A novel colorimetric sensor for measuring hydroperoxide content and peroxyl radical scavenging activity using starch-stabilized gold nanoparticles.

A novel colorimetric nanosensor was developed for evaluating peroxyl radical scavenging activity of phenolic antioxidants and for the detection of hydroperoxides formed during AAPH-induced oxidation of linoleic acid emulsions. Starch was used as a green reduction/stabilization agent for gold nanoparticles (AuNPs) synthesis in alkaline medium. When tert-butyl hydroperoxide (tert-BHP) was incubated with an excess of iodide ions in a 37 °C water bath for 90 min, triodide (I) was formed in an amount equivalent to tert-BHP concentration. Upon the addition of starch-stabilized gold nanoparticles (ss-AuNPs) solution to the incubation mixture, triiodide ions were rapidly adsorbed on the surface of AuNPs and caused their aggregation. A concomitant red shift (from 525 nm to 563 nm) of surface plasmon resonance (SPR) absorption of the nanoparticles was observed, absorbance linearly increasing with aqueous tert-BHP concentration. The method provided an LOD of 39 µM for tert-BHP, and was validated through linearity, precision and accuracy. The concentration of hydroperoxides estimated in linoleic acid peroxidation correlated well with those found by the reference ferric thiocyanate assay. Peroxyl radical scavenger antioxidants decreased the red-shifted SPR absorption of aggregated ss-AuNPs, thereby enabling an indirect estimation of antioxidant activity. This AuNPs-based colorimetric sensor is the first of its kind to directly determine peroxyl radical scavenging activity of polyphenols. The half-maximal inhibitive concentrations (IC) of selected antioxidant compounds were calculated by utilizing the decrease in absorbance with increasing concentration of scavengers, and compared to those of classical oxygen radical absorbance capacity (ORAC) assay. The proposed nanosensor was superior over FL-based ORAC in determining the peroxyl radical scavenging activity of the lipophilic antioxidant α-tocopherol. The percentage scavenging of real samples such as green tea infusion and synthetic serum were determined. The proposed assay can be used for estimating the peroxyl scavenging of various food and biological samples in terms of its low cost, ease of use and compatibility.