Cellulose Nanocrystal Self-Assembly
Overview of CNC Self-Assembly
Research Topic of the Department
Some of the most vibrant and dazzling colours in the natural world arise from photonic structures, such as the helicoidal arrangement of cellulose fibres in the fruits of Pollia condensata and Margaritaria nobilis. These natural examples inspire us to explore ways to use biopolymers as a sustainable source of colouration.
Cellulose is the most abundant biopolymer on Earth, as it is produced by virtually all plants, as well as many other organisms. Native cellulose is found in the form of semi-crystalline fibres, which provide mechanical strength to the plant cell wall. The crystalline cellulose can be extracted from biomass (e.g. wood pulp, cotton) by acid hydrolysis, which results in rod-like nanoparticles known as cellulose nanocrystals (CNCs).
Individual CNCs are less than a micron long - too small to be seen by the human eye, even under an optical microscope - but if CNCs are brought together at high enough concentrations, their collective behaviour can produce photonic structures that exhibit dazzling colour. This spontaneous formation of a highly ordered structure from simple components is known as self-assembly.
To initiate the self-assembly process, CNCs are first suspended in water at a low concentration and left to dry. As the water evaporates, the CNC concentration increases. At high enough concentration, the rod-shaped CNCs begin to assemble together to form a configuration known as a cholesteric lyotropic liquid crystal phase. While the CNCs are locally aligned with each other, the overall configuration is a left-handed helix-like structure with some pitch (periodicity). If the water is allowed to evaporate entirely, a solid film of cellulose is produced that preserves the cholesteric structure. Light interference within this periodic structure leads to reflection of certain wavelengths (colours) and not others, depending on the pitch: a film with a larger pitch reflects longer-wavelength light, so the colour is red-shifted, while a film with a smaller pitch reflects shorter-wavelength light, causing a blue-shift.
Using CNCs, we can therefore create colourful films, droplets and pigment particles from a sustainable feedstock. A more detailed technical overview to CNC self-assembly can be found in our comprehensive review article published in ACS Chemical Reviews https://doi.org/10.1021/acs.chemrev.2c00836
Outstanding Challenges
Greener Production of CNCs
While naturally-derived materials like CNCs are inherently sustainable, the conventional methods used to produce CNCs consume large quantities of strong acids and water. We are therefore exploring alternative methods to produce CNCs that minimise consumption of reagents, solvents and energy.
Standardised characterisation of CNC morphology
It is important to accurately characterise the key morphology properties of CNCs, such as their length and width, which depend on the chosen cellulose source and production method. We are developing protocols for rapid high-throughput characterisation of CNC morphology to speed up this process.
Manipulation of CNC surface chemistry
The interactions between CNCs in water and other solvents is highly dependent on the chemical groups on the nanoparticle surface. By exploring new surface functionalisations for CNCs beyond conventional sulfate and carboxylate groups, we aim to understand the role of surface chemistry in CNC self-assembly and develop novel functional materials.
Understanding Cellulose Assembly in the Plant Cell Wall and the Origin of Interactions between Cellulose and Hemicelluloses
The metallic colouration seen in the fruits of Pollia condensata and Margaritaria nobilis arises from the interactions between cellulose fibres and non-cellulosic cell wall components such as hemicellulose. We use atomistic simulations, alongside experimental research, to investigate the origins of these interactions and how they can contribute to the helicoidal arrangement of cellulose fibres in the plant cell wall.