Multi-mode optical detection of iodide based on the etching of silver-coated gold nanobipyramids

Abstract

In this paper, we developed a simple, rapid, selective and colorimetric method to detect iodide (I−) by etching silver-coated gold nanobipyramids (Au NBs@Ag nanostructure) in the presence of Cu2+. The detection is based on the reaction that Cu2+ could oxidize I− into I2, which sequentially etches the Ag shell of Au NBs@Ag nanostructure. When the Ag coating is thick and the Au NBs are buried in Ag nanorods, the decrease of the Ag shell thickness and aspect ratio results in the blue-shift and intensity decrease of the longitudinal localized surface plasmon resonances (LSPR) peak. The blue-shift and intensity decrease of the longitudinal LSPR peak have been employed to quantificationally detect the concentration of I−. The linear range of the detection could be improved by increasing the concentration of Cu2+ and reaction time. Under the optimized conditions, both the blue-shift and intensity decrease of the longitudinal LSPR have linear correlation with the concentration of I− in the range of 1.0–15μM. When the Ag coating is very thin, the etching of the Ag shell results in the red shift of the LSPR in the visible range, which further leads to the color of the colloid change from blue to pink. The etching-induced color change of the colloid also depends on the concentration of I−, which enables the visual detection of I−. Compared with other techniques, this method could facilitate multi-model quantitative detection of I− without any sophisticated instrument. Interference studies and real samples test show that I− could be specifically detected in dried kelp using this bimetallic optical nanoprobe.