Chinese scientists have demonstrated a different way to make multilayer thin films.

Multilayer films can be created by adding an ionic liquid to a polymer containing sulfonate and ammonium ions

Multilayer films - that can be as thin as a few atoms - are made by alternating layers of two different materials. Now, Suobo Zhang, Zhaohui Su and colleagues at the Chinese Academy of Sciences in Changchun have discovered a new way of building up these layers to make a more versatile film.

Conventionally ultrathin films are made using a layer-by-layer method that relies on electrostatic attraction. For multilayers created using polyanions and polycations, each new layer deposited over-compensates for the original surface charge. This results in charge-reversal on the new surface and enables the multilayer film to be built up.

Zhang and his colleagues found that using a polymer with two negatively-charged sulfonate and one positively-charged ammonium ion in each of its repeat monomer units makes it possible to build a multilayer from a neutral surface.

Zhang explains the assembly method. 'The distribution of positive and negative charges in the polymer makes it behave like a single negatively-charged unit over its entire length. It adsorbs positive surface charge but only to neutralise it, no charge-reversal step takes place. Instead, the multilayer is built up when ammonium ions in the inner salt pairs of the polymer, are replaced with cations from an ionic liquid in solution. The displaced ammonium ions create a positively charged surface to continue the process.'

Xi Zhang, professor of polymer chemistry and physics at Tsinghua University in Beijing, China, is enthusiastic about the work. 'Generating surface charge in this way by a chemical reaction within the polymer is new in multilayer assembly,' he says.

The method offers tremendous potential, says Suobo Zhang. 'It enables different polyelectrolytes and single charged functional molecules to be combined - or exchanged - within multilayer films. Also, because the assembly steps are carried out in aqueous solution, without organic solvents, the multilayers obtained are compatible with biosystems and biomolecules'.

Janet Crombie