One-step synthesis of metallic and metal oxide nanoparticles using amino-PEG oligomers as multi-purpose ligands: Size and shape control, and quasi-universal solvent dispersibility

Article abstract of DOI:10.1039/c0cc02615h

A one-step and room temperature synthesis toward metallic and metal oxide nanoparticles soluble both in water and organic solvent is reported. This was achieved using amino-PEG oligomers that make it possible to control the size and shape of the nanoparticles.

Full text of DOI:10.1039/c0cc02615h

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One-step synthesis of metallic and metal oxide nanoparticles using
amino-PEG oligomers as multi-purpose ligands: size and shape
control, and quasi-universal solvent dispersibilityw
Javier Rubio-Garcia,abc Yannick Coppel,ab Pierre Lecante,d Christophe Mingotaud,c
Bruno Chaudret,ab Fabienne Gauffre*c and Myrtil L. Kahn*ab
Received 16th July 2010, Accepted 4th November 2010
DOI: 10.1039/c0cc02615h
A one-step and room temperature synthesis toward metallic and
metal oxide nanoparticles soluble both in water and organic solvent
is reported. This was achieved using amino-PEG oligomers that
make it possible to control the size and shape of the nanoparticles.
transfer step.16 To reach this goal, we took advantage of the
versatility of organometallic chemistry to prepare either metallic
or metal oxide NPs. We reasoned that the water-soluble poly
(ethyleneglycol) (PEG) oligomers are also soluble in most
non-polar solvents, and could be used as a ligand for this
purpose. We targeted the commercially available amino-PEG
oligomers, a-(2-ethylamine)-methoxy(ethyleneglycol) and poly
(ethyleneglycol)bis (3-propylamine), referred to hereafter as
ethylamine-PEG750 and bis-propylamine-PEG1500, respec-
tively. The choice of an amine as the coordinating group was
motivated by our previous observations that alkyl amine ligands
lead to monocrystalline and well defined NPs.5,6 Indeed, in the
presence of amino-PEG oligomers, metallic and metal oxide
NPs were obtained in quantitative yields as powders which
could readily be solubilized in many non-polar and polar
solvents, including water.
Numerous methods for the preparation of nanoparticles (NPs)
have been described in the literature. The solution synthesis of
metallic NPs usually proceeds via the reduction of a precursor
whereas metal oxide NPs necessitate an oxidation step. The
various methods of synthesis performed in water present the
advantage of yielding NPs dispersible in water including both
metal oxide1,2 and metallic NPs.3 However, in most cases, large
particles and broad size distributions are obtained due to ionic
interaction between the growing nuclei. In addition, such
approaches generally necessitate high temperature conditions.4
Fine control over size and shape is of tremendous importance
for many applications because these parameters are critical
to both physical and chemical properties of NPs. Therefore,
one-step and room temperature (RT) synthetic procedures
allowing access to NPs dispersible in a large range of solvents
(in particular in water) are desirable, especially when consider-
ing formulation issues. In this context, it has been demonstrated
that the organometallic solution synthesis is particularly effec-
tive in producing ultrasmall NPs (o10 nm) with a very well
defined structure.5,6 This approach leads however to NPs which
are only dispersible in non-polar solvents. As a consequence,
strategies to produce water soluble NPs usually involve the
transfer of the NPs from the organic solvent, either via ligand
exchange with more hydrophilic capping molecules7–11 or via the
formation of interdigitated layers.12–15
In the present work, ZnO NPs were chosen as models for
metal oxide materials. NPs were obtained via RT hydrolysis
of the biscyclohexyl zinc precursor in the presence of
either bis(propylamine)-PEG1500 (of mean molecular weight
1500 g molÀ1) or ethylamine-PEG750 (of mean molecular
weight 750 g molÀ1) in a solvent (typically THF) (Scheme 1),
using a procedure similar to the one previously reported by our
group.17 The solution containing the Zn-precursor and the
ligand was first prepared in a glove box under argon and then
exposed to ambient moisture by opening the Schlenk tube. The
amount of ligand was varied from rlig = 0.05 to 2 (ratio of
ligand versus zinc precursor). After solvent evaporation, the
resulting powder could be dispersed in a large variety of polar
and non-polar solvents including water, acetonitrile, ethanol,
chloroform, tetrahydrofuran, and toluene (Fig. 1). In all cases
colloidal suspensions were obtained with concentrations
Here we report a very simple RT synthesis procedure,
yielding metallic or metal oxide NPs dispersible in almost
any solvent, and especially in water, without the need of a
ranging from
1 to 3 g . Transmission Electron
LÀ1
Microscopy (TEM) was used to observe the nanoparticles,
measure their size and estimate their dispersion state.18 Fig. 1b
presents typical TEM images of the ZnO NPs prepared using 1
equivalent of bis(propylamine)-PEG1500 and after dispersion
a CNRS, LCC (Laboratoire de Chimie de Coordination), 205,
route de Narbonne, F-31077 Toulouse, France.
E-mail: myrtil.kahn@lcc-toulouse.fr; Fax: +33 5 6155 3003;
Tel: +33 5 6133 3130
´
Universite de Toulouse, UPS, INPT, LCC, F-31077 Toulouse,
b
France
CNRS, IMRCP (Laboratoire des Interactions Moleculaires et
c
´
Reactivites Chimiques et Photochimiques), 118 Route de Narbonne,
´
F-31062 Toulouse, France.
´
E-mail: gauffre@chimie.ups-tlse.fr; Fax: +33.5 6155 8155;
Tel: +33 5 6155 6143
d
´
CNRS, CEMES (Centre d’Elaboration de Materiaux et d’Etudes
Structurales) 29, rue Jeanne Marvig, F-31055 Toulouse, France.
Fax: +33 5 62257999; Tel: +33 5 62257851
w Electronic supplementary information (ESI) available: WAXS, NMR
and photoluminescence experiments. See DOI: 10.1039/c0cc02615h
Scheme 1 Organometallic synthesis of metal oxide NPs using bis-
propylamine-PEG1500 molecule (n E 30) or ethylamine-PEG750
molecule (n E 15).
c
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the presence of both ethylamine-PEG750 and acetic acid.19
Importantly, all samples were soluble in a wide range of solvent
without the need for any surface modification.
Metallic NPs of Ru, Pt, and Pd were similarly obtained by
RT reduction of an organometallic precursor under
dihydrogen atmosphere in solution (S5, ESIw). Following the
organometallic procedure previously reported by our group,6
Ru(COT)(COD), Pt2(dba)3, or Pd2(dba)3 was first dissolved
in a solvent (e.g. THF) in the presence of the oligomer at
À100 1C. The mixture was then allowed to reach RT. Then H2
was supplied for 20 min at a pressure of 3 bars, and the mixture
was left to react overnight at RT. After releasing the H2, 30 mL
of pentane was added in order to precipitate the nanoparticles
(black powder). This solid was washed using 3 Â 30 mL of
pentane. The powder formed stable colloidal solutions in
various solvents (water, THF, toluene, acetonitrile, ethanol,
chloroform). Very small isotropic Ru NPs of diameter ca.
1.1 Æ 0.3 nm were obtained when ethylamine-PEG750
oligomer was used (Fig. 2c) while Ru NPs of 2.7 Æ 0.6 nm
were obtained with bis(propylamine)-PEG1500 oligomer
(Fig. 2d). Pd and Pt NPs were only obtained using a
mixture of ethylamine-PEG750 and carboxymethyl-PEG3000
oligomers. Isotropic Pd NPs of 1.7 Æ 0.6 nm were obtained
(Fig. 2e) while worm-like Pt NPs were obtained under the
same conditions (Fig. 2f). In all cases, TEM images show
individually isolated NPs, indicating an absence of coalescence.
All of the investigated solvents yielded similar results. The size
and shape also remained unchanged regardless of the solvent.
Fig. 1 (a) Photographs of dispersions of ZnO nanoparticles in various
solvents under UV-light illumination (l = 365 nm, B3.40 eV). rlig = 1,
bis-propylamine-PEG1500; concentrations of the dispersions: 1 to 3 g LÀ1
.
(b) TEM images of ZnO nanoparticles (rlig = 1) left from a THF
dispersion; right from a water dispersion. Histograms are given in the
inset. The mean diameter is evaluated by fitting with a Gaussian curve.
First and second values correspond to the center of the curve and to the
standard deviation, respectively.
in THF (Fig. 1b1) or in water (Fig. 1b2). As can be
seen, isotropic nanoparticles were obtained with a diameter
of 4.0 Æ 1.0 nm. Remarkably, no significant coalescence was
observed in THF or in water. Similar results were obtained for
all solvents investigated. Clearly, the size and shape were
solvent independent. In water, the ZnO nanoparticles were
chemically stable in the pH range 4–13 for more than one day.
The hydrodynamic diameters were measured by dynamic
light scattering using a Malvern zetasizer ‘‘NanoZS’’ at a
back-scattering angle of 1731. Data were interpreted using
the Non Negative Least Square algorithm. Dispersions were
sonicated for 30 minutes prior to measurements. The
hydrodynamic diameters of ZnO NPs synthesized using
bis-propylamine-PEG1500 (rlig = 1) were 19 Æ 4 nm and
66 Æ 8 nm in THF and water respectively. These values suggest
partial aggregation of the particles and/or oligomers. The zeta
potential of these ZnO NPs was measured using the ‘‘dip-in’’
cell system of the zetasizer. Values were relatively close to zero
(À11.9 and À4.6 mV in THF and in water respectively), which
shows that the system is stabilized by steric rather than
electrostatic repulsion forces. Such negative values most
likely resulted from hydroxy anions associated with the
oligomer. Indeed, the medium is alkaline (pH E 9 in water)
due to the presence of amino groups, and PEG chains are
known to be strongly hydrated. This was confirmed by zeta
potential measurements of the polymer alone (À10 mV and
À10.5 mV in THF and water respectively).
The size of the NPs could be finely controlled by adjusting
the amount of ligand introduced in the reaction medium. For
instance, as the amount of bis(propylamine)-PEG1500
increases from rlig = 0.05 to 2 equivalents, the mean size
of the NPs decreases from 7.2 Æ 2.3 nm to 3.9 Æ 0.8 nm
(S4, ESIw). Moreover, the size distribution becomes narrower
as the amount of ligand increases. Shape control was achieved
by modification of the experimental conditions (Fig. 2a and b).
Isotropic, rod-like, and even triangular nanoparticles could be
obtained. Typically, nanorods of ca. 20 Â 5 nm were formed
when ethylamine-PEG750 oligomer was used instead of
bis(propylamine)-PEG1500. Nano-triangles were formed in
Fig. 2 TEM images of ZnO NPs prepared in the presence of (a)
ethylamine-PEG750 (rlig = 0.5) from a THF dispersion; (b) ethyl-
amine-PEG750 (rlig = 2) and acetic acid from an anisole dispersion.
TEM images of Ru NPs prepared in the presence of (c) ethylamine-
PEG750 (rlig = 1) from a water dispersion; (d) bis(propylamine)-
PEG1500 (rlig = 1) from a water dispersion. TEM images of Pd (e) and
Pt (f) NPs prepared in the presence of ethylamine-PEG750 and
carboxymethyl-PEG3000 (rlig = 1) from a water dispersion.
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Interestingly, the dispersions of the metallic NPs were stable
for months at RT.
wavelength of 370 nm (S2a, ESIw). Similar results were
obtained when ethylamine-PEG750 oligomer was used for
the synthesis, also consistent with the coordination of this
amine to the surface of the nano-objects.
Wide Angle X-ray Scattering (WAXS) measurements on
samples sealed in Lindemann capillaries were performed in
conditions allowing for RDF analysis (Mo wavelength,
extended angular range, low background) in order to access
the crystallographic characteristics of the NPs. For all of the
samples, well defined diffraction peaks were observed, (S1, left,
ESIw). The sharp peaks for 2y o151 are attributed to the
crystalline ligand. This contribution (obtained in the same
conditions from pure ligand) was subtracted prior to Fourier
Transform (S1, right). For all samples, and especially for the
less crystalline samples such as Ru, correspondence (in real
space) of the experimental RDF spectrum with the calculated
one is a clear indication of the metallic character of the NPs.
The interaction between oligomers and the NPs surface was
investigated by NMR studies performed in deuterated water
(S2, ESIw). For metallic as well as metal oxide NPs, the oligomer
was observed to coordinate to the surface via the amine
function. However, the strength of this interaction depends on
the NPs surface. For metallic NPs the coordination of the
oligomer via the amine function is strong whereas it is
very weak for metal oxide NPs. In particular, the signals
of the protons in the a and b-positions of the NH2 function
(d 1H 2.67 ppm (CH2a) and 3.46 ppm (CH2b), respectively) were
significantly affected by the oligomer interaction with the
metallic surface. A complete disappearance of the CH2a and
CH2b 1H signals was even observed for Pt NPs. This is
attributed to a locally restricted motion of the oligomer close
to the NP surface and is associated with the presence of a Knight
shift in the case of Pt NPs.20 In the case of metal oxide NPs, the
exchange rate between free and bound oligomers was rapid
compared to the time scale of the NMR. In this case, the CH2a
and CH2b 1H signals were only weakly affected and only small
shifts of the 1H signals could be observed.
In summary, metallic or metal oxide nanocrystals that are
readily dispersible in water as well as in many other polar and
non-polar solvents were obtained by a simple one-step RT
procedure. This was achieved using organometallic reagents in
combination with (amino)-PEG ligands. It was shown that
such ligands can efficiently give access to well-defined
crystalline nanoparticles with a narrow size distribution. It
was possible to control the particle size and shape by varying
the experimental conditions.
Notes and references
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whereas at slightly higher energies
a blue emission is
observed. A white emission corresponding to the sum of the
two can also be observed for an intermediate excitation
c
990 Chem. Commun., 2011, 47, 988–990
This journal is The Royal Society of Chemistry 2011