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Luminescent, Fire-Resistant, and Water-Proof Ultralong Hydroxyapatite Nanowire-Based Paper for Multimode Anticounterfeiting Applications Ri-Long Yang,†,‡ Ying-Jie Zhu,*,†,‡ Fei-Fei Chen,†,‡ Li-Ying Dong,† and Zhi-Chao Xiong*,† †

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: Counterfeiting of valuable certificates, documents, and banknotes is a serious issue worldwide. As a result, the need for developing novel anticounterfeiting materials is greatly increasing. Herein, we report a new kind of ultralong hydroxyapatite nanowire (HAPNW)-based paper with luminescence, fire resistance, and waterproofness properties that may be exploited for anticounterfeiting applications. In this work, lanthanide-ion-doped HAPNWs (HAPNW:Ln3+) with lengths over 100 μm have been synthesized and used as a raw material to fabricating a free-standing luminescent, fire-resistant, water-proof paper through a simple vacuum filtration process. It is interesting to find that the luminescence intensity, structure, and morphology of HAPNW:Ln3+ highly depend on the experimental conditions. The as-prepared HAPNW:Ln3+ paper has a unique combination of properties, such as high flexibility, good processability, writing and printing abilities, luminescence, tunable emission color, waterproofness, and fire resistance. In addition, a well-designed pattern can be embedded in the paper that is invisible under ambient light but viewable as a luminescent color under ultraviolet light. Moreover, the HAPNW:Ln3+ paper can be well-preserved without any damage after being burned by fire or soaked in water. The unique combination of luminescence, fire resistance, and waterproofness properties and the nanowire structure of the as-prepared HAPNW:Ln3+ paper may be exploited toward developing a new kind of multimode anticounterfeiting technology for various high-level security antiforgery applications, such as in making forgery-proof documents, certificates, labels, and tags and in packaging. KEYWORDS: hydroxyapatite, nanowires, paper, luminescent, fire-resistant, water-proof, anticounterfeiting security labels,12 biomimetic microfingerprints,13 and magnetic responses14 can achieve increased levels of protection against counterfeiting. Nevertheless, specific and expensive instruments are needed to check facticity. From the security and low-cost points of view, the luminescence-based strategy offers improved security and economical reasonableness, and has become one of the most promising methods in the anticounterfeiting field. A typical example is the banknote, which shows luminescence patterns under ultraviolet (UV) light. Moreover, organic dyes, carbon dots, and semiconductor nanoparticles have been used as luminescent materials,15−17 but the long-term toxicities or broad emission bands hinder their further applications. In contrast, lanthanide-doped nanostructured materials provide unique spectral fingerprints and show several advantages, such as being difficult to duplicate, low toxicities and long lifetimes,

1. INTRODUCTION Paper-based materials are the most widely used merchandise in the industrial sector and everyday life. For thousands of years, paper has been used for various purposes, such as for writing and printing, recording and transmission of information, packaging, and decorating. With the development of modern technologies, a variety of paper-based functional materials have been fabricated and widely applied in various fields. Some application examples of paper include catalyst carriers,1 electrical actuators,2 microfluidic devices,3 biosensing platforms,4 adsorbent carriers,5 and anticounterfeiting labels.6 Recently, anticounterfeiting has become an important issue in various fields. Counterfeit banknotes, checks, certificates, and medicines are constantly found, leading to serious economic losses and even endangering human health.7−10 In recent years, considerable efforts have been devoted to developing anticounterfeiting technologies.11 The traditional methods including using watermarks, holograms, and barcodes have become ineffective because of their simple fabrication processes and easy duplication. Recent technologies based on plasmonic © XXXX American Chemical Society

Received: May 15, 2017 Accepted: July 11, 2017

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DOI: 10.1021/acsami.7b06835 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. (a) Digital images of the as-prepared HAPNW:5% Tb3+ paper: (i) a circular free-standing paper, (ii) the paper can be used as a writing paper, (iii) and (iv) the paper shows high flexibility, the scale bars in (a) are 1 cm; (b) SEM and (c) TEM images of the HAPNW:5% Tb3+ sample; (d) XRD patterns of the HAPNWs, HAPNW:5% Ln3+ (Ln = Tb, Eu), and the standard data of HAP; and (e) SEM and (f) TEM images of the HAPNW:5% Eu3+ sample.

used as a raw material to fabricating a free-standing paper through a simple vacuum filtration process. The effects of the experimental conditions on the structure, morphology, and photoluminescence are investigated in detail. The combination of HAPNWs and lanthanide ions endows the as-prepared paper with unique characteristics. More importantly, the multiple functions of luminescence, fire resistance, and waterproofness, as well as the nanowire structure of the as-prepared paper can greatly increase the security in advanced high-level anticounterfeiting, such as in making forgery-proof documents, certificates, labels, and tags and in packaging.

leading to their promising applications in the anticounterfeiting field.18 Luminescent paper is usually fabricated through directly printing luminescent nanostructured materials on the traditional plant cellulose paper.19−22 The luminescence stability is usually unsatisfactory because the nanostructured materials attach onto the cellulose paper via physical adsorption and can be easily degraded or erased by a solvent. Therefore, a complicated chemical modification is desirable.23−25 In addition, luminescent nanostructured materials tend to aggregate together and trigger the well-known aggregationcaused quenching phenomenon.26 It is highly desirable to achieve a homogeneous distribution of the luminescent materials in a matrix. In addition, the cellulose-based luminescent paper can be easily ruined by fire. Synthetic inorganic fibers and nanowires with high thermal stabilities are ideal alternatives. Until now, several inorganic fibers and nanowires have been used for making paperlike materials.27−32 Although some synthesized paper sheets have shown high thermal stabilities, they are not white in color and are unsuitable for anticounterfeit applications. It is noteworthy that a new kind of fire-resistant ultralong hydroxyapatite nanowire (HAPNW)-based inorganic paper with high-quality white color and excellent printable properties was reported by this research group.29,33 Soon after, several functional hydroxyapatite nanowire-based paper sheets were successfully prepared and displayed great potential applications in the biomedical, food, and environmental fields.34−37 The abovementioned innovative progresses provide an ideal alternative material for the fabrication of a new kind of luminescent, fireresistant, and water-proof paper for high-security advanced anticounterfeit applications. In this work, we have developed a new kind of HAPNWsbased luminescent, fire-resistant, and water-proof paper for promising multimode anticounterfeiting applications. Lanthanide-ion-doped HAPNWs (HAPNW:Ln3+) are prepared and

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Calcium chloride (CaCl2), sodium hydroxide (NaOH), and sodium dihydrogen phosphate (NaH2PO4· 2H2O) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Oleic acid, methylene blue, terbium chloride hexahydrate (TbCl3·6H2O), europium chloride hexahydrate (EuCl3· 6H2O), and sodium oleate were obtained from Aladdin Industrial Corporation (Shanghai, China). All chemical reagents were of analytical grade and used as received without further purification. 2.2. Preparation of HAPNW:Ln3+ (Ln = Eu, Tb). A facile solvothermal procedure was adopted. In brief, NaOH (10.500 g) aqueous solution (150 mL) was added into a mixture containing H2O (135 mL), methanol (60 mL), and oleic acid (105 mL) under mechanical stirring. Then, CaCl2 (3.163 g) and EuCl3·6H2O (0.550 g) or TbCl3·6H2O (0.560 g) aqueous solution (120 mL) and NaH2PO4· 2H2O (9.360 g) aqueous solution (180 mL) were successively added into the mixture. The molar ratio of Ln/(Ca + Ln) is 5 mol %. After being stirred for 30 min, the mixture was transferred into a 1 L Teflonlined stainless steel autoclave and then the autoclave was sealed and solvothermally treated at 180 °C for 24 h. The as-obtained product was washed with deionized water and ethanol three times, respectively, to remove the impurities. Finally, the HAPNWs were obtained and dispersed in deionized water for further use. 2.3. Preparation of Sodium Oleate-Modified Tb3+-Doped HAPNWs (HAPNW:Tb3+). The as-synthesized HAPNW:Tb3+ (100 mg) were dispersed in deionized water (100 mL). Then, sodium oleate (0.609 g) was added into the suspension. After being stirred for 2 h, B


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Figure 2. Excitation spectra: (a) the HAPNW:5% Tb3+ sample and (b) the HAPNW:5% Eu3+ sample, and the insets are the digital images of the corresponding HAPNW:5% Tb3+ and HAPNW:5% Eu3+ paper sheets under a UV lamp (∼365 nm). Photoluminescence emission spectra: (c) the HAPNW:Tb3+ sample and (d) the HAPNW:Eu3+ sample, and the insets show the curves of the relationship between the luminescence intensity and the content of Ln3+ (Ln = Tb, Eu) ions. the product was collected and washed with deionized water three times. 2.4. Fabrication of the Free-Standing HAPNW:Ln3+ Paper. A simple vacuum filtration process was adopted. 50 mg of the HAPNW:Ln3+ or sodium oleate-modified HAPNW:Tb3+ aqueous suspension was poured onto the filter paper in a sand core funnel with a diameter of 4 cm. After suction filtration, the free-standing HAPNW:Ln3+ or sodium oleate-modified HAPNW:Tb3+ paper was formed on the filter paper. The HAPNW:Ln3+ or sodium oleatemodified HAPNW:Tb3+ paper could be facilely peeled off from the filter paper after being dried at 60 °C. 2.5. Characterization. Scanning electron microscopy (SEM) images were taken with a field-emission scanning electron microscope (Hitachi S-4800; Japan). Transmission electron microscopy (TEM) images were obtained with a transmission electron microscope (Hitachi H-800; Japan). X-ray diffraction (XRD) patterns were obtained by an X-ray diffractometer (RINT 2200). Luminescence spectra were recorded on a fluorescence spectrometer (FluoroMax-4; France). The quantum yields of the as-prepared samples were determined using a fluorescence spectrometer with an integrating sphere (ISF-513). Fourier transform infrared (FTIR) spectra were measured by a FTIR spectrometer (FTIR-7600; Lambda Scientific Pty Ltd., Australia). The water contact angle was measured using an optical contact angle system (model SL200A/B/D), and each sample was tested at at least five different locations. For the bending test, the HAPNW:Tb3+ paper was cut into a strip with a width of 15 mm. After being fixed on the bending test instrument, the HAPNW:Tb3+ paper strip was bent to 135° and the process was repeated 500 times. The surface and cross section of the bent zone of the paper were observed by SEM. X-ray photoelectron spectra (XPS) were recorded on an Xray photoelectron spectrometer (PHI-5300; Physical Electronics, Inc.) with a Mg Kα source. The C 1s signal peak at 284.6 eV was used as the internal standard for the calibration of the binding energy.

3. RESULTS AND DISCUSSIONS 3.1. Preparation and Characterization of the HAPNW:Ln3+ Paper. A luminescent paper is commonly used as an anticounterfeiting material. Fabrication of a luminescent paper with specific chemical or physical properties can further improve security. Herein, we have prepared a new kind of HAPNW-based paper with luminescence, fire resistance, and waterproofness properties that may be exploited toward developing a novel multimode anticounterfeiting technology for various high-level security antiforgery applications. Figure 1a shows the digital images of the as-prepared HAPNW:Tb3+ paper. Similar to the undoped HAPNW paper and Eu3+-doped HAPNW (HAPNW:Eu3+) paper (Figure S1 in the Supporting Information), the HAPNW:Tb3+ paper presents a high-quality white color. The as-prepared paper is writable and printable. English words can be easily and clearly written on the paper by using a Chinese brush. Furthermore, the HAPNW:Tb3+ paper exhibits a free-standing property and it can be used and stored in a convenient manner. More importantly, the HAPNW:Tb3+ paper has high flexibility. A bending test was carried out to investigate the bending durability of the as-prepared HAPNW:Tb3+ paper (Figure S2b). The experiments show that the paper can be wellpreserved without any damage after repeated bending or rolling around a glass bar. In addition, a slight crease is observed on the surface of the HAPNW:Tb3+ paper strip after being bent by 135° 500 times, as shown in Figure S2c. The SEM image in Figure S2d shows that the layered structure of the cross section is a little tortuous, but the part not in the bending area is intact. In addition, the HAPNW:Tb3+ nanowires preserve well their morphology and are interwoven with each other as before (Figure S2e). These experimental results demonstrate that the C


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Figure 3. SEM images of the as-prepared undoped HAPNW and HAPNW:Tb3+ samples with different initial Tb3+ molar percentages: (a) undoped HAPNWs, (b) HAPNW:1% Tb3+, (c) HAPNW:2.5% Tb3+, (d) HAPNW:5% Tb3+, (e) HAPNW:7.5% Tb3+, and (f) HAPNW:10% Tb3+.

HAPNW:Ln3+ paper has high flexibility and good bending durability. SEM and TEM characterizations were performed to investigating the morphology of the samples. As shown in Figure 1b,c,e,and f, the as-prepared HAPNW:5% Tb3+ and HAPNW:5% Eu3+ samples consist of HAPNWs with diameters of about 20 nm. In many cases, the ultralong nanowires selfassemble into nanowire bundles and form a three-dimensional porous network structure. These ultralong nanowires and nanowire bundles possess lengths of over 100 μm and aspect ratios above 5000, resulting in the formation of a free-standing and highly flexible paper. The HAPNW:Ln3+ paper has no obvious difference in morphology from that of the undoped HAPNW paper (Figure S3). The XRD patterns of the HAPNW, HAPNW:5% Eu3+, and HAPNW:5% Tb3+ samples are shown in Figure 1d. All of the diffraction peaks of the three products can be indexed to hexagonal hydroxyapatite (JCPDS No. 09-0432). The as-prepared HAPNW:5% Eu3+ and HAPNW:5% Tb3+ exhibit high crystallinities, and no impurities are detected in the XRD patterns. In addition, the XPS spectra in Figure S4 show that the binding energies of the Ca, P, and O of the HAPNW:Ln3+ (Ln = Tb, Eu) samples are slightly higher than those of the undoped HAPNWs sample, suggesting a stronger ion interaction in the Ln3+-doped hydroxyapatite crystal structure. These results demonstrate that Ln3+ (Ln = Tb,

Eu) ions are doped in the host crystal lattice of hydroxyapatite by replacing the Ca2+ ions. The photoluminescence excitation and emission spectra of the as-prepared HAPNW:Ln3+ (Ln = Tb, Eu) samples at room temperature are shown in Figure 2. According to the excitation spectra in Figure 2a and b, the most intense excitation peaks at 378 and 393 nm are chosen as the excitation wavelengths of the HAPNW:Tb3+ and HAPNW:Eu3+ samples, respectively. Subsequently, the photoluminescence emission spectra were recorded at these wavelengths. In Figure 2c, four emission peaks at 489, 545, 587, and 620 nm appear in the photoluminescence emission spectrum of the HAPNW:Tb3+ sample at an excitation of 378 nm, which is ascribed to the typical transitions of Tb3+ ions (5D4 → 7FJ, J = 6, 5, 4, and 3).38,39 In addition, in the emission spectrum of the HAPNW:Eu3+ sample (Figure 2d), the characteristic peaks of Eu3+ ions are observed, which may be explained by the following transitions: 5D0 → 7F0 (576 nm), 5D0 → 7F1 (590 nm), 5D0 → 7F2 (616 nm), 5D0 → 7F3 (653 nm), and 5D0 → 7 F4 (700 nm).40 The digital images (insets in Figure 2a and b) show the photoluminescence color of HAPNW:Ln3+ (Ln = Tb, Eu) paper sheets under irradiation by a UV lamp (∼365 nm). The as-prepared HAPNW:5% Tb3+ paper and HAPNW:5% Eu3+ paper exhibit strong green and red colors, respectively. D


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Figure 4. Digital images of two kind of paper sheets before and after thermal treatment: (a) common commercial paper made of plant cellulose fibers, (b) HAPNW:Tb3+ paper, and (c) HAPNW:Tb3+ paper under UV irradiation (∼365 nm). (d) Whiteness change of the common commercial paper and the HAPNW:Tb3+ paper after thermal treatment at different temperatures.

The quantum yields of the as-prepared HAPNW:5% Tb3+ and HAPNW:5% Eu3+ samples are measured to be 6.76% and 13.29%, respectively. We further investigated the effects of the doped lanthanide ion concentration on the photoluminescence intensity, morphology, and crystalline phase of the product. The experimental results show that the doping concentration of Ln3+ (Ln = Tb, Eu) ions has no obvious effect on the positions of photoluminescence emission peaks, but the peak intensity can be adjusted by regulating the doping concentration of Ln3+ (Ln = Tb, Eu) ions. In Figure 2c, the photoluminescence intensity increases with increasing the doping concentration of Tb3+ ions within a certain range below 5 mol % but dramatically decreases with further increasing the doping concentration. This phenomenon may be attributed to the concentration quenching effect.41 At high concentrations, the energy transfer between the activating ions and the excitation energy will be lost at the quenching sites. As a result, the photoluminescence intensity decreases. For the photoluminescence emission spectrum of the HAPNW:Eu3+ sample, a similar trend can be found, as shown in Figure 2d. The photoluminescence intensity increases with increasing concentration of Eu3+ ions, reaching the maximum value at a doping concentration of 7.5 mol %, and then the intensity decreases when the doping level increases to 10 mol %. Furthermore, morphological changes are observed by SEM. As shown in Figure 3, the morphologies of the HAPNWs without doping and the HAPNW:Tb3+ with different doping concentrations from 1 to 5 mol % exhibit no obvious changes. The as-prepared products are composed of ultralong nanowires with lengths of over 100 μm and aspect ratios above 5000. However, with further increasing the doping concentration of Tb3+ ions to 7.5 and 10 mol %, the length and aspect ratio of the one-dimensional (1D) nanostructures decrease significantly and nanorods instead of ultralong nanowires are obtained. In the reaction system containing CaCl2, NaH2PO4·2H2O, NaOH, oleic acid, methanol, and water under sovlothermal conditions, calcium oleate is formed as the precursor,42,43 which then reacts with the phosphorus source to form hydroxyapatite crystal nuclei, and the resultant crystal nuclei could preferentially arrange and grow along the c axis, resulting in the formation of nanowires with long lengths and high aspect ratios.44,45 However, with increasing the doping level of Ln3+ (Ln = Tb, Eu) ions, the Ca2+ ions in the crystal lattice are substituted with

Ln3+ ions increasingly, further resulting in a severe imbalance of the charges.46 The excessive charges in the crystal lattice plane may block the active growth sites of the crystals along the c axis and subsequently lead to 1D nanostructures with short lengths and low aspect ratios. Similar results are observed for the HAPNW:Eu3+ samples with different doping levels (Figure S5). Figure S6 shows the XRD patterns of the as-prepared HAPNW:Ln3+ (Ln = Tb, Eu) with different doping levels. All of the samples exhibit similar diffraction peaks, which can be indexed to a single phase of hexagonal hydroxyapatite. No obvious impurities are detected in the HAPNW:Ln3+ (Ln = Tb, Eu) samples with different doping levels. With increasing the doping level of Ln3+ (Ln = Tb, Eu) ions (Figure S6), the diffraction peaks slightly shift to higher degrees. This may be due to the fact that Eu3+ and Tb3+ ions have smaller ionic radii and higher oxidation states than those of Ca2+ ions, leading to a shrinkage of the crystal lattice. In consideration of the photoluminescence intensity and the morphology of the sample, 5 mol % is chosen as the optimum doping concentration for HAPNW:Tb3+ and HAPNW:Eu3+. 3.2. Fire-Resistant Property of the HAPNW:Ln3+ Paper. The major drawbacks of traditional luminescent paper are its poor thermal stability and flammability. The traditional luminescent paper made from plant cellulose fibers can be easily destroyed by fire. Compared with other fluorescent materials, lanthanide-ion-doped inorganic materials have superior properties. For example, Ln3+-doped inorganic materials exhibit high thermal stability, low photobleaching, low toxicity, and large Stokes shifts.47−49 However, some semiconductor quantum dots, such as CdS, CdSe, and CdTe, have disadvantages, such as toxicity and low thermal stability.50 In this study, the representative HAPNW:Tb3+ are chosen as the raw material for fabricating the luminescent and fireresistant paper. The HAPNW:Tb3+ paper has high thermal stability and nonflammable properties. To verify the excellent thermal stability of the HAPNW:Tb3+ paper, several thermal stability tests are carried out and the common commercial paper made of plant cellulose fibers is used as the control sample. In Figure 4a, the common commercial paper made from plant cellulose fibers is carbonized and the color changes from white to black after thermal treatment at 300 °C for only 10 min. In contrast, there is no obvious change in the HAPNW:Tb3+ paper and the English words are still clearly observable after thermal treatment at 300 °C for as long as 60 E


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Figure 5. Fire resistance tests: (a) the common commercial paper made of plant cellulose fibers and (b) the HAPNW:Tb3+ paper.

Figure 6. (a) Water contact angles of the unmodified and modified HAPNW:Tb3+ paper sheets. The insets show the water droplets dyed by methylene blue on the unmodified (i) and modified (ii) HAPNW:Tb3+ paper sheets; (b) FTIR spectra: (i) the unmodified and (ii) modified HAPNW:Tb3+; (c) photoluminescence emission spectra of the unmodified (i) and modified (ii) HAPNW:Tb3+ samples; (d) the self-cleaning test for the modified HAPNW:Tb3+ paper at different stages of (i)−(iv).

Furthermore, the whiteness changes of the common commercial paper and HAPNW:Tb3+ paper after thermal treatment are also studied. Figure 4d shows that the whiteness of the common commercial paper sheets changes obviously after thermal treatment at 200 and 250 °C and dramatically decreases after being thermally treated at 300 °C for only 5 min. However, there is no obvious change in the whiteness of the HAPNW:Tb3+ paper even after thermal treatment at 300 °C for 60 min.

min, as shown in Figure 4b. The photoluminescence emission performance of the HAPNW:Tb3+ paper before and after thermal treatment is also investigated. From Figure 4c, it can be seen that the photoluminescence color and intensity of the HAPNW:Tb3+ paper before and after thermal treatment have no obvious changes. The emission spectra also support this result (Figure S7), and no appreciable change is found in the photoluminescence intensity or peak position of the HAPNW:Tb3+ paper before and after thermal treatment. F


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Figure 7. Digital images of photoluminescent paper sheets made from HAPNW:Eu3+ and HAPNW:Tb3+ with different weight ratios: (a) under visible light and (b) under UV irradiation (∼365 nm), the weight ratios of HAPNW:Eu3+ to HAPNW:Tb3+ from (i) to (v) are 10:0, 5:5, 3:7, 1:9, and 0:10, respectively; (c) HAPNW:Ln3+ (Ln = Tb, Eu) paper sheets with various shapes under UV irradiation (∼365 nm).

To further demonstrate the excellent fire-resistant performance of the HAPNW:Ln3+ paper, the common commercial paper made of plant cellulose fibers and the HAPNW:Tb3+ paper are heated by a spirit lamp following the same procedure. As shown in Figure 5a, the common commercial paper with a written English word and Arabic numeral is burnt to ashes immediately. In contrast, the HAPNW:Tb3+ paper exhibits no appreciable change and the written English words are clearly visible. Consequently, the HAPNW:Ln3+ paper can be used as the printing and writing medium with excellent thermal stability and fire resistance. In addition, the fire resistance property can effectively enhance the security for the anticounterfeiting performance of the HAPNW:Ln 3+ paper, making the HAPNW:Ln3+ paper more different to be duplicated. Although the HAPNW:Ln3+ paper has many fascinating properties, the mechanical properties of the pure HAPNW:Ln3+ paper are not satisfactory. In the fabrication of the traditional plant-cellulose-based paper in the paper making industry, many agents are added to the cellulose pulp to improve the mechanical properties of the paper. In our recent studies, an inorganic binder,29 a glass fiber,33 and a polymer37 were used for preparing a hydroxyapatite nanowire-based paper and the mechanical properties were significantly improved. For example, after adding sodium silicate as an inorganic binder and glass fibers as a strengthening agent, the tensile strength of the hydroxyapatite nanowire-based paper can reach as high as 15 MPa, which is close to that of the commercial printing paper.51 3.3. Waterproofness Properties of the Sodium OleateModified HAPNW:Ln3+ Paper. Owing to its unique 1D structure and physicochemical properties, the as-prepared HAPNW:Ln3+ paper can be functionalized with specific chemicals, resulting in desirable surface properties, such as superhydrophobicity, and greatly increasing the security for advanced high-level anticounterfeiting. To achieve superhydrophobic properties, a low-cost and biocompatible sodium

oleate is selected to modify the surface of the as-prepared HAPNW:Tb3+. As illustrated in Figure 6a, the modified HAPNW:Tb3+ paper still presents a white color but the surface wettability changes drastically from hydrophilic to superhydrophobic. A water droplet maintains a spherical shape instead of spreading out on the surface of the modified HAPNW:Tb3+ paper. In addition, the water contact angle of the unmodified HAPNW:Tb3+ paper is nearly 0°, showing high hydrophilicity. However, the water contact angle of the modified HAPNW:Tb3+ paper increases to 151.72 ± 1.27° after the surface modification. For the sliding angle test, the modified HAPNW:Tb3+ paper was glued onto the glass slice with an inclined angle of about 5°. The water droplet rolls down as soon as it drips to the surface of the modified HAPNW:Tb3+ paper (Figure S8). These experimental results reveal the superhydrophobic property of the modified HAPNW:Tb3+ paper. In addition, letters in different colors can be easily written on the modified HAPNW:Tb3+ paper by using an oil-based ink (Figure S9). Even after being rinsed or erased with water for 1 min, the written letters on the modified HAPNW:Tb3+ paper remain as clear as they were before. It can be concluded that the oil-based ink has an excellent adhesion performance on the modified HAPNW:Tb3+ paper. Moreover, FTIR analysis was performed to investigating the change of the samples before and after surface modification. In the FTIR spectrum of the HAPNW:Ln3+ sample (Figure 6b), there exist the typical absorption peaks of the PO43− group (1096, 1024, 959, 605, and 560 cm−1), hydroxyl group (3568 cm−1), and adsorbed water (3432 and 1637 cm−1). After being modified with sodium oleate, two strong characteristic peaks appear at 2925 and 2852 cm−1, which are attributed to the symmetrical and asymmetrical C−H stretching bands of the oleate group. These experimental results indicate that the HAPNW:Ln3+ paper can be modified with sodium oleate to achieve a superhydrophobic performance. The photoluminescence intensity of HAPNW:Tb3+ before and after surface modification is also investigated. As shown in G


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Figure 8. Illustration of the as-prepared HAPNW:Ln3+ paper used for multimode anticounterfeiting application.

it always requires complex fabrication processes. In this work, a new kind of HAPNW-based paper with luminescence, fire resistance, and waterproofness properties has been fabricated through a relatively simple approach. These unique properties and the nanowire structure of the HAPNW:Ln3+ paper can serve as a new kind of multimode anticounterfeiting technology and can greatly increase the security in high-level anticounterfeiting. The HAPNW:Ln3+ paper was investigated for application in the anticounterfeiting field (Figure 8). For anticounterfeiting applications, a superhydrophobic HAPNW:Eu3+ paper was fabricated through a simple vacuum filtration process and a star pattern was prepared on the paper using a predesigned mold and superhydrophobic HAPNW:Tb3+. As shown in Figure 8, the star pattern cannot be distinguished by the naked eye under visible light. However, the star pattern exhibits an obvious green color and the other areas display a red color under irradiation with a UV lamp (∼365 nm). This result reveals that the visibility of the predesigned pattern can be facilely turned on and off using a single UV lamp. The unique photoluminescence emission feature of the HAPNW:Ln3+ paper is promising for application in the anticounterfeiting field. In addition, the anticounterfeit effect of the HAPNW:Ln3+ paper is greatly enhanced by its unique physical properties. For example, waterproofness and fire resistance tests can be used to check the facticity. As shown in Figure 8, when the as-synthesized HAPNW:Ln3+ paper is immersed in water dyed with methylene blue and taken out from the water after a few seconds, the HAPNW:Ln3+ paper does not get wet and colored and keeps its white color as before. In addition, when the HAPNW:Ln3+ paper is exposed to fire, it can be well-preserved without obvious damage, owing to its excellent fire resistance properties. Furthermore, the unique nanostructure of the HAPNW:Ln3+ paper can also be used in high-level security anticounterfeiting. It is obvious that the nanowire structure of the HAPNW:Ln3+ paper is quite different from the microstructure of the common commercial

Figure 6c, after being modified with sodium oleate, the emission intensity of the HAPNW:Tb3+ paper increases slightly. This may be explained by the hydrophobic layer on the surface of the modified HAPNW:Tb3+, which can reduce the luminescence quenching effect caused by water molecules.52 In addition, the modified HAPNW:Tb3+ paper also exhibits an excellent self-cleaning performance. As shown in Figure 6d, the water droplets roll away from the surface of the paper and take away dirt (soil) simultaneously, finally making the paper clean. 3.4. Tunable Luminescence Color and Multimode Anticounterfeiting Applications of the HAPNW:Ln3+ Paper. Benefiting from the unique photoluminescence character, the as-prepared HAPNW:Ln3+ (Ln = Tb, Eu) paper exhibits a strong green or red color under irradiation with a UV lamp (∼365 nm). Furthermore, the HAPNW:Ln3+ paper with tunable photoluminescence properties can be obtained by varying the weight ratios of HAPNW:Eu3+ and HAPNW:Tb3+. As representative examples, five kinds of paper sheets are fabricated with the weight ratios of HAPNW:Eu3+ to HAPNW:Tb3+ ranging from 10:0 to 0:10. As shown in Figure 7a, all of the paper sheets display a white color under visible light with no obvious differences. However, this situation remarkably changes under irradiation with a UV lamp (∼365 nm). The five paper sheets exhibit abundant colors, including red, red-orange, orange, yellow-green, and green, respectively (Figure 7b). The photoluminescence phenomenon can be attributed to the simultaneous photoluminescence emissions of Tb3+ and Eu3+ ions. In addition, the as-prepared HAPNW:Ln3+ paper can be easily rolled up and randomly cut into desired shapes without obvious damage, owing to its good flexibility and processability (Figure 7c). A noteworthy strategy in the anticounterfeiting field is to apply multiple anticounterfeiting technologies into one product. Although multiple anticounterfeiting technologies can greatly enhance the anticounterfeit effect of the products, H


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ACS Applied Materials & Interfaces paper made from plant cellulose fibers. The above experimental results demonstrate that the as-prepared HAPNW:Ln3+ paper is a promising material for high-security advanced anticounterfeiting applications. To use this material in the anticounterfeiting field, we can process the HAPNW:Ln3+ paper sheets into desired shapes and glue them onto the products directly or onto the packages of the products. Moreover, the HAPNW:Ln3+ paper can also be used as a writing paper and printing paper for anticounterfeiting documents and certificates.

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4. CONCLUSIONS In summary, a novel kind of inorganic ultralong nanowire-based luminescent paper made from HAPNW:Ln3+ (Ln = Eu, Tb) has been developed. Benefiting from the luminescence ability of doped lanthanide ions and the high flexibility of HAPNW:Ln3+, the synthesized paper exhibits unique luminescence properties, tunable emission color, writability, and good processability. More importantly, the as-prepared HAPNW:Ln3+ paper possesses unique characteristics, such as fire resistance, waterproofness, and a nanowire structure; thus, it can significantly improve the anticounterfeiting performance. The unique advantages of the as-prepared HAPNW:Ln3+ paper may be exploited toward developing a new kind of multimode highsecurity anticounterfeiting technology for various applications, such as for making forgery-proof documents, certificates, labels, and tags and in packaging.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b06835. Digital images, SEM images, TEM images, XRD patterns, XPS spectra, and luminescence emission spectra of the as-prepared samples (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: 0086-21-52412616. Fax: 0086-21-52413122 (Y.-J.Z.). *E-mail: [email protected] (Z.-C.X.). ORCID

Ying-Jie Zhu: 0000-0002-5044-5046 Zhi-Chao Xiong: 0000-0002-0216-5699 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (21601199), the Science and Technology Commission of Shanghai (15JC1491001), and the Shanghai Sailing Program (16YF1413000) is gratefully acknowledged.

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REFERENCES

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