Sulfide-arrested growth of gold nanorods

The growth of gold nanorods can be arrested at intermediate stages by treatment with Na2S, providing greater control over their optical resonances. Nanorods prepared by the seeded reduction of AuCl4 in aqueous cetyltrimethylammonium bromide solutions in the presence of AgNO3 typically exhibit a gradual blueshift in longitudinal plasmon resonance, over a period of hours to days. This "optical drift" can be greatly reduced by adding millimolar concentrations of Na2S to quench nanorod growth, with an optimized sulfur: metal ratio of 4:1. The sulfide-treated nanorods also experience a marked redshift as a function of Na2S concentration to produce stable plasmon resonances well into the near-infrared. Sulfide treatment permitted a time-resolved analysis of nanorod growth by transmission electron microscopy, revealing two distinct periods: an initial growth burst (t < 15 min) that generates dumbbell-shaped nanorods with flared ends and a slower phase (t > 30 min) favoring growth around the midsection, leading to nanorods with the more familiar oblate geometry. The blueshift in plasmon resonance that accompanies the dumbbell-to-oblate shape transition correlates more strongly with changes in the length-to-midsection (LID,) ratio rather than the length-to-end width (L/D-2) ratio, based on the empirical relationship introduced by ElSayed and co-workers.