Artificial Nacre from Supramolecular Assembly of Graphene Oxide

Jun 11, 2018 - Inspired by the “brick-and-mortar” structure and remarkable mechanical performance of nacre, many efforts have been devoted to fabr...
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Artificial Nacre from Supramolecular Assembly of Graphene Oxide Yang Wang,†,‡ Ting Li,† Piming Ma,† Shengwen Zhang,† Hongji Zhang,† Mingliang Du,† Yi Xie,† Mingqing Chen,† Weifu Dong,*,† and Weihua Ming*,‡

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†

Key Laboratory of Synthetic and Biological Colloids, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, 1800 Lihu Road, Wuxi 214122, China ‡ Department of Chemistry and Biochemistry, Georgia Southern University, P.O. Box 8064, Statesboro, Georgia 30460, United States S Supporting Information *

ABSTRACT: Inspired by the “brick-and-mortar” structure and remarkable mechanical performance of nacre, many efforts have been devoted to fabricating nacre-mimicking materials. Herein, a class of graphene oxide (GO) based artificial nacre material with quadruple hydrogen-bonding interactions was fabricated by functionalization of polydopaminecapped graphene oxide (PDG) with 2-ureido-4[1H]-pyrimidinone (UPy) self-complementary quadruple hydrogenbonding units followed by supramolecular assembly process. The artificial nacre displays a strict “brick-and-mortar” structure, with PDG nanosheets as the brick and UPy units as the mortar. The resultant nanocomposite shows an excellent balance of strength and toughness. Because of the strong strengthening via quadruple hydrogen bonding, the tensile strength and toughness can reach 325.6 ± 17.8 MPa and 11.1 ± 1.3 MJ m−3, respectively, thus exceeding natural nacre, and reaching 3.6 and 10 times that of a pure GO artificial nacre. Furthermore, after further H2O treatment, the resulting H2O-treated PDG-UPy actuator displays significant bending actuations when driven by heat. This work provides a pathway for the development of artificial nacre for their potential applications in energy conversion, temperature sensor, and thermo-driven actuator. KEYWORDS: nacre, graphene oxide, polydopamine, supramolecular, actuator

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strategies have been devoted to prepare artificial nacre composites, including layer-by-layer deposition, self-assembly,15 freeze-casting,16,17 and spin-coating.18 Currently, graphene as a two-dimensional nanomaterial has emerged as a promising material for applications. Because of its excellent electrical and mechanical properties, graphene (and graphene derivatives) has attracted significant attention as an ideal candidate for graphene-based artificial nacre composites.19−25 However, the obtained graphene-based materials often show improvement for only one performance rather than

ollusk shells are mostly composed of brittle aragonite matrix (∼95 vol %) and contain a small volume fraction of organic materials (∼5 vol %).1 Among the materials found in mollusk shells, nacre is a mineralized biological composite.2−4 Nacre is a naturally occurring “brick-and-mortar” structure, which is composed of highly aligned aragonite platelets connected by an organic matrix that serves as glue between the platelets.5 The “brickand-mortar” hierarchical architecture with abundant interfacial interactions of nacre renders it with outstanding mechanical properties.6 To mimic nacre’s structure, CaCO3 crystals,7 nanoclay,8−11 Al2O3 flakes,12 flattened double-walled carbon nanotubes,13 and layered double hydroxides14 have been used as reinforcing platelets to fabricate artificial nacre. Various © 2018 American Chemical Society

Received: April 23, 2018 Accepted: June 11, 2018 Published: June 11, 2018 6228

DOI: 10.1021/acsnano.8b03025 ACS Nano 2018, 12, 6228−6235

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Figure 1. Illustration for the preparation of PDG-UPy artificial nacre.

onto the surface of polydopamine (PDA) modified GO (PDG) nanosheets by reacting isocyanate containing UPy with PDG to form UPy functionalized PDG (PDG-UPy), in which selfcomplementary quadruple hydrogen bonding interaction would form among UPy units. Then via self-assembly with a vacuum-assisted filtration process, PDG-UPy nanocomposite showing “brick and mortar” structure was prepared (Figure 1). The resultant artificial nacre showed excellent mechanical properties. Moreover, after a water treatment, the obtained H2O-treated PDG-UPy actuator became very sensitive to thermal transitions, which offers great potential applications in many fields such as smart devices for motion detection and energy harvesting.

comprehensive performance. It is known that interfacial interactions in nacre materials have an important effect on the resultant mechanical properties. Most of the reported graphene-based composites exhibit poor flexibility and instability because of weak interaction between the adjacent nanosheets. Many different kinds of interfacial interactions such as ionic bonding,26 hydrogen bonding,27 and covalent bonding28 have been designed to improve mechanical performances of graphene-based composites. For instance, metal ions (e.g., Ca2+, Fe3+, Al3+, Zn2+) are introduced into the interlayer of graphene, which increase the mechanical properties.29,30 GO-Poly(vinyl alcohol) composites have been formed through hydrogen bonding.31 An et al. have fabricated borate cross-linked GO paper.32 Most recently, Cheng et al. have prepared graphene-based composites via ionic bonding and covalent bonding.33 Hydrogen bonding is a more versatile motif that can enhance interactions; in a previous report,34 graphene-based composites with only single hydrogen bonds showed a relatively small enhancement compared with other methods because of its low bonding energy. The strategy based on hydrogen bonding interaction needs further improvement to obtain high-performance graphene-based composites. Quadruple hydrogen bonds have higher strength and directionality.35,36 2-Ureido-4[1H]-pyrimidinone (UPy) is a well-studied supramolecular building block pioneered by Meijer et al.36 The UPy units can dimerize strongly in a selfcomplementary array of four cooperative hydrogen bonds. This dimerization strength is stronger than most other noncovalent bonds.37 UPy motifs have been widely used in self-assembly, self-healing, and shape memory polymers. These quadruple hydrogen bonds can remain stable even under high temperature (up to 225 °C) in solid state.38 Therefore, we envision that quadruple hydrogen bonds may enhance mechanical properties of GO-based artificial nacre. In this work, we have demonstrated excellent mechanical properties GO-based artificial nacre through quadruple hydrogen bonding interfacial interaction. First, UPy was attached

RESULTS AND DISCUSSION The preparation of PDG-UPy nanocomposites is schematically illustrated in Figure 1. First, UPy-NCO was synthesized as previously reported39 and characterized to satisfaction by 1H NMR (Figure S1). GO was synthesized using an improved Hummers procedure.40 The exfoliated GO nanosheets were mixed with a dopamine solution, leading to PDA-modified GO (PDG) nanosheets. Next, the PDG nanosheets were decorated with UPy groups by reaction with UPy-NCO in dimethylformamide (DMF). Finally, the homogeneous dispersion was assembled into PDG-UPy artificial nacre via a vacuum-assisted filtration, with the UPy chain content ranging from 2.5 to 19.7 wt % (determined by thermogravimetric analysis, Figure S2 and Table S1), denoted as PDG-UPy-I (UPy content: 2.5 wt %, the same below), PDG-UPy-II (5.6 wt %), PDG-UPy-III (11.8 wt %), PDG-UPy-IV (14.5 wt %), and PDG-UPy-V (19.7 wt %), respectively. The UPy groups can form quadruple hydrogen bonding during the preparation. This quadruple hydrogen bonding of UPy units between the adjacent nanosheets plays an important role in nanocomposite properties, as detailed below. The surface morphologies and thickness of nanosheets were characterized by transmission electron microscopy (TEM) and 6229

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Figure 2. TEM images of (a) GO, (b) PDG, and (c) PDG-UPy-IV nanosheets. AFM images of (d) GO, (e) PDG, and (f) PDG-UPy-IV nanosheets and corresponding height profiles.

Figure 3. SEM images of the artificial nacre: (a, a′) GO, (b, b′) PDG, and (c, c′) PDG-UPy-IV nanocomposite.

disappeared completely, confirming the successful covalent bonding between UPy-groups and PDG (Figure S3). In order to study the effects of UPy on GO-based artificial nacre, a series of PDG-UPy nanocomposites have been prepared. The side-view morphologies of GO, PDG, and PDG-UPy-IV nanocomposite have been examined by the scanning electron microscopy (SEM). As shown in Figure 3a, the pristine GO nanocomposite showed a clear layered structure with abundant interlayer gaps, since only the weak interactions between adjacent GO nanosheets exist. After the dopamine reduction, the PDA layer improved the interlayer interfacial interaction and reduced the gaps (Figure 3b). In sharp contrast, it became evident (Figure 3c) that PDG-UPy

atomic force microscopy (AFM) (Figure 2). The fully exfoliated GO nanosheets with a smooth finish were observed, and wrinkles formed due to the thin thickness (∼0.88 nm). Compared with the smooth GO nanosheets, PDG nanosheets appeared to be relatively rough with the average thickness of 1.80 nm and showed a large amount of wrinkles caused by the PDA coating on the nanosheets. The PDA coating can be used as a versatile platform for further nanosheet functionalizations owing to the abundant catechol groups. After reaction with UPy-NCO, the thickness of PDG-UPy-IV nanosheets further increased to ∼3.32 nm. Furthermore, while UPy-NCO displayed the characteristic IR peak of isocyanate at 2281 cm−1, for the PDG-UPy nanosheets the isocyanate peak 6230

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Figure 4. (a) Representative stress−strain curves of GO, PDG, and PDG-UPy nanocomposite with the UPy content ranging from 2.5 to 19.7 wt % (denoted as PDG-UPy-I, PDG-UPy-II, PDG-UPy-III, PDG-UPy-IV, and PDG-UPy-V, respectively). The nanocomposites were dried in a vacuum oven at 45 °C for 24 h to eliminate water completely. Mechanical properties were characterized under ambient conditions (25 °C, 15% RH). (b) Comparison of mechanical properties of PDG-UPy-IV (STAR) and other GO-based materials, such as GO-PVA, GO-Mg2+, GO-GA, GO-PMMA, GO-borate, GO-PVA-borate, GO-Mg2+-PI, GO-Al2O3-PVA, GO-MoS2-TPU, GO-DWNT-PCDO, GO-SL, GO-CMCMn2+, GO/PDMS-PGMA, ai-GO-CNC, GO-AA-SCMC, GO-CMC. (c) Proposed fracture mechanism of nacre-inspired PDG-UPy nanocomposite under stress.

GO-PVA,31 GO-PMMA,31 GO-Mg2+,41 GO-GA,42 GOBorate,32 GO-PVA-Borate,43 GO-Mg2+-PI,44 GO-Al2O3PVA,45 GO-MoS2-TPU,46 GO-DWNT-PCDO,47 GO-SL,48 GO-CMC-Mn 2+ ,30 GO/PDMS-PGMA,49 ai-GO-CNC, 50 GO-AA-SCMC,51 and GO-CMC52 (Figure 4b). A prominent advantage of our PDG-UPy nanocomposites lies in the fact that both tensile strength and toughness have been synergistically enhanced (PDG-UPy-IV, Figure 4b), whereas in most of the previously reported composites only one of these two properties was improved. To understand the strengthening effect of the nacre-inspired PDG-UPy nanocompisites, a typical fracture model is proposed (Figure 4c). In the initial stretching process, the slippage first occurs between adjacent PDG nanosheets, and the adhered PDA layer may resist the sliding. As the loading further increased, the UPy chains between adjacent PDG nanosheets were gradually stretched. With even higher loading, the quadruple hydrogen bonding starts to dissociate, allowing the PDG nanosheets to slide away from each other. We selected UPy as our interaction motif because it offers an attractive combination of relatively high thermodynamic stability and rapid kinetic reversibility. Recently, UPy has been widely used to prepare reversible supramolecular materials including self-healing hydrogel because of its thermoresponsive property. 53 Because of this thermal responsiveness, the PDG-UPy nanocomposites may be good candidates for thermo-driven actuators. In fact, in dry states, these quadruple hydrogen bonds can remain stable without dissociation even under high temperature up to 225 °C due to strong interaction.38 However, the strength embodied in the UPy-based quadruple H-bonds can only be realized in strongly hydrophobic environments. The hydrated matrix can cause the disruption/dissociation of H-bonds. So, water molecules were introduced into the gallery regions between adjacent PDGUPy nanosheets, which are expected to alter the UPy-UPy

nanosheets were laminated densely together to form a highly aligned nacre-mimetic structure due to stronger interlayer interfacial interactions through quadruple hydrogen bonding. The mechanical properties of PDG-UPy nanocomposites are shown in Figure 4a and Table S1. The pure GO exhibited a tensile strength of 90.7 ± 1.7 MPa and a toughness of 1.1 ± 0.1 MJ m−3. The tensile strength and toughness of the resultant PDG noanocomposite were improved to 119.5 ± 5.6 MPa and 2.7 ± 0.1 MJ m−3, respectively, which can be attributed to the increased interfacial interaction between GO nanosheets and PDA. After introducing UPy groups, the mechanical properties of PDG-UPy nanocomposites were dramatically enhanced. For instance, PDG-UPy-I showed a tensile strength of 171.8 ± 10.2 MPa and a toughness of 4.0 ± 0.3 MJ m−3, respectively. As the UPy content increased in the nanocomposite from 2.5 to 14.5 wt %, both the tensile strength and toughness were further enhanced (Figure 4a); at a UPy content of 14.5 wt %, the tensile strength and roughness of PDG-UPy-IV were 325.6 ± 17.8 MPa and 11.1 ± 1.3 MJ m−3, respectively, which are about 3.6 and 10 times that of the pure GO due to the quadruple hydrogen bonding. When the UPy content was further increased to 19.7 wt % (PDG-UPy-V), the tensile strength did increase to 350.7 ± 13.9 MPa. However, the toughness of PDG-UPy-V became slightly lower than that of PDG-UPy-IV. A likely cause is that, at the UPy content of 19.7 wt %, the UPy units from the same GO surface might have formed quadruple H-bonds, instead of interacting with UPy units from a different GO surface. Many kinds of interactions have been used to improve the mechanical properties of GO-based materials, such as π−π conjugated interactions, hydrogen bonding, ionic bonding, covalent bonding, and synergistic interfacial interactions. Here, the performance of PDG-UPy nanocomposite with selfcomplementary quadruple hydrogen bonding is superior to natural nacre6 and many reported GO-based materials, such as 6231

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Figure 5. (a) Schemes for fabricating H2O-treated PDG-UPy-IV nanocomposites. (b) Performance of the H2O-treated PDG-UPy-IV actuator in thermal-induced bending. The heating stage temperature is 65 °C. (c) Schematic mechanism of the thermal-driven actuator. (The insets show the photographs of the H2O-treated PDG-UPy-IV ribbons under the heating stage.)

original shape because of the reassociation of UPy groups (Figure 5c). Because hydrogen bonds are susceptible to temperature and humidity, mechanical properties of the PDG-UPy-IV nanocomposite at different temperatures and water contents were also measured. From Figure S6 and Table S2, it is obvious that both the tensile strength and the toughness decreased as the water content increased. At room temperature (ca. 25 °C), the PDG-UPy-IV actuator (water content 8.2 wt %) had a tensile strength of 287.5 ± 15.3 MPa and a toughness of 8.5 ± 0.5 MJ m−3. As the water content was increased to 18.4 wt %, the tensile strength and toughness were reduced to 219.4 ± 19.0 MPa and 7.5 ± 0.8 MJ m−3, respectively. In addition, temperature has demonstrated significant impact on the tensile strength and toughness, as quadruple hydrogen bonding can be disrupted at high temperatures. For instance, the tensile strength and toughness of PDG-UPy actuator (water content 8.2 wt %) at 65 °C was 162.7 ± 13.2 MPa, and 6.1 ± 1.0 MJ m−3, respectively, much lower than at room temperature. Nevertheless, these values were still higher than those of natural nacre.

interactions (Figure 5a). To fabricate the H2O-treated PDGUPy actuator, the dry PDG-UPy-IV was placed at a relative humidity of 97% for 24 h at room temperature. The obtained H2O-treated PDG-UPy-IV actuator was then placed onto a heating stage. As shown in Figure 5b and Video S1, the H2Otreated PDG-UPy-IV actuator became very sensitive to heat. The bending angle of PDG-UPy-IV actuator with different water contents is shown in Figure S4. The bending angle rapidly reached ∼70° within 7 s as the PDG-UPy-IV actuator (water content 8.2 wt %) got into contact with the heating stage (65 °C). The original shape of the actuator was fully recovered when the heat was removed. Moreover, this actuating behavior can be repeated for at least 50 cycles (Figure S5). For comparison, dry PDG-UPy-IV did not show any obvious change on the heating stage. As water molecules percolate to the tightly aligned regions of PDG-UPy, new hydrogen bonding forms during the absorption stage, resulting in the decrease of quadruple hydrogen bonding. In our previous study, we have investigated thermal conductivity anisotropy of nacre-inspired materials, and cross-plane thermal transmission can be effectively suppressed due to thermal relaxation.54 In this way, thermal gradient forms along the lateral direction of the H2O-treated PDG-UPy actuator. The associated UPy groups in the high temperature layer could gradually shift to the disassociated state, whereas in the low temperature layer the quadruple H-bonds remained intact. The asymmetric expansion in the H2O-treated PDG-UPy actuator would lead to its bending toward the low temperature side. As the temperature decreased, the actuator gradually recovered its

CONCLUSIONS In summary, we successfully fabricated a PDG-UPy artificial nacre by a supramolecular assembly process. Thanks to the strong self-complementary quadruple H-bonds from UPy, the PDG-UPy nanocomposite shows excellent mechanical properties, with the tensile strength and toughness reaching 325.6 ± 17.8 MPa and 11.1 ± 1.3 MJ m−3, respectively, far superior to 6232

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1 H NMR spectrum of UPy-NCO; TGA curves of PDG, UPy-NCO, and PDG-UPy nanocomposites; tables of UPy content and mechanical properties of the nanocomposites; FTIR spectra of GO, PDG, UPy-NCO, and PDG-UPy; bending angles of the actuator as a function of water content; responsive of the actuator with 50 cycles; effect of water content and temperature on tensile strength and toughness for PDG-UPy-IV nanocomposite, and mechanical properties of the PDG-UPyIV nanocomposite with different water contents at various temperatures (PDF) Video S1: the actuator became very sensitive to heat (AVI)

the natural nacre. Additionally, after H2O treatment, the obtained H2O-treated PDG-UPy actuator became very sensitive to heat. The bending angle rapidly reached ∼70° within 7 s as the PDG-UPy actuator contacted the heating stage at 65 °C, and the actuator restored its original state when the heat was removed. This type of artificial nacre has great potential applications in many fields, including aerospace and smart devices for motion detection or energy harvesting.

METHODS Materials. Graphite powder was received from Qingdao Jinrilai Co. 1,6-Hexanediisocyanate (HDI, 98%), n-pentane, and 2-amino-4hydroxy-6-methylpyrimidine (99%) were purchased from J&K. Dopamine (98%) and tris(hydroxymethyl)aminomethane were purchased from Aladdin. Preparation of Polydopamine-Capped Graphene Oxide. GO was prepared from an improved Hummers procedure.40 PDG were synthesized according to the literature procedure.54 Synthesis of 2-Ureido-4[1H]-pyrimidinone Hexamethylene Isocyanate. UPy was synthesized according to the typical procedures.39 In brief, 2-amino-4-hydroxy-6-methylpyrimidine (5.0 g, 40 mmol, 1 equiv) and HDI (40.32g, 240 mmol, 6 equiv) were heated in a flask at 100 °C for 16 h under a nitrogen atmosphere. Pentane was added to the flask for precipitation after the mixture was cooled to the room temperature. The precipitate was filtered and washed with pentane followed by drying in vacuum at 40 °C for 12 h. Modification of PDG with UPy-NCO. PDG nanosheets were decorated with UPy-groups by reacting with UPy-NCO. The PDG (0.5 g) and 100 mL anhydrous DMF were charged into a threenecked 250 mL round-bottom flask under a nitrogen atmosphere. The mixture was stirred and sonicated for 30 min, followed by the dropwise addition of UPy-NCO (0.02, 0.03, 0.08, 0.10, and 0.15 g), dissolved in 5 mL anhydrous DMF. Then, dibutyltin dilaurate (0.1 mL) was added and the mixture was stirred at 100 °C for 16 h under N2 atmosphere. The reaction mixture was purified via filtration, the powder was thoroughly washed with DMF. After drying at 45 °C in vacuum overnight, PDG-UPy nanosheets were obtained. Fabrication of Artificial Nacre. The prepared PDG-UPy nanosheets were first dispersed in H2O. Next, the mixtures were self-assembled, via vacuum-assisted filtration, into PDG-UPy nanocomposites with different UPy contents, and then vacuum-dried at 60 °C for 12 h. Further treatment with water was performed by exposing PDGUPy-IV naocomposite to a relative humidity (RH) of 97% for 24 h at room temperature. The resulting sample was named H2O-treated PDG-UPy. The environmental RH was controlled by using saturated aqueous solution of ZnSO4 at room temperature. Characterization. 1H NMR spectra (400 MHz) were recorded on a Bruker Avance spectrometer with CDCl3 as the solvent. FTIR spectra were collected using a Nicolet 6700 FTIR spectrometer. Scanning electron microscopy (SEM) images were recorded on a Hitachi S-4800 instrument. Transmission electron microscopy (TEM) images were recorded on a JEOL JEM-2100 instrument. Thermal gravimetric analysis (TGA, Meteler-Toledo 1100SF) measurements were performed from 20 to 800 °C at 20 °C min−1 in nitrogen atmosphere. Atomic force microscope (AFM, Bruker MuLtimode 8) was used to detect the surface topography of the samples. Tensile properties of nanocomposites were measured by using a universal tensile tester (Instron 5967, USA) with a rate of 1 mm·min−1. The thickness range of the film used in the mechanical property test was about 8.0 ± 2.0 μm, which was estimated from the frozen-fractured cross-section of the sample via SEM.

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. Tel: +86-510-8532-6290. *E-mail: [email protected]. Tel: +1-912-478-5043. ORCID

Yang Wang: 0000-0001-7875-9111 Weifu Dong: 0000-0002-7432-8362 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51373070), the Fundamental Research Funds for the Central Universities (JUSRP51624A), MOE & SAFEA, 111 Project (B13025), the scholarship from China S c h o l a r s h i p Co u n c i l ( C S C ) , u n d e r G r a n t C S C N201706790065, Joint Preresearch Foundation of Ministry of Education of China (6141A02022228), and National FirstClass Discipline Program of Light Industry Technology and Engineering (LITE2018-19). REFERENCES (1) Wegst, U. G. K.; Bai, H.; Saiz, E.; Tomsia, A. P.; Ritchie, R. O. Bioinspired Structural Materials. Nat. Mater. 2015, 14, 23−36. (2) Gong, S.; Ni, H.; Jiang, L.; Cheng, Q. Learning from Nature: Constructing High Performance Graphene-Based Nanocomposites. Mater. Today 2017, 20, 210−219. (3) Ortiz, C.; Boyce, M. C. Bioinspired Structural Materials. Science 2008, 319, 1053−1054. (4) Mayer, G. Rigid Biological Systems as Models for Synthetic Composites. Science 2005, 310, 1144−1147. (5) Wang, J.; Cheng, Q.; Tang, Z. Layered Nanocomposites Inspired by the Structure and Mechanical Properties of Nacre. Chem. Soc. Rev. 2012, 41, 1111−1129. (6) Jackson, A. P.; Vincent, J. F. V.; Turner, R. M. The Mechanical Design of Nacre. Proc. R. Soc. London, Ser. B 1988, 234, 415−440. (7) Mao, L. B.; Gao, H. L.; Yao, H. B.; Liu, L.; Cölfen, H.; Liu, G.; Chen, S. M.; Li, S. K.; Yan, Y. X.; Liu, Y. Y.; Yu, S. H. Synthetic Nacre by Predesigned Matrix-Directed Mineralization. Science 2016, 354, 107−110. (8) Podsiadlo, P.; Kaushik, A. K.; Arruda, E. M.; Waas, A. M.; Shim, B. S.; Xu, J.; Nandivada, H.; Pumplin, B. G.; Lahann, J.; Ramamoorthy, A.; Kotov, N. A. Ultrastrong and Stiff Layered Polymer Nanocomposites. Science 2007, 318, 80−83. (9) Tang, Z.; Wang, Y.; Podsiadlo, P.; Kotov, N. A. Biomedical Applications of Layer-by-Layer Assembly: From Biomimetics to Tissue Engineering. Adv. Mater. 2006, 18, 3203−3224. (10) Wang, J.; Lin, L.; Cheng, Q.; Jiang, L. A Strong Bio-Inspired Layered PNIPAM-Clay Nanocomposite Hydrogel. Angew. Chem., Int. Ed. 2012, 51, 4676−4758.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.8b03025. 6233

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DOI: 10.1021/acsnano.8b03025 ACS Nano 2018, 12, 6228−6235