Luminophor

The layer width is taken from the relation d > 1,5 dg, where dg - thickness of a gas discharge gap. The employment of a resistive layer instead of electrode profiling can significantly simplify the device manufacture. The UV radiation is efficiently converted into a visible one by a number of photo-luminophors, e.g. Zn2Si04 Mn. For stroboscopic registration of fast-proceeding processes the luminophors with short period of luminescence are used, e.g anthracene etc. 

No information is available on the chemical nature of the luminophore, although the photoprotein must contain a chro-mophore to emit luminescence and fluorescence. Acid treatment of the protein, followed by extraction with organic solvents, did not yield coelenteramide or coelenteramine, indicating that this luminescence system is unrelated to coelenterazine. A flavin (FAD) was found in partially purified preparations of photoprotein, but not in highly purified preparations. 

Between 1923 and 1927, the concepts of quantum efficiency (number of photons emitted divided by number of photons absorbed by a sample) and quantum yield (fraction of excited molecules that emit) had been defined and values determined for many compounds by Vavilov (34). The quantum yield indicates the extent that other energy loss mechanisms compete with emission in an excited molecule. Although the quantum yield is influenced by the molecular environment of the emitter, for a given environment it depends on the nature of the emitting compound and is independent of concentration and excitation wavelength, at least at low concentrations (35). Tlius, it serves as another measurable parameter that can be used to identify the compounds in a sample and also, because of its sensitivity to the surroundings of the luminophore, to probe the environment of the emitter. 

Using the long-lifetime emission as a reference in intensity sensing by shortlifetime dye. This approach known as dual luminophore referencing (DLR) will be considered in the next section. 

Fluorescent chemical sensors occupy nowadays a prominent place among the optical devices due to its superb sensitivity (just a single photon sometimes suffices for quantifying luminescence compared to detecting the intensity difference between two beams of light in absorption techniques), combined with the required selectivity that photo- or chemi-luminescence impart to the electronic excitation. This is due to the fact that the excitation and emission wavelengths can be selected from those of the absorption and luminescence bands of the luminophore molecule in addition, the emission kinetics and anisotropy features of the latter add specificity to luminescent measurements8 10. 

Provided it is optically diluted, the relationship between a luminophore (= luminescence bearer ) concentration and the intensity of its emission is a linear one ... 

Fluorescence quenching may be dynamic, if the photochemical process is the result of a collision between the photoexcited indicator dye and the quencher species, or static, when the luminophore and the quencher are preassociated before photoexcitation of the former20. It may be easily demonstrated that dynamic quenching in isotropic 3-D medium obeys the so-called Stem-Volmer equation (2)21 ... 

The Stem-Volmer equations discussed so far apply to solutions of the luminophore and the quencher, where both species are homogeneously distributed and Fick diffusion laws in a 3-D space apply. Nevertheless, this is a quite unusual situation in fluorescent dye-based chemical sensors where a number of factors provoke strong departure from the linearity given by equation 2. A detailed discussion of such situations is beyond the scope of this chapter however, the optosensor researcher must take into account the following effects (where applicable) ... 

More fluorescence features than just the emission intensity can be used to develop luminescent optosensors with enhanced selectivity and longer operational lifetime. The wavelength dependence of the luminescence (emission spectmm) and of the luminophore absorption (excitation spectrum) is a source of specificity. For instance, the excitation-emission matrix has shown to be a powerful tool to analyze complex mixtures of fluorescent species and fiber-optic devices for in-situ measurements (e.g. 

The commercialization of inexpensive robust LED and laser diode sources down to the uv region (370 nm) and cheaper fast electronics has boosted the application of luminescence lifetime-based sensors, using both the pump-and-probe and phase-sensitive techniques. The latter has found wider application in marketed optosensors since cheaper and more simple acquisition and data processing electronics are required due to the limited bandwidth of the sinusoidal tone(s) used for the luminophore excitation. Advantages of luminescence lifetime sensing also include the linearity of the Stem-Volmer plot, regardless the static or dynamic nature of the quenching mechanism (equation 10) ... 

Further cases of non-photochemical fluorescent sensors include those where the luminophore undergoes a variation in the originally populated electronic excited state due to temperature or environmental changes (solvent, viscosity, ionic strength,...). 

The sensor for the measurement of high levels of CO2 in gas phase was developed, as well90. It was based on fluorescence resonance energy transfer between 0 long-lifetime ruthenium polypyridyl complex and the pH-active disazo dye Sudan III. The donor luminophore and the acceptor dye were both immobilized in a hydrophobic silica sol-gel/ethyl cellulose hybrid matrix. The sensor exhibited a fast and reversible response to carbon dioxide over a wide range of concentrations. 

A number of promising C02 sensors have been described13,33. The phase-fluorometric system employing dual luminophore referencing, which is rather complex (two dyes, internal buffer system, sol-gel polymer matrix), has demonstrated satisfactory working characteristics33. But it still has to prove its safety and cost-effectiveness, and requires extensive validation with real food samples. 

D.R. McMillin, Purdue University You indicated that emission can sometimes be detected from luminophors fixed near a platinum electrode. Isn t this surprising given the fact that metal electrodes are generally excellent quenchers of excited states ... 

The physical significance of 6a is that under defined experimental conditions it is the constant of proportionality between ICl, the observed intensity of chemiluminescence, and the rate of consumption of the initial luminophore (reactant L) i.e.,... 

SH Chen, D Katsis, AW Schmid, JC Mastrangelo, T Tsutsui, and TN Blanton, Circularly polarized light generated by photoexcitation of luminophores in glassy liquid-crystal films, Nature, 397 506-508, 1999. 

Mixed ligand 8-quinolinolato aluminum chelate luminophors... 

Dual lifetime referencing (DLR) is another powerful technique that enables referenced measurements in case of fluorescent indicators [23]. In this method, the analyte-dependent signal from an indicator is referenced against the signal from an inert luminophore. This can be realized in both the time domain [24] and in the frequency domain [25]. Often, a luminescent reference dye is embedded into gas blocking nanobeads to avoid oxygen quenching. Polymers with very low gas permeability such as poly(acrylonitrile) [24] or poly(vinylidene chloride-co-acry-lonitrile) [26] are the best choice here. 

Mohr GJ, Klimant I, Spichiger-Keller UE, Wolfbeis OS (2001) Fluoro reactands and dual luminophore referencing a technique to optically measure amines. Anal Chem 73 1053-1056... 

Santra S, Zhang P, Wang KM, Tapec R, Tan WH (2001) Conjugation of biomolecules with luminophore-doped silica nanoparticles for photostable biomarkers. Anal Chem 73 4988 1993... 

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