Preparation Of Photochromic Materials Based On SiO₂@SP Complexes And Its Anti-Counterfeiting Applications

The problem of counterfeiting has been increasingly stressed in the rapid development of the age.An ideal anti-counterfeiting technology is safe,practical and reliable,and can protect the safety of information.Photochromic materials are a class of materials being able to change color under the irradiation of light,which are rapidly developing anti-counterfeiting.Spiropyran is a photochromic material with the most commercial prospect because of its multi-stimulus responses,high color contrast,strong stability,reversible photochromic property and simple recognition suitable for public security.Spiropyran molecules exhibit distinct absorption spectral changes under ultraviolet radiation,as they experience structural isomerization from the unconjugated SP form to conjugated merocyanine（MC）form.In fact,the primary challenge in the solid-state photo-switching of SP molecules concerns tight molecular packing in aggregated molecules.Such tight molecular packing significantly impedes the photoisomerization,which occurs during photochemical reactions between the SP and MC forms.Consequently,providing a large free volume in the solid state is crucial to facilitate efficient photo-switching.In this paper,the high specific surface area and large pore volume of porous silica nanoparticles are used to provide freedom needed in the isomerization of spiropyran and improve its photochromic properties in the solid state.The effect of silica modification on the solid-state photochromic properties of spiropyrans has been systematically studied,and their applications in the field of anti-counterfeiting have been explored,which the materials provide a reference for the application of anti-counterfeiting in production practice.Its main contents include the following two parts:（1）Study on solid-state photochromic properties and anti-counterfeiting applications of spiropyrans in modified porous SiO₂ nanoparticlesThe effect of hydroxyl content on the surface of SiO₂ on the solid-state photochromic properties of spiropyran was studied.The films with different fading times were prepared by sol-gel method,achieving time-dependent information encryption and improving the anti-counterfeiting level.Firstly,SiO₂ nanoparticles N1-N4 with different-OH contents were synthesized by sol-gel method using trimethylchlorosilane as a modifier,and spiropyran was adsorbed by physical adsorption to obtain powder complexes NX@SP and thin films.The materials were characterized by fourier infrared spectroscopy（FT-IR）,thermogravimetric analysis（TGA）,powder X-ray diffraction（PXRD）,specific surface area and pore size analysis.The photochromic properties were characterized by UV-vis absorption spectrum.（2）Study on the preparation and properties of dynamic anti-counterfeiting materials based on carbon dots functioned SiO₂@SPThe effects of carbon dots functioned SiO₂ on the solid-state photochromic and fluorescence properties of spiropyran were studied,and its application prospects in the field of dynamic anti-counterfeiting were explored.Firstly,SiO₂ nanoparticles SNQ with blue fluorescence were synthesized by sol-gel method using carbon quantum dots as a modified agent,and the powder complex SNQ@SP1-4 were prepared by adsorption of spiropyran using physical adsorption method.The materials were successfully prepared and proved by fourier infrared spectroscopy（FT-IR）,thermogravimetric analysis（TGA）,powder X-ray diffraction（PXRD）,specific surface area and pore size analysis.Besides,the photochromic properties of the materials were explored using UV-vis absorption spectroscopy and fluorescence emission spectroscopy.Furthermore,dynamic anti-counterfeiting materials based on time-resolved encryption were prepared by using the different photochromism of SNQ@SP2 and SNQ@SP4 with different spiropyran loading.