Green Synthesis of Nitrogen‐Doped Carbon Quantum Dots in Natural Deep Eutectic Solvent for Sustainable Detection of Sn²⁺ Ions

ABSTRACT Nitrogen‐doped carbon quantum dots (N‐CQDs) were synthesized using a green hydrothermal method with a lignin precursor in a natural deep eutectic solvent (NADES) and used for selective Sn²⁺ ion detection. Uniformly distributed spherical N‐CQDs with a mean diameter of 4.3 ± 0.9 nm and intense excitation‐independent blue emission at 445 nm with a quantum yield of 21% were obtained. Results from structural studies using FT‐IR, XPS, XRD, and TEM/SAED confirm the successful doping of nitrogen in pyridinic, pyrrolic, and graphitic forms with sufficient surface ─COOH and ─NH functional groups for metal ion coordination. The sensor material shows a linear Stern–Volmer relationship in the concentration range from 0.5 to 60 µM with a high quenching constant ( K sv = 4.1 × 10⁴ M⁻¹) and low detection limits of 0.52 µM (NADES) and 0.80 µM (aqueous system). Temperature‐dependent Stern–Volmer plots confirm static quenching with surface complexes and photo‐induced electron transfer from Sn²⁺ ions to N‐CQDs. Improved sensing capability in the NADES system is due to better dispersion and reduced aggregation of the quantum dots and increased metal‐ligand binding affinity. The method demonstrated good repeatability (RSD = 3.1%), reproducibility (RSD = 4.5%), and satisfactory recoveries (95.2%–103.4%) in simulated environmental samples. The proposed NADES‐mediated platform offers a sustainable, sensitive, and reliable fluorescence strategy for environmental monitoring of Sn²⁺ ions.