Synthesis and properties of multi-functionalized graphene quantum dots with tunable photoluminescence and hydrophobicity from asphaltene and its oxidized and reduced derivatives

Graphene quantum dots (GQDs) with tunable photoluminescence (PL) and hydrophobicity were synthesized from an abundant natural carbon source containing nitrogen, sulfur, and oxygen heteroatoms. Asphaltene and its oxidized and reduced derivatives were used as precursors to produce GQDs in organic solvents (i.e., methanol, toluene, and chloroform) using a facile ultrasonication technique. Asphaltene surface chemistry was tuned by sequential oxidation and reduction to investigate the surface effects on GQD properties. Spectroscopic characterizations confirmed the presence of N, S, and O heteroatoms and different electron-donating and electron-withdrawing groups. Microscopic characterizations revealed that these crystalline carbon nanomaterials have mono-layered or multi-layered structures with lateral sizes in the range of ∼5–15 nm. The asphaltene-derived GQDs exhibit tunable PL with emission colors ranging from blue to orange, depending on the carbon precursor and the organic solvent. Solvent exchange studies also revealed that asphaltene and its derivatives contain hydrophilic and hydrophobic fractions, resulting in varied hydrophobicity of the synthesized GQDs. Adding to the appeal of the present work, PL quenching of GQD-silica hybrid materials upon exposure to nitro-aromatics confirms that these GQDs can be incorporated to different host materials for advanced sensing or optoelectronic applications.