Photochemical formation of colloidal gold particles in chitosan films

Thin films of chloroauric acid salt of chitosan (CTOAu films) were photolyzed with 253.7 nm light. On irradiating with UV light in air al room temperature, the CTOAu film first became yellow-blue colored due to the formation of colloidal gold. When irradiation was continued, the irradiated surface of the film finally changed into clear gold mirror. Primary photochemical products are assumed to be gold atom, quaternary ammonium chloride of chitosan and chlorine. With aids of UV-visible absorption spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy, aggregation of small gold particles to form colloidal gold and gold metal film has been investigated. It has been found that the aggregation state of small gold particles is controlled by reaction conditions; temperature, evacuation, the amount of chloroauric acid. When the CTOAu film was photolyzed in vacuum at room temperature, the gold particulate film was formed. Irradiation at liquid nitrogen temperature resulted in absorption spectrum characterized by the structureless absorption extending from lambda = 400 nm to shorter wavelength. However, an introduction of air after photolysis brought about the development of the broad colloidal absorption band.