Magnetic-based biomolecule detection using giant magnetoresistance sensors

Abstract

This paper presents a novel microfluidic chip for in-vitro detection of biomolecules tagged by magnetic microparticles (MAPs) suspended in a static fluid. The system consists of two microfluidic channels: a reference channel in which bare MAPs are suspended and a detection channel in which magnetically tagged biomolecules are suspended (LMAPs). The LMAPs are functionalized MAPs (of the same magnetic volume as the ones in the reference channel) with attached biomolecules. The overall, non-magnetic volume of the LMAPs is greater than that of the bare MAPs. Current carrying microconductors are positioned underneath the channels in order to impose a magnetic field gradient to the MAPs and LMAPs and move them from the inlet to the outlet of the channels without flow. The innovative aspect of the proposed method is that the induced velocity on the MAPs and LMAPs, while imposed to the same magnetic field gradient, is inversely proportional to their overall, non-magnetic volume. This is due to the enhanced Stokes drag force exerted on the LMAPs, resulting from the greater volume and altered hydrodynamic shape. This induced velocity is measured by utilizing Giant Magnetoresistance (GMR) sensor pairs fabricated underneath the first and the last microconductors. Detected differences in velocity between the LMAPs and the reference MAPs indicate the presence of biomolecules in the static liquid sample. We also present a novel method for signal acquisition and demodulation: expensive function generators, data acquisition devices, and lock-in amplifiers were substituted by a generic PC sound card and an algorithm combining the Fast Fourier Transform of the signal with a peak detection routine. Experiments with functionalized MAPs and magnetically tagged Escherichia coli (representing the LMAPs) were carried out as a proof of concept. In order to identify the detection limit of the GMR sensor, single MAP (2.8 μm diameter) detection was performed.