Nanoscale Self-Assembly: Seeing Is Understanding.

In the midst of the second world war, Derjaguin and Landau in Russia and Verwey and Overbeek in The Netherlands arrived independently at a theory to explain the kinetic stability of colloidal dispersions. The celebrated DLVO theory combined two types of forceslong-ranged electrostatic repulsion and short-ranged van der Waals attractionto predict the rates at which nanoand microparticles aggregate irreversibly in time. Knowledge of these and other colloidal forces has since been applied to direct the spontaneous self-assembly of colloidal materials with emerging applications in optics, electronics, and catalysis. Despite their critical importance, measuring the interactions between colloidal objects in situ remains a challenging endeavor. One approach uses video microscopy to collect particle configurations and/or dynamical trajectories from which to infer particle interactions with the help of statistical mechanical models. This approach was used to validate the DLVO theory for the electrostatic repulsion between charge microspheres; however, its extension to nanoscale particles has been limited by the spatial resolution of optical microscopy. Now, with the advent of in situ liquid phase transmission electron microscopy (TEM), it is possible to apply similar techniques to quantifyfor the first time interaction potentials between nanoscale components. In this issue, Alivisatos and co-workers demonstrate the power of this approach by measuring the interaction potential between pairs of charged nanorods. Their investigation reveals an interesting twist on the classic story of colloidal stability: anisotropic interactions between the rod-shaped particles lead them to aggregate selectively at their ends to form particle chains. The ability to watch this and other assembly processes in real time offers new insights into the forces and mechanisms underlying nanoscale self-assembly.