Chemistry Reference
In-Depth Information
De Heer has reviewed a range of top-down methods available for the produc-
tion of gas-phase metal cluster ions [
71
]. Of these, the laser vaporisation cluster
source, also known as the 'Smalley source', has been commonly used for the
production of gas-phase clusters for over 30 years [
72
]. Indeed this method has
been most widely used to generate bare gold cluster ions in the gas phase.
Readers interested in the design and operational aspects of these laser
vaporisation cluster source should refer to an excellent recent review by
Duncan [
72
].
2. The 'bottom-up' approach involves chemical methods whereby a metal salt or
complex, in the presence of an appropriate chelating ligand, is reduced to form
ligand-protected metal nanoclusters. Reduction commonly occurs via treatment
with a chemical reducing agent such as sodium borohydride, borane
tert
-
butylamine complex and citrate [
73
]. The addition of chemical reducing agents
to gold ions in solution provides a source of electrons whereby neutral gold
atoms are formed. When the onset of reduction occurs in the presence of an
appropriate chelating ligand, such as a phosphine or thiolate, the aggregation of
neutral gold atoms to bulk material is perturbed by the properties of the ligand in
a given environment. Size selectivity of a ligand is an inherent property that can
be optimised experimentally by varying factors such as the temperature, stir rate,
concentration, solvent, etc.
Gold cluster synthesis via the bottom-up method requires a deep understanding
of the assembly processes, governed by the interactions of the atomic and molecular
components, that leads to the formation of stable and monodisperse nanoclusters
[
74
-
77
]. This can lead to the rational design and fine tuning of the AuNCs
architecture. Although the bottom-up method is seen as the inverse method to the
top-down approach, it may also be used in addition to top-down techniques [
78
]in
particular via processes that involve ligand etching. There is a growing awareness
that there are three phases associated with AuNC synthesis via the bottom-up
method and that ESI-MS can be used to examine ionic species present during
these phases (Fig.
3b
):
1.
Pre-reduction phase
: To provide insight into the fundamental processes that
govern the formation of nanoclusters it is important to establish the identity and
relative abundance of ionic complexes that exist in solution before the initiation
of chemical reduction. These complexes are the 'molecular building blocks' that
can then interact to form particles of well-defined stoichiometry upon reduction.
ESI-MS has been used to examine the identity and relative abundance of the
cationic precursors that spontaneously assemble in solution prior to reduction.
2.
Reduction phase
: A key step in the synthesis of AuNCs in the condensed phase
involves the use of chemical reducing agents. Reducing agents which are
routinely used for AuNCs are sodium borohydride (NaBH
4
), a fast reducing
agent, and borane
tert
-butylamine complex (BTBC), a comparatively slow
reducing agent. Subsequently, ESI-MS can be used to monitor the abundance
and identity of AuNC ions present in solution over time from the onset of
reduction. This provides a tool
to monitor reactive cluster intermediates,
