Implications of intermolecular interactions on normal Raman and surface-enhanced Raman spectra using gold nanostars and polymers

Normal Raman and surface-enhanced Raman scattering (SERS) are powerful methodologies that are complementary to infrared absorption spectroscopy. While Raman spectroscopy provides unique molecular fingerprint information versus other techniques, this approach is limited by low signal-to-noise. SERS, in particular, is a surface sensitive spectroscopy that utilizes nanoparticles to increase molecular signals by 2−9 orders of magnitude, but molecular chromophores are required to interact with the metal surface at short, sub-2 nm distances, and as revealed in this dissertation, SERS enhancements are associated with nanomaterial morphology and physical stability as well as binding affinities between analytes and nanomaterials. In addition, SERS signals are sensitive to intermolecular interactions as these increase spectral complexity. Thus, this thesis focuses on understanding and overcoming challenges with small molecular detection using SERS. Specifically, understanding how vibrational spectral features of small molecules are impacted by nanomaterials, molecular orientation, and nanomaterial/polymer-molecule interactions are investigated. Impacts of binding affinity, incubation time, and the distance between the materials and target molecules are evaluated. First, gold nanostar stability is predicted and measured by monitoring the optical properties of Good’s buffer synthesized gold nanostars before and after functionalization with 6-mercaptohexanoic acid. DLVO theory reveals that nanostar dimension influences the attractive and repulsive interactions between nanostructures, which are measured experimentally using localized surface plasmon resonance (LSPR) spectroscopy. Next, the interactions of the Good’s buffer reagent, N-2-hydroxyethylpiperazine-N’-2-ethanesulfonic acid (HEPES), to gold nanostars are evaluated as a function of pH using both DFT calculations and experimental methods including SERS and LSPR spectroscopies. The adsorption of the weakly-binding molecule, benzene, to gold is used to investigate how HEPES and benzene adsorb to the nanostar surfaces through changes in Raman intensity, vibrational frequency, and extinction maximum wavelength. To further understand the implication of HEPES on SERS measurements of weakly binding analytes, the adsorption of benzene derivatives including benzoic acid, aniline, and phenol to nanoparticles as a function of surface pKa, surface orientation, and binding affinity are studied. While these approaches yield promising results, selectivity is lacking. This leads to a final investigation in which molecular imprinted polymers are synthesized, characterized, and used to provide modest selectivity and enrichment of small molecules such as acetaminophen, aspirin, and caffeine. The enrichment processes is monitored using normal Raman spectroscopy, and the observed Raman vibrational frequency shifts indicate that hydrogen bonding and other weak dispersion interactions between the molecules and polymers drive selectivity. In the future, if nanomaterials can be integrated with molecular imprinted polymers, the signal of enriched small molecules could be further improved, and the detection of small molecules in a complex system achieved.