Design of a heterometallic Zn/Ca-MOF decorated with alkoxy groups on the pore surface exhibiting high fluorescence sensing performance for Fe3+ and Cr2O72−

Abstract

A fluorescent heterometallic Zn/Ca-MOF sensor, named {[(CH₃)₂NH₂]₄[Ca₂Zn₄(L)₄]·4DMF}_n (1), has been designed based on a flexible ligand ((5,5′-(propanen-1,3-diyl)-bis(oxy)diisophthalic acid (H₄L))) under solvothermal conditions. Compound 1 features a novel (4,8)-connected 3D architecture with the linear trinuclear cluster [CaZn₂(COO)₈] bridged by eight carboxylate groups from different organic ligands. The introduction of Ca²⁺ into the Zn-MOF effectively improves the thermal and water stability of the heterometallic Zn/Ca-MOF, which encouraged us to evaluate its fluorescence sensing performance. Consequently, compound 1 exhibits highly selective sensing of Fe³⁺ and Cr₂O₇²⁻ through a luminescence quenching mechanism. The corresponding quenching coefficient (K_sv) values are 4.36 × 10³ M⁻¹ for Fe³⁺ and 1.15 × 10³ M⁻¹ for Cr₂O₇²⁻. The detection limits are estimated to be 18.8 μM for Fe³⁺ and 29.1 μM for Cr₂O₇²⁻, which are lower than most reported sensors. Furthermore, the DFT theoretical calculation simulates the modified channel A of alkoxy groups as recognition sites to explain its recognition mechanism for Fe³⁺.