The chemical, physical, and biological properties of bacteria developing resistance have been explored in animal based bacteria while plant bacteria have been largely neglected. Thus, the ability to probe changes in stiffness, adhesion, binding interactions and molecular traits of bacteria causing plant diseases is of great interest to develop a new generation of more potent, yet sustainable, pesticides. Our study aims to investigate the physical and chemical properties of bacterial systems, in particular their cell walls. Building upon this fundamental understanding of the cells, we also investigate the physicochemical responses associated to multivalent nanoparticle-based bactericide treatments on bacterial systems identified as pathogens in plant diseases. Here our efforts focus on developing a protocol for the fundamental understanding of Xanthomonas perforans, a strain known for causing bacterial spot in tomatoes and causing close to 50% losses in production. To support the design and accelerate the development of pesticides and treatments against this disease, we evaluate the changes bacteria undergo in the presence of the treatment. Using a silica nanoparticle-based treatment designed with a shell containing multivalent copper and quaternary ammonium, we compare bacteria pre- and post-treatment with infrared spectroscopy, atomic force microscopy (AFM)-based techniques, and TIRF microscopy. Statistical data analysis enables the identification of attributes that can potentially serve as markers to track the bacterial responses to the treatment in the future. Finally, we will discuss the exciting implications of this work, such as potential clues for the development of more potent treatments for resistant bacteria.

Polyelectrolyte based hydrogel fibers can mimic extracellular matrix and have applications such as drug delivery and tissue scaffolding. Metal ions play a critical role in hydrogel fiber stability via electrostatic interactions, but knowledge of how they modulate mechanical properties of individual polyelectrolyte polymers is lacking. In this study, electrospun polyacrylic acid with chitosan is used as a model system to evaluate ferric ion effect on nanofiber mechanics. Using dark field microscopy imaging and persistence length analysis, we demonstrate that ferric ions modulate the bending stiffness of nanofibers. Young's modulus of individual nanofibers is estimated at values of a few kilopascals, suggesting that electrospun nanofibers possibly exist in a hydrated state. Furthermore, Fourier Transform Infrared (FTIR) spectra indicate the effect of ferric ions on polyacrylic acid molecular bonds. Our results suggest that metal ions can regulate single nanofiber stiffness, thereby providing designs to fabricate hydrogels in a tunable fashion.

Understanding interactions between nanoparticles and biological systems is fundamental for the development of emerging nano-biotechnology applications. In this thesis, I present an investigation of zinc oxide (ZnO) nanoparticles interactions with biomolecules in two separate studies. The first section of my thesis covers tracking and detection of ZnO nanoparticles using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). ZinkicideTM is a bactericidal ZnO nanoparticle which has been developed for agriculture. The characterization of Zinkicide in biological media and in solution has been difficult due to its high dispersibility and ultra-small size. SDS-PAGE is considered a golden standard for protein qualitative interpretations. In this study, we have modified this typical protein assay and developed protocols for quantifying Zinkicide concentration, fluorescence intensity, and relative molecular weight changes in aqueous solutions. We found that SDS-PAGE is a novel and fundamental approach for assessing ZnO nanoparticles. The second part of my thesis is focused on investigating biological toxicity induced by nanoparticles. Recent studies have shown that nanoparticles have the capabilities to induce abnormalities on cellular networks including actin cytoskeleton. We have studied the effects of ZnO nanoparticles on filamentous actin assembly dynamics utilizing total internal reflection fluorescence (TIRF) microscopy imaging and biophysical analysis. The combination of these studies has provided pertinent information for the future development of nanoparticles designed for biological applications.

Two dimensional (2D) materials-based nanostructures have attracted much attention due to their unique properties which exhibit promising prospects for application in catalysis and energy storage devices. Control growth method is important in synthesizing these nanostructures. Chemical vapor deposition (CVD) is a powerful method that provides scalability and controllable way to grow high quality 2D materials-based nanostructures. Here, we report a novel CVD growth method of 2D molybdenum disulfide (MoS2) based different structures in which thin film of molybdenum trioxide (MoO3) was used as the source of molybdenum (Mo), while sulfur (S) powder was used for the chalcogen precursor. Precise control of Mo precursor which is hard to achieve with MoO3 powder promotes high quality growth of MoS2 based nanostructures. In particular, we observed different MoS2-based nanostructures under different growth conditions. The structures were characterized by a variety of techniques to identify their chemical composition and structural nature. Scanning electron microscopy showed the morphology of the vertical plates, nanocrystals, and triangles structures. Raman spectroscopy indicated that the MoS2 based vertical plates are composed of MoO2 and MoS2. Transmission electron microscopy confirmed the multilayer shell of MoS2 with MoO2 core in the nanocrystal structures. We have successfully grown these nanostructures using precise control of the precursor concentration in confined vapor phase. These nanostructures could be relevant in the application of electrocatalytic materials with insufficient long-range conductivity, such as water oxidation catalysts consisting of poorly conducting metal oxides. Confined vapor phase paves the way to control surface structures of MoS2 at the nanoscale to ultimately develop effective catalyst-based materials with high densities of active edge sites at the surface.

Anticancer drugs like gemcitabine (GEM) are used to treat cancers such as, pancreatic ductal adenocarcinoma (PDAC). However, the use of free gemcitabine yields challenges including cytotoxicity to healthy cells and poor circulation time. By encapsulating GEM in nanoparticles these challenges can be overcome. In this study poly(acrylic acid) (PAA)-GEM nanoparticles are fabricated by coupling GEM onto PAA. The particle formation is driven by the hydrophobic interaction of GEM, which collects in the core of the nanoparticle, forming a PAA shell. The nanoparticles were optimized by studying the PAA/GEM ratio and pH during fabrication. Characteristics of the nanoparticles including size, morphology and surface charge were investigated using dynamic light scattering (DLS), transmission electron microscopy (TEM) and zeta potential measurements. Conditions such as ionic stability and pH stability were optimized to achieve high drug loading efficiency. Cell uptake and cytotoxicity studies were used to determine the efficiency of the nanoparticles as drug delivery vehicle.

The complexity of biological soft matter at the sub-micrometer level is fundamentally correlated to the functionalities at the larger scale. Reflecting the level of heterogeneities in the properties of systems remains challenging when probing small scales, due to the mismatch between the area surveyed with the tools offering nanoscale resolution, such as atomic force microscopy (AFM), and the scale of natural variations inherent to biology. Hence, to understand the physiological and mechanical alterations that occur within a single cell relative to a cell population, a multiscale approach is necessary. In this work we show that it is possible to observe molecular, chemical and physical alterations in both plant and human cells with a multiscale approach. Biophysical and biochemical traits of cell populations are studied with Fourier Transform infrared spectroscopy (FTIR) and in turn, guide higher resolution discovery with Raman spectroscopy and nanoscale infrared spectroscopy using AFM (NanoIR) to access finer details. We illustrate this with three examples of biological soft matter systems: 1) a preliminary study of cellular interactions with naturally occurring vehicles applicable to human health, 2) a qualitative examination of antibiotics and new pesticide treatments in food crop systems, and 3) a fundamental investigation of the deconstruction mechanisms of plant cells during pre-treatments in preparation for biofuel production.

Development and application of engineered nanomaterials to a variety of industrial and medical fields have progressed rapidly. Metal nanoparticles are predominantly desirable for their enhanced catalytic properties due to their high surface area-to-volume ratio as a result of their size reduction. Incorporating metal nanoparticles into another nanomaterial creates a nanocomposite that exhibits novel and better properties. There is a search for a stable flexible substrate that can contain the aqueous reaction of metal reduction for nanoparticle formation. In this study, we developed nanocomposite mats that are comprised of hydrogel nanofibers of polyelectrolytes poly(acrylic acid) and poly(allylamine hydrochloride) and loaded metal nanoparticles. The nanofibers are fabricated by the electrospinning technique, and subsequently immersed into a metal salt solution to absorb the metal ions. The metal ions within the fibers are then chemically reduced to form metal nanoparticles inside and on the surface of the nanofibers. Redox studies on various organic compounds were conducted to observe the catalytic reduction by the nanocomposites. The proposed fabrication process is advantageous in terms of simplicity, controllability and versatility.

In this thesis, we present an efficacious way of tracking nanoparticle movement in plant tissue through the use of fluorescence lifetime imaging (FLIM) and spectroscopy as well as a review of nanoparticle uptake in plants and the proposed mechanisms governing them. Given the increasing number of nanomaterials in agriculture and society as a whole, proper imaging tools and proactive measures must be taken to track nanoparticle movement in plant tissues and create infrastructure and products to keep things sustainable and safe. Herein we report a ZnO comparable nanoparticle— a CdS:Mn/ZnS/ N-acetyl cysteine (NAC) quantum dot— which boasts longer lifetimes and suitable fluorescent properties above ZnO to properly delineate from plant tissue fluorescence of chlorophyll and cinnamic acids. In addition to FLIM mapping, quantum dot localization in plant vascular tissue was clearly seen and confirmed via characteristic emission spectra and time correlated single photon counting decay curves (TCSPC). Most quantum dots were seen to reside in the xylem. Plant age and structure was seen to affect uptake. QD size likely restricted extensive translocation. Inhibitive effects of QDs were likely water and mechanical stress. We surmise that travel of the cadmium quantum dots up the leaf and branch plant tissues is likely most governed by diffusion as the quantum dots bound to the cell structures create a diffusion gradient which aids travel up the leaf.

Interfacial interactions play a major role in a wide range of applications from our everyday life to high-tech industrial applications. These interactions act between almost all the surfaces around us. In this work, we particularly study interfacial interactions in solid/water/liquid and solid/air/solid systems. In the first system, we mainly explored hydrophobic interactions that can take place only in a liquid medium. Long-range hydrophobic attractions between mesoscopic surfaces in water play an important role in many colloid and interface phenomena. Despite having been studied in different ways, the origin of these forces has yet to be explained. While previous research has focused on solid/water/solid and solid/water/air scenarios, we investigated a solid/water/liquid situation to gain additional insight. Here we directly measure long-range interactions between a solid and a hydrophobic liquid separated by water using force spectroscopy, where colloidal probes were functionalized with graphene oxide (GO) to interact with immobilized heptane droplets in water. We detected attractions with a range of ≈0.5 µm that cannot be explained by standard Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. When the GO was increasingly reduced to rGO to become more hydrophobic, these forces increased in strength and ranged up to 1.2 μm. This suggests that the observed attractions are indeed a result of long-range hydrophobic forces. Based on our results, we propose nanoscale air bubbles attached to the colloidal probe and molecular rearrangement at the water/oil interface as possible origins of the observed interactions. This knowledge will be useful to understand and motivate the formation of Pickering emulsions using 2D materials and other amphiphilic/hydrophobic particles. We studied interfacial interactions in solid/air/solid systems to understand attraction and adhesion between 2D materials (graphene and hexagonal boron nitride) and polymers. These 2D materials and their derivatives are widely used in nanocomposites due to outstanding mechanical, thermal, and electric properties. The interfacial bonds in the nanocomposite should be strong to transfer these properties of the nanomaterials to the polymer. In this study, we present a simple method to directly measure the interactions between different types of polymers and graphene versus hBN using force spectroscopy technique. We use polymer colloidal probes which were fabricated in the lab to carry out force spectroscopy measurements on graphene and hBN. We have studied some of the widely used polymers for nanocomposites such as polystyrene, PMMA, and epoxy. The attraction and adhesion between polymers and 2D materials were quantitatively studied. The results suggest that we cannot predict adhesive forces based on known van der Waals forces and the direct measurement of adhesive forces is required. With these two studies carried out in liquid and air media, we have uncovered interfacial phenomena that can be used in many applications such as mass production of nanomaterials, Pickering emulsions, oil/water separation, and nanocomposites.

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