Particle size, surface charge, and leaf microtopography govern rainfastness of polymeric nano- and microparticles for agrochemical delivery.

Abstract

Polymeric nano- and microcarriers have emerged as promising platforms for foliar agrochemical delivery. However, insufficient adhesion and rainfastness on leaf surfaces remain major obstacles to their efficient application, and the influence of particle properties on carrier retention is still poorly understood. This study investigates the impact of particle size, surface chemistry, and surface charge of polymeric nano- and microparticles on their adhesion and rainfastness on grapevine leaves and correlates these particle properties with the leaf microstructure by a combination of confocal laser scanning microscopy (CLSM) and atomic force microscopy (AFM). We show that surface chemistry and surface charge strongly determine the strength of the interaction between nanoparticles and the leaf surface. Model polystyrene (PS) particles and biodegradable lignin nanocarriers with amino-functionalized and positively charged surfaces retained more than 80% of the fluorescence signal after simulated rainfall, while negatively charged carboxyl (-COOH) functionalized PS-COOH showed low retention with 10 ± 2% remaining after rain simulation. The lower adhesion and less homogeneous distribution of microparticles compared to nanoparticles highlight that the interplay between particle size and leaf microstructure governs foliar adhesion. CLSM and AFM revealed that the characteristic cuticular striations of grapevine leaves confine nanoscale particles within the microscale surface grooves. Importantly, biodegradable lignin nanocarriers wetted the leaves and retained ca. 60% of the fluorescence signal even without silicone-based superspreaders, which did not further improve their rainfastness, highlighting their potential as sustainable, microplastic-free foliar delivery systems. From a colloid and interface science perspective, these findings provide design principles for next-generation foliar spray formulations with improved rainfastness and reduced agrochemical losses, and they point to the opportunity of adapting particle interfaces and dimensions to the surface architectures of different crop species.

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