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Au–silica nanoparticles by ‘‘reverse’’ synthesis of cores in hollow silica
shells

Sara Cavaliere-Jaricot,*a Masih Darbandia and Thomas Nann*b
Received (in Cambridge, UK) 13th March 2007, Accepted 10th April 2007
First published as an Advance Article on the web 13th April 2007
DOI: 10.1039/b703811a

Core–silica shell nanoparticles were prepared in a ‘‘reverse’’
manner by nucleation and growth of Au cores within hollow
silica nanospheres.
Recently, interest in nanomaterials has been growing tremendously
due to their attractive and unique physical and chemical sizedependent
properties.1Metal nanoparticles, and gold in particular,
have attracted great attention since ancient times.2 This is due to
their promising new applications in fields such as materials science,
medicine, catalysis, and fluorescence-spectral engineering based on
surface-enhancement effects.3
The synthesis of such materials by wet chemical methods is
based on the reduction of an inorganic or organo-metallic
precursor in the presence of a stabilizing agent.2,4 The surface of
nanoscaled materials is very reactive, and needs to be protected by
surface ligands to avoid irreversible growth and agglomeration.
The choice of such ligands is critical for the monodispersity and
the quality of the resulting particles. Different compounds such as
surfactants or polymers were used for this purpose. Confinement
within nano-reactors such as micelles and dendrimers was utilised
to stabilise metal nanoparticles as well.5 A further method is
template synthesis, within porous host materials such as alumina
and mesoporous silica to confine nanoparticles and nanowires as
guests.6
An alternative and advantageous route to stabilise and
functionalise nanoparticles colloidally is to encapsulate them with
silica shells. Silica is an inert, robust and optically transparent
material. The silica shells not only enhance the colloidal stability,
but also control the distance between core particles within
assemblies by the thickness of the shell. Furthermore, this shell is
hydrophilic, biocompatible and easy to functionalise with several
groups using different silane coupling agents. These unique
characteristics and the possibility of bio-conjugation made silica
coating an attractive route for bio-analytical and medical
applications of metal nanoparticles. Extensive studies have been
performed on the homogeneous coating of metal nanoparticles
with silica shells. Typically, ligand-stabilised nanoparticles were
covered with a silica shell of varying thickness subsequently. Two
main synthetic pathways were developed. The first one is based on
the hydrolysis–condensation7 of a silica precursor onto the particle
in alcoholic medium (Sto¨ber method).8 The second is based on the
microemulsion method,9 leading to a complete shell with tunable
thickness. Liz-Marza´n, Mulvaney, and co-workers have extensively
studied metal–silica core–shell particles prepared by a liquid
phase procedure in which the use of a silane coupling agent was
necessary to provide the surface with silanol anchor groups.7 Other
authors have demonstrated that the coating of materials such as
gold or silver with silica shells can be accomplished without using
silane coupling agents. Xia and co-workers, for instance, prepared
silica-coated gold nanospheres and silver nanowires by hydrolysis
and condensation of tetraethyl orthosilicate (TEOS) in ethanol.10
Graf et al. used poly(vinylpyrrolidone) as a stabiliser to transfer
gold nanoparticles into ethanol and perform a direct coating with
TEOS.11
In this communication, we present the first synthesis of silica
coated gold particles (Au@SiO2) by means of hollow silica
particles used as nano-reactors. In recent years, such hollow
materials have attracted a lot of interest due to their low density,
low toxicity, large surface area, high chemical and thermal
stability, surface permeability, and the consequent possible
applications in drug release12 and catalysis.13 However, to the
best of our knowledge, they have not been used as reactors for
nanoparticle synthesis so far.
In the present work, the metal particles were grown in situ, in the
internal cavity of the previously prepared hollow silica particles.
The latter were synthesised from silica coated CdSe–ZnS
nanocomposites as published earlier.9 The etching of the core
material was possible in acidic or alkaline conditions,14 due to the
porosity of the silica shell.12 This strategy was exploited to transfer
a gold precursor and a reducing compound into the silica’s empty
cavity, giving rise to the Au@SiO2 nanocomposites. Firstly, we let
the metal precursor (HAuCl4) diffuse into the holes, and then we
added an excess amount of NaBH4 as reducing agent. The latter
could be diffused into the cavity of the silica spheres, which were
filled with Au3+ cations and reduced. The metal nanoparticle
formation took place by nucleation and growth of Au(0) atoms.
The silica shell avoided further growth of the nanoparticles, acting
as a template (as confinement factor)5, and finally stabilised them.
The final product is a gold nanometric particle surrounded by a
silica shell. The suggested chemical process is depicted in Scheme 1.
aFreiburg Materials Research Centre (FMF), Albert-Ludwig University
Freiburg, Stefan-Meier-Strasse 21, D-79104 Freiburg, Germany.
E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.;
Fax: +49 761 203-4700; Tel: +49 761 203-4759
bSchool of Chemical Sciences and Pharmacy, University of East Anglia
(UEA), Norwich, UK NR4 7TJ. E-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.;
Fax: +44 1603 592-003; Tel: +44 1603 593-985
{ Electronic supplementary information (ESI) available: Synthesis details
and size distribution histograms. See DOI: 10.1039/b703811a
Scheme 1 A suggested chemical process that leads to the formation of
Au@SiO2 from hollow silica nanoparticles.
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The yielded nanocomposites were characterised by transmission
electron microscopy (TEM), energy-dispersive X-ray analysis
(EDAX) and UV–vis spectroscopy. The synthesis was quite
reproducible. Some gold particle aggregates which eventually
formed outside the SiO2 shells were removed by further washing
and precipitation.
It is interesting to notice the importance of the silica pore size for
the success of such a reaction. Indeed, using a bigger molecule as
reducing agent (e.g. sodium citrate), we obtained large gold clusters
outside the shell (cf. supporting information). This result suggests
that the dimensions of the existing pores were too small for such a
compound to enter the cavity. The reaction could then only take
place in the solution outside the silica particles, containing HAuCl4
in excess, giving rise to big particles mildly stabilised by citrate.
TEM (Zeiss LEO 912 Omega operating at 120 kV) was used to
investigate the size and the structure of the pristine hollow empty
silica particles and the final nanocomposites. For the measurements,
drops of nanoparticle solution were dispersed onto a
carbon-coated copper grid. The core–shell structure of the hollow
(Fig. 1a) as well of the Au@SiO2 nanoparticles (Fig. 1b) is revealed
because of the different electron penetrability of the core and shell
material. Size distribution measurements performed randomly on
TEM micrographs showed for the gold nanoparticles an average
diameter of 10 nm, and for the silica shell an average thickness of
40 nm (cf. supporting information).
EDAX measurements (by EDX Oxford) confirmed the presence
of Si and O in the hollow particles and in the nanocomposites, and
moreover Au in the final nanocomposite materials in agreement
with the formation of Au@SiO2 (Fig. 2) (Cu, C, and Ti are
introduced from the TEM grid and the sample holder in the TEM
apparatus). Chemical elements related to the original CdSe–ZnS
quantum dot cores were not detected.
The UV–vis analysis of the silica encapsulated gold particles
presented a quite broad absorption peak with a lmax situated at
550 nm (Fig. 3). The latter was clearly attributed to the plasmon
band of Au(0). The position and the width of the signal were in
agreement with the TEM data, which showed the size dispersion of
the core gold nanoparticles around the average value of 10 nm.
The ensemble of measurements here reported allowed the
identification of the particles synthesised inside the hollow silica as
gold nanoparticles. Such a method is reproducible and can be
applied for the preparation of other silica encapsulated metal
particles by only changing the precursors. For instance, by using
AgNO3 it was possible to synthesise Ag@SiO2 (figures not shown).
In conclusion, a novel and original use of hollow silica particles
is described. It consists of growing monodisperse gold nanoparticles
into preformed empty silica shells, giving rise to Au@SiO2
nanocomposites. The method can be easily transferred to other
materials to obtain a variety of core–silica nanoparticles with
spherical shape. Their size can be controlled by tailoring the hole
size in the silica reactors. The silica shell increases the mechanical
and colloidal stability, enables transfer into polar solvents and
allows an easy functionalisation, widening the potential applications
of such nanocomposite materials.
We would like to thank Dr Ralf Thomann (Macromolecular
Chemistry Department of Freiburg) for TEM and EDAX
measurements.
Notes and references
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Fig. 1 TEM micrographs of hollow silica nanoparticles (a) and the
Au@SiO2 derived nanocomposites (b).
Fig. 2 EDAX spectra of hollow silica nanoparticles (a) and the
Au@SiO2 derived nanocomposites (b).
Fig. 3 UV–vis spectrum of the Au@SiO2 nanocomposites.
2032 | Chem. Commun., 2007, 2031–2033 This journal is
The Royal Society of Chemistry 2007
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