Role of the capping agent in the interaction of hydrophilic Ag nanoparticles with DMPC as a model biomembrane

The interaction of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) Langmuir monolayers as a model biomembrane with small silver nanoparticles (AgNPs) similar in size but coated with different capping molecules such as citrate (CIT-AgNPs) and 4-mercaptobenzoic acid (MBA-AgNPs), both negatively charged at physiological pH, is studied using a multi-technique approach. Both CIT-AgNPs and MBA-AgNPs expose carboxyl groups and have similar zeta potentials, but differ in the aliphatic or aromatic nature of the capping agent. Results show that AgNPs exhibit quite different behaviors: CIT-AgNPs weakly adsorb on DMPC, while MBA-AgNPs irreversibly adsorb on the interface and remain there upon monolayer compression. It is also shown that there is a cooperative effect of many ligands in the interactions between MBA-AgNPs and DMPC, as MBA molecules in solution are unable to strongly adsorb on the phospholipid monolayer surface. We propose an explanation based on the surface charge density and on the chemical nature of the capping molecule based on XPS studies and on DFT calculations.