Interesting optical variations of the etching of Au Nanobipyramid@Ag Nanorods and its application as a colorful chromogenic substrate for immunoassays

Common colorimetric immunoassays only show monochromatic intensity variations, and the absorbance intensity is utilized for the quantitative detection of the analytes. Herein, we demonstrate for the first time that Au Nanobipyramid@Ag Nanorods (Au NBP@Ag) is an ideal multicolor chromogenic substrate for immunoassays which combines the absorbance intensity with the Localized Surface Plasmon Resonance (LSPR) peak shifts are utilized for quantitative detection of the analytes, and vivid color variations are used for semi-quantitative detection. Our investigation reveals that unlike the single metallic nanoparticles such as gold nanorods which show one-way LSPR peak shifts, Au NBP@Ag Nanorods shows an interesting multi-way peak shifts: the LSPR peak would initially blue shift, and then red shift, finally blue shift again during the etching process. As a result, a series of vivid colors can be observed. The results indicate that the correlation linear range for the detection of Squamous cell carcinoma antigen (SCCA) was 2.5–105 ng/mL. Although only the detection of SCCA is demonstrated in this work, this method is easy to expand to detect other biomarkers since our method is based on conventional immunoassay strategy.