Biomedical Engineering Reference
In-Depth Information
lead to cross-reactions. Directed coupling with SPDP led to increased affi nities and
increased binding densities, as had been shown previously for other applications
[81]. The increased affi nity and valency of the particles led to a 16-fold increase in
the performance of the nanoparticle sensor for cell internalization. Although these
studies focused on targeting superparamagnetic nanoparticles for cell encapsula-
tion with endosomes, the dependence of performance on the bioconjuga-
tion method may be generalized for nanoparticle- based sensors [81] . Similar
observations were made for a cell-targeted biosensor that used multivalent-RGD-
decorated nanoparticles to bind cell-surface integrin proteins [85].
Several other reports have been made of the methods used for coupling
targeting groups to polymer-coated, superparamagnetic nanoparticles. Some
of these have utilized bifunctional nanoparticles, such as fl uorescently labeled
CLIO nanoparticles, to conduct parallel synthesis and high-throughput screening
(HTS) on large numbers of nanoparticles for cell recognition applications
[86]. Robotic systems have also been used to conjugate 146 different small
molecules (
500 Da) to fl uorescently functionalized CLIO nanoparticles [87], with
the average coupling ratios being 60 small molecules per nanoparticle. These
nanoparticle conjugates were also screened for eukaryotic cellular uptake, thus
demonstrating that nanoparticle surface modifi cation can target nanoparticles not
only to different cell types but also to different physiological states of the same cell
type [88] .
<
1.7.2
Measurement and Sensitivity Enhancement Methods
A variety of reports have introduced new methods for measuring MRSw assays to
improve measurement accuracy and sensitivity. One such report, made by a team
led by S. Taktak, utilized a biotin-avidin model system to examine the physical
characteristics of the MRSw biosensor system [61]. The model system was created
from biotinylated nanoparticles that agglomerated in the presence of the tetra-
meric protein avidin. Upon the addition of avidin, the T
2
changed from 100 ms to
40 ms after incubation at room temperature for 1 h. Taktak
et al.
showed that the
overall average cluster size increased linearly with the addition of avidin, and that
the observed
R
2
depended linearly on the average particle size [61]; these fi ndings
were similar to previous results which demonstrated avidin-coated particles and a
bi-biotinylated peptide [47]. This relationship corresponded to these nanoparticles
being within the motional averaging regime (i.e., on the left side of the curve in
Figure 1.7). This correlated well with observations for other MRSw systems [69],
and the proposed porous fractal nature of nanoparticle aggregates [60, 61]. Based
on these observations, Taktak
et al.
predicted that, as cluster size increases there
should be a decrease in the cluster magnetization and increases in the cluster
volume fraction [61].
In addition to exploring the fundamental physics that underlie MRSw, Taktak
et al.
introduced some new methods for improving assay performance. The fi rst
method consisted of tuning the dynamic range and sensitivity of the assay by
