Molecule-responsive sensors

There are many hosts with which metal cations can be detected by spectral changes. However, almost no effort has been invested to constmction of the hosts that exhibit spectral variations upon complexation with guest molecules. Under this situation, we have attempted to constmct host compounds that are spectroscopically responsive to molecules, and recently succeeded in constructing molecule-responsive sensors or indicators. This review gives a brief survey of our work. 

Due to their response mechanism the polyion-selective electrodes are not sensitive to the small fragments of polyionic macromolecules. Thus, if an enzyme cleaves the polyionic molecule these sensors can be used for detection of enzyme activity. Polycation protamine is rich in arginine residues that make it a suitable substrate for protease-sensitive electrochemical assays. Real-time detection of trypsine activity was demonstrated with the protamine-selective electrode as a detector [38],... 

A similar experiment was performed using fiber diameter as the independent variable. In this experiment, fibers with diameters ranging from 0.1 to 50 im were exposed to 90 mg/dL glucose and the response allowed to equilibrate. In this case, the results indicate that the oxygen response of the sensors is independent of diameter however, clearly, the glucose response strongly depends on sensor diameter. Approximately, 20% increase in intensity is observed when the fiber diameter is decreased from 5 to 1 0,m. The intensity increase was attributed to an increase in enzymatic activity per GOx molecule for sensor diameters >10 pm, the authors state that enzymes trapped within the inner core of the sensors become active only after enzyme molecules toward the sensors surface become deactivated. 

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. 

Entrapment of biochemically reactive molecules into conductive polymer substrates is being used to develop electrochemical biosensors (212). This has proven especially useful for the incorporation of enzymes that retain their specific chemical reactivity. Electropolymerization of pyrrole in an aqueous solution containing glucose oxidase (GO) leads to a polypyrrole in which the GO enzyme is co-deposited with the polymer. These polymer-entrapped GO electrodes have been used as glucose sensors. A direct relationship is seen between the electrode response and the glucose concentration in the solution which was analyzed with a typical measurement taking between 20 to 40 s. 

The high specificity required for the analysis of physiological fluids often necessitates the incorporation of permselective membranes between the sample and the sensor. A typical configuration is presented in Fig. 7, where the membrane system comprises three distinct layers. The outer membrane. A, which encounters the sample solution is indicated by the dashed lines. It most commonly serves to eliminate high molecular weight interferences, such as other enzymes and proteins. The substrate, S, and other small molecules are allowed to enter the enzyme layer, B, which typically consist of a gelatinous material or a porous solid support. The immobilized enzyme catalyzes the conversion of substrate, S, to product, P. The substrate, product or a cofactor may be the species detected electrochemically. In many cases the electrochemical sensor may be prone to interferences and a permselective membrane, C, is required. The response time and sensitivity of the enzyme electrode will depend on the rate of permeation through layers A, B and C the kinetics of enzymatic conversion as well as the charac-... 

Arbitrary the book can be divided into two complementary parts. The first one describes the physical and chemical basics leading to description of the method of semiconductor sensors. The mechanisms of underlying processes are given. These processes involve interaction of gas with the surface of semiconductor adsorbent which brings about tiie change of electric and physics characteristics of the latter. Various models of absorption-induced response of electric and physics characteristics of semiconductor adsorbent are considered. Results of numerous physical and chemical experiments carried out by the authors of this book and by other scientists underlying the method of semiconductor sensors are scrupulously discussed. The possibility of qualitative measurements of ultra-small concentrations of molecules, atoms, radicals as well as excited particles in gases, liquids and on surfaces of solids (adsorbents and catalysts) is demonstrated. 

Detectors are composed of a sensor and associated electronics. Design and performance of any detector depends heavily on the column and chromatographic system with which it is associated. Because of the complexity of many mixtures analysed and the limitation in regard to resolution, despite the use of high-resolution capillary columns and multicolumn systems, specific detectors are frequently necessary to gain selectivity and simplify the separation system. Many detectors have been developed with sensitivities toward specific elements or certain functional groups in molecules. Those detectors that exhibit the highest sensitivity are often very specific in response, e.g. the electron capture detector in GC or the fluorescence detector in LC. Because... 

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