Abstract

Colloidal silver nanoparticles were prepared by a conventional reduction of Ag+
ions with NaBH4 and with a novel approach using tannic
acid (tannin) as reducing agent of Ag+
ions. The formation of colloidal silver was followed by electronic absorption spectroscopy
monitoring the growth of the surface-plasmon resonance band of the nanoparticles. Colloidal silver in tannin is more stable than that
prepared with NaBH4 because tannin exerts also a protective colloid function permitting to reach much higher colloidal silver concentration
than in the case of NaBH4. Colloidal silver nanoparticles are readily reactive with ozone producing oxidized silver nanoparticles composed
by the mixed oxide Ag2O2 formulated as AgIAgIIIO2. If the ozonization is conducted in gentle conditions the oxidized silver nanoparticles
remain as colloidal suspension for a certain time. If instead mechanical shaking is applied, the particles coagulate immediately and separate
from the liquid medium as a precipitate.

Key words: : silver nanoparticles; colloidal silver; NaBH4; tannic acid (tannin); ozone; oxidized silver nanoparticles