Interactions Between Silver Nanoparticles/Silver Ions and Liposomes: Evaluation of the Potential Passive Diffusion of Silver and Effects of Speciation.

Abstract

Silver nanoparticles, used mainly for their antibacterial properties, are among the most common manufactured nanomaterials. How they interact with aquatic organisms, especially how they cross biological membranes, remains uncertain. Free Ag+ ions, released from these nanoparticles, are known to play an important role in their overall bioavailability. In this project, we have studied the uptake of dissolved and nanoparticulate silver by liposomes. These unilamellar vesicles, composed of phospholipids, have long been used as models for natural biological membranes, notably to study the potential uptake of solutes by passive diffusion through the phospholipid bilayer. The liposomes were synthesized using extrusion techniques and were exposed over time to dissolved silver under different conditions where Ag+, AgS2O3-, or AgCl0 were the dominant species. Similar experiments were conducted with the complexes HgCl 2 0 and Cd(DDC) 2 0 , both of which are hydrophobic and known to diffuse passively through biological membranes. The uptake kinetics of Ag+, HgCl 2 0 , and Cd(DDC) 2 0 show no increase in internalized concentrations over time, unlike AgS2O3- and AgCl0, which appear to pass through the phospholipid bilayer. These results are in contradiction with our initial hypothesis that lipophilic Hg and Cd complexes would be able to cross the membrane, whereas silver would not. Encapsulated tritiated water inside the liposomes was shown to rapidly diffuse through the lipid bilayer, suggesting a high permeability. We hypothesize that monovalent anions or complexes as well as small neutral complexes with a strong dipole can diffuse through our model membrane. Finally, liposomes were exposed to 5-nm polyvinylpyrrolidone-coated silver nanoparticles over time. No significant uptake of nanoparticulate silver was observed. Neither disruption of the membrane nor invagination of nanoparticles into the liposomes was observed. This suggests that the main risk caused by AgNPs for nonendocytotic biological cells would be the elevation of the free silver concentration near the membrane surface due to adsorption of AgNPs and subsequent oxidation/dissolution.