Biomass-derived carbon quantum dots: a novel and sustainable fluorescent "ON-OFF-ON" sensor for ferric ions.

Fluorescent carbon dot sensing probes have attracted much attention in recent times due to their amazing properties regarding chemical inertness, solubility, non-toxicity, optoelectronic behavior, and charge transport functionality. Herein, we report the green synthesis of lotus stem-derived carbon dots (LS-CQDs) from the naturally available lotus stem by a simple and economical hydrothermal method without the use of an oxidizing agent. HR-TEM and DLS measurements confirm the quasi-spherical shaped LS-CQDs, with a 2.5 nm average diameter. The LS-CQDs possess better aqueous dispersibility and stability due to the presence of hydrophilic hydroxyl, carboxyl, and amine surface functional groups, as manifested by FT-IR analysis. The LS-CQDs demonstrate excellent fluorescence properties that are sensitive to conditions of pH, time, and temperature. Furthermore, the prepared LS-CQDs display an interesting fluorescence "ON-OFF-ON" property. The LS-CQDs depict a selective and sensitive fluorescence quenching response in the presence of ferric ions. Moreover, the prepared LS-CQDs exhibit a quantum yield of about 0.44%. The LS-CQDs show an excellent sensing response with the limit of detection (LOD) equal to 0.212 ppm. The promising sensitivity and selectivity of LS-CQDs were utilized for the detection of ferric ions in the water samples collected from three polluted sources viz. lake water (Dal lake), underground water (tube well), and stream water. For all the collected water samples the results were reasonably good with the achievement of recovery factor above 1. Therefore, we strongly believe that the present study will serve as a good guiding star for the selective and sensitive detection of ferric ions from various polluted water bodies.