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.
--
Wei Hu
Ph.D. Candidate
Department of Materials and Engineering
Stanford University
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