Copper-Cross-Linked Alginate-Based Hydrogel Nanoparticles for Sustainable Slow-Release Fertilizer Applications

Controlled-release materials are desirable for many delivery applications and have been used to improve the efficiency of fertilizers and pesticides in crop management. Due to their potential to reduce application of toxic chemicals while prolonging exposure to active agents, controlled-release nanomaterials are currently being investigated for increasing agricultural production and preventing overfertilization. Hydrogels are underexplored as controlled-release nanomaterials and can deliver many types of cargo, from metal ions to small molecules. Alginate-based hydrogels are biocompatible and their internal carboxylic acids coordinate agriculturally valuable micronutrients like Cu²⁺, Zn²⁺, and Ca²⁺. Hydrogels comprising ionic and nonionic polymers can coordinate agriculturally valuable micronutrients, and the combination of ionic and nonionic polymers results in hydrogels with tunable release profiles. Alginate, for example, contains carboxylates that ionically cross-link with divalent cations like Cu²⁺, Zn²⁺, and Ca²⁺, while polar moieties on chitin enable nonionic coordination. To our knowledge, soft-material copper-loaded nanoparticles have not yet been applied as controlled-release materials for foliar delivery. In this work, we present the synthesis and micronutrient release characteristics of hydrogel nanoparticles containing Cu²⁺, which is coordinated by ionic and nonionic polymers. Hydrogel nanoparticles (HNPs) were prepared by liquid-liquid emulsion techniques and cross-linked with Cu²⁺ to form double-network hydrogels made from alginate and non-cross-linking chitin. Nanoparticles (100-300 nm in diameter) were characterized by cryogenic electron microscopy, nanoparticle tracking analysis, Fourier transform infrared spectroscopy, and thermogravimetric analysis. The copper release profiles of HNPs with different polymer compositions were compared. HNPs containing both chitin and alginate released 8-20 times more copper than HNPs with alginate alone, suggesting that the presence of non-cross-linking polymers improves copper release. Thus, HNP delivery characteristics can be tuned by manipulating intraparticle bond dynamics in the hydrogel polymer matrix.