Optical Property Regulation And Anti-Counterfeiting Application Of Cellulose Nanocrystalline Chiral Liquid Crystal Films

Cellulose nanocrystals（CNCs）are natural nanomaterials with short rod-shaped,which are mainly prepared by acid hydrolysis using natural cellulose,such as plants,animals or bacteria as raw materials.The CNCs suspension can form CNCs chiral nematic liquid films via evaporation induced self-assembly（EISA）processes,which endow them have potential applications in sensing,catalysis,anti-counterfeiting and pigment due to their the properties of photonic crystals,the excellent circular polarization ability and birefringence effects.However,it is still a great challenge to control its optical properties and improve its mechanical properties in practical applications.Herein,CNCs suspension was obtained by a method of sulfuric acid hydrolysis using absorbent cotton as raw material and formed CNCs chiral（cholesteric）liquid crystal films with tuable structure based on ultrasonic technology.Subsequently,the mechanical properties of chiral liquid crystal films were improved via introducing polyols or small molecular organic citric acid（CA）into CNCs suspensions.Additionally,a one-pot method was proposed to prepare Na Gd F4:Yb³⁺,Er³⁺@CDs oil-soluble multimode luminescent nanoparticles.The flexible CNCs/CA chiral liquid crystal luminescent nanoparticles prepared above were modified by coating method to obtain flexible and water-resistant multimode CNCs/CA/Na Gd F4:Yb³⁺,Er³⁺@CDs chiral liquid crystal composite film.Moreover,the application in anti-counterfeiting was explored.Specific research contents are as follows:The optical performance of CNCs chiral liquid crystal films were regulated by ultrasonic crushing technology,and the influence of low power and high-power ultrasonic treatment duration on the optical properties of CNCs chiral liquid crystal films were investigated.Atomic force microscopy（AFM）,scanning electron microscopy（SEM）,Zeta potential,ultraviolet visible spectrophotometer（UV-vis）and polarization microscopy（POM）were utilized to analyze the influence of ultrasonic condition on the morphology,size,surface charge density,the structure and properties of CNCs chiral liquid crystal films.The regulation mechanism of optical properties of CNCs chiral liquid films were elucidated during the process of self-assembly.The results showed that low power ultrasonic treatment did not change the morphology and size of CNCs.With the increase of ultrasonic treatment duration,the dispersion and stability of CNCs suspensions were improved,and the reflection wavelength of were redshifted,while the redshift gradient was not obvious.High power ultrasonic treatment could not only improve the dispersion and stability of CNCs suspensions,but also changed the morphology and size of CNCs.With the increase of ultrasonic power,CNCs changed from aggregates to nanorod bundles with a certain degree of twist.The pitch of self-assembled CNCs chiral liquid crystal film increased,and the reflection wavelength was redshifted.More importantly,the redshift gradient was more obvious.Subsequently,liquid crystal fragments with iridescence were prepared using the brittleness of chiral films,which showed excellent anti-counterfeiting,decorative,and coating properties,as well as good environmental protection and degradation properties.Polyols such as polyethylene glycol（PEG）,polyvinyl alcohol（PVA）,glycerol（Gly）and organic small molecule citric acid（CA）were introduced as plasticizers to improve the brittleness and toughness CNCs chiral liquid crystal films.The effects of different amounts of polyol addition and different polyol combinations addition on the optical and mechanical properties of CNCs self-assembled liquid crystal films were investigated in detail.The results showed that the addition of single PEG,PVA and Gly could change the pitch of the CNCs chiral liquid crystal films,causing the emission wavelength redshift,and thus realized the change of the structure color.However,the flexibility of the prepared hybrid film was not good.The addition of different polyols in combination had a greater effect on the pitch of CNCs liquid crystal films,and the structure color changes were more obvious,while there were still problems of poor flexibility.Different from polyols,the addition of CA could not only regulate the structural color of CNCs chiral liquid crystal films,but also improved flexibility.The flexible CNCs/CA chiral liquid crystal composite films obtained by the co-assembly of CNCs and CA can realize its advanced anti-counterfeiting application.Hydrophobic Na Gd F4:Yb³⁺,Er³⁺@carbon dots（CDs）nanomaterials with dual-mode luminescence were prepared by a one-step thermal decomposition method using oleic acid as a chelating agent and octadecene as a solvent,which was then combined with the flexible CNCs/CA composite film prepared in the previous chapter by coating technology.CNCs/CA/Na Gd F4:Yb³⁺,Er³⁺@CDs chiral liquid crystal composite films with flexible and water-resistant multi-mode luminescence were obtained.Furthermore,a flexible,water-resistant anti-counterfeiting label with multimode fluorescence,showing up-conversion luminescence,down-conversion luminescence and structural color,was designed and prepared.The results showed that the prepared dual-mode Na Gd F4:Yb³⁺,Er³⁺@CDs nanocrystals with spherical morphology had good dispersion and stability in non-polar solvents,and their average particle size is 14.5±0.2 nm.Under excitation of 365 nm ultraviolet light and 980 nm laser light,they could emit visible blue down-conversion fluorescence and green up-conversion fluorescence,respectively.After compounding with CNCs/CA flexible chiral liquid crystal film by coating technology,its flexibility and transparency remained unchanged,while its water-resistance was improved.The composite film had excellent fluorescence stability for Na Gd F4:Yb³⁺,Er³⁺up-conversion luminescence or down-conversion luminescence of CDs.Its excellent multimode luminescence performance and high luminescence stability enable the composite to achieve three levels of luminescence anti-counterfeiting.