Security Printing Research Articles

Designing vivid green Sr9Al6O18:Er3+ phosphor for information encryption and nUV excitable cool-white LED applications

Advanced forensic applications and optoelectronics both heavily rely on rare earth (RE) element activated luminous phosphor materials. We used latent fingerprints (LFPs) and white light-emitting diodes (w-LEDs) to detect Using an easy solution combustion technique, a single-phase green-emitting Sr9Al6O18:Er3+ (1–11 mol%) (SAO: Er) nanophosphor was created. The X-ray diffraction and Rietveld refinement were used to examine crystallographic data. The phosphors showed vivid-green luminescence when stimulated by 378 nm near ultraviolet (nUV). The concentration of Er3+ ions is observed to affect the photoluminescence emission (PLE) intensity, reaching its highest value at 5 mol% doping. The optimized phosphor is used in latent finger print (LFP), lip print (LP) and anti-counterfeiting (AC) applications. Under 378 nm excitation, multilevel features of the phosphor-attached LFPs could be clearly identified. The high colour contrast between the ridge and valley lines made for an excellent observation. We have developed simple brush and screen-printing methods for creating a variety of patterns using our custom anti-counterfeit security ink. In addition to this, the obtained AC tags exhibit high resolution and long-term durability. These characteristics clearly demonstrated that the current phosphor is an excellent luminescent material for data storage applications.

Security ink could prevent counterfeiting of passports

Security ink could prevent counterfeiting of passports

PEDOT:PSS: Smart Infrared Rewritable Materials

AbstractThermal radiation spectrum regulation plays an important role in energy and information fields. Effectively hiding targets and rendering it invisible to infrared imaging detectors are great challenges in past decades. Herein, a smart infrared rewritable and emissivity tunable polymer film is proposed based on PEDOT:PSS. The adjustment ability for the thermal emission is attributed to the mobile polaronic of PEDOT that can be effectively regulated by redox agents, resulting in an infrared emission modulation > 30%. The conformation and molecular stacking order of PEDOT also present reversible property, accompanied with the transformation between dication state and neutral state. Finally, a technique for direct writing and erasing process in the infrared region is implemented, based on the simple redox treating process. This work aims to provide a simple strategy on fabricating rewritable films based on conducting polymer PEDOT:PSS, which can be potentially applied in infrared optics security printing.

Development of photochromic carboxymethyl cellulose-based hydrogel toward dual-mode authentication stamp with self-healing properties

Ultraviolet-induced photochromism has proven to be an appealing encoding strategy to improve the anti-counterfeiting of commercial items. However, recent reports have highlighted significant disadvantages with photochromic inks such as high cost, low efficiency and poor durability. Herein, we present a self-healable ink for anti-counterfeiting applications from a hydrogel of carboxymethyl cellulose (CMC) combined with rare-earth activated aluminate (REA) nanoparticles (NPs). Self-healing security inks provide high durability. The REA@CMC hydrogel has the ability to efficiently self-heal under ambient conditions. Using UV light to promote photochromism, the REA@CMC ink showed excellent photostability. To create a composite ink with a wide range of emission characteristics, different quantities of REA NPs were used. CIE (Commission Internationale de L'éclairage) Lab established that stamping a homogeneous film onto a paper surface gives them a transparent layer that transformed into green upon exposure to ultraviolet spectrum. The morphological features of REA NPs were investigated to show a particle diameter of 3–7 nm. The morphology of the photochromic films were studied using different analytical methods. Both ink solution rheological properties and stamped paper sheet mechanical characteristics were examined. The luminous films peaked in green emission at 518 nm, whereas their excitation wavelength was at 370 nm.

Pyrene-naphthalimide Schiff base as a fluorescent pigment in water-based security ink

Fluorescence-based materials that are affordable and easy to use for commercial anti-counterfeiting applications are in high demand. While exploring new fluorescent pigments, a pyrene-naphthalimide Schiff base, 5-hydroxy-2-((pyren-1-ylmethylene)amino)-1H-benzo[de]isoquinoline-1,3(2H)-dione (NHPY) with yellow fluorescence under UV light was synthesized. An eco-friendly flexographic ink prepared with NHPY as the pigment was coated on a UV dull paper and further printed on security paper as well as packaging papers and boards. When exposed to UV light, the printed samples showed yellow fluorescence. The light fastness, gloss, colorimetric results, and abrasion resistance of the printed samples proved that NHPY is a suitable fluorescent pigment for security printing applications.

Visible security ink blended by Prussian blue analogues encapsulated gold (Au@PBA) nanoparticles for information hiding

Developing visible security ink with simple preparation, high optical stability and interference-free secret signals are critical to hiding security information. Here, two Prussian blue analogue encapsulated gold nanoparticles (Au@PBA NPs) with different triple-bond Raman emissions were synthesized to prepared novel red inkjet inks for security labels. And the ink formulation was well optimized to obtain suitable surface tension and viscosity. The labels contain public information in red as well as secret information that can only be acquired using Raman spectroscopy. With the visible public information on the labels, the secret information is not easy to be noticed. Also, the secret information written by the triple-bond Raman scattering is readily identifiable without interference. This method improves the security of secret information transmission, providing a new strategy for information hiding technology.

Lanthanide-Directed Luminescent “Soft” Coordination Polymer Gels: White Light Emission, Anticounterfeiting, and Thin-Film-Based Sensing

Stimuli-responsive light-emitting materials are of great interest owing to their potential use in encryption and multilevel security. In this context, herein, the synthesis of light-emitting multifunctional coordination polymer gels (CPGs) was accomplished by the self-assembly of europium and terbium acetate with a bifunctional (amide/carboxylic acid) gelator. The as-synthesized luminescent CPGs are stable up to 300 °C. The FESEM micrograph exhibits pebble-like morphology, and the TEM images reveal a sheet-like structure which is grown on top of each other. The optical properties of the as-synthesized CPGs are readily tuneable over a broad spectrum, including white-light emission, simply by modulating the europium and terbium ion stoichiometry. The as-synthesized luminescent materials have the potential to be used as invisible security inks for high-security information encoding. Additionally, water-resistant luminescent thin films were fabricated by incorporating Eu@BTT or Tb@BTT (BTT: 5,5′,5″-((benzene-1,3,5-tricarbonyl)tris(azanediyl))triisophthalic acid) into the poly(methyl methacrylate) (PMMA) polymer matrix for the naked eye test of nitro-explosive compounds (2,4,6-trinitrophenol and nitrophenol). The sensing mechanism is proposed based on the DFT and time-resolved photoluminescence studies.

Hydrophobic Carbon Dots Derived from Organic Pollutants and Applications in NIR Anticounterfeiting and Bioimaging.

In an effort to fulfill the strategy of sustainable development, Rhodamine B, a common and toxic organic pollutant in the textile industry, was reported for the first time as a single precursor to develop a kind of novel hydrophobic nitrogen-doped carbon dot (HNCD) through a green and facile one-pot solvothermal method. The HNCDs with an average size of 3.6 nm possess left and right water contact angles of 109.56° and 110.34°, respectively. The HNCDs manifest excitation wavelength-tunable and upconverted fluorescence from the ultraviolet (UV) to the near-infrared (NIR) range. Furthermore, the PEGylation of HNCDs enables them to be used as an optical marker for cell and in vivo imaging. Notably, the HNCDs with solvent-dependent fluorescence can be used for invisible inks with a wide range of light responses from UV-vis-NIR spectra. This work not only provides an innovative way to recycle chemical waste but also expands the potential application of HNCDs in NIR security printing and bioimaging.

Orange-red emitting MoO3:Sm3+ transparent nano-composite films for anti-counterfeiting and data secure applications

Poly(vinyl alcohol) (PVA) and chitosan (CS) based nanocomposites (NCs) filled by various weight percent of modified MoO3:Sm3+ (MS) nanoparticles (NPs). For the preparation, folic acid (FA) up to 150 mg were used via ultrasonication method then was cast to thin films. The changes in the structural properties of the prepared samples were examined by X-ray diffraction. FT-IR spectroscopy indicated the possible interactions of the MS NPs with the PVA/CS backbones through the absorption bands. MS NPs were homogeneously dispersed in the polymer matrix at nanoscale. The photoluminescence (PL) emission spectra of PVA/CS-MS NCs excited by 404 nm wavelength, with the key emission peaks centred at ∼ 563, ∼600 and ∼ 647 nm are mainly attributed to transitions from 4G5/2→6H5/2, 4G5/2→6H7/2, 4G5/2→6H9/2 respectively. The powder dusting method was used to visualize the latent fingerprints (LFPs) on various surfaces under 404 nm excitation. Additionally, PVA/CS matrix was masked on the developed fingerprints (FPs) for long term storage. The results clearly indicated that, the level 1–3 ridge features were stable up to 180 days with superior contrast and sensitivity. The optimized phosphor powder is used to fabricate unclonable security ink and brush mode technique was used to mark on various surfaces and exposed to UV 404 nm light. The results revealed that the present phosphor can garner significant interest for the use in personal identification and security encoding.

Rapid handheld time-resolved circularly polarised luminescence photography camera for life and material sciences

Circularly polarised luminescence (CPL) is gaining a rapidly increasing following and finding new applications in both life and material sciences. Spurred by recent instrumental advancements, the development of CPL active chiral emitters is going through a renaissance, especially the design and synthesis of CPL active luminescent lanthanide complexes owing to their unique and robust photophysical properties. They possess superior circularly polarised brightness (CPB) and can encode vital chiral molecular fingerprints in their long-lived emission spectrum. However, their application as embedded CPL emitters in intelligent security inks has not yet been fully exploited. This major bottleneck is purely hardware related: there is currently no suitable compact CPL instrumentation available, and handheld CPL photography remains an uncharted territory. Here we present a solution: an all solid-state small footprint CPL camera with no moving parts to facilitate ad hoc time-resolved enantioselective differential chiral contrast (EDCC) based one-shot CPL photography (CPLP).

Water-based fluorescent flexo-ink for security applications

UV-readable fluorescent ink formulations find versatile applications in various fields including information encryption, automated identification systems, security markers and optical devices. In this context, a new bithiophene-based chalcone (BTCF) that exhibits good solution phase and solid-state fluorescence was synthesized as a colourant for formulating an eco-friendly UV fluorescent ink. The molecule demonstrated good thermal stability and photophysical features including intramolecular charge transfer, confirmed through emission studies in THF–hexane mixtures with varying hexane content. The intense greenish yellow solid-state fluorescence emission displayed by BTCF was exploited by using it as a colourant in a water-based fluorescent ink formulation. Further, the ink was used to print a fast-drying solid patch on an UV dull paper substrate using flexography technique. The analysis of colorimetric, densitometric and rub resistance properties of the printed paper samples demonstrated good fluorescence, moderate photostability and good rub resistance, and hence could be used for security printing applications.Graphical abstract

Microwave-assisted synthesis of GdOF: Eu3+/Tb3+ ultrafine phosphor powders suitable for advanced forensic and security ink applications

The rare-earth-doped inorganic ultrafine oxyfluoride host matrices in forensic science, especially in latent fingerprint detection and anti‐counterfeiting applications, were still unexplored and may replace the existing technology owing to its high sensitivity. Herein, GdOF: Eu3+/Tb3+ ultrafine red and green phosphors were synthesized via a rapid, green microwave-assisted hydrothermal method at 150 °C. The phosphors synthesized by this novel method possess good luminescent intensity for the hypersensitive 5D0→7F2 transition of Eu3+ and 5D4→7F5 transition of Tb3+ ions as compared to the phosphors prepared via other conventional methods such as co-precipitation synthesis, sol–gel synthesis, and microwave-assisted co-precipitation synthesis. Further, an enhancement in the luminescent intensity of the ultrafine phosphor was noticed when the microwave parameters and pH values were tuned. The optimized red and green phosphors having high luminescence intensity, good color purity, and high quantum yields of 89.3%, and 71.2%, respectively, were used for the visualization of latent fingerprints on various substrates. These promising phosphors exhibited excellent visualization regardless of the background interference and limit the risk of duplication and are highly reliable. The security inks developed using these phosphors are highly efficient for anti-counterfeiting applications. These multifunctional properties of investigated phosphors can be explored for security applications.

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:Mn4+, 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.

Preparation of carbon dots-embedded fluorescent carboxymethyl cellulose hydrogel for anticounterfeiting applications

Fluorescent inks have been emerged as a desirable encoding technique to enhance anticounterfeiting printing of commercial goods. However, significant drawbacks with fluorescent inks, such as poor durability, low efficiency, and high cost. Herein, we describe the preparation of a self-healing authentication ink based on carboxymethyl cellulose (CMC) hydrogel immobilized with nitrogen-doped carbon dots (NCD) nanoparticles (NPs) for cutting-edge anticounterfeiting applications. Security inks that self-heal are very durable. Under ambient conditions, the prepared NCD@CMC hydrogel could self-heal with a high healing efficiency. It might stick to diverse surfaces such as plastic, glass and paper sheets. The self-healing composite ink demonstrated outstanding photostability under UV light. Straightforward and environmentally friendly method was applied on the agricultural waste of rice straw toward the production of NCD using hydrothermal carbonization in an aqueous medium, and in the presence of NH4OH as an inexpensive passivating agent. The quantum yield (QY) for NCD reached 24.09 %. Various concentrations of NCD NPs were employed to produce self-healable nanocomposite inks with a variety of emission properties. Stamping homogeneous films onto paper surfaces produced a transparent layer. The CIE Lab and emission spectra of prints independently verified the capability of NCD nanocomposite inks to vary their color to blue under UV illumination. To measure the particle diameter of the prepared NCD, their morphological characteristics were examined by transmission electron microscopy (TEM) to indicate diameters of 10–25 nm. Utilizing various analytical techniques, the morphology and chemical composition of the fluorescent prints were examined. We examined the mechanical qualities of the stamped papers as well as the rheological characteristics of the ink hydrogel. Due to their colorless appearance, the excitation band of the printed films was peaked at 364 nm, while their emission was peaked at 465 nm. The current smart ink holds high potential for numerous applications like smart packaging and authentication, and shows great promise as a practical and mass production approach for easily creating anticounterfeiting stamps.

Fabrication of stable solid fluorescent starch materials based on Hantzsch reaction

In this paper, a series of fluorescent starches were prepared simply and effectively by Hantzsch multi-component reaction (MRC). These materials showed bright fluorescence emission. Notably, due to the existence of polysaccharide skeleton, starch molecules can effectively inhibit the common aggregation induced quenching effect caused by the aggregation of conjugated molecules in traditional organic fluorescent materials. Meanwhile, the stability of this material is so excellent that the fluorescence emission of the dried starch derivatives would not destroy after boiling at a high temperature in some common solvents, and even brighter fluorescence can be stimulated in alkaline solution. In addition to fluorescence, starch was also endowed with hydrophobic property by one-pot method connecting long alkyl chains. Compared with native starch, the contact angle of fluorescent hydrophobic starch increased from 29° to 134°. Furthermore, the fluorescent starch can be prepared into film, gel and coating by different processing methods. The preparation of these Hantzsch fluorescent starch materials provide a new way for the functional modification of starch materials and has great application potential in detecting, anti-counterfeiting, security printing and other related fields.

Stimuli-responsive color-tunable BaLa2ZnO5:Bi3+ phosphor for the encryption and authentication of security patterns and latent fingerprint detection

Due to the outstanding optical qualities, stimulation-responsive luminous materials have drawn a lot of attention in the anti-counterfeiting application. In this work, the feasibility of applying BaLa2ZnO5:Bi3+ (BLZO:Bi3+) (0.5–4.0 mol%) phosphor, to multiple stimuli anti-counterfeiting (AC) and latent fingerprint visualization (LFP) is explored and discussed. The prepared luminous security ink from the optimized phosphor is effectively used for the multiple stimuli AC under 325, 355 and 375 nm excitation wavelengths. The results show that the fabricated luminous security ink is highly stable in ultraviolet (UV) light, various chemical environments, and ambient conditions. In addition, powder dusting method is utilized for LFPs detection on porous, non-porous and semi-porous surfaces under different excitation wavelengths. The multilevel features of the phosphor-attached LFP could be identified clearly in all the surfaces. The high-resolution LFPs imaging by scanning electron microscope (SEM) pictures, providing us more precise details on the width, distance, distribution, and forms of fingerprint (FP) pores. All the results reveal that the phosphor is a good potential choice for AC and LFP detection.

Water Soluble PMPC-Derived Bright Fluorescent Nitrogen/Phosphorous-Doped Carbon Dots for Fluorescent Ink (Anti-Counterfeiting) and Cellular Multicolor Imaging.

Here, a simple one-step hydrothermal-assisted carbonization process was adopted for the preparation of nitrogen/phosphorous-doped carbon dots from a water-soluble polymer, poly 2-(methacryloyloxy)ethyl phosphorylcholine (PMPC). By the free-radical polymerization method, PMPC was synthesized using 2-(methacryloyloxy)ethyl phosphorylcholine (MPC) and 4,4'-azobis (4-cyanovaleric acid). The water-soluble polymers, PMPC, that have nitrogen/phosphorus moieties are used to prepare carbon dots (P-CDs). The resulting P-CDs were thoroughly characterized by various analytical techniques such as field emission-scanning electron microscopy (FESEM) with energy-dispersive X-ray spectroscopy (EDS), high-resolution transmittance electron microscopy (HRTEM), X-ray diffraction (XRD), Raman spectroscopy, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), Ultraviolet-visible (UV-vis) spectroscopy and fluorescence spectroscopy to determine their structural and optical properties. The synthesized P-CDs displayed bright/durable fluorescence, were stable for long periods, and confirmed the enrichment of functionalities including oxygen, phosphorus, and nitrogen heteroatoms in the carbon matrix. Since the synthesized P-CDs showed bright fluorescence with excellent photostability, excitation-dependent fluorescence emission, and excellent quantum yield (23%), it has been explored as a fluorescent (security) ink for drawing and writing (anti-counterfeiting). Further, cytotoxicity study results advised for biocompatibility and thus were used for cellular multicolor imaging in nematodes. This work not only demonstrated the preparation of CDs from polymers that can be used as advanced fluorescence ink, a bioimaging agent for anti-counterfeiting, and cellular multicolor imaging candidate, but additionally prominently opened a new perspective on the bulk preparation of CDs simply and efficiently for various applications.

Polyaryl substituted imidazobenzothiadiazoles as multifunctional organic fluorescent materials for visualization of latent fingerprints and lysosome staining

Organic fluorescent materials have attracted increasing attention for their applications in various fields. Design and synthesis of multifunctional organic fluorescent emitters based on simple molecular structure are still highly demanded and challenging. In this paper, we designed and synthesized six organic fluorescent molecules with polyaryl substituted imidazobenzothiadiazole structure. The substituents on imidazole ring have important effects on the optical properties. Compounds with N, N-dialkyl aniline moieties on imidazole ring could show obvious solvatochromic properties with relatively large Stokes shifts and low quantum yields. However, the fluorescent emissions of other compounds with phenol, 4-methoxy phenyl or methyl groups on imidazole ring are almost unaffected by the solvent polarity, and they can exhibit higher fluorescent quantum yields. Under acidic conditions, the fluorescence emission of these compounds can be gradually extinguished. In addition, the fluorescence emission can be recovered under basic conditions. Significant changes in fluorescence emission indicate that these molecules can be used as anti-counterfeiting inks. Further investigation showed that the compound with methyl group on imidazole ring can be used for latent fingerprint development. High-contrast fingerprint images are obtained on various substrate surfaces. It is also found that the compound with phenol moiety on imidazole ring can be utilized for lysosomes imaging in living cells accurately with high efficiency. This work provides a series of imidazobenzothiadiazole-based multifunctional organic materials with potential applications in security inks, latent fingerprint imaging and lysosome staining.

Assessment of Thermochromic Packaging Prints’ Resistance to UV Radiation and Various Chemical Agents

Thermochromic inks, also known as color changing inks, are becoming increasingly important for various applications that range from smart packaging, product labels, security printing, and anti-counterfeit inks to applications such as temperature-sensitive plastics and inks printed onto ceramic mugs, promotional items, and toys. These inks are also gaining more attention as part of textile decorations and can also be found in some artistic works obtained with thermochromic paints, due to their ability to change color when exposed to heat. Thermochromic inks, however, are known to be sensitive materials to the influence of UV radiation, heat fluctuations, and various chemical agents. Given the fact that prints can be found in different environmental conditions during their lifetime, in this work, thermochromic prints were exposed to the action of UV radiation and the influence of different chemical agents in order to simulate different environmental parameters. Hence, two thermochromic inks with different activation temperatures (one being cold and the other being body-heat activated), printed on two food packaging label papers that differ in their surface properties were chosen to be tested. Assessment of their resistance to specific chemical agents was performed according to the procedure described in the ISO 2836:2021 standard. Moreover, the prints were exposed to artificial aging to determine their durability when exposed to UV radiation. All tested thermochromic prints showed low resistance to liquid chemical agents as the color difference values were unacceptable in all cases. It was observed that the stability of thermochromic prints to different chemicals decreases with decreasing solvent polarity. Based on the results obtained after UV radiation, its influence in terms of color degradation is visible on both tested paper substrates, but more significant degradation was observed on the ultra-smooth label paper.

High-throughput fabrication of large-scale metasurfaces using electron-beam lithography with SU-8 gratings for multilevel security printing

The field of metasurface research has rapidly developed in the past decade. Electron-beam lithography (EBL) is an excellent tool used for rapid prototyping of metasurfaces. However, Gaussian-beam EBL generally struggles with low throughput. In conjunction with the recent rise of interest in metasurfaces made of low-index dielectric materials, we propose in this study the use of a relatively unexplored chemically amplified resist, SU-8 with EBL, as a method for rapid prototyping of low-index metasurfaces. We demonstrate the use of SU-8 grating on silicon for cost-efficient fabrication of an all-dielectric multilevel security print for anti-counterfeiting purposes, which encrypt different optical information with different light illumination conditions, namely, bright-field reflection, dark-field reflection, and cross-polarized reflection. The large-scale print (1 mm2) could be exposed in a relatively short time (∼11 min) due to the ultrahigh sensitivity of the resist, while the feature size of ∼200 nm was maintained, demonstrating that SU-8 EBL resist serves as a good candidate for rapid prototyping of metasurface designs. Our results could find applications in the general area of increasing EBL patterning speed for a variety of other devices and structures.