Using differential confocal microscopy to detect the phase transition of the membrane of giant unilamellar liposomes

Giant unilamellar liposomes (diameter > 10 mu m) are important for cell-membrane research and controlled drug-delivery. Mechanical properties of unilamellar liposomes in different physiological conditions are crucial for their applications. For example, liquid-gel phase transition of the bilayer membrane under different temperatures determines the stability and activity of liposomes. Bending rigidity is the most closely related mechanical property to phase transition. Owing to the flexible nature of bilayer membranes, accurate measurements of the bending rigidity of membranes are difficult. Here we report an air-optical technique to directly measure the bending modulus of unilamellar liposomes. We use differential confocal microscopy, a far-field optical profilometry with 2-nm depth resolution, to monitor the thermal fluctuations and the deformation of unilamellar liposomes. From the amplitude changes of thermal fluctuations along with temperature we can directly determine the phase-transition temperature of the membrane structure. We then employ optical force to induce submicrometer deformation of the unilamellar liposomes. From the deformation we obtain their bending rigidity with simple calculation. We find the bending modulus decreases from 8-11 pico-erg to 0.5-0.9 pico-erg as the liposomes are heated across the phase-transition temperature. All the measurements are done without contacting the samples, and the shapes of liposomes remain the same after the experiments.