Poly(diallyl dimethylammonium)/peroxotungstic acid nanoparticles (PDDA@PWA NPs) were successfully synthesized via electrostatic self-assembly between the cationic polyelectrolyte PDDA and the anionic PWA species. The morphology, architecture, and composition of the PDDA@PWA NPs were systematically characterized by using transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The newly developed, white composite exhibited excellent selectivity and adsorption capacity when used as a solid-phase extraction (SPE) platform for the efficient separation of trace Cr(VI) ions from aqueous matrices. A portable fiber-optic reflectance spectroscopy (FORS) analysis was subsequently performed directly on the Cr(VI)-PDDA@PWA surface employing the diphenyl carbazide (DPC) reagent. Meanwhile, the Cr(VI)-PDDA@PWA platform displayed a color change from yellow to red-violet for trace levels of Cr(VI) ions as low as 0.035 μM. Under the optimized FORS conditions, a linear calibration curve was obtained for Cr(VI) concentrations ranging from 0.2 to 10 μM, based on the measured reflectance intensity of the DPC–Cr(VI) complex. For comparative analysis and method validation, the colored DPC–Cr(VI) complex was eluted from the nanoparticle surface using an Liebermann-Burchard reagent@N,N-dimethylformamide (LBR@DMF) solution. The absorbance of Cr(VI) was proportional in a concentration range of 0.2–6.0 μM with a detection limit of 0.01 μM. The synthesized PDDA@PWA NPs displayed excellent FORS sensing performance for the sensitive, selective, and on-site detection of Cr(VI) ions. The proposed approach was employed for Cr(VI) detection in real samples, yielding satisfactory apparent recovery rates (93–97%).
Avan et al. (2026) studied this question.