Despite the growing applications of silver nanoparticles, toxicity information on this nanomaterial is still deficient. Conclusions on the toxicity of silver nanoparticles vary and atomic level toxicity mechanisms are not yet achieved. Consequently, our group conducted combined computational and experimental toxicity studies of silver nanoparticles (AgNPs). Toxicity of 10 nm citrate stabilized AgNPs on HepG2 cells were investigated. Experimental results show that the 10 nm citrate stabilized AgNPs begin to be toxic to HepG2 cells at a dosage that exceeds 1 ppm and LD50 was observed at 3 ppm. Elevated reactive oxygen species levels were seen upon exposure to AgNPs with the maximum at the LD50 concentration of 3 ppm. Normal protein regulation of HepG2 cells were affected by exposure to AgNPs. TEM images of HepG2 cells exposed to AgNPs reveal that AgNPs can penetrate and agglomerate inside the cells. Our preliminary computational study was guided by one of the widely accepted toxicity mechanism of AgNPs in which the nanoparticles dissolute to Ag+. The computational model was composed of a 1:1 ratio of silver and phospholipid head. The silver employed are in atomic and anionic form while the phospholipid head are the phosphocholine (PC) and phosphoethanolamine (PE), which are abundant in HepG2 cells. Computational study shows that the presence of Ag+ results in partial oxidation of both the phospholipid heads. Our preliminary experimental and computational studies lead us to develop new computational methods that can accurately predict oxidation potentials (HOMO), reduction potentials (LUMO), and absorption spectra that can be used in studying toxicity mechanism of AgNPs through the oxidation pathway. Thus, computational methods for cyclic voltammetry and absorption spectroscopy that use DFT and TD-DFT, respectively, were improved to provide more accurate electronic and optical properties. Cyclopenta-fused polycyclic hydrocarbons (CP-PAHs) with available experimental data for HOMO, LUMO, ΔEgap and absorption spectra and have potential application as AgNP stabilizers were used in developing the improved computational methods for cyclic voltammetry and absorption spectroscopy. The improved computational method for cyclic voltammetry was developed by accounting for the anion species that occur experimentally and by using B3LYP the best density functional in predicting the HOMO, LUMO and ΔEgap of CP-PAHs with overall MAE of 014 eV. The best absorption spectra otef CP-PAHs were predicted using B3LYP for geometry optimizations followed by TD-CAMB3LYP with MAE of 29 nm. All calculations of CP-PAHs were implemented using the 6-311g (d,p) basis set and tetrahydrofuran (THF) as solvent. These two developed computational methods were tested on a group of methyl triphenyl amine (MTPA) derivatives with available experimental data for HOMO, LUMO, ΔEgap and absorption spectra and have potential application as AgNP stabilizers. The new computational methods for cyclic voltammetry and absorption spectroscopy also provided the most accurate predicted electronic and optical properties of MTPA derivatives. Among the ten density functionals employed, prediction of HOMO, LUMO and ΔEgap were most accurate using B3LYP and B3PW91 with overall MAE of 0.31 eV and 0.27 eV, respectively. Absorption spectra of MTPA derivatives were still best predicted using the B3LYP/TD-CAMB3LYP method with MAE of 13 nm. All calculations of MTPA were implemented using the 6-31+g (d,p) basis set and dichloromethane as solvent.

In this work, we use optical approaches to study and provide mechanistic insight into electrochemical reactions occurring at the surface of single nanoparticles. Correlated optical-electrochemical studies offer several advantages over single nanoparticle electrochemical studies including, higher spatial resolution, the ability to interrogate many nanoparticles at the same time and identify populations of inactive nanoparticles. Throughout this dissertation, two optical techniques are discussed in detail, dark-field scattering and super-localization imaging. In the first set of experiments, we describe calcite-assisted localization and kinetics (CLocK) microscopy, a multiparameter super-localization imaging technique. By placing a rotating birefringent calcite crystal in the infinity space of an optical microscope, CLocK provides immediate polarization and orientation information while still maintaining the ability to localize a single nanoparticle with < 10 nm resolution. Additionally, we demonstrate that the CLocK point spread function encodes kinetic information that we quantified to be an order of magnitude shorter than the integration time of the camera. In this work, CLocK provides new mechanistic insight into dynamic processes such as the dissolution of single gold nanorods as well as single-molecule surface-enhanced Raman scattering. In the second work, dark-field scattering was employed to monitor a proposed post-synthesis silver nanoparticle surface cleaning strategy to improve homogeneity across a population. Here, a sacrificial silver-sulfide sulfide shell is chemically grown on single silver nanoparticles to outcompete surface impurities. We demonstrate that upon electrochemical removal of the shell, a more reactive and reproducible silver surface can be achieved as revealed by enhanced electrodissoluion of the freshly cleaned silver nanoparticles. In these experiments, we additionally found a sulfide-dependent formation of multiple sulfide-species as well as mixed character sulfide shells on single nanoparticles themselves, thus demonstrating the sensitivity provided by optical microscopy at identifying multiple surface chemistries. Overall, the work in this dissertation highlights the ability of optical tools at revealing heterogeneity in single particle studies providing insight into structure-function relationships. / Physiology

Microbial community functional diversity enhances the degradation of organic matter and pollutants in the environment, but there is a growing concern that these ecosystem services may be altered by the introduction of emerging environmental contaminants including silver nanoparticles (AgNPs) into aquatic systems. We added 0, 25, 50, 75, 100, and 125 mg L−1 (nominal concentrations) of citrate-AgNP and polyvinylpyrrolidone-AgNP (PVP-AgNP) each to freshwater sediment and examined their antimicrobial effects on microbial communities using community-level physiological profiling. The results showed that citrate-AgNP decreased the overall microbial catabolic activity by 80% from 1.16 ± 0.02 to 0.23 ± 08 while PVP-AgNP decreased the catabolic activity by 51% from 1.25 ± 0.07 to 0.61 ± 0.19 at 125 mg L−1. Citrate-AgNP and PVP-AgNP caused a statistically significant reduction in substrate richness and substrate diversity that decreased microbial functional diversity. AgNPs decreased microbial catabolic capability and functional diversity at concentrations ranging from 0.12 ± 0.04 to 0.43 ± 0.07 mg Ag kg-1 which are lower than the predicted concentrations in freshwater sediment. To our knowledge, this is the first study to demonstrate inhibition of microbial functional diversity by citrate-AgNP and PVP-AgNP in a pathogen impaired stream. Citrate-AgNP caused greater inhibition of carbon substrate utilization but amino acids, carbohydrates, and carboxylic acids were the most affected carbon groups which led to a shift in the metabolic fingerprint pattern of the microbial community. AgNPs decreased the catabolic capability and the ability of the microbial community to degrade organic matter and a variety of pollutants in the environment.

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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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