A Water-Stable Lanthanide-Based MOF as a Highly Sensitive Sensor for the Selective Detection of Paraquat in Agricultural Products

The toxic agrochemical contamination of food and the environment is one of the most pressing concerns for human health and environmental sustainability. In this work, a water-stable 3D lanthanide-based MOF, [Tb2(H2btec)(btec)(H2O)]·4H2O (Tb-MOF) has been demonstrated as a turn-off luminescent sensor for the detection of poisonous paraquat. The ligand 1,2,4,5-benzenetetracarboxylic acid (H4btec) was incorporated into Tb-MOF to act as a detection recognition site for a selective interaction with paraquat, resulting in a dramatic luminescence quenching. The obtained Tb-MOF not only is water stable but also tolerates acidic and basic conditions. On top of that, the rapid detection of paraquat in aqueous solution with high sensitivity and selectivity can be obtained even in the presence of common contaminants in the environment. The linear range for paraquat detection was observed in the concentration range from 0 to 50 μM with a low limit of detection of 2.84 μM and a high quenching constant of 3.95 × 104 M–1. Tb-MOF can be applied to detect paraquat in water and agricultural samples with satisfactory recoveries from 85.59 to 105.20%. To gain insights into the molecular recognition, time-dependent density functional theory (TD-DFT) calculations were conducted to reveal the specific interactions between paraquat and the ligand in the backbone structure of the MOF. The quenching mechanism is attributed to a photoinduced electron transfer (PET) from the MOF to paraquat and the competitive absorption of excitation energy by paraquat. To demonstrate a practical implementation of the MOF-based sensor, a PVA/Tb-MOF@paper strip was fabricated, which can visually detect paraquat under irradiation with a UV lamp. These results indicate that Tb-MOF can be used as a potential sensor for the practical detection of paraquat in the environment and agricultural products.