Chitosan hydrogelation with a phenothiazine based aldehyde: a synthetic approach toward highly luminescent biomaterials

Pure organic luminescent hydrogels were synthesised using the condensation reaction of amino groups of chitosan with a photoactive aldehyde bearing a phenothiazine moiety. The hydrogels were structurally and supramolecularly characterized by FTIR and H-1-NMR spectroscopy, X-ray diffraction and polarized light microscopy. It was concluded that hydrogelation occurred due to the self-ordering of the chitosan segments grafted with pendant phenothiazine imines into ordered clusters, which play the role of cross-linking nodes. Scanning electron microscopy revealed microporous morphology with very thin pore walls. The ordered clusters immobilized in the polycationic chitosan network were responsible for high quantum efficiency, reaching the value of 51% in the xerogel state. The hydrogels formed flexible, transparent films with high mechanical toughness. Besides the remarkable optical and mechanical properties, the importance of the hydrogels is increased by their eco-design, based on chitosan from renewable resources and bio-friendly phenothiazine.