Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 1802−1809
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Achieving Multicolor Long-Lived Luminescence in DyeEncapsulated Metal−Organic Frameworks and Its Application to Anticounterfeiting Stamps Jianbin Liu,† Yixi Zhuang,*,† Le Wang,*,‡ Tianliang Zhou,† Naoto Hirosaki,§ and Rong-Jun Xie*,†,§ †
College of Materials, and Fujian Provincial Key Laboratory of Materials Genome, Xiamen University, Simingnan Road 422, Xiamen 361005, P. R. China ‡ College of Optical and Electronic Technology, China Jiliang University, Hangzhou 310018, P. R. China § Sialon Group, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan S Supporting Information *
ABSTRACT: Long-lived luminescent metal−organic frameworks (MOFs) have attracted much attention due to their structural tunability and potential applications in sensing, biological imaging, security systems, and logical gates. Currently, the long-lived luminescence emission of such inorganic−organic hybrids is dominantly confined to shortwavelength regions. The long-wavelength long-lived luminescence emission, however, has been rarely reported for MOFs. In this work, a series of structurally stable long-wavelength long-lived luminescent MOFs have been successfully synthesized by encapsulating different dyes into the green phosphorescent MOFs Cd(m-BDC)(BIM). The multicolor long-wavelength long-lived luminescence emissions (ranging from green to red) in dye-encapsulated MOFs are achieved by the MOF-to-dye phosphorescence energy transfer. Furthermore, the promising optical properties of these novel long-lived luminescent MOFs allow them to be used as ink pads for advanced anticounterfeiting stamps. Therefore, this work not only offers a facile way to develop new types of multicolor long-lived luminescent materials but also provides a reference for the development of advanced long-lived luminescent anticounterfeiting materials. KEYWORDS: metal−organic frameworks, long-lived luminescence, dye-encapsulated, energy transfer, anticounterfeiting rescence (i.e., persistent luminescence and delayed fluorescence) or room-temperature phosphorescence (RTP) is quite important for the development of advanced anticounterfeiting technologies without background fluorescence. Luminescent materials with long-lived fluorescence and phosphorescence have always been the interest of research due to their important applications in light-emitting diodes, sensing, security systems, and biological imaging.10−13 At present, these materials are mainly inorganic phosphors (e.g., SrAl2O4:Eu2+,Dy3+ or CaAl2O4:Eu2+,Nd3+). These materials, however, contain rare earth elements with limited resources and also lack tunable structure.14−17 Through sensitization of metal ions by organic linkers, organometallic complexes can also exhibit phosphorescence phenomena.18−21 Unfortunately, they have a relatively low quantum efficiency, extremely short phosphorescence lifetime (typically several milliseconds), and poor structural stability. It is therefore necessary to find new material systems with desired long-wavelength long-lived luminescence and excellent structural flexibility and stability.
1. INTRODUCTION Counterfeiting of documents, currency, food, and branded products is a global problem, causing enormous economic and security implications for the government, industry, and consumers.1−3 It is estimated that the size of the underground economy due to counterfeiting each year amounts to billions of dollars. The urgent need for developing a next-generation hightech anticounterfeiting system has triggered extensive research on advanced anticounterfeiting materials in the past decade. Optical functionality, owing to the features of being hard-tocopy and easy-to-authenticate (readout), is considered to be an ideal security element in the anticounterfeiting systems. So far, a variety of optical-functionality-based anticounterfeiting materials, including lanthanide-doped luminescent nanomaterials, quantum dots (semiconductor and carbon-based), and plasmonic nanomaterials have been successfully developed.4−9 These materials generally possess small/uniform particle size and high fluorescent efficiency, suggesting great promise in the anticounterfeiting application. However, the anticounterfeiting optical information in these materials is mainly based on the short-lived fluorescence emission, which suffers serious background fluorescence interference from the common printing media (e.g., office paper or banknotes). Therefore, discovering or developing excellent materials showing long-lived fluo© 2017 American Chemical Society
Received: September 5, 2017 Accepted: December 20, 2017 Published: December 20, 2017 1802
DOI: 10.1021/acsami.7b13486 ACS Appl. Mater. Interfaces 2018, 10, 1802−1809
Research Article
ACS Applied Materials & Interfaces
heating rate of 10 °C min−1. The UV−vis diffusive reflectance spectra were measured on a Shimadzu UV-3600-plus spectrophotometer, and the fluorescence spectra were recorded by using a Hitachi F-4600 spectrometer. The long-lived luminescence spectra were obtained from an Ocean Optics QE Pro charge-coupled device (CCD). The fluorescence quantum yield (QY) at room temperature was measured by using a Teflon-lined integrating sphere (F-M101, Edinburgh, diameter: 150 mm; weight: 2 kg) in a FLS980 fluorescence spectrometer. The long-lived luminescence QY was measured by using a Hitachi F-4600 spectrometer. Confocal laser scanning images were taken on a Leica TCS SP5 laser scanning confocal microscope equipped with a Leica TCS SP5 positive confocal microscope. The photos and supporting movies were respectively recorded by a Canon EOS 5D MarkII equipped with a Canon EF 100 mm f/2.8 Macro and a Canon EOS 700D equipped with a Canon EF 50 mm f/1.8 STM after the products were irradiated by either 254 or 365 nm ultraviolet light. Synthesis of MOF PM 1. PM 1 was synthesized following a literature procedure with some modifications. A mixture of Cd(NO3)2·4H2O (160 mg, 0.5 mmol), m-BDC (83 mg, 0.5 mmol), BIM (26 mg, 0.2 mmol), CH3COONa (287 mg, 3.5 mmol), and H2O (10 mL) were sealed into a 50 mL Teflon reactor, heated under autogenous pressure at 150 °C for 36 h. After the reaction mixture was slowly cooled to room temperature, colorless crystals were collected by filtration and washed with distilled water and ethanol in turn and then dried at 60 °C for 12 h (yield: 29.5%, based on Cd). Synthesis of Dye-Encapsulated MOF Composites PM 2−6. Dye-encapsulated MOF composites were synthesized in a similar procedure as the PM 1 crystal, except that different dye solutions in distilled water or ethanol with different concentrations were additionally injected into the precursor solution of PM 1. Namely, 200 μL ethanol solutions of 4-methylumbelliferone (BMU; 2 g·L−1), 200 μL aqueous solutions of Fluorescent Green B (FGB; 180 mg· L−1), 200 μL aqueous solutions of Rhodamine 123 (Rh 123; 200 mg· L−1), 200 μL aqueous solutions of Rhodamine 6G (Rh 6G; 100 mg· L−1), and 1500 μL aqueous solutions of Rhodamine B (Rh B; 100 mg· L−1) were injected into the precursor solution of PM 1 to yield PM 2, PM 3, PM 4, PM 5, PM 6, respectively. The products were washed thoroughly with distilled water and ethanol for several times to remove residual dyes on the surface of MOFs and dried at 60 °C for 12 h (yield: 26.5% for PM 2, 23% for PM 3, 26.1% for PM 4, 25.7% for PM 5, 25.8% for PM 6, based on Cd). Preparation of Ink Pads for the Anticounterfeiting Stamp. A mixture of 500 mg of PVP (polyvinylpyrrolidone, Mw = 58 000), 2 mL of Baifb skin care glycerin, and 2 mL of ethanol were sealed in a 10 mL glass vial and preserved after the ultrasonic treatment for 15 min. Subsequently, a colored ink pad was prepared by mixing 150 mg of ground PM 6 and 0.5 mL of the preservation solutions described above under ultrasonic treatment for 15 min.
Metal−organic frameworks (MOFs) are crystalline materials self-assembled from metal ions or clusters with organic ligands (linkers) to form a highly cross-linked structure with chemically adjustable porosities and a high internal surface area.22−24 Because they possess both the stable structure of inorganic crystals and the easy tailoring of organic materials, luminescent MOFs (LMOFs) are getting much more attention in recent years and find their potential applications in sensing, laser, bioimaging, photocatalysis, and so on.25−36 Recently, Qian et al. developed some fluorescence-color-tunable LMOFs based on a dye-encapsulated strategy for applications in microlasers, white light-emitting diodes, and temperature sensing.37−41 In addition, several LMOFs with secondtimescale RTP have also been successively reported and are thus suggested for applications in temperature/pH sensing.42−47 Unfortunately, these phosphorescent LMOFs mostly show the short-wavelength (ca. 520 nm) green RTP. Although the long-wavelength phosphorescence (650 nm) can be realized by introducing pyridine guest molecules into MOFs as demonstrated by Yan’s group, the crystal structure of MOFs is inevitably destroyed or distorted with the incorporation, and the phosphorescence is also unstable due to the volatilization of guest molecules when the temperature is above 50 °C.42 Additionally, a few red (near 615 nm) phosphorescent MOFs doped or filled with lanthanide Tb3+ ions or organometallic complexes have been reported, but they more or less have problems of low quantum efficiency, short lifetime, and poor phosphorescence stability and cannot meet the requirements for practical applications.48−50 To expand the application of such types of luminescent materials to other fields such as biological imaging and advanced security systems, longwavelength long-lived luminescent materials also need to be discovered. In this work, we successfully synthesized a series of colortunable and structurally stable LMOFs with second-timescale long-lived luminescence in the wavelength regions ranging from green, yellow, orange, to red. Multicolor long-lived luminescence was achieved by efficient RTP energy transfer from the host (phosphorescent MOFs) to the guest (dyes) in a dye-encapsulated MOF system. The strategy of RTP energy transfer shows a great potential to obtain enormous novel longlived luminescent materials by the different combinations of phosphorescent MOFs and dyes in the reported materials library. Also, we attempted to produce an anticounterfeiting stamp by using the novel dye-encapsulated MOFs with longwavelength long-lived luminescence. As the stamp has both short-lived fluorescence and long-lived luminescence (i.e., RTP from MOFs and delayed fluorescence from the encapsulated dyes sensitized by the triplet state of the ligands) with different and unduplicated spectral fingerprints, multiplex modes of anticounterfeiting can be readily achieved.
3. RESULTS AND DISCUSSION 3.1. Synthesis and Structure. Micrometer-sized crystals of Cd-based MOFs, Cd(m-BDC)(BIM) (hereafter referred to phosphorescent MOF PM 1), were synthesized via a solvothermal reaction following a literature procedure with some modifications.47 A mixture of Cd(NO3)2·4H2O, 1,3benzenedicarboxylic acid (m-BDC), benzimidazole (BIM), CH3COONa, and H2O were sealed into a 50 mL Teflon reactor and heated under autogenous pressure at 150 °C for 36 h. After the reaction mixture was slowly cooled to room temperature (RT), colorless crystals were collected by filtration and washed with distilled water and ethanol in turn and then dried at 60 °C for 12 h. Inspired by the one-pot synthesis strategy of guest molecules encapsulated in ZIF-8 crystals, such as caffeine,51 fluorescein,52 porphyrin,53 doxorubicin,54 and coronene,43 the thermally and chemically stable PM 1 emitting green RTP was selected as the host to encapsulate organic fluorescent dyes. Dye-encapsulated PM 1 composites were
2. EXPERIMENTAL SECTION Materials. All chemical reagents and dyes for the syntheses in this work were purchased from commercial sources and used without further purification. Characterization. Powder X-ray diffraction (PXRD) patterns of all composites were recorded with a Bruker AXS Advance diffractometer using Cu Kα radiation (λ = 1.5418 Å). Measurements were made in the 2θ range of 5−60° at room temperature with steps of 0.0163° (2θ) and a counting time of 0.1 s/step. The operating power conditions were 40 kV and 40 mA. Thermogravimetric analysis (TGA) experiments were carried out on a TG209F1 thermal analyzer under air atmosphere from room temperature to 800 °C with a 1803
DOI: 10.1021/acsami.7b13486 ACS Appl. Mater. Interfaces 2018, 10, 1802−1809
Research Article
ACS Applied Materials & Interfaces synthesized in a similar procedure as that for the PM 1 crystals (see Experimental Section2). The resulting crystals of dyeencapsulated PM 1 composites (hereafter referred to PM 2, PM 3, PM 4, PM 5, PM 6) were washed with distilled water and ethanol in turn to remove the excess dyes adsorbed on the MOF surface. To confirm the successful encapsulation of dyes into MOFs, typical PM 1 and PM 6 were chosen to be characterized by PXRD, TGA, and laser scanning confocal microscopy (LSCM). PXRD patterns of these dye-encapsulated MOFs are nearly identical to that of PM 1 (Figure S1), indicating that the one-pot synthesis process does not destroy the crystal structure. TGA results show that no substantial weight loss originates from the sublimation of Rhodamine B (Rh B) in PM 6 at about 290 °C (the temperature for Rh B starting to sublime), which implies that dye molecules are tightly restricted by the MOF matrix (Figure S2). A comparison of an Rh B aqueous solution and PM 6 dispersed in aqueous solution before and after centrifugation was conducted to verify the integration of dyes and MOFs. The simultaneous sedimentation of the dye Rh B and MOFs after centrifugation indicates that Rh B is tightly bonded to the MOFs (Figure 1a).
Scheme 1. Schematic Diagram for the Photophysical Processes in Dye-Encapsulated MOFsa
A = absorption; F = fluorescence; P = phosphorescence; DF = delayed fluorescence; ISC = intersystem crossing; ET = energy transfer.
a
of incident radiation by the ligands in the host matrix MOFs, possible energy transfer (blue and green ET) from MOFs to guest dyes may take place in two routes: (1) S1 → S0 (MOFs) to S′0 → S′n (dye) and (2) T1 → S0 (MOFs) to S′0 → S′1 (dye). Therefore, successful encapsulation of dyes into MOFs may result in the quenching of the original fluorescence (blue F) and phosphorescence (green P) emissions from the ligand and the appearance of additional fluorescence (red F) and delayed fluorescence (red DF) emissions from the dye. On the basis of this strategy, luminescent colors and relative intensities can be flexibly tuned by controlling the species and concentrations of dyes. As shown in Figure 2a, the synthesized PM 1−6 powders show a variety of colors under natural light, which suggests different absorptions in the visible region associated with the dye incorporation. Specifically, the UV−vis diffuse reflection spectra of PM 1−6 (Figure S3) evidence that the PM 1−6 powders have absorption bands in the visible region (>400 nm). The changes in these spectra also indicate the formation of host−guest systems between MOFs and dyes. The free m-BDC ligand emits the blue fluorescence centered at 394 nm under UV excitation (Figure S4) and immediately turns into green phosphorescence at 510 nm after the removal of UV excitation (Figure S5). After the ligands coordinated with metal Cd clusters to form PM 1, the fluorescence peak shifts to 401 nm upon excitation at 365 nm (Figure S6). Interestingly, after the removal of UV excitation, PM 1 emits bright green phosphorescence centered at 520 nm (Figure S7), which should be attributed to the radiative transition from the triplet state T1 to S0 of m-BDC ligands.55 The phosphorescence duration could be perceived for approximately 10 s by the naked eye (see the Movie S1). The above results are similar to those reported in the literature.47 With the integration of dyes into MOFs, the fluorescence color in PM 2−6 varies from blue to green, yellow, orange, and red under UV irradiation (Figure 2a, middle). As shown in the fluorescence spectra (Figure 2b, white solid line and pink solid line), the spectra of PM 2−6 show double-band characteristics: one peak from the original MOFs at approximately 400 nm and another from the dyes at longer wavelengths, respectively. The emission maximum for the long-wavelength fluorescence reveals an obvious guest-dependent feature covering a wide range from 410 nm for PM 2 to 630 nm for PM 6. Accordingly, the fluorescence of PM 2−6 shifts from blue to red in the Commission Internationale deL’ Eclairage (CIE) chromaticity diagram (Figure 2c), which proves the superiority of dye-encapsulated MOFs as luminophores with different
Figure 1. (a) Photographs of Rh B aqueous solution and PM 6 dispersed in aqueous solution before and after centrifugation. The colorless suspension solution of PM 6 after centrifugation (upper right) confirms the successful conjunction of MOFs and Rh B. (b) Confocal image of PM 6 at different depths recorded on a Leica TCS SP5 laser scanning confocal microscope (λex = 561 nm).
LSCM displays a uniform dye distribution in a single crystal, suggesting that the dye Rh B molecules are dispersed throughout the MOFs and not adsorbed onto the outside surface (Figure 1b). 3.2. Optical Properties and Tuning of Emission Colors. A schematic diagram for the illustration of photophysical processes in dye-encapsulated MOFs is shown in Scheme 1. The incident radiation is absorbed (purple arrow A in Scheme 1) by typically conjugated organic compounds called linkers or ligands. The fluorescence emission in the MOFs (blue F) is subsequently observed due to the electron radiative transition from the excited singlet state S1 to the ground singlet state S0 of ligands. Also, intersystem crossing (ISC) between the S1 and T1 states of the ligand occurs. Afterward, the phosphorescence emission in the MOFs (green P) occurs owing to the radiative transition from the triplet state T1 to S0 of ligands, and this emission usually has a lifetime in the range of several microseconds to seconds. For the host− guest system of dye-encapsulated MOFs, after the absorption 1804
DOI: 10.1021/acsami.7b13486 ACS Appl. Mater. Interfaces 2018, 10, 1802−1809
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ACS Applied Materials & Interfaces
Figure 2. (a) Photographs of PM 1−6 with a black plastic board as background under natural light (left), UV excitation on (middle), and UV excitation off (right). Exposure time was 1/2000 s for PM 1−6 (left), 1/10 s for PM 1, 1/100 s for PM 2, 1/50 s for PM 3, 1/60 s for PM 4 and PM 5, 1/125 s for PM 6 (middle), and 1/4 s for PM 1−6 (right), respectively. Other parameters were manually kept constant: ISO value = 3200, aperture value = 3.5, and white balance = sunlight. (b) Fluorescence emission spectra of different dye species (pink solid line), fluorescence (white solid line), and long-lived luminescence emission spectra (colored solid line) as well as (c) corresponding CIE chromaticity coordinates of PM 1−6 with different dye species excited at 365 nm under ambient conditions.
Figure 3. (a) Phosphorescence emission spectrum (green solid line) of PM 1 and the UV−vis absorption spectrum of Rh B in aqueous solution (red solid line). (b) The long-lived luminescence emission spectra and (c) the CIE chromaticity coordinates for PM 6 with different dye concentrations excited at 365 nm under ambient conditions. (d) Time-resolved emission decay curves for PM 1 and PM 6 monitored at 520 and 630 nm under ambient conditions.
fluorescence, the long-lived delayed fluorescence of the dye may be achieved by the efficient MOF-to-dye phosphorescence energy transfer in dye-encapsulated MOFs. Compared with PM 1 itself, PM 2−6 possess a higher photoluminescence quantum yield (Table S1). The fluorescence and long-lived luminescence quantum yields for PM 6 are estimated as 18.99 and 12.35%, which are higher than those of PM 1 (with quantum yields of 2.38% for fluorescence and 5.1% for phosphorescence). The increase in quantum yields can be
emissions by modulating dye species. Charmingly, after the removal of UV excitation, PM 3−6 exhibit MOF−dye dual long-lived luminescence emissions that last for a few seconds as traced by the naked eye and show yellowish green, yellow, orange, and red delayed fluorescence from the encapsulated dyes (Figure 2a,b, and Movie S1). These changes of the photoluminescence spectra are also indicative of the successful encapsulation of dyes into PM 1. As PM 1 itself shows a green phosphorescence emission while the dye only emits short-lived 1805
DOI: 10.1021/acsami.7b13486 ACS Appl. Mater. Interfaces 2018, 10, 1802−1809
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ACS Applied Materials & Interfaces
Figure 4. (a) Photographs of the anticounterfeiting stamp under natural light (left), UV excitation on (middle), and UV excitation off (right). Exposure time was 1/400 s for the left, 1/100 s for the middle, and 1/4 s for the right, respectively. Other parameters were manually kept constant: ISO value = 3200, aperture value = 2.8, and white balance = sunlight. (b) Fluorescence and (c) long-lived luminescence emission spectra. (d) Time-resolved emission decay curves monitored at 520 and 630 nm under ambient conditions.
in PM 6. The decay time of the MOFs themselves mainly depends on the phosphorescence energy transfer from the MOFs to Rh B, which is shortened in MOFs but elongated in Rh B. 3.3. Application to the Anticounterfeiting Stamp. Encouraged by the excellent long-lived luminescent performance and ultra high photostability of PM 2−6 (Figure S12), an anticounterfeiting ink pad was fabricated by mixing PM 6, polyvinylpyrrolidone (PVP), ethanol, and glycerol at an appropriate mass ratio (see Experimental Section2). Figure 4a presents the pattern (the logo of College of Materials, Xiamen University) sealed by using the homemade ink pad, and the high-quality pink pattern can be easily identified after the rapid evaporation of alcohol under natural light. In addition, when excited at 365 nm, the stamp shows the red fluorescence emission on a printing paper exhibiting the blue fluorescence background. As expected, it immediately turns into the dark orange long-lived luminescence emission after the removal of UV excitation, which can be perceived within several seconds by the naked eye (Figure 4a; Movie S2). The fluorescence spectrum also displays a strong MOF−dye dual fluorescence emission with a peak value intensity ratio of 7.11 (Ired/Iblue), and the calculated chromaticity coordinates are (0.57, 0.32) (Figure 4b). Similarly, in the long-lived luminescence spectrum, the strong MOF−dye dual long-lived luminescence emission (Ired/Igreen = 9.48) is also observed, with the CIE values being (0.58, 0.36) (Figure 4c). Also, in comparison with PM 6 itself, the emission of the ink pad is subjected to a slight blueshift, which is possibly due to the lower energy transfer efficiency caused by the partial structure destruction during the grinding for the ink pad preparation. The time-resolved emission decay curves upon excitation at 365 nm show that the lifetime is measured to be 293 and 363 ms when monitored at 520 and 630 nm, respectively (Figure 4d). PXRD patterns also confirm the stability of PM 6 crystallinity in the stamp forming solution for 5 months (Figure S13). From these results, we believe that the luminescent properties and crystalline integrity of PM 6 are well maintained in the ink pad.
ascribed to efficient light harvesting of dyes. It is unprecedented that the excellent long-wavelength long-lived luminescence emission can be realized by simply encapsulating dyes into MOFs, thus providing a new and facile strategy for the synthesis of long-wavelength long-lived luminescent MOFs materials. In order to get deeper insights into the energy transfer mechanism in PM 2−6, PM 6 is selected as a typical case for analysis. As shown in Figure S8, the absorption of the dye Rh B and the fluorescence of PM 1 show an obvious spectral overlap; therefore, the energy transfer mentioned above may occur from PM 1 to dyes when the two luminescent species are close to each other. Moreover, although Rh B has much less absorption at 365 nm, the intense emission of Rh B in PM 6 can be observed, which is due to the efficient sensitization of the ligand m-BDC and the isolation effect from the MOF matrix (Figure S9). Additionally, the fluorescence intensity and color of PM 6 can be finely tuned by just changing the dye concentration. Upon increasing the dye concentration from 0.006 to 0.15 mg, the emission intensity of MOFs increases and reaches its maximum at 0.07 mg, and the emission spectra redshift monotonically (Figure S10). Similarly, as shown in Figure 3a, due to the significant spectral overlap between the absorption of Rh B in PM 6 and the phosphorescence of PM 1, the phosphorescence energy transfer would occur from MOFs to Rh B effectively, leading to long-lived red delayed fluorescence assigned to Rh B (Figure S11). The delayed fluorescence color and intensity of PM 6 can be systematically tuned by altering the dye concentration under 365 nm excitation (Figure 3b,c). Upon increasing the dye concentration from 0.006 to 0.15 mg, the phosphorescence emission intensity of MOFs gradually quenches, while the emission intensity of Rh B rises first and then decreases accompanied by the redshift of the emission wavelength due to the partial dye aggregation caused by the excessive concentration. Phosphorescence decay curves upon excitation at 365 nm show that the lifetime monitored at 520 nm sharply decreases from 765 ms for PM 1 to 293 ms for PM 6 (Figure 3d). The changes in the emission lifetime also confirm the presence of the MOF-to-dye phosphorescence energy transfer 1806
DOI: 10.1021/acsami.7b13486 ACS Appl. Mater. Interfaces 2018, 10, 1802−1809
Research Article
ACS Applied Materials & Interfaces Typically, the highly designable attributes make luminescent materials a popular class of advanced anticounterfeiting agents, which give prescribed luminescent features upon exposure to a specific illumination wavelength. Compared to a single fluorescence-based anticounterfeiting, additional security information, such as rare spectra and lifetime features, can be provided by long-lived luminescence, thus making long-lived luminescent anticounterfeiting materials more difficult to be replaced by others. In addition, upconversion- or downconversion-based anticounterfeiting inks can usually be excited by a single wavelength, and they subsequently emit at only a single wavelength too. This anticounterfeiting measure appears to be comparatively less effective in protecting documents. On the other hand, the dye-encapsulated MOF PM 6 anticounterfeiting ink pad has at least two excitations and color emissions (viz. MOFs themselves and dyes, respectively). In particular, there is fluorescence but no phosphorescence emission for the PM 6 anticounterfeiting ink pad under 520 nm excitation (i.e., the excitation only for the dye). Herein such a distinctive and unduplicated spectral fingerprint makes the stamp a highly reliable anticounterfeiting measure that is very tough to break through. Predictably, upgraded levels of anticounterfeiting measures can be achieved if a combination of different-color long-lived luminescent MOFs is used or a single MOF material is encapsulated with a variety of dyes. Furthermore, because of their excellent luminescence properties, the security information can be easily detected by a simple time-resolved charge-coupled device (CCD), which is of great significance for practical applications. Clearly, such a dyeencapsulated MOF platform enables us to readily fabricate high-performance advanced anticounterfeiting materials by modulating the amounts and components of encapsulated dyes. We believe that this simple but facial synthetic strategy provides a new path for the development of advanced anticounterfeiting materials, which would be a new challenge for counterfeiting technology.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (Y.Z.) *E-mail:
[email protected] (L.W.) *E-mail:
[email protected];
[email protected] (R.J.X.) ORCID
Yixi Zhuang: 0000-0001-7290-1033 Rong-Jun Xie: 0000-0002-8387-1316 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51572232, 51561135015, 51502254, 61575182, 61405183) and the National Key Research and Development Program of China (2017YFB0404301).
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REFERENCES
(1) Volodin, B. L.; Kippelen, B.; Meerholz, K.; Javidi, B.; Peyghambarian, N. A Polymeric Optical Pattern-Recognition System for Security Verification. Nature 1996, 383, 58−60. (2) Meruga, J. M.; Cross, W. M.; May, P. S.; Luu, Q.; Crawford, G. A.; Kellar, J. J. Security Printing of Covert Quick Response Codes Using Upconverting Nanoparticle Inks. Nanotechnology 2012, 23, 395201. (3) Kumar, P.; Dwivedi, J.; Gupta, B. K. Highly Luminescent Dual Mode Rare-Earth Nanorod Assisted Multi-stage Excitable Security Ink for Anti-Counterfeiting Applications. J. Mater. Chem. C 2014, 2, 10468−10475. (4) Kumar, P.; Singh, S.; Gupta, B. K. Future Prospects of Luminescent Nanomaterial Based Security Inks: from Synthesis to Anti-Counterfeiting Applications. Nanoscale 2016, 8, 14297−14340. (5) Jiang, Y.; Li, G.; Che, W.; Liu, Y.; Xu, B.; Shan, G.; Zhu, D.; Su, Z.; Bryce, M. R. A Neutral Dinuclear Ir(III) Complex for AntiCounterfeiting and Data Encryption. Chem. Commun. 2017, 53, 3022−3025. (6) Andres, J.; Hersch, R. D.; Moser, J.-E.; Chauvin, A.-S. A New Anti-Counterfeiting Feature Relying on Invisible Luminescent Full Color Images Printed with Lanthanide-Based Inks. Adv. Funct. Mater. 2014, 24, 5029−5036. (7) Kumar, P.; Nagpal, K.; Gupta, B. K. Unclonable Security Codes Designed from Multicolor Luminescent Lanthanide-Doped Y2O3 Nanorods for Anticounterfeiting. ACS Appl. Mater. Interfaces 2017, 9, 14301−14308. (8) Jiang, K.; Zhang, L.; Lu, J. F.; Xu, C. X.; Cai, C. Z.; Lin, H. W. Triple-Mode Emission of Carbon Dots: Applications for Advanced Anti-Counterfeiting. Angew. Chem., Int. Ed. 2016, 55, 7231−7235. (9) Zheng, Y. H.; Jiang, C.; Ng, S. H.; Lu, Y.; Han, F.; Bach, U.; Gooding, J. J. Unclonable Plasmonic Security Labels Achieved by Shadow-Mask-Lithography-Assisted Self-Assembly. Adv. Mater. 2016, 28, 2330−2336. (10) Li, Y.; Gecevicius, M.; Qiu, J. Long Persistent Phosphors-from Fundamentals to Applications. Chem. Soc. Rev. 2016, 45, 2090−2136.
4. CONCLUSION In summary, we successfully synthesized novel dye-encapsulated MOFs that exhibit the long-wavelength (red) secondtimescale long-lived luminescence owing to the efficient MOFto-dye phosphorescence energy transfer. The dye-encapsulated MOFs were applied as anticounterfeiting stamps with multiple spectral features that are superior to others based on a single fluorescence. Additionally, a large number of organic fluorescent dyes with high quantum efficiency, such as cyanine, coumarin, and boron dipyrromethene, can also be encapsulated into various phosphorescent MOFs to systematically tune the long-lived luminescent emission color and quality. Furthermore, the facile approach for preparing dye-encapsulated MOFs enables the design and development of novel longlived luminescent materials as well as an advanced anticounterfeiting system or the possibility of use for potential biological imaging.
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phosphorescence spectra; 1931 CIE chromaticity coordinates, and fluorescence quantum yield data (PDF) Movie S1 shows the fluorescence and long-lived luminescence of PM 1−6 samples with different dye species (MPG) Movie S2 shows the fluorescence and long-lived luminescence of the anticounterfeiting stamp (MPG)
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b13486. Additional XRD and TGA data, UV−vis diffusion reflection and absorbance spectra, fluorescence and 1807
DOI: 10.1021/acsami.7b13486 ACS Appl. Mater. Interfaces 2018, 10, 1802−1809
Research Article
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