Aqueous Electrostatic Colloidal Crystallization

The complementarity demonstrated by atomic ions is an effective strategy to form a range of complex structures from relatively simple building blocks. Due to van der Waals forces, however, mixtures of oppositely charged colloids in water only form disordered aggregates. Consequently, formulating sets of complementary particles has required intensive surface chemistry, despite the availability of these naturally charged starting materials. Here we employ a straightforward technique to stabilize and tune the bonds between oppositely charged colloids to promote ordered self-assembly. After introducing surfactants that can reliably hold particles physically separated, the long-range electrostatic attraction is tuned via charge screening until particles spontaneously crystallize. Single crystals on the centimeter scale are produced in solution that become permanently fixed when introduced to deionized conditions, after which they become robust enough to survive drying for SEM analysis and further processing.