Chitosan-based gating microcapsules with thermo-pH dual stimuli-responsive for controlled nutrient release

Stimuli-responsive fertilizers show great potential for precision nutrient delivery but are often limited by single responsiveness and unsynchronized release with plant growth demands. This study developed a novel polysaccharide-based thermo-pH dual stimuli-responsive gating microcapsule (CSMC-g-PNM) via atom transfer radical polymerization (ATRP). N-isopropylacrylamide (NIPAM) and methacrylic acid (MAA) were sequentially grafted into the chitosan microcapsules (CSMC), with thermo-sensitive poly(N-isopropylacrylamide) (PNIPAM) acting as a macromolecular initiator for the subsequent grafting of MAA by reactivating dormant chain ends. Independent chain lengths of PNIPAM and PMAA were achieved by controlling grafting time, endowing the microcapsules with reversible and dual-responsive gating properties. The PNIPAM and PMAA chains acted as dual-responsive gating mechanisms, extending below the low critical solution temperature (LCST) (25 °C) or above pKa (pH 8), closing the gates to slow release. Conversely, when the temperature exceeds 40 °C or is below the pKa (pH 3), the chains contract, opening the gates to accelerate release. CSMC-g-PNM demonstrated sustained urea release (cumulative release 80.79% over 168 h), following First-order kinetics and Korsmeyer-Peppas diffusion-erosion mechanisms. Biodegradation studies affirmed the system’s environmental compatibility, showing a controlled weight loss of up to 19.74% after 35 days in soil. Furthermore, pot experiments demonstrated significant agricultural efficacy, with maize seedlings treated with CSMC-g-PNM-6 in acidic soil achieving a plant height of 44.1 cm, significantly outperforming the 33.1 cm observed with unmodified microcapsules. This work provides a robust and sustainable strategy for designing intelligent nutrient delivery systems, advancing the development of solutions for precision agriculture.