(120c) Magnetic Nanoparticle Characterization By Dark-Field Imaging and Bright-Field Absorbance

Paramagnetic particles extensively varying in composition, size and magnetization are used in biochemical separations, cell purification, cell labeling, cell physiology, endocytosis, endosome research, ferrofluids, drug targeting, analytical biochemistry, environmental analysis, magnetic resonance imaging, hyperthermia, in vivo diagnostics and others. The particle size uniformity and colloidal stability of paramagnetic particles have been the focus of most producers, whether research or commercial, but magnetization is never specified and is typically mentioned only in terms of per cent iron oxide content. Bulk measurements of magnetic susceptibility and saturation magnetization, using a SQUID magnetometer, vibrating sample magnetometer, Guoy scale, or Faraday scale provide an average value at best and do not account for the distributed nature of these variables or for particles with zero or very low susceptibility and saturation magnetization.

By means of particle tracking velocimetry in dark field illumination we measured multiple characteristics of several thousand individual particles per sample. The Hyperflux^TM velocimeter is utilized to provide quantitative video analysis of cells and particles using a high definition camera/microscope and capture of the images of the particle trajectories in an isodynamic field. Image analysis software converts the image data to the parameters of interest based on velocimetry calculations. By measuring calibrated magnetophoretic mobility and calibrated diameter of every particle we report apparent magnetic susceptibility and saturation magnetization on a particle-by-particle basis and provide distributions of susceptibility as well as saturation magnetization for numerous commercial products in a short period of time. The direct measurement of magnetophoretic mobility of the particles with size less than 400 nanometers is limited by optical resolution. In this research, to overcome the size limitation, two different approaches- optical density method and chain velocity method were studied to estimate the magnetophoretic mobility of magnetic nanoparticles of size < 400 nm. The optical density method is based on optical absorbance versus time measured in a bright field, using ImageJ image processing software. The chain velocity method takes advantage of chain formation of the superparamagnetic nanoparticles in an applied magnetic field, making particles visible in the dark field and extrapolates magnetophoretic mobility of nanoparticle chains. It was found that estimation of the distribution of magnetophoretic mobility and intrinsic magnetic properties of magnetic nanoparticles using dark-field particle tracking allows economical and time-efficient magnetic evaluation of a broad range of magnetic particle sizes, nano- and micro.

Key words: magnetophoretic mobility, dark field imaging, susceptibility distribution, saturation magnetization, paramagnetic nanoparticles