Imaging Large Iron-Oxide Nanoparticle Clusters by Field-Dependent Magnetic Force Microscopy

Abstract

Iron-oxide nanoparticles are intensively considered for high-performance biomedical applications, where simultaneous functionalities, such as magnetic state, large surface area for maximal protein/enzyme binding, high magnetization values to provide large signals, and good dispersion in liquid media, are usually required. In this context, the association of individual nanoparticles into large clusters is of particular interest. Here, we present a magnetic force microscopy (MFM) approach capable to image individual nanoparticulate clusters as large as 350 nm at room temperature and under variable magnetic fields. It is shown that an in situ removal of electrostatic interactions─particularly important for large particle sizes supported by dielectric substrates─in MFM experiments based on phase detection allows us to image the magnetic state of individual clusters. After taking into account the magnetization behavior of the microscope tip, the phase signal reveals a gradual and uniform rotation of the magnetization with the magnetic field and the absence of a hysteretic behavior for all investigated clusters.