Urea‐Based Carbon Quantum Dots for Recognition of Fe 3+ by Fluorescence Quenching and Pb 2+ 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 (Fe 3+ ) ions, UCQD proves its sensitivity by “Turn Off” fluorescence response, while it shows a “Turn On” sensing for Pb 2+ 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.