ABSTRACT
A general seeded-growth procedure to grow extremely
monodisperse core-shell silica spheres covering almost
the whole colloidal size range has been developed. The
core-shell seeds were grown in a water/ammonia in cyclohexane microemulsion made with a non-ionic sufactant:
polyoxyethylene nonylphenyl ether (NP5). The core-
shell seeds were obtained with radii (R) around 20 nm
and polydispersities d (relative standard deviations in size) as low as 3%. Synthesis [1] and characterization
[2] of the very monodisperse and spherical seed particles
as grown in the non-ionic microemulsion have been
described before. Also, the use of the microemulsion
procedure to make nano-crystalline CdS-core silica-shell
colloids by mixing multiple microemulsions has been
demonstrated [3]. We have found a way of extracting
these seeds out of the microemulsion without losing
colloidal stability by vacuum distillation and redispersion
of the particles and non-ionic surfactant in a mixture of
water, ethanol and ammonia in which the spheres can be
grown larger using alkoxysilanes. This further growth of
the seeds reduces the polydispersity as 1/R [4] resulting in extremely monodisperse spheres. The synthetic pro-
cedure can also be used to make more monodisperse fluor-
escent core-shell particles [5], which are very useful in
quantitative 3D confocal microscopy studies [6]. Further,
we have slightly modified the method developed by Liz-
Marzan [7] to prepare metal-core/silica-shell systems. The surface properties of the core-shell particles can be
modified in many ways, for instance, with short alkane
chains (octadecyl chains), longer polymer brushes (poly-
isobutyne) or silane coupling agents in order to change
the interaction potential between the spheres and
make the particles dispersable in a range of solvents [see
e.g., 8 and Refs. cited]. The core-shell nature of the
model particles gives additional control over, for
example, the optical properties of the colloids. Some
examples of particles synthesized from alkoxysilanes are
shown in the following [9]: polar molecules is energeti-
cally more favourable in comparison with hydrogen-
bonded structures. By experiment it has been established
that after thermal vacuum treatment of silica samples
at 470 K an adsorption complex contains two molecules
of H2O per silanol group and after that at 670 K it
has one molecule of water [2,3]. The formation of the
most stable coordination complexes is related to pen-
etration of adsorbate molecules into a surface layer with
the subsequent coordination in trans-positions to
(Si)OH groups. The channels for such a penetration are
hexagonal structural cavities typical of the (111) face of
b-crystobalite. Substitution of trimethylsilyl groups for hydroxyls
does not eliminate sites of strong adsorption of small
polar molecules. In the case of low surface coverages,
molecules of hydrogen fluoride that is distinguished for
the maximum affinity to silicon compounds do not react
with silanes and form strong complexes capable of
acting as intermediates of the fluorination reaction. The
field desorption mass-spectrometry makes it possible to
register three characteristic regions of the adsorbate
removal with desorption maxima, and the number of
these regions is in agreement with the number of different
types of adsorption complexes of H2O and CH3OH
(hydrogen-bonded complexes and coordination com-
plexes of cis-and trans-structure). Besides, it has been
also shown that during reactions with water-adsorbing
reagents one can perform a separate introduction of differ-
ent forms of adsorbed water. The x-ray photoelectron
spectroscopy detects an increase in the binding energy
for core 2s-and 2p-electrons of silicon atoms as SiO2 undergoes dehydration, and this increase is most sharp
in the range of the high-temperature desorption maxima.
