Journal of the American Chemical Society
Communication
Furthermore, the addition of Fe3O4·AC NPs to pre-formed
SiO2@Fe3O4 assemblies, followed by UV irradiation, resulted in
the thickening of the Fe3O4 shell (multilayers comprising at
least four layers of Fe3O4 NPs were observed), and, again, no
self-aggregation of Fe3O4 NPs was observed.18 It should be
emphasized, however, that the composition of the solvent
mixture played an important role in determining the outcome
of the light-induced self-assembly process: when the solvent
was too apolar (e.g., a 9:1 toluene−tetrahydrofuran mixture),
Fe3O4·AC NPs, in addition to occluding the surface of silica,
formed aggregates on their own (Figure S4), whereas in a much
more polar 4:1 toluene−ethanol mixture, iron oxide remained
in a disaggregated state, even upon exposure to UV light (due
to the polar solvent’s stabilizing effect on cis-AC; Figure S6).
Only in mixtures of intermediate polarity, e.g., 19:1 toluene−
ethanol, Fe3O4·AC selectively covered the surface of SiO2
(Figure 2b, left; also Figure S5).
To demonstrate the generality of this approach, we
attempted to encapsulate small (5 nm) gold NPs inside a
monolayer of Fe3O4·AC. Toward this end, we functionalized
Au NPs with an azobenzene-terminated thiol, AT (see SI,
section 2). The surface concentration of AT on Au (∼3.7
azobenzene units/nm2) was intentionally higher than that of
AC of Fe3O4 (∼2.0 azobenzenes/nm2), such that the energy of
interaction between two NPs would be, EAu−Au > EAu−Fe O
>
3
4
E
Fe O −Fe O . The higher azobenzene surface coverage on Au also
3 4 3 4
led to a faster aggregation of Au·AT as compared to Fe3O4·AC.
As a consequence, UV irradiation of a mixture of these two
types of particles resulted in selective formation of Au@Fe3O4
core−shell assemblies (Figure 2c).19 Again, no aggregates
comprising only Fe3O4 could be seen,18 although such
aggregates formed readily (Figure 2a) in the absence of Au·AT.
We further speculated that, since in the presence of external
magnetic field the diamagnetic (= SiO2; Au) core−para-
magnetic shell assemblies interact with each other via multiple
Fe3O4 NP dipole−dipole interactions (analogous to the
interactions between all-Fe3O4 NP assemblies9), these
structures could further assemble into one-dimensional
aggregates. (It should be noted that the magnetic dipole
interactions between assemblies comprising both diamagnetic
and SPM components are maximized when the SPM
components are located at their surfaces.) Indeed, application
of low-intensity magnetic field to a solution of these assemblies
results in the formation of linear aggregates, as shown in Figure
2b, right, and 2d. Notably, only a small, “catalytic” amount of
∼11 nm Fe3O4 NPs (corresponding to a loose monolayer; cf.
Figure 2b) is sufficient to induce further aggregation of these
core−shell assembliesfor example, we have determined that
assemblies comprising as little as 4% m/m of Fe3O4·AC NPs
(96% being the Au·AT “cargo”) form larger, linear aggregates
when external magnetic field is applied.
As these larger aggregates form, the magnetic force acting on
them becomes large enough to overcome the Brownian
force,20,21 and the aggregates move toward the regions of
maximum magnetic field, quickly concentrating near the
magnets. The resulting precipitate remains attracted to a
magnet and can readily be transported from one location to
another in nearly quantitative yield with speeds of up to ∼1
cm/s for up to 1 day in the dark, and for at least several days
under UV illumination. At the same time, the formation of all
of the aggregates described here is fully reversible due to their
dynamic nature: both the core−shell assemblies and their linear
Figure 2. (a) Light-controlled self-assembly of azobenzene-coated
Fe3O4 (Fe3O4·AC) nanoparticles. (b) UV light-induced occlusion of
silica particles with Fe3O4·AC (left) and magnetic field-induced
aggregation into linear structures (right). (c) Core−shell assemblies
prepared by exposing a mixture of Au·AT and Fe3O4·AC NPs to UV
light. (d) Linear structures obtained by exposing the core−shell
assemblies to a magnetic field.
1a, center), the strength of these aggregate−aggregate
interactions is sufficient to overcome the thermal energy, and
the spherical aggregates align into larger, linear assemblies. In
other words, the magnetic interactions in this system can be
turned “on” and “off” using UV and visible light, respectively.
The light-induced self-assembly process occurs as a result of
a combination of attractive electric dipole−dipole interactions
between the cis-azobenzene groups15 and the solvophobic
effect.16 Therefore, we hypothesized that hydrophilic particles
intentionally introduced to a UV-irradiated solution of
Fe3O4·AC NPs would serve as preferential attachment sites
for these NPs, whose self-aggregation would then be sup-
pressed. Proof-of-concept experiments were performed on
mixtures of Fe3O4·AC NPs and ∼90 nm silica particles17
dispersed in toluene containing 5% v/v ethanol. As Figure 2b,
left, shows, exposure of this mixture to UV light indeed resulted
in rapid (within 30 s) formation of SiO2@Fe3O4 assemblies,
and, notably, no aggregates of free Fe3O4·AC were found
(provided the Fe3O4·AC/SiO2 ratio was low enough18).
19565
dx.doi.org/10.1021/ja309633v | J. Am. Chem. Soc. 2012, 134, 19564−19567
Chovnik, Olga
