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www.rsc.org/nanoscale
ISSN 2040-3364
2040-3364(2010)2:1;1-T
COVER ARTICLEGraham et al.Mixed metal nanoparticle assembly and the effect on surface-enhanced Raman scattering
REVIEWLin et al.Progress of nanocrystalline growth kinetics based on oriented attachment
Volume 2 | N
umber 1 | 2010
Nanoscale
Pages 1–156
www.rsc.org/nanoscale Volume 2 | Number 1 | January 2010 | Pages 1–156
NanoscaleView Article OnlineView Journal
Page 2
Table of contents
Internalization of carbon nanotubes decorated with iron oxide nanoparticles by tumor cells
allows their magnetic manipulation and detection at single cell level by high resolution
magnetic imaging resonance, offering new opportunities for targeted therapy.
Endowing carbon nanotubes with superparamagnetic properties: applications for cell
labeling, MRI cell tracking and magnetic manipulations
Giuseppe Lamanna, Antonio Garofalo, Gabriela Popa, Claire Wilhelm, Sylvie Bégin-Colin,*
Delphine Felder-Flesch,* Alberto Bianco,* Florence Gazeau* and Cécilia Ménard-Moyon*
ToC figure
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Endowing carbon nanotubes with superparamagnetic properties:
applications for cell labeling, MRI cell tracking and magnetic
manipulations
Giuseppe Lamanna,a Antonio Garofalo,
b Gabriela Popa,
b Claire Wilhelm,
c Sylvie Bégin-
Colin,*b Delphine Felder-Flesch,*
b Alberto Bianco,*
a Florence Gazeau*
c and Cécilia
Ménard-Moyon*a
Abstract
Coating of carbon nanotubes (CNTs) with magnetic nanoparticles (NPs) imparts novel
magnetic, optical, and thermal properties with potential applications in the biomedical
domain. Multi-walled CNTs have been decorated with iron oxide superparamagnetic NPs.
Two different approaches have been investigated based on ligand exchange or "click
chemistry". The presence of the NPs on the nanotube surface allows conferring magnetic
properties to CNTs. We have evaluated the potential of the NP/CNT hybrids as contrast agent
for magnetic resonance imaging (MRI) and their interactions with cells. The capacity of the
hybrids to magnetically monitor and manipulate cells has also been investigated. The
NP/CNTs can be manipulated by remote magnetic field with enhanced contrast in MRI. They
are internalized into tumor cells without showing cytotoxicity. The labeled cells can be
magnetically manipulated as they display magnetic mobility and are detected at a single cell
level through high resolution MRI.
1. Introduction
The continuous growing of nanotechnology has brought to many innovations in medicine,
particularly revolutionizing the field of imaging and therapy.1 The main input of
nanotechnology in the biomedical field, at the present time, is that it allows a real progress
towards temporal and spatial site-specific drug delivery, local therapy, and imaging.2 In this
context, the association of the magnetic properties of iron oxide nanoparticles (NPs) to the
characteristics of carbon nanotubes (CNTs) can open new possibilities in the development of
multimodal imaging and therapy platforms. A targeted delivery will enhance the efficacy of a
treatment by localizing the complex at the site of disease, and it will permit activation and
spatial manipulation in vivo via magnetic stimulation.3 CNTs are promising for biomedical
applications as they are capable of crossing many biological and biophysical barriers.4
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Because of their high surface area, CNTs are efficient template for the assembly of
nanoparticles5 and versatile carrier for a wide variety of bioactive molecules.
6 Due to their
magnetic properties and mainly their very high transverse relaxivity, functionalized iron oxide
NPs are widely developed as negative contrast agent for magnetic resonance imaging (MRI),
for therapy with magnetically-induced hyperthermia, and for cell labeling.7 The development
of multimodal contrast agents by functionalization or combination of NPs with other
nanomaterials, such as CNTs, could allow reaching effective MRI contrast enhancement,
while combining multiple functionalities for therapy. Indeed, NP/CNT hybrids possessing
novel magnetic, optical, and thermal properties could offer the potential for imaging,
targeting, as well as hyperthermia.8 Fe3O4/CNT hybrids were shown to exhibit MRI
enhancement effect on cancer cells and allow strong MRI contrast in vivo.8b
The magnetic
properties of CNTs decorated with Fe3O4 NPs have also been exploited for targeting of cancer
cells.8e
Various preparation methods for the decoration of CNTs with NPs, leading to highly
anisotropic ferromagnetic nanomaterials, have been reported, but the synthesis of such type of
materials for theranostic applications still remains highly challenging. Few studies reported
the filling of CNTs with iron oxide NPs,9 whereas most studies described the coating of the
nanotube surface with NPs. For instance, CNTs have been decorated with iron oxide NPs via
in situ generation of the nanoparticles by solvothermal synthesis.8b,10
However, this method
requires rigorous conditions and the control of the particle size is rather difficult.
Alternatively, different strategies have been developed for the decoration of CNTs with pre-
formed iron oxide NPs based on non-covalent interactions11
(i.e. electrostatic self-assembly,
π-stacking)12,13
and sol-gel process.14
Other approaches involved covalent grafting of NPs on
CNTs by amidation.15
In this article, we explored two strategies for the efficient decoration of
multi-walled carbon nanotubes (MWCNTs) with controlled size iron oxide superparamagnetic
NPs based on ligand exchange or on chemoselective ligation (i.e. "click chemistry"). The
tethered iron oxide NPs imparted magnetic properties to CNTs. Then, we evaluated the
potential of the NP/CNT hybrids as contrast agent for MRI, their interactions with cells and
their capacity to magnetically monitor and manipulate cells.
2. Preparation of NP/CNT hybrids
2.1 Preparation of NP/CNTs by ligand exchange
Iron oxide superparamagnetic NPs were synthesized by thermal decomposition of iron
stearate in high boiling point solvent (i.e. octyl ether) and in the presence of a fatty acid (i.e.
oleic acid) to coat the NPs and improve their colloidal stability in organic solvents.16
The
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mean diameter determined from transmission electron microscopy (TEM) images is 10.0 ±
0.5 nm (see Supplementary Information, Figure S1). The colloidal stability of the suspension
of non-aggregated NPs was also confirmed by dynamic light scattering measurements. The
particle hydrodynamic diameter distribution is monomodal and is centered at 13.8 nm (Figure
S2). This value is slightly higher than the mean diameter measured from TEM images because
of the oleic acid surface coating.
To decorate the nanotube surface with iron oxide NPs, we first investigated an
approach based on ligand exchange by mixing ox-MWCNTs 1 and iron oxide NPs 2 coated
with oleic acid (Figure 1). This procedure led to decoration of the nanotube surface with high
loading of NPs via replacement of oleic acid in the coating layer of NPs by carboxyl moieties
present on the surface of oxidized MWCNTs.17
1
+
3
Fe
Fe
THF
O O
7
7
CH3
2
O
OH
O
O
Figure 1. Decoration of MWCNTs with iron oxide NPs by ligand exchange between NPs
coated with oleic acid and oxidized MWCNTs. For clarity, the representation of the NPs
displays only one oleic acid molecule in the coating layer.
After filtration and washings to remove excess of iron oxide NPs, the NP/CNT hybrids
3 were observed by TEM (Figure 2).
Figure 2. TEM images of NP/CNTs 3.
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Nearly all the NPs were bound on the nanotubes and only a few were found isolated
on the TEM grids. The NPs on the nanotube surface tended to form aggregates, likely due to
attractive forces between the NPs such as van der Waals and magnetic dipolar interactions, as
already observed during the deposition of NPs on mixed self assembled monolayers
displaying CH3 and COOH head groups.17
2.2 Preparation of NP/CNTs by click ligation
To better control the anchoring of NPs on CNTs and reduce NP agglomeration, we explored a
strategy based on chemoselective ligation by click chemistry. The so-called click approach
relying on the Cu(I)-catalyzed azide-alkyne Huisgen 1,3-dipolar cycloaddition (CuAAC)
reaction is efficient, highly selective and it can be performed in mild conditions.18
To the best
of our knowledge, only one study reported the decoration of CNTs with iron oxide NPs by
click reaction between azide-modified NPs and CNTs functionalized with a polymer
containing alkyne groups.19
To perform the chemoselective ligation through click chemistry,
we prepared NPs coated with a dendron bearing an azide group and CNTs functionalized with
alkyne moieties.
To improve biocompatibility and stability of iron oxide NPs, we recently developed
small-sized dendrons displaying at their focal point phosphonic acid groups, which are very
strong anchors for the surface of iron oxide NPs.20
Dendrons have a controlled molecular
structure and a high versatility. Therefore, they have found interest for the functionalization of
magnetic nanoparticles to prepare biocompatible and polyfunctional water-soluble NPs. We
have previously demonstrated that the covalent attachment of hydrophilic PEGylated
dendrons to iron oxide NPs through a phosphonate anchor leads to versatile MRI contrast
agents with high relaxivity.20b,20c
The coating of the NPs also confers good colloidal stability
in physiological media by electrostatic and steric repulsions, thus reducing agglomeration
effects, while preserving the magnetic properties of the nanoparticles. The phosphonate
linkers allow a higher grafting rate of molecules at the surface of the NPs, and a stronger
binding than carboxylate anchors.21
Therefore, we synthesized a dendron bearing a PEG-8
linker with a phosphonate function at the focal point and an azide group at the periphery to
allow click chemistry ligation with MWCNTs functionalized with alkyne moieties (Figure 3).
In a previous work we showed that the benzyl ethyl phosphonate 6 can be easily synthesized
in good yield from methyl gallate as starting material.20c
The hydroxyl function of the
octaethylene glycol chain 4 was activated by tosylation to give compound 5 in 95 % yield.
The corresponding tosylate was coupled to phenol derivative 6 by a Williamson reaction in
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acetone in the presence of potassium carbonate and a catalytic amount of potassium iodide to
give dendron 7. The treatment of the ethyl phosphonic acid with trimethylsilyl bromide
(TMSBr) afforded dendron 8 in very good yield. Finally, the NPs were functionalized with
azide-dendron 8 by ligand exchange between oleic acid and the phosphonate group of the
dendron. The dendronized NPs were purified by ultrafiltration to remove the excess of
ligands. Grafting of the dendron was confirmed by elemental analysis and FT-IR
spectroscopy. The IR spectrum displayed the characteristic bands of the dendron, in particular
the band of the azide function and the modification of the phosphonate bands, confirming the
direct anchoring of the dendron at the surface of the NPs (Figure S3).20,21
33
OH
P
OEt
OEt
O
O O
OO
MeO OMe
OTs
O O
N3
6
K2CO3 / KI
Acetone
OH
O O
N3
6
p-TsCl / Et3N
CH2Cl20 °C - r.t.
95 %
60 °C
6
5
4
60 %
33
7
7
O
P
OEt
OEt
O
O O
OO
MeO OMeO
N3
33
7
8
O
P
OH
OH
O
O O
OO
MeO OMeO
N3
TMSBr
CH2Cl2
95 %
0 °C - r.t.
Figure 3. Synthesis of azide-dendron 8.
In parallel, we prepared MWCNTs functionalized with alkyne moieties starting from
oxidized MWCNTs (Figure 4). The carboxylic acid functions of ox-MWCNTs 1 were
activated by forming the corresponding acyl chlorides by reaction with oxalyl chloride.
Subsequent amidation with propargylamine afforded the desired alkyne-MWCNTs 9.
1 9THF
O
OH
O
NH
a. (COCl)2
b. NH2
Reflux
Reflux
Figure 4. Synthesis of alkyne-modified MWCNTs 9.
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Thermogravimetric analysis (TGA) of pristine MWCNTs, ox-MWCNTs 1, and
alkyne-MWCNTs 9 was performed under inert atmosphere (Figure 5).
Figure 5. TGA of pristine MWCNTs (a), ox-MWCNTs 1 (b), and alkyne-MWCNTs 9 (c) in
N2 atmosphere.
As expected, the pristine MWCNTs were stable in inert atmosphere (less than 1 %
weight loss), while the weight loss increased for oxidized MWCNTs (9.1 % at 500 °C) and
even more for the alkyne-derivatized MWCNTs (10.6 % at 500 °C), in comparison to pristine
material, confirming the occurrence of the functionalization.
The click chemistry ligation between alkyne-MWCNTs 9 and iron oxide NPs
functionalized with an azide group 10 was done in the presence of copper sulfate and sodium
ascorbate (Figure 6).
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Fe
Fe
9
O
NH
33
7
10
O
PO
O
O
O O
OO
MeO OMeO
N3
THF / H2O3:1
Sodiumascorbate
CuSO4
+
O
NH
11
N N
N
3
O
P
O
O
O
O
MeO
MeO
O
O
O
3
7
O
Figure 6. Decoration of MWCNTs with iron oxide NPs by click chemistry. For clarity, the
representation of the NPs displays only one phosphonate anchor in the coating layer.
By adjusting the concentration of NPs in suspension, we observed by TEM that the
anchoring of iron oxide NPs on the nanotube surface was homogeneous and that the NPs
formed less aggregates on the nanotube surface (Figure 7), in comparison to the first approach
based on ligand exchange. One possible reason to explain the difference in terms of NP
aggregation between both strategies relies on the level of functionalization which is higher for
oxidized MWCNTs 1 compared to alkyne-derivatized MWCNTs 9. Indeed, based on the
weight loss obtained from TGA we estimated that the amount of functional groups per gram
of nanotubes was about 2.0 mmol of oxygen-containing species (mainly carboxylic acid
functions) for ox-MWCNTs 1 and 0.41 mmol of alkyne moieties for functionalized MWCNTs
9. Not all carboxylic functions on ox-MWCNTs 1 are available for further modification, as
previously demonstrated.22
This can explain the different efficiency in decorating the
functionalized CNTs with iron oxide NPs. The ligand exchange approach led to a higher
coverage of the oxidized nanotube surface by iron oxide NPs while the coating of the alkyne-
MWCNTs 9 was lower and better controlled due to chemoselective ligation based on click
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chemistry. Moreover, as previously underlined, functionalization of the iron oxide NPs with
the dendron having a phosphonate anchor allows to decrease the agglomeration effects of
azide-NPs 10, while the NPs 2 coated with oleic acid formed more aggregates.21
Figure 7. TEM images of NP/CNTs 11.
3. Magnetic properties of NP/CNT hybrids
3.1 Magnetic properties of NP/CNTs
We next investigated how conjugation of NPs confers superparamagnetic properties to CNTs.
For this study, we have decided to use the NP/CNT hybrids 3. The field-dependent
magnetization curve clearly demonstrates the magnetic responsiveness of the NP/CNT
complexes (Figure 8A) and the absence of hysteresis at 300 K. Initial susceptibility and
saturation magnetization were identical to those of isolated NPs, when normalized to iron
concentration. The saturation magnetization of NPs anchored on CNTs was about 53 emu/g
and was similar to the saturation magnetization of the NPs alone coated with oleic acid (~ 54
emu/g).16b,20e
This result indicates that the superparamagnetic properties of the NPs were
preserved after combination with CNTs.
One of the most interesting features of NPs is to confer MRI detectability to CNTs.
The efficacy of the NP/CNT hybrids as contrast agent for MRI was investigated by relaxivity
measurements at a frequency of 20 MHz (0.47 T). Longitudinal and transverse magnetic
resonance (MR) relaxivities are defined as the ability of the contrast agent to increase the
relaxation rates of the proton magnetization, 1/T1 and 1/T2, respectively. Proton relaxation
rates were linearly proportional to the iron concentration for the NP/CNT hybrids and for the
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NPs alone (Figure 8B and C). However, the relaxivities (i.e. the slope of the relaxation rates
versus iron concentration) were slightly different for NPs coupled to CNTs in comparison to
isolated NPs. The longitudinal relaxivity r1 decreased from 19 mM-1
·s-1
to 13 mM-1
·s-1
following the coupling of NPs to nanotubes, whereas the transverse relaxivity r2 slightly
increased (103 mM-1
·s-1
vs 88 mM-1
·s-1
for NPs alone). The reduction of r1 can be explained
by the gathering of NPs onto the nanotube surface, which can impair the proton accessibility
to magnetic centers. Conversely, the local magnetic field created by NPs assembled on CNTs
becomes larger than the one created by dispersed isolated NPs, thus enhancing transverse
relaxivity. As a direct consequence, the ratio r2 / r1, which appraises the efficacy of a negative
contrast agent, is almost twofold enhanced after conjugation to CNTs (8 vs 4.6).
Figure 8. A) Magnetization curves of NP/CNTs 3 at 5 and 300 K. B) Relaxation rate of
proton magnetization measured at 0.47 T (20 MHz) as a function of iron concentration in the
suspension of NP/CNTs 3. The slope of the linear dependence represents the relaxivity. C)
Relaxation rate of proton magnetization measured at 0.47 T (20 MHz) as a function of iron
concentration of an aqueous suspension of NPs alone.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 0.01 0.02 0.03
0
0.5
1
1.5
2
2.5
3
3.5
0 0.01 0.02 0.03
1/T
1(s
-1)
[Fe] (mM)
1/T
2(s
-1)
[Fe] (mM)
B
A
H (kOe)
M (
em
u/g
)
-80
-40
0
40
80
-4 -2 0 2 4
C
0
10
20
30
40
50
60
0 1 2 3
1/T
1(s
-1)
[Fe] (mM)
0
50
100
150
200
250
0 1 2 3
1/T
2(s
-1)
[Fe] (mM)
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3.2 Interactions of NP/CNT hybrids with cells
In order to examine the potential applications of the NP/CNT hybrids as new theranostic
system, we investigated their interactions, internalization, and trafficking in human prostatic
cancer cells (PC3). The cells were incubated with the NP/CNT hybrids 3 and 11 for 20 h in
complete culture medium at concentrations of 2, 5 and 10 µg/mL. Cell metabolic activity,
assessed by Alamar Blue test, was similar to that of control cells, irrespective of the synthesis
route to obtain the NP/CNT hybrids (Figure 9A). Thus, no toxic effects were observed under
the above conditions. Nevertheless, optical microscopy shows a dose-dependent uptake of
nanotubes, which appear as punctuate black areas mainly located inside the cells (Figure 9B).
Figure 9. A) Metabolic activity of PC3 cells after 20 h incubation with NP/CNT hybrids 3
and 11 at concentrations of 2, 5 and 10 µg/mL normalized to that of control unexposed cells.
B) Optical micrographs of PC3 cells after 20 h incubation with NP/CNT hybrids 3 at
concentrations of 5 and 10 µg/mL.
0
20
40
60
80
100
120
Série2
Série1
50 µm
5 µg/mL 10 µg/mL
A
B
3
11
Control 2 µg/mL 5 µg/mL 10 µg/mL
% M
eta
bo
lic
ac
tiv
ity
50 µm
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This intracellular localization was confirmed by TEM, which evidenced that most of
conspicuous NP/CNT hybrids were found as bundles into membrane-bound endosomes or
lysosomes (Figure 10A).
Figure 10. Electron micrographs of PC3 cells incubated with 10 µg/mL of NP/CNTs 3 for 20
h. A) Large bundles of NP/CNT hybrids can be observed into membrane-bound endosomes.
B) Individual MWCNTs coupled with NPs are also observed within multivesicular bodies.
Images at the bottom are a zoom-in version of the images at the top.
We could also observe individual MWCNTs into multivesicular bodies (secondary
lysosomes), suggesting a route for sorting of nanotubes (Figure 10B).23
Interestingly, once
processed by cells (over a 24 h period), the magnetic NPs were still linked on the nanotube
surface, discarding a potential detachment and segregation of both nanomaterials.
3.3 Magnetic modulation of the NP/CNT hybrids cellular uptake
The magnetic properties of the NP/CNT hybrids could be exploited to modulate their uptake
by cancer cells. When exposed to a circular neodymium magnet placed underneath the Petri
2 µm
200 nm 200 nm
500 nm
A B
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dish, cells showed enhanced CNT uptake in the area where the magnetic field gradient is
maximized, corresponding to the periphery of the magnet (Figure 11). This result
demonstrates the magnetic mobility of the NP/CNT hybrids 3, which could be manipulated by
remote magnetic forces to target a specific site and promote cell internalization. Such
magnetic manipulation could be a key advantage for targeted drug delivery or photothermal
therapy mediated by CNTs and for applications of CNTs in tissue engineering techniques,
where CNTs are used to guide the cell growth. Indeed, this paves the way for on demand
localization of CNTs and enhanced delivery to target cells.
Figure 11. Magnetic modulation of uptake of NP/CNTs by PC3 tumor cells. The cells were
incubated with 10 µg/mL of NP/CNTs 3 for 20 h in the presence of a magnet placed
underneath the culture dish. NP/CNTs were attracted in the area of highest magnetic field
gradient at the periphery of the magnet, resulting in higher cellular uptake (top micrograph)
compared to the regions outside the magnet (bottom micrograph).
3.4 Magnetic mobility and MRI monitoring of labeled cells
Due to the presence of NPs on their surface, the nanotubes can also confer magnetic
responsivity to cells that they target. After 20 h incubation with NP/CNT hybrids 11 at a
concentration of 10 µg/mL corresponding to 14 µM of iron, PC3 cells were trypsinized,
resuspended in culture medium and subjected to a uniform magnetic field gradient of 17 T/m
(for a mean field of 150 mT). As illustrated in Figure 12A, some of the cells show a directed
Magnet
100 µm
100 µm
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migration towards the magnet. The cell magnetophoretic mobility is c.a. 7 µm/s. This
indicates that not only CNTs can be manipulated by magnetic fields, but also the cells which
internalize them. Moreover, this single cell magnetophoresis experiment directly provides an
accurate quantification of the cellular uptake of the NP/CNT hybrids.24
Since in permanent
regime the magnetic force exactly balances the viscous force (proportional to the cell velocity
and cell diameter), it is easy to deduce the magnetization and magnetic load for each cell.
Figure 12B displays the histogram of magnetic load in cells showing magnetophoretic
mobility. An iron load of 0.97 ± 0.51 pg was found, corresponding to a NP/CNT mass of 12.1
± 6.3 pg per cell (expressed as mean ± standard deviation in the cell population). Here we
assumed that NPs conserve the same binding rate to CNTs, once they have been internalized
by cells, as suggested by TEM observation. Considering the rather low concentration of NPs
carried by CNTs (8 % in weight, 14 µM of iron in the incubation medium), we can stress that
the synthesized CNTs are very efficient to convey magnetic agents to cancer cells. Indeed, in
most studies reporting cell labeling with magnetic NPs, the cells are incubated with iron
concentration in the millimolar range. In this case a micromolar iron concentration was
sufficient to achieve magnetic manipulation of cells after uptake of the NP/CNT hybrids. We
have to note however that a large fraction of cells were not magnetic enough to be driven in
our magnetic field gradient. Single cell magnetophoresis method selects the cells above a
threshold uptake (Figure 12). Thus, we used a complementary method based on electron spin
resonance (ESR) to quantify the average NP load in a large number of cells (typically 105
cells). The characteristic ESR absorption signal provided by superparamagnetic NPs enables
to measure NP concentration in a biological sample (2 µL of cell pellet).24
We found an iron
uptake of 0.23 ± 0.03 pg per cell. Considering that all the cells (even the less magnetic) are
taken into account using this global measurement, it is still consistent with magnetophoresis
quantification. The heterogeneity of the cell labeling could possibly arise from the partial
dispersion of the NP/CNT hybrids which are internalized into cells mainly as large clusters.
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Figure 12. Single cell magnetophoresis of cells labeled with NP/CNTs. A) Magnetophoretic
mobility of PC3 cells labeled with NP/CNTs 11 (10 µg/mL for 20 h) in a magnetic field
gradient of 17 T/m. B) Histogram of iron load per cell transfered by CNTs.
Finally, we further evaluated the MRI detectability of cells loaded with the NP/CNT
hybrids. Following incubation with NP/CNTs 3 (10 µg/mL), PC3 cells were dispersed in
agarose gel at different densities and imaged using a high resolution cryoprobe adapted on a
4.7 T MRI scanner. Labeled cells could be detected individually as punctual signal voids on
three dimensional scans with 50 µm isotropic resolution (Figure 13). The loss of MR signal
was directly linked to the local magnetic field created by each cell and experienced by
surrounding protons.25
We indeed observed that the density of signal voids increased
proportionally to the cell density in agarose gel (in agreement with the quantification of cell
iron load, a part of cells were not detected by MRI, due to insufficient iron uptake). This
result demonstrates that decoration of CNTs with NPs enables to achieve high sensitive
detection of cell-internalized CNTs (with a detection threshold of less than 14 pg of NP/CNT
hybrids per cell) and high resolution depiction of their localization even at the cell level. Such
hybrid constructs could be valuable for MRI follow-up of CNT fate and for monitoring of
their therapeutic efficacy as drug delivery carrier.
0 s 4 s 8 s 12 sGra
dB
= 1
7T
/m
20 µm
Cell iron load (pg)
% c
ell
A
B
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Figure 13. High resolution 4.7 T MRI observation of cells dispersed in 300 µL agarose gel.
Cells labeled with NP/CNTs 3 can be detected as punctate signal voids. (Fisp-3D-sequence
with TE = 5 ms, TR = 20 ms, flip angle 25 deg, three-dimensional 50 μm spatial resolution).
The images show the superposition of 20 slices (total thickness 1 mm).
4. Conclusions
We demonstrated that an efficient, uniform, and stable coupling of magnetic iron oxide NPs
to MWCNTs confers unique functionalities to the nanotubes. Two approaches were explored
for the decoration of MWCNTs with iron oxide superparamagnetic NPs based on i) ligand
exchange between NPs coated with oleic acid and oxidized CNTs, and ii) click chemistry
between NPs functionalized with a dendron bearing an azide group and alkyne-derivatized
CNTs. Owing to their magnetic properties, these NP/CNT hybrids could be manipulated by
remote magnetic field and offer contrast enhancement in MRI. Moreover, they were
efficiently internalized by tumor cells without presenting evident toxicity effects. In turn, cells
labeled with NP/CNT hybrids showed magnetic mobility and were detected at a single cell
level through high resolution MRI. Conjugation of NPs on CNTs also provides new reliable
tools for quantification of CNTs in biological environment and follow-up of their
biodistribution and fate in vivo. On demand control of CNTs by magnetic fields combined
with non-invasive monitoring opens new prospects for targeted therapy mediated by CNTs
and potential biomedical applications including for example tissue engineering.
1x105 cells
5 mm
1x106 cellsAgarose Control
4.7 T MRI
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5. Experimental Section
Materials and Methods
MWCNTs, produced by the catalytic carbon vapor deposition (CCVD) process, were
purchased as purified from Nanocyl (Thin MWCNT 95+ % C purity, Nanocyl 3100® batch
no. 071119), with average diameter and length of 9.5 nm and 1.5 microns, respectively.
Reagents and solvents were purchased from commercial suppliers and used without further
purification. Kieselgel 60 F-254 commercial plates were used for analytical TLC. Silica gel
Kieselgel Si 60, 0.230-0.400 mm (Fluka) was used for column chromatography. 1H,
13C, and
31P NMR spectra were recorded using a Bruker 300 MHz spectrometer; the residual solvent
protons were used to reference the chemical shift in ppm. Coupling constants (J) are reported
in Hertz (Hz), and splitting patterns are designated as s (singlet), d (doublet), t (triplet), and m
(multiplet). The thermogravimetric analyses were performed using a TGA Q500 TA
instrument with a ramp of 10 ºC/min under N2 from 100 °C to 800 °C. The NPs before and
after anchoring on CNTs were characterized by TEM with a TOPCON 002B microscope
operating at 200 kV (point resolution 0.18 nm) and equipped with a GATAN GIF 200
electron imaging filter. MALDI-TOF was performed at Mass Facility Core of the University
of Strasbourg. MALDI-TOF analyses were carried out on a Bruker Daltonics MALDI-
TOF/TOF Autoflex II or III spectrometer in positive ion mode using dithranol as the matrix.
The benzyl ethyl phosphonate 6 was synthesized according to literature procedures.20c
Preparation of ox-MWCNTs 1
1 g of pristine MWCNTs was sonicated in a water-bath (20 W, 40 kHz) for 24 h in 150 mL of
sulfuric acid/nitric acid (3:1 v/v, 98 % and 65 %, respectively) at room temperature.22
Deionized water was then carefully added and the MWCNTs were filtered (Omnipore® PTFE
membrane filtration, 0.45 µm), re-suspended in water, filtered again until pH became neutral
and dried.
Synthesis of iron oxide nanoparticles 2 coated with oleic acid
Iron oxide NPs were synthesized by thermal decomposition of an iron complex in high
boiling point solvent. The synthesis was performed in the presence of fatty acids, which in
situ coat the NPs and improve their stability in organic solvents. A solution of iron stearate
(1.38 g, 2.22 mM) and oleic acid (1.25 g, 4.44 mM) in 20 mL of octyl ether was heated to 288
°C with a heating rate of 5 °C/min without stirring. The reflux was maintained for 2 h in order
to obtain a good monodispersity of nanoparticles. After cooling down to room temperature,
the solution was washed three times by addition of ethanol and by centrifugation (8000 rpm,
10 min). The oleic acid coated iron oxide nanoparticles 2 were solubilized in hexane for
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further use.
Preparation of NP/CNTs 3 by ligand exchange
Oxidized MWCNTs 1 (1 mg) were dispersed in 15 mL of THF using a sonication tip for 20
min (tip amplitude 20 W, using a discontinuous sonication program: pulsation time of 1 s ON
and 5 s OFF). The dispersion was added to a solution of 0.1 mg NPs coated with oleic acid 2
in 10 mL of THF and the mixture was magnetically stirred for 24 h. The suspension was
filtered using a 1 µm pore size membrane. The nanotubes on the filter were dispersed in THF,
sonicated in a water bath for 5 min, and filtered. This sequence was repeated with chloroform
and water to eliminate ungrafted NPs and free oleic acid molecules.
Tosyl-dPEG8-azide 5
To a solution of hydroxy-dPEG8-azide 4 (2.00 g, 5.0 mmol, 1.0 eq.) in 20 mL of CH2Cl2 at 0
°C were added sequentially 2.1 mL (15.0 mmol, 3.0 eq.) of triethylamine and 1.45 g (7.5
mmol, 1.5 eq.) of para-toluenesulfonyl chloride. After 40 h stirring at room temperature, the
reaction mixture was diluted with 50 mL of CH2Cl2. The organic phases were combined,
washed with brine, dried over MgSO4, filtered and concentrated under reduced pressure.
Purification by chromatography over silica gel column (dichloromethane/ethanol: 100:0 to
95:5) afforded 5 as a colourless oil in 95 % yield. 1H NMR (300 MHz; CDCl3), δH (ppm):
2.45 (3 H, s), 3.38 (2 H, t, J = 5.0 Hz), 3.55-3.70 (28 H, m), 4.15 (2 H, t, J = 4.9 Hz), 7.34 (2
H, d, J = 8.0 Hz), 7.81 (2 H, d, J = 8.0 Hz). 13
C NMR (75 MHz; CDCl3), δC (ppm): 21.10,
52.21, 66.80, 68.23, 69.00, 70.12, 79.76, 127.53, 128.52, 131.94, 144.30.
Benzyl ethyl phosphonate 7
To an equimolar solution of 5 (711 mg, 1.30 mmol, 1.0 eq.) and 6 (850 mg, 1.30 mmol) in 15
mL of acetone were added 894 mg (6.50 mmol, 5.0 eq.) of K2CO3 and 65 mg (0.39 mmol, 0.3
eq.) of KI. The reaction mixture was stirred at 60 °C for 72 h. After filtration over celite, the
solvent was evaporated and the residue was solubilized in CH2Cl2. The organic phase was
washed with a saturated solution of NaHCO3 and with brine, filtered and concentrated under
reduced pressure. Purification by chromatography over silica gel column
(dichloromethane/methanol: 95:5 to 90:10) afforded 7 as a yellow oil in 60 % yield. 1H NMR
(300 MHz; CDCl3), δH (ppm): 1.25 (6 H, t, J = 7.0 Hz), 3.05 (2 H, d, J = 21.2 Hz), 3.37 (6 H,
s), 3.42 (2 H, t, J = 5.1 Hz), 3.53-3.75 (50 H, m), 3.77 (2 H, t, J = 5.0 Hz), 3.84 (4 H, t, J =
5.0 Hz), 3.95-4.05 (4 H, m), 4.08-4.16 (6 H, m), 6.52 (2 H, d, J = 2.5 Hz). 13
C NMR (75
MHz; CDCl3), δC (ppm): 16.42 (JC-P = 6.0 Hz), 33.78 (JC-P = 139.1 Hz), 50.82, 58.95, 62.12
(JC-P = 6.4 Hz), 68.89, 69.71, 70.04, 70.51, 70.55, 70.61, 70.69, 70.75, 70.82, 71.91, 72.23,
109.54 (JC-P = 6.6 Hz), 126.97 (JC-P = 8.8 Hz), 137.28 (JC-P = 2.8 Hz), 152.57 (JC-P = 3.3 Hz);
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31P NMR (81 MHz; CDCl3), δP (ppm): 26.25. MALDI/TOF m/z: 1008.54 ([C45H87NO21P]
+,
30 %), 1033.21 (M, 10), 1056.51 ([M + Na]+, 60).
Azide-dendron 8
To a solution of 7 (500 mg, 0.48 mmol, 1.0 eq.) in 15 mL of CH2Cl2 at 0 °C was added
dropwise 0.70 mL (4.80 mmol, 10.0 eq.) of TMSBr. After overnight stirring at room
temperature, the volatiles were evaporated, the crude product was solubilized in methanol and
the solution was evaporated several times. Compound 8 was obtained as a brown oil in 95 %
yield without further purification. 1H NMR (300 MHz; CD3OD), δH (ppm): 3.08 (2 H, d, J =
21.3 Hz), 3.37 (6 H, s), 3.40 (2 H, t, J = 5.0 Hz), 3.52-3.75 (50 H, m), 3.77 (2 H, t, J = 4.9
Hz), 3.79 (2 H, t, J = 4.8 Hz), 3.88 (4 H, t, J = 4.8 Hz), 4.18 (4 H, t, J = 5.0 Hz), 6.65 (2 H, d,
J = 2.2 Hz). 13
C NMR (75 MHz; CD3OD), δC (ppm): 34.47 (JC-P = 135.1 Hz), 50.47, 57.84,
68.57, 69.48, 69.73, 69.95, 70.08, 70.17, 70.22, 70.27, 70.37, 71.58, 72.16, 108.97 (JC-P = 6.0
Hz), 128.78 (JC-P = 9.3 Hz), 136.76 (JC-P = 3.8 Hz), 152.31 (JC-P = 2.7 Hz); 31
P NMR (81
MHz; CDCl3), δP (ppm): 24.15. MALDI/TOF m/z: 952.47 ([C41H79NO21P]+, 60 %), 978.52
([M + H]+, 15), 1000.44 ([M + Na]
+, 25).
Preparation of alkyne-MWCNTs 9
A suspension of 30 mg of ox-MWCNTs 1 in 10 mL of oxalyl chloride was stirred at 62 °C for
24 h under Ar atmosphere. The excess of oxalyl chloride was evaporated under vacuum. The
nanotubes were then dispersed in 15 mL of dry THF and 520 mg of propargylamine were
added. The reaction mixture was heated at reflux for 48 h. After cooling to room temperature,
the suspension was filtered (Omnipore® PTFE membrane filtration, 0.10 µm) and the solid
was dispersed in DMF, sonicated for 15 min in a water bath and filtered. This sequence was
repeated twice with DMF, methanol, and diethyl ether. Finally, the nanotubes were dialyzed
against deionized water for 36-72 h (Spectra/Por®
dialysis membrane, MWCO: 12-14.000
Da) and lyophilized.
Preparation of azide-dendron-coated NPs 10
To a solution of NPs coated with oleic acid 2 (1 mg/mL) in 10 mL of THF was added azide-
dendron 8 (14 mg). The solution was stirred under vigorous stirring overnight. The resulting
azide-dendron-coated NPs 10 were purified by ultrafiltration to eliminate ungrafted dendron
molecules and free molecules of oleic acid. The ultrafiltration was performed using 100 mL of
THF under argon pressure (0.5 barr) in a Millipore ultrafiltration cell.
Preparation of NP/CNTs 11 by click chemistry
To a suspension of 1 mg of alkyne-MWCNTs 9 in a mixture of 12 mL THF and 4 mL water
were added azide-NPs 10, sodium ascorbate and copper(II) sulfate. The mixture was
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magnetically stirred for 24 h. The suspension was filtered and the nanotubes on the filter were
dispersed in THF, sonicated in a water bath for 5 min, and filtered. This sequence was
repeated with chloroform and water to eliminate ungrafted NPs and excess reagents.
MR relaxometry characterization
The nuclear magnetization relaxation times were measured at 0.47 T (20 MHz proton Larmor
frequency) and 37 °C using a Minispec PC120 spectrometer (Bruker, France). The T1
relaxation time was calculated from the inversion-recovery sequence, with 15 data points and
3 acquisitions for each measurement. The T2 relaxation time was obtained from a Carr-Purcell
Meilboom Gill (CPMG) spin-echo pulse sequence (100 data points, 3 acquisitions). T1 and T2
were determined three times for each sample with standard deviations of 2 % and 5 %,
respectively.
Cell culture and exposition to CNT-NPs
Human prostatic PC3 tumor cells were maintained as monolayer culture in Dulbecco's
Modified Eagle Medium (DMEM) supplemented with 10 % fetal bovine serum (FBS), 1 %
penicillin-streptomycin at 37 °C and 5 % of CO2. Cells were incubated with NP/CNTs at
concentrations of 2, 5 and 10 µg/mL for 20 h at 37 °C. After three rinsing steps with PBS,
cells were observed by optical microscopy or prepared for single cell magnetophoresis. To
assess the magnetic mobility of NP/CNTs and magnetically modulate cellular uptake, PC3
cells were cultured in Petri dishes and incubated with NP/CNTs at a concentration of 10
µg/mL for 20 h in the presence of a circular, 18 mm in diameter neodymium-iron-boron
permanent magnet placed under the Petri dish.
Cell viability test
In order to evaluate the acute toxicity of NP/CNTs on PC3 tumor cells, their metabolic
activity was assessed by the Alamar Blue test. One thousand cells were seeded in 48-well
plates and incubated with NP/CNTs the day after as described above. The labeled PC3 cells
were then washed twice in PBS and were incubated with 10 % Alamar Blue in culture
medium for 2 h. The fluorescence in cell medium due to the reduction of resazurin (oxidized
form) to resorufin by cell activity was quantified on a FLUOstar OPTIMA microplate reader
(excitation: 550 nm, emission: 590 nm) and compared to the control non-labeled cells. The
conditions were run in quadruplicate.
Transmission electron microscopy on cells
After exposure to NP/CNTs (10 µg/mL for 20 h), cells were rinsed and fixed with 5 %
glutaraldehyde in 0.1 mol/L sodium cacodylate buffer, post fixed with 1 % osmium tetroxide
containing 1.5 % potassium cyanoferrate. Cells were then gradually dehydrated in increasing
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concentrations of ethanol and embedded in Epon resin. Thin sections (70 nm) of cells were
examined with a Zeiss EM 902 transmission electron microscope at 80 kV (platform MIMA2,
INRA, Jouy-en-Josas, France).
Single-cell magnetophoresis
To quantify the iron load associated to single cells, cells were thoroughly washed after
labeling and trypsinized to obtain a dilute suspension. The magnetophoretic mobility of cells
towards a magnet creating a magnetic field B of 150 mT and a uniform field gradient, grad B,
of 17 T·m-1
was recorded by videomicroscopy, as described previously.24
Under these
conditions, the magnetic driving force acting on cells (M×grad B, where M is the cell
magnetic moment) is balanced by the viscous force (3πηdv, where d is the cell diameter, v is
the cell velocity and η is the medium viscosity). The cell magnetization and iron mass was
thus deduced from the velocity and diameter of each cell.
Electron Spin Resonance
Electron Spin Resonance was performed using a Varian ESR spectrometer operating at 9.26
GHz (X band) with the following parameters: microwave power = 1 mW, modulation
frequency = 100 kHz, modulation field = 10 Gauss. ESR spectra were recorded at room
temperature for 2 µL of NP suspension at different concentrations to obtain a calibration
curve or for 2 µL of cell suspension containing 105 cells labeled with NP/CNTs. The total
absorption signal was measured using a double integration of the ESR spectrum and was
directly proportional to the superparamagnetic iron amount in the sample.
High resolution MRI
MRI was performed using a 4.7 T preclinical MRI system (BioSpec 47/40 USR, Bruker) in
the Small Animal Imaging Platform Paris-Descartes PARCC-HEGP. High resolution MRI
was carried out using a cryogenic probe (CryoProbeTM
, Bruker) in 0.3 % low-melting-point
agarose gels in which cells labeled with NP/CNTs (10 µg/mL for 20 h) were dispersed at a
density of 105 and 10
6 for 300 µL of gel. Control agarose gel contained 10
5 cells that were not
exposed to NP/CNTs. Scans were run under a Fast Steady State Precession (FISP) protocol on
FID mode. Images were acquired using a T2* weighted sequence with the following
parameters: FOV of 9×9×9 mm; matrix of 180 x 180; voxel size of 50×50×50 µm; echo time
of 5 ms, repetition time of 20 ms; flip angle of 25° and bandwidth SW of 50 kHz.
Acknowledgements
This work was supported by ANR P2N (NANOTHER project 2010-NANO-008-398 04), by
the Région Ile de France (contract no E539), by CNRS, and by the University of Strasbourg.
A.B. wishes to acknowledge the CNRS financial support from PICS (Project for International
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Scientific Cooperation). The authors thank C. Péchoux and S. Chat for electron microscopy of
cells, G. Autret, O. Clément and J. Kolosnjaj for MRI experiments and image processing, R.
Di Corato for cell culture, C. Kiefer, E. Couzigné and E. Voirin for technical assistance.
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Notes and references
a CNRS, Institut de Biologie Moléculaire et Cellulaire, Laboratoire d’Immunopathologie et
Chimie Thérapeutique, UPR 3572, 67000 Strasbourg, France. Fax: +33 388 610680; Tel:
+33 388 417098;
E-mail: [email protected] ; [email protected]
b Institut de Physique et Chimie des Matériaux de Strasbourg IPCMS, UMR CNRS/UdS 7504,
67034 Strasbourg Cedex 2, France.
E-mail: [email protected] ; [email protected]
c CNRS/Université Paris Diderot, PRES Sorbonne-Paris Cité, Laboratoire Matière et
Systèmes Complexes (MSC), 75205 Paris cedex 13, France. E-mail: florence.gazeau@univ-
paris-diderot.fr
† Electronic Supplementary Information (ESI) available: additional TEM image, DLS
diagram, and FT-IR data. See DOI: 10.1039/b000000x/
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