This paper presents a microwave sensor using a pair of split-ring resonators (SRRs). The sensor is designed by loading a microstrip transmission line by two identical SRRs on its sides. Differential permittivity sensing is performed by loading the SRRs with dielectric samples. One transmission notch is observed for the identical loads, whereas the non-identical samples produce two split notches. The sensor’s operating principle is described through a circuit model analysis. A prototype of the designed sensor is fabricated and experimentally validated for verifying the differential sensing concept. The developed device can be used to compare or differentially characterize solid dielectric samples improving the robustness to environmental factors producing cross-sensitivity or miscalibration.

Differential Sensors Using Microstrip Lines Loaded With Two Split-Ring Resonators

The conventional resonant-type microwave microfluidic sensors made of planar resonators suffer from limited sensitivities. This is due to the existence of several distributed capacitors in their structure, where just one of them acts as a sensing element. This article proposes a very high-sensitivity microwave sensor made of a microstrip transmission line loaded with a shunt-connected series LC resonator. A large sensitivity for dielectric loadings is achieved by incorporating just one capacitor in the resonator structure. Applying sample liquids to the microfluidic channel implemented in the capacitive gap area of the sensor modifies the capacitor value. This is translated to a resonance frequency shift from which the liquid sample is characterized. The sensor performance and working principle are described through a circuit model analysis. Finally, a device prototype is fabricated, and experimental measurements using water/ethanol solutions are presented for verification of the sensing principle.

Ultrahigh-Sensitivity Microwave Sensor for Microfluidic Complex Permittivity Measurement

This chapter presents a summary of the issues discussed during the one day workshop on “Support Vector Machines (SVM) Theory and Applications” organized as part of the Advanced Course on Artificial Intelligence (ACAI ’99) in Chania, Greece [19]. The goal of the chapter is twofold: to present an overview of the background theory and current understanding of SVM, and to discuss the papers presented as well as the issues that arose during the workshop.

Support vector machines: Theory and applications

Limited sensitivity and sensing range are arguably the greatest challenges in microwave sensor design. Recent attempts to improve these properties have relied on metamaterial (MTM)-inspired open-loop resonators coupled to transmission lines (TLs). Although the strongly resonant properties of the resonator sensitively reflect small changes in the environment through a shift in its resonance frequency, the resulting sensitivities remain ultimately limited by the level of coupling between the resonator and the TL. This paper introduces a novel solution to this problem that employs negative-refractive-index TL MTMs to substantially improve this coupling so as to fully exploit its resonant properties. A MTM-infused planar microwave sensor is designed for operation at 2.5 GHz, and is shown to exhibit a significant improvement in sensitivity and linearity. A rigorous signal-flow analysis of the sensor is proposed and shown to provide a fully analytical description of all salient features of both the conventional and MTM-infused sensors. Full-wave simulations confirm the analytical predictions, and all data demonstrate excellent agreement with measurements of a fabricated prototype. The proposed device is shown to be especially useful in the characterization of commonly available high-permittivity liquids as well as in sensitively distinguishing concentrations of ethanol/methanol in water.

Strongly Enhanced Sensitivity in Planar Microwave Sensors Based on Metamaterial Coupling

Semiconductor photocatalysis has long been considered as a promising approach for water pollution remediation. However, limited by the recombination of electrons and holes, low kinetics of photocatalysts and slow reaction rate impede large-scale applications. Herein, we addressed this limitation by developing a triphase photocatalytic system in which a photocatalytic reaction is carried out at air-liquid-solid joint interfaces. Such a triphase system allows the rapid delivery of oxygen, a natural electron scavenger, from air to the reaction interface. This enables the efficient removal of photogenerated electrons from the photocatalyst surface and minimization of electron-hole recombination even at high light intensities, thereby resulting in an approximate 10-fold enhancement in the photocatalytic reaction rate as compared to a conventional liquid/solid diphase system. The triphase system appears an enabling platform for understanding and maximizing photocatalyst kinetics, aiding in the application of semiconductor photocatalysis.

Enhanced Photocatalytic Reaction at Air–Liquid–Solid Joint Interfaces

Limited sensitivity and sensing range are arguably the greatest challenges in microwave sensor design. Recent attempts to improve these properties have relied on metamaterial- (MTM-) inspired open-loop resonators (OLRs) coupled to transmission lines (TLs). Although the strongly resonant properties of the OLR sensitively reflect small changes in the environment through a shift in its resonance frequency, the resulting sensitivities remain ultimately limited by the level of coupling between the OLR and the TL. This work introduces a novel solution to this problem that employs negative-refractiveindex TL (NRI-TL) MTMs to substantially improve this coupling so as to fully exploit its resonant properties. A MTM-infused planar microwave sensor is designed for operation at 2.5 GHz, and is shown to exhibit a significant improvement in sensitivity and linearity. A rigorous signal-flow analysis (SFA) of the sensor is proposed and shown to provide a fully analytical description of all salient features of both the conventional and MTM-infused sensors. Full-wave simulations confirm the analytical predictions, and all data demonstrate excellent agreement with measurements of a fabricated prototype. The proposed device is shown to be especially useful in the characterization of commonly-available high-permittivity liquids as well as in sensitively distinguishing concentrations of ethanol/methanol in water.

Strongly Enhanced Sensitivity in Planar Microwave Sensors Based on Metamaterial coupling

A planar microwave resonator sensor is designed, customized, and fabricated to detect coating breaches in industrial steel pipelines. The sensor, which utilizes a ring-shaped resonator to maximize the sensitivity at its core, is tuned to 2.5 GHz with a quality factor of 280. In the setup, the sensor is grounded to a piece of steel pipeline with an Epoxy-100 coating, which provides the substrate beneath the microstrip structure. It is demonstrated that any change in the gap height between the substrate layer and the pipeline, from 0 to 3.5 mm, produces a significant resonant frequency variation and bandwidth change in the sensor's response. The sensor structure demonstrates sensitivity and selectivity to air and water penetration to the breach. The sensor structure described in this work is a compact, low-cost solution and has potential for further miniaturization in mobile applications which may serve as a method for pipeline breach detection.

A Microwave Ring Resonator Sensor for Early Detection of Breaches in Pipeline Coatings

Sensing of molecular analytes by probing the effects of their interaction with microwaves is emerging as a cheap, compact, label-free and highly sensitive detection and quantification technique. Microstrip ring-type resonators are particularly favored for this purpose due to their planar sensing geometry, electromagnetic field enhancements in the coupling gap and compatibility with established printed circuit board manufacturing. However, the lack of selectivity in what is essentially a permittivity-sensing method is an impediment to wider adoption and implementation of this sensing platform. By placing a polycrystalline anatase-phase TiO2 nanotube membrane in the coupling gap of a microwave resonator, we engineer selectivity for the detection and differentiation of methanol, ethanol and 2-propanol. The scavenging of reactive trapped holes by aliphatic alcohols adsorbed on TiO2 is responsible for the alcohol-specific detection while the different short chain alcohols are distinguished on the basis of differences in their microwave response. Electrodeless microwave sensors which allow spectral and time-dependent monitoring of the resonance frequency and quality factor provide a wealth of information in comparison with electrode-based resistive sensors for the detection of volatile organic compounds. A high dynamic range (400 ppm–10000 ppm) is demonstrated for methanol detection.

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Selective microwave sensors exploiting the interaction of analytes with trap states in TiO2 nanotube arrays

This work presents a proof of concepts of CO 2 gas monitoring by a microwave sensor operating in microwave regime and study its sensitivity enhancement using an adsorbent bed of commercially available Zeolite 13X, and two synthesized MOF-199 (MOF-199-M1 and MOF-199-M2). The sensing principle of the sensor is based on the change in the dielectric properties of the bed, in response to CO 2 concentration change in the dry CO 2 /He mixture. The sensor’s response is quantified in terms of change in the resonant frequency with respect to baseline for each material. The sensor shows maximum sensitivity of 24 kHz/% CO 2 for MOF-199-M2 and minimum of 10 kHz/% CO 2 for Zeolite 13 X. The sensitivity of the microwave senor to CO 2 concentration, with MOF-199-M2 as the bed is higher than MOF-199-M1, which can be related to the presence of different amount of unsaturated Cu 2+ ions in their frameworks. XPS and XRD studies revealed marginal structure difference between MOF-199-M1 and MOF-199-M2. MOF-199-M2 has higher adsorption capacity compared to Zeolite 13 X and MOF-199-M1 at CO 2 concentration >45 vol.% which demonstrates potential application of microwave sensors even at high CO 2 concentration (>45 vol.%) using MOF-199-M2. Further work needs to study the sensor’s performance in non-dry gas streams at different levels of humidity and its integration with different materials to address the practical challenges, and to improve the selectivity of the sensor.

Mohammad Abdolrazzaghi

Papers

Strongly Enhanced Sensitivity in Planar Microwave Sensors Based on Metamaterial Coupling

Strongly Enhanced Sensitivity in Planar Microwave Sensors Based on Metamaterial coupling

A Microwave Ring Resonator Sensor for Early Detection of Breaches in Pipeline Coatings

Selective microwave sensors exploiting the interaction of analytes with trap states in TiO2 nanotube arrays

Sensitivity enhancement in planar microwave active-resonator using metal organic framework for CO2 detection