Date of Award

5-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Committee Member

Prof. Ya-Ping Sun, Committee Chair

Committee Member

Prof. Rhett Smith, Co-Chair

Committee Member

Prof. Brian Dominy, Co-Chair

Committee Member

Prof. Leah Casabianca, Co-Chair

Abstract

Recently, carbon nanoparticles have emerged to represent a new class of zero-dimensional carbon nanostructures in contrast to the more beautiful and defined C60-fullerenes. Despite their undefined and seemingly uninspiring properties, surface passivation or functionalization reveals high performance optical properties intrinsic to the carbon nanoparticles, resulting in “core-shell” nanostructures dubbed carbon dots (CDots). Generally defined as small carbon nanoparticles with various surface passivation schemes (i.e. organic or biological molecules), CDots display bright and colorful fluorescence emissions in addition to high performance photoinduced redox, and other properties, rivaling those of the more traditional semiconductor quantum dots (QDs) while retaining the biologically and environmentally benign characteristics of carbon.

In this dissertation, CDots were synthesized through the surface functionalization of carbon nanoparticles with 2,2’-(ethylenedioxy)-bis(ethylamine) (EDA), forming a highly stable aqueous suspension of EDA-CDots. The resulting dispersion could be considered “solution-like”, allowing for the analysis and characterization of these CDots with solution phase spectroscopy techniques, and were shown to be highly fluorescent and structurally compact, with the brightest fluorescence emissions occurring over the spectral region covered by popular fluorescent proteins, such as green fluorescent proteins (GFPs).

In terms of photoexcited state properties, photoinduced redox interactions of these CDots with of nitrotoluenes were probed through fluorescence quenching using steady-state and time-resolved fluorescence spectroscopy techniques. The emission properties of EDA-CDots were efficiently quenched by nitrotoluenes, which, mechanistically, result from highly efficient diffusion-controlled electron-transfer interactions at low quencher concentrations.

Excitation wavelength dependent emission properties of CDots were systematically studied in steady-state and time-resolved fluorescence regimes. CDots were shown to exhibit characteristic emission properties with strong excitation wavelength dependence for fluorescence quantum yields, while the fluorescence lifetimes only exhibited weak excitation wavelength dependencies. In order to better understand CDots fluorescence emissions and a photoexcited state deactivation mechanisms, a model consisting of two sequential processes leading to fluorescence emissions has been constructed, in which one process is primarily responsible for the observed excitation wavelength dependent emissions.

In an effort to specifically tailor the optical properties of carbon dots, core modified CDots have recently been reported, such that red sensitive chromophores, such as Nile blue (NB), are incorporated into the core carbon structure of polyethylene glycol functionalized CDots. The resulting nanostructure exhibits enhanced optical properties beyond what should be expected for the combination of these two species. The modified core structure displays an electronically integrated photoexcited state with excellent optical properties, such as effective visible and near-IR photon-harvesting, corresponding bright fluorescent emissions, and efficient photoninduced electron transfer (PET) serving as both excellent electron donors and acceptors.

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