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Hexagonal NaYF4 nanocrystals (NCs) have been prepared by core-mediated hetero-shell growth process using polyvinylpyrrolidone (PVP) as chelating agents and cubic CaF2 as core. It is found that the NaYF4 NCs prepared by this strategy have narrow size distribution, small particle size and well water-soluble property, can be easily dispersed in water. Besides, by increasing the doping concentration of Tm3+ ions in Er3+-Yb3+ codoping system, the upconversion (UC) liminescence of CaF2@NaYF4: Yb/Er/Tm NCs was modulated from green to red by the naked eye. These NCs with red upconversion luminescence (UCL) and good water solubility show potential applications in biological labeling field.
© 2015 Optical Society of America
1. Introduction
Recently, upconversion luminescence of lanthanide-doped nanocrystals has attracted extensive research attention. Because these nanocrystals have unique optical properties and potential applications in fields such as color display, temperature sensors, UC lasers, DNA detection, photodynamic therapy and biological labeling [1–9 ]. As a kind of biological fluorescence label, the “optical windows” of biological tissue is an important influencing factor to be considered, its range is from 600 nm to 1100 nm [8,10–12 ]. When the wavelength of light locates in this region, the tissue absorption is minimum, and the penetration depth is maximum. As we know, fluorescent emissions of many ions or ion pairs locate in this range, but Yb3+ - Er3+ ion pairs are still the best choice because of their excellent UC efficiency. When Yb3+ - Er3+ ion pairs were excited by 980 nm laser, we can observe two strong emission peaks: green emission (550 nm) and red emission (660 nm). Obviously, the emission in green region is not conducive to biological probes. Therefore, avoiding the green emission and achieving strengthened red emission from the Yb3+ - Er3+ ion pairs are essential for the deep tissue imaging by fluorescent label [13,14 ].
Up to now, many methods have been reported to increase the luminescence intensity ratio of red/green UC emissions in Yb3+- Er3+ codoped system. As the particle size of Y2O3:Yb3+, Er3+ NCs decreases, the relative luminescence intensity ratio of red to green increases gradually which is induced by surface effects [15]. Bai et al. has modified the UC luminescence from green to red in Er3+/Yb3+ doped zeolites by tuning the concentration of Yb3+ ions [16]. Recently, some dopants, such as Mn2+ [12, 17,18 ], Au+ [19], Li+ ions [20], have been recognized as effective elements which can enhance the ratio of red/green. However, few papers have been found on tuning output color from green to red by increasing the doping concentration of Tm3+ ions in Yb3+ - Er3+ co-doping system. Although Yb/Er/Tm tridoped La2O3, NaYF4, BaYF5 and YF3 have been reported in some literatures, they focused on white light emission [21–24 ].
Herein, we have prepared a kind of water dispersed and small particle size hexagonal NaYF4 NCs with red upconversion fluorescence by a heterogeneous core-shell strategy. With the increase of doped Tm3+ ions concentration from 0 to 4 mol%, the intensity ratio of red/green UC emission increases dramatically from 0.1 to 6.2 in CaF2@NaYF4: Yb/Er/Tm Core-Shell NCs.
2. Experimental
2.1 Materials
All chemicals were analytical grade and used without further purification. Y(NO3)3·6H2O (99.999%), Yb(NO3)3·6H2O (99.999%), Er(NO3)3·6H2O (99.999%), Tm(NO3)3·6H2O (99.999%), were supplied by Yutai Qingda Chemical Technology Co., Ltd. China. Polyvinylpyrrolidone K-30 (PVP, 58000 g/mol), Ca(NO3)2·4H2O (AR), NaCl (AR), NaNO3 (AR), KF·2H2O (AR), NaF (AR) and ethylene glycol (EG, AR) were supplied by Beijing Fine Chemical Company.
2.2 Synthesis of the cubic CaF2 core
Polyvinylpyrrolidone K-30 (PVP, 0.5 g) was dissolved in EG (5 mL) to form transparent solution. Then, 1 mmol Ca(NO3)2 were added into PVP solution under strong stirring, KF (3 mmol) was dissolved in 2 ml EG and dropwise into above solution. Subsequently, the above solution was transferred into a polytetrafluoroethylene autoclave, and then heated at 180 °C for 2 hours. The content was taken out at room temperature and retained as core for next procedure.
2.3 Synthesis of the CaF2@NaYF4 core-shell hybrid nanocrystals
For synthesis of CaF2@NaYF4:20 mol%Yb, 2 mol%Er hybrid nanocrystals (HNCs), PVP (0.5 g) was dissolved in the above CaF2 core solution. Then 0.8 mmol Y(NO3)3, 0.18 mmol Yb(NO3)3, 0.02 mmol Er(NO3)3 and 1 mmol NaNO3 were added into above core solution respectively under strong stirring. 5 mmol KF was dissolved in 2 ml EG, and subsequently was added dropwise into above mixture. All the solution was then transferred into a polytetrafluoroethylene autoclave and reacted at 180 °C for 12 hours. The final product was obtained by centrifugation and washed with ethanol for several times. The other samples CaF2@NaYF4:20 mol%Yb, 2 mol%Er, x mol%Tm (x = 0, 0.5, 1, 1.5, 2, 2.5, 3, 4) were prepared by the similar process as described above.
2.4 Characterization
The crystal structures and phase purities were analyzed by a Rigaku RU-200b X-ray powder diffractometer (XRD) using nickel-filtered Cu-Kα radiation (λ = 1.5406 Å). The sizes and morphologies of the samples were investigated by transmission electron microscopy (TEM, Hitachi H-600). UC luminescence spectra were recorded using a Hitachi F-4500 fluorescence spectrophotometer which was equipped with a power-controllable 980 nm CW diode laser (Maximum power 2W).
3. Results and discussion
The CaF2 core NCs and the resulting CaF2@NaYF4 core-shell HNCs were synthesized by a facile solvothermal method using an amphiphilic surfactant, polyvinylpyrrolidone (PVP) as the chelating agent. To illustrate the corresponding crystalline phases of the core precursors and core-shell NCs, all these samples were examined by X-ray powder diffraction (XRD) to determine their crystal structures. As is shown in the Fig. 1(a) , all the diffraction peaks of the precursor can be well indexed to pure cubic phase CaF2 (JCPDS NO. 35-816), no other impurity peaks can be detected from this XRD pattern, indicating that the obtained precursor is cubic phase CaF2. In the further step, the CaF2 NCs, adopted as the precursors, together with the sodium and yttrium sources, to induce the growth of the hexagonal NaYF4 NCs. As evidenced by XRD pattern in Fig. 1(b), pure hexagonal NaYF4 NCs (JCPDS NO. 16-334) are fabricated.
Fig. 1 XRD patterns (a) CaF2 core; (b) CaF2@NaYF4:20 mol%Yb, 2 mol%Er HNCs; (c) CaF2 (JCPDS NO. 35-816); (d) β-NaYF4 (JCPDS NO. 16-334)
Compare with forming a new nanoparticle, reaction ions depositing on the surface of pre-existing particles has the less surface tension and less energy barrier to overcome, the formation of core-shell NCs is more advantageous [25–27 ]. Therefore, in the 12h shell growth process, very less new nanocrystals formed and most of the ions were coated on CaF2 core to forming the NaYF4 shell. This was demonstrated in Fig. 2 . After a 12 h growth process, Ca element, Na element and Y element can be observed by EDS analysis [Fig. 2(e)] and the mean size of NCs increased from 27 nm [Fig. 2(c)] to 33 nm [Fig. 2(d)]. In addition, the size distribution becomes narrower, this means core-shell growth process is successful, a 6 nm shell has been coated on the CaF2 core. Combine to the result of the Fig. 1, it demonstrates that the cubic phase CaF2 NCs can induce the growth of hexagonal phase NaYF4 shell, forming the core-shell hetero structure. Moreover, from the Fig. 2(e), we also can see strong O element and K element peaks. The O element may come from the PVP which was on the surface of the NCs. This can also be an indirect proof that PVP has been successfully modified to surface of NCs. The K element may come from the fluorine source KF.
Fig. 2 (a) TEM images of CaF2 core NCs; (b) TEM image of CaF2@NaYF4:20 mol%Yb, 2 mol%Er HNCs; (c) Size distribution of CaF2 core NCs; (d) Size distribution of CaF2@NaYF4:20 mol%Yb, 2 mol%Er HNCs; (e) EDX analysis of elemental composition of CaF2@NaYF4:20 mol%Yb, 2 mol%Er HNCs.
To further study the growth process of the CaF2@NaYF4 hetero structure, we replaced some reaction material or changed the reaction sequence to prepare the CaF2@NaYF4:20 mol%Yb, 2 mol%Er HNCs. The details of the reaction process and product crystallographic phase are shown in Table 1 . In addition, the corresponding XRD patterns, UC photographic and UCL spectra are shown in Fig. 3 . As shown in XRD patterns (left of Fig. 3), the sample (a) and sample (b) which were prepared with Sodium source NaNO3 or NaCl, together with Fluorine source KF can be induced by CaF2 NCs successfully and get the hexagonal phase NaYF4 shell. But the samples (c) and (d) are pure cubic phase NaYF4, which was synthesized with NaF as Sodium source and Fluorine source in the shell growth. The sample (e) was prepared by a non-core induce process, directly mixing all the reaction material of sample (a) in a one-step reaction process. The result product is cubic phase NaYF4. Therefore, CaF2 core NCs induce and KF as the fluorine source are two necessary conditions for this core-mediated hetero-shell process. In this growth process, α-NaYF4 and CaF2 have similar lattice Cell parameters (CaF2: a = 0.546 nm, α-NaYF4: a = 0.545 nm) and similar crystal structure, α-NaYF4 is very easy to continue to grow on the surface of CaF2 core NCs. When K+ ions exists in reaction solution, K+ ions replaced some Na+ ions lattice sites in the NaYF4 crystal, the ionic radius difference between K+ ions (0.138 nm) and Na+ ions (0.102 nm) cause the lattice distortion of the hetero interface, forming a low symmetric structure. Therefore, the subsequently deposited NaYF4 shell can epitaxial grow in hexagonal phase. As a result, K+ ions and CaF2 core are two necessary conditions in forming β-NaYF4 NCs [27–30 ]. Moreover, UCL spectra (right-top of Fig. 3) and the UC photographic (right-bottom of Fig. 3) of these samples are also shown that the product prepared with Fluorine source KF [sample (a) and sample (b)], exhibited enhanced UC luminescence intensity as compared to the other three products. The upconversion luminescence intensity of sample (a) or sample (b) is 23 times larger than that of sample (c) at 550 nm. This is more than the previously reported that green UC luminescence efficiency in hexagonal phase NaYF4: Yb, Er NCs is approximately 10 times stronger than that in the cubic phase form [31,32 ]. The reason to explain this phenomenon is that KF not only served as Fluorine source in NaYF4 shell growth process, but also a part of K+ was doped into NaYF4 crystal lattice, occupy the Na+ ions lattice sites in the crystal. A small amount of non-fluorescent dopant ions in the asymmetric crystal, the crystal symmetry around Er3+ ions were lower. It is generally favorable for higher UC emission luminescence intensity [33–36 ].
Table 1. The detail of reaction process and the Crystallographic phase of product.
Fig. 3 XRD patterns (left), UCL spectra excited by a 80 mW 980 nm laser (right-top) and corresponding UC photographic (right-bottom) of HNCs samples prepared with different reaction material or different reaction sequence: (a) NaCl as Sodium source, KF as Fluorine source; (b) NaNO3 as Sodium source, KF as Fluorine source; (c) NaF as Sodium source and KF as Fluorine source; (d) KF as Fluorine source in the core growth, NaF as Sodium source and Fluorine source in the shell growth; (e) All the reaction materials of sample (a) was mixed directly (non-core induce).
As shown in Fig. 4 , the XRD patterns of shell growth time increased from 6h to 24h, the diffraction peak intensities of the hexagonal phase increased as the reaction time longer. This evolution of diffraction peaks is also indicated the growth of hexagonal phase NaYF4 NCs.
Fig. 4 XRD patterns of CaF2@NaYF4:20 mol%Yb, 2 mol%Er HNCs. with shell growth time of (a) 6h; (b) 12h; (c) 24h.
Figure 5(a) is the UCL spectra of CaF2@NaYF4:20 mol%Yb, 2 mol%Er, x mol%Tm (x = 0, 0.5, 1, 1.5, 2, 2.5, 3, 4) nanocrystals excited by 980 nm laser respectively. As shown in Fig. 5(a), with the increase of Tm3+ concentration from 0 mol% to 4 mol%, red emission (650 nm, 4F9/2 → 4I15/2) was enhanced remarkably, green emission (525 nm, 540 nm, 2S3/2, 2H11/2 → 2I15/2) was quenched. Figure 5(b) shows the intensity ratio of red/green UC emission and the total luminescence integrated intensity. The ratio increases dramatically from 0.1 to 6.2 with the increase of doped Tm3+ ion concentrations, but the total luminescence integrated intensity did not reduce. This result indicates that the enhanced red UC fluorescence arises from the effective energy transfer between Er3+ ions and Tm3+ ions, not in the expense of a lower PLQY. Figure 5(c) is photostability of CaF2@NaYF4:20 mol%Yb, 2 mol%Er, 2 mol%Tm NCs. The sample was excited by 980 nm and its luminescence emission was collected by a 652 nm channel. The exposure time for imaging data collection is 0.5 s. As is shown in picture, the NCs exhibited neither blinking nor photobleaching over 30 min continuous laser excitation. And the UC photographic of CaF2@NaYF4:20 mol%Yb, 2 mol%Er, x mol%Tm (x = 0, 0.5, 1, 1.5, 2, 2.5, 3, 4) nanocrystals solution excited by 980 nm laser are shown in Fig. 5(d), the UC color output of emission was gradually modulated from green to yellow, then to red by the naked eye.
Fig. 5 (a) The UCL spectra of CaF2@NaYF4:20 mol%Yb, 2 mol%Er, x mol%Tm (x = 0, 0.5, 1, 1.5, 2, 2.5, 3, 4) nanocrystals excited by a 80 mW 980 nm laser; (b) Corresponding UC emission red/green intensity ratio and total luminescence integrated intensity; (c) Photostability of CaF2@NaYF4:20 mol%Yb, 2 mol%Er, 2 mol%Tm NCs. Emission was collected by a 652 nm channel. Time interval of imaging data collection = 0.5 s; (d) UC photographic of nanocrystals water solution excited by a 80 mW 980 nm laser.
Figure 6 describes schematically energy level diagrams of Er3+ and Tm3+ ions and possible UC processes excited by 980 nm. When NCs were excited by 980 nm, compared with Er3+, Yb3+ has the larger doping concentration and a much larger absorption cross section around 980 nm, therefore, the main pathway to populate the excited states of Er3+ is the energy transfer (ET) from Yb3+ to Er3+. As pictured in Fig. 6, two ET processes from Yb3+ to Er3+ ion excite the 4I15/2 level to 4I11/2 and 4F7/2 levels. Then two nonradiative relaxations processes:4F7/2→2H11/2 and 4F7/2→4S3/2 populate the 2H11/2 and 4S3/2 levels respectively. Subsequent back to ground level 4I15/2, green (525 nm, 540 nm 2S3/2, 2H11/2 → 2I15/2) emission can be observed. Moreover, when it stays at 4I11/2 level, it could nonradiative relaxations to 4I13/2 level, then another ET process excite the 4I13/2 level to 4F9/2 level. Then back to ground level, and emit red (660nm, 4F9/2 → 4I15/2) fluorescence. In addition, there are two non-resonant energy transfer (ET) processes between Tm3+ and Er3+ ions, (ET1) 3F4 → 3H6 (Tm3+) 4I11/2 → 4F9/2 (Er3+), (ET2): 4I13/2 → 4I15/2 (Er3+):3H6 → 3F4 (Tm3+). The color tuning of upconversion emission from green to red is achieved by these non-resonant energy transfer (ET) processes.
Fig. 6 Energy level diagrams of Er3+ and Tm3+ ions and possible energy transfer mechanism under 980 nm excitation.
4. Conclusions
In conclusion, a kind of water dispersed and small particle size hexagonal NaYF4 NCs are prepared by a new strategy. In this strategy, we use cubic CaF2 to induce heterogeneous growth of hexagonal phase shells NaYF4 and using an amphiphilic surfactant, polyvinylpyrrolidone (PVP) as the chelating agent to make our core-shell NC material have good water solubility and small particle size. In addition, we tune the doping concentration of Tm3+ ions to modulate the UC color output from green to yellow, then to red by the naked eye. The above features may make our NCs have a good prospect about the applications of lanthanide-based luminescent bioprobes.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (NSFC) (grants 61405016, 11274139, 61275189), China Scholarship Council, Jilin Province Education Department (2015107) and Natural Science Foundation of Jilin Province (2016).
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