Optodes biosensors

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... 

Construction of the optode for optical biosensor requires immobilization of sensitive compounds in the host matrix. There are several methods enabling molecules entrapment. One can use gels, polymers, saccharose, various meshes and membranes78. In case of fiberoptic indirect sensors optode must be attached to the fiber tip. Nowadays, there are two commonly used optode host materials sol-gel materials and polymers. 

S. L. R. Barker, Y. Zhao, M. A. Marietta, and R. Kopelman, Cellular Applications of a Fiber-Optic Biosensor Based on a Dye-Labeled Guanylate Cyclase, And. Chem. 1999, 71, 2071 M. Kuratli and E. Pretsch, S02-Selective Optodes, AnaL Chem. 1994,66, 85. 

Keywords Biosensors Dyes Fluorescence Glass Immunosensors Luminescence Nanocomposites Optodes Optosensors Ormosils PVC Silica Siloxanes Sol-gel... 

Both organic and inorganic polymer materials have been used as solid supports of indicator dyes in the development of optical sensors for (bio)chemical species. It is known that the choice of solid support and immobilization procedure have significant effects on the performance of the optical sensors (optodes) in terms of selectivity, sensitivity, dynamic range, calibration, response time and (photo)stability. Immobilization of dyes is, therefore, an essential step in the fabrication of many optical chemical sensors and biosensors. Typically, the indicator molecules have been immobilized in polymer matrices (films or beads) via adsorption, entrapment, ion exchange or covalent binding procedures. 

In this chapter, the fundamental electrochemical principles of potentiometry, voltammetry and/or amperometry, conductance, and coulometry will be summarized and clinical apphcations presented. Next, optodes and biosensors will be discussed. The chapter concludes with a discussion of in vivo and minimally invasive sensors. 

Earlier, the electrochemical detections were mostly employed in chemical sensors and biosensors, and until now they are most commonly used, especially in commercially available sensors, mainly for clinical and environmental analyses. An intensive development of optical sensors (optodes) in recent 30 years has resulted in numerous designs and commercial products, which are increasingly competitive to electrochemical sensors. A more limited importance, especially as mass production is concerned and applications in routine analyses, have thermal and mass-sensitive sensors and biosensors. 

It was mentioned before that fiber-optic chemical and biosensors are broadly classified into two categories extrinsic- and intrinsic-type sensors. In the extrinsic-type sensors, the fiber is acting as a link connecting optical signals to (and from) the active material (medium) positioned at the end of the fiber, such as the Optode case. In the intrinsic-type sensors, the fiber is modified in different ways, through construction of the sensing component, which will be explained next. 

Optoelectronic biosensors are advantageous in that they require no reference signed and no electrical shielding. Furthermore, the sensitivity of the optode is readily adaptable to the desired measuring range. 

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]. 

Two serious problems are encountered in the design, manufacture, and performance of in vivo sensors the lack of biocompatibility of the materials used and the poor long-term stability. The latter, however, plays only a minor role in the case of disposable optodes, which are in use only for the duration of a particular operation or test. Disposable sensor heads for clinical analytes seem to be the most promising candidates for practical use at present. Another problem results from the need for sterilization, which is difficult to solve in the case of biosensors with their thermally labile components such as enzymes. 

A pH-enzyme optode spectrophotometric flow injection system has been used for monitoring clinical hemodialysis. The role of the incorporated dialyzate urea detector is played by an optical flow-through biosensor based on Prussian Blue film with chemically linked urease forming a monomolecular layer of the enzyme. This pH-enzyme optode-FIA system is useful for selective determination of postdialyzate urea in the range of concentrations corresponding to its level in real clinical samples (2-16 mmol 1 ) at 15 samples of spent dialyzate per hour. 

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