Article pubs.acs.org/Langmuir
Plasmon Excitation of Supported Gold Nanoparticles Can Control Molecular Release from Supramolecular Systems Daniela T. Marquez, Adela I. Carrillo, and Juan C. Scaiano* Department of Chemistry and Centre for Catalysis Research and Innovation, University of Ottawa, 10 Marie Curie, Ottawa, Ontario, K1N 6N5, Canada ABSTRACT: Hybrid mesoporous silica materials containing gold nanoparticles (AuNPs) have been investigated as potential molecular delivery systems. The photophysical properties of AuNPs, particularly their plasmon band transitions, have been used to control the rate of the release of naproxen from the pores of mesoporous silica matrices. Two different approaches were employed to incorporate AuNPs into the silica network: that is, grafting (using 3aminopropyltriethoxisilane) and direct absorption. In this research, the antiinflamatory drug naproxen serves as a test molecule, showing how localized plasmon heating could be used to modify diffusion kinetics within mesoporous materials. Beyond naproxen release, the methodology developed could be employed to release other drugs, sensors, or active molecules, not just in medicine, but in many other fields where nanotechnology is leading to many innovative applications. The hybrid materials developed show a new simple system to efficiently control the release of active cargo from mesoporous silica matrices.
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INTRODUCTION Recent developments in the design of surface-functionalized mesoporous silica materials have revealed the potential of these solids as molecular delivery systems, such as in drug delivery,1−5 in what is sometimes referred as “cargo-controlled release”.6 These materials can be light-activated, and in this case can replace conventional photocages where release is based on chemical transformations, usually bond cleavage.7,8 The distinctive physical, chemical, and mechanical properties of mesoporous silica solids, that is, thermal stability, large surface area, and narrow pore size distribution, facilitate the use of these materials in different fields of chemistry such as adsorption, separation, drug release, or catalysis.9−13 Moreover, mesoporous silica materials show desirable features, such as biocompatibility2 and tunable pore sizes and surfaces, thus making them ideal for controlled delivery of functional molecules.4,14−17 MCM-41 is one of the most extensively studied mesoporous materials due to its unique hexagonal, uniform, and highly ordered pore structure.18 The facile synthesis of MCM-41 makes it a reproducible and readily accessible material. Besides the highly ordered pore distribution, this material has free hydroxyl groups, enabling ready functionalization with dyes, drugs, and other molecules.19 Since the first proposal for using MCM-41 as implantable systems made by Vallet-Regi et al. in 2001,3 researchers have focused on the design of more efficient mesoporous materials to be used in drug delivery processes.3,17,20 Because the mesopore diameter determines available space for guest drugs,19 different methods have been developed to control the pore size of these materials. In 1999, Di Renzo et al.21 © 2013 American Chemical Society
proposed the addition of hydrocarbons during the synthesis of the mesoporous silica. These organic molecules were solubilized in the hydrophobic region of the surfactant micelles, used as structure directing agent, thus, increasing the micellar size. Nevertheless, this particular synthesis method also affects the diffusion of the drug in the delivery host structure.22 Several gated systems have been proposed in which external stimulus (e.g., pH or light) can be used to open the gates and induce cargo release.6,17,19 The study of drug delivery processes has resulted into the development of complex supramolecular systems aimed at cancer therapies. Plasmonic nanomaterials, especially those that can convert near-infrared (NIR) light into heat, have been developed as photothermal agents for localized hyperthermia cancer therapy.23 Further, researchers are focused on the combination of hyperthermia and chemotherapeutic agents to optimize cancer therapy and achieve enhanced antitumor efficacy.20,24 In many cases, delivery systems require elaborate synthetic procedures. We have designed simple hybrid mesoporous materials based on the use of the surface plasmon band transition of gold nanoparticles (AuNPs) to control the rate of the release of naproxen, used as a test material. In our systems, light-induced cargo release is the result of changes in the dynamics of diffusion resulting from plasmonic heating within mesoporous channels shared by AuNPs and naproxen. These hybrid materials have been synthesized by using two different Received: May 23, 2013 Revised: July 17, 2013 Published: August 8, 2013 10521
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approaches. The first one consists of the anchoring of AuNPs to the mesoporous material by the use of 3-aminopropyltriethoxisilane (APTES) as a linker. Grafted materials were synthesized by functionalizing MCM-41 with APTES followed by the photochemical incorporation of AuNPs into the solids by using a one pot approach based on that developed by McGilvray and co-workers25,26 (Scheme 1). Grafting processes
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RESULTS AND DISCUSSION Materials Characterization. Figure 1 shows small-angle Xray diffraction patterns for MCM-41 before and after
Scheme 1. Photochemical Route for the Synthesis of AuNPs25a
Figure 1. Low angle XRD patterns of pure silica MCM-41 (a) grafted MCM-41 type silica with 3-aminopropyltriethoxisilane (MAp) (b) and MCM-APTES silica after photochemical incorporation of AuNPs (MApAu-3) (c).
a
Reprinted with permission from ref 25. Copyright 2006 American Chemical Society.
incorporation of AuNPs. The presence of the three distinctive (100), (110) and (200) X-ray diffraction peaks, typical for MCM-41 samples, confirmed that the hybrid materials maintain the 2D hexagonal mesopore arrangement even after subsequent incorporation of APTES and AuNPs into their structures. Hexagonally ordered mesoporosity is also confirmed by transmission electron microscopy (TEM) images shown in Figure 2. X-ray diffraction (XRD) intensity decreased upon incorporation of APTES and AuNPs into the materials structure. Moreover, the position of the main XRD reflection shifted to higher angles. This shift corresponds to an interplanar spacing reduction. The decrease in the interplanar spacing due to the loading of organic molecules into the mesoporous structure of MCM-41 materials caused by the loading of pendant groups has already been described in the literature.30 TEM analyses of the samples confirm that the structure of mesoporous materials is preserved upon functionalization/ incorporation of AuNPs. As expected, hybrid materials without APTES in their structure led to larger nanoparticles even though when overall low amounts of gold were incorporated (see Figure 2b). Moreover, smaller and highly dispersed AuNPs were obtained in the case of materials synthesized by using the grafting method and incorporating low amounts of metal (see Figure 2c). Hybrid solids with higher gold loading, that is, 5 and 10 wt %, lead to agglomeration making the higher loading of gold impractical (data not shown). Nitrogen adsorption/desorption isotherms of the hybrid silica materials and their corresponding pore size distributions are shown in Figure 3a and b, respectively. For comparison purposes, the isotherms of the pure support as well as the MAp materials are included. Isotherms of the solids after the loading of naproxen are also shown. All hybrid materials show type IV isotherms with a sharp nitrogen uptake at 0.35 − 0.45 P/P0, due to the capillary condensation of nitrogen inside the mesopores (Figure 3a). Based on the isotherms, textural parameters were calculated and listed in Table 1, being the average pore diameter around 2.7 nm in all cases, typical value for C16TAB templated materials.31 Incorporation of APTES and AuNPs
are widely used to functionalize silica solids because they preserve the porosity order in the material structure which leads to an enhancement of diffusion properties.27 For comparison, AuNPs were also immobilized into the same silica materials by simple absorption-deposition during their photochemical synthesis. This second approach differs from the first one due to the absence of a linker that anchors the AuNPs to the walls of MCM-41. A key advantage of the systems reported here is the possibility to exploit the surface plasmon resonance band of AuNPs to control the rate of the release of a drug, while avoiding the use of UV light or the need for elaborate synthetic procedures. The applicability of these hybrid materials in drug or molecular delivery systems was tested using naproxen as guest molecule. Naproxen is a nonsteroidal anti-inflammatory drug used to treat symptoms related to different conditions such as migraine, kidney stones and gouty arthritis;28 drug encapsulation can also minimize some side effects caused by naproxen.29 The nomenclature used to describe the samples is shown in Scheme 2. Scheme 2. Key to Sample Nomenclature
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Figure 2. TEM micrographs of (a) pure MCM-41, hybrid silica synthesized by using the absorption route (b) MAu-3 and silicas prepared following the grafting approach (c) MApAu-1, (d) MApAu-3.
into the structures leads to a reduction in surface area and pore volume suggesting a partial blocking of the porosity in the hybrid materials.32 The decrease in the textural parameters of the solids is more evident in the case of the grafted materials. Thus, the organic molecules responsible for this decrease are mainly anchored to the pore surface, lining the mesoporous channels of the solids. As expected, in the case of MAu-3 and MAu-3-Nap the surface area and pore volume turned out to be lower to that of the parent MCM-41 due to the inclusion of AuNPs and naproxen. Nevertheless, the change in these parameters was not as drastic as the one observed for the rest of the materials because of the lower naproxen loading compensating the decrease in textural parameters. Isotherms corresponding to grafted materials show a major decrease in both surface area and pore volume after naproxen loading than materials synthesized by absorption method indicating a higher loading for the grafted materials. This fact was confirmed by thermogravimetric analysis (Figure 4). Besides textural parameters, Table 1 shows the metal content of the different hybrid materials as well as the number of equivalents of gold precursor used with the aim to facilitate a comparison between each synthetic approach. The grafting method led to two materials with different metal loading (entries 4 and 6), whereas the absorption-deposition approach led to a hybrid mesoporous silica containing approximately 1% gold (MAu-3). Samples MApAu-1 and MAu-3 (entries 4 and 8) synthesized by grafting and absorption−deposition, respectively, have similar gold loading, but to obtain the latter three times more gold precursor was needed during the synthesis. These results highlight the role of APTES in overcoming the agglomeration tendency of AuNPs during the synthesis. By minimizing agglomeration, using APTES, it is possible to obtain materials with a higher metal loading. Because MAu-3 and MApAu-1 contain similar amounts of AuNPs, most of the
Figure 3. Representative nitrogen adsorption/desorption isotherms at 77 K (a, top) and their corresponding pore size distribution (b, bottom) of selected hybrid materials.
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Table 1. Metal Incorporation and Textural Parameters of Representative Hybrid Mesoporous Materials entry
sample
dp (nm)a
SBET (m2 g‑1)b
1 2 3 4 5 6 7 8 9
MCM-41 MAp MAp-Nap MApAu-1 MApAu-1-Nap MApAu-3 MAu-1 MAu-3 MAu-3-Nap
2.7 2.7 2.4 2.4 2.4 2.4
945 700 620 675 550 615
2.7 2.7
900 900
Vp (cm3 g‑1)c
metal loading (wt %)d
0.95 0.70 0.5 0.65 0.81 0.5 (0.81) 0.6 2.5 unsuccessful synthesis 0.90 0.77 0.90 (0.77)
naproxen loading (wt %)e
HAuCl4 equivf
30 10 10
7.5
1 1 3 1 3 3
a
Average mesopore diameters were estimated from the adsorption branch of the nitrogen isotherm using the BJH method. The differences observed are within the errors of these measurements. bThe BET surface area was estimated by multipoint BET method using the adsorption data in the relative pressure (P/P0) range of 0.05−0.30. cMesopore volume from the isotherms at relative pressure of 0.8. dGold abundance determined by ICP analyses. Values in parentheses are based on the line above, as it refers to the same host material. eNaproxen loading determined by TGA measurements. fEquivalents of HAuCl4 used in the synthesis of the materials (1 equiv = 1% weight).
Figure 4. Thermogravimetric analysis for several samples. Notice the marked weight loss at around 400 °C for the naproxen-containing sample.
results shown in this study are based on the comparison between these two hybrid materials. The uptake of naproxen by the mesoporous materials was performed by 12 h exposure of the mesoporous host to a hexane solution of the drug. The sample was “cleaned” by extensive washing, and finally the amount of naproxen determined by TGA studies of dry samples. Table 1 includes the naproxen loading for selected materials. The amount of naproxen loading decreases upon APTES functionalization; this is likely due to the reduced pore volume available. Moreover, a slight decrease in the naproxen loading is shown after incorporation of AuNPs in the MAp material. This may reflect blockage of some pores by the AuNPs. The presence of AuNPs in all the hybrid mesoporous materials was also confirmed by diffuse reflectance (DR) analyses of the solids (see Figure 5) showing an intense surface plasmon band (SPB) at 540 nm characteristic to AuNPs.25,33,34 Conveniently, plasmon excitation of these materials is possible under 532 nm LED irradiation. Experiments utilizing colloidal AuNPs suggest that temperatures around 500 °C for submicrosecond times are obtained.35 The temperatures reached upon excitation of the surface plasmon band of AuNPs incorporated in mesoporous materials have never been reported. However, the absence of naproxen decomposition in our experiments upon continuous wave irradiation at 532 nm suggests that smaller temperature changes occur in this case. Drug Release Studies. The percentage of naproxen released as a function of time for the new materials immersed into simulated body fluid (SBF) solution was monitored by
Figure 5. Diffuse reflectance spectra of hybrid materials as compared with the spectrum for pure MCM-41 mesoporous silica.
UV−visible spectroscopy. In order to study the effect of the presence of AuNPs on the release process, experiments were conducted in the dark and under 532 nm AuNP plasmon excitation. Figure 6 shows control experiments for samples without AuNP, where light has little or no effect on the release, as expected. Note also the relatively low level of naproxen release. Figure 7 shows the release profiles of the hybrid materials synthesized by using the grafting method and loaded with different amounts of AuNPs. The release is faster during the first hours of the assays, but decreases with time reaching a plateau. These results can be interpreted as two different molecular populations, one corresponding to naproxen adsorbed on the outside surface of the material and a second population located in the pores. IR spectroscopy was performed with the aim to identify and describe possible interactions between the second naproxen population and AuNPs. Unfortunately, the differences were too small to reach any significant conclusion. For system MApAu-3-Nap under dark conditions, the plateau is reached after 20 h, whereas for MApAu-1-Nap it is reached at half the time (10 h). This result could be related to the larger size of AuNPs obtained using 3% Au during synthesis in terms of a partial pore blockage. 10524
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Figure 6. Percent naproxen release curves in time for the control samples (a) M-Nap and (b) MAp-Nap. Black curves represent a release in dark conditions and red curves represent the release done under continuous 532 nm LED irradiation.
The speed of the release as well as the final percentage of naproxen delivered increases considerably under 532 nm irradiation indicating the presence of heating effect due to the excitation of the AuNP plasmon band. When the surface plasmon is excited, most of the energy absorbed is dissipated as heat that diffuses away from the surface of the nanoparticles to the surrounding medium.36 The local temperature increase and decrease of the local viscosity facilitates naproxen diffusion. With the aim to further establish the possibility of controlling the molecular release process in the hybrid systems prepared, one additional assay was carried out. Drug was delivered into SBF under dark conditions for a period of several hours. Then, light was turned on and the system was maintained under these conditions several hours (Figure 8). As anticipated, an increase in the rate of drug release was achieved when the sample was irradiated at 532 nm. Beyond demonstrating light-induced release, these data provide a method to “clean” the material in case that one wants exclusively or predominantly photochemical cargo release. For many health applications, the possibility of slow thermal and light accelerated release may prove useful. With the aim to confirm that only naproxen is being released and to rule out product decomposition or APTES release, NMR spectra were taken after the release under dark and 532 nm irradiation conditions, respectively. A comparison of these with a pure naproxen NMR confirmed that the only species released is naproxen and that no decomposition products are formed.
Figure 7. Percent naproxen release curves for MApAu-1-Nap (a), MApAu-3-Nap (b) and (c) for MAu-3-Nap. Black curves represent a release in dark conditions and red curves represent the release performed under continuous 532 nm LED irradiation. The fit curves are simply a visual aid. Note that even in the dark there is about 20% very fast release.
On the other hand, when redispersing the material used in a previous release in fresh SBF and continue the release for 48 more hours an additional 9% of naproxen was obtained suggesting the presence of equilibrium when the plateau is reached.
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CONCLUSIONS A number of reports over the past few years have established that plasmon excitation can lead to extreme temperatures at the surface of metal nanoparticles. These temperatures are sufficiently high to result in the cleavage of chemical bonds.35,37 Ultimately, heat is released to the medium, which in the case of mesoporous materials initially influences the guests and solvents contained within the channels in the supramolecular structure, in this case MCM-41. In this work, a new simple system based on the use of gold plasmon excitation for controlling drug delivery processes has been developed. Two types of hybrid mesoporous silicas with AuNPs in their structures were synthesized. Materials prepared by using an absorption−deposition approach led to agglomer10525
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mL Teflon lined stainless steel autoclave and heated at 80 °C for 24 h. After cooling to room temperature, the solid product was washed with water and ethanol, filtered out, and air-dried overnight at 60 °C. Finally, the surfactant was removed by calcination at 550 °C for 8 h (2 °C min−1) under a static air atmosphere. The grafting of the 3-aminopropyltriethoxisilane onto MCM-41 was carried out as follows: 1 g of MCM-41 was kept at 200 °C for 2 h in order to regenerate an adequate amount of hydroxyl groups on its surface. Then a solution of 50 mL of toluene and 466 μL of APTES was added to the activated material and refluxed for 12 h. Finally, the hot solution was filtered out and the solid sample was washed with fresh toluene and air-dried at 60 °C, leading to the MCM-APTES (MAp) solid. Incorporation of AuNPs into the silica network was carried out by using a new in situ synthesis method based on that proposed by McGilvray and co-workers.25,39 A desired amount of HAuCl4·3H2O was added to an aqueous mixture (160 mL) of the MAp solid (3.5 g). Then, the corresponding amount of Irgacure-2959 was added and the yellow solution was placed into a photoreactor and irradiated with 14 UVA lamps for 30 min. The final pink solution was then filtered off and washed several times with water to remove the nonreacted salt. The air-dried new synthesized hybrid materials were denoted as MApAu-#, where M points out the use of MCM-41 as solid support, Ap stands for 3-aminopropyltriethoxisilane incorporation, Au stands for AuNPs, and # stands for the amount of AuNPs incorporated into the material (nominal wt %). Synthesis of Hybrid Materials by Using an Absorption Approach. These materials were synthesized in a two-step process. First, MCM-41 type silica was prepared by using the reported synthesis that has been described above. Then, AuNPs were generated within the mesoporous silica solids following the same method already described. Final hybrid materials were denoted as MAu-#, where M points out the use of MCM-41 as solid support, Au stands for AuNPs, and # stands for the amount of AuNPs incorporated into the material (nominal wt %). Naproxen Loading. Naproxen was chosen as test molecule to study drug incorporation and delivery processes by using the new hybrid materials. The loading procedure of naproxen on the silica was based on that proposed by Vallet-Regi et al.3 Naproxen (0.4 g) was added to a hexane solution (15 mL) and the hybrid material (0.4 g). This solution was stirred for 12 h while preventing the evaporation of hexane. Then, the mixture was filtered off, washed several times with hexane in order to eliminate the not absorbed drug, and air-dried. The amount of drug absorbed into the solids was determined by thermogravimetric analysis. Drug Release Studies. Drug Release studies were performed by soaking 25 mg of the material in 7 mL of simulated body fluid (SBF). The concentration of the drug over time was monitoring by UV spectroscopy. Instrumentation. a. X-ray Diffraction Studies. Small-angle powder X-ray diffraction (XRD) analysis was carried out with a Rigaku Ultima IV diffractometer using a Cu Kα radiation (k = 1.541836 Å), operating at 40 kV and 30 mA, at a scanning velocity of 0.03° min‑1 in the 0.7° < 2θ < 10° range. b. N2 Adsorption−Desorption Isotherms. Textural properties of the solids were determined from N2 adsorption at 77 K in an AUTOSORB-6 apparatus. MApAu-# samples were previously degassed for 4 h at 373 K at 5 × 10−5 bar. Moreover, MAu-# materials were degassed for 4 h at 523 K at 5 × 10−5 bar because of the absence of organic groups degradation. The adsorption branch of the obtained isotherms was used to determine the pore size distribution using the Barret−Joyner−Helender (BJH) method. The surface area was calculated using the multipoint BET method in the 0.05−0.30 relative pressure ranges. Mesopore volume was measured at the plateau of the adsorption branch of the nitrogen isotherm, P/P0 = 0.8.31 Gas adsorption at higher P/P0 is mainly due to interparticle condensation. c. Transmission Electron Microscopy Analysis. The morphology of the mesoporous materials was characterized by transmission electron
Figure 8. Percent naproxen release curves for MAu-3-Nap (a) and for MApAu-1-Nap (b) in dark conditions (black) followed by 532 nm LED irradiation (red). During the limited LED exposure, the release follows reasonably well a linear dependence. Under prolonged exposure the release reaches a plateau, as expected and at close to 99% release efficiency.
ation and leaching of nanoparticles when higher concentration of reagents where used. Nevertheless, grafting materials showed AuNPs evenly dispersed in the silica structure when low moieties of AuNPs were incorporated. Drug delivery processes studied by using naproxen as guest molecule were tested. Results proved that the excitation of the gold plasmon increases the rate of release of the drug out the mesoporous of the studied materials as well as the final percentage of drug delivered. The versatility and simplicity of the proposed system makes it a very good candidate for the delivery of other substances in other type of applications besides biological systems.
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EXPERIMENTAL SECTION
Materials. Tetraethylorthosilicate (TEOS, 98%) and cetyltrimethylammonium bromide (C16TAB, 96%) were used as silica source and structure-directing agent, respectively. Aqueous ammonia solution (NH4OH, 30%), tetrachloroauric acid (HAuCl4), Irgacure-2959, and 3-aminopropyltriethoxysilane (APTES) were also used in the synthetic protocol to obtain the final hybrid mesoporous materials. Naproxen was purchased from Sigma-Aldrich and used as guest molecule as received. All chemicals were purchased from Aldrich and used as received without further purification, with the exception of Irgacure2959 that was a gift from Basf. Synthesis of Hybrid Materials by Using a Grafting Approach. A reported synthesis of MCM-41 type silica38 was followed to prepare the mesoporous silica materials for the postgrafting incorporation of 3-aminopropyltriethoxisilane on their structure. In a typical synthesis, 4.4 g of C16TAB was dissolved in a solution of 19.2 mL of ammonium hydroxide (NH4OH) in 400 mL of distilled water. Then, 23.3 mL of TEOS was added and the mixture was stirred until hydrolysis was complete. The molar composition of the synthesis gel was 1 SiO2/0.12 CTAB/1.41 NH4OH/280 H2O. After hydrolysis was concluded, the solution was transferred to a 100 10526
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microscopy (TEM). TEM analyses were carried out on a JEM-2010F microscope (JEOL, 200 kV, 0.14 nm resolution). For this purpose, samples were prepared by dipping a sonicated suspension of the material in ethanol on a carbon-coated copper grid. The digital analysis of the TEM micrographs was done using DigitalMicrographTM 3.6.1 by Gatan. d. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-AES). Metal loading in the materials was determined by ICP-AES on a Perkin-Elmer Analyst 300 absorption apparatus and plasma ICP Perkin-Elmer 40. An amount of 25 mg of every sample was digested in 1 mL of HF during 12 h prior to analysis by ICP-AES. e. UV−Vis Spectroscopy. Reflectance UV−vis (DR) data of pressed powders were recorded on a UV−vis spectrophotometer AGILENT 8453 with a Har-153 rick praying mantis accessory, in a wavelength range from 200 to 600 nm, and recalculated following the Kubelka− Munk function.40 The amount of naproxen released was monitored using 1 cm path length quartz cuvettes with a Varian Cary 100 spectrophotometer. f. NMR Spectroscopy. Analysis of the simulated body fluid after the release was performed on a Bruker Avance II 400 MHz NMR spectrometer.
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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 We gratefully acknowledge the Natural Sciences and Engineering Research Council of Canada and the Canadian Foundation for Innovation for generous support of this research. We thank Dr, Yun Liu for assistance with TEM measurements.
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REFERENCES
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