Direct optical

Various polymorphs have been reported for SnS with band gap widths in the range 1.0-1.5 eV, depending on the preparation method. The a-SnS (herzenbergite) is the most frequently occurring phase and is a p-type semiconductor with a direct optical transition at 1.3 eV and a high absorption coefficient (> 10" cm ). The orthorhombic S-SnS phase possesses a direct gap between 1.05 and 1.09 eV. 

In addition, typical methods of sensing are total internal reflection fluorescence or monitoring of fluorescence resonance energy transfer6,7. The second class is a direct optical detection principle which relies either on reflectometry or refractometry. The latter is connected to evanescent field... 

Figure 1. Classification of detection methods using either labelled system of applying direct optical detection.

Figure 11. Comparison of different assay types using a direct detection scheme were the receptors immobilized to the surface and the analyte is recognized at the surface (direct optical detection and using labelled systems), a competitive test scheme were labelled analyte molecules compete with the non-labelled sample, and thirdly a binding inhibition assay were analyte derivatives (ligand derivatives) are immobilized at the surface, in a preincubation phase the ligands block receptor molecules, non-blocked receptors go to the surface being either labelled or optically detected.

Optical read out will compete with micro electrode arrays. New developments in array detectors will open new perspectives. Direct optical detection techniques will add new possibilities to bioanalytical applications in addition to fluorescence measurements presently preferred in biochips read out. 

Detection by Direct Optical Spectroscopy, with Emphasis on Correlation Spectroscopy... 

Gas detection methods may be split into two groups, (i) direct methods, which monitor a physical parameter of the target gas, and (ii) indirect methods, which use a chemical reaction or indicator to show the concentration of the gas being sensed. This division of methods may be further split into optical sensing techniques and non-optical techniques. This review will predominantly focus on direct optical-spectroscopy fibre sensing techniques. 

E. Mallat, C. Barzen, R. Abuknesha, G. Gauglitz, and D. Barcelo, Part per trillion level determination of isoproturon in certified and estuarine water samples with a direct optical immunosensor. Anal. Chim. Acta 426, 209-216 (2001). 

E. Mallat, C. Barzeb, A. Klotz, A. Brecht, G. Gauglits, and D. Barcelo, River Analyzer for chlorotri-azines with a direct optical immunosensor. Environ. Sci. Technol. 33, 965—971 (1999). 

J. M. Wiesenfeld andR. K. Jam, Direct Optical Probing of Integrated Circuits and High-Speed Devices... 

Brecht, A. Piehler, J. Lang, G. Gauglitz, G., A direct optical immunosensor for atrazine detection, Anal. Chim.Acta 1995, 311, 289 299... 

Gauglitz, G., Direct optical sensors principles and selected applications, Anal. Bioanal. Chem. 2005, 381, 141 155... 

Huber, W. Bamer, R. Fattinger, C. Hubscher, J. Roller, H. Muller, F. Schlatter, D. Lukosz, W., Direct optical immunosensing (sensitivity and selectivity), Sens. Actuators B 1992, 6, 122 126... 

C. Allain, D. Ausserre, and F. Rondelez, Direct optical observation of interfacial depletion layers in polymer solutions, Phys. Rev. Lett. 49, 1694-1697 (1982). 

The results show that the dynamics of the reaction zone in a turbulent flame can be traced by the evolution of the volatiles oxidation intensity field. Ignited in the ball-shaped volume, the turbulent flame expands as a relatively wide spherical layer containing strong nonuniformities of the reaction rate. Similar nonuniformities were also detected in the experiments by means of direct optical registration of the flame-ball dynamics [10]. Thus, the numerical results qualitatively reflect the influence of dust concentration nonuniformities that existed... 

Enantiomers have very similar chemical properties, but they rotate polarized light in opposite directions (optical activity, see pp. 36,58). The same applies to the enantiomers of lactic acid. The dextrorotatory L-lactic acid occurs in animal muscle and blood, while the D form produced by microorganisms is found in milk products, for example (see p.l48). The Fischer projection is often used to represent the formulas for chiral centers (cf.p. 58). 

The surface sensitivity is ensured by detecting the decay products of the photoabsorption process instead of the direct optical response of the medium (transmission, reflection). In particular one can measure the photoelectrons, Au r electrons, secondary electrons, fluorescence photons, photodesorbed ions and neutrals which are ejected as a consequence of the relaxation of the system after the photoionization event. No matter which detection mode is chosen, the observable of the experiment is the interference processes of the primary photoelectron with the backscattered amplitude. 

In summary, when using the indirect" technique for optical trace analysis, all of the points mentioned above have to be considered and have to be validated when setting up a standard operation procedure" (SOP), in order to conform to good laboratory practice (GLP) analysis methods. This means an extra work load compared with validation of the direct optical trace analysis procedure. Sometimes there is no way of getting round this so-called less elegant , more cumbersome and more error prone indirect technique. However, if it is performed correctly and judged critically, it is still a good method and should easily allow optical trace analysis down to 0.1% and lower. 

The theoretical description of photochemistry is historically based on the diabatic representation, where the diabatic models have been given the generic label desorption induced by electronic transitions (DIET) [91]. Such theories were originally developed by Menzel, Gomer and Redhead (MGR) [92,93] for repulsive excited states and later generalized to attractive excited states by Antoniewicz [94]. There are many mechanisms by which photons can induce photochemistry/desorption direct optical excitation of the adsorbate, direct optical excitation of the metal-adsorbate complex (i.e., via a charge-transfer band) or indirectly via substrate mediated excitation (e-h pairs). The differences in these mechanisms lie principally in how localized the relevant electron and hole created by the light are on the adsorbate. 

ALEXANDRU ET AL. Organic Direct Optical Recording Media... 

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