Angewandte Chemie International Edition 2006, 45, 1737–1741
Hard Shell – Soft Core
Polymeric microspheres have a shell whose density is dependent on the temperature
Contact: Walter Richtering, RWTH Aachen (Germany)
Registered journalists may download the original article here:
Temperature-Sensitive Core–Shell Microgel Particles with Dense Shell
Nanoparticles whose outer shells and inner cores are
made of different materials are useful for many industrial and
biomedical applications. In order for nanoparticles to be used as
sensors or for the controlled release of substances held within their
shell, for example, an important requirement must be met: the shell must
be more dense than the core to form a barrier for the external medium.
W. Richtering and I. Berndt in Aachen, in collaboration with J.S. Pedersen in Århus, Denmark, have now found an elegant solution to this problem. In a two-step process,
they synthesized polymeric microspheres with a core made of
poly-N-isopropylacrylamide and a shell of
poly-N-isopropylmethacrylamide. Both polymers are known for a particular
characteristic: they swell in water, forming microgels. Because of the
different polymer building blocks used in the shell and core, these
differ in the absorption of water.
At 70 °C, the temperature at which the microspheres
are synthesized, both polymers are densely packed. They cannot take up
much water and thus no substances dissolved in the water either. When
they are cooled to 25 °C, the core and shell have the highest water
content and the lowest density. Dissolved molecules can pass through the
shell into the core, where they disperse. Things get especially
interesting when the temperature is raised to 39 °C: at this
temperature, only the swelling properties of the shell are changed. It
expels water, shrinks together, and becomes denser than the core.
Substances dissolved in the core can no longer pass through the shell
and are now locked inside.
Aside from their sensitivity to temperature,
polymeric microgels with core–shell architectures have another advantage
too. Selection of the basic components and the reaction conditions also
allow other properties of the polymers to be controlled. In addition,
the synthesis of particles with multiple shells is also a possibility;
these could separate different reaction chambers within the particle.
The possible applications are many and varied.