Influence of pH and electrolyte on the deposition of cerium oxide nanoparticles on supported lipid bilayers

Abstract

The adhesion of cerium oxide nanoparticles (n-CeO₂) to cell membranes is likely a crucial step for initiating the toxic effects of n-CeO₂ on biological cells. To investigate the propensity of n-CeO₂ to adhere to model cell membranes, 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) supported lipid bilayers (SLBs), the deposition attachment efficiencies of n-CeO₂ on SLBs were derived using a quartz crystal microbalance. The impact of solution pH and the valence and concentration of the electrolyte on the attachment efficiency was consistent with the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. The isoelectric points of DOPC membranes and n-CeO₂ are pH 4.2 and 7.2, respectively. At pH 8.0, the attachment efficiency between negatively charged SLBs and n-CeO₂ increased with increasing NaCl and CaCl₂ concentrations. The critical deposition concentrations of NaCl and CaCl₂ are 11 and 0.62 mM, respectively. At pH 2.0 and 4.0, both n-CeO₂ and SLBs were positively charged. The attachment efficiency increased as the NaCl and CaCl₂ concentrations increased due to charge screening by Cl⁻ anions. At pH 5.5, n-CeO₂ and SLBs were oppositely charged in NaCl solutions, hence resulting in favorable attachment. In the presence of CaCl₂, however, the adsorption of Ca²⁺ cations reversed the charge of SLBs to become positive and thus led to unfavorable deposition. Negligible detachment of n-CeO₂ from SLBs was observed upon exposure to low ionic strength solutions or pH changes, indicating irreversible attachment.