Optical sensors recognition elements

Optical sensors based on spectroscopy of guided waves have been demonstrated for detection and identification of numerous chemical and biological analytes. The choice of detection format for a particular application depends on the size of target analyte molecules, binding characteristics of available biomolecular recognition element, and range of analyte s concentrations to be measured. 

Figure 1.6 shows the major types of recognition elements and transducing systems used for implementing (bio)chemical sensing. A detailed discussion is provided in other sections of this chapter and the selected examples described throughout this book. Recently, Wolfbeis reported a systematic review of recognition elements based on enzymes, ion-carriers and molecular interactions used in optical sensors [5]. 

One very useful technical classification of sensors establishes two categories according to the relationship between the recognition element and the transducer. These two essential elements can be connected optically (e.g. 

This technique has also been employed for the preparation of a catalytic imprinted membrane by coating a cellulose membrane with a polymer incorporating particles imprinted with the transition-state analogue of a dehydrofluorination reaction [264]. The application of such an MIP composite membrane as the recognition element in an optical sensor has been reported for digitoxin analysis in serum samples by embedding digitoxin-MIP particles in polyvinyl chloride film in presence of plasticizer by the dry inversion process [265],... 

Fig. 1 Chemical interaction mechanisms, basic components of the optical sensor instrumentation and their operation. Mechanisms direct measurement of chemical compounds that exhibit spectroscopic properties (1 A) and measurement of light originating from a chemical or a biological reaction in chemiluminescent or bioluminescent phenomena (IB) 2 optodes based on the interaction of indicators and labels with light, which are immobilized in a support and sensors that modify the intrinsic physical or chemical properties of a waveguide (refractive index, phase, etc.) as a result of the presence of the analyte (3A), a recognition element (35), an intermediate analyte (3C) or an indicator (3D)...

Fig. 4 Different optochemical configurations for flow-cell-based sensors. Notice that flow cells are expendable. S source, D detector, Of optical fiber, a Non-guided sensors the transmission of a non-guided collimated radiation beam is measured, b Radiation is guided though optical fibers from the source to the recognition element and from this position to the detector...

The sensor developed by Gawley and co-workers is based on de Silva s modular approach comprises an azacrown ether linked to a fluorophore by a short link. The molecule combines the aza[18]crown-6 recognition element with a fluorescent coumaryl group attached by a methylene spacer. In tests it was shown to bind to saxitoxin with a binding constant on the order of 105 M 1, even in a phosphate buffer at physiological pH and concentrations of sodium and potassium, and could detect levels of the toxin down to 1(T7 M. Subsequent modification of the sensor allowed it to be linked to the surface of a quartz slide to allow the fluorescent response to concentrations of saxitoxin between 10-4 and 10-6 M to be detected via a fibre optic system. This level of sensitivity is comparable to the current mouse bioassay that requires the inoculation of a large number of animals to determine the concentration of saxitoxin present in the test sample [25],... 

Recognition element Sample preparation Sensor SPR SPR optical platform ... 

In recent years, first prototypes of SPR biosensor systems integrating these elements have been reported and their application for detection of biological analytes in the field have been demonstrated. Undoubtedly, advances in the development of the key elements - sample preparation technology, microfluidics, biomolecular recognition elements, SPR optical platform - will further stimulate this effort and eventually lead to in-field SPR sensor systems becoming a commercial reality. 

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