Research On The Optical Response Properties Of MnO₂ Hollow Spheres And Their Anti-counterfeiting Application

With the development of society,the number of counterfeit products is increasing,which not only influences property security and social stability,but even endangers people’s health,it is urgent to develop advanced and effective anti-counterfeiting technology.However,traditional anti-counterfeiting strategies such as barcodes,two-dimensional code,and laser holograms have disadvantages such as high cost,difficulty in identification and easy to be copied,which are difficult to satisfy the need.Smart responsive optical materials with abundant,accurate,measurable optical signals,and controllable response mechanisms,have potential application prospects in the field of anti-counterfeiting.Structural color materials,as one of the typical optically responsive materials,with a unique periodic structure.The colors are derived from interactions such as interference,diffraction and scattering that occur between incident light and microstructures,and could change colors by adjusting the refractive index or lattice parameters under the stimulation of external physical or chemical factors such as solvents,magnetic fields,heat and mechanical forces,occupy an important position in the applications fields of anti-counterfeiting and optical encryption.As one of the most easiest,economical and environmentally friendly stimulation sources,water vapor has attracted much attention for its use in stimuli-responsive structural color materials.However,the currently developed water vapor stimuli-responsive structural color materials have difficulty in combining vivid colors with high stability,and the preparation process is complicated and time-consuming.Moreover,the currently stimuli-responsive structural color anti-counterfeiting materials only have a single response mode and couldn’t satisfy the growing demand for anti-counterfeiting in the future.Making to address the above issues,the following studies were carried out in this thesis:（1）Inorganic oxide MnO₂ with high refractive index,wide absorption range and stable properties was used as the research object to designed and prepared inorganic solid-based water vapor stimuli-responsive structural color optical anti-counterfeiting materials with vivid colors and high stability.Hollow MnO₂nanospheres with homogeneous size and mono-disperse were synthesized by the hard template method and surface re-dox method,and assembled into thin films with bright purple,blue,and green structural colors.It is demonstrated that the hollow MnO₂nanospheres film have unique responsiveness to dynamic water vapor.The peak of reflectance spectrum of the film red-shifted 60 nm within 600 ms at a relative humidity of 100%and a flow rate of 3L/min,while the reflectance spectrum did not change significantly under static water vapor at a flow rate of 0 L/min.This responsiveness comes from the formation of the quasi-multilevel pore structure by randomly stacked nanosheets on the surface of hollow MnO₂nanospheres,which could induce rapid capillary condensation of dynamic water vapor under Laplace pressure and thus induce changes in the effective refractive index and structural color of the system.The color-switching process could maintain good stability under 100 cycles.Using this property,encrypted models with hidden patterns were prepared to encrypt and decrypt information under the stimulation of high humidity dynamic water vapor such as the human breath.The results show that the hollow MnO₂nanospheres films could produce a rapid response to dynamic water vapor with high humidity and have potential applications in the fields of anti-counterfeiting and optical encryption.（2）To improve the security of anti-counterfeiting,it is an effective method to construct dual-mode optical anti-counterfeiting materials with multiple optical signals.Therefore,based on the synthesis of the above hollow MnO₂nanospheres,Cs Pb Br₃nanocrystals were confined inside the hollow spheres to prepare Cs Pb Br₃@MnO₂yolk-shell structural composite material,and to construct dynamic water vapor stimuli-responsive structural color-fluorescent dual-mode optical anti-counterfeiting system with correlated response mechanism,fast response rate and high stability.Under the stimulation of dynamic water vapor,the MnO₂shell layer rapidly condenses water vapor and produces the structural color switch.Meanwhile,the enriched water simultaneously induces the structural transformation of luminescent Cs Pb Br₃nanocrystals to non-luminescent Cs Pb₂Br₅within 1 second.When the water evaporates,both the structural color and fluorescence of the composite film could return to the initial state.The process has good stability at 100 cycles.The experimental data and theoretical calculation show that the construction of the yolk-shell structure composite could effectively improve the response rate and reversibility of the water-stimulated fluorescence response compared with a single Cs Pb Br₃,and the structural color change in the bright field combined with the fluorescence change in the dark field to give the system multiple response optical properties,significantly improve the security of anti-counterfeiting.The potential of the Cs Pb Br₃@MnO₂yolk-shell structure composite for advanced optical anti-counterfeiting applications is further confirmed by the fabrication of a"dual-mode and four-channel"signal optical encryption model.