University Ph.D. Oral Examination - Wei Hu

Wei Hu mse.whu at stanford.edu
Thu May 22 14:27:36 PDT 2008


High-Moment Synthetic Magnetic Nanoparticles for Biomedical Applications

Wei Hu
Materials Science & Engineering
Advisor: Prof. Shan X. Wang

Thursday, June 5th, 2008
9:30 AM (Refreshments served at 9:15 AM)
McCullough Bldg. Room 335

Abstract:

Superparamagnetic nanoparticles are widely used in biology and  
medicine for applications which include biomolecule purifications and  
cell separations, magnetic resonance imaging (MRI) contrast agents,  
and bio-magnetic sensing. These nanoparticles are usually synthesized  
by chemical routes, which are powerful but the size of nanoparticles  
are typically below 20 nm due to the superparamagnetic limit. Beyond  
this size, it is difficult to attain monodispersity and the onset of  
ferromagnetism results in coercivity, remanent magnetization and  
consequently magnetically induced agglomeration. Magnetic  
nanoparticles with higher moments are often desired to produce large  
signals or to avoid restrictive requirements for high magnetic field  
gradients in separations. One conventional solution is to incorporate  
numerous magnetic nanoparticles into larger composites using matrices  
comprised of dextran or silica. However, there are still limitations  
associated with controlling the monodispersity, magnetic response and  
variations in the number and size of the embedded nanoparticles.

In this talk, I?ll present the physical fabrication of sub-100 nm  
monodisperse disk-shape synthetic nanoparticles with high  
magnetization ferromagnetic multilayers (e.g. Co-Fe alloy) using  
nanoimprint lithography (NIL) and high vacuum deposition, followed by  
release and stabilization of nanoparticles in solution.  
Antiferromagnetic interlayer interactions are exploited to achieve  
zero remanence and thus these nanoparticles are termed synthetic  
antiferromagnetic (SAF) nanoparticles, which posses magnetic moments  
well above those typical of superparamagnetic nanoparticles.

Unlike the chemical synthesis of magnetic nanoparticles, physical  
fabrication enables accurate control of particle shape, size and  
composition, and thus synthetic nanoparticles possess a lot of  
interesting properties which are not readily accessible to  
conventional superparamagnetic nanoparticles. For example, I  
demonstrate SAF nanoparticles with adjustable saturation fields, which  
are desired for multiplex magnetic labeling in biodetection or  
multiplex cell sorting. Their high magnetic moments afford great ease  
for magnetic manipulation in solutions with only modest field  
gradients, which is highly desired for magnetic sorting. Metallic  
synthetic nanoparticles strongly scatter light and can be individually  
tracked in solution under optical microscopy.

To further evaluate their application potential for biomedicine, we  
performed bio-magnetic detection with streptavidin functionalized SAF  
nanoparticles. A low concentration of analyte DNA molecules at 10 pM  
was clearly detectable. MRI measurements of nanoparticle enhanced  
proton transverse relaxation revealed that SAF nanoparticles are  
promising as contrast enhancement agents. In addition, hysteresis  
measurements indicate that magnetic nanoparticles with vortex domain  
structure (a second type of synthetic nanoparticles) could be  
efficient heating elements for magnetic nanoparticle hyperthermia.

Last but not least, large scale fabrication of SAF nanoparticles with  
low cost and high throughput is achieved using self-assembled stamps  
and a polymer sacrificial layer with the assistance of batch-process  
thermal evaporation. This fabrication technique is ideal for producing  
multi-modal nanoparticles by exploiting layers with unique magnetic,  
optical, radioactive, or electronic properties.







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