“…The peak amplitude of χ″(ω) is shifted to a lower frequency when magnetic particles in solution bind target molecules, increasing the effective hydrodynamic radius and, thus, the Brownian rotational diffusion time ( Figure A-1). For particle radii of approximately 25-350 nm, a change in susceptibility can be directly measured using relatively simple equipment via changes in the inductance and/or resistance detected by an antenna coil in the presence of a small external variable frequency AC magnetic field (Hoffmann et al, 2007). At larger radii, peaks in χ″(ω) may disappear altogether, providing the opportunity for indirect detection of even larger analytes such as viruses, spores and bacteria.…”