Potentiometric probes distance

Several of the procedures described in the previous sections can be advantageously carried out with double barrel tips. Such a probe consists of two capillaries (see Sec. V.B), one of which acts as the potentiometric sensor, while the other is used to determine the tip-substrate distance. For example (79), a gallium microdisk was combined with an ion-selective (K+) potentiometric probe to image K+ activity near the aperture of a capillary (see Fig. 7). Similarly (77), a double barrel tip with one channel as an open Ag/ AgCl micropipette for solution resistance measurement and the other channel as an ion-selective neutral carrier-based microelectrode for potentiometric measurements was successfully used to image concentration distributions for NH4 (Fig. 8) and Zn2+ (Fig. 9). While dual-channel tips facilitate the approach of the substrate and permit a direct determination of the absolute tip-substrate distance, their difficult fabrication severely limits their use. Reference 80 compares the above methods. 

To perform potentiometric SECM, one needs to combine a high impedance voltmeter as described above with an SECM (homebuilt or commercial). The use of amperometric mode SECM is still desirable as it can be used to position the pipette and determine the tip-substrate distance. A more detailed discussion concerning the positioning of potentiometric probes and the acquisition of approach curves will be presented in the next section. 

To use potentiometric probes in SECM, it must be possible to exactly evaluate the tip-substrate distance. Different methods to evaluate the distance dependence are presented... 

Analogous to SECM amperometric methods, SECM potentiometric methods are based on the measurement of electrode potentials (tip and substrate, AE-j- and AEg, respectively) as a function of various parameters, including tip-substrate distance (d), XY location, and time. Again, a potentiometric tip responsive to the desired solution component is necessary to perform any SECM potentiometric experiment. To use potentiometric probes in SECM, it must be possible to exactly evaluate the tip-to-substrate distance. Different electrochemical methods to evaluate the potential-distance dependence have been investigated, for example, utilization of metal/metal oxide electrodes as both amperometric and potentiometric tips for pH measurement," double barrel tips having one... 

An alternative approach to obtain/(mol/cm /s) that is also suitable for potentiometric probes converts the i d) data points of an approach curve over the center of the enzyme-modified surface to concentrations as function of normalized distance C(H) and fits Equation 11.8 to this using an independently measured diffusion coefficient D of P and r ... 

At smaller values of the tip-substrate separation distance, d, feedback and hindering effects, as those observed in the feedback modes at amperometric tips, perturb the transport processes. The tip then starts to interfere with the source, which complicates the quantitative data analysis without numerical modeling.Such perturbing effects are not observed when passive probes such as potentiometric " or biosensor microelectrodes are used, but then the positioning of these substance-selective sensors is difficult. An alternative would be to consider the use of the scanning ion conductance microscopy (SICM). Indeed, recently, the SICM afforded the opportunity to image and quantify precisely local K+ and Cl ionic fluxes. ... 

Potentiometric SCEM requires an independent determination of the absolute distance between the microelectrode and the sample surface. This is motivated by two reasons. First, the probes tend to be very fragile and it is important to restrict their travel to avoid touching the sample, and second, the potentiometric response of the tip does hold the tip-substrate distance dependence found in the amperometric mode. As a result, quantitative information regarding concentration profiles can only be extracted with the knowledge of the tip-substrate distance. The latter is considered in subsequent sections. 

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