A colorimetric method is described for the determination of ferrous ion (Fe2+) with high sensitivity and selectivity. The method is based on catalytic etching of gold nanorod (NR). In an acid condition, Fe2+ reacts with H2O2 to produce superoxide radical (O2•-) that etches gold NRs from the low energy surface along the longitudinal direction preferentially. As a result, the changes in the absorption spectrum and color of gold NR can be measured and also can be detected visually. Under the optimal conditions, the assay has very low detection limit (13.5 nM) and a linear response in a concentration range of 75 to 1 μM. The method was applied to the determination of Fe2+ in spiked samples of fetal bovine serum and also transferred to a kind of test stripe for use in fast practical applications. A unique colorimetric sensing method is demonstrated for the colorimetric detection of Fe2+, again based on the oxidation of gold nanorods which leads to the blue-shift of the absorption. Graphical abstract A unique colorimetric sensing method was shown for the colorimetric detection of Fe2+. Fe2+reacts with H2O2 to generate superoxide radical that oxidize gold nanorods. This leads to a color change from blue-green to pink.
Keywords: Colorimetric method; Etching; Gold nanorod (gold NR); Superoxide radical (O2 •−); Test stripe.