Electrodes chemically selective

If metallic electrodes were the only useful class of indicator electrodes, potentiometry would be of limited applicability. The discovery, in 1906, that a thin glass membrane develops a potential, called a membrane potential, when opposite sides of the membrane are in contact with solutions of different pH led to the eventual development of a whole new class of indicator electrodes called ion-selective electrodes (ISEs). following the discovery of the glass pH electrode, ion-selective electrodes have been developed for a wide range of ions. Membrane electrodes also have been developed that respond to the concentration of molecular analytes by using a chemical reaction to generate an ion that can be monitored with an ion-selective electrode. The development of new membrane electrodes continues to be an active area of research. 

Figure 3. Components of an ion-selective electrode chemical sensor (left) and photographs of electrode body (right) showing electrode barrel with silver-silver chloride electrode, and screw-on electrode tip with end-clip for attaching the PVC membrane containing immobilised molecular receptors that will selectively bind specific target species.

We begin by pointing out that this concept of covering an electrode surface with a chemically selective layer predates chemically modified electrodes. For example, an electrode of this type, the Clark electrode for determination of 02, has been available commercially for about 30 years. The chemically selective layer in this sensor is simply a Teflon-type membrane. Such membranes will only transport small, nonpolar molecules. Since 02 is such a molecule, it is transported to an internal electrolyte solution where it is electrochemically reduced. The resulting current is proportional to the concentration of 02 in the contacting solution phase. Other small nonpolar molecules present in the solution phase (e.g., N2) are not electroactive. Hence, this device is quite selective. 

Research into chemically modified electrodes has led to a number of new ways to build chemical selectivity into films that can be coated onto electrode surfaces. Perhaps the simplest example is the use of the polymer Nafion (see Table 13.2) to make selective electrodes for basic research in neurophysiology [88]. Starting with the pioneering investigations by Ralph Adams, electrochemists have become interested in the electrochemical detection of a class of amine-based neurotransmitters in living organisms. The quintessential example of this class of neurotransmitters is the molecule dopamine, which can be electrochemically oxidized via the following redox reaction ... 

Dual-electrode LCEC is very useful for the selective detection of chemically reversible redox couples. In this case, two electrodes are placed in series (Fig. 27.1 OB). The first electrode acts as a generator to produce an electroactive species that is detected more selectively downstream at the second electrode, which is set at a more analytically useful potential. One excellent example of the use of a dual-electrode detector for electrochemical derivatization is the detection of disulfides [34]. In this case, the first electrode is used to reduce the disulfide to the corresponding thiol. The thiol is then detected by the catalytic oxidation of mercury, described earlier. Because of the favorable potential employed at the second electrode, the selectivity and sensitivity of this method are extremely high. In addition, thiols can be distinguished from disulfides by simply turning off the generator electrode. 

Because enzymes present such an attractive possibility for achieving chemical selectivity, enzyme electrodes were the first enzymatic chemical sensors (or first biosensors) made. The early designs used any available method of immobilization of the enzyme at the surface of the electrode. Thus, physical entrapment using dialysis membranes, meshes, and various covalent immobilization schemes have been... 

In general, traditional electrode materials are substituted by electrode superstructures designed to facilitate a specific task. Thus, various modifiers have been attached to the electrode that lower the overall activation energy of the electron transfer for specific species, increase or decrease the mass transport, or selectively accumulate the analyte. These approaches are the key issues in the design of chemical selectivity of amperometric sensors. The long-term chemical and functional stability of the electrode, although important for chemical sensors as well, is typically focused on the use of modified electrodes in energy conversion devices. Examples of electroactive modifiers are shown in Table 7.2. 

An effective means of achieving chemical selectivity is to modify the electrode with either a synthetic catalyst or an enzyme. Let us first consider the ampero-... 

There is an inherent similarity between the spectrum and an electrochemical current-voltage curve that is important from the point of view of chemical selectivity. In both cases, the x-axis (voltage or wavelength) is directly related to the energy. In electrochemistry, this energy corresponds to the transfer of electrons between the analyte and the electrode. It is related to the standard electrochemical potential. In optical interactions, molar absorptivity is probabilistically related to the excitation energy of the molecule. 

Cammann, K., Working with Ion-Selective Electrodes. Chemical Laboratory Practice, Springer Verlag, Berlin, 1979. 

Anodic chlorination of toluene and anisole using graphite electrodes, chemically modified with a-cyclodextrin, results in higher para selectivity in comparison to chlorination with NaOCl in presence of a-cyclodextrin in solution384. Similar results are observed with a Pt electrode38. ... 

As for the permeability measurements, most techniques based on the analysis of transient behavior of a mixed conducting material [iii, iv, vii, viii] make it possible to determine the ambipolar diffusion coefficients (- ambipolar conductivity). The transient methods analyze the kinetics of weight relaxation (gravimetry), composition (e.g. coulometric -> titration), or electrical response (e.g. conductivity -> relaxation or potential step techniques) after a definite change in the - chemical potential of a component or/and an -> electrical potential difference between electrodes. In selected cases, the use of blocking electrodes is possible, with the limitations similar to steady-state methods. See also - relaxation techniques. 

The loading is crucial for the response characteristics and the stability of the enzyme electrode. The choice of the enzyme determines the chemical selectivity of the measurement due to the specificity of the signal-producing interaction of the enzyme with the analyte. The selection of the indicator electrode is largely determined by the species involved in the enzyme reaction. 

The dependence of the etching selectivity of X-8000K2 relative to PIQ (a polyimide type resin from Hitachi Chemical Co.,) on O2 RIE conditions was examined. RIE power, O2 pressure, flow rate and the distance between the electrodes were selected as the parameters to determine the RIE condition. For these parameters, values of 100 W, 10 m Torr, 10 SCCM and 6 cm were selected as a standard condition. The changes in the etching rates are shown in Figure 3a, in which one parameter is varied and the remaining three parameters are fixed. 

A specific aim of this chapter is to compare the similarities and differences in reactivity that distinguish photoelectrochemical conversions from those that take place on poised electrodes or upon treatment with homogeneously dispersed redox reagents. In this chapter, therefore, we consider specifically how reactions that take place under controlled ultraviolet irradiation on a redox-active heterogeneous suspension permit specific chemical selectivity and control of oxidation and reduction level. 

The ion-selective membrane is the heart of an ISE as it controls the selectivity of the electrode. Ion-selective membranes are typically composed of glass, crystaUine, or polymeric materials. The chemical composition of the membrane is designed to achieve an optimal permselectivity toward the ion of interest. In practice, other ions exhibit finite interaction with membrane sites and will display some degree of interference for, determination of an analytei ion. In chnical practice, if the interference exceeds an acceptable level, a correction is required. 

Various additives are used for the Ni-Cd batteries to improve the battery performance. Additives are selected based on their special functions to improve the electrode structure and/or electrode chemical and electrochemical properties. For example, cadmium hydroxide Cd(OH)2 is added to the cathode to prevent phase segregation and to help maintain a single phase of the solid solution during the transfer between Ni(OH)2 and NiOOH in charge and discharge processes. Because Cd(OH)2 is isomorphous with both Ni(OH)2 and NiOOH, this structural functionality can improve the cycle life of the battery. Cd or CdO can increase the overpotential of oxygen evolution... 

The first group, which is developed in this chapter, use ion selective electrodes (ISE). The principle of these chemical sensors is to create an electric cell in which the analyte behaves in such a way that the potential difference obtained relates to its concentration. Measurement of pH, probably the most common and best known electroanalytical method, is part of this group. Most of the measurements concern the determination of ions in aqueous solution, though particular electrodes with selective membranes also allow the determination of molecules. The sensitivity of these methods is very great for certain ions but matrix is sometime responsible for lack of reliability in these measurements. In such cases, complexometric or titrimetric methods must replace direct potentiometry. It remains however for potentiometry multiple applications in which the instruments range from low-cost pH meters to automatic titrators. 

With regard to biosensors and analytical chip systems, challenging problems of electrochemical detection strategy are deterioration in selectivity and stability of biological functional objects like enzymes and chronic passivation of the underlying electrodes. Insufficient selectivity or specificity is an intrinsic limitation of electrochemical detection, which has been addressed by combining chemically or biologically specific receptors. 

An enzyme layer imparts chemical selectivity to the electrode. 

We want to focus on the modification of substrate electrodes with metal particles of mesoscopic dimensions and their characterization with STM. For such chemically heterogeneous surfaces, the chemical information complementing the structural investigation is required. The different chemical compositions of the constituents, namely the substrate and the deposit, is often manifest in different characteristic mesoscopic structures of both. Tlius, although the chemical selectivity of STM is rather poor, this chemical information can often be concluded fiom the mesoscopic structure. 

Semiconductor electrodes ion-selective field effect transistors (ISFETs) are semiconductor devices used to measure ionic species in solution. They are sometimes called chemical field-effect transistors or ChemFETs. The transistor is coated with silicon nitride, which is in contact with the test solution via an analyte-sensing membrane and also connected to a reference electrode. A variation in the concentration of the analyte ions changes proportionally with the voltage of the ISFET. ISFETs are rugged, have a faster response time than membrane electrodes and can be stored dry. 

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