Biocompatible magnetite nanoparticles trapped at the air/water interface.

In the last decade, iron oxide nanoparticles (NPs)—known as super-paramagnetic iron oxide nanoparticles (SPION)—have received significant attention due to their applicability in a variety of domains. In the biomedical field, iron oxide nanoparticles are very promising as drug-delivery systems, magneticresonance-imaging contrast enhancers (clinical diagnosis), inflammation-responsive or anti-cancer agents, or for labeling and cell separation. In addition to their potential medical applications, magnetic nanoparticles are of great interest in materials science, for the development of magnetic data-recording media and nanocomposite permanent magnets, as well as in catalysis and colloid chemistry, where ferrofluids represent a topic of high interest. This study is focused on Fe3O4 NPs grafted with the thermosensitive and biocompatible copolymer MEO2MA/OEGMA, [10] whose interfacial behavior is yet unknown. Many papers have been dedicated to the formation, organization, and stability of Langmuir films of iron oxide (Fe2O3 or Fe3O4) NPs and their Langmuir–Blodgett transfer. The system studied herein is completely different from the iron oxide NPs already studied at the air/water interface due to the unique properties of the polymer used. A monodisperse population of uncharged Fe3O4 cores with a diameter of 6.4 nm, which are grafted with catechol-terminated copolymers of 2-(2-methoxyethoxy) ethyl methacrylate (MEO2MA) and oligo(ethylene glycol) methacrylate (OEGMA), has been investigated (Figure 1). Due to the presence of oligo(ethylene glycol) side groups on the surfaces, the Fe3O4@MEO2MA90-co-OEGMA10 NPs (90 and 10 represent the molar fractions of MEO2MA and OEGMA, respectively) can be dispersed in water, exhibiting a high colloidal stability against salt and, at the same time, they can be well dispersed in organic solvents such as chloroform, ethanol or toluene. This specific copolymer in water (no salt) is hydrophilic and becomes hydrophobic at temperatures above 40 8C. The experimental details are presented in the Supporting Information. Since the polymer and the polymer-dressed NPs behave exactly in the same way, showing that the polymer dictates the surface activity, we will describe here mainly the results obtained with the polymer-dressed NPs because this system can be investigated with a larger number of methods giving complementary information. The surface activity of the Fe3O4@MEO2MA90-co-OEGMA10 NPs is based on the amphiphilic character of the copolymer shell. This (oligo ethylene glycol) methyl ether methacrylate polymer has a graft structure (Figure 1) composed of an apolar carbon–carbon backbone which leads to a competitive hydrophobic effect and multiple oligo(ethylene glycol) side chains of which ether oxygen atoms form stabilizing hydrogen bonds with water. Moreover, the ethylene oxide motif can adopt a configuration with the oxygen atoms on one side of the molecule and with the two methylene groups on the other, thus giving the molecule both a hydrophilic and a hydrophobic surface. The hydrophobicity can be tuned by changing the molar fraction of the two monomers. The polymer can be dispersed both in water and in chloroform; therefore, the corresponding films have been prepared either by adsorption from aqueous bulk solution or by spreading from a chloroform solution at the interface. Using, for example, a NP bulk concentration of 1.5 10 3 mgmL , a constant surface pressure value of approximately 23 mNm 1 is reached after 20 h (Figure 2A). Compression isotherms of Langmuir layers formed by spreading certain amounts of NPs are shown in Figure 2B. By compression, the surface pressure increases continuously to 25 mNm 1 (critical pressure pc of the polymer as well as the NP film). During further compression, a plateau region appears at which the surface pressure increases only slightly up to a maximum value of 27 mNm . It is very important to highlight that no hysteresis of the compression/ expansion isotherms is observed when the interfacial film is compressed to surface pressures below the critical pressure of the Langmuir layer (Figure 2C), suggesting that no loss of material from the interface occurs. Based on this observation, we can calculate the interfacial concentration of NPs corresponding to the critical pressure. The critical concentration amounts to (7.7 0.6) 10 4 mgcm . This value is in good agreement with the NP interfacial concentration of 8.2 10 4 mgcm , which can be calFigure 1. Schematic representation of the Fe3O4 @ MEO2MA90-co-OEGMA10 NPs