Turning Agroforestry Waste into Value-Added Fluorescent Carbon Quantum Dots for Effective Detection of Fe3+ in an Aqueous Environment

In the context of the circular economy, the high quantity of agroforestry waste should be transformed into sustainable and high-value materials to abate pollution, CO2 emissions, and expensive waste disposal. Herein, the agroforestry waste of apple leaves was initially used as a precursor to extract the value-added nanomaterial carbon quantum dots (CQDs) by way of an easy hydrothermal strategy without complicated purification processes, as extracted CQDs doped with N and P possess a typical graphite-like structure, a fine particle size of 2.0 nm, and excitation-dependent photoluminescence (PL) behavior. The doping of N and P endows CQDs with a much higher quantum yield (18.1%), good water solubility, high fluorescence stability, and specific recognition ability for the detection of Fe3+. The fluorescence of CQDs could be quickly quenched by Fe3+ within 1 min and recovered with the addition of ascorbic acid, suggesting the recyclability of the prepared CQD-based fluorescent probe. Systematic analyses support that a synergistic mechanism of static fluorescence quenching and inner filter effect was involved in the detection of Fe3+ by CQDs, showing a linear range between 0 and 160 μM and a limit of detection (LOD) of 4.0 μM. Furthermore, the feasibility of detecting Fe3+ by CQDs in practice was verified by tap water/lake water samples. The present work evinces that apple leaves are useful in producing green and low-cost CQDs as a promising fluorescent probe for sensitive, rapid, and selective detection of Fe3+ in an aqueous environment.