Membrane electrodes potentiometric biosensors

The use of additional membranes, which selectively convert nonionic analytes into ionic species that can be determined via ISEs is another common approach. An abundance of ingenious designs make use of biocatalysts for the development of potentiometric biosensors. Much of the earlier designs have made use of enzymes as the molecular recognition element. The products that are associated with such enzyme-catalyzed reactions can be readily monitored with the potentiometric transducer by coating the traditional electrodes with the enzyme. 

Composite potentiometric sensors involve systems based on ion-selective electrodes separated from the test solution by another membrane that either selectively separates a certain component of the analyte or modifies this component by a suitable reaction. This group includes gas probes, enzyme electrodes and other biosensors. Gas probes are discussed in this section and chapter 8 is devoted to potentiometric biosensors. 

Enantioselective, potentiometric membrane electrodes (EPMEs) are proposed for the potentiometric detection of the enantiomers [2,10]. The advantages of utilization of these electrodes over amperometric biosensors and immunosensors are a longer lifetime, a large working concentration range, no dilution required for the samples and possibility of decreasing of limit of detection by utilization of KC1 0.1 mol/L as internal solution [2],... 

Ion-selective membrane electrodes as amperometric and potentiometric biosensors cannot be successfully used for ion monitoring in water. Their main characteristic is detection of an ion in the sample continuously and without any prior separation. The sampling process for a solid sample is reduced at its dissolution in distilled water. Due to the complexity of the matrix for wastewater or for seawater samples, there are a number of interfering inorganic and organic ions. Using biosensors for water analysis, one can obtain the total quantity of organic substances that are contained in a class it is practically impossible to discriminate the content of every compound from within the same class. 

The most sensitive enantioselective separation technique is capillary zone electrophoresis. Here, the detectors utilized are not sensitive enough to be able to detect the enantiomers. In the case of sensors, amperometric biosensors have been found to be most sensitive.264 A better enantioselectivity was found for potentiometric, enantioselective membrane electrodes because a direct interaction between the chiral selector and enantiomer occurred.282 285... 

Immunoreactions have proved to exhibit good enantioselectivity. They can yield both a sensitivity higher these biosensors and an enantioselectivity higher than potentiometric, enantioselective membrane electrodes. 

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. 

Guilbault and Montalvo were the first, in 1969, to detail a potentiometric enzyme electrode. They described a urea biosensor based on urease immobilized at an ammonium-selective liquid membrane electrode. Since then, over hundreds of different applications have appeared in the literature, due to the significant development of ion-selective electrodes (ISEs) observed during the last 30 years. The electrodes used to assemble a potentiometric biosensor include glass electrodes for the measurement of pH or monovalent ions, ISEs sensitive to anions or cations, gas electrodes such as the CO2 or the NH3 probes, and metal electrodes able to detect redox species some of these electrodes useful in the construction of potentiometric enzyme electrodes are listed in Table 1. 

Detection limits for enzyme-based potentiometric biosensors are governed by the iimate detection limits of the membrane electrode toward the product of the enzyme reaction, as well as the kinetics of the enzymatic reaction (Michaelis-Menten constant and turnover number of the enzyme), and mass transfer rate of substrate into the enzymatic layer. Carr and Bowers developed in detail the theory of how such parameters affect the response curves of such sensors [48]. Generally, enzyme-based potentiometric biosensors respond to target substrates over the concentration range of 0.01 mM to 10 mM. In the case of the urea example mentioned earlier, limited selectivity of the nonactin-based ammonium membrane electrode over potassium ions k ... 

In practice, potentiometric biosensors are used just like any other membrane electrodes or gas-sensing probe. Calibration plots are prepared using known standards of the biomolecule analytes (usually prepared in a given buffer), and exposing the sensors to these solutions until a steady state emf value can be recorded (usually in 1 to 2 minutes). Plots of emf versus log [substrate] are prepared and used to obtain concentration data for unknown samples (either diluted in buffer or nondiluted) that are exposed to the same sensor under exactly the same measurement conditions. 

In potentiometric biosensors the biological recognition reaction causes a modulation of a redox potential, a transmembrane potential, or the activity of an ion. So the potentiometric biosensors utilize the measurement of a potential at an electrode in reference to another electrode (Bard and Faulkner, 1980). Mostly, it is comprised of a permselective outer layer and membrane or sensitive surface to a desired species (a bioactive material), usually an enzyme. The enzyme-catalyzed reaction generates or consumes a species, which is detected by an ion-selective electrode. Usually a high-impedance voltmeter is used to measure the electrical potential difference or electromotive force (EMF) between two electrodes at near-zero current. The basis of this type of biosensor is the Nemst equation, which relates the electrode potential (E) to the concentration of the oxidized and reduced species. For the reaction aA + ne bB, the Nemst equation can be described as the following. 

Biosensors are measuring devices that produce a signal in proportion to the concentration of a defined group of substances through a suitable combination of a selective biological system (e.g., enzyme, antibody, membrane, organelle, cell or tissue) and a physical transmission device (e.g., potentiometric or ampero-metric electrode, optical or optoelectronic receiver). Examples include ... 

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