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Characterization of the size, shape, crystallinity and surface charge of C60

aggregates formed in aqueous systemsLaura K. Duncan, Joerg R. Jinschek, Linsey C. Marr & Peter J. Vikesland

Virginia Polytechnic Institute and State UniversityThe aggregates formed by this method are heterogeneous in size (20 nm and larger) and shape (angular to round), but are crystalline in structure – exhibiting a face centered cubic (FCC) system. In addition, particle shape and surface charge change when C60 is mixed in the presence of electrolytes (NaCl, CaCl2) or sodium citrate at concentrations from 1 to 100 mM. These changes in solution composition affect aggregate formation and stability and suggest that C60 fate and transport will be a function of the composition of the solution.

Figure 1: Aqu/nC60 solutions after 1, 5 and 14 days stirring (left to right).

Dynamic Light Scattering (DLS)

Zeta potential increases and the particle surfaces become less negatively charged as NaCl and CaCl2 are added to solution. The effects of CaCl2 are much more dramatic due to its divalent nature. This change in zeta potential is caused by a reduction in the electrostatic double layer surrounding the particles.

Funding for this research is provided by a National Science Foundation Grant and the Via Fellowship. The authors would like to thank Stephen McCartney help with the TEM.

about 100 to 500 nm. Hydrodynamic diameter (Zave) and a polydispersity index (PDI) were calculated using the cumulants algorithm which assumes a single exponential decay. The PDI is a measure of standard deviation of the Gaussian distribution generated about the Z-ave and are described in the following equation:Large PDI values indicate either a very broad monomodal distribution or the presence of multiple peaks within a distribution, while small PDI values indicate narrow monomodal distributions. THF/nC60 has a PDI of 0.029, which is far smaller than the PDI for aqu/nC60 of 0.139. The higher PDI value indicates a more heterogeneous distribution of particle in the aqu/nC60 system.

Abstract

The discovery that negatively charged aggregates of C60 are stable in aqueous environments has elicited concerns regarding the potential environmental and health effects of these aggregates. Although many previous studies have used aggregates synthesized using intermediate organic solvents, this work employed an aggregate production method believed to more closely emulate the fate of fullerene upon accidental release – extended mixing in water.

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The fullerene nanoparticles (nC60) were produced using two basic methods, aqu/nC60 and THF/ nC60, previously described in the literature.1

Aqu/nC60 was synthesized using a procedure modified from Cheng et al.2 C60 powder was added to filtered

Introduction

Materials and Methods

TABLE 1. Polydispersity index, intensity-weighted mean, volume-weighted mean, and of nC60 solutions prepared with different methods. Values are the average of two or

more solutions.

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mix/nC60 171 0.139 194Peak 1 = 26

Peak 2 = 150Peak 3 = 289

THF/nC60 219 0.029 224 233

THF/nC60 incorporates more recent modifications1 of the original procedure developed by Deguchi et al.3 C60 powder was added (25 mg/L) to THF and the resulting solution was purged with argon to remove dissolved oxygen, stirred overnight and then filtered. 250 mL of filtered nanopure water were added at a rate of 1 L/min to an equal volume of THF-C60, while stirring rapidly. The THF was then removed using a rotary evaporator and the final solution, containing C60 aggregates in water, was filtered though a 0.45 μm cellulosic filter.

Sodium & Calcium Chloride

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Conclusions

1. Brant, J. A.; Labille, J.; Bottero, J. Y.; Wiesner, M. R., Characterizing the impact of preparation method on fullerene cluster structure and chemistry. Langmuir 2006, 22, (8), 3878-3885.

2. Cheng, X.; Kan, A. T.; Tomson, M. B., Naphthalene adsorption and desorption from aqueous C60 fullerene. Journal of Chemical & Engineering Data 2004, 49, (3), p 675-683.

3. Deguchi, S.; Alargova, R. G.; Tsujii, K., Stable dispersions of fullerenes, C60 and C70, in water. Preparation and characterization. Langmuir. 2001, 17, (19), p 6013-6017.

4. Dravid, V. P. L., S.L.; Kappes, M.M., Transmission electron microscopy of chromatographically purified solid state C60 and C70. Chemical Physics Letters 1991, 185, (1,2), 75-81.

Fullerene was added to sodium citrate solutions (0.01 to 100 mM) and stirred for two weeks. At low concentrations (1 mM) the citrate had a stabilizing effect reflected by a decrease in the zeta-potential from about -47 mV for nanopure water to -65 mV for 1 mM solutions of citrate (Figure 5). At higher citrate concentrations the stabilizing effect was overcome by a reduction in the repulsive electrostatic forces as the ionic strength of the solutions increased.

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Acknowledgments

nanopure water at a concentration of 0.8 mg/mL and mixed with a magnetic stirrer for two weeks. The solutions darken from clear to brown (Figure 1) over the two week period as the fullereneparticles gain a negative charge and become stable in solution. A series of experiments was conducted to characterize the fullerene nanoparticle formation process. Size distributions of the smaller particles in solution were obtained by filtering the solution with a 0.45 μm cellulosic filter and measuring the particle size with dynamic light scattering (DLS). In another series of three experiments the C60 was first pulverized (to decrease variability in solution concentration caused by variation in initial particle size) and then added to solutions containing NaCl, CaCl2, sodium citrate or filtered nanopure water. UV-Vis

Figure 2: The UV-Vis absorbance of C60 in nanopure water increases over the two week stirring period.

spectroscopy was used to monitor the concentration of the water stable particles over the two week stirring period (Figure 2). After two weeks of stirring the solutions were settled for one hour and average particle size and zeta potential were measured using DLS.

Size distributions for aqu/nC60 and THF/nC60 were obtained using a Malvern Zetasizer 3000HS equipped with a helium/neon laser ( = 633 nm) and a 10 mm wide measuring cell. Samples were sonicated briefly prior to measurement. Five size measurements were made on each sample to ensure accuracy and repeatability. The instrument was set to automatically select an appropriate runtime based on sample characteristics. The refractive index1 of C60 was set as 2.2 and temperature was held at 25 C internally by the instrument. Data were analyzed using the Contin algorithm. Size distributions for the two nC60 preparation methods were analyzed using a size range from 20-2000 nm.

Results: The volume weighted distribution (Figure 3) indicates that aqu/nC60 contains a heterogeneous assortment of particles ranging in size from 20 nm to 500 nm, while THF/nC60 has a more narrow size range of

a)

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Figure 3. Intensity and volume weighted size distributions for aqu/nC60 and THF/nC60 in nanopure water after filtration (0.45 μm).

PDI = (σ/ Zave)2

Figure 4. HRTEM images of aqu/nC60 acquired at 200 kV with a FEI Titan scanning/transmission electron microscope (S/TEM). (a) Overview image of aqu/nC60 shows that particles are heterogeneous in size and shape. (b) Visible lattice fringes reveal the crystalline nature of the small particles. (c) Lattice image of a face centered cubic (FCC) crystal in [112] orientation with inset Fast Fourier Transform (FFT) image produced from the selected region.

A high-resolution transmission electron microscope (HRTEM) was used to investigate the morphology and crystallinity of the small particles in solution. Samples were prepared by placing one drop of solution on a carbon coated copper mesh grid and allowing the sample to air dry. Ultrathin carbon grids with a holey carbon support were selected to allow greater visibility of the carbon nanoparticles.

[112]

Results: Particle sizes observed in the HRTEM images are consistent with DLS measurements which indicate the presence of both large and very small particles (down to 20 nm in diameter). Although isolated particles of various sizes were found, the majority of particles

were in aggregate form. Some of the particle aggregation is likely to have occurred during drying of the samples. Diffraction patterns from two aqu/nC60 crystals in [112] and [011] orientations were analyzed, and indicate that the crystals are in the face centered cubic (FCC) crystal class, which is consistent with C60 in bulk crystalline form4. Lattice fringes, created by the crystal structure, were observed in both small and large particles. The larger particles have more complex and angular shapes than the small ones due to pronounced faceting. The smaller particles could be products of advanced weathering of large particles or nanocrystals formed during the two weeks of stirring.

Figure 5. Zeta Potential of aqu/nC60 stirred in solutions of sodium citrate (0.01- 100 mM). TEM images are inset to show the change in particle morphology from irregular to spherical as citrate concentration increases.

Aqu/nC60 concentration of water stable particles continues to increase over the course of the two week stirring period.The particles have the same crystal structure as bulk C60 and may form as a result of weathering of larger particles to small ones or new crystal formation. The presence of electrolytes and citrate can increase or decrease the stability of aqu/nC60 and may therefore have a strong effect on the environmental fate and transport of C60.

Figure 6. The zeta potential of aqu/nC60 stirred in solutions of NaCl and CaCl2 increases with electrolyte concentration.

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