Scanning Electrochemical Microscopy. 28. Bon-Selective Neutral CarriermBased Microelectrode Potent iometry

Electrodes for scanning ion-selective potentiometric microscopy (or potentiometric SECM) were developed and used to image local concentration profiles of NH4+, K+, and Zn2' ions. Neutral carrier-based micropipet ionselective electrodes with high spatial resolution (-2-3 pm) and fast response time (-300 ms) were fabricated and employed as potentiometric SECM tips, and highresolution ion-selective images around micrometer-scale objects were obtained for selected model systems. This technique was also used to map enzyme activity and to measure concentrations within the diffusion layer of an ultramicroelectrode. Several methods are used to determine the absolute distance between tip and target on the basis of measurement of solution resistance using As/ AgCl micropipet electrodes or potentiometric measurement of the steadystate concentration profile using ionselective microelectrodes; the corresponding theories of both are presented. Dual-channel tips are also introduced for ion-selective potentiometric microscopy. In these, one channel operates as an ion-selective electrode and the other as a distance sensor. Quantitative concentration profiles can be obtained with knowledge of absolute tip position.