Label-free carbon dots from water hyacinth leaves as a highly fluorescent probe for selective and sensitive detection of borax

Herein, we reported a simple strategy for the synthesis of highly fluorescent carbon-based nanomaterials from one of the world’s most invasive aquatic plants and investigated their uses as practical, low-cost borax sensors, which have not been reported elsewhere to date. The carbon dots (CDs) were synthesized through acid-treatment-assisted pyrolysis with water hyacinth leaves as a precursor. Quasi-spherical CDs in solution were shown to produce blue emission, at a relatively high quantum yield of 27%. Their use as a borax probe was demonstrated with a detection limit of 1.5 μM. The selectivity and sensitivity of CDs toward borax in the presence of interferences confirmed their unique and selective sensing properties. A portable paper-based device was fabricated for use in on-site borax detection and was shown to have a detection limit 11.85 μM. Real fish ball samples were tested, and the CDs exhibited excellent borax recovery, in the range 98.8–101.8%. Our computational findings based upon density functional theory (DFT) and time-dependent density functional theory (TD-DFT) suggested that selective fluorescence quenching arose from charge transfer between the CDs and borax via their favorable lowest unoccupied molecular orbitals. Both experimental and computational findings therefore confirmed the novel sensing properties of the CDs.