Techniques to identify the magnetic susceptibilty of subcellular organelles: a feasibility analysis.

Abstract

Magnetic susceptibility is the degree of magnetization in response to an applied magnetic field. While the expected magnetic susceptibility of subcellular components is on the order of 9.05x10-6, variations in subcellular magnetic susceptibility as low as 10% can create forces in the pN range within a cell when high intensity magnetic fields are imposed. The objective of this study is to investigate the feasibility of two techniques to measure the magnetic susceptibility of subcellular organelles. The sensitivity and accuracy of resonance frequency shifts of microcantilever beams and magnetophoresis are evaluated to ensure magnetic susceptibility measurements can be obtained within the expected range for subcellular organelles. The resonance frequency detection method was tested using a bismuth particle and permanent magnets (B*dB/dz = 0.54 T2/m) to determine an uncertainty of 0.013 Hz. To determine the technique feasibility for application to subcellular organelles, a theoretical examination of resonance frequency shifts due to the application of magnetic field and gradient products varying from 0.54 T2/m (permanent magnets) to 1300 T2/m (superconducting magnet) was conducted for silicon, silicon nitride and polymer beams of varying dimensions. Results indicate that only a polymer beam can produce a detectable resonance frequency shift that is greater than the experimentally determined uncertainty. However, an uncertainty smaller than 0.009 Hz is required to sense the differences in resonance frequency shifts due to a 10% variance in magnetic susceptibility. In summary, the microcantilever beam approach cannot achieve sufficient sensitivity to detect the predicted differences among different subcellular organelle types. ii Tests implementing magnetophoresis for polystyrene test particles with a 100 μm diameter explored the sensitivity and accuracy effects of varying fluid flow speeds of 0.63, 1.09, and 1.44 mm/sec, different particle radius to channel depth ratios (r/a) of 0.043 and 0.199, and a magnetic field and gradient product (B*dB/dz) of 38.91 T2/m. Particle speed due to the magnetic field was recorded by a CCD camera with 32.68 pixels/mm resolution. The percent uncertainties of the experimental magnetic susceptibilities are 12.3, 18.3 and 22.4% (in order of flow speed). The trial runs were also used to determine that the smallest detectable magnetic velocity for the detection method is 3.16 ± 15 μm /sec and the smallest detectable particle diameter is 31 μm. The trial runs indicate that a balance of a larger r/a ratio and a slower flow speed is ideal to optimize consistency in flow velocities and calculated magnetic susceptibilities while minimizing uncertainty. A theoretical examination of organelle magnetic velocities due to the application of magnetic field and gradient products of -38.91 T2/m and -1300 T2/m indicates that the nuclei and large mitochondria are detectable in either magnetic field while the small mitochondria is detectable only in the larger magnetic field. In addition, all organelles have diameters smaller than 31 μm rendering them undetectable by the current visualization method. The following recommendations are necessary to detect the magnetic susceptibilities of subcellular organelles through magnetophoresis with sufficient accuracy: use the larger B*dB/dz (-1300 T2/m), reorient the channel or incorporate a level so that gravity is not a factor, and use a higher quality velocity detection method.