Eco-engineered functionalized mesoporous silica from rice husks for selective detection of Fe(iii) in aqueous media

By utilizing rice husk (RH) waste as a renewable source of silica, this study presents a sustainable approach for engineering eco-friendly sensors for Fe(iii) detection. Through calcination and acid-leaching, mesoporous silica platforms (MSP) were created from RH, producing high-purity, mesoporous, and uniform morphology materials perfect for functionalization. Schiff base ligands, (E)-2-(((4-bromophenyl)imino)methyl)naphthalene-1,6-diol (BMN) and (E)-2-(((2-hydroxyphenyl)imino)methyl)naphthalene-1,6-diol (HMN), were immobilized onto MSP to create the BMN@MSP and HMN@MSP sensing systems. Upon coordination with Fe(iii), both BMN@MSP and HMN@MSP exhibit LMCT related changes in their UV-Vis absorption spectra (primarily intensity changes and, for HMN, more pronounced spectral modulation near ∼300 nm). These spectral changes are recorded with a UV-Vis spectrophotometer and are used to construct calibration curves and to determine the limits of detection and quantification, with HMN@MSP achieving a superior detection limit of 0.19 ppb and remarkable selectivity. Additionally, the sensors showed fast response times (<100 s), robust performance over pH 3.5-4.5, and excellent reusability over six cycles. Real water analyses (tap, river, lake) confirmed reliable Fe(iii) quantification with recovery rates of 92.5-99.1%, in strong agreement with ICP-MS results. Density Functional Theory (DFT) calculations further validated the enhanced affinity of HMN toward Fe(iii), attributed to its electron-rich structure and reduced HOMO-LUMO gap. Overall, this work promotes green analytical chemistry through the valorization of agricultural waste, promoting circular economy principles, and offering a low-impact, sustainable pathway for environmental monitoring.