The particle surface must be engineered to have at least two distinct chemistries with a defined boundary.

A method developed at the University of Connecticut, USA, involves the manipulation of polymer tethers on the surface of gold and silver nanoparticles to generate asymmetric metal nanoparticles directly in an aqueous solution. In their paper in Small, they describe two techniques: one to create metal nanobowls, and one to create Janus-type metal dimers.