Silica optical fibers, coatings

Solid-phase microextraction (SPME) — is a procedure originally developed for sample preconcentration in gas chromatography (GC). In this procedure a small-diameter fused silica optical fiber, coated with a liquid polymer phase such as poly(dimethylsiloxane), is immersed in an aqueous sample solution. The -> analytes partition into the polymer phase and are then thermally desorbed in the GC injector on the column. The same polymer coating is used as a stationary phase of capillary GC columns. The extraction is a non-exhaustive liquid-liquid extraction with the convenience that the organic phase is attached to the fiber. This fiber is contained in a syringe, which protects it and simplifies introduction of the fiber into a GC injector. Both uncoated and coated fibers with films of different GC stationary phases can be used. SPME can be successfully applied to the analysis of volatile chlorinated organic compounds, such as chlorinated organic solvents and substituted benzenes as well as nonvolatile chlorinated biphenyls. 

Solid-phase extraction devices and applications are evolving rapidly, and novel techniques that stretch the classical definition of SPE are becoming routine. Pawliszyn introduced solid-phase micro extraction (SPME) in 1989,5,14 and a commercial apparatus is available from Supelco (Bellefonte, PA). The SPME apparatus is merely a modified syringe that houses a fused silica optical fiber coated with an immobilized polymer film. The fiber can be exposed for extraction and then retracted for insertion or removal from the sample vial or instrument. Both manual and autosampler devices are available and each can be adjusted for proper fiber depth. Several coatings are available with varying thickness including polydimethylsiloxane, polyacrylate, polydimethylsiloxane/divinylbenzene, and carbowax/divinylben-zene. In contrast to SPE, which is an exhaustive extraction approach, SPME will extract only a fraction of an available analyte, hence it is not suitable for the isolation of impurities and degradants in most applications.15... 

Solid-phase microextraction (SPME) is a sampling and concentration technique used to increase the sensitivity of HS methods. This technique is utilized for arson analysis and environmental monitoring purposes and also for clinical and forensic procedures. Short, narrow diameter, fused-silica optical fibers coated with stationary phase polymers are either immersed in the sample or the HS and compounds are adsorbed or absorbed (depending on... 

The SPME apparatus looks like a modified syringe (see Fig. 5) consisting of a fiber holder needle and a fiber assembly, the latter equipped with a 1-2 cm long retractable SPME fiber. The fiber itself is a thin fused-silica optical fiber, coated with a thin polymer film (such as polydimethylsiloxane, PDMS), as shown... 

Modified fiber-optic-based sensors can be used for sensing pollutants, explosives, drugs, pharmaceuticals, and miscellaneous organics (Yeh et al. 2006). Optical fibers coated with porous silica can be used to detect the presence of chlorinated hydrocarbons. Alternatively, these compounds can also be detected using fiber-optic-coupled surface plasmon resonance methods. Aromatic compounds were detected by evanescent wave absorption spectroscopy. Suitably modified fiber-optic array tips can be used to detect presence of explosive materials (Wolfbeis 2000). 

Fig. 5.11. (a) Spin-coated polymer 1 films (40-80 nm thick, with refractive index (n 1.70) are coated on a transparent 200-nm-thick film of chemical vapor deposited parylene (n 1-67), forming a two-layer index-matched waveguide on glass (n 1.45). (b) Spin-coated polymer 1 films (40nm) coated on DFB gratings fabricated from PDMS. (c) Ring-mode laser structure produced by dip-coating polymer 1 on a 25 mm diameter silica optical fiber... 

Fig. 5.12. Photostability of a thin film of polymer 1 coated on a silica optical fiber. A small amount of photobleaching is observed ( 3% of total signal) over a 20 s time interval. During TNT measurements the sample was exposed to laser excitation for approximately 10s (0.5 s for 25 measurements). Because the measured quenching due to TNT is much greater, the signal attenuation due to photobleaching can be neglected...

In the described material different properties have been integrated, such as capability of UV curing, availability of chemically sensitive moieties and a flexible structure suitable for coating silica optical fibers. 

Vitreous silica is important not only in the traditional ceramic and glass uses but also in its application in optical fibers, microelectronics, and catalysis. A detailed understanding of the bulk and the surface structure of the material is needed to improve optical fiber coatings, microelectronic devices, and catalytic supports. 

Acoustic and optical gas sensors, respectively, were developed by coating CNTs onto the chips of a quartz crystal microbalances (QCM) and silica optical fibers (SOFs). As shown in Figure 7.7c, CNT-coated QCM crystals were roughly 1-2 orders of magnitude more sensitive than uncoated QCM crystals. Also, it was danonstrated that CNT-coated SOFs had similar low limits of detection and enhanced sensitivity. 

Even cationic species such as the (hydrated) hydrogen ion (H ) can be lured into the hydrophobic silicone films from water medium as long as they contain appropriate additives to allow penetration of the analyte. For instance, Nivens et al. [140,141] have reported fabrication of silicone/ sol-gel films using a hydroxy-terminated poly(dimethyl siloxane (PDMS-OH), tetraethoxysilane (TEOS) and 3-ami-nopropyltriethoxysilane (APTES) mixture in ethanol-water. Tetramethylammonium hydroxide (TMAOH) is added to the mixture to entrap the fluorescent pH indicator dye 8-hydroxypyrene-l,3,6-trisulfonic acid trisodium salt (HPTS) [102], after base-catalyzed polycondensation of the silanes and the PDMS-OH. Silica optical fibers are stripped from their jacket at the distal end and dip-coated with the indicator gel before the end of gelation (24 h). The optical fluorosensor is able to measure between pH 6.0 and 8.5 for 6 months if stored in buffer medium. 

Optical fibers are widely used in the telecommunication industry for fhe fransmission of digital pulses of voice, video, and dafa. In order to keep losses in signal strength at a minimum, the fiber (usually a doped silica glass) has to be coated with a material of lower refractive index than its own refractive index. This layer is protected by a "buffer," which acts as a cushion. The buffer is encased in one or more protective layers (see Figure 7.18). Both the primary and protective coatings are very often UV radiation curable. 

In the supported systems the catalyst can be coated on the walls of the reactor, supported on a solid substrate or deposited around the case of the light source. Many are the supported materials used in literature, such as glass beads, and tubes [69], silica-based materials [70], hollow beads, membranes [71], optical fibers, zeolites, activated carbon, organic fibers [72], and so on. 

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