Magnetoresistive Biosensors use a new detection method for molecular recognition reactions based on two recently developed techniques and devices: Magnetic markers and XMR –sensors, where XMR means either GiantMagneto- (GMR) or Tunneling-MagnetoResistance (TMR). The markers are specifically attached to the target molecules, and their magnetic stray field is picked up by the embedded magnetoresistive sensor as a change of the electrical resistance. Compared to established, e.g. fluorescent, detection methods, magnetic biosensors have a number of advantages, including low molecular detection limits, flexibility and the direct availability of an electronic signal suitable for further automated analysis. This makes them a promising choice for the detection units of future widespread and easy to use lab-on-a-chip systems or biochips.
Both the measurement technique using XMR-sensors as well as new developments in the preparation of magnetic carriers are discussed here. Different configurations are discussed and the results for Giant Magnetoresistance sensors are compared to an analysis of the same biological systems marked with fluorescence dyes. Down to a concentration of about 10 pg/μl of, e.g., DNA molecules, the magnetoresistive technique is competitive with nowadays standard analysis methods. The capability of the TMR sensors to detect even single markers is additionally demonstrated by a model experiment using the tip of a magnetic force microscope to meamic the presence of a magnetic particle on top of the sensor surface.
The magnetic carriers (beads) usually detected by the sensors consist of paramagnetic magnetite particles embedded in a polymer matrix with sizes from some μm down to about 100nm. They are linked to, e.g., DNA or proteins (often by a avidin-biotin bond) and thereby enable highly specific detection of complementary molecules. These magnetic particles often suffer from their broad size distribution and the relatively small magnetic moment. With the new colloidal synthesis of superpara- or ferromagnetic Co, CoFe and FePt nanocrystals by, e.g., pyrolythic decomposition of CVD precursor molecules, magnetic markers with superior magnetic moments, smaller size and size distribution can be produced. Here, the question about their potential to replace magnetite is addressed. Starting from a magnetic analysis of the corresponding magnetophoretic mobility of Co and FeCo based alloys their synthesis and resulting microstructural and magnetic properties as function of the underlying particle size distribution and the stability of the oleic acid ligand are discussed.
Moreover, the magnetic particles offer an additional feature: They can be manipulated on chip via currents running through specially designed line patterns. We show, that this manipulation can be performed in a precise and reproducible manner, enabling locally enhanced concentration or even the measurement of binding forces with very low loading rates.