Planar Hall Sensor for Influenza Immunoassay

The title of this thesis is Planar Hall sensor for influenza immunoassay. The thesis considers fabrication, characterization and demonstration of planar Hall sensors for influenza immunoassay detection. The goal of this research project is, first of all, to design a magnetic sensor capable of detecting magnetic beads. These beads are polystyrene spheres with sizes ranging between a few hundred nanometers to a few micrometers, and can be magnetized in an applied field. In order to integrate this sensor into a device for clinical tests the sensing principle has to be sensitive as well as reliable. A signal-to-noise study for various sensor types, GMR, spin-valve, and planar Hall sensors, points towards the planar Hall effect as a promising sensing principle for DC detection. DC detection will probably be easier to handle on a chip than AC detection. A second goal is to demonstrate a relevant application, where conventional techniques have failed. Statens Serum Institut has provided antibodies and antigens for the demonstration of influenza detection. The theoretical analysis of single bead signal shows that the planar Hall sensor has potential for single bead detection. The active area of the sensor has to be designed to match the specific bead, and using areas below 1μm×1μm offers the possibility of detecting a single 50 nm bead. The theoretical detection limit as a function of sensor size presents great possibilities for the planar Hall sensor, the main reason being its low noise level. Following the single bead study, theoretical investigation of the specific sensor and bead combination used in the experimental part of this thesis is presented. The fabricated sensors 20μm×20μm are used with 250 nm Nanomag-D beads. These sensors and beads are used for the influenza experiments. The approximate theoretical signal from a monolayer of beads is β ≈ 0.024(ξoutside−ξsensor), where ξ is the layer coverage. β is the field produced by the beads normalized to the applied field and can be compared to the experimental data. Furthermore, the effect of screening the positive contribution from the total signal by placing a simple barrier adjacent to the sensor cross is evaluated. The barrier can be constructed in SU-8, which is used for attachment of biochemical species. Theoretically, the signal from a monolayer of magnetic beads can be enhanced by this procedure. Additionally, the capture of beads by fringing fields produced at the voltage leads can be reduced. The fringing fields capture beads at areas insignificant to the measurements but biologically active material would be lost at these capture sites. Next, design, fabrication and characterization of planar Hall sensors for the influenza