Unravelling the Mechanism behind Charge Reversal at Silica Nanoparticle-Model Cell Membrane Interfaces.

Vibrational sum frequency generation spectroscopy is used to understand the interactions of silica nanoparticles (SNPs) with a model cationic membrane (1,2-dipalmitoyl-3-(trimethylammonium)propane, DPTAP) by monitoring changes in the interfacial water and lipid structure at pH ∼ 2 and pH ∼ 11. Our study reveals that, at pH ∼ 11, SNPs are attracted to DPTAP due to electrostatic forces, causing changes in the interfacial water structure and lipid membrane. At high concentrations of SNPs (≥70 pM), the interfacial charge reversed from positive to negative, inducing the formation of new hydrogen-bonded structures and reorganization of water molecules. Conversely, negligible changes are observed at pH ∼ 2 due to nearly neutral charge of the SNPs. Molecular dynamics simulations demonstrated that the interfacial potential due to model membrane and SNPs dictates the water structure at the interface. These results elucidate the fundamental mechanism governing interfacial interactions and could have implications in drug delivery, gene therapy, and biosensing.