Langmuir 2008, 24, 12123-12125
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Synthesis of Hexagonal-Phase Core-Shell NaYF4 Nanocrystals with Tunable Upconversion Fluorescence Hai-Sheng Qian† and Yong Zhang*,†,‡ DiVision of Bioengineering, Faculty of Engineering, Blk E3A-04-15, National UniVersity of Singapore, 7 Engineering DriVe 1, Singapore 117574, and Nanoscience and Nanotechnology InitiatiVe, National UniVersity of Singapore, 9 Engineering DriVe 1, Singapore 117576 ReceiVed July 24, 2008. ReVised Manuscript ReceiVed September 23, 2008 Hexagonal-phase core-shell-structured NaYF4:Yb,Tm@β-NaYF4:Yb,Er and β-NaYF4:Yb,Tm@β-NaYF4:Yb,Er@βNaYF4:Yb,Tm nanocrystals were synthesized by a seeded growth approach. β-NaYF4:Yb,Tm nanocrystals with 20 nm diameter were used as seed crystals to induce the growth of β-NaYF4:Yb,Er and then β-NaYF4:Yb,Tm crystals, resulting in the formation of core-shell-structured nanocrystals with upconverting lanthanide ions Tm and Er doped in the core and shell, respectively.
1. Introduction Because the shape, phase, size, and components of inorganic nanocrystals are important elements in varying their electrical, optical, and other properties,1 rational control over those elements has become a hot research topic in recent years.2 Compared to down-conversion fluorescent materials, upconversion (UC) fluorescent nanoparticles that convert near-infrared (NIR) light to visible light by emitting high-energy photons after absorbing low-energy photons have the following advantages: high light penetration depth in tissues, no photodamage to living organisms, weak autofluorescence from cells or tissues, and low background noise and high sensitivity for detection.3 Upconverting lanthanidedoped inorganic crystals has been extensively studied over the past two decades owing to wide applications in solid-state lasers,4 flat-panel displays,5 solar cells,6 and nanobiotechnology.7 It has been reported that hexagonal-phase NaYF4 (β-NaYF4) crystals are the most efficient host materials for upconverting lanthanide ions owing to the low phonon energy of the crystal lattice.8 Much effort has been expended to synthesize NaYF4 nanocrystals * Corresponding author. E-mail:
[email protected]. Fax: (+65)68723069. † Division of Bioengineering. ‡ Nanoscience and Nanotechnology Initiative. (1) (a) Lieber, C. M. Solid State Commun. 1998, 107, 607. (b) Alivisatos, A. P. Science 1996, 271, 933. (c) Peng, X. G.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (2) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayer, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (3) (a) Heer, S.; Kompe, K.; Gudel, H. U.; Haase, M. AdV. Mater. 2004, 16, 2102. (b) Suyver, J. F.; Aebischer, A.; Biner, D.; Gerner, P.; Grimm, J.; Heer, S.; Kramer, K. W.; Reinhard, C.; Gudel, H. U. Opt. Mater. 2005, 27, 1111. (4) Heine, F.; Heumann, E.; Danger, T.; Schweizer, T.; Huber, G.; Chai, B. Appl. Phys. Lett. 1994, 65, 383. (5) Downing, E.; Hesselink, L.; Ralston, J.; Macfarlane, R. Science 1996, 273, 1185. (6) Shalav, A.; Richards, B. S.; Trupke, T.; Kramer, K. W.; Gudel, H. U. Appl. Phys. Lett. 2005, 86, 013505. (7) (a) Li, Z. Q.; Zhang, Y. Angew. Chem., Int. Ed. 2006, 45, 7732–7735. (b) Wang, F.; Tan, W. B.; Zhang, Y.; Fan, X. P.; Wang, M. Q. Nanotechnology 2006, 17, R1. (c) Wang, L. Y.; Li, Y. D. Chem. Commun. 2006, n/a, 2557. (d) Wang, L. Y.; Yan, R. X.; Huo, Z. Y.; Wang, L.; Zeng, J. H.; Bao, J.; Wang, X.; Peng, Q.; Li, Y. D. Angew. Chem., Int. Ed. 2005, 44, 6054. (e) Wang, L. Y.; Li, Y. D. AdV. Mater. 2007, 19, 3304. (f) Yi, G. S.; Lu, H. C.; Zhao, S. Y.; Yue, G.; Yang, W. J. Nano Lett. 2004, 4, 2191. (8) (a) Burns, J. H. Inorg. Chem. 1965, 4, 881. (b) Aebischer, A.; Hostettler, M.; Hauser, J.; Kramer, K.; Weber, T.; Gudel, H. U.; Burgi, H. B. Angew. Chem., Int. Ed. 2006, 45, 2802. (c) Suyver, J. F.; Aebischer, A.; Garcia-Revilla, S.; Gerner, P.; Gudel, H. U. Phys. ReV. B 2005, 71, 125123. (d) Bril, A.; Sommerdijk, J. L.; Jager, A. W. De J. Electrochem.Soc. 1975, 122, 660. (e) Suyver, J. F.; Grimm, J.; Kramer, K. W.; Gudel, H. U. J. Lumin. 2005, 114, 53.
with different sizes and morphology, for instance, cothermolysis of Na(CF3COO) and RE(CF3COO)(3) in oleic acid/oleylamine/ 1-octadecene9 and hydrothermal or solvothermal methods.10,11 CeF3:Tb/LaF3 core-shell nanoparticles have been synthesized by using the polyol method.12 Core-shell NaYF4 nanoparticles such as R-NaYF4:Yb,Er/R-NaYF4,β-NaYF4:Yb,Er/R-NaYF4 nanoparticles have also been made to improve the upconversion efficiency.13 Recently, a method was reported for synthesizing core-shell β-NaYF4:Yb,Er(Tm)/β-NaYF4 nanoparticles with both the core and shell composed of hexagonal-phase NaYF4 crystals.14 However, no upconverting lanthanide ions were doped into the NaYF4 shell. These nanoparticles have great potential to be used in bioimaging and biodetection as fluorescent probes. However, the use of nanoparticles for the multiplexing detection of biomolecules requires multicolor fluorescent probes. So far only a few lanthanide ions such as Tm and Er are doped into NaYF4 nanocrystals to emit observable blue, green, or red upconversion fluorescence. It is possible to codope Er and Tm into the same NaYF4 nanocrystals and tune the fluorescence emission by changing the Er/Tm ratio; for instance, cubic-phase NaYF4 nanocrystals doped with Yb3+, Tm3+, and Er3+ were prepared and fluorescence emission wavelengths were tuned from the visible to NIR regions through combinations of different upconverting lanthanide ions and changes in their concentrations.15 (9) (a) Mai, H. X.; Zhang, Y. W.; Si, R.; Yan, Z. G.; Sun, L. D.; You, L. P.; Yan, C. H. J. Am. Chem. Soc. 2006, 128, 6426. (b) Mai, H. X.; Zhang, Y. W.; Sun, L. D.; Yan, C. H. J. Phys. Chem. C 2007, 111, 13730. (c) Boyer, J. C.; Vetrone, F.; Cuccia, L. A.; Capobianco, J. A. J. Am. Chem. Soc. 2006, 128, 7444. (d) Boyer, J. C.; Cuccia, L. A.; Capobianco, J. A. Nano Lett. 2007, 7, 847. (e) Yi, G. S.; Chow, G. M. AdV. Funct. Mater. 2006, 18, 2324. (f) Shan, J. N.; Qin, X.; Yao, N.; Ju, Y. G. Nanotechnology 2007, 18, 445607. (10) (a) Wang, L. Y.; Li, Y. D. Nano Lett. 2006, 6, 1645. (b) Wang, L. Y.; Li, Y. D. Chem. Mater. 2007, 19, 727. (c) Zeng, J. H.; Li, Z. H.; Su, J.; Wang, L. Y.; Yan, R. X.; Li, Y. D. Nanotechnology 2006, 17, 3549. (d) Zeng, J. H.; Su, J.; Li, Z. H.; Yan, R. X.; Li, Y. D. AdV. Mater. 2005, 17, 2119. (e) Liang, X.; Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. D. Inorg. Chem. 2007, 46, 6050. (f) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. D. Inorg. Chem. 2006, 45, 6661. (g) Li, C. X.; Quan, Z. W.; Yang, J.; Yang, P. P.; Lin, J. Inorg. Chem. 2007, 46, 6329. (11) Zhang, F.; Wang, Y.; Yu, T.; Zhang, F. Q.; Shi, Y. F.; Xie, S. H.; Li, Y. G.; Xu, L.; Tu, B.; Zhao, D. Y. Angew. Chem., Int. Ed. 2007, 46, 7976. (12) Wang, Z. L.; Quan, Z. W.; Jia, P. Y.; Lin, C. K.; Luo, Y.; Chen, Y.; Fang, J.; Zhou, W.; O’Connor, C. J.; Lin, J. Chem. Mater. 2006, 18, 2030. (13) Mai, H. X.; Zhang, Y. W.; Sun, L. D.; Yan, C. H. J. Phys. Chem. C 2007, 111, 13721. (14) Yi, G. S.; Chow, G. M. Chem. Mater. 2007, 19, 341. (15) Wang, F.; Liu, X. G. J. Am. Chem. Soc. 2008, 130, 5642.
10.1021/la802343f CCC: $40.75 2008 American Chemical Society Published on Web 10/08/2008
12124 Langmuir, Vol. 24, No. 21, 2008
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In this work, a method has been developed to synthesize β-NaYF4:Yb,Tm@β-NaYF4:Yb,Er core-shell nanocrystals with Tm and Er doped in the core and shell, respectively. β-NaYF4: Yb,Tm nanocrystals with 20 nm diameter (core) were used as seed crystals to induce the growth of β-NaYF4:Yb,Er and β-NaYF4:Yb,Tm crystals, resulting in the formation of β-NaYF4: Yb,Tm@β-NaYF4:Yb,Erorβ-NaYF4:Yb,Tm@β-NaYF4:Yb,Er@ β-NaYF4:Yb,Tm nanocrystals with core-shell structure. The nanocrystals emit multicolor NIR-to-visible upconversion fluorescence upon excitation at a wavelength of 980 nm.
2. Experimental Section 2.1. Synthesis of β-NaYF4:Yb,Tm Nanocrystals. All chemicals were of analytical grade and used without further purification. The synthesis of monodisperse β-NaYF4:Yb(25%), Tm (0.3%) nanocrystals has been reported in detail.16 In a typical procedure for the synthesis of β-NaYF4:Yb/Tm, 0.75 mmol of YCl3, 0.25 mmol of YbCl3, and 0.003 mmol of TmCl3 were added to a 100 mL flask and dissolved in 2 mL of DI water to form a clear solution after vigorous stirring. After 6 mL of oleic acid and 15 mL of 1-octadecene were added, the solution was heated to 100 °C for 10 min and then to 156 °C for 30 min and then cooled to room temperature. A solution of 4 mmol of NH4F (0.1482 g) and 2.5 mmol of NaOH (0.1 g) in 10 mL of methanol was added, and then the solution was kept at 50 °C for 30 min. After methanol was evaporated, the solution was heated to 300 °C under an argon atmosphere for 1.5 h and then cooled to room temperature. The nanocrystals were precipitated with 10 mL of acetone, collected after centrifugation, and redispersed in 6 mL of cyclohexane. 2.2. Synthesisofβ-NaYF4:Yb,Tm@β-NaYF4:Yb,ErNanocrystals. YCl3 (0.80 mmol), YbCl3 (0.2 mmol), and ErCl3 (0.02 mmol) were added to a 100 mL flask and dissolved in 2 mL of DI water to form a clear solution after vigorous stirring. Oleic acid (6 mL) and 1-octadecene (15 mL) were subsequently added, and the solution was heated to 100 °C for 10 min and then to 156 °C for 30 min to form a clear yellow solution and then cooled to 80 °C. A solution of 1 mmol of NaYF4:Yb,Tm nanocrystals in 6 mL of cyclohexane was added to the solution. After the removal of cyclohexane, a solution of 4 mmol of NH4F (0.1482 g) and 2.5 mmol of NaOH (0.1 g) in 10 mL of methanol was added, and then the solution was kept at 50 °C for 30 min. After the methanol was evaporated, the solution was heated to 300 °C under argon for 1.5 h and cooled to room temperature. The nanocrystals were collected after centrifugation and then redispersed in 6 mL of cyclohexane. 2.3. Synthesis of β-NaYF4:Yb,Tm@β-NaYF4:Yb,Er@β-NaYF4: Yb,Tm Nanocrystals. YCl3 (0.75 mmol), YbCl3 (0.25 mmol), and TmCl3 (0.003 mmol) were added to a 100 mL flask and dissolved in 2 mL of DI water to form a clear solution after vigorous stirring. After 6 mL of oleic acid and 15 mL of 1-octadecene were added, the solution was heated to 100 °C for 10 min and then to 156 °C for 30 min and then cooled to room temperature. A solution of β-NaYF4:Yb,Tm@β-NaYF4:Yb,Er nanocrystals in 6 mL of cyclohexane was added to the solution. A solution of 4 mmol of NH4F (0.1482 g) and 2.5 mmol of NaOH (0.1 g) in 10 mL of methanol was added, and then the solution was kept at 50 °C for 30 min. After the methanol was evaporated, the solution was heated to 300 °C under an argon atmosphere for 1.5 h and then cooled to room temperature. The nanocrystals were precipitated with 10 mL of acetone, collected after centrifugation, and redispersed in 6 mL of cyclohexane. 2.4. Characterization. Transmission electron microscope (TEM) images were recorded on a JEOL-2010 TEM operated at an acceleration voltage of 200 kV. X-ray powder diffraction (XRD) measurements were made on a Siemens D5005 X-ray powder diffractometer equipped with Co KR radiation (λ ) 1.78897 Å; the diffraction patterns are different from those obtained with Cu KR radiation). Fluorescence spectra were recorded on a Hitachi F-500 (16) Li, Z. Q.; Zhang, Y. Nanotechnology. 2008, 19, 345606.
Figure 1. TEM images of monodisperse NaYF4:Yb,Tm nanocrystals (a, b) and core-shell NaYF4:Yb,Tm@ NaYF4:Yb,Er nanocrystals (c, d).
fluorescence spectrophotometer equipped with a commercial CW IR laser (980 nm).
3. Results and Discussion Small hexagonal-phase NaYF4 (β-NaYF4) nanocrystals were synthesized following a protocol reported previously.16 Figure 1a,b showed that the nanocrystals were monodisperse and ca.20 nm in diameter. These nanocrystals were codoped with Yb (25%) and Tm (0.3%) (upconverting lanthanide ions) and used as seed crystals to further grow β-NaYF4 crystals on the surface in which lanthanide ions Yb and Er were doped, to form core-shell NaYF4: Yb,Tm@ NaYF4:Yb,Er nanocrystals, with Tm and Er ions doped into the core and shell, respectively. Core-shell nanocrystals can be produced by controlling the nucleation and growth of nanocrystals in the solution.17 The experimental conditions were kept the same when the β-NaYF4:Yb,Er crystals was grown on the β-NaYF4:Yb,Tm nanocrystals. As shown in the TEM image of NaYF4:Yb,Tm@NaYF4:Yb,Er nanocrystals in Figure 1c,d, uniform, monodisperse nanocrystals with a diameter of ca. 26 nm, which is larger than that of the seed nanocrystals, were obtained, suggesting that NaYF4:Yb,Er crystals were grown on the surface of NaYF4:Yb,Tm nanocrystals (seeds) and no small NaYF4:Yb,Er nanocrystals were formed. X-ray diffraction was used to examine the structure of the core-shell nanocrystals (Supporting Information Figure S1). All of the diffraction peaks could be indexed to hexagonal-phase NaYF4 crystals with cell parameters of a ) 5.960 Å and c ) 3.510Å, which is in good agreement with the data reported in the JCPDS standard card (28-1192). The X-ray photoelectron spectra (XPS) of the as-prepared NaYF4:Yb,Tm@ NaYF4:Yb,Er core-shell nanocrystals were measured to examine the composition of the crystal surface (Supporting Information Figure S2). Peaks at 283.5 eV, 530.2 eV, 157.1 eV, 682.2 and 1069.1 eV could be assigned to the binding energies of C1s, O1s, Y3d5/2, F1s and N1s respectively. The peaks at 168.2 eV for Er4d5/2 and 184.1 eV for Yb4d5/2 were very weak and the peak at 175 eV for Tm4d5/2 was not observed (Supporting Information Figure S2b), suggesting that the Tm ions were doped in the nanocrystals, not on the surface. The formation of the core-shell nanocrystals is illustrated in Figure 2a. NaYF4 core-shell nanocrystals are formed by a twostep growing process in which preformed NaYF4 nanocrystals serve as nuclei for the formation of new NaYF4 crystals on the (17) (a) Peng, X. G.; Schlamp, M. C.; Kadavanich, A.; Alivisatos, A. P. J. Am. Chem. Soc. 1997, 119, 7019. (b) Talapin, D. V.; Nelson, J. H.; Shevchenko, E. V.; Aloni, S.; Sadtler, B.; Alivisators, A. P. Nano Lett. 2007, 7, 2951. (c) Habas, S. E.; Lee, H.; Radmilovic, V.; Somorjai, G.; Yang, P. D. Nat. Mater. 2007, 6, 692. (d) Banin, U. Nat. Mater. 2007, 6, 625.
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Figure 2. (a) Schematic illustration of the formation of core-shell NaYF4: Yb,Tm@NaYF4:Yb,Er (AB) and NaYF4:Yb,Tm@NaYF4:Yb,Er@NaYF4: Yb,Tm (ABA) nanocrystals. TEM images of NaYF4:Yb,Tm (b) and ABA (c) are also given.
surface. Upconverting lanthanide ions Tm and Er are doped into the core and shell, respectively. The as-obtained NaYF4: Yb,Tm@NaYF4:Yb,Er (AB) nanocrystals are used as seed crystals to grow another layer of NaYF4:Yb,Tm crystals to form NaYF4:Yb,Tm@NaYF4:Yb,Er@NaYF4:Yb,Tm (ABA) nanocrystals with a sandwich structure. TEM images of the nanocrystals with the sandwich structure are given in Figure 2. The nanocrystals have an average size of 30 nm diameter, which is larger than that of the NaYF4:Yb,Tm@NaYF4:Yb,Er nanocrystals. Furthermore, the nanocrystals are single crystals, as seen in the high-resolution TEM image in Figure 2d. Recently, it was reported that an enhancement in the fluorescence intensity of NaYF4:Yb,Er and NaYF4:Yb,Tm nanocrystals could be achieved by coating the nanocrystals with a undoped NaYF4 shell.14 However, upconversion nanocrystals with multicolor emission could not be obtained by simply codoping Er and Tm in the same NaYF4 nanocrystals. The upconversion fluorescence of Yb,Er- or Yb,Tm-doped NaYF4 nanocrystals has been studied.18 The absorber Yb ions absorb NIR light, followed by energy transfer to the emitter Er or Tm ions that emit visible light. For Er and Tm codoped nanocrystals, the fluorescence from Tm is quenched by Er, and Er fluorescence is enhanced, probably as a result of the preferential energy transfer from Yb to Er. As such, only Er fluorescence can be observed. Fluorescence spectra of a 1 wt % solution of NaYF4:Yb,Er/Tm nanocrystals and AB nanocrystals in cyclohexane were obtained at room temperature and shown in Figure 3. The emission peaks at 450, 475, 409, 520, 541, and 653 nm were assigned to the 1D2 f 3F4 and 1G4 f 3H6 transitions of Tm and the transitions from 1 4H , 4H 4 4 4 9/2 11/2, S3/2, and F9/2, to I15/2 of Er, respectively. The intensity of the emission peaks corresponding to the transitions of 4H11/2, 4S3/2, and 4F9/2 to 4I15/2 of Er was greatly enhanced for the AB nanocrystals. As a comparison, the emission peaks of Tm for Er/Tm-codoped NaYF4 nanocrystals were not observed, suggesting that the Tm fluorescence was quenched (Figure 3a). Furthermore, the intensity of the emission peaks of Er was much lower than that of AB nanocrystals. The fluorescence spectra of NaYF4:Yb,Tm, NaYF4:Yb,Er, AB, and ABA nanocrystals were obtained and are shown in Figure 3b. As compared to that of AB nanocrystals, the intensity of Er emission peaks of ABA nanocrystals is much higher, suggesting that Er fluorescence (18) Wang, F.; Chatterjee, D. K.; Li, Z. Q.; Zhang, Y.; Fan, X. P.; Wang, M. Q. Nanotechnology 2006, 17, 5786.
Figure 3. (a) Fluorescence spectra of Tm/Er-codoped NaYF4 nanocrystals and core-shell NaYF4:Yb,Tm@NaYF4:Yb,Er (AB) nanocrystals in cyclohexane. Fluorescence spectra of core-shell NaYF4:Yb,Tm@NaYF4: Yb,Er@NaYF4:Yb,Tm (ABA), NaYF4:Yb,Tm@NaYF4:Yb,Er (AB), NaYF4:Yb,Er, and NaYF4:Yb,Tm nanocrystals are also given in plot b.
was greatly enhanced after coating an extra layer of NaYF4 crystals onto AB nanocrystals.
4. Conclusions Hexagonal-phase core-shell NaYF4:Yb,Tm@NaYF4:Yb,Er nanocrystals were synthesized with Tm and Er doped in the core and shell, respectively. Different from the Tm/Er-codoped NaYF4 nanocrystals in which Tm fluorescence was quenched, both Tm and Er fluorescence were observed from the core-shell nanocrystals. Remarkable enhancement in the fluorescence intensity was also observed for the emission peaks corresponding to the transitions of 4H11/2, 4S3/2, and 4F9/2 to 4I15/2 of Er. NaYF4: Yb,Tm@NaYF4:Yb,Er@NaYF4:Yb,Tm nanocrystals with a sandwich structure were also synthesized, and the fluorescence from Er was greatly enhanced. The nanocrystals emit multicolor NIR-to-visible upconversion fluorescence upon excitation at a wavelength of 980 nm. Acknowledgment. We acknowledge financial support from A*STAR BMRC (R-397-000-624-305) and the National University of Singapore. We also thank Dr. Haitao Zhang from the Department of Materials Science and Engineering, National University of Singapore, for help with the XPS characterization. Supporting Information Available: XRD patterns and spectra and photographs of nanocrystals. This material is available free of charge via the Internet at http://pubs.acs.org. LA802343F
