Biomimetic CaCO3-chitosan hybrid pigments inspired by sea urchin spines biomineralization: a photostable colored additive for UV-protective self-supported chitosan films

To reduce the use of toxic and polluting synthetic pigments and dyes, which are still common in industry, marine biomimicry has inspired the development of functional hybrid materials to enhance the photoresistance of natural pigments. This is a key factor limiting their industrial use. Herein, we propose to develop a new type of photostable hybrid pigment, inspired by the biomineralization process of calcium carbonate (CaCO₃), which traps natural organic dyes within sea urchin spines. Alizarin (polyhydroxyanthraquinone, AZ) was selected to mimic organic chromophores present in sea urchins, i.e. polyhydroxynaphthoquinone. The biomacromolecules chitosan (CS) and α-poly-l-lysine (PLL) were used to imitate the organic matter involved in CaCO₃ biomineralization in sea urchins, thanks to their abundance of amine groups (-NH₂). Using a straightforward coprecipitation method under gentle conditions, it was shown that AZ dye molecules became embedded in a CaCO₃ shell, thereby shielding them from photodegradation. It has been demonstrated that chitosan effectively aids in the coprecipitation process by stabilizing the vaterite polymorph, while α-poly-l-lysine has little impact on CaCO₃ formation. To extend the potential of these hybrid pigments for applications such as UV-protective films for food packaging, self-supporting CaCO₃/chitosan (CaCO₃/CS) composite films and tinted hybrid pigment/chitosan (CaCO₃-AZ/CS) films were developed, leveraging the excellent film-forming properties of chitosan. Different quantities of CaCO₃(-AZ) particles were incorporated into the formulations to tailor the films' mechanical and optical characteristics.