Facile Synthesis of Highly Photoluminescent Ag2Se Quantum Dots as

May 24, 2013 - Facile Synthesis of Highly Photoluminescent Ag2Se Quantum Dots as a New Fluorescent Probe in the ... Mater. , 2013, 25 (12), pp 2503–...
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Facile Synthesis of Highly Photoluminescent Ag2Se Quantum Dots as a New Fluorescent Probe in the Second Near-Infrared Window for in Vivo Imaging Bohua Dong, Chunyan Li, Guangcun Chen, Yejun Zhang, Yan Zhang, Manjiao Deng, and Qiangbin Wang* Division of Nanobiomedicine and i-Lab, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, People’s Republic of China S Supporting Information *

ABSTRACT: A facile solvothermal method is reported to synthesize highly photoluminescent Ag2Se quantum dots (QDs) with emission at 1300 nm in the second near-infrared window. After surface modification of C18-PMH-PEG, the Ag2Se QDs possess bright photoluminescence, good watersolubility, high colloidal stability and photostability, as well as decent biocompatibility, which are further successfully performed in in vivo deep imaging of organs and vascular structures with high spatial resolution. This new NIR-II fluorescent nanoprobe with small sizes, ideal optical properties, and decent biocompatibility opens up exciting opportunities for future biomedical applications. KEYWORDS: Ag2Se quantum dot, near-infrared, fluorescence, in vivo imaging

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quantum yield (QY) is a critical issue needing to be addressed.8 Therefore, it is urgent to develop a new type of bright and biocompatible NIR-II nanoprobe with an emission wavelength centered at 1300 nm for in vivo deep tissue/organ imaging. Regarding β-Ag2Se, as a narrow band gap semiconductor (0.15 eV),18,19 its emission can be readily tuned in the NIR-II region by tailoring its sizes. Without a heavy metal component, β-Ag2Se with an optimized emission peaking at 1300 nm might be an ideal candidate for in vivo imaging. In fact, the controlled synthesis of Ag2Se QDs with bright NIR-II emission in a facile manner is much less developed than other semiconductor nanocrystals, such as CdSe,20−24 CdTe,25−28 Ag2S,12−14,29,30 etc. There are few reports about the successful synthesis and biological application of Ag2Se NIR-II QDs, due to the low QY, poor stability, and so on.31,32 For example, Heiss et al. synthesized Ag2Se QDs with two fluorescent wavelengths of 1030 and 1250 nm in an organic phase by tuning the sizes of Ag2Se with a relatively low QY of 1.7%.31 Pang and co-workers prepared the water-soluble Ag2Se QDs with an ultrasmall size in aqueous media; nevertheless, the obtained QDs had fluorescence located in the NIR-I range (700−850 nm) with a QY of ∼3%.32 To the best of our knowledge, there is no report on the synthesis of Ag2Se QDs with bright emission centered at 1300 nm and their application in in vivo imaging. It

INTRODUCTION Fluorescence in the second near-infrared (NIR-II) region with wavelengths from 1000 to 1400 nm has exhibited great promising in in vivo epifluorescence imaging, in comparison to that in the first NIR window (NIR-I) with wavelengths from 650 to 950 nm, due to the much reduced photon absorption and scattering by tissues,1−14 which offers deeper tissue penetration and higher spatial and temporal resolution. Up to date, the most promising NIR-II emissive nanoprobes are limited to single-walled carbon nanotubes (SWNTs)5−11 and Ag 2 S quantum dots (QDs). 12−14 The Dai group has demonstrated that SWNTs are a sensitive NIR-II fluorescent probe for in vivo imaging. For example, they recently reported that the SWNTs can be effectively used in in vivo real-time epifluorescence imaging of mouse hind limb vasculatures with high spatial and temporal resolution.11 More recently, our group developed a new type of NIR-II nanoprobe, Ag2S QDs, with emission centered at 1200 nm, which have been successfully employed in cellular imaging13 and xenograft tumors imaging14 with a high signal-to-noise ratio. Studies have indicated that the NIR-II emission centered at 1300 nm is the optimal wavelength for in vivo imaging with the deepest tissue penetration due to the lowest photon absorption and tissue scattering.1,9 Although CdHgTe,15 PbS,16 and PbSe17 QDs have been previously reported, however, the intrinsic toxicity of the heavy metal elements deters their potential practice in in vivo imaging. SWNTs have demonstrated their success in in vivo imaging, whereas the low © XXXX American Chemical Society

Received: March 12, 2013 Revised: May 19, 2013

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Figure 1. Characterizations of Ag2Se QDs. (a) TEM and HR-TEM (inset) images of monodispersed Ag2Se QDs. (b) XRD pattern of Ag2Se QDs. (c) XPS spectrum of Ag2Se QDs showing the composition of both Ag and Se. High-resolution XPS spectra of Ag3d (d) and Se3d (e). in cyclohexane. The Ag2Se QDs with various sizes and emissions can be synthesized by tuning the molar ratio of initial Ag and Se precursor. Synthesis of C18-PMH-PEG. C18-PMH-PEG was synthesized following a literature procedure.33 In a typical synthesis, 143 mg of 5k PEG was reacted with 10 mg of C18-PMH (PEG/C18-PMH monomer molar ratio = 1:1) in 5 mL of dichloromethane for 24 h in the presence of 11 mg of N-(3-dimethylaminopropyl)-N-ethylcarbodiimide hydrochloride (EDC, Sigma-Aldrich) and 6 μL of triethylamine. After 24 h of stirring, the dichloromethane was blown dry by N2. The final solid was dissolved in water, forming a transparent solution, which was dialyzed against distilled water for 48 h in a dialysis bag with a molecular weight cutoff (MWCO) of 14 kDa to remove unreacted PEG. After lyophilization, the final product in a white solid with a yield of 80% was stored at −20 °C for future use. Synthesis of Water-Soluble C18-PMH-PEG-Ag2Se QDs. A 50 mg portion of C18-PMH-PEG was added in 5 mL of chloroform containing as-prepared DT-Ag2Se QDs, and then the solution was stirred for 12 h at room temperature. After 12 h of stirring, the chloroform was blown dry by N2, and then residue was dissolved in water. All the DT-Ag2Se QDs were transferred into the water with a yield of ∼100% after capping the C18-PMH-PEG. Characterization. The transmission electron microscope (TEM) and high-resolution TEM (HR-TEM) images were examined by a Tecnai G2 F20 STwin TEM (FEI, U.S.A.) operated at 200 kV. X-ray photoelectron spectroscopy (XPS) was performed on a Perkin-Elmer PHI 5000C ESCA X-ray photoelectron spectrometer using Al Kα radiation (1486.6 eV) as the exciting source. The bonding energies of Ag2Se QDs obtained from the XPS analysis were corrected for specimen charging by referencing the C 1s to 284.80 eV. Powder X-ray diffraction (XRD) patterns of the as-obtained products were recorded on a Bruker D8 Advance powder X-ray diffractometer, using Cu Kα radiation (λ = 1.5406 Å). A UV/vis/NIR absorption spectrum was performed on a Lambda-750 spectrometer (Perkin-Elmer) at room temperature. The NIR-II fluorescence spectra of Ag2Se QDs were collected on an Applied NanoFluorescence spectrometer at room temperature with an excitation laser source of 785 nm. Fourier transform infrared spectroscopy (FTIR) measurements were taken on

still remains a great challenge for controlled synthesis of Ag2Se QDs with desirable fluorescence in a simple and convenient method, let alone the in vivo imaging study. In this work, we present a facile solvothermal synthesis of Ag2Se QDs with emission centered at 1300 nm. After the surface modification, the Ag2Se QDs possess bright NIR-II emission, good water solubility, high colloidal stability and photostability, as well as decent biocompatibility, which are further successfully performed in in vivo deep imaging of organs and blood vessels with higher spatial resolution in comparison with the classic NIR-I fluorescent dye of indocyanine green (ICG).

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EXPERIMENTAL SECTION

Chemical Materials. Selenium powder (99.95%), sodium borohydride (>96%), and silver nitrate were purchased from Sinopharm Chemical Reagent (Shanghai, China). The 1-Dodecanethiol (DT, 98%), oleylamine (OAM, 80−90%), and poly(maleic anhydride-alt-1-octadecene) (C18-PMH) were purchased from SigmaAldrich. 5k PEG polymer was purchased from Beijing KaiZheng Bio Inc. All other reagents were of analytical grade and used without further purification. Preparation of Silver Stock Solutions. The silver precursor solution was prepared by dissolving 0.04 mmol of silver nitrate in the mixture of 3 mL of OAM and 7 mL of toluene, and then was heated to form an optically clear solution. Synthesis of Oil-Soluble DT-Ag2Se QDs. In a typical reaction, 7 mL of toluene containing 0.04 mmol of Ag-OAM was loaded into a 50 mL autoclave; then, 3 mL of DT was added, followed by addition of 7 mL of NaHSe solution (0.01 mmol). The mixture solution was kept stirring for 5 min and then heated to 180 °C and aged for 1h. Once the reaction was finished and cooled down to room temperature, the products with a size of 3.4 nm and an emission centered at 1300 nm were purified with excess ethanol by centrifugation (12 000 rpm, 5 min) with a yield of 70%. The final products were dispersed and stored B

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a Nicolet 6700. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) (Optima 7300DV, Perkin-Elmer) was used to measure the concentrations of Ag2Se QDs. Dynamic laser scattering measurement of Ag2Se QDs was performed on a zetasizer Nano ZS (Malvern). Cell Culture. The mouse fibroblast L929 cell line was purchased from the Cell Bank of Type Culture Collection of Chinese Academy of Sciences and was grown in RPMI 1640 medium (Hyclone). Human mesenchymal stem cells (hMSCs) were a gift from the Orthopedic Institute of Soochow University and were cultured in low-glucose Dulbecco′s modified Eagle medium (L-DMEM, Hyclone). All culture media were supplemented with 10% FBS, 1% penicillin/streptomycin solution (Gibco). Both of the L929 cells and hMSCs were maintained in 5% direct heat Autoflow CO2 air-jacketed incubators at 37 °C and were culture expanded with a medium change every 3 days. MTT Assay. In vitro cytotoxicity studies of C18-PMH-PEG-Ag2Se QDs on L929 cells and hMSCs were performed by using a MTT cytotoxicity assay. Cells were cultured overnight in 96-well plates (2 × 103 cells per well) to allow cell attachment, and then incubated with 200 μL of fresh cell media containing 0, 10, 20, 50, and 100 μg/mL of C18-PMH-PEG-Ag2Se QDs for 24 h, respectively. The cell viabilities were measured by the standard MTT assay. Apoptosis and Necrosis Assay. The apoptosis and necrosis of L929 cells and hMSCs induced by C18-PMH-PEG-Ag2Se QDs were measured by using an apoptosis and necrosis assay kit. In brief, both of the L929 cells and hMSCs were plated into a six-well plate at a density of 2 × 105 cells per well overnight and then treated with 0, 50, and 100 μg/mL of C18-PMH-PEG-Ag2Se QDs for 24 h. The cells were harvested, washed twice with PBS, resuspended in 500 μL of PBS, and incubated with antiannexin V-FITC and PI. Single-cell suspensions were analyzed by FACS within 1 h. In Vivo Imaging. All nude mice were obtained from Suzhou Belda Bio-Pharmaceutical Co. and raised in an animal facility under filtered air (22 ± 2 °C) and fed with a standard pellet diet and pure water. The study was performed with the Guidelines for the Care and Use of Research Animals. A 200 μL portion of C18-PMH-PEG-Ag2Se QDs at a 0.6 mg/mL concentration (a dose of 6 mg/kg per mouse) was injected intravenously. During injection and imaging, the mice were anesthesized using a 2 L/min oxygen flow with 2% Isoflurane. NIR-II fluorescence images were collected using a two-dimensional InGaAs array (Photonic Science) for collecting photons in NIR-II. The excitation light was provided by an 808 nm diode laser and filtered by an 850 nm short-pass filter and a 1000 nm short-pass filter. The excitation power density at the imaging plane was 15 mW/cm2, much lower than the safe exposure limit of 329 mW/cm2 at 808 nm determined by the International Commission on Nonionizing Radiation Protection. The emitted light from the animal was filtered through a 1100 nm long-pass filter coupled with the InGaAs camera for NIR-II imaging. NIR-I fluorescence images of the mice injected with 200 μL of ICG at 3.7 mg/mL were collected by a modified Maestro in vivo imaging system (CRi Inc.) using a 980 nm optical fiber-coupled laser (Hi-Tech optoelectronics Co., Ltd.) as the excitation source. The laser power density was about 0.4 W/cm2 (beam size ∼ 20 cm2) during imaging with an exposure time of 20 s. An 850 nm short-pass emission filter was applied to prevent the interference of excitation light to the CCD camera.

powder X-ray diffraction (XRD) pattern verifies the product as β-Ag2Se (orthorhombic structure, JCPDS no. 24-1041), which is consistent with the HR-TEM observation. X-ray photoelectron spectroscopy (XPS) is further employed to determine the composition of the products. The survey-scan and narrowscan spectra of the Ag 3d and Se 3d are shown in Figure 1c−e. The binding energy of Ag 3d5/2 and 3d3/2 are depicted at 367.56 and 373.59 eV, respectively, and the peak at 53.53 eV is identified as Se 3d, indicative of the components of Ag and Se. Fourier transform infrared spectroscopy (FTIR) analysis confirms the anchoring of DT on the surface of Ag2Se QDs (Figure S3, Supporting Information). The as-prepared hydrophobic DT-Ag2Se QDs are well-dispersed in cyclohexane and maintain high photoluminescence (PL) for several months (Figure 2a,b). Interestingly, it is observed that the absorption spectrum of Ag2Se QDs is featureless with no first discrete absorption feature evident (Figure 2a), which is different from the II−VI QDs that usually exhibit an evident discrete absorption feature. To the best of our knowledge, the absorption spectrum of I−VI Ag2Se QDs typically does not

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RESULTS AND DISCUSSION Figure 1a shows a typical transmission electron microscopy (TEM) image of the as-synthesized Ag2Se QDs with an average size of 3.4 nm in diameter. (See Figure S1, Supporting Information, for size distribution analysis. Different-sized Ag2Se QDs can be obtained by modifing the reaction conditions; see Figure S2, Supporting Information.) The high-resolution TEM (HR-TEM) image (inset in Figure 1a) depicts the singlecrystalline nature of the Ag2Se QDs with the lattice spacing of ca. 0.24 nm, which agrees well with the distances between adjacent facets (013) of orthorhombic Ag2Se (β-Ag2Se).32 The

Figure 2. Optical properties of Ag2Se QDs before (a, b) and after (d, e) C18-PMH-PEG coating. (a) A digital camera white light optical image and a PL image of the DT-Ag2Se QDs suspended in cyclohexane at a concentration of 0.6 mg/mL. (b) Absorbance and PL spectra of the DT-Ag2Se QDs in cyclohexane. (c) The scheme for C18-PMH-PEG polymer coating on the surface of DT-Ag2Se QDs. (d) A digital camera white light optical image and a PL image of the C18-PMH-PEG-Ag2Se QDs suspended in water at a concentration of 0.6 mg/mL. (e) Absorbance and PL spectra of the C18-PMH-PEGAg2Se QDs in water. C

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Figure 3. Stability assay of C18-PMH-PEG-Ag2Se QDs. (a) The Ag2Se QDs at a concentration of 0.03 mg/mL are stable in 1x PBS, fetal bovine serum (FBS), and cell culture media (DMEM) over 7 days. (b) No statistical PL intensity change of Ag2Se QDs after 7 days of storage in different media at room temperature. (c) The photostabilities of Ag2Se QDs and ICG after continuous irradiation with an 808 nm laser diode at 123.8 mW/ cm2 power densitiy for 2 h.

show feature absorption peaks.18,31,32 We attribute this featureless absorption spectrum to two possible reasons: the special electronic property and the size/shape inhomogeneity of the Ag2Se QDs.34 A deep understanding of this observation will be needed in the future. For the bioimaging purpose, a PEG drafted amphiphilic polymer, amphiphilic poly(maleic anhydride-alt-1-octadecene)methoxy poly(ethylene glycol) [C18-PMH-PEG],33,35 is employed to render the Ag2Se QDs hydrophilic through the strong interaction between the hydrophobic chains of C18PMH-PEG and DT molecules and leaving the hydrophilic chain on the outer surface of Ag2Se QDs (Figure 2c). As a result, the C18-PMH-PEG-coated QDs are well protected to retain their optical properties, for example, absorption and emission spectra (Figure 2d,e). The successful coating of C18PMH-PEG on Ag2Se QDs’ surface is evidenced by the dynamic laser scattering measurement (DLS, Figure S4, Supporting Information) and FTIR analysis (Figure S5, Supporting Information). The hydrodynamic radius of C18-PMH-PEGAg2Se QDs obtained by DLS is about 21.5 nm, which is larger than that of the DT-Ag2Se QDs (2.3 nm) dispersed in cyclohexane, indicating the polymer coating on the surface of Ag2Se QDs. A new band at 1110 cm−1 in the FTIR spectrum of C18-PMH-PEG-Ag2Se QDs is related to the stretching vibration of the ether bond of PEG chains, which further confirms the successful surface modification. The TEM and HR-TEM images (Figure S6, Supporting Information) indicate that C18-PMH-PEG-Ag2Se QDs maintain their single-crystalline nature and are well-dispersed in aqueous solution without any agglomeration. The QY of the C18-PMH-PEG-Ag2Se QDs

is determined to be 29.4% with the previous PEG-Ag2S QDs as a standard,14 which is much higher than that of the reported Ag2Se QDs,31,32 SWNTs,5−11 and PEG-Ag2S QDs,14 indicative of their great potential in in vivo imaging. Besides high fluorescence QY, high colloidal stability and photostability are prerequisites of Ag2Se QDs for in vivo imaging. Thus, the stability of the obtained C18-PMH-PEGAg2Se QDs is carefully investigated. Three common media in biological study, phosphate buffered saline (PBS), fetal bovine serum (FBS), and Dulbecco′s modified eagle media (DMEM), are used as the dispersion media to testify the colloidal stability of C18-PMH-PEG-Ag2Se QDs, respectively. As shown in Figure 3a, the C18-PMH-PEG-Ag2Se QDs in different media maintain high colloidal stability after 7 days of storage at ambient temperature. The photostability of the QDs is analyzed under exposure to daylight and a continuous irradiation with a 808 nm laser, respectively. Figure 3b presents that the C18PMH-PEG-Ag2Se QDs keep their PL intensity over 7 days of storage at ambient temperature. Figure 3c further illustrates the excellent photostabiliy of C18-PMH-PEG-Ag2Se QDs, which preserve more than 90% of the initial PL intensity after 2 h of irradiation of a 808 nm laser at a power density of 123.8 mW/ cm2. In contrast, ICG, a classic NIR-I fluorescent dye, loses 70% of its PL intensity over the 2 h irradiation. On the basis of the above observations, we conclude that the C18-PMH-PEGAg2Se QDs possess bright PL, high colloidal stability and photostability, which are very promising as a new type of NIRII probe for in vivo imaging. A preliminary cytotoxicity study, a methyl thiazolyl tetrazolium (MTT) and apoptosis/necrosis assay, has been D

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Figure 4. Cytotoxicity assay (a, b) and in vivo imaging (c−e) of C18-PMH-PEG-Ag2Se QDs. MTT assay (a) and apoptosis/necrosis results (b) of L929 cells and hMSCs after 24 h of treatment with different concentrations of Ag2Se QDs. (c) In vivo imaging of live mice in supine position by tail injection of 200 μL of ICG (3.7 mg/mL). (d) In vivo imaging of live mice in supine position by tail injection of 200 μL of C18-PMH-PEG-Ag2Se QDs (0.6 mg/mL). (e) A zoom-in image of the selected zone in (d). (f) A cross-sectional intensity profile measured along the red-dashed line in (e) with its peak fitted to Gaussian functions. The fluorescence signal from the Ag2Se QDs is easily distinguishable from the endogenous autofluorescence without any image processing.

100 μg/mL, the populations of L929 cells and hMSCs that undergo apoptosis and necrosis remain constant without a significant difference in comparison with those of the control experiments without addition of Ag2Se QDs. These data reveal the remarkably negligible cytotoxicity of C18-PMH-PEG-Ag2Se QDs. The above results clearly present that the C18-PMH-PEGAg2Se QDs possess bright NIR-II emission centered at 1300 nm, decent biocompatibility, and a suitable hydrodynamic radius of 21.5 nm, which prompt us to evaluate its feasibility as an NIR-II probe for in vivo imaging. To compare the performance of Ag2Se QDs in in vivo imaging, ICG is employed as a reference. As shown in Figure 4c, the mouse tail injected with ICG (a dose of 37 mg/kg per mouse) as an imaging agent exhibits a very weak signal and a faint vascular structure due to the substantial scattering and absorbance of the photons in the NIR-I region by the tissues. In contrast, after administration of C18-PMH-PEG-Ag2Se QDs by tail intra-

executed to evaluate the biocompatibility of C18-PMH-PEGAg2Se QD prior to its practice in in vivo imaging. Two cell lines, mouse fibroblast L929 cells and human mesenchymal stem cells (hMSCs), are chosen for the cytotoxicity study. The purpose of using the normal cell and stem cell, instead of the widely used cancer cell, is to evaluate the cytotoxicity of C18PMH-PEG-Ag2Se QDs more objectively, especially for the hMSC, which is very sensitive to its culture environment. These two types of cells are exposed to C18-PMH-PEG-Ag2Se QDs at different concentrations of 0−100 μg/mL for 24 h, respectively. The MTT results illustrate that both the L929 cells and hMSCs retain their viability under the culture with different concentrations of C18-PMH-PEG-Ag2Se QDs without statistic variation (Figure 4a). In terms of the induced apoptosis and necrosis of L929 cells and hMSCs by C18-PMH-PEG-Ag2Se QDs, the flow cytometry analysis is performed after 24 h of treatment. As shown in Figure 4b and Figure S7 (Supporting Information), with the concentrations of Ag2Se QDs at 50 and E

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Science and Technology (2011CB965004), the National Science Foundation of China (21073225), the National Science Foundation of Jiangsu Province, China (BK2012007), and the CAS/SAFEA International Partnership Program for Creative Research Teams.

venous injection (a dose of 6 mg/kg per mouse, which is onesixth of the dose of ICG, and half the dose of the Ag2S in vivo imaging14), the liver, spleen, and vascular network of the whole mouse are clearly visualized within a few minutes post injection under an 808 nm illumination (Figure 4d; see the Supporting Information for details). It shows that C18-PMH-PEG-Ag2Se QDs are preferentially associated with the lumen periphery of the vasculatures and are distributed throughout the large and small vessels. Moreover, the minimal autofluorescence and the low absorption and scattering of NIR-II emission at 1300 nm afford a maximal penetration depth for deep tissue imaging with high feature fidelity, which help the imaging of the organs buried deep in the body, such as the liver and spleen (Figure 4d and Figure S8, Supporting Information). High-ordered branches of blood vessels can also be well-defined with high spatial resolution, in which a small vessel with a diameter of 123 μm can be unambiguously observed without the help of a zoom-in lens. Inspired by these exciting observations, we expect that the C18-PMH-PEG-Ag2Se QDs are very promising in realtime visualization of in vivo vascular structures and vascular related disease imaging, especially for evaluation of ischemic necrosis of tissues and organs after transplantation and screen antiangiogenic drugs.

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CONCLUSIONS In summary, a new NIR-II emissive nanoprobe, Ag2Se QDs, has been successfully synthesized through a facile solvothermal method. By optimizing the reaction conditions, the optimal wavelength for in vivo imaging is achieved with the emission of Ag2Se QDs centered at 1300 nm. Functionalizing the surface with an amphiphilic C18-PMH-PEG polymer, the obtained C18-PMH-PEG-Ag2Se QDs exhibit bright NIR-II fluorescence, good water solubility, high colloidal stability and photostability, as well as decent biocompatibility. The in vivo experiment by administration of C18-PMH-PEG-Ag2Se QDs by tail intravenous injection achieves high spatial resolution imaging of organs buried deep in the body and vascular structures down to ∼100 μm. This new NIR-II fluorescent nanoprobe with small sizes, ideal optical properties, and decent biocompatibility opens up exciting opportunities for future biomedical applications.

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ASSOCIATED CONTENT

S Supporting Information *

Additional details of the DT-Ag2Se QDs, TEM images, FTIR spectra, dynamic light scattering measurement of DT-Ag2Se QDs and C18-PMH-PEG-Ag2Se QDs, the flow cytometry analysis of C18-PMH-PEG-Ag2Se QDs induced apoptosis and necrosis on the L929 cells and hMSCs, and more in vivo imaging of Ag2Se QDs are included (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Q.W. acknowledges funding by the Chinese Academy of Science “Bairen Ji Hua Program” and “Strategic Priority Research Program” (XDA01030200), the Chinese Ministry of F

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dx.doi.org/10.1021/cm400812v | Chem. Mater. XXXX, XXX, XXX−XXX