Transcript Slide 1

Materials and Methods
The fullerene nanoparticles (nC60) were produced using
two basic methods, aqu/nC60 and THF/ nC60, previously
described in the literature.1
Virginia Polytechnic Institute and State University
Dynamic Light Scattering (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.
60
35
Aqu/nC60
THF/nC60
50
40
30
30
20
10
25
Aqu/nC60
THF/nC60
20
15
100
1000
10
100
1000
Diameter (nm)
Diameter (nm)
Figure 3. Intensity and volume weighted size distributions for aqu/nC60 and
THF/nC60 in nanopure water after filtration (0.45 μm).
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.
Intensity-weighted
Mean (nm)
Volume-weighted
mean (nm)
Results: The volume weighted distribution
mix/nC60
171
0.139
194
(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
THF/nC60
219
0.029
224
233
a more narrow size range of
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
PDI = (σ/ Zave)2
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.
Nanoparticle crystallinity and shape
a)
3
2
1
0
0 2 4 6 8 10 12 14
Time Stirring (days)
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.
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.
b)
-60
0.01
0.1
1
10
100
1000
Citrate Concentration (mM)
10
PDI
200 nm
-80
0.001
0
Zave
(nm)
-40
10
5
0
-20
Experiment 1a
Experiment 1b
Experiment 2
Experiment 3
50 nm
Peak 1 = 26
Peak 2 = 150
Peak 3 = 289
4
Absorbance
Absorbance
Aqu/nC60 was synthesized using a procedure modified
from Cheng et al.2 C60 powder was added to filtered
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 Figure 1: Aqu/nC solutions after 1, 5
60
period as the fullerene and 14 days stirring (left to right).
particles 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
spectroscopy was used to
Day 2
4
Day 5
monitor the concentration
Day 9
of the water stable
Day 14
3
particles over the two
2
week
stirring
period
(Figure 2). After two
1
weeks of stirring the
0
solutions were settled for
200 300 400 500 600 700 800
one hour and average
Wavelength (nm)
Figure 2: The UV-Vis absorbance particle size and zeta
of C60 in nanopure water increases potential were measured
over the two week stirring period. using DLS.
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.
0
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.
Sodium & Calcium Chloride
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.
Zeta Potential (mV)
The 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.
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.
0
-10
-20
-30
-40
-50
-60
-70
1 mM NaCl
10 mM NaCl
30 mM NaCl
70 mM NaCl
100 mM NaCl
Zeta Potential (mV)
Introduction
Sodium Citrate
Zeta Potential (mV)
Laura K. Duncan, Joerg R. Jinschek, Linsey C. Marr & Peter J. Vikesland
% Volume
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.
Characterization of the size, shape,
crystallinity and surface charge of C60
aggregates formed in aqueous systems
% Intensity
Abstract
0
-5
-10
-15
-20
-25
-30
1 mM CaCl2
5 mM CaCl2
10 mM CaCl2
Figure 6. The zeta potential of aqu/nC60 stirred in solutions of
NaCl and CaCl2 increases with electrolyte concentration.
Conclusions
Results: Particle sizes
c)
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
[112]
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.
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.
Acknowledgments
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.
References
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.