Surface modification of magnetic nanoparticles capped by oleic acids: characterization and colloidal stability in polar solvents

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Reference

Surface modification of magnetic nanoparticles capped by oleic acids: characterization and colloidal stability in polar solvents. S-Y Lee, MT Harris, J Colloid Interf Sci 293, 401-408 (2006).

Summary

A magnetic nanoparticle with a oleic acid on the surface produces azelaic acid (i) and 1-nonanal(ii) when reacted with a stream of ozone.
Interfacial ligand exchange reaction of aleic acid with betaine-HCl.

Oxonolysis and interfacial ion exchange were used to make acid-linked magnetic nanoparticles more soluble in either polar solvents or hydrophilic solutions. Oxonolysis cleaves the alkene oleic acid molecule to produce azelaic acid, which ends in a carboxyl group, and 1-nonanal, which ends in a carbonyl group. Azelaic acid and 1-nonanal made the nanoparticles soluble in ethanol, a polar solvent, because both molecules have terminal polar groups and contain shorter carbon chains than ones in oleic acid. Interfacial ligand exchange reaction consisted of adding betaine-HCl to nanoparticles suspended in ethanol, shaking for 40 h, 10 min of sonication, phase separation, and, finally, precipitation from ethanol. The reaction made the magnetic particles hydrophilic and, therefore, able to form a monodisperse solution in water. The structure of the nanoparticle coating was verified using Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Colloidal stability was quantified using a UV-visible spectrophotometer, and magnetic properties were explored by measuring the isothermal magnetization curve of a solution containing the nanoparticles.

Suspensions of nanoparticles that have sat untouched for one month. (A) Unmodified oleic acid-coated particles precipitated out of the ethanol solution. (B) Ozonized nanoparticles were lyophilic and remained suspended in ethanol. (C) Ligand exchanged particles remained suspended in water.

Soft Matter Connection

SEM of (a) micro-patterned magnetic nanoparticle Langmuir-Blodgett film on a silicone wafer, and (b) the corresponding photoresist pattern. Scale bar = 20 um.

The ability to create stable suspensions of magnetic particles in polar solvents has numerous industrial applications. For instance, such suspensions can be used to create magnetic thin films. Gue et al (J Am Chem Soc 125: 630) showed that it is possible to use soft lithography to pattern magnetic nanoparticles into a thin film. Specifically, they spread oleic acid-stabilized nanoparticles monodispresed in hexane at the air-surface interface. The particles acted as a surfactant, and formed a monolayer because their concentration was well below the critical micellar concentration (creating a Langmuir-Blodgett film). Polydimethylsiloxane (PDMS) stamps were dipped into the Langmuir-Blodgett film, and thereby coated with a nanoparticle monolayer. Standard microcontact printing techniques could then be used to pattern a silicone wafer with a magnetic monolayer.

Other researchers have proposed methods for increasing the stability of hydrophobic nanoparticles in hydrophilic or more polar solvents beyond the ozonolysis and ligand exchange reaction presented in this paper. One technique has been the the formation of a surfactant bilayer around the nanoparticles. For example, Shen et al. (Langmuir 15: 447) made iron oxide particles were coated with a bilayer of fatty acids.

Synthesis of a surfactant bilayer around magnetic nanoparticles.