# Scaling of Transverse Nuclear Magnetic Relaxation due to Magnetic Nanoparticle Aggregation

 Title: Scaling of Transverse Nuclear Magnetic Relaxation due to Magnetic Nanoparticle Aggregation Author: Brown, Keith A.; Vassiliou, Christophoros C.; Issadore, David Aaron; Berezovsky, Jesse; Cima, Michael; Westervelt, Robert M. Note: Order does not necessarily reflect citation order of authors. Citation: Brown, Keith A., Christophoros C. Vassillou, David Issadore, Jesse Berezovsky, Michael J. Cima, and R. M. Westervelt. 2010. Journal of Magnetism and Magnetic Materials 322(20): 3122-3126. Full Text & Related Files: SPIO_JMMM_rev1v2.pdf (3.365Mb; PDF) Abstract: The aggregation of superparamagnetic iron oxide (SPIO) nanoparticles decreases the transverse nuclear magnetic resonance (NMR) relaxation time $$T_{2}^{CP}$$ of adjacent water molecules measured by a Carr-Purcell-Meiboom-Gill (CPMG) pulse-echo sequence. This effect is commonly used to measure the concentrations of a variety of small molecules. We perform extensive Monte Carlo simulations of water diffusing around SPIO nanoparticle aggregates to determine the relationship between $$T_{2}^{CP}$$ and details of the aggregate. We find that in the motional averaging regime $$T_{2}^{CP}$$ scales as a power law with the number $$N$$ of nanoparticles in an aggregate. The specific scaling is dependent on the fractal dimension $$d$$ of the aggregates. We find $$T_{2}^{CP} \propto N^{-0.44}$$ for aggregates with $$d=2.2$$, a value typical of diffusion limited aggregation. We also find that in two-nanoparticle systems, $$T_{2}^{CP}$$ is strongly dependent on the orientation of the two nanoparticles relative to the external magnetic field, which implies that it may be possible to sense the orientation of a two-nanoparticle aggregate. To optimize the sensitivity of SPIO nanoparticle sensors, we propose that it is best to have aggregates with few nanoparticles, close together, measured with long pulse-echo times. Published Version: doi:10.1016/j.jmmm.2010.05.044 Terms of Use: This article is made available under the terms and conditions applicable to Open Access Policy Articles, as set forth at http://nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of-use#OAP Citable link to this page: http://nrs.harvard.edu/urn-3:HUL.InstRepos:5364374

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Peer reviewed scholarly articles from the Faculty of Arts and Sciences of Harvard University