Tuning the wettability and photoluminescence of graphene quantum dots via covalent modification

Tuning of the photoluminescence of graphene quantum dots (GQDs) through modulation of the bandgap and chemical doping is important for the GQDs use in optoelectronic devices. Herein, tuning of dual properties, i.e., wettability (hydrophilic to hydrophobic) and photoluminescence, of graphene quantum dots is demonstrated. We synthesized graphene quantum dots (GQDs) and alkylated graphene quantum dots (C-12-GQDs) by simple chemical exfoliation (top-down technique) of graphite nanopowder. The synthesized GQDs are highly hydrophilic due to the presence of oxygenated functional moieties. We successfully functionalized the synthesized GQDs covalently with dodecyl amine (DDA) and simultaneously reduced them with glycine to render hydrophobicity on the GQD surface. Herein, glycine plays a dual role as a chemical functionalizer and reducing agent. All the characterizations of GQDs and C-12-GQDs were carried out by UV-visible absorption spectroscopy, scanning electron microscopy, Fourier transform infrared spectroscopy, dynamic light scattering, Raman spectroscopy, and contact angle analysis. Interestingly, after covalent modification, we also observed the tuning of photoluminescence (from green to blue) of GQDs. Thus, we can easily tune the dual properties, i.e., wettability and photoluminescence, of GQDs by simple covalent modification with long-chain alkyl (-C12H27) groups.