The microwave near-field detection technique is of interest to many researchers for characterizing materials because of its high sensitivity. It is based on sensing buried objects by producing an evanescent field.The advantage of evanescent fields is their capability to interrogate electrically small objects. In the past, near-field probes have been designed to sense magnetic materials. For dielectric materials, a near-field probe that senses the permittivity of the materials is important. This work presents a novel design of a near-field probe that generates a dominant electric eld. The probe is an electrically small dipole measuring approximately 0.07?? in length operating at 216.3 MHz. The antenna is matched to a 50??? system using two chip inductors distributed symmetrically on the dipole. The numerical and measurement results show that the proposed design is highly sensitive and capable of sensing subsurface object. The proposed design is compact, lightweight and applicable for microwave applications.

<p dir="ltr">Per- and polyfluoroalkyl substances (PFAS), once considered to be emerging micro-pollutants, are now a very present class of pervasive and persistent micropollutant. Frequently referred to as “forever chemicals”, once they’re in the environment, they do not break down owing to the strength of their network of carbon-fluorine bonds. Their persistence is of particular concern, as they have been shown to have a plethora of negative health effects on living things including low infant birth weights, dyslipidemia, and cancer, to name a few. Due to both their persistence and negative health effects, the ability to rapidly test waters (<i>i.e.,</i> drinking water, river water, lake water, etc.) is of critical importance. The current “gold-standard” method for testing waters is the collection and transport of a sample to a centralized facility where chromatography and mass spectrometric methods can be performed for the separation, identification, and quantification of PFAS; however, this method is not able to be used for real-time analyses and is not sufficient for efficiently informing consumers or remediation efforts. An in-field detection method that is capable of providing real-time analyses is needed.</p><p dir="ltr">Electrochemistry stands well-poised to offer a suite of techniques that can be used for in-field detection. Electrochemistry is cost-effective, easy to perform and analyze, and readily portable; however, it lacks specificity and typically requires an electroactive analyte. These limitations can be overcome through the use of a surface functionalization strategy which adds specificity through the imprinting of the analyte of interest and monitors the change in signal from an alternate mediator molecule. Molecularly imprinted polymers (MIPs) are the chosen surface functionalization strategy that will be used and discussed in this work. While MIPs overcome the specificity and requirement of an electroactive analyte limitations and have been previously demonstrated for the detection of perfluorooctane sulfonate (PFOS), they traditionally require the use of added buffers and one electron mediators, which are not found in natural waters. Thus, to expand MIP-based electrochemical detection to in-field use strategies must be developed and employed to mitigate these concerns.</p><p dir="ltr">This work provides significant strides forward in enabling in-field, MIP-based electro-chemical sensing. We take advantage of ambient dioxygen present in river water to quantify one of the more harmful PFAS molecules, perfluorooctane sulfonate (PFOS), from 0 to 0.5 nM on a MIP-modified carbon substrate. Differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS) generated calibration curves for PFOS in river water using oxygen as the mediator. Importantly, we show that electrochemical impedance spectroscopy is superior to voltammetric techniques: like ultramicroelectrodes, this technique can be used in low-conductivity matrices like river water with high reproducibility. Further, impedance provides a PFOS limit of detection of 3.4 pM. We also demonstrate that the common interferents humic acid and chloride do not affect the sensor signal. The use of dioxygen is predicated on the assumption that there will be consistent ambient dioxygen levels in natural waters. This is not always the case in hypoxic groundwater and at high altitudes. To overcome this challenge, and further advance the strategies that will enable in-field electroanalysis of PFAS, we demonstrate that dioxygen can be generated in solution through the hydrolysis of water. The electrogenerated dioxygen can then be used as a mediator for molecularly imprinted polymer (MIP)-based electroanalysis. We demonstrate that calibration curves can be constructed with high precision and sensitivity (LOD > 1 ppt). We also demonstrate the development and use of a universal multiplexer and electrode array, which can enable high throughput, in-field electroanalysis for a wide variety of compounds. In this work, we demonstrate it specifically for detecting PFOS from 0.05 to 0.05 nM and lead at a concentration of 1 nM.</p><p dir="ltr">Additionally, in this work, we lay the groundwork for the future direction of developing a more fundamental understanding of MIPs to be able to fine-tune their selectivity and performance. Preliminary data and experimental approaches are shown for using nanoparticle deposition and visualization, with scanning electrochemical microscopy, to characterize surface reactivity and binding site distribution, functional group studies to better understand what groups and molecular interactions affect the binding of the analyte to the MIP the most, and using cyclic voltammetry to determine the capacitance and resistance of the polymer. Further approaches are outlined to relate the conditions under which the polymer was created to the polymer’s characteristics and then the polymer’s performance. Future improvements to make the in-field use of the multiplexer more efficient are also shown. In total, this work shows the feasibility and nearness of in-field, MIP-based electrochemical detection for PFAS by advancing the strategies and hardware necessary to do so.</p>

Microwave near-field probes (MNPs) confine evanescent fields to regions that are substantially smaller than the wavelength at the operation frequency. Such probes are able to resolve subwavelength features, thus providing resolution much higher than the classical Abb?? limit. These abilities of MNPs are primarily due to the evanescent nature of the field generated at the tip of the probes. In the past, MNPs with ultra-high resolution were designed by tapering a resonant opening to provide high field concentration and high sensitivity. The limitations of these MNPs were subject to low surface roughness and practical realization challenges due to their geometrical features and vibration control constraints. Metamaterials with their ability to enhance evanescent fields, lead to the speculation that they could potentially increase the sensitivity of near-field probe. Periodically arranged metamaterial unit elements such as split-ring-resonators (SRRs) can create negative permeability media. Placing such material layer in the proximity of a probe leads to enhancement of the evanescent waves. Guided by this remarkable feature of metamaterials, I proposed an MNP consisting of a wire loop concentric with a single SRR. The evanescent field behavior of the probe is analyzed using Fourier analysis revealing substantial enhancement of the evanescent field consistent with metamaterial theory predictions. The resolution of the probe is studied to especially determine its ability for sub-surface detection of media buried in biological tissues. The underlying physics governing the probe is analyzed. Variations of the probe are developed by placement of lumped impedance loads. To further increase the field confinement to smaller region, a miniaturized probe design is proposed. This new probe consists of two printed loops whose resonance is tunable by a capacitor loaded in the inner loop. The sensing region is decreased from ??/20 to ??/55, where ?? is the wavelength of the probe???s unloaded frequency. The magnetic-sensitive nature of the new probe makes it suitable for sensing localized magnetostatic surface resonance (LMSR) occurring in electrically very small particles. Therefore, I proposed a sensing methodology for detecting localized magnetostatic surface (LMS) resonant particles. In this methodology, an LMS resonant sphere is placed concentrically with the loops. A circuit model is developed to predict the performance of the probe in the presence of a magnetic sphere having Lorentz dispersion. Full-wave simulations are carried out to verify the circuit model predictions, and preliminary experimental results are demonstrated. The Lorentzian fit in this work implies that the physical nature of LMSR may originate from spin movement of charged particle whose contribution to effective permeability may be analogous to that of bound electron movement to effective permittivity in electrostatic resonance. Detection of LMSR can have strong impact on marker-based sensing applications in biomedicine and bioengineering.

Les circuits à base de jonctions Josephson comme les filtres à interférences quantiques, nommés SQIF (Superconducting Quantum Interference Filter), sont des capteurs très sensibles au champ magnétique. Les éléments de base d’un tel circuit sont les SQUID (Superconducting Quantum Interference Device). Aussi performants dans la détection de champ magnétique, ces derniers ne permettent pas de réaliser des mesures absolues. De plus, la nécessité d’un asservissement par une boucle à verrouillage de flux limite la bande de fréquence d’utilisation. Les SQIF n’ont pas cette limitation et permettent les mesures absolues de champ magnétique. Leur capacité à combiner une taille compacte, une très bonne sensibilité et une large bande fréquentielle d’utilisation fait de ces capteurs des sérieux concurrents aux antennes classiques. Des mesures expérimentales avec des SQIF HTS faits par la technologie de jonctions irradiées montrent qu’il est possible de réaliser la détection de signaux radiofréquence jusqu’au moins 5 GHz en configuration de champ proche et en environnement non magnétiquement blindé. Afin de réaliser l’adaptation d’impédance et améliorer les caractéristiques DC de ces capteurs, différentes géométries de réseau sont étudiées. L’étude permet de définir les paramètres d’importance dans la conception de circuits SQIF afin de réaliser des détecteurs radiofréquence performants. / Superconducting Quantum Interference Filters (SQIF) are Josephson circuits very sensitive to magnetic field. They are made of arrays of SQUIDs (Superconducting QUantum Interference Devices). The latter, when operated alone, doesn’t allows absolute magnetic field measurements and have to be used with a flux locked loop, which limits the frequency band of operation. SQIFs doesn’t have such limitations and they offer the possibility to combine compactness, sensitivity and wide band of frequency at the same time. Because of this, SQIFs are serious concurrents to classical antennas in microwave applications. Experimental measurements made with HTS SQIFs and irradiated Josephson junctions shows that it is possible to detect microwave signals up to 5 GHz in an unshielded environment, and near field configuration. To perform better detection, it is important to match impedance of circuits. In the goal to do this and to improve DC characteristics, different network geometries are studied. At the end this study allows to define which parameters are important in the design of SQIF circuits for microwave detection.

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