Where it comes from
There are several ways to obtain nanocellulose. Still, it all starts with extracting common cellulose from wood. Wood is made up of small cellulose fibers, bound together by a polymer called lignin. Lignin can be removed by chemical means, so that only cellulose is left; normally, the result will look much like a wet towel. Apart from processing the many types of plant matter (from wood to marine flora), cellulose can be obtained from certain bacteria that secrete it.
After we have the cellulose, the next step is to break it down to nanofibrils that are about a thousand times smaller than cellulose fibers. As result, we get hydrogen-bonded chains of cellulose molecules; some of such “clusters” of molecules are separated by non-crystalline regions. If we dissolve them, we obtain the cellulose nanocrystals we've been talking about; usually, this is done with the help of strong acids.
Nanocrystalline cellulose production is nothing new; the material has already been around for about 50 years. Yet, it was only in the past 10 years that researchers focused on studying it more thoroughly, and learned that it can replace color filters (so that they will no longer contain toxic colorants), act as a biocompatible sensor for medical purposes, used as a barrier material, as well as have many other applications.
Optical properties of nanocellulose. Research at SCAMT laboratory
Nanocrystalline cellulose reveals its optical properties if obtained by the means of hydrolysis using a 65% sulfuric acid solution. This way, all of the non-crystalline regions are broken down to glucose, and negatively charged sulfonic groups form on the surface of nanocrystallites. The presence of this charge allows them to self-arrange in spiral structures, much like molecules cholesteric liquid crystals. By changing the spiral’s pitch, scientists can change the nanocrystals diffraction properties, i.e. control its optical properties.
What does this mean? DIffraction is the phenomenon that has to do with different changes in the propagation of light waves. As result of this process, a lightwave can enter a diffraction grid being of one wavelength and leave the grid with another, thus changing the light’s color.
Today, the scientists aspire to learn to change the diffraction grid of a cellulose nanocrystal. So, what are the possible ways of doing that? One is to introduce different electrolyte compounds during the process of obtaining the nanocellulose “spirals”, so that they would get sorbed on the crystals’ surfaces and thus change the distance between the layers of nanocellulose, thus changing the spiral’s pitch.
“Such objects will exhibit chromacity under polarized light. Thanks to that, we will be able to create fundamentally different filters that won’t require chemical pigments for their production. This means they they will be eco-friendly. We will be able to produce them by means of jet printing we apply in our laboratory,” explains Elena Eremeeva, a Master’s student working at ITMO’s SCAMT .
Recently, a research team from McGill University succeeded in controlling the chromacity of nanocrystalline cellulose in visual light. Practically, that allows to use this method to color cellulose items in papermaking industry, as well as any other industry where color sensing by naked eye is of essence.
The optical properties of nanocrystalline cellulose can be used to create new sensors; as of now, the concept of a humidity sensor looks most promising. Nanocrystalline cellulose has a large surface area and great absorption properties. In this particular case, the absorption of liquid will result in changes in the crystal’s structure, which will affect its optical properties, such as changing the wavelength of a reflected wave of polarized light.
Yet, Ms. Eremeeva stresses that there are substances that cannot be detected using nanocrystalline cellulose. Their molecules are too big, and they won’t be able to enter the crystals’ pores and thus change their structure.
Nanocellulose in flexible electronics
Nanocellulose’s properties allow it to be effectively applied in flexible electronics. In this case, cellulose nanofibrils can be used as substrate for electronic components. For instance, one can produce a hybrid material that can be used as an electrode for a superconductor by means of layer-by-layer deposition of gold and oxide nanoparticles on cellulose fibers. Superconductors are used as voltage sources, and may well become an alternative to batteries.
Such substrates have the advantage of being readily biodegradable, i.e. you can dispose of them by simply dissolving them in a solvent. But the problem here is that scientists still have to improve the substrate’s thermal and chemical stability, even when it comes to humidity (water). Development of coatings that can make the substrates hydrophobic can help solve this issue.
Harder than steel
Nanocellulose fibers have the shape of elongated tubes. If we were to merge them together, we would obtain a material harder than steel or aluminium. For instance, scientists from the DESY research center (Germany) have already created a ten-centimeter-long sample of such a nanocellulose thread. In order to do that, they put the nanocellulose fibers in parallel with a stream of salted water. In these conditions, the fibers could form connections, and after drying them, the scientists obtained a sturdy and flexible material. As of now, researchers are working on making this biodegradable compound even more durable so that it can be applied in different industries.
Nanocellulose can also be used as a strengthening agent in various composites. For example, a composite made of calcium carbonate and nanocellulose has properties similar to those of a crustacean carapace. In some composites, nanocellulose becomes transparent, so it might well be used for protective glazing.
Nanocellulose replacing plastic
A research team from Sweden is working on a biodegradable counterpart to the commonly used plastic. If special additives are used to augment nanocellulose fibers with hydrophobic and air-proof properties, it becomes possible to use them for long-term storage of food products and such.
At the University of Wisconsin-Madison (USA), scientists are working on making plastic a better material for the car industry by adding nanocellulose to it. Another research team from Saudi Arabia is developing a nanocellulose-based biodegradable material for bumpers; the cellulose for its production is obtained from the refuse of banana industry.
Nanocellulose in medicine and ecology.
There are already projects in which nanocellulose is used as a scaffold that provides for faster bone regeneration. Also, nanocellulose looks promising as a component of composites for enhanced wound healing. Scientists also consider using this material as a drug deliverer due to its biocompatibility.
There’s also research on using nanocellulose in ecology. A sponge made of nanocellulose-based aerogel is great at absorbing oil, and can be used to clean water bodies from oil, as it does not absorb water.
