Chemistry Reference
In-Depth Information
The Au
55
cluster has another special property: it belongs to the so-called very
stable full-shell clusters, species having an icosahedral or a hexagonal close-packed
cuboctahedral shape, consisting of a distinct number of atoms per shell around
1 central atom, namely 10
n
2
+ 2 atoms (
n
¼
number of shell). The smallest full-
shell Au cluster with
n
¼
1 consists of 1 + 12 atoms, well known since 1981 with
icosahedrally structured Au
13
cores [
3
,
16
]. Numerous full-shell clusters of other
metals have become known in the meantime, for instance the four- and five-shell
clusters Pt
309
[
17
] and Pd
561
[
18
].
.
Whereas the ligand-protected Au
13
cluster has
still typical molecular properties, the Au
55
species is just at the borderline between
molecule and bulk and is therefore often called as “metal in the embryonic state.”
The answer to the aforementioned question, why electronic and structural
properties of clusters or nanoparticles are of relevance with respect to bioresponse,
is not very surprising. First of all, the stability of metal particles in a biological
medium is of high importance because less stable species would decompose,
forming smaller or larger species with different properties. So, a stability
guaranteeing structure and size is a condition for reliable results. The electronic
structure is responsible for the chemical activity of a particle and so influences
fundamentally the interactions with other materials. Some examples of these two
factors are given.
Au
55
and various smaller and larger gold nanoparticles and even bulk gold
surfaces have been investigated with respect to their activity against oxygen in an
oxygen plasma by means of X-ray photoelectron spectroscopy (XPS) [
19
]. Except
Au
55
, all other species became partially or quantitatively oxidized! Why not Au
55
?
This is because of its structure. Oxidation of the cuboctahedral structure would
afford too much energy so that oxidation, even under the exceptional conditions in
an oxygen plasma, does not happen.
The special stability of the Au
55
nucleus becomes additionally visible when
again AuNPs, smaller and larger than Au
55
, are contacted with indium vapor. Bulk
gold forms an alloy of the composition AuIn
2
. All other NPs react easily with In
vapor, but again not Au
55
. It remains unreacted [
19
].
Theoretical [
20
] and experimental results [
21
] show that very small AuNPs are
most active in oxygenation catalysis. Especially particles below ~3.5 nm are the
most active species, indicating that electronic properties play a decisive role in
chemical processes. Au
55
(PPh
3
)
12
Cl
6
with a core diameter of 1.4 nm, supported on
inert materials, turned out to be the most active catalyst for the selective oxidation
of styrene by dioxygen [
21
]. In this case, particles larger than ~2 nm are
completely inactive. This finding is of high importance considering the cell
toxicity of Au
55
species due to oxidative stress (reactive oxygen species, ROS)
[
22
]. Though larger AuNPs also cause oxidative stress, the 1.4 nm cluster is the
most active particle. Oxidative stress is caused by activation of dioxygen when
touching the metal surface however, without oxidation of Au atoms. Details will be
given in Sect.
3
.
Another result indicating the special role of Au
55
clusters shall briefly be
mentioned. It is the relaxation behavior of excited electrons, compared with two
other particle sizes [
23
]. If 0.7 nm (Au
13
), 1.4 nm (Au
55
), and 15 nm AuNPs are
