Multiple Optical Responses And Anti-counterfeiting Application Based On Uniaxial Orientation Structure Of Cellulose Nanocrystals

Cellulose nanocrystals（CNCs）are one-dimensional rod-like nanoparticles extracted from biomass resources,such as wood,sea squirt,and bacteria.CNCs present various excellent properties,like high aspect ratio,specific strength,crystallinity,biodegradability,and renewability,making them a viable substitute for non-renewable rigid nanomaterials.Especially,owing to their asymmetric dimensional effects and surficial steric structures,CNCs can self-assemble into chiral nematic liquid crystals or with uniaxial orientation structures.Such self-assembly materials show extraordinary optical traits of circular polarization,structural colors,and photoluminescence,exhibiting great potential in the fields of optical anti-counterfeiting,sensing,polarization,display,and optoelectronics.With further modulation of the uniaxial-orientation assembled structure,the photoluminescence induced by the crystallization of cellulose can be enhanced to the commercial level.The photoluminescence also improves the security level of information encryption based on the homologous multiple luminescence of cellulose.The photoluminescence of cellulose originates from the coupling and splitting of electronic energy levels of adjacent oxygen atoms in its crystalline structure.However,the surficial,interior,and interparticle hydrogen bonding networks of CNCs are relatively stable and simplex,which makes it challenging to control the coupling and splitting degrees of the electronic energy levels.This results in a narrow color gamut of assembly/crystal-induced photoluminescence of CNCs,making it easily identifiable and duplicable.Meanwhile,the hydrogen bonding networks of the CNC-based assembly materials are prone to collapse under the influence of external forces or solvents,which may result in the loss of encrypted information.These limitations hinder the practical applications of CNC-based uniaxial orientation materials in the fields of information encryption and optical anti-counterfeiting.This study aims to widen the photoluminescent color gamut of CNC uniaxial-orientation assembled materials and improve their structural and information encryption stability.The approach is based on the simulation and experimental testing of the photonic bandgap of CNC uniaxial orientation arrays.Then,the matter-transformation methods of the optical materials of uniaxially oriented CNC assembly are explored.The mechanisms of the multiple luminescence and spontaneous emission enhancement are further explained,and the photoluminescent characteristics of the CNC uniaxial orientation materials are then controlled.The color gamut of homologous and structurally similar photoluminescent materials derived from CNC uniaxial assembly materials is thus expanded,and we even achieve excitation-dependent long afterglow luminescence.To address array stability and long-range ordering issues,a method of oligoclonal element micro-nano-scale assembly of CNCs in a non-homogeneous suspension system is developed.A surface coating strategy is further applied to improve the stability of micro-nano assembly arrays under force and alkaline environments.The potential of the excitation-dependent long afterglow luminescent materials and micro-nano assembly materials in multi-level optical information encryption and optical anti-counterfeiting coatings is verified.The specific research contents and results are as follows:（1）Finite element analysis is used to understand the photonic bandgap of CNC uniaxial orientation assembly materials and their inverted opal structure.Then,a series of CNC/silica co-assembly membranes are prepared using the vertical assembly method of CNCs and the sol-gel method of silica.The equivalence of the precursors is controlled to regulate the orderliness of CNC uniaxial orientation in the co-assembly membranes.Rod-like CNC particles in assembly membranes display oriented structures.Meanwhile,the average angle of assembly membranes increases from 5.52°to 8.64°along with the increase in silica precursor.The results show that the increase in silica precursor does not significantly affect the assembly behavior of CNCs,but slightly reduces the orderliness of the CNC assembly array.The matter-transformation strategy is employed to convert CNCs into carbon dots（CDs）in-situ in the silica matrix.The pore structure of the resulting inverted opal structure material replicates CNC uniaxial orientation arrays.The CDs exhibit up-converted luminescence and excitation-dependent long afterglow luminescence,producing blue and cyan long afterglow after the irradiation of 254 nm and365 nm,respectively.The lifetime of blue long afterglow increases with the improvement in assembly orderliness,whose lifetime reaches as long as 1.5 s.The lifetime gradually increases with the decrease in temperature,reaching 2.030 s,indicating that the long afterglow originates from the triplet state emission of CDs.The inverted opal structure of silica protects the triplet-state emission of CDs,and the presence of multiple triplet excited states in CDs results in the dependence of luminescence on excitation.Additionally,the overtone of phosphorescence emission at 420 nm of CDs is located at the edge of the lowest frequency band of CNC uniaxial orientation arrays.Compared with pure CD,The PL intensity of the inverted opal materials shows an improvement of 5times.Meanwhile,its lifetime improves to twice when compared with the inverted opal materials whose uniaxial arrays are destroyed.The results show that the slow-photon effect that occurs at the bandgap edge enhances the CDs’spontaneous emission and lifetime of long afterglow.However,this effect only occurs when the bandgap edge matches with the emission band.The phosphorescence emission of CDs at 480 nm does not show a significant enhancement because it is far away from the bandgap edge.（2）For the brittleness of materials above mentioned and structural defects in long-range ordered assemblies,the micro-nano-scale assembly mechanism of CNCs in the crystalline porous framework of ZIF-8 is investigated.The result shows that the homogeneous nucleation and growth of ZIF-8 benefit the micro-nano-scale assembly behavior of CNCs,which results in an improvement of 36.7%for PL intensity and an emission quantum yield of 60.0%.However,heterogeneous nucleation results in significant defects and weaker enhancement in photoluminescence intensity,whose improvement is only 23.6%.The main driving force of CNC micro-nano-scale assembly is further explored,demonstrating that the electrostatic interactions between CNCs and ZIF-8 are the dominant force for CNC assembly in the micro-environment constructed by ZIF-8.Depending solely on the hydrogen bonds between CNCs and ZIF-8 leads to the fact that a large amount of CNC particles remain free outside the ZIF-8 framework.Subsequently,various shell structures such as silica,titanium dioxide,and polydopamine are coated on the surface of ZIF-8@CNC using sol-gel methods or polymerization method via interfacial interactions between precursors and ZIF-8@CNC.The results indicate that the titanium dioxide shell significantly improves the photoluminescence stability of ZIF-8@CNC in an alkaline environment,whose PL intensity only decreases by 8%after being processed for 24h.Meanwhile,the titanium dioxide shell increases the Young’s modulus of ZIF-8@CNC to twice,reaching 2058 MPa.Furthermore,the derivative optical properties of the coated micro-nano-scale assembly materials are studied,via a thermal transformation strategy.After the thermal process,silica@ZIF-8@CNC exhibits excitation-dependent long afterglow,with phosphorescence emission at445 nm and 506 nm,and lifetime of 0.370 s and 0.253 s,respectively.However,Ti O₂@ZIF-8@CNC only displays weak photoluminescence after the thermal treatment.（3）The excitation-dependent long afterglow materials and micro-nano assembly materials prepared in（1）and（2）are applied in multiple information encryption and optical anti-counterfeiting.Combining luminescent materials with different time dependence and excitation dependence,a multiple information encryption label is prepared.Information is able to be hidden in misleading information,allowing for step-by-step information decryption under different optical environments including natural light,UV light,and long afterglow luminescence.In addition,luminescent inks are prepared by mixing micro-nano-scale assembly materials with a resin matrix,and their applications in optical anti-counterfeiting and personalized identification are verified using screen printing techniques.The results show that the layers printed with the luminescent ink are able to be identified easily under UV light.In summary,this study develops the single-structure multi-color luminescence strategy based on the CNC uniaxial orientation materials,and establishes a theoretical basis for the construction,spontaneous emission enhancement,and regulation of those materials.A new micro-nano-scale assembly method of CNC and corresponding surface coating methods are proposed.The applications of those CNC uniaxial orientation materials in multi-level information encryption labels and luminescent ink are further verified.