Biomedical Engineering Reference
In-Depth Information
2.4
Rotating Discs
In many cases, methods that allow magnetic labels to be detected with great sen-
sitivity cannot be confi gured to interrogate even low-density 2-D arrays of probe
molecules, because of their size. SQUID sensors consist of two Josephson tunnel
junctions in parallel, in association with a pick-up coil (antenna) that focuses the
signal from the magnetic labels onto them. Small currents are able to pass through
the junctions without developing a voltage, but when the current exceeds a certain
critical value then a voltage is developed. The magnitude of this critical current
depends on the ambient magnetic fi eld in a way that allows trace amounts of
magnetic label to be detected, although in order to operate the SQUID sensors
must be cooled with liquid nitrogen. The size of the cooling apparatus, and the
need to insulate biological molecules from contact with very low temperatures,
make it impractical to interface SQUID sensors directly with each individual spot
in a 2-D array. Nonetheless, Tsukamoto and colleagues overcame this problem by
locating individual tests around the perimeter of a rotating disc, as shown in
Figure 2.6 [34] . A magnetic fi eld was applied to samples outside the magnetic
shield, and the residual magnetism of the labels was detected as they passed over
the SQUID. This allowed the bound labels to be distinguished from the unbound
Figure 2.6
Schematic representation showing how liquid
samples organized around the rim of a disk are rotated
(in the direction of the red arrow) to locate individual samples
in close proximity to a high-temperature SQUID sensor.
