Rapid Microwave-Assisted Green Synthesis of N,S Co-Doped Red-Emissive Carbon Dots From Coconut Fiber Waste for Label-Free Ultrasensitive Creatinine Detection.

Carbon dots (CDs) are versatile fluorescent nanomaterials with tunable optical properties and biocompatibility. Here, we report a rapid, sustainable microwave-assisted synthesis of nitrogen and sulfur co-doped red-emissive carbon dots (N,S-CFCDs) from alkaline-pretreated coconut fibers, a renewable carbon source. The synthesis was completed in 5 min at 90°C under 300 W microwave power, achieving a high yield of ∼85%. Nitrogen and sulfur codoping effectively tuned the optical properties, shifting emission from blue in undoped CDs to intense red in N,S-CFCDs. TEM, FTIR, XPS, Raman, and UV-Vis analyses confirmed uniformly distributed nanoparticles (∼5 nm) with partially graphitized cores and N,S-enriched surface functionalities, resulting in strong red photoluminescence (λ_em ≈ 680 nm), a quantum yield of ∼19%, and excellent aqueous stability. Leveraging these properties, N,S-CFCDs were employed as label-free fluorescent probes for creatinine detection, exhibiting concentration-dependent quenching with a low detection limit of 0.12 nM, excellent linearity, and high selectivity against interfering biomolecules. Analytical performance in human serum, urine, and artificial cerebrospinal fluid showed recoveries of 95%-99.8%. This work demonstrates a sustainable strategy for converting biomass waste into high-value red-emissive nanoprobes, uniquely integrating synergistic N,S co-doping and green analytical validation to develop a high-performance platform for bioimaging, diagnostics, and environmental sensing.