Chemistry Reference
In-Depth Information
Fig. 1 Schematic presentation of the size-dependent properties of gold, spanning the size range
from individual atoms via ligand-stabilized clusters and colloidal particles up to macroscopic bulk
metal. The shaded areas indicate size ranges, where specific properties and features of gold occur.
Atoms, molecules, and molecular clusters exhibit an electronic structure with discrete electronic
states, while colloidal particles as well as bulk gold exhibit collective electronic states. In the size
range of clusters and colloids, biological effects may occur due to complementarity in shape to
biomolecules and cellular subunits. In this size range the surface plasmon resonance (SPR)
dominates the optical properties and most of the size-dependent catalytic properties of gold
nanoparticles occur. The biocompatibility increases with sizes above the typical cluster size.
The marked areas are not drawn to scale
The final section of this article (Sect.
5
) deals with perspectives and challenges
of the above findings especially in medicine. Diagnostic and therapeutic aspects as
well as drug delivery systems are already partially realized or can be prognosticated
(Fig.
1
).
2 Size-Dependent Properties of Metal Nanoparticles
The physical and chemical properties of defined materials, for instance elements or
chemical compounds in the solid state, depend on size and shape of the particles
under consideration. The size dependency of the properties usually becomes obvi-
ous in the nanometer regime. Nanotechnology is based on such effects which can
have very different appearance. Whereas the bright color of bulk gold is based on
relativistic effects [
7
-
9
], the red, purple, and blue color of gold nanoparticles
(AuNPs) in the size regime of ca. 15 and 50 nm is based on the so-called surface
