Small Thiols Stabilize the Shape of Gold Nanorods

The stabilization of the optical properties of anisotropic plasmonic particles upon thermal annealing and laser irradiation is an important issue in many biomedical applications. Here, we address the effect of small thiols on the thermal and photostability of gold nanorods. As-synthesized colloids were treated with mixtures of thiolated polyethylene glycol and methylbenzenethiol with molar ratios ranging from 0, for pure PEG, to 20 and then incubated in an oven at sub-boiling conditions. We found that small thiols dramatically enhance the thermal stability of gold nanorods. For instance, after 1 h at 90 °C samples with pure PEG lost more than 70% of optical absorbance at their initial peak position, while particles coated with a thicker layer of methylbenzenethiol remained nearly unchanged. We ascribe this effect to a modulation of the activation barrier for surface diffusion of gold atoms. Additionally, we addressed the translation of this effect to the photostability of gold nanorods irradiated under conditions of interest in photoacoustic imaging and found that small thiols delay relevant damage thresholds by as much as a factor of 2. Our findings will help researchers design novel tags that overcome the limitations related to thermal and photoinstabilities in a broad variety of applicative contexts.