Cylindrical Dielectric Resonator Antenna-Based Sensors for Liquid Chemical Detection

High-Sensitivity Slot-Loaded Microstrip Patch Antenna for Sensing Microliter-Volume Liquid Chemicals with High Relative Permittivity and High Loss Tangent

This paper proposes a microwave sensor based on a high-sensitivity slot-loaded rectangular microstrip patch antenna (MPA) for measuring microliter-volume liquid chemicals with high relative permittivity and high loss tangent. A rectangular single-ring complementary split ring resonator (SR-CSRR) slot with a bottom-edge center split (BCS) was inserted along the upper radiating edge of the patch to enhance the relative permittivity sensitivity of the MPA. The first resonant frequency of the proposed SR-CSRR-BCS slot-loaded MPA showed the highest sensitivity compared to the resonant frequencies of the MPAs with other commonly used slots for varying the relative permittivity of the planar substrate type material under test from 1 to 10 when placed above the patch. After designing the scaled SR-CSRR-BCS slot-loaded MPA with the unloaded first resonant frequency at 2.5 GHz, a hollow acrylic cylindrical liquid container with an inner volume of approximately 18.6 μL was placed at the top-edge center of the SR-CSRR-BCS slot to achieve maximum sensitivity. A quarter-wavelength transformer was applied between the patch and the feed line of the MPA to improve the impedance mismatch that occurs when liquid chemicals with a high loss tangent are placed in the container. Water, methanol, and ethanol were carefully selected for test liquids to cover a broad range of relative permittivity and high loss tangents. The proposed SR-CSRR-BCS slot-loaded MPA was designed and fabricated on a 0.76 mm-thick RF-35 substrate, and a reference RS-loaded MPA was designed and fabricated for comparison. The shift in the first resonant frequency of the input reflection coefficient characteristic was used for the sensitivity comparison, and the container was filled with 15 μL of the liquids at 25 °C. The measured sensitivity (%) of the proposed SR-CSRR-BCS slot-loaded MPA for water was 0.45%, which was higher than other antenna-based microwave sensors in the literature.

Near-field mapping of high permittivity dielectric microwave resonator modes via optically induced conductance

In this paper, we demonstrate a straightforward, low-cost, and high resolution optical-based method to measure the three-dimensional relative electric field magnitude in microwave circuits without the need to monitor reflected laser beams or the requirement of photoconductive substrates for the device under test. The technique utilizes optically induced conductance, where a focused laser beam excites electron-hole-pairs (EHPs) in a semiconductor thin film placed in the near-field of a microwave circuit. The generated EHPs create localized loss in the resonator and modulate the transmitted microwave signal, proportional to the local microwave electric field. As a proof of principle, several different modes of a high permittivity (ɛ ∼ 80) cylindrical dielectric resonator are mapped.

A Substrate Integrated Waveguide Resonator Sensor for Dual-Band Complex Permittivity Measurement

This paper presents a novel dual-band substrate integrated waveguide (SIW) sensor that is designed to measure the complex permittivities of liquids or solid powders at two industrial, scientific, and medical (ISM) frequencies simultaneously. Resonant frequencies and quality factors are obtained from S-parameter measurements with the proposed SIW sensor, and applied to reconstructing the permittivities of materials under test through an artificial neural network. The water–ethanol mixed liquids were measured with the proposed sensor. The maximum deviations of the measured permittivities at 2.45 and 5.8 GHz are within 3% of literature results. The measurement by the proposed SIW sensor with artificial neural network reconstruction is accurate and efficient.

RFID-Based Microwave Biosensor for Non-Contact Detection of Glucose Solution

Due to the increasing number of diabetic patients, early monitoring of glucose levels is particularly important; therefore, glucose biosensors have attracted enormous attention from researchers. In this paper, we propose a glucose microwave biosensor based on RFID and achieve a non-contact measurement of the concentration of glucose solutions. The Reader is a complementary split-ring resonator (CSRR), and the Tag is comprised of a squared spiral capacitor (SSC). A polydimethylsiloxane microfluidic quantitative cavity with a volume of 1.56 μL is integrated on the Tag to ensure that the glucose solution can be accurately set to the sensitive area and fully contacted with the electromagnetic flux. Because the SSC exhibits different capacitances when it contacts glucose solutions of different concentrations, changing the resonant frequency of the CSRR, we can use the relationship to characterize the biosensing response. Measurement results show that bare CSRR and RFID-based biosensors have achieved sensitivities of 0.31 MHz/mg·dL⁻¹ and 10.27 kHz/mg·dL⁻¹, and detection limits of 13.79 mg/dL and 1.19 mg/dL, respectively, and both realize a response time of less than 1 s. Linear regression analysis of the abovementioned biosensors showed an excellent linear relationship. The proposed design provides a feasible solution for microwave biosensors aiming for the non-contact measurement of glucose concentration.

Smart Superhydrophobic Textiles Utilizing a Long-Range Antenna Sensor for Hazardous Aqueous Droplet Detection plus Prevention

This paper demonstrates the feasibility of a long-range antenna sensor embedded underneath a liquid repellent fabric to be employed as a wearable sensor in personal protective fabrics. The sensor detects and monitors hazardous aqueous liquids on the outer layer of fabrics, to add an additional layer of safety for professionals working in hazardous environments. A modified patch antenna was designed to include a meandering-shaped resonant structure, which was embedded underneath the fabric. Superhydrophobic fabrics were prepared using silica nanoparticles and a low-surface-energy fluorosilane. 4 to 20 μL droplets representing hazardous aqueous solutions were drop-cast on the fabrics to investigate the performance of the embedded antenna sensor. Long-range (S₂₁) measurements at a distance of 2-3 m were performed using the antenna sensor with treated and untreated fabrics. The antenna sensor successfully detected the liquid for both types of fabrics. The resonant frequency sensitivity of the antenna sensor underneath the treated fabric exhibiting superhydrophobicity was measured as 370 kHz/μL, and 1 MHz/μL for the untreated fabric. The results demonstrate that the antenna sensor is a good candidate for wearable hazardous aqueous droplet detection on fabrics.

A New Approach to Determining Liquid Concentration Using Multiband Annular Ring Microwave Sensor and Polarity Correlator

This article presents a new approach to determining liquid concentration using a new microwave sensor and polarity correlator. The sensor design incorporates an annular ring resonator having inside three parallel lines, a trapezoid ground plane and a co-planar waveguide (CPW) tapered feeder, which altogether achieve multiple frequency bands. Multiple bands of interest are obtained at the lower end of the microwave spectrum, i.e., from 1–6 GHz, as this region is widely accepted in analyzing various liquid samples. The sensor size is 71 × 40 × 1.6 mm3 with material selection based on an economically available FR4 substrate. The sensor is realized and experimentally validated for its sensitivity by utilizing in-lab prepared aqueous solution samples. Further, liquid concentration is determined by adopting a polarity correlator, which is applied to the sensor’s responses obtained at different values.

A Submersible Printed Sensor Based on a Monopole-Coupled Split Ring Resonator for Permittivity Characterization

This work presents a non-invasive, reusable and submersible permittivity sensor that uses a microwave technique for the dielectric characterization of liquid materials. The proposed device consists of a compact split ring resonator excited by two integrated monopole antennas. The sensing principle is based on the notch introduced by the resonators in the transmission coefficient, which is affected due to the introduction of the sensor in a new liquid material. Then, a frequency shift of the notch and the Q-factor of the proposed sensor are related with the changes in the surrounding medium. By means of a particular experimental procedure, commercial liquids are employed to obtain the calibration curve. Thus, a mathematical equation is obtained to extract the dielectric permittivity of liquid materials with unknown dielectric properties. A good match between simulated and experimental results is obtained, as well as a high Q-factor, compact size, good sensitivity and high repeatability for use in sensing applications. Sensors like the one here presented could lead to promising solutions for characterizing materials, particularly in determining material properties and quality in the food industry, bio-sensing and other applications.