Long-term stability of surfactant-free gold nanostars

This work investigates the long-term stability of suspensions of surfactant-free gold nanostars with mean diameter of 78 +/- 13 nm (measured from tip to tip across the nanostar). We monitored the optical and morphological properties of the nanostars over the course of several days after synthesis by observing the changes in the UV-visible absorption spectra and mean radius of curvature of the nanostar tips. An aging process can be observed, evident in the blunting of the nanostar tips, leading to a blue shift in the absorption maximum. Stability is greatly improved by depositing on the nanostars a monolayer of mercaptopropionic acid (MPA), possibly because of the formation of the gold-sulfur (Au-S) bond that limits the mobility of the Au atoms. Capping the nanostars with MPA is an easy additional step for extending the stability of the nanostars in suspension without significantly affecting the original plasmonic resonance band.