Surface charge effects on the interaction between a solid-supported model lipid membrane and AuNPS studied by SFG spectroscopy

In this study, we use the selectivity and sensibility of sum-frequency generation spectroscopy (SFG) to interfacial media (Chen and Chen, Biochimica et biophysica acta 1758:1257–1273, 2006) in order to characterize the interactions, and their effects, between a cell membrane model, here made of a li-pid bilayer, and gold nanoparticles. With the aim of giving to this study a sound basis, we did first realize separately the SFG characterization of the membrane model and the gold nanoparticles. Here, the membrane model is a bilayer made of phosphatidylcholine – one of the major components of living membranes – adsorbed on a CaF₂ or SiO₂ prism. The gold nanoparticles have a diameter of 6, 12 or 38 nm and a negative surface state of charge. We highlighted that as long as it stays in water, the membrane model is SFG inactive, due to the globally centrosymmetric nature of the lipid bilayer and to the SFG selection rule, which forbids the emergence of a SFG signal from centrosymmetric media. On the other hand, once it has been exposed to air, the bilayer gives a SFG signal. This fact is explained by the folding (flip-flop) of a part of the bilayer on itself at the contact with the air, due to its hydrophobic nature. Concerning the nanoparticles, they generate a SFG signal coming from the surfactant giving them their state of charge while preventing them binding together. That signal has been measured at the interface with a CaF₂ prism. On the other hand, in the case of a SiO₂ prism, no signal coming from the nanoparticles has been detected. The same results have been observed when the lipid bilayer was present on the prisms. This is explained by the fact that at neutral pH, the SiO₂ has a distinctly negative state of charge (Yeganeh et al. Phys Rev Lett 83:1179–1182, 1999), repelling so the nanoparticles beyond the SFG field of detection at the interface. As for CaF₂, it has a positive state of charge at neutral pH (Schrodle et al. J Phys Chem C 111:10088–10094, 2007), which leads to andetection. Furthermore, since it has been shown (Lin et al. ACS Nano 4:5421–5429, 2010) that in pH-neutral water, phosphatidyl-choline bilayers have a negative state of charge, the positive nanoparticles are mostly expected to interact with these bilayers. To sum up, we have experimentally confirmed that negative nanoparticles weakly interact with phosphatidylcholine bilayer, even with a substrate taking on a positive state of charge in contact with water. On the basis of these results, the next step will be to confirm the interaction of positive nanoparticles with the bilayer, knowing that this interaction should be even more important if the substrate takes on a negative state of charge in water