  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.
