  Colloidal approaches allow for the synthesis of Au nanoclusters (NCs) with
atomic precision and sizes ranging from a few to hundreds of atoms. In most of
the cases, these processes involve a common strategy of thiol etching of
initially polydisperse Au nanoparticles into atomically precise NCs, resulting
in the release of Au-thiolate complexes as byproducts. To the best of our
knowledge, neither the removal of these byproducts nor the mass spectra in the
relevant mass region were shown in previous studies. A thorough analysis of
inorganic byproducts in the synthesis of Au25 NCs reveals that published
protocols lead to Au25 NCs in vanishingly small quantities compared to their
byproducts. Three purification methods are presented to separate byproducts
from the desired Au25 NCs which are proposed to be applicable to other
promising Au NC systems. Additionally, critical factors for a successful
synthesis of Au25 NCs are identified and discussed including the role of
residual water. An important finding is that the etching duration is very
critical and must be monitored by UV-Vis spectroscopy resulting in synthetic
yields as high as 40%.
