Improved functionalization of oleic acid-coated iron oxide nanoparticles for biomedical applications

Abstract

Superparamagnetic iron oxide nanoparticles can provide multiple benefits for biomedical applications in aqueous environments such as magnetic separation or magnetic resonance imaging. To increase the colloidal stability and allow subsequent reactions, the introduction of hydrophilic functional groups onto the particles' surface is essential. During this process, the original coating is exchanged by preferably covalently bonded ligands such as trialkoxysilanes. The duration of the silane exchange reaction, which commonly takes more than 24 h, is an important drawback for this approach. In this paper, we present a novel method, which introduces ultrasonication as an energy source to dramatically accelerate this process, resulting in high-quality water-dispersible nanoparticles around 10 nm in size. To prove the generic character, different functional groups were introduced on the surface including polyethylene glycol chains, carboxylic acid, amine, and thiol groups. Their colloidal stability in various aqueous buffer solutions as well as human plasma and serum was investigated to allow implementation in biomedical and sensing applications. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s11051-012-1100-5) contains supplementary material, which is available to authorized users.

Fig. 1

Overview of the chemical reactions during the functionalization of iron oxide nanoparticles with silanes. The formation of the silanol molecule occurs by reaction with water. Subsequent polycondensation renders a silane network on the surface of the nanoparticle

Fig. 2

Transmission electron microscopy image of oleic acid-stabilized iron oxide nanoparticles. The scale bar represents 20 nm. The inset shows the size distribution: 9.3 ± 1.6 nm

Fig. 3

Vibrating sample magnetometry signal of the nanoparticles showing a hysteresis curve. The data points are fitted by a Langevin function to determine the magnetic core size

Fig. 4

X-ray powder diffraction spectrum of the oleic acid-coated iron oxide nanoparticles. The solid and dashed vertical drop-down lines represent the peak positions of a reference magnetite and maghemite spectrum, respectively (AMCSD 0007824 and 0007899)

Fig. 5

Zeta potential values for PEG-, carboxylic acid-, and amine-coated nanoparticles in various pH solutions. Every data point was derived from 10 measurements by the software