CTAB Stabilizes Silver on Gold Nanorods

We present a study that allows us to explain the chemical changes behind the often observed but so far ununderstood drift of the plasmon resonance of chemically prepared gold nanorods in microfluidic devices. We systematically monitored the evolution of the plasmon scattering signal from thousands of single nanoparticles in parallel, from different gold nanorod batches, prepared by both, silver-assisted and silver-free seeded growth. By varying the experimental conditions, we confirmed the presence of a silver surface layer for nanorods prepared by the silver-assisted method. Although the silver layer is initially protected by the surfactant CTAB, it gets oxidized as soon as CTAB desorbs because of a reduced concentration in the flowing solution. We propose that these findings should be taken into account at the time of devising technological applications of gold nanorods, including the design of new nanorod synthetic routes, but in particular for applications that depend on the precise plasmon resonance position, for the plasmon sensing capabilities and to consider the nanoparticle surface reactivity.