Oleic acid-grafted chitosan polymeric micellar aggregates for coumarin-6 encapsulation

This study reports the synthesis and characterization of oleic acid-grafted chitosan (CS-g-OA), an amphiphilic graft designed for the encapsulation of hydrophobic compounds in polymeric micellar aggregates. The grafted chitosan derivative was synthesized via DCC-mediated amide coupling between chitosan (nominal molecular weight = 137 kDa and deacetylation degree (DD) = 87%) and oleic acid, yielding a degree of substitution (DS) of 8.2% by proton nuclear magnetic resonance (¹H NMR). Covalent grafting was confirmed by complementary Fourier-transform infrared spectroscopy (FTIR), Ultraviolet-visible spectroscopy (UV-Vis), and ¹H NMR data validated against a physical mixture control. CS-g-OA self-assembled in aqueous medium at a critical micellar concentration (CMC) of 574.8 μg/mL. Dynamic light scattering (DLS) revealed hydrodynamic diameters of 335.5 ± 78 nm (polydispersity index (PDI) = 0.285) and 491 ± 7 nm (PDI = 0.200) for empty and coumarin-6 (C6)-loaded aggregates, with Zeta potentials of +20.5 ± 0.458 mV and +38.5 ± 1.1 mV, respectively. SEM confirmed spherical morphology with dry-state diameters of 238 ± 6.05 nm and 310 ± 19.45 nm. C6 was encapsulated as a hydrophobic fluorescent model compound with an encapsulation efficiency (%EE) of 29%, quantified by UV-Vis spectrophotometry (R² = 0.9994), and fluorescence microscopy confirmed its compartmentalization within discrete micellar entities. These results provide systematic structural and colloidal characterization of CS-g-OA aggregates based on intermediate-to-high molecular weight chitosan, a design space overlooked in the literature, and identify DS and MW optimization as the primary direction for improving encapsulation performance.