A dual-mode MOF sensor for Al3+ detection based on ratiometric fluorescence red-shift and portable hydrogel and smartphone-based analysis

Abstract

A three-dimensional MOF, {[Cd(bpdc)(L1)]·5H₂O}_n (1) (H₂bpdc = biphenyl-4,4′-dicarboxylic acid; L1 = 1,1′-((2,4,6-trimethyl-1,3-phenylene)bis(methylene))bis(2-methyl-1H-imidazole)), was synthesized under hydrothermal conditions and comprehensively characterized. Luminescence studies reveal that complex 1 exhibits a distinct fluorescence red-shift response toward Al³⁺, enabling selective discrimination of Al³⁺ with a detection limit of 47.1 nM. Notably, the red-shift process is accompanied by the evolution of dual emission bands, which is favorable for ratiometric fluorescence sensing. In contrast, complex 1 shows pronounced fluorescence quenching in the presence of Fe³⁺, allowing selective detection of Fe³⁺ with a detection limit of 161 nM. Complex 1 shows good thermal and chemical stability, fast response behavior, and satisfactory reusability. Importantly, complex 1 represents the first example of an Al³⁺ ion probe combining red-shifted fluorescence with ratiometric detection based on the MOF material. Furthermore, a dual-mode sensing platform integrating ratiometric fluorescence and smartphone-based RGB colorimetric analysis was developed, enabling rapid on-site visualization and quantitative detection of Al³⁺ in environmental water samples.