When our data for counterion distributions were fitted to the
Debye−Hückel description, we found the agreement to be very
good. The interface between NP-bound and loosely associated
counterions was observed to be around 2.4 nm from the AuNP
center of mass, and a fit to the ion density distributions at
distances larger than this one resulted in values of 0.27 and 0.20
nm for the Debye length in AuNP+ and AuNP−, respectively.
The Debye lengths are small, and about an order of magnitude
smaller than the NP size, allowing us to use the Smoluchowski
theory for zeta potential determination. Consideration in this
spirit results in zeta potentials of about 26 mV for AuNP+ and
59 mV for AuNP−. The results therefore do not support the
idea of coagulation for the NPs studied here.