Pine Research Group
Colloids are microscopic particles, typically 10-6 m in diameter (1/100 of the diameter of a human hair) or smaller, that are suspended in a liquid, typically water. Colloids are often spherical, although other shapes such as rods and ellipsoids are common. The forces between spherical colloids are almost always isotropic, that is, they are the same in all directions radially outward from the particle.
Recently, we developed a method to make “patchy colloids.” These are nearly spherical colloids that have small patches on their surfaces. The patches are coated with a special “molecular glue” made from short strands of DNA, the molecule from which genetic material is made. The glue can be designed to be sticky only for temperatures below about 40°C (body temperature). The glue is typically designed in many different variants that are only very selectively sticky to each other.
Making patchy colloids: First make colloidal clusters
We start with solid colloidal spheres that are all the same size and then encapsulate them in droplets of oil. The oil droplets are dispersed in water. The movie on the right shows such an oil droplet that has a number of plastic (polystyrene) spheres in it. If we let the oil evaporate into the water, the droplet squeezes the colloidal spheres into a compact cluster. Press on the arrow to run the movie and watch a cluster form. It turns out that there are 7 spheres in this particular droplet. The cluster that forms when the droplet evaporates is a ring of 5 spheres that form a pentagon, with a sixth sphere above the ring and a seventh below the ring (not visible) near the center of the pentagon. This movie was obtained using a conventional optical microscope that forms images with light. A sharper picture of the same kind of cluster is visible at the far right of the figure below. That picture was obtained with an electron microscope.
Colloidal clusters viewed using an electron microscope (see other page for more information). Scale bar: 500 nm.
Making patchy colloids: Second swell clusters with styrene monomer, then polymerize.
The polystyrene spheres in the clusters above act like tiny sponges when exposed certain liquids like styrene, the molecule from which polystyrene is made. Adding just the right amount, we first swell the polystyrene spheres with styrene until the spheres can hold no more, at which point a styrene droplet begins to appear around the cluster. By carefully controlling how much styrene liquid we add, we make a single droplet of styrene with the extremities of the cluster it encapsulates sticking out. The styrene is then polymerized making the liquid droplet become solid and resulting in the patchy particles seen below.
Patchy particles with 1, 2, 3, 4, 5, 6, & 7 patches from left to right.
Making patchy colloids: Third attach sticky-ended DNA to patches.
The patches have amadine molecules on the their surface to which we attach double stranded DNA with single stranded sticky ends. The sticky-ended DNA molecules on the patches of different particles are designed to stick to each other. Because the particles stick to each other only where there is sticky DNA, which is only on the patches, the particles form directional bonds, as shown below.
Various patchy particles forming colloidal molecules by directional bonding. (a) Several examples of two 1-patch particles binding to each other to form an AB structure. (b) Two 1-patch particles binding to a central 2-patch particle to form an AB2 structure (like carbon dioxide). (c) Three 1-patch particles binding to a central 3-patch particle to form an AB3 structure. (d) Four 1-patch particles binding to a central 4-patch particle to form an AB4 structure (like methane). (e) A chain of 2-patch particles binding to each other. Scale bars = 2 micrometers.