Chemical surface modifications for improved mid-infrared evanescent field-sensing systems

Among optical sensing schemes, mid-IR evanescent field sensing systems are gaining attention because of their inherent molecular selectivity. Two distinct approaches of chemical surface modifications for mid-IR waveguides were studies in this thesis and their influence on the sensing system performance was thoroughly investigated: (i) the formation of thin diamond like carbon (DLC) films on ATR ZnSe crystals providing highly inert waveguides and (ii) the application of organically modified sol-gels as thin porous membrane at the surface of planar and fiberoptic mid-IR waveguides with the final aim of preparing molecule selective recognition layers. DLC membranes with thicknesses in the sub-μm range were deposited via pulsed laser deposition (PLD) onto the surface of ZnSe crystals, characterized and used as sensing element for direct analysis of strongly oxidizing agents in aqueous solution. The DLC films show high chemical and mechanical stability in contact with hydrogen peroxide, peroxyacetic acid and ammonium persulfate. Furthermore, good repeatability of experiments, complete reversibility and limits of detection in the sub-percentage concentration have been demonstrated for the applied sensing system. In the second part of this thesis organically modified sol-gels were formed as thin membranes at the surface of mid-IR evanescent field waveguides (planar ZnSe ATR-crystals; silver halide optical fibers) and their applicability as physico-chemical recognition element was investigated. The results prove the capability of sol-gel coated mid-IR waveguides to detect aromatic compounds containing nitrile-, carbonyl- and nitro-functional groups down to ppm and sub-ppm concentration ranges in lab experiments as well as simulated real world experiments. Principle component regression (PCR) was found to be a promising approach to significantly reduce sensing response tune as reliable signal evaluation after approximately 30 min is enabled. While base-catalyzed sol-gel processing enables significantly improved average porosity of the sol-gel based membrane, molecular templating approaches for organically modified sol-gels do not result in enhanced selectivity. The obtained results indicate that surface modification of mid-infrared evanescent field waveguides via diamond like carbon and organically modified sol-gels contributes to an enhanced applicability of mid-infrared sensing systems in the realm of in situ analysis for process control and environmental screening in harsh environments.