Harmonic Simulation Study of Simultaneous Nanoparticle Size and Viscosity Differentiation

The signal harmonics of magnetic nanoparticles in solution under an ac magnetic field are highly sensitive to their physical properties and the surrounding environment. Here, we use Monte Carlo simulations of coupled stochastic Langevin equations to demonstrate a possible technique that can simultaneously characterize different particle sizes and viscous environments. When combined with a constant bias field, the relative amplitudes of the harmonics change significantly, and are shown to be uniquely sensitive to changes in particle size and viscosity. Spectroscopic experiments verify the size-dependent harmonic behavior. With this technique, particles with sizes differing by only ~1-2 nm in diameter can theoretically be identified, while separately quantifying changes in viscosity. This concurrent characterization could be implemented in magnetic particle imaging to create a colorized image.