Interest in this paper follows from the versatility of the method, devised by Frank Caruso and colleagues, which can coat a range of particles, such as polymers, inorganic crystals, or biological colloids, with a variety of layers of synthetic polymers or natural proteins. Multiple coatings can be applied layer by layer, while still maintaining the integrity of the particles as colloids, and they can be given unique magnetic, optical, or catalytic properties. Such is the potential of the method, that the German chemists have set up a company to exploit the new technology.

Interest in coated colloids is coming from a wide range of industries, and possible applications are seen in such diverse areas as cosmetics, pharmaceuticals, and electronics. "I can foresee applications in the areas of encapsulation, drug delivery, surface coatings, electronics, photonics, catalysis, sensing and separations," says Caruso. "We can encapsulate compounds such as drugs, we can impart new functions to particles, we can put biomarkers on them and even get them to target certain surfaces through biomolecular recognition."

So how do you coat a colloidal particle with multiple layers? Normally those who are interested in coating surfaces with multiple thin films imprint them onto a flat surface, such as silicon, glass, or gold. Colloid particles pose a different kind of problem but films can be built up in sequence from solution, provided they are alternate layers of positively charged and negatively charged polymers, and this is how it is done: each layer is attracted to the surface by the oppositely charged layer that is already in place.

The group used layers of a polymeric form of allylamine, which is positively charged at many sites, and a polymer form of styrene sulfonate, which is negatively charged. To analyze the way these deposit from solution onto colloidal latex particles, the team used a technique known as single particle light scattering measurement (SPLS). The buildup of alternate layers of negative and positive layers could be followed by this method for at least 21 layers of particle coating. As each layer deposits, the particles get larger, thereby affording nanometer control over their size.

The interactions between the oppositely charged layers were studied by means of fluorescing molecules which are attracted to the polymer films, and whose fluorescence changes when they attach themselves to the layers. The fluorescent probes that so bind show changes that depend on the number of layers that surround the particle, although this effect levels off after 11 layers have been accreted. Caruso interprets the result as showing that not all charges on the electrolyte films are tied up in directly attracting their oppositely charged counterpart, and that the alternate layers overlap and interpenetrate one another.

Paper #6 also addresses the problem of whether salt ions from solution become embedded in the films. Recent studies have failed to find such ions and it has led to the suggestion that there is an exact charge balance between the negative groups on one polymer layer and the positive groups on the adjacent layers. Caruso also shows that there are no significant amounts of salt ions present in the layers in his coated colloids.