Schematic of the novel nanofiber-based aerogels. Credit: Tobias Benselfelt.
Schematic of the novel nanofiber-based aerogels. Credit: Tobias Benselfelt.

Lightweight multifunctional alternatives made from natural resources could mitigate some of the waste problems associated with non-biodegradable materials. To this end, researchers at KTH Royal Institute of Technology and Lund University in Sweden have developed a novel and sustainable approach to the preparation of cellulose nanofiber-based aerogels, which could be useful in a range of applications [Rostami et al., Materials Today (2021), ].

“We have been inspired by nature's method of designing hierarchical complex biomaterials using biodegradation routes,” says first author Jowan Rostami of KTH. “Plants provide us with intriguing building blocks such as lignin, hemicelluloses, and mechanically robust cellulose nanofibrils (CNFs) to develop advanced materials for niche applications.”

The team have developed a way of preparing bio-based, low-density nanofibrous materials using a straightforward ice-templating, solvent exchange, and air-drying process, which is cheap, quick and free-from toxic chemicals. Initially, CNFs are combined with small amounts of alginate and calcium carbonate (CaCO3) particles to form a gel. An ice-templating process forces the nanofibrils together, subsequently creating a porous structure. The frozen hydrogel then undergoes solvent exchange with an acetic acid solution containing acetone. The acid dissolves the CaCO3 particles, leading to increased mesoporosity and the release of Ca2+ ions, while water exchanges with the acetone. Some of the ions act as crosslinkers between the CNFs and alginate to produce a more robust wet-stable aerogel, while the rest are lost as carbon dioxide. The resulting materials are wet-resilient with very low density and high compressive modulus. They can, moreover, be combined with metal-organic frameworks (MOFs) to create structures with a very high specific surface area that can be readily biofunctionalized. The researchers demonstrate the potential of the new material for biosensing using a commonly used protein binding assay.

“The proof-of-concept colorimetric protein binding assay carried out in a direct enzyme-linked immunosorbent assay (ELISA) format shows the potential of the cellulosic aerogel as a new three-dimensional bioassay platform for point-of-care testing,” explains Rostami.

As well as biosensing, the researchers believe that the novel aerogels could also be used for controlled release of therapeutic agents in new medical treatments and diagnostics, as well as for water purification, separation, or catalysis.

“We are convinced that this work will drive a trend in the field towards the utilization of natural resources for aerogel preparation, and their subsequent biofunctionalization with MOFs will pioneer high value applications for these bio-based materials,” says Rostami.

The researchers are now working on fine tuning the properties of the bio-based aerogels, as well as the structural and chemical characteristics of the MOFs, to expand the usefulness of the current system for other purposes, such as fire retardancy and energy storage applications.