A Green Approach to N, S-doped Carbon Dots from Jengkol Peel (Archidendron pauciflorum) for Ratiometric Sensing of Amaranth Dye

The development of environmentally safe and cost-effective carbon nanoparticle-based ratiometric fluorescence sensors is critical for identifying hazardous synthetic food dyes. N, S-doped carbon dots (N, S-CD) were synthesized via a simple hydrothermal method using jengkol peel waste as the carbon source, with sodium thiosulfate and urea as co-dopants. The material exhibited spherical morphology with an average diamter 7.41 nm and a quantum yield of 20.5 %. Structural analysis confirmed the presence of graphitic structures containing hydroxyl, carboxyl, nitrogen, and sulfur functional groups on the particle surface, which improved dispersibility and created a conjugate system for fluorescence emission. With ideal emission/excitation peaks at 515/370 nm, N, S-CD exhibits excitation-dependent fluorescence. This N, S-CD based sensor has a detection limit of 7.5 nM (range 2.5 - 20 μM) and responds to amaranth with dual-emission fluorescence (λem 515/646 nm, λex 370 nm) through a FRET process. Validation on food samples revealed 94 - 98 % recovery, demonstrating the potential of biomass-based sensors for food safety analysis. HIGHLIGHTS N,S doped carbon dots were successfully synthesized from jengkol peel waste through a green hydrothermal method, resulting in spherical nanoparticles with an average diameter of 7.41 nm. The dual-doping strategy significantly enhanced fluorescence properties, increasing quantum yield from 4.5 % (CD) to 20.5 % (N, S-CD) with optimal emission at 515 nm when excited 370 nm. The N, S-CD sensor exhibits excellent analytical performance for amaranth dye detection with a LOD of 7.5 nM and linear range of 2.5 - 20 μM through a ratiometric approach (F515/F646). The sensing mechanism involves fluorescence resonance energy transfer (FRET), providing dual-emission signals that shift from cyan to yellow under UV light as amaranth concentration increases. High selectivity was demonstrated against common food additives and metal ions, with actual samples (strawberry syrup and tomato juice) showing 94 - 98 % recovery rates.