Razi University

      Lead (Pb2+) is a highly toxic, non-biodegradable heavy metal that bioaccumulates in the food chain, and even very low levels of exposure pose serious risks to human health. Conventional techniques for its determination, such as atomic absorption spectroscopy and ICP-MS, are accurate but require expensive instrumentation, skilled operators, and long analysis times, which limits on-site and routine monitoring. In this work, a fluorescent "turn-off" sensor based on polymer dots (Pdots) was developed for the selective and quantitative determination of Pb2+ in aqueous media. The Pdots were prepared through a simple, one-step, and green hydrothermal route (160 °C, 4 h) using xanthan gum, a low-cost natural polysaccharide, together with o-phenylenediamine (o-PD) as the fluorophore precursor and 15-crown-5 as a capping agent and lead-recognizing ionophore. FT-IR spectroscopy confirmed the formation of new C=N, C–N, and amide linkages among the precursors, UV-Vis spectroscopy indicated the formation of an extended conjugated system, and FESEM-EDS revealed a uniform, quasi-spherical morphology with a homogeneous distribution of carbon, nitrogen, and oxygen. The Pdots exhibited a strong emission at 420 nm under excitation at 342 nm. The sensing conditions were systematically optimized, and the best response was obtained at a xanthan gum to o-PD ratio of 50:50, pH 6, a probe volume of 90 µL, and an incubation time of 3 min. The addition of Pb2+ quenched the fluorescence of the Pdots, which is attributed to the interaction of Pb2+ with the crown ether cavity and the amine and carboxyl groups on the Pdot surface. The calibration curve showed two linear ranges, 1–6.5 ppm (I0 ? I = 23.60 C + 22.02, R2 = 0.98) and 15–46.3 ppm (I0 ? I = 1.48 C + 209.92, R2 = 0.97), with a limit of detection of [LOD value] µg/L and a limit of quantification of [LOQ value] µg/L. The applicability of the sensor to a complex biological matrix was evaluated by determining Pb2+ in human serum samples using the standard addition method, with recoveries of [recovery range value] %. The results show that combining a natural biopolymer as a template and passivating agent with a crown ether as a selective ionophore provides a rapid, simple, low-cost, and environmentally friendly platform for the optical determination of lead ions, with potential for environmental monitoring and clinical analysis. Keywords: Fluorescent sensor; Polymer dots; Xanthan gum; o-Phenylenediamine; 15-Crown-5; Lead ion (Pb2+); Hydrothermal synthesis; Fluorescence quenchin