Fabrication of highly ordered magnetic bio-Au nanoclusters by using SlaA-layer ghosts of Sulfolobus acidocaldarius as a template

The regularly structured paracrystalline surface layers (S-layers) of prokaryotic microorganisms are promising biological templates for production of metal nanoclusters for biotechnological applications. Formerly, we have demonstrated that some bacterial S-layers can effectively bind noble metals and can be used for preparation of palladium, platinum, and gold nanoclusters by using different reduction agents (1, 2) or electron beam (3).
In this work Au-nanoclusters were produced on the extremely stable (against high temperatures, acidity, proteases, and mechanic stress) S-layer of the acidothermophilic Archaeon Sulfolobus acidocaldarius. One additional important characteristic feature of this S-layer, called recently SlaA (4) is the presence of the sulfur containing amino acid cystein in its primary structure. This is in contrast to the above mentioned bacterial S-layers and is of great importance for the effective binding and deposition of Au cations. The purified SlaA-layer fraction was treated initially with tetrachloroauric (III) acid solution. Afterwards the deposited Au(III) cations were reduced to Au(0) by using dimethylaminoburane. The analysis of the obtained Au-nanoclusters by using Transmission Electron Microscopy combined with Energy Dispersive X-ray analyses (TEM-EDX) demonstrated that the nanoclusters have a size in the range of 2-3.5 nm and that they are associated with sulfur atoms (Fig. 1). As measured via SQUID Magnetometer they possess magnetic properties (Fig. 2). No magnetic properties were observed in the case of the Au-nanoparticles formed on the bacterial S-layers possessing no sulfur. We suggest that the sulfur atoms are possibly contributing to the observed magnetic properties. However, the number of sulfur atoms implicated in the Au complexation is extremely low (only a few atoms per nano-particle). On the other hand, the size of the nanoparticles formed on the archaeal SlaA-layer is significantly bigger (about 3 nm) in comparison to those formed on the bacterial S-layer (less then 1 nm). The size difference in the two kinds of nano-particles can be explained with the different size of the pores of the two S-layer lattices, where the initial deposition of the metal cations occurs. The archaeal SlaA-layer possesses p3 lattice symmetry and significantly larger pores (4) then the bacterial S-layer which has p4 symmetry (1, 2). The size of the Au-nanoclusters may play also a role for their magnetic properties. Efforts to understand the reasons for the observed unexpected magnetic behavior of the newly constructed bio-Au nanoparticles are in progress.
To our knowledge, this is the first report on constructing of highly ordered Au nanoparticles on an archaeal S-layer. These bio-Au structures possess some advantageous properties, such as high stability to high temperatures, acidic and mechanical stress, in comparison to those formerly constructed on bacterial S-layers. Moreover, in contrast to Au as a bulk noble metal and to bacterial bio-Au, the archaeal bio-Au nanoparticles possess unique magnetic properties.
References
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