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Despite the increasing number of applications of nano-sized particles (NP), there is a lack of systematic basic experimental studies on the physical basics of the interactions between NP and cell membranes. Here, we follow a bottom-up approach and investigate the intake of silica NP by giant unilamellar vesicles. We observe a massive nanoparticle uptake by fluid phase vesicles, but only above a specific ionic strength of the surrounding buffer solution. The uptake rates increase for decreasing NP size and increasing NaCl concentration. A correlation of ionic strength and adhesion force between the lipid membrane and the NP can explain this dependency. We discuss these effects employing a model which considers NP diffusion and an effective membrane permeability due to uptake-induced pores. Our findings contribute to a deeper understanding of the physics behind NP-membrane interactions as well as endocytotic particle uptake in living cells. [ABSTRACT FROM AUTHOR]