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Article Cite This: ACS Omega 2018, 3, 6683−6691
Morphological Evolution of Gold Nanoparticles into Nanodendrites Using Catechol-Grafted Polymer Templates Ho Yeon Son,†,§ Kyeong Rak Kim,†,§ Cheol Am Hong,*,† and Yoon Sung Nam*,†,‡ †
Department of Materials Science and Engineering and ‡KAIST Institute for the NanoCentury, Korea Advanced Institute of Science and Technology, Daejeon 34141, Republic of Korea
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S Supporting Information *
ABSTRACT: Morphology, dimension, size, and surface chemistry of gold nanoparticles are critically important in determining their optical, catalytic, and photothermal properties. Although many techniques have been developed to synthesize various gold nanostructures, complicated and multistep procedures are required to generate three-dimensional, dendritic gold nanostructures. Here, we present a simple method to synthesize highly branched gold nanodendrites through the well-controlled reduction of gold ions complexed with a catechol-grafted polymer. Dextran grafted with catechols guides the morphological evolution as a polymeric ligand to generate dendritic gold structures through the interconnection of the spherical gold nanoparticles. The reduction kinetics, which is critical for morphological changes, is controllable using dimethylacetamide, which can decrease the metal−ligand dissociation and gold ion diffusivity. This study suggests that mussel-inspired polymer chemistry provides a simple one-pot synthetic route to colloidal gold nanodendrites that are potentially applicable to biosensing and catalysis.
1. INTRODUCTION Gold nanoparticles (AuNPs) have received much attention due to their wide range of potential applications, including bioimaging, sensing, therapeutics, drug and gene deliveries, and catalysis.1−6 The applicability of AuNPs depends upon the optical, electronic, and chemical properties, which are strongly correlated with particle size, shape, and surface chemistry.7−13 In particular, dendritic gold nanostructures have been reported to exhibit enhanced surface-enhanced Raman scattering sensitivity and catalytic activities due to increasing local electric field and large surface-to-volume ratio.14−17 Various reducing and capping agents have been studied to induce the formation of AuNPs using gold ion precursors (e.g., chloroauric acid (HAuCl4)). It is now well established that a critical factor in determining the size and shape of AuNPs is the controlled balance of competitive nucleation and growth rates on specific crystalline facets.18−20 The balance is profoundly affected by the strength of metal−ligand interactions and the diffusivity of ionic precursors. Several phenolic compounds (e.g., hydroquinone, tannin, and catechol) can serve as reducing agents for noble metal precursors.3,21−27 Catechol has a reduction potential (Eo) of about 530 mV versus reversible hydrogen electrode at a neutral pH, and this modest reduction power is a crucial feature of catechol as an antioxidant in diverse biological processes.28 In particular, catechol has an exceptional binding affinity to various metallic substrates via metal−catechol complexation.29,30 In the presence of tetrachloroauric ions (AuCl4−), catechol can undergo rapid oxidative conversion into © 2018 American Chemical Society
catecholquinone through the transfer of two electrons to the ionic precursor.22−24 However, a previous study showed that the controlled synthesis of AuNPs using catechol is very challenging in water due to the very fast reduction kinetics, resulting in irregular, relatively large particles.22 Hence, the electron transfer kinetics needs to be reduced to modulate the morphology of gold nanostructures when catechol is used as a reducing agent while its reduction potential should be maintained. The use of an organic solvent as a reaction medium can be a solution for this issue. As an instance, N,N-dimethylformamide (DMF) could serve as a reductive medium for the synthesis of AuNPs at elevated temperatures, 150−160 °C, with an excess amount of poly(vinyl pyrrolidone) (PVP) as a stabilizer.31 The nucleation and growth of AuNPs are influenced by the presence of water, which can facilitate the dissociation of AuCl4−. As a result, the amount of water added to DMF plays a crucial role in determining the size and morphology of the synthesized AuNPs. The use of nonaqueous reaction media could be advantageous when catechol serves as a reducing ligand owing to the lower relative permittivity of media and the high solubility of catechol in a broad range of organic solvents. The chemical diversity of the side functional groups of catechol also provides a unique opportunity for controlling the reduction kinetics of gold ion precursors. Another useful Received: March 21, 2018 Accepted: June 8, 2018 Published: June 20, 2018 6683
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Figure 1. Graphical scheme of the overall synthesis using pyrocatechols and a dextran grafted with catechols as a reducing polymeric ligand to modulate the morphology of gold nanostructures.
Figure 2. (a) UV−vis absorption spectra of pyrocatechol incubated in 1 mM NaOH for 10 min. UV−vis absorption spectra ((b) inset: digital photographs of the sample suspensions), time-course profiles of intensities at 275 nm (circle) and 400 nm (square) (c), and transmission electron microscopy (TEM) images (d) of 0.4 mM HAuCl4 reacted with 2 mM pyrocatechol at 90 °C in water at various reaction times.
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Figure 3. UV−vis absorption spectra (a) and the maximum absorbance and the corresponding wavelength (b) of 0.4 mM HAuCl4 in the mixtures of water and DMA.
2. RESULTS AND DISCUSSION
property of catechol as a ligand is that it can be easily introduced to a polymeric backbone as a pendant group, as already implemented using various polymeric materials for bioadhesive and mineralization applications.24,29,32−34 Catechol-grafted polymers could lead to distinctive morphologies of AuNPs because the reduction sites of gold ion precursors are confined by the motion of the polymer chain and affected by the grafting density of catechol along the backbone.36 This situation is very much different from that of conventional polymeric stabilizers, such as PVP and poly(vinyl alcohol), where the nucleation and growth of AuNPs are mainly determined by the diffusional collisions of gold ion precursors with reducing agents in the bulk phase. Polymeric stabilizers are subsequently adsorbed to the surface of the synthesized AuNPs for interfacial stabilization, which can result in the formation of complex gold nanostructures. Although polymers and surfactants have been utilized for the templated growth of branched and dendritic gold nanostructures, their synthetic routes are complicated, or the final morphologies are limited to only short-armed “urchin-like” structures.37−39 Recently, the DNA-templated synthesis of branched gold nanostructures for photothermal applications was suggested, but their size and structural variation were also very limited.40 Therefore, a simple and fast synthetic method is still desirable for the generation of more diverse dendritic AuNPs and even more complicated gold architectures for practical applications. In this work, we present a one-pot synthesis of highly branched gold nanodendrites using a catechol-grafted dextran as a reducing polymeric ligand to control the morphogenesis of gold nanostructures (Figure 1). Dextran is a well-known polymer template because it exhibits excellent antifouling performances for biological applications.41−49 For instance, dextran-coated supermagnetic iron oxide (e.g., FeriDEX) has been widely used for magnetic resonance imaging,41−44 targeted imaging,45 cell tracking,46,47 and for both drug and nucleic acid deliveries.48,49 Catechol grafted to the dextran backbone was coordinated with AuCl4− through ligand exchange. The decomposition kinetics of the Au ion−catechol complex into metallic nanostructures was controlled using a mixed solvent of dimethylacetamide (DMA) and water at different volumetric ratios to change the relative permittivity of the reaction medium. The concentration of catechol-grafted dextran and its mixture with free catechol was controlled to generate interconnected gold nanostructures.
The reduction of AuCl4− into AuNPs can be easily performed using various reducing agents (e.g., sodium borohydride, quinones, ascorbic acid, etc.) as AuCl4− has a relatively low reduction potential.50 Accordingly, catechol can also spontaneously transfer two electrons to AuCl4− with the simultaneous oxidation of catechol to catecholquinone, as previously demonstrated.22−24,28 The conversion of catechol to quinone can be easily monitored by the spectral changes in the UV range. For instance, free pyrocatechol had an absorption peak around 280 nm, but the absorption peak was shifted to ∼290 nm with the increased intensity as catechol was oxidized to catecholquinone (Figure 2a). Other catechol derivatives, such as 3,4-dihydroxyphenylacetic acid (DHPA), 4-tert-butylcatechol, and dopamine, also showed a similar spectral shift via oxidation (Figure S1). Before synthesizing gold nanostructures using a catechol-grafted polymer template, we first examined the reduction of AuCl4− into solid AuNPs using pyrocatechol as a reducing agent with deionized water as a medium. When pyrocatechols were mixed with AuCl4−, during the first 5 min of incubation at 90 °C, the catecholquinone peak around 275 nm was dramatically increased, while a new absorption shoulder with a peak at 326 nm was generated from the metal-to-ligand (σ*−π) charge transfer (MLCT) transition of the tetrachloroauric ion (AuCl4−). The MLCT peak is originally located around 280 nm, being overlapped with the catecholquinone peak. As the catechol oxidation released free hydrogen ions to the medium, lowering the medium pH, the MLCT peak was red-shifted.51 Pyrocatechol can be coordinated with metal ions at room temperature, performing ligand exchange with a characteristic peak around 400 nm corresponding to the ligand-exchanged intermediate complexes of catecholquinone and Au1+ ion (Figure 2b,c). The Au− catecholquinone complex was transiently generated and then reduced to solid AuNPs.22 The generated AuNPs were quickly aggregated into large, irregular-shaped particulates having a diameter of around 100 nm (Figure 2d). After 5 min, there was no significant change in the absorption intensity of the Au− catecholquinone complex, whereas the catecholquinone peak continued to be increased. As a result, a thicker polymeric layer, presumably generated by catechol condensation, was formed on the surface of AuNPs, as indicated in Figure 2d. At room temperature, the reaction was quite slow, but the tendency was the same as at 90 °C (Figure S2). 6685
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Figure 4. Digital photographs (a), UV−vis absorption spectra (b), time-course profiles of peak intensities at 326 nm (circle) and 550 nm (square) (c), and TEM images (d) of AuNPs obtained by reduction of 0.4 mM HAuCl4 with pyrocatechol in an 85:15 mixture of DMA and water at 90 °C.
occurred in water.22 In DMA, which has a high dipole moment (3.86 D), the aggregation of AuNPs was efficiently prevented because the gold−chloride complex was effectively stabilized. During the formation of AuNPs, the competition between particle nucleation and growth can also be partially controlled by diffusion. In a low-viscosity solution, the diffusion is fast, and the reduced Au species can migrate to the surface of nuclei before they form a new nucleus, resulting in a wide size range of AuNPs because of a different nucleation and growth history. However, in the more viscous DMA (1.96 cP at 25 °C), relatively uniform AuNPs were synthesized because of the relatively lower diffusivity of reduced Au species. Therefore, we concluded that neither DMA nor water could make a good medium for the reduction of gold precursor ions. In water, the reduction kinetics is too fast to be controlled by the experimental parameters, whereas the metal−ligand dissociation does not happen efficiently in DMA. We prepared a mixture of DMA and water to control the relative permittivity, which could make a good reaction medium for the catechol-mediated synthesis of gold nanostructures. The addition of 15 vol % water to DMA reduced the AuCl4− peak at 326 nm (Figure 4a−c). This change indicates that water facilitated the reduction of Au3+ to Au1+ and Au0 through the electron transfer from catechols, whereas Cl− ligands were exchanged with catechol. The catecholquinone/Au1+ complexes were eventually reduced to AuNPs as evidenced by the increased surface plasmon resonance (SPR) peak at 550 nm. The apparent color of the obtained samples gradually turned from colorless to pink, revealing the formation of AuNPs. Accordingly, the MLCT peak at 326 nm decreased, whereas the SPR peak increased over time. Figure 4d shows the variation of the size and shape of AuNPs synthesized with pyrocatechol in an 85:15 (v/v) mixture of DMA and water for 4, 15, and 30 min. At the early stage of Au ion reduction (4 min), small nanoparticles of about 5 nm in diameter were aggregated into about 20 nm AuNPs. The increased size and density of spherical AuNPs were observed in the early stage of
The metal-to-ligand interaction is fundamentally the electrostatic force, which is influenced by the relative permittivity of the medium according to Coulomb’s law. Accordingly, the relative permittivity of reaction media can significantly affect the dissociation and reduction kinetics of the gold precursors. In our work, the dissociation of the gold precursors was monitored by the characteristic absorption peak of the MLCT transitions of AuCl4− in a mixed medium of DMA and water with different ratios (Figure 3). In deionized water, the ionic precursor solution (pH 3.5) has a maximum peak of absorption around 293 nm, corresponding to the unresolved MLCT transition. As the volume fraction of DMA increased, the MLCT transition peak was gradually red-shifted, and the absorptivity was increased. The increased interaction between Au3+ and Cl− by DMA can explain the red-shift of the MLCT peak because DMA has a relatively lower relative permittivity (e.g., 37.8) compared to that of water (80.4). Therefore, DMA can suppress the dissociation of AuCl4−. The same phenomena were observed for HAuCl4 in DMF and dimethyl sulfoxide (DMSO) (Figure S3). The suppression effect was very strong: when AuCl4− was mixed with pyrocatechol in DMA, no spectral change was observed even at 90 °C, indicating no reduction of gold precursors into solid nanoparticles (Figure S4a). The reduction of AuCl4− was surprisingly inhibited in DMA even when sodium borohydride (Eo = −1.24 V) was employed as a reducing agent (Figure S4b). The results indicate that the inhibition was not caused merely by the insufficient reduction potential of pyrocatechol in DMA. The increased electrostatic association of AuCl4−, created by the low relative permittivity of DMA, efficiently suppresses the dissociation of the metal ion from the ligands. Water has a very high relative permittivity, intermediate dipole moment (1.85 D), and viscosity (1.02 cP at 25 °C). Because of the higher relative permittivity of water than that of DMA (37.8), AuCl4− can be more readily dissociated in water and interact with catechols effectively. As a result, uncontrolled rapid nucleation and growth of AuNPs 6686
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Figure 5. Schematic illustration and TEM images of AuNPs synthesized with 0.4 mM HAuCl4 and dextran-g-ct in an 85:15 (v/v) mixture of DMA and deionized water: The catechol concentrations were 2 mM (a), 1 mM (b), and 0.17 mM (c−e).
the reaction, and finally, polyhedral AuNPs with >250 nm in diameter were generated in 30 min. When the amount of water was increased to 50 vol %, the AuCl4− peak was not observed because the Cl− ligand might be rapidly replaced with catechol, and the quinone peak was raised with increased absorbance in the range of 300−400 nm (Figure S5). The catecholquinone/ Au1+ complex peak was detected within 30 s and then decreased continuously while the SPR peak was generated. Next, we chemically grafted dopamine to the backbone of dextran via urethane linkage (denoted “dextran-g-ct”) and used dextran-g-ct as a polymeric template with a reducing power for the synthesis and steric stabilization of gold nanostructures. The hydroxyl groups of dextran were activated with 1,1′carbonyldiimidazole (CDI), and then the active intermediates were reacted with the primary amine groups of dopamine (Figure S6a). The presence of the catechol moiety grafted to the backbone of dextran was confirmed using 1H NMR spectrum (Figure S6b). The 1H NMR chemical shifts in DMSO-d6 were assigned as follows: 2.5−3.0 ppm for dopamine linked to dextran (t, −CH2CH2−), 3.0−3.8 and 4.3−5.0 ppm for dextran, and 6.4−6.75 ppm for catechol. The substitution degree of the catechol moiety was about 5.9 mol % determined from the 1H NMR spectrum, calculated using the
glucose peak of dextran and the benzene peak of catechol. Light absorption at 280 nm from catechol also indicated the successful conjugation of catechol to dextran (Figure S6c). In the preliminary study, we synthesized a higher catecholgrafting density of ∼10 mol %, which is more reactive to a large number of gold ions per the polymer chain. However, even during the storage at 4 °C, it quickly caused an insolubility problem due to π−π interaction, causing the reproducibility problem.32,35 To avoid the polymerization of catechol on the dextran, we decreased the grafting density to 5.9 mol % and confirmed that no insoluble aggregates were generated for more than a few months. We also chose a low-molecularweight dextran, 6 kDa, because it can be readily dissolved in the reaction media used in this work. To examine the morphological control of gold nanostructures using dextran-g-ct, we used different concentrations of dextran-g-ct to synthesize gold nanostructures. Spherical AuNPs with an average diameter of 7.2 ± 1.1 nm were synthesized at a high concentration of dextran-g-ct that contains 2 mM of catechol moiety grafted to the dextran backbone (Figures 5a and S7). The catechol moiety of dextran-g-ct can reduce gold ions to solid AuNPs that were sterically stabilized by dextran-g-ct adsorbed on the surface of 6687
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Figure 6. Bright-field (a) and high-angle annular dark field (HAADF) (b) STEM tilt series of gold nanodendrites synthesized with 0.4 mM HAuCl4 and 0.17 mM catechol grafted on dextran-g-ct in an 85:15 (v/v) mixture of DMA and deionized water. Scale bar: 50 nm.
Figure 7. TEM images of AuNP clusters synthesized from 0.27 mM HAuCl4 with a mixture of dextran-g-ct (0.11 mM catechol) and 1.33 mM dopamine in an 85:15 (v/v) mixture of DMA and deionized water.
AuNPs as a capping agent. The d-spacings of adjacent planes were 2.4 Å, which correspond to the (111) planes of the Au crystal. Unlike spherical AuNPs synthesized using high concentrations of dextran-g-ct, gold nanodendrite structures were generated at low concentrations of dextran-g-ct (1 and 0.17 mM catechol-based concentrations) (Figure 5b), which seem that several AuNPs were connected along the polymer templates, dextran-g-ct. The anisotropic growth of gold nanostructures could be caused by limited ligand protection (LLP), as suggested by the previous studies.52−55 The LLP domain did not provide sufficient ligand protection to induce crystal growth or directional attachment. As the concentration of dextran-g-ct was decreased, the surface of AuNPs became less stable because the number of small capping agents adsorbed on the surface was not enough. The branched gold nanostructures were generated presumably due to the connection of these less stable AuNPs for reducing the surface energy, whereas the catechol moieties of dextran-g-ct might be adsorbed on the surfaces of several AuNPs and induce the reduction of gold ion along the dextran-g-ct chains between the AuNPs. The TEM images of the branched gold nanostructures show that the d-spacings of adjacent planes were 2.4 and 2.0 Å,
corresponding to the (111) and (200) planes of the Au crystal, respectively (Figure 5e). The UV−vis absorption spectra show the SPR peaks of gold nanodendrites (Figures S8 and S9). The difference of absorbance resulted from the conversion efficiency of gold ions to solid gold nanostructures. The absorbance was increased with the increased concentration of dextran-g-ct because of the increased amounts of AuNPs reduced by the high concentration of catechols. The threedimensional (3D) morphology of the gold nanodendrites was confirmed using TEM tomography. Figure 6 shows the brightfield and high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) tilt series of the gold nanodendrites, which were synthesized with 0.4 mM HAuCl4 and 0.17 mM catechol grafted on dextran-g-ct in an 85:15 (v/v) mixture of DMA and deionized water. We further investigated the kinetics of the formation of AuNPs using a mixture of dextran-g-ct and dopamine (Figure 7). The clusters of small AuNPs were formed with an average size of 66.7 ± 22.1 nm (Figure S10), which were distinctive from the morphology of AuNPs synthesized using pyrocatechol only (Figure 4). The result indicates that the use of dextran-g-ct can effectively slow down the reduction rate of 6688
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in 4 mL of anhydrous DMSO was also added to prevent the oxidation of dopamine during the reaction. The conjugation reaction was carried out with magnetic stirring at 300 rpm at ambient temperature for 24 h. The product was isolated by repeated precipitation in a 6:4 mixture of isopropanol and hexane. 4.4. Synthesis of Gold Nanostructures Using Dextrang-ct. A HAuCl4 solution (0.8 mM) and three different concentrations of dextran-g-ct solutions (containing 4, 2, and 0.34 mM catechol) were prepared in an 85:15 (v/v) mixture of DMA and deionized water. The HAuCl4 solution (500 μL) was magnetically stirred for 1 min at 90 °C in an oil bath, followed by the immediate addition of 500 μL of the dextran-gct solution. The final concentrations were 0.4 mM for HAuCl4 and 2, 1, and 0.17 mM for the dextran-g-ct solutions. AuNP clusters were synthesized using a mixture of dextran-g-ct and dopamine as the reducing and capping agents. The solutions of HAuCl4 (0.8 mM), dopamine (4 mM), and dextran-g-ct (0.34 mM catechol) were prepared in an 85:15 (v/v) mixture of DMA and deionized water. The HAuCl4 solution (500 μL) was magnetically stirred for 1 min at 90 °C in the oil bath, followed by the immediate addition of 500 μL of the dextran-gct solution and 500 μL of the dopamine solution. The mixture was magnetically stirred for 10 min at 90 °C in the oil bath for reaction. After the reaction, the solution was centrifuged and washed three times. 4.5. Characterization. Dextran-g-ct was analyzed using nuclear magnetic resonance (1H NMR, Bruker Avance, 400 MHz) and UV−vis spectrophotometry (UV-1601, Shimadzu, Japan). The morphology of the prepared nanoparticles was observed using transmission electron microscopy (TEM, JEOL JEM-3011 HR, Japan) at an acceleration voltage of 300 kV. The 3D tomography images of gold nanostructures were observed using TEM (FEI Talos F200X) at an acceleration voltage of 200 kV.
gold ions. The AuNP clusters were covered with a very thin organic layer, which also indicates that the small AuNPs were generated quickly by dopamine and stabilized by dextran-g-ct that are adsorbed on the several AuNPs and formed the stable spherical aggregates with the cover of a very thin layer. The absorbance of AuNPs synthesized with a mixture of dextran-gct and dopamine shows that the reduction process was promoted by dopamine and controlled efficiently by dextran-gct.
3. CONCLUSIONS This study demonstrated that the morphology of gold nanostructures could be readily controlled using a catecholgrafted polymeric template in a mixture of water and an organic solvent. The size, morphology, and reduction kinetics of AuNPs could be adjusted by changing the ratio of dimethylacetamide to water and the functional groups of catechol. Although AuNPs were quickly aggregated in water, stable about 2 nm AuNPs were obtained in the mixture of DMA and water as 85 and 15 vol %, respectively. The functional groups of catechol also dramatically affected the formation of AuNPs. In contrast to DHPA and 4-tertbutylcatehcol, dopamine that undergoes oxidative polymerization showed faster kinetics while synthesizing AuNPs bigger than 50 nm. As a polymeric template, dextran-g-ct was synthesized by conjugating catechols to the backbone of dextran with a grafting density of 5.9 mol %. Gold nanodendrites were synthesized at a low concentration of dextran-g-ct, whereas spherical AuNPs coated with dextran-g-ct were produced when the concentration of dextran-g-ct was increased. The synthesized gold nanodendrites exhibited excellent dispersion stability in water presumably because dextran stabilized the surfaces of gold nanodendrites with the strong binding affinity of catechols to metals. This study demonstrated that a mussel-inspired modification of polymer templates is an efficient means to synthesize colloidal gold nanodendrites, which are potentially attractive for applications to biosensing, catalysis, and photothermal therapy.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b00538. Additional UV−vis absorption spectra of other species of catechol and catecholquinone; UV−vis absorption spectra and TEM images of HAuCl4 reacted with pyrocatechol in water at room temperature; additional absorption spectra of HAuCl4 reacted with pyrocatechol in other solvents; the scheme of the synthesis and 1H NMR spectrum of dextran-g-ct; UV−vis absorption spectra and size distribution histogram for AuNPs synthesized using different concentrations of dextran-gct and dopamine (PDF)
4. EXPERIMENTAL SECTION 4.1. Chemicals and Materials. Hydrogen tetrachloroaurate (III) trihydrate (HAuCl4·3H2O), dopamine hydrochloride, CDI, hydroquinone, and dextran (molecular weight ≈ 6 kDa from Leuconostoc spp.) were purchased from SigmaAldrich. Milli-Q water was used as deionized water in all experiments. 4.2. Synthesis of AuNPs Using Catechols. HAuCl4 was first dissolved in DMA, and deionized water was added to adjust the concentration (0.72 mM) and the DMA/water ratio. The mixture (5.5 mL) was magnetically stirred for 1 min at 90 °C, followed by the immediate addition of a catechol solution (4.5 mM in 4.5 mL DMA). The final concentrations were 0.4 mM for HAuCl4 and 2 mM for catechol. 4.3. Synthesis of Catechol-Grafted Dextran. Dextran-gct was synthesized according to our previous reports.26,29 Dextran (1.0 g, 6 mmol based on the glucose unit) was dissolved in 7 mL of anhydrous DMSO. CDI (98.5 mg, 0.6 mmol) in 3 mL of anhydrous DMSO was added to the dextran solution, followed by incubation at ambient temperature for 2 h. Dopamine hydrochloride (186 mg, 1.2 mmol) dissolved in 2 mL of anhydrous DMSO was then added to the CDI-activated dextran solution. Hydroquinone (668.5 mg, 6 mmol) dissolved
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Phone: +82-42-350-3311. Fax: +82-42-350-3310 (C.A.H.). *E-mail:
[email protected] (Y.S.N.). ORCID
Yoon Sung Nam: 0000-0002-7302-6928 Author Contributions §
H.Y.S. and K.R.K. contributed equally to this work.
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Basic Science Research Program and Nano Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (Grant Nos NRF-2016R1A2B4013045 and NRF2017M3A7B4042235).
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DOI: 10.1021/acsomega.8b00538 ACS Omega 2018, 3, 6683−6691
