Surfactant-Free Synthesis of Spiky Hollow Ag-Au Nanostars with Chemically Exposed Surfaces for Enhanced Catalysis and Single-Particle SERS

Spiky/hollow metal nanoparticles have applications across a broad range of fields. However, current bottom-up methods to produce spiky/hollow metal nanoparticles rely heavily on the use of strongly adsorbing surfactant molecules, which is undesirable since these passivate the product particle surfaces. Here we report a high-yield surfactant-free synthesis of spiky hollow Au-Ag nanostars (SHAANs). Each SHAAN is composed of more than 50 spikes attached to a hollow ca. 150 nm diameter cubic core, which makes SHAANs highly plasmonically and catalytically active. Moreover, the surfaces of SHAANs are chemically exposed which gives them significantly enhanced functionality compared to their surfactant-capped counterparts, as demonstrated in surface-enhanced Raman spectroscopy (SERS) and catalysis. The chemical accessibility of the pristine SHAANs also allows the use of hydroxyethyl cellulose as a weakly-bound stabilizing agent. This produces colloidal SHAANs which remain stable for more than 1 month while retaining the functionalities of the pristine particles and allow even single-particle SERS to be realized.