(Nanowerk Spotlight) Nanophotonic materials that manipulate light through optical interference have long enticed researchers with their potential for vibrant structural coloration, angle-dependent effects, and external stimulus-responsiveness. Researchers have proposed promising applications in the areas of security labels, sensing, and anticounterfeiting. However, researchers have struggled to scale and cost-effectively produce high-quality nanophotonic films, an enduring challenge.
Cellulose nanocrystals (CNCs) are an emerging sustainable photonic material composed of elongated cellulose nanoparticles derived from plants. When dispersed in water, CNCs have the unique ability to self-assemble into a chiral nematic phase, which is a type of liquid crystal. This self-assembly results in the formation of a structure that reflects light, producing vivid structural colors. This property makes CNCs of particular interest in various applications, including in the field of photonic materials, where they are being explored for their potential in sustainable and biodegradable optical devices. But widespread application requires developing industrial-scale deposition methods, an enduring challenge.
Inkjet printing is one scalable approach that has garnered attention. It is already widely used in graphics and labeling, enabling precise spatial control to deposit micro-scale droplets in an efficient, high-throughput manner compatible with roll-to-roll printing presses. However, inkjet printing relies on rapid drying, often seconds or minutes per drop. This requirement is fundamentally incompatible with the longer minutes-to-hours timescale needed for CNCs to slowly self-assemble into the highly ordered ‘helicoidal’ arrangement that produces vivid structural coloration after solvent evaporation.
Researchers at the University of Cambridge have now reported an innovative method for scalable inkjet printing of self-assembling cellulose photonic films. Their key innovation is printing aqueous CNC suspensions through a layer of immiscible oil. The oil encapsulates the droplets, drastically slowing water loss to enable sufficient time for internal nanostructure formation.
a) Photograph and b) schematic of the printing process; whereby: i) actuation of a solenoid within the translatable printhead dispenses a drop of aqueous CNC suspension, ii) the drops pass through a layer of dodecane oil and wet onto a glass substrate, iii) within the pinned drop the cholesteric domains merge and align to the substrate surface, iv) subsequent loss of water (through the oil layer) results in a photonic microfilm. The color reflected by each microfilm is principally determined by the initial formulation of the dispensed drop, but is also dependent on the angle and polarization of illumination and/or detection. For scale, the Petri dish in a) has a diameter of 9 cm. c,d) Printed “R G B” text using red, green, and blue inks. The text was printed using nominal drop volumes: c) 100 nL with a 2.4 mm inter-drop spacing and d) 10 nL with a 1.2 mm spacing, such that the font size is consistent. The white dashed circles indicate the microfilms reported in the high magnification images. These were collected in epi-illumination and through either a left- or right-circular polarized filter (LCP and RCP, respectively), confirming polarization-selective reflection. (Reprinted with permission by Wiley-VCH Verlag)
Varying the CNC concentration or adding polymer dopants allows tuning the final color of each printed microdot across the whole visible spectrum. This enables multicolored patterns, complex images, and even full color prints by combining dots of different formulations. Unlike previous small-scale casting, this inkjet approach allows on-demand control over the color and location of each 0.4-1.2 mm dot during a single automated print run.
Notably, while the vivid colors stem from the baked-in nanostructure, post-treatment allows realizing additional optical effects. For example, washing with solvents or UV light exposure degrades certain additives, revealing ‘hidden’ images encrypted into the array. The strong vertical alignment of rod-like CNCs also leads to polarization effects that could help differentiate genuine labels.
This unique effect originates from the fact that the appearance of these structural colors is not intrinsically linked to particle size, but rather to the self-assembly process, which is independent of the nanoparticle dimensions. This permits using a single CNC stock solution to achieve the full color spectrum through formulation alone. It also bestows a unique set of interacting optical effects beyond iridescence and polarization alone.
The Cambridge researchers demonstrate printing complex patterns like entire pictures at a resolution comparable to commercial dot matrix displays. However, current limitations include slight droplet misalignments when printing high droplet rates through thick oil layers. Regardless, the study illustrates pathways to fully exploit many unique advantages of self-assembled cellulosic photonic films.
It also tackles a major production challenge that has hindered past attempts to scale these materials. The oil-encapsulation concept helps reconcile the disparity between the rapid drying needed for printing and slow self-assembly timescales. Translating this to solid encapsulants like polymer varnishes could enable handling prints immediately, facilitating incorporation into high-throughput label printing.
The potential applications are extensive. The vibrant colors produced purely from cellulose could expand sustainability. Films could help prevent counterfeiting of luxury goods or pharmaceuticals through unique visual effects like encrypted images, polarization-selectivity, and humidity-induced color changes. Even niche uses like anticounterfeit labels on products otherwise unlikely to justify holograms may now be economically viable.
However, real-world commercial translation remains ongoing work. Researchers need to optimize printable ink rheology and develop varnishes with suitable humidity control and permeation rates. Nanostructuring kinetics may require acceleration for competitive industrial processes. Nonetheless, the unique advantages self-assembling photonic nanoparticle arrays confer over pigments or other approaches suggest this new scalable production concept could ripple across various technologies that leverage light for sensing, encryption, and human-machine interfaces.
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