New Perspective on Planar Inductive Sensors: Radio-Frequency Refractometry for Highly Sensitive Quantification of Magnetic Nanoparticles

We demonstrate how resonant planar coils may be used as sensors to detect and quantify magnetic nanoparticles reliably. A coil's resonant frequency depends on the adjacent materials' magnetic permeability and electric permittivity. A small number of nanoparticles dispersed on a supporting matrix on top of a planar coil circuit may thus be quantified. Such nanoparticle detection has application detection to create new devices to assess biomedicine, food quality assurance, and environmental control challenges. We developed a mathematical model for the inductive sensor response at radio frequencies to obtain the nanoparticles' mass from the self-resonance frequency of the coil. In the model, the calibration parameters only depend on the refraction index of the material around the coil, not on the separate magnetic permeability and electric permittivity. The model compares favourably with three-dimensional electromagnetic simulations and independent experimental measurements. The sensor can be scaled and automated in portable devices to measure small quantities of nanoparticles at a low cost. The resonant sensor combined with the mathematical model is a significant improvement over simple inductive sensors, which operate at smaller frequencies and do not have the required sensitivity, and oscillator-based inductive sensors, which focus on just magnetic permeability.

Automated summary

We demonstrate the use of resonant planar coils as sensors for reliable detection and quantification of magnetic nanoparticles. The coil's resonant frequency depends on the adjacent materials' magnetic permeability and electric permittivity. By dispersing a small number of nanoparticles on a supporting matrix on top of a planar coil circuit, the nanoparticles can be quantified. This nanoparticle detection has applications in biomedicine, food quality assurance, and environmental control challenges. We developed a mathematical model that accurately calculates the nanoparticles' mass from the self-resonance frequency of the coil at radio frequencies. The model relies only on the refraction index of the material around the coil, making it superior to other sensors that rely on separate magnetic permeability and electric permittivity measurements. The sensor can be scaled and automated for portable devices, providing a low-cost solution for measuring small quantities of nanoparticles. This resonant sensor, combined with the mathematical model, represents a significant improvement over other types of inductive sensors.