Multi-leveled Static-dynamic Anticounterfeiting Based on the Stimuli

Functional Inorganic Materials and Devices

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Multi-leveled Static-dynamic Anticounterfeiting Based on the Stimuli-responsive Luminescence in A Niobate Structure Jika Sang, Jinyu Zhou, Jiachi Zhang, Hui Zhou, Huihui Li, Zhipeng Ci, Shanglong Peng, and Zhaofeng Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03562 • Publication Date (Web): 10 May 2019 Downloaded from http://pubs.acs.org on May 10, 2019

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ACS Applied Materials & Interfaces

Multi-leveled Static-dynamic Anticounterfeiting Based on the Stimuli-responsive Luminescence in A Niobate Structure

Jika Sanga†, Jinyu Zhoua†, Jiachi Zhanga*, Hui Zhoub, Huihui Lia*, Zhipeng Cia, Shanglong Penga, and Zhaofeng Wangb*

a

National & Local Joint Engineering Laboratory for Optical Conversion Materials and Technology, Lanzhou University, Lanzhou 730000, P. R. China b

State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China

*Authors to whom correspondence should be addressed: Dr. Jiachi Zhang, Email: [email protected] (J. Zhang) Dr. Huihui Li, Email: [email protected] (H. Li) Dr. Zhaofeng Wang, Email: [email protected] (Z. Wang)

†

These authors contributed equally to this work.

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Abstract Anticounterfeiting is a highly required technique to protect the product and the consumer rights in modern society. The conventional luminescent anticounterfeiting is based on down-conversion luminescence excited by an ultraviolet light, which is easy to be faked. In this work, we realized six luminescent modes in a niobate-based structure, (LiNbO3: RE3+, RE3+ = Pr3+, Tm3+, Er3+, Yb3+), in which the photo-stimulated luminescence of LiNbO3: Pr3+, persistent luminescence of Li0.99NbO3: Pr3+, and up-conversion luminescence color evolution of LiNbO3: Er3+ were first presented. Based on the above luminescent modes of LiNbO3: RE3+, multi-leveled anticounterfeiting devices were developed. By employing the mechanoluminescence

and

persistent

luminescence,

we

achieved

a

dual-mode

anticounterfeiting which could display the luminescent patterns without any direct irradiation. In addition, another dual-mode anticounterfeiting based on photo-stimulated luminescence and up-conversion luminescence excited by a near infrared light was realized, which could display the anticounterfeiting patterns in both static and dynamic states. To obtain an even higher anticounterfeiting level, the down-conversion luminescence, thermoluminescence, photo-stimulated luminescence and up-conversion luminescence were simultaneously applied in a food trademark. This four-mode anticounterfeiting trademark could not only show a static-dynamic luminescence that is hard to be faked, but also allow consumers to distinguish the food freshness. The presented multi-leveled anticounterfeiting strategies could be employed to resolve the counterfeit issues in various fields.

2


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Keywords: Niobate structure; Rare earth; Stimuli-responsive luminescence; Static-dynamic anticounterfeiting; Multi-leveled anticounterfeiting

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1. Introduction Counterfeiting is an ever-growing global issue involving banknotes, documents, artworks, enterprises and individuals.1-3 During the past decades, a variety of technologies, such as marker, plasmon, magnetism, hologram and luminescence, have been developed for anticounterfeiting.4-6 Among them, the colorful luminescence has been widely adopted because of its advantages of visibility and facile design.7-11 Along with the development and popularization of luminescence science, the traditional mono-mode anticounterfeiting, mainly based on down-conversion luminescence (DCL; a process of converting one high-energy photon to one low-energy photon via an excitation-emission pathway), is far from the requirements for practical applications. Therefore, more complicated anticounterfeiting techniques by increasing the luminescent modes have been developed. For example, Liu et al. utilized both DCL and up-conversion luminescence (UCL; a process of converting two or more low-energy photons to one high-energy photon via an excitation-emission pathway) in NaYF4: Er3+/Tm3+/Yb3+, and achieved a dual-mode luminescent

anticounterfeiting.12

Xu

et

al.

employed

the

DCL,

UCL

and

thermoluminescence (ThL; a type of luminescence activated by heat) in perovskite nanocrystals, and obtained a triple-mode anticounterfeiting.13 In our previous work, mechanoluminescence (ML; a type of luminescence activated by various mechanics) was also introduced, and a dual-responsive anticounterfeiting device activated by both ultraviolet (UV) light and mechanical strain were fabricated.14 It should be noted that luminescence has a variety of modes, including steady, transient and dynamic luminescence. 4


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The present multi-mode luminescence for anticounterfeiting always lacks of dynamic luminescent modes, such as persistent luminescence (PersL; a spontaneous luminescence after removing the irradiation source) and photo-stimulated luminescence (PSL; a type of luminescence originated from de-trapped carriers with assistance of photon energy), which could further relate to the time dimension. One can conclude that by an appropriate combination of steady, transient and dynamic luminescence, the anticounterfeiting level should be maximally improved. Lithium niobate (LiNbO3) is a human-made dielectric material that does not exist in nature.15 It has a trigonal crystal structure with a space group of R3c, and Li+ ions locate at the octahedral sites (LiO6) which could be substituted by rare earth (RE) ions according to the Hume-Rothery rules.16-17 The substituted RE ions have rich energy levels, allowing electrons to be excited to generate luminescence. In addition, because of the charge difference between Li+ and RE3+ ions, the substitution would arouse intrinsic defect states in the structure of LiNbO3, which is supposed to be beneficial to enrich the luminescent modes, e.g., PersL, PSL, ThL and ML. Inspired by the above considerations, in this work, we prepared RE ion doped LiNbO3 materials (LiNbO3: RE3+, RE3+ = Pr3+, Tm3+, Er3+, Yb3+) and investigate their luminescent properties. The results suggest that we have obtained six types of luminescence for LiNbO3: RE3+, including the steady modes (PL and UCL), transient mode (ML), and dynamic modes (ThL, PersL and PSL). Based on the combination of the above luminescent modes, multi-leveled anticounterfeiting devices were designed and fabricated. Particularly, the 5



combination of DCL, UCL, ThL and PSL could not only realize a static-dynamic anticounterfeiting that is hard to be faked, but also endow it discernibility for product freshness, greatly protecting the consumer rights.

2. Experimental 2.1 Preparation of the LiNbO3: RE3+ (RE3+ = Pr3+, Er3+, Tm3+, Yb3+) powders LiNbO3: RE3+ samples were synthesized by the solid-state reaction (SSR). First, stoichiometric raw materials, Li2CO3 (99.98%), Nb2O5 (99.95%), Pr6O11 (99.99%), Er2O3 (99.99%), Tm2O3 (99.99%) and Yb2O3 (99.99%), were mixed homogeneously in an agate mortar. Then, the mixtures were transferred into a corundum crucible and sintered at 900-1200 ˚C for 8 h in air. 2.2 Fabrication of multi-leveled anticounterfeiting devices 2.2.1 Mono-mode anticounterfeiting based on DCL The tricolor DCL materials, LiNbO3: 1%Pr3+ (red), LiNbO3: 1%Tm3+ (blue) and LiNbO3: 1%Er3+ (green), were employed. First, shallow grooves of a rainbow river were made on the surface of a rubber mold (5.7 cm × 4.5 cm) by a photolithography technique. Then, the tricolor LiNbO3-based phosphors were separately mixed with polydimethylsiloxane (PDMS) precursor (SYLGARD 184, base resin and curing agent weight ratio of 10:1) with a weight ratio of 1:1 to form viscous slurries. The slurries were then filled in the shallow grooves of rainbow river image on the surface of the rubber mold. Subsequently, pure PDMS precursor was dropped on all area of the rubber mold. After curing in an oven at 60 6


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ºC for 5 h, the elastic “Rainbow River” film was peeled off carefully from the mold, and the multicolored anticounterfeiting device based on DCL was fabricated. 2.2.2 Dual-mode anticounterfeiting based on ML and PersL A metal school badge of Lanzhou University (5.0 cm × 2.2 cm) was chosen as the mold. The LixNbO3: 1%Pr3+ (x=0.99 and 1) powders were mixed with PDMS precursor separately with a weight ratio of 1:3 to obtain viscous slurries. The Li0.99NbO3: 1%Pr3+@PDMS slurry was filled in the character area of the school badge, and the LiNbO3: 1%Pr3+@PDMS slurry was filled in the rest rectangular area. After curing in an oven at 60 ºC for 5 h, the film was peeled off and the dual-mode anticounterfeiting device based on ML and PersL was fabricated. 2.2.3 Dual-mode anticounterfeiting based on UCL and PSL The epoxy base resin and curing agent with a weight ratio of 3:1 were mixed in a Petri dish. Then, the LiNbO3: 1%Pr3+, 0.1%Er3+ powders were mixed in the above epoxy precursor with a weight ratio of 1:3. After injecting the above LiNbO3: 1%Pr3+, 0.1%Er3+@epoxy precursor into a cylinder-shaped rubber molds (diameter: 4 cm; depth: 3 cm), it was placed in an oven at 60 ºC for 2 h to reach a semi-cured state. Then, another mixture of LiNbO3: 1%Tm3+, 1%Yb3+/1%Pr3+@epxoy precursor with a weight ratio of 1:3 was covered on the semi-cured LiNbO3: 1%Pr3+, 0.1%Er3+@epoxy. Finally, the above bilayered LiNbO3: 1%Pr3+, 0.1%Er3+@ LiNbO3: 1%Tm3+, 1%Yb3+/1%Pr3+@epoxy was cured in an oven at 60 ºC for 10 h, and the dual-mode anticounterfeiting device based on UCL and PSL was obtained. 7



2.2.4 Four-mode anticounterfeiting based on DCL, ThL, UCL and PSL The shallow grooves of the trademark of Lanzhou hand-pulled noodles were made on the surface of a rubber mold (5.7 cm × 4.5 cm) by a photolithography technique. The PDMS base resin and curing agent (weight ratio = 10:1) were mixed in a Petri dish. Then, 0.05 g of LiNbO3: 1% Pr3+, 0.1%Er3+, 0.05 g of LiNbO3: 1%Tm3+, 1%Yb3+, 0.02 g of LiNbO3: 0.1%Er3+ (sintered at 1150 ºC) and 0.02 g of LiNbO3: 3%Er3+ (sintered at 900 ºC) powders were separately mixed with the PDMS precursor in a weight ratio of 1:1 to form slurries. The as-prepared PDMS slurries were then filled in the shallow grooves of the trademark. Subsequently, pure PDMS precursor was dropped on all area of the rubber mold. After the mold was put into an oven at 60 ºC for 12 h, the elastic trademark film was peeled off carefully from the mold, and the four-mode anticounterfeiting device based on DCL, ThL, UCL and PSL was fabricated. 2.3 Toxicity tests The in vitro toxicity tests of the as-prepared samples were conducted by 2,3-(4,5-Dimethyl-2-thiazolyl)-2,5-diphenyl Tetrazolium Bromide (MTT) assay using rat intestinal epithelial cells (IEC-6). First, IEC-6 cells were seeded in 24-well plates at a density of 4×104 cells/well, and incubated overnight. Then, the sterilized samples (PDMS, LiNiO3: Tm3+, LiNiO3: Er3+, LiNiO3: Pr3+, and LiNiO3: Tm3+, Yb3+) were added into the corresponding wells. The size of PDMS was 3 mm×3 mm×1 mm in each well, and the concentration of niobate-based powders was controlled to 5 mg/ml. After cultured for 24 h, 100 l of MTT reagent (5 mg/ml) was added into each well, which were further incubated 8


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at 37℃ for 4 h. Then, dimethyl sulfoxide (DMSO) was employed to solubilize the formazan crystals. By use of a microplate reader (Bio-Rad, Philadelphia, PA, USA), the optical density at a wavelength of 490 nm was obtained, from which the cell viability was calculated. 2.4 Characterizations The crystallinity and phase purity of the samples were determined by X-ray diffractometer (XRD, D2 PHASER) with Ni-filtered Cu Kα radiation (λ = 1.54184 Å), operating at 15 mA and 30 kV with a scanning speed of 15° min−1 for phase determination and a step scanning rate of 2° min−1 for Rietveld refinement. The crystal structure was drawn by a software Diamond (Version 3), and the Rietveld refinement was performed by employing the General Structure Analysis System (GSAS) software. The valence state of RE ions were identified by X-ray photoelectron spectroscopy (XPS) on a PHI-5702 instrument, and the experimental data was fitted by a software of XPS peak 4.1. The size and morphology of the as-prepared samples were measured by scanning electron microscopy (SEM, Hitachi S-4800, Japan). The absorption spectra were measured by a UV-visible (UV-vis) spectrophotometer (lambda 950, PerkinElmer Inc.) using BaSO4 as a reference. The DCL and PersL spectra were obtained from an FLS-920T fluorescence spectrophotometer (Edinburgh Instruments) equipped with a Xe 900 arc lamp (450W). The PSL and UCL spectra were measured with the above spectrophotometer equipped with an external 980 nm semiconductor laser (Changchun New Industries Optoelectronics Tech. Co., Ltd.; power density: 1.5 W/cm2) as the excitation source. The ThL spectra measurements were 9



operated using the FLS-920T fluorescence spectrophotometer equipped with a cartridge heater, and the temperature was controlled by a standard TAP-02 high temperature controller. For the measurements of PersL, PSL and ThL spectra, the samples should be pre-irradiated by a UV lamp (254 nm) for 10 min. The curves of ThL intensity vs. temperature was measured by a FJ-417A TL meter (Beijing Nuclear Instrument Factory, Beijing, China) at a heating rate of 1 K/s from 298 K to 673 K.

3. Results and discussion LiNbO3: RE3+ (RE3+ = Pr3+, Tm3+, Er3+, Yb3+) powders were synthesized by SSR, which were composed of particles with a size around 500 nm (Figure S1). Figure 1a shows the XRD patterns of the as-prepared samples, confirming that they were of single phase. According to the Rietveld structure refinements (Figure S2), LiNbO3 belongs to a trigonal crystal system with a space group of R3c, lacking of inversion symmetry (as shown in the inset of Figure S2).15 Figure 1b shows the high resolution XPS profiles of the doped RE3+ ions (RE3+ = Pr3+, Tm3+, Er3+, Yb3+), confirming that RE ions have been successfully doped in LiNbO3 in trivalent state.18-19 The doped RE3+ ions are suggested to substitute Li+ ions at octahedral sites (LiO6, top inset of Figure 1a) according to the Hume-Rothery rules, which would further create two negative cation vacancies [VLi'] for charge balance (as shown in the bottom inset of Figure 1a).16-17 The defects created in the LiNbO3 structure could act as traps, contributing to the PersL, PSL, ThL and ML performance.8 By applying the refined data, it is calculated in Figure S3 that the material shows a direct band gap of 3.396 eV at 10


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the G point of Brillouin zone (in the left side of Figure S3).20 The calculated partial density of states (PDOS) predominantly originate from the O 2p orbitals and d/s levels of cations, and thus the levels of intrinsic defects should be near the bands acting as proper traps.21 When excited by a 365 nm light, the Tm3+, Er3+ or Pr3+ mono-doped LiNbO3 shows intense DCL with the color of blue, green and red, respectively, as shown in Figure 1c and Figure S4. In contrast to the delayed emission, the above tricolor DCL possesses microsecond lifetime (Figure S4) and would disappear immediately after the stoppage of excitation. Based on the DCL features of Tm3+, Er3+ or Pr3+ mono-doped LiNbO3, a multicolor luminescent image of “rainbow river” was drawn on a PDMS substrate by using a phosphor/PDMS slurry (Figure 1d), which represented the most basic anticounterfeiting mode in the present market.

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Figure 1. (a) XRD patterns and (b) XPS curves of LiNbO3: RE3+ (RE3+ = Pr3+, Tm3+, Er3+, Yb3+) powders, the inset of (a) illustrates the intrinsic polyhedron structure; (c) DCL spectra and luminescent photos of Tm3+, Er3+ or Pr3+ mono-doped LiNbO3; (d) the fabricated DCL-based “rainbow river” image drawn on a PDMS substrate, representing the basic anticounterfeiting mode in the present market.

In addition to DCL, the RE-doped LiNbO3 could exhibit many other types of luminescence. For LiNbO3: Pr3+, ML, PSL and ThL were also obtained, as shown in Figure 2a and 2b. When the chemical composition of LiNbO3: Pr3+ was adjusted to Li0.99NbO3: Pr3+, PersL could further appear with the optimal pre-excitation wavelength of 290 nm (as shown in Figure 2c). Similar with the PersL in Li0.99NbO3: Pr3+, the intensity of ML, PSL and ThL in LiNbO3: Pr3+ shows gradual degradation when experiencing continuous stimuli of the mechanical cycle, 980 nm irradiation or heating (Figure 2a and the inset of Figure 12


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2b). Such luminescent intensity degradation could be totally recovered by further irradiating the LiNbO3: Pr3+ sample under a 254 nm UV light. The above phenomenon suggests that the ML, PSL, ThL and PersL in LiNbO3: Pr3+ and Li0.99NbO3: Pr3+ should be all originated from the consumption of the trapped carriers in structure.22 To determine the trap information, the curves in terms of the ThL intensity vs. operating temperature in LiNbO3: Pr3+ and Li0.99NbO3: Pr3+ were measured, as shown in Figure 2d and 2e, respectively. The results confirm that whatever in LiNbO3: Pr3+ or Li0.99NbO3: Pr3+, there are two types of traps in the structure, i.e., shallow traps and deep traps. It has been widely accepted that PersL should be attributed to the spontaneous release of the carriers from shallow traps to conduction band (the inset of Figure 2e).23 However, no PersL was observed for LiNbO3: Pr3+, indicating that the electron transfer channels from the shallow traps to conduction band in LiNbO3: Pr3+ were prevented (the inset of Figure 2d). As a result, the luminescent mechanisms of ML, PSL and ThL in LiNbO3: Pr3+ could be concluded as follows: (i) under mechanical/photon/heat stimuli, carriers are released from traps with energy directly transferred to the luminescent centers; (ii) after accepting energy, the luminescent centers could produce the observed ML/PSL/ThL via electron-hole recombination. Based on the ML of LiNbO3: Pr3+ and PersL of Li0.99NbO3: Pr3+, a dual-mode anticounfeiting device with a relatively higher level was fabricated (Figure 2f). The Li0.99NbO3: Pr3+ with PersL and LiNbO3: Pr3+ with ML were filled in the character area and the rest rectangular area, respectively. Under daylight or the continuous irradiation of 254 13



and 365 nm, the implanted information is invisible. However, after the irradiation stopped, the PersL of Li0.99NbO3: Pr3+ appeared with displaying the anticounfeiting information of the Chinese characters of “Lanzhou University”. The above PersL induced patterns could last more than 20 s, which is long enough to observe/determine the embedded anticounterfeiting information. In addition, there is red luminescence induced by rubbing in the rest area of the device, endowing the device another anticounfeiting mode.

Figure 2. (a) ML intensity of LiNbO3: Pr3+ vs. compression cycles with and without the assistance of UV irradiation; the inset shows the corresponding compression induced ML photo; (b) PSL and ThL spectra of LiNbO3: Pr3+; the inset exhibits the corresponding intensity variation on dependence of time; (c) PersL spectrum of Li0.99NbO3: Pr3+; the insets show the corresponding PersL photos and PersL excitation spectrum; ThL intensity vs. operating temperature in (d) LiNbO3: Pr3+ and (e) Li0.99NbO3: Pr3+; the insets in (d) and (e) are the proposed luminescent channels in LiNbO3: Pr3+ and Li0.99NbO3: Pr3+, respectively; (f) optical and luminescent photos of the as-fabricated anticounterfeiting school badge.

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When Er3+ ions or Tm3+/Yb3+ ion pairs were doped, the niobate structure could further exhibit characteristic UCL under the excitation of 980 nm. The UCL intensity depending on the power of the near infrared (NIR) laser in Figure S5 demonstrates that the UCL of Er3+ and Tm3+ in LiNbO3 could be attributed to the two-photon and three-photon processes, respectively.24-25 The UCL of Tm3+ doped in LiNbO3 can only be observed after co-doping Yb3+, suggesting that the UCL mechanism of Tm3+ is associated with the energy transfer process from Yb3+ to Tm3+.24 The corresponding UCL mechanisms of LiNbO3: Er3+ and LiNbO3: Tm3+, Yb3+ are illustrated in Figure S6. Figure 3a exhibits the UCL spectra of LiNbO3: 0.1%Er3+ samples synthesized at different temperatures. It indicates that with the increase of the reaction temperature, the UCL ratio of red to green (Ired/Igreen) of LiNbO3: 0.1%Er3+ gradually decreases, and thus the entire UCL color gradually changes from yellow to green (in the left side of Figure 3b). In the UCL of LiNbO3: Er3+, there is a cross-relaxation: 4F7/2 + 4I11/2 → 4F9/2 + 4F9/2, which is beneficial to the red emission (as shown in Figure S7a).26-27 Therefore, the decrease of Ired/Igreen ratio for LiNbO3: 0.1%Er3+ by increasing the reaction temperature should be attributed to the decrease of the above cross-relaxation rate. To provide evidence for this explanation, we measured the Raman spectra of LiNbO3: 0.1%Er3+ synthesized at 950 and 1200 oC (Figure S7b). To fill the energy difference between 4F7/2↔4F9/2 and 4I11/2↔4F9/2 (ΔE≈240 cm-1), phonons with a matched energy should participate in the cross-relaxation to enhance the red emission. From the Raman spectra, it is obvious that for the sample synthesized at 950 oC, there is a high density of phonons with energy of 239.85 cm-1, which 15



matches well with the energy difference (ΔE≈240 cm-1). It suggests that the phonons with energy of 239.85 cm-1 could effectively assist the cross-relaxation, and in this case LiNbO3: 0.1%Er3+ could exhibit UCL with a relatively higher Ired/Igreen ratio. For the sample synthesized at 1200 oC, the density of phonons with energy around 239.85 cm-1 is greatly reduced. In this case, the cross-relaxation rate should be decreased, and LiNbO3: 0.1%Er3+ could exhibit UCL with a relatively lower Ired/Igreen ratio. Likewise, when the reaction temperature is fixed, the UCL properties of LiNbO3: Er3+ could also be affected by the doping concentration of Er3+. With increasing the content of Er3+ from 0.1% to 3%, LiNbO3: Er3+ shows an increased Ired/Igreen ratio, resulting in the UCL color changes from green to yellow (Figure S8 and the right side of Figure 3b). The luminescent investigations regarding to the LiNbO3: RE3+ (RE3+ = Pr3+, Tm3+, Er3+, Yb3+) demonstrate the existed six luminescent modes, i.e., DCL, ML, ThL, PSL, PersL and UCL. Since PSL and UCL were both activated by a 980 nm NIR irradiation, another dual mode anticounterfeiting device with static-dynamic luminescence was fabricated as shown in Figure 3e. Here, LiNbO3: Pr3+ and LiNbO3: Tm3+, Yb3+ were introduced in the top layer of epoxy resin, while LiNbO3: Pr3+, Er3+ powders were composited in the bottom layer. By employing the 980 nm irradiation, both of the top layer and the bottom layer of the fabricated device could exhibit intense blue and green UCL, respectively. When the device was charged by 254 nm for 10 min first and then irradiated by a 980 nm light, PSL of LiNbO3: Pr3+ could appear with time-dependent intensity variation. As a result, in this case, the top layer of the anticounterfeiting device could 16


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exhibit a dynamic emitting color variation from light red to blue, while the bottom layer could display a varied luminescent color from wine red to green. Such strategy realizes a much higher anticounterfeiting level with static-dynamic luminescence by simultaneously utilize the UCL and PSL modes.

Figure 3. (a) UCL spectra of LiNbO3: 0.1%Er3+ samples synthesized at various reaction temperature; (b) Ired/Igreen variation in the UCL of LiNbO3: Er3+ dependent on the reaction temperature and doping concentration; (c) UCL spectra of LiNbO3: 1.5%Tm3+and LiNbO3: 1.5%Tm3+, 2.5%Yb3+; (d) Static-dynamic luminescence evolution of the fabricated anticounterfeiting device based on UCL and PSL.

The aforementioned anticounterfeiting strategies in Figure 1, 2 and 3 are of mono-/dual-modes. We further developed a four-mode anticounterfeiting device which is hard to be faked (Figure 4). The four-mode anticounterfeiting was applied to protect the trademark of “Lanzhou hand-pulled noodles” by using the DCL, ThL and PSL of LiNbO3: Pr3+ as well as the UCL of LiNbO3: Er3+ and LiNbO3: Tm3+, Yb3+, as shown in Figure 4a. 17



The as-fabricated four-mode anticounterfeiting film shows a uniform thickness of ca. 0.59 mm, which possesses good flexibility and waterproof ability (Figure 4b). The anticounterfeiting patterns of the trademark could be fully displayed when irradiated by a 365 or 980 nm light, in which the emitting color shows an obvious difference (Figure 4c-4e). When experienced by a heat treatment, partial of the patterns could also be exhibited (Figure 4f). It is noted that when irradiated by a 980 nm light, PSL of LiNbO3: Pr3+ could also appear in the trademark, and therefore the luminescent patterns show a gradual color evolution with increasing the irradiation time, as shown in the top insets of Figure 4g. Since the PSL has been validated to come from the release of the trapped carriers in niobate structure which would be consumed spontaneously (as shown in Figure S9),28 the above dynamic color evolution based on PSL would lose its efficacy when the sample was placed for more than 10 h. In this case, when the anticounterfeiting trademark was excited by a 980 nm light, it could only exhibit UCL without the functionality of dynamic color change (a series of photos are presented in the bottom of Figure 4d). As a result, the above luminescent design could not only realize a four-mode static-dynamic anticounterfeiting for the trademark of “Lanzhou hand-pulled noodles”, but also endow it discernability for food freshness, which could greatly protect the consumer rights.

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Figure 4. (a) Design strategy of the four-mode anticounterfeiting film applied for a trademark of “Lanzhou hand-pulled noodles”; (b) cross-section SEM micrograph, flexibility and waterproof ability of the as-fabricated film; optical photographs of the as-fabricated four-mode anticounterfeiting film under (c) daylight, (d) 365 nm excitation, (e) 980 nm excitation, and (f) heat treatment; (g) luminescent color evolution of the four-mode anticounterfeiting film before or after been placed for more than 10 h to determine the product freshness.

Because the above developed multi-leveled anticounterfeiting devices would be used in daily consumer products, we further conducted the toxicity tests of the as-prepared samples, i.e., PDMS, LiNiO3: Tm3+, LiNiO3: Er3+, LiNiO3: Pr3+, and LiNiO3: Tm3+, Yb3+, as shown in Figure S10. The results showed that there was no significant difference in cell viability between the experimental groups and the control group. It suggests that the synthesized materials in this work have nearly no cytotoxicity, and therefore, it is safe to use. 19



Conclusions In summary, the luminescent properties of LiNbO3: RE3+ (RE3+ = Pr3+, Tm3+, Er3+, Yb3+) were investigated, and six luminescent modes including DCL, ML, ThL, PSL, PersL and UCL have been demonstrated. By utilizing the ML of LiNbO3: Pr3+ and PersL of Li0.99NbO3: Pr3+, a dual-mode anticounterfeiting device with higher level was fabricated, which could display luminescence without any direct light irradiation. Another dual-mode anticounterfeiting based on PSL and UCL was also developed, which could exhibit a static-dynamic luminescence under the irradiation of 980 nm. Furthermore, to achieve a more complicated anticounterfeiting, four mode of luminescence, i.e., DCL, ThL, PSL and UCL, were applied together in a trademark of “Lanzhou hand-pulled noodles”. The as-fabricated four-mode anticounterfeiting trademark could not only show a static-dynamic luminescence that is hard to be faked, but also allow consumers to distinguish the freshness of the product, greatly protecting the consumer rights.

Acknowledgements Z. Wang would like to thank the support from CAS Pioneer Hundred Talents Program. J. Zhang and H. Li appreciate the support from the National Natural Science Foundation of China (10904057 and 51402139), the Science and Technology Projects of Gansu Province (18JR3RA270). The authors would like to thank Prof. D. Xue for the kind discussion.

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Supporting Information. SEM image (Figure S1), Rietveld structure refinements, crystal structure, calculated band gap, Brillouin zone, PDOS and the experimental optical gap of LiNbO3 material (Figure S2, S3); DCL lifetime curves and CIE color coordinates of the mono-doped LiNbO3 phosphors (Figure S4); UCL intensity variations dependent on the NIR laser powers (Figure S5); Proposed UCL mechanism (Figure S6); Raman spectra of LiNbO3: 0.1%Er3+ samples (Figure S7); UCL spectra of LiNbO3: 0.1%Er3+ samples with various Er3+ doping concentration (Figure S8); ThL intensity vs. operating temperature curves of LiNbO3: Pr3+ after placing for various time (Figure S9); Cell viability test (Figure S10).

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Multi-leveled Static-dynamic Anticounterfeiting Based on the Stimuli

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