ConspectusPolysaccharides are a class
of biopolymers that are widely exploited
in living organisms for a diversity of applications, ranging from
structural reinforcement to energy storage. Among the numerous types
of polysaccharides found in the natural world, cellulose is the most
abundant and widespread, as it is found in virtually all plants. Cellulose
is typically organized into nanoscale crystalline fibrils within the
cell wall to give structural integrity to plant tissue. However, in
several species, such fibrils are organized into helicoidal nanostructures
with a periodicity comparable to visible light (i.e., in the range
250–450 nm), resulting in structural coloration. As such, when
taking bioinspiration as a design principle, it is clear that helicoidal
cellulose architectures are a promising approach to developing sustainable
photonic materials.Different forms of cellulose-derived materials
have been shown
to produce structural color by exploiting self-assembly processes.
For example, crystalline nanoparticles of cellulose can be extracted
from natural sources, such as cotton or wood, by strong acid hydrolysis.
Such “cellulose nanocrystals” (CNCs) have been shown
to form colloidal suspensions in water that can spontaneously self-organize
into a cholesteric liquid crystal phase, mimicking the natural helicoidal
architecture. Upon drying, this nanoscale ordering can be retained
into the solid state, enabling the specific reflection of visible
light. Using this approach, colors from across the entire visible
spectrum can be produced, alongside striking visual effects such as
iridescence or a metallic shine. Similarly, polymeric cellulose derivatives
can also organize into a cholesteric liquid crystal. In particular,
edible hydroxypropyl cellulose (HPC) is known to produce colorful
mesophases at high concentrations in water (ca. 60–70 wt %).
This solution state behavior allows for interesting visual effects
such as mechanochromism (enabling its use in low-cost colorimetric
pressure or strain sensors), while trapping the structure into the
solid state enables the production of structurally colored films,
particles and 3D printed objects.In this article, we summarize
the state-of-the-art for CNC and
HPC-based photonic materials, encompassing the underlying self-assembly
processes, strategies to design their photonic response, and current
approaches to translate this burgeoning green technology toward commercial
application in a wide range of sectors, from packaging to cosmetics
and food. This overview is supported by a summary of the analytical
techniques required to characterize these photonic materials and approaches
to model their optical response. Finally, we present several unresolved
scientific questions and outstanding technical challenges that the
wider community should seek to address to develop these sustainable
photonic materials.