Detection of ferrous ion by etching-based multi-colorimetric sensing of gold nanobipyramids

Colorimetric sensing methods based on non-spherically symmetric gold (Au) nanoparticles have become a powerful tool in the field of biomedical detection due to their intriguing plasmonic properties. In this study, Au nanobipyramids (Au NBPs) were used as colorimetric sensing probes to detect ferrous ions (Fe²⁺) through tip etching. The quick etching of Au NBPs along the longitudinal direction by superoxide radicals generated by the reaction of Fe²⁺ and H₂O₂ led local surface plasmon resonance (LSPR) to blue shift and produced vivid color change that could be used for visual inspection. Under the optimal reaction conditions, the peak shift of the Au NBPs and the logarithm of the concentrations of Fe²⁺ had a linear relationship in the range of 10 nM to 10 μM, with a very low detection limit of 1.29 nM. During the etching process, a different end shape of the Au nanoparticles results in a different process for the morphology transition, which makes the degree of spectral change and detection sensitivity significantly different. In the presence of trace amounts of Fe²⁺ (<1000 nM), the detection sensitivity of Au NBPs with sharp ends which rely on aspect ratio and truncation is nine times higher than that of gold nanorods with round ends which only rely on aspect ratio. Although the color change of larger-sized Au NBPs was not clear during detection, the LSPR peak shift was more severe. Therefore, the system provides different modes for detecting Fe²⁺ according to Au NBPs with different sizes and characteristics.