Biomedical Engineering Reference
In-Depth Information
FIGURE 2.7
TEM of ~6.5 nm AuNP.
From Ocean NanoTech.
Colloidal citrate AuNPs are made up of particles varying in diameter from
~ 5 to 150 nm. Typical diagnostic assays use gold conjugates with size ranging
between 20 and 40 nm indicating a large surface area having a very high surface
energy. By controlling the percentage of the trisodium citrate aqueous solution,
the diameter of the resulting AuNPs may range from 15 to 150 nm. AuNPs with
diameter between 6 and 15 nm are formed by reducing the HAuCl
4
solution with
an aqueous sodium ascorbate solution. Particles with diameters smaller than 5 nm
are produced by reduction with either white or yellow phosphorus in diethyl ether.
Any colloidal suspension with high surface energy can lose stability when proper
processing conditions are not maintained during production and use. Depending
upon the ratio of the gold to citrate, the AuNPs can grow to different sizes. Col-
loidal AuNPs are composed of a core made of pure gold that is surrounded by a
surface layer of adsorbed AuCl
−2
ions. This state confers a negative charge to the
colloidal gold resulting in electrostatic repulsion that prevents particle aggrega-
tion. The introduction of electrolytes compresses the ionic double layer, thereby
reducing electrostatic repulsion. This results in destabilization that causes particle
aggregation that is accompanied by a color change and eventual sedimentation of
the gold. Chlorides affect colloidal AuNPs more strongly than iodides.
During the synthesis of citrate AuNPs prior to the addition of the reduc-
ing agent, gold ions exist in solution. Upon the addition of the reducing agent,
sodium citrate, gold atoms begin to form until the concentration reaches super-
saturation that results in precipitation through nucleation. When this is reached,
all the gold atoms remaining in solution continue to bind to the nucleation sites
