In order to better understand how this method enables the
formation of uniform coatings with control over composition
and loading of NPs, we took a closer look at the interacting
components during the self-assembly process. Many studies
have explored the parameters that influence NP self-assembly,
such as shape of the particles, surface chemistry, choice of
solvent and its ionic strength [37–43]. The processes taking
place during the formation of these PS@NPs may be governed
by a number of different mechanisms. It might appear
that heterocoagulation plays a significant role in the selfassembly
of NPs onto the surfaces of PS spheres. The high
ionic strength of our mixture though weakens the polar
interactions of PVP with the PS spheres [44]. As well, the
addition of increasing amounts of PVP while preparing the
NP coatings either improves [32, 33] or has no effect (data not
shown) on the self-assembly process, which is counter to the
effects seen by Li et al when coating Au NPs onto PS spheres
via heterocoagulation [9]. The self-assembly observed in this
current study is attributed to the interactions of PVP stabilizing
groups with the surfaces of the PS spheres. The
dominant interactions are likely hydrogen bonding and electrostatic
interactions. This conclusion is based in part on the
reported conformation of PVP molecules in solution and the
acidic conditions necessary for a successful assembly of the
NP coatings [43, 44]. Additional observations that support
this conclusion include: (i) unsuccessful attempts to prepare
high-quality NP coatings by the same conditions using different
surface chemistries on the NPs (e.g., trisodium citrate)
(ii) the ability to prepare NP coatings with PS spheres with other functionalities that enable H-bonding (e.g., carboxyl
functionalized PS at pH 3, figure S1); (iii) uniformity of NP
coatings regardless of the type of NP; and (iv) homogeneity
of the composition over the surfaces of the PS spheres for a
multi-component coating. Small amounts of PVP appear to
remain associated with the PS@NPs after purification (figure
S4), suggesting that PVP interacts strongly with the PS
spheres (enduring the purification process) and may stabilize
the NP coatings on the surfaces of the PS spheres. The even
distribution of NPs over the surfaces of the PS spheres is most
likely due to coulombic repulsion between neighboring NPs
[37]. Altogether, these observations suggest that PVP plays a
key role in the self-assembly process, through simultaneous
interactions with the surfaces of both the NPs and PS spheres
to create stable, tunable and uniform PS@NPs