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This study reports the synthesis, structural characterization, adsorption studies, nanoscale interaction, and photocatalytic application of pure and Fe-doped ZnS quantum dots for the degradation of the antibiotic cefalexin in aqueous solution. Nanoparticles were synthesized via the microwave-assisted method, and Fe doping was introduced at a 1% molar ratio. HRTEM images confirmed quasi-spherical morphology and high crystallinity, with particle sizes averaging 2.4 nm (pure) and 3.5 nm (doped). XRD analysis showed a consistent cubic ZnS structure. UV-vis spectra showed strong absorption at 316 nm for both samples, and PL measurements revealed emission quenching upon Fe doping. Photocatalytic tests under UV light demonstrated significantly higher degradation rates of 10 ppm cefalexin with Fe-doped ZnS, reaching near-complete removal within 90 min. Adsorption experiments revealed higher affinity and adsorption capacity of Fe-doped ZnS toward cefalexin compared to pure ZnS, as demonstrated by the Freundlich isotherm analyses, contributing significantly to enhanced photocatalytic degradation performance. High-resolution QTOF LC-MS analysis confirmed the breakdown of the β-lactam and thiazolidine rings of cefalexin and the formation of low-mass degradation products, including fragments at m/z 122.0371, 116.0937, and 318.2241. These findings provide strong evidence for the structural destruction of the antibiotic and validate the enhanced photocatalytic performance of Fe-doped ZnS. 

This study demonstrates the potential application of polyethyleneimine (PEI)-modified quantum dots for sensing heavy metals, specifically copper ions, in aqueous matrices. Quantum dots were synthesized in aqueous phase, revealing a spherical morphology, a size of less than 5 nm, and a face-centered cubic crystalline structure. The presence of PEI on the quantum dots' surface significantly enhanced their photoluminescence within the range of 400 nm to 650 nm. To assess the sensor capabilities of PEI-capped quantum dots, two copper precursors (CuSO4 and CuNO3) were utilized at concentrations ranging from 0 ppm to 38 ppm. A systematic decrease in fluorescence intensity with increasing copper concentration was observed, establishing a quantitative relationship. These findings underscore the potential of PEI-modified quantum dots as efficient and selective sensors for copper ions. The concentration-dependent response in fluorescence intensity reflects the sensitivity of the system, suggesting promising applications in the field of heavy metal detection.