Doctor of Philosophy

dissertationThe optoelectronic properties of nanoscale metal and semiconductor material systems are notably sensitive to their corresponding physical structure. Contemporary synthesis techniques enable careful control of nanoparticle con figurations and therefore provide a wide array of systems where the eff ects of physical morphology on the interaction between nanoscale materials and light can be carefully probed. The investigated properties are immediately relevant to light-harvesting and ultra-sensitive trace-analysis and sensing applications. In this work, the structure-property relationships of both individual semiconductor nanocrystal heterostructures and aggregates of plasmonic silver nanoparticles in rough metal fi lms are probed. The semiconductor heterostructures behave as model light-harvesting systems where optical energy absorbed by one portion of the structure is funneled, on the nanoscale, to a model light-harvesting center, in analogy to photosynthesis. In the plasmonic silver nanostructures, collective optical excitation of the conduction electrons - plasmons - con es electromagnetic radiation to well beyond the traditional diraction limit of light in nanoscale regions called "hot spots." Within these hot spots, light-matter interactions are greatly enhanced and thus enable trace-sensing applications such as Raman scattering from a single molecule. Thorough application of relatively simple single particle spectroscopy techniques is combined with high resolution electron microscopy to elucidate the subtle details on how physical structure controls the optical properties of both material systems. There are four main results of this work. (1) The linear and nonlinear optical response of rough silver fi lms is shown to be enhanced by the excitation of surface plasmon polaritons. (2) The enhanced nonlinear response of rough metal films is conjectured to originate from metal clusters, and the observation of stark fluctuations in their efficiency of second-harmonic generation is reported for the fi rst time. (3) The presence of and enhanced emission from silver clusters of only a few atoms plays an important role in the intrinsic optical response of the silver films with considerable implications for surface-enhanced Raman scattering. (4) The e ffects of physical anisotropy on the electronic states of semiconductor nanocrystals are explicitly identifi ed through correlated optical and electron microscopy of single particles. These eff ects are shown to have important rami cations in the internal energy-transfer process of single nanocrystals

Michael Borys (Bristol- Myers Squibb) Incorporation of QbD elements into the development and characterization of a second generation process

QbD principles are readily incorporated into mammalian cell processes to streamline process development and characterization. A key enabler of the implementation of these principles has been widespread adoption of platform technologies by the industry. This allows easy and efficient navigation of the QbD roadmap laid out in the A-Mab case study over the course of the development lifecycle of a product. Here we examine the case of a 2nd generation process for a legacy product that was originally developed and approved using the traditional approach to process development and characterization. The goal of the 2nd generation process was to achieve several fold increases to productivity while achieving similar process performance across scales. Furthermore, comparability profiles of quality attributes must be maintained to ensure treatment efficacy and patient safety, and to streamline the regulatory approval process. To meet these constraints, it was necessary to make significant deviations from the platform process. This presentation outlines some of the challenges encountered during process development, tech transfer, and process characterization and how QbD principles were incorporated at each of the stages. Specifically, advanced metabolomics and proteomics methods were used to understand and eliminate differences in process performance after tech transfer to manufacturing scale and small scale bioreactor operations were optimized to ensure an appropriate scale down model. Risk assessments were used to guide process characterization efforts and custom DOE approaches were used to minimize bioreactor experiments. The experimental data were then fit to models to understand the design space and used to establish quantitative criteria to guide parameter classification. The models were verified through additional experiments and raw material variability was accounted for to improve robustness. The examples provided here demonstrate the advantages of incorporating QbD principles into the development cycle of biologics processes even in situations of compressed timelines and off-platform processes

Intermittency in second-harmonic radiation from plasmonic hot spots on rough silver films

Journal ArticleSurface enhancement of electromagnetic fields in plasmonic hot spots formed on rough silver films enables the observation of second-harmonic generation (SHG) from single metal nanoparticles. Nonlinear light scattering from these particles exhibits blinking in analogy to luminescence from single quantum dots, molecules and atoms; and fluctuations in single molecule surface-enhanced Raman scattering. Hot spots also display multiphoton white light emission besides SHG. In contrast to SHG, white light emission is stable with time, demonstrating that it is not the plasmonic field enhancement which fluctuates but the nonlinear polarizability (x(2)) of the emitting species

Exciton storage in CdSe/CdS tetrapod semiconductor nanocrystals: Electric field effects on exciton and multiexciton states

CdSe/CdS nanocrystal tetrapods are interesting building blocks for excitonic circuits, where the flow of excitation energy is gated by an external stimulus. The physical morphology of the nanoparticle, along with the electronic structure, which favors electron delocalization between the two semiconductors, suggests that all orientations of a particle relative to an external electric field will allow for excitons to be dissociated, stored, and released at a later time. While this approach, in principle, works, and fluorescence quenching of over 95% can be achieved electrically, we find that discrete trap states within the CdS are required to dissociate and store the exciton. These states are rapidly filled up with increasing excitation density, leading to a dramatic reduction in quenching efficiency. Charge separation is not instantaneous on the CdS excitonic antennae in which light absorption occurs, but arises from the relaxed exciton following hole localization in the core. Consequently, whereas strong electromodulation of the core exciton is observed, the core multiexciton and the CdS arm exciton are not affected by an external electric field

A polarizing situation: Taking an in-plane perspective for next-generation near-field studies

This mini-review provides a perspective on recent progress and emerging directions aimed at utilizing and controlling in-plane optical polarization, highlighting key application spaces where in-plane near-field tip responses have enabled recent advancements in the understanding and development of new nanostructured materials and devices

A carbon dioxide stripping model for mammalian cell culture in manufacturing scale bioreactors

Achieving adequate CO2 stripping rates in large scale bioreactors is an important consideration during the scale up of animal cell cultures to large scale bioreactors due to the use of relatively low power input and gas sparging rates. It has previously been reported that cell growth, productivity, and product quality attributes such as glycosylation can be significantly impacted when cells are exposed to high CO2 environments. CO2 stripping models that depend on the CO2 mass transfer coefficient have been applied to simulate CO2 profiles in cell cultures using varied sparger types, reagents for pH adjustment, gas flow rates, and agitation speeds. These models were reported as being validated for a cell culture after cell exponential growth phase. However, in recent years, cell culture processes have been improved to enhance productivity in part through a longer exponential growth phase to achieve higher viable cell densities, making those models less relevant. The current CO2 stripping models were tested in several improved cell culture processes and resulted in predicted CO2 profiles not fitting the measured CO2 profiles. A modified CO2 stripping model was then developed, of which CO2 stripping is independent of the CO2 mass transfer coefficient. Instead, CO2 stripping is a function of gas flow rates, the residence time of bubbles in the liquid, the time of bubbles being saturated with CO2, and CO2 concentrations. The model was validated with two CHO cell culture processes that achieved different peak viable cell density (approximately 7 × 106 cells/mL and 12 × 106 cells/mL) in 25,000-L and 5,000-L manufacturing bioreactors, respectively. The CO2 stripping model was also applied to optimize cell culture conditions to reduce CO2 level in cell cultures in the manufacturing scale bioreactors

GEN-1 immunotherapy for the treatment of ovarian cancer

GEN-1 is a gene-based immunotherapy, comprising a human IL-12 gene expression plasmid and a synthetic plasmid delivery system, delivered intraperitoneally (ip.) to produce local and persistent levels of a pleiotropic immunocytokine, IL-12, at the tumor site in patients with advanced ovarian cancer. The goal of local and persistent IL-12 delivery is to remodel the highly immunosuppressive tumor microenvironment to favor immune stimulation while avoiding serious systemic toxicities, a major limitation of recombinant IL-12 therapy. Safe and sustained local production of IL-12 and related immunocytokines at the tumor site could produce potentially more favorable immunological changes in the tumor microenvironment and antitumor responses than a bolus systemic delivery of recombinant IL-12. Treatment safety, clinical benefits and biological activity of GEN-1 ip. in patients with ovarian cancer and in representative animal models are described

Surface plasmon delocalization in silver nanoparticle aggregates revealed by subdiffraction supercontinuum hot spots

The plasmonic resonances of nanostructured silver films produce exceptional surface enhancement, enabling reproducible single-molecule Raman scattering measurements. Supporting a broad range of plasmonic resonances, these disordered systems are difficult to investigate with conventional far-field spectroscopy. Here, we use nonlinear excitation spectroscopy and polarization anisotropy of single optical hot spots of supercontinuum generation to track the transformation of these plasmon modes as the mesoscopic structure is tuned from a film of discrete nanoparticles to a semicontinuous layer of aggregated particles. We demonstrate how hot spot formation from diffractively-coupled nanoparticles with broad spectral resonances transitions to that from spatially delocalized surface plasmon excitations, exhibiting multiple excitation resonances as narrow as 13 meV. Photon-localization microscopy reveals that the delocalized plasmons are capable of focusing multiple narrow radiation bands over a broadband range to the same spatial region within 6 nm, underscoring the existence of novel plasmonic nanoresonators embedded in highly disordered systems

Electrically driven photon emission from individual atomic defects in monolayer WS2.

Quantum dot-like single-photon sources in transition metal dichalcogenides (TMDs) exhibit appealing quantum optical properties but lack a well-defined atomic structure and are subject to large spectral variability. Here, we demonstrate electrically stimulated photon emission from individual atomic defects in monolayer WS2 and directly correlate the emission with the local atomic and electronic structure. Radiative transitions are locally excited by sequential inelastic electron tunneling from a metallic tip into selected discrete defect states in the WS2 bandgap. Coupling to the optical far field is mediated by tip plasmons, which transduce the excess energy into a single photon. The applied tip-sample voltage determines the transition energy. Atomically resolved emission maps of individual point defects closely resemble electronic defect orbitals, the final states of the optical transitions. Inelastic charge carrier injection into localized defect states of two-dimensional materials provides a powerful platform for electrically driven, broadly tunable, atomic-scale single-photon sources