Recent studies have demonstrated that nanoparticle interactions with cellular membranes can lead to membrane disruption and changes in cellular function. While interaction mechanisms are still not well understood, results have shown that nanoparticles can partition into lipids can cause changes in the physical properties of membranes including lipid packing, phase separation, and elasticity. This study aims to investigate the effects of polyethylene glycol (PEG)-coated silver nanoparticles on lipid packing using dynamic surface pressure measurements. Langmuir monolayer techniques were employed to measure the surface pressure-area (π–A) isotherms of the lipid monolayer as a function of nanoparticle concentration. Monolayers were formed by depositing solutions of dioleoylphosphocholine/dioleoylphosphoglycerol (DOPC/DOPG; 1:1 mole ratio) at the air-water interface to mimic a bacterial membrane monolayer. A suspension of PEG-coated silver NPs was injected below the lipid monolayer in the water subphase and the interaction with the monolayer was examined as a function of time.
Our results show that nanoparticles themselves were surface-active and competed with the lipids for adsorption at the air/water interface. In the absence of lipids, the nanoparticles increased the surface pressure up to 45 mN/m upon compression before the nanoparticle surface layer collapsed. In the presence of lipids, the nanoparticles and lipids acted cooperatively to increase the surface pressure, and at high nanoparticle concentrations, corresponding to the nanoparticles completely covering the interface, the π–A isotherms more closely resembled that for nanoparticles than for lipids. Dynamic measurements of the change in surface pressure at a constant area supported these observations. Low nanoparticle concentrations led to an increase in surface pressure due to increased lipid packing and the inherent surface activity of the nanoparticles, and high concentrations led to monolayer collapse due to lipid desorption from the interface. Dynamic changes in surface pressure followed a logarithmic curve and occurred over 3 h, which suggests that nanoparticle adsorption and monolayer restructuring occurred over long timescales. This work shows that surface-active nanoparticles can compete with lipid adsorption at air/water interfaces in model bacterial membrane monolayers. In this case nanoparticle surface-activity appears to be the driving force for monolayer insertion as opposed to direct lipid-nanoparticle interactions (for example, electrostatic attraction).