Thermoelectric power generation could be crucial in a sustainable energy future through waste heat energy recycling. Typical nanoengineering of thermoelectric materials requires energy intensive and expensive methods of device fabrication. We employ a facile solution processed device fabrication method that can easily be scaled up for manufacturing while maintaining its cost effectiveness. In this work, we establish and qualify a thermopower measurement system for thin film and bulk devices. To qualify the measurement system, we show that trends in the signs and magnitudes of the Seebeck coefficients of known silicon standards are correct. We also show that PbSe nanocrystal thin film devices exhibit a large Seebeck coefficient (~1500-2000 μV/K) prior to alumina infilling and a reduced Seebeck coefficient (~800-1200 μV/K) post alumina infilling. With this measurement tool, we are able to view differences in thermopower before and after atomic layer deposition treatment within an oxygen-free atmosphere.

Bacterial biofilms cause persistent and deadly infections in medical settings, which are resistant to conventional antibiotic doses. Medical biofilms are often multi-species communities of bacteria which are maintained through chemical signaling and metabolite exchange. An improved understanding of the interactions between bacteria governing community behavior will facilitate the discovery of drug targets for biofilm prevention and eradication. In this thesis, I describe coculture interactions between different species of bacteria and a method to track metabolic states of bacteria in different environments.

One example of such coculture interactions is illustrated in the competition for iron in an in vitro infection model. Salmonellaenterica serovar Typhimurium (STm) causes acute infections in the gut, but STm infections are abolished in coculture with a probiotic bacterium, Escherichia coli Nissle (EcN). EcN outcompetes STm in the gut via a class of bactericidal compounds, called microcins, which are conjugated to iron scavenging siderophore molecules. In vitro biofilm models show that STm uses EcN siderophores to acquire iron, and that siderophore conjugation is an anti-cheating strategy employed by EcN to outcompete STm for nutrients.

In another example of novel coculture interactions, E. coli and Pseudomonas aeruginosa have competitive interactions governing biofilm establishment and dispersal. The E. coli biofilm dispersal is triggered by P. aeruginosa quorum sensing (QS) compounds. However, E. coli biofilms grown on periodic microcturctures, resembling the stomach micro-villi, are shown to modulate this pathway by metabolite accumulation in engineered microenvironments. Substrate structures induce changes in E. coli biofilm morphology, which in turn increase the concentration of indole, a constitutively produced metabolite within the biofilm. Moreover, the monoculture biofilms grown on microstructured substrates are significantly more susceptible to antibiotics than monocultures on flat substrates. FLIM is a label free, non-invasive technique and has immense potential for use as a means to probe interactions in microbial communities. Fluorescence lifetime imaging microscopy (FLIM), demonstrates that this increased antibiotic susceptibility is due to changes in cellular metabolism induced by an altered microenvironment. The research in this thesis demonstrates more long term and permanent strategies for biofilm infections and will provide guidelines and inspiration for improved diagnostics, and treatments for biofilm infections.

The increasing demand for fossil fuels and the need to reduce greenhouse gases require clean energy sources and more efficient utilization of energy. Thermoelectric materials provide a means toward achieving these goals since they convert heat, including waste heat, directly into an electric potential difference. Metal-semiconductor heterostructures can work as Schottky barriers in thermoelectric materials to increase thermoelectric efficiency.

In this project, nickel silicide phases were introduced into silicon nanowires (SiNWs) to build up the Schottky barrier. SiNW arrays were fabricated using a metal-assisted chemical process, creating SiNWs about 200 nm in diameter and 30ìm in length. Different methods were adopted for nickel deposition: electroless nickel deposition, electro nickel deposition, E-beam deposition, and thermal evaporation. The samples were examined by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that depositing nickel on SiNWs in an aqueous solution without electricity is a simple way to deposit nickel particles, and the morphology of nickel particles depends on the concentration of the deposition bath. However, an aqueous solution will cause oxidation of the SiNWs and hinder the formation of nickel silicide. To solve this problem, depositing nickel on SiNWs in organic solutions inside an oxygen-free glove box is a way to prevent oxidation, and nickel can diffuse into silicon substrates easily via annealing when there no oxidation layer on the surface of SiNWs. The dominant phase formed in these samples is NiSi2 after being annealed at 650°C for one hour in a tube furnace.

At present, very little is known about the development of tolerance in response to osmotic stress. Although cells undergo similar adaptations to both antibiotic and osmotic stress, there exists a need to elucidate the relationship, observed through growth rate patterns, metabolic perturbations, and membrane alterations, in response to simultaneous exposure to both stresses. A better understanding of these interactions will produce new strategies for treating persistent and recalcitrant bacterial infections. We show here that osmotic stress cross-protects E. coli to bactericidal antibiotics, conferring tolerance to antibiotics.

The colloidal synthesis and surface modification of germanium and germanium-based nanomaterials is a developing area of research with potential applications in a variety of fields. Colloidal germanium nanocrystal syntheses have improved significantly in recent years, but there has been little exploration into the synthesis of Ge-based nanomaterials, especially metal-germanium nanocrystals. A solution-phase synthesis of nickel germanide nanocrystals was developed to produce colloidal particles possessing an unexpected Ni¬19Ge12 crystal phase. The mechanism of this synthesis was explored to show that the Ni19Ge12 phase formed over a wide range of experimental conditions, but the use of a trioctylphosphine cosolvent allowed formation of a Ni1Ge1 phase that was not thermally stable. Many proposed applications of colloidal Ge nanocrystals require thorough particle surface modification. To this end, surface ligand exchange of Ge nanocrystals was examined and improved through the development of a high-temperature sulfur treatment. Finally, Ge nanocrystal films were cast from solution and their electrical conductivity was characterized.

Nanomaterials derived from silicon and germanium have demonstrated several desirable properties for a variety of materials applications, such as optoelectronics, spintronics, catalysis, and thermoelectrics. Currently, the synthetic library for the production of technologically relevant Group IV nanomaterials comprised of silicon–germanium alloys, metal-silicides, and metal germanides are extremely limited, particularly with regard to solution-phase approaches. The work presented in this dissertation demonstrates the expansion of existing protocols to produce a variety of new Group IV-based nanomaterials using solution-phase syntheses. For the first time, silicon–germanium alloy nanocrystals are produced from a solution-phase approach. In addition, a new, generalizable scheme for the production of a variety of metal germanide nanocrystals is demonstrated through the use of metal(0) precursors. This methodology is then adapted to existing electroless etching methodologies used to produce silicon nanowire arrays, leading to the production of heterostructured silicon/metal silicide nanowires.

Proteins and peptides may be a promising route to develop conductive materials capable of interfacing biological components with electronics for bioelectronic applications. Amino acid sequences can drive the self-assembly of supramolecular nanostructures, whose high surfaces area, soft material surfaces may be suitable for interfacing with biology. These sequences can be coupled with conjugated small molecules or metals to drive the aqueous self-assembly of water soluble, electronically-conductive nanomaterials, or these sequences, by themselves, can form electronically-conductive supramolecular structures. In this work, we demonstrate long-range, non-redox, and non-thermally activated electron transport in biological protein nanofibers purified from Geobacter sulfurreducens, and we replicate these unique conduction properties in non--stacked, self-assembled nanofibers constructed from a de novo peptide sequence. We demonstrate that natural amino acids can be used to form supramolecular nanofibers with high conductivities, comparable to conjugated materials, in the absence of delocalization. This micrometer-scale transport does not fit within the framework of long-range conduction mechanisms in organic materials, and thus suggests a rethinking of current electron transport models. We also characterize electronic and electrostatic properties in two other amino acid based supramolecular systems, demonstrating how amino acids can be used to tune the self- assembly and electronic properties of conductive materials. These findings suggest that amino acid based materials may be a viable platform for designing functional bioelectronic materials.

Biology provides many sources of inspiration for synthetic and multi-functional nanomaterials. Naturally evolved proteins possess a range of unique functions and self-assembly behavior. Their flexible design, potential for stimuli responsiveness, and the ability for some proteins to undergo electron transport functions as part of metabolic processes make proteins ideal building blocks of bioelectronic interfaces. In biosensor and electrocatalysis applications, bioelectronic materials are designed to interconvert biological signals or processes into electronic signals and vice versa. Some examples of electronically conductive microbial protein materials motivate the use of peptides and proteins for responsive bioelectronic interfaces. In this work, we designed a peptide fiber structure that exhibits stimuli-responsive self-assembly and the capacity to transport electrical current over micrometer long distances. Hierarchically ordered nanofibers of α-helical coiled-coil peptides were assembled using a pH responsive structure ordering motif comprising of two neighboring lysine residues. Cryo-EM structures of these assemblies reveal a novel organization of α-helical peptides in a cross coiled coil arrangement that exhibits electrical conductivity. Both solid-state and solution-based electrochemical characterization show that these fibers are conductive. Single-fiber conductive AFM determined that the solid-state electrical conductivity is on a similar order of magnitude with conductive cytochrome filaments from bacteria. Solution deposited fiber films approximately doubled the electroactive surface area of the electrode, confirming their conductivity in aqueous environments. Furthermore, these fibers can be used as scaffolds for redox enzyme immobilization for electrochemical biosensing. Progress towards immobilized enzymatic devices will be discussed. This work further expands the field of stimuli-responsive and multifunctional nanomaterials by using peptide assemblies as structural supports for future implementation in bioelectronic and bioelectrochemical materials.