Thesis advisor: Dunwei Wang / Solar water splitting refers to the reaction that converts solar energy into chemical fuel. It is an attractive means to store solar energy. This process, analogous to nature photosynthesis, uses semiconductor to capture and convert solar irradiation and, as such, is called artificial photosynthesis. Despite its promising prospect, the lack of materials that can satisfy all requirements to achieve efficient solar water splitting becomes an important challenge. In this thesis, we aim to develop a strategy of forming heteronanostructure to tackle the challenge faced by metal oxide-based photoanode for water oxidation. The challenge associated with metal oxide-based photoanodes and current approach to alleviate the challenge is first discussed. We propose a strategy of combining multiple components to form heteronanostructure to meet the challenges, in particular the charge transport issue. By introducing a dedicated charge transporter, we fabricate various heteronanostructure including TiO₂/TiSi₂, Fe₂O₃/TiSi₂ and Fe₂O₃/AZO nanotubes to improve the charge collection and therefore overall efficiency. Additionally, the growth of several important metal oxides by atomic layer deposition is developed and its utilization as photoanode for water splitting is studied for the first time. Because this strategy is based on the rational design and synthesis of materials, it has the potential to produce electrodes with a combination of properties that have not been exhibited simultaneously by single-component materials. In addition, the strategy is highly versatile and can incorporate the latest developments produced by parallel efforts. We are confident that the rational design and synthesis of materials such as the strategy proposed here will play an increasingly more important role in energy research. / Thesis (PhD) — Boston College, 2012. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

Thesis advisor: Dunwei Wang / Semiconductor devices offer promise for efficient conversion of sunlight into other useful forms of energy, in either photovoltaic or photoelectrochemical cell configurations to produce electrical power or chemical energy, respectively. This dissertation examines ionic and electronic phenomena in some candidate semiconductors and seeks to understand their implications toward solar energy conversion applications. First, copper sulfide (Cu₂S) was examined as a candidate photovoltaic material. It was discovered that its unique property of cation diffusion allows the room-temperature synthesis of vertically-aligned nanowire arrays, a morphology which facilitates study of the diffusion processes. This diffusivity was found to induce hysteresis in the electronic behavior, leading to the phenomena of resistive switching and negative differential resistance. The Cu₂S were then demonstrated as morphological templates for solid-state conversion into different types of heterostructures, including segmented and rod-in-tube morphologies. Near-complete conversion to ZnS, enabled by the out-diffusion of Cu back into the substrate, was also achieved. While the ion diffusion property likely hinders the reliability of Cu₂S in photovoltaic applications, it was shown to enable useful electronic and ionic behaviors. Secondly, iron oxide (Fe₂O₃, hematite) was examined as a photoanode for photoelectrochemical water splitting. Its energetic limitations toward the water electrolysis reactions were addressed using two approaches aimed at achieving greater photovoltages and thereby improved water splitting efficiencies. In the first, a built-in n-p junction produced an internal field to drive charge separation and generate photovoltage. In the second, Fe₂O₃ was deposited onto a smaller band gap material, silicon, to form a device capable of producing enhanced total photovoltage by a dual-absorber Z-scheme mechanism. Both approaches resulted in a cathodic shift of the photocurrent onset potential, signifying enhanced power output and progress toward the unassisted photoelectrolysis of water. / Thesis (PhD) — Boston College, 2013. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

>Magister Scientiae - MSc / During the course of this study iron oxide nanoparticles, which have been researched for drug-targeted delivery, were synthesised via the co-precipitation method and characterised using various methods. This study focused on the role of relevant capping agents for the inhibition of agglomeration of the particles; chitosan, polyvinyl alcohol (PVA) and poly lactic glycolic acid (PLGA) were the capping agents of interest. The study is an assessment of the effects brought about the different capping agents used for this work. The prepared particles were then capped with the different capping agents followed by the loading of the drug curcumin. Various analytical methods were used to analyse the particles such as High resolution transmission electron microscopy (HR-TEM), Superconducting quantum interference device (SQUID), Fourier Transform Infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Thermogravimetric analysis (TGA) and zeta potential. PVA, chitosan and PLGA capped SPIONS were successfully prepared and verified by FT-IR spectrometry, various sizes were prepared almost ranging the same for the successfully prepared particles verified by XRD. The resultant particles were found to be spherical with an average particles size between 13- 22 nm. From the study it was concluded that the addition of the different capping agents resulted in the reduction of the intensity of the peaks in XRD, it was also found out the presence of the capping agents did not alter the crystalline phase of the particles. From the study it was also observed that higher saturation magnetization was experienced where PVA was used as the capping agents.

Hematite (α-Fe2O3) is a common, naturally occurring iron oxide, found throughout the earth's crust and atmosphere. Hematite is of interest to the scientific community because it is able to incite a reaction that produces hydrogen gas (H2), which is a form of clean energy (Bora et al., 2013). The composition of hematite in nature is also used to make inferences about conditions on early earth's surface (Guo et al., 2013). Hematite is useful for clean energy production and as an environmental indicator partly because of its apparent stability. However, some evidence suggests that hematite might not be as stable as previously thought. Many iron oxides undergo Fe atom exchange when they come into contact with aqueous Fe(II), as often occurs in nature (Pedersen et al., 2005, Jones et al., 2009, Gorski et al., 2012, Handler et al., 2009). This atom exchange can result in elements and nutrients being taken up or released from the iron oxides as they recrystallize (Frierdich & Catalano, 2012, Cwiertny et al., 2008, Boland et al., 2014). Although atom exchange has not been directly shown in hematite, it has been demonstrated that trace metals are released from hematite in the presence of aqueous Fe(II), implying that exchange may be occurring (Frierdich et al., 2011). Here, we directly demonstrate Fe atom exchange between hematite and aqueous Fe(II). This work provides knowledge concerning the surface chemistry of hematite that has important implications for clean energy production and the environment.

This dissertation presents the synthesis and application of nanodiamond based materials for electrochemical biosensors. In this research work, nanodiamond particles have been used to prepare doped and undoped nanocrystalline diamond films, and conducting polymer composites for enhanced biosensing. The performance of the synthetized materials towards sensing applications was evaluated against glucose amperometric biosensing. Besides, cholesterol biosensing was attempted to prove the capabilities of the platform as a generic biosensing substrate. Biosensors have been proved to provide reliable detection and quantification of biological compounds. The detection of biological markers plays a key factor in the diagnosis of many diseases and, even more importantly, represents a major aspect in the survival rate for many patients. Among all of the biosensors types, electrochemical biosensors have demonstrated the best reliability to cost ratio. Amperometric biosensors, for example, have been used for decades as point of care sensing method to monitor different conditions such as glucose. Despite the amount the research presented, the sensitivity, selectivity, stability, low cost and robustness are always driving forces to develop new platforms for biosensor devices. In the first phase of this dissertation, we synthesized undoped and nitrogen doped nanocrystalline diamond films. The synthetic material was thoroughly studied using different material characterization techniques and taken through a chemical functionalization process. The functionalization process produced a hydrogen rich surface suitable for enzymatic attachment. Glucose oxidase was covalently attached to the functionalized surface to form the biosensing structure. The response of the biosensor was finally recorded following voltammetry and amperometric techniques under steady state and dynamic conditions. The experimental results demonstrated that conductivity induced by the doping process enhanced the sensitivity of the sensing structure with respect to the undoped substrate. Also, the functionalization procedure showed strong bonding to avoid enzyme leaching during the measurements. Later, in the second phase of this dissertation, the nanodiamond particles were used as filler for conducting polymer composites. The objective for developing these composite materials was to overcome the high resistivity observed for nanocrystalline films. The experimental results demonstrated that the inclusion of nanodiamond particles increased the sensitivity of the overall structure towards the quantification of glucose with respect to the nanocrystalline films and the bare polymer. Besides, the experiment showed a noticeable enhancement in the signal-to-noise ratio and the mechanical stability of the sensing platform due to the nanodiamond addition. The best structures from the previous experiments were further grafted with iron oxide nanoparticles to attempt signal amplification. Initial experiments with nanodiamond based composited showed similar current for low glucose concentrations for two different active electrochemical sensing areas. This observation indicates that more area is still available to transport signal and to enhance even further the sensing action. Oxidation of iron oxide nanoparticles after initial enzymatic decomposition of glucose has been proved to provide higher current for the same glucose concentration; thus, creating amplification effect for the signal. Finally, the toxicity of the nanomaterial synthesized during this dissertation was evaluated in mammalian cells. The advances in biosensing techniques indicate the potential application of amperometric platform for continuous implantable devices; hence, the toxicity of the materials becomes a key aspect of the platform design.

An exogenous contrast agent has been developed to enhance the contrast achievable in Thermoacoustic Tomography (TAT). TAT utilizes the penetration depth of microwave energy while producing high resolution images through acoustic waves. A sample irradiated by a microwave source expands due to thermoelastic expansion. The acoustic wave created by this expansion is recorded by an ultrasonic transducer. The water content in biological samples poses an obstacle, as it is the primary absorber of microwave radiation. The addition of an exogenous contrast agent improves image quality by more effectively converting microwave energy to heat. The use of iron oxide nanoparticles in MRI applications has been explored but super paramagnetic iron oxide nanoparticles (SPION) have benefits in microwave applications, as well. Through ferromagnetic resonance, SPION samples more effectively convert microwave energy into heat. This transduction to heat creates significantly larger thermoacoustic waves than water, alone. Characterization of the SPION samples is executed through TAT, TEM, XPS, EDS, and a vector network analyzer with a dielectric probe kit. Onedimensional and phantom model imaging with an iron oxide nanoparticle contrast agent provide a two-fold improvement in contrast at current system configurations. Further enhancement is possible through adjustments to the nanoparticles and TAT system.

In many regions of the world, groundwater is contaminated with a high level of arsenic that must be treated before it can be safely used as drinking water. In situ immobilization of arsenic from groundwater within subsurface environment could have major advantages over the conventional above-ground chemical coagulation-precipitation treatment process. In this study, we develop a novel technique that can in situ emplace iron oxides onto the sand grain surface of porous media under mild chemical and temperature conditions. The technique involves sequential injections of a preconditioned ferrous iron solution and an oxidant solution and then orchestrate the advective-diffusive transport of the two reagents in porous media to create an overlapped reaction zone where ferrous iron is oxidized and precipitated on the sand grain surfaces. We demonstrate through bench-scale column tests the feasibility of using this technique to create a large-scale iron oxide-enriched reactive barrier in subsurface environment for in situ removal of arsenic. A sand filter with a fresh iron oxide coating can treat thousands of pore volumes of water contaminated with dozens of ppb arsenic before the coating needs to be regenerated. Arsenic breakthrough curves through the sand filter suggest that both reversible adsorption and irreversible precipitation are responsible for removing arsenic from the water. Unlike conventional excavate-and-fill permeable reactive barriers, the treatment capacity of our in situ created barrier can be in situ regenerated and replenished with a fresh coating.

Studying dynamics of magnetization relaxation in excited magnetic materials is important both for understanding the rates and pathways of magnetization relaxation and for the potential use in spin-based electronics and data storage devices in the future. Previous studies have demonstrated that the size of nanocrystals is an important factor for energy relaxation in quantum dots and metal nanoparticles. Since magnetization relaxation is one of energy relaxation pathways, the size of nanocrystals may be also an important factor for magnetization relaxation in nanoscale magnetic materials. The goal of this study is to have a better understanding of magnetization relaxation in nanoscale magnetic materials. In particular, we focused on the correlation between the nanocrystal size and the rates of spin-lattice relaxation (SLR), a magnetization relaxation pathway, in magnetic nanocrystals. The size-dependent magnetization relaxation rate after optically induced demagnetization in colloidal Fe3O4 nanocrystals was measured by using time-resolved Faraday rotation (FR). Fe3O4 nanocrystals were chosen as the model system to study the correlation between the size of nanocrystals and the rates of SLR due to the wellestablished synthetic procedure of making nanocrystals with various sizes and narrow size dispersion. Faster SLR rates were observed in smaller Fe3O4 nanocrystals. The results suggested the surface of nanocrystals have higher efficiency of SLR than the interior region by using a simple model to analyze the SLR rates of Fe3O4 nanocrystals with various sizes. Higher efficiency of SLR at the surface may be due to the stronger spin-orbit coupling at the surface relative to the interior region. In addition to magnetization dynamics studies, the effect of oxidation on static FR in iron oxide nanocrystals (between Fe3O4 and y-Fe2O3) was studied. The results indicated FR signal is linearly correlated to the strength of optical transition between Fe2 and Fe3 in Fe3O4 for a given size of nanocrystals.

In an effort to develop broadly applicable photoswitchable catalysts, we have reported a method for modulating N-heterocyclic carbene (NHC) donicity using light by incorporating a photochromic diarylethene (DAE) into the backbone of a NHC scaffold. UV irradiation of 4,5-dithienylimidazolone or an analogous NHC-Ir(CO)₂Cl complex effected a photocyclization between the two thiophene rings, which led to a change in the electron donating ability of the NHC scaffold. Subsequent exposure to visible light reversed the photocyclization reaction. The concept of photo-modulating NHC donicity in this manner enabled photoswitchable NHC organocatalysis. The catalytic activity of a DAE-annulated imidazolium pre-catalyst in transesterification and amidation reactions was successfully switched between the active and nearly inactive states ([kappa]vis/[kappa]UV = 100) upon alternate UV ([lambda]irr = 313 nm) or visible ([lambda]irr > 500 nm) irradiation. The photoswitchable NHC organocatalysis was later extended to facilitating ring-opening polymerizations of cyclic esters, the rates of which were controlled via external light stimuli. Additionally, a photochromic dithienylethene-annulated N-heterocyclic carbene (NHC)-Rh(I) complex was synthesized and enabled photoswitching of the catalytic activity in series of hydroboration reactions. All of the examples demonstrate extremely rare instances of photomodulating a catalyst's activity by tuning its electronic properties. Furthermore, by taking advantage of the versatility of NHCs in both organo- and organometallic catalysis, we have developed novel photoswitchable catalysts for a variety of applicable transformations. Nanoparticles that can be transported in subsurface reservoirs at high salinities and temperatures are expected to have a major impact on enhanced oil recovery and electromagnetic imaging. We have developed an approach that will facilitate nanopaticle transport through porous media at high salinity by adsorbing or grafting rationally designed co-polymers on platform nanoparticles. Notably, co-polymers of acrylic acid with either 2-acrylamido-2-methylpropanesulfonate or styrenesulfonate have been electrostatically adsorbed or covalently grafted onto iron oxide nanoclusters. The presence of sulfonate groups on the iron oxide surface enabled long-term colloidal stability of the particles in extremely concentrated brine (8% wt. NaCl + 2% wt. CaCl₂) at elevated temperatures (90 °C) and minimized their adsorption on model mineral surfaces. / text

Magnetic and plasmonic properties combined in a single nanostructure provide a synergy that is advantageous in a number of biomedical applications, such as contrast enhancement in multimodal imaging, simultaneous capture and detection of circulating tumor cells, and photothermal therapy of cancer. These applications have stimulated significant interest in development of magneto-plasmonic nanoparticles with optical absorbance in the near-infrared region and a strong magnetic moment. In this dissertation, we addressed this need to create a novel immunotargeted magneto-plasmonic nanoparticle platform. The nanostructures were synthetized by self-assembly of primary 6 nm iron oxide core-gold shell particles, resulting in densely packed spherical nanoclusters. The close proximity of the primary particles in the nanoclusters generates a greatly improved response to an external magnetic field and strong near-infrared plasmon resonances. A procedure for antibody conjugation and PEGylation to the hybrid nanoparticles was developed for biomedical applications which require molecular and biocompatible targeting. Furthermore, we presented two biomedical applications based on the immunotargeted hybrid nanoparticle platform, including circulating tumor cell (CTC) detection and cell-based immunotherapy of cancer. In the CTC detection assays, rare cancer cells were specifically targeted by antibody-conjugated nanoparticles and efficiently separated from normal blood cells by a magnetic force in a microfluidic chamber. The experiments in whole blood showed capture efficiency greater than 90% for a variety of cancers. We also explored photoacoustic imaging to detect nanoparticle-labeled CTCs in whole blood. The results showed excellent sensitivity to delineate the distribution of hybrid nanoparticles on the cancer cells. Thus, these works paves the way for a novel CTC detection approach which utilizes immunotargeted magneto-plasmonic nanoclusters for a simultaneous magnetic capture and photoacoustic detection of CTCs. In another application, we introduced a novel approach to label cytotoxic T cells using the magnetic nanoparticles with an expectation to enhance T cell recruitment in tumor under external magnetic stimulus. A series of in vitro experiments demonstrated highly controllable manipulation of labeled T cells. Thus, these results highlight the promise of using our nanoparticle platform as a multifunctional probe to manipulate and track immune cells in vivo and further improve the efficacy of cell-based cancer immunotherapy. / text