Highly Selective and Sensitive Luminescence Turn-On-Based Sensing of Al³⁺ Ions in Aqueous Medium Using a MOF with Free Functional Sites

A new metal–organic framework [Co­(OBA)­(DATZ)_0.5(H₂O)] {OBA = 4,4′-oxybis­(benzoic acid) and DATZ = 3,5-diamino-1,2,4-triazole}, 1, was synthesized by hydrothermal reaction. Single-crystal X-ray data of 1 confirmed two-dimensional rhombus grid network topology with a free nitrogen site of triazole ring and two amine groups of each DATZ. Photoluminescence study of 1 in aqueous medium shows blue emission at 407 nm upon excitation at 283 nm. This emissive property was used for the sensing of Al³⁺ ions in aqueous medium through very high luminescence turn-on (6.3-fold) along with the blue shifting (∼24 nm) of the emission peak. However, luminescence studies in the presence of other common metal ions such as Mg²⁺, Zn²⁺, Ni²⁺, Co²⁺, Mn²⁺, K⁺, Na⁺, Ca²⁺, Cd²⁺, Hg²⁺, Cu²⁺, Fe²⁺, Fe³⁺, and Cr³⁺ in aqueous medium shows luminescence quenching in varying extent. Interestingly, the luminescence turn-on-based selectivity of Al³⁺ ions in aqueous medium was achieved even in the presence of the highest quenchable metal ion, Fe³⁺. Furthermore, very high sensitivity was observed in the case of Al³⁺ ions with a limit of detection of Al³⁺ of 57.5 ppb, which is significantly lower than the higher limit of U.S. Environmental Protection Agency recommendation of Al³⁺ ion for drinking water (200 ppb).