Device amperometric sensors

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. 

Methods to electrically wire redox proteins with electrodes by the reconstitution of apo-proteins on relay-cofactor units were discussed. Similarly, the application of conductive nanoelements, such as metallic nanoparticles or carbon nanotubes, provided an effective means to communicate the redox centers of proteins with electrodes, and to electrically activate their biocatalytic functions. These fundamental paradigms for the electrical contact of redox enzymes with electrodes were used to develop amperometric sensors and biofuel cells as bioelectronic devices. 

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. 

Enzyme sensors can measure analytes that are the substrates of enzymatic reactions. Thermometric sensors can measure the heat produced by the enzyme reaction [31], while optical or electrochemical transducers measure a product produced or cofactor consumed in the reaction. For example, several urea sensors are based on the hydrolysis of urea by urease producing ammonia, which can be detected by an ammonium ion-selective ISE or ISFET [48] or a conductometric device [49]. Amperometric enzyme sensors are based on the measurement of an electroactive product or cofactor [50] an example is the glucose oxidase-based sensor for glucose, the most commercially successful biosensor. Enzymes are incorporated in amperometric sensors in functionalised monolayers [51], entrapped in polymers [52], carbon pastes [53] or zeolites [54]. Other catalytic biological systems such as micro-organisms, abzymes, organelles and tissue slices have also been combined with electrochemical transducers. 

Two widely used devices that operate on the voltammetric principle are the oxygen electrode and the glucose electrode. These are sometimes referred to as amperometric sensors. 

Abdel-Hamid et al. [122] used a flow-injection amperometric immunofll-tration assay system for the rapid detection of total E. coli and Salmonella. Disposable porous nylon membranes served as a support for the immobilization of anti- ]. coli or anti-Salmonella antibodies. The assay system consists of a flow-injection system, a disposable filter-membrane, and an amperometric sensor. A sandwich immunoassay specifically and directly detected 50 cells ml total E. coli or 50 cells ml Salmonella. The immunosensor can be used as a highly sensitive and automated bioanalytical device for the rapid quantitative detection of bacteria in food and water. 

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. 

It is commonly assumed that application of these methods in sensors has started from invention of oxygen Clark electrode,2 and in biosensors from first glucose biosensor.3 At present, main sensor application of amperometric and voltammetric detections include, with wide use of oxygen Clark electrode, amperometric sensors based on modification of working electrodes with various materials, and biosensors employing practically all biorecognition species. With the very wide use of the term sensors, applications of voltammetric detections include also miniaturized screen-printed devices for stripping determinations of, e.g., heavy metal ions. 

Amperometric Sensors A number of voltammetric systems are produced commercially for the determination of specific species of interest in industry and research. These are usually based on measuring the limiting current at a constant applied potential and relating the measured current to concentration. This technique is often called amperometry. Amperometric devices are sometimes called electrodes but are, in fact, complete voltammetric cells and are better referred to as sensors. Two of these devices are described here. 

Optodes provided with non-fluorescent esters of fluorophores have been used for the determination of external enzyme activities. The fluorophores are liberated by the enzymes and then seen by the optical Ober [214], As ecamples of p(02)-modulated optical biosensors, a glucose probe [213] and an ethanol probe [216] can be mentioned sensors based on glucose, alcohol, and other oxidases were reviewed by Opitz and Lttbbers [217]. The advantages of these 02-dependent optical biosensors are that, unlike corresponding amperometric sensors, they do not consume O2 and that they are strictly diffusion limited in their response. Fiber-optical devices are also available for the determination of substrates of dehydrogenases the NADH fluorescence produced by the immobilized enzyme is measured as a function of time [218, 219]. 

An important aspect in the development of sensor technology is the need for mass-produced and low-cost disposable transducers [48]. This is especially relevant for environmental and biomedical analysis. For electrochemical sensors, screen-printed electrodes fulfill this need, and the ease of preparation and low cost of MIPs make them attractive as recognition elements for such devices. A first report on this topic demonstrated that an imprinted polymer could be coated onto screen-printed carbon electrodes, and the resulting devices could be used as an amperometric sensor [33]. 

R.H.-independent signal output has been achieved in thefour-probe type sensor shown in Fig. 36.4, where two additional Ag probes are inserted in the proton conductor bulk (AA) beneath the Pt electrodes. One of the Pt electrodes is covered by a layer of AA sheet, which acts as a sort of gas diffusion layer. The short-circuit current flowing between the two Pt electrodes is proportional to H2 concentration but dependent on R.H., just as in the previous amperometric sensor. On the other hand, the difference in potential between the two Ag probes (inner potential difference, AE g) with the outer Pt electrodes short-circuited is shown to be not only proportional to H2 concentration but also independent of R.H. as shown in Fig. 36.3b and Table 36.2. This mode of sensing has no precedence, and is noted as a new method to overcome the greatest difficulty in using proton conductor-based devices, i.e. their R.H. dependence. 

Electrochemical biosensors or electrochemical sensors for monitoring biologically important species are the fastest developing area of this sensor field. There are now commercially available devices for clinical, food, and environmental applications. Suitable target species for amperometric sensors are thus electroactive species (i.e., capable of being oxidized or reduced electrochemically), with the oxidation/reduction potential being as near zero volts as possible, as the number of possible inter-ferents will increase with the magnitude of the external potential. 

The determination of oxygen, mainly in dissolved form in aqueous samples, is by far the largest field of application for amperometric sensors. Because of their reliability, ease of use, and the fact that results are available almost instantaneously, these devices are widely used. 

If two such electrodes are separated by a thin layer of only zirconia, the application of a potential will lead to the pumping of oxygen from the cathode to the anode. This device can be used as an amperometric sensor for oxygen if a diffusion barrier restricts the flux of oxygen to the cathode. Note that similar devices are also often used as potentiometric sensors according to the Nernst equation (i.e., the lambda-probe in cars with catalytic converters). In this case one side of the cell has to act as a reference, e.g., by using ambient air. 

The widespread use of electrochemical devices in clinical chemistry began with blood gas analysis. In blood gas analyzers, blood pH and CO2 concentration are measured potentiometrically while O2 concentration is measured by an amperometric sensor. The Severinghouse-type CO2 sensor incorporates a small electrometric cell for pH measurements filled with bicarbonate solution that is separated from the sample by a gas-permeable membrane. The CO2 activity of the sample and the bicarbonate solution are allowed to equilibrate. The resulting change of pH in the small amount of bicarbonate solution is linearly related to the logarithm of CO2 partial pressure in the sample. 

Point-of-care testing has now become feasible with the introduction of the handheld i-STAT instrument and similar portable devices, such as the optically based Reflotron. The i-STAT (Fig. 2) contains both potentiometric and amperometric sensors integrated into sensor arrays that are included within disposable cassettes [87-89]. Eight tests are possible with the most advanced cassette, the ECg+, and these are sodium, potassium, chloride. 

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