Amperometric sensors response

FIGURE 6-24 Response pattern of an amperometric sensor array for various carbohydrates. The array comprised carbon-paste electrodes doped with CoO (1), Cu20 (2), NiO (3) and Ru02 (4). (Reproduced with permission from reference 84.)... 

C. Malitesta, F. Palmisano, L. Torsi, and P. Zambonin, Glucose fast-response amperometric sensor based on glucose oxidase immobilized in an electropolymerized poly(o-phenylenediamine) film. Anal. Chem. 

Sensor A device having a response (ideally) for one particular analyte. Poten-tiometric sensors are typically ion-selective electrodes, while amperometric sensors rely on Faraday s laws. 

Selectivity can be viewed as the fraction of the overall response of a sensor to the species of interest in the presence of interfering species. For multiple electroactive species detectable by amperometric sensors we can write... 

Fig. 7.18 Response of fuel cell amperometric sensor to methane in air (adapted from Stetter and Li, 2008)...

Note that this equation describes the relationship between concentration C of the component and the sensor response X. It is purposely written backwards by comparison with the usual notation used with linear sensors (e.g., optical, amperometric, etc.) discussed earlier. This convention helps to define P as the matrix of regression coefficients. 

Sensing performance for H-,. Sensing performance of the amperometric sensor was examined for the detection of H2 in air. Figure 3 shows the response curve for 2000 ppm H2 in air at room temperature. The response was studied by changing the atmosphere of the sensing electrode from an air flow to the sample gas flow. With air the short circuit current between two electrodes was zero. On contact with the sample gas flow, the current increased rapidly. The 90% response time was about 10 seconds and the stationary current value was 10yUA. When the air flow was resumed, the current returned to zero within about 20 seconds. 

Figure 3. Response curve of the amperometric sensor to 2000 ppm H2 in air at room temperature.

Figure 12. Response curves of the modified amperometric sensor to CO.

In this chapter, the development of an amperometric sensor will be explained and discussed. The principle of the analysis method will be based on the results described in Chapter4 this means that use will be made of the oxidation reaction of hydrogen peroxide in the prewave, and that the concentration will be determined using the rate equation. In addition to measurement of the electrical current response, temperature and pH will therefore also be measured. Accordingly, it is interesting to start with an investigation of the temperature influence. 

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). 

The construction and response of amperometric biosensors for glucose, acetylcholine, and glutamate based on these polymeric materials are described, and the dependence of sensor response on the polymer structure is discussed. 

The covalent attachment of electron transfer mediators to siloxane or ethylene oxide polymers produces highly efficient relay systems for use in amperometric sensors based on flavin-containing oxidases. It is clear from the response curves that the biosensors can be optimized through systematic changes in the polymeric backbone. The results discussed above, as well as those described previously (25-32), show that the mediating ability of these flexible polymers is quite general and that it is possible to systematically tailor these systems in order to enhance this mediating ability. 

The same transducers covered with the polyHEMA membranes can serve as a basis for several other amperometric sensors. The optimum trade-off between rapid response (thin membrane) and stirring independency (thicker membranes) should be found for each particular application (77). In many cases, the hydrogel membrane... 

Choline oxidase and acetylcholineesterase Enzymes immobilized on a nylon net attached to H202-selective amperometric sensor. ChO is used for choline and AChE and ChO for acetylcholine. Rectilinear response in the range of 1-10 pM. Response time 1-2 min. Interferences occur from ascorbic acid, primary amines, and most seriously from betaine aldehyde. [64]... 

It has been shown that the thickness of the polypyrrole (PPy) film has a significant effect on the electrode performance.17 Figure 6 shows the dependence of the response of PPy/PQQ modified electrode to 10 mM DMAET (A) and 10 mM DEAET (B) as a function of PPy film thickness. As film thickness increases the oxidation current increases for both DMAET and DEAET, presumably due to increases in the amount of PQQ loaded in the PPy film. The maximum current for the oxidation of PQQH2 is observed when 200 nm films are used. When the PPy film thickness was larger then 200 nm a decrease in the sensor response was observed, which could be due to increased resistance (R ) of the thicker film. The optimum 200 nm PPy film thickness was used to characterize the performance of the electrode for amperometric detection of thiols. 

Amperometric sensors monitor current flow, at a selected, fixed potential, between the working electrode and the reference electrode. In amperometric biosensors, the two-electrode configuration is often employed. However, when operating in media of poor conductivity (hydroalcoholic solutions, organic solvents), a three-electrode system is best (29). The amperometric sensor exhibits a linear response versus the concentration of the substrate. In these enzyme electrodes, either the reactant or the product of the enzymatic reaction must be electroactive (oxidizable or reducible) at the electrode surface. Optimization of amperometric sensors, with regard to stability, low background currents, and fast electron-transfer kinetics, constitutes a complete task. 

Porphyrins are often employed in sensors on account of their ability to act as cation hosts and, with a suitable metal ion coordinated, as redox catalysts. Electropolymerised poly(metalloporphyrin)s have been used as potentiometric anion-selective electrodes [131] and as amperometric electrocatalytic sensors for many species including phenols [132], nitrous oxide [133] and oxygen [134]. Panasyuk et al. [135] have electropolymerised [nickel-(protoporphyrin IX)dimethylester] (Fig. 18.10) on glassy carbon in the presence of nitrobenzene in an attempt to prepare a nitrobenzene-selective amperometric sensor. Following extraction of the nitrobenzene the electrode was exposed to different species and cyclic voltammetric measurements made. A response was observed at the reduction potential of nitrobenzene (the polyporphyrin film acts only to accumulate the analyte and not in a catalytic fashion). Selectivity for nitrobenzene compared with w-nitroaniline and o-nitroto-luene was enhanced compared with an untreated electrode, while a glassy carbon-... 

Fig. 10.4. Schematic representation of the diflferent processes that control the response of an enzyme sensor. The scheme exemplifies an amperometric sensor with an oxidoreductase electrochemically connected to the electrode through a redox mediator.

Amperometric sensing is based on the record of the current response of an electrode in contact with the system to be analyzed under the application of a given potential input. Amperometric sensors operate under conditions where mass transport is limiting. 

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