Sustainable Agrochemical Delivery Systems Based on Cellulose Nanocrystals and Chitosan

Abstract

Bio-derived materials offer key advantages such as sustainability, biodegradability, non-toxicity, high loading capacity, and tunable physicochemical properties, making them excellent candidates for the design of environmentally friendly agricultural formulations. Surfactants, commonly used additives in pesticide formulations, possess good interfacial control performance. Their combination with bio-derived materials not only offers new insights into the development of sustainable pesticide formulations but also broadens their application with novel functionalities. In this thesis, we focus on the synergistic interactions of the bio-based materials and surfactants in (a) improving pesticide encapsulation efficiency, (b) controlling the behavior of pesticide-containing droplets on hydrophobic plant surfaces, and (c) facilitating the development of water-based and sustainable pesticide formulations.

We developed a strategy to enhance pesticide loading and droplet deposition by mixing small amounts of sodium dodecyl sulfate (SDS) (0.1 wt%) and cationically modified cellulose nanocrystals (PCNC). The reduced surface tension, increased viscosity and adhesion, and electrostatic and hydrogen interactions between SDS/PCNC complexes and plant surfaces resulted in a low retraction velocity, excellent spreading and resistance to air turbulence. The improved loading content was facilitated by the hydrophobic domain of PCNC and SDS micelles. However, such formulations exhibit limited effectiveness on superhydrophobic surfaces. To address this, we developed an advanced pesticide formulation capable of effectively controlling droplet splashing and rebound on superhydrophobic surfaces. CNC modified with tannic acid and copper ions was selected as nanocarriers for hydrophobic pesticides, methylcellulose (MC) served to enhance the viscous dissipation and mechanical integrity of the liquid, and surfactant Aerosol OT (AOT) was indispensable in improving its affinity toward non-wetting surfaces and mitigating capillary forces. The resulting formulation reduced surfactant usage to 0.1% and successfully formed a network structure, offering several advantages. These include excellent wetting capacity on superhydrophobic surfaces, a deposition efficiency of 88.92%, which is 17 times higher than that of water, enhanced resistance to wind and rain erosion, and improved insecticidal efficacy. Notably, this "ideal" pesticide formulation can be stored in a solid form, effectively overcoming the challenges associated with the storage of emulsion-based pesticide formulations.

However, the above systems are derived from fossil fuels, raising concerns about their sustainability and safety. Therefore, we explore a sustainable and effective alternative, where we developed a biosurfactant (QCS) derived from biodegradable, sustainable, and abundantly available chitosan. QCS exhibits excellent capability in modulating surface and interfacial properties and adjusting liquid rheology. These features help suppress droplet splashing and rebound on hydrophobic surfaces, improve pesticide deposition, and increase resistance to environmental erosion. Compared with the pesticide formulation using fossil-based surfactants (SDS and AOT) and CNC, the delivery system composed only of QCS is simpler and can inhibit droplet splash on hydrophobic plant surfaces, including superhydrophobic ones. Moreover, QCS alone could reduce the surface tension to 27.35 mN/m and markedly increase the liquid viscosity, without the need for additional polymers or CNC. It could also enhance deposition efficiency to approximately 62.0%. Beyond foliar sprays, QCS, with its excellent film-forming ability and abundant functional groups, is also suitable for soil-based delivery systems, such as mulch films and hydrogels. Both showed strong water retention capabilities, meeting the water-conservation needs of arid and resource-limited agricultural environments. The broader implications of this work align with key United Nations Sustainable Development Goals. These systems exhibit sustained release profiles, with cumulative release ranging from 90% within 48 h to 42.4% after 500 h. QCS offers diverse and sustainable pesticide delivery options. This work introduces a versatile biosurfactant that not only supports sustainable agriculture but also holds promises for broader applications in surfactant-reliant fields such as detergents, drug delivery, and biomedical formulations.