Structural and Mechanistic Insights into a Cu-Adenine MOF for Selective Fluorescence Sensing of Antibiotics: Experimental and TDDFT Approaches

An adenine–succinate-based metal–organic framework, [Cu(adenine)(succinate)]ₙ (CuAS), was synthesized via a solvothermal method and structurally characterized by single-crystal X-ray diffraction, PXRD, IR, TGA, and elemental analysis. CuAS forms a three-dimensional framework featuring two directional channels along the a - and b -axes, with amino groups oriented toward the cavities. The material exhibits solid-state fluorescence at 440 nm (λₑₓ = 330 nm) and, when dispersed in methanol, emits at 430 nm (λₑₓ = 350 nm), consistent with intraligand transitions of adenine. CuAS was employed as a fluorescent sensor for antibiotics, showing high selectivity toward nitrofurantoin (NFT) and nitrofurazone (NFZ), with strong fluorescence quenching driven mainly by photoinduced electron transfer (PET). Stern–Volmer analysis revealed K _sv values of 5.98 × 10⁴ M⁻¹ (NFZ) and 5.88 × 10⁴ M⁻¹ (NFT), with detection limits of 0.136 ppm and 0.110 ppm, respectively. DFT and TDDFT calculations support the PET mechanism, indicating that π-stacked interactions between adenine and NFZ/NFT facilitate electron transfer upon excitation. In contrast, bulky antibiotics show minimal quenching due to steric hindrance. These findings provide molecular-level insights into fluorescence sensing mechanisms and establish CuAS as a promising selective sensor for nitrofuran antibiotics.