Magneto-optical investigation of scattering by superparamagnetic nanoparticles using multi-harmonic analysis

摘要

Magnetic nanoparticles (MNPs) are widely used in imaging, magnetic hyperthermia, drug delivery, etc. We present a continuation of our study of dilute aqueous suspensions of magnetite nanoparticles using an AC Faraday rotation setup. Our previous results focused on analyzing the first and higher odd harmonics of the magnetic field frequency yielding the average particle magnetic moment μ , particle density n , and Verdet constant of the sample. Here, we investigate the leading even harmonic (2f) signal to analyze scattering by constructing a scattering ratio. This signal is independent of the polarization state of the incident light and is therefore not relatable to Faraday rotation ( FR ). The evidence of MNP clustering along with the geometric shadowing effect may explain the scattering response as a function of magnetic field frequency and amplitude. Investigation of clusters in SNPs is critical to establish their effective moment and size as a function of magnetic field profile. This analysis can provide a consistency check on the odd harmonic analysis, in addition to providing a physical picture of the underlying process of cluster formation. We have measured the scattering response of magnetite (Fe 3 O 4 ) nanoparticles that vary in size from 15 to 25?nm along with two different types of SNPs with the same core diameter (20?nm) but different functionalization layers (Oleic acid and PEG). To explain the change in light intensity as a function of magnetic field, we are developing a model that employs a field-dependent cross-section which we hope to relate to cluster size.