Surface-Enhanced Raman and Hyper-Raman Scattering: From Fundamentals of Nonlinear Light Scattering to Practical Analytical Detection

This dissertation is split into two parts: surface-enhanced Raman scattering (SERS) as an analytical tool for small molecule detection and surface-enhanced hyper-Raman scattering (SEHRS) for elucidating photophysical properties and its use as a bioimaging technique. Specifically, four studies will be presented and their contributions will be discussed.

The first study utilizes a capture agent for SERS detection of phenylalanine (Phe) and Phe-terminated peptides. Here, a macrocycle, cucurbit[7]uril (CB[7]), binds to Phe, increasing the local concentration of Phe surface of AgNPs. This host-guest interaction increases the sensitivity of Phe SERS detection. This scheme is then tested on Phe-terminal peptides, including insulin, to show its versatility for sensing larger biomolecules. Given the extensive binding chemistry of CB[7], this scheme provides a framework for increasing sensitivity of Phe detection and the detection of Phe-terminated biomolecules in SERS.

Part 2 then starts with a study of the lower-wavenumber SEHRS vibrational modes of rhodamine 6G (R6G). Single-molecule SEHRS detection uses R6G and its isotopologue because of its high SEHRS cross-section. R6G's isotopologue displays a different SEHRS response only in the lower-wavenumber vibrational modes, below 1000 cm^-1. The character of these vibrational modes have dependencies on the resonance conditions and molecular surface orientation, making them challenging to understand. Accurately characterizing the complex SEHRS profile of R6G and its isotopologue in the lower-wavenumber region is important for understanding the design of efficient two-photon chromophores.

The next study analyzes the resonant SEHRS response of chalcogen-substituted rhodamine-analogs. The relationship of structure to SEHRS response is elucidated through experiment resonant SEHRS scans and computations to analyze the effect of chromophore symmetry. Symmetry is shown to greatly influence the SEHRS response across multiple analogs. Experimental on-resonance SEHRS in conjunction with time-dependent density functional calculations provides a powerful basis for elucidating the complex vibronic landscape of rationally-designed two-photon chromophores.

The final study uses short-wave IR (SWIR) SEHRS to image a nanoparticle-loaded mouse spleen. In a proof-of-principle study, we spatially map the SEHRS response from dye-functionalized nanoparticles in a mouse spleen under efficient collection conditions. We observe SEHRS to have a higher signal variation than SERS, showing its advantages as an imaging method. Introducing SEHRS as a two-photon imaging method presents a means of spatially mapping tissue with a molecular fingerprint.