Lock and key colloids

The key to self-assembly Many functional materials can be created by directing the assembly of colloidal particles into a predetermined structure. Control over particle assembly usually involves tagging them with molecules such as DNA that can recognize and bind each other. But new work shows that shape complementarity — the construction of colloids using a lock-and-key recognition mechanism — offers a simple and effective alternative control mechanism. The keys are colloidal spheres, and monodisperse colloidal particles with a spherical cavity are the locks. The two will spontaneously and reversibly bind via the depletion interaction if their sizes match. This procedure yields complex colloidal structures held together by flexible bonds, and offers a simple yet general means to program and direct colloidal self-assembly. Many functional materials can be created by directing the assembly of colloidal particles into a desired structure. Control over particle assembly usually involves the use of molecules such as DNA that can recognize and bind each other. Here, a simple and effective alternative is described. Colloidal spheres serve as keys, and monodisperse colloidal particles with a spherical cavity as locks. These will spontaneously and reversibly bind to each other via the depletion interaction if their sizes match. New functional materials can in principle be created using colloids that self-assemble into a desired structure by means of a programmable recognition and binding scheme. This idea has been explored by attaching ‘programmed’ DNA strands to nanometre- 1 , 2 , 3 and micrometre- 4 , 5 sized particles and then using DNA hybridization to direct the placement of the particles in the final assembly. Here we demonstrate an alternative recognition mechanism for directing the assembly of composite structures, based on particles with complementary shapes. Our system, which uses Fischer’s lock-and-key principle 6 , employs colloidal spheres as keys and monodisperse colloidal particles with a spherical cavity as locks that bind spontaneously and reversibly via the depletion interaction. The lock-and-key binding is specific because it is controlled by how closely the size of a spherical colloidal key particle matches the radius of the spherical cavity of the lock particle. The strength of the binding can be further tuned by adjusting the solution composition or temperature. The composite assemblies have the unique feature of having flexible bonds, allowin