Spin valve sensor for biomolecular identification: Design, fabrication, and characterization

Biomolecular identification, e.g., DNA recognition, has broad applications in biology and medicine such as gene expression analysis, disease diagnosis, and DNA fingerprinting. Therefore, we have been developing a magnetic biodetection technology based on giant magnetoresistive spin valve sensors and magnetic nanoparticle (<20 nm in diameter) biomolecular labels in an effort to provide a highly sensitive, quantitative, portable, and cost-effective biomolecular identification device. This dissertation is concentrated on the design, modeling, fabrication, and characterization of the spin valve sensors, aiming to prove the magnetic biodetection concept and demonstrate the feasibility and sensitivity of the magnetic nanoparticle detection by the spin valve sensors.; The intended magnetic nanoparticle labels are superparamagnetic at room temperature with zero magnetic remanence, and thus need to be magnetically excited in order to generate magnetic fields detectable by the field-sensitive spin valve sensors. Either DC or AC magnetic excitation can be applied, and we have designed several nanoparticle detection schemes. An analytical model has been developed for the magnetic nanoparticle detection, assuming the equivalent average field of magnetic nanoparticles and the coherent rotation of spin valve free layer magnetization. Micromagnetic simulations have also been performed for the spin valve sensors. The analytical model and micromagnetic simulations are found consistent with each other and are in good agreement with experiments.; The prototype spin valve sensors have been fabricated at both micron and submicron scales. We demonstrated the detection of a single 2.8-mum magnetic microbead by micron-sized spin valve sensors. Based on polymer-mediated self-assembly and fine lithography, a bilayer lift-off process was developed to deposit magnetic nanoparticles onto the sensor surface in a controlled manner. With the lift-off deposition method, we have successfully demonstrated the room temperature detection of monodisperse 16-nm Fe3O 4 nanoparticles in a quantity from a few tens to several hundreds by submicron spin valve sensors, proving the feasibility of the nanoparticle detection. As desired for quantitative biodetection, a fairly linear dependence of sensor signal on the number of nanoparticles has been confirmed. The initial detection of DNA hybridization events labeled by magnetic nanoparticles further proved the magnetic biodetection concept.