Anticounterfeiting Quick Response Code with Emission Color of

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Functional Inorganic Materials and Devices

Anti-Counterfeiting Quick Response Code with Emission Color of Invisible Metal-Organic Frameworks as Encoding Information Yong-Mei Wang, Xue-Tao Tian, Hui Zhang, Zhong-Rui Yang, and Xue-Bo Yin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06901 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Applied Materials & Interfaces

Anti-Counterfeiting Quick Response Code with Emission Color of Invisible Metal-Organic Frameworks as Encoding Information Yong-Mei Wang, Xue-Tao Tian, Hui Zhang, Zhong-Rui Yang, Xue-Bo Yin*,

State Key Laboratory of Medicinal Chemical Biology and Tianjin Key Laboratory of Biosensing and Molecular Recognition, College of Chemistry, Nankai University, Tianjin, 300071, China E-mail: [email protected]; Fax: +86-22-23503034 Prof. Xue-Bo Yin Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin, 300071, China

KEYWORDS: Anti-counterfeiting, Metal-organic frameworks, Quick response code, Trichromatic emission, Fluorescence at single excitation, Color encoding

ABSTRACT Counterfeiting is a global epidemic that is compelling the development of new anti-counterfeiting strategy. Herein, we report a novel multiple anti-counterfeiting encoding strategy of invisible fluorescent quick response (QR) codes with emission color as information storage unit. The strategy requires red, green, and blue (RGB) light-emitting materials for different emission colors as encrypting information, single excitation for all of the emission for practicability, and ultraviolet (UV) excitation for invisibility under daylight. Therefore, RGB light-emitting nanoscale metal-organic frameworks (NMOFs) are designed as inks to construct the colorful light-emitting boxes for information encrypting, while three black vertex boxes were used for positioning. Full-color emissions are obtained by mixing the trichromatic NMOFs inks through inkjet printer. The encrypting information capacity is easily adjusted by the number of light-emitting boxes with the infinite emission colors. The information is decoded with specific excitation light at 275 nm, making the QR codes 1

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invisible under daylight. The composition of inks, invisibility, inkjet printing, and the abundant encrypting information all contribute to multiple anti-counterfeiting. The proposed QR codes pattern holds great potential for advanced anti-counterfeiting.

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INTRODUCTION Counterfeiting is a global epidemic that affects customers, companies, and governments to result in enormous financial loss and adversely affects human health.1,2 Anti-counterfeiting techniques that are hard to duplicate and easy to authenticate are therefore urgently required for the protection of valuable documents, luxury products, currency, drugs, and certificates. Various anti-counterfeiting codes have been designed and fabricated through lithography, laser engraving, screen-printing, and inkjet printing.3-7 Inkjet printing offers the advantages including designability, cost, throughput, and ease of fabrication. However, the security and universality of the traditional encoding strategy remain important tissues because the visible codes are easily duplicated by counterfeiters. The design and development of new encrypting strategies is becoming a burning issue for the purpose of anti-counterfeiting. Fluorescent organic dyes, quantum dots, upconversion nanoparticles, and carbon dots have been prepared, but the fluoresecent materials are only used as fluorescent inks to realize traditional quick response (QR) code or anti-counterfeiting mark.5,8-13 We here propose a novel QR code encoding strategy with fluorescent emission color as information storage unit for the purpose of anti-counterfeiting and information storage. The QR code consists of adjustable colorful light-emitting boxes as infornation storage unit and three black vetex boxes for positioning. All the boxes are of the same size and arranged in a large square pattern. The emission color of the invisible boxes is used as readable information storage unit and the arrangement of the boxes encrypted the anti-counterfeiting information, in contrast to the traditional dot size-dependent matrix code and line width-dependent universal product barcode. The key to the strategy is to design and prepare materials with trichromatic fluorescence and full-color emissions are obtained from the trichromatic materials after mixing through an inkjet printer. Additionally, all of the emissions, including the trichromatic fluorescence, should be excited with a single-excitation wavelength for the practicality of the

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fluorescent QR code. Moreover, all of single- or mixed-inks should be stable and invisible under daylight, so UV light is the optimal excitation light source. Metal-organic frameworks (MOFs) are the hybrid materials from organic ligands and metal nodes.14 MOFs with different emission color and proper size for inkjet printing application could be achieved by rational selection of the building blocks.15-18 Moreover, MOFs are highly stable compared with molecular dyes. Thus, MOFs are good candidates to realize our fluorescent QR code encoding strategy. Dyes have been encapsulated into the green-emissive MOFs to obtain long-live luminescent MOFs; the emissions ranging from green to red were observed because of the differnt MOF-to-dye phosphorescence energy transfer.19 Red emission with blue background was achieved when excited at 365 nm, but became dark orange long-lived emission after removal of the excitaion. The emission color change

before

and

after

the

excitaion

was

considered

as

anti-counterfeiting.19

Electrochemically-assisted microwave deposition was used to prepare the pattern of Eu-MOFs and Tb-MOFs as promising anti-counterfeiting barcode.20 Hierarchical Ln-MOF crystals are prepared for high-throughput, multiplexed coding capability in a 3D maneuver way.21 Mixed-metal Ln-MOFs were designed with β-diketonate as ligand in combination of specific

choice

of

lanthanide

ions

and

their

ratios.22

Their

wavelength-

and

temperature-dependent emission realized the “chameleon” multistage anti-counterfeiting.22 Eu3+, Tb3+, Gd3+, or Nd3+ and mellitate were used to generate Red-emissive MOF, Green-emissive MOF, Blue-emissive MOF, and NIR-emissive MOF.23 The authors studied the possibiliy of the MOF series as inks and their potential for invisible security labeling/encoding.23 We herein propose the new QR code encoding strategy with the emission color of nanoscale MOFs (NMOFs) as signal storage unit for multiple anti-counterfeiting as illustrated in Scheme 1. Red and–green light-emitting NMOFs are synthesized with 5-boronoisophthalic acid (5-bop) and isophthalic acid (1,3-BDC) as mixed ligands and with europium (Eu3+), and 4


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terbium (Tb3+) as metal nodes, respectively (Scheme 1a). By carefully tuning the type and ratio of the precursors, 200-nm Eu-NMOFs and Tb-NMOFs were obtained with red and green fluorescence, respectively. The antenna effect governs the emission of the Ln-NMOFs, and thus the ligands are excited to produce their triplet state and sensitize the Ln3+ ions.24 Thus, both the red and green emissions are achieved under single-wavelength excitation of 275 nm. The nanosize Uio-66-NH2 (a kind of MOFs) has strong blue emission at the excitation of 275 nm. According to the standard Red-Green-Blue (RGB) theory, we obtained invisible full-color fluorescent images through inkjet printing with the Eu-NMOFs, Tb-NMOFs, and Uio-66-NH2 inks under 275 nm excitation (Scheme 1b and c). The printed QR codes consist of invisible light-emitting boxes and three visible vertex black boxes as locators with the same size arranged in a square pattern (Scheme 1d). The decryption is easily realized by a 275 nm UV light excitation.

Scheme 1. (a) Preparation of MOF-based RGD light-emitting inks; (b) Cartridge fabrication; (c) Design and printing; (d) The input, encrypted, and decrypted objects. Multiple anti-counterfeiting is realized (Scheme 1d). The NMOF materials have specific compositions and their invisibility makes counterfeiting duplication difficult without the authenticated excitation light source at specific wavelength. Infinite arrangement of the 5



colorful light-emtting boxes of QR codes is the most powerful anti-counterfeiting. The information capacity is easily adjusted by the emission color and unit number. The use of smaller fluorescent boxes results in more encoding unit, larger information storage capacity, and stronger anti-counterfeiting ability. The anti-counterfeiting technique that employs the new encoding strategy substantially increases the difficulty of duplication and satisfies the requirements of high-level security protection for different applications. Moreover, the accessibility of a single UV excitation light source makes the fluorescent QR code extensively practical. RESULTS AND DISCUSSION Preparation and characterization of Ln-NMOFs The emission properties of Ln-MOFs are extensively reported,25,26 but their size is critical for real inkjet printing application. Reducing the size to nanoscale dimensions can avoid blocking the nozzle of inkjet printer and improve the printing resolution. The Ln-MOFs prepared with 5-boronoisophthalic acid (5-bop) and Ln3+ (including Eu3+, Tb3+, and Dy3+ ions) had small size of 100 nm but irregular morphology (Figure S1a-c, Supporting Information) and weak fluorescence. However, isophthalic acid (1, 3-BDC) and Ln3+ produced Ln-MOFs with size of 500 nm even at low concentration (Figure S1d-f, Supporting Information). We therefore finally adopted 5-bop and 1,3-BDC as mixed ligands to synthesize spherical Ln-MOFs particles (Scheme 1a). The 1:1 (molar ratio) mixed ligands together with the three kinds of Ln3+ produced nanoscale spherical Ln-MOFs with average size of 200 nm (Figure 1a-c). The size is less than 1/50 of the nozzle diameter (~10 µm) and thus is suitable for inkjet printing.27 The Ln-NMOFs dispersed well in different solvents for real application as shown in transmission electron microscope (TEM) images in the insets of Figure 1a-c, including ethanol and water.

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Figure 1. Scanning electron microscope and transmission electron microscopy (insets) images of (a) Eu-NMOFs, (b) Tb-NMOFs, (c) Dy-NMOFs. Coordination environments of the two types of independent Eu3+, Tb3+, and Dy3+ ions in the building units of (d) Eu-SMOFs, (e) Tb-SMOFs and (f) Dy-SMOFs. Fourier transform infrared spectra (FTIR) of 1,3-BDC, 5-bop, and Ln-MOFs were recorded (Figure S2, Supporting Information). C=O stretching vibration (νC=O) peaks at 1695 cm-1 were observed in the ligands, but not in any of the Ln-NMOFs, therefore carboxylate groups coordinated with Ln3+ to form the Ln-NMOFs. The B-O absorption peak was observed at 1313 cm-1 for both 5-bop and Ln-NMOFs, clearly indicating the successful preparation of Ln-NMOFs and that the boric groups did not coordinate with Ln3+ ions. As the single crystal of Ln-NMOFs with the mixed ligands is difficult to synthesize, we prepared the single-crystal Ln-SMOFs with 1,3-BDC as the single ligand, and Eu3+, Tb3+, and Dy3+ as metal nodes to reveal the crystallization and the composition of Ln-NMOFs. Single-crystal X-ray diffraction results of Eu-SMOFs,28 Tb-SMOFs, and Dy-SMOFs show monoclinic systems with similar space groups of P21/n or P21/c (Table S1-3, Supporting Information). The fundamental building unit of the Eu-SMOFs contained two types of independent Eu atoms, three deprotonated 1,3-BDC ligands, and two coordinated H2O molecules with the formula of Eu2(1,3-BDC)3(H2O)2. The Tb-SMOFs and Dy-SMOFs had the same configuration, but with one more crystal water molecule in the channel, as the 7



formula of Tb2(1,3-BDC)3(H2O)2 • H2O and Dy4(1,3-BDC)6(H2O)4 • (H2O)2 (Figure 1d–f). Eu1/Tb1/Dy1 and Eu2/Tb2/Dy2 atoms were arranged alternately and linked by the discrete carboxylate group of 1,3-BDC, resulting in a similar infinite one-dimensional rodlike chain. Adjacent one-dimensional chains then connected with each other through the ligands to form two-dimensional layer. Two-dimensional layers further connected with each other for the three-dimensional extended framework with a one-dimensional channel (Figure S3-5, Supporting Information). Powder X-ray diffraction (PXRD) results of Eu-NMOFs, Tb-NMOFs, and Dy-NMOFs matched well with the simulation result obtained from the single-crystal data of the Eu-SMOFs, Tb-SMOFs, and Dy-SMOFs; the wide peaks are attributed to the nanosize of the MOFs (Figure S6, Supporting Information). All Ln-NMOFs are thus isostructural and phase-pure with high crystallinity. Optical properties of Ln-NMOFs The emission properties of Ln-NMOFs were investigated in combination of the fluorescence spectra of 1,3-BDC, 5-bop, and Eu3+ (Figure S7, Supporting Information). Upon the optimal excitation at 275 nm, 5-bop had strong emission at 340 nm, while 1,3-BDC and Eu3+ had weak emission (Figure S7, Supporting Information). Eu-NMOFs, Tb-NMOFs, and Dy-NMOFs emitted the characteristic red, green, and blue emission, respectively, under excitation at 275 nm (Figure 2a-d). The emission of the Ln3+ ions was therefore efficiently sensitized and the Ln-MOFs with RGB trichromatic fluorescence were obtained. Full-color emissions are expected from simple mixing of the NMOFs according to RGB theory. The antenna effect governs the emission of the Ln-NMOFs. Ligands are excited by photons to reach their singlet state, and then triplet state forms through intersystem crossing process (Figure 2e). The energy transfer from the triplet state to the Ln3+ ions results in the emission of Ln3+ ions. Energy gaps between the singlet and triplet states affect the emission behavior. The energy gaps between the triplet states of both 5-bop and 1,3-BDC and the excited state of Ln3+ ions are larger than 5000 cm-1 (Table S4, Supporting Information), so the 8


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intersystem crossing is effective.24 Eu3+ and Tb3+ were therefore sensitized efficiently as shown by procedures “I” and “II” in Figure 2e for their red and green emissions. Eu-NMOFs had red fluorescence with characteristic sharp peaks at 579, 590, 618, 655, and 701 nm attributed to the transitions of the Eu3+ ions [5D0→7FJ (J = 0-4)].25 Tb-NMOFs showed intense green fluorescence with peaks at 488, 544, 584, and 622 nm attributed to the transitions of [5D4→7FJ (J = 3-6)] in Tb3+ ions.25

Figure 2. Luminescence spectra of (a) Eu-NMOFs, (b) Tb-NMOFs, and (c) Dy-NMOFs under 275 nm excitation. (d) CIE chromaticity coordinates of the emissions from (1) Eu-NMOFs, (2) Tb-NMOFs, and (3) Dy-NMOFs. (e) Schematic illustration of the absorption, migration, and emission from Ln-NMOFs depicted with antenna effect (A: absorption at 275 nm; F: ﬂuorescence; P: phosphorescence; ISC: intersystem crossing; ET: energy transfer; S: singlet; T: triplet.). “I”, “II”, and “III” represent the energy transfer procedure from T1 of 1,3-BDC and 5-bop to Eu3+, Tb3+, and Dy3+. Dy-NMOFs had weaker blue emission because the transfer of intramolecular energy between the triplet state of the ligands and the Ln3+ ions also affects the luminescence properties.29 The gap between the triple states of 5-bop (23923 cm-1) or 1,3-BDC (22831 cm-1) and the excited state of Dy3+ (22000 cm-1) is so small (Table S4, Supporting Information) that energy transfer and back-transfer occur simultaneously as shown by procedure “III” in Figure 2e. A broad but weak emission was thus observed from Dy-NMOFs. The corresponding Commission International de L’Eclairage (CIE) chromaticity coordinates of the fluorescence of Eu-NMOFs, Tb-NMOFs, and Dy-NMOFs are (0.644, 0.3313), (0.2982, 9



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0.5917), and (0.1947, 0.1708), which are in red, green, and blue areas, respectively (Figure 2d). We expect that full color emissions are obtained through mixing the RGB light-emitting Ln-NMOFs under single excitation of UV light. Full-color emission through physically mixing of NMOFs According to the respective primary RGB emission properties of the Ln-NMOFs, we try to creat a wide array of emission colors. Figure 3 shows the secondary emission colors of yellow, cyan, and magenta by 1:1 (molar ratio) physically mixing of two species among Eu-NMOFs, Tb-NMOFs, and Dy-NMOFs. With a mixing ratio of 1:2 or 2:1, emission colors between the primary and secondary emission colors were observed (Figure 3). The corresponding CIE chromaticity coordinates of the mixed emission colors are in line with our expectations, with a triangle having red, green, and blue vertices forming over a wide region to cover the different colors (Figure 3d). The results indicate the realization of full-color emission by simple mixing of the three Ln-NMOFs.

Figure 3. Luminescence spectra of mixed solutions of (a) Eu-NMOFs and Tb-NMOFs, (b) Eu-NMOFs and Dy-NMOFs, and (c) Tb-NMOFs and Dy-NMOFs with the ratios of 1:0, 0:1, 1:1, 1:2, and 2:1 under 275 nm excitation, and (d) CIE chromaticity coordinates of the emissions from the single or mixed Ln-NMOFs. Owing to the different energy transfer efficiency between the ligands and Ln3+, the blue emission from Dy-NMOFs was weak and not pure enough to match with the emissions of Eu-NMOFs and Tb-NMOFs for practical inkjet printing application. Coincidentally, Uio-66-NH2,

as

a

kind

of

MOFs

prepared

with

Zr4+

ions

and

2-amino-1,4-benzenedicarboxylate (BDC-NH2), emits strong blue fluorescence centered at 10


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450 nm under the excitation of 275 nm.30 By rational adjustment of reaction conditions, we prepared 90 nm Uio-66-NH2 with a uniform morphology of octahedron (Figure S8, Supporting Information)). TEM results reveal the well dispersed nanoscale Uio-66-NH2 (Figure S9, Supporting Information). To study the emission of Uio-66-NH2, we also prepared Uio-66 with Zr4+ ions and 1,4-benzenedicarboxylate (BDC). Figure S10 (Supporting Information) shows the fluorescence profiles of BDC, BDC-NH2, Uio-66, and Uio-66-NH2. The emission of Uio-66-NH2 derived from the ligand, in which the introduction of the amino group in BDC-NH2 enhanced the emission intensity of parent BDC because the amino group provided a lone pair to form a big π bond with the benzene ring.30 The nanoscale Uio-66-NH2 emits purer blue fluorescence with CIE coordinate of (0.1462, 0.1097) compared with Dy-NMOFs (0.2190, 0.1978) (Figure S11, Supporting Information).

Figure 4. Luminescence spectra of mixed solutions of (a) Edu-NMOFs and Tb-NMOFs, (b) Eu-NMOFs and Uio-66-NH2, and (c) Tb-NMOFs and Uio-66-NH2 with ratio of 1:0, 0:1, 1:1, 11



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1:2, and 2:1 under the excitation at 275 nm. (d) CIE chromaticity coordinates for the luminescence of the mixed NMOFs. (e) Images of the mixed inks under 275-nm UV light. “R” represents the red ink of Eu-NMOFs, “G” represents the green ink of Tb-NMOFs, and “B” represents the blue ink of Uio-66-NH2. On the basis of the excellent emission colors of Eu-NMOFs, Tb-NMOFs, and Uio-66-NH2, we tested the mixing emission colors of the three kinds of RGB light-emitting NMOFs. Figure 4 shows that the emission color of mixed NMOFs, with Uio-66-NH2 as blue emission source, had pure CIE coordinates than that illustrated in Figure 3d. The images of mixed NMOFs under excitation at 275 nm show a wide emission color region (Figure 4e). Light-emitting materials are thus not limited to Ln-NMOFs, and other kinds of MOFs or fluorescent materials, which satisfy the size, excitation and emission conditions, could also be used as potential fluorescence inks in the proposed encoding strategy. Table 1. The CIE coordinates of emission spectra of the mixed Eu-NMOFs and Tb-NMOFs with ratio of 1:0, 0:1, 1:1, 1:1.001, 1:1.01, 1:1.1, 1:2, 1:1.3, 1:1.4, and 1:1.5 under the excitation at 275 nm.

R:G

CIE

x

CIE y

Peak (nm)

Maximum Peak Intensity

1:0

0.6079

0.3232

618

1934

0:1

0.2997

0.5864

547

1937

1:1

0.4661

0.4415

618

1014

1:1.001

0.4659

0.4421

617

976.8

1:1.01

0.4631

0.4447

617

982.5

1:1.1

0.4563

0.4516

547

966.3

1:1.2

0.4534

0.4546

547

1010

1:1.3

0.4458

0.4600

546

1035

1:1.4

0.4357

0.4671

547

1061

1:1.5

0.4330

0.4663

548

1121

Resolution of emission color Although the NMOFs have excellent ability to create various emission colors, the ability to differentiate the created emission colors is crucial to realize the new anti-counterfeiting 12


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encoding strategy as the emission color is used as encoding information. We selected the mixture of red light-emitting Eu-NMOFs and green light-emitting Tb-NMOFs as example. Figure S12 shows that the single Eu-NMOFs and Tb-NMOFs, emit strong red and green fluorescence under excitation of 275 nm, respectively. When mixing the two NMOFs with different ratios, the mixed NMOFs emit yellow fluorescence with similar intensity (Figure S12). However, the emissions were easily differentiated from each other according to their CIE coordinates. A subtle coordinate change of (△0.0002, △0.0006) was observed from the fluorescence of mixed NMOFs between the ratio of 1:1 and 1:1.001 with volume increase of only 1 µL (Table 1). The results validate the excellent ability of NMOFs to produce various fluorescence color with high resolution and showed the feasibility of using the emission color as information encoding unit. The abundant emission colors and high resolution contribute to a large capacity of encoding information for safe and precise encryption. Writing application with invisible RGB light-emitting NMOFs inks The RGB light-emitting inks were first tested for their practicability as writing inks.9,22 We filled the pens with the NMOFs fluorescent inks. Figure 5a and b show that the pens filled with the inks are nearly transparent under daylight, and emit red, green, and blue fluorescence under UV light. We wrote the acronym of ‘Nankai University’ with the pens directly. The letters N, K, and U were absolutely invisible under daylight (Figure 5c), but displayed bright red, green, and blue letters respectively under UV lamp irradiation (Figure 5d), and thus can be used as invisible anti-counterfeiting labels. Figure 5e shows the SEM image of the letter ‘N’ in Figure 5c. It is clearly seen that the Eu-NMOFs spheres are homogeneously dispersed on the paper. The high-resolution SEM image validated that the morphology of the NMOFs spheres remained intact (inset of Figure 5e), demonstrating the high chemo-stability and excellent fluidity of the NMOFs inks for high practicability.

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Figure 5. Images of pens filled with Eu-NMOFs, Tb-NMOFs, and Uio-66-NH2 inks under (a) daylight and (b) UV light. Images of the handwritten acronym of ‘Nankai University’ (c) under daylight and (d) UV light irradiation, where “N” is written with the red-emitting pen, “K” is written with the green-emitting pen, and “U” is written with the blue-emitting pen. (e) SEM image of the red-emitting ink on paper. Emission-color-encoded QR code for multiple anti-counterfeiting The RGB light-emitting NMOFs was successfully inkjet printed on non-fluorescent printing paper. To achieve the emission color encoding, the NMOFs inks were mixed through inkjet printer for invisible emission information storage unit boxes and three positioning boxes of the same size arranged in a square pattern. The left column of Figure 6a–c shows the input QR code patterns, where the three black boxes are designed for position recognition of the QR code and the other colorful boxes are information storage units. The information storage capacity can be adjusted by the number of colorful light-emitting encoding boxes, and thus we designed QR codes with 3×3, 4×4, and 5×5 boxes in a fixed area of 3×3 cm2 (Figure 6a–c). When the printed paper was exposed to room light, only the three black positioning boxes were observed (the middle column in Figure 6a-c). When excited with a 275 nm UV light, the fluorescent QR codes were decrypted obviously and found to be identical to the designed input patterns (the right column in Figure 6a–c). We thus obtain different anti-counterfeiting information by simply changing the emission colors of the designed boxes. Infinite encoding information can be obtained through changing the arrangement the infinite colorful light-emitting boxes. The use of smaller boxes within the QR code results in more encoding units and larger information storage capacity. The information storage capacity is thus easily adjusted with the encoding strategy. 14


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The colorful light-emitting inks were also used to encode a barcode with emission color as barcode encoding unit (Figure 6d). The encoding lines were invisible and isometric with equal width, and thus they could only be decrypted by specific excitation light, which greatly improves the anti-counterfeiting ability. Meanwhile, the invisible inks were applied to other anti-counterfeiting labels on different kinds of papers, as shown in Figure S13 and S14 (Supporting Information). The stability of the printed QR code is critical in terms of long-term practicability and decryption. We took photographs of printed QR code patterns stored in room temperature without any light avoidance after a day, a week, and a month and observed almost the same fluorescence efficiency as illustrated in Figure S15 (Supporting Information). The results illustrate the high chemo- and photo-stability of the fluorescent inks for anti-counterfeiting QR code.

Figure 6. Images of the QR code patterns of input patterns, and printed output patterns under daylight and UV light with the unit numbers of (a) 3×3, (b) 4×4, and (c) 5×5. (d) Images of a barcode printed with the RGB-emitting NMOFs inks under daylight (upper) and 275-nm UV light (bottom). 15



CONCLUSION In summary, we developed a new encoding strategy of invisible fluorescent QR code for multiple anti-counterfeiting. In a simple one-pot hydrothermal procedure with mixed ligands and metal ions, nanoscale MOFs inks were prepared with distinctive red, green, and blue emissions. Morevoer, the single and mixed-MOFs inks are excitied at single wavelength of 275 nm. The fluorescent inks were used to print the anti-counterfeiting QR code with emission color as information storage. The single-wavelength excitation for all the emissions improved the practicality and the UV excitation amd made the QR code invisible under daylight. The material composition, invisibility, and arrangement of the fluorescence boxes contribute to the multiple anti-counterfeiting. The light-emitting materials are not limited to the NMOFs prepared in this work; any other fluorescent materials that satisfy the requirements of size, single-wavelength excitation, and full-color emission, could be used to realize the proposed encoding strategy of anti-counterfeiting QR code. We believe that the proposed fluorescent QR code encryption and decryption strategy will open up a potential avenue in anti-counterfeiting and security-protecting applications. In this work, we discussed the new encoding strategy in terms of the selection and preparation of fluorescent materials, the design of fluorescence encryption and decryption, and the printing technique. The traditional QR codes of the matrix barcode and universal product barcode are very useful for identification and dissemination. Our new encoding strategy could further broaden the application of the QR code to the field of anti-counterfeiting.

EXPERIMENTAL SECTION Materials Europium (III) chloride hexahydrate (EuCl3▪6H2O, 99.99%), terbium (III) chloride hexahydrate (TbCl3▪6H2O), and dysprosium (III) chloride hexahydrate (DyCl3▪6H2O) were 16


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obtained from Sigma–Aldrich, Shanghai, China. Isophthalic acid was brought from Shanghai Macklin Biochemical Co. Ltd. 5-Boronoisophthalic acid (5-bop) and isophthalic acid (1,3-BDC) was purchased from HWRK Chem Co., Ltd., Beijing, China. Dimethylformamide (DMF) and other solvents were from Concord Reagent Co., Tianjin, China. Ultra-pure water was prepared with an Aquapro system (18.25 MΩ). All reagents were used as purchased without further purification. Instrument and Characterization The scanning electron microscopy (SEM) images were recorded with JSM-7500F, Japan. The transmission electron microscopy (TEM) images and EDX were recorded with Tecnai G2 F20, FEI Co. (America), and operated at an accelerating voltage of 200 kV. UV-Vis absorption spectrum was recorded by a U-3900 visible spectrophotometer (Hitachi, Japan). The steady-state ﬂuorescence experiments were performed on a FL-4600 Fluorescence Spectrometer (Hitachi, Japan) equipped with a plotter unit and a quartz cell (1 cm × 1 cm). The slit width was 5 and 5 nm for excitation (275 nm) and emission, respectively. The infrared spectra were measured by the Bruker TENSOR 27 Fourier transform infrared spectroscopy. XRD patterns were recorded by a D/max-2500 diffractometer (Rigaku, Japan) using Cu-Kα radiation (λ = 1.5418 Å). The printing patterns are printed by modified HP Deskjet 1111 inkjet printer. The images are taken by a mobile phone of Huawei P10. Synthesis of the Ln-NMOFs Eu-NMOFs were synthesized by simple solvothermal method. A mixture of EuCl3▪6H2O (7.3 mg, 0.02 mmol), 1,3-BDC (2.1 mg, 0.01 mmol) and 5-bop (1.7 mg, 0.01 mmol) were vigorously stirred for 1 h in DMF/H2O (7:3) solution. Then, the mixed solution was transferred into a Teflon vessel in a stainless steel autoclave, heated at 120 °C for 6 h. The mixture was cooled to room temperature. The white powder was collected after centrifugation, washed thoroughly with DMF and ethanol, and dried at the room temperature. Tb-NMOFs and Dy-NMOFs were synthesized in the similar procedure as Eu-MOF 1, except for the use of TbCl3▪6H2O (7.5 mg, 0.02 mmol) and DyCl3▪6H2O (7.5 mg, 0.02 mmol) as the metal sources, respectively. Synthesis of the Uio-66-NH2 Uio-66-NH2 was prepared by hydrothermal method. A mixture of ZrCl4 (77 mg), 1,4-BDC-NH2 (48 mg), and benzoic acid (610 mg) were vigorously stirred in 6 mL DMF and 55 µL HCl (12 M) solution. Then, the mixed solution was transferred into a Teflon vessel in a stainless steel autoclave, heated at 120 °C for 48 h. after the mixture was cooled to room

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temperature, the light yellow powder was collected after centrifugation, washed thoroughly with DMF and ethanol, and dried at the room temperature. Preparation of luminescent NMOFs inks for printing Eu-NMOFs (25 mg), Tb-NMOFs (15 mg), Dy-NMOFs or Uio-66-NH2 (20 mg) were separately added to a 1 mL solution mixed with 45% ethanol, 45% ethylene glycol, 5% glycerol, and 5% diethylene glycol. Then, 3 mg/L SDS was added to the mixture to obtain an appropriate surface tension and dynamic viscosity for an inkjet printer. Finally, the obtained mixture was in alternation with sonication and vortex for 10 minutes and stored in 4 °C for use. Preparation of luminescent NMOFs pens for writing M&G rollerball pens with ball diameters of 0.5 mm were used in the present study. The original ink was first removed with running water and ethanol. Then, the ink barrel and rollerball tip were cleaned thoroughly in ethanol in an ultrasonic bath. The prepared Ln-NMOFs inks was injected into the cartridge by micro-syringe. Preparation of anti-counterfeiting patterns through inkjet printing The color cartridge of the inkjet printer was refitted firstly. Remove the orginal cartridge cover and wash away the original color inks with water and ethanol until there is no ink residue. Then, inject the RGB light-emitting inks into the cartridge with a syringe respectively, and cover the cartridge cover to print various anti-counterfeiting patterns.

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ASSOCIATED CONTENT Supporting Information Supplementary figures and tables. The Supporting Information is available free of charge via Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding authors *[email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by NSFC (No. 21675090, 21435001) and 973 Program (No. 2015CB932001).

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Table of Contents/Abstract graphic

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Anticounterfeiting Quick Response Code with Emission Color of

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