A Zn/Cd/Ni Metal–Organic Framework-Based Solid and Flexible Fluoroprobes for Highly Selective Detection of 4-Nitroaniline in Real Water Samples

The combination of diazabicyclic compound i.e. 1,4-diazabicyclo[2.2.2]octane (DABCO) and dicarboxylate linker 1,4-dicarboxylic acid are employed to self-assemble with three divalent metal ions such as Zn²⁺, Cd²⁺, Ni²⁺ led to the formation of highly stable pillar layered luminescent metal–organic frameworks. The emissive response of as-synthesized MOFs was studied in organic solvents with different functionalities resulting in the development of fluoroprobes having excellent recognition ability for 4-nitroaniline with detection limit upto micromolar. All the MOFs display a good linear relation between the fluorescence intensity and concentration of 4-NA in the range of 10–90 µmol L⁻¹. The values of K_sv for Zn-MOF, Cd-MOF, and Ni-MOF are found to be 1.75 × 10⁴ mol⁻¹L, 1.25 × 10⁴ mol⁻¹L, and 28.0 × 10⁴ mol⁻¹L, respectively. The calculated detection limit values are 19.7 µmol L⁻¹, 27.6 µmol L⁻¹, and 1.19 µmol L⁻¹ respectively. Moreover, the study was further extended to fabricate the solid membrane-based fluoroprobe using a linear chitosan polysaccharide and act as an efficient solid fluoroprobe for the detection of 4-NA. This proposed synthesis of polymeric membrane facilitates the MOF@chitosan fluorophore to transfer its fluorescence-emissive nature to a solid state.