Phase Transitions in Single Nano-Vesicles

Abstract

Phase transitions in nano-scale lipid vesicles are believed to be influenced by the morphology as well as the finite size of the system. Both the effect of finite size and the asymmetric lateral stresses in the bilayer are believed to broaden the phase transition in small vesicles which has been confirmed by bulk studies. However, it still remains to be resolved whether the measured phase transition of the ensemble results from a broadening of the phase transition in individual vesicles or to heterogeneities among single vesicles exhibiting sharp phase transitions. We investigate the transition from gel (ordered) to fluid (disordered) state in single nano-sized lipid vesicles, composed of only ∼104 molecules, by measuring the anomalous permeability which peaks at the phase transition. We study single vesicles by immobilizing them on functionalized substrates. The vesicle size and efflux/influx for each vesicle is accurately quantified which allows us to study the effect of curvature on phase transitions. To assess the width of the phase transition we probe the pore size at different temperatures using chromophores of different molecular radii. Our data suggest a revaluation of the currently accepted concepts about the nature of permeation at Tc and of phase transitions in nano-scale systems.