Potentiometric probes response time

The above authors coimmobilized choline oxidase and AChE on a nylon net which was fixed to a hydrogen peroxide probe so that the esterase was adjacent to the solution. The apparent activities were 200-400 mU/cm2 for choline oxidase and 50-100 mU/cm2 for AChE. The sensitivity of the sequence electrode for ACh was about 90% of that for choline, resulting in a detection limit of 1 pmol/l ACh. The response time was 1-2 min. The parameters of this amperometric sensor surpass those of potentiometric enzyme electrodes for ACh (see Section 3.1.25). Application to brain extract analysis has been announced. 

The response time is one of the most critical characteristics of the potentiometric probe in continuous analyzers and in in situ monitoring. Most often the activity step method is used for its determination (Fig. 4). The response time is affected by several factors such as the properties of the ion-selective... 

To characterize potentiomebic probes, one must evaluate their response function, selectivity coefficient, response time, and ohmic drop. The response function of a potentiometric probe is a calibration curve of the measured membrane potential with the log of the concentration (or activity) of the primary ion must be acquired. To use concentration values, the activity coefficients must be known and many of than are tabulated. Another option is to use (when possible) dilute solutions such that the activity coefficients maybe neglected. 

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... 

Denuault et al. reported the use of potentiometric tips to probe ion fluxes near surfaces. The 10 and 50 pm diameter silver microdisk electrodes were used to probe the concentration profile due to the diffusion of Ag+ to and from a planar Ag electrode. A similar experiment was carried out with a 50 jm diameter microcylinder substrate electrode. The tip response was recorded at various locations above the substrate while simultaneously applying a potential square wave to the substrate to induce the dissolution and plating of silver. These experiments demonstrated the principle of the technique and confirmed the possibility of obtaining spatially resolved information. Moreover, these experiments clearly showed that the tips response time was in fact very good (tgo below 10 ms) (see Figure 10.14). Undoubtedly, this was due to their very low impedance. 

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