Sensing electrode sensitivity

Selectivity of Gas-sensing Electrode The selectivity of the gas-sensing electrode may be enhanced by making use of such an internal electrode which is particularly sensitive enough to certain species other than the H+ ion. 

Example Nitrate-sensing electrode is employed to cater for a cell which will be sensitive exclusively to nitrogen dioxide (N02). The equilibrium of such a reaction may be represented as follows ... 

The measurement of pH is carried out using a sensing electrode, which is sensitive to hydrogen-ion activity and a reference electrode. Combination electrodes incorporating both of these electrodes are also suitable for most applications. Separate reference and sensing electrodes are normally used only for high-precision research applications. 

Bovine serum albumin (BSA) and cyclic AMP (cAMP) are determined by a competitive binding enzyme immunoassay (315). With urease as label, an ammonia gas-sensing electrode is used to measure the amount of urease-labeled antigen bound to a double-antibody solid phase by continuously measuring the rate of ammonia produced from urea as substrate. The method yields accurate and sensitive assays for proteins (BSA less than 10 ng/mL) and antigens (cAMP less than 10 nM), with fairly good selectivity over cGMP, AMP, and GMP. 

White et al. synthesized nanometric La2Cu04 through three techniques auto-ignition, Pechini method, and coprecipitation (White et al., 2008). The NPs were used to fabricate sensing electrodes for NO, and the effect of electrode microstructure on the sensitivity and response time was studied. The response times of the sensors were exponentially dependent on electrode grain size. Sensors with fine-grained electrodes were able to produce a steady-state and consistent voltage at lower temperatures, which improved their response sensitivity. 

In the recent years, the occurrence of anilines, chloroanilmes, and benzidines in environmental waters has started to be of concern due to their widespread use in various industrial processes. Detection of these compounds is generally carried out with LC-ECD working in the amperometric mode using a glassy-carbon electrode. Problems encountered are similar to those for phenolic compounds and, therefore, confirmation is always necessary by UV, DAD, or, preferably, MS detection [6]. These compounds are also CE amenable and this approach is really promising in the sense of sensitivity and selectivity because humic and fulvic material can be separated from the analytes due to their different migration kinetics. 

As low concentrations of dopamine are released and rapidly cleared from the extracellular space, the sensing electrodes must be sensitive, and selective and respond quickly[6]. For in vivo detection of neurotransmitters such as dopamine, physically small electrodes are advantageous due to their small size and high sensitivity to catecholamines[34]. There are currently no electrodes small enough to measure dopamine concentrations within the approximately 100-nm synapse, but considerable developments are being made in minimizing electrode size to approach synapses as closely as possible and also to minimize... 

For compendial purposes, pH is defined as the value given by a suitable, properly standardized, potentiometric instrument (pH meter) capable of reproducing pH values to 0.02 pH unit using an indicator electrode sensitive to hydrogen ion activity, the glass electrode, and a suitable reference electrode. The instrument should be capable of sensing the potential across the electrode pair and, for pH sfandardization purposes, applying an adjustable potential to the circuit by manipulation of "standardization," "zero," asymmetry, or "calibration" control. 

FIGURE 3.19 Dependence of gas sensitivity ( Z air - Z gas) on concentration of NO (or NOj) at 650°C and 700°C for the YSZ-based device using a ZnCr204-SE. (Reprinted from Miura, N., Nakatou M. and Zhuiykov, S., Impedancemetric gas sensor based on zirconia sobd electrolyte and oxide sensing electrode for detecting total NO at high temperature, Sens. Actuators B, Chem. 93 (2003) 221-228, with permission from Elsevier Science.)... 

Elumalai, P. et al.. Dependence of NOj sensitivity on thickness of oxide-sensing electrodes for mixed-potential-type sensors using stabilized zirconia. Ionics 12 (2006) 331-337. 

Wang, J. et al.. Improvement of NO2 sensitivity of mixed-potential-type zirconia-based sensor by the addition of WO3 to NiO sensing-electrode, in Proc. llth Int. Meeting on Chemical Sensors, Brescia, Italy, 11-17 July 2006, 32. 

FIGURE 5.14 Cross-sensitivities to various gases, such as CO, CjHg, NO, and NO2 (400 ppm each), at 800°C in humid conditions for the sensor attached with a 10 wt % CuO-added synthesized NiO-SE. (From Plashnitsa, V.V., Ueda, T., and Miura, N., Improvement of NOj sensing performances by an additional second component to the nano-structured NiO sensing electrode of YSZ-based mixed-potential-type sensor, Int. J. Appl. Ceram. Tech. 3 (2006) 127-133. With permission.)... 

Kobos et al. (1988) described the adsorption of urease on a fluorocarbon membrane for the construction of a urea sensor. The spontaneous adsorption was enhanced by a factor of 7 by perfluoroalkylation of the amino groups of the enzyme. The enzyme membrane was attached to an ammonia gas-sensing electrode. The urea sensor thus prepared exhibited a sensitivity of 50 mV per decade of urea concentration and a response time of 3 min. Only a small amount of enzyme could be adsorbed on the limited membrane surface, so the sensor was stable for only 7 days. 

The cells were immobilized on an ammonia gas-sensing electrode. The potential of the resulting microbial sensor was linearly dependent on the concentration of NTA between 0.1 and 1 mmol/1. Intermediates of NTA metabolism, such as glycine and serine, were measured with the same sensitivity. 

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