*ACS Nano*: Magnetoresponsive Nanocellulose Hydrogels: A Novel Strategy for Dynamic Structural Control and Optical Encoding  

In nature, the iridescent colors of butterfly wings and metallic luster of fish scales originate from the interference and diffraction of light by periodically arranged microstructures, a phenomenon known as "structural color." Although significant progress has been made in synthesizing structural color materials, achieving dynamic control remains challenging.

Recently, a collaborative study by Prof. Fan Yimin (Nanjing Forestry University), Prof. Orlando J. Rojas (University of British Columbia), and Dr. Lu Yi, published in *ACS Nano* under the title "Magnetoresponsive Cellulose Nanofiber Hydrogels: Dynamic Structuring, Selective Light Transmission, and Information Encoding," developed a magnetic field-responsive hydrogel system by combining magnetic nanoparticles with oxidized nanocellulose. This system enables real-time modulation of light reflection behavior and fixation of microstructures through gelation, providing new approaches for optical information encoding and encryption. Prof. Fan Yimin, Prof. Orlando J. Rojas, and Dr. Lu Yi served as co-corresponding authors, with Dr. Xu Junhua (Nanjing Forestry University) as the first author.  

一. Preparation of Nanocellulose and Loading of Magnetic Nanoparticles

Nanocellulose (CNF), a nanoscale fibrous material extracted from plant cell walls, exhibits high specific surface area, strength, and biodegradability. This study employed TEMPO oxidation to prepare oxidized nanocellulose (TOCN): under alkaline conditions, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) selectively oxidizes primary hydroxyl groups on cellulose surfaces to carboxyl groups (-COOH), imparting a negative surface charge (charge density ~0.96 mmol/g).  

To achieve dynamic control under magnetic fields, the team loaded magnetic Fe₃O₄ nanoparticles (MNPs) onto TOCN via co-precipitation. Specific steps included:  

1. Ion adsorption: Fe²⁺ and Fe³⁺ were added to the TOCN dispersion at a 1:2 molar ratio, with carboxyl groups binding to metal ions via coordination.  

2. In-situ precipitation: Ammonia solution was added dropwise to pH >10, inducing co-precipitation of Fe²⁺/Fe³⁺ as magnetic nanoparticles on TOCN surfaces, thereby magnetically modifying the nanofibers.  

3. Stability verification: XPS and FTIR confirmed the absence of covalent bonds between MNPs and TOCN, with binding primarily relying on electrostatic and coordination interactions (Fig. 1).  

Fig. 1: Preparation of magnetoresponsive M-TOCN and surface loading mechanism.

Under a magnetic field, MNP-loaded TOCN (M-TOCN) exhibited rapid orientation response (response time <1 s). SEM and SAXS analyses confirmed that magnetically modified nanofibers could achieve highly ordered alignment along the external magnetic field (orientation rate: 82.5±2.5%). This alignment significantly influenced optical behavior: when fibers were parallel to the sample surface, light reflectivity was highest; when vertically aligned, reflectivity decreased by over 60% (Fig. 2).

  Fig. 2: M-TOCN fiber alignment states under different magnetic fields and corresponding reflectance spectra.  

二. Gelation Fixation of Dynamic Structures 

Magnetic field-induced fiber orientation is reversible, requiring pH-responsive gelation for structural fixation. In M-TOCN, carboxyl groups are fully ionized under alkaline conditions (pH >8), maintaining a sol state; when pH drops below 8, protonation weakens electrostatic repulsion, enabling fiber crosslinking via hydrogen bonds and van der Waals forces to form hydrogels.  

This study innovatively employed acetic acid vapor diffusion to control gelation:  

1. Sol-state modulation: Initial dispersion pH = 9 ensured fiber fluidity.  

2. Gradient gelation: Samples were exposed to acetic acid vapor, which diffused from surface to interior, creating a pH gradient (surface pH reached the gelation point first).  

3. Simultaneous magnetic locking: During gelation, an applied directional magnetic field "froze" fiber orientation within the gel network (Fig. 3). 

 Fig. 3: Gradient gelation induced by acetic acid vapor and magnetic field-assisted fiber alignment fixation.  

三. Optical Encoding and Encryption Applications 

Through stepwise gelation and magnetic programming, M-TOCN-based hydrogels functioned as "optical canvases" for three functional applications:  

1. Angle-sensitive patterns: Molds covering the gel surface allowed region-specific magnetic fields to create patterns whose brightness changed with viewing angle. For example, traffic signs could gradually brighten as vehicles approached, providing distance cues (Fig. 4).  

Fig. 4: Dynamic brightness changes in patterns with viewing angle.  

2. Multilayer information encoding: Leveraging spatiotemporal gradients of acetic acid diffusion, fibers with different orientations were sequentially fixed, forming barcode-like stripes of bright/dark regions. Binary encoding (bright=1, dark=0) converted text (e.g., "NFU," "UBC") into optical signals (Fig. 5).  

Fig. 5: Binary-encoded stripes from layered gelation and their decoded information.  

3. Polarization encryption: Embedding multiple fiber layers with distinct alignment directions allowed hidden messages to be revealed only under specific polarizer angles (e.g., 45° displayed "NFU," -45° displayed "UBC," while other angles showed interference patterns). This "optical password lock" holds great potential for anti-counterfeiting (Fig. 6).  

Fig. 6: Polarization response of multi-oriented fiber layers and encrypted information retrieval.  

四. Future Prospects: From Lab to Life

This technology opens new avenues for smart materials:  

- Dynamic displays: Low-energy magnetic screens or smart windows.  

- Information storage: Multi-encrypted optical labels for pharmaceuticals or luxury goods authentication.  

- Biomedicine: Optical sensing for wound healing monitoring, leveraging biocompatibility.  

Conclusion  

By integrating the biomimetic properties of nanocellulose with magnetic responsiveness, this study achieved dynamic control of microstructures and precise design of optical functionalities. This breakthrough not only deepens understanding of structural color materials but also pioneers new pathways for multifunctional smart materials. As co-corresponding author Prof. Orlando J. Rojas noted, *"We are combining nature's structural wisdom with human precision to truly 'animate' materials."* In the near future, such "dancing" nanocellulose hydrogels may appear in smartphone screens, bank cards, or even bandages—demonstrating that the most sophisticated technologies often emerge from observing and mimicking nature.

From butterfly wings to magnetically controlled gels, this "dance of light and shadow" showcases the allure of materials science and the boundless possibilities of tomorrow's smart world.  

Paper link：https://doi.org/10.1021/acsnano.4c18542