Azo-based-functionalized rice husk silica nanocomposites for dual-mode sensing of Hg(II) and Ag(I): Experimental insights and DFT-elucidated mechanism

Abstract

A cost-effective dual-mode sensor was fabricated by covalently immobilizing para red 1-((4-nitrophenyl)diazenyl)naphthalen-2-ol on rice-husk-derived mesoporous silica to yield PR@RHS, a nanocomposite that couples agricultural-waste valorization with high-performance metal-ion detection. Thorough physicochemical characterization confirmed retention of the amorphous silica framework (broad XRD halo at 2θ≈21.35°) and preservation of well-defined mesoporosity (BET surface area 285 m²/g), while FTIR analysis revealed the emergence of C = N and N = N stretching bands, evidencing successful azo-based grafting. Density Functional Theory calculations clarified that Hg(II) coordinates through bidentate ON-chelation, inducing a bathochromic shift, whereas Ag(I) binds linearly to the azo nitrogens, producing a hypsochromic shift and distinct spectral fingerprints. Under optimized conditions (20 mg material in 10 mL solution, pH 4.0 for Hg(II) and pH 3.5 for Ag(I), 25 °C), PR@RHS achieved limits of detection of 13.67 ± 0.45 ppb (RSD = 3.3 %) ppb for Hg(II) and 10.25 ppb ± 0.38 ppb (RSD = 3.1 %) for Ag(I) with rapid response times of 140 s and 90 s, respectively, and negligible interference from 13 common metal ions even at 100 ppm. The probe retained ≥80 % of its initial signal after five EDTA/thiourea regeneration cycles and ≥81 % activity after 12-month room-temperature storage, highlighting outstanding durability. Practical utility was demonstrated by quantitative mercury screening in commercial skin-lightening creams (recoveries 95.8–101.2 %, RSD ≤ 3.8 %) and high-efficiency silver recovery from electronic-waste leachates (overall yield 93.85 % for e-waste). These results establish PR@RHS as a sustainable, reusable platform that bridges environmental monitoring and resource reclamation through green nanotechnology.