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The localized surface plasmon resonance (LSPR), arising due to the collective oscillation of free electrons in metal nanoparticles, is a sensitive probe of the nanostructure and its surrounding dielectric medium. Synthetic strategies for developing surfactant free nanoparticles using ultrafast lasers providing direct access to the metallic surface that harvest the localized surface plasmons will be discussed first followed by the applications. It is well known that the hot carriers generated as a result of plasmonic excitation can participate and catalyze chemical reactions. One such reaction is the dissociation of hydrogen. By the virtue of plasmonic excitation, an inert metal like Au can become reactive enough to support the dissociation of hydrogen at room temperature, thereby making it possible to optically detect this explosive gas. The mechanism of sensing is still not well understood. However, a hypothesis is that the dissociation of hydrogen may lead to the formation of a metastable gold hydride with optical properties distinct from the initial Au nanostructures, causing a reversible increase in transmission and blue shift in LSPR. It will also be shown that by tracking the LSPR of bare Au nanoparticles grown on a substrate, the adsorption of halide ions on Au can be detected exclusively. The shift in LSPR frequency is attributed to changes in electron density rather than the morphology of the nanostructures, which is often the case. / Chemistry

Droplets are ubiquitous microscopic systems - ranging in size from several nanometers to ~100 micrometers – that undergo abundant environmental interactions. Researchers have shown that droplets can impact both earth climate and air quality through physical and chemical processes. Droplets released from the human respiratory system, either suspended in air or deposited on surfaces, can carry pathogens (e.g., influenza viruses, the SARS-CoV-2 virus), and are thus important for disease transmission. The need to understand the role of droplets in environmental processes requires appropriate tools for droplet characterization. We used Raman and surface-enhanced Raman spectroscopy (SERS) based imaging as such tools due to their capacity for simultaneous collection of abundant molecular information inside droplets and their potential to collect detailed images of droplet component distributions. We imaged pH and chemical moiety distributions inside droplets over a wide range of: 1) droplet compositions; 2) surrounding environmental conditions (relative humidity, temperature); and 3) droplet morphologies. This dissertation describes measurement of droplet pH in droplets containing mixtures of phosphate buffer (PB), one of the most commonly used biological solvents, and ammonium sulfate (AS), arguably the most abundant chemical species in atmospheric droplets, at room temperature. We observed a pH gradient inside PB droplets while a homogeneous pH distribution was found inside AS droplets, thus showing a significant pH effect due to droplet composition. We attributed the contrasting pH distribution in the two droplet systems to different ionic interactions at the air-water interface. In addition, we obtained AS droplet images at 223K to investigate ice nucleation upon freezing. We observed variable nucleation behavior in AS droplets as a function of concentration, a finding with implications for atmospheric cloud nucleation. We also investigated virus deposition during sessile droplet evaporation using gold nanoparticles. SERS imaging enabled development of correlations between virus viability and droplet deposition pattern and related them in terms of the coffee-ring effect. Suppression of the coffee-ring effect can reduce virus infectivity on surfaces during droplet evaporation. These works collectively exhibit the potential of Raman and SERS imaging for droplet characterization. / Doctor of Philosophy / Droplets are ubiquitous in the environment. Small droplets can form clouds and fogs, and are often micro- to nano-scale in size. Droplets can either grow or shrink in the environment when they absorb or lose water. Similarly, reactions may happen when droplets contain various species. Droplets in human breath exhalate may contain pathogens, such as the SARS-CoV-2 virus that is the cause of the COVID-19 pandemic. If the virus stays viable in droplets, no matter where the droplets are located, the virus will remain infectious and may be transmitted to others through contact. The studies in this dissertation were conducted to determine the distributions of soluble and insoluble components inside droplets and to elucidate how the observed distributions correlate with important droplet properties and environmental processes. We used two methods to observe droplets: Raman spectroscopy and surface-enhanced Raman spectroscopy (SERS). Molecules are constantly vibrating, these vibrations result in characteristic Raman signals that can be monitored. Both Raman and SERS provide such measurements, except that SERS has greater sensitivity due to the signal enhancement provided by gold or silver nanoparticles. In this dissertation, we obtained images of droplets with variable compositions at both room temperature and -50 °C. We also examined virus survival inside droplets during droplet drying. Using the collected images, we related the component distribution inside a droplet to its acidity, and evaluated virus survival in terms of droplet drying patterns. The images demonstrate that Raman and SERS imaging are promising tools for the study of droplets.