Luminescence behaviour of nitrogen-doped graphene quantum dots via solvent variation for ferric ion sensing

In this work, the effect of solvent in the luminescence behaviour of nitrogen-doped graphene quantum dots (NGQDs) during their synthesis was evaluated. NGQDs were prepared via solvo/hydrothermal methods using citric acid and melamine, as precursors, in a molar ratio of 4 : 3. This study demonstrates that the solvent plays a significant role in tuning the luminescence emission of the synthesised nanomaterials. This modulation is linked to variations in surface functionalization, particularly with regards to the distribution and nature of oxygen- and amine-groups. This behaviour has been confirmed by X-ray photoelectron spectroscopy, and Fourier-transform infrared spectroscopy. Atomic force microscopy analysis showed that the synthesised NGQDs exhibited a graphitic carbon core with an average size centred in the range 2-10 nm, regardless of the solvent used (water, ethanol, acetone, or DMF). Also, the synthesised NGQDs in these solvents exhibited bright visible emission between 450 and 550 nm upon excitation at 365 and 405 nm, respectively. Their performance as selective fluorescence turn-off probes for ferric ions in acidic aqueous medium was evaluated for all synthesis conditions in order to identify the most suitable route to obtain sensitive and efficient luminescent sensors for metal ion detection in acidic conditions.