Fiber-optic evanescent wave spectroscopy

Raichlin Y. Katzir A. (2008). Fiber-Optic Evanescent Wave Spectroscopy in the Middle Infrared. Applied Spectroscopy, Vol.62, No.2, p>p. 55A-72A, ISSN 1943-3530 Rappoport, Z. Marek, I. (2004). The chemistry of organolithium compounds, Wiley, ISBN 0-470-84339-X, Chichester... 

Messica A., Greenstein A., and Katzir A., Theory of fiber-optic, evanescent-wave spectroscopy and sensors, App/. Opt, 35,2274-2284 (1996). 

D. S. Blair, L. W. Burgess and A. M. Brodsky, Study of analyte diffusion into a silicone-clad fiber-optic chemical sensor by evanescent wave spectroscopy, Appl. Spectr., 49 [11], 1636-1645 (1995). 

Optical biosensor technology has realized remarkable developments in the last decades [126-129]. This section will specifically focus on IR fiber sensors based on ChG materials. We will first describe the mechanism of evanescent wave spectroscopy and summarize the essential properties of ChG glasses when used in the framework of fiber-based sensing. Finally we will present several techniques commonly employed for the shaping of ChG glasses into fiber-optic sensors and survey their applications. 

The use of fiber-optics in the preparation of the optical sensors allows performing spectroscopy measurements at sites that are inaccessible to conventional spectroscopy and also over large distances. Moreover, the possibility of using evanescent wave spectroscopy and spatially resolved lifetime spectroscopy makes FOCS (fiber optics chemical sensors) a very interesting option for building the sensors. Comprehensive reviews [183,184] on fiber-optic chemical sensors are available, where the most interesting applications of sol-gel coatings for optical sensors are described. 

A fiber-optic device has been described that can monitor chlorinated hydrocarbons in water (Gobel et al. 1994). The sensor is based on the diffusion of chlorinated hydrocarbons into a polymeric layer surrounding a silver halide optical fiber through which is passed broad-band mid-infrared radiation. The chlorinated compounds concentrated in the polymer absorb some of the radiation that escapes the liber (evanescent wave) this technique is a variant of attenuated total reflection (ATR) spectroscopy. A LOD for chloroform was stated to be 5 mg/L (5 ppm). This sensor does not have a high degree of selectivity for chloroform over other chlorinated aliphatic hydrocarbons, but appears to be useful for continuous monitoring purposes. 

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). 

Fiber-optic techniques offer increased sensitivity relative to conventional bulk optic approaches. For example, evanescent wave (EW) spectroscopy is significantly more sensitive than bulk attenuated total reflection spectroscopy. Fiber-optical EW spectroscopy is the only technique suitable for use with highly absorbing or scattering media. 

MS Braiman, RE Jonas. Evanescent-wave IR spectroscopy of singje-bilayer membranes coated on chalcogenide fibers sensitivity improvements using a diamond rod coupler between fiber and source . Proceedings of Spie - the International Society for Optical Engineering, vol. 1796,, pp.402-11. USA, 1993. 

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