Magnetic particle rotation

Magnetic particle rotation in contrast to the AC susceptibility where the MNP are subject to a sinusoidal AC field - the particles are exposed to a rotating magnetic field up into the MHz regime and can be used to characterize both single nanoparticles as well as the larger multicore particles. Due to the viscous drag in the carrier medium, the MNPs cannot follow the rotating magnetic field instantaneously but a phase lag between magnetic field and sample net magnetic moment will evolve. If the particle rotation is magnetically detected, e.g., by sensitive fluxgate sensors measurements can be performed on small single-core and multi-core MNPs of any shape.

 

The analysis of the phase lag between rotating field and the MNP’s magnetization utilizing an empirical model by Yoshida et al. based on the Fokker-Planck equation allows one to study structural and magnetic particle properties. Also fundamental magnetic properties such as remanent particle moment and coercivity of the nanoparticles can be determined from this technique. A variation of the amplitude of the rotating magnetic field revealed distinct differences between the AC field and the rotating field mode. The spread of the phase spectra measured for various field amplitudes provides direct information on the core’s magnetic moments. 

 

If large multi-core particles close to a rigid surface or free in solution are used, particle rotation can be detected by microscopy and magnetization dynamics can be determined. The torque applied to the large particles is intimately linked to the nanoparticle size distribution inside. When the particles are attached to a rigid surface by a protein system (e.g. in an immunoassay as used in biosensors) this method can be applied for fundamental studies on protein mechanics and for the improvement of specificity in biosensor technology.

 

For multicore particles two different mechanisms can result in a net torque on the particle. At high frequencies, torque is generated by the phase lag between the magnetization and the rotating field while at low angular frequencies, torque is generated by the alignment of the applied field with the remanent magnetization of the nanoparticles in the tail of the nanoparticle size distribution. These rotating multicore particles have several new and interesting applications. When the particles are attached to a rigid surface by a protein system (e.g. in an immunoassay as used in biosensors) torque can be used to probe the angular stiffness of a ligand-receptor pair. Experiments show examples of the twisting behaviour of a ProteinG coated magnetic multicore particle bound to an immunoglobulin (IgG) physisorbed on a substrate. Comparison of different ligand receptor pairs showed large differences in angular stiffness that promises improved specificity in biosensor applications.

 

From a more fundamental point of view the application of torque allows to study protein mechanics and reveal structure-function relationships. The association of a large number of rotating magnetic particles on functionalized surfaces can be used to measure the particle-surface interaction potential. This provides a unique tool to investigate the non-fouling properties of functionalized surface coatings and investigate the particle association for specific ligand receptor pairs as applied in immunoassays.

 

In the NanoMag consortium protocols will be developed to characterize multicore particles in terms of their remanent magnetic moment and coercivity as well as the maximum torque that can be applied by a rotating magnetic field. The size distribution of the nanoparticles inside the multicore particles will be characterized. The improved control on the particle properties will be used to further investigate the application of torque in fundamental studies on protein mechanics, structure-function relationships of proteins and the application of the rotating particles probe for studying interaction potentials. If the particle rotation is magnetically detected by sensitive fluxgate sensors measurements the method can also be used on smaller single-core and multi-core MNPs.