Harvesting Nature's Brilliance: Iron Ion (Fe³⁺) Sensing by Plumeria obtusa Leaf-Derived Carbon Quantum Dots

The focus of the present study is the synthesis and characterization of Plumeria obtusa leaf-derived carbon quantum dots (PCQDs) and their sensing capabilities for environmentally hazardous heavy metal ions. The hydrothermally synthesized PCQDs were morphologically and optically characterized using TEM and FESEM image analysis, XRD, UV–Vis absorption, photoluminescence, and FTIR spectroscopy. Dual emission was observed which confirms the existence of two emissive species with two lifetime decay profiles: one from the singled layer graphene core state and another from the passivated state of synthesized PCQDs. Furthermore, the applicability of PCQDs as fluorescent sensors for Fe³⁺ metal ions has been investigated up to 1.3 µM Fe³⁺ ion concentration. “Turnoff” fluorescence is observed in the presence of Fe³⁺ ions suggesting complex formation between PCQDs and Fe³⁺. Additionally, fluorescence lifetime decay curve showed that the average lifetime of PCQDs decreased with addition of Fe³⁺ metal ion. This reduction indicates that the Fe³⁺ ions introduced new energy levels or trap states within or around the quantum dot. These states provide alternate pathways for excited electrons and holes to recombine non-radiatively. The proposed optical techniques, along with principal component analysis, were successfully applied to detect dissolved metal ions in real samples such as paint, river water, and rainwater. This validates the effectiveness of the PCQD sensor as a reliable optical sensor for detecting water-dissolved metal ions, which contribute to water pollution.