Self-Assembled Fullerene Nanostructures Lok Kumar Shrestha1＊ , Rekha Goswami Shrestha2, Jonathan P

REVIEW Self-Assembled Fullerene Nanostructures Lok Kumar Shrestha1＊ , Rekha Goswami Shrestha2, Jonathan P. Hill1 and Katsuhiko Ariga1, 3 1 World Premier International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044 JAPAN 2 ‌Department of Pure and Applied Chemistry, Faculty of Science and Technology, Tokyo University of Science 2641 Yamazaki, Noda, Chiba 278- 8510, JAPAN 3 PRESTO & CREST, JST, 1-1 Namiki, Tsukuba 305-0044 JAPAN

Abstract: This review briefly summarizes recent developments in fabrication techniques of shape-controlled nanostructures of fullerene crystals across different length scales and the self-assembled mesostructures of functionalized fullerenes both in solutions and solid substrates.

Key words: ‌fullerene, self-assembly, nanostructures, liquid-liquid interfacial precipitation, nanowhiskers, nanotubes, nanosheets

1 INTRODUCTION compose an icosohedra（l Ih）symmetric closed cage struc-

Design of nanostructured materials whose properties ture of the C60 molecule（diameter～0.8 nm）. In C60, each can be tailored across different length scales so that they carbon atom is bonded to three other carbon atoms can be utilized in different functional systems and nanode- through sp2 hybridized bonds with the tendency for double vices fabrication is a current topic of great interest in the bonds not to be present at the pentagonal rings resulting in

ﬁeld of materials nanoarchitectonics. Self-assembly is one poor electron delocalization, i.e. C60 is not a superaromatic of the special techniques applied for the organization of molecule. As a result, it behaves as an electron deficient functional molecules such as fullerenes, amphiphiles, pro- molecule. The electron-accepting capability of C60 both in teins and peptides with molecular level precision in the solid state and solution have permitted special uses in the preparation of functional systems. This technique enables formation of charge-transfer complexes and bulk hetero- the arrangement of building blocks into a variety of nano- junctions with suitable donors for the production of photo- structures on the micro to macroscopic length scales. current. The formation of charge-transfer salts with a Using the concept of materials nanoarchitectonics in com- number of donor groups or doping with metals has led to 4） 5） bination with self-assembly, various building units have ferromagnetic or superconducting materials . C60 exhibits been assembled or organized/reorganized into hierarchic strong absorption bands in the UV region and weaker but functional nanostructures. Assembly of the ideal zero-di- significant bands in the visible region. Functionalized C60 mensional building block fullerene（C60）into higher dimen- retains these characteristics but, in addition, the absorp- sional crystalline nanostructures and assembly of function- tion of the derivatives extends further into the near-IR alized fullerenes into different mesostructures are good region demonstrating that C60 and its derivatives are very examples of the materials nanoarchitectonics concept. easily excited by low-energy light. This, combined with the 1） Soon after its discovery in 1985 , C60 received tremen- fullerenes’ high electron afﬁnities, makes these materials dous attention due to its unique structure and properties. appealing for use in photoinduced electron transfer6） and 7） After several years of intensive research activity, C60 has also for applications in medicinal chemistry . Guldi and regarded as a molecule having tremendous potential as a Prato8） have recently reviewed the excited-state properties building block for molecular engineering, novel materials of C60 derivatives. synthesis, supramolecular chemistry, and also medicinal C60 has also demonstrated excellent capabilities in the chemistry2, 3）. The 20 hexagonal and 12 pentagonal rings quenching of various free radicals compared to conven-

＊Correspondence to: Lok Kumar Shrestha, World Premier International Center for Materials Nanoarchitectonics (WPI-MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba 305-0044 JAPAN E-mail: [email protected] Accepted April 25, 2013 (received for review March 31, 2013) Journal of Oleo Science ISSN 1345-8957 print / ISSN 1347-3352 online http://www.jstage.jst.go.jp/browse/jos/ http://mc.manusriptcentral.com/jjocs

541 L. K. Shrestha, R. G. Shrestha, J. P. Hill et al.

9） 27, 28） tional antioxidants . Many fullerene-based compounds duction of tailored nanostructured crystalline C60 . Mi- have shown tremendous practical values biological yazawa and coworkers have developed a liquid-liquid inter- 10） systems although the highly hydrophobic character of C60 facial precipitation（LLIP）method for growing low （it is water insoluble）limits its application in biomaterials’ dimensional nanowhiskers or hollow tubular structures ﬁeld. This problem has been addressed by many materials from solution under ambient conditions29, 30）. It should be chemists and water soluble fullerenes have been produced noted that if diameters of the 1D rod structure is in the by appropriate substitution or functionalization with ionic range of hundreds of nanometers to a few microns and and nonionic functional groups11）. Furthermore, amphiphil- lengths reach several hundreds of microns then they are

ic fullerene derivatives have also been produced that can referred to as C60 nanowhiskers. In contrast to CNTs, C60 self-assemble into different mesostructures in solution or nanowhiskers do not contain any internal hollow spaces. 12） on a substrate . Water-soluble fullerene derivatives led to Rather, whiskers are composed of many C60 molecules the discovery of the interactions between organofullerenes bonded through a combination of van der Waals interac-

and DNA, proteins, and living cells. C60 can also be encap- tions and chemical bonds. Miyazawa et al. have also ex- sulated in supramolecular structures containing a host tended this method to produce 2D hexagonal and polygo- moiety, such as cyclodextrin, surfactants, gels, or poly- nal shaped nanosheets31, 32）. Very recently, we extended mers13）. this method to produce giant microcrystals with macro- Needless to say signiﬁcant advances in fullerene chemis- pores at their surfaces and also succeeded in fabricating, try have lead to a number of interesting developments in- for the first time, hexagonal-shaped crystalline fullerene cluding carbon nanotube（s CNTs）14）. This functional mole- with bimodal pore structures33, 34）. Furthermore, we utilized cule requires assembly into well-ordered one dimensional this method for the unusal formation of 3D highly crystal-

（1D）or two dimenaiona（l 2D）forms to promote its elec- line cubic-shape crystals of C60-fullerene-Ag（I）organome-

tronic and optical properties and to construct electronic or tallic heteronanostructure［C6｛0 AgNO3｝5］, which showed photonic devices15－17）. It is therefore essential to know the irreversible structural rearrangement upon gentle washing

intermolecular ordering of C60 molecules at the micro and with aliphatic alcohols resulting in the formation of unique-

macroscopic level for the production of dimension-con- ly structured formations of well-oriented C60 microcrys- 18） 35） trolled nanostructures of C60 . tals . In this review, we brieﬂy summarize the recent develop- In solution-based crystal formation methods, the crystal ment of the production of low to higher dimensional nano- formation mechanism is suggested to be driven by super-

structured fullerene（C60）crystals over different length saturation related to the low C60 solubility in alcohols

scales and focussing on solution-based approaches and dif- （known as an antisolvent or a poor solvent for C60）. There-

ferent self-assembled mesostructures of functionalized fore, the solubility of C60 is a key parameter for good and fullerenes. poor solvent selection. Rods or needles grow in arbitrary directions through the LLIP process at the interface of sat-

urated C60 in toluene and isopropyl alcoho（l IPA）. In depth investigations on the production of structure controlled 2 NANOSTRUCTURED CRYSTALLINE FULLERENES crystalline fullerene have conﬁrmed the fact that the mor-

Nano-sized C60 crystals have received much attention phology and size of crystalline assembly of C60 largely

and are considered to be intermediate states between mol- depend on the synthetic route, concentration of C60 in solu-

ecules and bulk materials. Hierarchical assembly of C60 into tion, crystallization temperature, and mixing ratio of anti- nanocrystal（s or nanoparticles）offers unique properties solvent and solvent particularly in the precipitation with potential to be used in device fabrication. Bulky parti- method. Formation of 1D rods and 2D plate-like morpholo-

cles without any uniﬁed shape of pristine C60 have the in- gies from slow evaporation of C60 solution and vapor depo- teresting property of forming crystalline assemblies of sition, respectively, can be taken as a good example. In the various well-deﬁned morphologies and sizes depending on following section, we discuss recent advances for the lower the method of molecular assembly. Various synthetic ap- to higher dimensional nanostructured crystalline fullerene proaches have been explored to make 1D or 2D nanostruc- by solution-driven assembly methods.

tures of C60. Templating, slow evaporation, and vapor-solid A very simple method for producing so-called zero-di-

processes are the commonly employed methods to produce mensional C60 nanoclusters is the solvent-exchange method 19－24） C60 crystals . Similarly other processes, such as layer- in which C60 molecules dissolved in a good solvent assemble by-layer assembly and electrophoretic deposition have also into particle type（pseudo sphere or irregular shape parti- been applied to assemble fullerene nanoparticles25, 26）. cles）nanocrystals upon solvent removal or during solvent In addition to the aforementioned methods, precipitation exchange. Recently this method has been extended to the

either in the bulk or at interface is also important and is fabrication of higher dimensional C60 nanocrystals by opti- the most extensively employed method used for the pro- mizing synthetic conditions. Work of Deguchi et al.36） can

542 J. Oleo Sci. 62, (8) 541-553 (2013) Self-Assembled Fullerene Nanostructures

be taken as a good example of the formulation of stable pared by a drop-drying process at room temperature. They aqueous dispersions of fullerene（s C60 and C70）consisting of found a critical correlation between the solvent geometry nearly spherical shaped nanoclusters by simply injecting a and the geometry of the self-assembled C60 crystals. Al- saturated solution of fullerene in tetrahydrofuran（THF） though a clear mechanism of such crystal formation was into water followed by THF removal by purging gaseous ni- not clear, they found that pseudo nD solvent guides C60 trogen. At that time, it was the first example of the stable molecules to self-assemble into n-1D morphology. That is, dispersion of C70 in water. From transmission electron mi- pseudo 3D（p3D）solvents guide C60 molecules to self-as- croscopy（TEM）observations they found fairly monodis- semble into p2D hexagonal disk structures, and that p2D perse clusters of C60 and C70 displaying low dimensional and p1D solvents result in p1D C60 wire and p0D C60 dot morpholog（y see Fig. 1）. High resolution transmission elec- structures, respectively. They found that when a C60 solu- tron microscopy（HR-TEM）revealed the polycrystalline tion in p3D tetrahedral carbon tetrachloride was dropped nature of the clusters. They found that the dispersion is and dried, 2D hexagonal nanosheets were observed. On the very easy to perform and exhibits excellent colloidal stabil- other hand, when a C60 solution in linear chain n-hexane ity without any stabilizing agent. No precipitation was ob- （p1D structure）was drop-dried, spherical 0D C60 crystals served even after storage in the dark at room temperature were observed. over long period（s at least 9 months of preparation）. It was We noticed that morphology of fullerene crystals fabri- found that the surface of the cluster is negatively charged cated using solvent-exchange method is not very well and the electrostatic repulsion between the negatively defined in particular for lower or 0D morphology. Most re- charged cluster surfaces is important for the stability of the ported examples have been mostly particle type and were dispersions. The method proposed by Deguchi et al. is a not of perfect 0D morphology. Masuhara and coworkers40） simple route to produce stable dispersions of colloidal C60 have recently reported fine fullerene crystals with unique and C70 in water, which are certainly suitable for the study shapes including perfect 0D fullerene nanoballs using a re- of biomedical applications of fullerenes. precipitation method. The 0D spherical nanostructures In solvent-exchange methods, THF is the most common- having average size～100 nm were fabricated by injecting ly employed solvent for the production of colloidal disper- 200 μL of 1 mM C60 solution in m-xylene into 10 mL of IPA sions of C60 nanocrystals. Rapid mixing of a THF solution of solution at 353 K and aging the mixture at room tempera-

C60 into water followed by evaporative removal of the THF ture（see Fig. 2a）. On the other hand, nanoballs having controls the particle size by varying the liquid-phase addi- sizes in the range of 300-500 nm were fabricated by inject- tion rate. Various other solvents have been used with or ing 200 μL of C60 solution in pyridine（0.3 mM）into 10 mL without THF for the production of sub-100 nm low dimen- ethanol solution at room temperatur（e Fig. 2b）. The nano- sional C60 nanoclusters. Other examples can be found else- spheres and nanoballs were apparently monodisperse. where37, 38）. Recently Park et al.39） investigated the critical Herein, we discuss 1D fullerene nanowhiskers and nano- effect of solvent geometry on the morphology of C60 pre- tubes. Fullerene nanowhiskers are generally rodlike or needlelike 1D crystals having diameters＜1 μm and length several tens of μm to hundreds of nm. These structures are

the most investigated nanostructures of C60 and have been prepared by several different approaches. Slow evaporation is one of the methods used to produce 1D nanorods. In this

method, C60 solutions in organic solvents are evaporated at a controlled rate at a particular temperature. The aspect ratio can be controlled by adjusting the solvent evaporation

Fig. 1 Low dimensional fullerene nanocrystals fabricated by the so-called solvent exchange method:

(a) TEM image C60 nanocrystal, and (b) the Fig. 2 Schematic model demonstrating 0D fullerene

same for C70. Reproduced with permission from nanostructures: (a) nanospheres obtained from Langmuir 17, 6013-6017 (2001). © (2001) IPA/m-xylene system, and (b) nanoballs fabri- American Chemical Society. cated from ethanol/pyridine systems.

543 J. Oleo Sci. 62, (8) 541-553 (2013) L. K. Shrestha, R. G. Shrestha, J. P. Hill et al.

kinetics, physical properties of solvent（s vapor pressure, solubility of C60 etc.）, temperature, etc. Usually, very long high aspect ratio 1D rods of C60 crystals can be synthesized using this method. Bulk precipitation and drop drying method also yields 1D crystal. Volume ratio of poor solvents with C60 solutions, concentration of C60, and solvents nature have been pointed out as crucial parameters in the control of the dimensions of 1D rods prepared by the precipitation method. On the other hand, as mentioned earlier, in the drop-drying method solvent structure is crucial to control the geometry of C60 crystals produced. Drop-drying Fig. 3 A body-centered tetragonal (bct) model of 1D of C60 solution prepared in so-called p2D solvents usually C60 nanowhisker polymerized via “2 + 2” cyclo- results in the formation of p1D C60 wires. For example, 1D addition of C60 molecules. Reprinted with per- C wires were formed from a C solution prepared using 60 60 mission from J. Mater. Res. 17, 83-88 (2002), © m-xylene, a representative p2D benzene series solvent 2002, Materials Research Society. with methyl functional groups meta to each other. Similar

1D C60 wires were obtained upon drop-drying C60 solutions cules in C60 nanowhiskers was found to be shortened by a of mesitylene and 1,3-dichlorobenzene, which have similar few percent along the growth axis than that of pristine C60 structures to m-xylene, except for the multiplicity and crystals based on a transmission electron microscope type of functional groups. These results of Park et al.39）（TEM）observations. This shortening is anticipated to be conﬁrmed that C60 molecules are speciﬁcally guided to self- caused by the polymerization of C60 caused by electron assemble into 1D morphologies upon evaporation of beam irradiation. benzene series solvents, as long as they have functional These C60 nanowhiskers usually have a face-centered groups 120° apart, regardless of the polarity, size and cubi（c fcc）crystal structure with growth direction parallel number of the functional groups. The method of Park and to［110］and the overall morphology of C60 crystals is found coworkers may provide guidelines for the production of to depend on the combinations of antisolvent and solvent, multidimensional fullerene nanostructures required for their volume ratios, and also the presence of water41）. nanodevice fabrication. However, the solvated hexagonal close packed（hcp）struc-

Recently, use of the LLIP method is becoming more ture of C60 nanowhisker has also been occasionally ob- widespread for the production of nanostructures of C60 served depending on the combination of antisolvent and particularly for growth of 1D nanowhisker of C60 crystals solvent. Solvated crystals grown at an interface of IPA with due to the ease of synthesis under ambient conditions. a saturated solution of C60 in m-xylene can be taken as one 42） Furthermore, the possibility of doping various elements example . These solvated hcp C60 nanowhiskers were into nanowhiskers and easy production of multicomponent transformed into fcc structure upon evaporation of solvent/ fullerene nanowhiskers composed of different kinds of or heating the sample. Minato and Miyazawa pointed out fullerene molecules are another advantage of the LLIP that the kinetics of hcp-to-fcc structure transformation method. The basic concept of the LLIP method is to use a depend on vapor pressure of solvent; higher vapor pres- liquid-liquid interface as nucleation site for crystals. Mi- sures lead to more rapid transformation. For example, the yazawa et al.29） used an interface between a concentrated hcp to fcc structural transformation occurred more rapidly solution of C60 in toluene and isopropyl alcoho（l IPA）to in toluene than in m-xylene. Entrapment of solvent mole- produce a needlelike morphology of C60 with single crystal- cules in the crystals in the initial production stage is ex- line structure having diameters of about 250 nm and pected to be responsible for the formation of hexagonal lengths in the range of several tens of microns. The largest solvated C60 nanowhiskers. Solvents could be easily ratio of length to diameter reached 4000. It has been found removed by heat treatment. that the intermolecular distance within C60 nanowhiskers is Investigations have shown that fabrication of fullerene shorter along the growth axis than in pristine C60 crystals nanotubes is rather difficult compared to fullerene indicating the formation of strong bonding between C60 nanowhiskers. Generally, two types of tubular structures of molecules. It was anticipated that the fullerene nanowhis- fullerene crystals have been observed. One is partially kers polymerize via a“ 2＋2” cycloaddition in the close- hollow around the edge of 1D structures and the other is 29） packed［110］direction . An initial model of C60 nanowhis- hollow throughout. Masuhara et al. found that, in addition kers where the C60 molecules are chained by the“ 2＋2” to the crystal size, overall concentration of C60 also causes cycloaddition along the whisker growth axis was proposed the formation of a hollow structure although the tubes by Miyazawa and is presented in Fig. 3. This model is based were short～2-5 μm. They observed hollow structures on the fact that the intermolecular distance of C60 mole- upon injecting 300 μL C60 solution in m-xylen（e 2 mM）into

544 J. Oleo Sci. 62, (8) 541-553 (2013) Self-Assembled Fullerene Nanostructures

10 mL of 2-propanol, otherwise, solid C60 rods were ob- 43） served upon injection C60 solution below 300 μL. Ji et al. also observed single crystalline submicrometer C60 tubes with highly uniform size by using a solvent-induced and surfactant assisted self-assembly technique. Although they pointed out that length and length-to-width ratio of the tubes can be controlled by varying the concentration of C60 in the stock solution and the mixing ratio of antisolvent and C60 solution. Lengths of nanotubes were found to be in the range of a few micron（s ～5 μm）. A major contribution to the production of long fullerene nanotubes came from Miyazawa and coworkers. They fabricated very lon（g length in the range of few hundreds microns）single crystalline nanotubes of not only C60 but also of C70 using LLIP method. They also found that the C60 nanotubes sometimes grow to extreme lengths into the millimeter length scale with outer diameters of several hundreds of nanometers and inner diameters of several tens of nanometers to a few hundreds of nanometers. These nanotubes showed good uniformity and linearity. The same group succeeded to fabricate vertically aligned fullerene microtubes by slowly injecting IPA solution into a saturated solution of C60 in toluene through an alumina membrane44）. The length of the vertically aligned C60 microtubes reached about 500 μm and their planar density and diameter could be controlled by changing the growth conditions, such as the injection rate of IPA and the amount of the C60 solution.

Regarding the dimensions of 1D nanostructures of C60 Fig. 4 SEM images of C60 rods (a-d) and tubes (e-g) at crystals, usually volume ratio of antisolvent and C60 solu- different concentration of C60 in m-xylene and at tion and concentration of C60 in the stock solution play a different volume ratio of IPA and C60 solution: crucial role. At fixed volume ratio, increasing C60 concen- (a) Typical SEM images of C60 rods at 1:1 vol- tration decreases the length and width of derived C60 rods –1 43） ume ratio and 0.75 mgmL C60 in m-xylene, (b) and tubes. Ji et al. carefully addressed the effect of C60 –1 concentration and volume ratio of antisolvent and C solu- C60 rods from 1.0 mgmL C60 in m-xylene, (c) 60 low and (d) high magnification SEM images of tion on the dimensions of 1D C60 crystals using IPA and m- –1 from 1.2 mgmL C60 in m-xylene, (e) SEM im- xylene systems. At fixed 1:1 volume ratio of IPA and C60 so- ages of C tubes obtained at volume ratio of 1:2 lution in m-xylene, increasing C60 concentration from 0.75 60 –1 －1 to 1.2 mgmL formed C60 rods with average length ca. 25 and 1.0 mgmL C60, (f) tubular structure ob- –1 to 3.0 μm and width 447 to 406 nm, respectively. On the tained at volume ratio of 1:2 and 1.2 mgmL －1 other hand, at fixed concentration of 1.2 mgmL increas- C60, (g) show a tubular feature obtained at vol- –1 ing volume ratio of IPA and C60 solution in m-xylene from ume ratio of 1:3 and 1.2 mgmL C60, and (h)

1:1 to 1:3 modulated the shapes of C60 crystal from rods to TEM image demonstrating tubular structure. tubes. The lengths of rods and tubes were essentially the Adopted with permission from J. Phys. Chem. C same at ca. 3 μm but their widths increased from 406 to 111, 10498-10502 (2007) © 2007, American 811 nm（see Fig. 4）. Chemical Society. Miyazawa’s group proposed a modification of the LLIP method to produce fullerene nanowhiskers and nanotubes found in the range 200-500 nm. They also succeeded to having lengths hundreds of μm in a relatively short period control the diameter of C60 nanowhiskers by changing the of time～2 h45）. They divided nucleation stage and crystal area of the liquid-liquid interface, which could be done by growth stage. For nucleation stage, saturated C60 solution simply altering the reaction glass bottle size. Increasing the was mixed with a small amount of alcohol and for the internal diameter of the glass bottle, i.e., increase in the growth stage C60 was precipitated in the nucleated solution area of liquid-liquid interface, increases the overall diame- by injecting alcohol to the bottom of the solution at a con- ter of nanowhiskers. For example, the mean diameter of trolled flow rate using a syringe pump. The widths were C60 nanowhiskers grown at an IPA and toluene interface

545 J. Oleo Sci. 62, (8) 541-553 (2013) L. K. Shrestha, R. G. Shrestha, J. P. Hill et al.

was increased from 222 to 579 nm upon increasing the in- added to the C70/mesitylene solution, was expected to be 46） ternal diameter of the glass bottle from 17 and 32.4 mm . the key factor for C70 cube formation. Due to the low solu-

Details of the dimension controlled fabrications of C60 bility of C70 in IPA, the local concentration of C70 in the me- nanowhiskers or nanotubes from a variety of antisolvents sitylene cavities is increased upon addition of IPA in C70/ and solvents and using different methods have been re- mesitylene solution. Therefore, instant nucleation of C70 cently summarized elsewhere47）. occurs in the mesitylene cavities. This mixed-solvent-medi- Next we brieﬂy discuss 2D crystals. Fabrication of a 2D ated nucleation phenomenon is similar to the traditional plate like morphology of fullerene crystals particularly by recrystallization process and the synthesis of various self- solution based methods has been sparsely reported in crystallized C60 structures. Once C70 molecules are localized comparison to the 1D structures. Physical vapor deposition in the cavity of mesitylene, which is surrounded by IPA, methods have been mostly used to make 2D thin ﬁlms of mesitylene guides C70 molecules to self-crystallize into 48） C60 . Highly selective disk-shaped 2D single crystalline C60 cubes. They found that the average size of the C70 cubes reported by Shin et al.49） is another good example of vapor decreases with increasing the amount of IPA and the con- deposition processes for synthesis of 2D crystals of C60. A centration of C70 in the starting solution. Their results

C60 solvate formed upon storing C60 solution in trichloro- support the hypothesis that the addition of IPA induces bromomethane（BrCCl3）in dark for a week, which showed local cavities of mesitylene in which C70 molecules are lo- interpenetrated hexagonal shape single crystals of C60. calized and their self-crystallization is guided. It should be

2BrCCl3, may be the ﬁrst example of fabrication of 2D C60 noted that the size of the cavity should become smaller as crystals based on a solution method50）. The average size of the amount of IPA increases as a result; the average size of the hexagonal shaped 2D crystals was about 100 μm with the resulting cube crystal is then also smaller. The decrease thicknesses of a few microns. The group of Miyazawa re- in the size of the cube with the C70 concentration is due to ported the first example of very thin（so called transpar- the increased number of nucleation sites. The average size ent）hexagonal shaped C60 nanosheets by the LLIP method of C70 cubes was ca. 2.05 μm. 31） at an IPA and carbon tetrachlorid（e CCl4）interface . They Another contribution to the fabrication of cube-shape C70 could tailor the average size of the nanosheets by simply crystals came from Yao et al.52）. They fabricated cube shape changing the type of alcohols though thickness remained C70 microcrystals by using a fast solvent-assisted evapora- essentially unchanged. For example, hexagonal shape tion method by selectively using aromatic solvents with nanosheet with a diameter～7.5 μm was observed in the halogen radicals as a controller. The C70 cubes were of

IPA/CCl4 system, whereas the diameter decreased to～2.5 rather polydispersed sizes ranging from several hundred μm and 500 nm, respectively with ethanol and methanol. nanometers to tens of micrometer（s ＞30 μm）. It was found The same group also fabricated polygonal shaped 2D crys- that when the size of the cubic crystals was larger than～ tals including hexagons and rhombuses by altering solvent 10 μm, the cube crystals formed usually have a concave molecular structures32）. As mentioned earlier, Park et al.39） pyramid and some of them have a polycrystalline flower- produced 2D crystals of C60 by a drop-drying method from like shape. a so-called pseudo 3D geometry solvent such as CCl4 and To our knowledge, a perfect cubic shape 3D morphology

SnCl4, which also supports the results of Miyazawa and co- of C60 crystal is still absent from the literature. However, workers. Figure 5 shows representative SEM images of 2D assembly of 1D nanorods or nanotubes of C60 has been crystals produced by using LLIP method. shown to yield 3D nanostructures53）. We have also found a Regarding the 3D morphology of fullerene crystals, Choi similar aggregation of 1D nanorods or nanotubes into a and coworkers51） recently reported high-definition cube- flower-like morphology in the presence of surfactants in shaped crystals of C70, which were synthesized by the LLIP the antisolvent. These 3D ﬂower-shaped C60 crystals have and precipitation methods from a mixture of a solution of hexagonal close packed structure and show intense photo 54） C70 in mesitylene and IPA. Formation of local mesitylene luminescence（PL）compared to pristine C60 . Our group cavities, which are instantaneously generated when IPA is very recently succeeded to fabricate highly crystalline

cube-shaped crystals of C60-fullerene-Ag（I）organometallic

heteronanostructur［e C6｛0 AgNO3｝5］, which undergoes irreversible structural rearrangement upon exposure to low molecular weight aliphatic alcohols（in particular 1-butanol）yielding a uniquely structured formation of well- 35） oriented C60 nano/microcrystal（s‘bucky cubes’） . The Fig. 5 SEM images of 2D C60 crystals grown at liquid- mechanism of rearrangement represents a supramolecular liquid interface as typical examples. Adopted analogue of topotactic processes more commonly associat- with permission from J. Am. Chem. Soc. 131, ed with some purely inorganic materials, such as maghe- 6372-6373. © 2009 American Chemical Society matite, where chemical changes can occur with addition or

546 J. Oleo Sci. 62, (8) 541-553 (2013) Self-Assembled Fullerene Nanostructures

loss of materials. Hence, C6｛0 AgNO3｝5, undergoes a super- topotactic transformation from a crystalline organometallic complex to well-ordered cube-shaped arrays of needle-like

C60 crystals which reflect the original cubic crystal morphology an internal structure.

3 SELF-ASSEMBLY‌ OF FUNCTIONALIZED FULLERENES Herein, we discuss the self-assembly of functionalized fullerenes into different mesostructures in solution. As we have mentioned earlier, owing to its highly hydrophobic character, fullerene C60 is insoluble in water, which limits its practical applications. However, this extremely hydrophobic C60 molecule can be made water soluble by func- tionalizing with charged groups such as carboxylic acids or amines and, hence, its theoretical importance and potential applications are improved55, 56）. Ravi et al.57） recently reviewed several synthetic routes proposed to functionalize

C60 to increase its solubility and processibility. Derivatizing C with different functional groups, solubilizing and encap- 60 Fig. 6 (a) Molecular structure of bola-amphiphilic C , sulating through the formation of complexes with surfac- 60 (b) schematic model of vesicle, and (c-f) elec- tants, cyclodextrins, calixerenes, Tween-20, phospholipids, tron micrographs of the derived vesicles at dif- and liposomes, and producing charge-transfe（r CT） ferent magnifications. Adopted with permission complex with organic compounds bearing electron donat- ing groups through electron donor and acceptor interaction from Langmuir 16, 3773-3776 (2000) © 2000 are the most commonly employed synthetic routes to func- American Chemical Society. tionalize C60. After suitable functionalization or modifica- 60） tion of C60 into an amphiphilic molecule its self-assembly ers obtained experimental evidence of nanoscale aggre- behavior in different solvent systems has been investigated gates formed by e,e,e-tris-malonic acid-C6（0 or C3）in water, in detail with emphasis on its potential impact for the de- which was commonly assumed to exist as discrete mole- velopment of functional materials. For example, Jeng et cules solubilized in aqueous solution. Figure 7 shows the 58） al. observed monodisperse spherical type aggregates molecular structure of C3 and electron micrographs of the having a radius of gyration of about 2.0 nm in dilute solu- aggregates at different magnifications. The aggregates tions of starlike hexaanionic fullerene derivatives, C6［0 CH2］4 were polydisperse having sizes in the range of 40-80 nm.

SO3Na］（6 star-ionomer）. SAXS and SANS studies showed The size was independent of concentration and tempera- that the interparticle interaction between the aggregates is ture, but strongly dependent on pH between pH 4-8. The much weaker and insensitive to the temperature and con- aggregates increased in size with decreasing pH. For in- centration characteristics of hard-sphere type interactions stance, the average hydrodynamic diameter of the aggre- common in surfactant micelles. The hydrophobic behavior gates formed by C3 increased from about 90 to 380 nm at a of star-ionomer was expected to be responsible for the for- ﬁxed concentration 5 mgmL－1 at 25℃. This increase with mation of monodisperse aggregates. Sano et al.59） have also pH has been explained by considering the effect of proton synthesized novel amphiphilic C60 derivatives with two am- dissociation of the C3 malonic acid groups. They also con- monium headgroup（s so called bola-amphiphilic C60: sche- ﬁrmed that it is the aggregates of C3, which are responsible matic molecular structure is presented in Fig. 6a）and ob- for their neuroprotective action in cells, not the individual served self-assembled spherical vesicles with average C3 molecules. diameter in the range of 20-50 nm from the dilute aqueous Geckeler et al.61） synthesized water-soluble mono-6-aza- solution of this bola-amphiphilic C6（0 3-10 mM）（see Fig. 6）. 6-deoxy-β -cyclodextrin-C6（0 C60 is covalently bonded to Vesicular dispersion was obtained by applying simple ultra- β-cyclodextrin via a bridging nitrogen atom）, which exhib- sonication to the mixture. This novel aggregation was an- ited unusual aggregation behavior in solution. The average ticipated to be caused by the hydrophobic interaction pro- particle size increased from 0.55 to 3.255 μm with decreas- duced by the fullerene moieties exposed to water ing concentration of the solution from 0.216 to 0.01 mM. molecules by the disordered alkyl tails. Wilson and cowork- This unusual phenomenon was related to the diffusivity of

547 J. Oleo Sci. 62, (8) 541-553 (2013) L. K. Shrestha, R. G. Shrestha, J. P. Hill et al.

assembled fullerene derivatives can be found elsewhere and in the references therein64, 65）. Nakamura and coworkers recently reported that fullerene anions featuring a nonpolar/polar/nonpolar ternary motif spontaneously form vesicles exposing their nonpolar fluorous chains to the aqueous environment. Average diameter of vesicles was ca. 36 nm66）. They found that these vesicles tightly yet nonco- valently bind fluorous molecules on their surfaces. In contrast to lipid vesicles that easily lose their structural integ- rity upon removal from aqueous solution, their vesicles were very robust and retained their spherical shape even on a solid substrate under high vacuum. They looked like nanometer-sized hollow Teflon balls. When the vesicle solution was coated and dried on a hydrophilic surfac（e Indium tin oxide）, it become water-insoluble and made the surface water-repellen（t contact angle ca. 111.7°）similar to a Teflon surface demonstrating the possible utility of the vesicle surface as a scaffold for molecular display, the interior for molecular delivery, and the solution for macroscopic surface modification. In subsequent work, Nakamura and Fig. 7 Cryo-TEM micrograph of the aggregates formed coworkers synthesized twenty potassium complexs of

by the C3 solution at different magnifications. fullerene anion amphiphiles having overall structure non- Inset of panel (a) shows schematic molecular polar/polar/nonpolar and investigated their abilities to form structure of C3. Adopted with permission from bilayer vesicles in water67）. The complexes were penta- Nano Lett. 4, 1759-1762 (2004) © 2004 Ameri- ［（4-substituted）phenyl］［60］fullerene anions, where the can Chemical Society. 4-substituents included alkyl groups ranging from methyl to icosanyl groups and perfluoromethyl, perfluorobutyl, the particles. Dilution led to a reduction in the viscosity and perfluorooctyl groups. They found that despite the hy- and a corresponding increase in the diffusivity of the parti- drophobicity of the fullerene moiety（nonpolar part）and cles, which facilitated the interaction between clusters alkyl/perfluoroalkyl chain（s second nonpolar part）, all com- leading to cluster-cluster aggregation. The group of Naka- pounds except for the one with perfluoromethyl groups mura investigated association behavior of potassium salt of were soluble in water due to the centrally located fullerene 62） pentaphenyl fullerene（Ph5C60K）in water . Using a laser cyclopentadienid（e polar part）and spontaneously formed a light scatterin（g LLS）technique they found that in aqueous vesicle of 25-60 nm diameter with a narrow unimodal size － solution the hydrocarbon anions Ph5C60 associate into bi- distribution. The vesicles were stable upon heating to 90℃ layers forming stable spherical vesicles with an average or standing over a year in air, as well as on a solid substrate radius of gyration of about 17 nm. The critical aggregation in air or in vacuum, maintaining their spherical form. The concentration was found to be extremely low（＜10－7 moles vesicle membrane consists of an interdigitated bilayer of per liter）. However, the average aggregation number of as- the amphiphile molecules, in which the nonpolar fullerene sociated particles in these large spherical vesicles was part is at the interior and the other nonpolar part is about 1.2×104. The same group has shown that the attach- exposed to water. Note that the exposure of nonpolar parts ment of five aromatic groups to one pentagon of a C60 mol- of an amphiphile to water is an unusual phenomenon in ecule produces deeply conical molecules, which can stack surfactant science and it is beyond common sense. into polar columnar assemblies63）. The stacking was ex- However, this fact was well supported by experiments. pected to be driven by attractive interactions between the These vesicles, in particular those bearing icosanyl chains, spherical fullerene moiety and the hollow cone formed by exhibited the smallest water permeability coefficient ever the five aromatic side groups of a neighboring molecule in found for a self-assembled membrane in water. Figure 8 the same column. Interestingly they found that the packing shows SEM images of vesicles on indium tin oxide（ITO） pattern is maintained upon extending the aromatic groups surface formed by the fullerene anion bearing five perfluo- by attaching flexible aliphatic chains, which thereby result- rooctyl chain（s Rf8K）as a typical example. As can be seen ed in the formation of compounds with thermotropic and in Fig. 8a, the Rf8K vesicles uniformly cover a wide area of lyotropic liquid crystalline properties demonstrating the the ITO substrate. A closer look at the SEM imag（e Fig 8b） possibility of designing a range of new polar liquid crystal- reveals that the vesicles sit on the grain boundaries of the line materials. Liquid crystalline properties of selected self- ITO surface to maximize contact with the surface. The 30°

548 J. Oleo Sci. 62, (8) 541-553 (2013) Self-Assembled Fullerene Nanostructures

between adjacent nanowires at the nanometer length scale by the molecular design. They presented an approach to

direct the assembly of C60 groups on graphite in a predict- able way using alkanes as structure-directing components,

covalently bonded to C60 and explored the polymorphism of fullerene assemblies by using the two different intermo-

Fig. 8 (a) SEM images of Rf8K vesicles on ITO under lecular forces, namely π-π interactions of C60 and van der ca. 10–5 Pa. (b) Magnified view of panel a, and Waals interactions of aliphatic chains. The same group also (c) a different area of the same sample as viewed reported novel micron size flowerlike supramolecular as- with a 30° tilt of the sample stage. Adopted with semblies of fullerene-based molecules bearing long alkyl 71） permission from J. Am. Chem. Soc. 133, 6364- chains . In the following work, Nakanishi and coworkers 6370 (2011) © 2011 American Chemical Soci- observed that a novel N,N-dimethylfulleropyrrolidinium ety. iodid（e ionic fullerene derivative derived from further functionalization of the N-methylfulleropyrrolidine）form a tilted side view（Fig. 8c）conﬁrms this observation and also variety of functional and polymorphic self-assembled indicates that the vesicles largely retain their spherical structures both from solution and on substrate72）. They ob- structures under the rather harsh SEM conditions of 10－5 served that these ionic fullerene derivatives due to π-π, van Pa, which demonstrates the unusual robustness of the der Waals, and electrostatic interactions hierarchically or- fullerene vesicle. The average diameter of the Rf8K vesicle ganized into ﬂake-like microparticles with high water repel- particles was ca. 35.6 nm. The attractive features of these lency, doughnut-shaped objects with rough surfaces, and

vesicles are that they are morphologically stable, monodis- long 1D C60 nanowire（s ＞1μm）. The flakelike microparti- persed, nanosized, spherical hollow objects, which can be cles due to surface roughness in nanometer length scale prepared by a simple method without any purification. showed water repelling properties giving a static water They can also be used to modify solid surfaces. It is antici- contact angle of～140°, which is about 10° lower than that pated that these nonpolar/polar/nonpolar type amphiphiles observed from microﬂowers formed by the parent fullerene will open a new avenue in research on nano-sized compart- amphiphile, i.e. N-methylfulleropyrrolidine. The authors ment structures in solution and on solid surfaces. mentioned that the difference in water repellency is due to In the study of supramolecular assemblies of fullerene different surface roughnesses. Figure 9 shows typical SEM derivatives, Georgakilas et al.68） found that ionic fullerene derivatives, which are relatively soluble in polar solvents, self-organize into different morphological nanostructures such as spheres, nanorods, and nanotubes in water depending on the side chain appendage of the fullerene spheroid. Guldi et al.12） discussed the organization of fullerene derivatives in bulk, at interfaces, and on substrates. They highlighted that the proper combination of the hydrophobic fullerene core with hydrophilic functional groups （both ionic and nonionic）potentially leads to organized structures whose sizes range from nanometer to micrometer length scales. The main driving force of such spontane- ous assembly is the amphiphilic character of the fullerene derivatives. They pointed out that, similarly to the conven- tional surfactant systems, supramolecular assembly, dimensions and the functions of the fullerene derivatives depend on the following parameters:（ a）hydrophilic-hydrophobic balance,（ b）the effect of the environment, typically provid- ed by solvents,（ c）the interface at which the aggregation occurs, and（d）the solvation process. The influence of these effects on the fullerene supramolecular assemblies can be found elsewhere69）. Fig. 9 Typical SEM and AFM images of flakelike mic- Nakanishi et al.70） reported 1D lamellae of fullerene de- roparticles and 1D nanowires of N,N-dimethyl- rivatives bearing long alkyl chains. The 1D structure epi- fulleropyrrolidinium iodide. Adopted with per- taxially oriented along the highly oriented pyrolytic mission from Langmuir 27, 7493-7501 (2011). graphite（HOPG）lattice. They could tune the distance © 2011 American Chemical Society.

549 J. Oleo Sci. 62, (8) 541-553 (2013) L. K. Shrestha, R. G. Shrestha, J. P. Hill et al.

images of flake-like microparticles formed by N,N-dimethylfulleropyrrolidinium iodide by slow addition of excess methanol solution into its concentrated solution in dichloromethane and typical AFM images of nanowires of the N,N-dimethylfulleropyrrolidinium iodide obtained by spin- coating of its 10 μM solution in dichloromethane on HOPG substrate. As can be seen in AFM images, N,N-dimethylfulleropyrrolidinium iodide forms extremely large areas of 2D lamellar domains composed of the 1D C60 nanowires with lengths of over 1.0 μm. The periodic separation of the nanowires is ca. 7.3 nm, which is almost exactly twice the molecular length of N,N-dimethylfulleropyrrolidinium iodide in a fully extended conformation（3.7 nm）. Moreover, a cross-sectional line analysis reveals that the average height difference within the lamellae is 0.38 nm, which is close to the difference between the diameter of C60 and the height of a flat- lying alkyl chain layer on the HOPG surface. These results indicate that the organization of Fig. 10 (a) SEM image of whiskers casted from hybrid N,N-dimethylfulleropyrrolidinium iodide within nanowires toluene solution (0.5 mgmL–1) of 1:1 C : is similar to that of N-methylfulleropyrrolidine, with the C 60 60 C PPPOH, (b) side view of an individual moieties organized in a zigzag conformation and the ali- 12 70） nanowhiskers showing multiple fibers, (c) HR- phatic chains localized in between . In the study of supramolecular assembly of fullerene gly- TEM image, and (d) magnified HR-TEM of se- coconjugates in aqueous solution, Kato et al.73） found small lected area. Adopted with permission from ACS supramolecular aggregate（s spherical micelles type）having Nano 2, 120-127 (2008). © 2008 American particle size of about 4 nm. These spherical micelles with Chemical Society. an extremely narrow size distribution represent supramolecular sugar balls, which have potential for biomedical ap- solubilization of C60 in water by complexation with disac- plications. Nurmawati et al.74） outlined a synergistic self-as- charides in mixed homogeneous solvent system. The com- sembly of fullerene（s C60）and functionalized poly（p- plexation of hydrophobic water-insoluble C60 dissolved in phenylene）（PPP）to develop nanofibers with high aspect nonpolar solvent toluene and extremely water-soluble di- ratios. Nanostructured PPP-C60 hybrids were prepared by saccharides dissolved in polar solvent DMSO resulted in a direct casting of a dilute solution on solid substrates and unique self-assembled highly crystalline water-soluble C60- on water under ambient conditions. When a solution of the disaccharide complex with average particle size ca. 60 nm. mixture of C60 and PPP polyme（r C12PPPOH）（1:1, 0.5 Their results highlight the potential application of water- －1 mgmL in toluene）was cast onto a solid substrate, unidi- soluble C60-disaccharide complex in biomedical applica- rectional crystallization of nanohybrid material having tions. nanofibrous morphology with high aspect ratios was observed（see Fig. 10）. Nanofibers that have an average diameter of about 35 nm and length of 30-50 μm aggregate to form whiskers. 4 CONCLUDING REMARKS The observed high aspect ratio of 1050 is comparable to In this review we have briefly summarized recent devel- typical aspect ratios of CNTs. The HR-TEM images shown opments in the production of shape-controlled fullerene in Fig. 10c, d shows regular lattice fringes in the nanonee- nano/microcrystals by solution-based approaches. Facile dles spanning along the growth axis. They explained that syntheses of fullerene crystals having different morphology planarization of the polymer backbone through potential such as fullerene spheres, fullerene nanowhiskers and O-H-O hydrogen bonds and optimum alkyl chain crystalli- nanotubes, fullerene nanosheets, and assembly of fullerene zation played an important role in the molecular level or- nanorods or nanotubes in three-dimensional assemblies are dering of C12PPPOH during the whisker formation. The first discussed. Moreover, supramolecular assembly of function- step of the multiscale assembly of the whisker formation alized fullerene（s fullerene derivatives）into different meso- involves a molecular level polymer C60 assembly driven by structure（s micelles, liquid crystals, and vesicles）both in π-π stacking of the conjugated backbone of the polymer solution and substrate was discussed. and the fullerene, resulting in the nanofibers. Murthy and coworkers75） have recently demonstrated first example of

550 J. Oleo Sci. 62, (8) 541-553 (2013) Self-Assembled Fullerene Nanostructures

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