Green and Functional Aerogels by Macromolecular and Textural Engineering of Chitosan Microspheres

Tremendous interest was recently devoted to the preparation of porous and functional materials through sustainable route, including primarily the use of renewable biopolymers instead of petroleum-sourced synthetic chemicals. Among the biopolymers available in enormous quantity, chitosan - obtained by deacetylation of chitin - stands as the sole nitrogen-containing cationic amino-sugar carbohydrate. This distinctively provides chitosan derivatives with plenty of opportunities in materials science. Particularly, its pH switchable solubility allowed the preparation of three-dimensional entangled nanofibrillated self-standing microspheres. These porous hydrogels behave as nano-reactors to confine exogenous nanoobjects within the polysaccharide network, including sol-gel metal alkoxide species, organometallic derivatives and isotropic and oriented nanoparticles. Besides, the interfacial interplay of chitosan with lamellar clay and graphene oxide allowed the penetration of the biopolymer inside of the galleries, which result in a complete delamination of the layered nanomaterials. The preserved gelation memory of chitosan in these formulations provides a way to access porous microspheres entangling exfoliated nanometric sheets. CO₂ supercritical drying of functional hydrogel beads enabled efficient removal of water and other liquid solvents without wall collapsing, allowing large-scale preparation of millimetric hydrocolloidal microspheres with an open macroporous network. These functionalized lightweight biopolymer aerogels find applications in heterogeneous catalysis, sensing, adsorption, insulation and for the design of other sophisticated porous nanostructures. Beyond their tailorable molecular and textural-engineering, the possibility for macroscopic shaping of these intriguing nanostructures opens many new opportunities, especially in additive-manufacturing for soft and hybrid robotics.