- Email: [email protected]
Hydrothermal synthesis of hydrophilic NaYF4:Yb,Er nanoparticles with bright upconversion luminescence as biological label
Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Hydrothermal synthesis of hydrophilic NaYF4:Yb,Er nanoparticles with bright upconversion luminescence as biological label Yan Ma a,b,n, Meilian Chen a,b, Mingyong Li a,b a b
College of Computer and Information Science, Chongqing Normal University, Chongqing 400047, China Institute of Computational Chemistry, Chongqing Normal University, Chongqing 400047, China
art ic l e i nf o
a b s t r a c t
Article history: Received 10 August 2014 Accepted 8 October 2014
A prerequisite for biomedical applications of rare-earth upconverting nanophosphors (RUNPs) is to prepare hydrophilic RUNPs with strong upconverting luminescence. Herein, we developed a simple onepot synthesis strategy for hydrophilic NaYF4:Yb,Er nanophosphors by hydrothermally treating water/ ethylene glycol mixture containing different rare-earth ions, BF4 , Na þ and polyethylene glycol (MW ¼6000, abbreviated as PEG-6000). NaYF4:Yb,Er sample consisted of irregular spherical nanoparticles with average size of 50 nm, and it could be easily dispersed in water due to the presence of PEG6000. Importantly, the aqueous dispersion of NaYF4:Yb,Er nanoparticles exhibited bright upconversion luminescence and could be used for imaging HeLa cells. Therefore, these hydrophilic NaYF4:Yb,Er nanoparticles have great potential as luminescent labeling materials for biological applications. & 2014 Published by Elsevier B.V.
Keywords: Hydrothermal synthesis NaYF4 Nanoparticles Luminescence Biological label
1. Introduction Rare-earth upconverting nanophosphors (RUNPs) have attracted increasing attention in biomedical fields due to their unique properties, such as tunable multicolor emission, superior photostability, deep tissue penetration, suppression of autofluorescence and low toxicity [1–3]. Generally, a prerequisite for RUNPbased biomedical applications is to obtain hydrophilic RUNPs with strong upconverting luminescence. For obtaining hydrophilic RUNPs, two types of synthetic strategies have typically been developed. Strategy one is two-step controllable synthesis method, and the first step is to prepare RUNPs with hydrophobic organic ligands (such as oleic acid, oleylamine), for example, by the hydrothermal route assisted with oleic acid [4,5], thermolysis of rare-earth trifluoroacetate [6,7]. The second step is to convert hydrophobic RUNPs into hydrophilic ones, for example, by SiO2/ amphiphilic copolymer encapsulation [8–10], ligand exchange [11], ligand oxidization [12] and acid treatment [13]. However, these two-step synthesis processes may be costly and/or complicated, and precise control of some factors may be difficult. Strategy two is the one-pot synthesis of hydrophilic RUNPs, for example, by hydrothermal synthesis assisted with hydrophilic polymer [14], glycol-mediated solvothermal synthesis [15,16]. However, sizes and luminescence efficiencies of these hydrophilic RUNPs are not
n Corresponding author at: College of Computer and Information Science, Chongqing Normal University, Chongqing 400047, China. Tel.: þ 86 23 65910270. E-mail address: [email protected] (Y. Ma).
well optimized and thus RUNPs are still unsatisfied. Therefore, it is still necessary to develop simple methods for the one-pot preparation of hydrophilic RUNPs with small size and excellent luminescence. In addition, it has been revealed that BF4 ions is stable at room temperature but it can decompose slowly to produce F ions, which can be used as F source to prepare rare-earth nanoparticles [17,18]. Recently, we reported a facile one-pot synthesis strategy for hydrophilic NaYF4:Yb,Er@NaYF4:Yb active-core/active-shell nanoparticles by two-step injection of ethylene glycol (EG) solution containing BF4 , Na þ , Poly(vinylpyrrolidone) (PVP) and different rare-earth ions into EG at 180 1C [19]. To further extend the synthesis method, in the present work, we develop a simple onepot synthesis strategy for hydrophilic NaYF4:Yb,Er nanoparticles by hydrothermal treatment of water/EG mixture containing different rare-earth ions, BF4 , Na þ and polyethylene glycol (MW¼ 6000, abbreviated as PEG-6000). NaYF4:Yb,Er nanoparticles have the size of about 50 nm and can be easily dispersed in water. Importantly, the aqueous dispersion of NaYF4:Yb,Er nanoparticles exhibits bright upconversion luminescence and can be used as biological labels.
2. Experimental section All reagents were analytically pure and used without further purification. In a typical synthesis for NaYF4:Yb,Er nanoparticles, LnCl3 (total amounts: 1 mmol; 0.78 mmol YCl3 6H2O þ0.20 mmol
http://dx.doi.org/10.1016/j.matlet.2014.10.042 0167-577X/& 2014 Published by Elsevier B.V.
Please cite this article as: Ma Y, et al. Hydrothermal synthesis of hydrophilic NaYF4:Yb,Er nanoparticles with bright upconversion luminescence as biological label. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.10.042i
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Y. Ma et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
YbCl3 6H2O þ0.020 mmol ErCl3 6H2O) and polyethylene glycol (MW¼6000, abbreviated as PEG-6000, 1.0 g) were firstly dissolved in the mixture of water (20 mL) and ethylene glycol (EG, 20 mL). Subsequently, 1-butyl-3-methylimidazolium tetrafluoroborate (BMI BF4, 1 mL) and NaBF4 (1.2 mmol) as F and Na sources were added to the above solution. Then the solution was magnetically stirring until a homogeneous solution was formed. The resulting solution was transferred to a 50 mL autoclave, sealed, and hydrothermally treated at 180 1C for 10 h. The system was cooled to room temperature naturally, and white precipitate was collected by centrifugation at 10,000 rpm, and washed with deionized water and ethanol several times. Finally, the precipitate was dried under vacuum at room temperature for 24 h. The morphology of NaYF4:Yb,Er nanoparticles was determined by scanning electron microscope (SEM; Hitachi S-4800) and transmission electron microscope (TEM; JEM-2010 F). X-ray diffraction (XRD) measurement was made with a Bruker D4 X-ray diffractometer. Fourier transform infrared (FTIR) spectrum was measured using an IRPRESTIGE-21 spectrometer (Shimadzu). Upconverting luminescence spectrum was measured by using a JASCO FP-6600 spectrometer, but the excitation source was an external 980 nm laser device (Beijing Hi-Tech Optoelectronic Co., China). NaYF4:Yb,Er nanoparticles were used as biological luminescent labels to image HeLa cells, as described elsewhere [2]. Briefly, HeLa cells were incubated in phosphate buffer solution (PBS) containing 60 μg/mL NaYF4:Yb, Er nanoparticles at 37 1C for 4 h under 5% CO2, and then washed with PBS solution sufficiently to remove excess nanoparticles. The imaging in vitro was performed on modified laser scanning upconversion luminescence microscope (Olympus).
These cells were excited by 980 nm laser, and luminescence signals were detected in two channels: green channel (500– 560 nm) and red channel (600–700 nm).
3. Results and discussion In the present study, the solution containing Ln3 þ and BF4 ions is clear and stable at room temperature. When the solution was hydrothermally treated at 180 1C for 10 h, BF4 ions could decompose slowly to produce F ions, which is similar to the previous report [17–19]. Then F ions would react with rare-earth ions (Ln3 þ , Ln ¼ Y, Yb, Er) and Na þ , resulting in the formation of NaYF4: Yb,Er nanoparticles. In the course of the reaction, PEG-6000 and EG molecules were coated onto the outer face of the in-situ generated NaYF4:Yb,Er nanoparticles. The morphology, phase and surface ligand of NaYF4:Yb,Er nanoparticles were investigated. Fig. 1A shows SEM images of NaYF4:Yb,Er nanoparticles. Obviously, this sample consists of irregular spherical nanoparticles with average size of 40–70 nm. Further information is obtained from TEM images (Fig. 1B), and it confirms that NaYF4:Yb,Er particles are uniform and have an average diameter of 50 nm. Furthermore, the high-resolution TEM image (the inset in Fig. 1B) also indicates clear lattice fringes, and this interplanar d-spacing is determined to be 0.297 nm, agreeing with the (1 1 0) lattice fringes of hexagonal phase NaYF4 (JCPDs card 16-0334). In addition, XRD pattern (Fig. 1C) shows well-defined peaks, indicating the high crystallinity. Their peak positions and intensities confirm the existence of both cubic (JCPDs card 77-2042) and hexagonal (JCPDs card 16-0334) phase
Fig. 1. SEM image (A), TEM image (B), XRD pattern (C) and FT-IR spectrum (D) of hydrophilic NaYF4:Yb,Er nanoparticles. The standard XRD patterns of cubic phase (JCPDs: 77-2042) and hexagonal phase (JCPDs: 16-0334) of NaYF4 are also supplied.
Please cite this article as: Ma Y, et al. Hydrothermal synthesis of hydrophilic NaYF4:Yb,Er nanoparticles with bright upconversion luminescence as biological label. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.10.042i
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NaYF4, which is similar to the previous report [15]. The capping ligand on their surface was identified by FT-IR spectrum (Fig. 1D). The sample exhibits a broad band at about 3440 cm 1, due to the O–H stretching vibration of adsorbed water and/or PEG-6000. The 2923 and 2854 cm 1 transmission bands are respectively assigned to the asymmetric (νas) and symmetric (νs) stretching vibrations of methylene (CH2) units in PEG-6000 chain [20]. The band at 1635 cm 1 should be related to the bending modes of the hydroxyls of adsorbed water. The band at about 1097 cm 1 is corresponding to C-O stretching vibration coordinating to metal
Fig. 2. Upconverting luminescence spectrum of aqueous dispersion containing 1.0 mg/mL NaYF4:Yb,Er nanoparticles. The inset shows the luminescent photo of the aqueous dispersion under excitation of 980 nm laser.
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cations [15], suggesting the formation of chemical bonds between PEG-6000 and inorganic components. Therefore, it can be deduced that there are PEG-6000 ligands on the surface of NaYF4:Yb,Er sample. As a result of the presence of PEG-6000 ligands on their surface, NaYF4:Yb,Er nanoparticles could be readily dispersed in water. The aqueous dispersion containing 1.0 mg/mL NaYF4:Yb,Er nanoparticles shows strong yellow-green luminescence under 980 nm laser excitation (the inset of Fig. 2). The corresponding upconversion spectrum was recorded (Fig. 2), and there are three distinct Er3 þ emission bands, which is similar to what has been reported previously for these materials [4–12]. The double green emissions between 514 and 534 nm and between 534 and 560 nm are attributed to the 2H11/2-4I15/2 and 4S3/2-to 4I15/2 transitions, respectively. A red emission is observed between 635 and 680 nm due to the transition from 4F9/2 to 4I15/2. Most importantly, hydrophilic NaYF4:Yb,Er nanoparticles with bright upconversion luminescence have great potential as biological luminescent labels. Herein, as an applied example of bioimaging, live HeLa cells were labeled with NaYF4:Yb,Er nanoparticles (60 μg/mL), and the feasibility of imaging of live cells was demonstrated (Fig. 3). Obviously, HeLa cell treated with NaYF4:Yb,Er nanoparticles for 4 h shows intense upconverting luminescence signals at 500–560 nm (green channel, Fig. 3a) and 600–700 nm (red channel, Fig. 3b) under 980 nm laser excitation. In addition, brightfield image of the cell was also measured (Fig. 3c). The overlay (Fig. 3d) of the upconverting luminescence (green and red channels) and brightfield image indicates that signal distributions of the upconverting luminescence is strongly correlated with the HeLa cell, which may result from the adsorption/endocytosis of nanoparticles in/on HeLa cells [2].
Fig. 3. Images of a live HeLa cell labeled with NaYF4:Yb,Er nanoparticles: (a) green channel image when emission was collected at 500–560 nm, (b) red channel image when emission was collected at 600–700 nm, (c) brightfield image, (d) the overlay of upconverting luminescence (green and red channels) and brightfield images. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Please cite this article as: Ma Y, et al. Hydrothermal synthesis of hydrophilic NaYF4:Yb,Er nanoparticles with bright upconversion luminescence as biological label. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.10.042i
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Based on the above results, one can find that there are some advantages for the present hydrophilic NaYF4:Yb,Er nanoparticles, including: (1) the present one-pot synthesis strategy is very simple, compared with two-step controllable synthesis method [8–13]; (2) BF4 ions are used to decompose slowly to produce F ions, resulting in the growth of hydrophilic NaYF4:Yb,Er nanoparticles with relatively small diameter; (3) NaYF4:Yb,Er nanoparticles exhibit bright upconversion luminescence and can be used for imaging HeLa cells. Therefore, these hydrophilic NaYF4:Yb,Er nanoparticles have great potential as luminescent labeling materials for biological applications.
KJ120617) and Chongqing Education Science “The 12th Five Year “ planning 2013 Education Technology special Project. References [1] [2] [3] [4] [5] [6] [7]
4. Conclusions NaYF4:Yb,Er nanoparticles have been prepared by hydrothermal treatment of water/EG mixture containing different rare-earth ions, BF4 , Na þ and PEG-6000 at 180 1C for 10 h. NaYF4:Yb,Er sample have the diameter of about 50 nm and are hydrophilic. Furthermore, the aqueous dispersion containing NaYF4:Yb,Er nanoparticles exhibits bright upconversion luminescence and can be used as biological luminescent labels. Acknowledgments This work was financially supported by the Chongqing Education Committee Science and Technology funded Projects (NO:
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
Wang F, Liu X. Accounts Chem Res 2014;47:1378. Zhou J, Liu Z, Li F. Chem Soc Rev 2012;41:1323. Chen P, Yu S, Xu B, Wang J, Sang X, Liu X, et al. Mater Lett 2014;128:299. Zhang F, Wan Y, Yu T, Zhang F, Shi Y, Xie S, et al. Angew Chem Int Edit 2007;46:7976. Zeng S, Wang H, Lu W, Yi Z, Rao L, Liu H, et al. Biomaterials 2014;35:2934. Mai HX, Zhang YW, Si R, Yan ZG, Sun LD, You LP, et al. J Am Chem Soc 2006;128:6426. Huang P, Zheng W, Zhou S, Tu D, Chen Z, Zhu H, et al. Angew Chem Int Edit 2014;53:1252. Li Z, Zhang Y, Jiang S. Adv Mater 2008;20:4765. Yi G-S, Chow G-M. Chem Mater 2007;19:341. Wang X, Liu K, Yang G, Cheng L, He L, Liu Y, et al. Nanoscale 2014;6:9198. Ma C, Bian T, Yang S, Liu C, Zhang T, Yang J, et al. Anal Chem 2014;86:6508. Chen ZG, Chen HL, Hu H, Yu MX, Li FY, Zhang Q, et al. J Am Chem Soc 2008;130:3023. Wang L, Dong H, Li Y, Xue C, Sun L-D, Yan C-H, et al. J Am Chem Soc 2014;136:4480. Wang F, Chatterjee DK, Li Z, Zhang Y, Fan X, Wang M. Nanotechnology 2006;17:5786. Chen ZG, Tian QW, Song YL, Yang JM, Hu JQ. J Alloy Compd 2010;506:L17. Song YL, Tian QW, Zou RJ, Chen ZG, Yang JM, Hu JQ. J Alloy Compd 2011;509:6539. Zhang L, Li W, Hu X, Peng Y, Hu J, Kuang X, et al. Prog Nat Sci 2012;22:488. Chen C, Sun L-D, Li Z-X, Li L-L, Zhang J, Zhang Y-W, et al. Langmuir 2010;26:8797. Chen M, Ma Y, Li M. Mater Lett 2014;114:80. Xu W, Tian Q, Chen Z, Xia M, Macharia DK, Sun B, et al. J Mater Chem B 2014;2:5594.
Please cite this article as: Ma Y, et al. Hydrothermal synthesis of hydrophilic NaYF4:Yb,Er nanoparticles with bright upconversion luminescence as biological label. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.10.042i
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Hydrothermal synthesis of hydrophilic NaYF4:Yb,Er nanoparticles with bright upconversion luminescence as biological label Yan Ma a,b,n, Meilian Chen a,b, Mingyong Li a,b a b
College of Computer and Information Science, Chongqing Normal University, Chongqing 400047, China Institute of Computational Chemistry, Chongqing Normal University, Chongqing 400047, China
art ic l e i nf o
a b s t r a c t
Article history: Received 10 August 2014 Accepted 8 October 2014
A prerequisite for biomedical applications of rare-earth upconverting nanophosphors (RUNPs) is to prepare hydrophilic RUNPs with strong upconverting luminescence. Herein, we developed a simple onepot synthesis strategy for hydrophilic NaYF4:Yb,Er nanophosphors by hydrothermally treating water/ ethylene glycol mixture containing different rare-earth ions, BF4 , Na þ and polyethylene glycol (MW ¼6000, abbreviated as PEG-6000). NaYF4:Yb,Er sample consisted of irregular spherical nanoparticles with average size of 50 nm, and it could be easily dispersed in water due to the presence of PEG6000. Importantly, the aqueous dispersion of NaYF4:Yb,Er nanoparticles exhibited bright upconversion luminescence and could be used for imaging HeLa cells. Therefore, these hydrophilic NaYF4:Yb,Er nanoparticles have great potential as luminescent labeling materials for biological applications. & 2014 Published by Elsevier B.V.
Keywords: Hydrothermal synthesis NaYF4 Nanoparticles Luminescence Biological label
1. Introduction Rare-earth upconverting nanophosphors (RUNPs) have attracted increasing attention in biomedical fields due to their unique properties, such as tunable multicolor emission, superior photostability, deep tissue penetration, suppression of autofluorescence and low toxicity [1–3]. Generally, a prerequisite for RUNPbased biomedical applications is to obtain hydrophilic RUNPs with strong upconverting luminescence. For obtaining hydrophilic RUNPs, two types of synthetic strategies have typically been developed. Strategy one is two-step controllable synthesis method, and the first step is to prepare RUNPs with hydrophobic organic ligands (such as oleic acid, oleylamine), for example, by the hydrothermal route assisted with oleic acid [4,5], thermolysis of rare-earth trifluoroacetate [6,7]. The second step is to convert hydrophobic RUNPs into hydrophilic ones, for example, by SiO2/ amphiphilic copolymer encapsulation [8–10], ligand exchange [11], ligand oxidization [12] and acid treatment [13]. However, these two-step synthesis processes may be costly and/or complicated, and precise control of some factors may be difficult. Strategy two is the one-pot synthesis of hydrophilic RUNPs, for example, by hydrothermal synthesis assisted with hydrophilic polymer [14], glycol-mediated solvothermal synthesis [15,16]. However, sizes and luminescence efficiencies of these hydrophilic RUNPs are not
n Corresponding author at: College of Computer and Information Science, Chongqing Normal University, Chongqing 400047, China. Tel.: þ 86 23 65910270. E-mail address: [email protected] (Y. Ma).
well optimized and thus RUNPs are still unsatisfied. Therefore, it is still necessary to develop simple methods for the one-pot preparation of hydrophilic RUNPs with small size and excellent luminescence. In addition, it has been revealed that BF4 ions is stable at room temperature but it can decompose slowly to produce F ions, which can be used as F source to prepare rare-earth nanoparticles [17,18]. Recently, we reported a facile one-pot synthesis strategy for hydrophilic NaYF4:Yb,Er@NaYF4:Yb active-core/active-shell nanoparticles by two-step injection of ethylene glycol (EG) solution containing BF4 , Na þ , Poly(vinylpyrrolidone) (PVP) and different rare-earth ions into EG at 180 1C [19]. To further extend the synthesis method, in the present work, we develop a simple onepot synthesis strategy for hydrophilic NaYF4:Yb,Er nanoparticles by hydrothermal treatment of water/EG mixture containing different rare-earth ions, BF4 , Na þ and polyethylene glycol (MW¼ 6000, abbreviated as PEG-6000). NaYF4:Yb,Er nanoparticles have the size of about 50 nm and can be easily dispersed in water. Importantly, the aqueous dispersion of NaYF4:Yb,Er nanoparticles exhibits bright upconversion luminescence and can be used as biological labels.
2. Experimental section All reagents were analytically pure and used without further purification. In a typical synthesis for NaYF4:Yb,Er nanoparticles, LnCl3 (total amounts: 1 mmol; 0.78 mmol YCl3 6H2O þ0.20 mmol
http://dx.doi.org/10.1016/j.matlet.2014.10.042 0167-577X/& 2014 Published by Elsevier B.V.
Please cite this article as: Ma Y, et al. Hydrothermal synthesis of hydrophilic NaYF4:Yb,Er nanoparticles with bright upconversion luminescence as biological label. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.10.042i
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Y. Ma et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
YbCl3 6H2O þ0.020 mmol ErCl3 6H2O) and polyethylene glycol (MW¼6000, abbreviated as PEG-6000, 1.0 g) were firstly dissolved in the mixture of water (20 mL) and ethylene glycol (EG, 20 mL). Subsequently, 1-butyl-3-methylimidazolium tetrafluoroborate (BMI BF4, 1 mL) and NaBF4 (1.2 mmol) as F and Na sources were added to the above solution. Then the solution was magnetically stirring until a homogeneous solution was formed. The resulting solution was transferred to a 50 mL autoclave, sealed, and hydrothermally treated at 180 1C for 10 h. The system was cooled to room temperature naturally, and white precipitate was collected by centrifugation at 10,000 rpm, and washed with deionized water and ethanol several times. Finally, the precipitate was dried under vacuum at room temperature for 24 h. The morphology of NaYF4:Yb,Er nanoparticles was determined by scanning electron microscope (SEM; Hitachi S-4800) and transmission electron microscope (TEM; JEM-2010 F). X-ray diffraction (XRD) measurement was made with a Bruker D4 X-ray diffractometer. Fourier transform infrared (FTIR) spectrum was measured using an IRPRESTIGE-21 spectrometer (Shimadzu). Upconverting luminescence spectrum was measured by using a JASCO FP-6600 spectrometer, but the excitation source was an external 980 nm laser device (Beijing Hi-Tech Optoelectronic Co., China). NaYF4:Yb,Er nanoparticles were used as biological luminescent labels to image HeLa cells, as described elsewhere [2]. Briefly, HeLa cells were incubated in phosphate buffer solution (PBS) containing 60 μg/mL NaYF4:Yb, Er nanoparticles at 37 1C for 4 h under 5% CO2, and then washed with PBS solution sufficiently to remove excess nanoparticles. The imaging in vitro was performed on modified laser scanning upconversion luminescence microscope (Olympus).
These cells were excited by 980 nm laser, and luminescence signals were detected in two channels: green channel (500– 560 nm) and red channel (600–700 nm).
3. Results and discussion In the present study, the solution containing Ln3 þ and BF4 ions is clear and stable at room temperature. When the solution was hydrothermally treated at 180 1C for 10 h, BF4 ions could decompose slowly to produce F ions, which is similar to the previous report [17–19]. Then F ions would react with rare-earth ions (Ln3 þ , Ln ¼ Y, Yb, Er) and Na þ , resulting in the formation of NaYF4: Yb,Er nanoparticles. In the course of the reaction, PEG-6000 and EG molecules were coated onto the outer face of the in-situ generated NaYF4:Yb,Er nanoparticles. The morphology, phase and surface ligand of NaYF4:Yb,Er nanoparticles were investigated. Fig. 1A shows SEM images of NaYF4:Yb,Er nanoparticles. Obviously, this sample consists of irregular spherical nanoparticles with average size of 40–70 nm. Further information is obtained from TEM images (Fig. 1B), and it confirms that NaYF4:Yb,Er particles are uniform and have an average diameter of 50 nm. Furthermore, the high-resolution TEM image (the inset in Fig. 1B) also indicates clear lattice fringes, and this interplanar d-spacing is determined to be 0.297 nm, agreeing with the (1 1 0) lattice fringes of hexagonal phase NaYF4 (JCPDs card 16-0334). In addition, XRD pattern (Fig. 1C) shows well-defined peaks, indicating the high crystallinity. Their peak positions and intensities confirm the existence of both cubic (JCPDs card 77-2042) and hexagonal (JCPDs card 16-0334) phase
Fig. 1. SEM image (A), TEM image (B), XRD pattern (C) and FT-IR spectrum (D) of hydrophilic NaYF4:Yb,Er nanoparticles. The standard XRD patterns of cubic phase (JCPDs: 77-2042) and hexagonal phase (JCPDs: 16-0334) of NaYF4 are also supplied.
Please cite this article as: Ma Y, et al. Hydrothermal synthesis of hydrophilic NaYF4:Yb,Er nanoparticles with bright upconversion luminescence as biological label. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.10.042i
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Y. Ma et al. / Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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NaYF4, which is similar to the previous report [15]. The capping ligand on their surface was identified by FT-IR spectrum (Fig. 1D). The sample exhibits a broad band at about 3440 cm 1, due to the O–H stretching vibration of adsorbed water and/or PEG-6000. The 2923 and 2854 cm 1 transmission bands are respectively assigned to the asymmetric (νas) and symmetric (νs) stretching vibrations of methylene (CH2) units in PEG-6000 chain [20]. The band at 1635 cm 1 should be related to the bending modes of the hydroxyls of adsorbed water. The band at about 1097 cm 1 is corresponding to C-O stretching vibration coordinating to metal
Fig. 2. Upconverting luminescence spectrum of aqueous dispersion containing 1.0 mg/mL NaYF4:Yb,Er nanoparticles. The inset shows the luminescent photo of the aqueous dispersion under excitation of 980 nm laser.
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cations [15], suggesting the formation of chemical bonds between PEG-6000 and inorganic components. Therefore, it can be deduced that there are PEG-6000 ligands on the surface of NaYF4:Yb,Er sample. As a result of the presence of PEG-6000 ligands on their surface, NaYF4:Yb,Er nanoparticles could be readily dispersed in water. The aqueous dispersion containing 1.0 mg/mL NaYF4:Yb,Er nanoparticles shows strong yellow-green luminescence under 980 nm laser excitation (the inset of Fig. 2). The corresponding upconversion spectrum was recorded (Fig. 2), and there are three distinct Er3 þ emission bands, which is similar to what has been reported previously for these materials [4–12]. The double green emissions between 514 and 534 nm and between 534 and 560 nm are attributed to the 2H11/2-4I15/2 and 4S3/2-to 4I15/2 transitions, respectively. A red emission is observed between 635 and 680 nm due to the transition from 4F9/2 to 4I15/2. Most importantly, hydrophilic NaYF4:Yb,Er nanoparticles with bright upconversion luminescence have great potential as biological luminescent labels. Herein, as an applied example of bioimaging, live HeLa cells were labeled with NaYF4:Yb,Er nanoparticles (60 μg/mL), and the feasibility of imaging of live cells was demonstrated (Fig. 3). Obviously, HeLa cell treated with NaYF4:Yb,Er nanoparticles for 4 h shows intense upconverting luminescence signals at 500–560 nm (green channel, Fig. 3a) and 600–700 nm (red channel, Fig. 3b) under 980 nm laser excitation. In addition, brightfield image of the cell was also measured (Fig. 3c). The overlay (Fig. 3d) of the upconverting luminescence (green and red channels) and brightfield image indicates that signal distributions of the upconverting luminescence is strongly correlated with the HeLa cell, which may result from the adsorption/endocytosis of nanoparticles in/on HeLa cells [2].
Fig. 3. Images of a live HeLa cell labeled with NaYF4:Yb,Er nanoparticles: (a) green channel image when emission was collected at 500–560 nm, (b) red channel image when emission was collected at 600–700 nm, (c) brightfield image, (d) the overlay of upconverting luminescence (green and red channels) and brightfield images. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Please cite this article as: Ma Y, et al. Hydrothermal synthesis of hydrophilic NaYF4:Yb,Er nanoparticles with bright upconversion luminescence as biological label. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.10.042i
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Based on the above results, one can find that there are some advantages for the present hydrophilic NaYF4:Yb,Er nanoparticles, including: (1) the present one-pot synthesis strategy is very simple, compared with two-step controllable synthesis method [8–13]; (2) BF4 ions are used to decompose slowly to produce F ions, resulting in the growth of hydrophilic NaYF4:Yb,Er nanoparticles with relatively small diameter; (3) NaYF4:Yb,Er nanoparticles exhibit bright upconversion luminescence and can be used for imaging HeLa cells. Therefore, these hydrophilic NaYF4:Yb,Er nanoparticles have great potential as luminescent labeling materials for biological applications.
KJ120617) and Chongqing Education Science “The 12th Five Year “ planning 2013 Education Technology special Project. References [1] [2] [3] [4] [5] [6] [7]
4. Conclusions NaYF4:Yb,Er nanoparticles have been prepared by hydrothermal treatment of water/EG mixture containing different rare-earth ions, BF4 , Na þ and PEG-6000 at 180 1C for 10 h. NaYF4:Yb,Er sample have the diameter of about 50 nm and are hydrophilic. Furthermore, the aqueous dispersion containing NaYF4:Yb,Er nanoparticles exhibits bright upconversion luminescence and can be used as biological luminescent labels. Acknowledgments This work was financially supported by the Chongqing Education Committee Science and Technology funded Projects (NO:
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Please cite this article as: Ma Y, et al. Hydrothermal synthesis of hydrophilic NaYF4:Yb,Er nanoparticles with bright upconversion luminescence as biological label. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.10.042i
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