Ligand Emission-Based Turn-On Sensing of Glyphosate via Competitive Coordination with NH₂-MIL-88B (Fe) MOF Nanocrystals

Abstract

Glyphosate is one of the most effective herbicides and is extensively used in agriculture to improve crop production. However, its toxicity and carcinogenicity stimulate the development of potent, rapid, and highly selective sensors for glyphosate. In this study, we have successfully developed a rational and effective method for detecting glyphosate concentrations in the nanomolar range through luminescence enhancement. For this purpose, we synthesized NH2-MIL-88B (Fe) (MIL = Material Institute Lavoisier) metal–organic framework (MOF)-based nanocrystals (NCs) using the hydrothermal method and applied it for glyphosate detection in an aqueous medium. The MOF NCs contain coordinative ligands (2-aminoterephthalic acid) and metal clusters. The Fe clusters act as luminescence quenchers, suppressing emission from the MOF due to photoinduced electron transfer (PET) from the ligand to Fe3+ ions. The phosphate group of glyphosate competes with the carboxylate of the ligand in the MOF. As a result, the coordination between ligand molecules and Fe metal clusters in the MOF weakens, causing the ligand molecules to separate from the Fe cluster. This process diminishes the PET and regenerates the ligand emission at 450 nm. The luminescence enhancement exhibits high selectivity toward glyphosate and shows minimal interference from various pesticides and metal ions. The limit of detection was calculated as 198 nM, and the performance of the detection probe was evaluated in drinking water. Finally, MOF NCs-coated circular paper strips were prepared and successfully applied to detect glyphosate at femtomolar concentrations.