Size-Dependent Mechanical Property Measurement of 1,2-dioleoyl-SN-3-phosphocholine Liposome Using Atomic Force Microscopy

Most studies have reported on the effects of bending rigidity using giant unilamellar liposomes. However, few studies have been conducted on nanosized liposomes. We studied the properties of DOPC liposomes of various sizes using atomic force microscopy (AFM). Two different topographies (convex and planar) of DOPC liposomes on a mica surface were observed in the tapping mode in the fluid. The topographic change of the liposomes could be attributed to the interaction force between the AFM tip and the liposome. Small DOPC liposomes (below 1 mu m) showed a bending modulus (k(bend)) ranging between 10(-18) and 10(-20) J. The bending modulus was found to be size-dependent; it decreased as DOPC liposome size increased. In this study, images of unilamellar DOPC liposomes were obtained in the contact mode. The bending moduli of lipid vesicles from 100 to 1300 nm were measured using an AFM force curve. The bending modulus tended to increase as liposome size decreased. From experimental and theoretical perspectives, we showed that the value of the bending modulus is proportional to 1.5 power of the radius of the liposome.