Potentiometric probes limit

Although somewhat dependent on the method of electrode construction, the Rs of a potentiometric probe depends on the active tip area (24,25). One limiting factor for the ultimate size of the probe used in potentiometric measurements is the electrical characteristics. As Rs approaches 1013 fl, the noise level of the measurement will not allow greater than 1% accuracy, assuming very naively that the only noise source is the resistor Johnson noise. The noise level of small tips suggests that potentiometric tips of 0.05 /xm diameter are close to the smallest useful size. 

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. 

The minimum concentration is often related to the minimum amount of electroactive substance that can be measured with the electrochemical probe. Generally, with ISE or gas potentiometric probes the limit of concentration measurable is 10 moll The calibration graphs are therefore often S-shaped, leveling off at high concentration due to the or fCm-related maximum concentration measurable and at low values by the performances of the potentiometric base probe (Figure 1). 

Other applications snch as the measuranent of local concentrations in environmental analy-sis44,45 using potentiometric probes as detectors in various types of scanning electrochemical microscopies inclnding probes with dnal functionality are emerging, but are stiU largely limited to micrometer-size electrodes due to inherent difficulties of using nanoelectrodes. 

Instead of immobilizing the antibody onto the transducer, it is possible to use a bare (amperometric or potentiometric) electrode for probing enzyme immunoassay reactions (42). In this case, the content of the immunoassay reaction vessel is injected to an appropriate flow system containing an electrochemical detector, or the electrode can be inserted into the reaction vessel. Remarkably low (femtomolar) detection limits have been reported in connection with the use of the alkaline phosphatase label (43,44). This enzyme catalyzes the hydrolysis of phosphate esters to liberate easily oxidizable phenolic products. 

In scanning electrochemical microscopy (SECM) a microelectrode probe (tip) is used to examine solid-liquid and liquid-liquid interfaces. SECM can provide information about the chemical nature, reactivity, and topography of phase boundaries. The earlier SECM experiments employed microdisk metal electrodes as amperometric probes [29]. This limited the applicability of the SECM to studies of processes involving electroactive (i.e., either oxidizable or reducible) species. One can apply SECM to studies of processes involving electroinactive species by using potentiometric tips [36]. However, potentio-metric tips are suitable only for collection mode measurements, whereas the amperometric feedback mode has been used for most quantitative SECM applications. 

By coupling this biocatalytic activity with a C02 probe, L-glutamate was successfully determined potentiometrically in the concentration range from 4.4 x 10-4 to 4.7 x 10-3 mol 1 1 with a limit of detection (LD) of 2.0 x 10 4moll 1 and a slope of 48mV per concentration decade. 

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. 

Promethazine sensing devices based on MIPs include an MIP-based potentiometric sensor which was reported to be applicable in the concentration range of 5.0 X 10" - 1.0 X 10 M, with an LOD 1.0 X 10 M [409] and an MIP-modified carbon phase electrode with two linear response ranges of 4 x 10 -lx 10 M and 1 x 10 - 1 X 10" M and a detection limit of 2.8 x 10 M [381]. Propranolol detection has also been reported through MIP-based phosphorescent probes using tetrabromobisphenol A and diphenylmethane 4,4 -diisocyanate as functional monomers in tetrahydrofuran [380], and an MIP-based ion-selective sensor with a narrow linearity range of 10 -10" M [360]. 

---