Security Printing Research Articles

Multicolor-tunable biomass thermochromic dyes utilizing tea polyphenols color developer for temperature-controlled linen fabric

Linen, an important industrial crop, is commonly used in the textile field. The development of temperature-controlled linen fabrics is important for the high-value utilization of cellulose-based industrial products. The linen fabrics with color and performance changes controlled by temperature are prepared via the vacuum impregnation process utilizing biomass three-component thermochromic dyes. Currently, three-component thermochromic dyes based on the bisphenol A (BPA) color developer have received special attention due to high saturation color and reversible color-changing performance. However, the toxicity and carcinogenicity of BPA limit its applications in security inks, decorative coatings, plastics, etc. Exploiting excellent and safe biomass color developers for three-component thermochromic dyes remains a challenge. Tea polyphenols (TP) are proposed as alternatives to BPA to prepare the blue, red, yellow, and green three-component thermochromic dyes. According to the laws of subtractive color mixing, multicolor-tunable thermochromic dyes are obtained by color matching of red, yellow, and blue dyes to span multiple shades. Biomass thermochromic dyes are adopted to fabricate temperature-controlled linen fabrics by the vacuum impregnation process. Temperature-controlled linen fabrics can change color reversibly between the original color and achromatic color. The color-changing occurs due to the ring opening and closing of leuco dyes triggered by the solid-to-liquid transition of solvent. The phase transition temperature of the thermochromic dyes absorbed on the linen fabric is 46 ℃. As the temperature increases, the contact angle of temperature-controlled linen fabrics decreases from 129° to 29°, resulting in a change from hydrophobic to hydrophilic with 49 J g−1 of thermal energy stored. The linen fabrics exhibit temperature-controlled changes in color, surface wettability, strength, and thermal insulation. This eco-friendly method with biomass thermochromic dyes for the preparation of color-changing linen fabrics provides a new approach, which is contributing to the development of cellulose-based smart color-changing textiles.

Organic mediated solvothermal synthesis, enhanced luminescence dynamics, photometric and Judd-Ofelt analyses of Zn2+ modified Gd2O3: Eu3+, Li+ nanophosphors for deep UV LEDs and security inks

Highly luminescent red emitting Zn2+ modified Gd2O3:Eu3+, Li + nanophosphors with superior colour purity were prepared by the organic mediated solvothermal combustion of metal-citrates in diethylene glycol. Structure, morphology, spectroscopic and luminescence dynamics of the nanophosphors were found to be sensitive to Zn2+ concentration. Simultaneous incorporation by Zn2+ and Li+ ions in Gd2O3:Eu3+ resulted in better emission features and improved photophysical radiative properties. Enhanced photoemission has resulted from the cooperative and combined roles caused by Li+ and Zn2+ ions, improvement of crystallinity, modifications of the crystal field and reduced symmetry around Eu3+ ions in Gd2O3. Photoemission intensity of the optimal zinc compensated phosphor with composition Gd1.71Eu0.1Li0.15Zn0.04O3 was found to be 1.82 times that of Gd1.75Eu0.1Li0.15O3 and it exhibited a lifetime of 1.467 ms with the highest colour purity of 97.06 %. Luminescence decay profiles of all the synthesized phosphors were found to follow single exponential behaviour. Judd-Ofelt analysis revealed that the emission intensity parameters were found to be optimum for Gd1.71Eu0.1Li0.15Zn0.04O3 phosphor, indicating the highest asymmetry of local environment around Eu3+ ions in this material. Better radiative and Judd-Ofelt intensity parameters obtained indicate that this highly luminescent red emitting phosphor can act as a promising material for high resolution phosphor coated displays, deep UV excited phosphor converted LEDs and for developing defence security inks.

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.

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.

Conventional Non-Fluorescent Polymers: Unconventional Security Inks for Data Storage and Multidimensional Photonic Cryptography.

Traditional security inks relying on fluorescent/phosphorescent molecules are facing increasing risks of forgery or tampering due to their simple readout scheme (i.e., UV-light irradiation) and the advancement of counterfeiting technologies. In this work, a multidimensional data-encryption method based on non-fluorescent polymers via a "lock-key" mechanism is developed. The non-fluorescent invisible polymer inks serve as the "lock" for data-encryption, while the anti-rigidochromic fluorophores that can distinctively light up the polymer inks with programed emissions are "keys" for decryption. The emission of decrypted data is prescribed by polymer chemical structure, molecular weight, topology, copolymer sequence, and phase structure, and shows distinct intensity, wavelength, and chirality compared with the intrinsic emission of fluorophores. Therefore, the data is triply encrypted and naturally gains a high-security level, e.g., only one out of 20000 keys can access the only correct readout from 40000000 possible outputs in a three-polymers-based data-encryption matrix. Note that fluorophores lacking anti-rigidochrimism cannot selectively light up the inks and fail in data-decryption. Further, the diverse topologies, less well-defined structures, and random-coiled shapes of polymers make it impossible for them to be imitated. This work offers a new design for security inks and boosts data security levels beyond the reach of conventional fluorescent inks.

Achieving Stable and Switchable Ultralong Room-Temperature Phosphorescence from Polymer-Based Luminescent Materials with Three-Dimensional Covalent Networks for Light-Manipulated Anticounterfeiting.

Developing polymer-based organic afterglow materials with switchable ultralong organic phosphorescence (UOP) that are insensitive to moisture remains challenging. Herein, two organic luminogens, BBCC and BBCS, were synthesized by attaching 7H-benzo[c]carbazole (BBC) to benzophenone and diphenyl sulfone. These two emitters were employed as guest molecules and doped into epoxy polymers (EPs), which were constructed by in situ polymerization to achieve polymer materials BBCC-EP and BBCS-EP. It was found that BBCC-EP and BBCS-EP films exhibited significant photoactivated UOP properties. After light irradiation, they could produce a conspicuous organic afterglow with phosphorescence quantum yields and lifetimes up to 5.35% and 1.91 s, respectively. Meanwhile, BBCS-EP also presented photochromic characteristics. Upon thermal annealing, the UOP could be turned off, and the polymer films recovered to their pristine state, showing switchable organic afterglow. In addition, BBCC-EP and BBCS-EP displayed excellent water resistance and still produced obvious UOP after soaking in water for 4 weeks. Inspired by the unique photoactivated UOP and photochromic properties, BBCC and BBCS in the mixtures of diglycidyl ether of bisphenol A (DGEBA) and 1,3-propanediamine were employed as security inks for light-controlled multilevel anticounterfeiting. This work may provide helpful guidance for developing photostimuli-responsive polymer-based organic afterglow materials, especially those with stable UOP under ambient conditions.

Heteroatom doped carbon dots from green sources as metal ion sensor and as fluorescent ink

In this study, novel eco-friendly nitrogen-doped carbon dots (NCDs) were prepared via a green, efficient, and one-pot synthesis method from Sida cordifolia root as the carbon source and triethylene tetraamine as the nitrogen source for the detection of metal ions. The amorphous, nanosized carbon dots with 17.8 % quantum yield exhibited exceptional stability towards different experimental conditions. These NCDs acted as a nanoprobe for the sensing of Co(II) and Cu(II) via fluorescence and electrochemical techniques, respectively. These blue fluorescent carbon dots detects Co(II) with a detection limit as low as 0.11 μM and Cu(II) with 0.03 nM. The inner filter effect (IFE) acted as the primary fluorescence quenching mechanism. Also, the developed sensor could detect metal ions in different water samples. Moreover, NCDs were used as fluorescent security ink for writing and drawing. The findings of this study open up a new platform for the effective and reliable sensing of metal ions.

Tunable Excitation Polarized Upconversion Luminescence and Reconfigurable Double Anti‐Counterfeiting from Er3+ Doped Single Nanorods

AbstractExcitation polarized upconversion luminescence (EPUL) from lanthanide ions has attracted considerable attention due to its wide applications in microfluidics, single particle tracking, security inks, and cell internal viscosity testing. However, controlling the degree of excitation polarization (DOEP) of the EPUL remains a significant challenge. Here, the modulation of the DOEP (from 0 to 0.5) of the EPUL from Er3+ doped single nanorods by changing the concentration of doped Er3+, Yb3+, or Mn2+ is systematically studied. By analyzing the lifetimes and disproportionate changes in luminescence intensities, it is found that optimizing Er3+, Yb3+, or Mn2+ concentration can reduce non‐radiative transition and population density in excited states, leading to the enhancement of the DOEP under a good alignment of transition dipoles. Furthermore, the possibility of anti‐counterfeiting based on such tunable EPUL is illustrated. Three kinds of fine patterns with a small size of 10 µm are realized by assembling the single nanorods accurately via optical tweezers. The patterns and their EPUL guarantee double protection for the feasibility of anti‐counterfeiting. The findings of this study offer insights into the EPUL from lanthanide ions and provide a microscale platform via the EPUL for the application of multidimensional information encoding and reconfigurable double anti‐counterfeiting.

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.

Effect of precursors on ZnO nanoparticles to enhance the level-III ridge details of LFPs and anti-counterfeiting applications

Comparative analysis for the combustion-based synthesis of zinc oxide nanoparticles (NPs) utilizing various precursors of zinc was reported. l-ascorbic acid extraction was used as a fuel for the combustion process. The sizes, morphologies, structural details and purity of samples were examined using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffractometry (XRD). XRD pattern matches with the standard JCPDS card of ZnO with good crystallite size. The average crystallite size of the NPs was confirmed the nano-regime. Variations in morphological structures and the energy band gap variation with the change in precursors was reported. Using a powder-dusting technique, on separate surfaces, level-III features of the fingerprints produced by these NPs without any background noise. The suggested powder composition was easy to make and effective for producing latent finger prints on a variety of dry and wet surfaces. In the area of counterfeit prevention, the fluorescent ink has several potential applications. Invisible security ink was developed for anti-counterfeiting applications based on the UV fluorescence feature of ZnO NPs. The fluorescent invisible security ink for screen printing was evaluated and proved to be stable on a variety of microporous papers. The new strategy adopted here lowers the cost of ink by using rare earth free ZnO NPs.

Bithiophene-naphthalene chalcone as a fluorescent pigment in eco-friendly security ink formulation

Approaches to prevent document/product forgery using eco-friendly printing inks and security printing techniques on flexible substrates are two vital areas of research that demand coherent advancements. In this context, a new bithiophene-naphthalene chalcone (BTNP) was synthesized and characterized as a fluorescent pigment for use in security ink. BTNP exhibited good solid-state and solution phase fluorescence with intramolecular charge transfer confirmed using theoretical studies and emission spectra collected in THF/THF–hexane mixtures. The aggregation-induced emission of BTNP was established using solution phase studies in THF/THF–water mixtures. The strong solid-state yellow emission of BTNP prompted its use as a pigment in the preparation of an environment-friendly UV fluorescent formulation, devoid of any volatile organic compounds or hazardous air pollutants. The screen prints obtained on a UV dull paper substrate utilizing BTNP ink revealed good fluorescence, photostability, colorimetric, densitometric, and rub resistance characteristics, which showcase the potential applicability of the BTNP formulation in security printing. The low cytotoxic nature of the chalcone as observed in the MTT assay could also be exploited for the use of formulation in inkpads.Graphical abstract

Heat-induced ʻparallelʼ to ʻorthogonalʼ switching of fluorophores and reversible luminescence changes in divinylbenzene-benzoxazole crystals

Organic crystals that exhibit reversible solid-state luminescence triggered by heat stimulus have garnered great research interest in recent years. In this work, we demonstrate thermoresponsive, reversible color and photoluminescence switching in divinylbenzene-benzoxazole derivatives. The fluorescence switching is caused by differential chromophore packing and the resulting charge transfer On/Off states induced by heat. It is noteworthy that the reversibility, range of colors, and transition temperature associated with the luminescence switching are dependent on the length of the alkyl chains. Our results suggest that controlling charge-transfer interactions in chromophore assemblies may offer a versatile approach for creating functional organic materials with switchable, high-contrast solid-state luminescence. These findings have implications for the development of sensors, data storage devices, dynamic optical materials, and security inks.

Light-Responsive Fluorescent Security Ink Based on Calixpyridinium–Protoporphyrin IX Disodium Salt Supramolecular Nanoassemblies

In this work, the coordination effects of protoporphyrin IX disodium salt (PpIX) with CuII, FeII, and ZnII were systematically studied, and the host–guest interactions of calixpyridinium with PpIX and the three metalloporphyrins were further explored. The bonding ratio between calixpyridinium and PpIX was different from that between calixpyridinium and the other three metalloporphyrins. The reduction of the electronegativity of PpIX caused by the coordination of metal cations may be the reason for the change of the bonding ratio. The reduction of the electronegativity of PpIX caused by the coordination of metal cations also leads to a weaker host–guest interaction with calixpyridinium. The supramolecular nanoassemblies prepared by the host–guest interactions of calixpyridinium with PpIX and the three metalloporphyrins under alkaline conditions could emit a bright fluorescence due to the pH-dependent luminescence characteristics of calixpyridinium and PpIX. Among calixpyridinium and the three metal ions, calixpyridinium alone had the best effect on promoting the photoactivity of PpIX. Its fluorescence gradually disappeared after being irradiated by an incandescent lamp not only in aqueous solution but also on a filter paper. More interestingly, the supramolecular nanoassemblies formed by calixpyridinium and the photoreaction product of PpIX had a reversible temperature responsiveness, which could lead to a reversible fluorescence on–off control. As a result, this material was successfully applied as an intelligent light-responsive fluorescent security ink.

Photoluminescent pyrene-based ionic liquid derived ratiometric organo nanosensor for rapid and selective detection of picric acid

Due to the rising menace of illicit actions and pollution aroused by explosive nitroaromatic compounds (NACs), the growth of an adept sensor for detecting these NACs is highly essential. Herein, in this communication, a photoluminescent IL-assimilated group of uniform materials based on organic salt (GUMBOS) and nano-GUMBOS has been fabricated by integrating pyrene butyrate with a quaternary phosphonium IL (PbIL) via a simple ion exchange process. Neat PbIL shows a bright cyan color photoluminescence under a 365 nm UV lamp irradiation and is employed as photoluminescence security ink and picric acid (PA) detection among the tested NACs. By simple reprecipitation method, we have developed water-suspended crystalline pyrene assimilated nanoparticles, nPbIL, characterized by various analytical techniques. The PbIL-derived water-suspended nanomaterials, nPbIL, display a robust cyan color excimer-like emission, which turns blue (monomeric) due to adding PA over the other related NACs. This ratiometric cyan to blue photoluminesce change is due to the displacement of the anionic pyrene moiety of the nPbIL by the picrate anion. The fabricated organo nanosensor is enormously discerning and responsive towards PA with a detection limit of 0.77 nM and is found to be superior to many available in the literature. Additionally, a fluorogenic paper strip-based test kit experiment has been demonstrated to detect and quantify PA selectively in aqueous solvents amongst the other tested NACs. The present contribution evokes a novel approach to developing different IL-based chemosensors for detecting and quantifying various target analytes.

A Cu2+ triggered reversible photochromic system: the structure photochromic response relationship study and potential applications.

Fifteen rhodamine B hydrazide hydrazone (RhBHH) derivatives (compounds a-o) with various substituent groups at different position and their photochromic property triggered by Cu2+ were studied to illustrate the structure photochromic response relationship (SPRR). Three of them (compounds f-h) with a para-position hydroxyl group and two meta-position halogen substituents display Cu2+-triggered photochromic which is significantly different from the previous reports. It was found that halogen atoms, which were generally considered to have no remarkable regulation effect, exhibited great influences on the photochromic behaviour of RhBHH derivatives. Detail photochromic properties of the developed photochromic system were revealed by using compound g as the model substrate, and only Cu2+ displayed high selective trigger effect. Good reversible photochromic phenomenon was observed after stimulated with visible light irradiation and dark (or heat) bleaching consecutively. Furthermore, this photochromic system could be used in the preparation of photochromic glass, special security ink, molecular logic gate and two-dimensional code for security information storage.

Water-based composite ink from modified chitosan and ruby nanoparticles for advanced anticounterfeiting applications

There is a huge demand for sophisticated materials and innovative strategies to create cost-effective and real-life solutions in security printing field to combat the grave threat due to forgery or counterfeiting of documents and products. In this study, nano ruby is dispersed in an eco-friendly, water-based screen-printing formulation containing modified chitosan as both luminescent colourant and resin to achieve superior security features against counterfeiting. Biopolymer chitosan is modified using 4-bromo-3-hydroxy-1,8-naphthalic anhydride through a green imidation reaction to obtain N-(6-bromo-5-hydroxy-1H-benzo[de]isoquinoline-1,3(2H)-dione) chitosan (CSNA) that display yellow fluorescence, good thermal stability, and low cytotoxicity. CSNA is used as a fluorescent resinated dye to formulate CSNA ink, devoid of any hazardous air pollutants or volatile organic compounds. To improve the security feature, ruby nanoparticles are dispersed in the CSNA ink to achieve nanoruby-CSNA (CSNAR) composite ink, which is further screen-printed on paper substrates. The CSNAR ink film exhibited advanced level security features, wherein the forger can detect only a bright yellow fluorescence from the prints under 365 nm UV light. However, the user can validate the originality of the print by viewing red and yellowish-orange emissions under specific excitation wavelengths, which is unique to the CSNAR ink, and therefore difficult for the forger to replicate. The good adhesion, lightfastness and abrasion resistance of the print combined with the multi-level security features open a new avenue for potential applications of CSNAR security inks/films in document/product authentication and packaging/pharmaceutical anti-counterfeiting applications.

A new Milarite type KMLS:Eu3+ orange-red-emitting phosphor for pc-white LEDs and Forensic applications

In this article, we present the synthesis and characterization of Eu3+-activated KMLS:Eu3+ phosphor for pc-WLEDs, latent fingerprint detection, and security ink applications. The structural phase, luminescent property, and colour purity of the as-prepared phosphor were investigated. XRD profiles and Rietveld refinement analysis confirm the hexagonal crystal system of the as-prepared phosphor. Concentration quenching effect was not observed up to 17 mol% of Eu3+. The site occupancy preferences of Eu3+ were investigated using the bond-valence model. The CIE-coordinate of optimal Eu3+ is estimated to be close to the standard NTSC value, and color purity is found to be above 90% for all the Eu3+ concentrations. Both Judd-Ofelt (J-O) analysis and asymmetric ratios revealed that the local environment around Eu3+ is asymmetric in nature. The radiative properties, including the stimulated emission cross section and optical gain parameters, of the highest concentration of Eu3+ (17mol%) were determined. Further, the results of thermal quenching analysis show that 91% of the luminescence intensity can be maintained at 150 °C, demonstrating good thermal stability. The multilevel ridge characteristics of the latent fingerprint stained with as-prepared phosphor could be clearly visualized under 254 nm UV excitation. All of the above results indicate that as-prepared phosphor materials can be suitable for use in forensic applications as well as solid state lighting applications.

Enhance photoluminescence properties of Ca-Eu:Y2O3@SiO2 core–shell nanomaterial for the advanced forensic and LEDs applications

The divalent (Ca2+)-doped Eu:Y2O3@SiO2 core–shell luminescent nanophosphors have been synthesised by a cost-effective combustion technique. Various characterizations were carried out to confirm the successful formation of the core–shell structure. The TEM micrograph reveals the thickness of the SiO2 coating over Ca-Eu:Y2O3 as ∼25 nm. The optimal value of silica coating over the phosphor has been obtained as 10 vol%(TEOS) of SiO2, with this value increasing fluorescence intensity by 34 %. Phosphor exhibits CIE coordinates as x = 0.425, y = 0.569 and a CCT value as ∼2115 K with color purity and the respective CRI of 80 % and 98 %, respectively, which make the core–shell nanophosphor suitable for warm LEDs, and other optoelectronic applications. Further, the core–shell nanophosphor has been investigated for the visualisation of latent finger prints and as security ink. The findings point towards the prospective future application of nanophosphor materials for anti-counterfeiting purposes and latent finger prints for forensic purposes.

Bisphenol A monitoring during anaerobic degradation of papers with thermochromic prints in soil

Pseudoestrogene bisphenol A (BPA) can be important ingredient of thermochromic inks, increasingly used materials in thermal printing paper, security printing, advertising, design and as temperature indicators in medicine and food industry. BPA mass fraction in thermochromic inks can be up to several percent. Hence, disposal of items with thermochromic prints pose a risk of environmental pollution. In this work BPA mass fraction was monitored during anaerobic degradation of papers with thermochromic prints in soil in both matrices: papers and soil. The degradation conditions simulated deeper layers of waste at a landfill site. Six types of papers with prints of thermochromic ink containing 2% of BPA were subjected to anaerobic degradation over up to 150 days. Initial mass fractions of BPA in papers decreased form (126–460) μg/g to (<QL – 45) μg/g after 150 days. BPA amounts were reduced 10 to 50 times depending on the paper type: least for synthetic paper and most for wood-free coated. For soil analysis new HPLC-UV method was developed and validated. The method was linear from 0.75 ng/g to 0.6 μg/g of BPA in soil with correlation coefficient of 0.9994. Method precision was 4.4%, accuracy 83% and detection limit 0.9 ng/g. Expectedly, amount of BPA in soil was increasing during the experiment. Mass fractions of BPA in soil were from not detected in earlier stage of degradation to (4.9–23.2) ng/g after 150 days. Final BPA amounts in soil were similar to those found in industrial, urban and agricultural soils worldwide. Hence, BPA from papers with thermochromic prints was notably decomposed, and contaminated soil had the capacity to absorb and decompose BPA even under anaerobic conditions. After 150 days of anaerobic degradation, only up to 1.86% of BPA contained in paper prints was found in soil, whilst, on average, 4% of initial BPA remained in paper.

Modified chitosan as a multi-functional material in eco-friendly formulation for anti-counterfeit applications

Renewable resources and waste materials can be used as viable raw materials to prepare security formulations devoid of volatile organic compounds and would essentially offer environmental protection. Therefore, chitosan (deacetylated chitin), which is a marine waste material is modified through simultaneous N-conjugation with pyrene-1-carboxaldehyde (PCA) as an aggregation caused quenching fluorophore and fluorene-2-carboxaldehyde (FCA) to obtain a smart fluorescent chitosan-based Schiff base (FCSSB). The covalent attachment could immobilize pyrene onto the chitosan, and the subsequent diluting effect of the biopolymer framework could hinder the aggregation of pyrene and, thereby, the self-quenching of fluorescence. The modified FCSSB could serve as a multi-functional material; both as a binder and a fluorophore in an eco-friendly formulation using aqueous acetic acid as the mild solvent and glycerol as the viscosity regulator. The formulation is used as a security ink to obtain single/multilayer coating/s and further prints using flexography technique. The prints that appeared invisible in daylight exhibited a blue fluorescence under 370 nm UV illumination. Moreover, the ink film presented good rub resistance and colour stability after different cycles of UV exposure. Therefore, FCSSB formulation can be readily utilized as a security ink for information decryption and could also be processed into products of diverse forms, including ink pads, coatings, printed patterns, and films. This simple, but effective approach provides additional impetus to real-life applications of biopolymers from natural/waste materials in anti-counterfeiting.