Optical Nanomaterials and Enabling Technologies for High-Security-Level Anticounterfeiting

Upconversion luminescence through dynamically regulating the depletion of Ho3+: 5I6 level for speed sensing

In this work, a strategy for dynamically adjusting the upconversion luminescence (UCL) color of NaGdF4:Yb3+/Ho3+/Ce3+/Sc3+ is reported based on a phosphor wheel. It has been demonstrated that the rotation-dependent UCL mainly originated from the regulation of depletion mode for the Ho3+: 5I6 level. Due to the dominant linear decay, a high-pure red UCL is observed under the steady-state excitation. However, as the proportion of the steady-state excitation decreases, the green-red emission intensity ratio gradually increases, followed by the color conversion from red to green. An approximate physical model is proposed to understand the dependence of IG/IR on rotation speed. We not only report a UCL material that shows potential application in velocity sensing but also provide new insights into wheel-based dynamic UCL regulation.

Dual-Mode Multicolor Display Based on Structural and Fluorescent Color CdS Photonic Crystal Hydrogel

In this study, we prepared a multicolor structural-fluorescent CdS-PEGDA photonic crystal hydrogel (SFC-CPH) with a dual display mode, which has two different optical states: structural color mode and fluorescent color mode. SFC-CPH displays structural color mode under visible light and fluorescent color mode under ultraviolet light. Initially, monodisperse CdS colloidal particles were synthesized via a hydrothermal method, leading to the self-assembly of a photonic crystal template. The high refractive index of CdS contributes to the photonic crystals' low-angle dependence and vivid structural colors. Then, a variety of fluorescent molecules were doped into poly(ethylene glycol) diacrylate (PEGDA) hydrogel and combined with photonic crystals with distinct structural colors to prepare three distinct colors of SFC-CPH. We also investigated the optical characteristics and surface properties of these photonic crystal hydrogels. Based on these dual-mode display characteristics, we designed several dual-mode display patterns and a method for information encoding. The unique property of this photonic crystal hydrogel material suggests its substantial potential for applications in information storage, security, and encoding, offering innovative avenues in the realm of information display.

Rare-Earth-Metal-Free Solid-State Fluorescent Carbonized-Polymer Microspheres for Unclonable Anti-Counterfeit Whispering-Gallery Emissions from Red to Near-Infrared Wavelengths

Colloidal carbon dots (CDs) have garnered much attention as metal-free photoluminescent nanomaterials, yet creation of solid-state fluorescent (SSF) materials emitting in the deep red (DR) to near-infrared (NIR) range poses a significant challenge with practical implications. To address this challenge and to engineer photonic functionalities, a micro-resonator architecture is developed using carbonized polymer microspheres (CPMs), evolved from conventional colloidal nanodots. Gram-scale production of CPMs utilizes controlled microscopic phase separation facilitated by natural peptide cross-linking during hydrothermal processing. The resulting microstructure effectively suppresses aggregation-induced quenching (AIQ), enabling strong solid-state light emission. Both experimental and theoretical analysis support a role for extended π-conjugated polycyclic aromatic hydrocarbons (PAHs) trapped within these microstructures, which exhibit a progressive red shift in light absorption/emission toward the NIR range. Moreover, the highly spherical shape of CPMs endows them with innate photonic functionalities in combination with their intrinsic CD-based attributes. Harnessing their excitation wavelength-dependent photoluminescent (PL) property, a single CPM exhibits whispering-gallery modes (WGMs) that are emission-tunable from the DR to the NIR. This type of newly developed microresonator can serve as, for example, unclonable anti-counterfeiting labels. This innovative cross-cutting approach, combining photonics and chemistry, offers robust, bottom-up, built-in photonic functionality with diverse NIR applications.

Recent advances in photoresponsive printing inks for security encoding applications

Counterfeiting of banknotes, important documents, and branded goods continues to be a major worldwide problem for governments, businesses, and consumers. This problem has serious financial, security, and health implications. Due to their stability for printing on various substrates, the photochromic anticounterfeiting inks have received important interest. There have been various photochromic agents, such as polymer nanoparticles, quantum and carbon dots, and organic and inorganic fluorophores and luminophores, which have been broadly used for antiforging applications. In comparison to organic agents, inorganic photochromic materials have better stability under reversible/long-term light illumination. Recently, the remarkable optical characteristics and chemical stability of photoluminescent and photochromic agents have led to their extensive usage anticounterfeiting products. There have been also several strategies to tackle the rising problem of counterfeiting. Both of solvent-based and water-based inks have been developed for security encoding purposes. Additionally, the printing methods, including screen printing, labeling, stamping, inkjet printing, and handwriting, that have been used to apply anticounterfeiting inks onto various surfaces are discussed. The limitations of photoluminescent and photochromic agents and the potential for their future preparation to combat counterfeiting were discussed. This review would benefit academic researchers and industrial developers who are interested in the area of security printing.

Multicolor Photochemical Printing Inside Polymer Matrices for Advanced Photonic Anticounterfeiting

Conventional security inks, generally directly printed on the data page surface, are vulnerable to counterfeiters, thereby raising the risk of chemical structural deciphering. In fact, polymer film-based data pages with customized patterns embedded within polymer matrix, rather than printed on the surface, emerge as a promising solution. Therefore, the key lies in developing fluorophores offering light dose-controlled fluorescent color inside polymer matrices. Though conventional fluorophores often suffer from photobleaching and uncontrolled photoreactions, disqualifying them for this purpose. Herein a diphenanthridinylfumaronitrile-based phototransformers (trans-D5) that undergoes photoisomerization and subsequent photocyclization during photopolymerization of the precursor, successively producing cis- and cyclo-D5 with stepwise redshifted solid-state emissions is developed. The resulting cyclo-D5 exhibits up to 172 nm emission redshift in rigidifying polymer matrices, while trans-D5 experiences a slightly blueshifted emission (≈28 nm), cis-D5 undergoes a modest redshift (≈14 nm). The markedly different rigidochromic behaviors of three D5 molecules within polymer matrices enable multicolor photochemical printing with a broad hue ranging from 38 to 10 via an anticlockwise direction in Munsell color space, yielding indecipherable fluorescent patterns in polymer films. This work provides a new method for document protection and implements advanced security features that are unattainable with conventional inks.

Programmable multimode optical encryption of advanced printable security inks by integrating structural color with Down/Up- conversion photoluminescence

Optical information encryption with high encoding capacities can significantly boost the security level of anti-counterfeiting in the scenario of guaranteeing the authenticity of a wide scope of common and luxury goods. In this work, a novel counterfeiting material with high-degree complexity is fabricated by microencapsulating cholesteric liquid crystals and triplet-triplet annihilation upconversion fluorophores to integrate structural coloration with fluorescence and upconversion photoluminescence. Moreover, the multimode security ink presents tailorable optical behaviors and programmable abilities on flexible substrates by various printing techniques, which offers distinct information encryption under different optical modes. The advanced strategy provides a practical versatile platform for high-secure-level multimode optical inks with largely enhanced encoding capacities, programmability, printability, and cost-effectiveness, which manifests enormous potentials for information encryption and anti-counterfeiting technology.

Fine Optimization of Colloidal Photonic Crystal Structural Color for Physically Unclonable Multiplex Encryption and Anti-Counterfeiting

Robust anti-counterfeiting techniques aim for easy identification while remaining difficult to forge, especially for high-value items such as currency and passports. However, many existing anti-counterfeiting techniques rely on deterministic processes, resulting in loopholes for duplication and counterfeiting. Therefore, achieving high-level encryption and easy authentication through conventional anti-counterfeiting techniques has remained a significant challenge. To address this, this work proposes a solution that combined fluorescence and structural colors, creating a physically unclonable multiplex encryption system (PUMES). In this study, the physicochemical properties of colloidal photonic inks are systematically adjusted to construct a comprehensive printing phase diagram, revealing the printable region. Furthermore, the brightness and color saturation of inkjet-printed colloidal photonic crystal structural colors are optimized by controlling the substrate's hydrophobicity, printed droplet volume, and the addition of noble metals. Finally, fluorescence is incorporated to build PUMES, including macroscopic fluorescence and structural color patterns, as well as microscopic physically unclonable fluorescence patterns. The PUMES with intrinsic randomness and high encoding capacity are authenticated by a deep learning algorithm, which proved to be reliable and efficient under various observation conditions. This approach can provide easy identification and formidable resistance against counterfeiting, making it highly promising for the next-generation anti-counterfeiting of currency and passports.

Heteroepitaxial Growth to Construct Hexagonal/Hexagonal β-NaYF4:Yb,Tm/Cs4PbBr6 Multi-Code Emitting Core/Shell Nanocrystals

Synthesis of upconversion nanoparticles (UCNPs)-metal halide perovskites (MHPs) heterostructure is garnered immense attentions due to their unparalleled photophysical properties. However, the obvious difference in their structural forms makes it a huge challenge. Herein, hexagonal β-NaYF4 and hexagonal Cs4PbBr6 are filtrated to construct the UCNP/MHP heterostructural luminescent material. The similarity in their crystal structures facilitate the heteroepitaxial growth of Cs4PbBr6 on the surface of β-NaYF4 NPs, leading to the formation of high-quality β-NaYF4:Yb,Tm/Cs4PbBr6 core/shell nanocrystals (NCs). Interestingly, this heterostructure endows the core/shell NCs with typically narrow-band green emission centered at 524 nm under 980 nm excitation, which should be attributed to the Förster resonance energy transfer (FRET) from Tm3+ to Cs4PbBr6. It is noteworthy that the FRET efficiency of β-NaYF4:Yb,Tm/Cs4PbBr6 core/shell NCs (58.33%) is much higher than that of the physically mixed sample (1.84%). In addition, the reduced defect density, lattice anchoring effect, as well as diluted ionic bonding proportion induced by the core/shell structure further increase the excellent water-resistance and thermal cycling stability of Cs4PbBr6. These findings open up a new way to construct UCNP/MHP heterostructure with better multi-code luminescence performance and stability and promote its wide optoelectronic applications.

Photorewriting, Time-Resolved Encryption, and Unclonable Anticounterfeiting with Artificial Intelligence Authentication via a Reversible Photoswitchable System

In the fields of photolithographic patterning, optical anticounterfeiting, and information encryption, reversible photochromic materials with solid-state fluorescence are emerging as a potential class of systems. A design strategy for reversible photochromic materials has been proposed and synthesized through the introduction of photoactive thiophene groups into the molecular backbone of aryl vinyls, compounds with unique aggregation-induced emission properties, and solid-state reversible photocontrollable fluorescence and color-changing properties. This work develops novel photochromic inks, films, and cellulose hydrogels for enhancing the security of information encryption and anticounterfeiting technologies. They achieve rapid and reversible color change under ultraviolet light irradiation. Dependent upon the rate of color change, higher levels of time-resolved security can be achieved. This feature is important for enhancing the confidentiality of encrypted information and the reliability of security labels. Color-changing cellulose hydrogels, inks, and films consisting of three photochromic fluorescent molecules have quick photoactivity, great photoreversibility and photostability, and good processability, making them ideal for time-delayed anticounterfeiting and smart encryption. Furthermore, specialized algorithms are used to construct convolutional neural networks, and image analysis is performed on these systems, thus solving the current problem of the time-consuming information decryption process. This artificial intelligence method offers new opportunities for enhanced data encryption.

Multilevel Information Encryption Based on Thermochromic Perovskite Microcapsules via Orthogonal Photic and Thermal Stimuli Responses

Increasing modal variations of stimulus-responsive materials ensure the high capacity and confidentiality of information storage and encryption systems that are crucial to information security. Herein, thermochromic perovskite microcapsules (TPMs) with dual-variable and quadruple-modal reversible properties are designed and prepared on the original oil-in-fluorine (O/F) emulsion system. The TPMs respond to the orthogonal variations of external UV and thermal stimuli in four reversible switchable modes and exhibit excellent thermal, air, and water stability due to the protection of perovskites by the core-shell structure. Benefiting from the high-density information storage TPMs, multiple information encryptions and decryptions are demonstrated. Moreover, a set of devices are assembled for a multilevel information encryption system. By taking advantage of TPMs as a "private key" for decryption, the signal can be identified as the corresponding binary ASCII code and converted to the real message. The results demonstrate a breakthrough in high-density information storage materials as well as a multilevel information encryption system based on switchable quadruple-modal TPMs.

Dynamic multicolor emissions of multimodal phosphors by Mn2+ trace doping in self-activated CaGa4O7

The manipulation of excitation modes and resultant emission colors in luminescent materials holds pivotal importance for encrypting information in anti-counterfeiting applications. Despite considerable achievements in multimodal and multicolor luminescent materials, existing options generally suffer from static monocolor emission under fixed external stimulation, rendering them vulnerability to replication. Achieving dynamic multimodal luminescence within a single material presents a promising yet challenging solution. Here, we report the development of a phosphor exhibiting dynamic multicolor photoluminescence (PL) and photo-thermo-mechanically responsive multimodal emissions through the incorporation of trace Mn2+ ions into a self-activated CaGa4O7 host. The resulting phosphor offers adjustable emission-color changing rates, controllable via re-excitation intervals and photoexcitation powers. Additionally, it demonstrates temperature-induced color reversal and anti-thermal-quenched emission, alongside reproducible elastic mechanoluminescence (ML) characterized by high mechanical durability. Theoretical calculations elucidate electron transfer pathways dominated by intrinsic interstitial defects and vacancies for dynamic multicolor emission. Mn2+ dopants serve a dual role in stabilizing nearby defects and introducing additional defect levels, enabling flexible multi-responsive luminescence. This developed phosphor facilitates evolutionary color/pattern displays in both temporal and spatial dimensions using readily available tools, offering significant promise for dynamic anticounterfeiting displays and multimode sensing applications.

Physical Unclonable Functions Based on Photothermal Effect of Gold Nanoparticles

Physical unclonable functions (PUFs) based on uncontrollable fabrication randomness are promising candidates for anticounterfeiting applications. Currently, the most popular optical PUFs are generally constructed from the scattering, fluorescent, or Raman phenomenon of nanomaterials. To further improve the security level of optical PUFs, advanced functions transparent to the above optical phenomenon have always been perused by researchers. Herein, we propose a new type of PUF based on the photothermal effect of gold nanoparticles, which shows negligible scattering, fluorescent, or Raman responses. The gold nanoparticles are randomly dispersed onto the surface of fused silica, which can enhance the photothermal effect and facilitate high contrast responses. By tuning the areal density of the gold nanoparticles, the optimized encoding capacity (2319) and the total authentication error probability (3.6428 × 10-24) are achieved from our PUF due to excellent bit uniformity (0.519) and inter Hamming distances (0.503). Moreover, the intra-Hamming distance (0.044) indicates the desired reliability. This advanced PUF with invisible features and high contrast responses provides a promising opportunity to implement authentication and identification with high security.

Multisite Fine-Tuning in Hybrid Cadmium Halides Enables Wide Range Emissions for Anti-Counterfeiting

Achieving tunable emissions spanning the spectrum, from blue to near-infrared (NIR) light, within a single component is a formidable challenge with significant implication, particularly in tailoring multicolor luminescence for anti-counterfeiting purposes. In this study, we demonstrate a broad spectrum of emissions, covering blue to red and extending into NIR light in [BPy]2CdX4 : xSb3+ (BPy=Butylpyridinium; X=Cl, Br; x=0 to 0.08) through precise multisite structural fine-tuning. Notably, the multicolor emissions from [BPy]2CdBr4 : Sb3+ manifest a distinctive pattern, transitioning from blue to yellow in tandem with the host [BPy]2CdBr4 and further extending from yellow to NIR with its homologous [BPy]2CdCl4 : Sb3+, resulting in the simultaneous presence of intersecting and independent emission colors. Detailed modulation of chemical composition enables partial luminescence switching, facilitating the creation of diverse patterns with multicolor luminescence by employing [BPy]2CdX4 : xSb3+ as phosphors. This study for the first time successfully implements several groups of tunable emission colors in a single matrix via multisite fine-tuning. Such an effective strategy not only develops the specific relationships between tunable emissions and adjustable compositions, but also introduces a cost-effective and straightforward approach to achieving unique, high-level, plentiful-color and multiple-information-storage labels for advanced anti-counterfeiting applications.

Rewritable Photoluminescence and Structural Color Display for Dual-Responsive Optical Encryption

Optical encryption using coloration and photoluminescent (PL) materials can provide highly secure data protection with direct and intuitive identification of encrypted information. Encryption capable of independently controlling wavelength-tunable coloration as well as variable light intensity PL is not adequately demonstrated yet. Herein, a rewritable PL and structural color (SC) display suitable for dual-responsive optical encryption developed with a stimuli-responsive SC of a block copolymer (BCP) photonic crystal (PC) with alternating in-plane lamellae, of which a variety of 3D and 2D perovskite nanocrystals is preferentially self-assembled with characteristic PL, is presented. The SC of a BCP PC is controlled in the visible range with different perovskite precursor doping times. The perovskite nanocrystals developed in the BCP PC are highly luminescent, with a PL quantum yield of ≈33.7%, yielding environmentally stable SC and PL dual-mode displays. The independently programmed SC and PL information is erasable and rewritable. Dual-responsive optical encryption is demonstrated, in which true Morse code information is deciphered only when the information encoded by SCs is properly combined with PL information. Numerous combinations of SC and PL realize high security level of data anticounterfeiting. This dual-mode encryption display offers novel optical encryption with high information security and anti-counterfeiting.

Short-wave Infrared Photoluminescence Lifetime Mapping of Rare-Earth Doped Nanoparticles Using All-Optical Streak Imaging

The short-wave infrared (SWIR) photoluminescence lifetimes of rare-earth doped nanoparticles (RENPs) have found diverse applications in fundamental and applied research. Despite dazzling progress in the novel design and synthesis of RENPs with attractive optical properties, existing optical systems for SWIR photoluminescence lifetime imaging are still considerably restricted by inefficient photon detection, limited imaging speed, and low sensitivity. To overcome these challenges, SWIR photoluminescence lifetime imaging microscopy using an all-optical streak camera (PLIMASC) is developed. Synergizing scanning optics and a high-sensitivity InGaAs CMOS camera, SWIR-PLIMASC has a 1D imaging speed of up to 138.9 kHz in the spectral range of 900-1700 nm, which quantifies the photoluminescence lifetime of RENPs in a single shot. A 2D photoluminescence lifetime map can be acquired by 1D scanning of the sample. To showcase the power of SWIR-PLIMASC, a series of core-shell RENPs with distinct SWIR photoluminescence lifetimes is synthesized. In particular, using Er3+ -doped RENPs, SWIR-PLIMASC enables multiplexed anti-counterfeiting. Leveraging Ho3+ -doped RENPs as temperature indicators, this system is applied to SWIR photoluminescence lifetime-based thermometry. Opening up a new avenue for efficient SWIR photoluminescence lifetime mapping, this work is envisaged to contribute to advanced materials characterization, information science, and biomedicine.

Tristate Photonic Crystal Film with Structural, Fluorescent, and Up-Conversion Luminescent Color for Multilevel Anticounterfeiting

Counterfeit items are growing worldwide, affecting the global economy and human health. Anticounterfeiting tags based on a physical microstructure or chemical materials have enjoyed long-term commercial success due to their visualization and inexpensive production. However, conventional anticounterfeiting tags can be readily imitated. Herein, we have overcome this limitation by assembling colloidal nanospheres and two luminescent micromaterials into a composited photonic crystal (PhC) and achieved massive scale-up fabrication of multilevel anticounterfeiting PhC films in just several minutes of thermal rolling. The fabricated PhC film exhibits three optical states, including angle-dependent structural color (reflectivity = 66%) under white light, emits green light under 980 nm light, and emits red light under ultraviolet light. Multilevel anticounterfeiting colorful images were obtained by further use of masking templates, which integrate colors from both physically colored microstructures and chemical luminescent materials. Besides, the thermal-rolling process also shows excellent feasibility for assembling microunits with different sizes into high-quality functional PhC films.

Light-Responsive Supramolecular Liquid-Crystalline Metallacycle for Orthogonal Multimode Photopatterning

The development of multimode photopatterning systems based on supramolecular coordination complexes (SCCs) is considerably attractive in supramolecular chemistry and materials science, because SCCs can serve as promising platforms for the incorporation of multiple functional building blocks. Herein, we report a light-responsive liquid-crystalline metallacycle that is constructed by coordination-driven self-assembly. By exploiting its fascinating liquid crystal features, bright emission properties, and facile photocyclization capability, a unique system with spatially-controlled fluorescence-resonance energy transfer (FRET) is built through the introduction of a photochromic spiropyran derivative, which led to the realization of the first example of a liquid-crystalline metallacycle for orthogonal photopatterning in three-modes, namely holography, fluorescence, and photochromism.

Preparation of Self-Healing Anthocyanidin-Containing Thermochromic Alginate Ink for Authentication Purposes

Thermochromic inks have proven to be a promising security encoding approach for making commercially available products less susceptible to forgery. However, thermochromic inks have been plagued with poor durability. Thus, self-healable hydrogels can be used as self-repair inks with better durability. Herein, we combined hybrid cellulose nanofibers (CNFs) and sodium alginate (SA) with anthocyanidin(Cy)-based Brassica oleracea L. var. capitata extract in the existence of mordant (ferrous sulfate) to create a self-healing ink for authentication. CNFs were used as a reinforcement agent to enhance the mechanical strength of the sodium alginate hydrogel. Both durability and thermal stability were ensured using self-healing inks. Red cabbage was used to extract Cy-based chromophore as an environmentally friendly spectroscopic probe for immobilization into SA. Using varying concentrations of anthocyanidin, self-healable composite hydrogels (Cy@SA) with thermochromic properties were provided. Using the CIE Lab color coordinate system, homogeneous purple (569 nm) films were printed onto a sheet surface. Upon heating from 25 to 70 °C, the purple color changed to red (433 nm). Transmission electron microscopy was applied to study anthocyanidin/mordant (Cy/M) nanoparticles (NPs). The properties of the applied prints were analyzed using several methods. Both the hydrogel and stamped sheets were tested for their mechanical and rheological characteristics, respectively. Research on the nanocomposite ink (Cy@SA) antibacterial properties and cytotoxicity was also conducted.

2D Information Security System Based on Polyurethane Inverse Photonic Glass Structure

Information security has become a major global problem in recent years. Thus, people continue to exert much effort in developing new information security technologies based on encryption and storage. In this study, a 2D information security technology based on polyurethane optical devices with inverse photonic glass structure (PU-IPG) is introduced. Based on 1) the swelling and plasticizing effects of various solvents on PU-IPG and 2) the capillary force that can produce geometric deformation on micro/nanostructures when solvents evaporate, a 2D information security system with two modules of decryption (structural color information display) and anticounterfeiting (structural color transformation) is successfully constructed. The spraying method adopted can be simple and fast and can provide a large area to build photonic glass templates, which greatly improves the capacity and category of information in the encryption system. The prepared PU-IPG optical devices can produce large-area multicolor output capability of information. These devices also have excellent mechanical properties, strong cycle stability, environmental friendliness, and low price. Therefore, the preparation strategy has great reference value and application prospects in the field of information security.

The Combination of Upconversion Nanoparticles and Perovskite Quantum Dots with Temperature-Dependent Emission Colors for Dual-Mode Anti-Counterfeiting Applications

Novel and high-security anti-counterfeiting technology has always been the focus of attention and research. This work proposes a nanocomposite combination of upconversion nanoparticles (UCNPs) and perovskite quantum dots (PeQDs) to achieve color-adjustable dual-mode luminescence anti-counterfeiting. Firstly, a series of NaGdF4: Yb/Tm UCNPs with different sizes were synthesized, and their thermal-enhanced upconversion luminescence performances were investigated. The upconversion luminescence (UCL) intensity of the samples increases with rising temperature, and the UCL thermal enhancement factor rises as the particle size decreases. This intriguing thermal enhancement phenomenon can be attributed to the mitigation of surface luminescence quenching. Furthermore, CsPbBr3 PeQDs were well adhered to the surfaces and surroundings of the UCNPs. Leveraging energy transfer and the contrasting temperature responses of UCNPs and PeQDs, this nanocomposite was utilized as a dual-mode thermochromic anti-counterfeiting system. As the temperature increases, the color of the composite changes from green to pink under 980 nm excitation, while it displays green to non-luminescence under 365 nm excitation. This new anti-counterfeiting material, with its high security and convenience, has great potential in anti-counterfeiting applications.

Visualizing temperature inhomogeneity using thermo-responsive smart materials

Despite the substantial progress made, the responsiveness of thermo-responsive materials upon various thermal fields is still restricted to monochromatic visualization with single-wavelength light emission. This stems from a poor understanding of the photophysical processes within the materials and the unvarying optical performance of luminescent centers' response to various ambient temperatures. Conventional techniques to assess the inhomogeneities of thermal fields can be time-consuming, require specialized equipment and suffer from inaccuracy due to the inevitable interference from background signals, especially at high temperature. To this end, we overcome these limitations for the first time, to flexibly visualize temperature inhomogeneities by developing a thermochromic smart material, SrGa12-xAlxO19:Dy3+. Two distinct modes of thermochromic properties (steady-state temperature-dependent luminescence and thermally stimulated luminescence) are investigated. It is revealed that the abundant colors (from yellow, green to red) and amazing color-changing features are due to the superior optical integration of the host (SrGa12-xAlxO19) and dopant (Dy3+) emissions under specific thermal stimulations. We suggest that this thermo-responsive smart material can be used to realize highly efficient and simple visualization of invisible thermal distribution in industry and beyond.

Enhancement of upconversion photoluminescence in phosphor nanoparticle thin films using metallic nanoantennas fabricated by colloidal lithography

Lanthanide-doped upconversion nanoparticles (UCNPs), as multifunctional light sources, are finding utility in diverse applications ranging from biotechnology to light harvesting. However, the main challenge in realizing their full potential lies in achieving bright and efficient photon upconversion (UC). In this study, we present a novel approach to fabricate an array of gold nanoantennas arranged in a hexagonal lattice using a simple and inexpensive colloidal lithography technique, and demonstrate a significant enhancement of UC photoluminescence (UCPL) by up to 35-fold through plasmon-enhanced photoexcitation and emission. To elucidate the underlying physical mechanisms responsible for the observed UCPL enhancement, we provide a comprehensive theoretical and experimental characterization, including a detailed photophysical description and numerical simulations of the spatial electric field distribution. Our results shed light on the fundamental principles governing the enhanced UCNPs and pave the way for their potential applications in photonic devices.

Dual-signalled magneto-optical barcodes with lanthanide-based molecular cluster-aggregates

A proof-of-concept for magneto-optical barcodes is demonstrated for the first time. The dual-signalled spectrum observed via magnetic circular dichroism spectroscopy can be used to develop anti-counterfeiting materials with extra layers of security when compared with the widely studied luminescent barcodes.

Three-Dimensionally Printed, Vertical Full-Color Display Pixels for Multiplexed Anticounterfeiting

Information encryption strategies have become increasingly essential. Most of the fluorescent security patterns have been made with a lateral configuration of red, green, and blue subpixels, limiting the pixel density and security level. Here we report vertically stacked, luminescent heterojunction micropixels that construct high-resolution, multiplexed anticounterfeiting labels. This is enabled by meniscus-guided three-dimensional (3D) microprinting of red, green, and blue (RGB) dye-doped materials. High-precision vertical stacking of subpixel segments achieves full-color pixels without sacrificing lateral resolution, achieving a small pixel size of ∼μm and a high density of over 13,000 pixels per inch. Furthermore, a full-scale color synthesis for individual pixels is developed by modulating the lengths of the RGB subpixels. Taking advantage of these unique 3D structural designs, trichannel multiplexed anticounterfeiting Quick Response codes are successfully demonstrated. We expect that this work will advance data encryption technology while also providing a versatile manufacturing platform for diverse 3D display devices.

Water-Based Additive-Free Chromic Inks for Printing of Flexible Photochromics and Electrochromics

Digital inkjet printing has become one of the most convenient and efficient technologies for coating chromic materials on flexible substrates with complicated patterns. However, the development of water-based, additive-free chromic inks for inkjet printing still remains a challenge. Herein, three ammonium-functionalized colorless viologen derivatives AV, APV, and AQV with excellent water solubilities are utilized as chromes in the chromic inks due to their excellent photochromic and electrochromic properties. Water, ethanol, and ethylene glycol are selected as cosolvents, and their contents in this ternary solvent system have been optimized to achieve comprehensive rheological properties. With the H2O:EtOH:EG weight ratio of 8:1:7, the chromic ink based on AV realizes a viscosity of 4.69 mPa·s, a surface tension of 45.13 mN/m, and a Z value of 3.87. Without adding any additive, the as-prepared chromic inks can be printed on flexible substrates, such as paper and poly(ethylene terephthalate) (PET) films, by a conventional inkjet printer with inherent high resolutions. The printed patterns are initially invisible due to the colorless characteristics of the chromic inks. Interestingly, the printed films are responsive to both light and electric stimuli. Upon irradiation by UV light, a series of sentences with font sizes from 5 to 12 points and four quick response codes with different lattice resolutions clearly appear on the printed paper. Meanwhile, after printing on an indium tin oxide-coated PET substrate, electrochromic devices (ECDs) can be facilely fabricated by covering a hydrogel electrolyte on the printed films. Upon application of different potentials, the assembled ECDs exhibit "Peking Opera facial makeup" patterns with different colors. Therefore, the developed water-based additive-free chromic inks can be utilized for information display and encryption applications.

General Synthesis of Carbon Dot-Based Composites with Triple-Mode Luminescence Properties and High Stability

Carbon dot (CD)-based luminescent materials have attracted great attention in optical anti-counterfeiting due to their excellent photophysical properties in response to ultraviolet-to-visible excitation. Hence, there is an urgent need for the general synthesis of CD-based materials with multimode luminescence properties and high stability; however, their synthesis remains a formidable challenge. Herein, CDs were incorporated into a Yb,Tm-doped YF3 matrix to prepare CDs@YF3:Yb,Tm composites. The YF3 plays a dual role, not only serving as a host for fixing rare earth luminescent centers but also functioning as a rigid matrix to stabilize the triplet state of the CDs. Under the excitation of 365 nm ultraviolet light and 980 nm near-infrared light, CDs@YF3:Yb,Tm exhibited blue fluorescence and green room-temperature phosphorescence of CDs and upconversion luminescence of Tm3+, respectively. Due to the strong protection of the rigid matrix, the stability of CDs@YF3:Yb,Tm is greatly improved. This work provides a general synthesis strategy for achieving multimode luminescence and high stability of CD-based luminescent materials and offers opportunities for their applications in advanced anti-counterfeiting and information encryption.

Analysis of Crystallographic Structures and Properties of Silver Nanoparticles Synthesized Using PKL Extract and Nanoscale Characterization Techniques

In this cutting-edge research era, silver nanoparticles impose a substantial impact because of their wide applicability in the field of engineering, science, and industry. Regarding the vast applications of silver nanoparticles, in this study, the crystallographic characteristics and nanostructures of silver nanoparticles extracted from natural resources have been studied. First, biosynthetic silver nanoparticles were synthesized using the Pathor Kuchi leaf (PKL) extract as a mediator, and their crystal structures and characteristics were analyzed by UV-visible absorption spectroscopy, Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and energy-dispersive X-ray (EDX) analysis. The average crystallite size of the synthesized silver nanoparticle was determined to be 20.26 nm, and also the lattice strain, intrinsic stress, and dislocation density were measured to be 2.19 × 10-3, 0.08235 GPa, and 3.062045 × 10-3/nm2, respectively. Further, the prepared sample of silver nanoparticles shows four peaks in the X-ray diffraction pattern, which correspond to the (111), (200), (220), and (311) face-centered cubic (FCC) crystalline planes. The outstanding finding of this work was that when the lattice parameters of the precursor were increased, the volume of the material did not considerably change, but the particle size decreased. Second, it was clearly demonstrated that this straightforward method is a clean, cost-effective, environmentally sustainable, nontoxic, and efficient route for the synthesis of silver nanoparticles (Ag NPs) using PKL leaf at ambient temperature, which also satisfies the green chemistry requirements. Finally, this study demonstrates the scope for the production of silver nanoparticles using low-cost natural resources.

Hydrochromic Perovskite System with Reversible Blue-Green Color for Advanced Anti-Counterfeiting

The intrinsic instability of halide perovskites toward to external stimulus, has created a competitive advantage for designing stimuli-responsive materials. However, the external environment tuning reversibly fluorescence emission of perovskite system is still limited. In this work, humidity is verified to act as a new option to modulate the emission properties of mixed-halide perovskite. The perovskite nanocrystals (PNCs) photoirradiated in dichloromethane are easily and stably redispersed in water, and emit bright fluorescence which is quite different from the original. Moreover, the perovskites confined on glass slide can reversibly switch their fluorescence between blue and green colors under moisture. It is demonstrated that the factors of different solubilities of CsCl and CsBr in water, the structural transformation of perovskites and the confine of glass matrix play key roles in the reversible transformation. Finally, the combination of hydrochromic CsPb(Brx Cly )3 and water-resistant CsPb(Brx Cly )3 -polymethyl methacrylate have been applied in advanced anti-counterfeiting, which greatly improves the information security. This work not only give an insight into the effects of humidity on fluorescence and structures of PNCs, but also offer a new class of hydrochromic PNCs materials based on reversible emission transformation for potential application in sensors, anti-counterfeiting and information encryption.

Co-multiplexing spectral and temporal dimensions based on luminescent materials

Optical multiplexing is a pivotal technique for augmenting the capacity of optical data storage (ODS) and increasing the security of anti-counterfeiting. However, due to the dearth of appropriate storage media, optical multiplexing is generally restricted to a single dimension, thus curtailing the encoding capacity. Herein, the co-multiplexing spectral and temporal dimensions are proposed for optical encoding based on photoluminescence (PL) and persistent-luminescence (PersL) at four different wavelengths. Each emission color comprises four luminescence modes. The further multiplexing of four wavelengths leads to the maximum encoding capacity of 8 bits at each pixel. The wavelength difference between adjacent peaks is larger than 50 nm. The well-separated emission wavelengths significantly lower the requirements for high-resolution spectrometers. Moreover, the information is unable to be decoded until both PL and PersL spectra are collected, suggesting a substantial improvement in information security and the security level of anti-counterfeiting.

Triple-Mode upconversion emission for dynamic multicolor luminescent anti-Counterfeiting

Lanthanide (Ln³⁺) luminescent materials play a crucial role in information security and data storage owing to their excellent and unique optical properties. The advances in dynamic colorful luminescent anti-counterfeiting nanomaterials enable the generation of a high-level information encryption. In this work, a superior thermal, optical wavelength and excitation power triple-mode stimuli-responsive emission color modulation is demonstrated in a lanthanide-doped nanostructured luminescent material. The plentiful emission colors are manipulated by modulating the composition of a fluoride core-shell nanostructure with different Ln³⁺ at different doping concentrations. The nanomaterials display remarkable excitation wavelength/power-dependent color change, along with temperature-dependent color variation in the range from 298 K to 437 K, with a good relative sensitivity S_r of 1.1387% K^-1 at 398 K. The universal optical modulation, combined with the excellent optical and structural stability of the luminescent nanoparticles, renders many advantages for the anti-counterfeiting application. This work explores a universal strategy for the manipulation of triple-mode stimuli-responsive dynamic luminescence and demonstrates its good potential for anti-counterfeiting application.

Microcavity- and Microlaser-Based Optical Barcoding: A Review of Encoding Techniques and Applications

Optical microbarcodes have recently received a great deal of interest because of their suitability for a wide range of applications, such as multiplexed assays, cell tagging and tracking, anticounterfeiting, and product labeling. Spectral barcodes are especially promising because they are robust and have a simple readout. In addition, microcavity- and microlaser-based barcodes have very narrow spectra and therefore have the potential to generate millions of unique barcodes. This review begins with a discussion of the different types of barcodes and then focuses specifically on microcavity-based barcodes. While almost any kind of optical microcavity can be used for barcoding, currently whispering-gallery microcavities (in the form of spheres and disks), nanowire lasers, Fabry-Pérot lasers, random lasers, and distributed feedback lasers are the most frequently employed for this purpose. In microcavity-based barcodes, the information is encoded in various ways in the properties of the emitted light, most frequently in the spectrum. The barcode is dependent on the properties of the microcavity, such as the size, shape, and the gain materials. Various applications of these barcodes, including cell tracking, anticounterfeiting, and product labeling are described. Finally, the future prospects for microcavity- and microlaser-based barcodes are discussed.

Photochromic Tungsten Oxide Quantum Dots-based Fluorescent Photoswitches towards Dual-mode Anti-counterfeiting Application

Development of new anti-counterfeiting technology to increase the difficulty of imitation and decoding is becoming increasingly important, but still remains challenging yet. In this work, we report the design of new fluorescence photoswitches based on photochromic tungsten oxide quantum dots (WO₃ QDs) for dual-mode anti-counterfeiting applications. Complexing photochromic WO₃ QDs with fluorescent gold nanoclusters (AuNCs) enables the construction of a photoswitchable fluorescence system (WO₃-AuNCs) based on fluorescence resonance energy transfer. Detailed spectral and photophysical characterization showed that WO₃ QDs well-retain the photochromic properties within the WO₃-AuNCs composite. Importantly, photoresponsive and highly reversible switching of both color and fluorescence signals was successfully achieved by simply alternating the irradiation with UV and visible light. Potential utility of photoswitchable WO₃-AuNCs composite as novel dual-mode anti-counterfeiting materials has been successfully demonstrated, including photoswitchable ink, rewritable paper and number encryption. Compared with other anti-counterfeiting materials, the present photochromic WO₃ QDs-based fluorescent switches are easily synthesized and handled, and they can provide dual security mode (color and fluorescence). This work provides a generable WO₃ QDs-assisted strategy of fabricating advanced fluorescence photoswitches for versatile optical counterfeiting applications.

Multimodal dynamic and unclonable anti-counterfeiting using robust diamond microparticles on heterogeneous substrate

The growing prevalence of counterfeit products worldwide poses serious threats to economic security and human health. Developing advanced anti-counterfeiting materials with physical unclonable functions offers an attractive defense strategy. Here, we report multimodal, dynamic and unclonable anti-counterfeiting labels based on diamond microparticles containing silicon-vacancy centers. These chaotic microparticles are heterogeneously grown on silicon substrate by chemical vapor deposition, facilitating low-cost scalable fabrication. The intrinsically unclonable functions are introduced by the randomized features of each particle. The highly stable signals of photoluminescence from silicon-vacancy centers and light scattering from diamond microparticles can enable high-capacity optical encoding. Moreover, time-dependent encoding is achieved by modulating photoluminescence signals of silicon-vacancy centers via air oxidation. Exploiting the robustness of diamond, the developed labels exhibit ultrahigh stability in extreme application scenarios, including harsh chemical environments, high temperature, mechanical abrasion, and ultraviolet irradiation. Hence, our proposed system can be practically applied immediately as anti-counterfeiting labels in diverse fields.

Dynamic Flow Control over Optical Properties of Liquid Crystal-Quantum Dot Hybrids in Microfluidic Devices

In this paper, we report developing approaches to tuning the optical behavior of microfluidic devices by infusing smart hybrids of liquid crystal and quantum dots into microchannel confinement. We characterize the optical responses of liquid crystal-quantum dot composites to polarized and UV light in single-phase microflows. In the range of flow velocities up to 10 mm/s, the flow modes of microfluidic devices were found to correlate with the orientation of liquid crystals, dispersion of quantum dots in homogeneous microflows and the resulting luminescence response of these dynamic systems to UV excitation. We developed a Matlab algorithm and script to quantify this correlation by performing an automated analysis of microscopy images. Such systems may have application potential as optically responsive sensing microdevices with integrated smart nanostructural components, parts of lab-on-a-chip logic circuits, or diagnostic tools for biomedical instruments.

Directional Self-Assembly of Facet-Aligned Organic Hierarchical Super-Heterostructures for Spatially Resolved Photonic Barcodes

Organic multicolor heterostructures with spatially resolved luminescent colors and identifiable patterns have exhibited considerable potential for achieving micro-/nanoscale photonic barcodes. Nevertheless, such types of barcodes reported thus far are exclusively based on a single heterostructure with limited coding elements. Here, a directional self-assembly strategy is proposed to achieve high-coding-capacity spatially resolved photonic barcodes through rationally constructing organic hierarchical super-heterostructures, where numerous subheterostructure blocks with flat hexagonal facets are precisely oriented with their specific facets via a reconfigurable capillary force. The building blocks were prepared through a one-pot sequential heteroepitaxial growth, which enables the effective modulation of the structural and color characteristics in coding structures. Significantly, a directional facet-to-facet attraction between particles via facet registration leads to the formation of well-defined 1D super-heterostructures, which contain multiple coding elements, thus providing a good platform for constructing the high-coding-capacity photonic barcodes. The results may be useful in fabricating organic hierarchical hybrid super-heterostructures for security labels and optical data recording.

Admirable stability achieved by ns2 ions Co-doping for all-inorganic metal halides towards optical anti-counterfeiting

Optical materials play a momentous role in anti-counterfeiting field, such as authentication, currency and security. The development of tunable optical properties and optical responses to a range of external stimuli is quite imperative for the growing demand of optical anti-counterfeiting technology. Metal halide perovskites have attracted much attention of researchers due to their excellent optical properties. In addition, co-doping methods have been gradually applied to the research of metal halide perovskites, by which more abundant luminescence phenomena can be introduced into the host perovskite. Herein, the ns² ions of bismuth (Bi³⁺) and antimony (Sb³⁺) ions co-doped zero-dimensional Cs₂SnCl₆ metal halide with an excitation-wavelength-dependent emission phenomenon is synthesized as an efficient multimodal luminescent material, the luminescence of which is tunable and covers a wide region of color. What's more, a dynamic dual-emission phenomenon is captured when the excitation wavelength changes from 320 nm to 420 nm for Cs₂SnCl₆:Bi_0.08Sb_0.12 crystals. Moreover, the Bi³⁺ and Sb³⁺ doped metal halide material shows great enhancement in solvent resistance and thermal stability compared to the pristine Cs₂SnCl₆. The admirable stability and distinguishable photoluminescence (PL) phenomenon of this all-inorganic metal halide has great potential to be applied in optical anti-counterfeiting technology. Furthermore, the co-doping method can accelerate the discovery of new luminescence phenomena in original metal halide perovskites.

Butterfly-Inspired Tri-State Photonic Crystal Composite Film for Multilevel Information Encryption and Anti-Counterfeiting

Counterfeiting is a worldwide issue and has long troubled legitimate businesses, while nowadays anti-counterfeiting materials and technology are still insufficient to combat the escalating counterfeit behaviors. Inspired by hindwing structure of Troides magellanus, a new kind of anti-counterfeiting material taking advantage of both physical and chemical structures to display multiple optical states is prepared. The chemical units (luminescent lanthanide) are blended with physical units (monodispersed colloidal particles) and mediating molecules, which are then assembled into a photonic crystal structure at room temperature in less than 10 s through a new assembly technique called molecule-mediated shear-induced assembly technique (MSAT). The as-prepared photonic crystal films feature three unique optical states, each displaying structural, fluorescent, and phosphorescent color under different lighting conditions, which integrates colors from both physical and chemical origins. Furthermore, by incorporating different luminescent materials into different parts of the photonic crystal pattern, a high-level information encryption system is designed to be capable of carrying three distinct types of information. Thanks to this powerful tool of MSAT, it is now possible to assemble different-sized, even irregular non-spherical units with monodispersed spherical units into high-quality photonic crystal films, which provides easy access to incorporating new features into photonic crystal systems.

Synthesis and Application of Fluorescent Polymer Micro- and Nanoparticles

Fluorescent polymer particles have witnessed an increasing interest in recent years, owing to their fascinating physicochemical properties as well as wide-ranging applications. In this review, the state-of-the-art research progress of fluorescent polymer particles in the past five years is summarized. First, the synthesis protocols for fluorescent polymer particles, including emulsion polymerization, precipitation polymerization, dispersion polymerization, suspension polymerization, nanoprecipitation, self-assembly, and post-polymerization modification, are presented in detail. Then, the applications of the resulting beguiling particles in anticounterfeiting, chemical sensing, and biomedicine, are illustrated. Finally, the challenges and opportunities that exist in the field are pointed out. This review aims to offer important guidance and stimulate more research attention to this rapidly developing field.

Multi-stimuli-responsive and dynamic color tunable security ink for multilevel anticounterfeiting

Luminescent security features have been used for anticounterfeiting for a long time. However, constant effort is required to strengthen these security features to be ahead of counterfeiters. Here, we developed a multi-stimuli-responsive luminescent security ink containing Tb(ASA)₃Phen, K₂SiF₆:Mn^4+,and NaYF₄:Yb³⁺/Er³⁺luminescent materials in PVC gold medium. Tb(ASA)₃Phen complex shows a broad excitation band in the UV region; upon UV light radiation it shows strong greenish emission of Tb³⁺ions through the antenna effect. K₂SiF₆:Mn⁴⁺, on the other hand, has three excitation bands with maxima at 248, 354, and 454 nm which emit red light after excitation through these bands. NaYF₄:Yb³⁺/Er³⁺is used as an upconverting nanophosphor showing green emission under 976 nm laser excitation. Thus, the multi-stimuli-responsive luminescent security ink shows greenish, red, and green emissions under 367 nm, 450 nm, and 976 nm excitations, respectively. Furthermore, the distinct lifetimes of the activators in Tb(ASA)₃Phen and K₂SiF₆:Mn⁴⁺, i.e. 0.1708 ms and 8.165 ms, respectively, under 380 nm excitation make this ink suitable for dynamic anticounterfeiting as well. The ink shows a change in the emission color with time delay, after the removal of the 380 nm excitation source, from greenish yellow (at 0 delays) to reddish color after a delay of 7.5 ms. These unique optical features along with excellent photo-, chemical- and environmental stability make this ink useful for advanced-level anticounterfeiting.

Orthogonal luminescence lifetime encoding by intermetallic energy transfer in heterometallic rare-earth MOFs

Lifetime-encoded materials are particularly attractive as optical tags, however examples are rare and hindered in practical application by complex interrogation methods. Here, we demonstrate a design strategy towards multiplexed, lifetime-encoded tags via engineering intermetallic energy transfer in a family of heterometallic rare-earth metal-organic frameworks (MOFs). The MOFs are derived from a combination of a high-energy donor (Eu), a low-energy acceptor (Yb) and an optically inactive ion (Gd) with the 1,2,4,5 tetrakis(4-carboxyphenyl) benzene (TCPB) organic linker. Precise manipulation of the luminescence decay dynamics over a wide microsecond regime is achieved via control over metal distribution in these systems. Demonstration of this platform's relevance as a tag is attained via a dynamic double encoding method that uses the braille alphabet, and by incorporation into photocurable inks patterned on glass and interrogated via digital high-speed imaging. This study reveals true orthogonality in encoding using independently variable lifetime and composition, and highlights the utility of this design strategy, combining facile synthesis and interrogation with complex optical properties.

Magnetic and optical studies of a new family of multidimensional and multiproperty PO-lanthanide(III) derived systems

A new family of lanthanide compounds has been synthesized using 1,2-bis(diphenylphosphino)ethane dioxide (dppeO₂) as an O-donor ligand through the phosphoryl group to lanthanide(III) cations and structurally, magnetically and optically studied. Depending on the lanthanide, two different topologies appear: the two-dimensional structure [Ln^III(dppeO₂)_1.5(NO₃)₃(H₂O)_0.5]_n (Ln = Ce (1), Sm (2) and Dy (6)) and the one-dimensional structure [Ln(dppeO₂)(NO₃)₃DMF]_n (Ln = Eu (3), Gd (4) and Tb (5)). Some of the Ln-derived complexes have been used as structural probes, while others have been synthesized to use the specific characteristics of each cation to take advantage of their magnetic/luminescence properties. Complex 6 presents slow relaxation of the magnetization while 2, 3 and 5 present emitting properties in the visible range.

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.

Dual-Color Photoluminescent Functionalized Nanoparticles for Static-Dynamic Anticounterfeiting and Encryption: First Collaboration of Spiropyran and Coumarin

Increasing the security of anticounterfeiting materials has been the most important challenge in recent years, and the development of dual-color photoluminescent inks with multi-level security, static/dynamic emission, and dynamic color change is an important solution to overcome this problem. In this study, the multi-functionalized copolymer nanoparticles containing different functional groups (with a concentration of 20 wt %), including ester, carboxylic acid, hydroxyl, epoxide, amide, and amine groups were synthesized successfully by the emulsion polymerization method. The results showed that the particle size and morphology of nanoparticles are affected by the polarity of functional groups. The prepared multi-functionalized copolymer nanoparticles were modified physically with spiropyran (photochromic and red fluorescence emission) and coumarin (cyan emission) derivatives to develop dual-color photoluminescent polymer nanoparticles with application in static-dynamic photoluminescent anticounterfeiting inks, which have multi-level security. The investigation of optical properties indicates that the kinetics of photochromism and photoluminescence properties of samples containing spiropyran is dependent on the local polarity on the surface of polymer nanoparticles. Hence, an increase in the polarity (functionalization with amide, carboxylic acid, and hydroxyl groups) has resulted in fast photochromism, high-intensity photoluminescence emission and increased the efficiency of the photoswitchable color change of emission from cyan to pink. Dual-color photoluminescent anticounterfeiting inks were prepared by mixing polymer nanoparticles containing spiropyran with polymer nanoparticles containing coumarin, in different ratios (1:1, 1:3, 1:5, 1:8, and 1:10). Obtained results showed that prepared samples have cyan emission under UV light of 254 nm (static mode), and a dynamic photoswitching of fluorescence emission from cyan to pink (as a function of irradiation time) was also observed under UV-light irradiation of 365 nm, which is well known as a dynamic mode of emission. The responsivity and intensity of dynamic photoluminescence emission are dependent on the local polarity of the surface functional groups, in which the samples based on amide functionalized copolymer nanoparticles displayed high-intensity emission in the static mode and high-intensity photoswitchable dual-color emission in the dynamic mode, in the case of all ratios of colloid solution mixtures. Printing security tags on cellulose paper by dual-color photoluminescent inks indicates advantages such as maximum printability, resolution, brightness, and static-dynamic photoluminescence emission with high intensity for inks based on amide functionalized nanoparticles. The static-dynamic dual-color photoluminescent anticounterfeiting ink with unique properties and multi-level security was reported for the first time by the collaboration of spiropyran and coumarin. This study can open a new approach and window to the future of advanced and high-security anticounterfeiting technologies.

Janus Microdroplets with Tunable Self-Recoverable and Switchable Reflective Structural Colors

Microdroplets made from chiral liquid crystals (CLCs) can display reflective structural colors. However, the small area of reflection and their isotropic shape limit their performance. Here, Janus microdroplets are synthesized through phase separation between CLCs and silicone oil. The as-synthesized Janus microdroplets show primary structural colors with ≈14 times larger area compared to their spherical counterparts at a specific orientation; the orientation and thus the colored/transparent states can be switched by applying a magnetic field. The color of the Janus microdroplets can be tuned ranging from red to violet by varying the concentration of the chiral dopant in the CLC phase. Due to the density difference between the two phases, the Janus microdroplets prefer to orientate the silicone oil side up vertically, enabling the self-recoverable structural color after distortion. The Janus microdroplets can be dispersed in aqueous media to track the configuration and speed of magnetic objects. They can also be patterned as multiplexed labels for data encryption. The magnetic field-responsive Janus CLC microdroplets presented here offer new insights to generate and switch reflective colors with high color saturation. It also paves the way for broader applications of CLCs, including anti-counterfeiting, data encryption, display, and untethered speed sensors.

NIR-II Responsive Upconversion Nanoprobe with Simultaneously Enhanced Single-Band Red Luminescence and Phase/Size Control for Bioimaging and Photodynamic Therapy

Lanthanide based upconversion (UC) nanoprobes have emerged as promising agents for biological applications. Extending the excitation light to the second near-infrared (NIR-II), instead of the traditional 980/808 nm light, and realizing NIR-II responsive single-band red UC emission is highly demanded for bioimaging application, which has not yet been explored. Here, a new type of NIR-II (1532 nm) light responsive UC nanoparticles (UCNPs) with enhanced single-band red UC emission and controllable phase and size is designed by introducing Er³⁺ as sensitizer and utilizing Mn²⁺ as energy manipulator. Through tuning the content of Mn²⁺ in NaLnF₄ :Er/Mn, the crystal phase, size, and emitting color are readily controlled, and the red-to-green (R/G) ratio is significantly increased from ≈20 to ≈300, leading to NIR-II responsive single band red emission via efficient energy transfer between Er³⁺ and Mn²⁺ . In addition, the single band red emitting intensity can be further improved by coating shell to avoid the surface quenching effect. More importantly, NIR-II light activated red UC bioimaging and photodynamic therapy through loading photosensitizer of zinc phthalocyanine are successfully achieved for the first time. These findings provide a new strategy of designing NIR-II light responsive single-band red emissive UCNPs for biomedical applications.

Lanthanide-Doped Upconversion Nanoparticles: Exploring A Treasure Trove of NIR-Mediated Emerging Applications

Lanthanide-doped upconversion nanoparticles (UCNPs) possess the remarkable ability to convert multiple near-infrared (NIR) photons into higher energy ultraviolet-visible (UV-vis) photons, making them a prime candidate for several advanced applications within the realm of nanotechnology. Compared to traditional organic fluorophores and quantum dots (QDs), UCNPs possess narrower emission bands (fwhm of 10-50 nm), large anti-Stokes shifts, low toxicity, high chemical stability, and resistance to photobleaching and blinking. In addition, unlike UV-vis excitation, NIR excitation is nondestructive at lower power intensities and has high tissue penetration depths (up to 2 mm) with low autofluorescence and scattering. Together, these properties make UCNPs exceedingly favored for advanced bioanalytical and theranostic applications, where these systems have been well-explored. UCNPs are also well-suited for bioimaging, optically modulating chemistries, forensic science, and other state-of-the-art research applications. In this review, an up-to-date account of emerging applications in UCNP research, beyond bioanalytical and theranostics, are presented including optogenetics, super-resolution imaging, encoded barcodes, fingerprinting, NIR vision, UCNP-assisted photochemical manipulations, optical tweezers, 3D printing, lasing, NIR-II imaging, UCNP-molecule nanohybrids, and UCNP-based persistent luminescent nanocrystals.

Triply Hiding Optical Information via Excitation-Dependent Allochroic Photoluminescence Based on Cellulose Derivates

Optical encryption technologies are widely used in information security, whereas the technology with one single optical secret key can be easily cracked. Here, a triple encryption is reported, which hides patterned information in excitation-dependent allochroic materials with long afterglow, enhancing the security level. The allochroic materials are based on a uniaxial co-assembly structure of cellulose nanocrystals (CNCs) and silica. The assembled CNCs present blue emission with quantum yield of 19.8% under 367 nm UV radiation. The blue emission is maintained in the inverse structure when CNCs are calcinated and converted to carbon dots (CDs). The inverse uniaxial-assembly structure improves the CD emission by 6.7 times. The assembly structure can even improve the phosphorescence of CDs, leading to excellent excitation-dependent allochroic properties. Specifically, the materials maintain a cyan long afterglow luminescence at 480 nm after removing 365 nm UV light, whose lifetime is 0.492 s. Changing the excitation wavelength to 254 nm, a UV emission at 343 nm can be obtained, alongside a blue long afterglow luminescence of 420 nm, whose lifetime is 1.574 s. Combining with blue afterglow materials, optical encryption labels are prepared, which hide different patterned information in three scenarios: natural light, UV light, and afterglow luminescence.

Development of Thermochromic Ink Using the Anthocyanidin-Based Red-Cabbage Extract for Anticounterfeiting Applications

Temperature-driven colorful switching inks have been an interesting security encoding method to improve the anticounterfeiting properties of commercially available merchandise. Recently, thermochromic inks have faced many disadvantages, such as low efficiency, high cost, and low durability. In the current study, we developed self-healable ink from poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPSA) integrated with the anthocyanidin(ACY)-based red-cabbage extract in the presence of ferrous sulfate for authentication purposes. Self-healable inks have been able to guarantee durability and thermal stability. Environmentally friendly, ACY-based chromophore was extracted from Brassica oleracea L. var. Capitata (red-cabbage) to serve as a spectroscopic probe immobilized into PAMPSA. The prepared self-healable nanocomposite ink (PAMPSA-ACY) displayed temperature-induced chromism with high reversibility and thermal stability. Different self-healable nanocomposite inks of thermochromic features were prepared employing different ratios of the ACY-based red-cabbage extract. As described by Commission Internationale de L'éclairage Lab coordinates, homogeneous films were stamped on the paper surface to show a purple color (631 nm) able to switch color into red (458 nm) with the increase in temperature from 25 to 65 °C, respectively. Transmission electron microscopy, infrared spectra (FT-IR), energy-dispersive X-ray, and scanning electron microscopy were utilized to inspect the morphological behavior and chemical compositions of thermochromic prints. Both mechanical and rheological properties of ink-printed paper substrates and ink solution were also investigated. Both of antimicrobial activity and cytotoxicity study of the nanocomposite ink (PAMPSA-ACY) were also evaluated. Various industries can take the advantage of the current ink as a competent approach for anticounterfeiting purposes.

Synthesis of a Tetrahedral Metal-Organic Supramolecular Cage with Dendritic Carbazole Arms

In recent years, incredible endeavors have been devoted to the design and self-assembly of discrete metal-organic cages (MOCs) with expanding intricacy and functionality. The controlled synthesis of metal-organic supramolecular cages with large branched chains remains an interesting and challenging work in supramolecular chemistry. Herein, a tetrahedral metal-organic supramolecular cage (ZnII4L4) containing 12 dendritic carbazole arms is unprecedentedly constructed through coordination-driven subcomponent self-assembly and characterized in different ways. Interestingly, tetrahedral supramolecular Cage-1 exhibited the potential for aggregation-induced emission (AIE) performance and stimulus-responsive luminescence features, and it achieved color-tunable photoluminescence due to the introduction of dendritic carbazole arms. Crucially, owing to the great photophysical properties of Cage-1 in solution, Cage-1 was enabled to act as a fluorescent ink for the vapor-responsive recording and wiping of information.

Adapting UFF4MOF for Heterometallic Rare-Earth Metal-Organic Frameworks

Heterometallic metal-organic frameworks based on rare-earth metals (RE-MOFs) have potential in a number of applications where energy transfer between nearby metal atoms is required. This observation implies that it is important to understand the level of local mixing that is achieved between metals of different types during synthesis of RE-MOFs. Density functional theory calculations can give quantitative information on the relative energy of different configurations of RE-MOFs, but these calculations cannot be applied to the full range of medium- and long-range orderings that are possible in heterometallic materials. This limitation can be overcome using force field (FF)-based calculations if appropriate FFs are available. We show that an existing generic FF for MOFs, UFF4MOF, does not accurately predict energies of mixing in heterometallic Nd/Yb MOFs and introduce a modified FF to address this shortcoming. The resulting FF is used to explore metal orderings in large simulation volumes for a Nd/Yb MOF, illustrating the complexities that can arise in the structure of heterometallic RE-MOFs.