A new quinoline based luminescent Zr(iv) metal–organic framework for the ultrasensitive recognition of 4-nitrophenol and Fe(iii) ions

Abstract

A Zr(IV) based luminescent metal–organic framework (MOF; 1) was synthesized by a solvothermal method using a mixture of ZrCl₄, quinoline-2,6-dicarboxylic acid (H₂QDA) ligand and trifluoroacetic acid (modulator) having a 1 : 1 : 10 molar ratio in N,N-dimethylformamide (DMF). For activation, the methanol-exchanged sample was heated under high vacuum at 120 °C for 1 day. Different techniques like X-ray powder diffraction (XRPD), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR) spectroscopy and energy dispersive X-ray (EDX) analysis were applied to fully characterize 1. The activated form of 1 (called 1′) has the formula [Zr₆O₆(OH)₂(CF₃COO)₂(C₁₁H₅NO₄)₄(H₂O)₄]. It exhibited quick response and great selectivity for the fluorimetric sensing of 4-nitrophenol (4-NP) in acetonitrile and Fe³⁺ ions in water. The probe maintained its high selectivity for 4-NP and Fe³⁺ ions even in the presence of potentially intrusive nitroaromatics and metal ions, respectively. The detection limits for 4-NP and Fe³⁺ were found to be 1.40 ppt and 0.71 ppb, respectively. These values are the lowest among the existing MOF probes for Fe³⁺ and 4-NP. The electron and energy transfer processes along with the electrostatic interactions of 4-NP with the MOF can be the possible reasons for the selectivity for 4-NP. The fluorescence resonance energy transfer (FRET) process can be the major reason behind the quenching mechanism for Fe³⁺ ions. The recyclability test indicates that 1′ is a promising probe for the long-term detection of 4-NP and Fe³⁺ ions.