Efficient strategy to increase the surface functionalization of core-shell superparamagnetic nanoparticles using dendron grafting

Article abstract of DOI:10.1039/b717027k

Core-shell γ-Fe₂O₃/polymer 300 nm superparamagnetic nanoparticles, grafted by fluorescent dendrons using a convergent approach, showed an increase in their surface functionalization compared to grafting using a linear analogue. The R

Full text of DOI:10.1039/b717027k

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LETTER
www.rsc.org/njc | New Journal of Chemistry
Efficient strategy to increase the surface functionalization of core–shell
superparamagnetic nanoparticles using dendron graftingw
a
a
a
b
´
Karine Heuze,* Daniel Rosario-Amorin, Sylvain Nlate, Manuel Gaboyard,
d
Anthony Boutercd and Rodolphe Clerac
´
Received (in Montpellier, France) 5th November 2007, Accepted 21st January 2008
First published as an Advance Article on the web 4th February 2008
DOI: 10.1039/b717027k
Core–shell c-Fe2O3/polymer 300 nm superparamagnetic nano-
particles, grafted by fluorescent dendrons using a convergent
approach, showed an increase in their surface functionalization
compared to grafting using a linear analogue.
a decade and offers a wide range of applications in various
areas of chemistry, life and materials science.8 However, only a
few papers have been published in the field of dendrimers
grafted or immobilized onto MNPs. Some of them report the
synthesis of MNPs stabilized by dendritic architectures,9 and
most deal with either protein immobilization10 or catalysis.11
In the case of dendron-functionalized core–shell MNPs, the
general synthetic approach reported is a divergent route, in
which the dendron is built step by step on the MNPs’ surface.
In this Letter, we describe the synthesis of dendrons functio-
nalized by a fluorescent tag and their grafting onto core–shell
MNPs by a convergent approach.12 In contrast to the diver-
gent grafting approach, this method uses well-defined den-
drons, since the dendritic parts are synthesized and
characterized before their grafting on the MNPs’ surface.13
We show that this way of functionalization of core–shell
MNPs leads to a high percentage of grafting, as demonstrated
by the use of dendrons bearing fluorescent units on the MNPs’
surface. As expected, the resulting dendron-grafted MNPs
possess a larger number of fluorescent sites than those grafted
with their linear analogue.
In recent years, superparamagnetic iron oxide nano-
particles1 have shown great potential in many applications
related to biotechnology and nanomaterials, such as biomedi-
cal applications (MRI, efficient enzyme or protein immobiliza-
tion as an ELISA test, magnetically controlled transport of
drugs)2 and more recently as supported catalysts.3 Most
functionalized superparamagnetic nanoparticles (MNPs) are
synthesized directly at the oxide surface (Fe3O4 or Fe2O3),
but the key to their recent development has been the tremen-
dous progress made in the surface chemistry of these nano-
materials, along with surface functionality needs and
biocompatibility purposes for biotechnology applications.
Two coating processes of superparamagnetic iron oxide
nanoparticles have mainly been reported: (i) with a polymer
shell or (ii) with a silica shell.2–5 It is worth noting that new
strategies have been explored, such as the direct functionaliza-
tion of MNPs through phosphonic acid bridges3i for the
immobilization of catalysts and the synthesis of nanoparticle
heterodimers.6
Dendrimers7 are a class of macromolecules developed in the
1990s with a highly branched three-dimensional architecture.
They have been used in many fields of chemistry (medical
applications, nanoscience, catalysis) because of their intriguing
properties. They are synthesized in an iterative sequence of
reaction steps, in which each additional iteration leads to a
higher generation material. The immobilization of dendrimers
or dendritic structures on insoluble organic or inorganic
supports (polymer, resin, silica surfaces) has been studied for
In order to demonstrate the widespread use of this grafting
method, two dendritic structures have been studied. The D1
dendron’s backbone is mainly formed of hydrophobic compo-
nents (alkyl chain and aromatic ring) and the D2 backbone is
mainly composed of hydrophilic components (polyether chain
and polyamido amine chains) (Scheme 1). Indeed, the polarity
of the dendron is often of great importance to the specific
applications of the final material.
Monomer M and two dendrons, D1 and D2, bearing a
fluorescent tag (fluorescein), have been synthesized (Scheme 1)
and grafted onto core–shell superparamagnetic nanoparticles.
On the one hand, D1 has been synthesized from the coupling
of triaminophenol 214 with the Boc-protected 6-iodo-1-
aminohexane 3, followed by the coupling between FITC
(fluorescein isothiocyanate) and amino groups of the dendron.
On the other hand, D2 has been prepared according to the
general method of Tomalia and co-workers15 for the propaga-
tion of PAMAM (poly(amidoamine)) dendrimer generations.
The first step is a Michael-type addition of the free amino
group of a mono Boc-protected 2,20-(ethylenedioxy)diethyl-
amine 1 to methyl acrylate, forming the amino propionate
ester. Then, amidation of the ester groups with ethylenedia-
mine forms the first generation of the amino dendron. A
repetition of these two steps (Michael-type addition followed
by amidation) produces the four amino group-terminated
a
´
´
Institut des Sciences Moleculaires, Universite Bordeaux 1, UMR
CNRS No 5255, 351 cours de la Liberation, 33405 Talence cedex,
France. E-mail: k.heuze@ism.u-bordeaux1.fr; Fax: +33 556 40 00
69 94; Tel: +33 556 40 00 62 74
b
c
´
Ademtech, SA Parc scientifique Unitec, 4 allee du Doyen Georges
Brus, 33600 Pessac, France
ENITAB, IECB - Universite Bordeaux 1, UMR CNRS No 5248
´
CBMN, 2 rue Robert Escarpit, 33607 Pessac, France
d Universite´ Bordeaux 1, CNRS, Centre de Recherche Paul Pascal
(CRPP), UPR CNRS No 8641, 115 avenue du Dr Schweitzer,
33600 Pessac, France
w Electronic supplementary information (ESI) available: Character-
ization details, confocal and optical microscopy analyses of the sample
described in Table 1, entry 9, and magnetization curves for D2-grafted
MNPs. See DOI: 10.1039/b717027k
ꢀc
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Scheme 1 Synthesis of M, D1 and D2: (i) FITC, MeOH, 50 1C, 20 h; (ii) KOH, DMF, ꢁ20 1C to RT, 2.5 h; (iii) FITC, MeOH, 45 1C, 24 h; (iv)
FITC, MeOH, 50 1C, 20 h.
Table 1 Grafting conditions of M, D1 and D2 onto Carboxyl-
Adembeads 300 nm nanoparticles in water
dendron 5. The final step is the coupling between FITC and
the amino groups of the dendron 5, leading to D2.
Next, the covalent grafting of M, D1 or D2 onto core–shell
g-Fe2O3 was investigated. Core–shell g-Fe2O3/polymer 300 nm
superparamagnetic nanoparticles (Carboxyl-Adembeads
300 nm, Ademtech, France, 300 mmol COOH gꢁ1 of particles)
were used as the MNP starting material.16 These particles are
usually used for in vitro diagnostics (particularly in the fields of
immunology and virology).
Entry
Dendron or monomer
Quantity mmol mgꢁ1 particles
1
2
3
4
5
6
7
8
9
M
M
M
D1
D1
D1
D2
D2
D2
0.5
1
2
0.5
1
2
0.5
1
2
They are well defined particles (dispersity below 20%), highly
magnetic materials (70% iron oxide), and show a controlled
polymeric surface (functionalized with carboxylic acid groups).
The general grafting method used has been optimized from
typical procedures,2 involving the functionalization of these
core–shell g-Fe2O3 particles with biological molecules. Our
grafting procedure is based on amide bond formation between
an amine group and a carboxylic acid group. In accordance
with both typical grafting procedures for such superparamag-
netic nanoparticles and the solubility of dendrons D1 and D2,
two solvent conditions (DMF or aqueous) were investigated
for the grafting experiments. For both procedures, the first
step is in situ-deprotection of the amino group by TFA
(trifluoroacetic acid). Next, the grafting method has been
optimized by adjustment of the dendron loading (Table 1
and Table 2). In an aqueous medium, all the nanoparticles
remained in a colloidal state (except Table 1, entry 9), pre-
suming a generally good integrity of the nanoparticles’ poly-
mer shell during the grafting procedure.z It is noteworthy that
at the highest loading of D2 (Table 1, entry 9), clear aggregation
of the nanoparticles is observed by confocal and analytical
microscopy. In a DMF medium (Table 2), particles seem to be
degraded, as was confirmed further by TEM and flow cytometry
analysis. This result confirmed that the better grafting procedure
occurred in the aqueous medium. Fluorescence measurements
revealed that the grafting rate in the aqueous medium was about
10%, based on the particles polymer shell COOH loading (300
mmol COOH gꢁ1 of particles).y These results have not been
corrected from the quenching phenomenon induced by the local
environment of the chromophores, so the grafting rate has been
particularly underestimated for dendritic species (especially in
the case of D2).
TEM microscopy of both D1- and D2-grafted MNPs reveals
that the particle shape and size remained unchanged, since the
size distribution analysis of the final material was comparable
with the size distribution of the starting nanoparticles
(240–360 nm), at least in the case of grafting experiments in
the aqueous medium (Fig. 1, left). On the other hand, it is
z Colloidal state was observed by confocal and optical microscopy for
each sample. An example of sample in a colloidal state is given in
Fig. 2.
y See experimental section for details.
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Table 2 Grafting conditions of D1 and D2 onto Carboxyl-Adem-
beads 300 nm nanoparticles in DMF
Entry
Dendron
Quantity mmol mgꢁ1 particles
1
2
5
6
7
8
D1
D1
D2
D2
D2
D2
0.094
0.47
0.15
0.75
11.5
3.75
noteworthy that a slight degradation of the polymer shell is
observed in the TEM images of nanoparticles grafted in the
organic medium (Fig. 1, right), confirming that DMF is not an
appropriate solvent in the grafting process of these particles.
Confocal microscopy analysis (Fig. 2) of D1- and D2-grafted
nanoparticles shows a strong fluorescence emission at a fluor-
escein wavelength. In the case of D1-grafted nanoparticles, a
comparison of the images from the optical and fluorescence
modes indicates that all the nanoparticles located in the focal
plane are fluorescent, highlighting the homogeneous grafting at
the surface of the nanoparticles. Flow cytometry measurements
(Fig. 3) also show a sharp shape to each signal, at least for the
grafting in the aqueous medium (Fig. 3B). Moreover, a compar-
ison of these histograms reveals that the more the molecule is
loaded during the grafting experiment, the stronger the recorded
fluorescence signal. This result indicates a high percentage of
grafting for a high molecule loading, especially for monomer
molecules. In the case of grafting in the organic medium
(Fig. 3A), the shape of the signal is slightly broader for higher
dendron loadings, indicating a possible adsorption phenomenon
of dendrons on the nanoparticles’ surface. A comparison of the
fluorescence signal recorded during flow cytometry (Fig. 3C) of
M-, D1- and D2-grafted MNPs (Table 1, entries 2, 5 and 8,
respectively) shows a significant dendritic effect. In fact,
M-modified MNPs bear less fluorescent units (11.7 RFU)
(RFU = relative fluorescence unit) than D1- (31.8 RFU) and
D2- (28.6 RFU) modified MNPs. Again, these results have not
been corrected for the quenching phenomenon, so the D2-
modified MNPs’ RFU value must be an underestimate. Any-
how, this result confirms the crucial role of dendron molecules in
increasing the functionalization ability of such grafted materials.
The magnetization curves of D1 and D2 dendron-grafted
MNPs have been recorded between 1.8 K and 300 K (Fig. 4
and Fig. S2w). Both samples and the original nanoparticles
have virtually identical magnetic properties, in accordance
with superparamagnetic behaviour. Below 150 K, a hysteresis
effect appears on the M vs. H data, and has been followed as a
Fig. 2 Confocal microscopy and optical microscopy images (48 ꢂ 48
mm). A: Confocal microscopy image of g-Fe2O3 Carboxyl-Adem-
beads 300 nm. B: Confocal microscopy image of g-Fe2O3 Carboxyl-
Adembeads 300 nm grafted with D1. C: Optical microscopy image of
g-Fe2O3 Carboxyl-Adembeads grafted with D1. D: Comparison of B
and C images. Grafting conditions are described in Table 1, entry 4.
function of temperature (inset Fig. 4). The thermal variation
of the coercive field (which reaches 150 Oe at 1.8 K) is strictly
the same, confirming that the MNPs remain intact and that
the size of the magnetic core of the particle is quasi-identical.
This result is of crucial importance, since many applications of
MNPs require integrity of their magnetic properties, even after
surface chemical modification.
In summary, we have prepared core–shell superparamagnetic
nanoparticles functionalized by fluorescein-modified dendrons.
The fluorescence measurements of these nanoparticles confirm
the ability of dendronized molecules to increase the surface
functionalization of such MNPs, in contrast to their linear
analogue. This alternative way of functionalization (convergent
approach) of core–shell MNPs by well-defined dendritic mole-
cules allows the tunable functionalization of the MNPs’ surface.
Also, the rate of functionalization increases with the generation
of the dendron. We believe that this convergent approach for the
grafting of MNPs would be a convenient way to better inves-
tigate dendritic effects,17 which may be of great interest in the
fields of biochemistry or catalysis.
Financial Support from the Centre National de la
Recherche Scientifique (CNRS), the Universite Bordeaux 1,
´
the region Aquitaine and the Agence Nationale de la Re-
cherche (ANR-05-JCJC-0245-01) is gratefully acknowledged.
We thank Odile Babot at University Bordeaux 1, ISM for
TGA measurements.
Experimental
Reagents
Fig. 1 TEM images of grafted g-Fe2O3 Carboxyl-Adembeads 300 nm
nanoparticles. Grafting conditions: Left D2: 0.15 mmol mgꢁ1 of
particles in water. Right D1: 0.1 mmol mgꢁ1 of particles in DMF.
All reagents and solvents purchased were of analytical grade
and used as received without further purification.
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Fig. 4 Magnetization vs. applied magnetic field curves of D1 dendron
grafted g-Fe2O3 Carboxyl-Adembeads 300 nm nanoparticles (grafting
conditions are described in Table 1, entry 4). Inset: expanded view of
the main figure (M vs. H) in the low field domain between ꢁ1500 and
1500 Oe, highlighting the hysteresis effect observed below 150 K.
Synthesis of D1
(a) First step. Coupling of triaminophenol dendron 2 with
mono Boc-protected 6-iodo-1-aminohexane 3: Triaminophe-
nol dendron 214 (1.99 g, 7.12 mmol) was dissolved in dry DMF
and cooled to ꢁ20 1C. KOH (0.8 g, 14.3 mmol) was then
added to the solution, and the mixture stirred for 15 min. A
solution of mono Boc-protected 6-iodo-1-aminohexane 3 in
DMF (6 mL) was added to the reaction mixture, and the
temperature was increased slowly to RT (over 2.5 h). Water
was added and the final product extracted with CH2Cl2. The
organic layer was washed with water, dried over Na2SO4, and
evaporated under reduce pressure to yield 4 as a colourless oil
1
(2.38 g, 70%). H NMR (d, CDCl3): 7.17 (d, 2H, CHar), 6.79
(d, 2H, CHar), 4.54 (br, 1H, NH), 3.91 (t, 2H, CH2O), 3.10 (q,
2H, CH2NH), 2.60 (t, 6H, CH2NH2), 1.75 (q, 2H, CH2CH2O),
1.59 (m, 6H, CH2), 1.55–1.43 (m, CH2CH2N), 1.42 (s, 9H,
CH3), 1.42–1.20 (m, 4H, CH2CH2) and 1.17 (m, 6H, CqCH2).
13C NMR (d, CDCl3): 156.85 (COar), 156.1 (CQO), 139.1
(Cq(ar)), 127.4 (CHar), 114,0 (CHar), 79.1 (Cq(t-Bu)), 67.7
(CH2O), 43.0 (CH2NH2), 42.1 (CarCq), 40.1 (CH2NH), 34.8
(CH2CH2NH2), 30.1 (CH2CH2O), 29.4 (CH2CH2NH), 28.5
(CH3), 28.0 (CqCH2), 26.7 (CH2) and 25.9 (CH2).
Fig. 3 Flow cytometry histograms of M, D1 and D2 grafted onto
Carboxyl-Adembeads 300 nm nanoparticles. A: In DMF (see entries
in Table 2). B: In water (see entries in Table 1). C: Comparison of M,
D1 and D2 (see Table 1, entries 2, 5 and 8, respectively).
(b) Second step: A mixture of 4 (310 mg, 0.648 mmol) and
FITC (756 mg, 1.94 mmol) was dissolved in MeOH (8 mL) and
stirred at 45 1C for 24 h. The solvent was evaporated and the
resulting precipitate washed with CH2Cl2 and MeOH to give D1
as an orange powder (630 mg, 59% yield). 1H NMR (d, d6-
DMSO): 10.1 (br, 6H, OH), 8.16 (m, 3H, CHar), 7.52 (m, 3H,
CHar), 7.17 (d, 3H, CHar), 6.84–6.56 (m, 18H, CHar) 3.85 (m,
2H, CH2O), 3.54 (m, 6H, CH2NHCQS), 2.99 (m, 2H, CH2NH),
1.75 (m, 6H, CH2–Cq), 1.62 (m, 2H, CH2–CH2O) and 1.41 (m,
21H, CH3Boc + CH2–CH2). Elemental analysis for C90H83-
N7O18S3ꢃ11H2O: calc. C, 58.59; H, 5.74; N, 5.31; S, 5.21; found
C, 58.77; H, 5.57; N, 5.92; S, 5.48%. The presence of water
molecules in the solid was confirmed by TGA measurements.
Synthesis of M
Mono Boc-protected 2,20-(ethylenedioxy)diethylamine 1 (77 mg,
0.31 mmol) and FITC (120 mg, 0.31 mmol) were dissolved in
MeOH (5 mL), and the mixture was heated at 50 1C for 20 h.
The solvent was removed and the solid washed with CH2Cl2 and
pentane. 131 mg (66% yield) of M was isolated as an orange
1
solid. H NMR (d, d6-DMSO): 8.28 (s, 1H, CH), 7.71 (d, 1H,
CH), 7.15 (d, 1H, CH), 6.55 (m, 6H, CH), 3.66 (m, 2H,
CH2NHCS), 3.53 (m, 4H, CH2O), 3.37 (m, 4H, CH2O), 3.06
(q, 2H, CH2NHBoc) and 1.35 (s, 9H, CH3). 13C NMR (d, d6-
DMSO): 180.5 (CQS), 168.5 (CO–O), 178.7 (CONH), 155.9
(CQOBoc), 160.3 (CO–O), 155.5 (COBoc), 160.3, 152.1, 141.3,
129.0, 127.3, 124.3, 116.9, 113.0, 109.9, 102.2 (Cq + CH), 77.5
(CqBoc), 69.4 (CH2O), 43.6 (CH2NHCS), 39.4 (CH2NHBoc) and
28.1 (CH3Boc). MS (CI): m/z calc. 636.7, found 636.3 (100)
[MꢁH]+.
Synthesis of D2
We followed the general propagation procedure of dendrimers
pioneered by Tomalia and co-workers.15 A Michael-type
ꢀc
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addition of methyl acrylate and mono Boc-protected 2,20-
(ethylenedioxy)diethylamine produced the amino propionate
ester, followed by amidation of the resulting ester groups with
ethylenediamine. A repetition of the two steps (Michael addi-
tion and amidation) afforded dendron 5, which bears four
terminal amine groups. Next, 5 (200 mg, 0.21 mmol) and FITC
(334 mg, 0.86 mmol) were dissolved in MeOH (5 mL), and the
mixture heated at 50 1C for 20 h. The red precipitate was
washed with toluene and CH2Cl2, and dried under vacuum. We
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1
obtained 425 mg of D2 (80% yield). H NMR (d, d6-DMSO):
8.27 (m, 4H, CHar), 7.75 (m, 4H, CHar), 7.17 (d, 4H, CHar),
6.64–6.55 (m, 24H, CHar), 3.47–3.36 (m, 8H, CH2O), 3.16 (m,
14H, CH2NHCQO), 2.64 (m, 16H, CH2N), 2.19 (m, 12H,
CH2CO) and 1.36 (s, 9H, CH3Boc). Elemental analysis for
C
125H128N18O30S4ꢃ10 H2O: calc. C, 56.21; H, 5.59; N, 9.44;
found C, 56.63; H, 5.59; N, 10.81%. The presence of water
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reagent; CHMC (1-cyclohexyl-3-(2-morpholinoethyl) carbodii-
mide metho-para-toluene sulfonate) being used instead of EDC
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14 S. Nlate, unpublished results.
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Fluorescence analyses to determine the grafting rate of
M, D1 and D2
Standard solutions of M, D1 and D2 in water containing free
nanoparticles were prepared and measured (fluorescence inten-
sity vs. Nmole mgꢁ1 particles). Solutions of M, D1 and D2 grafted
onto MNPs (see grafting conditions in Table 1, entries 2, 5 and
8, respectively) were analyzed. Grafting rates were calculated
based on the particles’ polymer shell COOH loading (300 mmol
COOH gꢁ1 of particles) and were found to be 8.4, 11.4 and 9.1%
for M, D1 and D2 grafting, respectively.
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New J. Chem., 2008, 32, 383–387 | 387