INVESTIGATING THE EFFECTIVE HYDRODYNAMIC SIZE OF DEXTRAN COATED IRON 1 2 1 OXIDE NANOPARTICLES, Vikas Gumber , Prem P. Vaishnava* , Gavin Lawes* , Rajesh K. 1 1 1 3 1 1 Regmi , Correy C. Black , Ambesh Dixit , Vaman M. Naik , Ratna Naik , Sudakar Chandran , 1 Wayne State University , Department of Physics and Astronomy, Detroit, MI 48202, Kettering 2 3 University , Flint, MI 48504, University of Michigan-Dearborn , Department of Natural Sciences, Dearborn, MI 48128, firstname.lastname@example.org, email@example.com 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 1 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 6πηR 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 2 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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