Fine-tuning of gold nanorod dimensions and plasmonic properties using the Hofmeister effects

Gold nanorods (Au NRs) present unique optical and electronic properties that depend on their morphology. Their applications in sensing and therapeutics require easy synthesis with precise control over their dimensions. Here, we report a method for the synthesis of highly pure and monodisperse Au NRs with fine-tuneable dimensions and longitudinal localised surface plasmon resonance by addition of Hofmeister salts into the growth medium. The control of Au NR formation relies on the double interaction between salt-gold and salt-surfactant (cetyl trimethylammonium bromide, CTAB). With the addition of Hofmeister salts (i.e. NaNO3, NaBr, NaCl and NaHSO4) we can fine-tune the aspect ratio of Au NRs in the range of 3.3 to 4.8 with a precision of 0.1 and the longitudinal absorption band from 777 to 960 nm. In addition, we have studied the physical changes in the CTAB micelles induced by the salts using rheology, electron microscopy and light-scattering techniques. We report for the first time cryo-electron microscopy imaging of the micelles under Au NR growth conditions. With the comprehensive characterization of CTAB micelles in the growth solution, this study provides a deeper understanding of the anisotropic growth of metallic crystals.