Specific Microelectrodes

The nonactin electrode exhibited a sensitivity of 150 1 for to Na and a preferential selectivity for NH over (Rechnitz, 1973). 

A chloride-sensitive microelectrode is made in an identical manner, except that Corning 477315 chloride ion exchanger is used in place of the potassium ion exchanger. 

There is a wide variety of ion-specific electrodes currently available and it would appear that new techniques will develop rapidly in the future. A promising area is the synthesis of series of cyclic polyethers by Pedersen (1967). These selectively bind alkali metal ions (Izatt et al, 1969). Such compounds hold promise for sensitive alkali-metal-ion-selective electrode membranes, and may well become the basis of some of the commercially available proprietary electrodes. 

Charlton, G., Read, D., and Read, J., 1963, Continuous intrarterial pOj in normal man using a flexible microelectrode, J. Appl. Physiol 18 1247-1251. 

Monitor and control of blood and tissue oxygen tensions, Trans. Am. Soc. Artificial Internal Organs 2 41-48. 

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The magnitude of the ohmic drop at a microelectrode can be evaluated quite readily for case 1 from a knowledge of the specific solution resistance (obtained from conductivity measurements such as in Table 12.1) and the expressions for the voltammetric current for the specific microelectrode employed. Case 2 is also straightforward if the free concentration of ions exceeds that of the electroactive species. However, the situation is somewhat more complicated for the third class. In this case, and in case 2 for fully associated electrolyte, migration as well as diffusion can affect the observed voltammetric signals. In all three cases, the situation may be further complicated by a change in structure of the double layer. However, this is ignored for now, and is considered in the section on very small electrodes. 

Walker (1973) has reported a technique for producing ion-specific microelectrodes which use liquid ion exchangers. The basic electrode is shown schematically in Figure 6.15. A liquid ion exchanger is formed by an organic electrolyte dissolved in a low-dielectric-constant organic solvent. 

Vyskocil, F., Kriz, N., Modifications of single and double-barrel potassium specific microelectrodes for physiological experiments. Pflugers Arch Eur J Physiol 1972,4(3), 265-276. 

Conventional colloid chemistry and elaitrochemistry have always been clo ly related with each other, the keywords electrophoresis, double layer theory, and specific adsorption describing typical asp ts of this relationship. In more ro nt times, new aspects have arisen which again bring colloid chemistry into contact with modem developments in electrcolloidal particles as catalysts for electron transfer reactions and as photocatalysts. In fact, the similarity between the reactions that occur on colloidal particles and on compact electrodes has often been emphasized by calling the small particles microelectrodes . 

The main classes of plasticizers for polymeric ISEs are defined by now and comprise lipophilic esters and ethers [90], The regular plasticizer content in polymeric membranes is up to 66% and its influence on the membrane properties cannot be neglected. Compatibility with the membrane polymer is an obvious prerequisite, but other plasticizer parameters must be taken into account, with polarity and lipophilicity as the most important ones. The nature of the plasticizer influences sensor selectivity and detection limits, but often the reasons are not straightforward. The specific solvation of ions by the plasticizer may influence the apparent ion-ionophore complex formation constants, as these may vary in different matrices. Ion-pair formation constants also depend on the solvent polarity, but in polymeric membranes such correlations are rather qualitative. Insufficient plasticizer lipophilicity may cause its leaching, which is especially undesired for in-vivo measurements, for microelectrodes and sensors working under flow conditions. Extension of plasticizer alkyl chains in order to enhance lipophilicity is only a partial problem solution, as it may lead to membrane component incompatibility. The concept of plasticizer-free membranes with active compounds, covalently attached to the polymer, has been intensively studied in recent years [91]. 

Electronically active chips (e.g.. Nanogen s NanoChip Electronic Microarray) are true microchips in which microelectrodes (pads) become elements of the array (Figure 2.13). The microelectrodes are covered with materials that allow immobilization of probes. Each electrode is individually addressable so that specific probes can be attached to different electrodes. Hybridization is accelerated by electromotive force (emf) on the target. Enhanced stringency is also achieved by modulation of the emf (Heller et al., 2000). 

The simplest specific example to examine is for a microelectrode with a spherical geometry [53]. The resistance (R) of a spherical microelectrode was given in Chapter 7. However, for a sphere whose radius is much smaller than the distance from the sphere to the counter electrode, the resistance is... 

Microelectrode arrays containing AChE were also utilised within a flow injection system [40]. A system was developed where a sample was separated and flushed simultaneously through eight cells, each containing a screen-printed electrode and fitted with a separate bespoke mini-potentiostat (Fig. 15.3). This allowed multiple measurements to be made on a single water sample using multiple electrodes, each specific for a different pesticide due to inclusions of different AChE mutants in each of the electrodes. Pattern-recognition software could then be utilised to deduce the pesticide levels in a potentially complex sample. 

This chapter focuses on the approach we followed for developing a novel electrochemical sensor platform based on disposable polymer microchips with integrated microelectrodes for signal transduction. It presents the development of the so-called Immuspeed technology, which is dedicated to quantitative immunoassays with reduced time-to-results as well as sample and reagent volumes. Prior to presenting the specific characteristics of Immuspeed, the basic principles integrated in this platform are first presented and illustrated with reference to... 

The electrode size is another important variable to analyze since the use of microelectrodes is very relevant for experimental electrochemical studies enabling the reduction of capacitative and ohmic drop effects, as indicated in Sect. 2.7. Specifically, it is of great interest to check the behavior of the system when the size of the electrode is reduced. In Fig. 4.20, the influence of the electrode radius on the... 

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