A sustainable optical nanosensor based on biogenic mesoporous silica and a chromophore for ultra-trace Fe³ ⁺...

In this work, a highly responsive chemo-sensor was designed for the trace determination of Fe³ ⁺ ions in industrial wastewater. The sensing platform is constructed by anchoring the 3-allyl-5-[(4-nitrophenylazo)-2-thioxothiazolidin-4-one (HL5) ligand onto mesoporous nanosilica prepared through a green, rice-husk-derived synthesis route. The resulting material provides a well-defined porous framework that enhances ligand loading and facilitates rapid metal-ion diffusion. The structural, morphological, and surface characteristics of both the nano silica support and the final HL5 sensor were thoroughly investigated using Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), Brunauer-Emmett-Teller (BET) analysis, and X‑ray Diffractometry (XRD). Key spectrophotometric variables, including pH, contact time, sensitivity, and selectivity, were systematically optimized to achieve maximum analytical performance. Under the optimized conditions, the sensor reliably detected Fe³ ⁺ at ultra-trace levels. Method validation following ICH guidelines confirmed excellent linearity, precision, and quantification capability, with a detection limit of 12.95 µg/L. The HL5-modified nanosilica also demonstrated notable regeneration potential; after treatment with 0.1 M Ethylenediaminetetraacetic acid (EDTA), the sensor retained its functionality for up to eight consecutive cycles. Application studies showed that the developed system performs effectively in diverse matrices, including industrial effluents and pharmaceutical iron formulations. Overall, the proposed chemo-sensor offers a selective, sensitive, and reusable platform suitable for real-world iron monitoring.