Responsive Fluorescent Nanorods from Coassembly of Fullerene

Article pubs.acs.org/Macromolecules

Responsive Fluorescent Nanorods from Coassembly of Fullerene (C60) and Anthracene-Ended Hyperbranched Poly(ether amine) (ANhPEA) Zhilong Su, Bing Yu, Xuesong Jiang,* and Jie Yin School of Chemistry & Chemical Engineering, State Key Laboratory for Metal Matrix Composite Materials, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China S Supporting Information *

ABSTRACT: We herein demonstrated a novel multiresponsive fluorescent nanorod based on C60, which is fabricated through the versatile coassembly of fullerene C60 and anthracene-ended hyperbranched poly(ether amine) (ANhPEA). The supramolecular nanorods (C60@AN-hPEA) can be further cross-linked through photodimerization of anthracene, and the size of the obtained nanorods is 2−12 μm in length and 50−90 nm in diameter. C60@AN-hPEA nanorods are amphiphilic, responsive, and fluorescent. The fluorescence of C60@AN-hPEA nanorods in aqueous solution is responsive to temperature and pH. The C60@AN-hPEA-1/4 nanorods exhibit the interesting temperature-enhanced fluorescence, while the fluorescence intensity of AN-hPEA without C60 decreases with the increasing temperature. Detailed fluorescence study revealed that the temperature-enhanced fluorescence behavior of C60@AN-hPEA-1/4 nanorods might be ascribed to the static quenching of the excited anthracene by C60.

1. INTRODUCTION Extensive efforts have been devoted toward the development of one-dimensional (1D) nanostructures such as wires, rods, belts, and tubes due to their unique size-dependent electronic and optical properties.1−5 Among these novel nanostructures, functional nanorods or nanowires based on fullerene have attracted much attention for its potential applications in material science 6−11 and medicinal chemistry. 12−14 1D nanostructure of fullerene can be prepared by solvent evaporation, 6,15 liquid−liquid interfacial precipitation (LLIP),16−19 and templates.20,21 Liu reported for the nanorods of C60 with the enhanced luminescence by evaporation of mxylene.15 Miyazawa et al. prepared C60 nanowhiskers by the LLIP method in the C60-saturated m-xylene and isopropyl alcohol system.18 The poor solubility and instability of these C60 nanostructures, however, limited their application, especially in aqueous solution. To obtain the C60 nanostructures with good solubility, many efforts have been done including self-assembly of C60 derivatives with hydrophilic groups or polymeric chains.22−24 Munoz synthesized dendrofullerenes endowed with the carboxylic groups which can aggregate into micelles, rods, or vesicles in aqueous solution.25 Cassell reported that C60-N,N-dimethylpyrrolidinium iodide self-assembled to nanorods or vesicles depending on the treatment of solution.26 However, these approaches always involved in grafting functional groups or polymeric chains to C60 through covalent bond, which might damage the π−π conjugation of C60 and thus probably disrupt the physical and chemical properties of C60.27,28 Coassembly of pristine C60 and © 2013 American Chemical Society

polymers has some advantages such as keeping the perfect properties of C60 and providing the possibility to be further functionalized.27−30 Another important issue for nanostructure of C60 is the photochemistry of fullerene, which has been exhaustively investigated.31,32 Allotropic forms of carbon such as fullerenes, carbon nanotubes, and graphene can form the stable complexes with π-rich compound by π−π stacking.33−35 This interaction between fullerene and organic compounds can lead to the quenching of electronic excited states of the chromophores and can even promote photoinduced electron transfer between fullerene and the organic probe molecule.35,36 The quenching of electronic excited states of pyrene, (2,2′-bipyridyl)ruthenium(II) dichloride (Ru(bpy)32+), and porphyrins by fullerene has been reported.37−39 The stability of these supramolecular assemblies is also of great importance in the practical application. Most of these nanostructures may only exist under a certain case and disassemble under some extreme conditions. As a result, these assemblies often need to be crosslinked to maintain the nanostructure and enhance the stability for application in some fields.40−42 Herein, we reported for the photo-cross-linked supramolecular nanorods fabricated by the coassembly of two components: anthracene-ended hyperbranched poly(ether amine) (AN-hPEA) and pristine C60. The obtained nanorods Received: January 28, 2013 Revised: April 20, 2013 Published: May 1, 2013 3699

dx.doi.org/10.1021/ma400181z | Macromolecules 2013, 46, 3699−3707

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Scheme 1. (a) Structure and Model of AN-hPEA and C60; (b) Strategy of Coassembly of AN-hPEA and C60 To Form Nanorods

of C60@AN-hPEA are responsive and fluorescent. These responsive supramolecular nanorods of C60 are of our great interest because of their potential application as the smart material in sensors and optical devices. In the design of the coassembly of C60 and AN-hPEA, AN moieties have two functions: AN possesses the strong π−π interaction with C60, and part of AN groups can be photodimerized to make the coassemblies of C60@AN-hPEA cross-linked. Because of the well-known π−π stacking interaction between fullerene and AN,43 the formation of the supramolecular coassemblies between C60 and AN-hPEA (C60@AN-hPEA) can be expected. The strategy for the coassembly of C60 and AN-hPEA is illustrated in Scheme 1, and the resulting supramolecular coassemblies of C60@AN-hPEA can take a rodlike shape. The obtained C60@AN-hPEA nanorods were further cross-linked through the photodimerization of AN groups and can be dispersed in water very well. Compared with the assembly of one-component C60-based polymers,44,45 coassembly of two components pristine C60 and AN-hPEA has some advantages such as easy syntheses and convenient change of the ratio between C60 and polymer. Fujita described the formation of spherical C60 nanoclusters with the controlled size and morphology through coassembly of C60 carboxylic acid and block copolymer.46 However, the coassembly of pristine C60 and amphiphilic hyperbranched polymers was rarely reported. To the best of our knowledge, this is the first time to fabricate the responsive C60-based 1D nanostructure via the coassembly of pristine C60 and the amphiphilic responsive hyperbranched polymers. The obtained C 60 @AN-hPEA nanorods are responsive to temperature in aqueous solution and exhibited the interesting temperature-enhanced fluorescence.

DMF using cellulose membrane with a molecular weight cutoff of 3500. The solution was then centrifuged at 6000 rpm for 15 min, and the precipitation C60@AN-hPEA-1/4 nanorods was obtained. C60@ AN-hPEA-1/2, C60@AN-hPEA-1/8, and C60@AN-hPEA-1/16 assemblies were prepared similarly, while C60 and AN-hPEA in feed was 1:2, 1:8, and 1:16 (w/w) with the same centration, respectively. The processes were carried out under an N2 atmosphere at 20 °C. Measurements. Transmission Electron Microscopy (TEM). The TEM, HR-TEM images, and SAED pattern of coassemblies were obtained using a JEM-2100 (JEOL Ltd., Japan) transmission electron microscope operated at an acceleration voltage of 200 kV. The samples were prepared by dropping the cross-linked C60@AN-hPEA solution onto copper grids coated with a thin carbon film, and the excess solution was removed by filter paper. Then the samples were dried at 25 °C for 24 h. No staining treatment was performed for the measurement. Scanning Electron Microscopy (SEM). The SEM images of coassemblies were obtained using a Sirion 200 (FEI Company, Netherlands) field emission scanning electron microscope operated at an acceleration voltage of 5 kV. The samples were prepared by dropping the cross-linked C60@AN-hPEA solution onto silica wafer and dried at 25 °C for 24 h. Then the samples were sputter-coated with gold. Atomic Force Microscopy (AFM). The AFM images of assemblies were obtained using a NanoscopeIII (Digital Instruments). The surface morphologies of samples were acquired in tapping mode. The samples were prepared by dropping the cross-linked C60@AN-hPEA solution onto mica sheet and dried at 25 °C for 24 h. UV−vis Spectra. The UV−vis spectra of assemblies were carried out with a UV-2550 spectrophotometer (Shimadzu, Japan). The solutions were equilibrated for 15 min before measurement, and the concentration of the cross-linked C60@AN-hPEA is 0.1 mg/mL. Fluorescence Spectra, Fluorescence Decay, and Quantum Yield. The fluorescence spectra of coassemblies were recorded using a QM40\TM-3 fluorescence spectrofluorometer (PTI Company). The solutions were equilibrated for 10 min before measurement, and the concentration of the cross-linked C60@AN-hPEA for fluorescence spectra is 0.1 mg/mL. The excitation wavelength is 381 nm. The fluorescence decay of assemblies was recorded using the same instrument. The solutions were equilibrated for 10 min before measurement, and the concentration of the cross-linked C60@ANhPEA for fluorescence spectra is 0.1 mg/mL. The excitation wavelength is 355 nm, and the emission wavelength is 435 nm to minimize the interference. The quantum yields of coassemblies were determined by a comparative method with quinine sulfate in 0.5 M H2SO4 as standard (λex = 381 nm, ΦF = 0.66). TGA Analysis. TGA analysis of coassemblies was carried out on Q5000IR thermogravimetric analyzer (TA); the samples were run

2. EXPERIMENTAL SECTION Materials. AN-hPEA was synthesized according to previous work of our group.47 Fullerene C60 (>99.9%) was purchased from Huawen Chemical Co. Ltd. (Zhengzhou, China) and used without further purification. Preparation of C60@AN-hPEA Nanorods. 3.0 mg of C60 and 12.0 mg of AN-hPEA were dissolved in 20 mL of benzene, and then the mixture was slowly stirred for 36 h. After 2 mL of N,Ndimethylformide (DMF) was added, the solution was evaporated slowly with a gently stir to remove benzene. Then, 20 mL of Milli-Q water was added slowly to the crude product and equilibrated for 6 h. The nanorods were cross-linked by irradiation at 365 nm for 15 min through an ultraviolet LED lamp (Uvata, 8.4 mW/cm2). After crosslinking, the solution was dialyzed against water for 24 h to remove 3700


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Figure 1. (a) SEM image of the obtained C60@AN-hPEA-1/4 nanorods. Inset: enlarged view of C60@AN-hPEA-1/4 nanorods. (b) Length distribution of the C60@AN-hPEA-1/4 nanorods determined by SEM (100 nanorods counted). under flowing N2 gas at a heating rate of 20 °C/min. The samples were prepared by freeze-drying of the solution of coassemblies. Laser Scanning Confocal Microscope (LSCM). Fluorescence images of coassemblies were viewed with a Leica TCS-SP5 laser scanning confocal microscope (Leica, Germany) equipped with UV lasers. The sample was prepared by dropping the cross-linked C60@ AN-hPEA solution onto coverslip and then dried at 25 °C for 24 h.

nanorods is more than 60. It should be noted that a certain amount of dot was also found in SEM images, which might be ascribed to that part of AN-hPEA that did not take part in coassembly, and formed nanoparticles by itself.47 The rodshaped morphology of C60@AN-hPEA-1/4 coassemblies was further confirmed by AFM images, which revealed that the diameter of C60@AN-hPEA-1/4 nanorods is around 70 nm (Figure 2a). The fluorescent image taken by LSCM revealed

3. RESULTS AND DISCUSSION Preparation and Characterization of Nanorods. ANhPEA was synthesized by introducing AN moieties to the periphery of the hydrophilic hPEA. AN-hPEA was chosen as one component for construction of C60-based nanostructures due to its novel characteristics. The amphiphilic AN-hPEA is comprised of the hydrophilic and responsive backbone of hyperbranched poly(ether amine) (hPEA) and the hydrophobic ended groups of anthracene (AN). The hydrophobic AN moiety of AN-hPEA is expected to interact with C60 through π−π interaction and hydrophilic−hydrophobic interaction, resulting in the formation of the supramolecular coassemblies of C60 and AN-hPEA in aqueous solution. Additionally, the photodimerization of the part of AN moieties can lead to the cross-linking of coassemblies, and the residual AN can make the resulting coassemblies fluorescent. The whole process for fabrication of coassemblies of C60@AN-hPEA is illustrated in Scheme 1. C60 and AN-hPEA were dissolved in benzene, and the mixture was stirred at room temperature for 36 h. DMF was then added dropwise into benzene solution of C60 and AN-hPEA. After evaporation of benzene, water was added dropwise into the DMF solution of C60 and AN-hPEA. As a result, the coassemblies were obtained. Because DMF is good solvent for the AN-hPEA, but precipitant for C60, C60 are expected to precipitate along the evaporation of benzene. As AN possesses the strong interaction with C60, AN-hPEA are expected to assist the formation of C60 crystals. Further assembly may take place along the addition of water as water is good solvent only for the hydrophilic hPEA backbone. The obtained coassemblies are expected to be comprised of the hydrophobic AN and C60 as the inner layer and the hydrophilic hPEA as the outer layer. Upon the exposure of UV-light at 365 nm, the coassemblies are cross-linked through photodimerization of AN. The obtained cross-linked coassemblies of C60@AN-hPEA1/4 were observed by SEM, TEM, AFM, and LSCM. The rodshaped morphology of C60@AN-hPEA-1/4 coassemblies with the length from 2 to 12 um can be found in the whole SEM image (Figure 1). The average diameter of C60@AN-hPEA-1/4 nanorods is about 100−110 nm according to SEM images. The aspect ratio between length and diameter of the obtained

Figure 2. (a) AFM image of C60@AN-hPEA-1/4 nanorods and its height profile along the two lines of the AFM image. (b) LSCM image of C60@AN-hPEA-1/4 nanorods.

that the obtained C60@AN-hPEA-1/4 nanorods exhibit the strong blue fluorescence, which should be ascribed to the emission of AN moieties of AN-hPEA (Figure 2b). TEM images (Figure 3) also revealed the rodlike morphology of C60@AN-hPEA-1/4 nanorods, and the diameter of C60@ANhPEA-1/4 nanorods determined by TEM is almost the same as the height of the nanorods according to AFM. The diameter determined by SEM and AFM is larger than that by TEM, which might be caused by the flattening of nanorods with the soft hydrophilic outer layer of hPEA-AN on the hydrophilic substrates such as silicon and mica. The high-resolution TEM (HR-TEM) image revealed the highly ordered crystallized aggregation of C60 in the inner of nanorods, which might be the key factor to the formation of nanorods during the coassembly of C60 and AN-hPEA.15,48 The crystals were found to display dense packing along the growth axis with average lattice plane spacing of 0.84 nm. The SAED pattern of the coassemblies can be ascribed to the (210) crystallographic planes of a hexagonal close-packed (hcp) structure,49,50 which is also confirmed by the XRD spectrum (Figure S1). Both LSCM and TEM supported that the C60@AN-hPEA-1/4 nanorods are com3701


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AN in the coassembly, a control experiment of coassembly of C60 and hyperbranched poly(ether amine) without AN groups (hPEA) was carried out. The resulting “assemblies of C60/ hPEA” precipitated at the bottom of the bottle after evaporation of benzene, and few nanorods were found in SEM images (Figure S3). This control experiment indicated that π−π interaction between two components C60 and ANhPEA plays a key role in the formation of the nanorods. The benzene solution of C60 and AN-hPEA was transferred into DMF through solvent evaporation to get a dark yellow solution. After water was added dropwise into DMF solution, the aqueous solution of C60 and AN-hPEA was obtained. Compared with the UV−vis spectrum of DMF solution, the absorbance of AN moiety decreases in aqueous solution (Figure 5c). This can be explained by that the hydrophobic AN moieties were suppressed onto the surface of C60 core upon the addition of water. SEM images (Figure S4) revealed that the rodlike coassemblies were formed after evaporation of benzene, which is in accordance with our strategy. To get the stable coassemblies formed with C60 through noncovalent interaction, the coassemblies always need to be cross-linked to enhance its stability.54 The obtained coassemblies of C60@AN-hPEA-1/4 nanorods were further cross-linked through photodimerization of AN moieties to enhance its stability. The process of AN photodimerization was traced by UV−vis and fluorescent spectra (Figure 6). After exposure to 365 nm UV-light for 15 min, the dimerization degree of AN reaches a balance of about 70% (Figure 6a,b). The residue AN moieties leads to the strong fluorescence of the obtained coassemblies. The fluorescence of C60@AN-hPEA-1/4 nanorods is not always weakening during UV irradiation, which is strengthened at the beginning and then becomes weak with the increasing UV irradiation time (Figure 6c). This phenomenon was often found in coassemblies of anthracene-containing polymers. The self-quenching effect of AN decreases with the decreasing concentration of AN first, while the fluorescence decreases with the further decreasing concentration of AN.47,55 Taking both cross-linking density and fluorescence intensity of C60@AN-hPEA-1/4 nanorods into consideration, the exposure time was chosen as 15 min. To test the effect of cross-linking on the stability of coassemblies, C60@AN-hPEA-1/4 nanorods before and after cross-linking were transferred into the good organic solvent tetrahydrofuran (THF). The cross-linked coassemblies kept their morphology well and can dispersed stably in THF (Figure S5a,b), while the un-cross-linked coassemblies disassembled and participated at the bottom of the bottle (Figure S4c,d), indicating that the cross-linking can enhance the stability of C60@AN-hPEA-1/4 nanorods.

Figure 3. (a)TEM image of C60@AN-hPEA-1/4 nanorods; inset is the enlarged view. (b) HR-TEM image of the inner of C60@AN-hPEA-1/4 nanorods. (c) SAED pattern of the inner of C60@AN-hPEA-1/4 nanorods.

prised of AN-hPEA and C60, which indicating that our strategy to fabricate C60@AN-hPEA coassemblies is feasible. To understand the mechanism of coassembly of AN-hPEA and C60, the whole process of preparing of C60@AN-hPEA-1/4 nanorods was traced through UV−vis and fluorescent spectra (Figures 4 and 5). As shown in Figure 4, the purple benzene solution of C60 and AN-hPEA turned brown after being stirred at room temperature for 36 h. The characteristic absorptions of AN at 349, 366, and 386 nm decreased obviously, while a new broad absorption from 400 to 500 nm appeared (Figure 5a). After staying for 36 h, the fluorescence intensity of benzene solution of C60 and AN-hPEA became weaker (Figure 5b). All these phenomena supported that some interaction between C60 and AN moieties should take place. There are two possible interactions between C60 and AN moieties: one is the supramolecular π−π interaction, and the other is the [4 + 2] Diels−Alder cycloaddition. The π-rich compounds such as anthracene and porphyrin have been proved to interact with the unsaturated carbon materials such as fullerene, carbon nanotube, and graphene through supramolecular π−π interaction.48,51 Also, it is well-known that AN can undergo [4 + 2] Diels−Alder cycloaddition with C60;52,53 however, no absorption around 704 nm (a characteristic absorption for C60 and AN D−A adducts)52,53 was observed according to Figure 5a. This may imply that D−A reaction did not happen between C60 and AN moieties in our system, which was further confirmed by the 1H NMR study. No obvious difference in 1H NMR spectra of C60 and AN-hPEA in C6D6 was observed after staying for 36 h (Figure S2). Therefore, π−π interaction between AN moieties and C60 is the dominant driving force in the formation of coassemblies of C60@AN-hPEA. To understand the role of

Figure 4. Photographs of the whole preparation process of C60 nanorods: (a) 3.0 mg of C60 and 12.0 mg of AN-hPEA in 20 mL of benzene; (b) after stirring for 36 h; (c) addition of DMF and then evaporation of benzene; (d) addition of water to DMF solution and then exposure of UV-light. 3702


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Figure 5. UV−vis (a) and florescence (b) study of the conjugate process. Inset: enlarged view between 400 and 800 nm. The solution was diluted to 0.1 mg/mL by toluene for UV−vis and fluorescence study. (c) UV−vis spectra of the water-addition process. The solution after addition of water was diluted to 0.1 mg/mL by water (red line). The solution before addition of water was diluted to 0.1 mg/mL by DMF (black line).

Figure 6. (a) UV−vis spectra of the UV cross-linking process (baseline corrected). Inset: enlarged view from 320 to 400 nm. Samples of different time were taken and diluted to 0.1 mg/mL by THF for UV−vis study. (b) Dimerization degree of AN at different irradiation time calculated from (a). (c) Fluorescence emission spectra of the UV cross-linking process. Inset: plot of the fluorescence emission intensity of C60@AN-hPEA-1/4 nanorods at 414 nm vs irradiation time. Samples of different time were taken and diluted to 0.1 mg/mL by H2O for fluorescence study.

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Figure 7. TEM images the coassemblies of C60@AN-hPEA-1/16 (a), C60@AN-hPEA-1/8(b), and C60@AN-hPEA-1/2(c). Insets: enlarged views of the coassemblies.

To verify the feasibility of our strategy to fabricate supramolecular coassemblies based on C60, we changed the feed ratio between two components of C60 and AN-hPEA in coassembly. TEM images reveal morphologies of these coassemblies: C60@AN-hPEA-1/16, C60@AN-hPEA-1/8, and C60@AN-hPEA-1/2 (Figure 7). Rod-shaped coassemblies can be obtained when the ratio of C60 is above 12.5% (w/w). When reducing the content of C60 further, only nanoparticles can be found. The morphology changes from nanoparticles to nanorods with the increasing content of C60. According to TEM images, the diameter of the obtained nanorods increased from 50 to 90 nm with the increasing content of C60, which is in agreement with DLS data. Although DLS is not suitable for determining the size of 1D nanostructure, it can still provide some information. As shown in Figure S6, the size of coassemblies of C60@AN-hPEA increases with the content of C60. Additionally, the peak at 180 nm in DLS of C60@ANhPEA-1/4 can be ascribed to the nanoparticles of AN-hPEA during the addition of water, which is also revealed in the other analyses such as the dark dots in SEM image and blue dots in LSCM image. Thermogravimetric analysis (TGA) was carried out to obtain the information regarding the component of the obtained coassemblies of C60@AN-hPEA. The final weight retention of C60@AN-hPEA-1/4 is a little higher than the ideal value (Figure S7), suggesting that part of AN-hPEA did not take part in the coassembly. Multiresponsive Behavior of C60@AN-hPEA Coassemblies. The obtained nanorods of C60@AN-hPEA can be well dispersed in aqueous solution due to the hydrophilic outer layer of hPEA, which is comprised of PEO short chains. ANhPEA possesses the sharp response to temperature and pH in aqueous solution.47 Therefore, the dispersion of C60@ANhPEA nanorods in water is expected to responsive to temperature. As shown in Figure 8a, C60@AN-hPEA-1/4 nanorods were dispersed well in water to form the almost transparent solution at room temperature. After being heated over 85 °C, C60@AN-hPEA-1/4 nanorods began to aggregate and then precipitated from the aqueous solution at the bottom. It should be noted that this process is reversible. After cooling to room temperature, the precipitation of C60@AN-hPEA-1/4 nanorods can be dispersed into water again. Such a phenomenon can be explained as follows: with the increasing temperature, hydrogen bonds between water molecules and PEO chains are destroyed, which makes the outer layer of the nanorods less hydrophilic, resulting in the aggregation of the nanorods of C60@AN-hPEA-1/4. As mentioned previously, the residue anthracene moieties lead to the fluorescence of the coassemblies of C60@AN-hPEA. As shown in Figure 8b, the aqueous solution of C60@ANhPEA-1/4 nanorods emits strong blue fluorescence under 365

Figure 8. (a) Photograph of the temperature-responsive behaviors of C60@AN-hPEA-1/4 nanorods: at 20 °C(left); heated to 85 °C (right). (b) Photograph of C60@AN-hPEA-1/4 nanorods under 365 nm irradiation. The concentration of C60@AN-hPEA-1/4 nanorods is 0.75 mg/mL in water.

nm irradiation. The temperature-dependent fluorescent spectra of C60@AN-hPEA-1/4 nanorods in aqueous solution are shown in Figure 9 as well as AN-hPEA as a comparison. C60@ANhPEA-1/4 nanorods exhibited the temperature-enhanced fluorescence in aqueous solution, while the fluorescence of AN-hPEA aqueous solution decreased with the increasing temperature. The temperature-weakened fluorescence of ANhPEA can be easily understood because the exited anthracene can be quenched by the amino groups in the hydrophilic outer layer of hPEA more efficiently at the higher temperature. With the increasing temperature, the hydrophilic outer layer of hPEA shrinks, which makes amino groups closer to anthracene. Therefore, the excited anthracene is quenched more efficiently, resulting in the weaker fluorescence at the higher temperature. In comparison to AN-hPEA, the abnormal temperatureenhanced fluorescence behavior of C60@AN-hPEA-1/4 nanorods might result from the presence of C60. To understand this abnormal fluorescence behavior, we measured the fluorescent quantum yield and lifetime of aqueous solution of C60@AN-hPEA-1/4 nanorods at different temperatures. As shown in Figure 10a, the fluorescence lifetime of C60@AN-hPEA-1/4 nanorods is 4.40 ns at room temperature, which is shorter than that of AN-hPEA (4.77 ns). The fluorescent lifetime of C60@AN-hPEA-1/4 nanorods becomes even shorter at higher temperature, while the fluorescent quantum yield increases with the increasing temperature. In comparison to AN-hPEA, the shorter lifetime of C60@ANhPEA-1/4 nanorods should be ascribed to the additional quenching of the excited AN by C60. To explain the abnormal temperature-enhanced fluorescent behavior, we proposed that C60 can quench the excited AN through two ways in C60@ANhPEA-1/4 nanorods: dynamic and static quenching (Scheme 2). The dynamic quenching of the excited AN by C60 leads to the shorter lifetime (τ) of C60@AN-hPEA-1/4 nanorods. As an 3704


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Figure 9. Fluorescence emission spectra of C60@AN-hPEA-1/4 nanorods (a) and AN-hPEA (b) in aqueous solution at different temperatures. Red insets: polynomial curve-fitting of plots of the fluorescence emission intensity at 414 nm vs temperature. The concentration of is 0.1 mg/mL in water.

Figure 10. (a) Effect of temperature on the fluorescence decay of C60@AN-hPEA-1/4 nanorods. (b) Quantum yield and fluorescence decay of C60@ AN-hPEA-1/4 and AN-hPEA at various temperatures.

Scheme 2. Proposed Quenching Mechanism of C60 to Anthracene

excellent excited-state energy acceptor,38 the ground state of C60 can accept electron from the excited state of π-rich compounds such as AN and pyrene to form a charge transfer complex (CTC), resulting in the static quenching of the excited AN. Usually, the static quenching has no effect on the lifetime of the excited state of AN. According to the fluorescent decay of C60@AN-hPEA-1/4 nanorods at different temperatures, the lifetime decreased from 4.40 to 3.67 ns as temperature increased from 20 to 60 °C, which should be resulted from the dynamic quenching by amino groups and C60. However, with the increasing temperature, CTC tends to decompose to the excited AN and C60 and thus emits fluorescence again.56 This means that the static quenching of AN by C60 can be weaken at the higher temperature, leading to the temperatureenhanced fluorescence of C60@AN-hPEA-1/4 nanorods. In summary, both static and dynamic quenching of the excited AN by C60 took place in the temperature-responsive fluorescent behavior of C60@AN-hPEA coassemblies, and the dominant static quenching by C60 is the key factor to the interesting temperature-enhanced fluorescence, which supports again that

C60@AN-hPEA coassemblies should be comprised of both C60 and AN-hPEA. Additionally, the fluorescence of C60@AN-hPEA-1/4 nanorods can also be affected by pH value. As shown in Figure S8, the fluorescence emission intensity decreases while pH increases. This phenomenon can be ascribed to the fact that the fluorescence quenching effect of AN by tertiary amino groups is weakened when amino groups are protonated.47,55 As a result, the fluorescence of C60@AN-hPEA-1/4 nanorods is stronger at a lower pH.

4. CONCLUSION In summary, a novel multiresponsive fluorescent nanorod based on C60 was developed through the coassembly of fullerene C60 and AN-hPEA, followed by the further cross-linking through photodimerization of anthracene. The strong supramolecular interaction between C60 and AN moieties plays a key factor during the process of coassembly. Because of the hydrophilic outer layer of hPEA, the obtained coassemblies of C60@AN3705


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(17) Sathish, M.; Miyazawa, K. I.; Hill, J. P.; Ariga, K. J. Am. Chem. Soc. 2009, 131 (18), 6372−6373. (18) Minato, J.-I.; Miyazawa, K. I. Carbon 2005, 43 (14), 2837−2841. (19) Jun-ichi, M.; Kun’ichi, M.; Tadatomo, S. Sci. Technol. Adv. Mater. 2005, 6 (3−4), 272. (20) Liu, H.; Li, Y.; Jiang, L.; Luo, H.; Xiao, S.; Fang, H.; Li, H.; Zhu, D.; Yu, D.; Xu, J.; Xiang, B. J. Am. Chem. Soc. 2002, 124 (45), 13370− 13371. (21) Guo, Y. G.; Li, C. J.; Wan, L. J.; Chen, D. M.; Wang, C. R.; Bai, C. L.; Wang, Y. G. Adv. Funct. Mater. 2003, 13 (8), 626−630. (22) Kawauchi, T.; Kumaki, J.; Kitaura, A.; Okoshi, K.; Kusanagi, H.; Kobayashi, K.; Sugai, T.; Shinohara, H.; Yashima, E. Angew. Chem., Int. Ed. 2008, 47 (3), 515−519. (23) Pantoş, G. D.; Wietor, J.-L.; Sanders, J. K. M. Angew. Chem., Int. Ed. 2007, 46 (13), 2238−2240. (24) Li, H.; Hollamby, M. J.; Seki, T.; Yagai, S.; Möhwald, H.; Nakanishi, T. Langmuir 2011, 27 (12), 7493−7501. (25) Muñoz, A.; Illescas, B. M.; Sánchez-Navarro, M.; Rojo, J.; Martín, N. J. Am. Chem. Soc. 2011, 133 (42), 16758−16761. (26) Cassell, A. M.; Asplund, C. L.; Tour, J. M. Angew. Chem., Int. Ed. 1999, 38 (16), 2403−2405. (27) Huang, Z.; Kang, S.-K.; Banno, M.; Yamaguchi, T.; Lee, D.; Seok, C.; Yashima, E.; Lee, M. Science 2012, 337 (6101), 1521−1526. (28) Jenekhe, S. A.; Chen, X. L. Science 1998, 279 (5358), 1903− 1907. (29) Laiho, A.; Ras, R. H. A.; Valkama, S.; Ruokolainen, J.; Ö sterbacka, R.; Ikkala, O. Macromolecules 2006, 39 (22), 7648−7653. (30) Zhang, X.; Takeuchi, M. Angew. Chem., Int. Ed. 2009, 48 (51), 9646−9651. (31) Lee, E.; Kim, J.-K.; Lee, M. J. Am. Chem. Soc. 2009, 131 (51), 18242−18243. (32) Hasobe, T.; Sandanayaka, A. S. D.; Wada, T.; Araki, Y. Chem. Commun. 2008, 29, 3372−3374. (33) Guldi, D. M.; Rahman, G. M. A.; Zerbetto, F.; Prato, M. Acc. Chem. Res. 2005, 38 (11), 871−878. (34) de Miguel, M.; Alvaro, M.; García, H. Langmuir 2012, 28 (5), 2849−2857. (35) Takai, A.; Chkounda, M.; Eggenspiller, A.; Gros, C. P.; Lachkar, M.; Barbe, J.-M.; Fukuzumi, S. J. Am. Chem. Soc. 2010, 132 (12), 4477−4489. (36) Maeyoshi, Y.; Saeki, A.; Suwa, S.; Omichi, M.; Marui, H.; Asano, A.; Tsukuda, S.; Sugimoto, M.; Kishimura, A.; Kataoka, K.; Seki, S. Sci. Rep. 2012, DOI: 10.1038/srep00600. (37) Zhou, Z.; Sarova, G. H.; Zhang, S.; Ou, Z.; Tat, F. T.; Kadish, K. M.; Echegoyen, L.; Guldi, D. M.; Schuster, D. I.; Wilson, S. R. Chem.− Eur. J. 2006, 12 (16), 4241−4248. (38) Martin, R. B.; Fu, K.; Sun, Y.-P. Chem. Phys. Lett. 2003, 375 (5− 6), 619−624. (39) D’Souza, F.; Chitta, R.; Sandanayaka, A. S. D.; Subbaiyan, N. K.; D’Souza, L.; Araki, Y.; Ito, O. J. Am. Chem. Soc. 2007, 129 (51), 15865−15871. (40) Ryu, J.-H.; Chacko, R. T.; Jiwpanich, S.; Bickerton, S.; Babu, R. P.; Thayumanavan, S. J. Am. Chem. Soc. 2010, 132 (48), 17227− 17235. (41) Wu, E. C.; Andrew, J. S.; Cheng, L.; Freeman, W. R.; Pearson, L.; Sailor, M. J. Biomaterials 2011, 32 (7), 1957−1966. (42) Wang, J.; Shen, Y.; Kessel, S.; Fernandes, P.; Yoshida, K.; Yagai, S.; Kurth, D. G.; Möhwald, H.; Nakanishi, T. Angew. Chem., Int. Ed. 2009, 48 (12), 2166−2170. (43) Kishi, N.; Li, Z.; Yoza, K.; Akita, M.; Yoshizawa, M. J. Am. Chem. Soc. 2011, 133 (30), 11438−11441. (44) Guldi, D. M.; Zerbetto, F.; Georgakilas, V.; Prato, M. Acc. Chem. Res. 2004, 38 (1), 38−43. (45) Wang, X. F.; Zhang, Y. F.; Zhu, Z. Y.; Liu, S. Y. Macromol. Rapid Commun. 2008, 29 (4), 340−346. (46) Fujita, N.; Yamashita, T.; Asai, M.; Shinkai, S. Angew. Chem., Int. Ed. 2005, 44 (8), 1257−61. (47) Yu, B.; Jiang, X.; Yin, J. Soft Matter 2011, 7 (15), 6853−6862.

hPEA can be well dispersed in aqueous solution and emit fluorescence due to the presence of AN moiety. The fluorescence of C60@AN-hPEA nanorods is responsive to temperature and pH in aqueous solution and exhibit the interesting temperature-enhanced fluorescence. Detailed fluorescence studies revealed that the dominant static quenching of the excited AN by C60 might be the reason to the temperatureenhanced fluorescence behavior of C60@AN-hPEA-1/4 nanorod. These advantages make it a potential application as the smart material in sensors and optical devices.

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ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S8. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel +86-21-54743268; Fax +86-21-54747445; e-mail [email protected] (X.J.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Nature Science Foundation of China (21174085, 21274088), Science & Technology and Education Commission of Shanghai Municipal Government (12ZZ020), and the Shanghai Leading Academic Discipline Project (B202) for their financial support. X. S. Jiang is supported by the NCET-12-3050 Project.

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REFERENCES

(1) Dong, H.; Jiang, S.; Jiang, L.; Liu, Y.; Li, H.; Hu, W.; Wang, E.; Yan, S.; Wei, Z.; Xu, W.; Gong, X. J. Am. Chem. Soc. 2009, 131 (47), 17315−17320. (2) Kim, F. S.; Ren, G.; Jenekhe, S. A. Chem. Mater. 2010, 23 (3), 682−732. (3) Leong, W. L.; Vittal, J. J. Chem. Rev. 2010, 111 (2), 688−764. (4) Wu, Y.; Xiang, J.; Yang, C.; Lu, W.; Lieber, C. M. Nature 2004, 430 (6995), 61−65. (5) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. Adv. Mater. 2003, 15 (5), 353−389. (6) Geng, J.; Zhou, W.; Skelton, P.; Yue, W.; Kinloch, I. A.; Windle, A. H.; Johnson, B. F. G. J. Am. Chem. Soc. 2008, 130 (8), 2527−2534. (7) Mateo-Alonso, A.; Iliopoulos, K.; Couris, S.; Prato, M. J. Am. Chem. Soc. 2008, 130 (5), 1534−1535. (8) Matsuo, Y.; Ichiki, T.; Nakamura, E. J. Am. Chem. Soc. 2011, 133 (25), 9932−9937. (9) Li, H.; Tee, B. C. K.; Cha, J. J.; Cui, Y.; Chung, J. W.; Lee, S. Y.; Bao, Z. J. Am. Chem. Soc. 2012, 134 (5), 2760−2765. (10) Ji, H.-X.; Hu, J.-S.; Wan, L.-J.; Tang, Q.-X.; Hu, W.-P. J. Mater. Chem. 2008, 18 (3), 328−332. (11) Nakanishi, T.; Miyashita, N.; Michinobu, T.; Wakayama, Y.; Tsuruoka, T.; Ariga, K.; Kurth, D. G. J. Am. Chem. Soc. 2006, 128 (19), 6328−6329. (12) Bakry, R.; Vallant, R. M.; Najam-Ul-Haq, M.; Rainer, M.; Szabo, Z.; Huck, C. W.; Bonn, G. K. Int. J. Nanomed. 2007, 2 (4), 639−649. (13) Da Ros, T.; Prato, M. Chem. Commun. 1999, 8, 663−669. (14) Jeong, J.; Jung, J.; Choi, M.; Kim, J. W.; Chung, S. J.; Lim, S.; Lee, H.; Chung, B. H. Adv. Mater. 2012, 24 (15), 1999−2003. (15) Wang, L.; Liu, B.; Yu, S.; Yao, M.; Liu, D.; Hou, Y.; Cui, T.; Zou, G.; Sundqvist, B.; You, H.; Zhang, D.; Ma, D. Chem. Mater. 2006, 18 (17), 4190−4194. (16) Cha, S. I.; Miyazawa, K. I.; Kim, J.-D. Chem. Mater. 2008, 20 (5), 1667−1669. 3706


Macromolecules

Article

(48) Rahman, G. M. A.; Guldi, D. M.; Campidelli, S.; Prato, M. J. Mater. Chem. 2006, 16 (1), 62−65. (49) Shrestha, L. K.; Hill, J. P.; Tsuruoka, T.; Miyazawa, K. I.; Ariga, K. Langmuir 2012, DOI: 10.1021/la304549v. (50) Yao, M.; Andersson, B. M.; Stenmark, P.; Sundqvist, B.; Liu, B.; Wagberg, T. Carbon 2009, 47 (4), 1181−1188. (51) Guldi, D. M. Chem. Soc. Rev. 2002, 31 (1), 22−36. (52) Sarova, G. H.; Berberan-Santos, M. N. Chem. Phys. Lett. 2004, 397 (4−6), 402−407. (53) Kräutler, B.; Müller, T.; Duarte-Ruiz, A. Chem.Eur. J. 2001, 7 (15), 3223−3235. (54) Badamshina, E.; Gafurova, M. J. Mater. Chem. 2012, 22 (19), 9427−9438. (55) Mori, H.; Tando, I.; Tanaka, H. Macromolecules 2010, 43 (17), 7011−7020. (56) Wang, G.-W.; Saunders, M.; Cross, R. J. J. Am. Chem. Soc. 2000, 123 (2), 256−259.

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Responsive Fluorescent Nanorods from Coassembly of Fullerene

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