Visualization of the atomic structure of the Au102(p-MBA)44 particle (left) and the partially ligand-exchanged Au102(p-MBA)40(p-BBT)4 (right). The exchanged ligand bromo benzene thiol (p-BBT) is schematically shown in the middle and the observed ligand exchange sites by red and blue on the right. |
Nanometer-scale
gold particles are currently intensively investigated for possible
applications as catalysts, sensors, biolabels, drug delivery devices,
biological contrast agents and as components in photonics and molecular
electronics. The particles are prepared in a solution from gold salts
and their reactive gold cores can be stabilized with various organic
ligands.
Particularly
stable particles can be synthesized by using organothiol ligands that
have a strong chemical interaction to gold, producing precise
compositions in the size range of 1 to 3 nm. Modification of the
protecting molecular overlayer is a key step in almost all
applications. A detailed structural atomistic understanding of the
processes of the exchange reaction has been lacking.
Now, professors Chris Ackerson in the Colorado State University in Ft. Collins, USA, and Hannu Häkkinen
at the Nanoscience Center of the University of Jyväskylä, Finland,
report the first structural study on the atomistic processes of a
ligand-exchange reaction of a well-defined gold nanoparticle that has
102 gold atoms and 44 ligand sites in the molecular overlayer. The study
was published in the Journal of the American Chemical Society
on 21 July 2012. Prof. Häkkinen’s work is funded by the Academy of
Finland and prof. Ackerson’s work is funded by the Colorado State
University and the American Federation for Aging Research.
The studied particle has a chemical formula of Au102(p-MBA)44
and it was made by using a water-soluble thiol (para—mercapto benzoic
acid, p-MBA) as the stabilizing molecule. The X-ray crystal structure of
this particle was first reported as the cover article of Science in 2007 by the group of Roger D. Kornberg
from Stanford University. Häkkinen led an international team of
researchers that published a theoretical analysis of this and other
thiol-stabilized gold nanoparticles in 2008 in the Proceedings of the National Academy of Sciences.
In
the new study, Ackerson’s group succeeded in making heterogeneous
crystals of samples of Au102 particles that had undergone a
ligand-exchange reaction where the p-MBA thiols in the molecular
overlayer had been partially exchanged to a similar thiol containing a bromine atom, the so-called para—bromo benzene thiol (p-BBT), under a
fast 5-minute reaction. The analysis of the heterogeneous crystals
showed which ligand sites in the overlayer are the most likely to be
changed during the short reaction time, i.e., from which sites the
exchange process starts. Surprisingly, only 4 sites out of the 44
possibilities showed occupation by the exchanged ligand (see Figure).
Theoretical analysis performed by Häkkinen’s group gave insight into the
atomistic details of possible reaction mechanisms. Evidence from
experiment and theory indicates that the Au102(p-MBA)44 nanoparticle has
a thiol overlayer where almost every thiol ligand site has its own
reaction rate due to a highly heterogeneous structure of the overlayer.
“The
Au102(p-MBA)44 nanoparticle has a structure reminiscent of a protein,
with a rigid inorganic gold core analogous to the alpha-carbon backbone
of a protein core and chemically modifiable functional groups in the
low-symmetry molecular overlayer,“ says Prof. Ackerson.
“When
ligand exchange reactions are better understood, we hope to fully
control the surface functionalization of the Au102 and similar
water-soluble gold nanoparticles. The implications in biology for a
fully controllable synthetic surface the size of a protein are
profound,” says Prof. Häkkinen.
The other researchers involved in the work are Christine Heinecke, Thomas Ni and Andrea Wong from Ft. Collins and Sami Malola and Ville Mäkinen
from Jyväskylä. The massively parallel computations needed for
interpretation of the experimental observations were made in the Louhi
supercomputer at CSC—the Finnish IT Center for Science in Espoo.
