Engineering Homogeneous Doping in Single Nanoparticle To

May 29, 2014 - Very little attention is paid to the dopant ion distribution in the single nanoparticle level. This raises the question of whether the ...
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Engineering Homogeneous Doping in Single Nanoparticle To Enhance Upconversion Efficiency Xiaomin Li, Rui Wang, Fan Zhang,* and Dongyuan Zhao* Department of Chemistry and Laboratory of Advanced Materials, Fudan University, Shanghai 200433, P. R. China S Supporting Information *

ABSTRACT: Upconversion nanoparticles (UCNPs) have shown considerable promises in many fields; however, their low upconversion efficiency is still the most serious limitation of their applications. Herein, we report for first time that the homogeneous doping approach based on the successive layer-by-layer method can greatly improve the efficiency of the UCNPs. The quantum yield as high as 0.89 ± 0.05% is realized for the homogeneous doping NaGdF4:Yb,Er/NaYF4 UCNPs, which is nearly 2 times higher than that of the heterogeneous doping NaGdF4:Yb,Er/NaYF4 UCNPs (0.47 ± 0.05%). The influences of spatial distributions and local relative concentrations of the dopants on the optical properties of UCNPs were investigated in the single particle level. It was found that heterogeneous doping indeed existed during the spontaneous growth process of the nanoparticles. The heterogeneous doping property can further induce many negative effects on the optical properties of UCNPs, especially the luminescent efficiency. The spatial distributions and local relative concentrations of the dopants can be well controlled by the successive layer-by-layer homogeneous doping method on the monolayer level and homogeneously distributed in the single particle level. Furthermore, by using homogeneous doping NaGdF4:Yb,Tm as initial core, the multicolor emission intensity of NaGdF4:Yb,Tm/NaGdF4:A (A = Tb3+, Eu3+) core/shell nanoparticles can also exhibit 20%−30% improvement. We believe that such a homogeneous doping model can open the door to improve the upconversion optical properties by engineering the local distribution of the sensitizer, activator, host, etc., in a microcosmic and provide a track for engineering the high quality UCNPs with advanced nanostructure and optical properties. KEYWORDS: Upconversion, lanthanide, homogeneous doping, synthesis ∼20% and ∼2%, respectively.4,11,17,20,22,23,28 The higher or lower doping concentration does not seem to be beneficial to the promotion of the efficiency, meaning that the higher doping concentration can induce the cross-relaxations between the dopants and the lower doping concentration can cause the reduction of the luminescent center.18,20 Up to now, the doping concentration of the products is usually denoted by the macroscopic reactant feed ratio in the experiment (macroscopic doping concentration). Very little attention is paid to the dopant ion distribution in the single nanoparticle level. This raises the question of whether the doping elements are statistically distributed in the single nanoparticle level. Homogeneous distribution of dopants can minimize concentration quenching and thus enhance the optical properties. Conversely, the particles may possess quite different local doping concentrations at different positions of single nanoparticle even though the macroscopic doping concentration is maintained constant. The heterogeneous doping of the dopants can induce a series of negative effects on the UCL of the particles, such as the concentration quenching caused by cross-relaxations between the dopants, energy transfer barrier between the sensitizer and activator, and so on.

U

pconversion nanoparticles (UCNPs) have attracted a great deal of attention in biological applications due to their unique upconversion luminescence (UCL), such as high chemical stability, low toxicity, and high signal-to-noise ratio.1−9 These features suggest that UCNPs are becoming a new generation platform to achieve a reliable performance in the context of highly complex bioimaging applications.10−15 Despite the gains, improvements are still needed to optimize optical properties for potential practical applications. The most important parameter that needs improvement is the luminescent efficiency of UCNPs,14,16,17 which is still much lower compared with that of the bulk materials (3−5%).14,16,18−20 In order to make up this deficiency, a major improvement has been realized through growing an inert shell with similar lattice constants around the core, which could protect the luminescent lanthanide ions in the luminescent core from nonradiative decay caused by surface defects as well as vibrational deactivation from solvents or surface-bound ligands.4,11,17,21−30 However, the absolute quantum yield (QY) of the UCNPs is still low to meet the practical bioapplications even with a very thick shell coating.31 So, it is still a great challenge to develop more efficient strategies for the UCL efficiency enhancement. Nevertheless, besides the surface defects induced low efficiency, some other factors influencing the optical properties of the UCNPs are rarely investigated until now. In general, in order to get highly efficient UCL, the optimum doping concentration for the Yb3+, Er3+ codoped system is © 2014 American Chemical Society

Received: April 12, 2014 Revised: May 23, 2014 Published: May 29, 2014 3634

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Figure 1. TEM images of the HEC NaGdF4:Yb,Er nanoparticles obtained during the spontaneous growth at different reaction time by using the most commonly used one pot heating-up method: (A) 5, (B) 30, and (C) 100 min. (D) The total doping concentration of Yb3+ and Er3+ ions determined by the EDX at different growth stage of the spontaneous growth process. The evolutions of Yb3+ and Er3+ doping concentrations during the spontaneous growth process (E) and SLBL doping process (F) determined by ICP-AES. The doping concentrations are evaluated with molar concentration.

In this work, the influences of spatial distributions and relative concentrations of the dopants on the optical properties of UCNPs were investigated for the first time in the single nanoparticle level. We demonstrated that heterogeneous doping indeed existed in the UCNPs prepared by the regular synthesis approaches reported previously [referred as heterogeneous doping core (HEC) nanoparticle]. The heterogeneous doping can induce the cross-relaxation and energy transfer barrier between dopant ions because of the local relative enrichment of the dopants. In order to avoid the deficiency, we developed a homogeneous doping strategy to fabricate the UCNPs [i.e., homogeneous doping core (HOC) nanoparticle] based on the one-pot successive layer-by-layer (SLBL) method.17 Most importantly, the overall UC emission intensity and lifetime of the HOC UCNPs were remarkably enhanced compared with the HEC UCNPs. The absolute QY as high as 0.89 ± 0.05% is realized for the NaGdF4:Yb,Er/ NaYF4 UCNPs by homogeneous doping, which is about 2 times higher than that of the HEC based core/shell UCNPs (0.47 ± 0.05%). Furthermore, by using homogeneous doping NaGdF4:Yb,Tm nanoparticles as initial core, the multicolor emission intensity of NaGdF4:Yb,Tm/NaGdF4:A (A = Tb3+, Eu3+) can also exhibit 20%−30% enhancement compared with that of HEC based nanostructure, suggesting that the homogeneous doping approach can not only be used for the efficiency enhancement of lanthanide ion-doped nanoparticles but also provide a track for engineering the high quality UCNPs with advanced nanostructure and optical properties. The one-pot heating-up method is considered to be one of the most commonly used ways to synthesize the UCNPs.11,24,27 With Yb3+, Er3+ codoped β-NaGdF4 as a model system, the size dependent doping concentration during the growth process of the particles obtained from this regular synthesis approach was investigated by transmission electron microscopy (TEM) and energy dispersive X-ray spectrometer (EDX). TEM images of the UCNPs obtained at different reaction time show that the

Figure 2. TEM images, energy migration cartons, and energy transfer mechanisms of the NaGdF4:Yb,Er/NaGdF4 (A), NaGdF4:Yb,Er/ NaGdF4:Er (B), and NaGdF4:Yb,Er/NaGdF4:Yb (C). (D) Proposed energy transfer mechanism in HEC nanoparticles. 3635

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Figure 3. (A) Synthetic procedures for the HOC and HOC/S UCNPs based on the SLBL doping strategy. (B) Spontaneous growth process of the UCNPs by using the one pot heating-up method. (C−J) TEM, HAADF-STEM, HRTEM images, and corresponding FFT and SAED patterns of the obtained NaGdF4:Yb,Er HEC, HOC nanoparticles and NaGdF4:Yb,Er/NaYF4 HEC/S, HOC/S nanoparticles: (C) HOC, (D) HOC/S, (E) HOC/S, (F) HOC/S, (G) HEC, (H) HEC/S, (I) HEC/S, and (J) HEC/S. The color bar on the top this figure indicating the doping concentrations of the dopants.

size of the particles gradually increases from ∼2 to 10 nm with the reaction spontaneously going on (Figure 1A−C). EDX results show that the UCNPs synthesized under the regular synthesis approach are heterogeneous doping during the spontaneous growth process, and the total (Yb3+ and Er3+) local relative doping concentration of the dopants in a single particle is gradually decreased as the particle growth radially (Figure 1D). We assume that the heterogeneous doping property should result from different reactivity of rare earth ions during the reaction, which further leads to varied nucleation and growth rate. Because of the limitation of detectability and sensitivity of EDX, we further used the inductively coupled plasma atomic emission spectrometry (ICP-AES) to investigate the distributions of Yb3+ and Er3+ of UCNPs during the radial growth process, respectively. The results show that the Yb3+ doping concentration gradually decreases from ∼29% to 19% from inside to outside, while the radial distribution of Er3+ is nearly maintained constant (∼2.2%) (Figure 1E). In other words, the concentration of Yb3+ ions at the inner layer is much higher than that of the outer layer of the particle, which is consistent with the decrease of the red (4F9/2−4I15/2)/green (2H11/2−4I15/2, 4 S3/2−4I15/2) ratio of the emission spectra during the nanocrystal spontaneous growth process (Figure S1).33 The distribution of Er3+ ions is relatively more uniform in the whole particle, but it is surplus in the outer layer of the particle as compared with the distribution of Yb3+. In order to certify the effect of the spatial distributions of the doped elements in UCNPs on the UC optical properties, we further designed three core/shell structures: NaGdF4:Yb,Er/NaGdF4 (Figure 2A and Figure S2), NaGdF4:Yb,Er/NaGdF4:Er (Figure 2B and Figure S2), and NaGdF4:Yb,Er/NaGdF4:Yb (Figure 2C and Figure S2), which were prepared by SLBL protocol developed by our group. It can be seen that all the nanoparticles with different nanostructures have uniform spherical morphology

and nearly the same diameter of about 13 nm. When Er3+ ions are doped in outer layer of the particles, the UC emission intensity is sharply declined as compared with the NaGdF 4 :Yb,Er/NaGdF 4 (Figure S2). In stark contrast, the NaGdF4:Yb,Er/NaGdF4:Yb UCNPs exhibit the most optimum emission, which is consist with the literature.23 We assume that this obvious difference in the intensity of the UC emission can be attributed to the energy transfer between the Er3+ activator and the surface defect. It means that when the Er3+ ions are relatively enriched in outer layer of the particles, the absorbed energy can be transferred to surface and quenched by the defects through the outer layer enriched Er3+ ions (Figure 2B).34 For the NaGdF4:Yb,Er/NaGdF4:Yb UCNPs, the Yb3+ enriched outer layer can also absorb and transfer the near-infrared (NIR) radiation to the Er3+ ions, further enhancing the total UC emission (Figure 2C).23 To sum up, the relative enrichment of the activator (Er3+) in the outer layer of the particle is not good for the improvement of the optical property, which is just opposite to that of the sensitizer (Yb3+). Therefore, compared with the Yb3+ and Er3+ ion distributions in the HEC nanoparticles, we can conclude that the heterogeneous distribution of the doped sensitizer and activator is not optimum for the improvement of the efficiency of the UCNPs (Figure 2D). First, part of the energy absorbed by Yb3+ would be cross-relaxed because of the enrichment of Yb3+ at the inner layer of the particle. Second, the relatively surplus of Er3+ (compared with Yb3+) in the outer layer can induce the energy transfer between the Er3+ activator and the surface quencher. The last but not the least, energy transfer barrier between Yb3+ and Er3+ can occur due to inconsistent spatial distribution of Yb3+ and Er3+. It has been demonstrated that the SLBL protocol can realize the monolayer (ML) control of the thickness and components by simply tuning the amounts and species of the precursors,17 which allows us to fabricate the homogeneous doping UCNPs with controllable doping concentration at the ML scope. 3636

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the same as that of the HOC and HOC/S. Considering the large difference in atomic number between Y (39) and Gd (64), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) was employed to identify the shell thickness of the core/shell structures. All the HAADFSTEM images of HOC/S (Figure 3E) and HEC/S (Figure 3I) nanoparticles show a discernible contrast, indicating the formation of the core/shell structures. The thicknesses of the shells are well controlled at 5−6 nm. The high-resolution TEM (HRTEM) images along with the corresponding fast Fourier transform (FFT) results of the resultant HOC/S (Figures 3F1 and 3F2) and HEC/S (Figures 3J1 and 3J2) show similar lattice fringes and FFT patterns. It indicates that the obtained nanoparticles retain the same hexagonal crystal structure, consistent with the results of selected area electron diffraction (SAED) (Figure 3F3 and 3J3) and X-ray diffraction (XRD) patterns (Figure S5). On the premise of the above identical conditions (same size, shell thickness, crystal structure, etc.), the obtained HEC/S and HOC/S UCNPs are comparable. The UC spectra of the HEC/S and HOC/S nanoparticles under 980 nm excitation show three emission peaks at around 520, 550, and 650 nm, corresponding to the 2H11/2−4I15/2, 4S3/2−4I15/2, and 4F9/2−4I15/2 transitions of Er3+, respectively. Obviously, the HOC/S shows significant luminescence enhancement compared with the HEC/S UCNPs (Figure 4A). According to the method reported by Boyer and van Veggle,16 the absolute UC QY for HOC/S was obtained to be 0.89 ± 0.05% at an excitation power density of 50 W/cm2, which is nearly 2 times higher than that of the heterogeneous doping UCNPs (0.47 ± 0.05%). We assume that this enhancement results from the homogeneous distribution of the dopants, which further induces the reduction of the cross-relaxations and increase the efficiency of the energy transfer between the Yb3+ and Er3+. According to the relative emission intensity comparison of the obtained UCNPs (Figure 4B), we can conclude that the difference in emission intensity between the HEC/S and HOC/ S UCNPs is not caused by the surface defect, but by difference of the spatial distribution of the dopants, because the luminescence intensity of the HOC UCNPs is always stronger than that of HEC UCNPs before and after overcoating with the passivation shell. In order to compare the emission intensity of the obtained HEC/S and HOC/S UCNPs at a single particle level, dilute samples of HEC/S and HOC/S UCNPs were dispersed on pretreated glass substrate and imaged in an optical microscopy equipped with a electron-multiplying charge-coupled device (EMCCD) camera while excited with a tightly focused 980 nm laser (Figure S6). The local in situ spectral analysis of the luminescent spots shows that HOC/S particle exhibits about 1.5 times intense emission than the HEC/S particle at a single particle level. In terms of UCNPs, the lifetime shows positive correlation with the UC QY. Therefore, the luminescence decay curves of the obtained HEC, HOC, HEC/S, and HOC/S UCNPs were also compared (Figure 4D).13 It can be seen that the lifetimes of the core/shell structured HEC/S and HOC/S show obviously increment compared with the corresponding HEC and HOC UCNPs, which result from the suppression of nonradiative processes caused by surface defect. Furthermore, the HOC (414 μs) UCNPs show much longer lifetime than HEC (234 μs), indicating that these unwanted cross-relaxations between dopant ions are significantly suppressed in HOC nanoparticles.35 In order to fulfill the requirements of bioapplications, the UCNPs were transferred to water solution by the ligand exchange method with polyvinylpyrrolidone (PVP).36 Both HEC/S and HOC/S UCNPs show good performance of

Figure 4. (A) UC emission spectra of NaGdF4:Yb,Er/NaYF4 HEC/S, and HOC/S nanoparticles under identical experimental conditions. Inset shows the absolute QYs of the HEC, HOC, HEC/S, and HOC/ S nanoparticles. (B) Comparison of the relative emission intensity of the HEC, HOC, HEC/S, and HOC/S nanoparticles. (C) Single particle imaging and the corresponding in situ local spectral analysis of the HEC/S, and HOC/S nanoparticles. The spectra were obtained from one of the luminescent spots. (D) UC luminescence decay curves of the HEC, HOC, HEC/S, and HOC/S nanoparticles. (E) Comparison of emission intensity loss in polar solvents with different amounts of water for HEC/S and HOC/S nanoparticles. (F) In vivo UCL imaging of mice after subcutaneous injections of water-soluble HEC/S and HOC/S NaGdF4:Yb,Er/NaYF4 core/shell nanoparticles.

As shown in Figure 3A, the ultrasmall β-NaGdF4:Yb,Er with the mean size of ∼2.5 nm was synthesized as the initial seeds with the doping concentrations of ∼22% Yb3+ and ∼2.5% Er3+. The seeds were then coated with NaGdF4:Yb,Er ML successively by using the SLBL method. By precisely controlling the ratio of the precursors, the doping concentration of each ML could be well controlled at nearly constant value. The spherical concentric shell model (see Supporting Information) was employed to calculate the amount of the shell precursor necessary for the growth of each ML (Table S1).28 Because of the delicate ML growth, the doping concentrations can be well controlled at ∼22% for Yb3+ and ∼2.5% for Er3+ (Figure 1F) during the UCNPs growth process. It means that the dopants are homogeneously distributed in the particle during SLBL doping process. In order to compare the optical properties of the obtained HEC and HOC UCNPs objectively, we further deposited an ∼5 nm thick NaYF4 passivation shell on the surface of the obtained HOC and HEC nanoparticles to eliminate the influence of surface defect (Figure 3A,B; the obtained core/shell UCNPs are referred as HOC/S and HEC/S). TEM images of the initial seeds before (Figure S3) and after the SLBL doping process (HOC, Figure 3C) and the HOC/S nanoparticles (Figure 3D) show the mean diameter of 2.50, 10.53, and 20.11 nm, respectively. For the HEC and HEC/S (Figure 3G,H and Figure S4), the particle diameters are nearly 3637

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Figure 5. (A) UC emission spectra of NaGdF4:Yb,Tm/NaGdF4:Tb and NaGdF4:Yb,Tm/NaGdF4:Eu nanoparticles based on HEC (left) and HOC (right) under identical experimental conditions. The emission spectra were normalized to maximum Tm3+ emission at 475 nm. (B) Energy transfer mechanism in NaGdF4:Yb,Tm/NaGdF4:A core/shell structure. (C) Luminescence photographs of representative samples with multicolour emissions in cyclohexane solution under irradiation of a 980 nm laser. From left to right the sample is corresponding to HOC/S NaGdF4:Yb,Tm/ NaYF4, HOC/S NaGdF4:Yb,Er/NaYF4, HOC-NaGdF4:Yb,Tm/NaGdF4:Eu, and HOC-NaGdF4:Yb,Tm/NaGdF4:Tb, respectively.

separately incorporated in the shells can be increased accordingly. Therefore, as a proof-of-concept experiment, we further fabricated core/shell UCNPs with the Yb3+/Tm3+ and the A3+ activator (A = Tb, Eu) separately incorporated into the core and shell layer of the particle (NaGdF4: 20% Yb, 0.5% Tm/NaGdF4: 15% A) by using the HOC and HEC as cores, respectively (Figures S8 and S9). The results show that all the samples exhibit the character emission of the Tb3+, Eu3+ activators (Figure 5A). Furthermore, when the HOC nanoparticles were used, the emission peaks of both Tb3+ and Eu3+ were stronger than that of HEC based NaGdF4:Yb,Tm/ NaGdF4:A UCNPs (∼30% and ∼20% improvements for Tb3+ and Eu3+, respectively). So, it can be demonstrated that Tm3+ in the HOC can accumulate more energy than that of the HEC, meaning the improvement the energy transfer between the Yb3+ and Tm3+. Because of the optimized distribution of the dopants, highly efficient multicolor emission can be obtained by tuning the activator (Figure 5C), suggesting that the homogeneous doping model not only can be used for the UC efficiency improvement of lanthanide ion-doped nanoparticles but also provide a track for engineering the high quality UCNPs with advanced structure and optical properties. In summary, we represent for first time that the homogeneous doping approach base on the SLBL method can greatly improve the efficiency of the UCNPs. The QY as high as 0.89 ± 0.05% is realized for the homogeneous doping NaGdF4:Yb,Er/NaYF4 UCNPs, which is nearly 2 times higher than that of the heterogeneous doping NaGdF 4:Yb,Er/NaYF4 UCNPs (0.47 ± 0.05%). The influence of spatial distributions and local relative concentration of the dopants on the optical properties of UCNPs were investigated in the single particle level. It was found that heterogeneous doping indeed existed

resistant for luminescence quenching by water (Figure 4E), which can be attributed to the well passivation with the uniform NaYF4 shell. PVP-modified water-soluble HEC/S and HOC/S NaGdF4:Yb,Er/NaYF4 core/shell UCNPs were then employed as luminescent probe for in vivo imaging (Figure 4F). Dramatically enhanced UCL signal was observed at the HOC/S subcutaneous injection site compared with the HEC/S injection site, further certifying the superiority of HOC UCNPs in the bioapplications. It should be noted that this luminescence enhancement of the HOC is not restricted to the Yb3+, Er3+ codoped systems, similar results can also be observed in the NaGdF4:Yb3+,Tm3+ HOC UCNPs, and the enhancement factor for the emission at 450 nm can reach as high as 4.13 compared with the HEC UCNPs (Figure S7). Up to now, considerable efforts have been devoted to tuning UC emission over a broad spectral range for applications in multicolor labeling and multiplexed biodetection. Most recently, Liu et al. have shown that through Gd-mediated energy migration and core/shell engineering, efficient UC emission could be realized for lanthanide activators (Tb3+, Eu3+, Dy3+, and Sm3+) without long-lived intermediary energy states.29 A migrator Gd3+ extracts the excitation energy from high-lying energy states of Tm3+, followed by random energy hopping through the migratory ion sublattice and trapping of the migrating energy by an activator ion. Besides the important role of the migratory ion, we believe that the spatial distribution of the Yb3+ and Tm3+ in the cores is also one of the significant factors in the energy transfer process. In other words, the more energy is collected by Tm3+, the more energy can be migrated to the activator located in the outer shells. If the homogeneous doping of Yb3+ and Tm3+ can be realized in this system, the multicolor emission intensity of the activator (Tb3+, Eu3+) 3638

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(13) Liu, Q.; Sun, Y.; Yang, T.; Feng, W.; Li, C.; Li, F. J. Am. Chem. Soc. 2011, 133, 17122. (14) Li, X.; Zhang, F.; Zhao, D. Nano Today 2013, 8, 643. (15) Jayakumar, M. K. G.; Idris, N. M.; Zhang, Y. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 8483. (16) Boyer, J. C.; van Veggel, F. C. J. M. Nanoscale 2010, 2, 1417. (17) Li, X.; Shen, D.; Yang, J.; Yao, C.; Che, R.; Zhang, F.; Zhao, D. Chem. Mater. 2013, 25, 106. (18) Auzel, F. Chem. Rev. 2004, 104, 139. (19) Page, R. H.; Schaffers, K. I.; Waide, P. A.; Tassano, J. B.; Payne, S. A.; Krupke, W. F.; Bischel, W. K. J. Opt. Soc. Am. B 1998, 15, 996. (20) Krämer, K. W.; Biner, D.; Frei, G.; Güdel, H. U.; Hehlen, M. P.; Lüthi, S. R. Chem. Mater. 2004, 16, 1244. (21) Chen, G. Y.; Shen, J.; Ohulchanskyy, T. Y.; Patel, N. J.; Kutikov, A.; Li, Z.; Song, J.; Pandey, R. K.; Ågren, H.; Prasad, P. N.; Han, G. ACS Nano 2012, 6, 8280. (22) Ostrowski, A. D.; Chan, E. M.; Gargas, D. J.; Katz, E. M.; Han, G.; Schuck, P. J.; Milliron, D. J.; Cohen, B. E. ACS Nano 2012, 6, 2686. (23) Vetrone, F.; Naccache, R.; Mahalingam, V.; Morgan, C. G.; Capobianco, J. A. Adv. Funct. Mater. 2009, 19, 2924. (24) Wang, F.; Wang, J.; Liu, X. Angew. Chem., Int. Ed. 2010, 49, 7456. (25) Liu, X.; Kong, X.; Zhang, Y.; Tu, L.; Wang, Y.; Zeng, Q.; Li, C.; Shi, Z.; Zhang, H. Chem. Commun. 2011, 47, 11957. (26) Yi, G. S.; Chow, G. M. Chem. Mater. 2007, 19, 341. (27) Su, Q.; Han, S.; Xie, X.; Zhu, H.; Chen, H.; Chen, C. K.; Liu, R. S.; Chen, X.; Wang, F.; Liu, X. J. Am. Chem. Soc. 2012, 134, 20849. (28) Zhang, F.; Che, R.; Li, X.; Yao, C.; Yang, J.; Shen, D.; Hu, P.; Li, W.; Zhao, D. Nano Lett. 2012, 12, 2852. (29) Wang, F.; Deng, R.; Wang, J.; Wang, Q.; Han, Y.; Zhu, H.; Chen, X.; Liu, X. Nat. Mater. 2011, 10, 968. (30) Abel, K. A.; Boyer, J. C.; Andrei, C. M.; van Veggel, F. C. J. M. J. Phys. Chem. Lett. 2011, 2, 185. (31) Chen, O.; Zhao, J.; Chauhan, V. P.; Cui, J.; Wong, C.; Harris, D. K.; Wei, H.; Han, H. S.; Fukumura, D.; Jain, R. K.; Bawendi, M. G. Nat. Mater. 2013, 12, 445. (32) Dong, C.; Pichaandi, J.; Regier, T.; van Veggel, F. C. J. M. J. Phys. Chem. C 2011, 115, 15950. (33) Wang, F.; Liu, X. J. Am. Chem. Soc. 2008, 130, 5642. (34) Yuan, D.; Shun, Y. G.; Chow, G. M. J. Mater. Res. 2009, 24, 2042. (35) Liu, Y.; Tu, D.; Zhu, H.; Li, R.; Luo, W.; Chen, X. Adv. Mater. 2010, 22, 3266. (36) Johnson, N. J. J.; Sangeetha, N. M.; Boyer, J. C.; van Veggel, F. C. J. M. Nanoscale 2010, 2, 771.

during the spontaneous growth process of the nanoparticles. The heterogeneous doping property can further induce many negative effects on the optical properties of UCNPs, especially the luminescent efficiency. The spatial distributions and local relative concentrations of the dopants can be well controlled by the SLBL homogeneous doping method on the monolayer level and homogeneously distributed in the single particle level. Furthermore, by using homogeneous doping NaGdF4:Yb,Tm as a core, the multicolor emission intensity of NaGdF4:Yb,Tm/ NaGdF4:A (A = Tb3+, Eu3+) core/shell nanoparticles can also exhibit 20%−30% improvement. We believe that such a homogeneous doping model can open the door to improve the UC optical properties by engineering the local distribution of the sensitizer, activator, host, etc., in a microcosmic and provide a track for engineering the high quality UCNPs with advanced nanostructure and optical properties.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details; Table S1 and Figures S1−S12. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (F.Z.). *E-mail [email protected] (D.Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by NSFC (grants 21322508, 21101029, 21273041, and 21210004), China National Key Basic Research Program (973 Project) (No. 2013CB934100, 2012CB224805, and 2010CB933901), the Shanghai Rising-Star Program (12QA1400400), Program for New Century Excellent Talents in University (NCET) and the State Key Laboratory of Pollution Control and Resource Reuse Foundation (No. PCRRF12001).

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dx.doi.org/10.1021/nl501366x | Nano Lett. 2014, 14, 3634−3639