Biodegradable and pH-Responsive Nanoparticles Designed for Site-Specific Delivery in Agriculture

While pH-responsive materials have been extensively studied in the realm of biological and medicinal fields, less attention has been given to the use of pH-responsive materials within agricultural science. Responsive nanoparticles have significant potential as vehicles for the delivery of pesticides and nutrients to plants, due to the higher pH of the delivery site (phloem) compared to its surrounding environment. While the most well-known and studied pH-responsive polymers (PAA, PMAA, PDMAEMA, etc.) contain non-degradable polymethylene backbones, biodegradability is particularly important for delivery to plants. The construction of a nanoparticle suitable for delivery to plants thus becomes more complicated—the particle must be biodegradable, responsive to basic pH, and capable of encapsulating hydrophobic, and ideally, hydrophilic compounds as well.

Polysuccinimide (PSI) was chosen as a starting material due to the biodegradable and hydrophilic nature of its derivatives, namely poly(aspartic acid) (PASP) and poly(hydroxyethyl aspartimide) (PHEA). Nanoparticles of assorted size and functionalization were prepared via the nanoprecipitation technique, incorporating either hexylamine (HA) or 2-(2-aminoethoxy)ethanol (2AEE) nucleophiles. Nanoparticle size distributions were approximated via dynamic light scattering (DLS), and size trends were explored by modifying functionalization and organic phase dilution. It was seen that dilute organic phase solutions led to small average hydrodynamic radii following nanoprecipitation, while degree of functionalization held little influence. Because nanoparticle size depended on dilution rather than functionalization, we were able to tune the pH responsiveness of the nanoparticles while retaining the required diameter of 30 nm or less. To demonstrate their potential as controlled-release delivery vehicles, functionalized nanoparticles were loaded with a model hydrophobic molecule and subsequently exposed to buffer solutions of varied pH. It was found that above pH 7, PSI-nanoparticles hydrolyzed and released encapsulated materials. The release rate significantly increased with increasing pH, and decreased with increasing degree of functionalization. Finally, plant toxicity studies suggested that the polymer materials exhibit little to no toxic effects at biologically relevant concentrations.