Optimizing the geometry of an in vitro tunneling magnetoresistance biosensor using an immobilized ferrimagnetic nanoparticle agent

Abstract

Spatial interactions of the magnetic field produced by an immobilized ferrimagnetic nanoparticle agent on the sensing layer have been numerically calculated considering the longitudinal and transverse components of the magnetic field. Based on the calculations, the sensor geometry to obtain the maximum sensing performance of an in vitro exchange biased tunneling magnetoresistance (TMR) based biosensor can be determined prior to fabrication. Numerical analysis considering the one-dimensional (longitudinal) component of the magnetic field along the magnetization of the ferrimagnetic nanoparticle agent found that the geometrical parameters of the biosensor can be determined by simply considering the effective distance (δ). The effective distance (δ) is determined by the remnant magnetization and radius of the ferrimagnetic nanoparticle agent and the detectable field limit (BDL) relevant to the exchange bias field of the TMR sensor. The optimized sensor geometry in terms of the critical length (lc) and width (wc) is approximated as wc / lc ≈3.14. For a more accurate optimization of the sensor geometry, the two-dimensional magnetic field distributions on the sensing surface are numerically analyzed by employing the astroid curve model (or Stoner-Wohlfarth model). According to the numerical results using this model, the effective sensing area is found to have been extended due to the transverse component of the magnetic field. The extended effective sensing area in the optimized sensor geometry resulting from the spatial field interaction caused by the two-dimensional magnetic field is expected to enhance the output signal of the in vitro TMR based biosensor. However, in considering the two-dimensional magnetic field distribution, the undetectable area formed in the vicinity of the center of the optimized sensing area due to the spatial field interaction is a problem in designing a more accurate sensor geometry. Making a TMR sensor with a larger exchange bias field and introducing an especially designed sensor structure with a magnetic shield with a high permeability are suggested as solutions to this technical problem. © 2008 American Institute of Physics.