PEC bioanalysis represents a unique and dynamically developing methodology that offers an elegant route for sensitive biomolecular detection. Building on the principle of EC analysis, PEC biosensors harness the unique properties of optically active species to enhance analytical performance. Owing to the current based outputs evolved in both PEC and EC bioanalysis, they can be miniaturized and potentially integrated with handheld and portable analyzers, making them uniquely positioned as tools to build effective POC diagnostics. The commercialization of PEC technology for building POC diagnostics, however, heavily depends on enhancing the stability of the photoelectrodes upon repeated use, lowering the limit-of-detection (LOD) of the PEC biosensor used, enhancing the efficiency of signal transduction and the ability of the device to detect minute amounts of biomolecular target in complex biological matrices. In order to address these constraints, we first developed a new solution-based strategy integrating inorganic semiconductive species (titanium dioxide) in an organic framework to construct photoelectrodes with enhanced signal baselines and adequate stability for the cyclic measurements required in biosensing. These transducers were subsequently used to investigate the interaction mechanisms (wavelength dependency, coverage density dependency and spatial dependency) between plasmonic NPs (Au) and the photoelectrodes —chosen as model materials—with the goal of enabling predictive dual-signal modulation and enhanced limit-of-detection in PEC biosensors. The understanding gained was used to design a dual-signal PEC transduction strategy—operated at a single excitation wavelength and on a single electrode—to detect nucleic acid sequences in urine without direct target labeling, target amplification or target enrichment. Here, Au NP terminated biobarcodes served as dynamic signal amplifiers that enabled a low limit-of-detection (5 fM), a wide linear range (1 fM – 100 pM), and the ability to discern between single and double base-mismatched nucleic acid sequences. In parallel, we also detail the development of an EC biosensor featuring dynamic DNA motifs, capable of reagentless, sensitive and specific detection of N-PEDv (nucleocapsid protein of porcine epidemic diarrhea virus)—a protein target with emerging global significance—in both buffer (LOD ~ 1.08 μg mL-1) and urine (LOD ~ 1.09 μg mL-1) Ultimately, this work presents innovations in material architecture and programmable dual-signal transduction that enhance the performance metrics of biosensors; thus, presenting the potential to design POC molecular diagnostics of the future. / Thesis / Doctor of Engineering (DEng) / To address critical limitations in the field of point-of-care molecular diagnostics, it is vital to develop new tools integrating bio-recognition systems with programmable photoelectrochemical and electrochemical signal transduction that enables the design of more effective biosensors. In photoelectrochemical (PEC) systems, plasmonic materials such as gold nanoparticles are often featured to either amplify or attenuate signal response. While there is a significant amount of literature regarding the interaction of gold nanoparticles (Au NPs) with semiconductive systems, the exact nature of the interaction between the two particles has not yet been fully mapped out. In this thesis, we examine various degrees of freedom—including surface coverage density and separation distance—that influence the design of effective photoelectrochemical systems. The understanding gained is further harnessed to design dual-signal photoelectrochemical systems featuring titanium dioxide (TiO2) photoelectrodes and Au linked dynamic deoxyribonucleic acid (DNA) motifs to enable predictive modulation in response to target identification. An electrochemical (EC) analogue featuring structure switching DNA motifs and redox tagged barcodes was also developed. The resultant PEC and EC biosensing assays were critically examined, and their analytical performance was subsequently evaluated in terms of limit-of-detection, sensitivity, and specificity. Ultimately, new classes of bioassays featuring integrated biorecognition and dual-signalling capability for oligonucleotide (i.e., nucleotide sequences and aptamers) based biomolecular detection in urine and saliva were realized.

No description available.

Artificial photosynthesis is broadly defined as the process of solar energy conversion into chemical fuels and represents a promising route towards alleviating the global energy crisis. In this context, the development of photocathodes for the use in photoelectrochemical cells is an attractive approach for the storage of solar energy in the form of a chemical energy carrier (e.g. H$_{2}$ and CO$_{2}$-reduction products from H$_{2}$O and CO$_{2}$). However, molecular catalyst-based photocathodes remain scarcely reported and typically suffer from low efficiencies and/or stabilities due to inadequate strategies for interfacing the molecular component with the light-harvesting material, with benchmark systems continuing to rely on precious metal components. In this thesis, the straightforward preparation of a p-silicon|mesoporous titania|molecular catalyst photocathode assembly that is active towards proton reduction in aqueous media is first established. The mesoporous TiO$_{2}$ scaffold acts as an electron shuttle between the silicon and the catalyst, while also stabilising the silicon from passivation and enabling a high loading of molecular catalysts. When a Ni bis(diphosphine)-based catalyst is anchored on the surface of the electrode, a catalytic onset potential of +0.4 V vs. RHE and a high turnover number of 1 $\times$ 10$^{3}$ was obtained from photoelectrolysis under UV-filtered simulated solar irradiation at 1 Sun after 24 hours. Notwithstanding its aptitude for molecular catalyst immobilisation, the Si|TiO$_{2}$ photoelectrode showed great versatility towards different types of catalysts and pH conditions, highlighting the flexible platform it represents for many potential reductive catalysis transformations. The Si|TiO$_{2}$ scaffold was extended towards solar CO$_{2}$ reduction via the immobilisation of a novel phosphonated cobalt bis(terpyridine) catalyst to achieve the first precious metal-free, CO$_{2}$-reducing molecular hybrid photocathode. Reducing CO$_{2}$ in both organic-water and purely aqueous conditions, the activity of this photocathode was shown to be affected by its environment and reached record turnover numbers for CO production by a molecular photocathode under optimal conditions, maintaining stable activity for more than 24 hours. Critically, in-depth electrochemical and in situ resonance Raman and infrared spectroelectrochemical investigations provided key insights into the nature of the surface-bound Co complex under reducing conditions. While demonstrating the power and precision offered by such in situ spectroelectrochemical techniques, these studies ultimately alluded to a catalytic mechanism that contrasts with that reported for the in-solution (homogeneous) catalyst. Overall, this affords a distinct mechanistic pathway that unlocks an earlier catalytic onset and enables photoelectrochemical activity. Finally, in the context of improving product selectivity in molecular-based CO$_{2}$ reduction, polymers based on the cobalt bis(terpyridine) motif were synthesised and immobilised on inverse opal-type electrodes designed specifically to accommodate large molecules. Rational design of the polymers' co-monomers was aimed towards the provision of an artificial environment for the active complex that would influence product selectivity, which was ultimately demonstrated by the improvement of a H$_{2}$:CO product ratio of 1:2 (molecule) to 1:6 (polymer). Further studies of this all-in-one system included modulating its degree of cross-linkage as well as a CO$_{2}$ reducing demonstration photocathode on a Si|inverse-opal TiO$_{2}$ scaffold.

Thesis(PhD)--University of South Australia, 1998

No description available.

Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 208-216). Available also in a digital version from Dissertation Abstracts.

Photoelectrochemical studies were performed on bismuth vanadate (BiVO₄) to understand chemical and physical properties of the photocatalysts, and to improve the photoactivity for water oxidation. Scanning electrochemical microscopy (SECM) was used to screen various dopants for BiVO₄, to calculate the photoconversion efficiencies to chemical energy at BiVO₄ electrodes, and to study the water oxidation intermediate radicals at the surface of BiVO₄. Tungsten and molybdenum doped BiVO₄ (W/Mo-BiVO₄) shows a photocurrent for water oxidation that is more than 10 times higher than undoped BiVO₄. Photoelectrochemical measurements and material analysis were done to discuss the factors that affect performance of BiVO₄. Finite elements analysis was also performed to explain the electron-hole transport and electrochemical reactions at W/Mo-BiVO₄ electrodes in solutions. Addition of conductive or electron accepting materials, e.g. reduced graphene oxide, into BiVO₄ was tried to study the electron-hole transport phenomena in the metal oxide electrodes. Surface adsorbed radicals produced during the water oxidation at W/Mo-BiVO₄ were interrogated by using SECM that the surface coverage and decay kinetics of adsorbed hydroxyl radicals at W/Mo-BiVO₄ were measured. The quantum efficiencies of the injected photon conversion to chemical energy were obtained from the photoelectrochemical measurements by using SECM. SECM techniques and finite elements analysis were also used to measure the faradaic efficiency of water oxidation at W/Mo-BiVO₄ under irradiation. Finally, unbiased water splitting to generate hydrogen and oxygen from water splitting only using photon energy at W/Mo-BiVO₄ electrodes was demonstrated in a dual n-type semiconductor or Z-scheme device. / text

Photoelectrochemical cells (PEC) and microbial fuel cells (MFC) are two promising environmental technologies with the purposes of energy production and pollutant degradation. In this study, p-type Cu2O thin film electrodes were synthesized by electrodeposition on the ITO glass. The influences of various electrodeposition conditions, including the deposition potential, temperature, electrolyte pH, substrates and deposition duration on the morphology and the photoelectrochemical properties of the Cu2O films were investigated. The so-called p-type micro-crystal Cu2O thin film photocathodes were synthesized at -0.4 V, 70 °C and pH 10. An innovative composite Cu2O/TiO2 photoelectrode was developed by dip-coating TiO2 on the surface of the Cu2O film. The outer TiO2 layer would help reduce the electron-hole recombination and hence improve the catalyst stability. The photocatalyst was shown to be capable of photocatalytic degradation of model pollutants. Under simulated solar irradiation, methylene blue, acridine orange, and bromocresso brilliant blue G were effectively degraded in the Cu2O-based PEC. The composite Cu2O/TiO2 photoelectrode could further enhance the photodegradation of the dyes. For the study on MFC with the saline wastewater-inoculated MFCs, an electricity output of 581 mW/m2 could be achieved at a NaCl concentration of 200 mM. Based on the characterization of the bioande using the electrochemical impedance spectroscopy (EIS) technique, the R(QR)(QR) model, instead of the conventional R(QR) model, was found to fit well with the EIS data of the carbon cloth bioanode. The results support the two-interface-based physical model for the description of the bioanode, including an interface on the flat electrode and the other for the porous biofilm matrix. The new model was employed to monitor the biofilm formation and development on the carbon clothe anode during the MFC start-up. In addition, photocatalytic MFC was developed by using the Cu2O film as the photocathode for the MFC. With the simulated solar light illumination, the PMFC open circuit voltage could be increased by 200 mV comparing to the MFC test. Moreover, the cathode material (Cu2O) is much less expensive than Pt used by common MFCs. / published_or_final_version / Civil Engineering / Master / Master of Philosophy

Capture of the energy in sunlight relies mainly on the use of light-absorbing semiconductors, in solar cells and in water-splitting devices. While solar cell efficiency has increased dramatically since the first practical device was made in 1954, production costs for the most-efficient solar absorbers, III-V semiconductors, remain high. This is largely a result of use of expensive, slow growth methods which rely on hazardous gas-phase precursors. Alternative growth methods are necessary to lower the cost for III-V materials for use in solar cells and improve the practicality of water-splitting devices. The research goal of this dissertation is two-fold: to expand the capabilities of close-spaced vapor transport, an alternative growth method for III-Vs to demonstrate its compatibility with current technologies; and to explore the fundamental chemistry of close-spaced vapor transport as a growth method for these materials. This dissertation surveys plausibly lower-cost growth methods for III-V semiconductors (Chapter II) and presents in-depth studies on the growth chemistry of two ternary III-Vs: GaAs1-xPx (Chapter III) and Ga1-xInxP (Chapter IV). Finally, the growth of GaAs microstructures which could be utilized in a water-splitting device is studied (Chapter V). This dissertation includes previously published and unpublished co-authored material. / 2019-01-09

The recurring theme of this dissertation is the correlation between FeS2 surface chemistry and key electrical and electronic properties of FeS2. Efforts have been made to identify and characterize the FeS2 surface, investigate the photoelectrochemistry of FeS2 photoanodes under anhydrous and anoxic conditions, and investigate the influence of deliberate surface chemistry on FeS2 photoelectrochemistry. Infrared reflection-absorption spectroscopy (IRRAS) was used to investigate a thin adsorbate layer on pyrite. The results showed that the combination of angle-dependent studies and computational efforts are a powerful tool for characterizing the pyrite surface. The photoelectrochemistry of FeS2 photoanodes was investigated in an I¯/I3¯ acetonitrile electrolyte acetonitrile electrolyte. The results revealed that the non-aqueous system was suitable for strictly anhydrous and anoxic photoelectrochemical studies. A model was proposed to explain the observed influence of concentration of dissolved I2 on the photovoltage. A central component of the proposed model was that shunting was assumed to take place at physically distinct regions of the electrode and that mass-transport to and from these regions could be treated separately from mass-transport to the regions responsible for the rectifying behavior of the FeS2/liquid junction. The implication of the agreement between experimental and calculated J-E curves is that macroscopic photoelectrochemical investigations may underestimate the quality of FeS2 photoanodes due to the presence of defects. The influence of surface treatments on FeS2 photoelectrochemistry was further studied using non-coordinating redox species. A statistically significant increase of photovoltage was observed after treating FeS2 surfaces with KCN. X-ray photoelectron spectroscopy was used to study chemical bond formation between the electron donating ligands and iron(II) centers on the pyrite surface. The results were discussed in terms of charge recombination models and surface coordination chemistry. Unfinished work is also presented. Cathodic polarization in acidic media is a prerequisite for any detectable photoresponse. The exact function of the electrochemical activations was further investigated by electropolishing pyrite electrode under different experimental conditions including etchant identity and applied bias. The results suggested that the electrochemical treatment removes the damaged surface layer caused by mechanical polishing, and might also stabilize the surface states. Further experiments can be focus on anhydrous etching of pyrite photoanode. The research presented in this dissertation guides future studies of thin film FeS2 photovoltaics.