Fluorescent turn-off sensor based on sulphur-doped graphene quantum dots in colloidal and film forms for the ultrasensitive detection of carbamate pesticides

Materials that are highly sensitive, easy-to-use, label-free and ultrafast detection of analyte molecules are in great demand. In the present global scenario, there is immense value to be gained from the development of materials that easily detect harmful substances like pesticides or insecticides in fruits and vegetables, environmental pollutants, or other hazardous chemicals. Herein, we demonstrated fluorescent sulphur-doped graphene quantum dots (S-GQDs)-based sensor for the ultrasensitive and selective detection of carbamate pesticides. Green fluorescent S-GQDs with uniform particle size distribution (~5 nm), high crystallinity, monolayer graphene thickness, high quantum yield (27.8%) and high production yield (85%) were synthesised by microwave-assisted sonochemical method using graphene oxide as starting material in five min. KMnO4 is used as oxidising agent and H2SO4 as the precursor for sulphur doping. Sulphur doping is confirmed from XPS and FTIR analysis. Sulphur-doping increases the fluorescence intensity, lifetime and quantum yield of GQDs. This heteroatom doping strategy also results in tuning the optical properties of S-GQD when compared to the undoped system. The green fluorescent S-GQDs in aqueous phase exhibits a detection limit of 0.45 ppb for carbofuran and 1.6 ppb for thiram. Further, S-GQD was incorporated to poly (vinyl alcohol) [PVA] matrix for the preparation of flexible solid-state fluorescent sensing platform. S-GQD imparts fluorescent properties to the polymer film, and thus the film exhibited solid-state fluorescence. PVA/S-GQD flexible film exhibits a LOD of 60 ppb and 210 ppb for carbofuran and thiram, respectively. The selectivity of S-GQD based fluorescent sensor towards carbamate pesticide was investigated. Finally, the developed fluorescent sensor has demonstrated to detect carbofuran in real sample with ppb level sensitivity.