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In this work, molecular imprinting was combined with direct fluorescence detection of the pesticide Glyphosate (GPS). Firstly, the solubility of highly polar GPS in organic solvents was improved by using lipophilic tetrabutylammonium (TBA+) and tetrahexylammonium (THA+) counterions. Secondly, to achieve fluorescence detection, a fluorescent crosslinker containing urea-binding motifs was used as a probe for GPS-TBA and GPS-THA salts in chloroform, generating stable complexes through hydrogen bond formation. The GPS/fluorescent dye complexes were imprinted into 2–3 nm fluorescent molecularly imprinted polymer (MIP) shells on the surface of sub-micron silica particles using chloroform as porogen. Thus, the MIP binding behavior could be easily evaluated by fluorescence titrations in suspension to monitor the spectral changes upon addition of the GPS analytes. While MIPs prepared with GPS-TBA and GPS-THA both displayed satisfactory imprinting following titration with the corresponding analytes in chloroform, GPS-THA MIPs displayed better selectivity against competing molecules. Moreover, the THA+ counterion was found to be a more powerful phase transfer agent than TBA+ in a biphasic assay, enabling the direct fluorescence detection and quantification of GPS in water. A limit of detection of 1.45 μM and a linear range of 5–55 μM were obtained, which match well with WHO guidelines for the acceptable daily intake of GPS in water (5.32 μM).
The development of methodologies to sense glyphosate has gained momentum due to its toxicological and ecotoxicological effects. In this work, a red-emitting and polymerizable guanidinium benzoxadiazole probe was developed for the fluorescence detection of glyphosate. The interaction of the fluorescent probe and the tetrabutylammonium salt of glyphosate was studied via UV/vis absorption and fluorescence spectroscopy in chloroform and acetonitrile. The selective recognition of glyphosate was achieved by preparing molecularly imprinted polymers, able to discriminate against other common herbicides such as 2,4-dichlorophenoxyacetic acid (2,4-D) and 3,6-dichloro-2-methoxybenzoic acid (dicamba), as thin layers on submicron silica particles. The limits of detection of 4.8 µM and 0.6 µM were obtained for the sensing of glyphosate in chloroform and acetonitrile, respectively. The reported system shows promise for future application in the sensing of glyphosate through further optimization of the dye and the implementation of a biphasic assay with water/organic solvent mixtures for sensing in aqueous environmental samples.