Colloidal solutions of 1 mM gold nanoparticles (AuNPs) were synthesized by gamma-ray Co-60 and electron beam irradiation using 1 percent water soluble chitosan (WSC) with different molecular weight (M w) as stabilizer. The AuNPs size measured from TEM images was of 7.1 and 15.1 nm for electron beam and gamma-ray Co-60, respectively. The AuNPs sizes of 9.8, 15.1 and 22.4 nm stabilized by different WSC Mw of 155 x 103, 78 X 103 and 29 x 103 g mol-I, respectively, were also synthesized by gamma -ray Co-60 irradiation. Antioxidant activity of AuNPs with different size from 7.1 to 20.0 nm was investigated using free radical 2,2'-azino-bis (3-ethylbenzothiazoline-6sulphonic acid) (ABTSH). Results indicated that the smaller size of AuNPs exhibited higher antioxidant activity. In particular, the antioxidant efficiency was of nearly 100, 75, 65, 52 and 30 percent for 7.1,9.8,15.1, 20.0nm AuNPs and WSC 0.1 percent, respectively, at the same reaction time of 270 min. Thus, due to the compatibility of WSC and the unique property of AuNPs, the pure colloidal AuNPs/WSC solutions synthesized by irradiation method can be potentially applied in biomedicine, cosmetics and in other fields as well.
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