Monitoring of Superparamagnetic Particle Sizes in the Langevin Law Regime

In this work, aqueous solutions of magnetite nanoparticles (NPs) are studied. Magnetite NPs are very useful in biomedicine for magnetic resonance imaging (MRI), for drug delivery therapy, and also for hyperthermia. In order to predict the NP efficiency in these applications, it is crucial to accurately characterize their size distribution and their magnetization. Magnetometry, through the dependence of NP magnetization on the magnetic induction (MB curve), can provide interesting information on these physical properties. In this work, the extraction of the NP size distribution and magnetization from experimental MB curves of aqueous solutions of magnetite NPs is discussed. The results are compared to TEM and XRD characterizations. It is shown that an expression taking into account the size distribution better fits the results than the commonly used simple Langevin function. The size distributions obtained by magnetometry seem comparable to those obtained by TEM measurements. However, a closer look at the results shows some nonnegligible discrepancies: the size distributions obtained by magnetometry vary with the temperature and are closer to the TEM ones at room temperature. Our study suggests that it could be explained by the nonnegligible anisotropy energy of the NPs at low temperature and the lack of NP Brownian rotation below the freezing point of water. This demonstrates that care must be taken when interpreting the results obtained by magnetometry of magnetite NPs: only the size and size distribution obtained at room temperature should be used.

Approach to a Limiting Cycle in Thermally Relaxing Media

The thermally activated dynamics of an uniaxial superparamagnetic particle are studied here from the point of view of approach towards a steady state minor hysteresis loop representing a limiting cycle. The particle is initially at an arbitrary state, and is driven by a sufficiently small external applied ac magnetic field till the limiting minor hysteresis loop is reached with a desired precision. The number of periods required to reach the limiting cycle is found to satisfy an Arrhenius-type law, with an exponential dependence on the inverse ambient temperature. We tentatively associate this result with energy dissipation, its origin, however, remains quite unclear.

Circle-to-circle amplification on a digital microfluidic chip for amplified single molecule detection

We developed a novel highly efficient superparamagnetic particle extraction and shuttling protocol and integrated the isothermal circle-to-circle DNA amplification on a digital microfluidic chip for ultra-sensitive DNA detection.