Reminder: University Oral Examination Announcement for Guanxiong Li

Guanxiong Li lgx at
Mon Oct 4 09:07:12 PDT 2004

************ REMINDER ************
University Oral Examination

Title: Spin Valve Sensor for Biomolecular Identification: Design,
Fabrication and Characterization

Guanxiong Li
Department of Materials Science and Engineering

Date: Tuesday, October 5, 2004
Time: 1:30 pm
 Place: McCullough 335
(Refreshments will be served at 1:15 pm)

Biomolecular identification, e.g. DNA recognition, has broad applications in
biology and medicine such as gene expression analysis, disease diagnosis and
DNA fingerprinting. We have been developing a magnetic biodetection
technology based on the giant magnetoresistive spin valve sensors and the
magnetic nanoparticles (< 20 nm in diameter) as biomolecular labels in an
effort to provide a highly sensitive, quantitative, portable and
cost-effective biomolecular identification device. This dissertation is
concentrated on the design, modeling, fabrication and characterization of
the spin valve sensors, aiming to prove the magnetic biodetection concept
and demonstrate the feasibility of the magnetic nanoparticle detection by
the spin valve sensors.

The intended magnetic nanoparticle labels are superparamagnetic at room
temperature with zero magnetic remanence. Therefore, the nanoparticles need
to be magnetically excited in order to generate magnetic fringing fields
detectable by the field-sensitive spin valve sensors. DC or AC magnetic
excitation can be applied, for which we have designed several nanoparticle
detection schemes. An analytical model has been developed for the magnetic
nanoparticle detection, based on the two assumptions of (1) the equivalent
average field of magnetic nanoparticles and (2) the coherent rotation of
spin valve free layer magnetization. Micromagnetic simulations have also
been performed for the spin valve sensors. The analytical model is found
consistent with the micromagnetic simulations and can be used as an
efficient design tool.

The prototype spin valve sensors have been fabricated at both micron and
submicron scales. We first demonstrated that the micron-sized spin valve
sensors successfully detected a single 2.8-um magnetic microbead. To
demonstrate the detection of magnetic nanoparticles, we developed a bilayer
liftoff process, based on a polymer-mediated self-assembly and fine
lithography, to deposit magnetic nanoparticles onto sensor surface in a
controlled manner. With this liftoff process, we successfully detected the
monodisperse 16-nm Fe3O4 nanoparticles in a quantity from tens to hundreds
using submicron spin valve sensors at room temperature. A linear dependence
of the spin valve sensor signal on the number of nanoparticles has been
found, as desired for the quantitative biodetection. The detection limit was
found to be 14 Fe3O4 nanoparticles with the current detection scheme.
Preliminary detection results on the nanoparticle-labeled DNA hybridization
events will also be presented.

Materials Science and Engineering
Stanford University

Office: 208 McCullough Bldg.
Phone: 650-723-2939
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