# Synthesis and Application in Anticounterfeiting of RGB Upconversion Nanoparticles

Authors: Yongling Zhang; Peng Lv; Zhengkun Qin
Journal: Journal of Nanomaterials
 (2019)

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## Abstract

In this paper, we synthesized~105±15 nm cubic phase KMnF3:Yb3+,Er3+ NPs, ~195±10 nm hexagonal phase NaYF4:Yb3+,Tm3+ NPs, and ~200±10 nm hexagonal phase NaYF4:Yb3+,Er3+ NPs. Under the excitation of 980 nm, the strong red upconversion luminescence (4F9/2⟶4I15/2, the peak is located at ~653 nm) of Er3+ ions of KMnF3:Yb3+,Er3+ NPs can be seen, the strong blue upconversion luminescence (1D2⟶3F4, 1G4⟶3H6; the peaks are located at ~451 nm and ~477 nm, respectively) of Tm3+ ions of NaYF4:Yb3+,Tm3+ NPs can be observed, and the strong green upconversion luminescence (4I11/2⟶4I15/2, 4S3/2⟶4I15/2; the peaks are located at ~524 nm and ~541 nm, respectively) of Er3+ ions of NaYF4:Yb3+,Er3+ NPs can be found. The mixed white upconversion luminescent materials can be obtained by adjusting the doping ratio of the above NPs. Cyclohexane solution of red, blue, green, and the mixed white NPs can be used as ink, and “JLNU” is written on paper with a pen. Without 980 nm radiation, nothing could be seen. Under 980 nm irradiation, the colored letters “JLNU” can be seen. Red, blue, green, and the mixed white upconversion NPs can be used in security anticounterfeiting.


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## Body

## 1. Introduction

Lanthanide-doped nanoparticles (NPs) have been widely used in many fields due to sharp emission peaks, stable chemical properties, and a long fluorescence lifetime, such as optical waveguide amplifiers, biomarkers, bioimaging, and three-dimensional display [1–7]. In recent years, lanthanide-doped NPs have shown great application prospects in anticounterfeiting applications [8–10]. Li et al. [11] synthesized NaGdF4:Yb,Er@NaYF4:Yb@NaGdF4:Yb,Nd@NaYF4@NaGdF4:Yb,Tm@NaYF4 NPs by high temperature thermal decomposition. Excited at 980 nm, the prepared NPs show ultraviolet and blue upconversion luminescence coming from Tm3+ ions. Under the excitation of 796 nm, the prepared NPs show green and red upconversion luminescence coming from Er3+ ions. The multishell NPs can be applied to dual anticounterfeiting security design. Lei et al. [12] prepared Na3ZrF7:Yb,Er NPs and Na3ZrF7:Yb,Ho NPs. Under the excitation of 980 nm, these NPs can display upconversion luminescence of Er3+ and Ho3+ ions, which can be applied to security anticounterfeiting. Han et al. [13] fabricated NaYF4:2%Er,0.5%Tm@NaYF4 core-shell NPs, and the prepared NPs show different fluorescent colors by controlling the pulse duration and repetition frequency of the pump light. This optical property of NaYF4:2%Er,0.5%Tm@NaYF4 core-shell NPs can be applied to antifake printing. Based on the application of lanthanide-doped NPs in anticounterfeiting, we have done the following research work. In our work, cubic KMnF3:18%Yb3+,1%Er3+ NPs, hexagonal NaYF4:18%Yb3+,0.5%Tm3+ NPs, and hexagonal NaYF4:18%Yb3+,2%Er3+ NPs with oleic acid functional groups on the surface were successfully fabricated. We characterize the crystalline phases (XRD) and morphologies (SEM) of the three NPs. Meanwhile, we also monitored the fluorescence spectra of the three NPs under 980 nm excitation. Interestingly, we obtained the mixed white NPs by adjusting the doping ratio of the three NPs. The fluorescence spectra of white NPs stimulated by 980 nm can be obtained. These four NPs, which emit upconversion luminescence of red, blue, green, and mixed white, can be applied to anticounterfeiting.


## 2. Experimental

### 2.1. Preparation of Red, Blue, and Green Nanoparticles

The preparation process of red upconversion KMnF3:Yb3+,Er3+ NPs is shown as follows: oleic acid, ethanol, and deionized water were poured into 50 ml beakers as reaction solvents with volumes of 10 ml, 5 ml, and 5 ml, respectively. 12 mmol KOH was poured into the aforementioned beaker, and the reaction solution was stirred for half an hour. Next, YbCl3·6H2O, ErCl3·6H2O, and MnCl2 with moles of 0.072 mmol, 0.008 mmol, and 0.32 mmol, respectively, were poured into the aforementioned beaker in sequence. Then, the reaction solution was stirred for another half an hour. 3.5 mmol KF was poured into the aforementioned beaker under stirring conditions. Finally, all the solutions in the beaker were placed in the hydrothermal reactor and kept at 200 degrees for 12 hours. The reaction product was centrifugally washed with a mixture of ethanol and cyclohexane [14, 15].The preparation process of blue upconversion NaYF4:Yb3+,Tm3+ NPs and green upconversion NaYF4:Yb3+,Er3+ NPs is the same. The specific preparation process is shown below: 4 ml oleic acid, 6 ml octadecene, and 0.4 mmol ReCl3·6H2O were poured into a 50 ml two-necked flask with round bottom and kept at 150 degrees for an hour with stirring. Argon needed to be introduced into the reaction system. After the reaction solution was cooled, 8 ml methanol solution containing 1 mmol NaOH and 1.2 mmol NH4F was poured into the aforementioned round bottom. The reaction system was stirred for half an hour. And the temperature of the reaction solution was raised to 50 degrees with stirring. In this process, argon was needed to flow into the atmosphere. Finally, the reaction solution was heated to 300 degrees and kept an hour with stirring. Argon was needed to flow into the atmosphere. The reaction product was centrifugally washed with a mixture of ethanol and cyclohexane [16, 17].


### 2.2. Characterizations

The X-ray powder diffraction (XRD) of red, blue, and green NPs was achieved via Rigaku RU-200B (λ=1.5406 Å, scanning range: 10°-70°). The scanning electron microscope (SEM) of red, blue, and green NPs was obtained through JSM-7500F, JEOL, Japan. All upconversion spectra in this paper were collected via the Hitachi F-4500 fluorescence spectrophotometer. All color photographs of this paper were taken by Nikon D3200.


## 2.1. Preparation of Red, Blue, and Green Nanoparticles

The preparation process of red upconversion KMnF3:Yb3+,Er3+ NPs is shown as follows: oleic acid, ethanol, and deionized water were poured into 50 ml beakers as reaction solvents with volumes of 10 ml, 5 ml, and 5 ml, respectively. 12 mmol KOH was poured into the aforementioned beaker, and the reaction solution was stirred for half an hour. Next, YbCl3·6H2O, ErCl3·6H2O, and MnCl2 with moles of 0.072 mmol, 0.008 mmol, and 0.32 mmol, respectively, were poured into the aforementioned beaker in sequence. Then, the reaction solution was stirred for another half an hour. 3.5 mmol KF was poured into the aforementioned beaker under stirring conditions. Finally, all the solutions in the beaker were placed in the hydrothermal reactor and kept at 200 degrees for 12 hours. The reaction product was centrifugally washed with a mixture of ethanol and cyclohexane [14, 15].The preparation process of blue upconversion NaYF4:Yb3+,Tm3+ NPs and green upconversion NaYF4:Yb3+,Er3+ NPs is the same. The specific preparation process is shown below: 4 ml oleic acid, 6 ml octadecene, and 0.4 mmol ReCl3·6H2O were poured into a 50 ml two-necked flask with round bottom and kept at 150 degrees for an hour with stirring. Argon needed to be introduced into the reaction system. After the reaction solution was cooled, 8 ml methanol solution containing 1 mmol NaOH and 1.2 mmol NH4F was poured into the aforementioned round bottom. The reaction system was stirred for half an hour. And the temperature of the reaction solution was raised to 50 degrees with stirring. In this process, argon was needed to flow into the atmosphere. Finally, the reaction solution was heated to 300 degrees and kept an hour with stirring. Argon was needed to flow into the atmosphere. The reaction product was centrifugally washed with a mixture of ethanol and cyclohexane [16, 17].


## 2.2. Characterizations

The X-ray powder diffraction (XRD) of red, blue, and green NPs was achieved via Rigaku RU-200B (λ=1.5406 Å, scanning range: 10°-70°). The scanning electron microscope (SEM) of red, blue, and green NPs was obtained through JSM-7500F, JEOL, Japan. All upconversion spectra in this paper were collected via the Hitachi F-4500 fluorescence spectrophotometer. All color photographs of this paper were taken by Nikon D3200.


## 3. Results and Discussion

The solvothermal strategy was used to synthesize KMnF3:18%Yb3+,1%Er3+ NPs, and the detailed synthesis steps of KMnF3:Yb3+,Er3+ NPs have been given in the experimental part (Section 2.1). Figure 1 displayed the XRD and SEM of KMnF3:Yb3+,Er3+ NPs. From the analysis of the results, synthetic KMnF3:Yb3+,Er3+ NPs are cubic KMnF3 crystals, because all diffraction peaks of synthetic KMnF3:Yb3+,Er3+ NPs and the cubic KMnF3 standard card (JCPDS: 82-1334) are in the same position (Figure 1(a)). The synthetic KMnF3:Yb3+,Er3+ NPs could be assigned a cubic structure, and its size is ~105±15 nm (Figure 1(b)). The high temperature thermal decomposition strategy was adopted to prepare NaYF4:18%Yb3+,0.5%Tm3+ NPs, and the operation steps of the experiment were introduced in the experimental part (Section 2.1). Figure 2 exhibits the XRD and SEM of NaYF4:Yb3+,Tm3+ NPs. As can be seen from Figure 2(a), the prepared NaYF4:Yb3+,Tm3+ NPs could be assigned a hexagonal structure. That is because the positions of all diffraction peaks of NaYF4:Yb3+,Tm3+ NPs and the hexagonal NaYF4 standard card (JCPDS: 28-1192) are identical. Besides, the morphology of NaYF4:Yb3+,Tm3+ NPs is hexagonal, and its size is ~195±10 nm (Figure 2(b)). The synthesis process of NaYF4:18%Yb3+,2%Er3+ NPs is the same as that of NaYF4:Yb3+,Tm3+ NPs. Figure 3 shows the XRD and SEM of NaYF4:18%Yb3+,2%Er3+ NPs. It has been found from Figure 3(a) that the position of the diffraction peak of NaYF4:Yb3+,Er3+ NPs is exactly the same as that of the hexagonal NaYF4 standard card (JCPDS: 28-1192), hereby certifying that the prepared NaYF4:Yb3+,Er3+ NPs could be assigned a hexagonal structure. Another conclusion can be drawn from Figure 3(b) that NaYF4:Yb3+,Er3+ NPs are ~200±10 nm.Figure 1
(a) The XRD of KMnF3:Yb3+,Er3+ NPs and a standard card (JCPDS: 82-1334). (b) The SEM image of KMnF3:Yb3+,Er3+ NPs.
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(b)Figure 2
(a) The XRD of NaYF4:Yb3+,Tm3+ NPs and a standard card (JCPDS: 28-1192). (b) The SEM image of NaYF4:Yb3+,Tm3+ NPs.
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(b)Figure 3
(a) The XRD of NaYF4:Yb3+,Er3+ NPs and a standard card (JCPDS: 28-1192). (b) The SEM image of NaYF4:Yb3+,Er3+ NPs.
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(b)Upconversion fluorescence spectra of KMnF3:18%Yb3+,1%Er3+ NPs, NaYF4:18%Yb3+,0.5%Tm3+ NPs, and NaYF4:18%Yb3+,2%Er3+ NPs excited by 980 nm are shown in Figures 4(a)–4(c). The fluorescence photographs of the corresponding NP cyclohexane solution are embedded in each figure. Figure 4(d) displays the CIE color coordinates of the corresponding NPs excited by 980 nm. In the fluorescence spectra of KMnF3:18%Yb3+,1%Er3+ NPs, an emission peak at ~653 nm can be clearly seen as a result of 4F9/2⟶4I15/2 radiation transition, which is due to the existence of efficient energy transfer between Er3+ and Mn2+: 2H11/2, 4S3/2+6A1⟶4I15/2+4T1, 2H9/2+6A1⟶4I13/2+4T1, and 4I15/2+4T1⟶4F9/2+6A1 [14, 15] (the energy-level diagram of KMnF3:Yb3+,Er3+ NPs excited with a 980 nm laser is shown in Figure S1). Therefore, KMnF3:Yb3+,Er3+ NPs can emit red fluorescence, which can be demonstrated by digital photos and the CIE color coordinates of KMnF3:Yb3+,Er3+ NPs. From Figure 4(b), it appears that NaYF4:Yb3+,Tm3+ NPs have three emission peaks: a strong emission peak at 477 nm (1G4⟶3H6) and two weak emission peaks at 451 nm (1D2⟶3F4) and 644 nm (3F3⟶3H6). All emission peaks come from radiation transition of Tm3+ ions in NaYF4:Yb3+,Tm3+ NPs (the energy-level diagram of NaYF4:Yb3+,Tm3+ NPs excited with a 980 nm laser is shown in Figure S2). Compared with the fluorescence emission intensity at 477 nm, the emission at 451 and 644 nm can be neglected, so NaYF4:Yb3+,Tm3+ NPs show blue fluorescence emission, which can be seen from the photographs and the CIE color coordinates of NaYF4:Yb3+,Tm3+ NPs. As you can see in Figure 4(c), in the fluorescence spectra of NaYF4:Yb3+,Er3+ NPs, a strong green upconversion emission peak (~541 nm) can be seen, which is attributed to the energy-level transition process of 4S3/2⟶4I15/2. Three weak upconversion emission peaks (~524 nm, ~410 nm, and ~657 nm) can be seen, which are attributed to the energy-level transition process of 4I11/2⟶4I15/2, 2H9/2⟶4I15/2, and 4F9/2⟶4I15/2 (the energy-level diagram of NaYF4:Yb3+,Er3+ NPs excited with a 980 nm laser is shown in Figure S3). Because these three emission peaks are too weak, the fluorescence color of NaYF4:Yb3+,Er3+ NPs show a green upconversion fluorescence. It can be proven in the digital photos and the CIE color coordinates of NaYF4:Yb3+,Er3+ NPs.Figure 4
(a) The upconversion spectrum of (a) KMnF3:18%Yb3+,1%Er3+ NPs, (b) NaYF4:18%Yb3+,0.5%Tm3+ NPs, and (c) NaYF4:18%Yb3+,2%Er3+ NPs in 980 nm excitation. Embedded in every figure is the picture of the corresponding NP cyclohexane solution without or with 980 nm excitation. (d) The CIE color coordinates of the NPs.
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(d)In the following experiments, we first mixed KMnF3:Yb3+,Er3+ NPs, NaYF4:Yb3+,Tm3+ NPs, and NaYF4:Yb3+,Er3+ NPs in the mass ratio of 42 : 54 : 4, respectively, and then dissolved the mixed NPs into cyclohexane. Under 980 nm irradiation, the mixed NPs can emit white upconversion luminescence; the fluorescence spectra are shown in Figure 5(a). Three distinct emission peaks can be observed in Figure 5(a); comparing with Figure 4, we can see that the first fluorescent emission is from Tm3+ ions of NaYF4:Yb3+,Tm3+ NPs with wavelengths ranging from 446 nm to 494 nm, the second fluorescent emission comes from Er3+ ions of NaYF4:Yb3+,Er3+ NPs with wavelengths ranging from 515 nm to 564 nm, and the third fluorescent emission is from Er3+ ions of KMnF3:Yb3+,Er3+ NPs with wavelengths ranging from 632 nm to 681 nm. Mixed NPs exhibit white upconversion luminescence, which can be demonstrated by photographs of mixed NPs and the CIE color coordinates of mixed NPs under 980 nm irradiation.Figure 5
(a) The upconversion spectrum of the mixed white NPs (mixing of KMnF3:Yb3+,Er3+ NPs, NaYF4:Yb3+,Tm3+ NPs, and NaYF4:Yb3+,Er3+ NPs with a mass percentage of 42%, 54%, and 4%, respectively) in 980 nm excitation. Embedded in the figure is the picture of white NP cyclohexane solution without or with 980 nm excitation. (b) The CIE color coordinates of the mixed white NPs.
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(b)Interestingly enough, we dissolve red KMnF3:Yb3+,Er3+ NPs, blue NaYF4:Yb3+,Tm3+ NPs, green NaYF4:Yb3+,Er3+ NPs, and the mixed white NPs (mixing of KMnF3:Yb3+,Er3+ NPs, NaYF4:Yb3+,Tm3+ NPs, and NaYF4:Yb3+,Er3+ NPs with a mass percentage of 42%, 54%, and 4%, respectively) into the cyclohexane as ink and write directly on the paper with this ink. Without 980 nm laser irradiation, no pattern or text can be seen on the paper. Under 980 nm laser irradiation, four letters of “J,” “L,” “N,” and “U” will appear on the paper, which are written using our own synthetic red, green, blue, and white ink. Under 980 nm laser excitation, the photographs are shown in Figures 6(a)–6(d). Furthermore, green upconversion NPs were used as ink for writing the letters “JLNU.” Under 980 nm irradiation, we can clearly see the letters “JLNU” appear on paper, as shown in Figure 6(e). Significantly, these four nanoparticles can be used in anticounterfeiting.Figure 6
The anticounterfeiting photographs of (a) KMnF3:Yb3+,Er3+ NPs, (b) NaYF4:Yb3+,Tm3+ NPs, (c) NaYF4:Yb3+,Er3+ NPs, and (d) the mixed white NPs (mixing of KMnF3:Yb3+,Er3+ NPs, NaYF4:Yb3+,Tm3+ NPs, and NaYF4:Yb3+,Er3+ NPs with a mass percentage of 42%, 54%, and 4%, respectively). (e) NaYF4:Yb3+,Er3+ NPs, under 980 nm irradiation.
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## 4. Conclusion

We prepared KMnF3:18%Yb3+,1%Er3+ NPs, NaYF4:18%Yb3+,0.5%Tm3+ NPs, and NaYF4:18%Yb3+,2%Er3+ NPs encapsulated by oleic acid ligands. Under 980 nm excitation, KMnF3:Yb3+,Er3+ NPs can emit strong red upconversion luminescence which comes from 4F9/2⟶4I15/2 radiation transition, NaYF4:Yb3+,Tm3+ NPs can send strong blue upconversion emission which comes from 1G4⟶3H6 radiation transition, and NaYF4:Yb3+,Er3+ NPs can give strong green upconversion emission which comes from 4S3/2⟶4I15/2 radiation transition. Then, the white upconversion NPs can be obtained by mixing the three NPs. These NPs are used as fluorescent anticounterfeiting materials in product safety design.



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*Source: 3872591-2019-12-20.xml*