Optical Tracking of Monodisperse Magnetite Nanoparticles

Arjun Prakash1, Michail Stamatakis1, Christopher J. Jones2, J.T Mayo2, Matteo Pasquali1, and Vicki L. Colvin2. (1) Department of Chemical and Biomolecular Engineering, Rice University, 6100 S Main St, Houston, TX 77005, (2) Department of Chemistry, Rice University, 6100 S Main St, Houston, TX 77005

Magnetic separation using aqueous nanomagnetite dispersions have tremendous applications in environmental pollution, biotechnology and manufacturing processes1,2.In this regard we have recently reported highly efficient Arsenic removal using low field magnetic separation3. This is achieved by utilizing the high surface to volume ratio of the nanoparticles, which is maximized on uniform dispersion. Therefore, understanding the aggregation of these particles is the key to explain the mechanism behind highly effective magnetic separation at low field. Despite being superparamagnetic, magnetite nanoparticles are known to aggregate even in the absence of a magnetic field owing to their dipolar interactions4,5. Here, we report diffusion measurements of nanoparticles under Brownian motion, to decipher the size of the particles and hence their degree of aggregation in the absence of a magnetic field by using Stokes-Einstein diffusion equation. An optical tracking system was used via which nanoparticles of 12nm core diameter were tracked using epifluorescence microscopy. Interestingly, the particles with concentration on the order of 10-7M were found to be monodisperse, in confirmation with their superparamagnetic behavior. The data is supported by corresponding Small Angle X-ray Scattering (SAXS) and Dynamic Light Scattering (DLS) analysis along with Cryogenic Transmission Electron Microscopy (Cryo-TEM) images. Also, DLS measurements under magnetic field showed that the particles aggregated with time irrespective of initial concentration, followed by re-dispersion into the medium on magnetic field removal without the loss of colloidal stability. This is in confirmation with a reversible aggregation phenomenon, on field application and subsequent removal to understand the separation efficiencies observed3. The results not only explain the trend observed by previous studies4,6, but also provide an insight into the length scales of dipolar as well as van der Waals interactions between the magnetic nanoparticles.

Key words: Nanomagnetite, aggregation, diffusion.


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