Synthesis, characterisation, and antifungal activity of salicylic acid-grafted chitosan and its nanoparticles against Fusarium solani and Fusarium proliferatum

Abstract

Chitosan (CS) possesses versatile biological activities, but its applicability is limited by poor water solubility. This study developed water-soluble salicylic acid-grafted CS (SA-g-CS) derivatives and their nanoparticles (SANP) and evaluated their physicochemical properties and antifungal activity. SA was grafted onto CS at different SA-to-CS mass ratios via free-radical grafting, followed by the formation of SANP from the selected SA-g-CS through ionic gelation with sodium tripolyphosphate. The grafting of SA onto CS was characterised using Fourier transform infrared (FTIR), ultraviolet-visible (UV-Vis), and proton nuclear magnetic resonance (1H NMR) spectroscopy. The grafted SA content ranged from 102.0 to 324.1 mg/g, with SA-g-CS (0.75:1) exhibiting the highest grafting efficiency (43.2%) and grafted SA content (324.1 ± 7.5 mg/g). SA grafting also improved the water solubility of CS. Antifungal activity increased with concentration, and at 5.0 mg/mL, SA-g-CS (0.75:1) achieved 75.5% and 68.8% inhibition against Fusarium solani and Fusarium proliferatum, respectively. SANP prepared from SA-g-CS (0.75:1) exhibited an average size of 145.9 nm, a polydispersity index (PDI) of 0.125. and a ζ-potential of +16.4 mV. Despite its smaller particle size, SANP exhibited lower antifungal activity than SA-g-CS, which was associated with the reduction in ζ-potential from +34.7 to +16.4 mV following nanoparticle formation. These findings demonstrate that SA grafting improves the water solubility and antifungal performance of CS, whereas subsequent nanoparticle formation does not necessarily enhance antifungal activity, highlighting the importance of surface properties in the design of SA-g-CS-based antifungal materials.