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Shape and aggregation control of nanoparticles: not shaken, not stirred.

The aggregation of nanoparticles during synthesis, particularly the effect of mechanical agitation, is investigated from a viewpoint of nucleation using a conjugated polymer, polyaniline, as an example. Homogeneous nucleation of polyaniline results in nanofibers, while heterogeneous nucleation leads to granular particulates. Mechanical agitation, which is a common method for disrupting aggregates, instead dramatically triggers aggregation during the synthetic process and favors the formation of granular particulates. Correlating the shape and aggregation of polyaniline nanoparticles with the mode of nucleation, a new aggregation mechanism is proposed in which aggregation is triggered by heterogeneous nucleation. The mechanism may be quite general as indicated by experiments with other materials such as silica nanoparticles. Highly dispersible polyaniline nanofibers can now be reproducibly prepared from a conventional reaction simply by not mechanically agitating the reaction and carrying it out at an elevated temperature. This work may prove to be of great value in reproducibly synthesizing nanoparticles with well-controlled sizes and shapes and in effectively preventing aggregation in chemical, pharmaceutical, and materials production processes.