Polyelectrolyte capsules have
been recently proposed as a novel type of nano-engineered multifunctional
materials. These capsules are made by layer-by-layer adsorption of oppositely
charged polyelectrolytes on the surface of colloidal template particles of 0.05
– 20 mm diameter with sequential removal of the
template core. A great variety of materials including synthetic and natural
polyelectrolytes, proteins, multivalent ions, organic nanoparticles, lipids
were used to build walls of hollow capsules. Many of them were functionalized
to provide special surface properties of technical or biological relevance. The
possibility of tailoring different functionalities, impregnating inorganic and
organic substances both inside capsule volume and in polyelectrolyte shell,
controlled release of encapsulated material provided continuous scientific and
industrial interest for employing capsules as microcontainers and
microreactors.
General
methodological approach for fabrication of nanocomposite
polyelectrolyte-inorganic microspheres consists in preliminary, before the
nanoparticle synthesis, capturing one of the reagents (or reagent-generating
agent like enzyme or inorganic catalyst) inside the capsule. Varying pH, ionic
strength of the solution and using mixtures of two and more solvents, it makes
possible to switch open/closed state of pre-formed capsule shell and to
introduce desired reagent inside. Then the second reagent is added, that
results in product synthesis either in capsule volume or in the shell. By this
approach we have synthesized different magnetite and purely metal particles,
fluorites of rare-earth elements, hydroxyapatites and metal nanoparticles in
this micron-sized volumes. Inorganic
nanoparticles incorporated to polyelectrolyte shell makes possible the remote
activated release by ultrasound or infrared radiation.
Smart polymers
involved in capsule build-up exhibit reversible sensitivity to environmental
conditions, i.e. capable of undergoing sharp physical or chemical modifications in
response to external stimuli such as temperature, pH, ions, etc. They
constitute an important class of materials, useful in a wide range of
technological applications: conversion of chemical energy into mechanical
energy, controlled drug delivery, absorbents for solvent extractions, separation
processes, sensors or actuators. Here we present the results obtained with
hollow and filled capsules prepared with stimuli-responsive polymers and
capsules filled with different polymers responsive to ions, pH and temperature.
The possibilities for practical applications on living systems are illustrated.