Synthesis of Silver Nanoparticles. A typical reaction is as
follows, which is similar to the well-known Turkevich method.20
A 100-mL sample of aqueous trisodium citrate (7.0 mM) was
prepared in a 250-mL flask, containing controlled amounts
of citric acid or NaOH at room temperature. The pH value of
various reaction solutions was adjusted by further addition of
nitric acid or NaOH. The solution with different pH values (from
5.7 to 11.1) was first brought to a boil while being stirred, and
then 1.0 mL of 0.1 M aqueous silver nitrate was added to start
the reaction. The color of the reaction solution was observed to
change generally from colorless to yellow, then turbid. At last
no more change took place, indicating the reaction was
completed. The stepwise method was carried out by fast
nucleation at higher pH value (shown in the text), and the
solution was suddenly adjusted to pH 6.1 by addition of HNO3
at the end of the nucleation stage.