Transduction, principles

Later on, such S-layer-based sensing layers were also used in the development of optical biosensors (optodes), where the electrochemical transduction principle was replaced by an optical one [97] (Fig. 10c). In this approach an oxygen-sensitive fluorescent dye (ruthenium(II) complex) was immobilized on the S-layer in close proximity to the glucose oxidase-sensing layer [97]. The fluorescence of the Ru(II) complex is dynamically quenched by molecular oxygen. Thus, a decrease in the local oxygen pressure as a result of... 

With consideration to the quality of a bio or chemosensor it is therefore necessary to discuss the transduction principle, the quality of the sensitive layer and the data evaluation. These three issues will be discussed in this paper, with the focus on interferometric principles in the area of non-specific measurements using chemosensors and specific interaction processes in the case of biosensing. 

After discussing various sensitive layers later, both principles will be applied to monitor effects in biomolecular or chemical sensitive layers. The applications will demonstrate the feasibility of the methods as well as their advantages and their disadvantages. Therefore various applications are given, sometimes even for the same analyte, in order to demonstrate the normal approach in sensor development to select the best transduction principle for a specific application. 

To make the most advantageous glucose biosensor, it is important to combine the best transduction principle with the best immobilization protocol. As mentioned, the most progressive way to couple the oxidase and the electrode reaction is a low potential detection of hydrogen peroxide. Among available H202 transducers, Prussian blue is the most advantageous one. 

The geometry shown here corresponds to a semi-infinite planar diffusion. Other geometries (e.g., radial geometries) typical for microsensors can be used. The enzyme-containing layer is usually a hydrogel, whose optimum thickness depends on the enzymatic reaction, on the operating pH, and on the activity of the enzyme (i.e., on the Km). Enzymes can be used with nearly any transduction principle, that is, thermal, electrochemical, or optical sensors. They are not, however, generally suitable for mass sensors, for several reasons. The most fundamental one is the fact... 

The selection, preparation, and properties of a selective layer depend largely on the type of transducer at which they will be used, as well as on the application. Those aspects are discussed in the context of the individual transduction principles. Only certain common features and procedures are included in this section. 

The transduction principle in amperometric sensors is proportionality between the bulk concentration of the analyte and the mass transport limited current. There are two ways to keep the proportionality constant really constant. What are they ... 

The detection and quantification of the presence of biomolecules at the surface is based on specific interactions taking place in the evanescent field, generated by the total internal reflectance or by the surface plasmon resonance. The latter is the key transduction principle in the optical bioanalysis and biosensing area (Narayanaswamy and Wolfbeis, 2004). Launched in the early 1980s in Sweden,... 

Sensing chemical species is a much more difficult task than the measurement of mechanical variables such as pressure, temperature, and flow, because in addition to requirements of accuracy, stability, and sensitivity, there is the requirement of specificity. In the search for chemically-specific interactions that an serve as the basis for a chemical sensor, investigators should be aware of a variety of possible sensor structures and transduction principles. This paper adresses one such structure, the charge-flow transistor, and its associated transductive principle, measurement of electrical surface impedance. The basic device and measurement are explained, and are then illustrated with data from moisture sensors based on thin films of hydrated aluminum oxide. Application of the technique to other sensing problems is discussed. 

For the application of label-free optical transduction principles like SPR or RIfS, a chiral receptor bound to a transparent polymer layer is required. As various types of these polymers have already been applied to chromatographic separation processes, a substantial wealth of knowledge was achieved during the last few decades. Stationary materials like bonded amide selectors or cyclodextrins were adopted as sensor coatings. Several different applications of these materials in various fields of interest have been reported in the literature [17]. 

Both enantiomers of Chirasil-Val receptors have been applied to optical and piezoelectrical sensors [20]. In this chapter only the result of the optical transduction principle is presented. 

Both enantiomers of methyl and ethyl lactate ester and several amino acid derivatives were successfully discriminated by both optical transduction principles. In Fig. 8 the results of SPR measurements of both enantiomers of the valine esters with L-Chirasil-Calix are shown. In accordance with gas chromatographic investigations, the L-enantiomer of the amino acid derivatives shows stronger interaction with an L-Chirasil-Calix sensitive layer. This leads... 

SPR spectroscopy and RIfS were used as optical transduction principles for the detection of halogenated diethers as chiral guests with modified cyclodextrin as the stationary phase [7]. 

Investigation with the two optical transduction principles mentioned before confirmed the preferential enrichment of the S-enantiomer resulting in more than ninefold higher signals, which is the highest separation factor found for chiral separation on cyclodextrins up to now. The good agree-... 

According to the diversity of possible applications many applications of biosensors have been published, e. g. [145,317,462,463]. Often they concentrate especially on a certain group of biosensors but high redundancy is obvious. In the following sections, different sensor types, ordered by the transduction principle, are introduced. 

Chemical sensors are small devices for the detection and quantification of gaseous or solvated species. This is an active research area based on the need to obtain increasing amounts of data in chemical and food process streams as well as environmental monitoring. Most sensors consist of an appropriate transduction principle such as the quartz-crystal-microbalancc (QCM) and a chemically sensitive layer that imparts the desired chemical response behaviour. Most often a chemically selective response is desirable. Zeolite molecular sieves offer size- and shape-selective adsorption behaviour that can be combined with appropriate transduction concepts in order to construct chemically selective sensor devices. 

The successful application of this transduction principle to the PCR-free detection of highly repeated sequences was a starting point for the eventual detection of target DNA present as a single copy per genome [24],... 

Stratagem 2 Select a transduction principle with the greatest... 

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