Potentiometric sensors detection

Several other approaches for detecting nucleic acids are reported in the literature, based, for example, on the light-addressable potentiometric sensor (LAPS) (Kung et al. 1990) or on acoustic wave devices (Su et al. 1996). 

Dill K., Song J.H., Blomdahl J.A., Olson J.D., Rapid, sensitive and specific detection of whole cells and spores using the light-addressable potentiometric sensor, J. Biochem. Biophys. Meth. 1997 3 161-166. 

A.P. Soldatkin, D.V. Gorchkoh, C. Martelet, and N. Jaffrezic-Renault, New enzyme potentiometric sensor for hypochlorite species detection. Sens. Actuat. B 43, 99-104 (1997). 

Detection of Li+ in artificial serum with a voltammetric Li-selective electrode in a flowthrough system was demonstrated [64], Lithium salts such as lithium carbonate have been extensively used for treatment of manic depressive and hyperthyroidism disorders. The therapeutic range of Li concentration is generally accepted to be 0.5-1.5mM in blood serum. The authors used normal pulse voltammetry in which a stripping potential was applied between pulses in order to renew the membrane surface and expel all of the extracted ions from the membrane, similar to galvanostatically controlled potentiometric sensors described above. Unfortunately, the insufficient selectivity... 

Besides, potentiometric sensors with ion-selective ionophores in modified poly(vinyl chloride) (PVC) have been used to detect analytes from human serum [128], Cellular respiration and acidification due to the activity of the cells has been measured with CMOS ISFETS [129], Some potentiometric methods employ gas-sensing electrodes for NH3 (for deaminase reactions) and C02 (for decarboxylase reactions). Ion-selective electrodes have also been used to quantitate penicillin, since the penicillinase reaction may be mediated with I or GST. 

One other difference lies in the type of detection technique used, which dictates the flow-cell design. Thus, a distinction can be made in this respect between optical (absorptiometric, luminemetric) sensors, which make measurements of the bulk solution where the flow-cell is immersed, and electroanalytical (amperometric, potentiometric) sensors, where measurements are based on phenomena occurring at the electrode-solution interface. 

Several classical ion-selective electrodes (some of which are commercially available) have been incorporated into continuous systems via suitable flow-cells. In fact, Lima et al. [112] used a tubular homogeneous crystal-membrane (AgjS or AgCl) sensor for the determination of sulphide and chloride in natural and waste waters. However, the search for new active materials providing higher selectivity and/or lower detection limits continues. Thus, Smyth et al [113] tested the suitability of a potentiometric sensor based on calix[4]arene compounds for use in flow injection systems. They found two neutral carriers, viz. methyl-j3-rerr-butylcalix[4]aryl acetate and... 

In the method proposed by van Staden for the determination of three halides, these are separated in a short colunm packed with a strongly basic ion-exchange resin (Dowex i-X8) that is placed in an FI manifold. A laboratory-made tubular silver/silver halide ion-selective electrode is used as a potentiometric sensor. Van Staden compared the response capabilities of the halide-selective electrodes to a wide concentration range (20-5000 pg/mL) of individual and mixed halide solutions in the presence and absence of the ion-exchange column. By careful selection of appropriate concentrations of the potassixun nitrate carrier/eluent stream to satisfy the requirements of both the ion-exchange column and the halide-selective electrode, he succeeded in separating and determining chloride, bromide and iodide in mixed halide solutions with a detection limit of 5 /xg/mL [130]. 

FIGURE 1.3 Schematic drawing of a light addressable potentiometric sensor (LAPS) for pathogen detection. (Adapted from reference Gehring et al., 1998). 

Cytosensor Microphysiometer technology has been used to detect perturbation in mammalian cells (Hafner, 2000). The system measures small changes in extracellular acidification using a light addressable potentiometric sensor. If the metabolism is interfered with, acid excretion will be affected which could be sensitively measured by LAPS. In principle, this system should be suitable for monitoring pathogen interaction with mammalian cells. 

Tu, S. 1., Uknalis, J., and Gehring, A. (1999). Detection of immunomagnetic bead captured Escherichia coli 0157 FI7 by light addressable potentiometric sensor. /. Rapid Methods Automat. Microbiol. 7, 69-79. 

In potentiometric sensors, an electrical potential between the working electrode and a reference electrode is measured at zero current conditions in a solution containing ions that exchange with the surface. The first potentiometric MIP sensor was prepared in 1992 by Vinokurov (1992). The substrate-selective polyaniline electrode was electrosynthesized with polypyrrole, polyaniline, and aniline-p-aminophenol copolymers. The development of an MIP-based potentiometric sensor was reported in 1995 by Hutchins and Bachas (1995). This potentiometric sensor has high selectivity for nitrite with a low detection limit of (2 + l)x 10 M (Fig. 15.10). 

Ion-selective sensors for chloride are commercially available [103]. Both fundamental and practical information concerning the theory, design, and operation of chloride-selective electrodes is available from a recent textbook [57]. Sensor membranes for potentiometric chloride detection have been formed from Ag2 S (as ion... 

Recent research in the field of polymer membrane ion-selective electrodes [389-391], has revealed that their se-lectivities [392-396] and limits of detections [394-397] could be improved by several orders of magnitude. The review of Bakker and Pretsch [398] summarized recent progress in the development and application of potentiometric sensors with low detection limit in the range 10-8-10-11 M. 

Potentiometric Sensors Inthe field of ion-selective electrodes, considerable progress has been achieved in the last few years. By buffering the primary ions concentration on a low level in the internal solution, ionic fluxes in the membrane are affected [424-426]. Thus, primary ion leakage into sample solution is hindered, resulting in a tremendous shift of detection limits to lower values for Pb +-selective electrodes, the detection limit up to 10 M level has been achieved for internal solution electrodes [424, 427] and below 10 M for all-solid-state electrodes with conducting polymer solid... 

A potentiometric sensor for the determination of hydroxyzine in tablets and biological fluids has been reported by Javanbakht et al. [121]. This is probably the first carbon paste electrode described in the literature based on MIPs and potentiometric detection. The polymer was prepared with a very general composition, MAA as functional monomer, EDMA as the cross linker, and chloroform as porogen. Response range was found to be 0.01-100 mM, with a moderate response time (SOTO min). The method was applied to the analysis of hydroxyzine in tablets, spiked human serum, and human urine. 

It is important to note that if the detected species is hydrogen ion, then all aci-dobasic species are mutually interfering. Improved selectivity can be obtained by the judicial choice of the internal potentiometric element and to some extent by the selective permeability of the hydrophobic membrane. Thus, for example, for selective detection of HCN (pKa = 3.32) the internal element should be a potentiometric sensor selective to CN and the pH of the internal electrolyte should be at least two pH units above the pKa value (e.g., pH >5.5). In that case, practically all HCN is dissociated. 

By far the most important practical use of this sensor is for automotive applications, namely for the control of the air to fuel ratio. It compares favorably with the surface conductivity or high temperature potentiometric sensor (Logothetis, 1987). Other gases could be detected on the same principle provided that the right materials for the electrochemical pump were used. The electrode materials/solid electrolytes used for the construction of potentiometric high temperature sensors (see Table 6.7) could serve as guidance. 

Traditionally, potentiometric sensors are distinguished by the membrane material. Glass electrodes are very well established especially in the detection of H+. However, fine-tuning of the potentiometric response of this type of membrane is chemically difficult. Solid-state membranes such as silver halides or metal sulphides are also well established for a number of cations and anions [25,26]. Their LOD is ideally a direct function of the solubility product of the materials [27], but it is often limited by dissolution of impurities [28-30]. Polymeric membrane-based ISEs are a group of the most versatile and widespread potentiometric sensors. Their versatility is based on the possibility of chemical tuning because the selectivity is based on the extraction of an ion into a polymer and its complexation with a receptor that can be chemically designed. Most research has been done on polymer-based ISEs and the remainder of this work will focus on this sensor type. 

Potentiometric sensors for trace analysis realized to date with detection limits and a brief description of membrane and inner solution composition... 

K.R. Rogers, J.C. Fernando, R.G. Thompson, J.J. Valdes and M.E. Eldefrawi, Detection of nicotinic receptor ligands with a light addressable potentiometric sensor, Anal. Biochem., 202(1) (1992) 111-116. 

W. Parak, M. George, J. Domke, M. Radmacher, J. Behrends, M. Denyer, and H. Gaub, Can the light-addressable potentiometric sensor (LAPS) detect extracellular potentials of cardiac myocytes IEEE Trans. Bio-med. Eng., 47 (2000) 1106-1113. 

S.I. Tu, J. Uknalis and A. Gehring, Detection of immunomagnetic bead captured E. coli 0157 H7 by light addressable potentiometric sensor, J. Rapid Methods Autom. Microbiol., 7(2) (1999) 69-79. 

Immobilization of bioactive material on/in the electrode allows combining bio-reaction selectivity with sensitivity of electrochemical detection. Irrespective of reaction in the biosensor, the electrochemical response is measured, in particular, as current at the given potential (amperometric sensor) or electrode potential (potentiometric sensor). 

An interesting additional feature of using potentiometric sensors in array mode has been pointed out, which is to profit from the extra information to gain confidence in measurements, that is to perform redundant analysis. This strategy can be the basis for automated fault detection of the sensory elements [48] and also be an aid for more robust calibrations [49]. 

The objective of this protocol is the fabrication of a light-addressable potentiometric sensor (LAPS) for the detection of the pH value and the cadmium-ion concentration in aqueous solutions. For the pH-sensitive LAPS, use, e.g., Ta205 as a sensor membrane, and for the cadmium-selective LAPS, use a Cd2+-selective chalcogenide glass thin film as a heavy metal-sensitive material. The electrochemical sensor characterisation of the LAPS structure perform current vs. voltage (I/V) and constant current (CC) measurements. 

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