The effect of lithium chloride on the attenuation of cognitive impairment in experimental hypoglycemic rats

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Cite This: ACS Appl. Mater. Interfaces 2019, 11, 35294−35304

Dual-Mode, Color-Tunable, Lanthanide-Doped Core−Shell Nanoarchitectures for Anti-Counterfeiting Inks and Latent Fingerprint Recognition Jun Xu, Beibei Zhang, Lei Jia,* Yanping Fan,* Rujie Chen, Tinghui Zhu, and BaoZhong Liu College of Chemistry and Chemical Engineering, Henan Polytechnic University, Jiaozuo, Henan 454000, China

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ABSTRACT: With the rapid development of information in modern society, the research and development of advanced anti-counterfeiting technology is becoming more and more important to protect the security and comprehensiveness of information. Therefore, fluorescent ink as an anti-counterfeiting technology and fingerprint recognition technology as a ″human information identification card″ has attracted the attention of many research groups. Herein, dual-mode (upconversion and downconversion) lanthanide-doped luminescent nanoarchitectures were developed using Y2O3:Er3+,Yb3+ nanoparticles as a core and layered lanthanide hydroxides nanomaterials as a shell. Under the irradiation of 980 nm near-infrared light, the nanoarchitectures emitted a bright upconverted red light emission. Meanwhile, under the irradiation of 254 nm UV light, the nanoarchitectures can directly emit multicolor luminescence (from green to yellow-green, yellow, orange, and red) by changing the suitable ratios of Tb3+/ Eu3+ ions. The information can only be extracted when the irradiation of two kinds of excitation light sources existed at the same time, which can improve the difficulty of illegal imitation and enhance the level of anti-counterfeiting. Furthermore, these luminescent nanoarchitectures were investigated for visual latent fingerprint recognition on various substrates with high definition, high sensitivity, and high antiinterference. These results indicated that the nanoarchitectures reported in this study may have great application prospects in information security and identity recognition. KEYWORDS: dual-mode luminescence, layered lanthanide hydroxide, multicolor fluorescence, anti-counterfeiting, latent fingerprint fluorescence intensities, long and stable fluorescence lifetimes, and have been favored by many scientific researchers. In particular, lanthanide ions not only show the traditional Stökes type (that is, downconversion (DC)) emission but also have efficient anti- Stökes type (that is, upconversion (UC)) emission.11,12 The principle of UC emission is that the materials absorb low-energy photons and emit high-energy photons, while the principle of DC emission is just the opposite of UC. The UC lanthanide ion pairs are always Yb3+/Er3+ and Yb3+/Tm3+ ions, which usually display red, green, or blue colors under the irradiation of 980 nm.13 Recently, Liu et al. successfully fabricated the dual-mode luminescent NaYF4:Yb,Tm@SiO2/ carbon dot nanocomposites, which displayed good properties.14 The well-known DC materials, such as Tb3+ (green)- and Eu3+ (red)-ion-doped materials, can emit different colors according to the type of doping under the excitation of ultraviolet light.15 Dual-mode luminescence can integrate different fluorescent colors of UC and DC into the same luminous platform, which

1. INTRODUCTION Counterfeiting is an increasingly serious and long-standing global problem, which is also more common in our daily life.1 Documents and valuable items, such as banknotes, diplomas, and certificates, are widely and illegally copied in every country and pose a serious security threat to institutions, including individuals, companies, and communities.2,3 Advances in modern science and technology have contributed to counterfeiting and fraud. Therefore, anti-counterfeiting has been paid more and more attention by all countries in the world, and governments are also trying to develop advanced anticounterfeiting technology, which can protect important documents from being replicated.4,5 The application of fluorescence technology to high-tech anti-counterfeiting is one of the most effective technical methods. With the deepening of research, more and more optical materials, such as lanthanide-doped nanocomposites (NCs), quantum dots, and metal−organic frameworks (MOFs), have been used in the anti-counterfeiting marking of products.6−10 However, it is still a difficult point to produce large amounts of MOFs by chemical synthesis for anticounterfeiting, and semiconductor quantum dots have a certain degree of toxicity. Among the above many kinds of fluorescent materials, the fluorescent materials containing lanthanide elements have the advantages of low toxicity, strong © 2019 American Chemical Society

Received: June 23, 2019 Accepted: September 4, 2019 Published: September 4, 2019 35294

DOI: 10.1021/acsami.9b10989 ACS Appl. Mater. Interfaces 2019, 11, 35294−35304

Research Article

ACS Applied Materials & Interfaces

laser irradiation, which was ascribed to the 4F9/2 → 4I15/2 transition of Er3+ ions. Doping Tb3+ and Eu3+ ions into the appropriate matrix materials can be used as DC luminescent shells. In recent years, layered lanthanide hydroxides (LLHs) with the general formula Ln2(OH)5X·nH2O (Ln = lanthanides and X = interlayer organic or inorganic anions) have emerged as novel and promising matrix materials for DC luminescence, owing to their low toxicity, high stability, and multicolor fluorescence tunability. As anionic clays,24,25 LLHs have unique layered structures, similar to layered double hydroxides (LDHs),26−28 which consist of double lanthanide cations in the host layers and the exchangeable interlayer anion layers to neutralize the electric charge.29−33 Therefore, a core−shell nanoarchitecture was developed via in situ growth of the positively charged LLHs Ln2(OH)5(NO3)·nH2O (Ln = Gd3+, Tb3+, and Eu3+ ions) on the surface of the negatively charged surface silica modified Y2O3:Er3+,Yb3+ (Y2O3:Er3+,Yb3+@SiO2) nanomaterials. However, the parity transition and spinforbidden transition of lanthanides resulting in the low 4f−4f excitation efficiency considerably limit real application of LLHs in optical purposes. In addition, the layer structure of LLHs contains a large number of water molecules and hydroxyl groups, which can provide a nonradiative attenuation pathway and inhibit the emission of lanthanide activators to a great extent.34,35 Therefore, anionic organic sensitizer pyromellitic acid (PMA) was inserted into the interlayer through ion exchange with interlayer anions to improve the luminescent intensity of LLHs. The shell nanomaterials can directly emit different luminescent colors under a 254 nm lamp (from green to yellow-green, yellow, orange, and red) by adjusting the molar ratio of Tb3+/Eu3+ions. These dual-mode luminescent NCs effectively realized the anti-counterfeiting protection of valuables by manufacturing high-end luminous safety ink. In addition, the reported luminescent NCs here had the characteristics of high contrast, good selectivity, and small background interference, which showed high advantages in visual latent fingerprint recognition. The nanoarchitectures we designed may also provide a new method to design multifunctional nanomaterials for anti-counterfeiting applications and fingerprint recognition.

can improve the technical barrier of illegal counterfeiting and enhance the scientific and technological content of anticounterfeiting technology. In the previous dual-mode anticounterfeiting studies, a variety of rare-earth (RE) elements with UC and DC properties are often uniformly mixed into the same material. Meanwhile, lanthanide cross-relaxation often occurs, which leads to the decrease of luminescence intensity.16,17 An improved method is to introduce UC and DC lanthanide elements into the inner layer or outer layer of the core−shell nanostructure, respectively. Recently, the Xu group reported lanthanide-doped NaYF4@NaGdF4 core−shell nanoparticles with dual-mode luminescence properties, which can also be used for anti-counterfeiting printing.18 In view of the abovementioned results, a dual-mode luminescent core− shell nanoparticle can be designed and fabricated for the application in the field of anti-counterfeiting. Simultaneously, a ″fingerprint″ is called ″human identification card″, which has inherent specificity. Fingerprints will also appear at some crime scenes and will also be used as evidence to prove or establish a crime.19 At the same time, because of its specificity, the fingerprint will also be used as an unlock tool or permission access tool, such as the current more popular fingerprint unlock or fingerprint identification. However, in most cases, the fingerprints we find are latent, and the basic principle of fingerprint detection is to reproduce the fingerprint clearly by technical means while maintaining the integrity of the fingerprint pattern.20 In the study of visual detection of fingerprint clarity, methods such as the iodophor method, ninhydrin colorimetric method, silver nitrate titration, and fluorescence method have been proposed.21 Among them, the photoluminescence method has the unique advantages of high sensitivity, simple operation, and suitable for on-site detection, so it has a broad application prospect in fingerprint detection.22 Herein, dual-mode luminescent core−shell nanoarchitectures were designed and synthesized by a facile and green route (Scheme 1) in this research. Lanthanide oxides (Y2O3), which are often considered to be excellent matrix materials for highefficiency UC luminescence due to their high chemical stability and low photon energy, were used as the inner core.23 The Y2O3:Er3+,Yb3+ core prepared in this work was monodispersed and showed strong red upconversion emission under 980 nm

2. EXPERIMENTAL DETAILS 2.1. Materials and Reagents. Gadolinium nitrate (Gd(NO3)3· 6H2O, 99.95%) (REO), europium nitrate (Eu(NO3)3·6H2O, 99.99%) (REO), and terbium nitrate (Tb(NO3)3·6H2O, 99.99%) (REO) were purchased from Shanghai Energy Chemical. Pyromellitic acid (PMA, ≥98%) was purchased from Adamas Reagent Co., Ltd. Yttrium(III) nitrate hexahydrate (Y(NO3)3·6H2O, 99.99%) (REO), ytterbium(III) nitrate pentahydrate (Yb(NO3)3·5H2O, 99.99%) (REO), and erbium(III) nitrate pentahydrate (Er(NO3)3·5H2O, 99.99%) (REO) were also from Adamas Reagent Co., Ltd. In addition, there were ammonia solution (25−28%), silicic acid tetraethyl ester (TEOS, >28.0%), urea (99%, Adamas Reagent), and ethanol (≥99.7%). 2.2. Measurements. Transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energy dispersive spectrometry (EDS), and EDS mapping were performed by using a Tecnai-G2-F30 microscope at an acceleration voltage of 300 kV. The ζ potential measurement of each sample was analyzed by a Malvern Zetasizer 2000 analyzer. The surface compositions of the samples were investigated by X-ray photoelectron spectroscopy (XPS; Thermo Scientific ESCALAB 250Xi). CHN elemental analyses were performed on an Elementar Vario EL analyzer. Powder X-ray diffraction patterns (PXRD) were recorded with a Rigaku Dmax 2400 diffractometer using Cu Kα radiation over the 2θ range of 5− 80°. Fourier transform infrared (FTIR) spectra of the materials were

Scheme 1. Schematic Diagram of the Construction of DualMode Luminescent Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1−xHPMA (x = 0, 0.1, 0.2, 0.5, 1) Nanoarchitectures

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DOI: 10.1021/acsami.9b10989 ACS Appl. Mater. Interfaces 2019, 11, 35294−35304

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Figure 1. TEM images of (a) Y2O3:Er3+,Yb3+, (b) Y2O3:Er3+,Yb3+@SiO2, and (c) Y2O3:Er3+,Yb3+@[email protected]; (d) EDS spectra of Y2O3:Er3+,Yb3+@[email protected] NCs. Inset: HRTEM images of Y2O3:Er3+,Yb3+ and Y2O3:Er3+,Yb3+@SiO2 shown in panels (a) and (b). measured on a Bruker V70 FTIR spectrometer in the 4000−400 cm−1 wavenumber range utilizing the standard KBr disk technique. Upconversion luminescence spectra were measured by using an Edinburgh FS5 with a high-power 980 nm laser diode. Downconversion luminescence spectra in water solution were carried out on a Varian CARY Eclipse fluorescence spectrometer at the 280 nm excitation wavelength. Lifetime decay measurements were carried out with an Edinburgh FS5 equipped with a cooled R928P photomultiplier tube. The quantum yields of the samples were determined by an absolute method using a barium sulfate coated integrating sphere on a Hamamatsu Instrument Quantaurus-QY C11347. Three parallel measurements were carried out for each sample so that the presented value corresponds to the arithmetic mean value. 2.3. Preparation of Y2O3:Er3+,Yb3+ Nanospheres. The synthesis of Y2O3:Er3+,Yb3+ nanospheres was reported in the previous literature.36 In a typical synthetic work, 1.057 g of Y(NO3)3·6H2O, 0.067 g of Yb(NO3)3·5H2O, and 0.042 g of Er(NO3)3·5H2O were accurately weighed and dissolved in 50 mL of distilled water. Then the above solutions were mixed to form a transparent solution. Under vigorous stirring, 12 g of urea (first ultrasonically dispersed in 50 mL of distilled water) was added into the mixed solution. The resulting mixture was heated at 90 °C for 5 h, and the precipitates were collected by centrifugation and washed with distilled water three times. The obtained Y2O3:Er3+,Yb3+ precursor were fired at 1000 °C for 2 h to form the Y2O3:Er3+, Yb3+ nanospheres. 2.4. Preparation of Y2O3:Er3+,Yb3+@SiO2 Nanoparticles. A layer of silica was uniformly deposited on the surface of the Y2O3:Er3+, Yb3+ nanospheres using the literature method.36 100 mg of Y2O3:Er3+,Yb3+ powders was ultrasonically dispersed in 80 mL of the prepared solution containing ethanol and deionized water (v/v = 3:1). Then 1.2 mL of aqueous ammonia was added into the above solution. After sonicating for 20 min, 0.323 mL of TEOS solution was slowly added, and the mixed solution was stirred for 17 h at 60 °C. The products were centrifuged and washed three times with a mixed solution of deionized water and ethanol solution. 2.5. Preparation of Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1−xH (x = 0, 0.1, 0.2, 0.5, 1). The Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1−xH (x =

0, 0.1, 0.2, 0.5, 1) was synthesized via facile hydrothermal treatment. Typically, 100 mg of Y2O3:Er3+,Yb3+@SiO2 nanoparticles was ultrasonically dispersed in 10 mL of distilled water. Gd(NO3)3· 6H2O (0.50 mmol), the mixture of Eu(NO3)·5H2O and Tb(NO3)3· 6H2O (0.50 mmol, the molar ratios of Eu(III)/Tb(III) = 0: 1, 0.1: 0.9, 0.2: 0.8, 0.5: 0.5, 1: 1), and KNO3 (4 mmol) were dissolved in 5 mL of distilled water and then mixed with the Y2O3:Er3+,Yb3+@SiO2 aqueous solution. After stirring for 20 min, KOH (0.5 M) solution was added dropwise to the mixture to adjust pH to 6.94. The obtained mixed solution was heated at 150 °C for 48 h in a Teflon autoclave, and the final products were collected by centrifugation and washed with ethanol three times. 2.6. Preparation of Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1−xHPMA (x = 0, 0.1, 0.2, 0.5, 1) Nanocomposites. First, 50 mg of Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1−xH (x = 0, 0.1, 0.2, 0.5, 1) was dissolved in 10 mL of deionized water forming a milky white suspension, which was treated with ultrasonication for half an hour. Second, the aqueous solution of NaOH (1 mmol) was added into PMA (0.25 mmol) for deprotonation (pH = 7.35). The above two solutions were subsequently mixed and treated hydrothermally at 180 °C for 5 h. After cooling, the obtained precipitates were collected, washed with ethanol three times to remove any impurity, and then dried in vacuum. 2.7. Preparation of the Security Inks. The hydrophilic security inks were fabricated by the following process: the optimal viscosity solution was first prepared with ethanol/water (v/v = 10:32), which was added into 18 mL of glycerol, and then stirred evenly. Subsequently, sodium dodecyl sulfate and the multicolor anticounterfeiting material were dispersed in 5 mL of the above solution under ultrasonication for 20 min. To print a pattern on black paper, the stamp of Henan Polytechnic University was touched with multicolor inks and then pressed on black paper. 2.8. Latent Fingerprint Visualization. All fresh latent fingerprints were taken from the same volunteer by rubbing on the forehead and then lightly pressing on the different substrates, including glass Petri dish, plastic Petri dish, plastic sealed bag, aluminum alloy, ceramic tile, mouse, and so on. In order to create latent fingerprints, 35296

DOI: 10.1021/acsami.9b10989 ACS Appl. Mater. Interfaces 2019, 11, 35294−35304

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ACS Applied Materials & Interfaces the nanopowder was first shaken onto the surface of the whole fingerprint and then gently swept along the fingerprint lines by using a soft feather brush until the fingerprint images were developed. The final fingerprint images were recorded under daylight or 254 nm UV irradiation.

which was consistent with the designed elemental distribution and further validated the synthesis of the core−shell structure. The ζ potential measurement of each sample was analyzed by a Malvern Zetasizer 2000 analyzer. The Y2O3:Er3+,Yb3+ nanoparticles had a positive ζ potential of +31.2 mV, which was beneficial to the coating of the negatively charged silica shell using a modified Stöber method. After modifying with the silica shell, the ζ potential decreased to −8.6 mV, which facilitated the deposition of the positively charged LGdEu0.5Tb0.5H precursor on their surface through electrostatic interactions. The ζ potential value for the obtained Y2O3:Er3+,Yb3+@[email protected] was +3.4 mV. These ζ potential measurements strongly implied a successful surface functionalization of both silica shell and LGdEu0.5Tb0.5H-PMA to the Y2O3:Er3+,Yb3+ nanoparticles. XPS characterization was further employed to investigate the element compositions of Y2O3:Er3+,Yb3+ and Y2O3:Er3+,Yb3+@ [email protected] (Figure 3a). It can be observed that the peaks at 157.0, 168.5, and 183.9 eV for Y2O3:Er3+,Yb3+ were assigned to Y-3d, Er-4d, and Yb-4d, respectively. Compared with pure Y2O3:Er3+,Yb3+, the intensities of the Y3d, Er-4d, and Yb-4d peaks of Y2O3:Er3+,Yb3+@SiO2@ LGdEuTbH-PMA decreased, while the Si-2p, Gd-3d, Eu-3d, and Tb-3d peaks appeared, indicating the formation of silicon and LGdEuTbH shells on the surface of the Y2O3:Er3+,Yb3+ core. The insertion of PMA in the Y2O3:Er3+,Yb3+@SiO2@ LGdEuTbH can be demonstrated by the XPS N1s spectra (Figure S1). The peak in the N1s spectra of Y2O3:Er3+,Yb3+@ SiO2@LGdEuTbH located at 406.7 eV was characteristic of NO3− with an oxidation state of +5.37 However, it disappeared after exchanging with PMA anions, indicating that the nitrate anions were replaced by PMA anions. This was consistent with the results from elemental analysis. The chemical composition of the corresponding intercalated materials was also determined by the contents of C, H, and N (Table S1). It was found that the content of the N element greatly reduced and the content of the C element correspondingly increased after exchanging with PMA anions, indicating that the PMA anions replaced the nitrate anions in the interlayer. The successful assembly of the Y2O3:Er3+,Yb3+@SiO2@ LGdEu0.5Tb0.5H-PMA nanoparticles can also be confirmed by the FTIR spectra. Figure S2 shows the FTIR spectra of a series of materials, such as Y2O3:Er3+,Yb3+, Y2O3:Er3+,Yb3+@SiO2, Y2O3:Er3+,Yb3+@[email protected], and Y2O3:Er3+,Yb3+@ [email protected]. Compared with Y2O3:Er3+,Yb3+, Y2O3:Er3+,Yb3+@SiO2 had obvious Si− O−Si characteristic peaks at 1078 and 801 cm−1 and an obvious Si−OH characteristic peak at 954 cm−1, indicating that the Y2O3:Er3+, Yb3+ core surface was successfully covered with a layer of SiO2 shell.38−41 The intense band at 1384 cm−1 corresponded to the ν3 mode of the nitrate species that appeared in the FTIR spectra of Y2O3:Er3+,Yb3+@SiO2@ LGdEu0.5Tb0.5H, while it correspondingly weakened after exchanging with PMA anions.29,42 Moreover, the symmetric stretching vibration of the −COO− group appeared at 1520 cm−1, which illustrated that PMA molecules successfully displaced the nitrate ions and intercalated into the LRH gallery. The successful insertion of PMA in the Y2O3:Er3+,Yb3+@ SiO2@LGdEuTbH can also be verified by the PXRD pattern. The PXRD patterns of Y2O3:Er3+,Yb3+, Y2O3:Er3+,Yb3+@SiO2, Y2O3:Er3+,Yb3+@[email protected], and Y2O3:Er3+,Yb3+@ [email protected] samples are

3. RESULTS AND DISCUSSION 3.1. Morphology and Structure Properties of Y2O3:Er3+,Yb3+@SiO2@ LGdEuxTb1−xH-PMA Core−Shell Nanocomposites. To realize efficient dual-mode luminescence in well-defined core−shell nanoarchitectures, Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1−xH-PMA (x = 0, 0.1, 0.2, 0.5, 1) NCs were successfully synthesized via a layer-bylayer self-assembly strategy. TEM and EDS were adopted to characterize the as-synthesized NCs in order to investigate the microstructure, composition, and shape evolution of these core−shell NCs. It was observed in Figure 1a that Y2O3:Er3+,Yb3+ nanoparticles were monodispersed and spherical in structure with smooth surfaces, and the diameter was around 120 nm. The lattice fringes shown in the HRTEM image (Figure 1a, inset) indicated the formation of polycrystalline Y2O3 with good crystallinity. The observed lattice fringes of 0.30, 0.27, and 0.43 nm from the HRTEM image can be assigned to the d spacings of (222), (400), and (211), respectively.36 After being coated by a thin silica layer (Figure 1b), the morphological features of the nanospheres became rough as compared with pristine Y2O3:Er3+,Yb3+ nanoparticles, and the average size of nanospheres was found to be around 130 nm. Due to Y2O3:Er3+,Yb3+@SiO2 nanospheres having a negatively charged surface, the positively charged LLH precursor can be deposited on their surface via electrostatic interactions, resulting in the formation of the core−shell structure. As shown in Figure 1c, the LGdEu0.5Tb0.5H-PMA shell layer fully covered the surface of the core layer. Moreover, the energy dispersive X-ray spectroscopy spectra (EDS, Figure 1d) revealed the presence of Y(III), Yb(III), Er(III), Si(IV), Gd(III), Tb(III), and Eu(III) in the synthesized core−shell NCs, further confirming that the Y2O3:Er3+,Yb3+@SiO2@ LGdEu0.5Tb0.5H-PMA core−shell nanoarchitecture was successfully obtained. Furthermore, the EDS mappings of elemental distributions of Y2O3:Er3+,Yb3+@[email protected] are shown in Figure 2. Figure 2b−d reveals that Y(III), Yb(III), and Er(III) were uniformly distributed in the core, and Figure 2e displays the elemental map of Si along the shape of the core, revealing that the core was homogeneously coated by the SiO 2 encapsulation layer. Figure 2f−h describes that Gd(III), Tb(III), and Eu(III) were uniformly distributed in the shell,

Figure 2. (a) STEM image and the corresponding STEM elemental mapping for (b) Y(III), (c) Er(III), (d) Yb(III), (e) Si(IV), (f) Gd(III), (g) Eu(III), and (h) Tb(III) of Y2O3:Er3+,Yb3+@SiO2@ LGdEu0.5Tb0.5H-PMA NCs. 35297

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Figure 3. (a) XPS survey of Y2O3:Er3+,Yb3+ (red) and Y2O3:Er3+,Yb3+@[email protected] (blue) samples; (b) PXRD patterns of Y2O3:Er3+,Yb3+ (A), Y2O3:Er3+,Yb3+@SiO2 (B), Y2O3:Er3+,Yb3+@[email protected] (C), and Y2O3:Er3+,Yb3+@[email protected] NCs (D).

Figure 4. (a) Upconversion luminescence emission spectra of Y2O3:Er3+,Yb3+@[email protected] NCs with excitation at 980 nm; (b) energy transfer mechanism diagram of upconversion emission for Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1−xH-PMA (x = 0, 0.1, 0.2, 0.5, 1).

respectively.36 The typical energy transfer mechanism of the upconversion emission in the Er3+/Yb3+ codoping core is shown in Figure 4b. First, the Yb3+ electrons located in the 2 F7/2 level (ground state) were excited by the absorption of 980 nm light photon energy to reach the 2F5/2 level (excited state), and then the energy was absorbed by the electrons of Er3+ ions, resulting in energy transitions toward the 4I11/2 level (excited state). Although Er3+ can be also directly excited under 980 nm laser irradiation, Yb3+ can absorb photon energy more easily. In addition, Er3+ electrons can receive one more photon from Yb3+ energy and be promoted from the 4I11/2 level to the 4F7/2 level by transferring energy. Finally, the electrons decayed to the 4F9/2 and 2H11/2/4S3/2 levels from the 4F7/2 level by photon nonradiative relaxation, which produced a red emission band and green emission band, respectively.46 Due to the green emission band being much weaker, red upconversion luminescence emission can be easily seen with the naked eye (inset in Figure 4a). Furthermore, the absolute UC quantum yield was measured to be 2.3% by using an integrating sphere. The photoluminescence decay time of Y2O3:Er3+,Yb3+@SiO2@ LGdEu0.5Tb0.5H-PMA NCs were measured at room temperature, and the obtained 4F9/2 lifetime value was 667 μs (Figure S3), which was compatible with the reported values.47 To fabricate the dual-mode luminescent NCs, we designed and modified color-tunable layered downconversion luminescent lanthanide hydroxides on the surface of Y2O3:Er3+,Yb3+@ SiO2. In order to demonstrate that inserted pyromellitic acid

displayed in Figure 3b. It can be seen that all samples had the main characteristic peaks at the planes (211), (222), (400), (440), and (622), which can be attributed to the standard diffraction peaks of cubic phase Y 2 O 3 (JCPDS #010831).36,43,44 Compared with the Y2O3:Er3+,Yb3+@SiO2, a strong (002) reflection at 9.7° appeared in the PXRD pattern of Y2O3:Er3+,Yb3+@[email protected], indicating the formation of the LGdEuTbH layered phase on the surface of Y2O3:Er3+,[email protected] The exchange reaction between NO3− and PMA anions in the interlayer space of LGdEuTbH led to the formation of LGdEuTbH-PMA. The (002) reflection of Y2O3:Er3+,Yb3+@[email protected] shifted toward the lower reflection angle, which was due to the increase of the interlayer distance. This expansion of basal spacing from ∼9.12 to ∼13.33 Å indicated that the PMA were inserted into the LGdEuTbH gallery. 3.2. Upconversion Photoluminescence and Downconversion Properties of the Y2O3:Er3+,Yb3+@SiO2@ LGdEuxTb1−xH-PMA (x = 0, 0.1, 0.2, 0.5, 1) Nanocomposites. In order to explore the upconversion luminescence properties, the upconversion luminescence emission spectra of Y 2O 3 :Er 3+ ,Yb3+ @SiO 2 @LGdEu0.5 Tb 0.5H-PMA core−shell NCs were studied under the excitation at 980 nm continuous wave laser diode (Figure 4a). The emission spectra consisted of two primary emission bands: one with a strong red emission band at 654−683 nm and another with a weak green emission band at 521−564 nm, which were ascribed to the 4 F9/2 → 4I15/2 and 2H11/2/4S3/2 → 4I15/2 transitions of Er3+, 35298

DOI: 10.1021/acsami.9b10989 ACS Appl. Mater. Interfaces 2019, 11, 35294−35304

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Figure 5. (a) Downconversion luminescence emission spectra (the upper right corner was the digital luminescent photographs of samples with different Tb3+/Eu3+ ratios under a 254 nm UV lamp), (b) CIE chromaticity diagram with (x, y) emission color coordinates, and (c) energy transfer mechanism diagram of Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1−xH-PMA (x = 0, 0.1, 0.2, 0.5, 1) NCs with UV excitation at 280 nm. (d) Photoluminescence decay curves of 5D0 of Eu3+ in samples Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1−xH-PMA (x = 0.2, 0.5, 1; λex = 280 nm, λem = 617 nm).

the emission intensity of Tb3+ ions reduced obviously even when the content of Eu3+ ions was small. Accordingly, the red emission of Eu3+ ions was becoming predominant. The quantum yield of NCs decreased with the decrease of the Tb3+/Eu3+ ratio, which can be attributed to both the reduced Tb3+ emission and the Tb3+-sensitized Eu3+ emission.32 In addition, the digital luminescent photographs of samples with different Tb3+/Eu3+ ratios (the upper right corner of Figure 5a) for these NCs were also displayed under UV irradiation (254 nm). As illustrated in Table S2 and Figure 5b, the 1931 Commission Internationale de l’éclairage (CIE) chromaticity coordinates of Y2O3:Er3+,Yb3+@SiO2@ PMALGdEuxTb1−xH (x = 0, 0.1, 0.2, 0.5, 1) NCs under excitation at 280 nm were calculated and marked in the CIE chromaticity diagram. When the concentration of Eu3+ ions increased, the luminescent color shifted to the red region, indicating that the fluorescence colors of the NCs can be adjusted by changing the ratio of Tb3+/Eu3+ ions. Importantly, the photoluminescence of Y2O3:Er3+,Yb3+@ SiO2@-LGdEuH-PMA NCs was weak in the absence of Tb3+. It was generally believed that the triplet energy level of PMA was 26310 cm−1,42,52 higher than the 5D0 level of Eu3+ (17300 cm−1, resonance energy level) and the 5D4 level of Tb3+ (20500 cm−1). The empirical rules supposed that energy transfer was favorable when the energy gap of Eu3+ or Tb3+ (T1 → 5DJ) was 2500−4500 cm−1. Therefore, the distance between the triplet energy level of PMA and the resonance energy level of Eu3+ ions was too high to transfer energy to the europium ion sufficiently. Without the terbium ion, the strong emission

(PMA) can improve the luminescence intensity of layered downconversion luminescent lanthanide hydroxides (LLHs), Y2O3:Er3+,Yb3+@SiO2@LGdTbH and Y2O3:Er3+,Yb3+@SiO2@ LGdEuH examples were used to determine the fluorescence spectra before and after inserting PMA (Figures S4 and S5). Due to the parity transition and spin-forbidden transition of lanthanides, no efficient energy can transfer to lanthanide ions and the lanthanides were excited directly via the intra-4f6 level, which made the emission intensity of LLHs very weak.48 After being inserted by PMA anions, PMA can efficiently transfer energy to the lanthanide ions. The characteristic emission intensities of Eu3+ and Tb3+ ions significantly enhanced. According to the ratio of Tb3+/Eu3+ ions, Y2O3:Er3+,Yb3+@ SiO2@LGdEuxTb1−xH-PMA (x = 0, 0.1, 0.2, 0.5, 1) NCs exhibited a variety of luminescent colors from green to red under 280 nm excitation (Figure 5a). The characteristic emission peaks at 593, 617, 651, and 699 nm can be assigned to the 5D0 → 7FJ (J = 0, 1, 2, 3, 4) transition of Eu3+ ions. Similarly, the sharp emission peaks observed at 489, 545, 586, and 621 nm can be attributed to the 5D4 → 7FJ (J = 6, 5, 4, 3, 2) transition of Tb3+ ions.49−51 For the samples containing both Tb3+ and Eu3+ ions, the emission spectrum contains the characteristic emission peaks of the two ions. According to the relative concentrations of Tb3+ and Eu3+ ions, the emission spectra of the samples were basically similar, but the relative emission intensity was slightly different. The concentration dependence of the absolute emission intensities of the Tb3+ and Eu3+ ions, the Tb3+/Eu3+ ratio, and DC quantum yields (Φ) are listed in Table S2 and Figure S6. It can be found that 35299

DOI: 10.1021/acsami.9b10989 ACS Appl. Mater. Interfaces 2019, 11, 35294−35304

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ACS Applied Materials & Interfaces of Eu3+ can be observed, which was mainly caused by the energy transfer from Tb3+ to Eu3+, and the mechanism is shown in Figure 5c. During this energy transfer process, electrons in the organic sensitizers (PMA) can capture photon energy under 280 nm excitation and transfer from the ground state (S0) to the first excited state (S1) and further to the lowest excitation triplet state (T1).42 Subsequently, the electrons can directly transfer to the lowest excited state (5D4) of Tb3+, while the lowest excited state (5D0) electrons of Eu3+ may be derived from T1 of the ligand and 5D4 of Tb3+. Finally, the excited electrons returned to the ground state of RE3+ ions, which resulted in red emission (5D0 → 7FJ transition at 616 nm) and green emission (5D4 → 7FJ transition at 545 nm), respectively. It can be further confirmed by the following fluorescence lifetime experiments. The decay curves of 5D0 emission of Eu3+ ions were monitored at 612 nm under the excitation at 280 nm. All these curves can be fitted by the biexponential equation I = I0 + A1 exp( −t /τ1) + A 2 exp(−t /τ2)

Figure 6. (a) Normal pattern with handwritten Arabic numeral of 1 in the red region (inset: photographs of the as-prepared ink under 254 (left) and 980 nm (right) excitation). (b) Photograph of the purple square region under 254 nm UV lamp illumination. (c) Photograph of the red circle region under 980 nm laser irradiation.

multicolor fluorescence anti-counterfeiting. By adjusting the proportions of Eu3+ and Tb3+ ions, a variety of fluorescence colors can be obtained for different anti-counterfeiting requirements. In order to illustrate the stability of ink, we selected Y2O3:Er3+,Yb3+@SiO2@LGdTbH-PMA NC doped ink as an example and tested the fluorescence stability. Upon exposure of the ink to ambient surroundings for 10 days, the fluorescence of the ink had almost no difference both under 980 and 254 nm UV light excitation (Figure S8), demonstrating that these dual-mode luminescent NCs can serve as excellent stable sources for enhancing anti-counterfeiting performance. 3.4. Detection of Latent Fingerprint. The Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1−xH-PMA NCs were selected to develop latent fingerprints because of their relatively small nanometer size and high fluorescence brightness. The preparation method of the fluorescent anti-counterfeiting nanomaterial we designed was relatively simple, more rapid, and effective. We only need to gently apply the NCs to the surface where there were suspicious fingerprints and then gently blew away the excess materials. In order to avoid the damage of the NCs to the potential fingerprint as much as possible, we used a very soft brush throughout the experiment. To explore the detection of potential fingerprints, the same volunteer provided a number of fingerprints deposited on different substrates, including hydrophilic and hydrophobic substrates. Moreover, the developing methods for latent fingerprints, including contrast, selectivity, sensitivity, and applicability, were investigated in detail. In general, fingerprint features can be divided into three levels of detail.53 Level 1 describes in detail the overall direction of the fingerprint ridge, which is not sufficient to be identified but can be used to exclude. Level 2 indicates in detail the specific ridge paths, including termination and bifurcation, which are the most discriminative features. Level 3 provides quantitative data for accurate fingerprint recognition, including ridge path deviations, pores, and edge shape. To affirm the contrast and selectivity of Y2O3:Er3+,Yb3+@ SiO2@LGdEuxTb1−xH-PMA (x = 1, 0.5, 0.2, 0) fluorescent NCs in latent fingerprint development, transparent glass was chosen as the representative smooth substrate because glass is very common in people’s daily life. In forensic investigations, the suspect can often be identified by fluorescence imaging of potential fingerprints on glasses.54 The fingerprints labeled

(1)

where I is the luminescence intensity, A1 and A2 are constants, t is the time, and τ1 and τ2 are rapid and slow lifetimes for exponential components, respectively. Since the multiexponential decay curves are usually observed in the solid composites, the average lifetime τav was calculated as τav = (A1τ12 + A 2 τ2 2)/(A1τ1 + A 2 τ2)

(2)

Calculated using above eq 2, the average decay times of Eu3+ ions are listed in Figure 5d. The average decay time of the Eu3+ ion increased with the increase of Tb (III) concentration, indicating that there was obvious energy interaction in the NCs.30 3.3. Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1−xH-PMA NCs as Anti-Counterfeiting Fluorescent Inks. The rapid development of modern science and technology and the increasingly rampant activities of counterfeiting and forgery have promoted the development of various anti-counterfeiting technologies. Among the many anti-counterfeiting technologies, fluorescent encryption anti-counterfeiting technology has become a widely used anti-counterfeiting technology because of its good concealment and strong anti-counterfeiting strength. In order to encrypt the anti-counterfeiting information, we dispersed the synthesized dual-mode multicolor luminescent Y2O3:Er3+,Yb3+@ SiO2@PMA-LGdTbH NCs in the glycerol and ethanol−water solutions to prepare anti-counterfeiting ink and then draw anti-counterfeiting numbers (Figure 6 and Figure S7). As seen in Figure 6, the number 1 was handwritten on the normal pattern by using the synthesized fluorescence security inks, and the markings were almost invisible under normal light. When 1 was irradiated with 254 nm light, it can emit green light, and the fluorescence of 1 was red under the irradiation by a 980 nm laser. The anti-counterfeiting information can be extracted under the irradiation of two kinds of excitation light sources (254 and 980 nm). Its characteristic was that only the two luminescence colors were completely correct to confirm the authenticity of anticounterfeiting printing, making it difficult to forge. Patterning plays an essential role in the anti-counterfeiting process, so the multicolor luminescence patterns (20, 40 mm in size) consisting of Chinese characters (Henan Polytechnic University) and the abbreviation HPU are demonstrated in Figure 7 at high resolution. More interestingly, the NCs we synthesized as fluorescent inks can also be used in the field of 35300

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Figure 7. Multicolor luminescence patterns by using Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1−xH-PMA (x = 1 (a), 0.5 (b), 0.2 (c), and 0 (d)) NCs under 254 nm UV irradiation. All the figures have the same size, and the scale bar is 0.5 cm.

with fluorescent NCs showed medium contrast when the background color was dark (Figure 8a), and the contrast

rescent powders55,56 showed lower contrast in complicated background environments. Therefore, Y2O3:Er3+,Yb3+@SiO2@ LGdEuxTb1−xH-PMA fluorescent NCs reported here can indeed effectively provide high contrast and selectivity for developing latent fingerprints. To affirm the sensitivity of fluorescent NCs in latent fingerprint development, we also enlarged the representative image of Y2O3:Er3+,Yb3+@[email protected] NCs in Figure 8f,g to investigate the details of latent fingerprints. The image in Figure 8f displays a well-resolved ridge flow and pattern structure (level 1). The four expanded images of the fingerprint are shown in Figure 8g, which can clearly visualize level 2 details, including termination (1), bifurcation (2), core (3), and island (4). In addition, the level 3 details called the sweat pores (1) were also visible because of their small size and optimal affinity, which was rare to see such a fine fingerprint analysis in other research works.57,58 Overall, these details can apparently lead to individualization and can thus demonstrate its feasibility for fingerprint recognition. It is worth mentioning that all the fingerprint details in this work were obtained without the help of external equipment or any chemical method, which would greatly improve the speed of potential fingerprint identification. To affirm the applicability of Y2O3:Er3+,Yb3+@SiO2@ LGdEuxTb1−xH-PMA fluorescent NCs in latent fingerprint development, various substrates commonly handled in daily life, including drink glass, black mouse, white ceramic tile, knife, wrench, and wood (Figure 9), were used as the substrates for verification. Furthermore, we also demonstrated latent fingerprint comparison images using the fluorescent NCs on a plastic Petri dish, black mouse, plastic sealing bag, aluminum alloy, and black ceramic tile. As shown in Figure S9, the images with magnification on substrates displayed bright fluorescent fingerprint patterns under a 254 nm UV lamp, and the details of fingerprint ridges can be visualized by naked eyes with high contrast, well selectivity, and low background interference, which provided direct evidence for the practicability of our synthesized fluorescent NCs for personal identification in forensics.

Figure 8. Images of latent fingerprints on glass Petri dish by using Y2O3:Er3+,Yb3+@SiO2@LGdEuxTb1−xH-PMA (x = 1, 0.5, 0.2, 0) NCs under bright light (a) and with UV irradiation (b−e). (f) Image of a fingermark labeled by Y2O3:Er3+,Yb3+@[email protected] NCs with UV irradiation and (g) the corresponding magnified images.

enhanced markedly because of the strong fluorescence intensity with different fluorescence colors (Figure 8b−e), which were consistent with the luminescence emission spectrum (Figure 5a). In addition, these fluorescence fingerprint images clearly exhibited the flow of the colored ridges and the colorless furrows (level 1) due to their small size and suitable affinity. Only the papillary ridges were stained by the fluorescent NCs, and no background staining were observed, making the ridge details of the developed fingerprints easy to be identified by the naked eye and thus indicating a high selectivity. It was interesting that the fluorescence color and intensity of fluorescent NCs can be adjusted by changing the proportion of rare-earth ions (Eu 3+ /Tb 3+ ). The rich fluorescence color tunability of rare-earth fluorescent nanomaterials as mentioned above had great potential application value for the ultrasensitive recognition of suspicious fingerprints because these fluorescent composites can reduce the background interference and improve the accuracy of fingerprint identification by changing the intensity and color of the fluorescence signal. In previous reports, conventional fluo-

4. CONCLUSIONS In summary, new dual-mode luminescent nanoarchitectures were well designed and applied in the information security and 35301

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application on different printed matter, photographs of Y2O3:Er3+,Yb3+@SiO2@LGdTbH-PMA doped inks at indicated time periods under 980 and 254 nm excitation, fingerprint identification on different matrices, elemental analysis of the nanocomposites, the color coordinates of CIE chromaticity diagram, luminescence intensities of various transitions, the intensity ratios (ITb/IEu) and DC quantum yields of the nanocomposites (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.J.). *E-mail: [email protected] (Y.F.). ORCID

Lei Jia: 0000-0003-2082-0287 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the National Natural Science Foundation of China (Nos. 51773052, 51871090, U1804135, and 51671080), the Program for Science & Technology Innovation Talents in Universities of Henan Province (19HASTIT040), the Plan for Scientific Innovation Talent of Henan Province (194200510019), and the Program for Innovative Research Team (in Science and Technology) in the University of Henan Province (16IRTSTHN005).

Figure 9. Latent fingerprints on drinking glass (a,a’), mouse (b,b’), white ceramic tile (c,c’), knife (d,d’), wrench (e,e’), and wood (f,f’) labeled by multicolor Y2O3:Er3+,Yb3+@SiO2@ LGdEuxTb1−xH-PMA (x = 1, 0.5, 0) NCs under bright light (a−f) and under 254 nm UV irradiation (a’−f’).

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(1) You, M.; Zhong, J.; Hong, Y.; Duan, Z.; Lin, M.; Xu, F. Inkjet Printing of Upconversion Nanoparticles for Anti-Counterfeit Applications. Nanoscale 2015, 7, 4423−4431. (2) Yoon, B.; Lee, J.; Park, I. S.; Jeon, S.; Lee, J.; Kim, J. M. Recent Functional Material Based Approaches to Prevent and Detect Counterfeiting. J. Mater. Chem. C 2013, 1, 2388−2403. (3) Cui, Y.; Hegde, R. S.; Phang, I. Y.; Lee, H. K.; Ling, X. Y. Encoding Molecular Information in Plasmonic Nanostructures for Anti-Counterfeiting Applications. Nanoscale 2014, 6, 282−288. (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) Liu, Y.; Ma, X.; Lin, Z.; He, M.; Han, G.; Yang, C.; Xing, Z.; Zhang, S.; Zhang, X. Imaging Mass Spectrometry with a LowTemperature Plasma Probe for the Analysis of Works of Art. Angew. Chem., Int. Ed. 2010, 49, 4435−4437. (6) Abargues, R.; Rodriguez-Canto, P. J.; Albert, S.; Suarez, I.; Martínez-Pastor, J. P. Plasmonic Optical Sensors Printed from AgPVA Nanoinks. J. Mater. Chem. C 2014, 2, 908−915. (7) 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. Nat. Nanotechnol. 2012, 23, 395201. (8) Mills, A. Oxygen Indicators and Intelligent Inks for Packaging Food. Chem. Soc. Rev. 2005, 34, 1003−1011. (9) Deisingh, A. K. Pharmaceutical Counterfeiting. Analyst 2005, 130, 271−279. (10) 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.

information recognition. The nanoarchitectures, which were the composites of upconversion fluorescent materials Y2O3:Er3+,Yb3+ and downconversion fluorescent materials LGdEuTbH-PMA, were capable of producing multicolor fluorescence emission under the excitation of 254 and 980 nm wavelengths, respectively. This kind of safe ink formed by the intelligent integration of nanoarchitectures with dual-mode luminescence functions can provide a strong anti-counterfeiting effect for files or banknotes that need to be protected. Meanwhile, the nanoarchitectures were successfully used as effective fluorescence markers to establish highly sensitive potential fingerprints on a variety of substrates with great contrast, high selectivity, and low background interference. In a word, the nanoarchitectures with dual-mode fluorescence properties reported here may have a potential value in security applications.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b10989. XPS N1s spectra of Y2O3:Er3+,Yb3+@SiO2@LGdEuTbH before and after exchanging with PMA anions, infrared spectra of the product of each step, photoluminescence decay curves, DC luminescence emission spectra, concentration dependence of the intensity ratio of Tb3+ (545 nm) to Eu3+ (616 nm), anti-counterfeiting 35302

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DOI: 10.1021/acsami.9b10989 ACS Appl. Mater. Interfaces 2019, 11, 35294−35304