J. Phys. Chem. B 2007, 111, 4315-4319
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ARTICLES Aqueous Dispersions of C60 Fullerene by Use of Amphiphilic Block Copolymers: Preparation and Nonlinear Optical Properties G. Mountrichas,† S. Pispas,*,† E. Xenogiannopoulou,‡,§ P. Aloukos,‡,§ and S. Couris‡,§ Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou AVenue, 11635 Athens, Greece, Institute of Chemical Engineering and High Temperature Chemical Processes, Foundation for Research and Technology-Hellas, P.O. Box 1414, 26504 Patras, Greece, and Physics Department, UniVersity of Patras, 26500 Patras, Greece ReceiVed: December 21, 2006; In Final Form: February 16, 2007
The dispersion of the otherwise insoluble C60 fullerene in water is discussed. Amphiphilic block copolymers, namely, polystyrene-b-polyethylene oxide (PS-PEO), were found to be able to disperse C60 in aqueous solutions, where the polymer forms micelles with a hydrophobic PS core. The preparation protocol of the final solutions was found to play a crucial role in the ability of the block copolymer to disperse the C60 molecules. The C60 containing aggregates were studied using optical spectroscopy, light scattering, and scanning electron microscopy. In addition, their optical limiting action and nonlinear optical properties under visible nanosecond laser excitation were studied and compared with that of C60-toluene solutions.
Introduction Solution behavior of block copolymers is of intense interest in the scientific community because of their ability to selforganize in selective solvents (i.e., solvents that are good for one block but precipitants for the other).1 The aforementioned ability of these polymers serves in a wide gamut of applications including drug delivery systems,2 nanotemplating,3 and solubilization of otherwise insoluble materials.4 Significant interest is presented for amphiphilic polymers able to self-assemble in aqueous solution because of their potential use mainly in nanomedicine, as well as in environmentally friendly procedures and applications. Typically, block copolymers consisting of poly(ethylene oxide) (a water-soluble and biocompatible polymer) and a hydrophobic block (where the chemical nature of which depends on the application of interest) are used in an extensive spectrum of applications.5 Otherwise, polymers with a water-soluble polyelectrolyte block instead of poly(ethylene oxide) are used.6 On the other hand, since the discovery of fullerene,7 a nanotechnology revolution has been started. Moreover, the development of a large-scale production method has enforced heavy studies on the properties of this material, while many of these properties are novel or unusual. The low solubility of fullerene in a plethora of solvents, especially in water,8 is the major limiting factor on further studies and applications for this pioneering material, considering that the exclusive fullerene’s properties are strictly related to the isolation of individual molecules. Many interesting approaches on the solubilization * Corresponding author. Tel.: +30210-7273824. Fax: +30210-7273794. E-mail:
[email protected]. † National Hellenic Research Foundation. ‡ Foundation for Research and Technology-Hellas. § University of Patras.
of C60 have been presented in the literature. Strong efforts have been made by chemical modification of fullerene.9 However, alternations on the unique structure of this molecule were observed to have a destructive influence on the desired properties. Instead of chemical modification, the use of ultrasound10 and block copolymers4a,b were employed for the solubilization of this molecule without changing its chemical structure. Nevertheless, nondestructive solubilization of fullerenes in aqueous media through polymer encapsulation has not yet been reported. In the present paper, we describe a novel and general method for the preparation of stable aqueous dispersions of C60 fullerene in different concentrations by the use of amphiphilic block copolymers, namely, poly(styrene-b-ethylene oxide). The preparation protocol, as well as the characterization of the dispersions by scanning electron microscopy, UV-vis spectroscopy, and dynamic light scattering, is discussed. Moreover, polymers with different molecular weights and composition are used to elucidate the influence of the molecular characteristics on the dissolution protocol. In addition, the nonlinear optical response (NLO) and optical limiting (OL) efficiency of these aqueous dispersions of C60 were investigated in the visible spectral region, employing 8 ns laser pulses at 532 nm, and were compared with the corresponding properties of equal transmission C60-toluene solutions. Experimental Procedures Block Copolymer Synthesis. The synthesis of two block copolymers, with different molecular weights and composition, has been realized by anionic polymerization high vacuum techniques.11 sec-Butyllithium was employed as the initiator and benzene as the solvent. Styrene was the first monomer to be polymerized at 25 °C followed by the polymerization of ethylene
10.1021/jp068796x CCC: $37.00 © 2007 American Chemical Society Published on Web 04/06/2007
4316 J. Phys. Chem. B, Vol. 111, No. 17, 2007 oxide in the presence of phosphazine, at an elevated temperature, 40 °C. The polymers, denoted as SEO-1 and SEO-2, were characterized by size exclusion chromatography, 1H NMR, and attenuated total reflectance FTIR (ATR-FTIR) to obtain the exact molecular characteristics and to confirm the uniformity of the sample. SEO-1 and SEO-2 have molecular weights, Mw ) 24 600 and Mw ) 2550, and polydispersities, I ) 1.06 and I ) 1.07, while the portion of polystyrene is 35 and 12 wt %, respectively. Dispersion Preparation Protocol. Distilled water and analytical grade toluene (from Aldrich) were used to prepare the micellar solutions. Polymer solutions, with the encapsulated C60 (obtained from Aldrich) in the core, were prepared by a threestep process. The first step was the addition of a predetermined amount of C60 fullerene (saturated solution in toluene) in a micellar solution of the diblock copolymer in toluene (∼2 × 10-2 g/mL), a solvent where the formation of micelles with the PEO as the core block is favored, at temperatures lower than 25 °C. In the second step, evaporation of toluene at room temperature took place, leading to film formation. Finally, redissolution of the formed film in water (final polymer concentration 1 × 10-2 g/mL), with simultaneous inversion of the micellar structure (i.e., polystyrene is the core forming block), was realized by incubation of the solutions at 60 °C overnight. Solutions of SEO-1 and SEO-2 in water, with different C60 fullerene contents, were prepared. In the case of SEO-2, the solutions were filtered after 1 week through a 0.45 µm filter. Measurements. Dynamic light scattering measurements (at 90°) were performed on a ALV/CGS-3 Compact Goniometer System (ALV GmbH, Germany), using a JDS Uniphase 22 mW He-Ne laser, operating at 632.8 nm, and an avalanche photodiode detector, interfaced with a ALV-5000/EPP multi-tau digital correlator with 288 channels and a ALV/LSE-5003 light scattering electronics unit for stepper motor drive and limit switch control. Autocorrelation functions, from DLS measurements, were collected five times for each solution, and they were analyzed by the cumulants method and the CONTIN routine. Typically, correlation functions were collected for 20 s. Fits to the correlation functions were made using the software provided by the manufacturer. A “probability one to reject” of 0.5 was used routinely in the CONTIN analysis software. All the measurements were performed in diluted solutions at room temperature. UV-vis absorption spectra were recorded on a PerkinElmer (Lamda 19) spectrophotometer in the range of 300-700 nm. A scanning electron microscope (Zeiss, Supra 35VP) having a 1.5 nm resolution at 20 kV was used for studying the morphology of SEO aggregates incorporating C60 from aqueous solutions. Droplets of C60/SEO with different contents of C60 in H2O were cast on metal slabs suitable for SEM observation and then dried in a furnace at 60 °C. The specimens’ conductivity was improved by coating them with gold using a sputtering device. The details of the Z-scan and optical limiting experimental setups have been described in detail elsewhere.12 The laser source used was a 8 ns Q-switched Nd:YAG laser operating at its second harmonic at 532 nm at a repetition of 10 Hz. All prepared dispersions and solutions were placed into 1 mm quartz cells, and their UV-vis absorption spectra were always measured before and after irradiation to check for undesirable degradation of the studied samples. The methodology for the analysis of the observed nonlinear optical response has been described in detail elsewhere.13
Mountrichas et al. SCHEME 1: Three-Step Preparation Protocola
a From left to right: (a) fullerene and inverse micelles in toluene; (b) film of randomly dispersed C60 and microphase separated polymer; and (c) aqueous solution of C60 enclosed in micellar cores.
Results and Discussion Preparation Protocol. The preparation of the final solution was realized in a three-step protocol depicted schematically in Scheme 1. In the first step, the two components of the solution, polymer and C60, were mixed in toluene. It is well-known that toluene is a good solvent for C60 molecules, with a solubility of 2.8 mg/mL.8 Moreover, toluene is a selective solvent for the copolymer, in particular, it is a good solvent for polystyrene but poor for poly(ethylene oxide). Upon dissolution of the polymer in toluene, micellar structures are formed. Indeed, the solutions had an intense bluish tint, a typical observation in micellar systems. It must be mentioned that the characteristic bluish tint disappeared by increasing the solution temperature to ∼30 °C; thus, temperatures lower than 25 °C seem to be essential for the reproduction of the experiments. The amount of C60 solution that was added in the polymer solution was very small, leading to no appreciable changes in copolymer concentration. After mixing the two components, the solution turned brown, probably because of polymer-C60 interactions. In a second step, the solvent was evaporated slowly at room temperature. It is well-established that C60 is an excellent electron acceptor (EA ) 2.6-2.8 eV)14 spherical molecule. Donor-acceptor complexes are easily formed with electron donor molecules like polyethylene oxide, due to electrons at the ether oxygens. On the other hand, polystyrene can easily interact with fullerenes.4b,15 Therefore, a homogeneous dispersion of C60 in the polymer matrix is expected. Finally, dissolution of the polymer film was realized by the addition of water and by heating at 60 °C overnight. It must be noted that the use of ultrasonication was not performed to avoid any possible damage to the fullerene structure. In addition, it is worth mentioning that attempts involving codissolution of the polymer and C60 in water by ultrasonication have led to unstable mixed aqueous solutions. However, following the aforementioned three-step protocol, the solutions were stable for over a 9 month period, indicating the success of the protocol on the dispersion of fullerene in water. A series of solutions with different amounts of C60 were prepared. The amount of fullerene added each time was 1, 5, 10, and 20 wt % with respect to the polystyrene block. It is noticeable that films cast from solutions with a higher content of C60 are not fully dissolved in water; thus, the concentration of 10 wt % C60 with respect to polystyrene was assumed to be the upper encapsulation concentration limit in this work for both of the copolymers under study. In the particular case of SEO-1, a final concentration of 0.35 mg of C60/mL was achieved, while in the case of SEO-2, the solubility attained was ca. 0.12 mg of C60/mL. The difference can be attributed to the lower PS content of SEO-2 and possibly to its lower molecular weight, which results in a lesser ability to stabilize the hybrid structures in solution as well as to encapsulate fullerene. As compared to the data reported in ref
Aqueous Dispersions of C60 Fullerene
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Figure 1. Photographs of the C60 solutions with SEO-1. From left to right: 1 wt %, 5 wt %, and 10 wt % C60 with respect to polystyrene. Figure 3. Dynamic light scattering results of C60/SEO-1 solutions for different C60 concentrations (neat SEO-1 dissolved with the same protocol (solid line), 1 % wt C60 (dashed line), 5 % wt C60 (dotted line), and 20 % wt C60 (dashed-dotted line)).
Figure 2. UV-vis spectrum of a diluted C60/SEO-2 (10 wt % C60 to polystyrene) solution in water (solid line) and of net SEO-2 (dashed line).
10 on C60 dispersions in aqueous media, the solubility of the fullerene is enhanced in the presence of the amphiphilic block copolymers (it is more than two times higher for the case of sample SEO-1/C60 mixtures). The ability of this type of block copolymer, with a polystyrene and a polyethylene oxide block, to solubilize C60 can apparently be attributed to the chemical structure of the core forming block. The core forming PS block is a very friendly matrix for C60. From the known solubility parameters of C60 and PS (equal to 10 and 9.1 (cal/cm3)1/2 at 25 °C, respectively), the Flory-Huggins interaction parameter, χ, could be calculated. The resulting parameter, χ ) 0.015, indicates a strong interaction between C60 and PS.4a On the other hand, the well-solvated PEO chains provide stabilization to the hybrid aggregates. The homogeneous dispersions of C60 in aqueous solutions are stable for long periods. In Figure 1, a photograph of the solutions after standing for 9 months at room temperature is shown. The intensity of the characteristic brown color in the solution is clearly increased by increasing the fullerene content in the solutions. The existence of C60 molecules, enclosed in the SEO micelles, was confirmed by UV-vis spectroscopy. A characteristic UV-vis spectrum of the neat copolymer and of the 10 wt % C60 after dissolution is presented in Figure 2. The characteristic peak at 334 nm is attributed to the presence of C60 in the solution. Moreover, the previously mentioned characteristic peak of C60 originally appeared at 330 nm; however, a red-shift to 334 nm is observed, due to the presence of water as has been previously reported.16 Dynamic light scattering measurements gave the aggregate populations in solutions and their average diameter. In all cases, rather polydisperse populations are observed. The analysis of correlation functions using CONTIN gives more than one population almost in all solutions investigated, whereas the
polydispersity is increased upon increasing the fullerene content (Figure 3). In general, it should be mentioned that the higher the C60 content, the larger the mean apparent population diameter (estimated from cumulants analysis). Moreover, the use of a block copolymer with a lower overall molecular weight seems to lead to smaller aggregates, but the polydispersity remains high. The aggregates encapsulating C60 have been also studied on solid surfaces by SEM. The SEM image of a film resulting from the solution with 10 wt % C60 is shown in Figure 4A. In this image, the well-shaped spherical structure of the formed micelles is evident, as well as the presence of more than one population of aggregates. In Figure 4B, the image from a 1:20 diluted 10 wt % C60 solution is given, where the spherical structures and the heterogeneity of the diameter among the micelles can be seen more clearly. SEM images of solutions with 1 and 5 wt % C60 (not shown), as well as from solutions where SEO-2 has been used instead of SEO-1, lead to the same observations. The results from DLS and SEM agree well, leading to the conclusion of a multi-population micellar solution, where the micelle diameter varies with C60 content. This may be a result of the preparation protocol and particularly the step of redissolution of the films in water. This is corroborated by the fact that even in the case of the pure diblocks, aqueous solutions containing two populations of aggregates are obtained. This must be a consequence of the required phase inversion in the aggregate structure during a change of solvents going through the solid state. The larger aggregates may come from hydrophobic interactions or entanglement effects among block copolymer chains in the solid film state that are not completely overridden when redissolution in water takes place. Obviously, the situation becomes more dramatic when C60 is present in the solid films, increasing the hydrophobic interactions in the hybrid material. Secondary aggregation of initially formed aggregates can also be operative in these systems due to the hydrophobicity of the components. Z-scan measurements were performed to investigate the nonlinear optical response of the prepared C60 micelle dispersions and to compare it with that of C60-toluene solutions. As an example, the NLO response of SEO-1/5 wt % C60 is presented. The transmission of the initially prepared SEO-1/5 wt % C60 was about 62%. However, to obtain dispersions with transmissions comparable to that of the C60-toluene solutions studied here, dilutions in distilled water were prepared. In particular, dilutions of 1:2, 1:3, and 1:6 per volume of the SEO1/5 wt % C60 in water were prepared, having 76, 84, and 87% transmission, respectively. These dilutions had the same ratio
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Figure 4. SEM images of C60/SEO-1 10 wt % C60 to polystyrene. (A) Original solution and (B) diluted solution (1 to 20 mL final solution). Spherical micellar structures with different sizes are shown in the squares.
In Figure 6, the optical limiting actions of a SEO-1/5 wt % C60 dispersion, a SEO-1 water solution, and a 1.1 mM C60toluene solution are shown. All three samples exhibit the same linear transmittance of about 80%. As can be seen, the SEO1/5 wt % C60 dispersion exhibited a slightly less efficient optical limiting action as compared to that of a C60-toluene solution. Moreover, the threshold for achieving OL was relatively higher for the dispersions than that for the toluene solutions. Similarly, the water solutions of SEO-1 exhibit slightly less efficient optical limiting than that observed for the dispersions. Considering the previous experimental evidence and taking into account that the content of C60 present in the SEO-1/5 wt % C60 dispersions is significantly smaller than that of equal transmission C60-toluene solutions, it can be concluded that the water dispersions of C60 can be efficient optical limiters. Conclusion Figure 5. Nonlinear absorption parameter β vs incident laser intensity of various water diluted dispersions of SEO-1/5 wt % C60 and a 2.7 mM C60-toluene solution.
Figure 6. Optical limiting of a SEO-1/5 wt % C60 water dispersion, a C60-toluene solution, and a polymer SEO-1 water solution exhibiting the same linear transmittance of 80% at 532 nm.
of molecules of C60 versus SEO-1 (i.e., C60/SEO-1), but their linear absorption was reduced accordingly. In Figure 5, the intensity dependence of the nonlinear absorption parameters β of different aqueous dissolutions of SEO-1/5 wt % C60 and of a 3 mM C60-toluene solution is presented. As can be seen, the determined nonlinear absorption parameter β was found to remain constant as the incident laser intensity was increasing, while it was decreasing upon dilution (i.e., reducing the number density of C60). The determined values of β were found to be of the order of 10-10 m/W for all SEO-1/5 wt % C60 dilutions studied.
The ability of amphiphilic PS-PEO block copolymers to encapsulate C60 fullerene in water has been presented. The block copolymers under investigation consist of a hydrophobic block, polystyrene, and a water-soluble block, polyethylene oxide. The previous polymers were found to be able to disperse C60 fullerene in water by enclosing the fullerene in the micellar core. The dispersion was prepared by a three-step protocol that involves codissolution in toluene, casting, and redissolution in water with inversion of micellar structure. The C60-polymer dispersions were studied by optical spectroscopy, dynamic light scattering, and scanning electron microscopy. The existence of C60 in the final solution was confirmed using the previous techniques, while polydisperse micellar structures were observed upon the addition of C60 fullerene. Finally, the water dispersions of SEO-1/5 % wt C60 were found to exhibit similar nonlinear optical properties as compared to the C60-toluene solutions, while their optical limiting efficiency was found to be at least similar if not better. Acknowledgment. The NHRF group acknowledges financial support through the project Excellence in the Research Institutes (Phases I and II) of GSRT. Partial financial support through a GSRT PENED-2001 grant (395) and the EU NANOPHOS research project (IST-2001-39112) is also acknowledged. P.A. acknowledges support through a HERAKLEITOS-EPEAEK II grant of GSRT. References and Notes (1) (a) DeVelopments in Block Copolymer Science and Technology; Hamley, I. W., Ed.; John Wiley and Sons: New York, 2004. (b) Nanoscale Assembly: Chemical Techniques; Lockwood, D. J., Ed.; Springer Science and Business Media, Inc.: New York, 2005.
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