Multifunctional spiky branched gold-silver nanostars with near-infrared and short-wavelength infrared localized surface plasmon resonances.

We report a one-pot seedless green method for the synthesis of gold-silver (AuAg) spiky branched nanostars, with gold (90%) being the major component. Here, the zwitterionic surfactant lauryl sulfobetaine (LSB) is employed in the synthesis of bimetallic nanostars. The concentration of LSB plays an important role in determining the shape of nano-objects. A minimum LSB concentration of 50 mM is required for the formation of spiky branched nanostars, the size of which is controlled by increasing the LSB concentration. Two distinct intense localized surface plasmon resonances in the near-infrared (NIR) and short-wavelength infrared ranges are observed. The molecular structure of LSB causes LSB molecules to assemble into spherical micelles that act as a soft template for the growth of the nano-objects. An analysis of the mechanisms behind the formation of the nanostars suggests that there is a rapid growth of spikes followed by the formation of a spherical core at the center. AuAg nanostars with evenly spaced spikes and low branching demonstrate great potential as efficient nanocatalysts, surface-enhanced Raman scattering-active substrates and for photothermal therapy, active in both the visible and NIR regions.