Biomedical Engineering Reference
In-Depth Information
of these nanoparticles, in terms of magnetic relaxation biosensing, will clarify the
apparent discrepancy in the observed change in T
2
.
1.6.2
Detecting Proteins
The general applicability of MRSw design introduced with DNA detection was
quickly demonstrated for other types of analyte. A team led by J. M. Perez, whilst
at MGH, demonstrated the capability of MRSws for measuring proteins by attach-
ing biotinylated anti-GFP polyclonal antibodies to the surface of avidin-function-
alized, dextran-coated SPIO nanoparticles. When GFP was introduced to a solution
of anti-GFP decorated nanoparticles (45 nm diameter), there was a time- and dose-
dependent response in the measured T
2
values. After about 30 min, however, the
T
2
signal stabilized, indicating that the clustering reaction had reached completion.
Concentrations of GFP as low as single nanomolar were detected with T
2
changes
on the order of 10- 20 ms [45] .
Subsequent studies led by S. Taktak at MGH demonstrated that proteins could
be detected by decorating the nanoparticles with a ligand for which a multivalent
protein target had a selective binding affi nity. This biosensor was created by func-
tionalizing superparamagnetic nanoparticles with the ligand biotin, such that
there were
70 biotins per nanoparticle [61]. Addition of the tetravalent protein
avidin led to nanoparticle clustering and a change in average particle size, from
30 nm to 150 nm. There was a concomitant increase in
R
2
, from 37 m
M
− 1
s
− 1
to
132 m
M
− 1
s
− 1
, which corresponded to a decrease in T
2
of 135 ms to 38 ms at an iron
concentration of 0.2 m
M
. Taktak
et al.
extensively characterized the biophysical
characteristics of their avidin MRSw system by introducing the concept of report-
ing analyte titrations in terms of the ratio of moles of analyte to moles of nanopar-
ticles. According to their observations, the linear T
2
response for avidin spanned
0.4 and 1.2 avidin per nanoparticle equivalent. This approach, which relies on
knowing the moles of iron atoms per nanoparticle, has been shown to be particu-
larly useful for determining the point at which cluster formation leads to unstable
aggregates [61, 69] .
Taktak
et al.
reported T
2
and
T
2
*
measurements at 1.5 T, 4.7 T, and 9.4 T to
confi rm that their system was in the motional averaging regime, which was con-
sistent with the linear dependence of
R
2
on average particle size (Figure 1.7). From
these data and the available theory, the group predicted that there was a decrease
in cluster magnetization as the cluster size increased, and an accompanying
increase in volume fraction of the clusters during agglomeration. These results
were consistent with the clusters being porous fractal aggregates, as determined
by other theoretical and experimental observations [59, 61]. The use of this avidin
MRSw sensor for the development of new sensitivity enhancement methods is
discussed in greater detail below.
The protein hormone human chorionic gonadotropin (hCG) was detected by
antibody-decorated nanoparticles by a team in Michael Cima's laboratory at the
MIT [69]. hCG functions as a clinical pregnancy marker, and is also overexpressed
∼
