Photo-induced surface-enhanced Raman scattering for biochemical sensing

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Surface-enhanced Raman scattering constitutes the backbone of optical fingerprinting of many defence-relevant molecules, from plastic explosives to diseases agents, with the hallmark of providing both chemical specificity and high sensitivity, down to the picomolar regime and beyond. The latter is mainly achieved via an electromagnetic enhancement of Raman scattering, utilizing localised surface plasmon excitations in nanostructured metallic films or on metallic colloids. Since Raman scattering scales with the fourth power of the local field, large enhancements up to typical factors 108 to 1011 are possible.Much more elusive is an additional chemical enhancement of Raman scattering, facilitiated via charge transfer from the metallic nanostructures to the molecules under investigation. In a recent pioneering study, Parkin and Maier demonstrated that this chemical enhancement can be induced via UV illumination of titania substrates coated with metallic nanocolloids (Nature Communications 7, 12189, 2016). Crucially, the additional enhancements works for a large number of molecules, from plastic explosives to TNT and large biomolecules.With this studentship project we want to investigate the physical origin of this effect, named PIERS - photo-induced enhanced Raman scattering - further. We want to understand the physical mechanism of charge transfer, discover ways of tuning it, and optimize the conditions for chemical enhancement of Raman scattering utilizing a variety of metallic colloids under different illumination conditions. The effect will then be benchmarked for a number of defence-relevant substances at dstl.