Time–temperature–solvent modulated Zn–Fe bimetallic MOFs: correlating structural dynamics with optical, redox, and dopamine sensing performance

Abstract

Engineering synergistic metal centres inside metal–organic frameworks (MOFs) provides an effective approach to simultaneously adjust structural, magnetic, and electronic properties within a single architecture. We present a multivariate solvothermal method that utilizes the time–temperature–solvent (T–T–S) synthesis space to fabricate zinc–iron bimetallic metal–organic frameworks (ZnFe-MOFs) based on the Fe^III–MIL-88B (MIL – Materials of Institute Lavoisier) structure. The systematic alteration of reaction duration (12–48 hours), temperature (100–140 °C), and solvent polarity (N,N-dimethyl formamide (DMF)/ethanol) facilitates precise regulation of lattice expansion–contraction (“breathing”), metal-site distribution, and crystallographic orientation. Powder X-ray diffraction (PXRD) revealed that Zn^II incorporation enhances structural stability and induces preferential growth along the (101) plane. Electronic modulation through Zn²⁺ substitution effectively inhibits Fe³⁺ mediated non-radiative decay processes, resulting in a significant increase in ligand-centered fluorescence. Meanwhile, glutathione depletion assays, electron paramagnetic resonance (EPR) spectroscopy, and magnetic force microscopy verify the retention of Fe-centered redox activity and the integrity of nanoscale magnetic domains. The designed optical–redox framework results in superior electrocatalytic efficacy: ZnFe-MOF-modified electrodes provide a detection limit of 0.043 µM, a linear range of 10–1000 µM, and a sensitivity of 66.24 µA µM⁻¹ cm⁻² for dopamine oxidation, exhibiting significant selectivity against prevalent interferents. These findings demonstrate that bimetallic synergy-driven structural modulation provides an effective design strategy for metal–organic frameworks, enabling enhanced optical emission, retention of magnetic functionality, and improved redox-active sensing performance.