Fall 2010 IAB
September 15 to 17
Microfluidic Bioassays with Not-Quite-Superparamagnetic Microspheres
In the magnetic biosensor project, we aim to quantify environmental exposure by combining microfluidics and biochemistry with sensitive CMOS Hall-effect electronics to detect magnetic microspheres. Such small beads can be used to advantage as labels in immunoassays, where the microspheres stick to the sensor surface, their binding revealing the presence of specific biomolecules. Since this works even when the fluid is not optically transparent, and since biological fluids are usually not magnetic, magnetic labeling is potentially very useful. However, permanent magnet labels are clearly not suitable for bioassay purposes since these would attract and clump together in a fluid. Instead such tests usually employ superparamagnetic beads, comprising magnetic nanoparticles small enough to continually flip their magnetization due to thermal fluctuations. An externally applied field is needed to magnetize the beads. The Hall-effect sensors are thus presented with a challenging task: measuring the small change in magnetic field due to the presence or absence of a microsphere by carefully compensating for the large magnetizing field. This makes the detection susceptible to drift. It would be beneficial if the magnetizing field could be switched off during the measurement, and the bead would remain magnetized for a short time so its presence could be detected without background.
Here, we present measurements of the magnetic relaxation of commonly used commercially available superparamagnetic microspheres and show that a possible detection advantage is highly dependent on a suitable choice of beads.