparation and characterization of commonly used amperometric tips of different geometries as well as their operation modes will be described in this section. 

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