Photoresponsive, ionically cross-linked chitosan-azobenzene capsules for controlled cargo release

Photoresponsive capsules capable of controlled cargo release under mild conditions are of growing interest in soft matter and colloidal systems. In this study, we report the fabrication of visible light responsive, ionically cross-linked chitosan microcapsules using a tetra-ortho-fluoro azobenzene dicarboxylate cross-linker: disodium (E)-4,4'-(diazene-1,2-diyl)bis(3,5-difluorobenzoate) (NaFDAB) and chitosan. Chitosan was selected as a biocompatible and biodegradable shell material, and NaFDAB was added as a cross-linker to provide stability via strong ionic interactions with protonated chitosan amines, while also providing visible-light photoresponsiveness to the capsules. Capsules were prepared via a simple oil-in-water emulsion method, yielding spherical capsules with mechanically stable shells as confirmed by optical microscopy, field emission scanning electron microscopy (FE-SEM), and attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. UV-Vis spectroscopy demonstrated reversible trans-cis photoisomerisation of NaFDAB under green and blue light illumination. Nile Red as a model cargo was used to explore controlled release and monitored using fluorescence spectroscopy. Release occurred only under green-light irradiation, while minimal release was observed under ambient light conditions. Encapsulation efficiencies of approximately 45-50% were achieved. While controlled release of up to ∼28% was observed under green-light irradiation, pulsed irradiation experiments revealed controllable on-off release behaviour, consistent with diffusion-controlled transport governed by light-modulated shell permeability. These findings demonstrate a mild, effective and scalable strategy for photo-controlled cargo delivery using visible light.