CARBON QUANTUM DOTS: BRIDGING THE GAP BETWEEN CHEMICAL STRUCTURE AND MATERIAL PROPERTIES

Carbon quantum dots (CQDs) are the latest generation of carbon nanomaterials in applications where fullerenes, carbon nanotubes, and graphene are abundantly used. With several attractive properties such as tunable optical property, edge-functionalization, and defect-rich chemical structure, CQDs have the potential to revolutionize optoelectronics, electro- and photocatalysis, and biomedical applications. Chemical modifications through the addition of heteroatoms, chemical reduction, and surface passivation are found to alter the band gap, spectral position, and emission pathways of CQDs. Despite extensive studies, fundamental understanding of structure-property relationship remains unclear due to the inhomogeneity in chemical structure and a complex emission mechanism for CQDs. This dissertation outlines a series of works that investigate the structure-property relationship of CQDs and its impact in a variety of applications. First, this relationship was explored by modifying specific chemical functionalities of CQDs and relating them to differences observed in optical, catalytic, and pharmacological performance. While a number of scientific articles reported that top-down or bottom-up synthesized CQDs yielded similar properties, the results herein present dissimilar chemical structures as well as photoluminescent and metal sensing properties. Second, the role of nitrogen heteroatoms in top-down synthesized CQD was studied. The effect of nitrogen atoms on spectral position and fluorescence quantum yield was considerably studied in past reports; however, thorough investigation to differentiate various nitrogen related chemical states was rarely reported. By finely tuning both the quantity of nitrogen doping and the distribution of nitrogen-related chemical states, we found that primary amine and pyridine induce a red-shift in emission while pyrrolic and graphitic nitrogen produced a blue-shift in emission. The investigation of nitrogen chemical states was extended to bottom-up synthesized CQDs with similar results. Finally, top-down, bottom-up, nitrogen-doped and chemically reduced CQDs were separately tested for their ability to act as photodynamic anti-cancer agents. This series of experiments uncovered the distribution of reactive oxygen species produced during light exposure which elucidated the photodynamic mechanisms of cancer cytotoxicity. The results presented in this dissertation provide key insight into engineering finely-tailored CQDs as the ideal nanomaterial for a broad range of applications

Enhanced Acidity of Acetic and Pyruvic Acids on the Surface of Water

Understanding the acid–base behavior of carboxylic acids on aqueous interfaces is a fundamental issue in nature. Surface processes involving carboxylic acids such as acetic and pyruvic acids play roles in (1) the transport of nutrients through cell membranes, (2) the cycling of metabolites relevant to the origin of life, and (3) the photooxidative processing of biogenic and anthropogenic emissions in aerosols and atmospheric waters. Here, we report that 50% of gaseous acetic acid and pyruvic acid molecules transfer a proton to the surface of water at pH 2.8 and 1.8 units lower than their respective acidity constants pKa = 4.6 and 2.4 in bulk water. These findings provide key insights into the relative Bronsted acidities of common carboxylic acids versus interfacial water. In addition, the work estimates the reactive uptake coefficient of gaseous pyruvic acid by water to be γPA = 0.06. This work is useful to interpret the interfacial behavior of pyruvic acid under low water activity conditions, typically found in haze aerosols, clouds, and fog waters

Interfacial Oxidative Oligomerization of Catechol.

The heterogeneous reaction between thin films of catechol exposed to O3(g) creates hydroxyl radicals (HO•) in situ, which in turn generate semiquinone radical intermediates in the path to form heavier polyhydroxylated biphenyl, terphenyl, and triphenylene products. Herein, the alteration of catechol aromatic surfaces and their chemical composition are studied during the heterogeneous oxidation of catechol films by O3(g) molar ratios ≥ 230 ppbv at variable relative humidity levels (0% ≤ RH ≤ 90%). Fourier transform infrared micro-spectroscopy, atomic force microscopy, electrospray ionization mass spectrometry, and reverse-phase liquid chromatography with UV–visible and mass spectrometry detection provide new physical insights into understanding the surface reaction. A Langmuir–Hinshelwood mechanism is accounted to report reaction rates, half-lives, and reactive uptake coefficients for the system under variable relative humidity levels. The reactions reported explain how the oligomerization of polyphenols proceeds at interfaces to contribute to the formation of brown organic carbon in atmospheric aerosols

Enhanced Acidity of Acetic and Pyruvic Acids on the Surface of Water

Understanding the acid–base behavior of carboxylic acids on aqueous interfaces is a fundamental issue in nature. Surface processes involving carboxylic acids such as acetic and pyruvic acids play roles in (1) the transport of nutrients through cell membranes, (2) the cycling of metabolites relevant to the origin of life, and (3) the photooxidative processing of biogenic and anthropogenic emissions in aerosols and atmospheric waters. Here, we report that 50% of gaseous acetic acid and pyruvic acid molecules transfer a proton to the surface of water at pH 2.8 and 1.8 units lower than their respective acidity constants pK_a = 4.6 and 2.4 in bulk water. These findings provide key insights into the relative Bronsted acidities of common carboxylic acids versus interfacial water. In addition, the work estimates the reactive uptake coefficient of gaseous pyruvic acid by water to be γ_(PA) = 0.06. This work is useful to interpret the interfacial behavior of pyruvic acid under low water activity conditions, typically found in haze aerosols, clouds, and fog waters

Die Biegestabprothese: ein experimenteller Ansatz zur metaphysären Hüftendoprothetik

The aim of our study was to develop a femoral component for total hip arthroplasty that would exclusively anchor in the metaphysis of the femoral neck. To forego trochanteric fixation, the load needs to be transferred to the metaphysis at as many points as possible. A computer simulation model suggested that an implant with a central cylinder and 16 rods aligned along a thread would be the preferable solution. To evaluate primary implantation stability, 14 fresh frozen cadaver femora were used. A special instrument set was developed to allow for centered implantation of the prosthesis without the need to dissect the greater trochanter. For our tests, we used two prototype implants: one made from titanium and the other from a CoCrMo alloy. For the measurement of micromotions at the medial proximal femur, sinusoid dynamic loading with a force between 300 N and 1700 N and a frequency of 1 Hz was employed. In a neutral position of 16 degrees adduction and 9 degrees ante-torsion, the average micronnotions measured were 119 mu m. Despite these convincing in vitro results with regards to primary stability, circular cut-out of the implant, followed by aseptic osteonecrosis, loosening might still occur in a clinical situation. Animal experiments are therefore required to further evaluate this new implant design