Assembly‐Induced Emission of Cellulose Nanocrystals for Hiding Information

Abstract

Fluorescent‐labeled cellulose nanocrystal (CNC) films have been used to record and protect information in paper materials, whereas the fluorescent materials usually suffer photobleaching. Herein, a strategy of solid‐state emission induced by the vertical assembly of CNCs was established. The assembly‐induced emission starts from the structural diffraction of CNC, whose wavelength is adjusted into the ultraviolet (UV) region for hiding information under natural light. The small diameter (≈10 nm) of CNCs then promotes the resonance between the vertically assembled CNCs and the UV light, leading to a strong blue emission with an emitting quantum efficiency as high as 13.90%. By introducing the vertical‐assembly film with a specific pattern into paper materials, an anti‐counterfeiting image is obtained under a UV radiation. Since CNCs are a kind of cellulose with high crystallinity, this material can be a wear‐resistant anti‐counterfeiting material for banknotes or other paper applications.

Reductive Amination Reaction for the Functionalization of Cellulose Nanocrystals

“…However, it was demonstrated by Arndt and Stevens [ 45 ], using UV-circular dichroism, that mono- and di-saccharides absorb only between 150 and 190 nm. A final interpretation was that by Gan et al [ 46 ], who hypothesized that the emission recorded from cellulose nanocrystals was the consequence of a Stokes scattering [ 47 , 48 ] enhanced by CNC-oriented assembly [ 49 ]. In our case, it is very difficult to understand which can be the origin of the luminescence observed in CNCs, as the FTIR spectra of our pristine CNCs also did not reveal the presence of aromatic structures.…”

Surface modification of cellulose nanocrystals towards new materials development

“…With the cooperation of multi‐field competing driving forces of chiral assembly, CNCs can realize achiral assembly by evaporation‐induced self‐assembly. The uniaxial assembly film shows a strong blue emission 42 . Degree of uniaxial orientation is related to the virtual transition emission and luminescent intensity.…”

Structural Color Materials for Optical Anticounterfeiting

“…[ 42–45 ] The chiral nematic structures of the CNC aqueous suspensions can remain after the solvent evaporation process, resulting in a freestanding structurally colored CNC film, whose stop bands could be tuned by ionic strength and the initial concentrations of the suspensions, external magnetic, and electric fields, etc. In addition, the CNC film, as a mesoporous template, has been utilized to fabricate many composites and porous materials of inverse structures, [ 46–48 ] to achieve polarization‐dependent encryption which will be further discussed in Section 3.3.…”