Nature is filled with countless examples of eye-popping colours that far outshine the often muted tones of pigmentary colour. Many of the most vibrant blues and greens, and even shades of pink and gold, used by plants and animals alike are a result of so-called structural colour. These colours offer many benefits to both flora and fauna, making fruit more enticing to encourage seed distribution, and allowing creatures to attract mates, or even to adaptively camouflage themselves in their environments. By studying how structural colour has arisen in nature, these same techniques and properties can be used as inspiration to develop man-made structures and coatings, which can be brilliantly colourful and unfading, anti-reflective and even stimuli responsive.

How does nature do it?

Structural colour is a general name given to optical phenomena involving the interaction of light with nanostructures in the form of reflection, refraction, and diffraction,  yielding distinct macroscopic colours or optically interesting features such as iridescence.1 Structural colour arises in nature, with particular prevalence among insect species, often in the form of ordered photonic structures, which consist of layers of materials with different refractive indices, which interact with light in the form of constructive interference, acting as photonic crystals.2 Refraction is effectively the bending of light when it passes between two mediums with different refractive indices. The angle with which the light bends is dependent on the wavelength, which is the same phenomenon that allows a prism to separate white light into its constituent colours. Constructive interference is the way in which the phase of oscillating waves can align such that the peaks add together, increasing the intensity of the resultant wave, this allows for selectivity of the resulting colour, based on the spacing in the photonic crystal. These crystals can be grouped according to their translational symmetries, i.e., the number of directions in which they repeat regularly, with 1D photonic crystals e.g., multilayer structures or thin films, such as those found in the elytra of the Japanese jewel beetle, being the most common. The directional dependence of the refracted light from such nanostructures can be mitigated by the introduction of disorder, with the extreme cases allowing for highly effective scattering of white light, as in the case of the white beetle, which has a reflection of white light above 70%.2 Nanostructures, particularly those which are hierarchically ordered, can also allow for selective interaction with polarised light, such as the chiral, helical, cellulose microfibrils which are organised in compartments on the surface of the marble berry pollia condensata, which reflect circularly polarized light.1

Jewel beetle colours and nanostructures.

The brilliant colours of the jewel beetle, and the nanostructures which give rise to them.5

Efforts to mimic structural colour in the lab

Due to the impressive optical capabilities of such structures and the high degree of evolutionarily driven complexity, there is interest in the generation of synthetic analogues of natural systems. A useful technique, in the synthesis of ordered polymeric 3D structures, is two-photon polymerization direct laser writing (TPP-DLW), wherein a near infrared laser with high peak power is focused within a UV-sensitive photoresist, photochemically promoting local reactions which require the absorption of two or more photons.3 Many reactions require the input of external energy to proceed, such as external heat, light, or even an electric current. Using light allows for only reactants in a very small region to have the necessary energy to react and polymerize. The use of cholesteric (i.e., those having long range order) reactive mesogens and other liquid crystals as part of the DLW process, can also allow for the generation of polymeric microstructures with tuned photonic band gaps.3 Another method involved in the controlled generation of nanomaterials is the self-assembly of colloidal particles, such as the transference of pre-assembled ordered colloidal monolayers generated with a liquid template to a solid substrate, layered deposition to produce multilayer architectures, convective assembly techniques in 3D which allow for the formation of structures such as synthetic opals, and the combination of multiple methods to produce desirable biomimetic hierarchical structures.1

HPC: A potential material for synthesis of nanostructures

Hydroxypropyl cellulose (HPC) is a cellulose derivative wherein hydroxypropyl groups are substituted on the glucose units, which can be used to form polymer chains.4 These modifications allow for solubility in water and organic solvents. Furthermore, at concentrations between 50-70 wt %, HPC forms a right-handed chiral nematic (denoting the parallel arrangement of molecules in a liquid crystal) liquid crystal phase. The helical stacking in this liquid crystal, allows for selective reflection of light which can be tuned by exploiting the response of the structure to changes in factors such as temperature, solvent environment, and concentration. Chirality is a property of structures whose mirror image cannot be rotated or repositioned to match the original structure. Have a look at your hands, they are clearly very close to being mirror images of each other, and no matter how you orient them, you cannot make them overlap – in this same way, we refer to structures such as helices as right or left-handed. Photonic gels constructed from HPC and other compounds such as poly(ethylene glycol)s or inorganic additives, are of interest, as intermolecular interactions with these additives may be used to modify and tune helical pitch, thereby changing colour, or to modulate the periodic structure of the helix.4

Possible applications

The generation and control of the nanostructures mentioned above, may allow for the generation of biomimetic surface coatings and layers, which interact with light in interesting ways. Optical switches, long-lasting vibrant colours, adaptive structural colours and even anti-reflective surfaces are all possible. Using columnar structures as found in the eyes of creatures such as moths, it is possible to generate anti-reflective coatings which may help to reduce glare and increase absorption of light, and through the use of polymers which have been functionalized to respond to specific biomarkers, inexpensive patches may be developed which provide easily visual medical alerts to a patient. By leveraging modern materials science and hundreds of millions of years of evolution, biomimetic materials may shape our world in many colourful ways.

References

1             Eric S. A. Goerlitzer, Robin N. Klupp Taylor, and Nicolas Vogel, “Bioinspired Photonic Pigments from Colloidal Self-Assembly,” Advanced Materials 30 (28), 1706654 (2018).

2             Thomas B. H. Schroeder, Jared Houghtaling, Bodo D. Wilts, and Michael Mayer, “It’s Not a Bug, It’s a Feature: Functional Materials in Insects,” Advanced Materials 30 (19), 1705322 (2018).

3             Tiziana Ritacco, Dante M. Aceti, Gianfranco De Domenico, Michele Giocondo, Alfredo Mazzulla, Gabriella Cipparrone, and Pasquale Pagliusi, “Tuning Cholesteric Selective Reflection In Situ Upon Two-Photon Polymerization Enables Structural Multicolor 4D Microfabrication,” Advanced Optical Materials 10 (2), 2101526 (2022).

4             Luyao Huang, Xianzhe Zhang, Lin Deng, Ying Wang, Yongmin Liu, and Hongli Zhu, “Sustainable Cellulose-Derived Organic Photonic Gels with Tunable and Dynamic Structural Color,” ACS Nano 18 (4), 3627-3635 (2024).

5             Edoardo De Tommasi, Emanuela Esposito, Silvia Romano, Alessio Crescitelli, Valentina Di Meo, Vito Mocella, Gianluigi Zito, and Ivo Rendina, “Frontiers of light manipulation in natural, metallic, and dielectric nanostructures,” La Rivista del Nuovo Cimento 44 (2021).

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