Design,Preparation And Anticounterfeiting Application Of Core-shell Nanomaterials With Orthogonal Three-primary-color Emission

Lanthanide-doped upconversion nanoparticles(Ln-UCNPs)have attracted much attention because of their distinct anti-stokes shifts photoluminescence.Thanks to the4f-configurational transitions and near-infrared(NIR)excitations of Ln-UCNPs,upconversion luminescence exhibits unique photophysical properties,including narrow-bandwidth emissions,long fluorescence lifetime and low background interference.Thus,the UCNPs have been widely used in anticounterfeiting,display,bio-medicine and many other fields.In addition,since the light absorbing ability,quantum transitions and energy transfer processes of the Ln-UCNPs can be precisely controlled in nanoscales,the Ln-UCNPs are more attractive than quantum dots and organic dyes in tuning multicolor emissions.For core and typical core-shell UCNPs,the common approaches of tuning three-primary color upconversion luminescence are mainly based on physically mixing several different luminescent nanomaterials,co-doping several luminescent ions,regulating the doping concentration of the lanthanide ions or changing excitation power density.However,these methods suffer from many drawbacks,including excitation crosstalk,multicolor luminescence interference,unwanted energy transfer,fluorescent re-absorption,and luminescence quenching.These processes will result in the overlapping of three-primary color,color unevenness or inconsistent color brightness,which greatly restricts the color purity of the UCNPs in the field of anti-counterfeiting and the limited resolution in true 3D color display application.Therefore,the development of a single nanomaterial with high color purity three-primary color upconversion luminescence is of great significance to promote the development of UCNPs in related application fields.Inspired by core-shell structure of quantum dots,the core-shell structured UCNPs can not only address some intrinsic shortcomings,such as surface-related quenching effects,low absorption cross section,fixed excitation wavelength,and low upconversion quantum yield,but also achieve tunable multicolor upconversion luminescence by regulating energy transfer paths.Especially,the orthogonal emissive UCNPs have attracted wide interest due to their excitations orthogonalized multicolor luminescence characteristic.This orthogonal emissive property can finely address the above-mentioned problems of traditional core UCNPs in tuning multicolor luminescence.Ultimately,the UCNPs with orthogonal emission property will improve the color purity of fluorescent anti-counterfeiting and the resolution of true3D color display.However,the current researches mainly focus on the orthogonal dual-color luminescence system,and the excitation wavelengths are restricted to 980and 808 nm.There is still a lack of research on how to construct a ternary-quadrature excitations-emissions system.This severely hinders the further development of orthogonal emissive UCNPs in related fields,especially in color-based fluorescence encoding and anticounterfeiting,as well as three-primary color-based true 3D color display.In this thesis,hexagonal NaYF₄ was chosen as a host matrix material due to their high stability and low phonon energy.The core-shell nanomaterials with ternary NIR excitation-responsive orthogonal three-primary color upconversion luminescence profiles have been constructed via designing core-shell architecture,and their potential applications in information security and anticounterfeiting have been preliminarily studied.The main research results of this dissertation can be described as follows.1.The penta-layered core-shell UCNPs with orthogonal three-primary color emissions property were synthesized for the first time with an automatic nanomaterial synthesizer via a layer-by-layer epitaxial growth method.The formation of multi-shelled structure was confirmed by transmission electron microscopy(TEM)and energy dispersive X–ray spectroscopy(EDS)analysis.2.By regulating the concentration of the doped lanthanide ions and thickness of the layer,the as-developed penta-layered core-shell UCNPs can generate red,green,and blue upconversion luminescence with high color purity under 1560,808 and 980nm NIR excitations,respectively.Moreover,CIE chromaticity coordinates of the three-primary color are well retained with changing excitation power densities.Through adjusting the relative location and number of luminescent layers in the core-shell UCNPs and luminescence decay dynamics analysis,we have revealed the rationale behind the multilayered core-shell structure,and demonstrated the necessity of each layer of core-shell structure in regulating energy transfer and three-primary color upconversion luminescence.This work not only offers a strategy to achieve orthogonal three-primary color upconversion luminescence at a single nanoparticle level but also provides the possibility for further applications.3.The encrypted nanomaterial with three-primary color fluorescence switching properties was developed by combining orthogonal three-primary color emissive UCNPs with ultra-thin MnO₂ nanosheets.Take advantage of formation and reduction of the MnO₂ nanosheets,the three primary color fluorescence of the UCNPs can be quenched and recovered reversibly,which can realize data encryption and decryption processes.This encrypted nanomaterial with encryption and decryption functions and three-primary color luminescence feature has great potential for use in the fields of anticounterfeiting and confidential information protection.4.Thymolphthalein dye has a pH-dependent color change.We have developed a novel dual-stimuli-responsive security ink by combining orthogonal three-primary color fluorescence properties of the UCNPs and pH-dependent color response behavior of the thymolphthalein dye.The information of pattern generated from this ink can realize reversible information transformation between visible and invisible states under pH stimuli.The information of pattern can generate three-primary color fluorescence under 1560/808/980 nm excitation.In addition,the resulting information show excellent stability,as evidenced by various chemical treatments.