Chemistry Reference
In-Depth Information
1
Introduction
Gold-thiolate nanomolecules are ultra-small (
2-nm) gold nanoparticles with a
fixed composition, containing a precise number of gold atoms and thiolate ligands.
These highly stable nanomolecules have a unique molecular formula and can be
characterized by conventional analytical and spectroscopic techniques. They can be
isolated in dried form and re-dispersed in solvents and are stable in air for several
years. A typical example includes Au
25
(SCH
2
CH
2
Ph)
18
. The molecular weight of
7,394 Da was independently verified using electrospray ionization mass spectrom-
etry (ESI-MS) and matrix-assisted laser desorption ionization mass spectrometry
(MALDI-MS). Ligand variations were also used to determine its composition. This
composition was also verified by X-ray crystallography and its structure was found
to contain a central Au atom, surrounded by 12 atoms on the vertices of an
icosahedron, protected by 6 [
<
] units. The UV-visible spec-
trum displays a distinct step-like fine features, reminiscent of molecular behavior.
Its redox properties were studied by cyclic voltammetry and displays molecule-like
behavior, with a large electrochemical gap between oxidation and reduction waves
[
1
]. The renewed interest in these thiolated gold nanomolecules, which are a form
of thiolated gold clusters is twofold: (a)
molecular definition
- Contrary to naked
metal clusters generated in gas phase, the solution phase synthesis of thiolate-
protected gold nanomolecules yield exclusively one core-sized molecule. For
example, the Au
25
(SR)
18
synthesis yields exclusively Au
25
(SR)
18
and there is no
formation of Au
23
,Au
24
,Au
26
,Au
27
species; (b)
Stability
- Unlike phosphine and
other ligated gold clusters, the thiolate-protected gold nanomolecules are ultra-
stable. That is, the Au-S bond is strong and therefore the number of ligands is fixed,
as opposed to the dynamic nature of ligands in the many Au-phosphine clusters and
other ligated systems. Moreover, the thiolated nanomolecules are stable in
air/oxygen, moisture, and high temperatures (up to 80
C) and can be dried and
re-dispersed in solvents.
SR-Au-SR-Au-SR
2 Overview of Various Thiolated Gold Nanomolecules
Typical examples of chemically well-defined gold nanomolecules protected by
HS-CH
2
-CH
2
-Ph ligand include Au
25
(SR)
18
, A
38
(SR)
24
, A
40
(SR)
24
,
Au
67
(SR)
35
,Au
103-105
(SR)
45-46
,Au
130
(SR)
50
,Au
144
(SR)
60
, and the 76.3 kDa
Au
~320
species.
