Silica, luminescence

Solid-surface room-temperature phosphorescence (RTF) is a relatively new technique which has been used for organic trace analysis in several fields. However, the fundamental interactions needed for RTF are only partly understood. To clarify some of the interactions required for strong RTF, organic compounds adsorbed on several surfaces are being studied. Fluorescence quantum yield values, phosphorescence quantum yield values, and phosphorescence lifetime values were obtained for model compounds adsorbed on sodiiun acetate-sodium chloride mixtures and on a-cyclodextrin-sodium chloride mixtures. With the data obtained, the triplet formation efficiency and some of the rate constants related to the luminescence processes were calculated. This information clarified several of the interactions responsible for RTF from organic compounds adsorbed on sodium acetate-sodium chloride and a-cyclodextrin-sodium chloride mixtures. Work with silica gel chromatoplates has involved studying the effects of moisture, gases, and various solvents on the fluorescence and phosphorescence intensities. The net result of the study has been to improve the experimental conditions for enhanced sensitivity and selectivity in solid-surface luminescence analysis. 

Solid-surface luminescence analysis involves the measurement of fluorescence and phosphorescence of organic compounds adsorbed on solid materials. Several solid matrices such as filter paper, silica with a polyacrylate binder, sodium acetate, and cyclodextrins have been used in trace organic analysis. Recent monographs have considered the details of solid-surface luminescence analysis (1,2). Solid-surface room-temperature fluorescence (RTF) has been used for several years in organic trace analysis. However, solid-surface room-temperature phosphorescence (RTF) is a relatively new technique, and the experimental conditions for RTF are more critical than for RTF. 

Several solid surfaces, such as filter paper, sodium acetate, and silica gel chromatoplates with a polyacrylate binder, have been used in solid-surface luminescence work (1,2). Experimentally it is relatively easy to prepare samples for analysis. With filter paper, for example, a small volume of sample solution is spotted onto the surface, the filter paper is dried, and then the measurement is made. In many cases, an inert gas is passed over the surface during the measurement step to enhance the RTF signal. For powdered samples, the sample preparation procedure is somewhat more involved. Commercial instruments can be readily used to measure the luminescence signals, and a variety of research instruments have been developed to obtain the solid-surface luminescence data (1,2). 

The use of doped and undoped silica aerogels as multifunctional host materials for fluorescent dyes and other luminescent materials for display and imaging applications has been reported.278 Results have been presented on the PL spectra of undoped silica aerogels and aerogels doped with Er3+, rhodamine, and fluorescein.278... 

Optical fibres composed of plastics are also transparent in the visible spectral region but optical losses reach 102 - 103 dB/km13. Their refractive index varies from 1.35 to 1.6 depending on the kind of polymer used (e.g. polymethymethacrylate PMMA -1.49). The chemical resistance is much worse than that of silica fibres and thermal stability is incomparable. On the other hand, low temperature processes of plastic fibre preparation allow us mix the starting polymer with organic dyes which enables the production of luminescent fibres suitable e.g. for fluorescence-based sensing13. 

Fluorescent silica nanoparticles, called FloDots, were created by Yao et al. (2006) by two synthetic routes. Hydrophilic particles were produced using a reverse micro-emulsion process, wherein detergent micelles formed in a water-in-oil system form discrete nanodroplets in which the silica particles are formed. The addition of water-soluble fluorescent dyes resulted in the entrapment of dye molecules in the silica nanoparticle. In an alternative method, dye molecules were entrapped in silica using the Stober process, which typically results in hydrophobic particles. Either process resulted in luminescent particles that then can be surface modified with... 

The sol-gel-entrapped microbial cells have shown excellent tolerance to different alcohols [99], The immobilized E. coli cells followed the Michaelis-Menten equation when quantified with the (3-glucosidase activity via the hydrolysis of 4-nitrophenyl-(3-D-galactopyranosdie [142], The sol-gel matrices doped with gelatin prevented the cell lysis, which usually occurs during the initial gelation process [143], Microorganisms are now widely used in the biosorption of different pollutants and toxicants. Bacillus sphaericus JG-A12 isolated from uranium mining water has been entrapped in aqueous silica nanosol for the accumulation of copper and uranium [144], Premkumar et al. [145] immobilized recombinant luminous bacteria into TEOS sol-gel to study the effect of sol-gel conditions on the cell response (luminescence). The entrapped and free cells showed almost the same intensity of luminescence (little lower), but the entrapped cells were more stable than the free cells (4 weeks at 4°C). This kind of stable cell could be employed in biosensors in the near future. 

Cobalt (II) has been determined by online measurements on seawater which has been passed through a column containing 8-quinolinol immobilised on silica gel, followed by chemical luminescence detection [244]. 

Cobalt Co(III) adsorbed on 8-quinolol adsorbed on silica Chemical luminescence - [244]... 

Recently, Gianotti et al. (96) reported photoluminescence and DRUV spectra of pure siliceous MCM-41 and Ti-MCM-41 containing Ti4+ species anchored to the inner walls of the siliceous MCM-41. They observed complex luminescence signals and concluded that these could be used for a clear distinction of the emission of tetrahedral Ti4+ ions from those of silica surface centers. 

In the silica nanomatrix, the low diffusion was also reported in both liquid and gas phases. In the gas-phase reaction, it was found that the luminescence signal of Ru(bpy)32+ doped in DDSNs remained stable when the air pressure increased from 1 to 8 psi, showing no quenching by oxygen in the air. When the air pressure was further increased to above 8 psi, a decrease in fluorescence emission intensity was observed. The results suggested a slow diffusion of quencher oxygen in the silica nanomatrix. 

Silica makes up 12.6 mass-% of the Earth s crust as crystalline and amorphous forms. It was found that both modifications show similar main luminescence bands, namely a blue one centered at 450 nm ascribed to which substitutes for Si, red centered at 650 nm linked with non-bridge O, and dark-red at 700-730 nm linked with Fe. Time-resolved luminescence of hydrous volcanic glasses with different colors and different Fe, Mn, and H2O contents were measured and interpreted (Zotov et al. 2002). The blue band with a short decay time of 40 ns was connected with T2( D)- Ai ( S) and Ai C G)- Ai ( S) ligand field transitions of Fe " ", the green band with a decay time of approximately 250 ps with a Ti( G)- Ai( S) transition in tetrahedrally coordinated Mn ", while the red band with a much longer decay time of several ms with T1 (4G)- Ai( S) transitions in tetrahedrally coordinated Fe ". We detected Fe " " in the time-resolved luminescence spectrum of black obsidian glass (Fig. 4.43d). 

Practically all world production of newly smelted aluminum was made from bauxite. The term bauxite is used for naturally occurring mixtures of aluminum monohydrate (boehmite or diaspore) and trihydrate gibbsite Al(OH)3, including impurities which are typically clay minerals, free silica, iron hydroxides and titania. The luminescence ofboehmite and diaspore and LIBS (Fig. 8.10) maybe used for the detection, sorting and commercial value evaluation of A1 minerals. 

From our research group Santra et al. [11,41,42] reported the development of novel luminescent nanoparticles composed of inorganic luminescent dye RuBpy, doped inside a sihca network. These dye-doped silica nanoparticles were synthesized using a w/o microemulsion of Tx-lOO/cyclohexane/ n-hexanol/water in which controlled hydrolysis of the TEOS leads to the formation of mono dispersed nanoparticles ranging from 5-400 nm. This research illustrates the efficiency of the microemulsion technique for the synthesis of uniform nanoparticles. These nanoparticles are suitable for biomarker application since they are much smaller than the cellular dimension and they are highly photostable in comparison to most commonly used organic dyes. It was shown that maximum liuninescence intensity was achieved when the dye content was around 20%. Moreover, for demonstration... 

Santra S, Liesenfeld B, Bertolino C, Dutta D, Cao Z, Tan WH, Moudgil BM, Mer-icle RA (2006) Fluorescence lifetime measurements to determine the core-shell nanostructure of FlTC-doped silica nanoparticles An optical approach to evaluate nanoparticle photostability. J Luminesc 117 75-82... 

Properties dependent on adsorption are not confined to conductivity. Luminescence of materials may be affected, as Ewles and Heap (7) have shown for the case of silica, for which the luminescent peak at 4000 A. was shown to be associated with the adsorption of the OID radical. Many workers have demonstrated the dependence of the contact potential on the adsorption of gases. For example, Brattain and Bardeen (8) have shown that the contact potential of germanium varies with the adsorption of water vapor. Photoconductivity may be dependent on the adsorption. For example, Bube has shown (9) that the adsorption of water vapor has a marked effect on the photoconductivity of cadmium sulphide. He concluded (10) that the effect was indirect surface changes affect the lifetime of the excess carriers, thus affecting the photoconductivity. Melnick (11), however, working with zinc oxide, has produced evidence that part of the photoconductivity in this case is directly associated with excitation from adsorption levels. 

The evidence that adsorption has an influence on the luminescence of zinc oxide is not as clear-cut as it is in the case of photoconductivity and conductivity. No experiments have been carried out, to the author s knowledge, directly correlating adsorption to luminescence, such as the work by Ewles and Heap (7) on silica, which showed correlations between its fluorescence and the adsorption of the hydroxyl radical. 

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