Studies of magnetic multi-core nanoparticles for biomedical applications

Licentiate thesis, 2009

In recent years, there has been growing interest in using magnetic multi-core nanoparticles in biomedical applications, and particularly in bioseparation and biosensing. In this thesis, two computer simulation techniques have been implemented to study the magnetic response of these particles, and a microfluidic platform has been fabricated to manipulate bio-functionalized magnetic particles in suspension for immunoassays. The motion of the magnetic multi-core nanoparticles subjected to a magnetic field gradient, such as when used in magnetic bioseparation, was investigated using a Brownian dynamics algorithm. Optical measurements based on absorbance change as a function of particle concentration have been carried out as well. Both simulations and experiments indicate that particle aggregation is an essential factor to accelerate the separation process. A Monte Carlo method based on the Metropolis algorithm has been used to simulate the equilibrium magnetic properties of the magnetic multi-core nanoparticles. Results show that the Langevin model, even when including the size distribution of the magnetic nanocrystals (MNCs) contained in the nanoparticles, is not sufficient to fully describe the magnetization of this type of particles. Other intrinsic properties, such as the magnetic anisotropy of the MNCs and the magnetic dipole-dipole interactions between MNCs, must also be taken into account. The effective magnetic moment μ_eff and initial magnetic susceptibility χ_0 were studied both at zero magnetic field and in the low field region. We show that the true value of χ_0 can easily be derived in a single step from the value of μ_eff computed at zero field and including all additional factors (e.g. size distribution, magnetic anisotropy, and dipolar interactions). These findings are particularly relevant for biosensing systems relying on the low-field magnetic response of bio-functionalized magnetic multi-core nanoparticles. Finally, a digital microfluidic chip based on electrowetting-on-dielectric (EWOD) was fabricated to manipulate microliter-sized water droplets containing a suspension of bio-functionalized magnetic multi-core nanoparticles. AC-susceptibility measurements were performed using a Superconducting Quantum Interference Device (SQUID) sensor.