Techniques for Surface Modification of Aqueous-Stable Superparamagnetic Iron Oxide Nanoparticles

Abstract

Background: The iron oxide nanoparticles involved in this study are unique in their superparamagnetic properties, defined as their ability to flip the direction of their magnetic field under influence of temperature. This property has a variety of environmental and biomedical uses. Indeed, the exchange of ligands on the surface of these particles enables exploration of such applications. The purpose of this study is to determine an efficient method of ligand exchange in order to standardize the surface modification of these iron oxide nanoparticles (IONPs). Namely, the primary methods of ligand exchange to be evaluated are shaking and sonication of reaction mixtures. As part of this method comparison, the exchange of oleic acid (OA) ligands for 3,4-Dihydroxyphenylacetic acid (DOPAC) ligands serves as a general model for method comparison. When comparing methods, both time and quantity of materials required are considered. The quality of the final product is also considered, assessed by factors such as oxidation state, colloidal stability, and extent of ligand exchange.

Methods: Three methods of ligand exchange are performed, after which their products are compared. The first method involves shaking the mixture overnight for a duration of 18 hr. The second method involves sonication for a duration of 30 min. The third method involves sonication of the reaction mixture for an additional 30 min. (duration of 60 min. in total).
Results: The products were analyzed using Fourier-transform infrared spectroscopy (FT-IR), zeta potential measurements, thermogravimetric analysis (TGA), and x-ray photoelectron spectroscopy. FT-IR measurements indicate that the one-time sonication method leads to the surface of the IONPs bearing the most residual oleic acid, a disadvantageous result. TGA analysis indicates that the twice-sonicated product is more favourable than the once-sonicated product.

Limitations: Larger data sets of FT-IR, TGA, zeta potential, and XPS must be collected before the best method may be confirmed. Zeta potential measurements must be repeated for the shaken product at a concentration that matches that of the other products. As such, a direct comparison may be made. TGA must also be repeated for the shaking product in order to eliminate possible inaccuracies. Namely, these could result from technical difficulties encountered in the measurement discussed above. While zeta potential measurements indicate that the twice-sonicated product has the highest colloidal stability, XPS measurements did not vary significantly enough between methods to suggest a most advantageous method.

Conclusion: According to the TGA and zeta potential measurements, the twice-sonicated product appears to be most favourable in terms of coverage. XPS suggests that all methods are comparable in terms of oxidation of the IONPs’ iron.