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(ξ_outside−ξ_sensor), 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 immunoassay is presented. The chip design match the prospect of detecting just a few 250 nm beads, and is prepared for use in biological experiments. First the fabrication and results with nickel sensors are presented. Based on these results, the work with exchange bias for constructing an easy direction instead of an easy axis in permalloy planar Hall sensors continues. The nickel sensors are fabricated in MIC’s clean room and constitute a compromise with respect to magnetic material, since only nickel is available in the clean room. However, the nickel sensors prove the concept of magnetic bead detection with planar Hall sensors. The permalloy sensors are optimized with respect to the magnetic material and design of an easy direction. In an easy axis two equivalent magnetic states are present, which give identical electric signals with opposite signs. Exchange coupling of the easy axis to an antiferromagnet yields an easy direction. Thus the magnetization has always the same starting point in the absence of an applied field. This principle is utilized in the optimized sensors. Using DC sensor currents, these sensors can measure magnetic beads without a field applied. The magnetic field generated by the current is sufficient to magnetize the beads, which will be an advantage when designing a point-of-care chip. Using lock-in technique, i.e. AC sensor currents, on average the field produced by the current vanishes. Hence, the bead field measured by the sensor is solely produced by the applied field. The planar Hall sensors’ electronic noise is determined in a setup where external noise is reduced. Exactly the low noise level contributes to the high sensor sensitivity. However, the setup can be improved in several ways. A preamplifier for the lock-in amplifier together with increasing the applied field and the sensor current, would yield a factor 50-100 on the bead signal-to-noise. This is sufficient to offer single 250 nm bead detection with the studied sensors. Hereafter, measurements of sensor signal versus antibody concentration are presented. Biotinylated influenza antibodies are immobilized on the surface of a planar Hall sensor. Afterwards magnetic beads with streptavidin surface coating bind to the biotin on the antibodies. The signal is increasing for increasing antibody concentration. Maximum value is β = 0.011 (normalized to the applied field), i.e. of the same order of magnitude as the theoretical estimate. Finally, the actual influenza immunoassay detection is presented. First fluorescence detection with directly labelled detector antibodies. Second magnetic detection, where the total sandwich assay is performed on top of the planar Hall sensors. The detection principle is thus magnetic beads with influenza antibodies on the surface. S/NC is obtained as the signal from the sample (S) divided by the signal from the negative control (NC). S/NC = 2 is found for the magnetic immunoassay, and S/NC = 5 for the fluorescence immunoassay. The fluorescence assay is optimized, the magnetic is not. The results obtained with the planar Hall sensors are promising for point-of-care diagnostics. However, the SU-8 design can be developed further.