Simultaneously fabrication of free and solidified N, S-doped graphene quantum dots via a facile solvent-free synthesis route for fluorescent detection

A facile one-step solvent-free synthesis approach is proposed for the simultaneous fabrication of free and solidified N, S-doped graphene quantum dots (N, S-GQDs) by using citric acid as precursor and L-cysteine as dopant. Graphene nucleus is firstly formed via the intermolecular dehydration of citric acid. N and S are then incorporated into the graphene structure by attacking the margin of graphene nucleus. The cross-linking among the graphene nucleus via the intermolecular condensation leads to the generation of free N, S-GQDs, while the intermolecular amidation between L-cysteine molecules and graphene nucleuses contributes to the solid-state fluorescence graphene quantum dots (SSF-GQDs). The free N, S-GQDs exhibit favorable photoluminescence behaviors such as high fluorescent quantum yield of 74.5%, stable photoluminescence within a wide range of pH and high tolerance to external ionic strength of up to 1.0 mol L-1 NaCl, making it excellent fluorescence probe for the sensitive detection of Fe3+ with a linear range of 0.01-3 mu M and a detection limit of 3.3 nM. The solidification of GQDs prevents the aggregation of GQDs efficiently and offers the solidified N, S-GQDs yellow green fluorescence, with a fluorescence quantum yield of 10.6%. This proposed protocol provides a novel avenue to fabricate diverse fluorescent graphene materials for different practical applications.