Antioxidant capacity index based on gold nanoparticles formation. Application to extra virgin olive oil samples

A simple gold nanoparticles (AuNPs) based colorimetric assay for the antioxidant activity determination has been developed. The AuNP formation is mediated by extra virgin olive oil (EVOO's) endogenous polyphenols; the reaction is described by a sigmoidal curve. The ratio K-AuNps/X-c(50) (slope of the linear part of the sigmoid/concentration at half value of the absorbance) was found to be the optimal parameter to report the antioxidant capacity with respect to the single K-AuNps or X-c(50) values. The obtained data demonstrated that the compounds with ortho-diphenols functionality are most active in reducing gold (III) to gold (0). Thus, intermediate activity was found for gallic acid, while tyrosol (mono-phenols) had a significant lower activity than the others antioxidant compounds (at least one order of magnitude). In the analysis of olive oil samples, a significant correlation among classical methods used to determine antioxidant activity and the proposed parameter was found with R values in the 0.96-0.97 range. (C) 2015 Elsevier Ltd. All rights reserved.