Anticounterfeiting and Security Applications of Rare-Earth Upconversion Nanophosphors

Yang Wang; Po Hu; Shuai Zhou; Jiajun Fu

doi:10.7536/PC200726

Progress in Chemistry ›› 2021, Vol. 33 ›› Issue (7) : 1221-1237 DOI: 10.7536/PC200726

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Anticounterfeiting and Security Applications of Rare-Earth Upconversion Nanophosphors

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Abstract

Compared with the traditional luminescent materials such as organic dyes and quantum dots, rare-earth upconversion nanophosphors (UCNPs) have the unique advantages of large anti-Stokes shift, shaper and multicolor emission bands, long fluorescence life, high photostability and chemical stability, no photoblinking and photobleaching. The combination of UCNPs, graphical coding and anticounterfeiting printing technologies enables the formulation of invisible fluorescent patterns that are difficult to counterfeit. This has become a research hotspot in the field of anticounterfeiting and security. In this paper, the upconversion mechanism and synthetic methods of UCNPs are first introduced. Afterwards, the research status of UCNPs in the field of anticounterfeiting and security is elaborated, and four forms of upconversion anticounterfeiting and security patterns are summarized (i.e. NIR single-wavelength excitation, NIR dual-wavelength excitation, NIR/UV dual-wavelength excitation and three-wavelength excitation). Finally, the problems and challenges that UCNPs are facing in the field of anticounterfeiting and security, and future development directions are also proposed.

Contents

1 Introduction

2 Overview of upconversion luminescence mechanism of UCNPs

3 Synthetic methods of UCNPs

3.1 Co-precipitation method

3.2 Hydrothermal/solvothermal method

3.3 Thermal decomposition method

3.4 Other synthetic methods

4 Application of UCNPs in the field of anticounterfeiting security

4.1 NIR single wavelength excited upconversion anticounterfeiting security pattern

4.2 NIR dual wavelength excited upconversion anticounterfeiting security pattern

4.3 NIR/UV dual wavelength excited upconversion anticounterfeiting security pattern

4.4 Three wavelength excited upconversion anticounterfeiting security pattern

5 Conclusion and outlook

Graphical abstract

Keywords

upconversion fluorescence / graphic coding / excitation form / anticounterfeiting and security

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Yang Wang, Po Hu, Shuai Zhou, Jiajun Fu. Anticounterfeiting and Security Applications of Rare-Earth Upconversion Nanophosphors. Progress in Chemistry, 2021, 33(7): 1221-1237 DOI:10.7536/PC200726

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1 Introduction

The proliferation of counterfeit and shoddy goods has forced governments and enterprises to continuously increase investment in the research and development and application of anti-counterfeiting technology. In 2017, the global anti-counterfeiting technology market reached 51.8 billion US dollars, and it is estimated that the global anti-counterfeiting packaging market will reach 208.4 billion US dollars by 2023^[1⇓⇓~4]. In addition, various identification cards such as identity cards, passports, medical and social security cards have been widely used in people's daily work and life, because these cards carry key personal information.Therefore, they have higher requirements for their authenticity, security and non-replicability, and need to use advanced anti-counterfeiting technology to enhance the technical difficulty of card counterfeiting. At present, the developed anti-counterfeiting technologies mainly include radio frequency identification technology, nuclear track technology, laser holographic image technology and fluorescent printing technology^[5]. Among them, fluorescent printing technology based on anti-counterfeiting ink has the advantages of easy operation, high throughput and low cost, so it is widely used in protecting high-value commodities, confidential documents, banknotes and certificates. Organic dye molecules and semiconductor quantum dots are commonly used as luminescent materials in traditional anti-counterfeiting inks, which are favored by people because of their high quantum yield and adjustable color, but their application in anti-counterfeiting security is limited because of their strong photobleaching and high biological toxicity^[6,7]^[8⇓⇓~11]. Compared with these traditional luminescent materials, Lanthanide Upconversion Nanoparticles (UCNPs) have the advantages of high optical stability, rich and narrow emission bands, long fluorescence lifetime and low biological toxicity, which have become one of the hot spots in the field of anti-counterfeiting security^[12,13].

UCNPs are a class of unique luminescent materials following the anti-Stokes mechanism, which can emit short-wavelength UV-visible light under the excitation of long-wavelength near-infrared light. The up-conversion luminescence activated by near-infrared light makes UCNPs have the characteristics of large light penetration depth, no photobleaching and scintillation, no background fluorescence and no damage to biological tissues^[14⇓~16]. Generally, UCNPs are composed of three parts, namely, the host material, the sensitizer ion and the activator ion. The sensitizer ion has a large absorption cross section to efficiently capture the excitation photon energy, and the activator ion emits a series of characteristic bands after being excited. The upconversion luminescence properties, such as luminescence color, emission intensity and fluorescence lifetime, can be precisely modulated in multiple dimensions by adjusting the composition and structure of UCNPs and the synthesis process parameters^[17,18]. Adjusting the types and contents of activator ions and sensitizer ions is an efficient strategy to realize the luminescence modulation of UCNPs. By organically combine that UCNPs, the graphic coding technology and the anti-counterfeit printing technology, the invisible fluorescent encryption label can be printed on the surface of a diversified substrate, so that the pattern is difficult to copy and counterfeit, and the security of information is ensured. Fluorescent functional inks based on UCNPs have been widely used for high-level precision anti-counterfeiting of important high-value items due to their stable and adjustable luminescent properties. For example, multi-band excited UCNPs transparent ink is used to mark anti-counterfeiting identification points on banknotes, and the authenticity of banknotes can be accurately identified by observing the luminescence of laser irradiation^[19]. Multi-color UCNPs are used to develop multi-dimensional fluorescent Quick Response (QR) codes and printed on the surface of drug capsules, which can directly read the key information of drugs through specific smartphone applications^[20].

In the past decade, UCNPs-based high-level anti-counterfeiting technology and graphic encryption technology have been widely reported, showing great potential for application in protecting brand products and important information. In this paper, the upconversion luminescence mechanism and synthesis methods of UCNPs are firstly summarized, and the research status of UCNPs in the field of anti-counterfeiting security in recent years is reviewed, and four forms of upconversion anti-counterfeiting security patterns (NIR single-wavelength excitation, NIR dual-wavelength excitation, NIR/UV dual-wavelength excitation and three-wavelength excitation) are summarized. Finally, the existing problems and challenges in the application of UCNPs in anti-counterfeiting security are analyzed and prospected.

2 Overview of Upconversion Luminescence Mechanism of UCNPs

Since the discovery of upconversion phenomenon by Auzel et al in the 1960s, the upconversion process has been a hot research topic in the field of luminescent materials. Upconversion luminescence is a typical nonlinear optical process, which emits a high-energy photon by absorbing two or more low-energy near-infrared photons in succession, so it is a photoluminescence phenomenon violating Stokes'law^{[21⇓⇓⇓~25]}. Because the transitions between the energy levels of rare earth ions belong to the parity-forbidden f-f group transitions, and the intermediate metastable energy levels have long lifetimes, which are conducive to the realization of efficient two-photon or multi-photon processes, most of the upconversion luminescent materials are rare earth ion-doped compounds, among which UCNPs are typical. After decades of exploration, people have a deep understanding of the process and mechanism of upconversion luminescence, which is mainly divided into five types: Excited-State Absorption (ESA), Successive Energy Transfer (SET) upconversion,

Cooperation Upconversion (CU), Energy Migration-mediated Upconversion (EMU) and Photon Avalanche (PA) are the most basic processes of Upconversion luminescence. The principle is that a single activator ion continuously absorbs two or more photons from the ground state Energy level to the higher energy excited state energy level, and then returns to the ground state through radiative transition to produce Upconversion luminescence (Fig. 1. I)^[26]^[24]. Different from the ESA mechanism, SET generally requires the participation of two types of ions (i.e. activator and sensitizer). The principle is shown in Figure 1.II. The excited sensitizer Ion (Ion1) makes the activator Ion (Ion2) jump to the higher excited state through continuous resonance energy transfer, thus realizing the upconversion luminescence radiation. At the same time, Ion1 returns to the ground state through radiationless relaxation. Compared with the direct absorption of the activator ions in the ESA process, SET significantly increases the number of electrons in the excited state of the activator ions, thus improving the upconversion efficiency^{[27⇓⇓~30]}. Both ESA and SET mechanisms require the activator ion to have an intermediate energy level, and the intermediate energy level has a long lifetime. For activator ions without intermediate energy levels, the up-conversion luminescence needs to follow the up-conversion process mediated by CU or EMU. The CU process is a special upconversion mechanism, which is characterized by the synchronous transfer of excitation energy from two adjacent sensitizer ions (Ion1 and Ion2) to the activator Ion (Ion3) in the ground state, so that the Ion occupies a higher excited state energy level and emits a higher energy photon (Fig. 1.III). In general, the probability of this process is low^[31]. The EMU process requires the simultaneous participation of four different types of dopant ions to achieve upconversion luminescence, namely, the sensitizer (Ion1), the accumulator (Ion2), the transporter (Ion3) and the activator (Ion4). Firstly, Ion1 transfers the captured excitation energy to the neighboring Ion2, and Ion2 transitions to the high excited state energy level after continuous energy absorption, and then transfers the energy to Ion3, which gradually transfers the excitation energy to other Ion3 by using its migration ability, and finally the energy is acquired by Ion4 to produce upconversion luminescence (Fig. 1.IV)^[32]. PA is a combination of ESA and Cross Relaxation (CR). The principle is shown in Figure 1.V. Ion2 in the intermediate metastable state is excited to a higher excited state after absorbing the pump energy, and then cross-relaxes with Ion1 which is still in the intermediate metastable state.The upconversion luminescence induced by the PA process is strongly dependent on the pump power, and only when the pump power is higher than the threshold, the upconversion luminescence with high intensity can be obtained^[12,33].

UCNPs are usually obtained from rare earth fluorides, oxides, oxyhalides and other matrix materials by doping trivalent activator ions and sensitizer ions. The Er³⁺, Tm³⁺ and Ho³⁺ of lanthanide ions have abundant ladder-like energy levels, which have long lifetimes due to the effective shielding of the 5s²5p⁶ electron shell. The energy difference between adjacent energy levels is very close, which is conducive to the continuous absorption of excitation photons, so they are easy to produce up-conversion luminescence phenomenon, and are the most widely used activator ions at present (Fig. 1. Ⅵ Energy Level Diagram). In order to improve the light absorption ability and luminescence efficiency of UCNPs, it is necessary to dope high concentration of sensitizer ions. An ideal sensitizer ion needs to meet two basic requirements: (1) the sensitizer ion has a large absorption cross section at the pump wavelength; (2) The energy difference between the excited state and the ground state of the sensitizer ion is very close to the energy difference between adjacent energy levels of the activator ion. Common sensitizer ions mainly include Yb³⁺ and Nd³⁺,In which the Yb³⁺ contains only two energy levels that differ by 10⁴cm^-1 (i.e., the ground state ²

F 7 / 2

and excited state ²

F 5 / 2

),Its characteristic absorption wavelength is well matched with the most commonly used 980 nm pump laser.And its excited state ²

F 5 / 2

is consistent with the metastable excited state energies of Er³⁺(⁴I_11/2), Tm³⁺(³H₅) and Ho³⁺(⁵I₆.Therefore, the Yb³⁺ can efficiently transfer the excitation energy to the most commonly used sensitizers in Er³⁺, UTm³⁺ and Ho³⁺ upconversion luminescence. Nd³⁺ is another excellent sensitizer ion, which has a strong ability to absorb 808 nm excitation light. The excited Nd³⁺ can efficiently transfer the pump energy to the Yb³⁺, and then the Yb³⁺ transfers the energy to the activator ion to produce upconversion luminescence. Upconversion luminescence is one of the most important physical and chemical properties of UCNPs, which includes two aspects of luminescence, namely intensity and color. There are many factors affecting the upconversion luminescence properties, such as the type and concentration of dopant ions, host lattice, particle size and so on, among which adjusting the type and content of dopant ions is a very efficient and convenient method. For the upconversion luminescence system co-doped with Yb³⁺-Ln³⁺(Ln³⁺=Er³⁺, Tm³⁺ and Ho³⁺,There is an optimal doping concentration (C_opt) for both sensitizer Yb³⁺ and activator Ln³⁺. When the doping concentration of the Yb³⁺ is increased from 0 to C_opt, the energy absorption efficiency is gradually improved, so that the luminescent intensity is increased; When the doping concentration of the Yb³⁺ is further increased, the Ln³⁺ will return most of the excitation energy to the Yb³⁺ (that is, energy return) and weaken the upconversion luminescence. When the doping concentration of Ln³⁺ increases from 0 to C_opt, the number of luminescence centers increases correspondingly, and the luminescence intensity naturally increases significantly. When the concentration is further increased to be higher than the C_opt, the strong interaction between adjacent Ln³⁺ leads to the concentration quenching effect, and the luminescence intensity decreases significantly. Generally speaking, when the doping concentration of sensitizer Yb³⁺ is 15% ~ 40%, the concentration of activator Er³⁺ is 2%~6%,Tm³⁺/Ho³⁺, and the doping concentration is 0.1% ~ 1%, the upconversion luminescence intensity is optimal. The luminescent color can also be adjusted by changing the concentration of the doped ions, and when the contents of the Yb³⁺ and the Ln³⁺ are adjusted, the relative intensity of each emission peak of the Ln³⁺ is changed, so that the modulation of the luminescent color is realized. In general, the system co-doped with Yb³⁺-Er³⁺ and Yb³⁺-Ho³⁺ mainly emits green light (Er³⁺:²H_11/2,⁴S_3/2→⁴I_15/2;Ho³⁺:⁵F₄,⁵S₂→⁵I₈). By properly increasing the concentration of Yb³⁺ or Er³⁺/Ho³⁺, the emission intensity ratio of red light to green light increases, and the emission color changes to yellow or even red light. The Yb³⁺-Tm³⁺ co-doped system has two high-intensity emission peaks in the blue region, corresponding to the ¹D₂→³F₄(450 nm) and ¹G₄→³H₆(475 nm) transitions, and the energy level transition probability of the red emission is very low and is extremely difficult to improve, so the system always emits blue visible light. In addition, the construction of UCNPs with core-shell structure is another effective way to control the luminescent properties of materials. Due to the small size, the UCNPs core particles have a large specific surface area, and there are more surface defects and surface ligands that can induce fluorescence quenching, resulting in more non-radiative relaxation loss of activator ions near the surface, so the emission intensity is often low. In addition, for UCNPs, the low phonon energy mode is blocked by the phonon confinement effect, which reduces the energy transfer from the sensitizer ion to the activator ion, and also reduces the upconversion luminescence intensity. The luminescent intensity of UCNPs can be significantly enhanced by coating the core particles with shell materials with similar lattice constants, and the luminescent color of the materials can be easily modulated. The shell of UCNPs can be divided into three types: inert shell, sensitized shell and active shell. The inert shell is composed of an undoped matrix material and can greatly inhibit the fluorescence quenching effect caused by the surface defects of the core particles, the surface ligands and the high-energy vibration of solvents, thereby achieving the purpose of enhancing the luminous intensity of the core particles; The sensitization shell is composed of a matrix material doped with sensitizer ions, which not only has the function of an inert shell, but also can endow the UCNPs with more sensitizer ions to enhance the absorption capacity and the transmission efficiency of pump light,And the sensitizer ions in the shell layer are spatially isolated from the activator ions in the core particles, so that the energy backtransfer effect induced by high-concentration sensitizer doping is inhibited, and the luminescence enhancement effect is stronger; The active shell is obtained by doping the host material with activator ions, and generally, the activator ions in the shell layer are different from the activator in the core particle and can be up-conversion luminescence centers (Er³⁺, Tm³⁺, Ho³⁺),It can also be commonly used down-conversion rare earth ions (Eu³⁺, Tb³⁺, Sm³⁺, Dy³⁺) and Mn²⁺. The active shell can ensure the upconversion luminescence of all activator ions in the core-shell structure, thereby realizing the modulation of the luminescent color of the system and simultaneously improving the overall luminescent intensity^{[34⇓⇓⇓⇓~39]}. In view of the adjustable luminescence properties of lanthanide ions and the regularity of their changes, UCNPs meeting specific luminescence requirements can be designed and constructed under the guidance of upconversion luminescence mechanism.Novel luminescent properties such as multicolor orthogonal up-conversion and power density-dependent color conversion can be achieved, which can help to produce high-security luminescent labels that are difficult to counterfeit, thus providing advanced, sophisticated and complex fluorescent anti-counterfeiting technology.

3 Synthesis of UCNPs

So far, researchers have developed a variety of synthesis methods for UCNPs, mainly including co-precipitation, hydrothermal/solvothermal, thermal decomposition, sol-gel, microemulsion and combustion methods.In addition, the synthesis of UCNPs by ion exchange has also been reported^[40]^[41]^[42,43]^[44]^[45]^[46]^[47]. Different synthesis techniques have their own advantages and disadvantages (Table 1), and the UCNPs obtained are also different in crystallinity, particle size and morphology, which affect the upconversion luminescence efficiency of the products^{[25,27,48,49]}.

3.1 Coprecipitation method

The coprecipitation method is a soft chemical method in which two or more metal ions are mixed in a solution, and then an appropriate precipitant is added to precipitate these metal ions simultaneously. It is a common and important method for the preparation of composite materials containing multiple metal elements, and is also suitable for the preparation of rare earth ion-doped UCNPs (Fig. 2a). Because of the mild reaction conditions, the low crystallinity of the precipitate and the weak luminescence intensity, it usually needs to undergo post-treatment reaction and calcination crystallization. Yi et al synthesized cubic NaYF₄:Yb/Er(α-NaYF₄) with narrow particle size distribution by co-precipitation method under the control of EDTA crystal growth. Because of the low quantum efficiency of α-NaYF₄, it can only emit very weak upconversion fluorescence under the excitation of near-infrared light^[50]. However, after calcination at 400 ~ 600 ℃, the cubic NaYF₄ was transformed into hexagonal NaYF₄, and the luminescent intensity was increased by about 40 times, and the morphology and size of the calcined particles were basically unchanged. Ningthoujam et al. And Grzyb et al. Synthesized GdPO₄∶Yb/Tb and YPO₄∶Eu/Bi nanorods in a water-glycerol system using polyethylene glycol as a coating agent^[51,52]. Before the sample was calcined, the fluorescence intensity of the product was weak due to the serious quenching effect of a large number of hydroxyl groups on the surface. After calcination at 800 ~ 900 ℃, the crystal structure of the samples changed from hydrated hexagonal phase to dehydrated monoclinic or tetragonal phase, and the emission intensity was significantly enhanced.

3.2 Hydrothermal/solvothermal method

Hydrothermal/solvothermal method refers to a soft chemical method that uses aqueous solution/organic solution as medium, and when the solution system is heated to a certain temperature, the pressure generated by itself promotes the chemical reaction of the substances in the system, thus generating a new phase (Fig. 2b). The method has the advantages of low synthesis temperature, mild conditions, simple operation, less air pollution, low energy consumption and the like. When preparing rare earth luminescent materials, the hydrothermal/solvothermal reaction system generally comprises a rare earth precursor (usually a rare earth nitrate or chloride), a reaction solvent and an organic additive (usually a surfactant). The physical and chemical properties of the reaction solvent affect the activity, solubility and diffusion rate of the rare earth precursor, and then affect the crystal form, morphology and size of the product. The application of hydrothermal method in the synthesis of UCNPs is limited because the low boiling point of water (100 ℃) can not meet the conditions for the nucleation and growth of some UCNPs, while the solvothermal method using high boiling point organic solvents (such as oleic acid, oleylamine and octadecene) is the most widely used. In addition, organic additives can coordinate with rare earth ions to precisely adjust the concentration of free ions in the system, which greatly affects the growth kinetics of UCNPs, and then controls the crystal phase, size, morphology and dispersity of the products. Li et al. First added the aqueous solution of rare earth nitrate to the microemulsion system of oleic acid/linoleic acid, water, ethanol and sodium hydroxide, gradually added the aqueous solution of NaF under stirring, then transferred to a reaction kettle, and adjusted the reaction temperature and time to control the morphology of nanoparticles^[53⇓~55]. Using this method, they synthesized a series of UCNPs with tunable size and controllable morphology (spherical, cubic and rod-like). Ethylenediaminetetraacetic Acid (EDTA) or citrate is most commonly use in that addition of hydrophilic ligands, and the EDTA or citrate introduce into the reaction system as a shape modifier plays a key role in the shape evolution of the final product. Li et al. Found that when NaREF₄(RE=Y/Yb/Gd/Lu) and LnF₃(Ln=La~Lu) rare earth phosphors were prepared by hydrothermal/solvothermal method, the crystal phase, shape and size of the products could be systematically controlled by adjusting the citrate addition, pH and fluorine source in the reaction system^[56,57]. Sun et al. Controlled the size and morphology of nanocrystals by EDTA and citrate ligands, and assisted hydrothermal synthesis of cubic and hexagonal NaYF₄∶Yb/Er nanocrystals^[28]. Subsequently, it was found that the particle size of UNCPs was related to the nucleation rate. Because the chelation constant of EDTA was larger than that of citric acid, the incorporation rate of anions in Ln³⁺-EDTA complexes was smaller than that of rare earth ions in citric acid.The particle size is mainly determined by the nucleation rate, the higher the nucleation rate, the smaller the particle size, so the particles synthesized by citrate are smaller than those synthesized by EDTA. Among the hydrophobic organic ligands, the most commonly used is oleic acid, which can be used as a surfactant to obtain various oil-soluble UCNPs. Wang et al. Reported a method to control the crystal phase and size of NaYF₄ nanocrystals simultaneously by coating with oleic acid and doping with different concentrations of Gd³⁺^[58]. Because the light lanthanide elements with large ionic radius have a high tendency of electron cloud distortion, and the ionic radius of Gd³⁺ is larger than that of Y³⁺, the crystal phase of NaYF₄ changes from the coexistence of cubic and hexagonal phases to pure hexagonal phase when the doping concentration of Gd³⁺ is increased.

3.3 Thermal decomposition method

Since Zhang first synthesized monodispersed lanthanum fluoride nanocrystals by thermal decomposition of lanthanum trifluoroacetate in 2005, the thermal decomposition method has been developed rapidly^[59]. Thermal decomposition is one of the effective ways to prepare UCNPs with monodispersity, high crystallinity and regular morphology. Generally, organic/inorganic precursors are dissolved in organic solvents with high boiling point, and then the precursors are rapidly decomposed and nucleated at high temperature to obtain rare earth nanocrystals by growth (Figure 2 C). Specifically, rare earth trifluoroacetates, oleates, acetates and chlorides are usually selected as precursors. Octadecene, with a boiling point near 315 ° C, is the most commonly used organic solvent to provide high temperature reaction conditions. In addition, the preparation of UCNPs by thermal decomposition requires the participation of surfactants with polar end-capping groups and long hydrocarbon chains, which mainly include oleic acid, oleylamine and tri-n-octylphosphine oxide.They are attached to the surface of nanocrystals by the coordination between the end capping groups and rare earth ions to improve the dispersibility of nanocrystals in organic solvents and prevent agglomeration^[59]. UCNPs with uniform size, high crystallinity and high luminescence efficiency can be obtained by adjusting the process parameters of the thermal decomposition method, such as solvent properties, precursor concentration, reaction temperature and time. Yi et al obtained hexagonal NaYF₄∶Yb/Er/Tm nanocrystals by thermal decomposition of trifluoroacetate in the oleylamine phase, proposed the nucleation and growth mechanism of hexagonal nanoparticles during the thermal decomposition process, and found that the luminescence intensity of hexagonal nanocrystals was significantly higher than that of cubic nanocrystals^[60]. Ye et al. Used a molten salt bath as a heat source to optimize the thermal decomposition method, and the reaction heating rate could reach 100 ℃/min to overcome the difference in decomposition temperature of different trifluoroacetates, and synthesized a series of nano-luminescent materials with different morphologies (including spherical, rod-like, hexagonal prism and hexagonal disk), and the upconversion emission could be adjusted by selecting appropriate rare earth doping ions^[61]. Liu et al. explored the coexistence and different roles of Oleate Anions (OA) and molecules (OAH) in crystal formation^[62]. It has been experimentally found that controlling the ratio of OA to OAH can be used to directionally inhibit, promote, or etch the crystalline faces of the nanoparticles. This control enables the selective grafting of shells with complex morphologies growing on the nanocrystal core, enabling a wide variety of monodisperse nanoparticles below 50 nm to be obtained. Through this programmable addition and subtraction engineering, a variety of three-dimensional shapes can be implemented using a bottom-up scalable approach. Chen et al. prepare NaYF₄∶Yb³⁺/Er³⁺/Tm³⁺ by thermal decomposition in oleylamine^[63]. Amino-functionalized hydrophilic UCNPs(NH₂-UCNPs) were synthesized by ligand exchange method using 2-aminoethyl dihydrogen phosphate as a surface modifier instead of the original oleylamine ligand. NIR was used to induce the transition of micelle core from hydrophobic to hydrophilic to control the rapid release of drugs. In addition, through the surface modification of UNCPs, UCNPs can have surface groups with different properties, which can combine or interact with other specific substances and greatly broaden their application fields.

3.4 Other synthetic methods

In addition to the most commonly used co-precipitation, hydrothermal/solvothermal and thermal decomposition methods, sol-gel, microemulsion and combustion methods have also been widely used to synthesize UCNPs. Sol-gel method generally uses highly chemically active rare earth compounds as precursors, including metal alkoxides, inorganic salts or complexes, which react uniformly in the liquid phase to form a stable transparent sol system. Subsequently, the colloidal particles were slowly cross-linked and aggregated into a three-dimensional network gel, which was dried and sintered to prepare UCNPs. Patra et al. prepared erbium nanoparticles with different crystal phases, sizes and luminescent intensities by changing the sintering temperature^[44]. With the increase of sintering temperature from 500 ℃ to 1000 ℃, the crystal phase gradually changes from anatase to rutile, and the size of the crystal also increases with the increase of sintering temperature.The upconversion luminescence intensity of the Er³⁺ reaches the maximum at 800 ℃. The microemulsion method uses droplets (particles) uniformly dispersed in a medium to contain various compounds, and chemical reactions are carried out in such a microreactor to obtain monodisperse UCNPs. Because hydrothermal treatment can greatly improve the crystallinity of the product, Xiong et al^[45]. The microemulsion II (water phase: NaF) is transferred into a hydrothermal kettle for reaction, amino modified NaYF₄∶Yb/Er nanocrystals are obtained through hydrothermal treatment, and folic acid molecules are directly modified for subsequent biological imaging research by utilizing the existence of amino groups on the surface of the nanocrystals; The combustion process is essentially an oxidation-reduction reaction in which rare earth nitrates act as oxidants and organic compounds act as reductants. Combustion method can prepare uniform multi-component nanocrystals in a short time (tens of seconds or minutes). Xu et al. Synthesized monoclinic Gd₂O₃∶Er³⁺ nano-phosphors doped with different concentrations of Er³⁺ by glycine-nitric acid method^[46]. As the doping concentration of the Er³⁺ increases, the phonon-assisted energy transfer process (⁴I_11/2+⁴I_11/2→⁴F_9/2+⁴I_13/2) becomes dominant, thus enhancing the red, blue/ultraviolet upconversion luminescence.

4 Application of UCNPs in the field of anti-counterfeiting security

In the past few years, researchers have proposed a variety of new synthesis methods and strategies to optimize the upconversion luminescence, which are widely used in the field of anti-counterfeiting of currency, drugs, high-end consumer goods and other items, as well as fingerprint, identity card, passport and other identification fields^{[64⇓⇓⇓⇓⇓~70]}. By combining the unique up-conversion luminescence with a variety of graphic coding methods, a variety of unique visual optical patterns can be designed and constructed, which can effectively realize large-capacity information encryption and provide a new idea for innovating anti-counterfeiting strategies^[71,72]^[73,74]. In this paper, the up-conversion anti-counterfeiting security patterns and their applications reported in recent years are summarized and analyzed, and four excitation modes are summarized, namely NIR single-wavelength excitation, NIR dual-wavelength excitation, NIR/UV dual-wavelength excitation and three-wavelength excitation.

4.1 Up-conversion anti-counterfeiting security pattern excited by NIR single wavelength

The composition and structure of UCNPs excited by NIR single wavelength are relatively simple, and the preparation steps are less and less difficult, so the fluorescent pattern excited by NIR single wavelength (808/980/1550 nm) is one of the most common strategies for anti-counterfeiting security applications. Printing two-dimensional code labels on commodity packaging is the most common means of anti-counterfeiting, so the combination of UCNPs and two-dimensional code technology to produce optical two-dimensional code is a potential anti-counterfeiting strategy. Blumenthal et al. mixed the synthesized β-NaYF₄∶3%Er/17%Yb nanocrystals with redissolved Polymethylmethacrylate (PMMA), and then performed double-layer deposition on high-coated paper through direct writing printing and screen printing technology to obtain a fluorescent two-dimensional code pattern, which emits green light under 980 nm excitation (Figure 3A)^[75]. Sangeetha et al. injected positive and negative charges into the PMMA film, soaked the film in the solution of surface-modified β-NaYF₄∶Gd³⁺/Er³⁺/Yb³⁺@NaYF₄, and obtained two-dimensional code patterns with uneven thickness by AFM nano-electrostatic printing technology.The profile height distribution closely matches the 545 nm emission intensity distribution under 980 nm excitation, demonstrating that the use of nano-xerography can improve the security of two-dimensional codes in terms of size and complexity (Figure 3 B)^[76]. Kaboli et al combined UCNPs with maskless lithography to prepare special anti-counterfeiting optically active surfaces^[77]. They first synthesized NaYF₄∶Yb³⁺/Er³⁺ and NaYF₄∶Yb³⁺/Tm³⁺ nanoparticles, mixed them and used negative lithography to make two-dimensional code tags that were invisible in sunlight but showed green fluorescence under 980 nm illumination (Fig. 3C). It is also one of the research hotspots in the field of anti-counterfeiting to use up-conversion luminescent ink to print fluorescent patterns for currency anti-counterfeiting. Wu Wei et al. Of Wuhan University synthesized uniform hexagonal NaYF₄∶Yb³⁺/Tm³⁺ nanocrystals on a large scale by hydrothermal method as fluorescent ink raw materials, and the number "100" on the screen-printed RMB copy paper emitted blue fluorescence under 980 nm irradiation (Fig. 3D)^[78].

In addition, protecting information security is an important part of cracking down on crimes and maintaining social order, and up-conversion fluorescent patterns can achieve reliable information encryption. The new perovskite-type tungstate-upconversion phosphor LiYMgWO₆∶30%Er³⁺ synthesized by Li Ting et al. Of Northwest University through high temperature solid phase method produces green and red luminescence under 1550 nm excitation.It was dispersed in a mixture of ethanol and glycerin to make a fluorescent ink, and then the paper was stamped with the seal of "Northwestern University", which was invisible in sunlight but appeared under 1550 nm excitation and emitted yellow-green light (Fig. 3e)^[79]. Biometric identification technology is widely used in the field of identity verification, and fingerprint identification technology is the most representative one. It is an important prerequisite for fingerprint recognition to acquire high definition and high resolution fingerprint images. At present, fluorescence technology has become a common means of fingerprint display and extraction, and UCNPs, as a new generation of fluorescent materials, have shown great potential in the field of fingerprint detection, but the low upconversion luminescence efficiency limits the application of UCNPs in this area. Therefore, it is urgent to improve the luminescence intensity of UCNPs by appropriate methods. Surface Plasmon (SP) modulation by combining noble metals with UCNPs is an effective way to improve the intensity of upconversion luminescence. Song Hongwei et al. Of Jilin University prepared island-shaped gold and silver alloy films by two-step PMMA template method to enhance the upconversion luminescence of NaYF₄∶Yb³⁺/Tm³⁺(Er³⁺)^[80]. By optimizing the Au-Ag molar ratio (Au_1.25-Ag_0.625) and the particle size of the NaYF₄ (NPs ≈ 7 nm), the upconversion luminescence intensity of the NaYF₄∶Yb³⁺/Tm³⁺ is enhanced by a factor of 180. Fingerprints can be effectively extracted by pressing directly on the composite membrane with a fingertip, and bright fingerprint ridges can be clearly displayed under 980 nm light, providing a reliable strategy for identification (Figure 3 f).

The monochromatic pattern is easy to be counterfeited, and the multicolor anti-counterfeiting pattern can be printed by using the single-wavelength excited UCNPs with different luminescent colors, so that the anti-counterfeiting capacity and the security of the pattern are greatly improved. Meruga et al synthesized β-NaYF₄∶Yb³⁺/Er³⁺ green fluorescent ink and β-NaYF₄∶Yb³⁺/Tm³⁺ blue fluorescent ink respectively, and then printed two-dimensional codes through aerosol inkjet printing technology^[81]. The QR code was not visible in sunlight, but the green substrate, the blue substrate, and the blue letters U, S, and D printed on the green substrate were revealed under 980 nm laser illumination (Figure 4A), and the text message "SDSM & T" was obtained by smartphone scanning. Then they synthesized red, green and blue up-conversion fluorescent ink, and printed color two-dimensional codes through aerosol inkjet printing technology^[82]. Different areas of the two-dimensional code are printed with one primary color ink or a plurality of primary color inks, so that red, green, blue, cyan, magenta, yellow and white fluorescent areas are displayed on the same pattern (fig. 4B). Lin Min et al. Of Xi'an Jiaotong University synthesized β-NaYF₄∶2%Er/18%Yb (green), β-NaYF₄∶10%Er/2%Tm (red) and NaYF₄∶25%Yb/0.3%Tm (blue) nanoparticles, which can be dissolved in ethanol/glycerol solution after modification to form fluorescent ink^[20]. And then prin a two-dimensional code pattern on that surface of an empty capsule shell by using the monochromatic ink through an ink-jet printing technology, and obtaining a medicine capsule with three layers of different monochromatic two-dimensional code superposed after the capsule shell is refilled with medicine. A specific mobile phone APP was used to directly convert the color fluorescent two-dimensional code obtained by 980 nm irradiation into three identifiable black and white two-dimensional codes to obtain key information such as the authenticity and validity of the drug (Figure 4 C). In addition, it is another reliable strategy for commodity anti-counterfeiting to use UCNPs fluorescent ink and flexible materials to make fluorescent anti-counterfeiting labels for commodity packaging. Wu Wei et al. Of Wuhan University synthesized NaYF₄∶Er³⁺/Tm³⁺, NaYF₄∶Yb³⁺/Er³⁺ and NaYF₄∶Yb³⁺/Tm³⁺ by hydrothermal method and made them into fluorescent ink, and then deposited the prepared ink on flexible Polyethylene Terephthalate (PET) substrate to obtain three kinds of anti-counterfeiting labels with high transmittance, strong fluorescent intensity and good light stability^[1]. These labels are colorless and transparent in sunlight, but show pure green or a pattern containing green and blue when excited at 980 nm (Fig. 4D), which improves the safety of the goods. Xu Jianxiong et al. of Hunan University of Technology successfully prepared Upconversion Hollow Microtubes (UCHMs) of tricolor Upconversion Hollow Microtubes by doping different lanthanide ions and adjusting the molar ratio of doped lanthanide ions, and dispersed them in the mixed solvent of ethanol, water and glycerol by functionalization of Polyacrylic Acid (PAA) to make fluorescent ink^[83]. They first printed a variety of patterns on A4 paper using monochrome ink, which were not visible in sunlight, but appeared in 980 nm light, including red patterns (horse, face and cup), green patterns (school emblem, triangular circle and star) and blue patterns (lotus flower, QR code, "Bao" character) (Fig. 4E). They then used a combination of three monochrome inks to print "Tai Chi" and windmill patterns that displayed RGB colors under 980 nm light.

The anti-counterfeiting fluorescent patterns described above emit fluorescence of a fixed color under the excitation of a single wavelength, and their coding ability and anti-counterfeiting security level are relatively low. By changing the laser power, temperature, observation angle and other factors to achieve the control of pattern color and configuration, the security level of currency, commodities and drugs can be further improved. Laser power is an important factor affecting the luminescent properties of UCNPs, and the adjustment of laser power will cause the intensity of each emission peak to change with different amplitudes, which will change the brightness or color of the pattern formed by UCNPs, and is one of the effective strategies to achieve high-level anti-counterfeiting. Gao et al. Reported efficient and stable single-band green Upconversion luminescence in Upconversion Quantum Yield microcrystals, and the absolute Upconversion Quantum Yield (UCQY) of the microcrystals was as high as 2.6%^[84]. They then irradiated the transparent "IMT" pattern (containing LYH phosphor) with 980 nm laser at 0.1, 0.5 and 1 W/cm², respectively, and found that the brightness of the highly stable pure green "IMT" fluorescent pattern increased with increasing excitation power (Fig. 5A).

Zhang Zhongping et al. Of the University of Science and Technology of China synthesized NaGdF₄∶Yb/Tm/Er@NaGdF₄∶Eu@NaYF₄ nanocrystals with a core-shell structure, coated the nanocrystals into etched micropores on a silicon wafer, and then irradiated each micropore with 980 nm laser of different powers by means of an up-conversion imaging system to successfully fabricate a display panel containing a color dot array (Fig. 5B)^[85]. They then printed bamboo, panda and rose patterns on A4 paper with NaGdF₄∶Yb/Tm/Er@NaGdF₄∶Eu@NaYF₄ solution as ink. Green bamboo and black and white panda patterns appeared when two areas of A4 paper were irradiated by laser with 3 and 15 W/cm² powers respectively. In addition, a rose pattern consisting of red petals and green stems and leaves appeared when the upper and lower parts of an area were irradiated with lasers of 3 and 30 W/cm², respectively. In addition to laser power, temperature is also an important factor affecting the luminescence properties of UCNPs, and the emission intensity usually decreases with the increase of temperature, because the probability of multiphonon-assisted nonradiative relaxation is high at higher temperatures, that is, thermal quenching is easy to occur^[86]. In view of the fact that defects in appropriate energy States can store energy, UCNPs with defect States can be designed to store excitation energy and re-emit photons through thermally activated pathways, thus enhancing upconversion luminescence. Xu Shiqing of China Metrology University reported that Na₃ZrF₇∶Yb/Er(Ho) nanocrystals with defects in appropriate energy States showed reverse thermal quenching effect of enhanced thermoluminescence intensity^[87]. Under 980 nm laser irradiation, "Er" and "Ho" printed with Na₃ZrF₇∶Yb/Er and Na₃ZrF₇∶Yb/Ho fluorescent inks, respectively, emit red and yellow light at room temperature (Figure 5C), and when heated to 413 K, the brightness of the pattern increases significantly, which enhances the security of the fluorescent anti-counterfeiting label. Jin Dayong et al. Found thermal phonons on the surface of UCNPs, which are more conducive to transferring energy from sensitizers to activators, thus enhancing the upconversion luminescence efficiency^[88]. When the temperature is increased from 303 K to 453 K, the intensity of the blue emission peak of the 9.7 nm Yb³⁺/Tm³⁺ co-doped nanoparticles is increased by a factor of 2000, and it is confirmed that the complex formed by oxygen and Yb³⁺ (Yb … O) plays a key role in this upconversion enhancement mechanism (Fig. 5d). In addition, the combination of photonic crystals and metal surface plasmon effect is also an effective means to improve the energy capture efficiency and enhance the upconversion luminescence, and the color of photonic crystals is different at different viewing angles. Song Hongwei et al. First deposited a Opal Photonic Crystals dielectric layer on the island silver film supported by PMMA Opal Photonic Crystals (OPCs), and then covered the surface with a layer of NaYF₄∶Yb³⁺/Er³⁺(Tm³⁺) to obtain PMIU hybrid film^[89]. By virtue of the synergistic effect of local surface plasmon resonance and photonic crystal effect, the local electromagnetic field is significantly amplified, so that the upconversion luminescence intensity is increased by hundreds of times. Subsequently, they transferred the hybrid film to a PET substrate for nano-printing. The results showed that the film bent under 980 nm laser irradiation exhibited various colors, and the ordered structure of the film was not destroyed and the transmission performance was almost unchanged after 50 times of bending, so it could be used for anti-counterfeiting of clothing trademarks (Fig. 5e). They also assembled C

u 2 - x

S NPs, MoO₃ and NaYF₄∶Yb³⁺/Er³⁺@NaYF₄∶Yb³⁺/Nd³⁺ on the surface of three-dimensional PMMA opal photonic crystals layer by layer to obtain composite films^[90]. Through nano-printing technology, two-dimensional codes, school emblems and other patterns are printed on the composite film (Figure 5F). Graded structural colors can be observed when the composite film is bent in sunlight. Under the illumination of 980 nm, the color of the pattern changes at different viewing angles and maintains high resolution, which improves the security and stability of its anti-counterfeiting application. In addition, Liu Xiaogang et al. Used dielectric beads to significantly enhance the photon upconversion process of UCNPs^[91]. Dielectric beads can enhance the local electric field strength and confine the light energy to the nanometer scale. Fig. 6a shows the luminescence of a Polydimethylsiloxane (PDMS) film embedded with UCNPs. Under 980 nm laser irradiation, the area covered by the microbeads emitted bright visible light, while only weak luminescence was captured in the area without microbeads. Huang Wei et al. Found that under the excitation of 980 nm pulsed laser, the luminescence color of NaYF₄∶Er/Tm(2/0.5%)@NaYF₄ core-shell nanocrystals changed from green to red by adjusting the pulse duration and repetition rate^[92]. The mechanism of this phenomenon is that the pulse laser first excites the emission level to an unstable upconversion state, and then cuts off the unfinished population process. Subsequently, they used a solution of gadolinium in cyclohexane as an ink, which was injected into the character marks on the PMMA plate. Different colored "IAM" patterns can be observed by irradiating the marked characters with a 980 nm pulsed laser and varying the pulse duration and frequency (Fig. 6B).

4.2 Up-conversion anti-counterfeiting security pattern excited by NIR dual-wavelength

The security level of the anti-counterfeiting pattern excited by the NIR single wavelength is relatively low, and the anti-counterfeiting pattern is easy to be imitated and replaced by other fluorescent materials. In order to improve the security level of anti-counterfeiting, researchers have invested a lot of effort to design dual-wavelength NIR-excited UCNPs for full-color display, anti-counterfeiting security and information encryption. One strategy is to change the color or configuration of the up-conversion anti-counterfeiting security pattern under the excitation of two near-infrared lights, which greatly improves the capacity and security level of the anti-counterfeiting code. Zhang Fan of Fudan University reported NaGdF₄∶Yb/Er@NaYF₄Yb@NaGdF₄∶Yb/Nd@NaYF₄@NaGdF₄∶Yb/Tm@NaYF₄ nanocrystals with orthogonal upconversion luminescence characteristics^[93]. The cyclohexane dispersion of the nanocrystals was then used as an encryption ink to write directly on paper. The letters on the paper are not visible in daylight, but a bright green "FD" can be observed under high power density excitation at 796 nm (Fig. 6 C). However, when the excitation wavelength is switched to 980 nm, the color of the pattern becomes blue and does not change with the excitation power density. Interestingly, due to the higher quantum yield of the near-infrared down-conversion luminescence than the up-conversion luminescence of UCNPs, the near-infrared down-conversion luminescence of Nd³⁺ and Yb³⁺ is dominant under low power density irradiation at 796 nm, and the pattern is invisible to the naked eye. Other UCNPs and their mixtures do not have such properties, ensuring the security of the anti-counterfeiting pattern. In order to realize multi-primary color space and dynamic time information coding, Liu Xiaogang et al. Doped the pulse width sensitive NaYF∶Nd/Yb@NaYF₄∶Yb/Tm@NaYF₄@NaYF₄∶Yb/Ho/Ce@NaYF₄ nanocrystals into PDMS. Under the co-irradiation of 980 nm and 808 nm pulsed laser, the nanocrystal-doped PDMS showed a three-dimensional color image with high spatial resolution and high precision, which is expected to be applied to full-gamut color display and anti-counterfeiting (Fig. 6d)^[94]. yuan Quan of Wuhan University synthesized β-NaYF₄∶Yb/Ho and β-NaYF₄∶Yb/Er@NaYF₄∶Yb@NaNdF₄∶Yb@NaYF₄∶Yb as luminescent ink 1 and ink 2. Ink 1 emits green light only under 980 nm irradiation, while ink 2 emits green light under 808 nm and 980 nm irradiation^[95]. Subsequently, the design scheme further verifies the feasibility of the two fluorescent inks applied to information encryption. Under 808 nm excitation, the error code "0512" printed by ink 2 was displayed, and the correct code "8888" printed by ink 1 was only visible under 980 nm illumination. Similarly, irradiation with the 808 nm laser shows "UCNPs" printed with INK 2, and irradiation with the 980 nm laser shows the encrypted information "UCNPs-INK" printed with ink 1 (Figure 6e). Due to its tunable optical properties, this functional fluorescent ink has great flexibility and security in information encryption. Yan Chunhua et al. synthesized NaGdF₄∶Yb/Tm@NaGdF₄∶Tb@NaYF₄@NaYF₄∶Yb/Er@NaYbF₄∶Nd@NaYF₄ nanocrystals, which showed orthogonal luminescence characteristics under 980 nm and 808 nm illumination, and found that the green light lifetime of Tb³⁺ (3.6 ms) was significantly longer than that of Er³⁺ (0.13 ms), so they could achieve time-gated fluorescence imaging in both wavelength and lifetime dimensions (Figure 6 f)^[96]. Kong Xianggui et al., Chinese Academy of Sciences, constructed Dye-sensitized NaYF₄∶25%Yb/2%Er@NaYF₄∶20%Nd nanoparticles (Dye-NPs) and NaYF₄∶10%Yb/2%Ho/10%Ce@NaYF₄ nanoparticles (Ho-NPs)^[97]. It was found that IR-806 dye could enhance NIR light absorption and transfer energy to UCNPs, thus sensitizing and controlling the red-green emission intensity ratio (R/G) of the NaYF₄∶Yb/Er@NaYF₄∶Nd and improving its luminescence intensity. Then, an ink made of Dye-NPs and Ho-NPs (mass ratio: 1:40) was simultaneously applied to the "100" position of the RMB copy paper. Since Ho-NPs do not absorb 808 nm laser light, the fluorescence emission under 808 nm excitation (1 W/cm²) comes from Dye-NPs and appears as green "100". However, when the 808 nm excitation power was increased to the 3.5 W/cm², the R/G emission ratio of IR-806 increased due to the enhanced sensitizing ability of IR-806, resulting in a yellow-green "100". Because IR-806 does not absorb 980 nm laser and the outer layer of the doped Nd³⁺ will hinder the sensitizing effect of the dye on the Yb³⁺, IR-806 dye can not enhance the luminescence of UCNPs under 980 nm excitation. In addition, because the amount of red-emitting Ho-NPs is much larger than that of Dye-NPs, "100" only shows bright red light (fig. 6G). Wu Wei et al. Synthesized three kinds of upconversion nanocrystals, namely, NaYF₄∶Yb³⁺/Tm³⁺@NaYF₄∶Nd³⁺@NaYF₄@NaYF₄∶Yb³⁺/Er³⁺, NaYF₄∶Yb³⁺/Tm³⁺@NaYF₄@NaYF₄∶Nd³⁺@NaYF₄∶Yb³⁺/Er³⁺ and NaYF4∶Er³⁺/Tm³⁺@NaYF₄∶Nd³⁺@NaYF₄@NaYF₄∶Yb³⁺/Er³⁺, by an improved high-temperature co-precipitation method. Under 808 nm and 980 nm excitation, they all showed excellent upconversion luminescence properties. By changing the mixing ratio of these three kinds of UCNPs, a full-gamut tunable fluorescent ink can be produced^[98]. Then they used screen printing technology to print the twelve zodiac signs and complex multi-color patterns on white cardboard, and pasted the transparent fluorescent label of Polyvinyl Chloride (PVC) obtained by screen printing on the wine bottle to prevent the counterfeiting of red wine (Fig. 6h).

4.3 Up-conversion anti-counterfeiting security pattern excited by NIR/UV dual wavelength

The combination of up-conversion luminescence and down-conversion luminescence is another widely studied dual-wavelength excitation anti-counterfeiting strategy, which further expands the existing fluorescent anti-counterfeiting technology and has high practical application value. This strategy usually includes two kinds of design schemes: one is to synthesize UCNPs with both up-conversion and down-conversion functions; The other is to combine UCNPs for up-conversion luminescence with quantum dots for down-conversion luminescence. Lu Lehui et al., Chinese Academy of Sciences, synthesized oleic acid-coated NaYF₄∶Yb/Er, NaYF₄∶Yb/Tm and NaYbF₄∶Er nanocrystals, and found that the coordination between rare earth ions and oleic acid carboxyl groups makes UCNPs have additional down-conversion luminescence ability^[99]. Then the cyclohexane dispersions of different nanocrystals were dropped on the fingertips of volunteers, and then the fingerprints were collected with transparent film. The clear fingerprint lines were observed by SEM, and it was found that the nanocrystals were uniformly distributed on the ridges. The fingerprint pattern shows blue/green/red light under 980 nm excitation and blue light under 365 nm excitation, which provides technical support for identifying counterfeit fingerprints (Figure 7 a). Xu Feng of Xi'an Jiaotong University synthesized β-NaYF₄∶Yb³⁺/Er³⁺ to make up-conversion fluorescent ink, and used 110UV invisible ink as down-conversion fluorescent ink to print different patterns on the same area of paper with two kinds of fluorescent ink^[100]. The green two-dimensional code is displayed under 980 nm excitation and the blue "BEBC" letter is displayed under UV excitation (fig. 7B). This combination of up-conversion and down-conversion luminescence modes significantly improves the level of anti-counterfeiting security. Chen Daqin et al. prepared Yb³⁺/Ln³⁺/Mn⁴⁺(Ln=Er/Ho/Tm) co-doped Na₃ZrF₇ phosphors by oil-water interface cation exchange method with Yb³⁺/Ln³⁺∶Na₃ZrF₇ as the exchange host, and then filled the synthesized Na₃ZrF₇∶Yb/Ho/Mn and Na₃ZrF₇∶Yb/Ho/Mn phosphors in "HDU" and "FJNU" respectively (Fig. 7c).Under UV excitation, the pattern shows Mn⁴⁺ red down-conversion luminescence, while under 980 nm excitation, "HDU" and "FJNU" emit yellow and red light, respectively, which greatly improves the security level of anti-counterfeiting^[101].

Dong Xueling, et al., China University of Geosciences, reported a dual-mode anti-counterfeiting strategy for UCNPs coupled with perovskite quantum dots. The letters "C & G" in the letters "CUGB" were printed with NaYF₄∶Yb/Tm@NaYF₄-CsPbI₃ ink, and the letters "U & B" were printed with ink NaYF₄∶Yb/Er@NaYF₄ (Fig. 7d)^[102]. Under 980 nm laser irradiation, "C & G" and "U & B" letters emit red and green light, respectively, while under UV excitation, the CsPbI₃ in the NaYF₄∶Yb/Tm@NaYF₄-CsPbI₃ component produces down-conversion luminescence and displays "C & G" with red light. Xu Shiqing et al. Constructed NaErF₄∶2%Eu@NaYF₄@NaYbF₄∶0.5%Tm@NaYF₄ core-shell nanocrystals and mixed them with commercial invisible ink to produce bimodal fluorescent ink^[103]. Under 365 nm excitation, "100" printed on Chinese banknotes showed blue fluorescence, while with increasing 980 nm excitation power, the color of "100" changed from green to indigo (Fig. 7e). The results show that the core-shell structure UCNPs with dynamic polychromatic modulation characteristics are very suitable for high security anti-counterfeiting.

4.4 Three-wavelength excited up-conversion anti-counterfeiting security pattern

Compared with the anti-counterfeiting pattern excited by single wavelength and double wavelengths, the anti-counterfeiting pattern excited by three wavelengths can have different color responses or pattern changes to the three excitation wavelengths, and the multi-level response mode greatly increases the anti-counterfeiting code capacity, so that the pattern has higher concealment and security level. Feng Wang et al. Of City University of Hong Kong synthesized NaYF₄@NaYbF₄∶Er@NaYF₄, NaYF₄∶Yb/Tm@NaYF₄∶Yb/Nd@NaYF₄ and NaGdF₄∶Ce/Tb nanocrystals, and produced polymer-based anti-counterfeiting patterns doped with these three kinds of UCNPs using standard photolithography techniques (Figure 7f), which emitted red, blue and green light under 980 nm, 808 nm and 254 nm illumination, respectively^[104]. In addition, Kong Xianggui of the Chinese Academy of Sciences and others produced fluorescent inks I, II and III doped with NaYF₄∶20%Yb2%Er@NaYF₄∶20%Yb@NaYF₄∶10%Nd, NaYF₄∶20%Yb,2%Er@NaYF₄ and NaErF₄@NaYF₄ nanocrystals respectively^[105]. When the excitation wavelength changes from 980 nm to 800 nm, the green character "898" displayed by ink I changes to the green character "436" written by ink II, and when the excitation wavelength changes to 1530 nm, the red character "175" written by ink III is displayed (Fig. 7G). This strategy greatly improves the anti-counterfeiting code capacity and security level.

5 Conclusion and prospect

Due to their unique luminescence properties, high chemical stability, strong photostability, long fluorescence lifetime, large anti-Stokes shift, no background fluorescence and low toxicity, UCNPs have shown great potential applications in many fields, especially in the biomedical field^[106~108]. In view of the tunable luminescence properties of UCNPs, UCNPs also have important applications in non-biomedical fields, such as anti-counterfeiting security, and have attracted wide attention of researchers. In this paper, the upconversion luminescence mechanism of UCNPs is firstly discussed, and the main synthesis methods of UCNPs are summarized.Then, the research status of UCNPs in the field of anti-counterfeiting security in recent years is discussed, and four forms of upconversion anti-counterfeiting security patterns (NIR single-wavelength excitation, NIR dual-wavelength excitation, NIR/UV dual-wavelength excitation and three-wavelength excitation) are summarized.

Although the synthesis technology of UCNPs and its anti-counterfeiting security applications have made considerable development and progress, there are still many challenges to apply the upconversion fluorescence pattern to the real anti-counterfeiting field: (1) Compared with the traditional downconversion luminescent materials,The upconversion efficiency of UCNPs is generally low, and the maximum upconversion quantum yield reported so far is only 7.6%^[109]. Low fluorescence efficiency has become the bottleneck of practical application of UCNPs, and how to improve the quantum yield and luminescence intensity of UCNPs is still the key research direction of rare earth luminescent materials. At present, the strategies to enhance the upconversion luminescence intensity mainly include non-rare earth ion doping, core-shell structure, photonic crystal, metal surface plasmon resonance and dye sensitization.The enhancement of these strategies is limited, and there is no recognized theoretical model to explain the enhancement mechanism. Therefore, the luminescence enhancement theory of UCNPs needs to be further improved and fully verified in experiments. (2) The current synthesis methods of UCNPs are expensive and the yield per batch is usually less than 200 mg, which can not meet the demand of raw materials in the downstream market and greatly hinder the practical transformation process of UCNPs. Therefore, there is an urgent need to develop new green, economic and high-yield technologies to achieve large-scale production of high-quality UCNPs, so as to promote the subsequent practical application of UCNPs in the field of anti-counterfeiting security. (3) The type and amount of solvents, binders and additives are very important for the production of up-conversion fluorescent ink. These parameters comprehensively determine the stability, viscosity and surface tension of the ink, and then affect its printing effect and pattern resolution. Therefore, researchers need to pay close attention to the optimal design of ink formulations to promote the development of fluorescent information encryption and anti-counterfeiting applications. (4) The realization of high-quality upconversion fluorescence patterns with low cost and large-scale printing is also the key to promote the practical application of UCNPs. At present, the reported anti-counterfeiting printing technologies mainly include inkjet printing, aerosol inkjet printing, screen printing and photolithography, etc. Aerosol inkjet printing and photolithography can realize the printing of fine fluorescent patterns, but the production cost is too high.However, it is difficult to obtain high-resolution luminescent patterns by using low-cost inkjet printing and screen printing technologies, and the contradiction between cost-effectiveness and high printing performance hinders the application of up-conversion inks. Therefore, it is necessary to propose an improved scheme based on the existing anti-counterfeiting printing technology to establish a pattern printing system with both low cost and high performance. (5) As the final link of anti-counterfeiting security technology, there are also some difficulties and challenges in the decoding and verification of up-converted fluorescent patterns, such as the difficulty of accurately reading high-quality up-converted fluorescent patterns.A single device can not decode the encrypted information of multi-dimensional anti-counterfeiting patterns, and the rapid scanning and real-time recording of patterns put forward higher requirements for the performance parameters of laser devices and reading devices. These disadvantages seriously hinder the promotion and application of up-conversion fluorescent ink, which needs to be paid attention to and solved by researchers. The popularity of smart phones and the miniaturization of optoelectronic devices bring great hope to solve the above problems. By seamlessly integrating various functional micro-optical/electronic components (such as laser diodes, electronic shutters, bandpass filters, digital image sensors, etc.) Into a compact smartphone, the integration of lensless imaging technology with smartphones can be achieved to facilitate the accurate acquisition of high-resolution fluorescence patterns; In addition, by assembling advanced superlens arrays, smartphones can read anti-counterfeiting pattern information under low-power illumination. More importantly, the network access function of smart phones makes it possible to track, trace and verify the authenticity of goods in real time.

Compared with the traditional rare earth down-conversion fluorescent materials, the basic research and application of UCNPs are still in the initial stage, and there are still complex challenges in exploring the up-conversion luminescence enhancement mechanism, optimizing the fluorescent pattern printing technology and accurately decoding the pattern information. To this end, researchers in related disciplines need to work closely together to tackle the key problems. It is believed that UCNPs with unique luminescent properties and photochemical stability will become a new generation of optical anti-counterfeiting materials and cause a major innovation in the field of anti-counterfeiting security.

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