Schematic View of A Multifunctional Magnetic Nanoparticle (MNP)

• Magnetic nanoparticles (MNPs), with the size between 1 nm and 100 nm, are one important nanomaterial for science and technology in the past two decades.

• With comparable sizes to biologically important objects (such as proteins, nucleic acids, etc.) , MNPs have demonstrated unique properties such as larger surface-to-volume ratio, excellent reactivity, exceptional magnetic response compared to their bulk materials

• MNPs can be easily conjugated with biologically important constituents for biological diagnostics and therapy purpose.


• In sufficiently small MNPs, magnetization can randomly flip direction under the influence of temperature. Which is called superparamagnetism.

• Superparamagnetic nanoparticles have negligible remanent magnetizations, which, could effectively prevent the aggregations.

• Their magnetization curves are generally analyzed using the Langevin function model extended by a Debye pre-factor accounting for the dynamic magnetic properties.

The Nonlinear Magnetic Response of SPIONs

(a) Dual driving field-based MPS; (b) MH response curve of SPIONs; (c) Time domain magnetization response of SPIONs; (d) Power spectrum of collected signal contains higher harmonic components from SPIONs: the 3rd, 5th, 7th, and higher odd harmonics; (e) Power spectrum of the dual driving fields applied to SPIONs.

The dual driving field-based MPS modality is based on frequency mixing at the nonlinear magnetization curves of SPIONs. In this method, two sinusoidal magnetic fields are applied to SPIONs, one with low frequency fL (10 - 50 Hz) and sufficiently high amplitude (several tens of mT) to periodically drive SPIONs into the magnetically saturated region, the other with high frequency fL (1 kHz and above) and low amplitude (several mT) to shift harmonic signals to the high frequency range.

Brownian and Néel Relaxation

Representations of energy barrier governing the rotation of magnetic moment of each SPION (a) without the interruption of external magnetic field; (b) with external magnetic field; (c) with external magnetic field and dipolar field; (d) Néel relaxation process is the rotation of magnetic moment inside a fixed SPION; (e) Brownian relaxation process is the physical rotation of SPIONs along with its magnetic moment; (f) Representations of the magnetic core, spin canting layer, and polymer coating layer.

MPS Benchtop System Setup

A MPS benchtop system consists of: a PC controls Digital Acquisition Card (DAQ) to generate two sinusoidal voltages with frequencies of fL and fH. These two sinusoidal voltages are sent to power amplifiers (Amp) and excitation coils. The high and low frequency excitation coils generate sinusoidal magnetic fields with amplitudes of AH and AL, respectively, according to the Biot-Savart law.

The SPOINs react in response to the externally applied magnetic fields, and the measured signal strength depends on the SPION dynamic behavior. A pair of pick-up coils collect the induced voltage signal from SPION suspension, pre-amplified and filtered by a band-pass filter before sending back to DAQ for harmonic signal analysis. The pick-up coils are divided into two parts: the top half has clockwise windings (also called receive coils), and the bottom half has counter-clockwise windings (also called balancing coils). The balancing coils are used to cancel out the induced voltage due to the time-varying excitation fields from two outer coils, namely, the high and low frequency excitation fields.