An azo-receptor immobilized mesoporous honeycomb silica framework as a solid-state chromogenic sensor for capturing ultra-trace cadmium ions from environmental/industrial samples†

Abstract

This work focuses on a pollution-free ultra-portable solid-state opto-chemosensor for sensing noxious Cd²⁺ in environmental, industrial and non-industrial samples. An amphiphilic heterocyclic azo-receptor, (E)-4-((4,5-dimethylthiazol-2-yl)diazenyl)-6-hexylbenzene-1,3-diol (DMTHBD), is meticulously interlaced with a mesoporous honeycomb structured silica monolith framework (MHSF). The aqua-compatible optical sensor (DMTHBD@MHSF) offers remarkable structural integrity, surface morphology and porosity. The MHSF and DMTHBD@MHSF materials are characterized using powder X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, high-resolution transmission electron microscopy, field-emission scanning electron microscopy, energy dispersive X-ray analysis, selected area electron diffraction, elemental mapping analysis, thermogravimetric/differential thermal analysis, Brunauer–Emmett–Teller (surface area) analysis and Barrett–Joyner–Halenda (pore size distribution) plots. The MHSF shows a uniform distribution of well-packed continuous mesopore channels that expedite the voluminous loading of receptor molecules on the MHSF and the analyte diffusion to the receptor chelating sites. The DMTHBD@MHSF sensor exhibits exclusive selectivity for ultra-trace Cd²⁺, with brilliant concentration correlative color metamorphosis in ≤50 s, using a minimal sensor dose (3 mg). A distinguishable solid-state hue transition from salmon pink to intense violet is spotted in the concentration range of 1–400 μg L⁻¹, with a linear signal response between 0 and 150 μg L⁻¹, with detection and quantification limits of 0.15 and 0.50 μg L⁻¹ of Cd²⁺, respectively. The renewable sensor demonstrates excellent stability/durability under harsh working conditions, with reliable performance even after prolonged storage. The practical applications of the proposed sensor are authenticated using diverse actual samples, with an average recovery of ≥99.43% for Cd²⁺ and a relative standard deviation of ≤1.65%.