Fucoidan–Chitosan Polyelectrolyte Complex as a Marine-Derived Colloidal Carrier Platform for Photoprotective Agents

Brown algae are a valuable source of bioactive secondary metabolites, particularly polyphenols and sulfated polysaccharides with photoprotective and antioxidant activities. Among them, fucoidan stands out for its biocompatibility, biodegradability, and demonstrated photoprotective effects, mainly through antioxidant and anti-photoaging properties, making it a promising natural component for UV-protective formulations. This study developed polyelectrolyte complex sub-micron particles based on fucoidan and chitosan (F/Cs) to encapsulate quercetin (Q) as a natural UV-active antioxidant. Fucoidan from Sargassum filipendula was extracted and fractionated by ultrafiltration. An RCBD was used to optimize pH and F/Cs mass ratio. The optimal blank formulation (F/Cs = 1:1, pH 5.0) yielded sub-micron colloidal carriers with a mean hydrodynamic diameter of 421 ± 23 nm (PDI 0.252 ± 0.059) with ζ = +43.5 ± 1.6 mV. Quercetin-loaded particles (F/Cs/Q = 1:1:0.5) presented 915 ± 87 nm (PDI 0.278 ± 0.093) and ζ = +54.6 ± 1.2 mV. UV–Vis spectra evidenced UVB and partial UVA absorption for fucoidan and broad UVA/UVB coverage for quercetin, preserved upon encapsulation. Antioxidant activity was retained post-encapsulation (EC50, DPPH: 0.094 mg/mL; ABTS: 0.0749 mg/mL). These results demonstrate the potential of fucoidan–chitosan colloidal systems as multifunctional, biodegradable carriers for natural photoprotective agents, supporting their application in next-generation dermatological and cosmeceutical formulations.