Biomedical Engineering Reference
In-Depth Information
observed by another research group [19], the reported limit of detection was 40
colony - forming unit s ( CFU ) in 10
l of blood [48].
The nature of the response curve observed by the authors was unlike any previ-
ously reported response curves. MRSw response curves typically approach a
change in T
2
of zero as the concentration of the target decreases. For this sensor,
the change in T
2
approached a maximum as the concentration of analyte approached
zero. In order to explain this abnormal binding curve, the authors hypothesized
that the change in T
2
was derived from a mechanism which was different than
that of nanoparticle agglomeration. According to their hypothesis, the change in
T
2
was a function of the proximity between superparamagnetic nanoparticles on
the surface of the target cells. Accordingly, at high cell concentrations, the nanopar-
ticles were distributed between many cells, thereby having a more distant inter-
particle proximity. In addition, at low cell concentrations the nanoparticles were
distributed between only a few cells, and thus had a close inter-particle proximity.
Although the group validated the specifi city of their observed T
2
response to the
desired target cell, they failed to conduct any independent tests and controls to
validate their proposed mechanism for cell detection. Consequently, further
investigations will be required to confi rm the source of these unprecedented T
2
response curves.
More recently, a group in the Weissleder laboratory reported the detection of
intact whole cells with MRSw biosensors; these included bacterial cells from
Staphylococcus aureus
and a variety of mammalian cells. The detection of
S. aureus
was achieved by derivatizing nanoparticles with vancomycin, which binds to
peptide moieties on the bacterial cell wall. Following a 15 min incubation of the
vancomycin-nanoparticles with increasing amounts of
S. aureus
(from 10
0
to 10
3
cells), a linear dose-response curve with a change in T
2
of 30 ms was observed [23].
The group also reported a limit of detection of 10 CFU in 10
μ
l of milk, and 40 CFU in 20
μ
l, and verifi ed that
the nanoparticles were indeed attaching to the cell surface by using TEM and
energy dispersive X-ray spectrometry (EDS). This observation, in combination
with a fairly extensive set of controls, indicated that the T
2
sensitivity arose from
a vancomycin-dependent interaction between the nanoparticles and the cell sur-
faces [23] .
Lee
et al.
also demonstrated the detection of mammalian cells and cell biomarker
profi ling. For this, mouse macrophages were detected via a multistep method that
consisted of incubating the cells with fl uorescein - conjugated, dextran - coated
nanoparticles. Following a 3 h incubation at 37 °C to allow the macrophages to take
up the dextran-coated nanoparticles, the nanoparticle-labeled cells were separated
from any unbound nanoparticles by multiple washing. The resultant solution,
after calibration with a hemocytometer, was used to determine the limit of detec-
tion for nanoparticle-labeled mouse macrophages; this proved to be a single cell
in 10
μ
l, or 100 cells per ml. This multistep approach differed from the method
used to detect
S. aureus
, in that the unbound nanoparticles were separated from
the cell-immobilized nanoparticles. A similar multistep approach was used to
profi le different types of cancer cell by means of various antibody-targeted nanopar-
ticles [23]. Although mammalian cell detection required the inclusion of washing
μ
