Chemical sensor action

Fig. 13.1 Schematic depiction of chemical sensor action. An optical or electrical signal reports the non-covalent binding of an analyte to the receptor site.

In dealing with the mechanism of action (and response) of a flowthrough (bio)chemical sensor, one should consider three different types of kinetics (Fig. 2.20), namely ... 

The fundamental action of a chemical sensor is the conversion of the information about the concentration of a chemical species into an electric signal. The relationship between the signal and the chemical concentration can be represented by an analytical function that embodies the sensor operation. 

Small leakage currents or a transistor-like action of the junction are sufficient to generate a small current that may cause undesired passivation. This can be circumvented by application of an additional potential to the etching layer, shown by the broken line in Fig. 4.16 a. This electrochemical etch-stop technique is favorable compared to the conventional chemical p+ etch stop in alkaline solutions, because it does not require high doping densities. This etch stop has mainly been apphed for manufacturing thin silicon membranes [Ge5, Pa7, Kll] used for example in pressure sensors [Hil]. 

NN applications, perhaps more important, is process control. Processes that are poorly understood or ill defined can hardly be simulated by empirical methods. The problem of particular importance for this review is the use of NN in chemical engineering to model nonlinear steady-state solvent extraction processes in extraction columns [112] or in batteries of counter-current mixer-settlers [113]. It has been shown on the example of zirconium/ hafnium separation that the knowledge acquired by the network in the learning process may be used for accurate prediction of the response of dependent process variables to a change of the independent variables in the extraction plant. If implemented in the real process, the NN would alert the operator to deviations from the nominal values and would predict the expected value if no corrective action was taken. As a processing time of a trained NN is short, less than a second, the NN can be used as a real-time sensor [113]. 

In this section we will describe what may be called electrochemical composition actors, in which chemical action—and not mechanical action as usually described by the term actor (or actuator)—is caused by electrical excitation. Unlike electrochemical composition sensors which detect chemical composition via electrochemical methods, they cause changes in chemical composition. It shall not remain unmentioned that at present interesting work is pursued also to realize mechanical action on an electrochemical stimulus ( artifical muscles ).48... 

Nanowires and nanobelts of inorganic oxides have been fashioned into chemically sensitive semiconductor devices. These include tin and zinc oxides [9], and indium oxide [30], Once again, ammonia and NO2 gases were used for initial demonstrations. Oxygen had very little effect on the sensing action. Because of the low concentrations detected and the speed of the response, it was suggested that single-molecule response could be within reach with these ultraminiaturized sensors. 

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