Magnetic Spectroscopy of Nanoparticle Brownian Motion
Charlie Tsai, PI: Dr. John Weaver
Research Experience for Undergraduates
Thayer School of Engineering, Dartmouth College
•Parameter reflects effect of applied magnetic field and randomizing
thermal fluctuations
Introduction
Magnetic Nanoparticles
•Promising: Nanometer scale allow interaction with body on
cellular or molecular level
•Optimize particles for uptake into cell or used to control cellular
functions
•Adding biocompatible or biologically active coating for clinical
applications
Magnetic Particle Imaging (MPI)
•Introduced in 2005 by Phillips Research Europe
•Produce 3D in vivo images through magnetic nanoparticle
concentration
•Takes advantage of non-linear magnetic response of magnetic
nanoparticles
•Static gradient fields and alternating magnetic fields allow
harmonic signal of magnetic nanoparticles to be localized.
Brownian vs. Neel Relaxation
•The relaxation time, , governs a particle’s ability to follow changes in
the applied field via the Neel and Brownian relaxation methods.
•Neel – internal rotation independent of viscosity
•Brownian – entire hydrodynamic volume rotates
•Need to cancel out the field
• Take into account iron on each side
• Adjust turns on coils to cancel out harmonics in core
• Add small piece of iron on one side
•Strongest signals: 1st, 3rd, 5th harmonic
•Sample should have huge signal
Results
Setting the pickup coil turns at 32, having 24 turns on the balancing coil
produced a minimum in the induced voltage. The pickup coil alone produced
an induced voltage of 129.58 mV, 16.85 mV, and 9.21 mV for the 1st, 3rd,
and 5th harmonics respectively. The balancing coil produced 147.40 mV,
29.35 mV, and 10.77 mV for the 1st, 3rd and 5th harmonics respectively. The
two coils in series produced 46.54 mV, 26.25 mV, and 16.67 mV for the 1st,
3rd and 5th harmonics respectively. From this data, it seems that minimizing
the voltage for the 1st harmonic (by far the largest signal), does not
necessarily reduce the 3rd and 5th harmonics significantly.
Setup and Methods
Nanoparticle Spectrometer
Conclusions and Further Research
•Our results indicate that such a system using an iron core is does seem
possible since the induced voltage in the receiving coil can be reduced to a
value much lower than either of the component coils.
New Receiving Coil Using Iron Core
Magnetic Spectroscopy of Nanoparticle Brownian
Motion (MSB)
•Harmonic spectrum of nanoparticles not only influenced by
concentration
•Weaver group shows that nanoparticle temperature, the
viscosity, and molecular binding influence ratio of harmonics
•Magnetic nanoparticle material, size, shape, and coating
determines static response to applied alternating magnetic field
method of characterizing nanoparticle environment.
•Want to localize the magnetic field so that detection can be made on
fewer nanoparticles
•Propose using a magnetic iron core that will pull in the magnetic fields
generated in the inner coils
•Iron core will pull in magnetic field to localize
Iron Core
•Next step would be increase the number of turns to hundreds of turns so
that the device can detect nanoparticle responses. However, it still remains
to be seen whether or not the signal from nanoparticles is significantly
improved from the current coil design without the iron core. This will tell us
whether or not the balancing coil is sufficient enough in canceling out the
magnetic field in the pickup coil. If the signal from the sample is much
greater than the induced voltage in the receiving coil system, then it may not
matter that the pickup coil and balancing coil do not cancel each other out
perfectly. However, if the signal is not significantly larger, a method to cancel
out the fields beyond just adjusting the turns in each coil is needed.
Acknowledgements
•Thayer School of Engineering, Dartmouth College
•Department of Radiology, Dartmouth-Hitchcock Medical Center
•National Science Foundation REU
•Department of Defense ASSURE
Magnetization Theory
•Hysteresis curve determines magnetization induced in material
by time varying magnetic field
•Treat them as independent, isotropic spins governed by
combination of statistical thermal fluctuations that randomize
the moments and applied magnetic field that align the moments
•Magnetization described by Langevin equation:
Sample
References
magnetic
Drive Field
thermal
Balancing Coil
Pickup Coil
• Rauwerdink, AM., Weaver, JB. “Harmonic Phase Angle as a Concentration-Independent Measure of Nanoparticle
Dynamics.” Medical Physics 37, 2010.
• Rauwerdink, AM., Weaver, JB. “Measurement of Molecular Binding Using the Brownian Motion of Magnetic
Nanoparticle Probes.” Applied Physics Letters 96, 2010.
• Rauwerdink, AM., Weaver, JB. “Viscous Effects on Nanoparticle Magnetization Harmonics.” Journal of Magnetism
and Magnetic Materials 322, 2010.
• Rauwerdink, AM., Hansen, E., Weaver, JB. “Nanoparticle Temperature Estimation in Combined AC and DC
Magnetic Fields.” Physics in Medicine and Biology 54, 2009.