A green synthesis of colloidal silver nanoparticles and their reaction with ozone

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