Chitin-silica nanocomposites by self-assembly.

Self-assembly of chemical entities is at the basis of many biological processes and increasingly in materials syntheses. Self-assembled surfactants and block copolymers are widely and successfully used to prepare ordered hybrid and mesoporous materials with outstanding properties. 7–9] A strong interest also rises for energy-saving chemical processes and reactants from renewable resources. 11] Self-assembled biomacromolecules and other biological objects have been used previously as liquid-crystal templates for the formation of mesoporous silica. Herein we present a novel and versatile colloid-based approach for the large-scale synthesis of a new family of hybrid bioorganic–inorganic nanocomposites with an unprecedented control in texture and morphology. This approach combines the self-assembly properties of polysaccharide chitin nanorods, with the flexibility of sol– gel processes involving siloxane oligomers. The resulting optical and mechanical properties of the chitin–silica nanocomposites can be tuned by varying the chitin volume fraction fCHI. Nanorod alignment inside these materials was achieved under moderate magnetic fields (9 T), generating highly oriented textures and providing an alternative method to that developed for surfactant-templated materials. Furthermore, sol–gel chemistry is amenable to a variety of processing techniques such as spray-drying, which allowed us to prepare micrometer-size chitin–silica particles with variable porosity in the calcined replicas. Our approach consists in the formation and processing of a stable suspension containing two colloids, a-chitin nanorods and siloxane precursors, both in a dispersed state. This is a challenging issue owing to differences in stability and reactivity of the colloids. Chitin nanorods purified from shrimp shells (L = 260 80 nm, D = 23 3 nm) are bundles of monocrystals (D = 3.2 0.6 nm) with amino groups at their surface (Figure 1a). They are stably dispersed in water by electrostatic repulsions in slightly acidic conditions. The simple addition of silica precursors would lead to uncontrolled chitin/silica precipitation owing to electrostatic interactions and/or rapid siloxane condensation. To avoid this, we prepared mixed alcoholic suspensions of the chitin nanorods with siloxane oligomers through repeated solvent exchange cycles. The siloxane oligomers were formed by controlled acid-catalyzed hydrolysis with a hydrodynamic diameter of about 4 nm and a degree of siloxane condensation c of 0.75 (Figure 1a). The resulting clear to translucent suspensions (depending on chitin concentration) are stable over a long period of time (several months). Upon evaporation, the colloidal suspension shows a marked increase in viscosity and turbidity, which is accompanied by a gradual rise in optical birefringence. In a standard procedure, the resulting pasty mixture was cast and further dried overnight (348 K), yielding hard bulk materials with condensation degrees c in the expected range of 0.8–0.9. Preliminary compression tests show that the elastic mechanFigure 1. a) Representation of the siloxane oligomers (upper panel), and of the chitin nanorods (core: poly[b-(1! 4)-2-acetamido-2-deoxy-dglucopyranose]) as bundles of monocrystals with amino groups at their surface (lower panel). The colloid sizes were estimated by DLS and TEM. b–d) TEM analysis: b) bulk nanocomposite (fCHI = 0.28); spray-dried microparticles (fCHI = 0.28) before (c) and after (d) calcination (porous replicas).