Self-assembling proteins as basis for new nano materials.

The investigation of the interactions of radionuclides with the biosphere revealed special adapted bacteria. Some of them were isolated from a uranium mining waste pile. These bacteria are covered with a protein envelope called S-layer as many other bacteria and most of archaea. In these cases, the S-layers bind toxic and radioactive heavy metal ions preventing an uptake and a damage of the cell. The proteins exhibit self assembling properties and form a two dimensional paracrystalline protein lattice. The S-layer can be isolated and dissolved into monomers under the influence of chaotropic agents. The purified proteins are able to recrystallize once the chaotrope is removed. Recrystallization occurs preferably at interfaces, but also in solution and is used in the present study to cover various surfaces with a single layer of highly ordered protein polymers. The protein layer features pores of uniform size and distribution. This lattice is useful for nanostructuring of surfaces. The protein lattice can be used as matrix to form nanoparticles. The different functional groups of the protein are potential binding sites for attaching further functional molecules like enzymes or fluorescence markers. The periodicity of the layer is suitable to immobilize molecules regularly and with high density.
One challenge is the complete coverage of the respective surface in an appropriate time. Another problem is the stability of the produced coating under harsh conditions. We have used AFM analysis on a variety of substrates in order to optimize the substrates pretreatment regarding S-layer coverage and time consumptions. While the recrystallization at cleaned silicon supports is too slow and incomplete, the pretreatment with polyelectrolytes enhances speed of the recrystallization process and allows a complete coverage of surfaces. The generated protein layer shows a thickness of about 9 nm which is in good agreement with data published in literature. These monolayers were used as matrix for the formation of Pt- and Pd-nanoparticles. The proteins are able to bind high amounts of Pt or Pd ions from solution. Subsequent reduction leads to the formation of nanoparticles. Afterwards most of the organic compounds can be removed by UV excitation without heating leaving the nanoparticles well arranged on the surface. Moreover the special properties of the S-layer proteins make them highly interesting to be used as a technology platform for the design of new nano-materials.