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Localized surface plasmon resonance (LSPR) describes the collective oscillation of conductive electrons in noble metal nanostructures, such as gold, silver and copper; or in selected doped semiconductor nanocrystals. Nanoplasmonics is emerging as a useful and versatile platform that combines the intense and highly tunable optical responses derived from LSPR with the intriguing materials properties at the nanoscale, including high specific surface areas, surface and chemical reactivity, binding affinity, and rigidity. LSPRs in plasmonic nanoparticles (NPs) can provide large optical cross-sections, and can lead to a wide variety of subsequent photophysical responses, such as localization of electric (E-)fields, production of plasmonic hot charge carriers, photothermal heating, etc. Plasmonic NPs can also be combined with other molecular or nanoscale systems into plasmonic heterostructures to further harvest the resonant E-field enhancement or to prolong the lifetime of plasmonic charge carriers. In this dissertation, we study the photophysical properties of plasmonic Ag and Au NPs, particularly E-field localization and hot charge carrier production performances; and illustrate how they can be optimized towards plasmonic photocatalysis, development of nano-antimicrobials, and surface-enhanced Raman spectroscopy (SERS) sensing. We demonstrate that with a lipid-coated noble metal nanoparticle (L-NP) model, the E-field localization properties could be optimized through positioning molecular photosensitizers or photocatalysts within a plasmonic “sweet spot”. This factor renders the plasmonic metal NPs efficient nanoantenna for resonant enhancement of the intramolecular transitions as well as the photocatalytic properties of the molecular photocatalysts. The enhanced photoreactivity have been applied to facilitate fuel cell half reactions for the enhancement of light energy conversion efficiencies; as well as to facilitate the release of broad-band bactericidal compounds that enables plasmonic nano-antimicrobials. Localized E-fields in L-NPs also enhance the inelastic scattering from molecules through SERS. This is utilized to obtain molecular-level information on the configuration of sterol-based, alkyne-containing Raman tags in hybrid lipid membranes, which enables spectroscopic sensing and targeted imaging of biomarker-overexpressing cancer cells at the single-cell level. In contrast to the localized E-field, plasmonic charge carrier generation mechanism relies on non-radiative decay pathways of the excited plasmons that lead to production of ballistic charge carriers. The plasmonic hot charge carriers directly participate in chemical redox processes with degrees of controllability over the nature of the charge carrier produced and direction of their transfers. The implementation and optimization of these mechanisms are explored, and the significances of some relevant applications are discussed.

Anisotropic plasmonic noble metallic nanoparticles (APMNs) have received enormous attention due to their distinct geometric features and fascinating physicochemical properties. Owing in large part to their tailored localized surface plasmon resonance (LSPR) and the intensive electromagnetic field at the sharp corners and edges, APMNs are exceptionally well suited for biomedical applications such as biosensing, bioimaging, diagnostics and therapeutics. Although a rich variety of surfactant-assisted colloidal routes have been developed to prepare well-defined APMNs, biomedical applications necessitate tedious and rigorous purification processes for the complete removal of toxic surfactants. In this dissertation, we aim to develop generic bioenabled green synthetic methodologies towards APMNs. By applying a series of thermodynamic, kinetic and seed quality control, a series of APMNs with varied morphologies such as branched nanostars and triangular nanoprisms have been successfully prepared. We first presented the preparation of gold nanostars (Au NSTs) through a two-step approach utilizing a common Good's buffer, HEPES, as a weak reducing agent. Single crystalline Au NSTs with tunable branches up to 30 nm in length were produced and the halide ions rather than the ionic strength played a significant roles on the length of the branches of Au NSTs. Then consensus sequence tetratricopetide repeat (CTPR) proteins with increasing number of repeats were used as model proteins to probe the effects of concentration as well as the protein shape on the morphology and resulting physicochemical properties of plasmonic gold nanoparticles. Since the underlying growth mechanism for the biomimetic synthesis of APMNs remains elusive and controversial, the other objective is to elucidate the molecular interactions between inorganic species and biopolymers during the course of NP evolution. Fluorescent quenching and 2D NMR experiments have confirmed the moderate binding affinity of CTPR to the Au(0) and Au(III). We observed that the initial complexation step between gold ions and CTPR3 is ionic strength dependent. Furthermore, we also found that NPs preferentially interact with the negatively charged face of CTPR3 as observed in 2D NMR. Knowledge of binding behavior between biospecies and metal ions/NPs will facilitate rational deign of proteins for biomimetic synthesis of metallic NPs. A modified seed-mediated synthetic strategy was also developed for the growth of silver nanoprisms with low shape polydispersity, narrow size distribution and tailored plasmonic absorbance. During the seed nucleation step, CTPR proteins are utilized as potent stabilizers to facilitate the formation of planar-twinned Ag seeds. Ag nanoprisms were produced in high yield in a growth solution containing ascorbic acid and CTPR-stabilized Ag seeds. From the time-course UV-Vis and transmission electron microscopy (TEM) studies, we postulate that the growth mechanism is the combination of facet selective lateral growth and thermodynamically driven Ostwald ripening. By incorporation of seeded growth and biomimetic synthesis, gold nanotriangles (Au NTs) with tunable edge length were synthesized via a green chemical route in the presence of the designed CTPR protein, halide anions (Br⁻) and CTPR-stabilized Ag seeds. The well-defined morphologies, tailored plasmonic absorbance from visible-light to the near infrared (NIR) region, colloidal stability and biocompatibility are attributed to the synergistic action of CTPR, halide ions, and CTPR-stabilized Ag seeds. We also ascertained that a vast array of biosustainable materials including negatively charged lignin and cellulose derivatives can serve as both a potent stabilizers and an efficient nanocrystal modifiers to regulate the growth of well-defined Ag nanoprisms using a one-pot or seeded growth strategy. The influential effects of reactants and additives including the concentration of sodium lignosulfonate, H2O2 and NaBH4 were studied in great detail. It implies that appropriate physicochemical properties rather than the specific binding sequence of biomaterials are critical for the shaped-controlled growth of Ag NTs and new synthetic paradigms could be proposed based on these findings. Last but not the least, we have demonstrated the resulting APMNs, particularly, Au NSTs and Ag NTs exhibit remarkable colloidal stability, enhanced SERS performance, making them promising materials for biosensing and photothermal therapy. Since the Ag nanoprisms are susceptible to morphological deformation in the presence of strong oxidant, they also hold great potential for the colorimetric sensing of oxidative metal cation species such as Fe3+, Cr3+, etc. / Ph. D.