Towards A Wideband Induction-Based AC Magnetometer for A Fast Characterization of Magnetic Nanoparticles.

Abstract

Magnetic susceptibility methods particularly, AC susceptibility measurement have shown high potential to provide a fast characterization and standardization technique of magnetic nanoparticles (MNPs) [1]–[4]. Depending on the applications, AC magnetometers have been developed to measure magnetic responses from MNPs where Neel and Brown relaxation parameters are mostly characterized and utilized in these applications. From the acquired Brownian and Neel relaxation, physical and magnetic properties of the MNPs can be estimated such as the hydrodynamic size and magnetic anisotropy energy ratio. However, the characterization frequency range of the Neel and Brownian relaxations plays an important role where a wider frequency range may reveal further information on the MNP properties. In this study, to realize a simple, wideband, and sensitive AC magnetometer for bio-sensing applications and characterization of magnetic nanoparticles (MNPs), we report the improvements in the detection unit of our previously developed induction-based AC magnetometer [5] for achieving a larger usable range of the detection frequency. In order to obtain a linear sensitivity response, the induction coil is typically operated below its self-resonance frequency where its inductive property becomes dominant and not affected by the parasitic capacitance. This self-resonance frequency is governed by the coil’s inductance and parasitic impedance, and increasing the self-resonance frequency by reducing the coil’s inductance will extend the usable frequency range of the coil. However, it should be noted that there is a trade-off between the coil’s usable frequency range and sensitivity where an increase of the self-resonance frequency may reduce the coil’s sensitivity at the low-frequency region. In the attempt to achieve a wide frequency range yet sufficiently sensitive pickup coil, we compared the sensitivity and usable frequency range of 6 different designs of pickup coils by investigating the effect of inductance and parasitic capacitance. The number of turns and coil section separation were varied from 400 to 200 turns, and 1 to 4 sections, respectively. We found that the usable frequency range was greatly affected by the pickup coil’s inductance due to the self-resonance phenomena compared to parasitic capacitance. A low noise amplification circuit was designed based on two AD8429 instrument amplifiers and fabricated on a 2-layer printed circuit board as shown in Fig. 1 (a). For the field excitation unit, the excitation coil was designed to produce a field uniformity of more than 90% and constructed using a Litz wire with 60 strands of 0.1-mm Cu wire. The acquisition of the magnetization signal was realized by implementing a generalized Goertzel algorithm to achieve fast signal and phase extractions [6] in comparison to the conventional use of Fast Fourier Transform and lock-in amplifier techniques. As a result, the developed magnetometer showed a sensitivity of 10-8 Am2/sqrt(Hz) at 6 Hz and a frequency range of up to 158 kHz. We then demonstrated the developed AC magnetometer by evaluating the effect of viscosity on the frequency response of thermally blocked, single-core nanoparticles (SHP30, Ocean Nanotech, USA). The viscosity of the carrier liquid was varied by different weights of glycerol mixed in a constant volume of purified water and the Fe-concentration was fixed at 0.5 mg/mL. The excitation frequency was swept from 5 Hz to 158 kHz at a maximum field of 0.55 mTpp to obtain discrete 51 measurement points within the acquisition time of 5 minutes. As a result, the peak for the imaginary part was observed to be shifted towards lower frequency values when the wt/V% of the glycerol solution was increased. From the real and imaginary magnetization curves, the hydrodynamic size and the average anisotropy energy ratio s were estimated to be 60.6 nm and 25, respectively. It can be expected that the developed AC magnetometer can be a valuable tool in providing a simple, fast, and reliable assessment of MNPs for bio-sensing applications.AcknowledgmentsThis work was supported by the Ministry of Higher Education of Malaysia under the grant number of FRGS/1/2019/TK04/UMP/02/4 (RDU1901154), the Research Management Center of Universiti Malaysia Pahang under the grant number of RDU1903100, and the “Strategic Promotion of Innovative R&D” of the Japan Science and Technology Agency (JST). **