Prof. Dr. Matthias Karg’s group “Colloids and Nanooptics” at the Institute of Physical Chemistry has developed a simple yet precise method for producing extremely ordered particle layers. The group is using miniature, soft and deformable spherical polymer beads with a hydrogel-like structure. Hydrogels are basically water swollen, three-dimensional systems. For instance, hydrogels are commonly found in babies’ nappies as super-absorbers with the ability to soak up large quantities of liquids.

Inside these hydrogel beads are minute silver or gold particles, merely a few nanometers in size, which Karg’s team synthesized at Heinrich-Heine University (HHU) using metal salts in a reduction process. “We can adjust the size of the gold particles very precisely, because the hydrogel shells are permeable to dissolved metal salts, allowing for successive overgrowth of the gold cores.” The structure of these core-shell particles can be approximately compared with that of a cherry, where a hard core is enclosed by soft pulp. However, the particles from the lab are approximately one hundred thousand times smaller.

The scientists based at Duesseldorf can then utilize a dilute solution of these hydrogel beads to create thin monolayers. They apply the beads to a water surface, where an extremely ordered and colorfully shimmering layer self-assembles. They move this layer from the water surface onto glass substrates. This transfer makes the whole glass substrate shimmer.