Surface binding affinity measurements from order transitions of lipid membrane-coated colloidal particles

Lipid bilayers can be assembled onto the surface of colloidal silica particles to form a continuous and fluid supported membrane coating. In this configuration, the collective behavior of the colloidal dispersion is governed by interactions between particles and exhibits a sensitive dependency on chemical features of the membrane surface. Protein binding to membrane surface receptors can trigger macroscopic changes in the colloidal order, which provides a label-free readout of such binding events. Here, the relationship between order in the colloidal dispersion and the surface concentration of bound protein is characterized quantitatively in terms of the radial pair distribution function. Using parallel fluorescence measurements for comparison, we construct a scalar measure of the distribution function that exhibits linear proportionality with surface protein binding. This is used to determine binding affinity based only on observations of the colloidal distribution.