Biomedical Engineering Reference
In-Depth Information
was indeed due to peroxidase activity [75]. These two MRSw sensors represented
the fi rst example of agglomerative format sensing enzyme activity, and the fi rst
example of a covalently crosslinked nanoparticle assembly.
A similar approach was used by a collaborative group at MIT, the Brigham &
Women's Hospital in Boston, the University of California at San Diego, and the
Burnham Institute. This group, which was led by Todd Harris of the MIT, used
an approach which differed from that of Zhao
et al.
to measure MMP-2 activity
[68]. Rather than detecting MMP-2 by monitoring its proteolysis of a divalent
peptide that has been activated to enable nanoparticle crosslinking, Harris
et al.
decorated two types of nanoparticle coated with biotin or avidin with a cleavable
peptide attached to polyethyleneglycol (PEG). The bulky PEG groups inhibited
binding between the biotin and streptavidin. The addition of MMP-2 led to cleav-
age of the peptide linker that attached PEG to the nanoparticles, thus exposing the
biotin and avidin coatings so that the nanoparticles could self-assemble into clus-
ters [68]. This indirect agglomeration approach was analogous to that used for
detecting peroxidases, namely that the nanoparticle surface groups are activated
by the presence of a target enzyme to facilitate particle agglomeration. Harris
et al.
reported a limit of detection of 170 ng ml
- 1
(9.4 U ml
- 1
) and a T
2
change of
150 ms at 4.7 T after a 3 h incubation. Their dispersed nanoparticles were 50 nm
in diameter prior to coating with the PEG-peptide, and could be separated magneti-
cally, which indicated that they were indeed different in nature to those used by
Zhao
et al.
The MMP-2 cleavable peptide was eight residues long, and PEG chains
of 2 kDa, 5 kDa, 10 kDa, and 20 kDa were tested. PEG sizes below 10 kDa did not
inhibit biotin-avidin-mediated particle agglomeration in the absence of MMP-2
[68]. The sensitivity of this sensor was within the concentration range of MMP-2
typically found in tumor cells.
These demonstrations of enzyme detection showed not only the feasibility of
detecting enzyme activity in both direct and indirect ways, but also the architec-
tural fl exibility of MRSws that would in turn allow designers to tailor these
nanoparticle assays in a specifi c manner. Although, to date such fl exibility has
enabled the broad application of MRSws, as their development continues it is most
likely that such fl exibility that will lead to not only a wide range of applications but
also excellent performance and sensitivity.
1.6.4
Detecting Viruses
One of the most impressive applications of MRSws has been the detection of the
herpes simplex virus (HSV-1) and adenovirus (ADV) [46]. Due to the multivalent
nature of these analytes, extremely low concentrations of virus could be detected
in serum; in fact, a limit of detection as low as fi ve viral particles in 10
l was
achieved, which is subattomolar in terms of viral concentration. These viral sensors
were constructed by decorating superparamagnetic nanoparticles with monoclonal
antibodies by means of a protein G coupling method. The monovalent antibodies
bound selectively to coat proteins on the surface of either HSV-1 or ADV. The
μ
