Using differential confocal microscopy to detect the phase transition of lipid vesicle membranes

We use differential confocal microscopy, a far-field optical profilometry with 2-nm depth resolution, to monitor the thermal fluctuations and the deformation of the bilayer membranes of lipid vesicles. From the linear relation between the mean-square amplitudes of fluctuations and temperatures, we can directly determine the phase-transition temperatures of lipid bilayers. We then employ femtonewton optical force to induce submicrometer deformation of the vesicle membranes. From the deformation we obtain the bending rigidity of membranes with a simple geometric analysis. The bending modulus changes by an order of magnitude as the temperature is changed across the transition temperature, such that we can detect the phase transition behaviors of the bilayer structures.