A large excess of sodium borohydride is needed both to
reduce the ionic silver and to stabilize the silver nanoparticles
that form. A 10-mL volume of 1.0 mM silver nitrate was
added dropwise (about 1 dropsecond) to 30 mL of 2.0 mM
sodium borohydride solution that had been chilled in an icebath.
The reaction mixture was stirred vigorously on a magnetic
stir plate. The solution turned light yellow after the
addition of 2 mL of silver nitrate and a brighter yellow (Figure
1A) when all of the silver nitrate had been added. The
entire addition took about three minutes, after which the stirring
was stopped and the stir bar removed. The clear yellow
colloidal silver shown on the left in the photograph in Figure
1, is stable at room temperature stored in a transparent
vial for as long as several weeks or months.