Fundamental studies of carbon-based nanomaterials: Exploring the interface between nanotechnology and separation science

A sample containing carbon nanoparticles (CNPs) was generated starting with the soot from the combustion of inexpensive paraffin oil in a flame. The complexity of the sample, however, required fractionation to isolate its components. Anion-exchange high-performance liquid chromatography (AE-HPLC) was used for the analysis and collection of soot-derived CNPs. The fractionated species were monitored by ultraviolet (UV) absorbance and laser-induced photoluminescence detection, providing the chromatographic UV absorbance and emission profiles of the separated sample. Chromatographic fractionation allowed for bulk measurements of electronic properties for individual fractions, and further analysis via transmission electron microscopy (TEM). TEM of fractionated species showed a predominant size of about 3-5 nm diameter particulates, including carbon dots (C-dots), irregularly-shaped/amorphous CNPs, and graphitic nanoribbons. A general trend between photoluminescence and elution time was observed; the later eluting species in the chromatogram exhibited photoluminescence at longer wavelengths than the early eluting species.;Graphite nanofibers were shown to be effective for synthesizing C-dots exclusively but also exist as a relatively complex mixture. An unprecedented reduction in such complexity by AE-HPLC revealed fractions of C-dots with unique photoluminescence properties. The wavelength-dependent photoluminescence commonly assigned as an inherent property of C-dots was not present in fractionated samples. While UV-visible absorption profiles reported for C-dots are typically featureless, fractions of C-dots were found to possess unique absorption bands, with different fractions possessing specific emission wavelengths. Furthermore, fractionated C-dots showed profound differences in emission quantum yield, allowing for brighter C-dots to be isolated from an apparent low quantum yield mixture. These more photoluminescent fractions of C-dots displayed improved biological compatibility and usefulness as cellular imaging probes. Further, considering that the surface characteristics of C-dots represent one of the major determinants of their photoluminescence properties, the surfaces of the higher quantum yield C-dot fractions were analyzed by means of X-ray photoelectron spectroscopy and infrared spectroscopy. Surface functional groups on the C-dot fractions that provided the highest photoluminescence quantum yield were identified. Although some surface functionalities were common to all fractions (e.g., carboxylate groups), others were unique to particular fractions (e.g., aliphatic C-H bonding). This strongly suggests that in the mixture of the as-synthesized C-dots, all nanoparticles did not have identical surface functionality, and they contributed differently to the observed photoluminescence. A better understanding was established of the properties of the highest quantum yield C-dot particles within a complex mixture.;Sub-10 nm CNPs consistent with graphene quantum dots (GQDs) were synthesized from cost-effective carbon fiber starting material using a top-down synthetic approach. To optimize the production of GQDs, the reaction was allowed to proceed for 7 days and HPLC was used to monitor the different GQD fractions produced over time. During the course of the reaction, one fraction of GQD species was identified with relatively high photoluminescence. The species was short-lived during the first hour of reaction and disappeared after 1 day of reaction. Isolation of this species during the first hour of reaction was crucial to obtaining these luminescent GQD species. The photoluminescent species showed similar properties to those derived from graphite nanofibers. As the reaction proceeded out to 1 week, CNP growth occurred, with sizes approaching 200 nm. The experimental evidence suggests that these larger species are formed from small GQD nanoparticles bridged together by covalent sulfur-based moieties after the dehydration of sulfonic acids to form mutli-layer graphene oxide nanosheets. The size of the nanosheets could be tailored by controlling how long the reaction was allowed to proceed. These results highlight the variety of CNP products, from sub-10 nm GQDs to ~200 nm sulfur-bridged graphene oxide nanosheets, that can be produced by careful control and probing of the reaction of carbon fiber starting material over time. (Abstract shortened by UMI.).