The reduction of Au3+ on the surface of the GNPs increases their
diameter to a final value that is determined by the diameter of
the seed particles and the amount of reduced Au3+. A typical
reaction for 35-40 nm GNPs was carried out as follows. To 270
mL of water, 30 mL of preformed seed gold particles was added,
followed by the addition of 3 mL of 0.2 M NH2OH‚HCl. The
mixture was stirred vigorously at room temperature, and 2.5 mL
of 25.4 mM HAuCl4‚3H2O was added dropwise to the mixture.
The color of the suspension changed to darker red concurrent
with the addition of the gold salt. The reaction was completed
within a minute. The resulting gold colloids were stored at 4 °C
in the dark to minimize photoinduced oxidation