Amperometric chemical sensors

This type of diffusion/reaction mechanism has been treated semi-analyti-cally by Albery et al. [42, 44, 45], under steady-state conditions and its applications to amperometric chemical sensors has been described by Lyons et al. [46]. In both models, only diffusion and reaction within a boundary layer is considered, while the effect of concentration polarisation in the solution is neglected. Thus, to apply the model to an experimental system it is necessary to be able to accurately determine the concentration of substrate at the polymer/solution interface. Assuming that the system is in the steady state, the use of the rotating disc electrode allows simple determination of the substrate concentration at the interface from the bulk concentration and the experimentally determined flux using [47]... 

The catalytic detection of ammonium ions has not been extensively investigated in contrast with the large variety of potentiometric and amperometric chemical sensors and optical sensors described in the literature [236], Similarly, the detection of ammonia in air has merited diflierent approaches in the field of chemical sensors. Screen-printed electrodes modified with Meldola s Blue and covered with a polycarbonate membrane constitute the basis of the catalytic detection of NHj. The measurement is based on the electrocatalytic reduction of NADH upon addition of glutamate dehydrogenase to a stirred solution containing NADH, 2-oxoglutarate and ammonium ions. The rate of current decrease (nA s ), measured at 50 mV, correlates to the concentration of ammonium ions in the sample. Recoveries of ammonium ions in spiked pond and tap waters at the level of 0.1 ppm are close to 100%, which demonstrates the feasibility of this assay for the detection of ammonium ions in waters [237],... 

Faradaic processes of electrode reactions, which are principle mechanism of obtaining analytical signal in amperometric sensors, significantly depend on working electrode material and state of its surface. The common working electrode materials include noble and seminoble metals, solid oxides of various elements and different kinds of carbon materials including carbon nanostructures. They are employed in conventional voltammetric measurements with various modes of electrode polarization, as amperometric chemical sensors, as well as for construction of amperometric biosensors. 

Lyons, M.E.G., Lyons, C.H., Michas, A., and Bartlett, P.N. 1992. Amperometric chemical sensors using microheterogeneous systems. Analyst 117, 1271-1280. 

E-tongues have reportedly been used to obtain data for sourness, bitterness, and astringency for foodstuffs such as beers, wines, and teas. This involved detecting sensory attributes of bitter, sweet, sour, fruity, caramel, artificial, burnt, intensity, and body using potentiometric/amperometric chemical sensors along with the same pattern recognition techniques described above for the e-nose technology. 

One important application of amperometry is in the construction of chemical sensors. One of the first amperometric sensors to be developed was for dissolved O2 in blood, which was developed in 1956 by L. C. Clark. The design of the amperometric sensor is shown in Figure 11.38 and is similar to potentiometric membrane electrodes. A gas-permeable membrane is stretched across the end of the sensor and is separated from the working and counter electrodes by a thin solution of KCl. The working electrode is a Pt disk cathode, and an Ag ring anode is the... 

In the area of consumer products, amperometric glucose sensors hold high potential. Industrially, process monitors for the manufacture of consumer chemicals are under development. However, replacement of defective reference electrodes, which in a laboratory environment may be trivial, may be prohibitively difficult m vivo or in an industrial process environment. 

Figure 1.11 — Average number of papers on (bio)chemical sensors published annually, based on data from Janata s biannual review. E electrochemical sensors ISEs ion-selective electrodes P potentiometric sensors A amperometric sensors C conductimetric sensors O optical sensors M mass sensors T thermal sensors. (Adapted from [23] with permission of the American Chemical Society).

In general, traditional electrode materials are substituted by electrode superstructures designed to facilitate a specific task. Thus, various modifiers have been attached to the electrode that lower the overall activation energy of the electron transfer for specific species, increase or decrease the mass transport, or selectively accumulate the analyte. These approaches are the key issues in the design of chemical selectivity of amperometric sensors. The long-term chemical and functional stability of the electrode, although important for chemical sensors as well, is typically focused on the use of modified electrodes in energy conversion devices. Examples of electroactive modifiers are shown in Table 7.2. 

In Fig. 2.10, the boundary between the enzyme-containing layer and the transducer has been considered as having either a zero or a finite flux of chemical species. In this respect, amperometric enzyme sensors, which have a finite flux boundary, stand apart from other types of chemical enzymatic sensors. Although the enzyme kinetics are described by the same Michaelis-Menten scheme and by the same set of partial differential equations, the boundary and the initial conditions are different if one or more of the participating species can cross the enzyme layer/transducer boundary. Otherwise, the general diffusion-reaction equations apply to every species in the same manner as discussed in Section 2.3.1. Many amperometric enzyme sensors in the past have been built by adding an enzyme layer to a macroelectrode. However, the microelectrode geometry is preferable because such biosensors reach steady-state operation. 

Berney, H. (2004) Impedometric and amperometric chemical and biological sensors. In V.M. 

Figure 9. I-V characteristics of amperometric oxygen sensor (700 °C). Reproduced with permission from Ref. 6. Copyright 1984 Japan Association of Chemical Sensors.

Fig. 37.11. Use of an NO microsensor for detection of the NO release from cultured endothelial cells. The sensor is a dual probe microsensor. The small sensor is a bare Pt UME used to position the sensor in the feedback mode. Onto the larger Pt electrode a polymer was deposited from an acrylic resin containing Ni(4-lV-tetramethyl) pyridyl porphyrin and served as amperometric NO sensor, (a) Schematic of the sensor, (b) optical microphotograph of the sensor surface, (c) Response of the NO sensor to the stimulation of the cells with bradykinin at different distances of the sensor to the surface of the cells. Reprinted with permission from Ref. [104], Copyright 2004, American Chemical Society.

Kerner W, Lindquist S-E, Pishko MV, Heller A. Amperometric glucose sensor containing glucose oxidase cross-linked in redox gels. In Turner APF, Alcock SJ (Eds), In Vivo Chemical Sensors Recent Developments. Cranfield Press, Cranfield UK, 1993. 

On-wafer membrane deposition and patterning is an important aspect of the fabrication of planar, silicon based (bio)chemical sensors. Three examples are presented in this paper amperometric glucose and free chlorine sensors and a potentiometric ISRET based calcium sensitive device. For the membrane modified ISFET, photolithographic definition of both inner hydrogel-type membrane (polyHEMA) and outer siloxane-based ion sensitive membrane, of total thickness of 80 pm, has been performed. An identical approach has been used for the polyHEMA deposition on the free chlorine sensor. On the other hand, the enzymatic membrane deposition for a glucose electrode has been performed by either a lift-off technique or by an on-chip casting. 

Of particular relevance to chemical sensor technology are the novel results of the electrochemical competition experiments. When an equimolar mixture ofNa+/K+orNa + /K + /Mg2 + cations is added to electrochemical solutions of (26), the ferrocene/ferricinium redox couple shifts anodically by an amount approximately the same as that induced by the K+ cation alone. This observation, together with the FABMS competition experimental findings, suggests that (26) is a first-generation prototype potassium-selective amperometric sensor, capable of detecting the K+ cation in the presence of Na+ and Mg2+ ions. 

Chemical sensors may be classified according to the operating principle of the transducer. Based on this classification, electrochemical sensors are such chemical sensors where the chemical information is transduced into an electrical signal. Electrochemical sensors can be divided further into -> amperometric sensors, -> conduc-timetric sensors, and -> potentiometric sensors, depending on which electrical property is actually recorded. 

Suleiman and Xu [128] described a novel reusable amperometric immuno-sensor for the determination of cocaine. Horseradish peroxidase and benzoylecgonine-antibody were co-immobilized on a chemically activated affinity membrane, which was then mounted over the tip of an oxygen electrode. The enzymatic electrocatalytic current response to the substrate is inhibited by the association of the antigen to the co-immobilized antibody. The calibration plot for cocaine was linear in the concentration range of lx 10 -1x10 M. 

Abstract Brief historic introduction precedes presentation of main types of transducers used in sensors including electrochemical, optical, mass sensitive, and thermal devices. Review of chemical sensors includes various types of gas sensitive devices, potentiometric and amperometric sensors, and quartz microbalance applications. Mechanisms of biorecognition employed in biosensors are reviewed with the method of immobilization used. Some examples of biomimetic sensors are also presented. 

Gorton, L, Bremle, G., Gsoregi, E., Jdnsson-Pettersson, G., and Persson, B. (1991) Amperometric glucose sensors based on immobilized glucose-oxidizing enzymes and chemically modified electrodes. Analytica Chimica Acta, 249, 43-54. 

One of the most important fields in which the rapidity of the analytical process is necessary is in vivo clinical analysis. The use of chemical sensors (amperometric or potentiometric) as array sensors has solved the problem of time, sensitivity, and selectivity. Because of the selectivity and sensitivity assured by capillary electrophoresis, it can be successfully used for highspeed DNA genotyping, as in microfabricated capillary array electrophoresis chips.237 Its capacity to analyze 12 different samples in parallel in less than 160 s has made it the method of choice for this type of analysis. 

Diffusion barriers are typically used where the chemical reaction responsible for sensing is slow relative to the diffusion rate of the analyte to the active portion of the sensor. As shown above, a gas-permeable membrane can act as a diffusion barrier in some cases for gas sensors. Diffusion barriers are also often used in amperometric enzyme sensors which exploit the native selectivity of an enzyme-substrate reaction to measure the concentration of the substrate. If the substrate is at a high concentration, the enzyme may become saturated, especially if the enzyme turnover rate is low. In this condition, the signal will plateau and no longer be dependent upon the substrate concentration. A diffusion barrier between the enzyme layer and the sample reduces the flux of the substrate to the enzyme and, thus, prevents saturation of the enzyme and increases the linear range of the sensor. 

Microfabrication and micromachining techniques have also been used in the manufacture of electrochemical sensors. This includes po and pco sensors. Zhou et al [9] describe an amperometric CO2 sensor using microfabricated microelectrodes. In this development, silicon-based microfabrication techniques are used, including photolithographic reduction, chemical etching, and thin-film metallization. In Zhou s study, the working electrodes are in the shape of a microdisk, 10 pm in diameter, and are connected in parallel. In recent years, silicon-based microfabrication techniques have been applied to the development of microelectrochemical sensors for blood gases, i.e. P02. Pcoj and pH measurements. 

Capacitance-based chemical sensors are in the class of devices that transduce analytes into electrical currents. Such sensors are typically comprised of a dielectric, chemically-sensitive film coated onto a substrate electrode these films pass low conduction current, making amperometric or conductimetric measurements less sensitive or attractive for signal transduction. To detect an analyte, changes in the chemically-sensitive film s capacitive properties (associated with its dielectric constant, charge uptake, or formation of interface dipole layers) are measured when an active species is present or generated. 

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