In Situ Direct Monitoring of the Morphological Transformation of Single Au Nanostars Induced by Iodide through Dual-Laser Dark-Field Microscopy: Unexpected Mechanism and Sensing Applications

Single nanoparticle imaging is a significant technique to help reveal the reaction mechanism and provides insight into the nanoparticle transformation. Here, we monitor the in situ morphological transformation of Au nanostars (GNSs) induced by iodide (I−) in real time using dark-field microscopy (DFM) with 638 nm red (R) and 534 nm green (G) laser coillumination. The two lasers are selected because the longitudinal localized surface plasmon resonance of GNSs is located at 638 nm and that for GNSs after transformation is at 534 nm. Interestingly, I− can interact with GNSs directly without the engagement of other reagents, and upon increasing I− concentrations, GNSs undergo color changes from red to orange, yellow, and green under DFM. Accordingly, green/red channel intensities (G/R ratios) are extracted by obtaining red and green channel intensities of single nanoparticles to weigh the morphological changes and quantify I−. A single nanoparticle sensor is constructed for I− detection with a detection limit of 6.9 nM. Finally, a novel mechanism is proposed to elucidate this shape transformation. I− absorbed onto the surface of GNSs binds with Au atoms to form AuI−, lowering the energy of its bond with other Au atoms, which facilitates the diffusion of this atom across the nanoparticle surface to low-energy sites at the concaves, thus deforming to spherical Au nanoparticles.