An Injection-Locked-Oscillation-Based SIW Shorted Coaxial-Line Active Sensor for Detecting the MNPs Content in Water-Based Ferrofluid

A novel shorted coaxial-line resonator (SCR) in substrate-integrated waveguide (SIW) configuration is integrated with a negative resistance circuit (NRC) to implement an ultrahigh Q-factor injection-locked-oscillation (ILO)-based active sensor for high-resolution detection to an ultralow magnetic nanoparticles (MNPs) content in water-based ferrofluid (WFF). Two microfluidic chips, one of which is etched for a dielectric sensing channel and the other for a magnetic sensing channel, are embedded into the highly concentrated time harmonic E-field and H-field distributions region of the SIW SCR ILO-based active sensor, respectively. Then, WFF with different MNP contents is injected into the dielectric and magnetic sensing channels, and its dielectric and magnetic properties are used to detect the MNPs content, respectively. During the solution replacement, the corresponding ILO frequency shifts are recorded in real time by the LabVIEW-based ILO frequency monitoring system, and then, the ILO frequency shifts in a stable convergence state are used to characterize the MNPs content in these two sensing channels. The minimum MNPs content in WFF detected by utilizing both electrical and magnetic properties is 0.25%. Besides, a radially magnetized cylindrical magnet is cleverly embedded in the hollow inner conductor of the SCR to capture MNPs flowing through the magnetic sensing channel. Then, the MNPs will be gradually concentrated in the whole magnetic sensing channel due to the enrichment effects, and the corresponding ILO frequency shift rate is used as the response parameter to further improve the minimum detectable MNPs content to 0.125%.