Urea-Based Carbon Quantum Dots for Recognition of Fe³⁺ by Fluorescence Quenching and Pb²⁺ by Fluorescence Enhancement

The present study focuses on synthesis of urea-based carbon quantum dots (UCQDs) using a nontoxic, low-cost hydrothermal method without consumption of any harmful chemical. The synthesized UCQDs are characterized via UV-Vis absorbance, photoluminescence (PL), and Fourier-transform infrared spectroscopy (FTIR). Distinct UV-Vis absorption peaks due to π-π* and n-π* transitions and the existence of excitation-independent PL emission peaks at ≈450 and ≈525 nm, validated the presence of carbogenic core and nitrogen-enriched surface states. For water-dissolved iron (Fe³⁺) ions, UCQD proves its sensitivity by "Turn Off" fluorescence response, while it shows a "Turn On" sensing for Pb²⁺ ions. Bensei Hildebrand analysis, Stern-Volmer plots, Job plot, and FTIR analysis established the sensing capability of UCQDs by the formation of UCQD: metal ion complexation. The high quantum yield (≈38%), high sensitivity, and low detection limit highlight UCQD's real-life applicability for environmental pollution monitoring. The successful detection and quantification of metal ions in real samples validate UCQD as potential future metal ion sensor.