Biosensors optical/electrochemical

After a brief section introducing smart biosensors, we proceed to describe biosensor systems that have been created and studied in our laboratory. They include optical, electrochemical, and piezoelectric-based systems designed to detect specific analyte molecules in solution. 

The detection principles for biosensors integrated on microfluidic chips are classified into several types, including optical, electrochemical, and mass-sensitive methods. The trend in the development of detectors has been to constantly pursue two key virtues sensitivity and selectivity. In this regard, the research issues on the detectors for microfluidic systems are not significantly different to those for conventional biosensors. 

Microdialysate samples have been analyzed using a variety of nonseparation-based analytical techniques including immunoassay, biosensors, and MS [1-4]. The main limitation to the use of these methods is that they are typically restricted to the measurement of a single analyte. For more complex samples, the detection of multiple substances is usually necessary. In this case, the dialysate sample is normally analyzed by conventional chromatographic or electrophoretic separation methods employing optical, electrochemical, or mass spectrometric modes of detection [5]. 

Biosensors are also classified according to the parameter that is measured by the physicochemical transducer of the biological event. Thus, classically biosensors are grouped into optical, electrochemical, acoustic and thermal ones. Optical transducers of most common enzyme biosensors are based on optical techniques such as absorption, reflectance, luminescence, chemi-luminescence, evanescent wave, surface plas-mon resonance, and interferometry. 

Based on the type of transducer used, biosensors can be classified as optical, electrochemical (amperometric or potentiometric), piezoelectric or thermal. Among all these the electrochemical biosensors are most widely used. 

Other biomolecules, such as antibodies [175-177], DNA [178-180], etc., have attracted much interest in the development of biosensors useful for detection of viruses and genetically transmitted diseases. Few reports have appeared on immobilisation in conducting polymers. A full range of optical, electrochemical and piezoelectric transduction modes aimed at detecting the base pair hybridisation between the immobilised... 

Chemical sensors can be of gas, liquid, and solid particulate sensors based on the phases of the analyte. Depending on the operating principle of transducer in a chemical sensor, it can be used as optical, electrochemical, thermometric, and gravimetric sensor. Chemical sensors also include a special branch referred to as biosensors for the recognition of biochemicals and bio-reactions. The use of biological elements such as organisms, enzymes, antibodies, tissues, and cells as receptors differentiates biosensors from conventional chemical sensors. 

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... 

Similarly to the above-mentioned entrapment of proteins by biomimetic routes, the sol-gel procedure is a useful method for the encapsulation of enzymes and other biological material due to the mild conditions required for the preparation of the silica networks [54,55]. The confinement of the enzyme in the pores of the silica matrix preserves its catalytic activity, since it prevents irreversible structural deformations in the biomolecule. The silica matrix may exert a protective effect against enzyme denaturation even under harsh conditions, as recently reported by Frenkel-Mullerad and Avnir [56] for physically trapped phosphatase enzymes within silica matrices (Figure 1.3). A wide number of organoalkoxy- and alkoxy-silanes have been employed for this purpose, as extensively reviewed by Gill and Ballesteros [57], and the resulting materials have been applied in the construction of optical and electrochemical biosensor devices. Optimization of the sol-gel process is required to prevent denaturation of encapsulated enzymes. Alcohol released during the... 

In the light of promising applications offered by these supramolecular glycoconju-gate structures, notably for the intrinsic optical and electrochemical properties of metal complexes, this underexploited research area is undoubtedly in its infancy and holds promise for such systems as biomarkers or sensitive biosensors. 

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