Photo-induced surface-enhanced Raman scattering for biochemical sensing

表面增强拉曼散射构成了许多防御相关分子的光学指纹的支柱，从塑料炸药到疾病制剂，其特点是提供化学特异性和高灵敏度，低至皮摩尔范围或更高。后者主要是通过电磁增强的拉曼散射，利用局部表面等离子体激元激发纳米结构的金属膜或金属胶体。由于拉曼散射与局部场的四次方成比例，因此可能有高达108到1011倍的典型增强。更难以捉摸的是拉曼散射的额外化学增强，通过从金属纳米结构到所研究的分子的电荷转移来促进。在最近的一项开创性研究中，Parkin和Maier证明，这种化学增强可以通过对涂覆有金属纳米胶体的二氧化钛基材进行紫外照射来诱导（Nature Communications 7，12189，2016）。至关重要的是，额外的增强作用适用于大量的分子，从塑料炸药到TNT和大型生物分子。通过这个学生项目，我们希望进一步研究这种效应的物理起源，称为PIERS-光诱导增强拉曼散射。我们希望了解电荷转移的物理机制，发现调整它的方法，并优化在不同光照条件下利用各种金属胶体进行拉曼散射化学增强的条件。随后，dstl将对一些国防相关物质的影响进行基准测试。

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.