Biomedical Engineering Reference
Fig. 7.5 Demonstration of negligible nonspecific binding of MNPs to the sensor surface. Shown
above are binding curves for carcinoembryonic antigen (CEA) using a traditional sandwich assay.
In this format, the final washing step (of three washing steps in total) is used to remove the
unbound magnetic nanotags. However, as a direct result of the minimal nonspecific binding, the
signal remains unchanged after the final washing step, permitting removal of the final washing step
without distorting the final signal [ 15 ]
MNPs. Importantly, the magnetically responsive sensors operate as proximity-based
detectors of the dipole fields from the magnetic tags. Therefore, unbound MNP
tags contribute negligible signal in the absence of binding, rendering this unique
nanosensor-MNP system ideal for real-time kinetic analysis [ 8 ].
It is apparent from plotting the magnetic moment per particle versus the applied
external magnetic field that there is very minimal coercivity in these particles
confirming their superparamagnetic nature (Fig. 7.6 ). In our experience, these MNPs
have performed the best in terms of the kinetics of binding to detection antibody,
minimal nonspecific binding, and high reproducibility.
In summary, choosing the magnetic label is a vital aspect to any magnetic
biosensor where important design trade-offs must be made. On the one hand, larger,
micron-sized magnetic particles are desirable as they will generate a high signal
per particle. Larger particles, however, are kinetically unfavorable since detection
requires diffusion of the magnetic tags to the surface-immobilized detection anti-
body. Furthermore, larger particles are undesirable because nonspecific binding
events of micron-sized particles will have a much greater effect on the overall signal
than will a nonspecific binding event of a nanoparticle. Accordingly, the optimal
balance is to use MNPs that are comprised of a cluster of small superparamagnetic
particles imbedded in a dextran polymer. This configuration will increase the
magnetic content of each particle while remaining superparamagnetic.