  A paradigm proposed recently by Boal et al. (A.K. Boal et al., Nature 404,
746-748, 2000) deals with the possibility to use inorganic nanoparticles as
building blocks for the design and fabrication of colloidal and supracolloidal
assemblies. It is anticipated that these constructs could be made of different
shapes, patterns and functionalities and could constitute the components of
future nanodevices including sensors, actuators or nanocircuits. Here we report
a protocol that allowed us to fabricate such nanoparticle aggregates. The
building blocks of the constructs were anionically coated iron oxide
nanocrytals (superparamagnetic, size 7 nm) and cationic-neutral block
copolymers. We have shown that the electrostatic interactions between charged
species can be controlled by tuning the ionic strength of the dispersion. Under
appropriate conditions, the control of electrostatics resulted in the
elaboration of spherical or elongated aggregates at the micrometer length
scale. The elongated aggregates were found to be rod-like, with diameters of a
few hundred nanometers and lengths between 1 and 50 micrometers. In addition to
their remarkable stiffness, the nanostructured rods were found to reorient
along with an externally applied magnetic field, in agreement with the laws of
superparamagnetism.
