Anti-counterfeiting Applications Research Articles

Fluorene-naphthalene Schiff base as a smart pigment in invisible ink with multiple security features for advanced anticounterfeiting and forensic applications

Smart functional materials with captivating optical properties are of immense importance due to their versatile applicability in anticounterfeiting and forensic science. A fluorene-naphthalene Schiff base (FNH) that displays aggregation induced emission, mechanofluorochromism and excitation wavelength dependent fluorescence inherent to the pristine and ground samples is synthesized. Water/solvent-based invisible security inks for flexo/screen printing were formulated using FNH as a smart pigment to check the originality of documents/branded products etc. The prints with good photostability, adherence to substrate and rub resistance are invisible in daylight showcasing multiple non-destructive and destructive techniques to authenticate the document. The inked area on UV dull paper substrate exhibits a weak emission, which is observed by the forger under UVA light. However, the user can validate the authenticity of the document by rubbing the print with hard objects, especially using a metal coin or glass rod to perceive a human eye detectable intensification in the orange fluorescence under the same illumination source. The intensity of the orange fluorescence reverts to the original, which enables the reuse of the security document after originality check. Yet another nondestructive authentication method is to observe a cyan fluorescence from the print and orangish yellow fluorescence from the rubbed printed region when shined with a 270–400 nm light source, whereas a cyanish green fluorescence both from the unrubbed and rubbed regions of the print when illuminated with a visible light source ranging from 420 to 480 nm. An additional verification through a destructive technique is to perceive red and yellow fluorescence of the ink film upon contact with THF and NaOH/KOH, respectively and a penetrating red fluorescence from the rear side of the THF-exposed printed area of the paper. The multi-level security features that cannot be easily replicated by the forger but allows a simple and easy validation process by the user are unique to FNH, used as a single pigment in the inks. Further, the applicability of the ground FNH in forensic science is established to distinctly observe Level I to II details of latent fingerprints.

Rare Earth Ion Doped Luminescent Materials: A Review of Up/Down Conversion Luminescent Mechanism, Synthesis, and Anti-Counterfeiting Application

With the rapid development of modern technology and information systems, optical anti-counterfeiting and encryption have recently attracted considerable attention. The demand for optical materials is also constantly increasing, with new requirements proposed for performance and application fields. Currently, rare earth ion doped materials possess a unique electronic layer structure, underfilled 4f5d electronic configuration, rich electronic energy level, and long-life excited state, which can produce a variety of radiation absorption and emission. The distinctive properties of rare earth are beneficial for using in diverse optical output anti-counterfeiting. Design is essential for rare earth ion doped materials with multiple responsiveness and multi-channel optical information anti-counterfeiting in the field of information security. Therefore, this mini review summarizes the luminescent mechanisms, preparation methods, performance characteristics and anti-counterfeiting application of rare earth doped materials. In addition, we discuss some critical challenges in this field, and potential solutions that have been or are being developed to overcome these challenges.

Luminescent Mechanism and Anti-Counterfeiting Application of Hydrophilic, Undoped Room-Temperature Phosphorescent Silicon Nanocrystals.

Silicon nanocrystals (SiNCs) have attracted extensive attention in many advanced applications due to silicon's high natural abundance, low toxicity, and impressive optical properties. However, these applications are mainly focused on fluorescent SiNCs, little attention is paid to SiNCs with room-temperature phosphorescence (RTP) and their relative applications, especially water-dispersed ones. Herein, this work presents water-dispersible RTP SiNCs (UA-SiNCs) and their optical applications. The UA-SiNCs with a uniform particle size of 2.8nm are prepared by thermal hydrosilylation between hydrogen-terminated SiNCs (H-SiNCs) and 10-undecenoic acid (UA). Interestingly, the resultant UA-SiNCs can exhibit tunable long-lived RTP with an average lifetime of 0.85 s. The RTP feature of the UA-SiNCs is confirmed to the n-π* transitions of their surface C═O groups. Subsequently, new dual-modal emissive UA-SiNCs-based ink is fabricated by blending with sodium alginate (SA) as the binder. The customized anticounterfeiting labels are also prepared on cellulosic substrates by screen-printing technique. As expected, UA-SiNCs/SA ink exhibits excellent practicability in anticounterfeiting applications. These findings will trigger the rapid development of RTP SiNCs, envisioning enormous potential in future advanced applications such as high-level anti-counterfeiting, information encryption, and so forth.

Functionalization of cellulose carbon dots with different elements (N, B and S) for mercury ion detection and anti-counterfeit applications

Mercury ion (Hg2+), as one of the most toxic heavy metal ions, accumulates easily in the environment, which can generate potential hazards to the ecosystem and human health. To effectively detect and remove Hg2+, we fabricated four types of carbon dots (CDs) using carboxymethyl nanocellulose as a carbon source doped with different elements using a hydrothermal method. All the CDs exhibited a strong fluorescence emission, excitation-dependent emission and possessed good water dispersibility. Moreover, the four fluorescent CDs were used for Hg2+ recognition in aqueous solution, where the CDs-N exhibited better sensitivity and selectivity for Hg2+ detection, with a low limit of detection of 8.29 × 10−6 mol/L. It was determined that the fluorescence quenching could be ascribed to a photoinduced charge-transfer processes between Hg2+ and the CDs. In addition, the CDs-N were used as a smart invisible ink for anti-counterfeiting, information encryption and decryption. Furthermore, the CDs-N were immersed into a cellulose (CMC)-based hydrogel network to prepare fluorescent hydrogels capable of simultaneously detecting and adsorbing Hg2+. We anticipate that this research will open possibilities for a green method to synthesize fluorescent CDs for metal ion detection and fluorescent ink production.

Upconversion luminescence based temperature sensing properties and anti-counterfeiting applications of GdNbO4:Tm3+/Yb3+ phosphor

Pure monoclinic phase GdNbO4:Tm3+/Yb3+ phosphor was prepared using solid state reaction method for upconversion emission, optical thermometry, latent fingerprints visualization and anti-counterfeiting applications. The prepared phosphor exhibits colour tunability with temperature variation (from blue to almost white) and good suppression of thermal quenching. The temperature sensing characteristic from thermally and non-thermally coupled levels of Tm3+ ion were investigated using the fluorescence intensity ratio (FIR) approach. The result shows that the sensitivity for non-thermally coupled level is about ∼ 45 times higher than thermally coupled level. Latent fingerprint detection on various surfaces and anti-counterfeiting ink application were also shown upon 980 nm laser diode excitation. Above results indicate that the prepared GdNbO4:Tm3+/Yb3+ phosphor have applicability in the field of frequency upconversion, optical thermometry, color tunable devices, latent fingerprint visualization and anti-counterfeiting applications.

PH Tunable Patterning of Quantum Dots.

Patterning of quantum dots (QDs) is essential for many, especially high-tech, applications. Here, pH tunable assembly of QDs over functional patterns prepared by electrohydrodynamic jet printing of poly(2-vinylpyridine) is presented. The selective adsorption of QDs from water dispersions is mediated by the electrostatic interaction between the ligand composed of 3-mercaptopropionic acid and patterned poly(2-vinylpyridine). The pH of the dispersion provides tunability at two levels. First, the adsorption density of QDs and fluorescence from the patterns can be modulated for pH > ≈4. Second, patterned features show unique type of disintegration resulting in randomly positioned features within areas defined by the printing for pH ≤ ≈4. The first capability is useful for deterministic patterning of QDs, whereas the second one enables hierarchically structured encoding of information by generating stochastic features of QDs within areas defined by the printing. This second capability is exploited for generating addressable security labels based on unclonable features. Through image analysis and feature matching algorithms, it is demonstrated that such patterns are unclonable in nature and provide a suitable platform for anti-counterfeiting applications. Collectively, the presented approach not only enables effective patterning of QDs, but also establishes key guidelines for addressable assembly of colloidal nanomaterials.

Design, Synthesis, and Characterization of a Novel Blue-Green Long Afterglow BaYAl3O7:Eu2+, Nd3+ Phosphor and Its Anti-Counterfeiting Application.

Herein, a series of novel long afterglow nanophosphors BaYAl3O7:Eu2+, Nd3+ was synthesized by the combustion method. The investigation encompassed the characterization of X-ray diffraction, morphology, chemical valence, elemental composition, and photoluminescence behavior of BaYAl3O7:Eu2+ and BaYAl3O7:Eu2+, Nd3+ nanoparticles. Under 365 nm excitation, BaYAl3O7:Eu2+ and BaYAl3O7:Eu2+, Nd3+ show emission bands centered at 497 nm and 492 nm, which are attributed to the 4f65d→4f7 transition of Eu2+ ions. The optimal samples of BaYAl3O7:0.03Eu2+ and BaYAl3O7:0.03Eu2+, 0.02Nd3+ have average fluorescence lifetimes of 850 ns and 1149 ns, respectively. The co-doping of Nd3+ ions as the trap centers produced long afterglow luminescence properties, and the afterglow time could reach up to 8 min. Furthermore, the fluorescent powder can be mixed with polyacrylic acid to prepare anti-counterfeiting inks; a clover pattern and snowflake pattern have been successfully printed using screen printing technology, proving its potential application in the field of anti-counterfeiting.

Benzofuran derived pH-responsive AIEgens: Design, synthesis and applications in monitoring the urea content in soil and food quality evaluation

The stimuli-responsive luminescent materials have received special attention because of the modulable physical or chemical properties under the influence of external stimuli such as pH, temperature, light, heat, pressure, and so on. Therefore, such materials have potential applications in mechano-sensing devices, security inks, memory devices, data storage, deformation detectors, and so on. We present here the design and synthesis of three pH-responsive luminogens L1-L3 with aggregation-induced emissive properties built over the benzofuran scaffold. The AIE active luminogens L1-L3 demonstrated reversible fluorescence switching when exposed to Trifluoroacetic acid (TFA) fumes followed by Triethylamine (TEA) or NH3 fumes. The pH-sensitive fluorescence behavior of L2 has been successfully used in developing fluorescent inks for anti-counterfeit applications, as well as to construct paper strip-based detecting tools for qualitatively detecting the urea content in the soil and evaluating the ripeness/maturity of the packed fruits (grapes) in a non-destructive manner. Furthermore, both L1 and L2 were found to be highly biocompatible and well internalized within the cytoplasm of MDA-MB-231 cancer cells, with very high fluorescence intensity.

Anti-counterfeiting and multifunctional applications of near white emitting La1.99MgTiO6:0.01Dy perovskite nano phosphors

A 1 wt% dysprosium doped double perovskite nano phosphor La2MgTiO6 is synthesised by a low-temperature combustion method. When excited with UV radiation (351 nm), the sample shows major characteristic emission peaks at 480 nm (4F9/2 →6H15/2) and 574 nm (4F9/2 →6H13/2) whereas, when excited by IR radiation (980 nm), an up-conversion emission peak at 654 nm (4F9/2 →6H11/2) is observed. The photoluminescence spectra of La1.99Dy0.01MgTiO6 show dominant blue emission, which obliges potential white LED applications with chroma coordinates (0.32, 0.33). On the other side, the samples annealed above 1223 K build suitable reading light that shows dominant yellow emission with a quantum efficiency of 69.25 %; which simplifies why samples with high-temperature heat treatment are pertinent yellow-emitting phosphor. Using the Tauc plot, an improvement of ΔEg= 14 meV is seen owing to annealing at 1223 K. The XRD, Raman, SEM, and TEM results signify the increase in crystallinity and crystallite size with annealing. Noticeably, the red emission upon up-conversion highlights the medical imaging purpose. The effective combination of up-down conversion emission can be successfully exploited for anti-counterfeiting applications; underlining the multifunctional application of a single perovskite nano phosphor for smart materials.

Multiresponsive, easy-reversible, and dual-visual Pt(II) salt nanostructures for advanced anti-counterfeiting application

Multiresponsive, easy-reversible, and dual-visual Pt(II) salt nanostructures for advanced anti-counterfeiting application

FAPbBr3@PbBr(OH) phosphor with high stability for anti-counterfeiting application via water induction

Organic–inorganic hybrid perovskite nanocrystals have become a very widely used as semiconductor light-emitting materials. However, perovskite nanocrystals face stability challenges, which is a key factor hindering their application. In this paper, by introducing water into the synthesis of formamidinium lead bromide (FAPbBr3) perovskite, ultra-stable FAPbBr3@PbBr(OH) fluorescent material was prepared. The photoluminescence intensity of the material after the addition of water increased 2.9 times compared with that before the addition of water. The excellent green fluorescence emission was still maintained after four cycles of wash–dry treatment. Meanwhile, it also exhibits good ultraviolet and thermal stability. The above enhanced performance of FAPbBr3 nanocrystals is attributed the protection of PbBr(OH). In addition, the prepared material can be used in anti-counterfeit patterns. The anti-counterfeit patterns have good color rendering and the luminous color has a high dependence on temperature. Both of these features make it very valuable for various fluorescent anti-counterfeiting labels.

Luminescent Papers with Asymmetric Complexes of Eu(III) and Tb(III) in Polymeric Matrices and Suggested Combinations for Color Tuning.

Complexes of lanthanide ions, such as Eu(III) (red light emission) and Tb(III) (green light emission), with proper ligands can be highly luminescent and color-tunable, also attaining yellow and orange emission under UV radiation. The ligands employed in this work were poly(sodium acrylate), working as polymeric matrix, and 1,10-phenanthroline, taking advantage of its antenna effect. Possibilities of color display were further enhanced by incorporating a cationic polyfluorene with blue emission. This strategy allowed for obtaining cyan and magenta, besides the aforementioned colors. Uncoated cellulose paper was impregnated with the resulting luminescent inks, observing a strong hypsochromic shift in excitation wavelength upon drying. Hence, while a cheap UV-A lamp sufficed to reveal the polyfluorene's blue emission, shorter wavelengths were necessary to visualize the emission due to lanthanide ions as well. The capacity to reveal, with UV-C radiation, a full-color image that remains invisible under natural light is undoubtedly useful for anti-counterfeiting applications. Furthermore, both lanthanide ion complexes and polyfluorenes were shown to have their luminescence quenched by Cu(II) ions and nitroarenes, respectively.

NIR activated Er3+/Yb3+ doped LiYF4 upconverting nanocrystals for photothermal treatment, optical thermometry, latent fingerprint detection and security ink applications

This work reports the upconversion (UC) emission studies on Er3+/Yb3+ doped LiYF4 nanoparticles for radiation-to-heat conversion, optical thermometry using thermally coupled and uncoupled levels, latent fingerprint detection and security ink applications. LiYF4:Er3+/Yb3+ nanoparticles have been synthesized using the thermal decomposition reaction method using oleic acid and 1-octadecane as reaction medium. Crystal phase, crystallinity and size have been investigated using XRD and TEM techniques. UC emission ability of the prepared phosphor was tested using a CW laser source having 980 nm excitation. Under 980 nm excitation, synthesized nanoparticles have shown emission bands around 522 nm, 550 nm and 668 nm corresponding to transitions from 2H11/2, 4S3/2, and 4F9/2 levels to 4I15/2 level of Er3+ ions, respectively. Further, the number of photons involved in the UC process has been calculated using a pump power UC study. Decay time analysis of the most intense green (550 nm) and red (668 nm) emission bands is performed to study the emission dynamics. A study on the radiation-to-heat conversion phenomenon has revealed photothermal conversion efficiency of 29.5% which is important for photothermal therapy. The green and red UC emissions from the sample under stimulation at very low pump power (35 mW) are used to study the optical temperature sensing characteristics. The findings of this study suggest that the Er3+/Yb3+ co-doped LiYF4 phosphor may be particularly useful for creating NIR to visible upconversion and temperature sensing. Moreover, latent fingerprint detection and security ink for anti-counterfeiting applications are also demonstrated.

Novel intense blue emitting Bi2Zr2O7:Ce3+ nanocomposites insertion into flexible polymer films for anti-counterfeiting, long-term storage fingerprints, and display device applications

The development of multifunctional organic/inorganic hybrid materials for anti-counterfeiting and food packaging applications can be achieved through the use of polymer nanocomposites containing poly(vinyl alcohol) (PVA) and chitosan (CS), as well as Ce3+-activated Bi2Zr2O7 nanocomposites (PCBC NCs). However, synthetic methods for these nanocomposites are often time- and energy-intensive, leading to heterogeneous films that require compatibilizers to disperse the organic matrix's inorganic nanoparticles (NPs). This study used a simple solution casting approach to impregnate the appropriate phosphor sample into PVA-CS (PC) flexible films, resulting in luminous ink for high-security anti-counterfeiting applications. Using this nanocomposite ink offers an even higher level of security, as the prints can be submerged in copper chloride (CuCl2) aqueous solutions to quench the luminous message. The masked fingerprints used for long-term storage (up to 12 months) exhibit stable level 1–3 ridge features with high contrast and sensitivity, which may improve printing quality and performance in the fight against counterfeiting. In addition, a mathematical model implemented in Python-based software was used to investigate the latent fingerprints of level 1–2 ridge features.

LaVO4: Eu3+ nano-islands onto silica for achieving fluorescence enhancement and their detection of Fe3+ ions and anti-counterfeiting applications

Rare earth (RE) composite fluorescent materials are favored by researchers in the field of anti-counterfeiting and ion sensing due to their fascinating optical properties. Ultra-small RE fluorescent nanoparticles are anchored on inorganic carriers by a simple preparation method to improve luminous intensity and hydrophilicity, which has not been explored yet. Herein, LaVO4: Eu3+ nano-islands anchored on silica with high fluorescence intensity and easy formation of stable colloidal solution is designed. Through a simple and mild hydrothermal approach, ultra-small LaVO4: Eu3+ nano-islands are highly dispersed on the surface of hierarchical hollow silica sphere (HHSS) to expose more luminescent centers. Remarkably, the stable HHSS@LaVO4: Eu3+ colloidal solution displayed highly sensitive and selective sensor for Fe3+ ions. The “island-sea synergy” structure formed by the LaVO4: Eu3+ nano-islands and the surrounding silica surface makes HHSS@LaVO4: Eu3+ to be an outstanding sensor for the effective detection of iron ions in water. In addition, HHSS@LaVO4: Eu3+ phosphor exhibit unique properties for anti-counterfeiting and encryption applications. These findings provide a promising strategy for the carrierisation of RE luminescent materials to improve optical properties and enable broader applications.

Optimized Tb3+-activated highly efficient green-emanating BaLa2ZnO5 nanophosphors in PDMS matrix for flexible display, anti-counterfeiting, and dermatoglyphics applications

Green-emitting BaLa2ZnO5:Tb3+ nanophosphors (NPs) were synthesized via a safe and cost-effective solution combustion process, with Tb3+ dopant content optimized at 4 mol% to avoid concentration quenching. The NPs exhibited strong green emission at 545 nm, a decay lifetime of 0.520 ms, and a quantum efficiency of 35.12%. Chromaticity generalization demonstrated their potential for advanced lighting devices, with coordinate values of (0.2669, 0.5274), a temperature of 7246 K, and color purity of 91.03%. The optimized NPs produced a proof-of-concept security ink and a translucent anti-counterfeiting (AC) film that could emit a powerful green light when exposed to UV light. A Python program was developed to enhance and extract features from fingerprints, emphasizing the importance of each step in producing a perfectly improved image. The minutiae FPs were color-coded to indicate bifurcation and termination, and the final stage of thinning and binarizing images revealed Level-I and Level-II features. The optimized BLZO:4Tb3+ NPs demonstrated great potential for solid-state lighting, flexible displays, anti-counterfeiting, and dermatoglyphics applications.

Dual-mode security authentication of SrAl2 O4 :Eu,Dy phosphor encapsulated in electrospun cellulose acetate nanofibrous films.

Photochromic inks have been an attractive authentication strategy to improve the anti-counterfeiting efficiency of commercial products. However, recent reports have showed significant disadvantages with photochromic inks, including poor durability and high cost. In this context, we develop novel photochromic nanofibers for advanced anti-counterfeiting applications. Lanthanide-doped strontium aluminate (LdSA) nanoparticles (NPs) were prepared and immobilized into electrospun cellulose acetate nanofibers (CANF). Authentication materials immobilized with inorganic photochromic agent can warranty durability and photostability. Therefore, the ultraviolet-stimulated photochromism of LdSA-encapsulated cellulose acetate nanofibers (LdSA@CANF) demonstrated high reversibility and photostability. A broad range of cellulose acetate nanofibers with unique emission characteristics were developed when applying different ratios of LdSA NPs. LdSA@CANF appeared colorless under visible daylight, whereas a green emission was monitored under UV-illumination. The shape and chemical content of the photochromic fibrous films were examined by various analytical techniques. The mechanical characteristics of LdSA@CANF-coated paper were investigated. The emission wavelength was detected at 514 nm to designate green color, whereas the excitation wavelength was detected at 369 nm to indicate transparency. The prepared cellulose acetate nanofibrous film can be described as an efficient strategy for anti-counterfeiting of commercialized items.

Tailor Traps in Bi3+-Doped NaGdGeO4 Phosphors by Introducing Eu3+ Ions to Switch Multimodal Phosphorescence Emission.

Featured with a tunable excitation/emission wavelength and excellent physicochemical stability, inorganic fluorescent materials are widely used in the fields of anti-counterfeiting. Here, we design a multi-stimuli-responsive dynamic fluorescence and phosphorescence anti-counterfeiting material by introducing Eu3+ ions in NaGdGeO4: Bi3+ to tailor the trap structure. The photoluminescence (PL), long persistent luminescence (LPL), and photo-stimulated luminescence (PSL) colors of NaGdGeO4: Bi3+, Eu3+ can be switched by varying the excitation modes (ultraviolet, near infrared, and X-ray light). Especially, the LPL and PSL colors of NaGdGeO4: Bi3+, Eu3+ vary with increasing decay and stimulation times. In addition, X-ray excitation ensures the specificity of the luminescence of NaGdGeO4: Bi3+, Eu3+ compared with ultraviolet excitation. This rapidly-changing-color fluorescent material offers the possibility of sophisticated anti-counterfeiting applications in the future.

Orange-red persistent luminescence in Mn2+-Doped Sr3Y2Ge3O12 films for dynamic anti-counterfeiting

The research focus in the field of anti-counterfeiting has shifted toward the secure luminescent label technology, which has gained prominence. The exceptional luminescence properties of persistent luminescence materials are important in the achievement of effective anti-counterfeiting measures. It is in this context that Sr3Y2Ge3O12 can serve as an excellent matrix material. In this study, it is found that Sr3Y2Ge3O12: 4.00% Mn2+ exhibits a luminescence intensity, persistence time, and stability that meet the practical requirements for applications. It is shown that the fabricated persistent luminescence film can be used in anti-counterfeiting applications, thereby, demonstrating the possibility to broaden the application scope of persistent luminescence materials. An emission band centered at 634 nm and a high-energy inflection point at 578 nm are observed in the samples excited at 260 nm. Fluorescence emission at 634 nm is also achieved through X-ray excitation. Through thermoluminescence, photoluminescence, and phosphorescence spectroscopy, a comprehensive investigation of the trap characteristics and inherent mechanisms of the long persistent phosphorescence has been conducted. The novel orange-red Sr3Y2Ge3O12: Mn2+ phosphor is extensively examined.

Mechano- and solvato-fluorochromic chalcone and its water-based flexo-ink for anticounterfeit applications

Pigments with unique optical features are always sought after in realizing superior security features to protect authentic documents/products against forgery. The present study reports the synthesis of triphenylamine-naphthalene chalcone (TPA-NC) with fascinating optical features: solvato-fluorochromism, mechano-fluorochromism, and aggregation-induced emission. The flexo print obtained on UV dull paper using water-based eco-friendly UV fluorescent ink with TPA-NC as pigment displayed good adhesion, resistance to abrasion, light fastness, and favourable electrical parameter values, which could be well utilized in security documents and developing OTFTs in banknotes to prevent them from being forged. Moreover, the printed paper samples could impart superior features including solvent fugitiveness, penetrating fluorescence, and mechanical stimuli responsive variations in emission color for achieving multilevel security. The security ink prepared using the thermally stable and biocompatible TPA derivative could generate a robust platform for tamper-detection, validation of genuine documents/products and in marking technology. Moreover, an ultra-high radio frequency identification (RFID) antenna is designed to resonate at 2.35 GHz. The hybrid arrangement has steady radiation characteristics for electronic anticounterfeiting applications.