Learning from butterflies: Folding lipid membranes to build photonic-crystal materials

In some butterfly wing scales, a cellular membrane is folded into a periodic nanostructure which generates color and iridescence by constructive interference of visible light. Inspired by this biological achievement, where protein binding is thought to mediate the energetics and dynamics of membrane folding, we aim to build photonic-crystal materials from self-assembly of small particles on artificial membranes. We start by characterizing the interactions between colloidal particles and a supported phospholipid membrane. We graft single-stranded DNA onto them, so that hybridization of complementary strands generates a specific, attractive force between the particles and the membrane. Using a total internal reflection microscope, we measure interactions with femtonewton resolution and kilohertz dynamics. We find that ligand-receptor affinity dramatically affects the energetics and dynamics of particle-membrane interactions: over a temperature range of a few degrees Celsius, adhesion strength varies by about 10 kT, while bound lifetimes and particle mobility change by orders of magnitude. These results may lead to better understanding of self-assembly of particles on fluid membranes, and ultimately enable self-assembling, membrane-based materials with remarkable optical properties.