Ratiometric fluorescence sensing of formaldehyde in food samples based on bifunctional MOF

A new fluorescence strategy was described for ratiometric sensing of formaldehyde (FA) with bifunctional MOF, which acted as a fluorescence reporter as well as biomimetic peroxidase. With the assistance of H₂O₂, NH₂-MIL-101 (Fe) catalyzes the oxidation of non-luminescent substrate o-phenylenediamine (OPD) to produce fluorescent product (oxOPD) with the maximum emission at 570 nm. Besides, intrinsic fluorescence of MOF (λem = 445 nm) was quenched by oxOPD through inner filter effect (IFE). However, FA and OPD reacted to generate Schiff bases, which competitively consumed OPD inhibiting the generation of oxOPD. Under the excitation wavelength of 375 nm, a ratiometric strategy was designed to detect FA with the fluorescence intensity ratio at 445 nm and 570 nm (F₄₄₅/F₅₇₀) as readout signal. This strategy exhibited a wide linear range (0.1-50 μM) and low detection limit of 0.03 μM. This method was confirmed for FA detection in food samples. In addition to establishing a new method to detect FA, this work will open new applications of MOF in food safety.