The diverse use of colloidal suspensions in materials such as paints, lubricants, food, pharmaceuticals and optoelectronic devices has fostered extensive development in
the fabrication of colloidal particles. These colloidal particles may be assembled into complex
materials, such as gels or solids, driven by interactions between the particles. For single
component repulsive particles, substantial work has been done to characterize the phase behavior of
the colloidal suspensions as a function of volume fraction and temperature. Much of this work
focused on the phase behavior of model hard-sphere suspensions where the interparticle
interaction is infinitely repulsive at contact and zero otherwise 1. In this paper, we explore the use of
specific lock-and key biomolecules to assembly of colloidal materials through attractive interactions,
holding promise for the assembly of novel structures at low total volume. Ideally, the
ability to control both the strength and specificity of the specific interaction should make it
possible to control the formation of a large variety of material structures with a diverse array of
potential applications.
Bio-adhesion molecules make readily feasible the possibility of making binary
colloidal alloys. The use of single-sized particles having identical properties ultimately
limits the variety of structures and useful materials that can be made. Although one may vary the
properties of the single particle, the range of material structures is always restricted by the
single particle size.
Bidisperse colloidal suspensions increase the flexibility in the structure and
applications of colloidal materials. In a binary suspension, one has control over the material
properties of each of the two colloidal species, the ratio of particle sizes, the volume fractions
of the two species, and the total volume fraction. These additional parameters greatly increase the
range of possible structures that can form. In fact, many more crystalline states can be formed
from bidisperse colloidal materials than from monodisperse materials. As a result, considerable
attention has been paid to the formation of binary alloys using mixtures of colloidal
particles. By placing the lock on one type of bead, and the key on the other, one can direct the ordered
assembly of unlike particles through the specificity of the lock-and-key interaction.
In this paper, we have used a relatively weak biological adhesion interaction to
assemble two types of colloidal particles. Our laboratory has worked with selectin/carbohydrate
interactions for many years; these molecules are found in the immune system and mediate blood
cell adhesion to blood vessel walls2. While specific, they are weak, with off-rates on the order
of several sec-1; thus one can expect that binding would be reversible at low densities. At high
densities, the adhesion would be irreversible. Thus, the structure of the colloidal system
containing these molecules would be tunable, depending on the density of molecules used.
We developed bidisperse biocolloidal suspensions consisting of 0.94 µm (further
denoted as “A” particles) and 5.5 µm (further denoted as “B” particles) polystyrene
microspheres coated with E-selectin (further denoted as
∂) and the carbohydrate sialyl-Lewisx sLeX
(further denoted as ß) molecules, respectively. Using this unique system, we have confirmed our
hypothesis that binary structures can be formed through attractive, heterotypic, biological
interactions.
Moreover, we have shown that a variety of binary structures, from colloidal
micelles (a single large particle coated with smaller particles) and binary colloidal
clusters (panel A, with a number ratio of NA/NB = 200), to clusters (panel B, NA/NB = 100), to elongated
chains (panel C, NA/NB = 10 & panel D, NA/NB = 2), can be made by decreasing the number fraction
of small to large particles. The adhesion is specific, and can be reversed by the addition of a calcium chelator, since selectin interactions depend critically on the calcium concentration3. We project that additional manipulation of the number of E-selectin (∂) and sLeX (ß) molecules per particle will allow us to use this same system to test our hypothesis that weaker
(low affinity) interparticle interactions can lead
Hammer, D.A., Hiddessen, A.L., Rodgers, S., Weitz, D., Engineering Novel Biocolloid Suspensions, Proceedings of the Fifth Microgravity Fluid Physics and Transport Phenomena Conference, NASA Glenn Research Center, Cleveland, OH, CP-2000-210470, pp. 1810-1812, August 9, 2000.