MHz Range Frequency-based Sensor for Magnetic Particle Detection Sara Chaychian * , Keyvan Dayyani, Wamadeva Balachandran, Fellow, IEEE, Yichuang Sun, Senior, IEEE Abstract— Design of rapid technique for magnetic particles detection based on inductive sensing is described in this paper. A fabricated 3D coil, oscillating at few MHz range frequencies, is used as a sensing element with its associated electronics for inductance measurements. Phase locked loop is used for sensor stability and accuracy in the detection system. The induced magnetic field, due to the presence of magnetic particles on the coil, alter the coil inductance and accordingly the output frequency signal. Different concentration and number of magnetic beads (µm to nm sizes) were tested with the sensor while oscillating at two different resonance frequencies and the results were compared together. The best detection performance was accomplished at 2.4 MHz via 9.5 μH inductor and at 7.2 MHz with 85 μH inductor. The best sensitivity achieved based on using 10 µL of iron-oxide nanoparticles (with the bead size of 10nm) and 81Hz frequency shift was attained regarding that number of particles. This system can be used in future for molecular DNA diagnostics. Index Terms—Analog and Mixed Signal System, Frequency, GMR, Induction Coil, Magnetic Particles (MPs), Magnetic Particle/Bead Detection, Magnetic Sensor, Medical Electronics, Point of Care Diagnostics, Phase Locked Loop (PLL), Superparamagnetic Iron Oxide Nanoparticles. I. INTRODUCTION he key to early stage identification and prevention of health problems is molecular detection (i.e. detecting protein, DNA or RNA of bacteria or virus) [1]. Recently with the pathogenic threat to the public health worldwide [2]-[3], it is quite essential to establish a rapid and sensitive detection method to identify these harmful microorganisms [4]. The molecular diagnostic method is much faster in producing the results (within minutes) than conventional clinical and culturing method, which normally takes few days. More importantly, the major advantage of molecular technique is its ability to diagnose very early stage of disease, before it starts developing in the body, damaging other organs, and showing external signs [5]. S Chaychian and Y Sun are with School of Physics, Engineering and Computer Science, University of Hertfordshire, Hatfield, UK. * Corresponding author email: S.Chaychian@herts.ac.uk K.D was with Brunel University London, College of Engineering, Design and Physical Sciences, London, UK. W.B. is with Brunel University London, College of Engineering, Design and Physical Sciences, London, UK. The procedure of disease identification with molecular method is available through Lab-On-A-Chip (LOAC) technology [6]. The general processes in a LOAC device (Figure 1) starts with sample collection and preparation from the patient, which can be of any biological entity including blood, urine, swab, etc. and according to the sample type, its quantity differs from 25µL up to 4mL. Next the sample's DNA needs to be extracted and amplified for achieving a detectable signal and the desired amount of DNA amount of it in the detection section. The whole system is connected via microfluidic network and is controlled by the central control system. Fig. 1. Overview of LOAC device processes [6] This research focus is on the detection section of the LOAC process. For detection, each DNA strand can be labelled with specific marker such as florescent or magnetic particles. The element under detection in biosensors are known as recognition element, which for example can be DNA of target virus. These elements are essential part of biosensors as they can determine and influence the sensor‘s stability, selectivity, and sensitivity. Biosensors in which their target elements are based on Nucleic Acid have some limitations such as low number of target DNA or RNA. However, these elements are more chemically stable than others such as antibodies [7]. Due to the high demand for fast and reliable diagnosis devices, new recognition elements have been studied and created recently using different types of materials to improve the biosensors’ detection limit, selectivity, and stability. Combination of magnetic particles with DNA is one of these examples. Justino et al. reviewed and discussed recent development in recognition elements for biosensors [7]. Pirzada et al [8] have provided a comprehensive review on different types of nanomaterials which can be used as a label or link for bio-sensing and detection with their applications. Utilizing Magnetic Particles (MPs) as labels, can increase the sensor selectivity, as MPs effect cannot be influenced by any T