A promising direct visualization of an Au@Ag nanorod-based colorimetric sensor for trace detection of alpha-fetoprotein

Abstract

The Ag coating-induced blue shift and enhancement of longitudinal plasmon of Au nanorods result in abundant and tunable optical absorptions in the visible region, which leads to the Au@Ag nanorod becoming a good candidate for colorimetric sensing. In this paper, we have reported an effective and simple approach for visualization detection of alpha-fetoprotein (AFP) based on Au@Ag core–shell nanorods. The AFP concentration-dependent colloid color of Au@Ag core–shell nanorods could change from green to gray, olivine to brown, and even blue to violet to pink, which are more varied color changes compared with the traditional red to blue color change. Besides, as the Ag shell thickness is increased, a dip was observed, which is located between two strong absorption peaks. The change of the dip is proposed as a new biosensor parameter for biomolecule detection based on absorption spectra. AFP concentration detection could be realized according to the change of the position and intensity of the dip, which has not been reported in the biomolecule detection field so far. Au@Ag core–shell nanorods with a small aspect ratio of inner Au nanorods are more suitable for direct read-out visualization, with a color change from blue to violet to pink. From the point of view of spectra analysis, Au@Ag core–shell nanorods with a greater aspect ratio of inner Au nanorods show higher spectral sensitivity. Furthermore, it is found that non-centrifuged Au@Ag core–shell nanorods are preferentially assembled in a side-by-side fashion with the addition of AFP, which will provide new approaches for other kinds of nanorods in controllable assembly fashions.