Self-assembly of iron oxide-poly(ethylene glycol) core-shell nanoparticles at liquid-liquid interfaces

Nanoparticles (NPs) play an increasingly important role in the fabrication of functional advanced materials. Two major steps need to be carried out in order to achieve control of the material properties. First of all, the properties of the single NPs have to be under control, especially in relation to colloidal stability; aggregation and corrosion negate all the benefits associated to the nanoscopic dimensions. Secondly, the assembly process has to be controlled to achieve a material with the desired properties. We propose here to use stabilized ceramic NPs consisting of a magnetite core, coated by a poly(ethylene glycol) (PEG) shell and study their assembly at polar/ non-polar liquid interfaces, en route to fabricating functional NP membranes. These NPs show extraordinary stability in aqueous solutions achieved by anchoring linear PEG chains through an end-terminating nitroDOPA group to their surface. Furthermore, the core and shell sizes of these NPs can be independently varied with ease. We first describe the details of the NP synthesis and stabilization in bulk solutions, discussing the PEG molecular weight needed to achieve bulk stability. Subsequently, we demonstrate self-assembly of these particles at liquid-liquid interfaces (SALI) into monolayers of stable properties. SALI has been chosen as path for the assembly given its suitability for fabricating two-dimensional materials. We report here results from pendant drop tensiometry which illustrate the kinetics of NP adsorption at the liquid-liquid interface and highlight the role played by the molecular weight of the PEG shell in the interfacial assembly. In particular we show that the requisites to ensure particle stability at a liquid interface are more stringent compared to the bulk case.