Charge Interactions of Unilamellar Vesicles in Aqueous Suspensions

This project reports the results of an experimental optical trapping study of the charge interactions between phosphor-lipid unilamellar vesicles. A 1064nm laser coupled into a high NA objective lens provided the optical trap. Using fluorescently labeled vesicles, we were able to monitor the particle number density by using a 532nm excitation beam aligned to be parfocal with the trapping beam through the same objective. Fluorescent signals from the focal region common to both beams were band-passed to a pinhole for confocal detection. Using the number density of the vesicles in the focal spot as a function of trapping intensity and a force balance model, we were able to calculate the effective trapping energy per vesicle as well as the osmotic virial coefficients for a system of lipid vesicles prepared with DOPG, cholesterol, and DiI. We measured the compressibility of these vesicle suspensions as a function of surface charge and ionic strength of the suspending medium. Compared to conventional scattering methods, this optical trapping method is advantageous, since it can be used for concentrated suspensions, yielding an in situ measurement of colloidal interactions.