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PREPARATION AND CHARACTERIZATION OF A NEW CLUSTERED {C70}n FULLERENE
MATERIAL
I.V. Mikheev1, D.S. Volkov1, M.A. Proskurnin1, N.V. Avramenko1, M.V. Korobov1
1Lomonosov Moscow State University, Chemistry Department, Moscow, 119991, Russia
mikheev.ivan@gmail.com
PACS 61.48.-c
A new clustered fullerene material {C70}„ has been prepared and studied using UV/vis spectroscopy, dynamic light scattering and liquid-liquid extraction. The material was prepared from aqueous fullerene dispersions of C70 by ultracentrifugation. The dispersions were obtained via an ordinary solvent-exchange procedure. Liquid-liquid extraction of {C70}„ from water to the organic phase without any extra reagents was observed for the first time. {C70}„ fullerene solid material was made and characterized with transmission electron microscopy and differential scanning calorimetry.
Keywords: fullerenes, solid material, extraction properties, DSC.
1. Introduction
Currently, technologies exist for the production of new fullerene materials [1]. Unique properties of fullerenes find a number of applications in various branches of science and technology. Of much importance are their use for the immobilization and transport of medicines [2-4]. At both cellular and macro levels, their aqueous solutions (aqueous fullerene dispersions, AFDs) are powerful antioxidants that adsorb free radicals and prevent aging and various pathologies like cancer, cardiovascular diseases etc. [2, 5, 6]. Fullerene clusters of certain sizes may create an antiviral cell barrier as well [7, 8]. Preparation of hydrophilic fullerene-based nanomaterials is on demand. Such materials may be prepared from C70 and higher fullerenes rather than from C60 only. An approach to this is to make AFDs from the corresponding fullerene followed by the treatment of the resulting colloidal solution. However, the methods of characterization and analytical control over the properties of such novel AFD-based materials are not well-developed yet. Thus, the preparation and physicochemical studies of the materials based on C60, C70 or higher fullerenes seems rather appropriate.
Thus, the aim of this study is to (i) improve the general procedure for preparing aqueous fullerene dispersions; (ii) to work out an approach for preparing a solid powdered material from fullerene C70; and (iii) to carry out the physicochemical characterization of the prepared clustered material.
2. Materials and Methods
Commercially available fullerene C70 99%+ (LLC "Neo Tech Production", Russia) was used throughout. Chemically pure benzene and toluene ("Reachim", Russia) and water (18.2 MQxcm) from a Milli-Q purification system (Millipore, France) were used. The investigations of dynamic light scattering (DLS) were made using a Malvern "Zetasizer nano ZS" instrument. Spectrophotometric measurements were made using a Varian Cary 50 spec-trophotometer. Ultrasonic treatment was performed with a piezoceramic emitter without
a concentrator immersed in the solution. The resonance frequency was 35 kHz, power up to 0.3 kW, volume 1-1.5 L. A DSC-30 TA Mettler Toledo differential scanning calorimeter was used to capture heating traces from 243 up to 313 K with a scanning rate of 5 K/min. High-resolution transmission electron microscopy (HRTEM) imaging was carried out using a JEM 2100F transmission electron microscope with accelerating voltage 200 kV.
AFD C70 was produced as reported elsewhere [9] with increased power of the sonica-tion system, thus when increasing the quantity of fullerenes, the sonication duration is two weeks. The absorption spectra of AFD C70 completely matched the spectra described in [10, 11]. Aqueous solutions of fullerenes (Fig. 1) were prepared by the following procedure. A weighed portion of fullerene (0.5-0.6 g) was placed into a 200-mL volumetric flask, dissolved in 100 mL of toluene and sonicated for 2 h at 353 K; next, the solution was diluted to mark with toluene. Next, the prepared toluene solution was placed into a conic flask and 1 L of Milli-Q water was added, and the solution was sonicated for 14 days (12 h per day) at the maximum power setting of 0.3 kW at 353 K until complete evaporation of toluene (checked with LC/MS). Next, the solution was heated up for 30 min, and boiled for 15 min and filtered through a Shott filter followed by a 0.45-^ m microporous membrane filter. The concentration of fullerene (130 ± 1 ppm, n =3, P = 0.95) was measured by performing the total organic carbon analysis (TOC). The TOC analyzer was calibrated with a series of potassium hydrogen phthalate (KHP) samples with the 1-200 ppm range for carbon. The least square fit of TOC versus peak area gave c, ppm=(10.2 ± 0.1)Sarea, a.u., P = 0.95, n = 7, r = 0.99863.
2.1. General procedure for preparing {C70}n fullerene material
Solid powered {C70}n materials were produced as follows. AFDs of C70 were concentrated by ultracentrifugation. The solution was centrifuged for 25 min at 30000 rpm (298 K). Next, mother liquor was decanted by injection syringe and concentrated quantity of AFD was heated up to fully evaporation at 378 K. The resulting absorption spectrum of AFD C70 is shown in Fig. 1.
wavelength, nm
Fig. 1. Absorption spectrum of prepared aqueous dispersions of fullerenes C70
2.2. Liquid-liquid extraction AFD C to toluene
Toluene without any extra reagents was added to AFD C70 at a volume ratio of 1 to 1. Next, we performed the ultrasonic treatment at the maximum power setting of 0.3 kW and 378 K for 2 h in a closed test tube to prevent toluene evaporation. Then, we centrifuged the suspension at 4000 rpm for 15 min. Finally, the absorbance spectra of aqueous and organic phases were registered and compared to a toluene solution of pristine C70 fullerenes and original AFD C70 (see Figs. 2 and 3).
2.3. Differential scanning calorimetry (DSC) traces
The samples studied were mechanical mixtures (~1 : 3 by weight) of the solid {C70}ra and the solvent (water or benzene), respectively. A sample of the tested substance was placed into a standard aluminum container and sealed. Calorimetric measurements were carried out at a heating rate of 5 K/min. A Ni-Au thermocouple was used as a detector of the heat flux. The temperature of the sample was measured by a platinum resistance thermometer. Pre-calibration was performed on the basis of data for the enthalpies of melting for indium, lead, and zinc. The calibration of the heat flux was carried out based on data for the enthalpy of melting of an indium standard sample. The enthalpy of melting of the samples studied was calculated from the squares of the corresponding peaks in the DSC trace. The temperature was measured with an accuracy of ±0.2 K.
2.4. DLS analysis and HRTEM
DLS particle size distributions and (—potentials in AFD were measured with a 100 averages of 15 s scans to gain a histogram with the accuracy ca. 5%. The TEM specimens were prepared by dispersing the particle suspensions in ethyl alcohol onto TEM carbon-cupper grids and by exsiccation in vacuum at 10"2 mm Hg.
3. Results and Discussion
3.1. Liquid-liquid extraction
For the first time, we observed the extraction of fullerenes from aqueous solutions to toluene with considerable distribution constant. This extraction distinguishes these experiments from previous reports for AFD C60, where direct extraction was not observed, and AFD C60 has been transferred to the organic phase only after addition of an inorganic salt [12]. We compared the absorption spectra of C70 extracted into toluene from AFD C70 and that of pristine C70 dissolved in toluene (Fig. 2) and those for the aqueous phase of AFD C 70 before and after a sonication treatment during the extraction (Fig. 3). The shapes of spectra in Fig. 2 fully match, and we did not observe any red or blue shifts between the pristine toluene solutions and that from the extraction procedure using dA/dA and d2A/dA2 derivative spectrophotometry in the UV/vis range.
Next, we calculated the recovery of the fullerene from AFD using the absorption spectra of AFD before and after the extraction (Fig. 3), which averaged ca. 97 ± 2%, (n =3, P = 0.95). Based on these results, one may assume that the C70 recovered from AFD is a monomer, not subjected to any chemical modification and C70 clusters in AFD are weakly bonded associates of C70 monomers covered by a hydrophilic shell.
3.2. DLS and HRTEM experiments
The size of AFD C70 was found to be ca. 105, 140, and 160 nm respectively for number, volume and intensity size distributions (Fig. 4). Moreover, we carried out the
0.9
0.0
300
350
400
4S0
500
550 000 650
wavelength, nm
350 400 450 500 550 600 050 700 750 800 850
wavelength, nm
Fig. 2. Absorption spectra of fullerenes C70 extracted from AFD (black solid line) and pristine fullerenes C70 dissolved in toluene (red dash line)
70
Fig. 3. Absorption spectra of AFD C70 before and after sonication treatment for 2 hours
measurements of (—potential for AFD C70, which was estimated to be —40.8 ± 0.9 mV, and a rather low polydispersity index (PDI) of 0.08 was calculated for a AFD concentration of 14.6 fjM.
HRTEM data of the fullerene nanoparticles shows that the material is in fact clustered and shows spherical particles. HRTEM data for the target solid material (see Fig. 5) correlates well with above-discussed DLS data. Thus, we suggest that AFDs are not clumped together into large particles during the evaporation of the residual water. The particle size within the solid clustered material was found to be ca. 134 nm. The average number of C70 molecules in the cluster in terms of the spherical model was estimated at ca. 5 x 105. The density of the cluster was assumed to be 1.7 g/cm3 as in the C70 crystal.
3.3. DSC experiments
Finally, we recorded thermal effects related to the melting of water or, in another case, of benzene in the samples studied (see 2.3). In both cases, two peaks were observed (Figs. 6 and 7) that we believe corresponded to the melting of water (benzene) in the bulk solvent and in the pores within the clustered {C70}n fullerene material. We assume that {C70}n is a powder consisting of nanoclusters similar to nanodiamond powders [13]. Such powders show reproducible porous structures that disperse the solvent. Nanoparticles have solvent-filled pores in the secondary structure, and the extra peak in the DSC trace from the low temperature side corresponds to the solvent melting in the pores. For comparison, DSC experiments were carried out for pristine fullerene C70 mixed with water or benzene. Pristine C70 does not form nanoclusters and the corresponding secondary structure. As expected, in both cases the single peak of melting of the bulk solvent was observed only (Figs. 8 and 9). The differences between two peaks of melting in Figs. 6 and 7, ATm, are 1.0 ± 0.4 K and 3.9 ± 0.4 K for water and benzene, respectively. For a similar {C60}mmaterial with water ATmwas previously reported as equal to 2 K [1].
The Gibbs-Kelvin equation was used to calculate the diameters of the nanophase in pores. We obtained diameters of ca. 40 ± 5 nm and ca. 30 ± 5 nm for water and benzene, respectively. These values are in agreement, which suggests that two solvents occupy the
Fig. 4. Number, volume, and intensity size distributions for the AFD C70 (black solid line) and dispersion diluted by a factor of 10 (red dashed line)
Fig. 5. TEM image of an AFD sample prepared by ul-tracentrifugation technique
Q, mV
-20 -10
10 20
T, c
Q, mET
T, C
Fig. 6. Typical DSC trace of {C70}„ material with two en-dothermic effects for melting of water
Fig. 7. Typical DSC trace of {C70}„ material with two en-dothermic effects for melting of benzene
same voids within the secondary structure formed by C70 aggregates. It is worth noting that solvents with different properties — water and benzene — interact with {C70}„ material in the same manner. Both solvents are dispersed onto the {C70}„ secondary structure. It is also noteworthy that benzene neither dissolves nor distracts C70 aggregates formed in an AFD.
Conclusions
The study clearly demonstrated evidence for the formation of a new C70 based material with unique properties. The material was produced in the form of aqueous fullerene
Q, mV
T, С
Fig. 8. Typical DSC trace of pristine C70 with water. A single endothermic effect observed corresponded to the melting of bulk water
Q, mV-10
T, С
Fig. 9. Typical DSC trace of pristine C70 with benzene. A single endothermic effect observed corresponded to the melting of bulk water
dispersions and as a dry powder. In the first case, it was characterized using UV-vis spectra, dynamic light scattering and liquid-liquid extraction while in the second by means of differential scanning calorimetry and transmission electron microscopy. We observed liquid-liquid extraction of C70 from aqueous fullerene dispersions to monomeric solution in toluene with admissible recovery characteristics. The data obtained allows one to put forward a qualitative schematic structure for the C70 clusters in aqueous fullerene dispersions and in the powder. The developed procedure may be used to prepare clustered materials and aqueous fullerene dispersions for other members of the fullerene family.
Acknowledgments
This work was supported by the Russian Foundation for Basic Research, grants nos. 12-03-00653-a and 12-03-31569-moLa and the Ministry of Science and Technology of Russian Federation, contract no. 16.740.11.0471 and by the grant for Leading Scientific Schools in Russia. We thank Dr. S. Savilov and his group at the scientific center of NanoChemistry and Chemistry of Atmosphere at the Department of Chemistry of MSU for HRTEM images. We thank P.I. Semenyuk at the A.N. Belozersky Institute of Physicochemical Biology of MSU for providing the DLS data.
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