A novel antioxidant capacity (AC) method was designed exploiting the formation and growth mechanism of green synthesized silver nanoparticles (SNPs) using kappa-carrageenan. Carrageenan, a natural polysaccharide derived from edible red algae, was used for the green biosynthesis of kappa-carrageenan capped SNPs (CAR-SNP). Silver ion (Ag+) was reduced by antioxidants to spherical SNPs around silver seeds formed by K-carrageenan. After reaction with antioxidants, symmetrically enlarged SNPs displayed a surface plasmon resonance (SPR) absorption band at 420 nm superimposed onto a monotonous background due to the underlying monodisperse CAR-SNPs. The surface functional groups of CAR-coated, negatively-charged SNPs may be responsible for a net SPR peak emerging after the facilitated chemical reduction of Ag+ with antioxidants. AC was determined using the proposed CAR-SNP method by measuring the linear increase in SPR absorbance of the enlarged SNPs in the presence of increasing antioxidant concentrations. The CAR-SNP based AC method was applied to selected antioxidant compounds, synthetic antioxidant mixtures, and tea infusions. The LOD and LOQ for trolox were found as 0.023 and 0.077 mu M, respectively, which were well below those found by conventional electron-transfer based AC assays having similar mechanism. The CAR-SNP based AC method was validated for performance parameters (linearity, additivity, precision, and recovery). AC values obtained by the proposed method for tea infusions were correlated with those obtained by the classical CUPRAC method. The CAR-SNP based AC method is compatible with green chemistry principles of nanoparticle synthesis and application, and provides simple, rapid, low-cost, and sensitive AC determination. (C) 2021 Elsevier B.V. All rights reserved.