Chemistry Reference
In-Depth Information
fragments which are based either on icosahedra or cuboctahedra. These unambig-
uous crystallographic determinations have therefore not only extended greatly the
range of molecular gold cluster compounds but also provided an important gener-
alisation that gold colloids may actually be based on a mixed valency combination
of a central core (Au(0)/Au(I)) and thiolato-Au(I) chelating ligands, which are
described as “staple motifs”.
The distinction between nanoparticles and clusters or colloids is a subtle one and
one which is becoming more clearly defined as a result of the new developments
which are articulated in these reviews. The view that nanoparticles constitute a new
class of material, which have properties which are fundamentally different from
those of discrete cluster molecules or the bulk metal, has gained ground in recent
years. Nanoparticles with dimensions of the order of 1-20 nm fall between small
molecular clusters with discrete energy levels and the bulk metal which has
continuous energy states. Many properties of metal nanoparticles, e.g. ionisation
energy, electron affinity, cohesive energy, absorption frequency electrical conduc-
tivity, melting points and chemical reactivity, should show a regular variation with
cluster size according to classical physics theory, i.e. they should follow a relation-
ship of 1/
R
, where
R
is the radius of the metal particle; however, quantum size
effects may result in important discontinuities when the particles have dimensions
lie in the nano-range [
2
,
3
]. These quantum size effects could have important
implications for the chemical, catalytic, biological and electronic properties of
these species.
2 Synthesis of Gold Colloids
2.1 Aqueous Solutions
Faraday produced colloids of gold using H[AuCl
4
] as the source of gold and
phosphorus vapour as the reducing agent. They are stabilised in solution because
the solvent molecules and negative charges associated with the reducing agent
inhibit the aggregation of the gold particles to form larger samples of bulk gold
which would precipitate from the solution. They are therefore kinetically stabilised
relative to the bulk metal and require stabilising anions and solvents which do not
readily re-oxidise the colloidal gold particles to gold(I). The preparation of gold
sols may be classified according to whether they are prepared in aqueous or
non-aqueous solvents and whether they are made by dispersion (physical) methods
or reductive chemical condensation processes. H[AuCl
4
] is still the most widely
used precursor for gold colloids in aqueous solutions, and the reducing agent and
concentrations are varied in order to vary the size and size distribution of the
resultant gold particles. Phosphorus initially preferred by Faraday [
4
] is still
employed for preparing colloids with mean diameters of 3-12 nm. Specifically a
gold sol with a mean diameter of 5 nm may be made from H[AuCl
4
] neutralised by
