Solid-state ion recognition strategy using 2D hexagonal mesophase silica monolithic platform: a smart two-in-one...

The possibility of a multifunctional and reversible solid-state colorimetric sensor is described for the identification and quantification of ultra-trace Cd²⁺ and Hg²⁺ ions, using a honeycomb-structured mesoporous silica monolith conjoined with an indigenous chromoionophoric probe, i.e., 4-hexyl-6-((5-mercapto-1,3,4-thiadiazol-2-yl)diazenyl)benzene-1,3-diol (HMTAR). The amphiphilic probe is characterized using NMR, FT-IR, HR-MS, and CHNS elemental analysis. The structural and surface properties of the monolithic template have been characterized using p-XRD, XPS, TEM-SAED, SEM-EDAX, FT-IR, TG-DTA, and N₂ isotherm analysis. The unique structural features and distinct analytical properties of the solid-state sensor proffer a strong response in selectively signaling the target analytes. The probe (HMTAR) exhibits a 1:1 stoichiometric binding ratio with the target ions (Cd²⁺ & Hg²⁺), with a visual color change from pale orange to dark red for Cd²⁺ (525 nm, λ_max), and to purple for Hg²⁺ (530 nm, λ_max), respectively, in the pH range 7.0-8.0. The influence of various analytical criteria such as pH, temperature, response kinetics, critical probe concentration, sensor quantity, matrix tolerance, linear response range, reusability, the limit of detection (LOD), and quantification (LOQ) has been investigated to validate the sensor performance. The proposed method displays a linear signal response in the concentration range 5-100 μg/L, with a LOD value of 2.67 and 2.90 μg/L, for Cd²⁺ and Hg²⁺, respectively. The real-world efficacy of the sensor material has been tested with real and synthetic water samples with a significant recovery value of ≥ 99.2%, to authenticate its data reliability and reproducibility (RSD ≤ 3.53%). Graphical abstract.