Electrochemical techniques potentiometric sensors

Electrochemical techniques anodic stripping voltammetry (ASV) and cathodic stripping voltammetry (CSV) for determining trace elements, and potentiometric sensors for determining dissolved gases (C02, N02, S02, NH3, H2S, HCN, and HF) as well as chloride, fluoride, cyanide, and sulfide. 

Different electrochemical sensors have been developed for cell concentration measurement. The most promising of these sensors are based on impedimetric measurements. A commercial version of a sensor that measures the frequency-dependent i)ermittivity is available from Aber Instruments Ltd [137-139]. Another type of electrochemical probe measures the potential changes in the cell suspension caused by the production of electroactive substances during cell growth [140-143]. To date, no on-line applications of these potentiometric sensors under real cultivation conditions have been reported. Other types of probes, such as amperometric and fuel-cell sensors, measure the current produced during the oxidation of certain compounds in the cell membrane. Mediators are often used to increase the sensitivity of the technique [143-145]. 

Potentiometric detectors and formation of new t3 pes of sensors including detectors without internal solution (all-solid-state). The composition of ion-selective layers was considerably and aptly modified. New solutions and additives in the membrane electrodes were used and tested looking for analogy with biological membranes. Special applications of such electrodes in medicine were studied. Development of methods of the flow analysis, which was introduced to analytical chemistry in the last quarter of the twentieth century. A number of original results in the methodology of that method were obtained at the University of Warsaw, especially in the use in detection of electrochemical techniques. Number of interesting and new results were obtained in electrophoretic methods. 

This chapter presents an overview of electrochemical techniques applied for the determination of surfactants. Special focus will be put on both potentiometric and amperometric sensors and biosensors. 

The disadvantages described above in terms of the irreversibility of the polyion response stimulated further research efforts in the area of polyion-selective sensors. Recently, a new detection technique was proposed utilizing electrochemically controlled, reversible ion extraction into polymeric membranes in an alternating galvanostatic/potentiostatic mode [51]. The solvent polymeric membrane of this novel class of sensors contained a highly lipophilic electrolyte and, therefore, did not possess ion exchange properties in contrast to potentiometric polyion electrodes. Indeed, the process of ion extraction was here induced electrochemically by applying a constant current pulse. 

Contrary to potentiometric methods that operate under null current conditions, other electrochemical methods impose an external energy source on the sample to induce chemical reactions that would not otherwise spontaneously occur. It is thus possible to measure all sorts of ions and organic compounds that can either be reduced or oxidised electrochemically. Polarography, the best known of voltammetric methods, is still a competitive technique for certain determinations, even though it is outclassed in its present form. It is sometimes an alternative to atomic absorption methods. A second group of methods, such as coulometry, is based on constant current. Electrochemical sensors and their use as chromatographic detectors open new areas of application for this arsenal of techniques. 

It is typically on the order of several hundred nanometers. In practice the minimum thickness for polymeric membranes is 50gm or greater, which is far more than one would expect from (6.53). This is apparendy due to the fact that these membranes hydrate in the bulk, thus increasing the dielectric constant. They also form a hydrated layer at the solution/membrane interface (Li et al 1996) which affects their overall electrochemical properties and selectivities. Macroscopic ISEs use relatively thick membranes ( 500jU.m). In contrast, it is desirable to use thin membranes in the construction of asymmetric solid-state potentiometric ion sensors, in order to make their preparation compatible with the thin-layer preparation techniques. 

In order to maintain the advantage of the microfabrication approach which is intended for a reproducible production of multiple devices, parallel development of membrane deposition technology is of importance. Using modified on-wafer membrane deposition techniques and commercially available compounds an improvement of the membrane thickness control as well as the membrane adhesion can be achieved. This has been presented here for three electrochemical sensors - an enzymatic glucose electrode, an amperometric free chlorine sensor and a potentiometric Ca + sensitive device based on a membrane modified ISFET. Unfortunately, the on-wafer membrane deposition technique could not yet be applied in the preparation of the glucose sensors for in vivo applications, since this particular application requires relatively thick enzymatic membranes, whilst the lift-off technique is usable only for the patterning of relatively thin membranes. 

Potentiometric probes are the oldest forms of electrochemical sensors. They can conveniently be used for studying many interesting chemical systems not accessible to voltammetric techniques. In particular, alkali and alkaline earth metal ion concentrations, of importance in biological systems,... 

Since in most cases only one enantiomer possesses a desired pharmacological activity, it is necessary to construct enantioselective sensors to improve the quality of analysis due to the high uncertainty obtained in chiral separation by chromatographic techniques.315 For this purpose, enantioselective amperometric biosensors and potentiometric, enantioselective membrane electrodes have been proposed.264 The selection of one sensor from among the electrochemical sensor categories for clinical analysis depends on the complexity of the matrix because the complexity of different biological fluids is not the same. For example, for the determination of T3 and T4 thyroid hormones an amperometric biosensor and two immunosensors have been proposed. The immu-nosensors are more suitable (uncertainty has the minimum value) for direct determination of T3 and T4 thyroid hormones in thyroid than are amperometric biosensors. For the analysis of the same hormones in pharmaceutical products, the uncertainty values are comparable. 

Micromachined and microfabricated electrochemical sensors have been used either per se, or as part of a sensor system, in many practical applications. This includes various biosensors and chemical sensors reported in research literature. An example of a practical electrochemical sensor is the yttria-stabilized zirconium dioxide potentiometric oxygen sensor used for fuel-air control in the automotive industry. Thick-film metallization is used in the manufacture of this sensor. Even though the sensor is not microsize, this solid electrolyte oxygen sensor has proven to be reliable in a relatively hostile environment. It is reasonable to anticipate that a smaller sensor based on the same potentiometric or the voltammetric principle can be developed using advanced microfabrication and micromachining techniques. 

In many instances electrochemical sensors can be used in whole blood or undiluted plasma. This caused some problems when potentiometric results were compared with other techniques, e.g., flame photometry, which use diluted plasma as analyte. The disagreements were at least... 

Nearly all chemical sensors useful for liquid samples can be utiUzed to indicate titrations. Besides the preferred potentiometric, other electrochemical probes are also used, mainly amperometric and conductometric sensors. The so-called biamperometric titration works with simple wire pairs. Photometric and thermometric indication techniques are less common than electrochemical methods. Miniaturization does not play an important role for titration probes. Classical arrangements predominate to this day. Commercial titration instruments are only slowly starting to make use of the achievements of modern sensor technology. As an example, optodes have achieved a certain popularity in recent years for titration applications. 

Several useful techniques for water analysis utilize electrochemical sensors. These techniques can be potentiometric, voltammetric, or amperometric. Potentiometry is based on the general principle that the relationship between the electrical potential of a measuring electrode and that of a reference... 

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