Diabetes mellitus is a metabolic disease with abnormally high concentration of glucose in blood in patients. Continuous glucose monitoring, which involves measuring glucose concentration in the patient throughout the day and night, can significantly reduce the risk of diabetes-related complications. Commercially available CGM sensors are not yet suited for long-term applications due to reliability and accuracy issues associated with the irreversible, consumptive nature of the underlying electrochemical reactions. Affinity sensing methods, which are based on reversible affinity binding between glucose and a recognition molecule, hold the potential to address these challenges in CGM applications. These methods do not involve the consumption of glucose and can offer improved stability and accuracy for CGM. When combined with impedance-based transduction methods, affinity sensors can also offer a high level of miniaturization, allow low-cost instrumentation, and are amenable to physical and functional integration. The affinity sensors investigated in this thesis include hydrogel-based affinity microsensors and graphene-based affinity nanosensors. We first present a dielectric affinity microsensor that consists of a pair of coplanar electrodes functionalized in situ with a glucose-responsive hydrogel for dielectrically based affinity measurement of glucose in subcutaneous tissue. We present a study of the effects of the choice of hydrogel compositional parameters on the characteristics of the hydrogel-based microsensor, allowing the identification of the optimal hydrogel composition for the microsensor to sensitively and rapidly respond to changes in glucose concentration. A differential design is then demonstrated, both in vitro and in vivo, to effectively minimize the influence of fluctuations in the environmental conditions, thereby allowing the hydrogel-based microsensor to function appropriately as a subcutaneously implanted device. In addition, we present a preliminary study on affinity nanosensors for non-invasive monitoring of glucose concentrations in physiological media such as tears. The affinity nanosensor is based on a chemically modified graphene field-effect transistor for the electrical measurement of glucose concentrations. The study explores the sensing mechanism of the nanosensors and demonstrates a device with high sensitivity and low limit of detection, which satisfies the requirement for monitoring glucose concentrations in tears. Experimental results demonstrate that these affinity micros and nanosensors are capable of measuring glucose concentrations with a suitable sensitivity and dynamic range for the intended physiological media, with potential applications to minimally invasive or non-invasive continuous glucose monitoring in diabetes care.

The rational combination of dielectric and metallic nano particles brings novel optical properties to conventional subwavelength structures. This thesis introduces the optoplasmonic geometries demonstrating versatile ability in both far and near field modification within nano scale. Template-assisted self-assembly approaches are applied creating nano entities with titanium dioxide and gold nano spheres. A top-bottom mono hybrid unit and interdigitated array are developed. With the examination of the elastic and inelastic response of these hybrid materials, physical models are simulated to depict the scenario of varied geometry and combination of nano particles. In contrast to solely metal or dielectric particle arrays, this type of artificial material not only enhances the near electric field intensity within the metal nano cluster hot spots, but also expands the overall volume of enhanced electric field. Further study reveals that the additional enhancement and redistribution of near field are derived from the coupling between the nano gold cluster plasmon resonance and the in-plane diffractive mode of the dielectric array. The redirected emission profile of the fluorescent dyes within the hybrid array is explored.

abstract: My research centers on the design and fabrication of biomolecule-sensing devices that combine top-down and bottom-up fabrication processes and leverage the unique advantages of each approach. This allows for the scalable creation of devices with critical dimensions and surface properties that are tailored to target molecules at the nanoscale. My first project focuses on a new strategy for preparing solid-state nanopore sensors for DNA sequencing. Challenges for existing nanopore approaches include specificity of detection, controllability of translocation, and scalability of fabrication. In a new solid-state pore architecture, top-down fabrication of an initial electrode gap embedded in a sealed nanochannel is followed by feedback-controlled electrochemical deposition of metal to shrink the gap and define the nanopore size. The resulting structure allows for the use of an electric field to control the motion of DNA through the pore and the direct detection of a tunnel current through a DNA molecule. My second project focuses on top-down fabrication strategies for a fixed nanogap device to explore the electronic conductance of proteins. Here, a metal-insulator-metal junction can be fabricated with top-down fabrication techniques, and the subsequent electrode surfaces can be chemically modified with molecules that bind strongly to a target protein. When proteins bind to molecules on either side of the dielectric gap, a molecular junction is formed with observed conductances on the order of nanosiemens. These devices can be used in applications such as DNA sequencing or to gain insight into fundamental questions such as the mechanism of electron transport in proteins. / Dissertation/Thesis / Doctoral Dissertation Physics 2019

Photon avalanching (PA), one of the more unique nonlinear optical processes due to its combination of efficiency and extreme response, first attracted attention from the optics community more than four decades ago. But interest waned as researchers found that it did not provide immediately useful features observed in other nonlinear optical systems, such as amplified coherent light generation from lasing or optoelectronic amplification and transduction afforded by light-stimulated electron avalanching. The material systems supporting PA were also found to be rather limited, with reports concentrating on fragile, bulk lanthanide-doped crystals. However, the inter-ionic energy transfer mechanisms responsible for PA and its extreme nonlinearity are, in principle, realizable in objects with dimensions at the nanoscale. Further, new applications for PA in nanomaterials including simple super-resolution microscopy have recently been proposed. These factors motivated my research on the development of the first-ever lanthanide-doped nanoparticles capable of supporting PA behavior. In this thesis, the optical properties of Tm³⁺-doped NaYF₄ nanocrystals are investigated with photoluminescence microscopy, spectroscopy and differential rate equation model simulations. First, the photon avalanching behavior of Tm³⁺-doped NaYF₄ nanocrystals is studied. Specifically, the excitation-power-dependent luminescence of 1%, 4%, 8%, 20%, and 100% Tm³⁺-doped NaYF₄ is measured. The slopes of log-log excitation intensity versus emission intensity plots show that photon avalanche is realized in the nanocrystals when Tm³⁺ content is 8% and above. Time-resolved luminescence and rate equation model fitting to the experimental data validate the existence of photon avalanche, showing luminescence rise times > 600 ms, and the ratio of the ³F₄-to-³F₃ excited state absorption to the ³H₆-to-³F₄ ground state absorption is > 10⁴, which are signatures of photon avalanche. The design-dependent shift of the photon avalanching threshold also shows that photon avalanche is the main excitation scheme for the nanocrystals and implies potential applications for ultra-sensitive nano-sensing with the help of extreme nonlinearity. Additionally, the steep nonlinearity leads to super-resolution microscopy of single 8% Tm³⁺-doped nanocrystals with resolution down to <70 nm using conventional confocal microscopy without sophisticated techniques. In the second part of the thesis, the photodarkening effect of Tm³⁺-doped NaYF₄ nanocrystals is studied. We have found that photodarkening behavior is observed in Tm³⁺-doped nanocrystals that exhibit the photon avalanche effect. Power-dependent luminescence of a single 8% Tm3+-doped nanocrystal reveals that photodarkened nanocrystals still support photon avalanche behavior, but the avalanching threshold is shifted to a higher value. A photodarkening mechanism is proposed based on the concentration-dependent and power-dependent luminescence properties, and optical spectroscopic data. Notably, photodarkened nanocrystals are found to recover their original brightness and behavior under Vis-NIR optical illumination. This so-called “photobrightening” allows novel photoswitching of the inorganic nanocrystals, which has never before been achieved. We observe robust single nanocrystal photoswitching over 1000 cycles without permanent photodegradation. In addition, rewritable photolithography of multiple patterns using NIR lasers at 700 nm and 1064 nm is demonstrated.

Indiana University-Purdue University Indianapolis (IUPUI) / Semiconductor nanocrystals (SNCs) are a class of material with one dimension <100 nm, which display size, shape, and composition dependent photophysical (absorption and emission) properties. Ultrasmall SNCs are a special class of SNCs whose diameter is <3.0 nm and are strongly quantum confined leading to a high surface to volume ratio. Therefore, their electronic and photophysical properties are fundamentally dictated by their surface chemistry, and as such, even a minute variation of the surface ligation can have a colossal impact on these properties. Since the development of the hot injection-method by Bawendi et al., the synthetic methods of SNCs have evolved from high-temperature, highly toxic precursors to low-temperature, relatively benign precursors over the last 25 years. Unfortunately, optimization of their synthetic methods by appropriate surface ligation is still deficient. The deficiency lies in the incomplete or inappropriate surface passivation during the synthesis and/or post-synthetic modification procedure, which due to the high surface to volume ratio of ultrasmall SNCs, is a significant problem. Currently, direct synthetic methods produce SNCs that are either soluble in an aqueous media or soluble in organic solvents therefore limiting their applicability. In addition, use of insulating ligands hinder SNCs transport properties and thus their potential application in solid state devices. Appropriate choice of surface ligation can provide 1) solubility, 2) stability, and 3) facilitate exciton delocalization. In this dissertation, the effects of appropriate surface ligation on strongly quantum confined ultrasmall SNCs was investigated. Due to their high surface to volume ratio, we are able to highly control their optical and electronic properties through surface ligand modification. Throughout this dissertation, we utilized a variety of ligands (e.g. oleylamine, cadmium benzoate, and PEGn-thiolate) in order to change the solubility of the SNC as well as investigate their optical and electronic properties. First delocalization of the excitonic wave function 1) into the ligand monolayer using metal carboxylates and 2) beyond the ligand monolayer to provide strong inter-SNC electronic coupling using poly(ethylene) glycol (PEG)-thiolate was explored. Passivation of the Se sites of metal chalcogenide SNCs by metal carboxylates provided a two-fold outcome: (1) facilitating the delocalization of exciton wave functions into ligand monolayers (through appropriate symmetry matching and energy alignment) and (2) increasing fluorescence quantum yield (through passivation of midgap trap states). An ~240 meV red-shift in absorbance was observed upon addition of Cd(O2CPh)2, as well as a ~260 meV shift in emission with an increase in PL-QY to 73%. Through a series of control experiments, as well as full reversibility of our system, we were able to conclude that the observed bathochromic shifts were the sole consequence of delocalization, not a change in size or relaxation of the inorganic core, as previously reported. Furthermore, the outstanding increase in PL-QY was found to be a product of both passivation and delocalization effects. Next we used poly(ethylene) glycol (PEG)-thiolate ligands to passivate the SNC and provide unique solubility properties in both aqueous and organic solvents as well as utilized their highly conductive nature to explore inter-SNC electronic coupling. The electronic coupling was studied: 1) as a function of SNC size where the smallest SNC exhibited the largest coupling energy (170 meV) and 2) as a function of annealing temperature, where an exceptionally large (~400 meV) coupling energy was observed. This strong electronic coupling in self-organized films could facilitate the large-scale production of highly efficient electronic materials for advanced optoelectronic device applications. Strong inter-SNC electronic coupling together with high solubility, such as that provided by PEG-thiolate-coated CdSe SNCs, can increase the stability of SNCs during solution-phase electrochemical characterization. Therefore, we utilized these properties to characterize solution-state electrochemical properties and photocatalytic activity of ternary copper indium diselenide (CuInSe2) SNCs as a function of their size and surface ligand chemistry. Electrochemical characterization of our PEG-thiolate-coated SNCs showed that the thermodynamic driving force (-ΔG) for oxygen reduction, which increased with decreasing bandgap, was a major contributor to the overall photocatalytic reaction. Additionally, phenol degradation efficiency was monitored in which the smallest diameter SNC and shortest chain length of PEG provided the highest efficiency. The information provided herein could be used to produce superior SNC photocatalysts for a variety of applications including oxidation of organic contaminants, conversion of water to hydrogen gas, and decomposition of crude oil or pesticides. Therefore, we believe our work will significantly advance quantitative electrochemical characterization of SNCs and allow for the design of highly efficient, sustainable photocatalysts resulting in economic and environmental benefits.

DNA nanotechnology represents a powerful medium for manipulating the nanoscale arrangement of functional components. The first 15 years of DNA explorations has fast reached into every area of science and technology. Our group has focused attention on the utility of DNA as a structural material by folding DNA into rigid DNA objects such as tetrahedron or octahedron. These objects form the basis for engineered self-assembly by activating vertices of the nano-objects to interact with each other allowing for DNA mediated interaction which can achieve long range ordered cellular structures. Application of DNA nanotechnology can be likened to generating a flexible platform leveraging the precision afforded by the DNA sequences of A,G,T,C, and mostly are limited to experiments that could be accomplished within a 1μm3 volume. To scale emergent properties on the nanoscale, DNA origami techniques need profound improvements in synthesis and tools for characterization. The roadmap to transition DNA origami from a test tube to practical applications required a number of developments undertaken in this body of work. Critical milestones included: 1. Knowledge of nucleation and growth of DNA crystals (Chapters 1-3) 2. Transitioning DNA origami structures to the solid state (Chapters 4-7) 3. Characterization techniques to evaluate hierarchically engineered objects (Chapters 8-9) In the first thrust we performed investigative studies into the growth and nucleation of DNA origami crystals investigating thermodynamics and kinetics via in-situ experiments, these results iteratively improved synthesis conditions of DNA origami superlattices to grow from ~1um to over 250um single crystals up to 10x faster compared to previous synthesis conditions. These developments worked in tandem to explore methods to transition DNA constructs to the solid state via sol-gel synthesis of silica. The conversion process was reduced from by a factor of 12 from 24 hours to 2hours for rapid evaluation of crystals leveraged by a number of projects. The silication of structures allowed for further expanding the library of chemical structures available through the integration of liquid infiltration, atomic layer deposition and direct metallization of structures. The rapid development of DNA superlattices into larger and more complex motifs required the development of characterization techniques which could evaluate hierarchically designed materials spanning from 3-4nm to over 100 um. We characterize bulk mechanical properties of silica nanolattices leveraging in-situ indenters to examine nanoscale failure mechanisms. To characterize superlattices real-space artifacts we developed tomographic techniques to explore the spatial and elemental distribution of engineered constructs along with adopting biological serial sectioning approaches to evaluate defects in the assemblies.

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