Anti-counterfeiting Materials Research Articles

Multi-mode photoluminescence inks based on lanthanide-doped multilevel core-shell microspheres for advanced anti-counterfeiting applications

Development of advanced luminescent nanomaterials and technologies is of great significance for anticounterfeiting applications in global economy, security, and human health. In this work, we have designed two types of anti-counterfeit materials. (1) The dual excitation mode core–shell microsphere is based on SiO2 core, with the GdVO4:Yb,Er upconversion luminescent inner layer, SiO2 shell separator, and Gd2O3:Eu@SiO2 downconversion luminescent layer are sequentially deposited on it via co-precipitation method. The microspheres have a five-layer core–shell structure with an overall diameter of ∼ 146 nm and enables the spatial separation of Er3+ and Eu3+, which prevents the cross-relaxation of Er3+. The microspheres were prepared into an upconversion/downconversion excitation mode luminescent ink, with inkjet-printed patterns that only extract information when excited by 254 nm and 980 nm excitation. (2) The core–shell-shell SiO2@AIPA-S-Si-Tb@SiO2 microspheres have been synthesized by using organic ligands ((HOOC)2C6H3NHCONH(CH2)3Si(OCH2CH3)3) (defined as AIPA-S-Si) connected with Tb3+ ions and silica through covalent bond. The microspheres have a sandwich structure with an overall diameter of ∼ 100 nm and show bright green light. The sandwich structure and five-layer core–shell structure was mixed proportionally to obtain orange-red, white, and yellow emission ink under 254 nm, green emission under 310 nm, and green emission under 980 nm excitation.

Fluorescent Materials Based on Spiropyran for Advanced Anti-Counterfeiting and Information Encryption.

In daily life, counterfeit and substandard products, particularly currency, medicine, food, and confidential documents, are capable of bringing about very serious consequences. The development of anti-counterfeiting and authentication technologies with multilevel securities is a powerful means to overcome this challenge. Among various anti-counterfeiting technologies, fluorescent anti-counterfeiting technology is well-known and commonly used to fight counterfeiters due to its wide material source, low cost, simple usage, good concealment, and simple response mechanism. Spiropyran is favored by scientists in the fields of anti-counterfeiting and information encryption due to its reversible photochromic property. Here, we summarize the current available spiropyran-based fluorescent materials from design to anti-counterfeiting applications. This review will be help scientists to design and develop fluorescent anti-counterfeiting materials with high security, high performance, quick response, and high anti-counterfeiting level.

Excitation-Dependent Emission in Sb3+-Doped All-Inorganic Rare-Earth Double Perovskites for Anticounterfeiting Applications.

Achieving high-efficiency tunable emission in a single phosphor remains a significant challenge. Herein, we report a series of Sb3+-doped all-inorganic double perovskites, Sb3+:Cs2NaScCl6, with efficient excitation-dependent emission. In 0.5%Sb3+:Cs2NaScCl6, strong blue emission with a high photoluminescence quantum yield (PLQY) of 85% is obtained under 265 nm light irradiation, which turns into bright neutral white light with a PLQY of 56% when excited at 303 nm. Spectroscopic and computational investigations were performed to reveal the mechanism of this excitation-dependent emission. Sb3+ doping induces two different excitation channels: the internal transition of Sb3+: 5s2 → 5s5p and the electron transfer transition of Sb3+: 5s → Sc3+ 3d. The former one generates excited Sb3+ ions, which can undergo efficient energy transfer to populate the host self-trapped exciton (STE) state, yielding enhanced blue emission. The latter one leads to the formation of a new STE state with the hole localized on Sb3+ and the electron delocalized on the nearest Sc3+, which accounts for the newly exhibited low-energy emission. The difference in the excitation pathways of the two emitting STE states results in the highly efficient excitation-dependent emission, making the doped systems promising anticounterfeiting materials.

Dual-Mode Hydrogels with Structural and Fluorescent Colors toward Multistage Secure Information Encryption.

Constructing an anti-counterfeiting material with non-interference dual optical modes is an effective way to improve information security. However, it remains challenging to achieve multistage secure information encryption due to the limited stimulus responsiveness and color tunability of the current dual-mode materials. Herein, a dual-mode hydrogel with both independently tunable structural and fluorescent colors toward multistage information encryption, is reported. In this hydrogel system, the rigid lamellar structure of poly(dodecylglyceryl itaconate) (pDGI) formed by shear flow-induced self-assembly provides the restricted domains wherein monomers undergo polymerization to form a hydrogel network, producing structural color. The introduction of fluorescent monomer 6-acrylamidopicolinate (6APA) as a complexation site provides the possibility of fluorescent color formation. The hydrogel's angle-dependent structural color can be controlled by adjusting the crosslinking density and water content. Additionally, the fluorescence color can be modulated by adjusting the ratio of lanthanide ions. Information of dual-mode can be displayed separately in different channels and synergistically overlayed to read the ultimate message. Thus, a multistage information encryption system based on this hydrogel is devised through the programed decryption process. This strategy holds tremendous potential as a platform for encrypting and safeguarding valuable and authentic information in the field of anti-counterfeiting.

Force/Acid-Activated and Radical-Regulated photochromic luminescence in a binuclear Zero-Dimensional complex for multi-level anti-counterfeiting

Traditional static optical anti-counterfeiting technology can only output a single emission signal, which is easy to replicate and difficult to meet the needs in current high-tech fields. Multi-level stimuli-responsive luminescent materials with internal logical relationship can improve the degree of anti-counterfeiting and resist against the flood of counterfeiting. In this work, we propose to use force and acid dual stimuli to restrict the dissipation path of photogenerated free radicals, which further lead to the radical-actuated photochromic luminescence (PCL) performance. Based on this strategy, a zero-dimensional (0D) dinuclear zinc complex Zn2(9-AC)4(IM)2 (1) with blue luminescent properties (450 nm) was designed and synthesized. The interaction between 9-AC and IM (as 9-AC/IM exciplex) can be cut off by force-induced rearrangement of structural units in the spatial dimensions or acid-induced protonation of 9-AC, achieving the luminescent transformation from 9-AC/IM exciplex (447 nm) to 9-AC/9-AC excimer (480 nm) or 9-HAC (485 nm). At the same time, the PCL performance was activated and the emission intensity quenching rate increased from 4.77 % to 93.86 % or 44.93 % under UV light irradiation, respectively. The solid-state ultraviolet–visible absorption spectra, electron paramagnetic resonance spectra and theoretical calculation confirmed that the activation of PCL performance is due to the fact that the 9-AC• radicals are more difficult to dissipate upon force- or acid-induced stimuli. Combining mechanochromic luminescence (MCL) and PCL, a dynamic anti-counterfeiting quick response (QR) code with multi-level and fast-responsive logic sequence have been designed, which are promising for the application in time-resolved information encryption. This work provides an effective strategy to construct multi-level stimuli-responsive luminescence as novel anti-counterfeiting materials by changing the photogenerated free radical dissipation pathway to activate PCL properties.

A Tunable Hydrophilic-Hydrophobic, Stimulus Responsive, and Robust Iridescent Structural Color Bionic Film with Chiral Photonic Crystal Nanointerface.

Bio-inspired in nature, using nanomaterials to fabricate the vivid bionic structural color and intelligent stimulus responsive interface as smart skin or optical devices are widely concerned and remain a huge challenge. Here, the bionic flexible film is designed and fabricated with chiral nanointerface and tunable hydrophilic-hydrophobic by the ultrasonic energy perturbation strategy and crosslinking of the cellulose nanocrystals (CNC). An intelligent nanointerface with adjustable hydrophilic and hydrophobic properties is constructed by the supramolecular assembly using a smart ionic liquid molecule. The bionic flexible film possessed the variable hydrophilic-hydrophobic, stimulus responsive, and robust iridescent structural color. The reflective wavelength and the helical pitch of the film can be easily modulated through the ultrasonic energy perturbation strategy. The bionic flexible film by covalent cross-linking has excellent robustness, good elasticity and flexibility. The tunable brilliant structural color of the chiral nanointerface is attributed to the surface charge change of the CNC photonic crystal, which is disturbed by ultrasonic energy perturbation. The bionic flexible film with tunable structure color has intelligent hydrophilic and hydrophobic stimulus response properties. The chiral bionic materials have potential applications in smart skin, optical devices, bionic materials, robots, anti-counterfeiting, colorful displays, and stealth materials.

A gelatin-based ionic conductor with a dense/loose alternating network enabled by surface active cellulose

Although conductive and stretchable ionic conductors are increasingly prepared for flexible sensing applications, such as health monitoring, tissue engineering, and electronic skin, achieving a balance between remarkable mechanical properties and high sensing sensitivity remains a significant challenge. In this work, a dense/loose alternating network was meticulously constructed through a pre-assembly process in sodium citrate solution and a subsequent re-assembly in FeCl3 solution. This approach ensured the robustness of the network structure and facilitated ion migration. Within the network, a gelatin network cross-linked via covalently and hydrogen bonds was significantly enhanced by a synergy of micro-enhancement and dynamic bidirectional crosslinking containing hydrogen bonds, imine bonds, and Fe coordinate bonds enabled by surface active cellulose. Benefiting from the dense network, the obtained composite hydrogels delivered a strong tensile strength (496.3 kPa), outstanding elastic modulus (917.6 kPa), and an excellent energy dissipation rate (55.6 %) at 80 % of strain. The loose polymer network can also endow Fe3+/Cl− ions with sensitive mobility, leading to an ultra-high sensing sensitivity (maximum GF: 20.06). These exceptional properties enable the ionic conductor to accurately detect a wide range of motions and pressures without being affected by environmental noise. With attributes such as reusability, biodegradability, strength, and sensitivity, this ionic conductor emerges as a reliable soft material with promising applications in the field of healthcare monitoring, wearable strain sensor sensors, and writing anti-counterfeiting materials.

Stimulus-responsive room-temperature phosphorescence with tunable color for time-resolved advanced dynamic anti-counterfeiting

Multi-stimuli responsive multicolored room-temperature phosphorescent (RTP) carbon quantum dots (CQDs) have recently attracted considerable attention, owing to their potential applications in advanced dynamic anti-counterfeiting. The design and development of carbon dots that exhibit both stimulus response characteristics and multicolor RTP emission remains a significant challenge in the field. In this work, we present a novel approach to control the dynamic phosphorescence color and stimulus response properties of CQDs coated on paper by fine-tuning the surface functional groups of the CQDs. Specifically, histidine and 2,4,6-triaminopyrimidine was separately used to prepare N, O co-doped CQDs (NOCQDs) and N-doped CQDs (NCQDs), respectively. These CQDs were further coated on filter paper substrates to achieve water and heating cascade stimulus–response RTP. These stimuli-responsive properties of NOCQDs can be modulated by deprotonating their surface oxygen-containing groups. Eventually, we achieved time-resolved multicolor dynamic anti-counterfeiting by exploiting the stimuli-response properties and lifetimes differences between NOCQDs and NCQDs, realizing the 5D information encryption in all five stimulus-, pattern-, time-, and color-dependent codes. These findings highlight the potential of NOCQDs as advanced anti-counterfeiting materials and lay a foundation for further exploration into the synthesis and properties of CQDs with tunable stimuli-response features and RTP emission.

Coating Red Phosphor on Green Luminescent Material for Multi-Mode Luminescence and Advanced Anti-Counterfeit Applications

Traditional fluorescent anti-counterfeiting materials usually exhibit fixed-wavelength excitation patterns and monochromatic luminescence, which are extremely easy to be counterfeited and have low security. Therefore, there is an urgent need to develop multi-mode fluorescent materials with enhanced security to address this issue. Here, SrAl2O4:1%Eu,2%Dy@Y2O3:Eu3+ core-shell structured phosphors were prepared via a sol-gel method. Coating SrAl2O4:Eu,Dy with Y2O3:Eu3+ red phosphor did not significantly change the crystal structure of SrAl2O4. Under UV excitation at 254 nm, SrAl2O4:1%Eu,2%Dy@Y2O3:Eu3+ exhibited red emission at 613 nm (5D0→7F2 transition of Eu3+), and a strong green afterglow was observed after removing the UV irradiation. However, blue-green emission at 496 nm was observed under UV excitation at 365 nm, followed by green afterglow upon removal of the light source. Varying the content of the Y2O3:Eu3+ shell yielded different emissions and afterglows. The prepared samples are sensitive to the excitation wavelength and duration and have multimodal luminescence properties, which can be used for anti-counterfeiting patterns. The outcomes in this work indicate that the phosphor is a promising fluorescent material for anti-counterfeiting.

Aggregation-induced-emission red carbon dots for ratiometric sensing of norfloxacin and anti-counterfeiting

The detection of trace antibiotic residues holds significant importance because it’s related to food safety and human health. In this study, we developed a new high-yield red-emitting carbon dots (R-CDs) with aggregation-induced emission properties for ratiometric sensing of norfloxacin. R-CDs were prepared in 30 min using an economical and efficient microwave-assisted method with tartaric acid and o-phenylenediamine as precursors, achieving a high yield of 34.4 %. R-CDs showed concentration-dependent fluorescence and aggregation-induced-emission properties. A ratiometric fluorescent probe for detecting the norfloxacin was developed. In the range of 0–40 μM, the intensity ratio of two emission peaks (I445 nm/I395 nm) towards norfloxacin show good linear relationship with its concentrations and a low detection limit was obtained (36.78 nM). In addition, complex patterns were developed for anti-counterfeiting based on different emission phenomenon at different concentrations. In summary, this study designed a novel ratiometric fluorescent probe for detection of norfloxacin, which greatly shortens the detection time and improves efficiency compared with high-performance liquid chromatography and other methods. The study will promote the application of carbon dots in anti-counterfeiting and other related fields, laying the foundation for the preparation of low-cost photosensitive anti-counterfeiting materials.

Host-guest doped room/high-temperature phosphorescence of diarylfuro[3,2-b]pyridine derivatives

Due to the high temperature that can easily lead to non-radiative transitions of triplet excitons, obtaining high-temperature phosphorescence (HTP) materials is challenging. Herein, using diarylfuro[3,2-b]pyridines as the guest molecules and polyacrylic acid (PAA) as the host molecule, the two-component doped materials exhibit green room-temperature phosphorescence (RTP) with afterglow times of 2–8 s, delayed lifetimes of 256–939 ms, and phosphorescence quantum efficiencies of 140.3–21.2 %, and green HTP emissions with 2 s afterglow and 208 ms delayed lifetime at 373 K. The HTP activity of the doped system originates from the rigid environment provided by PAA, planar and rigid molecular structures of diarylfuro[3,2-b]pyridines, and strong interactions between PAA and diarylfuro[3,2-b]pyridines. Furthermore, using organic small molecules as the reference host molecules, compared to benzophenone, 5-methylphthalic anhydride, and butane-1,2,3,4-tetracarboxylic acid with chemical structures similar to PAA result in better RTP/HTP properties, revealing that strong interactions between host and guest molecules play a very important role in ultralong phosphorescence emissions of these host–guest doped materials. Moreover, these doped materials based on diarylfuro[3,2-b]pyridines can be developed as advanced anti-counterfeiting and encryption materials. This result provides valuable reference for developing excellent RTP/HTP materials based on N,O-containing fused-ring compounds.

Color-changing Fluorescent Carbon Dots Synthesized by Simple Hydrothermal Method for Anti-counterfeiting Pattern Study

A simple hydrothermal method was employed to synthesize the carbon dots with dopamine hydrochloride used as the mainly carbon source and o-phenylenediamine as the nitrogen source. In the study, the fluorescent carbon dots which have good water solubility and alcohol solubility showed visible pink under daylight and orange under UV after purification by column chromatography, and dichloromethane: methanol (v: v =20:1) was selected as the separation solvents. Meanwhile, the pink fluorescent carbon dots (λmax =595 nm under 577 nm excitation) was observed that it can be changed to bright yellow (λmax =530 nm under 442 nm excitation), the conditions of different stocked solvents and time were explored in detail. The IR spectra for the pink carbon dots indicated its color-changing property due to the unsatisfaction group of its chemical structure. For UV and fluorescence characterization, the quantum yield of pink carbon dots is ∼11.0%. In addition, the application of color-changing carbon dots as anti-counterfeiting ink for high-quality anti-counterfeit patterns on paper was investigated, and the preliminary results showed good potential of the prepared carbon dots as effective and eco-friendly anti-counterfeiting materials.

Flexible multiple-stimulus-responsive photonic films based on layer-by-layer assembly of cellulose nanocrystals and deep eutectic solvents

Photonic crystal materials responding to environmental stimuli are important in sensors, intelligent detection, and anti-counterfeiting. However, traditional photonic materials usually lack tunable color and sufficient color suppleness, hindering their practical applications. Here, we report a facile strategy to achieve mechanical and color-tunable photonic films by doping deep eutectic solvents (DES) into cellulose nanocrystals (CNC) using layer-by-layer assembly. The amount of DES and several assembly layers were vital in determining the structure color and mechanical and response properties in the CNC photonic films. The experimental results revealed that DES could endow CNC suspension systems with high zeta potential, high viscoelasticity, and low transmittance. The tiny DES compensated for the insufficient flexibility of the CNC film, resulting in a wide color range, high solvent discrimination ability, and multi-color at several view angles. Moreover, photonic crystal film also played an important role in intelligent display and could be applied to traffic reflective signs to remind pedestrians and drivers of road safety and as smart labels for indoor humidity detection. These characteristics of the CNC photonic films could provide a promising strategy for developing advanced intelligent sensors, detection, and anti-counterfeiting materials.

Robust, self-healing AIE fluorescent supramolecular elastomers for smart anti-counterfeiting

Providing aggregation-induced emission (AIE) polyurethane with self-healing capability can increase the lifetime of the material and adapt it to multiple application scenarios, which is a huge challenge in the development of luminescent smart materials. Herein, a range of transparent fluorescence polyurethanes (FPUs) with prominent strength were designed and synthesized. The optimization of robust FPUs exhibits outstanding mechanical performance (33.5 MPa tensile strength, 120.2 MJ m−3 toughness) and self-healing ability (88.9 %, at 80 °C for 2 h). Furthermore, the aggregation of tetraphenylethylene structure in the hard segment causes the system to emit bright blue fluoresence. We successfully screen-printed designable patterns on the self-healing polyurethane substrates. The concealed patterns self-repair after damage, and can be clearly and robustly displayed under 365 nm light irradiation. The as-prepared multifunctional self-healing material, exhibiting stable AIE fluorescence, is anticipated to possess significant value in the domains of smart anti-counterfeiting materials, flexible display screens and intelligent optoelectronics.

Viscoelasticity-Controlled Relaxation in Wrinkling Surface for Multistage Time-Resolved Optical Information Encryption.

As counterfeit techniques continue to evolve, ensuring the security of conventional "static" encryption methods becomes increasingly challenging. Here, the viscoelasticity-controlled relaxation is introduced for the first time in a bilayer wrinkling system by regulating the density of hydrogen bond networks in polymer to construct a "dynamic" encryption material. The wrinkling surface can manipulate light during the dynamic relaxation process, exhibiting three stages with frosted glass, structural color, and mirror reflection. By regulating the viscoelasticity of skin layer through UV irradiation, the wavelength and the relaxation rate of the wrinkles can be controlled. As a result, dynamic wrinkling anti-counterfeiting patterns and time-resolved multistage information encryption are achieved. Crucially, the encryption material is developed as an anti-counterfeiting label for packing boxes in daily applications, allowing the encrypted information to be activated manually and identified by naked eyes, surpassing the existing time-resolved encryption materials in utilization potential. Besides, the dynamic hydrogen bond networks are extended to various dynamic interaction networks, demonstrating the versatility of the dynamic encryption strategy. This work not only provides an additional dimension for dynamic information encryption in daily practical use, but also offers theoretical guidance for the development of advanced optical anti-counterfeiting and smart display materials in the future.

Pt(II)/Pd(II)-Based Metallosupramolecular Architectures as Light Harvesting Systems and their Applications.

The development of artificial light-harvesting systems mimicking the natural photosynthesis method is an ever-growing field of research. Numerous systems such as polymers, metal complexes, POFs, COFs, supramolecular frameworks etc. have been fabricated to accomplish more efficient energy transfer and storage. Among them, the supramolecular coordination complexes (SCCs) formed by non-covalent metal-ligand interaction, have shown the capacity to not only undergo single and multistep energy migration but also to utilize the harvested energy for a wide variety of applications such as photocatalysis, tunable emissive systems, encrypted anti-counterfeiting materials, white light emitters etc. This review sheds light on the light-harvesting behavior of both the 2D metallacycles and 3D metallacages where design ingenuity has been executed to afford energy harvesting by both donor ligands as well as metal acceptors.

High temperature fluorescence and multi-mode anti-counterfeiting material based on Eu doping

Due to the influence of the thermal quenching effect, the luminescence intensity of luminescent materials sharply decreases in high-temperature regions, strongly affecting their application in optical anti-counterfeiting and optical temperature supervision. In this study, we report a multi-mode anti-counterfeiting phosphor doped with Eu3+ that exhibits fluorescence thermoluminescence enhancement. This phosphor can trap electrons and release them as luminescence under external stimulation (the light and heat). The compensatory luminescence at high temperatures is the reason for the unique fluorescence thermoluminescence enhancement characteristics of this phosphor. Therefore, the sensitivity and differential response of this phosphor to light and heat can be used to monitor anti-counterfeiting patterns or temperature. This work provides an example for the future application of optical anti-counterfeiting and temperature monitoring.

Moiré effect enables versatile design of topological defects in nematic liquid crystals

Recent advances in surface-patterning techniques of liquid crystals have enabled the precise creation of topological defects, which promise a variety of emergent applications. However, the manipulation and application of these defects remain limited. Here, we harness the moiré effect to engineer topological defects in patterned nematic liquid crystal cells. Specifically, we combine simulation and experiment to examine a nematic cell confined between two substrates of periodic surface anchoring patterns; by rotating one surface against the other, we observe a rich variety of highly tunable, novel topological defects. These defects are shown to guide the three-dimensional self-assembly of colloids, which can conversely impact defects by preventing the self-annihilation of loop-defects through jamming. Finally, we demonstrate that certain nematic moiré cells can engender arbitrary shapes represented by defect regions. As such, the proposed simple twist method enables the design and tuning of mesoscopic structures in liquid crystals, facilitating applications including defect-directed self-assembly, material transport, micro-reactors, photonic devices, and anti-counterfeiting materials.

Multicolour room temperature phosphorescence of carbon nitride nanoparticles in sodium borohydride and borax matrix

Carbon nitride based materials find applications in photocatalysis, sensing, and bioimaging. The fluorescence properties of carbon nitride based nanomaterials is widely studied. However, its phosphorescence phenomenon is relatively less explored. Herein, we report a simple and economic one step thermal synthesis of highly fluorescent pure carbon nitride nanoparticles (CNNPs) from organic solvent formamide at 220 °C, just above its boiling point without any further passivation or purification.The fluorescent CNNPs is dispersed in sodium borohydride and borax which act as suitable host matrix resulting in activation of persistent room temperature phosphorescence of CNNPs with high phosphorescence lifetimes of 1.144 s and 0.737 s respectively. Moreover, the CNNPs-Sodium borohydride composite display tricolour, blue, green and yellow-orange phosphorescence under 254 nm, 365 nm and 400 nm light respectively while CNNPs-Borax composite show dual blue and green phosphorescence under 254 nm and 365 nm UV light respectively. Hence, these CNNPs-Composite may be used as multicolour anti-counterfeiting materials in future.

Development of a long-term fluorescent invisible marker using blue-emitting carbon dots

The development of novel anti-counterfeiting materials and technologies is vital in order to combat counterfeit activities, as existing technologies such as optical watermarking, micro-printing, laser holography, barcodes, RFID, and QR codes are widely known to counterfeiters and easily replicated. In particular, several challenges and issues, including high costs, low yields during synthesis, and the photo-bleaching effect associated with the use of luminescent materials in anti-counterfeiting applications are still need to be addressed. Carbon Dots (CDs) is a new emergent in carbon family with excellent luminescence properties. In this report, we describe a fast, cost-effective, and simple one-step heating method for synthesizing CDs. An invisible marker was produced by incorporating blue emitting CDs into the reservoir of a commercial sketch pen. The writings using the marker were not visible under normal light but emitted blue fluorescence under UV light (365 nm). The marker retained its fluorescence for up to six months without any decrease visible to the naked eye.