Now, assuming 2.4 nm to be a safe (that is likely too large)
estimate for the position of the interface between NP-bound
and loosely bound ions, the ESP gives an approximate estimate
for the zeta potential: 26 mV for AuNP+ and −59 mV for
AuNP− (see Figure 8, Supporting Information). The different
numbers highlight that cationic and anionic AuNPs respond to
an external field with different strengths. Further, even if our
assumption of the location of the interface were partly
inaccurate, we can still conclude that the zeta potential in the
present systems without salt is about 25 mV, or larger than this
value, which is often considered as a threshold value for
coagulation/aggregation. Further, if there were salt, the Debye
length would decrease for increasing ion concentration, which
would show up as an increase in the zeta potential (see Figure
8, Supporting Information). Recent experimental data by
Verma et al. for gold nanoparticles protected by a number of
different organic ligands is in agreement with this view, since
they found23 the (absolute value of) zeta potential to vary
between ∼31 and 38 mV. Summarizing, our analysis predicts
that the AuNPs considered in this work do not coagulate.