Enrichment and Portable Fiber Optic Reflectance Spectroscopy Determination of Cr(VI) Using Peroxotungstate-Anchored...

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%).