DOI: 10.7274/28786124
title: Noble Metal Nanoparticles: From Applications Down to Fundamental Surface Properties

description: The field of noble metal nanoparticles (NMN) has seen widespread applications in fields such as biomedicine and catalysis due to the high degree of tunability of the nanomaterial. This dissertation leverages NMN to investigate different analytical and fundamental problems. The first portion of this dissertation discusses the development of two schemes for analyte detection using silver nanoparticles (AgNPs). One scheme focuses on the detection of enzyme activity and inhibition using two novel surface-enhanced Raman scattering (SERS) reporter molecules that are released by the efficient enzymatic cleavage of enzyme substrate molecules by ß-Glucosidase (ß-Glcase). The two reporter molecules employ nitrile functional groups for clear and sensitive detection through the SERS silent region. This study was able to show that detection of the reporter molecules was more than 10x faster than comparative UV-vis absorption studies. The other scheme in this part focused on a host-guest capture method for the sensitive and selective detection of trace fentanyl in complex mixtures. The host molecule, cucurbit[7]uril (CB[7]), was shown to bind fentanyl with high specificity in the presence of cocaine and diphenhydramine via NMR studies. To implement this scheme on the surface of AgNPs, a thiolated-CB[7] analog (CB[7]-SH) was synthesized which achieved a fentanyl LOD of 0.37 nM. Unfortunately, the specificity of CB[7]-SH for fentanyl detection was challenging problem to circumvent due to the native affinity of fentanyl to the surface of AgNPs. Therefore, molecular selectivity in analytical sensing schemes for SERS is a more complex problem than the community may realize.
The second part of this dissertation shifts towards a new set of molecules for creating self-assembled monolayers (SAMs) – N-heterocyclic carbenes (NHCs). While thiols have been the gold standard in SAM technology for decades, they limit the utility of functionalized NMN due to their instability in situationally relevant conditions. About a decade ago, NHCs were shown to have superior stability on gold surfaces and gold nanoparticles (AuNPs). Even though their stability has been well researched by many groups since that seminal study, there remain many fundamental questions about NHCs on AuNPs. One of those questions is related to how the structure of the NHC impacts the molecular orientation on the surface of the AuNP. The study in this part utilized SERS to investigate how four different NHCs would orient on the AuNP as a function of the size of the groups on the nitrogen atoms (“wingtip groups”). Interestingly, SERS was able to simultaneously detect the flat and vertical orientations of all the NHCs on the AuNPs, with a disproportionate preference for the flat orientation.
Finally, the last part of this dissertation looks towards a more fundamental study on the enhancement mechanisms of surface-enhanced spectroscopies. The processes by which the weak Raman signal is enhanced are known as the electromagnetic mechanism (EM) and the chemical mechanism (CM), with the EM being the main contributor to the overall enhancement. SERS studies of these mechanisms have still left many questions regarding the contributions of the CM, necessitating the usage of a new technique for investigating these effects. The non-linear analog, surface-enhanced hyper-Raman scattering (SEHRS), provides complementary vibrational information with an increased sensitivity to the CM. The study in this final part shows that SEHRS scans of resonant and non-resonant molecules produces different results as a function of excitation energy, suggesting that the majority contributor of hyper-Raman signal enhancement is different.