Trans. Nonferrous Met. Soc. China 30(2020) 3333-3346

Preparation of NaYF₄:Yb³⁺,Tm³⁺@NaGdF₄:Ce³⁺,Eu³⁺ double-jacket microtubes for dual-mode fluorescent anti-counterfeiting

Yin CHEN¹, Shao-wen XIE¹, Chao TONG¹, Hai-hu TAN¹, Li-jian XU¹, Na LI^1,2, Jian-xiong XU¹

1. Hunan Key Laboratory of Biomedical Nanomaterials and Devices, College of Life Sciences and Chemistry, Hunan University of Technology, Zhuzhou 412007, China;

2. Key Laboratory of Electrochemical Green Metallurgy Technology of Hunan Higher Education Institutions, College of Metallurgy and Materials Engineering, Hunan University of Technology, Zhuzhou 412007, China

Received 22 February 2020; accepted 4 September 2020

Abstract: Novel hydrophilic NaYF₄:Yb³⁺,Tm³⁺@NaGdF₄:Ce³⁺,Eu³⁺ double-jacket microtubes (DJMTs) with upconversion/ downconversion dual-mode luminescence were designed and prepared through epitaxial growth of NaGdF₄:Ce³⁺, Eu³⁺ shell onto the NaYF₄:Yb³⁺,Tm³⁺ microtube via poly(acrylic acid) (PAA) mediated hydrothermal method. It is demonstrated that PAA ligand played an important role in guiding the direct growth of NaGdF₄:Ce³⁺,Eu³⁺ shell onto the surface of NaYF₄:Yb³⁺,Tm³⁺ parent microtubes. The growth of NaGdF₄:Ce³⁺,Eu³⁺ shell experienced a crystal phase transition from β-NaGdF₄ and β-NaYF₄ mixture to β-NaYF₄@NaGdF₄ composite crystal, and morphology evolution from mixture of β-NaGdF₄:Ce³⁺,Eu³⁺ nanorods and β-NaYF₄:Yb³⁺,Tm³⁺ microtubes to NaYF₄:Yb³⁺,Tm³⁺@NaGdF₄: Ce³⁺,Eu³⁺ DJMTs. The formation mechanism of DJMTs was the dissolution-renucleation of β-NaGdF₄:Ce³⁺,Eu³⁺ nanorods and the growth of β-NaGdF₄:Ce³⁺,Eu³⁺ shell via the classical Ostwald ripening mechanism. The as-prepared DJMTs could exhibit blue upconversion and red downconversion luminescence, which was further made into environmentally benign luminescent inks for creating highly secured and fluorescent-based anti-counterfeiting patterns via inkjet printing.

Key words: lanthanide-doped fluoride; core-shell structure; dual-mode luminescence; inkjet printing; anti- counterfeiting

1 Introduction

Nowadays, counterfeit products have significant impact on the interests of consumers and cause huge business economic losses [1,2]. Developing of efficient anti-counterfeiting technology is becoming critical, aiming to distinguish forge products and protect intellectual property [3-5]. In recent decades, a variety of anti- counterfeiting products such as radio frequency identification (RFID) [6], laser holography [7], nuclear trackers [8], and luminescent materials [9-11], have been developed to combat the counterfeiting. Among them, luminescence materials and their corresponding anti- counterfeiting technologies are considered as one of the most perspective ways to boycott counterfeiting because of their diverse chemical nature, unique optical properties, and easy availability [12-15]. During the past years, some fluorescent materials including organic dye molecules [16], semi- conductor quantum dots [17], and carbon dots [18] have been developed and applied for anti-counterfeiting. However, these well-studied anti- counterfeiting technologies are relatively easy to be cracked by the skilled counterfeiters. Besides, the broad emission bands, low photostability and biological toxicity of these fluorescent materials are still drawbacks [19].

Recently, the lanthanide-doped fluoride luminescent (NaREF₄:Ln³⁺) materials are recognized as a competitive alternative for anti- counterfeiting application due to their advantages of environmental friendship, long fluorescence life, multicolor optical properties, and sharp emission bands [20-23]. Moreover, the strong dependence of the color output on the excitation source including power density, pulse duration, and excitation wavelength offers additional benefits in fine-tuning the emission profiles, which is essential for practical applications in anti-counterfeiting and high-capacity information storage [24-27]. Due to the rich energy level pattern of lanthanides, NaREF₄:Ln³⁺ materials can be divided into two categories of upconversion (UC) and down- conversion (DC) depending on the luminescence mechanism [28,29]. Generally, the upconversion process can absorb low energy photons and convert them into high energy photons, while down- conversion refers to the opposite process. Typically, the efficient upconversion NaREF₄:Ln³⁺ materials doped with lanthanide ion pairs such as Yb³⁺/Er³⁺ and Yb³⁺/Tm³⁺ in NaYF₄ host material can harvest pumping photons and emit multiple wavelengths [23,30,31]. Similarly, downconversion NaREF₄:Ln³⁺ materials can also emit different colors depending on the doping category such as Tb³⁺ (green), Eu³⁺ (red), and Ce³⁺ (yellow) under short wavelength excitation [32,33].

In the past years, NaREF₄:Ln³⁺ luminescent materials with single color emission have been studied extensively and applied in anti- counterfeiting filed [34,35]. Nevertheless, they are still easy to duplicate owing to their simplicity. On the contrary, multi-mode luminescent materials have shown their advantages since the anti- counterfeiting technique is more complicate and difficult to simulate. Specially, dual-mode luminescent NaREF₄:Ln³⁺ materials that integrate UC and DC emission with different emission colors have aroused significant interests in high anti-fake level of anti-counterfeiting techniques [36-39]. Previous approach concerning dual-mode NaREF₄:Ln³⁺ materials is simple mixing of DC and UC NaREF₄:Ln³⁺ luminescent materials together to produce dual-mode luminescence [40]. However, this approach is time-consuming and process- tedious, and it is difficult to control the spatial distribution of UC and DC materials because the synthesis processes of DC and UC NaREF₄:Ln³⁺ luminescent materials are spatially separated. To obtain stable dual-mode NaREF₄:Ln³⁺ luminescent materials, promising strategy is to integrate both DC and UC materials into a single structure. For example, several studies have reported the dual-mode NaREF₄:Ln³⁺ luminescent materials by doping multiple UC and DC lanthanide activators into the identical matrix materials [41]. However, the cross relaxation between the rare earth ions under multiple doping conditions inevitably leads to the depression of luminescence intensity. A modified method is dividing UC and DC ions into different layers of NaREF₄:Ln³⁺ luminescent materials using the core-shell structure to achieve dual-mode luminescent materials [42-46]. This spatially confined structure could effectively depress the deleterious energy depletion. For example, LIU et al [43] reported a novel strategy to prepare dual-mode luminescent NaGdF₄ nano- crystals consisting of NaGdF₄:Yb,Tm core and NaGdF₄:Eu shell. In this core-shell structure, the NaGdF₄:Yb,Tm core can emit UC luminescence of Eu³⁺ in the shells by using double sensitizers (Yb³⁺ and Tm³⁺). Meanwhile, the red DC luminescence of Eu³⁺ could be achieved under the excitation at 273 nm. DING et al [44] also synthesized dual- mode fluorescent nanocrystals by coating a NaGdF₄:Ce,Tb shell with DC emission on the surface of UC nanoparticles of NaGdF₄:Yb/Tm. Recently, our group synthesized dumbbell-shaped lanthanide-doped NaYF₄@NaGdF₄ core-shell nanoparticles with dual-mode fluorescence by coating NaGdF₄:Ln³⁺ shell onto NaYF₄:Ln³⁺ core nanospheres via a two-step thermal decomposition process [45]. The resultant NaYF₄@NaGdF₄ core-shell nanoparticles exhibited different up-/ down-conversion luminescence under irradiation of a 980 nm laser and a 254 nm UV light, respectively. However, most of these dual-mode NaREF₄:Ln³⁺ luminescent materials with core-shell structure are synthesized through thermal decomposition method using organometallic compounds as precursors, which provides hydrophobic nature and poor surface conjugation activity of the materials. More importantly, the NaREF₄:Ln³⁺ luminescent materials synthesized by thermal decomposition method often have low fluorescence quantum yield (low luminescent intensity) owing to the surface defect generated during the thermal decomposition  process [47-49].

To develop a facile and versatile strategy for the synthesis of hydrophilic dual-mode fluorescent materials with high luminescent intensity, the preparation of monodisperse hydrophilic NaYF₄: Yb³⁺,Tm³⁺@NaGdF₄:Ce³⁺,Eu³⁺ double-jacket microtubes (DJMTs) by coating NaGdF₄:Ce³⁺,Eu³⁺ shell onto NaYF₄:Yb³⁺,Tm³⁺ parent microtubes via a PAA-mediated hydrothermal process was reported. The morphology, structure, composition of the DJMTs were carefully characterized. Besides, the growing mechanism of the NaGdF₄:Ce³⁺,Eu³⁺ shell on the surface of the NaYF₄:Yb³⁺,Tm³⁺ parent microtube was systematically investigated. The resultant hydrophilic DJMTs were subsequently made into environmentally benign luminescent inks. Eventually, the concealed and dual-mode luminescent anti-counterfeiting patterns were successfully printed on the paper based on the as-prepared DJMT inks via inkjet printing technology.

2 Experimental

2.1 Chemicals and materials

YCl₃·6H₂O (99.9%), YbCl₃·6H₂O (99.9%), TmCl₃·6H₂O (99.9%), GdCl₃·6H₂O (99.9%), CeCl₃·6H₂O (99.9%), EuCl₃·6H₂O (99.9%), sodium fluoride (NaF), and glycerin were purchased from Shanghai Aladdin Chemistry Co., Ltd. (China). Poly(acrylic acid) (PAA) (average relative molecular mass 240000 g/mol, and 25 wt.% solution in water) was obtained from Acros. All other chemicals were of analytical reagent grade and used without further purification. All solutions used in this work were prepared with deionized water. The printing substrate of duplicating paper without fluorescence was purchased from Double A (1991) Public Co., Ltd.

2.2 Synthesis of NaYF₄:Yb³⁺,Tm³⁺ parent upconversion microparticles (UCMPs)

The NaYF₄:Yb³⁺,Tm³⁺ core UCMPs were synthesized via a poly(acrylic acid) (PAA) assisted hydrothermal process as discussed in our previous work [50]. Typically, 236.6 mg (0.78 mmol) YCl₃·6H₂O, 77.5 mg (0.20 mmol) YbCl₃·6H₂O, and 7.6 mg (0.020 mmol) TmCl₃·6H₂O were dissolved in 5 mL H₂O with magnetic stirring to form the rare earth chloride aqueous solution. Then, a mixture of 15 mL ethanol and 6 mL PAA (10 wt.%) aqueous solution containing PAA (2.8×10^-6 mol) was added into the rare earth chloride aqueous solution. After being stirred for 30 min, 8 mL (1.0 mol/L) of NaF (8.0 mmol) aqueous solution was slowly added into the above reaction system under vigorous stirring to form a well-dispersed milky solution. The pH value of the mixture was adjusted at 5.0 by hydrochloric acid (2.0 mol/L). The reaction mixture was stirred at room temperature for 1 h and then transferred into a Teflon bottle held in a stainless steel autoclave, sealed, and maintained at 200 °C for 24 h. After the completion of the hydrothermal reaction, the autoclave was cooled to room temperature naturally. The resulting precipitates were separated by centrifugation at 3000 r/min for 10 min, washed with ethanol/deionized water (volume ratio of 1:1) three times, and finally dried at 60 °C for 12 h to obtain the NaYF₄:Yb³⁺,Tm³⁺ UCMPs as a white powder.

2.3 Synthesis of NaYF₄:Yb³⁺,Tm³⁺@NaGdF₄: Ce³⁺,Eu³⁺ DJMTs

NaYF₄:Yb³⁺,Tm³⁺@NaGdF₄:Ce³⁺,Eu³⁺ DJMTs were obtained by epitaxial growth of NaGdF₄: Ce³⁺,Eu³⁺ shell onto the NaYF₄:Yb³⁺,Tm³⁺ UCMPs using hydrothermal reaction. Typically, 31.6 mg (0.085 mmol) GdCl₃·6H₂O, 3.7 mg (0.010 mmol) CeCl₃·6H₂O and 1.8 mg (0.005 mmol) EuCl₃·6H₂O were dissolved in 5 mL H₂O with magnetic stirring. Then, a mixture of 15 mL ethanol and 6 mL PAA (10 wt.%) aqueous solution containing PAA (2.8×10^-6 mol) was added into the above solution. After stirring for 30 min, 8 mL (1.0 mol/L) of NaF (8.0 mmol) aqueous solution was slowly added into the reaction system under vigorous stirring. The pH value of the mixture was adjusted at 5.0 by hydrochloric acid (2.0 mol/L). Then, 8 mL aqueous solution containing 0.1 g UCMPs was added in the reaction system and further stirred at room temperature for 1 h. Finally, the mixture solution was transferred into a Teflon bottle held in a stainless steel autoclave, sealed, and maintained at 200 °C for 24 h. After the completion of the hydrothermal reaction, the autoclave was cooled to room temperature naturally. The resulting precipitates were separated by centrifugation at 3000 r/min for 10 min, washed with ethanol/ deionized water (volume ratio of 1:1) three times, and finally dried at 60 °C for 12 h to obtain the NaYF₄:Yb³⁺,Tm³⁺@NaGdF₄:Ce³⁺,Eu³⁺ DJMTs as a white powder.

2.4 Formulation of hydrophilic dual-mode luminescent DJMTs inks

The hydrophilic dual-mode luminescent DJMT inks were fabricated by dispersing the as-synthesized DJMTs in a mixture solvent of ethanol, deionized water, and glycerol. In order to obtain luminescent inks with the optimal performance such as viscosity and surface tension, the mass ratio of ethanol, water, to glycerol was kept at 40:40:20. Typically, 0.1 g DJMTs were added to 10.0 g mixture solvent (4.0 g ethanol,  4.0 g deionized water, and 2.0 g glycerol). Then, the resulting mixture was vigorously stirred for 20 min and followed by 10 min sonication to achieve the well dispersion of DJMTs.

2.5 Characterization and instrument

The phase purity and crystallinity of the as-prepared particles were characterized by X-ray diffraction (XRD) using a Rigaku Model D/max-2500 diffractometer, with Cu K_α radiation in the 2θ range of 10°-80° with a step size of 0.02°. Fourier transform infrared spectroscopy (FTIR) was recorded on a Nicolet 380 spectrometer using 32 scans and a 4 cm^-1 resolution. Field emission scanning electron microscopy (FE-SEM) images were obtained with Hitachi S-3000N instrument. Transmission electron microscopy (TEM) was obtained from a JEM-1011 instrument operating at an accelerating voltage of 100 kV. X-ray photoelectron spectroscopy (XPS) spectra were obtained with a PHI 5000 Versa Probe system, using a monochromatic Al K_α X-ray source. The emission spectra of the products were carried out using a Hitachi F-4500 5J2-0004 spectrophoto- meter with an external CNI (5W) 980 nm IR fiber coupled laser system (Changchun New Industries Optoelectronics Tech. Co., Ltd.). The inkjet printing of the DJMT inks was performed on a Canon PIXMA ip2780 inkjet printer equipped with 25 pL cartridge (PG-815) and 2.5 pL cartridge (PG-816). The PG-815 cartridge was capable of generating ink droplets as small as about 36 μm in diameter. The photographs of the sample under the 980 nm laser irradiation (5 W) were obtained using a Nikon D7000 camera with an infrared filter. The exposure time was 15 s, aperture size used was F14 and photosensibility (ISO) was 3200.

3 Results and discussion

Scheme 1 Schematic diagram of synthesis and inkjet printing of dual-mode NaYF₄:Yb³⁺,Tm³⁺@NaGdF₄:Ce³⁺,Eu³⁺ DJMTs for anti-counterfeiting application

Scheme 1 briefly describes a general procedure for the synthesis and inkjet printing of NaYF₄:Yb³⁺,Tm³⁺@NaGdF₄:Ce³⁺,Eu³⁺ DJMTs for the application of dual-mode fluorescent anti- counterfeiting. To this end, NaYF₄:Yb³⁺,Tm³⁺ upconversion microtubes were firstly synthesized via a PAA mediated hydrothermal process. Subsequently, a NaGdF₄:Ce³⁺,Eu³⁺ shell with down- conversion luminescence was epitaxially grown onto the surface of NaYF₄:Yb³⁺,Tm³⁺ UCMPs via a secondary PAA mediated hydrothermal process. The resultant NaYF₄:Yb³⁺,Tm³⁺@NaGdF₄: Ce³⁺,Eu³⁺generally presented as a double-jacket microtube structure with upconversion/ downconversion dual-mode luminescence and hydrophilic surface. Then, the as-obtained NaYF₄:Yb³⁺,Tm³⁺@NaGdF₄:Ce³⁺,Eu³⁺ DJMTs were dispersed into a mixture solvent of ethanol, deionized water, and glycerol to produce environmentally benign dual-mode luminescent DJMT inks. Finally, the dual-mode luminescent DJMT inks were inkjet-printed on paper substrates for creation of pre-designed dual-mode luminescent anti-counterfeiting patterns.

3.1 Characterization of NaYF₄:Yb³⁺,Tm³⁺@ NaGdF₄:Ce³⁺,Eu³⁺ DJMTs

Field emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), and trans- mission electron microscopy (TEM) were adopted to characterize the morphology and structure of the as-synthesized NaYF₄:Yb³⁺,Tm³⁺@ NaGdF₄:Ce³⁺,Eu³⁺ DJMTs. As shown in Fig. 1, the XRD pattern of NaYF₄:Yb³⁺,Tm³⁺ UCMPs showed typical standard pattern of hexagonal-phase NaYF₄ (JCPDS: 28-1192), and no other phases were observed, indicating Yb³⁺ and Tm³⁺ ions were successfully doped into the β-NaYF₄ lattice [35]. Compared with the XRD pattern of NaYF₄: Yb³⁺,Tm³⁺ UCMPs, an additional weak diffraction peak at 2θ=42.71° (as indicated by the black arrow) emerged after the epitaxial growth of NaGdF₄: Ce³⁺,Eu³⁺ shell, which can be indexed as (201) planes of β-NaGdF₄ (JCPDS No. 27-0699) [51].

Fig. 1 XRD patterns of UCMPs (a) and DJMTs (b) in comparison with standard data of β-NaYF₄ (JCPDS No. 28-1192) and β-NaGdF₄ (JCPDS No. 27-0699)

Besides, the peak at 2θ=29.92° became broader than that on XRD pattern of NaYF₄:Yb³⁺,Tm³⁺ UCMPs core, which might be due to the merging of the diffraction peak at 2θ=29.65° indexed as the (110) planes of β-NaGdF₄ (JCPDS No. 27-0699) and the diffraction peak at 2θ=29.96° indexed as the (110) planes of β-NaYF₄ (JCPDS No. 28-1192). The results collectively suggested the formation of hexagonal-phase NaYF₄:Yb³⁺,Tm³⁺@NaGdF₄:Ce³⁺, Eu³⁺ composites.

Fig. 2 Typical FE-SEM, TEM and elemental mappings of as-prepared microparticles

Fig. 3 SEM image of DJMTs (a), merged element map (b) and different element maps of Y (c), Yb (d), Tm (e), Gd (f), Ce (g) and Eu (h) elements

Figures 2(a, b) exhibit the representative FE-SEM images of NaYF₄:Yb³⁺,Tm³⁺ UCMPs and NaYF₄:Yb³⁺,Tm³⁺@NaGdF₄:Ce³⁺,Eu³⁺ DJMTs. It can be seen that the NaYF₄:Yb³⁺,Tm³⁺ UCMPs were monodisperse hexagonal microtubes with length and diameter of around 2.0 and 0.6 μm, respectively (Fig. 2(a)). After implementing the epitaxial growth, the resultant NaYF₄:Yb³⁺,Tm³⁺@ NaGdF₄:Ce³⁺,Eu³⁺ DJMTs still retained the uniform hexagonal microtube morphology (no additional crystals were found), as shown in Fig. 2(b). The results potentially suggested the NaGdF₄:Ce³⁺,Eu³⁺ crystals were formed and coated on the surface of NaYF₄:Yb³⁺,Tm³⁺ UCMPs. SEM elemental mapping of a single DJMTs in Fig. 3 gave more evidences about the formation of NaYF₄: Yb³⁺,Tm³⁺@NaGdF₄:Ce³⁺,Eu³⁺ composite since all the lanthanide elements (Y, Yb, Tm, Gd, Ce and Eu) were located in a single NaYF₄:Yb³⁺,Tm³⁺@ NaGdF₄:Ce³⁺,Eu³⁺ DJMT. It should be noted that the coating amount of NaGdF₄:Ce³⁺,Eu³⁺ shell was very low, which can not lead to a significant size change detected by SEM observation. To further demonstrate the double-jacket microtube structure and make an observation of the size change, TEM elemental mapping was performed. Figure 2(c) shows TEM image of typical NaYF₄: Yb³⁺,Tm³⁺@ NaGdF₄:Ce³⁺,Eu³⁺ DJMTs, which represented a microtube with the length and diameter of ~2.0 μm and ~0.6 μm, respectively. From the TEM elemental mapping (Figs. 2(d-f)), it can be clearly seen that the Gd³⁺ ions (cyan) originated from NaGdF₄: Ce³⁺,Eu³⁺ were uniformly distributed at the outer shell of the microtube (Fig. 2(e)) and Y³⁺ ions (red) originated from the parent NaYF₄:Yb³⁺,Tm³⁺ microtube were located at the inner pot of the double-jacket microtube (Fig. 2(d)), demonstrating the core-shell structure of the NaYF₄:Yb³⁺,Tm³⁺@ NaGdF₄:Ce³⁺,Eu³⁺ DJMTs. Based on the elemental mapping image of Y³⁺ and Gd³⁺ ions (Fig. 2(f)), it can be estimated that the coating of NaGdF₄: Ce³⁺,Eu³⁺ onto the surface of NaYF₄:Yb³⁺,Tm³⁺ UCMPs resulted in the size of NaYF₄:Yb³⁺,Tm³⁺@ NaGdF₄:Ce³⁺,Eu³⁺ DJMTs with 100 nm increased in length and 40 nm increased in diameter.

Figure 4(a) shows the full XPS spectra of the NaYF₄:Yb³⁺,Tm³⁺ UCMPs and NaYF₄:Yb³⁺,Tm³⁺@ NaGdF₄:Ce³⁺,Eu³⁺ DJMTs. As shown, electron binding energy peaks at 1072.0, 685.0, 160.0 and 302.0 eV corresponding to the Na 1s, F 1s, Y 3d, and Y 3p in the NaYF₄:Yb³⁺,Tm³⁺ were detected in the full XPS spectrum of NaYF₄:Yb³⁺,Tm³⁺ UCMPs. Besides, electron binding energy peaks at 285.0 and 532.0 eV corresponding to the C 1s and O 1s originated from PAA molecules on the surface of NaYF₄:Yb³⁺,Tm³⁺ UCMPs were also observed, potentially suggesting that the surface of NaYF₄:Yb³⁺,Tm³⁺ UCMPs was decorated by PAA molecule. Similarly, the elements including Na 1s, F 1s, C 1s and O 1s were also detected in the full XPS spectrum of NaYF₄:Yb³⁺,Tm³⁺@NaGdF₄: Ce³⁺,Eu³⁺ DJMTs. Differently, instead of Y 3d and Yb 3d, electron binding energy peaks at 143.2 and 884.2 eV corresponding to the Gd 4d and Ce 3d in NaGdF₄:Ce³⁺,Eu³⁺ were detected. The disappearance of the Yb 3d peak was demonstrated by the comparison of the two XPS spectra in the range of 193-186 eV, as shown in Fig. 4(b). Furthermore, the characteristic peaks at 149.3 and 143.2 eV for Gd 4d, 902.8 and 884.2 eV for Ce 3d, and 1135.4 eV for Eu 3d_5/2 were also observed in the magnified XPS spectra range of 155-140 eV (Fig. 4(c)), 915-880 eV (Fig. 4(d)), and 1150- 1130 eV (Fig. 4(e)), respectively. These surface elemental changes demonstrated the core-shell structure of the NaYF₄:Yb³⁺,Tm³⁺@NaGdF₄: Ce³⁺,Eu³⁺ DJMTs, where NaGdF₄:Ce³⁺,Eu³⁺ was coated on the surface of the NaYF₄:Yb³⁺,Tm³⁺ microtube. The surface chemical component of NaYF₄: Yb³⁺,Tm³⁺@NaGdF₄:Ce³⁺,Eu³⁺ DJMTs was further characterized by FT-IR. As shown in Fig. 4(f), the characteristic peaks of PAA molecules at 2929 and 2861 cm^-1 were assigned to the asymmetric and symmetric stretching vibrations of the methylene (—CH₂) in the long alkyl chain. 1581 and 1454 cm^-1 associated with the asymmetric and symmetric stretching vibrations of carboxylate anions, and 1722 cm^-1 corresponding to the vibration of carbonyl group were clearly seen, indicating that the surface of NaYF₄:Yb³⁺,Tm³⁺@ NaGdF₄:Ce³⁺,Eu³⁺ DJMTs was decorated by PAA molecule, rendering its hydrophilic nature [52].

Fig. 4 XPS spectra of as-prepared UCMPs and DJMTs

Fig. 5 Comparison of UC luminescent properties of UCMPs and DJMTs (a), excitation and emission spectra of DJMTs with excitation spectrum monitored at 614 nm and emission spectrum at 254 nm (b), UC energy transfer of processes of Yb³⁺-Tm³⁺ in both UCMPs and DJMTs (c), and ultraviolet excitation energy absorbed by Ce³⁺ sensitizer and transferred to Gd³⁺, followed by energy transfer to lanthanide emitter Eu³⁺ in DJMTs (d)

The dual-mode luminescent property of the NaYF₄:Yb³⁺,Tm³⁺@NaGdF₄:Ce³⁺,Eu³⁺ DJMTs was studied by upconversion and downconversion fluorescence spectrum. It is generally accepted that constructing a core-shell structure can decrease the non-radiative decay of the upconversion materials caused by surface defects and vibrational deactivation from solvent molecules, which will improve the UC fluorescence intensity of the upconversion materials under NIR irradiation [53]. In this study, we compared the UC fluorescence properties of UCMPs with a sole core structure and DJMTs with a core-shell structure in aqueous solution under 980 nm laser. As shown in Fig. 5(a), both UCMPs and DJMTs exhibited typical UC fluorescent emission peaks of NaYF₄:Yb³⁺,Tm³⁺ located at the same positions of 450, 475 and 644 nm. These three peaks were assigned to the ¹D₂-³F₄, ¹G₄-³H₆, and ¹G₄-³F₄ transitions of Tm³⁺, respectively (Fig. 5(c)). Since the intensity of blue emission peaks was significantly higher than that of the red peak, both UCMPs and DJMTs would exhibit a dominant blue color under the 980 nm laser irradiation (as shown in the inset of Fig. 5(a)). Besides, the DJMTs emitted 1.76 times higher UC luminescence than UCMPs, confirming that the core-shell structure resulted in an enhanced UC luminescence. Figure 5(b) shows the excitation and emission spectra of the NaYF₄:Yb³⁺,Tm³⁺@NaGdF₄: Ce³⁺,Eu³⁺ DJMTs in aqueous solutions under the irradiation of 641 and 254 nm laser, respectively. As can be seen, the excitation spectra of DJMTs consisted of a broad band at 254 nm and a sharp line at 310 nm, which were ascribed to the ²F_5/2-5d transition of Ce³⁺ and the ⁸S_7/2-⁶P_7/2 transition of Gd³⁺, respectively, implying there was an efficient energy transfer from Ce³⁺ and Gd³⁺ to the Eu³⁺ [54]. On the emission spectrum under the irradiation of 254 nm laser, five emission peaks of Eu³⁺ as activator ion at 534, 554, 591, 614 and 691 nm were detected, corresponding to the transitions of ⁵D₁-⁷F₁, ⁵D₁-⁷F₂, ⁵D₀-⁷F₁, ⁵D₀-⁷F₂ and ⁵D₀-⁷F₄ of Eu³⁺, respectively (Fig. 5(d)). Clearly, the intensity of red emission peaks at 591 and 614 nm was significantly higher than that of other peaks. Therefore, the DJMTs showed a dominant red color under the irradiation of 254 nm laser (as shown in the inset of Fig. 5(b)).

3.2 Growing mechanism

In the synthesis, PAA played an important role in guiding the direct growth of NaGdF₄:Ce³⁺,Eu³⁺ shell onto the surface of NaYF₄:Yb³⁺,Tm³⁺ parent microtubes. A comparative experiment was conducted by similar hydrothermal process without the adding of PAA. The SEM image of the resultant crystals is shown in Fig. 6. As shown, the resultant crystals showed a mixture of microtubes and some irregular microparticles, as indicated by the stained part in Fig. 6. Nevertheless, uniform microtubes were obtained in PAA mediated hydrothermal growth of NaGdF₄:Ce³⁺,Eu³⁺ shell (Fig. 2(b)). The results suggested that the PAA played a critical role in determining the particle growth and shape evolution.

To understand the growing mechanism of NaYF₄:Yb³⁺,Tm³⁺@NaGdF₄:Ce³⁺,Eu³⁺ DJMTs, the hydrothermal reaction in different time intervals of 0, 3, 6, 9 and 12 h was conducted. The intermediate products were subjected to XRD and FE-SEM characterization. Figure 7 shows the XRD patterns of samples prepared at different reaction time intervals. As shown in Fig. 7(a), before the reaction, the NaYF₄:Yb³⁺,Tm³⁺ UCMPs were pure hexagonal phase NaYF_4. Upon the hydrothermal treatment for 3 h, a hexagonal phase NaGdF₄ (β-NaGdF₄) emerged in the presence of hexagonal phase NaYF₄ as evidenced by the small (110) peak at 2θ=29.92° and the (201) peak at 2θ=42.71° (Fig. 7(b)), suggesting that the sample was a mixture of hexagonal phase NaYF₄ and NaGdF₄. With the reaction prolonging, the peaks at 2θ=29.92° and 42.71° were weakened (as shown in Figs. 7(c, d)), potentially suggesting that prolonged hydrothermal treatment led to the dissolution of β-NaGdF₄. After 12 h of hydrothermal treatment, the (110) peak of β-NaGdF₄ at 2θ=29.92° can not be clearly seen, but the (110) peak of β-NaYF₄ at 2θ=29.92° was broadened, which may be due to the merging of the (110) peak of β-NaGdF₄ at 2θ=29.92° and the (110) peak of β-NaYF₄ at 2θ=29.92°. Besides, the (201) peak of β-NaGdF₄ at 2θ=42.71° turned to be a broad halo peak, as shown in Fig. 7(e). This phenomenon implied the formation of β-NaYF₄@β-NaGdF₄ composite materials. Based on the XRD results, we deduced that the growth of NaGdF₄:Ce³⁺,Eu³⁺ shell experienced a process of generation of β-NaGdF₄ and β-NaYF₄ mixture firstly, and the formation of β-NaYF₄@β-NaGdF₄ composite materials ultimately.

Fig. 6 FE-SEM image of DJMTs prepared without addition of PAA

Fig. 7 XRD patterns of intermediates obtained at different reaction time intervals

Further, the morphology of the intermediate products was checked to make a clear observation on the growth of the NaGdF₄:Ce³⁺,Eu³⁺ shell. The FE-SEM images of the intermediate products are shown in Fig. 8. Initially, UCMPs presented as uniform hollow microtubes, as described in Fig. 2(a). Upon 3 h of hydrothermal reaction, the resultant crystals contained not only the UCMPs but also many small microrods (~700 nm in length and ~200 nm in diameter) (Fig. 8(a)). The small microrods might be due to the fast nucleation of Ln³⁺ (Gd³⁺, Ce³⁺, Eu³⁺) with sodium and fluoride ion and the formation of NaGdF₄:Ce³⁺,Eu³⁺ crystals. The results were in good agreement with the XRD results where the hexagonal phase of NaGdF₄ (β-NaGdF₄) emerged. As the reaction proceeded, the small microrods gradually disappeared, as shown in Figs. 8(b, c). The gradual disappearance of the microrods suggested that there existed a dissolution-renucleation of β-NaGdF₄:Ce³⁺,Eu³⁺. This meant that the β-NaGdF₄:Ce³⁺,Eu³⁺ microrods gradually dissolved as crystal monomer with the proceeding of the reaction. In one aspect, the monomer tended to deposit on the surface of the UCMPs to decrease their surface energy, known as the Ostwald ripening phenomenon. In the other aspect, the PAA polymer on the prismatic planes ({1010} crystal planes) was more than that on the top/bottom facets ({0001} crystal planes) due to the large surface area, and the negatively charged PAA polymer would adsorb the monomer, which promoted the growth of the β-NaGdF₄:Ce³⁺,Eu³⁺ shell and the formation of DJMTs. The products can be indexed as uniform DJMTs after 12 h of hydrothermal reaction, as shown in Fig. 8(d). The growing process is schematically shown in  Scheme 1.

Fig. 8 FE-SEM images of intermediates obtained at different reaction time intervals

3.3 Inkjet printing and anti-counterfeiting application

Taking advantages of the dual-mode responsive fluorescence and good water-dispersion ability, the DJMTs are believed to be a promising luminescent material for anti-counterfeiting applications. As a proof-of-concept, dual-mode luminescent inks were prepared by dispersing DJMTs into a mixed solvent of ethanol, water, and glycerol (40:40:20, mass ratio). We used the dual-mode responsive fluorescence DJMTs ink to construct fluorescence patterns by inkjet printing technology. Initially, we printed a complicated pattern on normal printing paper. As shown in Fig. 9(a), no printing imprints and visible pattern were observed under daylight. However, a red pattern and a blue pattern can be read out under the irradiation of a 254 nm UV lamp and a 980 nm laser, respectively. To develop more advanced anti- counterfeiting technique, we intended to design a complex quick-response (QR) code using the dual- mode fluorescent DJMT ink and single-mode fluorescent UCMP ink. The design principle is schematically shown in Fig. 9(b). We divided a QR code into two sections. One contained three squares at the three corners of the QR code, and the left fragmentary QR code was set as the other section. The three squares were inkjet-printed on the paper substrate using the dual-mode fluorescent DJMT ink and the left fragmentary QR code was inkjet-printed by using the single-mode fluorescent UCMP ink at proper location, as shown in Fig. 9(b). The printing process can be controlled on a computer. Similarly, no visible pattern was observed under daylight, but three red squares can be read out under a 254 nm UV lamp and an integral QR code can be read out under a 980 nm laser, as shown in Fig. 9(c). We believed that this anti-counterfeiting technique was more complicated and would have great potential in high level of anti-counterfeiting applications.

Fig. 9 Photographs of formulated dual-mode responsive fluorescence DJMT ink and printed patterns under irradiation of 254 and 980 nm (a), and possible luminescence anti-counterfeiting application using UCMP and DJMT ink (b, c)

4 Conclusions

(1) Mono-dispersed hydrophilic NaYF₄: Yb³⁺,Tm³⁺@NaGdF₄:Ce³⁺,Eu³⁺ double-jacket microtubes (DJMTs) with upconversion/ downconversion dual-mode luminescence were successfully prepared by coating NaGdF₄:Ce³⁺,Eu³⁺ shell on the surface of NaYF₄:Yb³⁺,Tm³⁺ hollow microtube via hydrothermal method using PAA as structure directing agent and ligand.

(2) The growth of NaGdF₄:Ce³⁺,Eu³⁺ shell experienced a crystal phase transition from β-NaGdF₄ and β-NaYF₄ mixture to β-NaYF₄@ β-NaGdF₄ composite crystal, and morphology evolution from mixture of β-NaGdF₄:Ce³⁺,Eu³⁺ nanorods and β-NaYF₄:Yb³⁺,Tm³⁺ microtubes to NaYF₄:Yb³⁺,Tm³⁺@NaGdF₄:Ce³⁺,Eu³⁺ DJMTs. The formation mechanism of DJMTs was the dissolution-renucleation of β-NaGdF₄:Ce³⁺,Eu³⁺ microrods and the growth of β-NaGdF₄:Ce³⁺,Eu³⁺ shell via the classical Ostwald ripening mechanism under the hydrothermal condition.

(3) The as-prepared NaYF₄:Yb³⁺,Tm³⁺@ NaGdF₄:Ce³⁺,Eu³⁺ DJMTs can be made into luminescent ink and can be inkjet-printed on paper substrates for creation of dual light responsive patterns.

(4) An advanced anti-counterfeiting technique was developed by printing a complex quick- response (QR) code using the dual-mode fluorescent DJMT ink and single-mode fluorescent UCMP ink, which is more complicated and have great potential in high level of anti-counterfeiting applications.

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具有套管结构NaYF₄:Yb³⁺,Tm³⁺@NaGdF₄:Ce³⁺,Eu³⁺晶体的制备及在双模式荧光防伪中的应用

陈 殷¹，谢少文¹，童 超¹，谭海湖¹，许利剑¹，李 娜^1,2，许建雄¹

1. 湖南工业大学 生命科学与化学学院 生物医用纳米材料与器件湖南省重点实验室，株洲 412007；

2. 湖南工业大学 冶金与材料工程学院 电化学绿色冶金技术湖南省高等学校重点实验室，株洲 412007

摘  要：通过聚丙烯酸(PAA)调介的水热法在NaYF₄:Yb³⁺,Tm³⁺微管上外延生长NaGdF₄:Ce³⁺,Eu³⁺壳层，制备新型亲水性NaYF₄:Yb³⁺,Tm³⁺@NaGdF₄:Ce³⁺,Eu³⁺双模式荧光套管(DJMT)。研究表明，PAA配体在NaYF₄:Yb³⁺,Tm³⁺微管表面形成NaGdF₄:Ce³⁺,Eu³⁺壳层中起到重要的结构导向作用。NaYF₄:Yb³⁺,Tm³⁺@NaGdF₄:Ce³⁺,Eu³⁺双模式荧光双夹层微管的生长经历由β-NaGdF₄和β-NaYF₄两相混合物到β-NaYF₄@NaGdF₄复合晶体的晶相转变过程，同时，颗粒形貌由β-NaGdF₄:Ce³⁺,Eu³⁺纳米棒与β-NaYF₄:Yb³⁺,Tm³⁺微管的混合物演变为NaYF₄:Yb³⁺,Tm³⁺@NaGdF₄: Ce³⁺,Eu³⁺套管结构。DJMTs的生长机理遵循Ostwald熟化机制，生长过程中原位生成的β-NaGdF₄:Ce³⁺,Eu³⁺纳米棒经过溶解-再成核以及表面沉积等过程，在NaYF₄:Yb³⁺,Tm³⁺微管的表面附着β-NaGdF₄:Ce³⁺,Eu³⁺壳层。制备的DJMT具有蓝色上转换和红色下转换荧光特性，可制成环保的荧光油墨，通过喷墨打印实现高度安全的荧光防伪图案的输出。

关键词：稀土掺杂氟化物；核壳结构；双模式荧光；喷墨打印；防伪

(Edited by Bing YANG)

Foundation item: Project (51874129) supported by the National Natural Science Foundation of China; Projects (2018JJ3115, 2019JJ60049) supported by the Science Foundation of Hunan Province, China; Projects (19B153, 19B158) supported by the Scientific Research Fund of Hunan Provincial Education Department, China

Corresponding author: Na LI; Tel: +86-731-28123470; E-mail: lina@hut.edu.cn

DOI: 10.1016/S1003-6326(20)65465-6