Biomedical Engineering Reference
In-Depth Information
(a)
(b)
(c)
Figure 2.3
(a) Section through a two -
dimensional array of magnetoresistive sensing
elements. Each sensing resistor (SR) is
located under one spot of probe molecules in
the array; (b) Section through individual
sensing element in a giant magnetoresistive
(GMR) array. Key: 1 = silicon substrate;
2 = silicon nitride buffer; 3 = sensing resistor;
4 = gold fi lm for anchoring probe molecules;
5 = sensing surface with probe molecules
bound to magnetic labels. The inset shows an
enlarged section through the multilayer
structure of the sensing resistor; (c) Scheme
showing how the direction of the induced
dipole fi eld surrounding a magnetic label
(broken blue lines) opposes the applied fi eld
in the region where it interacts with the
sensing resistor. The resultant decrease in
fi eld strength experienced by the sensing
resistor leads to a decrease in electrical
resistance.
been captured by the sensing surface, a small sensing current is passed through
the resistor and an alternating magnetic fi eld applied in a direction perpendicular
to the surface. The latter magnetizes any bound superparamagnetic labels and
induces a dipole fi eld in their vicinity (Figure 2.3c); this in turn causes the direc-
tion of the magnetization in the ferromagnetic layers to rotate and to become more
aligned. This leads to a decrease in resistance which is proportional to the number
of magnetic labels captured by the sensing surface. The possibility of using the
GMR effect for detecting magnetic labels was fi rst described by Baselt and col-
leagues in 1998 [19], and subsequently the concept has been developed by other
research groups, notably at the Naval Research Laboratory in Washington DC,
where arrays of GMR sensors have been fabricated and used for multiplexed
immunoassays and nucleic acid assays [20, 21] .
Spin valve sensors
are also based on the GMR effect, but in this case each sensing
resistor is composed of only two ferromagnetic layers, separated by a nonmagnetic
metallic layer. The direction of the magnetization in the magnetic layer furthest
from the sensing surface is pinned by interfacing it with a strong antiferromag-
netic material. When magnetic labels bind to the sensing surface they cause the
