A novel sustainable metal organic framework as the ultimate aqueous phase sensor for natural hazards: detection of nitrobenzene and F− at the ppb level and rapid and selective adsorption of methylene blue

Abstract

Herein, a new cationic Cu(II)-based porous and water stable metal–organic framework (MOF), [{Cu(bipy)_1.5(H₂pdm)}·2NO₃·H₂O]_n (Cu-MOF-1), is synthesised via a slow evaporation process using pyridine-2,6-dimethanol (H₂pdm) and 4,4′-bipyridine (bipy). The MOF is characterized using Fourier-transform infrared (FTIR) spectroscopy, powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), magnetic analysis and single-crystal X-ray diffraction analysis. The structural unit of Cu-MOF-1 consists of two Cu(II) ions bridged by bipy and supported by H₂pdm. This material exhibits excellent sensing ability for nitrobenzene (NB) and fluoride ions (F⁻) in 100% aqueous medium with an ultra-low limit of detection of 0.093 and 1.203 ppb for NB and F⁻, respectively. The detection of nitro aromatic compounds (NACs) was found to be governed by photo-induced electron transfer (PET) and fluorescence resonance energy transfer (FRET) mechanisms, while vapour pressure played a major role in NB detection, with a high fluorescence quenching of 96.4%. Moreover, the MOF showed high water stability, significant recyclability and microporosity. The MOF was also employed for the adsorption and separation of methylene blue (MB) from a mixture of three dyes (MB, rhodamine-B and methyl orange). At equilibrium, the removal percentage of Cu-MOF-1 for MB was 98.23% and the mechanism of dye adsorption was also explored. Thus, the present MOF was determined to be a sustainable multifunctional material for the aqueous phase sensing of hazardous NB and fluoride ions, as well as an excellent dye adsorbent.