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Functional Inorganic Materials and Devices

Highly-efficient, Chemically-stable, UV/blue Light Excitable Biluminescent Security Ink to Combat Counterfeiting Akhilesh Kumar Singh, Satbir Singh, and Bipin Kumar Gupta ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18997 • Publication Date (Web): 03 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018

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ACS Applied Materials & Interfaces

Highly-efficient, Chemically-stable, UV/blue Light Excitable Biluminescent Security Ink to Combat Counterfeiting

†

Akhilesh Kumar Singh,* ,† Satbir Singh,†,‡ Bipin Kumar Gupta*,† CSIR - National Physical Laboratory, Dr. K S Krishnan Road, New Delhi, 110012, India ‡ Academy of Scientific and Innovative Research (AcSIR), CSIR-National Physical Laboratory Campus, Dr K S Krishnan Road, New Delhi, 110012, India. *Email: [email protected]; [email protected]

Table of Content

The work demonstrates a transparent biluminescent security ink to combat the menace of counterfeiting. The biluminescent security ink exhibits two broad absorption bands in UV and blue regions, and its corresponding excitations show strong red and green emission, respectively. This highly-efficient, photo-, chemically-stable biluminescent security ink promises a paradigm shift for a next-generation luminescence security feature to protect the currency and other merchandized items.

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Abstract A strategy has been demonstrated to design a biluminescent security ink using Eu(TTA)3Phen (ETP) and fluorescein for protecting the currency and other essential documents viz. passport, bank cheque, certificates against counterfeiting. Biluminescent security ink exhibits strong red and green emission under 367 and 445 nm excitations, respectively. As it is quite challenging to prepare a material that possesses two prominent (green and red) and distinguishable colors upon excitation with two separate LEDs sources, emitting at different wavelengths, the biluminescent security ink would be hard to counterfeit as compared to the existing luminescent security ink that exhibits single color under UV light exposure. To check its feasibility for security application the patterns printed out using biluminescent security ink were kept under a hot and humid atmosphere for 150 days. Also, the ETP and fluorescein fluorophores were exposed to UV light for a prolonged time, which do not show any sign of deterioration in their luminescence intensities. Furthermore, to check their chemical stability printed patterns were also exposed to chemicals which have potential to wiped-out ink viz. detergent, ethanol, acetone, and sodium hypochlorite (bleach) solution and noticed that it is well stable against these chemicals. Because of the reasons mentioned above and easy availability of 367 and 445 nm LEDs at low cost, authors believe that the application of this biluminescent security ink can trigger to realize the full potential of this advanced security feature in detecting fake currency. KEYWORDS: luminescence, lanthanide, security ink, fluorescein, anticounterfeiting


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1. INTRODUCTION Counterfeiting, i.e., producing unauthorized replicas of the real products is no longer limited to branded and easy-to-manufacture products but has spread in every walk of life such as pharmaceuticals, food, drink, toys, medical equipment, and automotive parts.1-6 After showing its detrimental effect on socioeconomics, it is spreading footprints in areas related to national securities.7-8 Therefore, securing currency and documents of national importance such as passport, VISA, bank cheques from being counterfeited becomes a top priority of governments across the globe.9-11 Combating menace of increasing counterfeiting requires new strategies and enhanced security measures.12-18 In the modern world, along with various security features that vary nation-to-nation, currencies of most of the developed/developing nations use luminescent security features that are invisible under daylight and show different colors under UV light exposure.19-20 Although it is an advanced security feature to detect currency, until date its use is mostly limited to bankers that might be because of its covert nature, unawareness among people and logistic issues to carry UV light source. Because of an upswing of counterfeiting in the contemporary scenario, it is imperative to enhance the security level of the currency by replacing the luminescent ink that shows single-color emission with a biluminescent ink that can emit different colors under different excitation wavelengths. At this juncture, it is important to mention that the development of a biluminescent material which exhibits emission of two different colors upon excitation with two different wavelengths LEDs is very challenging and rarely exist in the literature. It is primarily because of overlapping of absorption bands of two fluorophores or energy sensitization from one fluorophore to other causing luminescence quenching of one/both fluorophores. In the literature, various research groups have made efforts to develop single-, biluminescent materials for anticounterfeiting applications; however, the cost of excitation sources, inability to illuminate whole (big) pattern simultaneously (using NIR laser) and its logistic limits the scope of their potential application.17-18, 21 3 ACS Paragon Plus Environment


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The objective of the present investigation was to develop an efficient, photo- and chemically-stable biluminescent security ink that can be excited using low power, costeffective, compact, readily available excitation sources. To achieve this, we have chosen Eu(TTA)3Phen (ETP) and fluorescein as fluorophores which show strong red and green emission under 367 and 445 nm excitations, respectively. Noticeably, 367 and 445 nm LEDs are readily available in the market and can be used to make small hand-held light sources. 2. EXPERIMENTAL SECTION 2.1 Materials Europium (ΙΙΙ) oxide (99.999%, Alfa Aesar), 2-thenoyltrifluoroacetone (99%, Sigma-Aldrich), 1,10-phenanthroline (99.5%, Loba Chemie Pvt. Ltd.), ethanol (Changshu Yangyuan Chemical, China), sodium hydroxide (Qualigens, 99%), polyvinyl chloride gold medium (Commercial Techno Colours, Varanasi, India), hydrochloric acid [Aldrich, 35%], and fluorescein (free acid, dye content 95%) were used for synthesizing/developing biluminescent security ink. 2.2 Synthesis ETP was synthesized in the present investigation following the method described in our previous report.22 Europium oxide was dissolved in dilute hydrochloric acid to prepare europium

chloride.

For

the

synthesis

of

ETP

complex,

six

mmoles

of

2-

thenoyltrifluoroacetone, two mmoles of 1,10 Phenanthroline and 8 ml of 1N sodium hydroxide were dissolved in 25 ml of ethanol. In this solution, two mmoles of europium .

chloride hexahydrate (EuCl3 6H2O) in 10 ml of distilled water was added drop-wise, under constant stirring. During this progression, a white colored precipitate of the ETP complex was formed. The sample was dried under vacuum at 40 °C for 24 h. To analyze the purity of synthesized ETP complex, we have determined the carbon (C), hydrogen (H), nitrogen (N) and sulfur (S) content in the sample and compared it with the calculated mass (%). As sulfur is present in TTA and nitrogen in Phen molecules, the determination of sulfur and nitrogen


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content in the sample helps to identify the actual chemical structure of the complex. The calculated value of C, H, N, and S as per ETP chemical formula (C36H20EuF9N2O6S3) are 43.42%, 2.02%, 2.81% and 9.66%, which were experimentally found as 43.35%, 2.07%, 3.07% and 9.65%, respectively, that indeed confirm the formation of ETP complex. The preparation scheme of biluminescent security ink is shown in Figure S1. Briefly, fluorescein was dissolved in ethanol and was mixed along with ETP in polyvinyl chloride (PVC) gold medium at a (w/w) ratio of 2:3:375. After the evaporation of ethanol mixture was stirred for 30 minutes on a magnetic stirrer. Because of its high viscosity (~3000 mP) and polymeric nature the PVC gold medium (viscosity ~3000 mP) is highly compatible for forming robust patterns on different substrates. The biluminescent security ink was further used for screen-printing using the method demonstrated in Figure S2. Briefly, the printing object was replicated on a meshy cloth and fitted within a frame. Subsequently, it was kept above the printing substrate (black paper), and biluminescent security ink was poured onto it and spread across the printing code using a squeegee. After a minute the printed mesh was removed from the substrate and allowed to dry in the open atmosphere for about an hour. For comprehensive photophysical studies, equal amounts of ETP, fluorescein and biluminescent security ink were individually dispersed in PVC gold medium. ETP was mixed in powder form, whereas fluorescein was dissolved in ethanol before mixing with PVC gold medium and kept for evaporation of ethanol as described for biluminescent security ink in Figure S1 and were poured into Petri dishes of equal dimensions placed side by side on a flat surface to make films of equal concentration and thicknesses. 2.3 Instrumentation Elemental analysis was carried out using CHNS analyzer (Elementar Analysensysteme, Model: Vario Micro Cube). The UV-vis spectrum of biluminescent security ink was recorded using an Avantes light source (Avalight-DH-S-BAL) and detector (Avaspec-2048). The PL excitation/emission spectra were recorded employing the fluorolog-3 spectrofluorometer 5 ACS Paragon Plus Environment


(FL3-11, Horiba Jobin Yvon) equipped with a 450 W xenon flash lamp. PL mapping of screen-printed objects was performed using a WITech alpha 300R+ confocal PL microscope system using a 375 nm diode laser (wave-375-s-3v2-01001, Toptica) as an excitation source. Photo-stability of fluorophores was checked using JSGW UV light source emitting at 367 nm and spectrum were recorded using CCD detector (Avaspec-2048). 3. RESULTS AND DISCUSSION 3.1. Photophysics of ETP, fluorescein, and biluminescent security ink The detailed photophysics of ETP is already reported in our previous publication22, therefore, this section comprises only essential luminescent features of ETP, fluorescein and biluminescent security ink. The absorption spectrum of BLSI shown in Figure 1 (a) depicts two distinct absorption bands between 275-400 and 400-450 nm corresponding to absorption of ETP and fluorescein, respectively. Upon 367 nm excitation ETP and biluminescent security ink exhibits characteristic emission of Eu3+ ions [shown in Figure 1(b)] at 577, 589, 611, 650, 696 nm corresponding to 5D0 →7F0, 5D0 →7F1, 5D0 →7F2, 5D0 →7F3, 5D0 →7F4 transitions, respectively. Among all these transitions, 5D0 →7F1 is allowed by magnetic dipole mechanism, whereas others are forced/induced electric dipole transitions observed because of mixing of 4f orbital with other orbitals (5s and 5p) and crystal/ligand field. The hypersensitive 5D0→7F2 transition is the most intense and responsible for the bright red color.23 On the other hand, fluorescein shows a rather weak emission upon 367 nm excitation (the integrated emission intensity of ETP is seven times higher than fluorescein), which is further diminished in biluminescent security ink. With an increase in excitation wavelength the emission intensity of fluorescein increases, whilst of ETP decreases. At 400 nm excitation, the peak intensities of both the ETP and fluorescein becomes almost equal [see Figure 1(c)]. The emission spectrum monitored at 450 nm excitation, as shown in Fig. 1(d), exhibits a strong emission of


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fluorescein and negligibly weak emission of ETP. Noticeably, in the case of biluminescent security ink, the emission intensity of fluorescein decreases whereas that of ETP increases. To further study its photophysical properties we have monitored the excitation spectrum of fluorescein (λem = 520 nm), ETP (λem = 611 nm) and biluminescent security ink (λem = 611 nm) as shown in Figure 2. Fluorescein shows structural excitation between 300500 nm with anomalies ~ 312, 380, 446 and 487 nm. The maximum emission intensity of fluorescein is observed at 446 nm excitation. The excitation spectrum of ETP shows broadband between 250-400 nm with maximum ~365 nm. Biluminescent security ink excitation spectrum monitored at 611 nm emission wavelength shows an intense excitation band between 250-400 nm corresponding to ETP and a weak band ~446 nm which was absent in ETP. The inset to Figure 2 shows the chemical structure of the ETP complex. 3.2 Screen-printing of patterns using biluminescent security ink For anticounterfeiting applications, biluminescent security ink was prepared by dispersing ETP and fluorescein in PVC gold medium as described in the scheme shown in Figure S1 and the experimental section. Digital photographs of biluminescent security ink taken under daylight, 367 nm and 455 nm shown in Figure S3 reveal that the ink is stable for several days as these do not show any signs of luminescence quenching for both the fluorophors monitored till a week. Selecting appropriate dispersive medium for preparing luminescent ink is very important because sometimes dispersive medium leads to quenching of luminescence, segregation of fluorophore, etc.24 We have tried some other mediums too for making biluminescent security ink for example micro inks (HS 6653/1F9; PP base coat medium), ethyl cellulose and polydimethylsiloxane, however, they do not form stable biluminescent security inks. For the printing of security code on different substrates, the standard screen-printing technique was used as demonstrated in Figure S2 and further explained in the experimental section. Figure 3 shows digital photographs of security code printed over different substrates under daylight, 445 nm and 367 nm light sources. It can be 7 ACS Paragon Plus Environment


seen from the Figure 3(a) that the printed code is transparent with a yellowish tinge. Under 445 nm LEDs light pattern becomes green because of the emission mainly coming through fluorescein molecules, whereas under 367 nm it appears red as TTA strongly absorb in this region and transfer its excitation energy to the Eu3+ ions which produce its characteristics red emission.

The explicit images are shown in Figure 3(a)-(c) demonstrate that the

biluminescent security ink can be used for printing over papers of different colors, plastic (transparency) and metallic (aluminum foil) substrates. Further, it can be noted that the clarity of pattern under 445 nm excitation (visibility of green emission) are quite sensitive to the color of the substrate, whereas it is less sensitive for red emission under 367 nm excitation. As fluorescein and ETP both show high absorption cross-section it can be excited with lowintensity LEDs light sources. 3.3 PL Mapping The PL mapping of the selected area of the printed pattern was performed using 375 nm laser excitation to examine the distribution of the fluorophore further. The 3D image of PL mapping shown in Figure 4 (a) reveals that (ETP) fluorophore is quite homogeneously distributed over the printed region. Figure 4(b) shows a representative spectrum from the printed part that is of the ETP complex. We have plotted integrated emission intensities from 608.9 – 630.9 nm over a straight line shown by red color in Figure 4(a) that shows an excellent color contrast between printed and the background region. It also reveals a perfect homogeneity of fluorophore across the printed area. As 375 nm wavelength mainly excite ETP complex and we do not have any excitation wavelength between 400 nm to 532 nm in the PL mapping set-up, we could not monitor the distribution of fluorescein moiety. 3.5 Stability of biluminescent security ink (printed pattern) For using biluminescent ink for a security application, stability under ambient environment, resistance to photo-bleaching and chemical stability are prerequisite. To check the stability of biluminescent security ink under ambient atmosphere printed pattern were kept under ambient 8 ACS Paragon Plus Environment

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atmosphere for 150 days (with average maximum temperature ~38 °C and humidity ~75%) and digital photographs were taken under daylight, 367 nm, and 445 nm, before and after exposure to ambient atmosphere. Figure 5 depicts that there is no any noticeable deterioration in the luminescence intensities after the exposure of pattern in ambient atmosphere. Further, to check the photo-stability of fluorophores, which is a very crucial test especially for fluorescein, we exposed fluorescein dispersed in PVC gold medium and ethanol and ETP in PVC gold medium to 367 nm UV light for a prolonged time and recorded kinetic series of spectra at an interval of 16 s. Figure 6 and Figure 4S show that peak profiles of fluorescein and ETP have not changed after prolonged exposure of UV light which reveal that the organic moieties show excellent resistance to UV light. Further, it shows that fluorescein dispersed in PVC gold medium shows ~18% increase of PL intensity over 5 h, whereas PL intensity of fluorescein in ethanol and ETP in PVC gold medium drops 6h UV light soaking. It indicates that both the fluorescein and ETP exhibits good photo-stability. Lastly, to check the chemical stability of biluminescent ink printed patterns were exposed to chemicals that have potential to wipe-out printed ink, for example, detergent (soap solution), ethanol (polar solvent), acetone (partially polar), and sodium hypochlorite (bleach) solution (about 4% w/v available chlorine) and dried it. Among these chemicals, detergent and sodium hypochlorite are used in day-to-day life, whereas ethanol and acetone (ketones) are used for ink processing. Digital images were taken before and after the chemical exposure under 367 nm UV light, shown in Figure 7, do not reveal any noticeable change in luminescence intensity and patterns are fully intact.

4. CONCLUSION We have developed a biluminescent security ink and demonstrated its advantage over existing luminescent security ink used for currency, passport, bank cheque, and other essential 9 ACS Paragon Plus Environment


documents. On excitation with long UV wavelength (367 nm), it shows intense red emission of ETP, whereas it shows green emission under 445 nm excitation. The excitation spectrum of ETP shows band between 250-400 nm and the maximum excitation of fluorescein is observed at 445 nm that helps in exciting both fluorophors pretty independently. Also, the biluminescent security ink can be printed over various type of substrates including paper, metallic and plastic. Moreover, it exhibits good photo-, chemical stability and can withstand under a hot and humid atmosphere. The quite transparent nature of the printed object under ambient light and prominent red and green emission under 367 and 445 nm excitation indicates its feasibility as advanced security measures to combat counterfeiting. Strikingly, the 367 and 445 nm LEDs are easily available in the market and therefore can be used to make small hand-held light sources to detect fake currency. SUPPORTING INFORMATION Preparation scheme of biluminescent security ink; schematic diagram depicting steps involved in screen-printing; digital images of biluminescent security ink taken under daylight, 455 nm and 367 nm light taken on alternate days to demonstrate its stability; 3D plot showing a kinetic series of PL spectra of fluorescein dissolved in ethanol recorded under 367 nm excitation: FTIR spectra of ETP and fluorescein and their description. The Supporting Information is available free of charge at http://pubs.acs.org. ACKNOWLEDGMENTS The Authors wish to thank the NPL director, Dr. D. K. Aswal for his keen interest in this mission-mode project and constant encouragement. AKS acknowledge funding from CSIR, India as Senior Research Associateship (Pool No. 8953-A). Authors acknowledge Professor O. N. Srivastava, Professor S. B. Rai, BHU, Dr. Asit Patra, NPL and Professor K. Sreenivas, Director, USIC, DU for his interest in work and extending their lab facilities.


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Crime

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Figure 1. (a) UV-vis absorption spectrum of a polymer film containing biluminescent security ink. PL spectra of polymer films containing ETP, fluorescein, and biluminescent security ink recorded at excitation wavelengths of (b) 367 nm, (c) 400 nm, (d) 450 nm.

Figure 2. The excitation spectrum of polymer films containing (i) ETP, (ii) fluorescein and (iii) biluminescent security ink recorded by monitoring emission at 520, 611 and 611 nm, respectively. Inset to the Figure shows the chemical structure of the ETP complex.


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Figure 3. Digital images of the pattern printed over (a) transparency, (b) white paper, (c) aluminum foil, (d) brown paper, (e) blue paper, and (f) black paper took under daylight, 445 nm (using 495 nm cut-on filter) and 367 nm light.

Figure 4. (a) 3D image of PL mapping recorded over a selected area of the printed pattern using 375 nm laser. (b) Representative emission spectrum recorded in a pixel over printed region shown by a blue square. (c) Integrated CCD counts between 608.7 - 630.9 nm calculated along the red line shown in Fig. 4(a).



Figure 5. Digital images of printed pattern taken under daylight, 367 nm and 445 nm (with the aid of 495 nm filter) light on the fresh sample and after keeping it for 150 days in the open atmosphere (with maximum temperature ~38°C and humidity ~80 %).


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Figure 6. Showing kinetic series of PL spectra of (a) fluorescein, and (b) ETP dispersed in PVC gold medium under 367 nm excitation. The insets to the Figure 6 (a) and (b) shows the peak intensity of fluorescein and ETP as a function of time, respectively. It suggests that fluorophores are pretty stable under UV light.



Figure 7. Chemical stability of printed patterns: (a) digital image of the printed pattern under 367 nm (b) digital image was taken after exposure with (1) detergent, (2) ethanol, (3) acetone and (4) sodium hypochlorite solution (about 4% w/v available chlorine)


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Blue-Light-Excitable

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