With the deep integration of materials science and optical engineering, nanocellulose has become the core substrate for building high-performance structural color materials by virtue of its natural nanoscale, excellent self-assembly properties and green sustainable attributes, bringing revolutionary breakthroughs to the fields of anti-counterfeiting, optical films, flexible displays and other fields. As a natural polymer material derived from biomass such as wood, cotton, and straw, nanocellulose not only inherits the environmental protection genes of biomass, but also solves the industry pain points of traditional chemical dyes that are easy to fade, highly polluted, and difficult to control with its unique microstructure.

1. Nanocellulose : natural green nanomaterial

Nanocellulose is a nanoscale cellulose material prepared from natural biomass through physical, chemical or biological methods. It is mainly divided into two categories: cellulose nanocrystals ( CNC) and cellulose nanofibers ( CNF). Its core characteristics lay the foundation for structural color construction:

· Nanoscale and high aspect ratio : CNC has a diameter of about 5-50nm and a length of 100-500nm. ，CNF has a diameter of 20-200nm and a length of up to micron level. The high aspect ratio gives it excellent film-forming and structure-building capabilities.

· High crystallinity and mechanical properties : The crystallinity can reach 70%-90% , the Young's modulus is 3 more than twice that of steel, the CNF film tensile strength reaches 223MPa , and the performance can be bent thousands of times without obvious loss, ensuring the stability and durability of the structural color film.

· Surface activity and self-assembly ability : The surface is rich in hydroxyl groups and can easily form a hydrogen bond network. It can spontaneously form a chiral nematic liquid crystal structure at a specific concentration and ionic strength , which is the core prerequisite for its structural color production.

· Green and sustainable : The raw materials are renewable, biodegradable, non-toxic and residue-free, in line with the ' double carbon ' goal and the development trend of green materials.

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2. Structural Color: Optical Principles and Construction Mechanism of Nanocellulose

Structural color is a color completely produced by the selective scattering, interference and diffraction of light by the microstructure of the material. It is essentially different from the molecular absorption and color development of traditional dyes/pigments. It has four core advantages : non-fading, high stability, controllable, environmentally friendly and non-toxic . The brilliant colors of peacock feathers, butterfly wings, and shell nacre in nature are all typical structural color phenomena.

1. Color development mechanism of nanocellulose structural color

Nanocellulose (especially CNC) aqueous suspension can self-assemble to form a periodic spiral layered chiral nematic structure under evaporation induction. When the helical pitch (period) of this structure matches the wavelength of visible light (400-700nm), structural color is generated through Bragg reflection .

· The reflection wavelength (color) is determined by the pitch size: the smaller the pitch, the shorter the wavelength of the reflected light (blue-violet); the larger the pitch, the longer the wavelength (red-orange).

· Color purity and brightness are determined by the degree of structural orientation: the higher the degree of orientation, the brighter the color and the stronger the gloss.

· Color stability is determined by structural periodic stability: The hydrogen bond network of nanocellulose can lock the microstructure, prevent structural collapse caused by drying and humidity changes, and ensure long-term color stability.

2. Dual core functions of nanocellulose

In the structural color system, nanocellulose plays the dual role of structural building unit and stable reinforcing skeleton :

· Structural building unit : Self-assembled to form a highly ordered, period-controllable photonic crystal structure, which accurately controls the reflection and interference of light to achieve a full spectrum of structural colors from blue to red.

· Stable and reinforced skeleton : The nanofiber network locks the microperiodic structure through hydrogen bonding, significantly improving the mechanical strength, moisture resistance and thermal stability of the structural color film, and solving the problems of traditional structural color materials being prone to instability and poor durability.

3. Core advantages of nanocellulose structural color

Compared with traditional chemical coloring and inorganic structural color materials, nanocellulose structural colors have irreplaceable comprehensive advantages:

1. Ultimate color stability : Color originates from physical structure rather than chemical substances. It never fades, is light-resistant, is anti-aging , and can withstand daily friction and environmental changes for a long time.

2. Full-spectrum precise control : By controlling parameters such as CNC concentration, ionic strength, and drying temperature, the pitch size can be precisely controlled to achieve full-spectrum structural colors of blue, green, yellow, and red , and the color purity can reach more than 90% .

3. Unique optical effect : It has an angle-dependent iridescent effect , showing dynamic colors with radiant colors as the viewing angle changes, and also has a metallic luster. It is a visual effect that traditional dyes cannot achieve.

4. Green, safe and environmentally friendly : completely bio-based, degradable, non-toxic and without chemical residues, the production process has low energy consumption and no pollution, and meets the needs of environmental protection and sustainable development.

5. Excellent processing adaptability : It can be prepared into various forms such as films, coatings, and micro patterns. It is compatible with flexible substrates and is suitable for roll-to-roll large-scale production with controllable costs.

4. Core application scenarios: Empowering industrial upgrading with structural color

With its unique optical properties and green attributes, nanocellulose structural colors have achieved breakthrough applications in high-end anti-counterfeiting, optical films, flexible displays, intelligent sensing, high-end decoration and other fields, among which the anti-counterfeiting field is the core implementation scenario.

1. High-end anti-counterfeiting: 'optical fingerprint' that cannot be copied

Nanocellulose structural color film has three major anti-counterfeiting properties: angle-dependent dynamic iridescence, non-replicable microstructure, and invisible embedding for easy identification , making it an ideal material for high-end anti-counterfeiting.

· Document anti-counterfeiting : embedded in hidden locations such as ID cards, passports, visa pages, etc., transparent and invisible under natural light, showing an exclusive iridescent texture when illuminated by a light source at a specific angle. Unique patterns can be customized to prevent counterfeiting and copying.

· Bill and currency anti-counterfeiting : Used for anti-counterfeiting marks on checks, money orders, and high-end currencies. The colors change dynamically and are difficult to print and scan, significantly improving the anti-counterfeiting level.

· Luxury goods and high-end packaging : used in luxury packaging boxes, labels, and logos . Dynamic iridescence enhances brand recognition while preventing counterfeiting. It is suitable for scenes such as beauty, alcohol, and high-end clothing.