2. Photonic Structures in Cellulose-Based Materials
Natural structural colors serve as fascinating examples of nature’s ability to achieve functionality by shaping and molding inherently non-functional compounds into precisely defined structures 
. Similar principles of creating functionality through structural design have become fundamental in the field of nanomaterials and nanotechnology over the past 40 years 
. The resulting materials, often referred to as metamaterials, consist of conventional substances such as metals, semiconductors, ceramics, or polymers, yet they exhibit unconventional properties due to their nanostructures. For instance, by synthesizing semiconductors in the form of nanometer-sized spheres or rods, their electronic and optical properties can be dramatically altered. Similarly, transforming a transparent ceramic into a colored coating can be achieved by structuring it with a three-dimensional periodic architecture featuring a lattice spacing of a few hundred nanometers. Alternatively, a polymer film can be converted into a super-hydrophobic, self-cleaning surface by introducing hierarchically organized nano-to-micrometer-sized protrusions and filaments.
In nature, the most brilliant example of blue coloration stems from a helical arrangement of cellulose fibers in tropical fruits such as Pollia condensata
and Margaritaria nobilis 
. Mimicking these natural-colored photonic structures and getting inspiration for material design and sustainable processing using natural materials is an interesting area of research 
and cellulosic materials have gained considerable prominence due to their renewable nature, compatibility with living organisms, and ability to degrade naturally, making them an eco-conscious alternative 
Revol et al. made a surprising finding when they stumbled upon the fact that a solid film could retain the chiral nematic liquid crystalline arrangement observed in a suspension of cellulose nanocrystals 
. The researchers accomplished this by evaporating the water from a cellulose nanocrystal suspension with a concentration of approximately 3.5% by weight, resulting in a film with preserved chiral nematic order. Upon examination using polarized optical microscopy (POM), they observed that the films exhibited birefringence, and their structures were susceptible to disruption by shear forces. Films made from CNCs have unique optical properties, including iridescence and the ability to selectively reflect left circularly polarized light while transmitting right circularly polarized light 
. Similar to a suspension of cellulose nanocrystals, the chiral nematic orientation within the solid films yielded a positive signal when subjected to CD spectroscopy 
. The helical twist of the cellulose nanocrystals consistently displayed a left-handed nature, as confirmed by the positive CD signal for transmitted light and the generation of left-handed circularly polarized light upon reflection.
Films formed through the self-assembly of cellulose nanocrystals often exhibit captivating colors due to the helical arrangements of cholesteric liquid crystals. The orientation and pitch of these structures determine the photonic bandgap, with the ability to modify the pitch allowing for alterations in perceived color. When the helix’s pitch approximated the wavelengths of visible light, the CNC films exhibited striking iridescent structural colors. These chiral nematic CNC films can be considered as one-dimensional photonic crystals and this manipulation relies on Bragg-like reflections that generate various colors 
The precise optical properties that can be developed in CNC films depends on many parameters, which leads to the development of a variety of tunable photonic CNC materials and technologies. The effects of water evaporation (evaporation at different relative humidity) and initial CNC concentration have been investigated by several groups 
and it was possible to obtain CNC films covering most the visible spectrum 
. Later, Tran et al. 
modified the evaporation time of CNC suspensions and obtained CNC films in the suspensions´ slow evaporation resulted in blue-shifted films. Utilizing differential evaporation, CNC films with gradients could be designed. They also discovered that the application of a cellulose acetate mask on top of a drying CNC suspension led to patterns with higher resolutions 
. They also showed that the obtained colored patterns could be tuned, from red to blue, depending on the stage of self-assembly when the masks were applied.
Despite their capacity to generate films with vibrant colors, cellulose nanocrystals (CNCs) possess certain constraints that result in limited productivity. The process of self-assembly is highly susceptible to disruptions and may necessitate an extended period of several days for the complete evaporation of water. Chen et al. 
developed a protocol consisted in a preliminary treatment of CNC suspensions through ultrasonication. They discovered that the duration of ultrasonication, volume of the suspension, and the application of vacuum were determinant in the preparation of iridescent CNC films. The resulting films exhibit striking and vibrant colors, when compared with those obtained with slower water evaporation techniques. Another influent parameter that was investigated was the surface upon which the CNC suspension is cast on 
. Several surfaces were tested (including aluminum, silicon wafers, mica and polystyrene) and different optical properties were obtained, meaning that substrate surface properties, such as wettability and hydrophobicity, influence the self-assembly behavior of the CNCs.
Not only altering CNC suspensions can have impact on the characteristics of chiral nematic CNC films. Also, external factors, including temperature and additives, can have influence on the properties of CNC films. The introduction of energy through methods such as heating or sonication gave rise to CNC films with an enhanced helical pitch, allowing the production of films with adjustable chiral photonic properties 
As previously mentioned, cellulose possesses attractive qualities for optical and photonic applications, thanks to its refractive index, transparency, dielectric properties, and birefringence 
. These combined characteristics enable the development of relevant technologies in the field of photonics. The crystal structure of cellulose plays a crucial role in modulating its optical properties, resulting in vibrant colors and establishing it as a valuable contender for sustainable bio-based optical materials. The self-assembly of cellulose nanocrystals offers a promising and cost-effective approach to producing optical materials on a large scale 
. Given that sustainability and the circular economy are crucial concerns today, several scientists are exploring the potential use of cellulose derived from biomass and waste materials in photonic and its capacity to form chiral nematic structures upon drying, which exhibit fascinating optical and photonic properties 
. Recent investigations in this field have primarily concentrated on understanding the self-assembly dynamics of helicoidal structures and optimizing them to achieve desired polarization responses.
Cellulose nanocrystals in water suspensions behave as lyotropic liquid crystals forming a chiral nematic phase above a critical concentration. It is well known that such an organization can be retained in solid films and give rise to an intense colored appearance. In several studies, researchers have characterized their optical response via optical and scanning electron microscopy, imaging scatterometry, and angle-resolved reflectance measurements 
. Wilts et al. go further by showing that the experimental results can be well explained by computational modeling using the finite-difference time-domain method 
. They performed 3-D finite-difference time-domain (FDTD) calculations, using a commercial-grade Maxwell equation solver, Lumerical FDTD Solutions 8.16, to simulate the polarization-dependent light scattering from a liquid-crystalline, helicoidal stack of cellulose in the wavelength range 350–700 nm. They examined the variation in reflectance with changing angles of a cellulose film formed through self-assembly and their findings demonstrate that the significant disparity in circular polarization is maintained across a wide range of incident light angles. This reflectance behavior can be effectively explained through the use of finite-difference time-domain modeling. Previous research has indicated that the color characteristics of these films can be manipulated by adjusting the self-assembly conditions of CNCs. Together, these studies contribute to a more holistic understanding of the angle-dependent color appearance in helicoidal layers, which holds potential for the development of sustainable colored materials, such as responsive dyes or food colorants.
3. Cellulose-Based Composite Materials with Structural Color
As mentioned before, the self-assembly of CNCs into a chiral nematic structure is tolerant to additives, which has allowed incorporation of additives and enabled the creation of a range of interesting materials, such as thin-films 
, hydrogels 
, and organosilicas 
. Incorporating additives provides a means to fine-tune the optical and mechanical properties of the resulting CNC-based materials. For instance, pure CNC thin films are known for their toughness but lack flexibility, besides by introducing hydroxypropyl cellulose (HPC) or chitosan/chitin into the matrix, the cellulose-based composite material’s flexibility can be significantly improved 
. Previous research in this field has also explored the incorporation of inorganic materials like metallic nanoparticles 
, infiltration of proteins or amino acids 
or the addition of organic units through careful surface modification 
, resulting in materials that exhibit unique chiroptical properties and are capable of changing color under applied pressure 
The optical properties of films are also dependent on surface roughness, as no surface of a biobased material is completely flat and, as shown previously, its surface roughness will directly impact its interaction with light 
. In fact, surface patterning and successful production of highly precise structures in a predeterminate configuration can be employed to create light interactive-structures, where nanoscale and microscale patterns are generated to control diffraction, scattering, or light outcoupling. Various lithographic techniques, involving similar steps but differ in processing and curing specifics, can be applied. Initially, the material to be modified is uniformly distributed across a surface, followed by the application of a mask or mold, and finally, through chemical and/or physical treatments, the modified surface is obtained 
. Several protocols process of obtaining modified surfaces on structural colored cellulosic-based materials using photolithography 
, soft lithography 
, and nanoimprint lithography 
Wolfberger et al. 
described an easy and versatile efficient patterning method for cellulose thin films by means of photolithography and enzymatic digestion. Depending on the conditions of development, either negative and positive type cellulose structures can be obtained, offering lateral resolutions down to the single-digit micro meter range by means of contact photolithography. These photochemically structured cellulose thin films are successfully implemented as dielectric layers in prototype organic thin film transistors.
The research findings of Mihi et al. 
introduced a groundbreaking approach to fabricating photonic crystals and plasmonic structures using a derivative of cellulose through the nanostructuring method known as soft lithography. Through the periodic nanostructuring of the cellulose film, its transparency is effectively eliminated, leading to the emergence of vibrant colors in its reflective properties, contingent upon the specific pattern employed during the molding process. By leveraging this innovative technique, which is both highly scalable and cost-effective, as an alternative to the conventional self-assembly of cellulose nanocrystals, a superior nanostructure is rapidly and reproducibly generated on the polymer substrate 
. This process offers a wide spectrum of iridescent colors solely reliant on the size and morphology of the resultant structures. The resulting photonic crystals can be nanoimprinted onto diverse substrates to confer photonic capabilities on surfaces lacking this characteristic, such as paper. This technology exhibits immense potential as photonic ink and finds practical applications in domains like anti-counterfeiting technology, packaging, decorative paper, labels, and sensors, among others 
. When these structures are coated with a thin layer of metal, they acquire plasmonic properties while retaining their flexibility, thereby intensifying the colors displayed. Additionally, the biodegradability and water solubility of the cellulose derivative can be adjusted based on the specific type employed. These plasmonic structures are ideal for disposable sensors, enabling Raman emission, or enhancing the light emitted by a dye 
In industrial settings, thermal imprinting has been implemented using roll-to-roll processing, which can also be utilized in thermal nanoimprinting lithography. Mäkelä et al. conducted experiments using a laboratory-scale roll-to-roll imprinting system to create cellulose acetate films, CNF films, and TEMPO-CNF films with pillar structures imprinted using a Ni-mold 
. The formation of these structures was heavily influenced by the temperature, speed, and pressure applied during the process. The resulting films exhibited varying levels of surface roughness, leading to different levels of transparency. When white light passed through the microstructures, diffraction colors such as blue, red, and green were observed, showcasing the potential for applications in optics and electronics 
Considering the cost of production and application in lithography, high-throughput techniques like soft lithography and roll-to-roll lithography tend to be more cost-effective, while photo and e-beam lithography are often more expensive due to their limited scalability 
. However, it should be noted that different surface patterning methods present varying restrictions on the achievable feature size of the fabricated surface structures.