Microfluidic reflective-mode differential sensor based on open split ring resonators (OSRRs)

Abstract

This paper proposes a differential sensor based on a pair of open split ring resonators (OSRR) operating in reflection. The output signal is thus the differential reflection coefficient of both resonators, intimately related to their dielectric loading. Thus, for identical loads in both sensing resonators, the individual reflection coefficients are equal, thereby providing an ideally null output signal. By contrast, when unequal dielectric loads truncate the symmetry, the reflection coefficients are different, resulting in a differential output signal related to the level of asymmetry. In order to ease the measurement of the output signal, a rat-race hybrid coupler is used. The OSRR sensing loads are connected to the coupled ports of the hybrid coupler, whereas the input signal is injected to the Δ-port, and the output signal is collected at the isolated port (Σ-port). By this means, the output signal, i.e. the differential reflection coefficient between both sensing loads, is obtained from the transmission coefficient of a simple two-port structure. For experimental validation purposes, the sensor is applied to the measurement of isopropanol content in aqueous solutions, and for that purpose, the sensitive regions are equipped with microfluidic channels.

Planar Microwave Resonant Sensors: A Review and Recent Developments

“…Huge sensitivities and resolutions, useful to detect, e.g., small volume fractions or electrolyte concentrations in liquid solutions, have been achieved by means of these sensors. Although, typically, differential-mode sensors involve four-port measurements (these sensors are four-port devices), it has been demonstrated that by adding extra (microwave) circuitry, differential sensing by means of two-port devices (and measurements) can be achieved [68,69]. In [69], the differential sensor works in reflection (other reflective-mode sensors have been reported [70], also including single-ended sensors [71]).…”

“…Although, typically, differential-mode sensors involve four-port measurements (these sensors are four-port devices), it has been demonstrated that by adding extra (microwave) circuitry, differential sensing by means of two-port devices (and measurements) can be achieved [68,69]. In [69], the differential sensor works in reflection (other reflective-mode sensors have been reported [70], also including single-ended sensors [71]). The sensor reported in [72] is also remarkable, where symmetry truncation in a pair of slotted resonators generates quasi-microstrip to slot-mode conversion, useful for sensing.…”

“…Differential sensors based on a pair of uncoupled lines each one loaded with a resonant sensing element have been reported. The considered resonant elements are OCSRRs [62], SRRs [63], OSRRs [67,69], CSRRs [66], and DB-DGS resonators [64,65]. We report in this paper three prototype Figure 24 and the mathematical model developed in [58].…”

“…Differential sensors based on a pair of uncoupled lines each one loaded with a resonant sensing element have been reported. The considered resonant elements are OCSRRs [62], SRRs [63], OSRRs [67,69], CSRRs [66], and DB-DGS resonators [64,65]. We report in this paper three prototype examples, including a sensor for complex permittivity measurement of liquids (based on DB-DGSs) [64], a sensor for the determination of volume fraction of solute in liquid solutions (based on OSRRs) [69], and a sensor devoted to the measurement of extremely small concentrations of electrolytes in DI water [66].…”

“…The frequency position of such maximum slightly varies, but it is possible to obtain the value at a fixed frequency, particularly the resonance frequency of the resonant element. In this case, a single-frequency measurement suffices, and, in this case, the cross-mode transmission coefficient can be inferred by means of a reflective-mode differential measurement, involving a simple two-port measurement, as reported in [69]. The sensor device consists of two pairs of grounded OSRR-loaded lines connected to the coupled ports of a rat race coupler by means of two identical transmission line sections.…”