Stability of mercury-coated platinum microelectrodes upon touching a solid surface in scanning electrochemical microscopy (SECM) experiments

The features of experimental approach curves, recorded in scanning electrochemical microscopy measurements, and acquired with a series of sphere-cap tips while approaching a solid insulating substrate, are examined in detail. Sphere-caps are prepared by electrodeposition of liquid mercury onto a platinum microdisk substrate 10 mum radius, and having RG=R/a=10 (R is total tip dimension, which includes the shield thickness and the electrode radius a). From the data obtained, it is established that sphere caps with aspect ratio h/a>0.3 (h is the sphere cap height) are able to touch, or even be squeezed against the surface of the solid substrate. Under these latter conditions, for the sphere caps, the negative feedback process at low tip-substrate distances, is more pronounced than that of the corresponding naked microdisk. A good tip stability and no mercury loss is observed upon touching the substrate for sphere caps with h/a<1.3. This circumstance allows one to perform "contact" voltammetric measurements of lead ions adsorbed onto a thin layer chromatography plate. (C) 2003 Elsevier Science B.V. All rights reserved.