Colloidal Superparticles From Artificial Atoms - Molecular Self-Assembly: Advances and Applications - page 421

13.3.1 Hydrogen Bonding

The strength of a typical hydrogen bond is about 2

−

10 k T. On the basis

B

of hydrogen bonding, Rotello demonstrated the first successful

synthesis of spherical superparticles via a “brick and mortar”
strategy.

et al.

Specifically, 2 nm gold nanocrystals functionalized
with a thymine derivative serve as the bricks, and diaminotriazine-
functionalized polystyrene polymers serve as the mortar for cross-
linking the colloidal gold nanocrystals (Fig. 13.2a).

23,43

In a nonpolar
organic solvent, poly-Triaz folds into a compact structure owing to
solvophobic interactions as well as intramolecular hydrogen bonds
between the triazines (Fig. 13.2b).

43

44

Multivalent interactions of poly-

Triaz with Thy Au can induce the unfolding of the polymer, which

opens up further triazine units for interaction with additional Thy

−

Au
nanocrystals, and leads to the propagation of the assembly process.
Indeed, addition of the polymer to a concentrated dichloromethane
solution of thymine-functionalized gold nanocrystals leads to a
rapid particle aggregation, resulting in a black solid. In contrast,
no precipitation was observed upon addition of poly-Triaz to the
control colloid MeThy

−

Au, demonstrating that hydrogen bonding is
a prerequisite to particle aggregation.

−

To prevent noncontrollable particle aggregation, Rotello et

al. Au in dilute dichloromethane

solution, resulting in a slow precipitation proceeding over a 96 h
period. The resulting solids appear to be spherical superparticles
with sizes ranging from about 0.1

chose to mix poly-Triaz and Thy

−

m in diameter. These
superparticles were insoluble in nonpolar solvents but were
dispersible in polar solvents such as methanol or tetrahydrofuran
(Fig. 13.2c). The precipitation of superparticles in dichloromethane
is associated with solvophobic interactions, which should also
play a major role in forming the spherical shape of superparticles
because spheres exhibit a minimal surface energy. Small-angle
X-ray scattering (SAXS) measurements show that the resulting
superparticles exhibit a short-range order with an interparticle
edge-to-edge separation distance of 4.4 nm. In addition, Rotello

−

1

µ

et

al. showed that superparticle size in each synthesis is tunable by

controlling the growth temperature: A lower temperature results in
larger superparticles (Fig. 13.2d,e). This temperature dependence