Bio-Au nanoparticles on archaeal and bacterial S-layers

Gold nanoparticles with substantially different properties were produced by using two alternative S-layer templates. The first one was a bacterial template, representing sheets of the S-layer of Bacillus sphaericus; the second one was in a form of empty cells (ghosts) consisting of the so-called SlaA-layer of the thermoacidophilic archaeon Sulfolobus acido-caldarius. The archaeal SlaA-layer is resistant not only to high temperatures and acidity but also to detergents, that allowed to purify the SlaA-layer-ghosts keeping the shape of the cells. The production of the Au nanoparticles was performed according to [1,2] in a two-step procedure by using DMAB as a reducing agent.
We demonstrate that the SlaA-ghosts of S. acido-caldarius serve as a very efficient template for complete reduction of Au(III) to Au(0). In the case of using S-layer sheets of B. sphaericus only 40 % of the added Au(III) was reduced to Au(0) [2]. The size of the archaeal bio-Au nanoparticles was about 2.5 nm, while those of the bacterial ones was about 4 nm. The most stricking property of the archaeal bio-Au nanoparticles is, however, that that they are paramagnetic, in contrast to the bacterial ones and also to bulk gold, which are diamagnetic. As demonstrated by SQUID magnetometry, the archaeal bio-Au possesses an unusually large magnetic moment of about 0.1 µB/Au atom. HR-TEM combined with EDX analysis revealed that the archaeal Au nanoparticles are bound to sulfur atoms. The latter originate from the thiol groups of the cystein amino acid residues which are characteristic for the SlaA-layer of S. acidocaldarius but absent in the S-layer of B. sphaericus. Surprisingly, the magnetic moment of the archaeal bio-Au nanoparticles is substantially larger than the ones observed for thiol capped, chemically produced Au nanoclusters [3]. We suggest that the unusual shape and the biochemical characteristics of the SlaA-ghosts are responsible for the observed extraordinary properties of the archaeal bio-Au.

[1] Merroun et al. (2007) Mat. Sc. Tech. 27, 188-192.
[2] Jankovski et al. (2010) Spectroscopy 24, 177-181, 2010.
[3] Crespo et al. (2004) Phys. Rev. Lett. 93, 087204.