Nano graphene oxide-wrapped gold nanostars as ultrasensitive and stable SERS nanoprobes.

We report a facile method to synthesize highly branched gold nanostars wrapped with nano graphene oxide (nGO) sheets with or without the addition of Raman dyes, as nanoprobes with high SERS activity. Both cysteamine and nGO are added to gold nanostars; the positively charged amino groups help self-assembly of nGO flakes around the nanostars. This increases the Raman signal of nGO by 5.3 folds compared to samples in which nGO is in contact with the nanostars but does not wrap them. We also prepare dye-based SERS nanoprobes by sandwiching a typical Raman reporter such as Rhodamine B (RhB), Crystal Violet (CV) and Rhodamine 6G (R6G) between the nanostars and the nGO coating. The Raman signals of RhB are 5.2 times larger when sandwiched between nGO and nanostars than if the molecules are just adsorbed on the nanostar surface, and similar enhancements are observed for the other dyes. In addition to improving SERS efficiency, the wrapping greatly improves the stability of the dye-based nanoprobes, showing a reproducible Raman signal of RhB for over a week in simulated body fluids at 37 °C. High SERS signal, facile fabrication method and excellent stability make these nanoprobes highly promising for SERS-based biosensing and bioimaging applications.