Self-Assembled Nanoreactors - Cluster for Molecular Chemistry

1474 Chemical Reviews, 2005, Vol. 105, No. 4 Vriezema et al.
Figure 28. Catalysis inside star and hyperbranched polymers: (a) star polymer encapsulating a catalytically active
nanoparticle, 268 and (b) hyperbranched polyglycerol nanocapsule with catalytically active pincer Pt complexes. 270
Figure 29. Schematic picture of the hollow amphiphilic nanoreactor that was used by Jungmann et al. for the synthesis
of metal colloids. The gray part consists of dimethylsiloxane, methylsiloxane, and a quaternary ammonium salt; the outer
shell is built up from dimethylsiloxane and methylsiloxane only. (Adapted with permission from ref 273. Copyright 2003
American Chemical Society.)
phine) (PS-b-PPH), to prepare polymeric PdCl2 complexes.
272 They found that this hybrid system selfassembled
to yield finite structures upon dispersion
in THF; no gels were formed. Depending on the ratio
phosphine/Pd, different morphologies were obtained,
that is, disklike micelles, uni- and multilamellar
vesicles, and “perforated hulls”. Although not described
by the authors, these aggregates might be
useful catalysts in Heck reactions or other processes
that require Pd, with the advantage that the aggregates
can be easily recovered by filtration or
precipitation.
A variety of other systems have also been developed
for the construction of nanoreactors. Jungmann et al.
have prepared hollow amphiphilic poly(organosiloxane)
nanospheres by sequential condensation of silanes
and have used them as nanoreactors for the
synthesis of Ag, Au, and Pd nanoparticles (Figure
29). 273
After the hollow poly(organosiloxane) nanospheres
were loaded with salts of the noble metals, reduction
was performed with LiBEt3H, resulting in the formation
of 2-5 nm size metal colloids.
Polymer containing hollow spheres can also be
prepared in a self-assembling approach, viz., by
assembling polymers around spherical colloids. 274
This method, referred to as layer-by-layer (LbL)
technique, allows the formation of nanocapsules with
a well-defined constitution by coating the colloidal
templates with alternating layers of polyanions and
polycations. 275 The templates can be removed by
changing the pH or by using solvents selective for
the templates. 276 These hollow particles have recently
been applied as nanoreactors for a variety of reactions.
277 Adopting a ship-in-a-bottle approach, hollow
polymeric capsules were loaded with different monomers,
for example, styrene sulfonate, and consecutively
polymerized. The capsule wall proved permeable
for the monomers, whereas the polymers were
trapped inside. In this way, the physicochemical
properties of the capsule interior could be varied over
a broad range (ion strength, pH, viscosity, etc.). In a
similar manner, the cationic dye 1,1′-diethyl-2,2′cyanine
(DEC) was crystallized inside polymeric
capsules that contained poly(styrene sulfonate)
(PSS). 278 The PSS was introduced inside the capsules
by the ship-in-a-bottle approach (Figure 30).
The formed fluorescent DEC-PSS aggregates were
highly photosensitive, and light irradiation resulted
in destruction of the aggregates and redistribution
of PSS inside the capsules. Addition of tetraphenylborate
ions resulted in precipitation of fluorescent
DEC-BPh4 nanocrystals and release of PSS inside the
polymer shells.
The versatility of the LbL technique was further
demonstrated by coating human erythrocytes with
alternating layers of polyanions and polycations. 279
A protein destruction treatment was carried out to
decompose the cytoplasmic proteins of the erythrocytes,
which were removed by centrifugation or
filtration. The resulting shells were used for the
controlled precipitation or crystallization of organic
and inorganic materials. In another biomimetic approach,
CaCO3 was synthesized exclusively inside
micrometer-sized polyelectrolyte capsules. 280 Urea
hydrolysis, catalyzed by urease, led to the fermentative
formation of CO3 2- ions and the precipitation of
CaCO3, which completely filled the capsule interior.
The LbL approach was also adopted by Ghan et al.
to polymerize phenols within polyelectrolyte micro-