Polymer-capped magnetite nanoparticles change the 2D structure of DPPC model membranes

Abstract

The interaction of biocompatible and stimuli-responsive Fe3O4 nanoparticles (NPs) with DPPC model membranes was studied at the air–water interface. The pure NPs, pure DPPC and mixed DPPC–NP Langmuir layers have been characterized in situ by compression–expansion isotherms, infrared reflection–absorption spectrometry (IRRAS), grazing incidence X-ray diffraction (GIXD), total reflection X-ray fluorescence (TRXF), Brewster angle microscopy, and by atomic force microscopy after transfer onto a solid support. When dispersed in aqueous solution, the NPs are able to penetrate the phospholipid monolayer and to occupy a certain part of the interface defined by their own surface activity. The study reveals a largely phase separated system which actually masks an important interplay between the two compounds. Detailed GIXD measurements prove that the NPs are able to change the DPPC monolayer structure in a highly cooperative way by inducing a tighter in-plane packing. As IRRAS additionally supports, the effective size of the phospholipid polar head group is reduced due to partial dehydration and reorientation. The most surprising observation is the rigidification of the DPPC–NPs composite in a large range of surface pressures. Above the critical pressure, the phase separated NPs are squeezed out but the ones interacting with the DPPC molecules stay in the layer up to much higher lateral pressures. The interactions between NPs and DPPC stabilize the NPs at the interface. Only at very high pressures, all NPs are squeezed out. The amount of NPs in the monolayer has been quantified by TRXF. The desorption process is kinetically hindered.