Fabrication and DFT Calculation of Amine-Functionalized Metal–Organic Framework as a Turn-On Fluorescence Sensor for Fe³⁺ and Al³⁺ Ions

Abstract

Due to their important role in biological systems, it is urgent to develop a material that can rapidly and sensitively detect the concentration of Fe³⁺ and Al³⁺ ions. In this work, a brand-new Cd^II-based metal-organic framework [Cd(BTBD)₂(AIC)]<i>_n</i> (<b>JXUST-18</b>, BTBD = 4,7-bis(1<i>H</i>-1,2,4-triazol-1-yl)-2,1,3-benzothiadiazole and H₂AIC = 5-aminoisophthalic acid) with a 4-connected <i>sql</i> topology was designed and synthesized. The symmetrical Cd^II centers are linked by AIC^2- ligands with μ₃-η¹:η¹:η¹:η¹ coordination mode to form a [Cd₂(COO)₂] secondary building unit (SBU). The contiguous SBUs are further connected by BTBD ligands to form a two-dimensional (2D) layer structure. <b>JXUST-18</b> can remain stable in aqueous solutions with pH values of 3-12 or in boiling water. Luminescent experiments suggest that <b>JXUST-18</b> displays more than eightfold fluorescence enhancement in the presence of Fe³⁺ and Al³⁺ ions, and the detection limits for Fe³⁺ and Al³⁺ ions are 0.196 and 0.184 μM, respectively. Furthermore, the change in luminescence color is uncomplicatedly distinguishable with the naked eye under ultraviolet light at 365 nm. In addition, a series of devices based on <b>JXUST-18</b> including fluorescence test strips, lamp beads, and composite films were developed to detect metal ions <i>via</i> visual changes in luminescence color. Significantly, <b>JXUST-18</b> is a rare MOF-based turn-on fluorescence sensor for the detection of Fe³⁺ ions. The theoretical calculation suggests that the complexation of Fe³⁺/Al³⁺ ions and the -NH₂ group contributes to fluorescence enhancement.