Impacts of Liquid Level on Microwave Resonance Sensing with a Flexible Microfluidic Channel

Abstract

Permittivity sensing based on resonance tracking lays the fundamental principle for a variety of biochemical sensors, which has found vast applications in cancer biomarker detection, antigen-antibody analysis, and so on. Driven by continuous promotion of the detection limit, precise environmental control highlights its critical importance. Here, the impacts of liquid level on microwave resonance sensing are investigated, in which a flexible polydimethylsiloxane microfluidic channel and soft tubing are employed to control the liquid under test. The hydraulic pressure affects the effective permittivity of the liquid and the channel material, hence causing extra resonance shift signals. Both contactless and contacting sensing scenarios are studied in numerical simulations and experiments. It is demonstrated that the resonance frequency varies sensitively with the liquid level, and a sensitivity of 343 kHz mm −1 is measured. Meanwhile, a spoof localized surface plasmon resonator and its optimized excitation structure are employed and analyzed for a good figure of merit, addressing the detectability difficulties for high-permittivity and high-loss aqueous solutions. These results provide general guidelines for understanding and controlling the resonance sensors in aqueous environments and help to realize further lower detection limits.

Localized Spoof Surface Plasmon Skyrmions Excited by Guided Waves

“…The resonant frequencies of the LSSP skyrmions are f 0 , f 0 + Δf, f 0 + 2Δf, …, where f 0 is the fundamental resonance frequency, Δf is the free spectral range (FSR). The DGS is the theoretical innovation compared to the previous study [59][60][61][62] and is the reason for the excitation of the near-equidistant multi-resonant response of the LSSP skyrmions. Figure 2m,n demonstrates the feature of nearequidistant multi-resonant response and Figure 2o demonstrates the feature of subwavelength confinement, which verify the conclusions in the previous study.…”

“…The LSSP skyrmions excited by coplanar waveguide also have the favorable properties of topological robustness and near-equidistant multi-resonant response, which is same as the counterparts excited by microstrip line. Inspired by the sensors in the previous studies, [47,59,61,62] the LSSP skyrmions excited by microstrip line and coplanar waveguide are adopted to measure the relative permittivity of liquids, as shown in Figures S3 and S4 (Supporting Information), which have the advantages of planarization and integration. The LSSP Skyrmions can also be excited by two-conductor transmission line, and the structure only contains one space-coiling meta-structure; but considering that the two-conductor transmission line is not widely used in integrated circuits, the results are put in Figure S5 (Supporting Information).…”

“…Only <3 modes were excited, the near-equidistant multi-resonant response of the LSSP skyrmions [44] was not observed, and the LSSP skyrmions were not declared. [59][60][61][62] Therefore, the excitation of the LSSP skyrmions by guided waves is necessary to be studied.…”

“…For the LSSP skyrmions, there have been attempts to excite by microstrip lines. [59][60][61][62] They directly put the space-coiling meta-structures next to the transmission lines, so the coupling strength was relatively weak. Only <3 modes were excited, the near-equidistant multi-resonant response of the LSSP skyrmions [44] was not observed, and the LSSP skyrmions were not declared.…”

Exceptional Point in a Microwave Plasmonic Dipole Resonator for Sub‐Microliter Solution Sensing

“…Flexible tubing is used to pump solutions from the petri dish, with the syringe pump working in the withdraw mode. [22,51] The starting liquid volume in the petri dish is 8 mL, and the pumping time is 120 s for each measurement. A frequency splitting of 84 MHz occurs when an empty microfluidic channel is attached to the chiral SLSP resonator, as shown in Figure S6b (Supporting Information).…”