Fluorescent N, S co-doped carbon dots for tartrazine sensing and mechanistic perception of their radical scavenging activity

Blue fluorescent, biocompatible, nitrogen & sulphur co-doped carbon dots (NSCDs), prepared via microwave-assisted pyrolysis of tartaric acid and thiourea, have been explored as a fluorescent probe for selective and sensitive detection of the yellow-coloured food additive, tartrazine, in the aqueous medium through fluorescence quenching mechanism. The NSCDs (quantum yield ∼ 26%) exhibited static quenching in presence of tartrazine with the limit of detection (LOD) of 48 nM in the linear concentration range from 1 to 50 μM. In addition, NSCDs have also been investigated for their antioxidant activity by probing their ability to scavenge various reactive oxygen species (e.g., superoxide, peroxide, hydroxyl, and KMnO₄) along with their total antioxidant capacity and 2,2-diphenyl-1-picryl-hydrazyl-hydrate (DPPH) free radicals premised on UV–Vis spectroscopy. From pH-dependent UV–Vis and EPR spectroscopy studies, the scavenging of DPPH free radicals by the NSCDs was affirmed to follow the hydrogen atom transfer (HAT) mechanism in an acidic medium while the proton-coupled electron transfer (PCET) mechanism was predominant in alkaline medium.