Opening of 'Hole–in–Cube ... - ACS Publications

Intentional Closing/Opening of “Hole-in-Cube” Fullerene Crystals with Microscopic Recognition Properties Partha Bairi,† Kosuke Minami,† Jonathan P. Hill,† Katsuhiko Ariga,*,†,‡ and Lok Kumar Shrestha*,† †

World Premier International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan ‡ Graduate School of Frontier Science, The University of Tokyo, Kashiwa, Chiba 277-0827, Japan S Supporting Information *

ABSTRACT: We report production of highly crystalline fullerene C70 cubes possessing an open−hole structure at the center of each of their faces using a solution-based self-assembly strategy. The holes are isolated with a solid core at the interiors of the cubes. The open-hole structure of the cubes can be intentionally closed by introducing additional C70 and reopened by applying electron beam irradiation. The open-hole cubes exhibit preferential recognition of graphitic carbon particles over polymeric resin particles of similar dimensions due to the cubes’ sp2-rich carboniferous nature. KEYWORDS: fullerene crystals, self-assembly, hollow structure, hole manipulation, microscopic recognition

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substantial solubility in a wide variety of aromatic and other organic solvents,24 although they have only very limited solubilities (almost insoluble) in aliphatic alcohols. In taking advantage of this important physical property, a variety of crystalline nanostructures of fullerene have been prepared using solution-based and other techniques such as static liquid−liquid interfacial precipitation (LLIP), re-precipitation, solvent vapor annealing, and drop-drying.25−31 Investigations have shown that controlled crystallization of fullerene C70 into a homogeneously shaped form is relatively difficult compared to its counterpart fullerene C60 due to its ellipsoidal molecular shape and the coexistence of energetically similar phases at room temperature.32 Under the optimized conditions of the LLIP method, one-dimensional (1D) C70 nanorods or nanotubes and three-dimensional (3D) C70 cubes have been observed. However, manipulable micron-sized hollow structures of C70 crystals have not been prepared.33−38 Here, we report the self-assembly of C70 molecules into cubeshaped crystals with a single open-hole structure (of micron size) on each face of the cubes. The open-hole cubes (OHcube) have a solid core at their interiors and can be intentionally closed by adding excess C70. Interestingly, the closed hole of the cubes could be reopened upon electron beam

he concept of atomic and molecular level manipulation has been established and extensively studied using welldefined systems, and various functional materials and nanosystems have been designed.1−5 However, the manipulation of micron-sized hollow objects remains a challenging task. There exists only a few examples of the successful manipulation of microscale hollow objects composed of metals or polymers, and these have been used in applications such as catalysis of CO oxidation or encapsulation and release of dyes, biomolecules, and functional nanoparticles.6,7 Controlled organization and directed manipulation of microscale hollow objects are highly desired in biomedical applications for the encapsulation, protection, transport, and release of nano/ micron-sized objects, especially cells, bacteria, biomolecules, and functional nanoparticles. It is not possible to achieve these functions using nano-sized hollow objects. Therefore, fabrication of hollow micron-sized objects with controllable morphology composed of functional molecules such as fullerene C70 would be very useful in specific applications such as protection of environmentally sensitive biological objects, cellular scaffolds, and biomedical carriers, as well as for environmental pollutant sequestration, etc.8,9 There is a burgeoning interest in the production of shapecontrolled nano- or micron-sized fullerene crystals as these crystals, in self-assembled forms, offer enhanced optoelectronic properties and innovative functionality.10−23 It should be noted that fullerenes are the only allotrope of carbon that possess any © 2017 American Chemical Society

Received: March 6, 2017 Accepted: July 25, 2017 Published: July 25, 2017 7790

DOI: 10.1021/acsnano.7b01569 ACS Nano 2017, 11, 7790−7796

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ACS Nano irradiation or by local electron beam irradiation at the faces of the cubes (Scheme 1). Scheme 1. Schematic Illustration of Microscopic Recognition of Graphitic Carbon Particles over Polymeric Resin Particles of Similar Dimensions by the 3D sp2-Rich Holes of the OH-Cube with Intentional Closing and Reopening of the Open-Hole Structure

Figure 1. (a,b) SEM images of fullerene C70 OH-cubes obtained by the dynamic LLIP method using a mesitylene/TBA volume ratio of 1:3. (c) TEM image of a single OH-cube showing the open-hole structure with unconnected holes. (d) HR-TEM image of OH-cube (inset: electron diffraction pattern). (e) Histogram of the edge size distribution of OH-cubes and (f) histogram of open-hole diameter/ cube edge size.

Furthermore, we have successfully demonstrated the microscopic recognition properties of OH-cube for graphitic carbon over polymeric resin particles of similar size (Scheme 1). The assembly of functional C70 molecules into a morphology possessing manipulable microscale hollow structures and the related recognition of micron-sized objects has not previously been explored. We believe that this material could be useful in biological applications for selective encapsulation, protection, and transport and release of micron-sized objects.

and Figure S3) reveal that the OH-cubes are not hollow and the holes are not mutually connected. There are solid cores at the interiors of the OH-cubes with holes extending 1−1.5 μm deep from their surfaces. Extended lattice fringes of fullerene C70 in the high-resolution transmission electron microscopy (HR-TEM) image (Figure 1d) and a spot type selected area electron diffraction (SAED) pattern (inset Figure 1d) indicate the highly crystalline nature of the OH-cubes. We have succeeded in intentionally manipulating the openhole structure of the C70 cube. The open-hole structure could be closed by adding excess C70 solution, and the closed-hole of the cube could be reopened by electron beam irradiation (Figure 2). To prepare CH-cubes, we first prepared the OH-cube, and then a freshly prepared C70 solution (0.5 mL, 1 mg/mL) was added to the reaction mixture containing OH-cube. SEM images showed that after addition of the additional C70 solution, all the open holes of the cubes were closed, resulting in the formation of CH-cubes (Figure 2e) with a negligible population of the OH-cubes (Figure S4). CH-cubes have sharp edges and a narrow edge length distribution (Figure S5) with average edge length of ca. ∼4.1 ± 0.4 μm, which is greater than that found for OH-cubes (ca. ∼3.4 ± 0.4 μm). CH-cubes could also be prepared by varying the mixing ratio of C70 solution in mesitylene and TBA. When the same volume of C70 solution in mesitylene (1 mL) was added into 2 mL of TBA, CH-cubes of similar dimensions were obtained (Figures S6 and S7). TEM

RESULTS AND DISCUSSION An open-hole cube (OH-cube) was produced using the dynamic liquid−liquid interfacial precipitation method with mesitylene and tert-butyl alcohol (TBA) as good and poor solvents, respectively, for fullerene C70. In a typical crystallization, a solution of C70 in mesitylene (1 mL, 1 mg/mL) was rapidly added into TBA (3 mL), and the mixture was incubated at 25 °C for 12 h. Details of synthetic methods for the production of OH-cube, closed-hole cube (CH-cube), and hole reopening of the CH-cubes are given in the Experimental Section. Scanning electron microscopy (SEM) images (Figure 1a,b and Figures S1 and S2b−e) of the crystallized fullerene C70 reveal cube-shaped crystals with a single open-hole structure on each face of the cubes. The narrow crystal edge length distribution of the OH-cube (Figure 1e) indicates homogeneous nucleation and crystal growth. OH-cubes have average edge lengths of ca. ∼3.4 ± 0.4 μm. The histogram of open-hole diameters normalized to the size of the OH-cubes also indicates a narrow size distribution (Figure 1f). TEM images (Figure 1c 7791

DOI: 10.1021/acsnano.7b01569 ACS Nano 2017, 11, 7790−7796

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process. First, a solid core (the small cubes) is formed almost instantaneously after addition of C70 solution into TBA. Second, this “seed” functions as the nucleus for further crystal growth into OH-cubes. Because the direct contact of fullerene C70 with TBA is energetically unfavorable, mesitylene forms droplets almost immediately after the addition of C70 solution into TBA.48,49 Diffusion of mesitylene into TBA occurs, promoting the nucleation of C70 as the small-sized cubic cores guided by the mesitylene molecules.36,48,49 If crystal growth is hindered in the {100} direction, then cubic cores tend to grow mostly along corner {111} and edge {110} directions, leading to the formation of the open-hole cube.40,50,51,52 When the mesitylene/TBA ratio is fixed at 1:2, C70 molecules cannot nucleate due to the lower concentration of anti-solvent TBA. Therefore, a newly formed cube with open-hole structure is covered by the remaining C70 present in the reaction medium, producing the CH-cube (Figure S6). As mentioned earlier, CH-cubes can also be formed by the addition of excess C70 solution into a suspension of freshly prepared OH-cubes; that is, excess C70 formed a thin layer on the surface of OH-cubes (Figure 2e). Because the C70 layer on the cube surface is very thin (∼700 nm), it is not stable when exposed to a high-power electron beam (30 kV) and readily peels off during exposure probably because local heating of the layer causes its distortion (by curling), and, because it is only weakly appended to the cube surface, it becomes detached, thus opening the hole (Figure S11). Structural characterization of the cubes (OH- and CH-cubes) was performed using powder X-ray diffraction (pXRD), Fourier transform infrared (FT-IR), and Raman scattering spectroscopy. In contrast to pristine C70 (pC70), which possesses a hexagonal close-packed structure15,52 with lattice parameters of a = 1.08 and c = 1.74 nm, the pXRD patterns of OH-cubes and CH-cubes (Figure 3a) could be indexed to a simple cubic (sc)

Figure 2. Schematic illustration of the intentional closing and reopening of the open-hole structure of a fullerene C70 OH-cube and the corresponding SEM images: (a,d) OH-cube, (b,e) CHcube, and (c,f) CH-cube with reopened-hole structure on the visible faces.

and STEM images further confirmed the CH-cube structure (Figure S8). The open hole of the C70 cube is covered with a thin layer (about 700 nm) of C70. Extended lattice fringes of fullerene C70 with a spot type of SAED pattern indicates the highly crystalline nature of the CH-cube (Figure S9). CH-cubes could be intentionally reopened upon local electron beam irradiation (30 kV) at the surface of a CHcube for a short period of time (∼5−10 s). During irradiation, the thin layer of C70 peeled off from the CH-cube surface with some visibly still attached to the cube surface, as can be seen in the SEM image (Figures 2f and S10b). Longer duration electron beam irradiation (>10 s) causes the complete removal of the thin layer from the cube surface (Figure S11d,f). Generally, solvent or chemical etching mechanisms are invoked to explain such morphological evolution in inorganic cubic-shaped nanocrystals, where dissolution occurs from the faces of solid cube-shaped crystals depending on the surface energy and synthesis conditions.39−46 However, this is unlikely to be the case for our organic C70 crystals. If a solvent etching mechanism resulting in hole formation were operating, then the diameters of the open holes of the cubes ought to increase with time, and further washing with fresh solvent (mesitylene−TBA) mixture would result in a cubic frame-like structure.47 In the case of OH-cubes, the dimensions of the open-hole structure remain unchanged, and the cavities do not become connected (i.e., a hollow interior is not formed) even after 10 days of storage in the mother liquor (Figure S12). The edges of the OH-cubes become sharper following washing with fresh mesitylene−TBA solvent mixture with essentially no significant variation in the diameters of the open holes of the cubes (Figure S13). TEM imaging of crystals formed immediately (within a few seconds) after addition of a C70/mesitylene solution (1 mL) into TBA (3 mL) revealed the formation of smaller solid cubes lacking open-hole structures, and only a few OH-cubes are visible (Figure S14). The average edge lengths of these instantaneously formed cubes without holes is in the range of ca. 1−1.5 μm, which corresponds closely with the dimensions of the solid core of the OH-cubes formed 1 min after the addition of C70/mesitylene solution in TBA (Figure S15). Cross-sectional observation of a single OH-cube after focused ion beam sectioning clearly shows a solid core with dimensions around 1.5 μm (Figure S16). This suggests that the open-hole formation is not driven by a solvent etching mechanism but that growth of OH-cubes involves a two-step

Figure 3. (a) pXRD patterns of pC70, OH-cubes, and CH-cubes. (b) Corresponding FT-IR.

structure with a cell dimension of a = 1.05 nm.15,37 Due to the D5h symmetry group, 31 vibrational bands (21E1′ + 10A2″) are expected to be present in the infrared spectrum of fullerene C70.53,54 However, experimentally, all expected active modes could not be observed. The FT-IR spectrum of pC70 showed bands characteristic of C70 molecules at 458, 535, 564, 577, 642, 673, 795, 1086, 1134, 1250, 1291, 1320, and 1431 cm−1 without 7792

DOI: 10.1021/acsnano.7b01569 ACS Nano 2017, 11, 7790−7796

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ACS Nano any IR bands in the region of 2600 to 3600 cm−1. FT-IR spectra of as-prepared OH-cubes and CH-cubes also showed absorption peaks characteristic of C70 molecules. However, four additional FT-IR bands appeared in the range of 3200− 2600 cm−1 (Figure 3b). These bands correspond to the aromatic and aliphatic C−H stretching vibrations of mesitylene.55 The FT-IR band at 1371 cm−1 (CH3 symmetrical def.) is also due to mesitylene. Thus, FT-IR spectra confirmed that both OH- and CH-cubes are solvated solids containing solvent molecules in the crystal. Infrared bands assignable to the TBA molecule were not observed, confirming that TBA does not take part in crystallization, with only mesitylene participating as an auxiliary lattice component of the solid solvate. Thermogravimetric analyses show that weight losses commence above 80 °C and continue to ∼230 °C for both OH- and CH-cubes (Figure S17a). Weight losses due to the removal of mesitylene are ca. 20.3% (OH-cube) and 22.7% (CH-cube), suggesting that the ratio of mesitylene to C70 is in the range of 1.83−2.05; that is, the stoichiometry of OH- and CH-cube crystals is approximately C70·(mesitylene)2.37 Raman spectroscopy is a useful tool for the analysis of the structures of carbon materials.56,57 The D5h symmetry group of the C70 molecule allows 53 bands in the Raman spectrum. Raman scattering spectra of OH- and CH-cubes are identical to that of pC70 (Figure S17b), and a Raman peak shift was not observed. This suggests that the influence of solvent molecules on the rotation of C70 molecules is negligibly small in the solid solvates; for example, rotation of the fullerene C70 molecules is free in the crystal. Microscopic recognition of nano/micron-sized particles using supramolecular forces is very challenging, although molecular level recognition has been successfully demonstrated.58−60 Microscopic recognition offers great opportunities in the development of materials with advanced functions, which could be beneficial in addressing several frontier problems in a wide range of fields from environment to medicine.59 In this regard, we have succeeded in demonstrating the microscopic preferential recognition properties of OH-cubes for graphitic carbon particles over resorcinol−formaldehyde polymeric resin particles of similar dimensions. SEM observations reveal that most of the open holes of OH-cubes are occupied by the graphitic carbon particles (Figure 4a,b). Conversely, the open holes of the OH-cubes are not well occupied by the resin particles (Figure 4c,d). Only a few of the OH-cubes’ holes are

occupied by the resin particles, which could be a result of weaker interactions between polymeric resin particles and OHcube holes compared to those with the carbon particles. Note that, although loading increases with the concentration of both carbon and resin particles, loading efficiency of carbon particles is much higher than that of the resin particles at all concentrations studied (Figure 4e). This demonstrates the greater recognition ability of the holes of the OH-cubes toward graphitic carbon particles over the resin particles, perhaps caused by strong π−π interaction between 3D sp2 carbon-rich open holes and graphitic carbon particles. OH-cubes are prepared from conjugated π-electron-rich sp2 carbon source fullerene C70, so that they are expected to contain a conjugated π-electron surface. Therefore, it is expected that OH-cubes interact with graphitic carbon particles (sp2 carbon with conjugated π-electrons) through π−π interactions (supramolecular interaction). On the other hand, as the resorcinol− formaldehyde polymeric resin particles have lower sp2 character compared to that of graphitic carbon (Figure S18), strong π−π interaction is not expected between the resin and OH-cubes.

CONCLUSION In conclusion, we have prepared self-assembled fullerene C70 cube-shaped crystals with an open-hole structure at the center of each face using a dynamic precipitation method. Holes at the cubes’ surfaces are isolated with a solid core at the centers of the cubes. We have successfully demonstrated intentional manipulation of the open-hole structure where holes were closed by the growth of a thin sheet of fullerene with subsequent reopening by local irradiation using an electron beam. Additionally, we have succeeded to demonstrate microscopic recognition properties of the open holes of the cubes, which select graphitic carbon particles over the resorcinol−formaldehyde resin particles of similar dimensions because of strong π−π interactions between sp2 carbon-rich open holes of fullerene C70 and the graphitic carbon particles. We believe that these sp2 carbon-rich fullerene cubes with multiple micron-sized three-dimensional cavities can be used to develop important materials for micro/nano-encapsulation processes and so could be very useful in many applications such as controlled release of drugs, cosmetics, pigments, protection of biologically active species, and removal of pollutants. Cubes with an open-hole structure with the possibility of intentionally closing and reopening of the hole structure could be useful in smart selective drug/or biomaterial delivery applications. For example, hydrophobic drugs (or other guests) could be selectively loaded into the holes followed by covering with excess C70, thus protecting the guest from the external environment prior to its release upon exposure to an electron beam. EXPERIMENTAL SECTION Synthesis of OH-Cubes and CH-Cubes. Fullerene C70 cube with open holes (OH-cubes) and closed holes (CH-cubes) were synthesized using the dynamic LLIP method at 25 °C using mesitylene as a good solvent and TBA as an anti-solvent for C70. Pristine fullerene C70 (pC70) (30 mg) was dissolved in mesitylene (30 mL) by applying sonication in a water bath for about 1 h. Following sonication, the solution of C70 in mesitylene was collected by filtration and stored at 25 °C. For the synthesis of the OH-cube, TBA (3 mL) was placed in a clean and dry glass vial (13.5 mL) and C70 solution (1 mL) was added rapidly (using a micropipette). The mixture was then stored in an incubator at 25 °C. The solid precipitate (OH-cubes) was collected after 12 h using centrifugation. It should be noted that addition of

Figure 4. (a−d) SEM images showing different loading capacities of the OH-cubes toward carbon and polymer particles (0.05 mg/mL). (e) Plots of number of holes containing particle(s)/total number of holes vs particle concentration. 7793

DOI: 10.1021/acsnano.7b01569 ACS Nano 2017, 11, 7790−7796

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ACS Nano TBA (3 mL) into an aliquot of the same C70 solution (1 mL) resulted in the formation of a mixture of OH-cubes and nanorods. CH-cubes were prepared by closing the open-hole of the OH-cubes using additional C70. In a typical CH-cube synthesis, we added extra C70 solution (0.5 mL/1 mg/mL) into freshly prepared OH-cubes contained in their mother liquor (mesitylene/TBA = 1:3). The mixture was then gently shaken by hand for about 30 s and kept in an incubator at 25 °C for 12 h. The CH-cube could also be prepared by altering the mixing ratio of C70 solution in mesitylene and TBA. When C70 solution in mesitylene (1 mL) was added into only 2 mL of TBA, we found CH-cubes with dimensions similar to those obtained upon addition of additional C70 to OH-cubes. Loading of Particles into the Holes of OH-Cubes. Different concentrations of carbon or resin particles were added into a suspension of OH-cubes in a mesitylene/TBA mixture (1:3). The mixture was shaken by hand for approximately 1 min and stored at room temperature without agitation for 10 min. Characterizations. SEM images of OH-cubes, CH-cubes, and particle-loaded OH-cubes were obtained using a Hitachi model S-4800 field effect SEM operating at an accelerating voltage of 10 and 30 kV. SEM samples were prepared by dropping the suspension of cubes onto clean, dried silicon wafers followed by drying under reduced pressure at 60 °C. In order to avoid charging during SEM imaging, all OH-cube samples were coated with platinum (∼2 nm) by sputtering using a Hitachi S-2030 ion coater. TEM images and SAED patterns were obtained using a transmission electron microscope (JEOL model JEM2100F operated at 200 kV). TEM samples were prepared by dropping suspensions of OH- and CH-cubes onto standard carbon-coated copper grids. TEM samples were dried at 60 °C under reduced pressure for 12 h prior to TEM observations. FT-IR spectra of pC70 and OH- and CH-cubes were recorded using a Nicolet 4700 FT-IR instrument (ThermoElectron Corporation). Thermogravimetric analyses of the cubes were performed using a SII Instrument (model Exstar 600) under an argon gas atmosphere at a heating rate of 10 °C min−1. Powder X-ray diffraction patterns were recorded at 25 °C on a Rigaku RINT2000 diffractometer with Cu Kα radiation (λ = 0.1541 nm). Raman scattering spectra of pC70, OH- and CH-cubes, graphitized carbon particles, and resorcinol−formaldehyde polymeric particles were recorded using Jobin-Yvon T64000 Raman spectrometer. Raman samples were prepared by drop-casting suspensions of samples onto a clean silicon wafer (0.7 cm × 0.7 cm) followed by drying at 60 °C under reduced pressure. Samples were excited using a green laser of wavelength of 514.5 nm and 0.01 mW power.

ACKNOWLEDGMENTS This work was partially supported by JSPS KAKENHI Grant Number JP 16H06518 (Coordination Asymmetry) and CREST JST Grant Number JPMJCR1665. REFERENCES (1) Nakaya, M.; Tsukamoto, S.; Kuwahara, Y.; Aono, M.; Nakayama, T. Molecular Scale Control of Unbound and Bound C60 for Topochemical Ultradense Data Storage in an Ultrathin C60 Film. Adv. Mater. 2010, 22, 1622−1625. (2) Custance, O.; Perez, R.; Morita, S. Atomic Force Microscopy as a Tool for Atom Manipulation. Nat. Nanotechnol. 2009, 4, 803−810. (3) Meyer, G.; Moresco, F.; Hla, S. W.; Repp, J.; Braun, K. − F.; Folsch, S.; Rieder, K. H. Manipulation of Atoms and Molecules with the Low-Temperature Scanning Electron Microscopy. Jpn. J. Appl, Phys. 2001, 40, 4409−4413. (4) Avouris, P. Manipulation of Matter at the Atomic and Molecular Levels. Acc. Chem. Res. 1995, 28, 95−102. (5) Ariga, K.; Ji, Q.; Nakanishi, W.; Hill, J. P.; Aono, M. Nanoarchitectonics: A New Materials Horizon for Nanotechnology. Mater. Horiz. 2015, 2, 406−413. (6) Mandal, S.; Sathish, M.; Saravanan, G.; Datta, K. K. R.; Ji, Q.; Hill, J. P.; Abe, H.; Honma, I.; Ariga, K. Open-Mouthed Metallic Microcapsules: Exploring Performance Improvements at Agglomeration-Free Interiors. J. Am. Chem. Soc. 2010, 132, 14415−14417. (7) Im, S. H.; Jeong, U.; Xia, Y. Polymer Hollow Particles with Controllable Holes in Their Surfaces. Nat. Mater. 2005, 4, 671−675. (8) Nakamura, E.; Isobe, H. Functionalized Fullerenes in Water. The First 10 Years of Their Chemistry, Biology, and Nanoscience. Acc. Chem. Res. 2003, 36, 807−815. (9) Nudejima, S.; Miyazawa, K.; Okuda-Shimazaki, J.; Taniguchi, A. Observation of Phagocytosis of Fullerene Nanowhiskers by PMATreated THP-1 Cells. J. Phys.: Conf. Ser. 2009, 159, 012008. (10) Minami, K.; Kasuya, Y.; Yamazaki, T.; Ji, Q.; Nakanishi, W.; Hill, J. P.; Sakai, H.; Ariga, K. Highly Ordered 1D Fullerene Crystals for Concurrent Control of Macroscopic Cellular Orientation and Differentiation toward Large-Scale Tissue Engineering. Adv. Mater. 2015, 27, 4020−4026. (11) Krishnan, V.; Kasuya, Y.; Ji, Q.; Sathish, M.; Shrestha, L. K.; Ishihara, S.; Minami, K.; Morita, H.; Yamazaki, T.; Hanagata, N.; Miyazawa, K.; Acharya, S.; Nakanishi, W.; Hill, J. P.; Ariga, K. VortexAligned Fullerene Nanowhiskers as a Scaffold for Orienting Cell Growth. ACS Appl. Mater. Interfaces 2015, 7, 15667−15673. (12) Shrestha, R. G.; Shrestha, L. K.; Khan, A. H.; Kumar, G. S.; Acharya, S.; Ariga, K. Demonstration of Ultrarapid Interfacial Formation of 1D Fullerene Nanorods with Photovoltaic Properties. ACS Appl. Mater. Interfaces 2014, 6, 15597−15603. (13) Wu, K.-Y.; Wu, T.-Y.; Chang, S.-T.; Hsu, C.-S.; Wang, C.-L. A Facile PDMS-Assisted Crystallization for the Crystal-Engineering of C60 Single-Crystal Organic Field-Effect Transistors. Adv. Mater. 2015, 27, 4371−4376. (14) Wei, L.; Yao, J.; Fu, H. Solvent-Assisted Self-Assembly of Fullerene into Single-Crystal Ultrathin Microribbons as Highly Sensitive UV−Visible Photodetectors. ACS Nano 2013, 7, 7573−7582. (15) Bairi, P.; Minami, K.; Nakanishi, W.; Hill, J. P.; Ariga, K.; Shrestha, L. K. Hierarchically Structured Fullerene C70 Cube for Sensing Volatile Aromatic Solvent Vapors. ACS Nano 2016, 10, 6631− 6637. (16) Saran, R.; Stolojan, V.; Curry, R. J. Ultrahigh Performance C60 Nanorod Large Area Flexible Photoconductor Devices via Ultralow Organic and Inorganic Photodoping. Sci. Rep. 2015, 4, 5041. (17) Miao, Y.; Xu, J.; Shen, Y.; Chen, L.; Bian, Y.; Hu, Y.; Zhou, W.; Zheng, F.; Man, N.; Shen, Y.; Zhang, Y.; Wang, M.; Wen, L. Nanoparticle as Signaling Protein Mimic: Robust Structural and Functional Modulation of CaMKII upon Specific Binding to Fullerene C60 Nanocrystals. ACS Nano 2014, 8, 6131−6144. (18) Wakahara, T.; D’Angelo, P.; Miyazawa, K.; Nemoto, Y.; Ito, O.; Tanigaki, N.; Bradley, D. D. C.; Anthopoulos, T. D. Fullerene/Cobalt

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b01569. Additional SEM, TEM, and HR-TEM images, SIM images, histogram cube size distributions, thermogravimetric plot, and Raman data (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Kosuke Minami: 0000-0003-4145-1118 Jonathan P. Hill: 0000-0002-4229-5842 Katsuhiko Ariga: 0000-0002-2445-2955 Lok Kumar Shrestha: 0000-0003-2680-6291 Notes

The authors declare no competing financial interest. 7794

DOI: 10.1021/acsnano.7b01569 ACS Nano 2017, 11, 7790−7796

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DOI: 10.1021/acsnano.7b01569 ACS Nano 2017, 11, 7790−7796

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DOI: 10.1021/acsnano.7b01569 ACS Nano 2017, 11, 7790−7796