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Tuning the energy migration and new insights into the mechanism of upconversion Muthu Kumara Gnanasammandhan Jayakumar,†a Kai Huang†a and Yong Zhang*ab The past decade has seen extensive developments in the field of upconversion nanotechnology, which has found applications in various fields. Different applications require different emission wavelengths from
Received 7th April 2014 Accepted 22nd May 2014
nanoparticles, and significant research has been undertaken to fine tune individual emission peaks in shorter wavelengths without much success. Recently, a novel class of upconversion nanoparticles with an orthorhombic crystal structure has been developed, which enables high concentrations of activator
DOI: 10.1039/c4nr01865f
ions to be used without concentration quenching and also allows for the excellent tuning of shorter
www.rsc.org/nanoscale
emission wavelengths.
The past decade has seen an exponential growth in the eld of upconversion nanotechnology. Bulk materials exhibiting upconversion were discovered several years ago, but the eld gained momentum with the synthesis of lanthanide-based inorganic nanocrystalline materials.1,2 The nanoscale and its unique property to be excited by a near-infrared light source enabled its use in diverse elds such as bioimaging, diagnostics, cancer therapy, sensing, photovoltaics and photocatalysis.3–9 These upconversion nanoparticles (UCNs) are excited by a near-infrared (NIR) source, and they emit in different wavelengths across the UV, visible and NIR ranges. Different applications make use of different emission wavelengths of UCNs, and there is considerable interest to tune particular emission wavelengths for particular applications. Several groups have attempted to tune the emission wavelengths by varying parameters like the sensitizer ion concentration, activator ion concentration and crystal lattice. However, there have been no systematic and comprehensive studies to date that have delved into the theory behind the tuning of emission wavelengths. In 2004, Vetrone and co-workers studied the inuence of activator Yb3+-ion concentration in enhancing the red and green emission peaks of Y2O3 : Er3+/Yb3+ UCNs.10 This was followed by Liu Xiaogang's group investigating the effect of both activator and sensitizer ion concentrations in NaYF4 and LiYF4 host crystals to tune the emission peaks and also to obtain multicolored UCNs.11,12 A similar approach of changing the crystal lattice was reported by our group to tune the emission
a
Department of Biomedical Engineering, Faculty of Engineering, National University of Singapore, 117576, Singapore. E-mail: [email protected]; Fax: +65-68723069; Tel: +65-65164871
b
NUS Graduate School for Integrative Sciences & Engineering, National University of Singapore, 117456, Singapore † Equal contribution.
This journal is © The Royal Society of Chemistry 2014
wavelength and also to obtain multi-colored UCNs. Here, the sodium in traditional NaYF4 hosts was replaced with other alkali ions like potassium, which were either smaller or larger than sodium, to modify the host crystal lattice.13 A study by Paras Prasad's group in 2011 reported that a 43% increase was achieved in the 800 nm emission peak of NaYF4 : Yb3+/Tm3+ UCNs with the increase of the Yb3+ concentration, and they also synthesized sub-10 nm UCNs successfully with good photoluminescence.14 Even with all these developments, there was difficulty in tuning the shorter wavelengths of the spectral region, which was required by many applications. A major reason was the signicant quenching observed with an increase in the concentration of Yb3+ ions, although it is supposed to enhance the luminescence efficiency. This major hurdle was overcome by a recent study by Liu Xiaogang's group, which was published in Nature Materials.15 In their research, they proved that the ratio of the emission peaks and the concentration quenching phenomenon is determined by the distance between the Yb3+ ions. They synthesized KYb2F7 : Er UCNs, which separates Yb3+ ions as arrays of discrete clusters at the sublattice level; thus, the average distance between the Yb3+ clusters is larger than the Yb3+ distance within the clusters. With this special arrangement of Yb3+ ions, no concentration quenching was observed when changing the Yb3+ concentration from 18% to 98%. Thus, it was shown that the excitation energy migration actually depended on the type of crystal lattice, as well as the distance between the lattice points, rather than the concentration of Yb3+ ions. By comparing the luminescence of KYb2F7 : Er to that of NaYF4 : Yb3+/Er3+ and NaYbF4 : Er3+, they concluded that the back-energy-transfer (BET) from Er3+ to Yb3+ and energy migration between Yb3+ ions is dominated by the distance between Yb3+ ions, which in turn determines the ratio of the emission peaks and the concentration quenching phenomenon. The only minor problem with these nanoparticles is their
Nanoscale, 2014, 6, 8439–8440 | 8439
View Article Online
Published on 26 May 2014. Downloaded by York University on 13/08/2014 04:44:34.
Nanoscale
Opinion
Fig. 1
Schematic showing the migration of excitation energy in different types of crystal lattices (a), prospective mechanism of upconversion in NaYF4 : Yb/Er (18/2 mol%) (b), NaYbF4 : Er (2 mol%) (c), and KYb2F7 : Er (2 mol%) nanoparticles (d) [ETU: Energy transfer upconversion, BET: Back energy transfer]. Adapted with permission from ref. 12. Copyright © 2014, Macmillan Publishers Limited.
relatively large size in comparison to the conventional NaYF4based UCNs, which are typically synthesized with an average size of 30 nm and could be less attractive for biological applications. Apart from tuning the energy based on the arrangement of the sublattice and obtaining the ability to tune shorter wavelengths, they also observed a new mechanism of upconversion. In recent years, it has been noticed that researchers are attempting to determine new upconversion mechanisms by studying the effect of host crystals in the energy transfer processes as compared to the classical upconversion mechanisms that only focus on sensitizer and activator ions. In 2011, Wang et al. described a mechanism called energy migrationmediated upconversion (EMU) and involved Gd3+ ions, which serve as the host lattice for sensitizers and activators, into the energy transfer process and provided a way to introduce more kinds of lanthanide ions to be used as activators in the upconversion.16 Recently, they described the mechanism of energy clustering in the sublattice of Yb3+ clusters. By reserving the excitation energy in the Yb3+ clusters in the host lattice, activators can absorb more energy and be excited to higher energy excited states. Therefore, a four photon upconversion has been observed for the rst time due to the extra energy transfer from the Yb3+ clusters to activators (Fig. 1b). This four photon upconversion led to an enhancement of the violet emission peak. These ndings were supported with rigorous experimental results and simulations. Their conclusion provides an insight on various previous studies and shows that the distance between the activator ions is the major factor in tuning the emission peak rather than its concentration. The novel upconversion mechanism involving host crystals highlights the importance of host crystals in the eld of upconversion nanotechnology and provides a new viewpoint when selecting host materials. It is foreseen that the knowledge gained from this study would pave the way for developing multi-
8440 | Nanoscale, 2014, 6, 8439–8440
colored UCNs with the ability to precisely tune specic emission peaks for specic applications.
References 1 G. Yi, H. Lu, S. Zhao, Y. Ge, W. Yang, D. Chen and L.-H. Guo, Nano Lett., 2004, 4, 2191–2196. 2 Z. Li and Y. Zhang, Angew. Chem., 2006, 118, 7896–7899. 3 F. Wang, D. Banerjee, Y. Liu, X. Chen and X. Liu, Analyst, 2010, 135, 1839–1854. 4 W. Qin, D. Zhang, D. Zhao, L. Wang and K. Zheng, Chem. Commun., 2010, 46, 2304–2306. 5 N. M. Idris, M. K. Gnanasammandhan, J. Zhang, P. C. Ho, R. Mahendran and Y. Zhang, Nat. Med., 2012, 18, 1580–1585. 6 M. K. G. Jayakumar, N. M. Idris and Y. Zhang, Proc. Natl. Acad. Sci., India, Sect. A, 2012, 109, 8483–8488. 7 J. Zhou, Z. Liu and F. Y. Li, Chem. Soc. Rev., 2012, 41, 1323– 1349. 8 Q. Liu, W. Feng, T. Yang, T. Yi and F. Li, Nat. Protoc., 2013, 8, 2033–2044. 9 G. Chen, H. Qiu, P. N. Prasad and X. Chen, Chem Rev, 2014, 114, 5161–5214. 10 F. Vetrone, J.-C. Boyer, J. A. Capobianco, A. Speghini and M. Bettinelli, J. Appl. Phys., 2004, 96, 661–667. 11 F. Wang and X. Liu, J. Am. Chem. Soc., 2008, 130, 5642–5643. 12 J. Wang, F. Wang, J. Xu, Y. Wang, Y. Liu, X. Chen, H. Chen and X. Liu, C. R. Chim., 2010, 13, 731–736. 13 Q. Dou and Y. Zhang, Langmuir, 2011, 27, 13236–13241. ˚ 14 G. Chen, T. Y. Ohulchanskyy, R. Kumar, H. Agren and P. N. Prasad, ACS nano, 2010, 4, 3163–3168. 15 J. Wang, R. Deng, M. A. MacDonald, B. Chen, J. Yuan, F. Wang, D. Chi, T. S. Andy Hor, P. Zhang, G. Liu, Y. Han and X. Liu, Nat. Mater., 2014, 13, 157–162. 16 F. Wang, R. Deng, J. Wang, Q. Wang, Y. Han, H. Zhu, X. Chen and X. Liu, Nat. Mater., 2011, 10, 968–973.
This journal is © The Royal Society of Chemistry 2014
OPINION
Published on 26 May 2014. Downloaded by York University on 13/08/2014 04:44:34.
Cite this: Nanoscale, 2014, 6, 8439
View Journal | View Issue
Tuning the energy migration and new insights into the mechanism of upconversion Muthu Kumara Gnanasammandhan Jayakumar,†a Kai Huang†a and Yong Zhang*ab The past decade has seen extensive developments in the field of upconversion nanotechnology, which has found applications in various fields. Different applications require different emission wavelengths from
Received 7th April 2014 Accepted 22nd May 2014
nanoparticles, and significant research has been undertaken to fine tune individual emission peaks in shorter wavelengths without much success. Recently, a novel class of upconversion nanoparticles with an orthorhombic crystal structure has been developed, which enables high concentrations of activator
DOI: 10.1039/c4nr01865f
ions to be used without concentration quenching and also allows for the excellent tuning of shorter
www.rsc.org/nanoscale
emission wavelengths.
The past decade has seen an exponential growth in the eld of upconversion nanotechnology. Bulk materials exhibiting upconversion were discovered several years ago, but the eld gained momentum with the synthesis of lanthanide-based inorganic nanocrystalline materials.1,2 The nanoscale and its unique property to be excited by a near-infrared light source enabled its use in diverse elds such as bioimaging, diagnostics, cancer therapy, sensing, photovoltaics and photocatalysis.3–9 These upconversion nanoparticles (UCNs) are excited by a near-infrared (NIR) source, and they emit in different wavelengths across the UV, visible and NIR ranges. Different applications make use of different emission wavelengths of UCNs, and there is considerable interest to tune particular emission wavelengths for particular applications. Several groups have attempted to tune the emission wavelengths by varying parameters like the sensitizer ion concentration, activator ion concentration and crystal lattice. However, there have been no systematic and comprehensive studies to date that have delved into the theory behind the tuning of emission wavelengths. In 2004, Vetrone and co-workers studied the inuence of activator Yb3+-ion concentration in enhancing the red and green emission peaks of Y2O3 : Er3+/Yb3+ UCNs.10 This was followed by Liu Xiaogang's group investigating the effect of both activator and sensitizer ion concentrations in NaYF4 and LiYF4 host crystals to tune the emission peaks and also to obtain multicolored UCNs.11,12 A similar approach of changing the crystal lattice was reported by our group to tune the emission
a
Department of Biomedical Engineering, Faculty of Engineering, National University of Singapore, 117576, Singapore. E-mail: [email protected]; Fax: +65-68723069; Tel: +65-65164871
b
NUS Graduate School for Integrative Sciences & Engineering, National University of Singapore, 117456, Singapore † Equal contribution.
This journal is © The Royal Society of Chemistry 2014
wavelength and also to obtain multi-colored UCNs. Here, the sodium in traditional NaYF4 hosts was replaced with other alkali ions like potassium, which were either smaller or larger than sodium, to modify the host crystal lattice.13 A study by Paras Prasad's group in 2011 reported that a 43% increase was achieved in the 800 nm emission peak of NaYF4 : Yb3+/Tm3+ UCNs with the increase of the Yb3+ concentration, and they also synthesized sub-10 nm UCNs successfully with good photoluminescence.14 Even with all these developments, there was difficulty in tuning the shorter wavelengths of the spectral region, which was required by many applications. A major reason was the signicant quenching observed with an increase in the concentration of Yb3+ ions, although it is supposed to enhance the luminescence efficiency. This major hurdle was overcome by a recent study by Liu Xiaogang's group, which was published in Nature Materials.15 In their research, they proved that the ratio of the emission peaks and the concentration quenching phenomenon is determined by the distance between the Yb3+ ions. They synthesized KYb2F7 : Er UCNs, which separates Yb3+ ions as arrays of discrete clusters at the sublattice level; thus, the average distance between the Yb3+ clusters is larger than the Yb3+ distance within the clusters. With this special arrangement of Yb3+ ions, no concentration quenching was observed when changing the Yb3+ concentration from 18% to 98%. Thus, it was shown that the excitation energy migration actually depended on the type of crystal lattice, as well as the distance between the lattice points, rather than the concentration of Yb3+ ions. By comparing the luminescence of KYb2F7 : Er to that of NaYF4 : Yb3+/Er3+ and NaYbF4 : Er3+, they concluded that the back-energy-transfer (BET) from Er3+ to Yb3+ and energy migration between Yb3+ ions is dominated by the distance between Yb3+ ions, which in turn determines the ratio of the emission peaks and the concentration quenching phenomenon. The only minor problem with these nanoparticles is their
Nanoscale, 2014, 6, 8439–8440 | 8439
View Article Online
Published on 26 May 2014. Downloaded by York University on 13/08/2014 04:44:34.
Nanoscale
Opinion
Fig. 1
Schematic showing the migration of excitation energy in different types of crystal lattices (a), prospective mechanism of upconversion in NaYF4 : Yb/Er (18/2 mol%) (b), NaYbF4 : Er (2 mol%) (c), and KYb2F7 : Er (2 mol%) nanoparticles (d) [ETU: Energy transfer upconversion, BET: Back energy transfer]. Adapted with permission from ref. 12. Copyright © 2014, Macmillan Publishers Limited.
relatively large size in comparison to the conventional NaYF4based UCNs, which are typically synthesized with an average size of 30 nm and could be less attractive for biological applications. Apart from tuning the energy based on the arrangement of the sublattice and obtaining the ability to tune shorter wavelengths, they also observed a new mechanism of upconversion. In recent years, it has been noticed that researchers are attempting to determine new upconversion mechanisms by studying the effect of host crystals in the energy transfer processes as compared to the classical upconversion mechanisms that only focus on sensitizer and activator ions. In 2011, Wang et al. described a mechanism called energy migrationmediated upconversion (EMU) and involved Gd3+ ions, which serve as the host lattice for sensitizers and activators, into the energy transfer process and provided a way to introduce more kinds of lanthanide ions to be used as activators in the upconversion.16 Recently, they described the mechanism of energy clustering in the sublattice of Yb3+ clusters. By reserving the excitation energy in the Yb3+ clusters in the host lattice, activators can absorb more energy and be excited to higher energy excited states. Therefore, a four photon upconversion has been observed for the rst time due to the extra energy transfer from the Yb3+ clusters to activators (Fig. 1b). This four photon upconversion led to an enhancement of the violet emission peak. These ndings were supported with rigorous experimental results and simulations. Their conclusion provides an insight on various previous studies and shows that the distance between the activator ions is the major factor in tuning the emission peak rather than its concentration. The novel upconversion mechanism involving host crystals highlights the importance of host crystals in the eld of upconversion nanotechnology and provides a new viewpoint when selecting host materials. It is foreseen that the knowledge gained from this study would pave the way for developing multi-
8440 | Nanoscale, 2014, 6, 8439–8440
colored UCNs with the ability to precisely tune specic emission peaks for specic applications.
References 1 G. Yi, H. Lu, S. Zhao, Y. Ge, W. Yang, D. Chen and L.-H. Guo, Nano Lett., 2004, 4, 2191–2196. 2 Z. Li and Y. Zhang, Angew. Chem., 2006, 118, 7896–7899. 3 F. Wang, D. Banerjee, Y. Liu, X. Chen and X. Liu, Analyst, 2010, 135, 1839–1854. 4 W. Qin, D. Zhang, D. Zhao, L. Wang and K. Zheng, Chem. Commun., 2010, 46, 2304–2306. 5 N. M. Idris, M. K. Gnanasammandhan, J. Zhang, P. C. Ho, R. Mahendran and Y. Zhang, Nat. Med., 2012, 18, 1580–1585. 6 M. K. G. Jayakumar, N. M. Idris and Y. Zhang, Proc. Natl. Acad. Sci., India, Sect. A, 2012, 109, 8483–8488. 7 J. Zhou, Z. Liu and F. Y. Li, Chem. Soc. Rev., 2012, 41, 1323– 1349. 8 Q. Liu, W. Feng, T. Yang, T. Yi and F. Li, Nat. Protoc., 2013, 8, 2033–2044. 9 G. Chen, H. Qiu, P. N. Prasad and X. Chen, Chem Rev, 2014, 114, 5161–5214. 10 F. Vetrone, J.-C. Boyer, J. A. Capobianco, A. Speghini and M. Bettinelli, J. Appl. Phys., 2004, 96, 661–667. 11 F. Wang and X. Liu, J. Am. Chem. Soc., 2008, 130, 5642–5643. 12 J. Wang, F. Wang, J. Xu, Y. Wang, Y. Liu, X. Chen, H. Chen and X. Liu, C. R. Chim., 2010, 13, 731–736. 13 Q. Dou and Y. Zhang, Langmuir, 2011, 27, 13236–13241. ˚ 14 G. Chen, T. Y. Ohulchanskyy, R. Kumar, H. Agren and P. N. Prasad, ACS nano, 2010, 4, 3163–3168. 15 J. Wang, R. Deng, M. A. MacDonald, B. Chen, J. Yuan, F. Wang, D. Chi, T. S. Andy Hor, P. Zhang, G. Liu, Y. Han and X. Liu, Nat. Mater., 2014, 13, 157–162. 16 F. Wang, R. Deng, J. Wang, Q. Wang, Y. Han, H. Zhu, X. Chen and X. Liu, Nat. Mater., 2011, 10, 968–973.
This journal is © The Royal Society of Chemistry 2014
