Chemistry Reference
In-Depth Information
the addition of K
2
CO
3
followed by the addition of white phosphorus in diethyl
ether. The success of the procedure is indicated by the development of purple-
brown colouration in the aqueous layer. Other reducing agents which are commonly
used include sodium citrate and ascorbic acid. The sodium citrate recipe, which was
developed by Turkevich (in the 1950s) [
67
,
68
] and Frens (in the 1970s) [
69
,
70
], is
the most commonly cited method. The procedure is based on the addition of sodium
citrate to a boiling aqueous solution of H[AuCl
4
] which initially gives a grey colour
which changes to a lavender colour and with continued boiling becomes deep red.
Gold colloids with mean diameters that range from 12 to 147 nm may be made by
varying the concentration of sodium citrate. Smaller colloid diameters have been
achieved with alternative reducing agents such as sodium thiocyanate and hydro-
lysed tetrakis(hydroxymethyl)phosphonium chloride. The kinetic stabilisation of
gold colloids is so effective that some of Faraday's samples are still available for
viewing at the Royal Institution in London. The synthetic procedure is sufficiently
routine that several chemical suppliers sell gold colloid solutions with well-defined
mean diameters (e.g. see [
28
]). This commercialisation has encouraged their wide-
spread use in medical applications and catalysis. Recently, the molecular evolution
of spherical gold nanoparticles in the Turkevich/Frens reaction has been investi-
gated in more detail. Widespread networks of gold nanowires are formed initially as
transient intermediates and they are responsible for the dark colour of the reaction
solution before it takes up its characteristic ruby-red colour [
71
].
Larger particles are formed when less sodium citrate is added (possibly down to
0.05%, after which there simply would not be enough to reduce all the gold). The
reduction in the amount of sodium citrate will reduce the amount of the citrate ions
available for stabilising the particles, and this will cause the small particles to
aggregate into bigger ones (until the total surface area of all particles becomes small
enough to be covered by the existing citrate ions). It has been proposed that colloids
produced by the citrate synthetic route are stabilised at their surface by a combina-
tion of citrate and chloride anions. Addition of stronger Lewis bases may result in
the displacement of these anions and the disproportionation of the colloid to larger
aggregates and in the limit insoluble bulk gold particles and gold(I) complexes of
gold. For example the addition of pyridine to a gold sol causes the colloidal
particles into strings or chains of gold particles. The chelate effect has been widely
used in co-ordination chemistry to stabilise metal complexes and polymers and
surfactant molecules have been used in a similar manner to form a protective sheath
on the surface of colloidal particles. The protective sheath bonds more strongly to
the surface gold atoms and thereby assists the solvation of gold colloids. In
Victorian times gelatin and agar were commonly used, but currently organic
synthetic polymers are used. At times the polymer may combine a co-ordination
and reducing role. For example, poly(ethylenimine) reduces H[AuCl
4
] and adheres
to the surface of the colloidal particles and thereby generates particles with very
small average diameters [
71
].
In 2009 Perrault and Chan [
72
] developed a seed-based synthesis of gold sols
using hydroquinone to reduce H[AuCl
4
]. Gold nanoparticle acts as the seeds in
much the same way that reduced silver particles seed silver halide particles in the
