Chemistry Reference
In-Depth Information
is the second of the series of the so-called “full-shell clusters.” It consists of a
hexagonal close-packed structure like bulk gold and has a cuboctahedral shape.
Full-shell clusters follow the composition 10
n
2
+ 2 for the number of atoms in the
n
th shell. Particles following this rule are especially stable, as has been
demonstrated in a variety of ways. The stability of AuNPs has indeed an important
influence on bioresponse, because of the interaction mechanisms between NP and
the relevant biosystems. As ligand-free AuNPs cannot be used because of their
instability, only ligand-protected species are used. The interaction with any kind of
biomolecule and the metal core can only happen, if the original ligands are either
completely or at least partially released or replaced during the chemical processes.
This means that the protecting ligand shell must have a special nature. If ligand
molecules are not sufficiently labile to be removed from the particle's surface, the
interaction between the particle and the biological environment only occurs via the
protecting skin and not by the metal itself. Phosphine ligands in combination with
gold usually fulfill these conditions. Au
55
(Ph
2
PC
6
H
4
SO
3
Na)
12
Cl
6
, a water-soluble
derivative of the original compound Au
55
(PPh
3
)
12
Cl
6
, therefore plays the dominant
role in this article. Larger and smaller particles are introduced for comparisons as
well as to highlight the emerging field of applications for AuNPs in diagnostics and
therapy.
Size, stability, and electronic properties indeed determine bioresponse: Sect.
3
illustrates in vitro applications by discussing the interaction of AuNPs with proteins
and cells. While these applications utilize the size-dependent properties of AuNPs
as an analytical probe, the molecular mechanisms occur in the ligand shell of the
molecules. In contrast 1.4 nm-sized Au
55
with its weak-binding phosphine ligands
turned out to be very cytotoxic. This is demonstrated in series of tests with human
cancer cell lines. Two reasons for the cytotoxicity have been proposed: (1) the size
of the 1.4 nm Au
55
clusters fits perfectly to the height of the major grooves of DNA
(1.3-1.5 nm) and thus may block transcription of DNA and (2) it induces the
formation of reactive oxygen species (ROS) as a consequence of its electronic
properties, leading to oxidative damage of neighbored biomolecules and subcellular
units. Comparisons with smaller and larger AuNPs, decorated with the same ligand
molecules like Au
55
, clearly show a much less or even no toxicity, supporting the
assumption that Au
55
has a very special bioresponse.
Properties of AuNPs in vivo are introduced in Sect.
4
. These experiments inform
about distribution in a living body (exemplified by means of rat data), of course
depending on the kind of administration. Again, size dependency plays the domi-
nant role. Among a series of AuNPs from 1.4 up to 200 nm, only the 1.4 nm Au
55
species distribute in all relevant organs, whereas larger particles are accumulated up
to 97% in the liver. A further aspect of interest is the role of surface charge in
relation to biodistribution experiments with positively and negatively charged
2.8 nm Au particles show little differences. Positively charged species are some-
what less assembled in the liver than negatively charged particles. Furthermore, a
new model system for in vivo analyses, i.e., the zebrafish, is introduced with its
potential to analyze the properties of AuNPs in whole animal tests.
