The effect of the electrostatic attractive force between giant unilamellar vesicles (GUVs) and the SiO2 surface on the formation of a Ca2+-free supported lipid bilayer (SLB) was investigated by atomic force microscopy and fluorescence microscopy. When negatively charged GUVs were incubated for I h without Ca2+, the surface coverage of lipid bilayer was < 1% on the SiO2 surface. In contrast, a high coverage was obtained without addition of Ca2+ on the positively charged surface modified by aminopropyldimethylethoxysilane, and the coverage of SLBs decreased with increasing KCl concentrations. The thickness of the water layer under SLB was reduced by modification of APS. (c) 2006 Elsevier B.V. All rights reserved.