Our data show that NPs of this type cannot be considered as
distinct bodies, but on the contrary, due to long-range
interactions, they form complexes together with the ions and
solvent molecules surrounding them. This implies that in NP
solutions there are interactions between the nanoparticles due
to the ordering effects of water and ions around the NPs, which
give rise to long-range solvent-mediated interactions that
complement those due to hydrodynamics (conservation of
momentum). The significance of these effects is stressed by the
fact that nanomaterials in biological environments are rarely
pristine neutral particles, as instead (synthetic) NPs under
these conditions are usually charged or polar. The present
results may therefore have generic interest especially in
biological situations where synthetic nanomaterials interact
with and aim to access cells. The main barrier that they need to
overcome is the cell membrane characterized by a membrane
potential coupled to a cloud of salt ions. Therefore, the central
issue that is worth clarifying is the interaction between NP
complexes and cell membranes. On the basis of our results, the
characteristic length over which charged AuNPs may affect
biological molecules or complexes (such as lipid membranes) in
terms of water-mediated interactions is at least ∼10 nm.
Depending on the NP charge and the molecular composition of
the membrane, the reorganization of the lipid membrane
system that results from this interplay is expected to vary. Work
in this direction to clarify these issues is underway.