Potentiometric sensors measuring cell

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. 

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. 

Another type of sensor is a high-temperature solid-state potentiometric sensor for oxygen (>400°C) in industrial processes. These are based on the measurement of the potential of a concentration cell of the type... 

Measurement of the cell potential of a potentiometric sensor should be carried out under zero-current or quasiequilibrium conditions. A high input impedance electrometer is commonly used. The advantage of potentiometric sensor is that the sensor output and the cell potential do not depend on the electrode surface area. This can make the manufacturing of practical sensors simpler. A major disadvantage of... 

Potentiometric sensors are based on the measurement of the voltage of a cell under equilibrium-like conditions, the measured voltage being a known function of the concentration of the analyte. Potentiometric measurements involve, in general, Nernstian responses under zero-current conditions that is, the measurement of the electromotive force of the electrochemical cell. 

Amperometric sensing of gases is based on solid ion-conducting materials, as described for potentiometric gas sensors. Solid-state amperometric gas sensors measure the limiting current (ij) flowing across the electrochemical cell upon application of a fixed voltage so that the rate of electrode reaction is controlled by the gas transport across the cell. The diffusion barrier consists of small-hole porous ceramics. The limiting current satisfies the relationship ... 

With regard to in vivo gas-sensing devices, the majority of the work reported to date has involved oxygen-sensitive devices which operate as an electrolytic, not galvanic, type of electrochemical cell (i.e., current measured, not potential). Since such oxygen-sensing catheters are not based on ISEs, they will not be considered in this review. There has been, however, some limited work concerning the development of potentiometric sensors, particularly for in vivo COg measurements. One approach has been to devise... 

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]. 

The concept of electrode potentials, described here, has great advantages over considerations based on thermodynamic data calculated with measured potential differences of cells for application of solid-electrolyte potentiometric sensors it is simple to understand, results follow immediately and thus it is very helpful in practical cases. 

Detectors with Enhanced Gas Sensitivity. One frequent complaint regarding the use of potentiometric gas and ion-selective membrane electrodes for analytical purposes is lack of precision owing to the logarithmic response of such devices. Thus, uncertainty in measured potentials of ilmV will result in i-4% precision for sensors with slopes of 5 9 mV/decade and —8% for those devices based on response to divalent ions (e.g., the above sulfite and SO2 gas sensors). One novel approach suggested in the literature for enhancing response slopes of potentiometric sensors is to use several membrane electrode cells in series (28, 29). This arrangement results in response... 

The inner filling solution for the sensors is usually 0.01 M NaCl, which is also used to condition the potentiometric sensors. Electrochemical potential is measured with the following galvanic cell Ag/AgCl/bridge electrolyte/sam-ple solution/ion-selective membrane/inner filling solution/ AgQ/Ag. A high impedance pH-mV meter is used to measure the electrochemical potential. Selectivity coefficients are evaluated by the matched potential method (also known as method of mixed solutions), or via the separate solution method. 

Numerous applications have been developed in the field of chemical analysis using potentiometric measurements as indicators, including the production of potentiometric sensors and titration devices. In this chapter, we will focus on the defining principles of these potentiometric methods at zero current when these systems are in thermodynamic equilibrium, which is not necessarily true for all potentiometric measurements. In particular, the following description is confined to electrochemical cells with no ionic junction. In practice, these results will also be applied to many experimental cases in which ionic junction voltages can be neglected . 

The operation of potentiometric sensors is based on the measurements of concentration cell emf (see Chaps. 1 and 8), which makes it possible to extract the activity, concentration, or partial pressure of potential-determining species at the working electrode vs. RE. The WE potential may be established by a thermodynamic equilibrium or by a nonequdihrium steady state, whereas key requirements to the reference electrodes are related to their reversibility, stability, and, often, fast equilihratirMi on changing external conditions. The solid-state potentiometric sensors are used for a wide variety of technological applicatirms and probed species [2,3,5,15,18,86-91] their application for oxidic glass melts is addressed in Chap. 8. 

Furthermore, it was shown that it is possible to integrate and miniaturize indicator and reference electrodes while maintaining the characteristics of macro-conventional potentiometric sensors [21]. In consequence, it was shown that such electrodes can be placed together on a flat surface. In this way the miniature galvanic cells for flow-through modules, for direct measurements in small sample volumes can be obtained. 

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