Surface-Enhanced Vibrational Spectroscopy: A New Tool in Chemical IR Sensing?

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Abstract
In order to further improve the sensitivity of chemical IR sensors, research into a significant change in the signal generation mechanism was carried out. This effort was realized by generating the surface-enhanced IR absorption and Raman-scattering effect at silver- and gold-island-covered Ge- and ZnSe ATR surfaces for surface-enhanced infrared absorption (SEIRA) and Ag-layer-covered brass plates for surface-enhanced Raman scattering (SERS), respectively, and linking the optimized tools to the IR sensor concept. By careful optimization of the generation of silver-island and gold-island films on Ge reflection elements, intensity gains by a factor of 50 were achieved. Chemical interactions between the silver-island layer and the various analytes investigated up to now (p-nitro benzoic acid, as a model compound, and selected aromatic and chlorinated pesticides) are likely, as indicated by significant IR spectral changes; the substances are, however, bound to the surfaces in a reversible manner. This property of the novel SEIRA sensor can be favorably exploited in flow-through systems. In a similar way, by a specially optimized treatment of anodically oxidized brass plates in a silver bath, repeatably usable SERS targets for the trace analysis of pesticide mixtures in aqueous systems could be developed. A signal enhancement factor of 1.4 million was obtained with pyridine as the standard sample. The data shown in this work suggest that both SEIRA and SERS can be coupled to a flow injection system in a reversible way for the molecular-specific trace analysis of organic compounds in aqueous solutions.