Loading and Co-Solvent-Triggered Release of Okanin into/from Functional Microgels for Potential Plant Protection

The constantly growing world population leads to increasing demands for food, which challenges modern agriculture manifold. The treatment of pests, including weeds, can be hampered by rain, which leads to losses of up to 90% of the applied agrochemicals and potential environmental impact of the hazardous chemicals. Including long-term release or release-on-demand mechanisms is one way to reduce the environmental impact and improve the efficacy of agrochemicals. Here, we investigated by combining experiments, molecular simulations, and free energy computations poly(N vinylcaprolactam) (pVCL)-based microgels as a potential agrochemical carrier and used it to formulate okanin as a potential C4 plant-selective herbicide. Dynamic light scattering, scanning transmission electron, and atomic force microscopy revealed that pVCL microgels collapse and rigidify upon the loading of okanin. The simulations identified loosely adsorbed okanin and tightly bound okanin mediating inter-chain crosslinks. With increasing okanin concentration, stacking interactions of okanin occur with adsorbed and bound okanin. These findings can explain the experimentally observed collapse of the microgels and the rigidification of the microgels. Based on the atomistic insights, two poly(N vinylcaprolactam co glycidyl methacrylate) microgels were synthesized, for which a doubled loading capacity of okanin was found. Finally, we investigated the release-on-demand of okanin with the addition of green solvents. This work establishes a basis for the further optimization of pVCL-based microgels as a carrier for the delivery of polyphenolic agrochemicals.