A convenient and sensitive colorimetric iodide assay based on directly inducing morphological transformation of gold nanostars

We propose a convenient and easy colorimetric assay for highly sensitive detection of iodide by using gold nanostars (GNSs) as probes. The assay relies on that iodide directly changes the morphology of GNSs and alters their longitudinal localized surface plasmon resonance (LSPR) without surface modifications and the use of other reagents. Upon increasing iodide concentration, GNSs gradually transformed to sphere gold nanoparticles, the absorbance at longitudinal LSPR decreased, and solution color varied from greenish blue to red, as confirmed by the UV-Vis absorption spectroscopy and transmission electron microscopy. With this strategy, as low as 0.005 μM of iodide can be determined due to the specific properties of GNS with plenty of tips and corners and high surface-to-volume ratio. The detection was simply achieved by mixing testing samples and GNS solution. Many ions like CO32-, S2- did not interfere with iodide detection since only iodide can trigger GNS geometry change through an electron injection process. The iodide contents in river water, table salt, seaweed, and complex vitamin tablet were quantified with great accuracy. The proposed assay shows great promises for environment protection and food safety. Moreover, GNSs are useful in developing colorimetric assays for biochemical analysis or clinical diagnosis.