Solvent-free ionic gelation of carvacrol-loaded chitosan nanoparticles: fractional factorial optimization and pH-responsive release

Abstract Carvacrol-loaded chitosan nanoparticles (CCNPs) were prepared by oil-in-water emulsification followed by a fully aqueous, solvent-free ionic gelation. A two-level fractional factorial design (25−1) was used to model particle size, zeta potential, and encapsulation efficiency (EE), and a multi-response desirability function identified the optimum. The validated formulation showed a hydrodynamic size of 386.20 ± 50.42 nm, a zeta potential of 30.52 ± 0.67 mV, an EE of 57.93 ± 0.42 %, and a polydispersity index (PDI) of 0.392 ± 0.01 (overall desirability = 0.934), yielding a colloidally stable, water-dispersible nanosuspension. Carvacrol release over 72 h was pH-dependent and faster in acetate buffer (pH 3.0) than in phosphate-buffered saline (PBS, pH 7.4). Release profiles were best described by first-order and Higuchi models. Consistently, the time to 50 % DPPH inhibition (t50) was shorter at pH 3.0 than at pH 7.4 (39.8 h vs 45.9 h). Dose–response assays indicated a lower half-maximal inhibitory concentration (IC50) for CCNPs than for free carvacrol (0.40 vs 0.55 mg mL−1), whereas blank nanoparticles showed negligible scavenging (IC50 = 132.5 mg mL−1). In disc diffusion tests against foodborne pathogens, CCNPs produced larger inhibition zones than blank chitosan nanoparticles. Overall, a positively charged, stable, water-dispersible nanosystem with sustained, pH-responsive release and preserved antioxidant and antibacterial activities was obtained via a green, solvent-free process.