Conformational induced behaviour of copolymer-capped magnetite nanoparticles at the air/water interface

Abstract

Biocompatible and stimuli-responsive copolymer-capped Fe₃O₄ nanoparticles (NPs) were studied at the air/water interface, below and above their lower critical solution temperature (LCST). The NP layers have been characterized in situ by compression–expansion isotherms, infrared reflection–absorption spectrometry, X-ray reflectivity, and by transmission electron microscopy after being transferred onto solid support. The data obtained highlight the different interfacial behaviour of the NPs below the LCST (good dispersibility in the aqueous subphase) and above the LCST (lack of dispersibility in the subphase, high affinity for hydrophobic interactions, and agglomeration at the interface). Conformational transitions of the copolymer from pancake to brush and mushroom-like structures are the key factors of the stimuli-induced behaviour of the NPs at the air/water interface. These conformational changes of the copolymer shell are due to the ability of the ethylene oxide units to form hydrogen bonds with the water molecules of the subphase or which are trapped inside the mushroom-like structure. The red shift of the C–O–C band accompanied by the blue shift of the CH₂ scissor band gives comparative information about the degree of hydration of the ethylene oxide groups for the different conformations.