Molecular Impacts of Plasmonic Effects Resulting from Surface Enhanced Raman Analysis

Surface enhanced Raman spectroscopy (SERS) is a technique that utilizes plasmonic nanostructured metal surfaces to increase the Raman signal. The excitement of the localized surface plasmon resonance (LSPR) of gold nanoparticles leads to enhanced electric fields and increases the Raman signal can also result in the generation of highly energetic or “hot” carriers in the form of electrons or holes. These hot carriers have been further been shown to perform chemical reactions. This dissertation includes the investigation of specific analytes that undergo changes resulting from hot carriers. Chapter 1 introduces Raman and SERS as analytical techniques and explains the fundamentals involved in each technique. The generation of hot carriers through the SERS process is explained as well as how hot carriers have been previously identified to impact SERS signal. An explanation of previous literature and understanding is included to provide support for the studies elaborated on in latter chapters. Chapter 2 introduces evidence of the formation of a tryptophan radical anion species generated that drastically impacts the observed SERS signal. The difference in the SERS and Raman signals are studied through various changes made to the system. This study includes the impact on the SERS identification of tryptophan in proteins through studying a protein that contains a single tryptophan residue and analyzing the SERS signal. The formation of a tryptophan radical anion is accompanied by an electronic resonance that generates surface enhanced resonance Raman scattering, enabling more sensitive detection. Chapter 3 uses computational modelling to explore the vibrational changes observed in the SERS spectrum of tryptophan with gold nanoparticles. DFT calculations are combined with experiments on an analog containing the indole ring moiety. Spectral analysis of the simulated vibrations compared with experimentally observed modes identified specific Raman modes that are impacted by experimental changes and the formation of the radical anion. The indole moiety is identified as the chemical feature that supports the additional electron. The impact of the experimental parameters that impact hot carrier formation and transfer is reported in chapter 4. This chapter focuses on a traditional Raman setup examining a common reporter molecule, 4-mercaptopbenzoic acid (MBA), on gold nanoparticles and references results collected utilizing a wide field imaging setup. Traditional point-focused microspectroscopy provides spectral resolution for the identification of intermediates observed. Further DFT simulations identify additional radical species of MBA formed. However, additional fluctuations observed in the data suggest the formation of other species. Chapter 5 summarizes the main results, makes conclusions, and provides a perspective on the research explained in this dissertation. This research allows for better understanding of how SERS can be further utilized in fields such as plasmonic catalysis, and detection of proteins. Appendices are included to provide the optimized structure coordinates from DFT, how the DFT simulations were run, and brief reports of other results that further support the work shown here. Overall, this dissertation provides new understanding of the molecular impact of hot carriers generated by exciting the LSPR of gold nanoparticles for SERS through experimental work coupled with DFT simulations.