Magnetic Force Microscopy of Superparamagnetic Nanoparticles for Biomedical Applications

Abstract

In the last two decades, AFM became an important tool for surface characterization. Different variations of the AFM allow the characterization of additional surface properties, such as electric, magnetic, adhesion etc. This work aims at the magnetic characterization of single magnetic nanoparticles in the superparamagnetic state for biomedical application and further understanding of MFM signals. The magnetic detection of single nanoparticles on flat substrates is the first milestone in this work. Magnetic signals are often hidden by other signals making the evaluation rather difficult or even impossible. One of the strongest disturbing signals is found to be due to capacitive coupling of the tip with the substrate. The superposition of magnetic signals by electrostatic signals and the origin of capacitive coupling are investigated. The distance between the substrate and the tip increases while measuring above the nanoparticles. This results in a reduction of the attractive force due to capacitive coupling and therefore leads to a positive phase shift. These positive phase shifts can hide the magnetic signals which are always attractive in case of superparamagnetic nanoparticles. The effect of capacitive coupling is explained theoretically and is proven by measurements. Different approaches to minimize capacitive coupling are discussed and applied. The combination of different approaches allows the magnetic visualization of single separated SPIONs with 10 nm diameter. A new approach is investigated to reduce the capacitive coupling by introducing a dielectric layer into the system. The capacitive coupling is reduced significantly for a dielectric layer with a thickness above 300 nm. This allows a magnetic visualization of single separated SPIONs without any further preparations and additional parameters. Furthermore, the influences of the capacitive coupling effect on interleave measurements for non-magnetic and magnetic samples are discussed. Theory predicts an annular attraction around the magnetic nanoparticle and circular repulsion above the nanoparticle due to the superposition of magnetic forces and capacitive coupling. This behavior is proven by measurements of single SPIONs on a silicon substrate and on a substrate with spin-coated dielectric layer. Finally, in terms of biomedical applications, in a polymer matrix embedded nanoparticles are investigated with MFM. With the previous characterization of tip magnetization, qualitative measurements of embedded magnetic nanoparticles are possible. The magnetic core size of the nanoparticles is calculated using dipole-dipole approximation. The detection limits are shown and discussed for a magnetic tip with a high magnetic moment. The detection of single SPIONs on a flat substrate and embedded SPIONs is a step towards the qualitative measurements of modified SPIONs, SPIONs in cells and tissues using MFM technique.