Preparation and RGB upconversion optic properties of transparent anti-counterfeiting films

Dynamic modulation of multicolor upconversion luminescence of Er3+ via excitation pulse width

Lanthanide-doped upconversion (UC) luminescent materials display multicolor emissions, making them ideal for a variety of applications, such as multi-channel biological imaging, fluorescence encryption, anti-counterfeiting, and 3D display. Manipulating the UC emissions of the luminescent materials with a fixed composition is crucial for their applications. Herein, we propose a facile strategy to achieve pulse-width-dependent multicolor UC emissions in NaYF4:Yb/Er/Tm nanocrystals. Upon excitation with a 980 nm continuous-wave laser diode, Er3+ ions in NaYF4:20%Yb,15%Er,1%Tm nanocrystals exhibited UC emissions with a red-to-green (R/G) ratio of 11.3. Nevertheless, by employing a 980 nm pulse laser with pulse widths from 0.1 to 10 ms, the UC R/G ratio can be easily adjusted from 0.9 to 11.3, resulting in continuous and remarkable color transformation from green, yellow, orange, to red. By virtue of the dynamic luminescence color variation of these NaYF4:20%Yb,15%Er,1%Tm nanocrystals, we demonstrated their potential applications in the areas of anti-counterfeiting and information encryption. These findings provide deep insights into the excited-state dynamics and energy transfer of Er3+ in NaYF4:Yb/Er/Tm nanocrystals upon 980 nm pulse excitation, which may pave the way for designing multicolor UC materials toward versatile applications.

A Heat-Resistant Polymer Based on the Reversible Change in Polymer Skeleton Structure for Self-Anticounterfeiting

Heat-resistant polymer materials have been widely used in many fields, but their anticounterfeit is still a significant challenge. This work has successfully constructed a heat-resistant polymer material that can achieve self-anticounterfeit. In response to changes in the external environment, the color of polymer changes from yellow-green to red reversibly, which is due to the fact that polymer material's backbone undergoes isomerization. Therefore, this high-performance polymer material can not only be used in a high-temperature environment for a long time but also achieve its anticounterfeit and be used in advanced security applications.

A 3D multistage information encryption platform with self-erasure function based on a synergistically shape-deformable and AIE fluorescence-tunable hydrogel

The traditional stored information is statically shown on single 2D planes, which leads to low information storage capacity and secondary information leakage without the proper handling of decrypted information. Developing a 3D multistage information encryption platform with self-erasure function is highly desirable. Here, a novel bilayer hydrogel with synergistic deformation and fluorescence color (SDFC) change is designed for 3D multistage information encryption. The bilayer hydrogel consisting of a shape-deformable hydrogel layer and a fluorescence hydrogel layer with aggregation-induced emission (AIE) properties can exhibit pH-responsive SDFC change. Fluorescence information can be ionoprinted on the fluorescent hydrogel layer based on electrostatic interactions and dynamic covalent bonds. The 2D bilayer hydrogel encoded with information can synergistically produce predesigned 3D shape configuration and enhanced background fluorescence to wrap information, which is only readable after sequential shape recovery with the disappearance of background fluorescence. Furthermore, multistage information can be further obtained by stepwise decryption due to information with differential fluorescence fading rates. The displayed information is automatically self-erased in the end, avoiding the information secondary leakage. This study paves an avenue for broadening conventional 2D single-level information encryption platforms to 3D multistage counterparts with self-erasure and multi-decryption capabilities based on SDFC change of the bilayer hydrogel.

Novel Green Reversible Humidity-Responsive Hemiaminal Dynamic Covalent Network for Smart Window

Recently, smart windows have attracted widespread attention on account of their unique features, yet traditional smart windows still rely on external energy support to accomplish dynamic reversible switching, which not only confines usage but also causes waste of energy. For this purpose, we have prepared hemiaminal dynamic covalent network (HDCN) film with outstanding flexibility and strength by a simple and low-cost method, in which the modulus is 206.28 MPa and the elongation at break is 39.02%. Additionally, the transition from a transparent to an opaque state is achieved when the film is stimulated by humidity, and the dynamic transformation of the film to different phases of transparency is obtained when the film is exposed to different relative humidities (60-99%). Most importantly, HDCN film fulfills the modern green requirements and enables complete dissolution in a certain mildly acidic solution, avoiding environmental pollution when the material is discarded due to loss of function. The dynamic tunability of HDCN film demonstrates great advantages and potential in smart windows and anticounterfeiting.

Dual Band-Edge Enhancing Overall Performance of Upconverted Near-Infrared Circularly Polarized Luminescence for Anticounterfeiting

Circularly polarized luminescence (CPL) is one of the critical chiroptical properties for chiral nanomaterials, which exhibit wide potential applications in many research fields. However, it remains a big challenge for real application, limited by their small luminescence dissymmetry factor or low emission intensity. Here, an upconverted near-infrared circularly polarized luminescence (UC-NIR-CPL) system is constructed based on the chiroptical property of chiral liquid crystals, embedding with the lanthanide-doped upconversion nanoparticles. More importantly, a strategy for improving the overall performance of UC-NIR-CPL was proposed and realized by taking advantage of the "dual-band-edge enhancement effect", wherein the g_lum value was amplified, while the NIR emission intensity showed dual enhancement and the threshold of excitation was decreased. Based on the improved overall performance of the UC-NIR-CPL, which can be used as a distinctive covert light, a kind of photonic barcode with multiple encryptions was realized. These findings will upgrade the level of information encryption through improving the overall performance of CPL-active materials.

Water-borne, durable and multicolor silicon nanoparticles/sodium alginate inks for anticounterfeiting applications

Recently, water-borne fluorescent inks have attracted extensive attention in anti-counterfeiting applications due to their convenient implementation and eco-friendliness. However, due to poor service durability, the latent authorization information from the inks is easily damaged, and even disappears when encountering water. Moreover, most of the existing fluorescent inks are monochromic, toxic, and allergic to skin, thus are unsuitable for their sustainability during real-life applications. Herein, this work presents environment-friendly, durable, and multicolor fluorescent anti-counterfeiting silicon nanoparticles (SiNPs)/sodium alginate (SA) inks. The multicolor SiNPs are synthesized by a one-pot method with defined morphologies and optical properties. Subsequently, SA is employed as the binder to prepare the fluorescent inks with optimized rheological properties. Practicability results show that the SiNPs/SA inks not only exhibit excellent printability, but also impart authentic information with superior covert performance. More notably, spraying solution of calcium dichloride can further improve fluorescent fastnesses of the SiNPs/SA inks by ionic crosslinking.

Upconversion Nanoparticle-Integrated Bilayer Inverse Opal Photonic Crystal Film for the Triple Anticounterfeiting

Optical anticounterfeiting plays a vital role in information security because it can be recognized by the naked eye and is difficult to imitate. Herein, a hydrophilic modified upconversion nanoparticle (M-UCNP)-integrated bilayer inverse opal photonic crystal (IOPC) film was designed in which the luminescent M-UCNPs were deposited on the surface of the optimized bilayer structure with double photonic stop bands. The structure which can modulate light to produce structural colors can also enhance the upconversion luminescence (UCL) to improve the anticounterfeiting effect synergistically. On the one hand, the reflection colors from green to blue were observed in the specular angles on the front (540-layer) of the film. Meanwhile, the scattering colors under nonspecular angles from red to blue on the back (808-layer) appeared in the natural light. On the other hand, the bilayer structure in which the 808-layer functions as a "secondary excitation source" to improve the intensity of the excitation light on M-UCNPs and the 540-layer reflects the emission light of the M-UCNPs to enhance the UCL intensity endows the film with good night vision ability. Finally, the dual-mode structural colors and enhanced UCL of the patterned film work together to realize triple anticounterfeiting in banknotes.

A single-beam NIR laser-triggered full-color upconversion tuning of a Er/Tm:CsYb2F7@glass photothermal nanocomposite for optical security

The development of advanced luminescent materials is highly desirable for addressing the rising threat of forgery. However, it is challenging to achieve stable full-color upconversion (UC) tuning in the same matrix upon a single-beam light excitation so as to ensure that authentic items are irreproducible. Herein, hexagonal Er/Tm:CsYb₂F₇ nanocrystals (NCs) embedded inorganic glass via an in situ crystallization strategy is fabricated, which can emit blue, cyan, green, yellow, orange, red and near-infrared (NIR) UC emissions by simply modifying an incident 980 nm laser power. This UC tuning is attributed to the combination roles of the highly efficient laser-induced photothermal effect of the CsYb₂F₇ host and simultaneous emissions of Er and Tm activators. Importantly, the robust inorganic glass matrix endows Er/Tm:CsYb₂F₇ NCs with excellent water resistance and the ability to withstand high-power laser irradiation. Based on these unique characteristics, a proof-of-concept anti-counterfeiting experiment is designed. The results indicate that dynamic full-color UC luminescence patterns can be easily tuned by simply changing the power of the incident 980 nm laser. The present work not only confirms that the designed photothermal material can increase information security, but also provides a new idea for practical applications in the field of anti-counterfeiting.

Sustainable and invisible anti-counterfeiting inks based on waterborne polyurethane and upconversion nanoparticles for leather products

Abstract

Counterfeit leather products infringe the intellectual property rights of the business, cause enormous economic loss, and negatively influence the business enthusiasm for innovation. However, traditional anti-counterfeiting materials for leather products suffer from complicated fabrication procedures, photobleaching, and high volatile organic compound (VOC) emissions. Here, a sustainable and invisible anti-counterfeiting ink composed of waterborne polyurethane and water-dispersible lanthanide-doped upconversion nanoparticles (UCNPs) featuring ease of preparation, high photostability, non-toxicity, low VOC emissions, and strong adhesion strength for leather products is designed and synthesized. After decorating on the surface of leather products, the obtained patterns are invisible under normal light conditions. Upon irradiation at 808 nm, the invisible patterns can be observed by naked eyes due to the visible light emitted by 808 nm excited UCNPs. Our approach described here opens a new pathway to realize the long-term, stable anti-counterfeiting function of leather products.

Graphical Abstract

Dual mode emissions with enhanced green up-conversion luminescence by Gd3+ doping and down-conversion from Eu3+ in NaMnF3:Yb,Er@NaGdF4:Eu

NaMnF3:Yb,Er and NaMnF3:Yb,Er@NaGdF4:xEu (x = 0, 0.3, 0.6, and 0.9) samples were synthesized by a simple hydrothermal method. The as-obtained NaMnF3:Yb,Er particles show a single red emission under the irradiation of infrared light (980 nm), due to the energy exchange transfer process between Mn2+ ions and Er3+ ions. Notably, further growth of NaGdF4:Eu on the basis of NaMnF3:Yb,Er enhanced the green up-conversion luminescence of Er3+ ions, which gradually shifted from red to green with the increase of the Gd3+ ion doping amount. In addition, NaMnF3:Yb,Er@NaGdF4:Eu exhibits red color under ultraviolet excitation (397 nm). These results provide a possible means for controlling the luminescence properties of the lanthanide doped up-conversion system. The luminescent film synthesized with NaMnF3:Yb,Er@NaGdF4:Eu can show different luminescent colors under different light irradiation (near infrared and ultraviolet), which indicates that they have potential applications in multi-mode anti-counterfeiting.

Highly Versatile Upconverting Oxyfluoride-Based Nanophosphor Films

Fluoride-based compounds doped with rare-earth cations are the preferred choice of materials to achieve efficient upconversion, of interest for a plethora of applications ranging from bioimaging to energy harvesting. Herein, we demonstrate a simple route to fabricate bright upconverting films that are transparent, self-standing, flexible, and emit different colors. Starting from the solvothermal synthesis of uniform and colloidally stable yttrium fluoride nanoparticles doped with Yb3+ and Er3+, Ho3+, or Tm3+, we find the experimental conditions to process the nanophosphors as optical quality films of controlled thickness between few hundreds of nanometers and several micrometers. A thorough analysis of both structural and photophysical properties of films annealed at different temperatures reveals a tradeoff between the oxidation of the matrix, which transitions through an oxyfluoride crystal phase, and the efficiency of the upconversion photoluminescence process. It represents a significant step forward in the understanding of the fundamental properties of upconverting materials and can be leveraged for the optimization of upconversion systems in general. We prove bright multicolor upconversion photoluminescence in oxyfluoride-based phosphor transparent films upon excitation with a 980 nm laser for both rigid and flexible versions of the layers, being possible to use the latter to coat surfaces of arbitrary shape. Our results pave the way toward the development of upconverting coatings that can be conveniently integrated in applications that demand a large degree of versatility.

Fabrication of multicolor Janus microbeads based on photonic crystals and upconversion nanoparticles

In this study, a facile method to fabricate Janus microbeads based on photonic crystals and upconversion nanoparticles is designed. The Janus microbeads can be reversed under magnetic response and generate upconversion fluorescence under near-infrared light. Three kinds of core-shell upconversion nanoparticles (UCNPs) are prepared by the solvothermal method and are mixed with Fe₃O₄ nanoparticles and different sizes of colloidal spheres. The Janus microbeads are assembled according to the hydrophilic property of the mixture and the hydrophobic property of substrates. The upper parts of the Janus microbeads are photonic crystals assembled with colloidal spheres, and the other parts are Fe₃O₄. Meanwhile, UCNPs are distributed inside the Janus microbeads. Furthermore, the Janus microbeads are prepared into different lattice patterns using special templates. In the lattice patterns, the structural colors of Janus microbeads can be displayed and disappeared by magnetic field inversion, and under external NIR irradiation, Janus microbeads can generate upconversion fluorescence to achieve multiple color display. The Janus microbeads are also applied to both sides of the bank card, and various reading information methods are designed according to different response modes, which have important applications in pattern display, response materials, and anti-counterfeiting.

Screen printing of multi-mode emission NaYF4:Yb,Er(Tm)/NaYF4:Ce,Mn composites for anti-counterfeiting applications

NaYF₄:Yb,Er(Tm)/NaYF₄:Ce,Mn composites were prepared by a hydrothermal method, and an anti-counterfeiting printed pattern can display different colors under different ambient lights.

Temperature-dependent upconversion luminescence multicolor tuning and temperature sensing of multifunctional β-NaYF4:Yb/Er@β-NaYF4:Yb/Tm microcrystals

Multifunctional β-NaYF₄:Yb/Er@β-NaYF₄:Yb/Tm microcrystals with simultaneous white-light emission, temperature-dependent upconversion luminescence multicolor tuning and good temperature sensitivity.

Color-switchable hybrid dots/hydroxyethyl cellulose ink for anti-counterfeiting applications

Many anti-counterfeiting inks have been explored recently, most of them are commonly involved in weak fastness, high cost and long-term toxicity, impeding their real-life applications. Herein, an environment-friendly and inexpensive anti-counterfeiting ink with excellent fastness is reported. The untifake ink is developed by combining hybrid dots (silicon/carbon) with hydroxyethyl cellulose (HEC) binder. Interestingly, the HEC binder can effectively prevent from aggregation-induced quenching of hybrid dots. Subsequently, the customized patterns are successfully transferred onto different surfaces of various substrates including cotton fabric, cellulosic paper, glass, metal, silicon wafer and PET film, using the as-prepared ink by screen-printing technique, exhibiting that the hybrid dots/HEC ink possesses widespread practicability. Notably, fluorescent color of these patterns can be switchable by adjusting environmental pH-value, further imparting the as-prepared ink with excellent covert performance. This new fluorescent hybrid dots/HEC ink will be promising candidates for high-level anti-counterfeiting applications including food packaging, apparel and documents.

Photoluminescent and Chromic Nanomaterials for Anticounterfeiting Technologies: Recent Advances and Future Challenges

Counterfeiting and inverse engineering of security and confidential documents, such as banknotes, passports, national cards, certificates, and valuable products, has significantly been increased, which is a major challenge for governments, companies, and customers. From recent global reports published in 2017, the counterfeiting market was evaluated to be $107.26 billion in 2016 and forecasted to reach $206.57 billion by 2021 at a compound annual growth rate of 14.0%. Development of anticounterfeiting and authentication technologies with multilevel securities is a powerful solution to overcome this challenge. Stimuli-chromic (photochromic, hydrochromic, and thermochromic) and photoluminescent (fluorescent and phosphorescent) compounds are the most significant and applicable materials for development of complex anticounterfeiting inks with a high-security level and fast authentication. Highly efficient anticounterfeiting and authentication technologies have been developed to reach high security and efficiency. Applicable materials for anticounterfeiting applications are generally based on photochromic and photoluminescent compounds, for which hydrochromic and thermochromic materials have extensively been used in recent decades. A wide range of materials, such as organic and inorganic metal complexes, polymer nanoparticles, quantum dots, polymer dots, carbon dots, upconverting nanoparticles, and supramolecular structures, could display all of these phenomena depending on their physical and chemical characteristics. The polymeric anticounterfeiting inks have recently received significant attention because of their high stability for printing on confidential documents. In addition, the printing technologies including hand-writing, stamping, inkjet printing, screen printing, and anticounterfeiting labels are discussed for introduction of the most efficient methods for application of different anticounterfeiting inks. This review would help scientists to design and develop the most applicable encryption, authentication, and anticounterfeiting technologies with high security, fast detection, and potential applications in security marking and information encryption on various substrates.

Achieving Multimodal Emission in Zn4B6O13:Tb3+,Yb3+ for Information Encryption and Anti-counterfeiting

With the rapid development of technology, information security has always been considered a major challenge. In this work, the excellent combination of persistent luminescence, photoluminescence, up-conversion luminescence, and thermo-luminescence in a particular material Zn₄B₆O₁₃:Tb³⁺,Yb³⁺ synthesized via a solid-state reaction is reported, which can be used for the information encryption and anti-counterfeiting. Tb³⁺ ions were chosen as the emitting centers for multimodal emissions, and Yb³⁺ codoping can be used as electron traps and sensitizer to adjust trap distribution and efficient up-conversion luminescence in rare-earth-doped luminescent materials. Besides, the as-prepared luminescent materials exhibit high thermal stability and excellent water resistance. On the basis of these properties, the samples were used to print luminescent images through a screen printing process on the film and banknote. The luminescent image in a film is showing different patterns and on a banknote is showing green emissions under different stimulations. These multimodal emissions demonstrate that the as-prepared sample is suitable for advanced information encryption and anti-counterfeiting.

Putting Ink into Polyion Micelles: Full-Color Anticounterfeiting with Water/Organic Solvent Dual Resistance

Anticounterfeiting paintings are usually with limited colors and easy blurring and need to be dispersed in an environmentally unfriendly organic solvent. We report a set of water-based polyion micellar inks to solve all these problems. Upon complexation of reversible coordination polymers formed with rare earth metal ions Eu³⁺ and Tb³⁺ and the aggregation-induced emission ligand tetraphenylethylene-L₂EO₄ with oppositely charged block polyelectrolyte P2MVP₂₉-b-PEO₂₀₅, we are able to generate polyion micelles displaying three elementary emission colors of red (R) (Φ_Eu³⁺ = 24%), green (G) (Φ_Tb³⁺ = 7%), and blue (B) (Φ_TPE = 9%). Full-spectrum emission and white light emission (0.34, 0.34) become possible by simply mixing the R, G, and B micelles at the desired fraction. Strikingly, the micellar inks remain stable even after soaking in water or organic solvents (ethyl acetate, ethanol, etc.) for 24 h. We envision that polyion micelles would open a new paradigm in the field of anticounterfeiting.

Lanthanide-Doped Core@Multishell Nanoarchitectures: Multimodal Excitable Upconverting/Downshifting Luminescence and High-Level Anti-Counterfeiting

The development of luminescent materials with concurrent multimodal emissions is a great challenge to improve security and data storage density. Lanthanide-doped nanocrystals are particularly appropriate for such applications for their abundant intermediate energy states and distinguishable spectroscopic profiles. However, traditional lanthanide luminescent nanoparticles have a limited capacity for information storage or complexity to shield against counterfeiting. Herein, it is demonstrated that the combination of upconverting and downshifting emissions in a particulate designed lanthanide-doped core@multishell nanoarchitecture allows the generation of multicolor dual-modal luminescence over a wide spectral range for complex information storage. Precise control of lanthanide dopants distribution in the core and distinct shells enables simultaneous excitation of 980/808 nm focusing/defocusing laser and 254 nm light and produces complex upconverting emissions from Er, Tm, Eu, and Tb via multiphoton energy transfer processes and downshifting emissions from Eu and Tb via efficient energy transfer from Ce to Eu/Tb in Gd-assisted lattices. It is experimentally proven that multiple visualized anti-counterfeit and information encryption with facile decryption and authentication using screen-printing inks containing the present core@multishell nanocrystals are practically applicable by selecting different excitation modes.

Aerosol synthesis of TiO2:Er3+/Yb3+ submicron-sized spherical particles and upconversion optimization for application as anti-counterfeiting materials

Er³⁺/Yb³⁺-doped TiO₂ up-conversion (UC) phosphors were prepared by spray pyrolysis, and the UC luminescence properties were optimized by changing the calcination temperature and the concentration of Er³⁺ and Yb³⁺ dopants. TiO₂:Er³⁺/Yb³⁺ showed green and red emissions due to the ²H_11/2/⁴S_3/2 → ⁴I_15/2 transition and the ⁴F_9/2 → ⁴I_15/2 transition of Er³⁺ ions, respectively. The R/G ratio between red (R) and green (G) emissions does not change significantly with Er concentration but increases linearly with increasing Yb³⁺ concentration. The dependence of UC luminescence intensity on 980 nm IR pumping power showed that both the red and green UC luminescence of TiO₂:Er³⁺/Yb³⁺ occurred through a typical two-photon process. In terms of achieving the highest red UC emission intensity, the optimal Er³⁺ and Yb³⁺ contents are 0.3% and 7.0%, respectively. The UC intensity of TiO₂:Er³⁺/Yb³⁺ particles increases until they are calcined at temperatures up to 600 °C and then decreases rapidly above 800 °C. This is because when the calcination temperature is 800 °C and higher, not only does the phase transition of TiO₂:Er³⁺/Yb³⁺ occur from anatase to rutile, but also the Yb₂Ti₂O₇ impurity phase is formed. According to SEM and TEM/EDX analysis, the prepared TiO₂:Er³⁺/Yb³⁺ UC powders have an average particle size of 680 nm, a spherical shape with a dense structure, and Er and Yb are uniformly dispersed throughout the particles without local separation. A mark prepared using TiO₂:Er³⁺/Yb³⁺ powder was found to have a UC emission high enough to be visually observed when irradiated with a portable 980 nm IR lamp.

TiO₂:Er/Yb spherical particles were synthesized by spray pyrolysis and their luminescence was optimized for application as anti-counterfeiting materials.

Dynamic wrinkling pattern exhibiting tunable fluorescence for anticounterfeiting applications

A dynamic surface pattern with a topography and fluorescence in response to environmental stimulus can enable information recording, hiding, and reading. Such patterns are therefore widely used in information security and anticounterfeiting. Here, we demonstrate a dynamic dual pattern using a supramolecular network comprising a copolymer containing pyridine (P4VP-nBA-S) and hydroxyl distyrylpyridine (DSP-OH) as the skin layer for bilayer wrinkling systems, in which both the wrinkle morphology and fluorescence color can be simultaneously regulated by visible light-triggered isomerization of DSP-OH, or acids. Acid-induced protonation of pyridines can dynamically regulate the cross-linking of the skin layer through hydrogen bonding, and the fluorescence of DSP-OH. On selective irradiation with 450 nm visible light or acid treatment, the resulting hierarchical patterned surface becomes smooth and wrinkled reversibly, and simultaneously its fluorescence changes dynamically from blue to orange-red. The smart surfaces with dynamic hierarchical wrinkles and fluorescence can find potential application in anticounterfeiting.

Dual stimuli-responsive lanthanide-based phosphors for an advanced full-color anti-counterfeiting system

Dual stimuli-responsive lanthanide (Ln)-based phosphors were prepared for constructing a multi-level full color anti-counterfeiting system. The encrypted information can be visually read out under UV irradiation. Further triggered by water, not only are the fluorescence colors changed, but so are the patterns of the encrypted information. The Ln-based phosphors can be cost-effectively prepared in a simple way, presenting great potential application in constructing an advanced anti-counterfeiting system with a convenient authentication assay.

Dual stimuli-responsive lanthanide-based phosphors were prepared for constructing a multilevel full color anti-counterfeiting system, augmenting the security level for information protection.

Multi-mode optical coded patterns enabled by upconversion nanoparticles and photonic crystals

In this study, we designed the luminescent enhanced composites by combining upconversion nanoparticles (UCNPs) and photonic crystals (PCs), and prepared multi-mode coding patterns through utilizing the optical properties of both materials, respectively. We prepared UCNPs with different luminescence and the composite materials can be quickly obtained by spin-coating the UCNPs on the surface of the PCs. We discover that the composite materials have significant luminescent enhancement and the enhancement of upconversion is attributed to the Bragg reflection of the photonic band gap. We fabricated upconversion luminescent lattices based on the surface of the PCs by dropping and endowed with the meaning of the Morse code so that it can have more information in different modes. In addition, we further obtained the cipher table pattern according to the structural color of the PCs and the luminescence of the UCNPs. The results reveal great potential applications in the field of multi-mode optical imaging and anti-counterfeiting.

Tuning the NIR downconversion luminescence and photothermal conversion efficiencies of MNdxY1-xF4 (M = Na and Li) nanocrystals for use in anti-counterfeiting labels with opposite displays

Rare earth (RE3+ = Y3+ and Nd3+) alkali (M+ = Na+ or Li+) tetrafluoride nanocrystals adopt various morphologies and crystal structures depending on the M+-RE3+ size compatibility and Nd3+ concentration. This in turn affects the downconversion NIR luminescence and photothermal properties of the nanocrystals. For NaOH precursor, hexagonal NaNdxY1-xF4 nanocrystals are formed from the initially created cubic NaNdxY1-xF4 nanocrystals, whereas for LiOH precursor, tetragonal LiNd0.03Y0.97F4 nanocrystals are obtained. Due to the large size mismatch between Li+ and Nd3+, unstable LiNdxY1-xF4 undergoes phase separation to form either orthorhombic or hexagonal NdxY1-xF3 nanocrystals upon increasing the Nd3+ concentration. The latter dominates when Nd3+ is the majority rare earth element in the host matrix. NaNdxY1-xF4 nanocrystals display better luminescence and photothermal properties as compared to their Li+-based counterparts and the inverse relationship between emission and light-to-heat conversion efficiencies is exploited for anti-counterfeiting purposes. In this case, patterns deposited on different substrates (e.g., glass and Teflon) using Nd3+-concentrated nanocrystals, with efficient light-to-heat conversion and poor NIR luminescence properties, exhibit bright thermal and dim emission images when irradiated with 808 nm light. On the other hand, areas printed with Nd3+-diluted nanocrystals display dim thermal and bright emission images. Such anti-counterfeiting labels with opposite thermal and NIR emission displays provide enhanced security.

Inkjet-printed unclonable quantum dot fluorescent anti-counterfeiting labels with artificial intelligence authentication

An ideal anti-counterfeiting technique has to be inexpensive, mass-producible, nondestructive, unclonable and convenient for authentication. Although many anti-counterfeiting technologies have been developed, very few of them fulfill all the above requirements. Here we report a non-destructive, inkjet-printable, artificial intelligence (AI)-decodable and unclonable security label. The stochastic pinning points at the three-phase contact line of the ink droplets is crucial for the successful inkjet printing of the unclonable security labels. Upon the solvent evaporation, the three-phase contact lines are pinned around the pinning points, where the quantum dots in the ink droplets deposited on, forming physically unclonable flower-like patterns. By utilizing the RGB emission quantum dots, full-color fluorescence security labels can be produced. A convenient and reliable AI-based authentication strategy is developed, allowing for the fast authentication of the covert, unclonable flower-like dot patterns with different sharpness, brightness, rotations, amplifications and the mixture of these parameters.

Anti-counterfeiting technologies should ideally be unclonable, yet simple to fabricate and decode. Here, the authors develop an inkjet-printable and unclonable security label based on random patterning of quantum dot inks, and accompany it with an artificial intelligence decoding mechanism capable of authenticating the patterns.

Photoluminescent Oxygeneous-Graphitic Carbon Nitride Nanodot-Incorporated Bioderived Hyperbranched Polyurethane Nanocomposite with Anticounterfeiting Attribute

Anticounterfeiting materials are neo-advanced materials with utility in covert and security strategies. In this context, a photoluminescent, mechanically robust, and thermally stable hyperbranched polyurethane (PU) nanocomposite was fabricated with oxygeneous-graphitic carbon nitride nanodots. The nanocomposite was characterized using infrared, ultraviolet-visible, and photoluminescence spectroscopy, X-ray diffractometry, transmission electron microscopy, and thermogravimetric analysis. The processed nanocomposite demonstrated improved physico-mechanical stability as well as enhanced thermal stability than the pristine PU. The nanocomposite displayed remarkable photoluminescence under long ultraviolet light (365 nm), courtesy of dispersion of oxygeneous-carbon nitride nanodots in the polymer matrix, without any solid-state quenching. The nanocomposite was consequently employed as an ultraviolet light-detectable anticounterfeiting ink material having reinforcing ability.

Blue, green, and red full-color ultralong afterglow in nitrogen-doped carbon dots

Carbon dots (CDs) with tunable emission colors and multiple emission modes are highly desirable in advanced optical anti-counterfeiting. Some pioneering efforts to trigger additional long-lived emission modes, nevertheless, did not perfectly solve the issue of printability and color-tunability in practical applications. Herein, we developed an encapsulating-dissolving-recrystallization route for the synthesis of CD-based anti-counterfeiting inks, and accordingly realized blue, green, and red full-color afterglow emissions from these CD-based inks when printed on paper. The printed inks simultaneously possessed triple emission modes including fluorescence (FL), delayed fluorescence (DF), and room-temperature phosphorescence (RTP), among which the long-lived emissions (DF and RTP) could be selectively activated by using different excitation wavelengths. We believe that the proposed synthetic route in this work may promote the development of multicolor-encoded and multiple-mode-integrated optical anti-counterfeiting systems, and will expand the application of CD-based materials to the fields of sensing, photodynamic therapy and bio-imaging.

A facile synthesis of 1,3,6,8-pyrenesulfonic acid tetrasodium salt as a hydrosoluble fluorescent ink for anti-counterfeiting applications

In this work, 1,3,6,8-pyrenesulfonic acid sodium salt (PTSA) was successfully synthesized via a one-step sulfonating reaction. This method is more convenient, effective and eco-friendly than the traditional one. The as-prepared PTSA exhibits pure blue fluorescence under UV light. Due to its excellent fluorescent properties and water solubility, PTSA was used to prepare water-soluble invisible inks based on hydroxyethyl cellulose (HEC) aqueous solution. Notably, the resulting inks possessed acceptable stability after being stored for 30 days. Besides, the red/green/blue fluorescent inks were obtained by adding extra pigments, all of which exhibited excellent rheology and thixotropy properties. Subsequently, various patterns, including a QR code, the logo of Wuhan University, Chinese characters and so on, were printed on non-background paper through ink-jet and screen printing, and the as-prepared materials exhibited good water solubility and outstanding fluorescence performances, indicating that the fluorescent PTSA material is a promising candidate for anti-counterfeiting applications.

In this work, 1,3,6,8-pyrenesulfonic acid sodium salt (PTSA) was successfully synthesized via a one-step sulfonating reaction and exhibited the potential in anti-counterfeiting.

Reversible Surface Dual-Pattern with Simultaneously Dynamic Wrinkled Topography and Fluorescence

The reversible surface patterns with fluorescence and topography can possibly enable information recording and reading and provide an important alternative to realize the higher information security. We demonstrated a reversible dual-pattern with simultaneously responsive fluorescence and topography using an anthracene (AN) and naphthalene diimide (NDI) containing copolymer (PAN-NDI-BA) as the skin layer, in which the reversible photodimerization of AN can simultaneously control the cross-linking and CT interaction between AN and NDI. Upon irradiation with UV light and thermal treatment, the resulting pattern assumes a reversible change between smooth and wrinkled states, and its fluorescence changes reversibly from red to white to blue-green. The smart surfaces with dynamic hierarchical wrinkles and fluorescence were achieved by selective irradiation with photomasks and can be employed for potential applications in smart displays and anticounterfeiting.

Tunable quantum dot arrays as efficient sensitizers for enhanced near-infrared electroluminescence of erbium ions

Under electrical pumping conditions, high-efficiency Si-based near-infrared light generation and amplification on a chip have long been pursued for future optical interconnection technology. However, the overall performance of Si-based near-infrared electroluminescence (EL) devices, including the overall efficiency, turn-on voltage and stability under operational conditions, can rarely meet the requirements of monolithic optoelectronic integration. In this work, we designed a confined crystallization growth strategy for fabricating metal oxide quantum dot (QD) arrays embedded in Si-based films as sensitizers of Er³⁺ ions. Through the precise control of particle size and number density of QD sensitizers, the near-infrared photoluminescence (PL) emission of Er³⁺ ions can be enhanced by more than three orders of magnitude. More significantly, such hierarchical control over the regular arrangement of QD arrays not only considerably enhances the resonance energy transfer efficiency, but also offers an effective conduction path for carrier transport. Therefore, the corresponding near-infrared EL device exhibits a decreased turn-on voltage of 4.5 V, a high external quantum efficiency of 0.7%, and a long operational lifetime of more than 1000 hours, making this device superior to most Si-based on-chip near-infrared EL devices. This well-controlled metal oxide QD array represents an ideal sensitizer to effectively promote the EL emission of rare earth ions and reduce the turn-on voltage. Meanwhile, the analysis of the carrier transport mechanism paves the way for future research into resonance energy transfer under electrical pumping conditions.