Silver nanoparticles (AgNPs) are tiny particles of silver with nanoscale dimensions (between 1 and 100 nm) and unique antimicrobial properties. AgNPs are potential environmental contaminants increasingly applied in consumer products. The effects on nontarget biological systems are not clearly defined. Research has shown that AgNPs may inhibit the function of bacteria responsible for organic matter decomposition, nutrient cycling, and control of pathogens population in the aquatic system. AgNPs have recently been detected in a treated municipal wastewater raising concerns about their potential risk to aquatic organisms. The microbial community in the sediment has a greater risk of AgNPs exposure, as metals in aquatic systems settle in the sediment. Studies have shown that microbial community growth and carbon sources utilization patterns were altered in response to AgNPs exposure in marine estuarine sediments. The antimicrobial activity of AgNPs in freshwater sediments may be different due to the water chemistry. Few studies have evaluated the toxicity of AgNPs in freshwater sediments due to the complex nature of their water chemistry. The current study investigated microbial community’s responses to AgNPs in sediments collected from a local stream. Microbial growth and activity assays were performed to determine whether AgNPs pose a risk to the microbial community in freshwater sediments. We found that AgNPs inhibited microbial growth, enzyme activity, and catabolic capabilities (P < 0.05). The number of viable bacterial cells and the ability of the microbial community to utilize different carbon sources decreased at 0.431 and 0.538 mg AgNPs kg-1 sediment, which are found within the estimated AgNPs concentration range in sediments. AgNPs inhibited the activity of glucosidase, an enzyme responsible for carbohydrate metabolism, but the activity of alkaline phosphatase was not affected. The current study demonstrates that AgNPs can inhibit the growth and functional diversity of beneficial microorganisms, which may affect the quality of surface waters and their designated uses. These adverse effects are expected due to the demonstrated antimicrobial properties of AgNPs incorporated in several commercial products. Toxicological data generated from this study could be incorporated in ecological risk assessment by regulatory agencies to assess the impacts of AgNPs on ecosystem systems.

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In this dissertation, the laser spectroscopic methods, second harmonic generation (SHG) and ultrafast transient absorption, have been employed to study the reactions and dynamics in two different types of materials, namely, silver nanoparticles and micro-sized ultrathin crystalline oligoacenes. These two materials, although both are in small dimensions, represent two distinct types of systems with divergent characteristics: 1) systems in which interactions at the surface/interface are dominant, and 2) systems in which bulk interactions are dominant. Silver nanoparticles are an important member of the class of noble metal nanoparticles, and possess unique optical and chemical properties due to their ultrafine size and high surface-to-volume ratio. Strong SHG signal has been observed from silver nanoparticles dispersed in aqueous colloidal solution, in which the SHG signal is enhanced due to a resonance with the localized surface plasmon of silver nanoparticles. Further experiments proved that the SHG signal predominantly originates from the particle surface, in full agreement with the intrinsically interface-sensitive properties of SHG. With the surface origin of the signal now well established, SHG can be used to probe the adsorption and reactions of thiol molecules at the nanoparticle surface in situ and in real time. It is experimentally demonstrated that the free energy change, activation energy, as well as adsorption density of the reactions of a variety of neutral and anionic thiols at the particle surface can be measured by means of SHG. The reaction mechanisms at the molecular level have been deduced, and the neutral vs anionic thiols are found to exhibit qualitatively different reaction mechanisms that reflect the effect of their molecular interactions with the particle surface. Oligoacenes, such as pentacene and hexacene, constitute a family of organic semiconductors that exhibit remarkable optoelectronic properties. In contrast to the nanoparticles in which surface interactions are dominant, as the sizes of materials become larger, the bulk characteristics become more deterministic. Therefore, polarized linear absorption and transient absorption spectroscopies have been applied to study the excitonic properties of crystalline pentacene and the mechanism of singlet fission in crystalline hexacene, respectively. The polarized absorption spectra of crystalline pentacene have been obtained by measuring transmitted light normal to the ab herringbone plane of micro-sized ultrathin single crystals. The significant deviations between the spectral line shapes polarized along the b-axis and orthogonal to the b-axis provide detailed information on the anisotropic mixing nature of the Frenkel/charge-transfer excitons responsible for the pronounced Davydov splitting between the lowest-energy singlet states. Additionally, both singlet and triplet Davydov splittings were also observed from the linear and transient absorption experiments in micrometer-sized ultrathin hexacene single crystals. A two-step process of anisotropic singlet fission was uncovered from the kinetic data, in which singlet fission at different rates were deduced along the a- and b-axes. Both the spectral and kinetic features indicate that singlet fission in crystalline hexacene is an anisotropic and charge-transfer mediated many-molecule process. / Chemistry

Incorporation of silver nanoparticles (AgNPs) into an increasing number of consumer products has led to concern over the potential ecological impacts of their unintended release to the environment. Dissolution is an important environmental transformation that affects the form and concentration of AgNPs in natural waters; however, studies on AgNP dissolution kinetics are complicated by nanoparticle aggregation. Herein, nanosphere lithography (NSL) was used to fabricate uniform arrays of AgNPs immobilized on glass substrates. Nanoparticle immobilization enabled controlled evaluation of AgNP dissolution in an air-saturated phosphate buffer (pH 7, 25 °C) under variable NaCl concentrations in the absence of aggregation. Atomic force microscopy (AFM) was used to monitor changes in particle morphology and dissolution. Over the first day of exposure to ≥10 mM NaCl, the in-plane AgNP shape changed from triangular to circular, the sidewalls steepened, and the height increased by 6-12 nm. Subsequently, particle height and in-plane radius decreased at a constant rate over a 2-week period. Dissolution rates varied linearly from 0.4 to 2.2 nm/d over the 10-550 mM NaCl concentration range tested. NaCl-catalyzed dissolution of AgNPs may play an important role in AgNP fate in saline waters and biological media. This study demonstrates the utility of NSL and AFM for the direct investigation of un-aggregated AgNP dissolution. / Master of Science