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 (λₑm ≈ 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.