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Letter pubs.acs.org/NanoLett

Direct Imaging the Upconversion Nanocrystal Core/Shell Structure at the Subnanometer Level: Shell Thickness Dependence in Upconverting Optical Properties Fan Zhang,* Renchao Che, Xiaomin Li, Chi Yao, Jianping Yang, Dengke Shen, Pan Hu, Wei Li, and Dongyuan Zhao* Department of Chemistry, Laboratory of Advanced Materials, Fudan University, Shanghai 200433, P. R. China

Nano Lett. 2012.12:2852-2858. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/06/19. For personal use only.

S Supporting Information *

ABSTRACT: Lanthanide-doped upconversion nanoparticles have shown considerable promise in solid-state lasers, three-dimensional flat-panel displays, and solar cells and especially biological labeling and imaging. It has been demonstrated extensively that the epitaxial coating of upconversion (UC) core crystals with a lattice-matched shell can passivate the core and enhance the overall upconversion emission intensity of the materials. However, there are few papers that report a precise link between the shell thickness of core/shell nanoparticles and their optical properties. This is mainly because rare earth fluoride upconversion core/shell structures have only been inferred from indirect measurements to date. Herein, a reproducible method to grow a hexagonal NaGdF4 shell on NaYF4:Yb,Er nanocrystals with monolayer control thickness is demonstrated for the first time. On the basis of the cryo-transmission electron microscopy, rigorous electron energy loss spectroscopy, and high-angle annular dark-field investigations on the core/shell structure under a low operation temperature (96 K), direct imaging the NaYF4:Yb,Er@ NaGdF4 nanocrystal core/shell structure at the subnanometer level was realized for the first time. Furthermore, a strong linear link between the NaGdF4 shell thickness and the optical response of the hexagonal NaYF4:Yb,Er@NaGdF4 core/shell nanocrystals has been established. During the epitaxial growth of the NaGdF4 shell layer by layer, surface defects of the nanocrystals can be gradually passivated by the homogeneous shell deposition process, which results in the obvious enhancement in overall UC emission intensity and lifetime and is more resistant to quenching by water molecules. KEYWORDS: Nanocrystal, core/shell, upconversion, luminescence

U

nanoparticles with a platform to best achieve a reliable performance in the context of highly complex bioimaging applications. At present, there are many approaches for the production of rare earth doped core/shell UC nanoparticles with the luminescence stability for most of the existing bioapplications.11−13 However, only a few papers have reported precisely the link between the shell thickness of core/shell nanoparticles and their optical properties, mainly because the upconversion core/shell structures have only been inferred from indirect measurements. Basic analyses with conventional transmission electron microscopy (TEM) technology are only able to infer that a core/shell structure has formed, but it is difficult to eliminate the heterogeneous growth of shell precursors onto core nanocrystals because of similar lattice constants and very weak contrast between the core and shell materials. Elemental analysis techniques, such as energy dispersive X-ray (EDX), failed to provide information regarding the location of the

pconversion (UC) phosphor nanocrystals are emerging as an important class of nanomaterials owing to their wide applications in solid-state lasers,1 three-dimensional flatpanel displays,2,3 and solar cells4 and especially biolabels5,6 and bioimaging.7 Compared with the conventional biological labels, such as organic dye markers and quantum dots, upconversion nanomaterials have many advantages, including higher chemical stability, lower toxicity, and higher signal-to-noise ratio.8 Despite the considerable potential of upconverting nanomaterials for applications in bioscience, improvements are still needed to optimize upconversion optical properties for further commercialization. A major improvement has been realized that growing a shell with similar lattice constants around the core could protect the luminescent lanthanide ions in the core (especially those near the surface) from nonradiative decay caused by surface defects as well as vibrational deactivation from solvents or surface-bound ligands in the case of colloidal dispersions.8,9 These lanthanide doped core/shell UC nanocrystals showed increased photoluminescence efficiencies (QY) by 300% (QY ∼ 0.30 ± 0.10%) over core only nanocrystals (QY ∼ 0.10 ± 0.05%) with the same particle size.10 These results suggest that the use of shell coating can provide the UC © 2012 American Chemical Society

Received: February 1, 2012 Revised: April 14, 2012 Published: April 30, 2012 2852

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element inside the single nanocrystal. Although the evidence of hexagonal NaYF4@NaGdF4 core/shell structures had been provided with a X-ray photoelectron spectroscopy (XPS) technique,12 it is unable to draw conclusions on the overall thickness of the shells or whether isotropic shell growth had occurred on individual core nanocrystals because this technique is an ensemble measurement. Most recently, van Veggel and coworkers used high-angle annular dark-field (HAADF) combined with energy dispersive X-ray spectroscopy (EDS) to analyze the NaYF4@NaGdF4 core/shell structure.13 Although the anisotropic shell growth was demonstrated by the EDS line scanning investigations, it was still difficult to get the precise shell thickness because of the noticeable beam damage or bombardment under the electron beam irradiation (no less than 200 kV) at room temperature. It is well-known that the optical properties of the core/shell nanoparticles in the strong confinement regime are sensitive to even the small changes in core or shell dimensions. Therefore, the extraction of reliable data on the optical properties of such structures hinges upon unambiguous verification of their core/shell physical dimensions. Establishing this link is vital to further increase the luminescent efficiency and stability of the UC nanoparticles for biolabeling and bioimaging. In light of this, the current challenge is to synthesize heterostructures with a well-defined core and shell of variable thickness and to infer the upconversion core/shell structures from direct measurements. Here we use NaYF4:Yb,Er@NaGdF4 core/shell nanocrystals as a model system, which allows for facile control of the UC optical properties by varying the thickness of the NaGdF4 shell. A reproducible successive ion layer adsorption and reaction (SILAR) method based on the previous reports9−13 was used to grow hexagonal NaGdF4 shell on NaYF4:Yb,Er nanocrystals with monolayer (ML) control thickness without introducing detrimental structural defects that lead to consequent reduction of luminescent efficiency. On the basis of the cryo-transmission electron microscopy (cryo-TEM), rigorous electron energy loss spectroscopy (EELS), and HAADF investigations on the core/ shell structure under the low temperature (96 K), direct imaging the NaYF4:Yb,Er@NaGdF4 nanocrystal core/shell structure was realized for the first time. Furthermore, a strong linear link between the NaGdF4 shell thickness and the optical response of the hexagonal NaYF4:Yb,Er@NaGdF4 core/shell nanocrystals has been established. Establishing this link is vital to further increase the luminescent efficiency and stability of the UC nanoparticles for biolabeling and bioimaging. To precisely determine the effects of shell thickness on the optical properties of the resultant nanocrystals, ML control of shell thickness must be achieved. In order to realize these goals, we first synthesized hexagonal NaYF4:Yb,Er cores with the mean size of 15.70 ± 1.80 nm (Table 1). The nanocrystal cores were coated with the hexagonal NaGdF4 shell layer adhering to the SILAR procedure (see Supporting Information).14 The spherical concentric shell model (CSM) was employed to calculate the amount of the shell precursor necessary for the growth of each ML.14a,15 This model has been used extensively for the hexagonal CdS and ZnS shell deposition during the quantum dots core/shell nanocrystal synthesis process.15 Because of the highly symmetric structure of the CdS, the ML was inferred to a thickness equal to half the c-lattice parameter. Although the NaGdF4 shell adopted here is also a hexagonal structure (P63/m, a = 6.02 Å, c = 3.60 Å),3 the cation sites are of three types: (1) a one-fold site occupied by RE3+; (2) another site occupied randomly by 1/2Na+ and 1/2RE3+;

Table 1. Statistics Obtained from Analysis of the Histograms Corresponding to the Particle Sizes for Samples 1-7 at Various Stages of NaGdF4 Shell Grown sample

NaGdF4 ML

1 2 3 4 5 6 7

0 2 4 6 8 10 6

measured dm (nm)

predicted dp (nm)

dm − dp (nm)

σ (%)

± ± ± ± ± ± ±

− 17.14 18.58 20.02 21.46 22.90 20.02

− +0.28 +0.40 +0.56 +0.66 +0.93 +1.19

12.1 12.1 12.0 11.8 11.3 10.7 17.4

15.70 17.42 18.98 20.58 22.12 23.83 19.11

2.00 2.10 2.27 2.43 2.49 2.56 3.32

*

SILAR procedure was used for samples 1−6. One step shell coating (OSSC) procedure was used for sample 7.

and (3) a two-fold one randomly by Na+ and vacancies.3 Therefore, referral to a NaGdF4 ML can be taken to mean a thickness equal to the c-lattice parameter of the bulk material, 0.36 nm in the case of the hexagonal NaGdF4 (Figure S1, Supporting Information). Cryo-TEM images show the hexagonal core NaYF4:Yb,Er nanocrystals and NaYF4:Yb,Er@NaGdF4 obtained by coating with 2 MLs (∼0.86 nm), 4 MLs (∼1.64 nm), and 6 MLs (∼2.40 nm) of the hexagonal NaGdF4 shells, respectively (Figure 1a,f,k,p and Table 1). The sizes were recorded and then binned into intervals of 0.5 nm, and the resultant histograms fitted to Gaussian distributions using a Levenberg−Marquardt algorithm to extract the mean sizes and standard deviations (Figure 1b,g,l,q).14b Cryo-TEM images reveal that throughout the growth process the nanocrystals retain their initial quasispherical morphology, except in the latter stages of shell growth for the thickness larger than 1.44 nm (4 MLs) (Figure 1a,f,k,p) where the nanocrystals begin to adopt a distinctly hexagonal shape (Figure 1k). It can be seen that the particle size distributions (deviation in measured dm) broaden slightly with each additional ML (Table 1). The full width at half-maximum (fwhm) of the distributions for the final core/shell hexagonal NaYF4:Yb,Er@NaGdF4 with 10 MLs is typically around 0.42 nm, which is greater than those of the original cores. It is revealed that in all cases the values for standard deviations of the distribution (σ) decrease by ∼0.1−1.4% by the termination of shell growth and that the final standard deviation of the core/shell nanocrystal diameter is about 11.7% (Table 1). To minimize the effects of electron beam damage on the materials, cryo-high resolution TEM (cryo-HRTEM) images of the hexagonal NaYF4:Yb,Er cores and corresponding core/shell nanocrystals with different shell thickness were performed using a liquid nitrogen cooled cryo-stage installed on the same TEM stage (96 K) (Figure 1c,h,m,r). The difference in contrast between the inner and outer part confirms the surface coating with the increase of shell thickness. With 6 MLs, the lattice space (0.19 nm) of the shell can be clearly observed (Figure 1r), which corresponds to the (213̅0) plane of hexagonal NaGdF4 (JCPDS-PDF no. 027-0699). And the lattice space of the inner core is ∼0.52 nm, corresponding to the (101̅0) plane of hexagonal NaYF4 (JCPDS-PDF no. 016-0334). The corresponding heterostructured nanocrystals comprising the NaYF4:Yb,Er core and NaGdF4 shell with variable thickness can be further demonstrated with the high-resolution HAADF (Figure 1d,i,n,s). The crystal lattice of the core and shell can clearly be resolved by the different orientation, respectively. 2853

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Figure 1. Cryo-TEM images and histograms of the measured particle sizes of NaYF4: 20% Yb, 2% Er core (a,b) and overcoated with 2 ML (f,g), 4 ML (k,l), and 6 ML (p,q) of the NaGdF4 shell, respectively. The dashed line indicates the expected size of the nanocrystals, with the cyan box outlining the associated error due to the error in the initial size calibration curve.13b Cryo-HRTEM (c,h,m,r), HAADF-STEM (d,i,n,s), and HAADF (e,j,o,t) images of hexagonal NaYF4: 20% Yb, 2% Er core nanocrystals (c−e) and the corresponding core/shell nanocrystals with 2 ML (h−j), 4 ML (m−o), and 6 ML (r−t) of the NaGdF4 shell, respectively. The composite color map (e,j,o,t) shows the location of different elements. Blue and yellow indicate yttrium and gadolinium atoms, respectively. The color map is made by merging two chemical maps that only contain electron intensities due to inelastic scattering of the source electrons with gadolinium M4,5 and yttrium L2,3 electrons. The shell thicknesses from ∼0.86 to ∼2.44 nm could be determined clearly for the core/shell nanocrystals with different ML.

diffraction peaks become broad and shift toward lower angles to approach the values for a bulk NaGdF4 crystal (Figure 2d), consistent with smaller local crystal domains and the volume increase of NaGdF4 compared to NaYF4. Selected area electron diffraction patterns (SAED) (Figure 2f) were recorded from the NaYF4:Yb,Er@NaGdF4 superlattice pattern with 6 ML shell of NaGdF4 (Figure 2e). The observed values (OBS) for the relative ring d-spacing are tabulated along with the simulated one (SIM) based on a bulk NaGdF4 and NaYF4 crystals (Figure 2). The deviation between the OBS (NaYF4:Yb,Er@NaGdF4 nanocrystals) and SIM (bulk NaGdF4 and NaYF4 crystals) values for each set of miller indices (hkl values) agree to within 5.0% for NaGdF4 and 6.6% for NaYF4 (Figure 2), respectively, which is corresponding to the XRD data and confirms the hexagonal phase of the core/shell nanocrystals. Both the above XRD and SAED data demonstrate that the shells grow along an epitaxial manner. The effect of the shell thickness on the UC optical properties of the NaYF4:Yb,Er@NaGdF4 core/shell nanocrystals is plotted in Figure 3. Upon deposition of the first ML of NaGdF4, the increase in the intensity and lifetime is observed (Figure 3a−d), indicative of epitaxial surface passivation and increased electronic isolation of the core from its environment. Ideally, a uniform shell should be formed over nanocrystals after the deposition of every ML, any voids on the surface can affect the photostability and also induce the leakage of the core material. However, the Cryo-HRTEM results reveal that the anisotropic shell growth always results in the voids at the initial ML deposition (Figure S4). During the epitaxial and layer-by-layer growth of the NaGdF4 shell, surface defects of the nanocrystals can be gradually passivated, resulting in the obvious enhancement in overall UC emission (Figure 3a, b). Upon continuous

To further determine the chemical composition of the core/ shell nanocrystals, EELS spectrum imaging approach was used to get the elemental distribution, at every pixel of an image. From this, element-specific chemical maps were constructed (Figure 1e,j,o,t). In combination with scanning transmission electron microscopy (STEM), EELS studies have proven to be advantageous in the research of core/shell nanostructures16,17 and have also unveiled a single atom.18 Two-color composite chemical maps of the NaYF4:Yb,Er cores and the corresponding core/shell nanocrystals with different thickness reveal the obvious contrast between the cores (Figure 1e) and shells (Figure 1j,o,t). The color indicates electrons scattered by Gd (atomic number Z = 64) (yellow) and Y (atomic number Z = 39) (blue). Taking the nanocrystal with 6 MLs of NaGdF4 shell as an example (Figure 2a), the EELS spectra recorded from the probe location on the inferred NaYF4:Yb,Er core show the Y L2,3 edge and Yb M5 edge signals (Figure 2b and Figure S2, Supporting Information). The EELS spectrum taken from the probe location on the inferred NaGdF4 shell could be indicated as the Gd M4 and M5 edge signals (Figure 2c), which further demonstrated the NaYF4:Yb,Er@NaGdF4 core/shell structure. The corresponding chemical maps (insets in Figure 2a) reveal the presence of yttrium at the nanocrystals center and gadolinium at the periphery of nanocrystals. Moreover, it is observed that the NaYF4:Yb,Er@NaGdF4 core/shell are inclined to form the hollow structure under the electron beam irradiation at the regular operation temperature after an irradiation for 2 s (Figure S3, Supporting Information). As such, the low operation temperature (96 K) is necessary to get the high-quality imaging results with the HAADF-STEM. The XRD pattern of NaYF4:Yb,Er core nanocrystals can be indexed as a hexagonal phase of bulk NaYF4 crystal (Figure 2d).1,3 Upon growth of the NaGdF4 shell layer by layer, the 2854

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Figure 2. (a) HADDF image of a single NaYF4:Yb,Er@NaGdF4 nanocrystal. The insets in (a) show the corresponding chemical maps and reveal the presence of yttrium at the nanocrystals center (blue) and gadolinium at the periphery of nanocrystals (yellow). EELS spectra of yttrium L2,3 (b) and gadolinium M4,5 (c) edges taken from the probe location on the inferred NaYF4:Yb,Er core and NaGdF4 shell (a) using the nanocrystal with 6 MLs NaGdF4 shell. (d) XRD patterns of hexagonal NaYF4: 20% Yb, 2% Er core nanocrystals and the corresponding core/shell nanocrystals with 2 ML (h), 4 ML (m), and 6 ML (r) of the NaGdF4 shell, respectively. Upon layer-by-layer growth of the NaGdF4 shell, the peaks broaden and shift to approach the predicted values for a bulk NaGdF4 crystal. SAED patterns (f) recorded on the 8 ML NaYF4: 20% Yb, 2% Er @NaGdF4 nanocrystals superlattice (e). The observed relative ring spacing (d-spacing) (OBS) are tabulated along with the simulated (SIM) values based on a bulk NaGdF4 and NaYF4 crystals (bottom right). The deviation between the OBS (NaYF4:Yb,Er@NaGdF4 nanocrystals) and SIM (bulk NaGdF4 and NaYF4 crystals) values for each set of miller indices (hkl values) were indicated by σ values. The data obtained from the XRD and SAED patterns demonstrate that shell growth proceeds in an epitaxial manner.

In principle, there are two critical issues for increasing the quality of the core/shell nanocrystals during the growth of shell: (1) the elimination of the homogeneous nucleation of the shell materials and (2) homogeneous ML growth of the shell precursor onto all core nanocrystals in solution, yielding uniform shell coating.14a It has been extensively reported that the UC core/shell nanocrystals can be synthesized with the one-step shell coating procedure by controlling the volume of the shell precursor below the phase separation point, which can eliminate the homogeneous nucleation of the shell materials.12,21 However, it was found in our experiments that the heterogeneous shell deposition around the core nanocrystals could not be eliminated during the shell growth process. We compared the core/shell nanocrystals with 6 ML thickness shell synthesized by the SILAR and one-step coating procedures,12 respectively (see the Supporting Information). The uniform (Figure 4c,g) and uneven (Figure 4a,b,e,f) shell growth can be directly observed with the one-step shell coating procedure, and the uneven shell deposition can also be demonstrated by the EELS results (Figure S5, Supporting Information). The Y signal can be detected at one end of the nanocrystal, and the Gd signal can be detected at the center of the nanocrystal, further indicating the heterogeneous anisotropic growth. However, with the SILAR procedure, only uniform concentric symmetry featured core/shell nanocrystals can be observed (Figure 4d,h),

shell growth, a linear increase in intensity (Figure 3d) and lifetime (Figure 3c) is observed. For the biological applications, the UC nanocrystals are always transferred from the nonpolar solvent to the water solution by the surface encapsulation methods.19 Therefore, it is significant to investigate the shell thickness effect on the UC optical properties in the aqueous solution environment. In order to obtain quantitative information on the optimum shell thickness for the UC emission intensity in the aqueous solution, the bare core and overcoated core/shell nanocrystals were transferred into water/ethanol solutions with different amounts of water by surface encapsulation with amphiphilic lipids.20 Because of the high-energy stretching vibration (ca. 3500 cm−1) effect, the water molecule is regarded as a surface oscillator that significantly quenches the fluorescence of rare earth dopant ions.12 The PL of the core only nanocrystals underwent dramatic quenching with the increase of water content to 30%, around 90% of the overall emission intensity was quenched (Figure 3e). The core/shell nanocrystals were found to be substantially more resistant to quenching by water molecules with an increase in shell thickness. The relative PL intensities of quenched to unquenched results (I/I0) decreased with the increasing of H2O amount and remained more or less the same for NaYF4:Yb,Er@NaGdF4 core/shell nanocrystals possessing >4 ML. 2855

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Figure 3. Room-temperature UC luminescence spectra (a), photograph of colloidal solutions (b), and evalution statistics (d) of cyclohexane solutions comprising NaYF4: 20% Yb, 2% Er core nanocrystals and the corresponding core/shell nanocrystals with 1−6 ML of NaGdF4 shell under excitation with 980 nm laser (error bars show the statistical results of duplicate experiments). (c) UC luminescence decay curves of NaYF4: 20% Yb, 2% Er core nanocrystals and the corresponding core/shell nanocrystals with 1−6 ML of NaGdF4 shell. (e) Comparison of luminescence intensity loss for NaYF4: 20% Yb, 2% Er core nanocrystals and the corresponding core/shell nanocrystals with 1−6 ML of NaGdF4 shell in polar solvents with different amounts of water.

Figure 4. Cryo-HRTEM images of hexagonal NaYF4: 20% Yb, 2% Er@ NaGdF4 core/shell nanocrystals with 6 ML of NaGdF4 shell prepared with OSSC procedure (a−c) and SILAR procedure (d), respectively. The composite color maps (e−h) show the location of different elements. Blue and yellow indicate yttrium and gadolinium atoms, respectively. The color maps are made by merging two chemical maps that only contain electron intensities due to inelastic scattering of the source electrons with gadolinium M4,5 and yttrium L2,3 electrons. (i) Room temperature UC luminescence spectra of the cyclohexane solutions comprising hexagonal NaYF4: 20% Yb, 2% Er@ NaGdF4 core/shell nanocrystals with 6 ML prepared by OSSC (red line) and SILAR (black line) procedures. The related UC luminescence decay curves are shown in inset of (i). (j) Comparison of luminescence intensity loss for hexagonal NaYF4: 20% Yb, 2% Er@ NaGdF4 core/shell nanocrystals with 6 ML of NaGdF4 shell prepared from OSSC (red line) and SILAR (black line) procedures in polar solvents with different amounts of water. Indicating the surface defects of the core nanocrystals can be gradually passivated by the homogeneous shell deposition process with the SILAR procedure, which results in the obvious enhancement in overall UC emission (i) and more resistance to quenching by water molecules (j). With the OSSC procedure, the large volume of the shell precursor always induces the heterogeneous shell growth, and the ineffective core passivation cannot tolerate harsher processing conditions and environments (i,j).

suggesting that the layer-by-layer deposition is advantageous to realize the homogeneous shell growth. From the UC

luminescence, the higher emission intensity and longer lifetime can be observed for the core/shell nanocrystals prepared with 2856

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Nano Letters SILAR procedure, comparing to that from the one-step shell coating method (Figure 4i). When the materials were transferred into water/ethanol solution by surface encapsulation with amphiphilic lipids, a marked decrease in I/I0 value is observed for the nanocrystals prepared with one-step shell coating procedure (Figure 4j). These results demonstrate that the core nanocrystals can be passivated more efficiently by the SILAR procedure to decrease the core surface crystal defects and insulate the core from its environment. It has been demonstrated extensively that the epitaxial coating of as-made UC core crystals with a lattice-matched shell can passivate the core and decrease the crystal defects on the surfaces, which finally can significantly enhance the overall UC emission intensity of the materials.12,22 In the present work, at the beginning of the ML growth process with the SILAR procedure, we speculate that the shell precursor inclines to realize the nucleation at the defects on the surface of the core crystals. Therefore, the core cannot be passivated permanently and is extremely sensitive to the local environment because of the uneven shell coating (Figure S4, Supporting Information). During the epitaxial growth of the NaGdF4 shell layer by layer, surface defects of the nanocrystals can be gradually passivated by the homogeneous shell deposition process, which results in the obvious enhancement in overall UC emission intensity and lifetime and more resistance to quenching by water molecules (Figure 3a−c,f). However, with the one-step shell coating procedure, the large volume of the precursors always induces the heterogeneous shell growth, and the ineffective core passivation cannot tolerate harsher processing conditions and environments (Figure 4j). In conclusion, utilizing the cryo-TEM and rigorous EELS and HAADF techniques at a low temperature (96 K), direct imaging the NaYF4:Yb,Er@NaGdF4 nanocrystal core/shell structure has been successfully realized. Furthermore, a strong linear dependence of the NaGdF4 shell thickness on the optical response of the hexagonal NaYF4:Yb,Er@NaGdF4 core/shell nanocrystals has been demonstrated. During the epitaxial layerby-layer growth of the NaGdF4 shells, surface defects of the nanocrystals can be gradually passivated by the homogeneous shell deposition process, which results in the obvious enhancement of the overall UC emission intensity and lifetime and more resistance to quenching by water molecules. The highly luminescent and stable core/shell UC nanocrystals synthesized with SILAR procedure would further push forward the application of UC nanomaterials in biological labeling and imaging. Furthermore, we hope the present work can lead to an important understanding of the nanostructures and related properties for a wide variety of complex core/shell nanomaterials in future work.

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ACKNOWLEDGMENTS

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REFERENCES

The work was supported by the NSFC (grant no. 21101029), China National Key Basic Research Program, (973 Project) (no. 2010CB933901), and Fudan Startup Foundation for Advanced Talents.

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Method, characterization, and additional analysis of the materials prepared at different reaction conditions. This material is available free of charge via the Internet at http:// pubs.acs.org.

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T.; Kimoto, K.; Sawada, H.; Sasaki, T.; Omoto, K.; Tomita, T.; Kaneyama, T.; Kondo, Y. Nat. Chem. 2009, 1, 415. (19) Wang, F.; Banerjee, D.; Liu, Y.; Chen, X.; Liu, X. Analyst 2010, 135, 1839. (20) Dubertret, B.; Skourides, P.; Norris, D. J.; Noireaux, V.; Brivanlou, A. H. Science 2002, 298, 1759. (21) Wang, F.; Deng, R. R.; Wang, J.; Wang, Q. X.; Han, Y.; Zhu, H. M.; Chen, X. Y.; Liu, X. Nat. Mater. 2011, 10, 968. (22) Peng, X. G. Acc. Chem. Res. 2010, 43, 1387.

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