Thesis (Ph. D.)--University of Hong Kong, 2006. / Title proper from title frame. Also available in printed format.

In the past two decades much effort has been put in the characterization of the mechanical and surface properties at the nano-scale in order to conceive reliable N/MEMS (Nano and Micro ElectroMechanical Systems) applications. Techniques like nanoindentation, nanoscratching, atomic force microscopy have become widely used to measure the mechanical and surface properties of materials at sub-micro or nano scale. Nevertheless, many phenomena such us pile-up and pop-in as well as surface anomalies and roughness play an important role in the accurate determination of the materials properties. The first goal of this report is to study the infulence of these sources of data distortion on the experimental data. The results are discussed in the first experimental chapter. On the other hand, conceptors would like to adapt/tune the mechanical and surface properties as a function of the required application so as to adapt them to the industrial need. Coatings are usually applied to materials to enhance performances and reliability such as better hardness and elastic modulus, chemical resistance and wear resistance. In this work, the magnetron sputtering technique is used to deposit biocompatible thin layers of different compositions (titanium carbide, titanium nitride and amorphous carbon) over a titanium substrate. The goal of this second experimental part is the study of the deposition parameters influence on the resulting mechanical and surface properties. New materials such as nanocrystal superlattices have recently received considerable attention due to their versatile electronic and optical properties. However, this new class of material requires robust mechanical properties to be useful for technological applications. In the third and last experimental chapter, nanoindentation and atomic force microscopy are used to characterize the mechanical behavior of well ordered lead sulfide (PbS) nanocrystal superlattices. The goal of this last chapter is the understanding of the deformation process in order to conceive more reliable nanocrystal superlattices.

<p>In this work a modified polyol method was developed to synthesize in-house catalysts. The method was modified for maximum delivery of product and proved to be quick and efficient as well as cost effective. The series of IH catalysts were characterized using techniques such as UV-vis and FT-IR spectroscopy, TEM, XRD, ICP and CV.</p>

This thesis proposes to use optical rectennae (rectifying antennae) for generating electricity by harvesting solar energy. Rectennae have theoretical efficiency over 90%, well above that of current photovoltaic devices. They could be built by arraying nano- antennae combined with a self-assembled monolayer for DC current rectification. Gold nanoarrays were built to absorb light by plasmonic resonance by depositing a gold layer on CdSe tetrapods or directly growing nanowires via template synthesis; various alkanethiolates were explored as rectifying units. The tunneling properties of alkanethiolates on gold nanoarrays were examined by electrochemical analysis. Preliminary photocurrent tests show that electric currents can be induced at different optical frequencies depending on the aspect ratios of the nanoarrays. However, gold contributes as an enhancement rather than an active material. Furthermore, by fitting the impedance spectroscopy data with equivalent electric circuits, the calculated tunneling barrier of the self-assembled monolayer on gold nanoarrays is ten times lower than on gold film, suggesting that the monolayer formed on gold nanoarrays is defective and cannot serve as a practical rectifying barrier. This concept will need further investigation to lead to an applicable photovoltaic cell.

Functionalizing surfaces with amine groups through the hydrolytic condensation of aminotrialkoxysilanes is a typical approach when modifying silica particles for use in bioimaging, enzyme immobilization, and other applications. This processing step can be eliminated if amine-functionalized silica particles are directly prepared without using aminotrialkoxysilanes. Here, a one-pot, ambient-condition, water-phase method to synthesize silica-based nanoparticles (NPs) that present surface amine groups is described. The formation mechanism involves the electrostatic crosslinking of cationic polyallylamine hydrochloride by citrate anions and the infusion of the formed polymer/salt aggregates by silicic acid. The particles were unimodal with average diameters in the range of 40 to 100 nm, as determined by the size of the templating polymer/salt aggregates. Colorimetric analysis using Coomassie brilliant blue and zeta potential measurements confirmed the presence of surface amine groups of the hybrid silica/polymer NPs. Surface charge calculations indicated the hybrid NPs had a lower amine surface density than aminopropyltriethoxysilane-functionalized silica (0.057 #/nm 2 vs. 0.169 #/nm 2 at pH 7).

Using scanning tunneling microscopy (STM) we have explored complex surface adsorbed molecules, nanocars, on Au(111) and the parameters related to the direct translation of these molecules by the STM tip. Specifically, the molecules focused on here were functionalized with C 60 or trans ruthenium complexes. With low tunneling currents the molecules could be imaged at room temperature. Increasing the tunneling current allowed us to bring the tip closer to individual molecules and reposition them on the surface. Below specific current and bias voltage conditions the molecules remained stationary, while in other cases the tip interaction was strong enough to drastically damage or eject the molecule from the field of view. High temperature scans revealed the effect of the wheel activation energy relative to the underlying surface as the different wheeled nanocars began diffusing at different temperatures confirming the manipulation measurements.

This thesis introduces a comprehensive numerical model for the determination of the electrical properties of carbon nanotube reinforced polymer composites. Procedures of this model are based on a new spanning network identification algorithm and the resistor network method. First, realistic nanotube geometry is generated from input parameters defined by the user. The spanning network algorithm then determines the connectivity between nanotubes in the representative volume element. Next, interconnected nanotube networks are converted to equivalent resistor circuits. Finally, Kirchhoff's Current Law is used in conjunction with finite element analysis to solve for the voltages and currents in the system and calculate the effective electrical conductivity of the nanocomposite. The Monte Carlo method is used to eliminate statistical variation by simulating five hundred random geometries. The model accounts for electrical transport mechanisms such as electron hopping and simultaneously calculates percolation probability, identifies the backbone, and determines effective conductivity. The accuracy of the model is validated by comparison to both models and experiments reported in the literature.

A numerical model is presented for the determination of the electrical properties of carbon nanotube-based composites. The model incorporates several experimentally-based statistical distributions to account for the stochastic nature of the problem. These distributions include parameters such as nanotube length and diameter in addition to contact resistance. Using a Monte Carlo-based simulation technique, a random nanotube geometry is generated, checked for a percolation spanning network and then converted into a pseudo-3D resistor network for which the effective electrical conductivity is found. Each data point is the ensemble average of 500 or more simulations, each with a unique set of realized parameter values thereby reducing statistical variations of the solution. Studies are conducted to investigate the importance of incorporating the stochastic parameters and to characterize the impact of nanotube waviness and alignment on the effective composite properties. Electron tunneling distance is also included as a variable model parameter.

This thesis describes the development and optical characterization of near infrared (NIR) gold nanoshells for the use as luminescent contrast agents for applications in small animal blood vessel imaging. Two types of gold-silica nanoshells excitable by NIR lasers are investigated: Type 1 nanoshells can be excited with a sub-mum NW laser, whereas Type 2 nanoshells can be excited with a NW laser in the micrometer range. Using NIR microscopy as an imaging platform, ex vivo and in vivo experiments are conducted to determine the efficacy of these nanoshells as suitable contrast agents. Specifically, individual particles of Type 1 nanoshells are successfully imaged and shown to provide bright optical contrast for blood vessel imaging both ex vivo and in vivo, while the Type 2 nanoshells are clearly imaged within the blood vessels ex vivo. These positive results suggest a promising possibility of developing a new class of contrast agents for deep tissue imaging and improving the imaging depth of NIR imaging techniques.

<p>Silver nanoparticles (AgNPs) are increasingly being integrated into a wide range of consumer products, such as air filters, washing machines, and textiles, due to their antimicrobial properties [1]. However, despite the beneficial applications of AgNPs into consumer products, it is likely that their use will facilitate the release of AgNPs into wastewater treatment plants, thereby possibly negatively impacting key microorganisms involved in nutrient removal. For this reason, it is important to characterize the effects of AgNPs in natural and engineered systems and to measure the antimicrobial effect of AgNPs on wastewater microorganisms. Polyvinyl alcohol coated AgNPs have already been linked to decreased nitrifying activity [2] and it is important to determine if AgNPs coated with other materials follow similar trends. Furthermore, it is likely that, with repeated exposure to AgNPs microbial communities could evolve and develop resistance to silver. Thus, a long-term effect of silver nanoparticle exposure could be a reduction of the efficacy of such products in a similar fashion to the development of microbial antibiotic resistance [3]. Therefore, it is critical that the impacts of these materials be ascertained in wastewater treatment systems to prevent long-term negative effects. </p><p>The objectives of this dissertation were to: 1) characterize the effect of several different AgNPs on the ammonia oxidizing bacterium (AOB) Nitrosomonas europaea and investigate possible mechanisms for toxicity, 2) test the effects of consumer product AgNPs on a wide range of heterotrophic bacteria, 3) evaluate the effects of AgNPs on bench scale wastewater sequencing batch reactors, and lastly 4) assess the impacts on microbial communities that are applied with AgNP spiked wastewater biosolids. </p><p>First, Nitrosomonas europaea was was selected because wastewater nitrifying microorganisms carry out the first step in nitrification and are known to be sensitive to a wide range of toxicants [4].The antimicrobial effects of AgNPs on the AOB N. europaea were measured by comparing nitrite production rates in a dose response assay and analyzing cell viability using the LIVE/DEAD® fluorescent staining assay. AgNP toxicity to N. europaea appeared to be largely nanoparticle coating dependent. While PVP coated AgNPs have shown reductions up to 15% in nitrite production at 20 ppm, other AgNPs such as gum arabic (GA) coated showed the same level of inhibition at concentrations of 2 ppm. The first mechanism of inhibition appears to be a post-transcriptional interference of AMO/HAO by either dissolved Ag or ROS, in treatments where membranes are not completely disrupted but nitrite production decreased (2 ppm GA AgNP and 2 ppm PVP AgNP treatments). The disruption of nitrification is dependent on AgNP characteristics, such as zeta potential and coating, which will dictate how fast the AgNP will release Ag+ and ROS production Finally, total membrane loss and release of internal cellular matter occur. </p><p>In order to test the effects of AgNP products available to consumers, simple bacterial toxicity tests were carried out on well-studied heterotrophic bacteria. A model gram-positive and gram-negative bacterium (B. subtilis and E. coli, respectively) was selected to assess any differences in sensitivity that may occur with the exposure to AgNPs. A third model gram-negative bacterium (P. aeruginosa) was chosen for its biofilm forming capabilities. In addition to testing pure nanoparticles, three silver supplements meant for ingestion, were randomly chosen to test with these three bacteria. Growth curve assays and LIVE/DEAD staining indicate that the consumer product AgNPs had the most significant inhibition on growth rates, but not membrane integrity. Overall, P. aeruginosa was most negatively affected by all AgNPs with nearly 100% growth inhibition for all 2 ppm AgNP treatments. TEM imaging also confirmed cell wall separation in P. aeruginosa and internal density differences for E. coli. The effects on B. subtilis, a gram-positive bacterium, were not as severe but toxicity was observed for several AgNPs at concentrations greater than 2 ppm. Citrate AgNPs appeared to have the most impact on membrane integrity, while other mechanisms such as internal thiol binding might have been at work for other AgNPs. </p><p>The effects of varying concentrations of pure AgNPs on complex microbial wastewater reactors are currently being tested. Eight bench-scale sequencing batch reactors were set up to follow the typical "fill, react, settle, decant, idle" method with an 8 hour hydraulic retention time and constant aeration. Reactors were fed synthetic wastewater and treatment efficiency is measured by monitoring effluent concentrations of COD, NH4+, and NO3-. The reactors were seeded with 500 mL of activated sludge from a local wastewater treatment plant. After reaching steady state, the reactors were spiked with 0.2 ppm gum arabic and citrate coated AgNPs. Treatment efficiency was monitored and results showed significant spikes and ammonia and COD immediately following the first spike, but the microbial community appeared to adapt for future AgNP spikes. Microbial community analysis (terminal restriction fragment length polymorphism) showed confirmed this hypothesis. </p><p>Overall, this dissertation asserts that by examining AgNP coating type, Ag+ dissolution rates and Stern layer surface charge, it may be possible to predict which AgNPs may be more detrimental wastewater treatment, but not all AgNPs will have the same effect. The results obtained herein must be expanded to other types of AgNPs and microorganisms of ecological importance.</p> / Dissertation