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Single excitable dual emissive novel luminescent pigment to generate

advanced security features for anti-counterfeiting applications

Article  in  Journal of Materials Chemistry C · October 2019

DOI: 10.1039/C9TC04492B

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National Physical Laboratory

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Cite this:DOI: 10.1039/c9tc04492b

Single excitable dual emissive novel luminescentpigment to generate advanced security featuresfor anti-counterfeiting applications†

Amit Kumar Gangwar,ab Kanika,ab Garima Kedawat,b Girija Shankar Papanaiab andBipin Kumar Gupta *ab

A novel strategy has been devised to develop a single excitable dual emissive luminescent pigment for

the formulation of unclonable security ink. The new advanced security features of the ink have been

demonstrated successfully and make it highly suitable for the printing of valuable products, such as

currency, passports, pharmaceuticals, and so forth for protection against duplicity. The formulation of

the single excitable dual emissive luminescent pigment is based on the unexplored combinatory

concept of the fluorescence@phosphorescence phenomenon, which provides a new feature in the

composite. It emits intense red color (611 nm) upon illumination with a 254 nm UV lamp excitation

source and long persisting green color (532 nm) when the excitation source is switched off. To study

the feasibility of the single excitable dual emissive luminescent security ink for commercial applications,

the patterns printed using this ink were not only subjected to rigorous atmospheric conditions such as

hot, cold and humid conditions for six months, but also examined after treatment with various bleaching

agents (ethyl alcohol, ethyl acetate, xylene, acetone, soap solution and laundry detergent) to ensure

their chemical stability. The security features printed using the single excitable dual emissive luminescent

security ink were found to be very stable against all of these stringent conditions. Hence, the

exceptional results obtained using the single excitable dual emissive luminescent security ink offer a new

pathway to prevent counterfeiting by generating advanced security features.

Introduction

Counterfeiting is defined as the reproduction of an intellectualproperty right without receiving permission from the proprietor.1

It has grown to be a serious global threat nowadays and thisincludes the duplication of currencies, merchandise, electronicproducts, official documents, passports, pharmaceuticals, and soforth which causes enormous loss to the economy of any country,as well as a constant risk to the health and safety of consumersworldwide.1–3 To combat counterfeiting, various advancedsecurity technologies are being explored, such as radiofrequency identification (RFID),4 hologram attachment,5 isotopetracking2 and luminescent printing.6–8 RFID-containing chips canbe incorporated onto various merchandise, but informationstored in these chips is not entirely protected and can be

tampered with.9 Recently, the hologram technique was adoptedcommercially to authenticate products. However, the simplicityand ease of fabrication makes it a highly unsuitable meansagainst counterfeiters.10 Isotope-tracking based techniques areconstrained to limited areas, in which the radiation satisfies thesafety levels.11 The luminescence printing based method is one ofthe best approaches to combat counterfeiting, owing to its lowproduction costs at the commercial scale, being eco-friendly andhard to counterfeit, and so forth.12–14

In general, a single color emissive luminescent material at asingle excitation wavelength is used, however counterfeiting ofsuch materials is easy owing to their simplicity.14,15 Initially, wesynthesized several composite pigments to achieve noveladvanced security features to replace existing luminescentmaterial based security inks, which are excitable at two differentwavelengths and emit two different colors, respectively.3,16,17

Recently, some other groups also made efforts to achieve dualemissive luminescent materials based on different excitationwavelengths (dual emission using either two UV excitation wave-lengths or UV and NIR wavelengths).18,19 Furthermore, we havealso synthesized a tri-luminescent composite pigment, which isexcitable at three different wavelengths and offers three different

a Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India.

E-mail: bipinbhu@yahoo.com; Fax: +91-11-45609310; Tel: +91-11-45608284b Photonic Materials Metrology Sub Division, Advanced Materials and Device

Metrology Division, CSIR-National Physical Laboratory, Dr K. S. Krishnan Road,

New Delhi-110012, India

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9tc04492b

Received 13th August 2019,Accepted 10th October 2019

DOI: 10.1039/c9tc04492b

rsc.li/materials-c

Journal ofMaterials Chemistry C

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color emissions, respectively.20 In this paper, a novel perspectiveis introduced for enhancing the concealment features of theluminescent security ink to prevent imitation activities by intro-ducing a single excitable dual emissive luminescent pigment.

Herein, a facile, scalable and inexpensive synthesis approachfor a single excitable dual emissive luminescent pigment has beenreported, which introduces a new concept for the fluorescence@phosphorescence (NaYF4:Eu3+@SrAl2O4:Eu2+/Dy3+) phenomenon,that is emitting dual colors, red (611 nm) and green (532 nm) atthe single excitation wavelength of a 254 nm UV lamp excitationsource switch ON and OFF, respectively. The alternate ON andOFF dual color can be easily distinguished using the naked eyeunder ambient conditions (see Video S1 in the ESI†). The singleexcitable dual emissive luminescent pigment is composed bystrategic combinatory mixing of the pre-synthesized NaYF4:Eu3+

and SrAl2O4:Eu2+/Dy3+ phosphors. The rare-earth based dualmode luminescent compounds and multi-modal excitable lumi-nescent inorganic materials which have been reported by us havealso been reported by other groups.21–24 However, to the best ofour knowledge, single excitable and dual color emissive lumines-cent materials that are applicable in the anti-counterfeiting fieldhave not been reported as yet. Using the proposed strategy, it isfeasible to produce the pigment at a large scale and in a costeffective manner. Additionally, the unique feature of the pre-existing bi-luminescent materials makes the single excitabledual emissive luminescent pigment an advanced luminescentmaterial, for security ink formulation to generate strong securityfeatures.

ExperimentalMaterials

Y2O3 (99.9%), NaF (99.9%), Eu2O3 (99.99%), NaOH, HF (44% v/v),SrCO3 (99.9%), Dy2O3 (99.9%) and Al2O3 (99.9%) were purchasedfrom Sigma Aldrich Corp. All reagents were of analytical grade andused without any further purification.

Synthesis of NaYF4:Eu3+ phosphor

The NaYF4:Eu3+ phosphor was synthesized using the hydro-thermal method. All of the precursor materials: Y2O3, Eu2O3

and NaF were dissolved separately in de-ionized (DI) waterusing the stoichiometric formula NaY1�xF4:Eux (x = 0.01 to0.06). As oxides are not soluble in DI water, 5 mL HCl wasadded into the Y2O3 and Eu2O3 solutions under continuousmagnetic stirring at B50 1C. When all of the raw materials haddissolved and the solutions became transparent, all of thesolutions were placed together into a 500 mL beaker undervigorous stirring and subsequently 10% NaOH solution wasadded dropwise until the pH reached B13. Finally, the homo-genous solution was placed into the autoclave and heated at200 1C for 8 h. Subsequently, the temperature of the autoclavewas allowed to cool down to ambient temperature naturally.The as-obtained precipitates were separated by centrifugationand washed properly using DI water. Finally, the as-obtainedprecipitates from the hydrothermal process were dried at 70 1C

for 8 h. The optimum concentration of Eu was obtained on thebasis of the highest photoluminescent (PL) intensity. At theconcentration x = 0.03 of Eu, the highest PL intensity was revealed,as shown in Fig. S1a (ESI†). Thus, the NaY0.97F4:Eu0.03

3+ phosphorwas used for further single excitable dual emissive luminescentsecurity ink formation.

Synthesis of the long decay SrAl2O4:Eu2+/Dy3+ phosphor

SrAl2O4:Eu2+/Dy3+ phosphor was synthesized using the solid-state reaction technique. All of the raw materials; SrCO3, Al2O3,Eu2O3 and Dy2O3 were measured on the basis of the stoichio-metric formula Sr1�x�yAl2O4:Eux

2+/Dyy3+; x = 0.05 to 0.30, y =

0.1 to 0.6. Furthermore, these materials were mixed properlyand ground in a mortar and pestle using a minor amount ofethanol for the formation of a fine powder. Finally, the wellmixed powder was placed in an alumina crucible and heated ina tube furnace at B1200 1C for 3 h under a reduced atmo-sphere, containing 90% N2 and 10% H2 to achieve the requiredphosphor material. Then, the powder was allowed to cooldown naturally to ambient temperature in the furnace. Theas-obtained powder sample was ground finely. The optimumconcentration of Eu and Dy was obtained on the basis of thehighest PL intensity. At the concentration x = 0.15 and y = 0.3 ofEu2+ and Dy3+, respectively, the highest PL intensity was achieved,as shown in Fig. S1b (ESI†). Thus, the Sr0.55Al2O4:Eu0.15

2+/Dy0.33+

phosphor was used for further single excitable dual emissiveluminescent security ink formation.

Synthesis of the single excitable dual emissive compositeluminescent pigment

The single excitable dual emissive luminescent pigment wassynthesized by admixing of the as-synthesized NaYF4:Eu3+ andSrAl2O4:Eu2+/Dy3+ phosphors. The quantity of the as-synthesizedNaY0.97F4:Eu0.03

3+ and Sr0.55Al2O4:Eu0.152+/Dy0.3

3+ phosphors weremixed in the weight ratio of 3 : 1. Furthermore, these phosphormaterials were mixed in acetone separately under magneticstirring for 5 min. After that, these solutions were mixed in abeaker and then under continuous stirring until the evaporationof acetone. The protocol for the synthesis of the single excitabledual emissive luminescent pigment is shown in Fig. S2 (ESI†). Theas-obtained phosphor composite pigment was sintered at 400 1Cfor 3 h to obtain a fine white powder of the single excitable dualemissive luminescent pigment. We performed annealing at400 1C to decorate the rod structure (NaYF4:Eu3+) onto thespherical surface (SrAl2O4:Eu2+,Dy3+) without quenching the PLintensity, as shown in Fig. S3 (ESI†) and this also helps to give auniform dispersion of the composite pigment during ink for-mation in the polyvinyl chloride (PVC) medium. If we directlyperform the physical mixing of both the phosphors withoutannealing, then the individual phosphors separate during inkformation and as a result the desired property of the ink isaffected, as well as the individual phosphors that separate inthe PVC medium during ink formation. To check the scalabilityand reproducibility of the process of composition of the singleexcitable dual emissive luminescent pigment, we synthesized upto 2.54 kg of the single excitable dual emissive luminescent

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pigment. The actual amount of raw materials needed for thesynthesis of up to 2.54 kg of the single excitable dual emissiveluminescent pigment is described in Tables S1–S3 (ESI†).

Formulation of the single excitable dual emissive luminescentsecurity ink

The as-synthesized single excitable dual emissive luminescentpigment was dispersed in a commercially available PVC goldmedium. The single excitable dual emissive luminescent secur-ity ink was formulated by dispersing 500 mg of the singleexcitable dual emissive luminescent pigment in 200 mL ofthe PVC gold medium. To obtain a better dispersion of thesingle excitable dual emissive luminescent security ink, vigorousstirring and ultra-sonication at 35 kHz for 1 h were used.A standard screen printing technique was used to print thesecurity code on the non-fluorescent white bond paper, theprotocol is illustrated in Fig. S4 (ESI†). The appropriate selectionof a dispersion medium plays an important role in the excellentdispersion of phosphor material without formation of any clustersand agglomerates. The viscosity of the security ink is also anessential factor to allow better printing quality. Optical photo-graphs of the single excitable dual emissive luminescent securityink under daylight and a 254 nm UV lamp switched ON and OFFare shown in Fig. S5a–c (ESI†), respectively.

Measurement and characterization

Phase identification was obtained using X-ray powder diffrac-tion (XRD) (Rigaku, Miniflex, Cu Ka1; l = 1.5406 Å). Surfacemorphology and elemental characterization were performedusing scanning electron microscopy (SEM) (model LEO 440),which was equipped with an energy dispersive X-ray analysis(EDAX) facility. The PL study was performed by using anEdinburgh luminescence spectrometer (model F900) equippedwith a xenon lamp as the excitation source. To find the absoluteluminescence quantum yield of the phosphors, an integratingsphere equipped with an Edinburgh luminescence spectro-meter (model F900) instrument was used to measure theintegrated fraction of the luminous flux and radiant flux. ThePL intensity distribution mapping of the surface of the securitycode was conducted using a WITech alpha 300R+ confocal PLmicroscope instrument, equipped with a 375 nm diode laserexcitation source. A fluorescent camera (Kodak CC40R) wasused to take a photograph of the UV illumination of the printedobject. The rheological characteristics of the ink were measuredby using a Rheometer MCR 102 (Anton Paar, Ashland,VA, USA)operating in the rotational and oscillation modes.

Results and discussion

To validate the above mentioned concept, the single excitable dualemissive luminescent pigment (NaYF4:Eu3+ @SrAl2O4:Eu2+/Dy3+)was used for the production of single excitable dual emissiveluminescent security ink with low cost and industrial grade PVCgold used as the ink medium. The pattern printed using theformulated single excitable dual emissive luminescent security

ink emit an intense red and green colors under the excitationsource of a 254 nm UV lamp ON and OFF, respectively.

To investigate the NaYF4:Eu3+ and SrAl2O4:Eu2+/Dy3+ phos-phors without modification of their phase and structure in thesingle excitable dual emissive luminescent pigment, XRD analysiswas performed on the optimized samples (see details providedin the Experimental section). Fig. 1a–c shows the XRD patternsof the NaY0.97F4:Eu0.03

3+ phosphor, Sr0.55Al2O4:Eu0.152+/Dy0.3

3+

phosphor and the single excitable dual emissive annealed andunannealed luminescent pigments, respectively. The diffrac-tion peaks of the NaY0.97F4:Eu0.03

3+ phosphor (Fig. 1a) are wellindexed and when compared with JCPDS card no. 39-0723shows that the phosphor exhibits a cubic phase and spacegroup Pm%3m. The diffraction peaks are observed at angles2y = 16.301, 28.651, 33.101, 38.171, 41.961, 45.931, 51.821 and54.981, corresponding to the (100), (111), (200), (210), (211),(220), (310) and (311) crystal planes, respectively, which provesthe formation of a cubic phase. Fig. 1b depicts the diffractionpattern of the Sr0.55Al2O4:Eu0.15

2+/Dy0.33+ phosphor. The XRD

peaks are well matched with JCPDS card no. 74-0794 andconfirm the formation of a pure monoclinic crystal structurehaving the space group P21. No significant change is observedin the host crystal structure by doping with Eu2+ and Dy3+

co-dopants. The XRD patterns of the combinatory single excitabledual emissive annealed and unannealed luminescent pigmentsare illustrated in Fig. 1c. The XRD peaks designated with this *symbol are similar to the NaYF4:Eu3+ phosphor (JCPDS card no.39-0723), this . symbol is assigned to peaks that are similarto the SrAl2O4:Eu2+/Dy3+ phosphor (JCPDS card no. 74-0794).Additionally, no other peaks were found in the diffrac-tion pattern, which confirms the formation of the singleexcitable dual emissive annealed and unannealed luminescentpigments without structural modification of the individualphosphors.

Furthermore, the surface morphology of the NaYF4:Eu3+,SrAl2O4:Eu2+/Dy3+ phosphor and single excitable dual emissiveluminescent pigment were first characterized using SEM, asshown in Fig. 1d–f, respectively. Fig. 1d and e illustrates the rodlike structure of the as-synthesized NaY0.97F4:Eu0.03

3+ phosphorand the spherical morphology of the Sr0.55Al2O4:Eu0.15

2+/Dy0.33+

phosphor, respectively. Fig. 1f exhibits the uniform admixing ofthe NaY0.97F4:Eu0.03

3+ and Sr0.55Al2O4:Eu0.152+/Dy0.3

3+ phos-phors in the single excitable dual emissive annealed lumines-cent pigment. For better clarity, we also performed SEManalysis of the single excitable dual emissive unannealedluminescent pigment, as shown in Fig. S6 (ESI†). The resultsof the SEM analysis of the single excitable dual emissiveunannealed luminescent pigment clearly demonstrates theagglomeration of the NaYF4:Eu3+ phosphor, which is indicatedby the red encircled area marked in Fig. S6 (ESI†), while therods are stuck onto the spherical Sr0.55Al2O4:Eu0.15

2+/Dy0.33+

phosphors in the annealed sample as shown in Fig. 1f. EDAXwas conducted for the elemental analysis of the single excitabledual emissive annealed luminescent pigment as shown inFig. 1g, which reveals the presence of Na, Y, F, Eu, Sr, Al, Oand Dy elements in the single excitable dual emissive

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luminescent annealed pigment. Moreover, SEM analysis of thesingle excitable dual emissive annealed luminescent pigmentconfirmed the sticking of rod morphology (NaY0.97F4:Eu0.03

3+

phosphor) on the surface of the large spherical particles(Sr0.55Al2O4:Eu0.15

2+/Dy0.33+ phosphor), the schematic of which

is shown in Fig. 1h. The advantage of the preferred combi-nation of the rod and spherical type morphology is that therods stucked on the surface of the spherical particles allowsboth of the phosphors (NaYF4:Eu3+, SrAl2O4:Eu2+/Dy3+) to beequally exposed to the excitation wavelength (254 nm UV lamp)and each emits a uniform luminescent intensity of visiblelight without compromising the quantum yield of the other.Although in the case of the core–shell structure of suchpigments, the light intensity is reduced owing to the wallthickness of the shell material around the core structure.

In the present study, the dimension and quantity ratio of theNaY0.97F4:Eu0.03

3+ and Sr0.55Al2O4:Eu0.152+/Dy0.3

3+ phosphorsplay a vital role in the design of the highly luminescent singleexcitable dual emissive annealed luminescent pigment. There isplenty of exposed areas, because the rods of the NaY0.97F4:Eu0.03

3+

phosphor are not completely covered and easily become attachedto the spherical particles of the Sr0.55Al2O4:Eu0.15

2+/Dy0.33+ phos-

phor surface, as shown in Fig. 1h. In the case of the core–shelltype composite, it is hard to avoid the obstacle in the emission ofvisible light from the core of the phosphor owing to the wallthickness of the shell of the phosphor, as demonstrated in Fig. 1i.Subsequently, the PL properties of the NaYF4:Eu3+ phosphor,SrAl2O4:Eu2+/Dy3+ phosphor and the single excitable dual emissiveannealed luminescent pigment were examined (see detailsprovided in the Experimental section). As seen in Fig. 2a, the

Fig. 1 (a) XRD pattern of the NaY0.97F4:Eu0.033+ phosphor with the reference NaYF4 XRD pattern (JCPDS card no. 39-0723); (b) XRD pattern of the

Sr0.55Al2O4:Eu0.152+/Dy0.3

3+ phosphor with the reference SrAl2O4 XRD pattern (JCPDS card no. 74-0794); (c) XRD patterns of the single excitable dualemissive luminescent pigment of the annealed and unannealed composite; (d) SEM image of the NaY0.97F4:Eu0.03

3+ phosphor; (e) SEM image of theSr0.55Al2O4:Eu0.15

2+/Dy0.33+ phosphor; (f) SEM image of the single excitable dual emissive annealed luminescent pigment; (g) EDAX spectrum of the single

excitable dual emissive annealed luminescent pigment; and (h) and (i) schematic representation of the two types of composite structure based on therods attached around the surface of the spherical particle and core–shell type structure, respectively.

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PL excitation spectrum of the optimized NaY0.97F4:Eu0.033+

phosphor at a 611 nm emission wavelength indicates that abroad peak in the 230–280 nm region centered at 256 nm isobtained. This broad peak originates from the charge transferstate (CTS) and other small peaks arise due to the f–f transitionsin the 4f6 shell of the Eu3+ ion25 which are analogous to the 7F0

(ground state) of the excited states: 5H0 (323 nm), 5D4 (362 nm),5G2 (383 nm), 5L6 (393 nm), 5D3 (416 nm) and 5D2 (465 nm).26

The PL emission spectrum of the NaY0.97F4:Eu0.033+ phosphor is

shown in Fig. 2b, in which sharp peaks were found in the redspectral region. The observed peaks were due to the transitionbetween the 5D0 level and the different 7FJ ( J = 0–3) levels ofthe Eu3+ ion.27 The corresponding energy level diagram of theNaYF4:Eu3+ phosphor is illustrated in Fig. S7 (ESI†). The emissionpeaks at 585, 590 and 597 nm wavelength are due to the magneticdipole transition of 5D0 - 7F1.27 A high intense emission peak

Fig. 2 (a) Excitation spectrum of the NaY0.97F4:Eu0.033+ phosphor at an emission wavelength of 611 nm; (b) emission spectrum of the NaY0.97F4:Eu0.03

3+

phosphor at an excitation wavelength of 256 nm; (c) excitation spectrum of the Sr0.55Al2O4:Eu0.152+/Dy0.3

3+ phosphor at the emission wavelength of532 nm; (d) emission spectrum of the Sr0.55Al2O4:Eu0.15

2+/Dy0.33+ phosphor at an excitation wavelength of 374 nm; (e) excitation spectrum of the single

excitable dual emissive luminescent pigment at an excitation wavelength of 611 nm; (f) excitation spectrum of the single excitable dual emissiveluminescent pigment at the emission wavelength of 532 nm; and (g) the emission spectrum of the single excitable dual emissive luminescent pigment atdifferent excitation wavelengths (256, 374, 395, 400, 405 and 467 nm); the inset shows the highest peak intensities valued at 532 nm (I1) and 611 nm (I2)upon different excitation wavelengths.

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at B611 nm with an B86% quantum yield corresponds to thehypersensitive electric dipole transition of the 5D0 - 7F2

transition, which is higher than the 5D0 - 7F1 transition,indicating that the Eu3+ ions are located at the non-inversionsymmetric sites.28 The CIE color co-ordinates of the NaY0.97F4:Eu0.03

3+

phosphor corresponds to a red color emission at an excitationwavelength of 256 nm with the values X1 = 0.6431 and Y1 =0.3523 (see Fig. S8a, ESI†); marked by a black solid circle. Thetime-resolved photoluminescence (TRPL) decay profile of thephosphor was recorded with the help of the time correlatedsingle photon counting technique. The TRPL decay profile ofthe NaY0.97F4:Eu0.03

3+ phosphor and the corresponding fittedspectra were recorded at a 611 nm emission wavelength uponexcitation at a wavelength of 256 nm at room temperature, asshown in Fig. S9a and b (ESI†), respectively. The decay profileof the NaY0.97F4:Eu0.03

3+ phosphor is best fitted with a singleexponential function as given in eqn (1).

IðtÞ ¼ A exp �tt

� �(1)

The lifetime parameters generated from the exponential fittingof the decay curve are t = 0.972 ms, w2 = 1.477 and A = 100. Thelifetime of the NaY0.97F4:Eu0.03

3+ phosphor is t = 0.972 ms.Fig. 2c exhibits the excitation spectrum of the optimized

Sr0.55Al2O4:Eu0.152+/Dy0.3

3+ phosphor at a 532 nm emissionwavelength, showing a single broad excitation peak from 250to 437 nm with a centered peak at 374 nm with a B72%quantum yield. Furthermore, a highly intense green emissionpeak for the Sr0.55Al2O4:Eu0.15

2+/Dy0.33+ phosphor centered at

532 nm was obtained at the excitation wavelength of 374 nm,as given in Fig. 2d. The green emission corresponds to the4f6

5d1–4f7 states because the electronic dipole allowed thetransition of the Eu2+ ions located inside the SrAl2O4 host.29

There is no emission peak for the Eu3+ ions obtained in theemission spectra, which indicates that the Eu3+ ions werecompletely reduced to Eu2+ ions in the Sr0.55Al2O4:Eu0.15

2+/Dy0.33+.30

Although the 4f electrons in Eu2+ are well protected by theexternal shell, the 5d electrons are easily accessible owing tothe split in the crystal field strength activity.30 In addition, theDy3+ acts as an secondary activator and significantly prolongsthe afterglow duration by increasing the number of electrontraps and their trap depths in the vicinity of Eu2+.31 Theemission from a specific energy level can occur when theenergy gap to the next lower level is more than four or multipletimes the greatest phonon energy of the host lattice.32 Whenthe energy gap is smaller, an n multi-phonon relaxationhappens and the emission does not occur.30 The two mainfactors which effect the emission of a phosphor are, primarilythe capability of the host material in which the phosphor ionsare substituted, and secondary to this, the degree of covalenceof these ions with the oxygen ions environment.30 The electrontransfer model of the SrAl2O4:Eu2+/Dy3+ phosphor is illustratedin Fig. S10 in the ESI.† The CIE color co-ordinates of theSr0.55Al2O4:Eu0.15

2+/Dy0.33+ phosphor corresponding to the

green color emission at the 374 nm excitation wavelength withthe values X2 = 0.3212 and Y2 = 0.6143 is shown in Fig. S8a

(ESI†); which is marked by a black solid triangle. The TRPLdecay profiles of the Sr0.55Al2O4:Eu0.15

2+/Dy0.33+ phosphor and

the corresponding fitted spectra observed at the 532 nm emissionand 374 nm (xenon lamp) excitation wavelength are shown inFig. S11a and b (ESI†), respectively. The results reveal that afterexposure to the excitation source for 15 min, the intensity firstdecreases rapidly within 2 h and produces a stable long persistentluminescence for more than 4 h. This shows that the decay timeof the single excitable dual emissive luminescent annealedpigment does not indicate any significant change from the decaytime of the Sr0.55Al2O4:Eu0.15

2+/Dy0.33+ phosphor. This indicates

that when the material is exposed to the excitation source for15 min, the intensity of the afterglow decreases rapidly at theoutset within 2 h and henceforth produces a stable long persistentluminescence for more than 4 h. Thus, the SrAl2O4:Eu2+/Dy3+

phosphor has the property of long lasting visible persistentluminescence after the excitation source is switched off. Thedecay curves of all of the different concentrations ratio of theSr1�x�yAl2O4:Eux

2+/Dyy3+ (x = 0.05 to x = 0.3, y = 0.1 to 0.6)

phosphor were recorded at a 374 nm excitation wavelengthusing a xenon flash lamp for 15 min and are illustrated inFig. S11c (ESI†). The obtained results are consistent with the PLemission data.

Furthermore, the decay profile of the Sr0.55Al2O4:Eu0.152+/Dy0.3

3+

phosphor is best fitted with a double exponential characteristicfunction using the empirical eqn (2), as stated below.

IðtÞ ¼ A1 exp �t

t1

� �þ A2 exp �

t

t2

� �(2)

Here, A1 and A2 are the weighting constants parameters, t is thetime period and t1 and t2 are the attenuation parameters forthe exponential decay components. The lifetime parameters gen-erated after the exponential fitting are t1 = 5.14 min and t2 =35.07 min.

The down-conversion (NaYF4:Eu3+; at the 611 nm emissionwavelength) and down-shift (SrAl2O4:Eu2+/Dy3+; at the 532 nmemission wavelength) PL excitation spectra of the single excitabledual emissive annealed luminescent pigment are depicted inFig. 2e and f, respectively. The different PL emission spectra ofthe single excitable dual emissive annealed luminescent pigmentat various excitation wavelengths of 256, 374, 395, 400, 405 and467 nm are illustrated in Fig. 2g. All of the emission curves atdifferent wavelengths consist of two intense major peaks, one at532 nm, named I1 and the second at 611 nm, named I2, theemission peaks signify the presence of the SrAl2O4:Eu2+/Dy3+ andNaYF4:Eu3+ phosphors in the single excitable dual emissiveannealed luminescent pigment. The inset of Fig. 2g indicatesthe plot between the highest peak intensity, valued at 532 nm, and611 nm upon different excitation wavelengths. The ratio I2/I1 ismuch higher than the numerical value 1 at the 256 nm emissionwavelength, which indicates that the emission of red colordominates over the green color and the overall color emissionat 256 nm wavelength is depicted by the K symbol in the CIEcolor coordinates (Fig. S8b, ESI†). The ratio of the emissionintensities at different excitation wavelengths is shown inTable S4 (ESI†). The ratio of intensities at excitation wavelengths

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of 374 and 405 nm is o1, therefore the emission of green colordominates over the red color at these two excitation wavelengthsand the overall color emission at 374 and 405 nm wavelengths isindicated by the * and . symbols in the CIE color coordinates(Fig. S8b in the ESI†), respectively. The intensity ratios at excita-tion wavelengths of 395, 400 and 467 nm are 41, therefore thered color emission dominates over the green color emissionand the overall color emissions at wavelengths of 395, 400 and467 nm are denoted by the m, ’ and + symbols in the CIE colorcoordinates (Fig. S8b, ESI†), respectively. Hence, it is clear fromthe observed data of the intensity ratios that the combination ofthese two phosphors with an optimum concentration is a suitablechoice for the ON and OFF emission of the intense red and greencolors under a 254 nm UV lamp excitation source respectively,without causing a significant change in the PL intensities of theindividual phosphors. For greater clarity, a comparison of theintensities of the individual phosphors and the annealed compo-site pigment is shown in Table S5 (ESI†). The intensity values inTable S5 (ESI†) were taken from Fig. 2g and Fig. S1 (ESI†). It isclear from Table S5 (ESI†) that there is no significant changeobserved in the PL intensities of the individual phosphors and theannealed composite pigment. The decay profile and its corres-ponding fitted spectra of the single excitable dual emissiveannealed luminescent pigment (down-conversion, at the 611 nmemission wavelength and 256 nm excitation wavelength) is shownin Fig. S12a and b (ESI†), respectively. The lifetime parametersgenerated from the exponential fitting of the decay curve aret = 0.973 ms, w2 = 1.517, and A = 100. The lifetime of the singleexcitable dual emissive annealed luminescent pigment ist = 0.973 ms, which shows no significant change in lifetimefrom the lifetime of the individual NaY0.97F4:Eu0.03

3+ phosphor.

Fig. S11c and d (ESI†) indicates the decay profile and itscorresponding fitted spectra of the single excitable dual emissiveannealed luminescent pigment (down-shift at the 532 nm emis-sion wavelength and 374 nm excitation wavelength), respectively.The lifetime parameters generated from the exponential fittingof the decay curve are A = 17.61, A1 = 0.66, A2 = 0.34, w2 = 1.17,t1 = 5.27 min, t2 = 34.89 min and tav = 28.17 min.

Finally, demonstration of the single excitable dual emissiveluminescent security ink based on the single excitable dualemissive luminescent pigment against counterfeiting wasperformed successfully, which has not been reported so far tothe best of our knowledge. During the development of thesecurity ink, the PVC gold medium has been used as a mediumfor dispersion, because of its several vital qualities, viz., strongadhesive nature, low cost, water resistant and the ease ofprinting on different surfaces.3 As mentioned in the Experi-mental section, approximately 500 mg of the single excitabledual emissive luminescent pigment was incorporated into200 mL of PVC gold medium for the development of the singleexcitable dual emissive luminescent security ink. The opticalphotographs of the single excitable dual emissive luminescentpigment under daylight and the 254 nm UV lamp switched ONand OFF are shown in Fig. 3a–c, respectively. The audio-visualdemonstration of the single excitable dual emissive lumines-cent pigment under daylight and the 254 nm UV lamp switchedON and OFF can be clearly seen in the multimedia file (seeVideo S1 in the ESI†). Furthermore, for the feasibility test of thesingle excitable dual emissive luminescent security ink, animage was printed on a non-fluorescent white bond paperusing a standard screen printing technique (for the protocolsee details in the Experimental section) as shown in Fig. 3d.

Fig. 3 Optical photographs of (a) the single excitable dual emissive luminescent pigment under daylight; (b) the single excitable dual emissiveluminescent pigment under a 254 nm UV lamp; (c) the single excitable dual emissive luminescent pigment after the 254 nm UV lamp is switched off;(d) printed image under daylight; (e) printed image under a 254 nm UV lamp; and (f) the printed image after the 254 nm UV lamp is switched off.

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The printed image on the non-fluorescent white bond paperemits red and green colors under the 254 nm UV lamp excita-tion source switched ON and OFF respectively, as shown inFig. 3e and f. Additionally, to ensure and confirm the unifor-mity of the PL intensity distribution on the printed image, PLmapping was performed using a PL confocal microscope. ThePL mapping of the marked red square area shown in Fig. 4a wasconducted under the excitation of a 375 nm wavelength lasersource. Fig. 4b and c shows the 3D and 2D PL mapping of theprinted image of the marked region in Fig. 4a under theexcitation of a 375 nm wavelength source, which confirmsthe uniformity of the phosphor distribution on the printedimage throughout the entire surface. Moreover, the PL emis-sion spectra of the marked area, enclosed by the blue hollowrectangle in Fig. 4b were recorded to verify the consistency ofthe emission all over the printed image, as shown in Fig. 4d.The stability of the printed image under different weather andtime conditions was checked immediately after printing ( June2018, temperature B42 1C, humidity 45%); after three months(September 2018, temperature B28 1C, humidity 79%) and after6 months (January 2019, temperature B10 1C, humidity 35%), theoptical photographs of the printed image under daylight and the

254 nm UV lamp switched ON and OFF are shown in Fig. 5. Thereis no noticeable change observed in the emission of the printedimages, which was further confirmed through the PL spectrum, asshown in Fig. S13 (ESI†). The chemical stability of the singleexcitable dual emissive luminescent security ink has also beenobserved by examining the printed image after treatment withvarious bleaching agents (ethyl alcohol, ethyl acetate, xylene,acetone, soap solution and laundry detergent) at the markedplaces 1, 2, 3, 4, 5 and 6, respectively, as shown in Fig. 6. Thesechemicals have the tendency to rub out the printed security inkor reduce the PL intensity, but no significant change has beenobserved in the emission of color at the appropriate markedplaces.

Furthermore, to measure the viscosity of the formulatedsingle excitable dual emissive luminescent security ink, rheo-logical characterization was conducted as shown in Fig. S14a–d(ESI†). For uniform and effective printing, the viscosity of theink should be in the range of 1–1000 mPa s.33,34 The viscosity ofthe formulated single excitable dual emissive luminescentsecurity ink was measured with respect to the shear stressrate of the immediately synthesized ink formation as shownin Fig. S14a (ESI†). It shows that the viscosity of the single

Fig. 4 (a) Optical photographs of the printed image under daylight; (b) and (c) 3D and 2D PL mapping of the marked area in the printed image (a) under a375 nm excitation wavelength, respectively; and (d) the PL emission spectra of the marked area in (b).

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excitable dual emissive luminescent security ink at a low shearrate of 10 s�1 was B1000 mPa s, which further decreased toB650 mPa s at a higher shear rate. By increasing the shear rate,the viscosity becomes thinner and the ink flows owing to itspseudo-plastic nature. After six months, the single excitabledual emissive luminescent security ink does not show anymajor variation in the viscosity, which reveals its stable nature

under different atmospheric conditions, as shown in Fig. S14b(ESI†). The insets of Fig. S14a and b (ESI†) show that the singleexcitable dual emissive luminescent security ink is placed onthe sample holder of the rheometer to perform the rheologicalcharacterization of the single excitable dual emissive lumines-cent security ink. Another dynamic behavior of the ink is itsviscoelastic nature, which is defined by the solid-like/elastic

Fig. 5 Optical photographs of the printed image under different weather conditions in the presence of daylight and a 254 nm UV lamp switched ONand OFF.

Fig. 6 Optical photographs of the printed image when tested against the various bleaching agents at different places under daylight and a 254 nm UVlamp switched ON and OFF, respectively.

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(storage modulus; G0) and the liquid-like/viscous (loss modulus; G00)behaviors. Fig. S14c and d (ESI†) confirms the liquid-likebehavior of the single excitable dual emissive luminescentsecurity ink; immediately after synthesis and after six monthsstorage, respectively. It also shows that the value of G00 is higherthan G0 in the given range of strain, which is a very importantfactor for performing high aspect ratio screen printing.35

Hence, the viscosity and viscoelastic properties of the singleexcitable dual emissive luminescent security ink prove that theformulated single excitable dual emissive luminescent securityink meets the stringent criteria for printing applications.

In order to find the best compatible fluorescent material forthe formation of the single excitable dual emissive luminescentpigment, various types of luminescent materials have beenemployed. The feasibility test for the SrAl2O4:Eu2+/Dy3+ (A)phosphor with other materials such as Eu(TTA)3Phen (B),NaYF4:Eu3+ (C), YBO3:Eu3+ (D), Y2O3:Eu3+ (E) for the formationof the composite pigments was performed. The optical photo-graphs of these corresponding materials under daylight and a254 nm UV lamp are shown in Fig. S15a and b (ESI†), respec-tively. The fluorescein, NaYF4:Eu3+, YBO3:Eu3+ and Y2O3:Eu3+

phosphor produce the red color emission under the excitationof a 254 nm UV lamp and the SrAl2O4:Eu2+/Dy3+ phosphor givesthe green color emission under the excitation of a 254 nm UVlamp (Fig. S15b, ESI†). Afterwards, A was mixed with B, C, Dand E at the weight ratio of 3 : 1 to carry out a comparativestudy. Fig. S16a–c (ESI†) shows the optical photographs of thecomposite pigments under daylight and a 254 nm UV lampswitched ON and OFF, respectively. The combinations ofY2O3:Eu3+ phosphor, YBO3:Eu3+ phosphor, Eu(TTA)3Phen withSrAl2O4:Eu2+/Dy3+ phosphor give the green color emissionunder the excitation of the 254 nm UV lamp switched ON andOFF, which may be due to the presence of the SrAl2O4:Eu2+/Dy3+

phosphor in the composite pigments, while the composition ofthe NaYF4:Eu3+ phosphor with the SrAl2O4:Eu2+/Dy3+ phosphorgives the red and green color emission under the excitation of a254 nm UV lamp switched ON and OFF, respectively. Whilein the case of Eu(TTA)3Phen, a very broad excitation bandcentered at B365 nm was observed, which overlaps the excita-tion range of SrAl2O4:Eu2+/Dy,3+ and ultimately results in avery weak emission intensity at the excitation of 254 nmof Eu(TTA)3Phen, hence the emission of the green color(SrAl2O4:Eu2+/Dy3+ phosphor) dominates over the red color ofEu(TTA)3Phen. Thus, the NaYF4:Eu3+ phosphor was found to bea better and more legitimate candidate for the formation of thesingle excitable dual emissive luminescent pigment.

Conclusions

In summary, a single excitable dual emissive luminescent pig-ment has been successfully synthesized using a novel combina-tory concept of the fluorescence@phosphorescence phenomenonfor the formulation of unclonable single excitable dual emissiveluminescent security ink which possesses strong security featuresto combat counterfeiting of currency, passports, pharmaceuticals,

important documents and so forth. The single excitable dualemissive luminescent pigment has the novelty of producing red(NaYF4:Eu3+, 611 nm) and green (SrAl2O4:Eu2+/Dy3+, 532 nm)dual color emissions under the excitation source of a 254 nmUV lamp switched ON and OFF, respectively, which can easilybe recognized by the naked eye under ambient conditions. Theimmediately formulated single excitable dual emissive lumi-nescent security ink and the ink after six months of storageexhibit almost the same viscosity, which confirms that it can bestored for long durations without undergoing any significantchanges in its properties. Furthermore, the single excitabledual emissive luminescent security ink printed images exhibitchemical stability under rigorous atmospheric conditions suchas hot, cold and humid conditions for six months. In a nutshell, the development of a single excitable dual emissiveluminescent security ink with the combination of a singleexcitable dual emissive luminescent pigment and PVC goldmedium offers a new scalable and inexpensive opportunity togenerate unbreakable security features, which are essential forprotection against counterfeiting.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors wish to thank the NPL director, Dr D. K. Aswal, forhis keen interest in the in-house mission-mode project on‘‘Luminescent Security Ink of CSIR-NPL’’ and for the constantencouragement. Mr A. K. Gangwar gratefully acknowledges financialsupport from the University Grant Commission (UGC), Governmentof India. Ms Kanika acknowledges financial support from theCouncil of Scientific & Industrial Research (CSIR), Government ofIndia. The authors are grateful to the CSIR-TAPSUN program for theconfocal PL mapping facility.

Notes and references

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