A new family of chitin–silica nanocomposites has been synthesized by using a versatile colloid-based combination of self-assembly and sol–gel chemistry. Various textures and morphologies can be obtained by adjusting the evaporation-based processes or by applying external fields. After calcination, textures and birefringence are preserved in the resulting mesoporous silicas.
Nanocomposites can also serve as “molds” for the production of porous substances. These have potential application in the areas of gas storage, catalysis, or materials separation.
For their synthesis, the researchers chose to use a sol–gel process, a popular technique for the production of inorganic network structures. In the first step they needed to generate a sol: a suspension of finely divided nanoscopic particles in a solvent. Their challenge was to obtain co-suspension of the two different components, silicon dioxide precursors (siloxane oligomers) and chitin nanorods from shrimp shells (a renewable resource). However, these two components require different conditions to remain in stable suspensions without uncontrolled precipitation. The researchers produced an alcohol suspension by slowly replacing water with ethanol. Through slow removal of the solvent, a gel formed. Gels are gelatinous substances; they contain solid but loose, cross-linked, three-dimensional polymer structures.
The sol can be “poured” into a desired mold and dried or it can be spray-dried into spherical particles. This process results in a nanocomposite made of chitin rods that are fully embedded in a silicon dioxide matrix. The mechanism by which this occurs is based on a self-organized aggregation of the chitin molecules and weak attractive forces between chitin and siloxane oligomers.
The stability of the alcohol suspensions opens up a wide range of possibilities for the production of materials with controllable volume ratios, spatial arrangements, and morphologies. If a magnetic field is applied during preparation of the material, the chitin rods line up in parallel. If the nanocomposite is heated, the chitin rods can be burned off to
leave behind cavities. This forms a highly porous material with interesting properties.