As materials science accelerates its evolution towards ' green, high performance, intelligent response ' , functional nanomaterials from natural sources have attracted widespread attention. Among them, nanocellulose crystals ( CNC ) are becoming the key support structure for sustainable optical material technology due to their rigidity, high crystallinity and liquid crystal self-assembly capabilities, especially in application scenarios such as photonic crystals, self-response display, anti-counterfeiting identification and intelligent perception, showing cutting-edge breakthrough potential.

1. The optical structure foundation and functional principle of CNC

1. Chiral nematic phase liquid crystal behavior driven by self-assembly

CNC can spontaneously form a chiral nematic liquid crystal phase ( Chiral Nematic Liquid Crystal Phase ) in a certain concentration, pH and ionic strength environment . The basic mechanism is that nanorod-shaped crystals are stacked in a spiral manner, deflecting layer by layer at an angle, and finally forming a periodic rotating structure.

This spiral period ( pitch ) determines the wavelength of reflected light. According to the Bragg equation λ=nPcos⁡θlambda = nPcos heta , the structural color forming different bands can be adjusted.；

The helical structure is fixed during the drying process, and the solid CNC film is a kind of **' solid-state photonic crystal '。

2. Photon bandgap regulation capability

The one-dimensional photonic crystal structure formed by CNC self-assembly has the function of selective light reflection. Its reflection bandwidth is narrow, its reflectivity is high , and it does not depend on dye or pigment absorption . Therefore, it can be used as an environmentally friendly, physically unreplicable structural color material 。
and its reflection band can be accurately controlled through the following path:

parameter	How to influence
CNC concentration	Determine the critical point and spiral period of liquid crystal formation
Surface functional groups	Change the interface charge density and the repulsion between particles
Add ions	Shield charge, improve alignment density, and shorten screw pitch
Temperature /humidity	Regulating optical response through changes in the dielectric environment

This programmable reflective characteristic of light makes CNC a preferred unit for building dynamic optical materials.

2. Key application areas of CNC in optical materials

1. Structural color anti-counterfeiting and information encoding materials

The uniqueness of CNC structural color comes from its self-assembled micro-nano cycle structure , which is non-printed, non-dyeed, and is physically unreplicable, and is suitable for high-end bills, anti-counterfeiting labels, and brand logos.

Multi-color gradient anti-counterfeiting pattern can be formed through concentration /pH gradient；

It can be combined with QR code and photolithography mask to prepare an optically readable encoded film layer。

2. Wet /thermal/chemically responsive optical sensor

The CNC- based structure color film can make color responses to environmental parameters ( RH 、T 、VOC ) and build an intelligent visual sensing platform:

Visual infection monitoring in medical dressings (such as changes in humidity lead to color from green to blue);

Freshness perceived labels in food packaging ( color changes to gas and pH );

Signs in wearable electrons perceive skin membrane layer (based on sweat humidity or local temperature response).

3. Low refractive index optical coating and flexible photonic crystal

CNC has a low refractive index ( ~1.53 ) and high transparency ( >90% light transmittance), and is suitable as a light guide layer, anti-reflective layer, adjustable grating and other applications:

Combining TiO₂ 、SiO₂ nanoparticles or graphene to form bifunctional hybrid photonic crystals；

Build flexible display devices, curlable reflective mirror surfaces, micro laser reflection chambers, etc. CNC also has regulatory potential in the near-infrared (NIR) band and has entered the field of infrared stealth materials and light-thermal conversion films .

3. Key technical challenges and research frontiers

Although CNCs show high potential in optical materials, the commercial application of their optical properties still faces the following key issues:

Key Bottleneck	Technical Difficulties
Self-assembly consistency	Defects and distortions of liquid crystal arrangement during process amplification
Optical stability	Structural color attenuates or drifts under humid and heat /ultraviolet conditions
Structural complexity	Achieve multi-color, multi-layer, and multi-function integration still depends on high-cost nanoprocessing methods
Process compatibility	Limited adaptability to existing lithography /printing/roll-to-roll production systems

Current research trends include:

Monitor the CNC self-assembly process in situ (such as polarized microscopy + laser confocal);

Nanopatterned interface guides CNC orientation arrangement ( microcontact printing/inkjet patterning );

Coordinate with metal nanoparticles or semiconductor nanomaterialsthe multiphysical responsive photonic metamaterials 。

4. Conclusion and future prospects

The optical function of nanocellulose crystals ( CNCs ) stems from their natural structural accuracy and liquid crystal physical behavior, which organically fuses By crossing the intersection of multiple disciplines such as material design, interface regulation, self-assembly control and micro-nano processing, ' green materials ' with ' photon engineering ' . CNC is gradually moving from basic research to a new generation of sustainable optical materials platform。

In the future, CNC will form large-scale applications in the following fields:

Structural color inks replace traditional pigments;

Smart vision sensors are used in medical wearable devices;

Light guide layer flexible optoelectronic devices ;/ reflection regulation layer in

The basic module for displaying dynamic multi-band photon structureAR/VR functions by 。

Driven by the dual driving of 'plastic-free optics' and 'functional degradable materials', the CNC optical material system will release huge technological and commercial potential in the fields of industrial manufacturing, intelligent packaging, advanced display, precision identification, etc.