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OXIDE NANOPARTICLES, Vikas Gumber , Prem P. Vaishnava* , Gavin Lawes* , Rajesh K.
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Regmi , Correy C. Black , Ambesh Dixit , Vaman M. Naik , Ratna Naik , Sudakar Chandran ,
Wayne State University , Department of Physics and Astronomy, Detroit, MI 48202, Kettering
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University , Flint, MI 48504, University of Michigan-Dearborn , Department of Natural Sciences,
Dearborn, MI 48128,,

Ferromagnetic iron oxide nanoparticles with sizes ranging from 10 to 100 nm have been
investigated for several decades, in part due to their potential in a number of biomedical
applications. These applications include targeted-drug delivery, where the nanoparticles could
carry attached drug molecules to the disease site, hyperthermia, where the particles produce
local heating when exposed to an ac magnetic field, which could be used for treating tumors, and
as contrast agents for MRI, among others . The use of iron oxide nanoparticles for biomedical
applications is feasible because dextran and other polymer coated magnetite nanoparticles have
been shown to be biocompatible, biodegradable, and nontoxic. However, before magnetic
nanoparticles can be used for many of these applications, it is necessary to understand the
hydrodynamic response of these materials, the key element of which is measuring the
hydrodynamic radius of the composite molecule (Iron oxide coated with polymers). A number of
methods, including dynamical light scattering (DLS) and ac magnetic susceptibility (ACMS), have
been used by researchers for determining the hydrodynamic radius of these systems. However,
these different measurement techniques often yield diverging values, which are often larger than
the expected maximum possible size for the coated nanoparticles. One factor that can account
for these discrepancies is that some of these values depend on the viscosity of the carrier liquid
in which these nanoparticles are suspended. Any interaction between the carrier liquid (water in
the present study) and the composite nanoparticle could potentially alter the effective viscosity of
the solution, leading to values considerably larger than the bulk viscosity. The Stokes-Einstein
                   k BT
equation, D   =         , which is the basis for extracting the nanoparticle size for the two most
widely used techniques (DLS and ACMS), does not take into account the significant difference
between the bulk and nanoscale effective viscosity. We propose that this change in viscosity
could produce the experimental discrepancies observed. To test our hypothesis, we prepared
magnetite nanoparticles having a diameter of roughly 12 nm using a co-precipitation method.
These nanoparticles were then coated with dextran molecules of different molecular weights,
including: 5 kDa, 15-20 kDa, 60-90 kDa, and 670 kDa. The hydrodynamic radii obtained through
DLS, using the bulk value for viscosity, were 91 nm, 100 nm, 106 nm, and 132 nm, respectively.
Using the ACMS method, again with the bulk value for viscosity, we extracted effective sizes of
105 nm, 113 nm, 122 nm, and 136 nm, respectively. These sizes are approximately twice as
large as expected given the iron oxide nanoparticle size and surfactant molecular chain length, at
least for the lower molecular weight dextran. At higher molecular weights, there is known to be
complex folding of the dextran polymers, which dramatically changes the chain lengths .
Comparing the results of hydrodynamic studies of all the dextran coated Fe 3 O 4 nanoparticles,
particularly the 5 kDa dextran coated sample, with the expected particle sizes, we conclude that
the effective viscosity for the coated nanoparticles may be different than the bulk viscosity of the
carrier liquid. However, by increasing the effective viscosity by a factor of two in the Stoke-
Einstein equation as compared to the bulk value, one can obtain better agreement between the
hydrodynamically measured nanoparticle sizes and the predicted sizes.
    Pankhurst Q. A., Connolly J., Jones S. K., and Dobson J. Applications of magnetic
         nanoparticles in biomedicine. Journal of Physics D: Applied Physics. 36, 167 (2003).
    Kawaguchi T. and Hasegawa M. Structure of dextran ± magnetite complex: relation between
         conformation of dextran chains covering core and its molecular weight. Journal of
         Material Science: Materials in Medicine. 11, 31 (2000).

V. Gumber would like to acknowledge support from NSF-REU grant No. EEC-0552772.

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