A Zn(II)-based metal-organic framework as a turn-off/on fluorescent sensor for selective detection of ammonium ferric citrate and aspirin

Abstract

The design of selective and sensitive chemosensor for sensing of biomolecules and phenol derivatives remains a significant challenge. Here, we report on the synthesis of a highly robust and thermally stable Zn-metal organic framework, via a hydrothermal reaction using 1,4-naphthalenedicarboxylic acid (H₂L), 1,2-di(pyridin-4-yl)ethane (bpe), and Zn(ClO₄)₂·6H₂O. The resulting compound [Zn₃(L^2-)₂(bpe)₂] (Zn-MOF-1) was synthesized and characterized using elemental analysis, FT-IR, TGA, PXRD, XPS, and single-crystal X-ray diffraction. Single-crystal analysis reveals that Zn-MOF-1 features a three-dimensional (3D) layered framework stabilized by C-H···O hydrogen bonding and π-π stacking interactions. Notably, the structure incorporates both distorted octahedral and pentagonal bipyramidal zinc coordination geometries within the same framework-an uncommon feature. Topological analysis confirms a (3,6)-connected, two-fold interpenetrated dia (diamondoid) network, underscoring the complexity and uniqueness of the structure. Given the excellent luminescent properties of Zn-MOF-1, sensing experiments were conducted using various biomolecules. The results demonstrated selective molecular recognition of Ammonium ferric citrate (AF) and aspirin (ASP) by Zn-MOF-1, exhibiting a 'turn-off' luminescence response for AF and a 'turn-on' response for ASA. The limits of detection (LOD) were 4.573 × 10^-5 mol/L for AF and 4.215 × 10^-5 mol/L for ASA, with corresponding Stern-Volmer constants (K_sv) of 2563.29 M^-1 and 2781.25 M^-1, respectively. The practical applicability of this sensing approach was evaluated using real water samples. Real water sample tests gave recoveries of 95-102 %, confirming reliability. Time-resolved studies revealed a reduced fluorescence lifetime upon AF addition, confirming a dynamic quenching mechanism. Furthermore, analysis of the absorption and emission overlap suggested a contribution from the inner filter effect (IFE). After IFE correction, the results highlighted genuine molecular interactions between AF and Zn-MOF-1. This establishes Zn-MOF-1 as a sensitive and selective fluorescent sensor for environmental and bioanalytical applications.

Keywords: Ammonium ferric citrate (AF); Aspirin (ASA); Fluorescent sensor; Molecular recognition; Zinc-based metal-organic frameworks.