Waste to Wealth: Sustainable Synthesis of Green Tea-Derived Quantum Dots for Nanomolar Detection of Fe3+ Ions and Ascorbic Acid

Highly fluorescent carbon quantum dots are synthesized from green tea leaves via microwave pyrolysis and abbreviated as G-CQDs. This eco-friendly method utilizes a renewable, low-cost, and easily accessible carbon source, with water as the solvent, aligning with green and sustainable synthesis. High-resolution transmission electron microscopy analysis reveals an average particle size of ≈2 nm. The nitrogen content in green tea leaves facilitates surface functionalization of G-CQDs, confirmed by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy techniques. The G-CQDs exhibit excitation wavelength-dependent behavior and a high photoluminescence quantum yield of ≈45%, eliminating the need for surface passivation. The G-CQDs show excellent photostability and environmental stability when tested under varying pH, irradiation time, and ionic strength. Additionally, they demonstrate high selectivity and sensitivity for detecting Fe³⁺ ions with a detection limit of 2 nM through fluorescence quenching. The fluorescence is restored in the presence of ascorbic acid (AA) due to a redox reaction with Fe³⁺ ions, with a detection limit of 13 nM. This dual-sensing capability makes G-CQDs a promising nanoprobe for rapid and on-site detection of Fe³⁺ ions and AA.