Single molecule actuation and detection on a lab-on-a-chip magnetoresistive
R. C. Chaves,
and P. P. Freitas
INESC-MN/Institute for Nanosciences and Nanotechnologies, R. Alves Redol 9, 1000-029 Lisbon,
Portugal and Physics Department; Instituto Superior Te´cnico, Universidade Te´cnica de Lisboa,
1049-001 Lisbon, Portugal.
Laboratoire de Physique Statistique, Ecole Normale Supe´rieure, 24 rue Lhomond, Paris 75005, France
(Received 24 November 2010; accepted 31 January 2011; published online 17 March 2011)
On-chip magnetic tweezers based on current loops were integrated with magnetoresistive sensors.
Magnetic forces up to 1:0 6 0:3 pN are produced to actuate on DNA anchored to the surface of a
ﬂow cell and labeled with micrometer-sized magnetic beads. The levitation of the beads stretches the
immobilized DNA. The relative position of the magnetic beads is monitored using spin-valve sensors.
A bead vertical displacement resolution of 60 nm is derived for DNA molecular motor activity in a
tweezer steady current regime.
2011 American Institute of Physics. [doi:10.1063/1.3560853]
DNA is a molecule that is processed (copied, transcribed,
repaired, unwound, unlinked, etc.) by many enzymatic motors
that are essential to the cell.
These molecular motors bind to
DNA and by pulling, twisting, or unwinding it they modify its
end-to-end distance. This activity can be monitored in real
time by measuring the length of a DNA tether anchoring a
magnetic bead to a surface and pulled by an external magnetic
ﬁeld acting as magnetic tweezers.
Here we show that a
magnetoresistive sensor can be used to deduce the change in
the bead to sensor distance (i.e., the change in DNA length)
by measuring the dipole-ﬁeld of the magnetic bead. This elec-
trical readout with high throughput capability is an alternative
to the optical read-out which requires a microscope for the
existing magnetic tweezers systems.
The proposed single
molecule detection cell
shown in Fig. 1 includes an on-chip
version of a magnetic tweezer based on a current loop and
includes a magnetoresistive sensor to detect the vertical move-
ment of the magnetic bead attached to the stretched DNA.
A. The magnetic tweezer
To miniaturize the magnetic tweezer setup we have
implemented it using micro-lithographic techniques. A
u ¼ 16 6 0:1 lm inner diameter current loop on top of a
baked photoresist mesa 4:5 6 0:2 lm thick generates a mag-
netic ﬁeld gradient on a magnetic bead anchored by a single
DNA molecule to the bottom resulting in a vertical magnetic
force pointing to the center of the current loop.
For a paramagnetic bead with magnetic susceptibility
v ¼ 0:42 (S.I.) the magnetic dipole moment
m ¼ V
M ¼ V
¼ 1:4 Â 10
is the bead’s volume and the
is perpendicular to the sensor
(aligned along z, in order to not interfere with the detection
by the magnetoresistive sensor of the ﬁeld of the bead).
Indeed the bead generates a magnetic ﬁeld on the sensor
The sensor will be sensitive to the longitudinal component of
this so-called fringe ﬁeld.
The magnetic force acting on the bead is then,
The predicted gradient produced by the loop is
¼ 6:0 6 1:8Â 10
=m along the z axis
for a dc current I
¼ 40 mA and for the geometry of the
detection cell (see Fig. 1). On a magnetizable bead (diameter
u ¼ 3 lm), the resulting average vertical magnetic force is
¼ 1:0 6 0:3 pN along the z axis and
a distance 3:7 lm between the sensor free layer and the bead
center (and at 1:6 lm from the center of the loop).
These mesa current loops are also used as traps for the
magnetic beads during their injection. In addition, if the
DNA is labeled prior to immobilization, then DNA immobi-
lization can be magnetically assisted by the current loop pre-
venting adsorption and ﬁxing the bead position. The current
loop geometry was optimized to be symmetrical in order to
eliminate any in-plane transverse ﬁeld component at the
magnetoresistive sensor level.
B. Tapered current lines
The platform also contains tapered current lines in close
proximity to the sensor stripe (see Fig. 1) to generate an ac
magnetizing ﬁeld on the bead, and to further localize beads
trapped at the bottom of the mesa well. When activated, this
pair of current lines can be used to attract the magnetic beads
to the center of the detection area. At the sensor level, the
ﬁeld that is produced has only a vertical component and to a
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2011 American Institute of Physics109, 064702-1
JOURNAL OF APPLIED PHYSICS 109, 064702 (2011)