Specific adhesion of vesicles monitored by scanning force microscopy and quartz crystal microbalance

The specific adhesion of unilamellar vesicles with an average diameter of 100 nm on functionalized surfaces mediated by molecular recognition was investigated in detail. Two complementary techniques, scanning force microscopy (SFM) and quartz crystal microbalance (QCM) were used to study adhesion of liposomes consisting of 1, 2-dipalmitoyl-sn-glycero-3-phosphocholine and varying concentrations of N-((6-biotinoyl)amino)hexanoyl)-1, 2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine (biotin-X-DHPE). Monitoring the adhesion of the receptor-doped vesicles to avidin-coated gold surfaces by QCM (f(0) = 5 MHz) revealed an increased shift in resonance frequency with increasing biotin concentration up to 10 mol% biotin-X-DHPE. To address the question of how the morphology of the liposomes changes upon adhesion and how that contributes to the resonator's frequency response, we performed a detailed analysis of the liposome morphology by SFM. We found that, with increasing biotin-concentration, the height of the liposomes decreases considerably up to the point where vesicle rupture occurs. Thus, we conclude that the unexpected high frequency shifts of the quartz crystal (>500 Hz) can be attributed to a firm attachment of the spread bilayers, in which the number of contacts is responsible for the signal. These findings are compared with one of our recent studies on cell adhesion monitored by QCM.