Full-Color Tunable Fluorescent and Chemiluminescent Supramolecular Nanoparticles for Anti-counterfeiting Inks

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.

Inkjet printing of heavy-metal-free quantum dots-based devices: a review

Inkjet printing (IJP) has become a versatile, cost-effective technology for fabricating organic and hybrid electronic devices. Heavy-metal-based quantum dots (HM QDs) play a significant role in these inkjet-printed devices due to their excellent optoelectrical properties. Despite their utility, the intrinsic toxicity of HM QDs limits their applications in commercial products. To address this limitation, developing alternative HM-free quantum dots (HMF QDs) that have equivalent optoelectronic properties to HM QD is a promising approach to reduce toxicity and environmental impact. This article comprehensively reviews HMF QD-based devices fabricated using IJP methods. The discussion includes the basics of IJP technology, the formulation of printable HMF QD inks, and solutions to the coffee ring effect. Additionally, this review briefly explores the performance of typical state-of-the-art HMF QDs and cutting-edge characterization techniques for QD inks and printed QD films. The performance of printed devices based on HMF QDs is discussed and compared with those fabricated by other techniques. In the conclusion, the persisting challenges are identified, and perspectives on potential avenues for further progress in this rapidly developing research field are provided.

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.

Stimuli-fluorochromic smart organic materials

Smart materials based on stimuli-fluorochromic π-conjugated solids (SFCSs) have aroused significant interest due to their versatile and exciting properties, leading to advanced applications. In this review, we highlight the recent developments in SFCS-based smart materials, expanding beyond organometallic compounds and light-responsive organic luminescent materials, with a discussion on the design strategies, exciting properties and stimuli-fluorochromic mechanisms along with their potential applications in the exciting fields of encryption, sensors, data storage, display, green printing, etc. The review comprehensively covers single-component and multi-component SFCSs as well as their stimuli-fluorochromic behaviors under external stimuli. We also provide insights into current achievements, limitations, and major challenges as well as future opportunities, aiming to inspire further investigation in this field in the near future. We expect this review to inspire more innovative research on SFCSs and their advanced applications so as to promote further development of smart materials and devices.

Preparation and dynamic color-changing study of fluorescent polymer nanoparticles for individualized and customized anti-counterfeiting application

Preparing new fluorescent materials for individualized and customized anti-counterfeiting applications to meet needs from the rapid development of e-commerce is of great significance. This paper reports the preparation of dynamic color-changing fluorescent polymer nanoparticles (PNPs) by constructing a fluorescence resonance energy transfer (FRET) pair between aggregation-induced emission (AIE) structures and photochromic structures. At first, methyl methacrylate (MMA) was used as the main monomer and tetraphenylethylene (TPE, a typical AIE structure) modified methacrylate (TPE-MA) and photochromic spiropyran (SP) modified methacrylate (SP-MA) as minor monomers were copolymerized to obtain the ternary copolymer PMMA-TPE-SP. Then, two types of PNPs based on this terpolymer was prepared via the reprecipitation method, with and without the addition of an amphiphilic polymer as the surfactant. The photophysical study shows that the fluorescence color of PNPs dynamically changes from blue to light violet and finally to red under UV light irradiation, a process that can be reversed under visible light. The PNPs were alternately irradiated with UV light and visible light for 10 cycles, which proved their good photoswitching reproducibility. The PNPs prepared with addition of surfactant were found to have stronger fluorescence and better stability. Finally, the photochromic fluorescent inks were prepared based on these PNPs. Several anti-counterfeiting scenarios and modes were designed, exhibiting excellent photochromic behavior on cellulose paper, even after 120 days of long-term storage. With simple equipment, desirable anti-counterfeiting effects with dynamic fluorescence color changing was achieved. This study demonstrated a promising hard-to-imitate anti-counterfeiting encryption strategy, which can achieve multiple outputs with simple operation and can be personalized and customized as needed.

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.

Photostable yellow emissive carbon dots for iron-mediated reversible sensing of biothiols and cellular imaging

The different levels of biothiols in cells can not only screen cancer cells but also play a selective role in killing cancer cells. Therefore, accurate monitoring of biothiol in cancer cells is of great research significance. Herein, novel yellow emission CDs (Y-CDs) were prepared by a simple hydrothermal method using 2, 5-dihydroxyterephthalic acid (DHTA) as precursors. The Y-CDs as a highly efficient dual-mode sensor could detect Fe3+ and biothiols by colorimetric and fluorescence signals. Especially, with the addition of L-Cysteine, the quenched fluorescence could be quickly restored within 2 min and the detection limit was as low as 31.65 nM. Additionally, this sensor was utilized to sense biothiols in actual samples and living cells due to its eminent biocompatibility. Finally, the Y-CDs were successfully applied not only as fluorescent ink for message encryption but also as a portable solid hydrogels sensor for the detection of Fe3+ and biothiols. Therefore, these results suggested that Y-CDs could serve as a promising sensor for Fe3+ and biothiol detection in early cancer screening.

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.

Photo-Controlled Nano-Supramolecular Size and Reversible Luminescent Behaviors Based on Cucurbit[7]uril Cascaded Assembly

Supramolecular luminescent material with switchable behavior and photo-induced aggregation with emission enhancement is a current research hot spot. Herein, a size-tunable nano-supramolecular assembly with reversible photoluminescent behavior was constructed by noncovalent polymerization of diarylethene-bridged bis(coumarin) derivative (DAE-CO), cucurbit[7]uril (CB[7]), and β-cyclodextrin-grafted hyaluronic acid (HACD). Benefiting from the macrocyclic confinement effect, the guest molecule DAE-CO was included into the cavity of CB[7] to give enhanced fluorescence emission of the resulting DAE-CO⊂CB[7]2 with longer lifetime at 432 nm to 1.43 ns, thereby further enhancing fluorescence output and lifetime (1.46 ns) when further assembled with HACD, compared with the free DAE-CO (0.95 ns). In addition, DAE-CO, DAE-CO⊂CB[7]2, and DAE-CO⊂CB[7]2&HACD all possessed characteristics of aggregation-induced emission and reversible photo-switched structural interconversion, exhibiting an obvious photophysical activation phenomenon of self-aggregation into larger nanoparticles with increase in fluorescence emission intensity, lifetime, and size after irradiation, which could be increased step by step with the alternating irradiation of 254 nm (5 min) or >600 nm (30 s) repeated 7 times. These supramolecular assemblies were successfully used in the tumor cells' targeted imaging and anti-counterfeiting because of the capability of HACD for recognizing specific receptors overexpressed on the surface of tumor cells and the excellent photo-regulated switch ability of DAE-CO, providing an approach of constructing photo-induced emission-enhanced luminescent materials.

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.

A light-fueled dissipative aggregation-induced emission system for time-dependent information encryption

A light-fueled dissipative aggregation-induced emission (LDAIE) system is successfully fabricated based on reversible electrostatic interactions between cationic AIE luminogens (AIEgens) and anionic spiropyran (ASP) transformed from sulfonato-merocyanine photoacid (SMEH) upon 420 nm light irradiation. The novel LDAIE system can exhibit reversible and spontaneous AIE fluorescence on/off, showing potential in time-dependent information encryption with self-erasing ability. This work opens new opportunities to fabricate a unique fluorescent anti-counterfeiting platform with high-level security.

An Overview of Security Materials in Banknotes and Analytical Techniques in Detecting Counterfeits

Counterfeiting or forged imitation of banknotes is a perpetual practice engulfing global economies. This not only poses challenges for the material scientists to come forth with advanced security materials but also demands veracious forensic examination to detect counterfeits. The present article pursues novel efforts in summarizing a study that lays focus on the recent optical and analytical examinations being used for the characterization and detection of chemical profiles of authentic and counterfeited banknotes. The article briefs the trends in banknote materials, security paper manufacturing process, security inks used for printing, and types of the security printing process in banknote practices. Reported literature shows the introduction of new anti-counterfeiting materials viz. magnetically-responsive photonic anti-counterfeiting watermarks, and fluorescent nanoparticles that can be used as anti-counterfeiting inks, anti-stokes inks, metameric inks, etc. Analytical techniques such as IR spectroscopy, Raman spectroscopy, Mossbauer spectroscopy, X-ray diffraction, X-ray fluorescence, LIBS, XRF, ELDI-MS, EASI/DESI-MS, HPLC, VSC, AFM, etc. in conjunction with different chemometrics approaches have been critically discussed. The study also presents the future scope in banknote examination like the use of hyper spectral imaging and sensor-based counterfeit detection systems.

Controllable Aggregation-Induced Emission and Förster Resonance Energy Transfer Behaviors of Bistable [c2] Daisy Chain Rotaxanes for White-Light Emission and Temperature-Sensing Applications

Bistable [c2] daisy chain rotaxanes with respective extended and contracted forms of [c2]A and [c2]B containing a blue-emissive anthracene (AN) donor and orange-emissive indandione-carbazole (IC) acceptor were successfully synthesized via click reaction. Tunable-emission bistable [c2] daisy chain rotaxanes with fluorescence changes from blue to orange, including bright-white-light emissions, could be modulated by the aggregation-induced emission (AIE) characteristics and Förster resonance energy transfer (FRET) processes through altering water fractions and shuttling processes (i.e., acid/base controls). Accordingly, as a result of excellent fine-tuning AIE (at 60% water content of H₂O/THF) and FRET (with a compatible energy transfer of E_FRET = 33.2%) behaviors after the shuttling process (by adding base), the brightest white-light emission at CIE (0.31, 0.37) with a quantum yield of Φ = 15.64% was obtained in contracted [c2]B with good control of molecular shuttling to possess higher photoluminescence (PL) quantum yields and better energy transfer efficiencies (i.e., the manipulation of reduced PET and enhanced FRET processes) due to their intramolecular aggregations of blue AN donors and orange IC acceptors with a proper water content of 60% H₂O. Furthermore, dynamic light-scattering (DLS) and time-resolved photoluminescence (TRPL) measurements, along with theoretical calculations, were utilized to investigate and confirm AIE and FRET phenomena of bistable [c2] daisy chain rotaxanes. Especially, both bistable [c2] daisy chain rotaxanes [c2]A and [c2]B and noninterlocked monomer M could be exploited for the applications of ratiometric fluorescence temperature sensing due to the temperature effects on the AIE and FRET features. Based on these desirable bistable [c2] daisy chain rotaxane structures, this work provides a potential strategy for the future applications of tunable multicolor emission and ratiometric fluorescence temperature-sensing materials.

Methoxy substituent facilitated wide solvatofluorochromism, white light emission, polymorphism and stimuli-responsive fluorescence switching in donor-π-acceptor

Introducing methoxy substituent into triphenylamine-acetophenone based donor-π-acceptor fluorophore, 3-(4-(diphenylamino)phenyl)-1-phenylprop-2-en-1-one (1), produced strong solvatofluorochromism including white light emission, fluorescent polymorphs and mechano-responsive fluorescence switching. The unsubstituted and methoxy substituted compounds displayed strong solvent polarity mediated tunable emission in the solution. Interestingly, 3-(4-(diphenylamino)phenyl)-1-(4-methoxyphenyl)prop-2-en-1-one (2) and 3-(4-(diphenylamino)-2-methoxyphenyl)-1-(4-methoxyphenyl)prop-2-en-1-one (3) showed single molecule white light emission in DMSO and ethanol, respectively. 1-3 exhibited strong green/yellow fluorescence in the solid-state (Quantum yield (Φ_f) = 10 to 23%). 2 produced fluorescent polymorphs (green (2-G) and yellow (2-Y). Single crystal structural analysis revealed that donor and acceptor phenyl units adopted coplanar conformation in 2-G and 3 whereas twisted molecular conformation in 1 and 2-Y. Further, 2-G exhibited π…π interactions facilitated isolated dimers whereas 2-Y showed well separated molecules in the crystal lattice. Aggregation induced emission (AIE) studies showed morphological transformation induced fluorescence tuning for 2. The intramolecular charge transfer (ICT) from TPA to acetophenone was confirmed by computational studies. Mechanofluorochromic (MFC) studies of 1 showed only slight reduction of intensity without modulating fluorescence wavelength significantly but 2 and 3 exhibited visible emissive colour change from yellow to green and vice versa by crushing and heating. Both 2 and 3 also exhibited self-reversible fluorescence switching that was confirmed by PXRD pattern. Thus, methoxy group introduction resulted in obtaining white light emitting fluorescence molecules in the solution state and self-reversible fluorescence switching materials.

Design of a large Stokes shift ratiometric fluorescent sensor with hypochlorite detection towards the potential application as invisible security ink

Hypochlorite (ClO^-) as a well-known highly reactive oxygen species (ROS), is widely used as preservative and household disinfectant in daily life. Although many fluorescence imaging sensors for ClO^- have been reported, the development of ClO^- ratio fluorescence sensors with large Stokes shift is still quite limited. This sensor shows obvious benefits including minimizing environmental intervention and improving signal-to-noise ratio. In the present project, we report an innovative conjugated pyrene-based system, 1-B, as a chlorine fluorescence sensor. The detector exhibits ratio detection performance, large Stokes and emission shifts. Furthermore, the system has desired sensitivity as well as selectivity for ClO^-. Based on these excellent properties, the sensor 1-B was successfully used as ink to encrypt patterns and anti-counterfeiting information through inkjet printing technology. Compared with the existing probes, the probe shows some superior characteristics, which provides a promising tool for exploring the role of ClO^- response sensor in the field of anti-counterfeiting.

A Texture-Hidden Anti-Counterfeiting QR Code and Authentication Method

This paper designs a texture-hidden QR code to prevent the illegal copying of a QR code due to its lack of anti-counterfeiting ability. Combining random texture patterns and a refined QR code, the code is not only capable of regular coding but also has a strong anti-copying capability. Based on the proposed code, a quality assessment algorithm (MAF) and a dual feature detection algorithm (DFDA) are also proposed. The MAF is compared with several current algorithms without reference and achieves a 95% and 96% accuracy for blur type and blur degree, respectively. The DFDA is compared with various texture and corner methods and achieves an accuracy, precision, and recall of up to 100%, and also performs well on attacked datasets with reduction and cut. Experiments on self-built datasets show that the code designed in this paper has excellent feasibility and anti-counterfeiting performance.

High-contrast reversible multiple color-tunable solid luminescent ionic polymers for dynamic multilevel anti-counterfeiting

Dynamic color-tunable luminescent materials, which possess huge potential applications in advanced multilevel luminescence anti-counterfeiting, are of considerable interest. However, it remains challenging to develop simple high-contrast reversible multiple (triple or more than triple) color-tunable high-efficiency solid luminescent materials with low cost, facile synthesis, and good processability. Herein, by simply grafting charged multi-color AIEgen-based chromophores into polymers, a series of high-efficiency multiple color-tunable luminescent single ionic polymers are constructed through tuning feed ratios, counter anions and reaction solvents. Remarkably, some ionic polymers can not only achieve rare high-contrast reversible multiple color-tunable emission in solid states in response to different solvent stimuli, but also could realize excitation-dependent color-tunable emission. To the best of our knowledge, such charming multiple (triple or more than triple) color-tunable solid polymers responding to multiple external stimuli are still rare. Based on comparative studies of emission spectra, excitation spectra and fluorescence lifetimes before and after swelling, it could be inferred that solvent stimuli could induce microstructure changes of these ionic polymers and then change the aggregated-states of their corresponding AIE-active emission centers. Moreover, the different solvent stimuli could induce to produce different degrees of microstructure changes, resulting in their unique multiple color-tunable emission. More significantly, these smart color-tunable ionic polymers show great promise for applications in dynamic multilevel (three-level or even more than three-level) anti-counterfeiting.

Full-Color-Tunable Nanohydrogels as High-Stability Intracellular Nanothermometers

Full-color-tunable hydrogels with ultrahigh stability can be used in various fields, including intracellular temperature sensing. However, constructing full-color-tunable organic nanohydrogels with excellent biocompatibility and stability for intracellular temperature sensing remains a great challenge. Here, we report a full-color-tunable nanohydrogel with ultrahigh stability as an intracellular nanothermometer. Three types of temperature-sensitive polymers with red, green, and blue fluorescence were synthesized. Through easy mixing of these three polymers with regulation of the mass ratio, these polymers can be encoded to full-color-tunable fluorescent nanohydrogels, including nanohydrogels with white-light emission (NWLEs), with sizes of about 200 nm in aqueous media. Further study suggested that the as-obtained NWLEs exhibited good performance in intracellular temperature sensing because of their ultrahigh stability on their fluorescence properties and morphologies.

Multilevel Anti-Counterfeiting Based on Covert Structural Features Embedded in Femtosecond-Laser-Treated Gold Nanocluster/Graphene Hybrid Layer

The conventional nanoscale anti-counterfeiting scheme, exhibiting limited encoding capacity, faces growing challenges of being falsified with the advent of advanced high-resolution equipment. In this study, we propose a multilevel anti-counterfeiting device based on a femtosecond laser (fs-laser) treated plasmonic gold nanocluster/graphene (AuNC/Gr) hybrid structure integrated with a resonant cavity. The covert structural features encoded in random colored patterns, optical reflection spectra, and Raman spectra constitute three classes of anti-counterfeiting signatures, which originate from the AuNC-covered Gr, which initiates plasmonic and thermal couplings. The attendant inverted thermal distribution is presumed to confine the structural features to the AuNC-Gr interface while leaving no detectable traces on the surface of AuNC/Gr even under advanced high-resolution equipment. Therefore, the proposed approach achieves multilevel anti-counterfeiting accomplishing physically unclonable functions in the form of random colored patterns, reflection spectra, and Raman spectra. As the first report for realizing remarkable optical modulation (i.e., random colored patterns) without any surface trace or damage via fs-laser-AuNC/Gr interaction, our study also discloses the outstanding performance of Gr in fs-laser-induced optothermoplasmonic lithography on near-percolation metal films. Simultaneously, the demonstrated fs-laser-processed plasmonic hybrid structure in conjunction with a resonant cavity is anticipated to expand the encoding capabilities for nanoscale anti-counterfeiting while avoiding the risk of being imitated because of the covert structural features.

Dynamic Fluorescent Anti-Counterfeiting Labels Based on Conjugated Polymers Confined in Submicron Fibrous Membranes

Developing a new anti-counterfeiting strategy is of great significance to combating the global counterfeiting problem. Here we report the construction of a dynamic fluorescence response system for anti-counterfeiting by combining the photochromism induced by the ring-opening of spiropyran (SP) to merocyanine (MC) with the fluorescence resonance energy transfer (FRET) between the conjugated polymer and MC. After elucidating the design principle, a new conjugated polymer, PPETE-SP, consisting of a poly[p-(phenylene ethynylene)-alt-(thienylene-ethynylene)] (PPETE) backbone with pendant SP, was synthesized and characterized. With poly(methyl methacrylate) (PMMA) as the matrix, the PPETE-SP/PMMA fibrous membrane was prepared via electrospinning. Under the irradiation of UV light, the fluorescent color of the membrane dynamically changed from green to light green, then light pink, and finally pink, and this process was reversible under visible light. The fluorescence emission switch was examined for 10 cycles and proved to have good repeatability, indicating that the membrane can be directly used as an anti-counterfeiting label for multiple verifications. The FRET efficiency was found to be about 61% based on the FRET study with confocal laser scanning microscopy. The covalent bonding between PPETE backbone and SP, the confinement of PPETE-SP chains in the fibrous membrane, as well as employing PMMA as the matrix were demonstrated to be crucial in realizing the photochromism and the FRET. Different anti-counterfeiting modes were proposed, providing rich selections for operation of verification. Such facile-to-operate and hard-to-imitate dynamic fluorescent responsive materials are very promising for use in practical anti-counterfeiting applications.

Background color dependent photonic multilayer films for anti-counterfeiting labeling

A new concept for anti-counterfeiting security films that utilize the humidity from human breath to reveal a QR code on color-tunable one-dimensional (1D) PC films is presented. The 1D PC film was fabricated on a transparent polyethylene terephthalate (PET) substrate via sequential alternate layer deposition of photo-crosslinkable poly(2-vinylnaphthalene-co-benzophenone acrylate) (P(2VN-co-BPA)) and quaternized poly(4-vinylpyridine-co-benzophenone acrylate) (P(4VP-co-BPA)) (P4QP-51%). The films exhibited remarkable color transitions with reliable reversibility and reproducibility. Films placed on a black background exhibited the full visible spectrum color in a high humidity environment. Additionally, films placed on a white background displayed three different composite colors, including yellow, magenta, and cyan. These films with vivid color transitions in a high humidity environment can be applied as anti-counterfeiting films. A hidden QR code was also laser printed on the initial PC film to enhance the film's anti-counterfeiting security capabilities. These colorimetric 1D PC films can be used as anti-counterfeiting labels and for information storage.

Multicolor Light Mixing in Optofluidic Concave Interfaces for Anticounterfeiting with Deep Learning Authentication

Anticounterfeiting technology has received tremendous interest for its significance in daily necessities, medical industry, and high-end products. Confidential tags based on photoluminescence are one of the most widely used approaches for their vivid visualization and high throughput. However, the complexity of confidential tags is generally limited to the accessibility of inks and their spatial location; generating an infinite combination of emission colors is therefore a challenging task. Here, we demonstrate a concept to create complex color light mixing in a confined space formed by microscale optofluidic concave interfaces. Infinite color combination and capacity were generated through chaotic behavior of light mixing and interaction in an ininkjet-printed skydome structure. Through the chaotic mixing of emission intensity, wavelength, and light propagation trajectories, the visionary patterns serve as a highly unclonable label. Finally, a deep learning-based machine vision system was built for the authentication process. The developed anticounterfeiting system may provide inspiration for utilizing space color mixing in optical security and communication applications.

Dual-color dynamic anti-counterfeiting labels with persistent emission after visible excitation allowing smartphone authentication

A significant impediment to the deployment of anti-counterfeiting technologies is the reliance on specialized hardware. Here, anti-counterfeiting labels are developed that are both excited and detected using a smartphone. The persistent luminescence pattern and color changes on the timescale of hundreds of milliseconds to seconds. The labels can be authenticated by comparing still images from the red and green channels of video acquired at known times after flashlight excitation against expected reference patterns. The labels are based on a green-emitting SrAl₂O₄: Eu²⁺,Dy³⁺ (SAED), and red-emitting CaS:Eu²⁺ phosphors whose lifetimes are varied: (i) for SAED from 0.5 to 11.7 s by annealing the commercial material in air; and (ii) CaS:Eu²⁺ from 0.1 to 0.6 s by varying the dopant concentration. Examples of anti-counterfeiting labels exhibiting changing emission patterns and colors on a seven-segment display, barcode, and emoji are demonstrated. These results demonstrate that phosphors with visible absorption and tunable persistent luminescence lifetimes on the order of hundreds of milliseconds to seconds are attractive for anti-counterfeiting applications as they allow authentication to be performed using only a smartphone. Further development should allow richer color shifts and enhancement of security by embedding further covert anti-counterfeiting features.

Dicarboxylate Modulating Molecular-Ionic Platinum Compounds with Variable Stacking and Photoluminescence

Under solvothermal conditions, 10 molecular-ionic platinum compounds [Pt(NIA)₂]·(L)·nH₂O (L = dicarboxylate) were synthesized. In the reaction, acetonitrile undergoes trimerization in situ to generate N-(1-iminoethyl)acetamidine (NIA), which coordinates to Pt^II ions in forming the N-(1-iminoethyl)acetamidine platinum cation, while the organic carboxylates act as anions. Structural analysis shows that carboxylate ligands regulate the mode of packing of [Pt(NIA)₂] in those compounds. Photoluminescence studies show that the photoluminescence behaviors of those compounds also depended on the carboxylate ligands. 1-4, 6, and 7 show blue light emission with fluorescence emission wavelengths of 437-440 nm despite the different carboxylate ligands used. 5 and 8 show green emissions with maximum intensity peak positions of 522 nm. Compared with that of 5 and 8, the emission of 9 and 10 has the same red shifts with peak positions of 567 and 528 nm. The variable-temperature photoluminescence studies reveal that 8 and 10 show two different thermal quenching (TQ) zones in the range of 80-420 K, while the emission intensity of 9 shows negative thermal quenching at low temperatures of 80-220 K and TQ in the range of 220-420 K.

Reversible Transformation between Cs3Cu2I5 and CsCu2I3 Perovskite Derivatives and Its Anticounterfeiting Application

Lead halide perovskites have promising values in photoelectronic and photovoltaic applications, but the toxicity of lead is a hard barrier. Copper halide perovskite derivatives (CHPDs), as a lead-free substitution of lead halide perovskites, also exhibit excellent photoelectric properties. Here, we present a facile one-step route for the synthesis of blue-emissive Cs₃Cu₂I₅ (emission at 440 nm) and yellow-emissive CsCu₂I₃ (emission at 552 nm) CHPDs in ethanol at room temperature. Triggered by ethanol or CsI, a reversible chemical transformation accompanied by emissive color change between Cs₃Cu₂I₅ and CsCu₂I₃ CHPDs was achieved. The reversible transformation mechanism was discussed, and this transformation was employed for effective anticounterfeiting.

Luminescent monomeric and dimeric Ru(ii) acyclic carbene complexes as selective sensors for NH3/amine vapor and humidity

A new class of luminescent bis(bipyridyl) Ru(ii) pyridyl acyclic carbene complexes with environmentally-sensitive dimerization equilibrium have been developed. Owing to the involvement of the orbitals of the diaminocarbene ligand in the emissive excited state, the phosphorescence properties of these complexes are strongly affected by H-bonding interactions with various H-bonding donor/acceptor molecules. With the remarkable differences in the emission properties of the monomer, dimer, and H-bonded amine adducts together with the change of the dimerization equilibrium, these complexes can be used as luminescent gas sensors for humidity, ammonia, and amine vapors. With the responses to amines and humidity and the corresponding change in the luminescence properties, a proof-of-principle for binary optical data storage with a reversible concealment process has been described.

Shedding light on predicting and controlling emission chromaticity in multicomponent photoluminescent systems

Predictable colour tuning in multicomponent photoluminescent (PL) systems is achieved using mixtures of simultaneously emitting organic molecules. By mitigating the potential for energy transfer through the control of concentration, the resulting emission chromaticity of five dichromic PL systems is approximated as a linear combination of the emitting components and their corresponding brightness (χ i , ϕ i , and I ex,i ). Despite being limited to dilute solutions (10-6 M), colour tuning within these systems was controlled by (1) varying the composition of the components and (2) exploiting the differences in the components' excitation intensities at common wavelengths. Using this approach, white light emission (WLE) was realized using a pre-determined mixture of red, green, and blue emitting organic molecules. Based on these results, materials and devices with built-in or programmable emission colour can be achieved, including highly sought-after WLE.

Static-Dynamic Fluorescence Patterns Based on Photodynamic Disulfide Reactions for Versatile Information Storage

Dynamic fluorescence patterns with variable output in response to external stimulus can make the current information storage technologies more flexible and intelligent. Yet it remains a great challenge to create such dynamic patterns because of the complicated synthesis process, high cost, limited stability, and biocompatibility of the functional fluorophores. Herein, a facile approach is presented for creating dynamic fluorescence patterns using the photodynamic surface chemistry based on disulfide bonds. By this method, high-resolution (≈20 µm) multicolor dynamic fluorescence patterns that are low-cost and dynamically rewritable can be easily fabricated using classical fluorophores such as fluorescein, rhodamine, and dansyl acid. Owing to the spatio-temporal controllability of light, the fluorescence patterns can be partly or entirely erased/rewritten on demand, and complex gray-level fluorescence images with increased information capacity can be easily generated. The obtained fluorescence patterns exhibit little changes after storing in air and solvent environments for 100 days, demonstrating their high stability. In addition, static patterns can also be created on the same disulfide surface using irreversible disulfide-ene chemistry, to selectively control the dynamicity of the generated fluorescence patterns. The authors show the successful application of this strategy on information protection and transformation.

Aggregation induced emission based active conjugated imidazole luminogens for visualization of latent fingerprints and multiple anticounterfeiting applications

Aggregation-induced emission based organic heterocyclic luminogens bearing conjugated electronic structures showed much attention due to its excellent fluorescence in aggregation state. In this communication, a novel conjugated blue light emitting imidazole molecule is synthesized by one pot multicomponent reaction route is reported for the first time. The prepared molecule exhibits a strong fluorescence in aggregation state with exceptional properties, such as high purity, inexpensive, eco-friendly, large scale production, high photostability, etc. By considering these advantages, a new fluorescence based platform has been setup for in-situ visualization of latent fingerprints and its preservation by spray method followed by Poly(vinyl alcohol) masking. A clear and well defined fluorescence fingerprint images are noticed on variety of surfaces by revealing level 1-3 ridge features upon ultraviolet 365 nm light exposure. The dual nature of binding specificity as well as excellent fluorescence properties permits the visualization of latent fingerprints for longer durations (up to 365 days) with superior contrast, high sensitivity, efficiency, selectivity and minimal background hindrance. We further fabricated unclonable invisible security ink for various printing modes on valuable goods for protection against forging. The developed labels are displaying uniform distribution of ink and exceptional stability under various atmospheric environments. The development of long preservative information using aggregation-induced emission based luminogen opens up a new avenue in advanced forensic and data security applications.

Borisov S. Carbazole- and Fluorene-Fused Aza-BODIPYs: NIR Fluorophores with High Brightness and Photostability

Three new aza-BODIPY dyes incorporating fused fluorene or carbazole moieties have been prepared. The dyes show significant enhancement of photophysical properties compared to the parent 1,3,5,7-tetraphenyl aza-BODIPY (TPAB): a bathochromic shift of the absorption maximum (up to 2700 cm-1 ) and emission maximum (up to 2270 cm-1 ); an almost threefold increase in molar absorption coefficients (to ca. 230 000 M-1  cm-1 ) and a significant increase in the fluorescence quantum yield to 49-66 %. Owing to the combination of these properties, the new aza-BODIPY dyes belong to the brightest NIR dyes reported. The dyes also show excellent photostability. Due to their outstanding properties, the new dyes represent a promising platform for further exploration in biomedical research. A pH indicator containing only one fused carbazole unit was also prepared and shows absorption and emission spectra that are bathochromically shifted by about 110 and 100 nm, respectively, compared to the indicator dye based on the TPAB chromophore.

Emission Color Tuning and White Light Generation from a Trimolecular Cocktail in Cationic Micellar System with Promising Applicability in the Anticounterfeiting Technology

The development of photoluminescent (PL) systems, displaying multiple stimuli-responsive emission color tuning, has been the pressing priority in the recent times due to their huge role in contemporary lighting and anticounterfeiting technologies. Acknowledging this importance, we present a simple and eco-friendly PL system showing emission color tuning in response to different stimuli, that is, the composition of the system, pH, excitation wavelength, and the temperature with the plus point of getting significantly pure white light emission (WLE). The novel system is fabricated from the aqueous mixture of three organic fluorophores, umbelliferone (UMB), fluorescein (FLU), and Rhodamine-B (RB). By varying the fluorophore composition in the mixture at pH 12, nearly pure WLE with a Commission Internationale d'Eclairage (CIE) 1931 profile of (0.33, 0.33) was obtained at the excitation wavelength of 365 nm, the sustainability of which was ensured by employing the micellar self-assemblies of tetradecyltrimethylammonium bromide (TTAB) molecules. Similar WLE was obtained under mildly acidic conditions (pH 6) but at the excitation wavelength of 330 nm. By proper tuning of pH and the wavelengths of the system to use it as a fluorescent ink, we found a remarkable and highly applicable phenomenon observed for the first time, that is, triple-mode orthogonal emission color tuning with white light ON/OFF switching. We validate the vital applicability of this phenomenon in protecting the authenticity of the document with its hard-to-counterfeit property. The applicability of this phenomenon is also explored by synthesizing PVA-based fluorescent films from the tri-fluorophore mixture. Moreover, the emission color of the PL system was explored lucidly for its temperature dependence owing to the thermal responsiveness of RB emission, where the PL system proves to be a full-color RGB system.

Light-Responsive Luminescent Materials for Information Encryption Against Burst Force Attack

Optical encryption with easy operation, multichannel and high security has been one of the most significant technologies for information security. Stimuli-responsive luminescent materials have emerged as an ideal candidate for optical encryption, owing to its smart responsive property and high security. Herein, a type of light-responsive multicolor luminescent materials for high-security information encryption, which are fabricated by combining sensitizer, consumption unit, and emitter is developed. Different types of sensitizers to achieve different stimulus light responses, and multicolor light-responsive luminescent can be obtained by varying the composition of perovskite nanocrystals emitter can be selected. Both stimulus light and emission color can be used as distinguishable encoding dimensions, which enable multiplexed encoding with high capacity and complexity. Importantly, the controllable consumption can be manipulated by varying the concentration of consumption unit, so the programmed information encoded in different channels can be selectively read and erased simultaneously by varying stimulus light. The method makes the encryption information highly resistive to brute force trial-and-error attacks, which achieves high security level of information protection.

Simple Vanilla Derivatives for Long-Lived Room-Temperature Polymer Phosphorescence as Invisible Security Inks

Developing novel long-lived room-temperature polymer phosphorescence (RTPP) materials could significantly expand their application scope. Herein, a series of RTPP materials based on eight simple vanilla derivatives for security ink application are reported. Attributed to strong mutual hydrogen bonding with polyvinyl alcohol (PVA) matrix, vanilla-doped PVA films exhibit ultralong phosphorescence emission under ambient conditions observed by naked eyes, where methyl vanillate shows the longest emission time up to 7 s. Impressively, when vanilla-doped PVA materials are utilized as invisible security inks, and the inks not only present excellent luminescent emission stability under ambient conditions but also maintain perfect reversibility between room temperature and 65°C for multiple cycles. Owing to the unique RTPP performance, an advanced anticounterfeiting data encoding/reading strategy based on handwriting technology and complex pattern steganography is developed.

Energy transduction through FRET in self-assembled soft nanostructures based on surfactants/polymers: current scenario and prospects

The self-assembled systems of surfactants/polymers, which are capable of supporting energy funneling between fluorophores, have recently gained significant attraction. Surfactant and polymeric micelles form nanoscale structures spanning a radius of 2-10 nm are generally suitable for the transduction of energy among fluorophores. These systems have shown great potential in Förster resonance energy transfer (FRET) due to their unique characteristics of being aqueous based, tendency to remain self-assembled, spontaneous formation, tunable nature, and responsiveness to different external stimuli. This review presents current developments in the field of energy transfer, particularly the multi-step FRET processes in the self-assembled nanostructures of surfactants/polymers. The part one of this review presents a background and brief overview of soft systems and discusses certain aspects of the self-assemblies of surfactants/polymers and their co-solubilization property to bring fluorophores to close proximity to transduce energy. The second part of this review deals with single-step and multi-step FRET in the self-assemblies of surfactants/polymers and links FRET systems with advanced smart technologies including multicolor formation, data encryption, and artificial antenna systems. This review also discusses the diverse examples in the literature to present the emerging applications of FRET. Finally, the prospects regarding further improvement of FRET in self-assembled soft systems are outlined.

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

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

Efficient Artificial Light-Harvesting System Based on Supramolecular Peptide Nanotubes in Water

Artificial light-harvesting systems in aqueous media which mimic nature are of significant importance; however, they are often restrained by the solubility and the undesired aggregation-caused quenching effect of the hydrophobic chromophores. Here, we report a generalized strategy toward the construction of efficient artificial light-harvesting systems based on supramolecular peptide nanotubes in water. By molecularly aligning the hydrophobic chromophores along the nanotubes in a slipped manner, an artificial light-harvesting system with a two-step sequential Förster resonance energy transfer process is successfully fabricated, showing an energy transfer efficiency up to 95% and a remarkably high fluorescence quantum yield of 30%, along with high stability. Furthermore, the spectral emission could be continuously tuned from blue through green to orange, as well as outputted as a white light continuum with a fluorescence quantum yield of 29.9%. Our findings provide a versatile approach of designing efficient artificial light-harvesting systems and constructing highly emissive organic materials in aqueous media.