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T. Herrmannsdo¨erfer et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e821–e823
e822
produce Pd(0)-nanoparticles. The metallized protein sam-
ples were again centrifuged and finally dried in a vacuum
oven (48 h at 80 1C).
of nanograins with a larger diameter of about 10 nm as
reported in Ref. [4]. Fig. 1 also demonstrates the strong
dependence of the susceptibility of the Pd nanograins on
the 3d impurity concentration.
The transition-metal nanoclusters were investigated
using mass spectroscopy in order to determine the mass
ratio of Pd and the protein. In the course of our
investigation, we have produced mass ratios of Pd and
the S-layer ranging from 5:1 to 1:3. We use typical sample
masses of a few milligrams for all measurements described
below. Extended X-ray absorption fine structure spectro-
scopy (EXAFS) performed at the ESRF (Grenoble,
France) was used to measure the cluster size. The analysis
of EXAFS data allows a determination of the coordination
numbers (e.g., of Pd neighbours of Pd atoms in the first
Pd–Pd coordination shell), for which we find five for the Pd
nanoclusters instead of 12 for bulk Pd. This then results in
a mean grain diameter of about 1.5 nm [3] for the Pd
nanoclusters on the protein S-layer [1].
We have investigated the magnetic properties of
nanoclusters of the transition metals, Pd and Pt, by
means of SQUID magnetometry. We observe a clear
reduction of the susceptibility compared to the one
obtained from bulk Pd (see Fig. 1), which develops
spin fluctuations and is nearly ferromagnetic. The
maximum of the susceptibility of bulk Pd at T ¼ 80 K
is attributed to spin fluctuations, i.e., ferromagnetic
correlations of the 4d magnetic moments on a small length
and short time scale. While the Stoner enhancement
(w ¼ SwPauli) is large and reaches a value of 10 in bulk Pd,
the Stoner enhancement factor in the pure Pd nanoclusters
almost vanishes to S ¼ 1, i.e., the 4d electrons interact
much less with each other in the nanograins. Our data
do not support former observations of ferromagnetic
contributions arising from oriented facets of the surface
The decrease of the magnetic d-band susceptibility
agrees with the model of Fay and Appel [5], which
considers the effect of spin–orbit scattering on the magnetic
and superconducting properties of granular nearly ferro-
magnetic metals. In the case of micro- [6] and sub-
microgranular [7] Pt, the weakened magnetism of the 5d
electrons has been found to play a crucial role for the
earlier observed occurrence of superconductivity by adjust-
ing the balance between electron–phonon interactions and
competing magnetic interactions. As the superconducting
transition temperature has been observed to increase with
decreasing Pt grain size [7], we now focus our interest on
the search of superconductivity in the transition-metal
nanoclusters deposed on the biological template. For
this, we are preparing to extend our investigations to the
mK-temperature range.
Further to that, we have started to produce nanoclusters
of superconducting elements such as Al, Sn, Ga and Pb
on the S-layer protein sheets of B. sphaericus JG-A12.
Recent improvements of our preparation method, in
particular processing of highly pure chemical agents under
anaerobic conditions in
a nonmagnetic quartz-glass
environment, made it possible to stabilize metallic Pb
nanograins on the S-layer and to prevent them
from agglomeration as well as from oxidation. The
superconducting properties of these separated Pb clusters
are under investigation. First data yield a superconducting
critical field of the size of several T which is strongly
enhanced compared to the corresponding critical magnetic
field of 0.09 T of bulk Pb. Recently, Ovchinnikov and
Kresin [8] have predicted the possibility of strengthening
the superconducting properties in metallic nanoclusters. As
a first experimental proof, Hagel et al. [9] have observed a
clear increase of the superconducting critical parameters in
a crystalline Ga84-cluster compound.
The S-layer proteins have the capacity to bind selectively
and reversibly high amounts of metals, making them
interesting also for technological applications. Their
usability for biosynthesis is not limited to elements only.
Besides metal oxides, e.g., FeO, also binary magnetic
compounds such as PtFe or CoNi and semiconducting
nanoclusters, e.g., PbSe [10], can be produced.
References
[1] K. Pollmann, J. Raff, M. Merroun, K. Fahmy, S. Selenska-Pobell,
Biotechnol. Adv. 24 (2006) 58.
Fig. 1. Magnetic molar susceptibility of nanogranular Pd deposed on a
protein S-layer (mass ratio 1:1) of two samples with different 3d impurity
concentrations compared to bulk Pd. Dashed lines indicate low-
temperature susceptibilities without the low-temperature increase resulting
from the magnetization of the magnetic 3d impurities. In addition, the
diamagnetic contribution per mole Pd of the protein S-layer is shown
(dotted line).
[2] K. Fahmy, M. Merroun, K. Pollmann, J. Raff, O. Savchuk,
C. Hennig, S. Selenska-Pobell, Biophys. J. 91 (2006) 996.
[3] Y. Sun, A.I. Frenkel, R. Isseroff, C. Shonbrun, M. Forman, K. Shin,
T. Koga, H. White, L. Zhu, Y. Rafailovitch, J.C. Sokolov, Langmuir
22 (2006) 807.
[4] T. Shinohara, T. Sato, T. Taniyama, Phys. Rev. Lett. 91 (2003)
197201.
Herrmannsd?erfer
