Pillar[5]arenes as Modulators for the Glyphosate and 2,4-D Herbicidal Activity: The Effect of Self-Assembly on Phyto- and Ecotoxicity

The widespread use of herbicides in agriculture results in their accumulation in the environment, which has a negative impact on non-target biota. One way to reduce environmental risks while maintaining the effectiveness of plant protection products is to apply supramolecular chemistry principles to agricultural practices. Although pillar[n]arenes are used in the production of sensors and antidotes for pesticides, their influence on the herbicidal properties and ecotoxicity of herbicides toward aquatic organisms and higher plants has hardly been studied. The effect of pillar[5]arenes on the herbicidal activity of 2,4-dichlorophenoxyacetic acid (2,4-D) and glyphosate (Glyp), as well as the ecotoxicity of the resulting binary systems toward Ceriodaphnia affinis and Paramecium caudatum, was assessed for the first time. The association constants of pillar[5]arenes with Glyp (logKa = 3.92–4.06) were an order of magnitude higher than the corresponding values for 2,4-D (logKa = 2.66–3.06) with the stoichiometry of 1:1. The formation of stable associates (143–177 nm) with negative zeta potential values (from −20.9 to −7.8 mV) was demonstrated for the pillar[5]arene/herbicide systems. Low phytotoxicity of pillar[5]arenes against Chlorella vulgaris was shown. The addition of pillar[5]arenes to 2,4-D reduced the wheat (Triticum aestivum L.) germination index by 4.5-fold compared to the pure herbicide. Forming associates between decamethoxypillar[5]arene and Glyp increased the LC10 by more than twofold compared to the individual herbicide against Paramecium caudatum and Ceriodaphnia affinis. It was demonstrated that combining pillar[5]arenes with Glyp can reduce ecotoxicity while partially preserving or selectively modifying phytotoxicity. The results obtained in this study are encouraging for the development of materials and supramolecular systems that could boost agricultural efficiency while reducing its environmental impact.