Rapid-scan electron paramagnetic resonance using an EPR-on-a-Chip sensor

Electron paramagnetic resonance (EPR) spectroscopy is the method of choice to investigate and quantify paramagnetic species in many scientific fields, including materials science and the life sciences. Common EPR spectrometers use electromagnets and microwave (MW) resonators, limiting their application to dedicated lab environments. Here, we present an improved design of a miniaturized EPR spectrometer implemented on a silicon microchip (EPR-on-a-chip, EPRoC). In place of a microwave resonator, EPRoC uses an array of injection-locked voltage-controlled oscillators (VCOs), each incorporating a 200 μm diameter coil, as a combined microwave source and detector. The individual miniaturized VCO elements provide an excellent spin sensitivity reported to be about 4 × 109spins/√Hz, which is extended by the array over a larger area for improved concentration sensitivity. A striking advantage of this design is the possibility to sweep the MW frequency instead of the magnetic field, which allows the use of smaller, permanent magnets instead of the bulky and powerhungry electromagnets required for field-swept EPR. Here, we report rapid scan EPR (RS-EPRoC) experiments performed by sweeping the frequency of the EPRoC VCO array. RS-EPRoC spectra demonstrate an improved SNR by approximately two orders of magnitude for similar signal acquisition times compared to continuous wave (CW-EPRoC) methods, which may improve the absolute spin and concentration sensitivity of EPR-on-a-Chip at 14 GHz to about 6 × 107 spins/√Hz and 3.6 nM⁄√Hz, respectively

Monitoring the state of charge of vanadium redox flow batteries with an EPR-on-a-Chip dipstick sensor

The vanadium redox flow battery (VRFB) is considered a promising candidate for large-scale energy storage in the transition from fossil fuels to renewable energy sources. VRFBs store energy by electrochemical reactions of different electroactive species dissolved in electrolyte solutions. The redox couples of VRFBs are VO2+/VO2+ and V2+/V3+, the ratio of which to the total vanadium content determines the state of charge (SOC). V(iv) and V(ii) are paramagnetic half-integer spin species detectable and quantifiable with electron paramagnetic resonance spectroscopy (EPR). Common commercial EPR spectrometers, however, employ microwave cavity resonators which necessitate the use of large electromagnets, limiting their application to dedicated laboratories. For an SOC monitoring device for VRFBs, a small, cost-effective submersible EPR spectrometer, preferably with a permanent magnet, is desirable. The EPR-on-a-Chip (EPRoC) spectrometer miniaturises the complete EPR spectrometer onto a single microchip by utilising the coil of a voltage-controlled oscillator as both microwave source and detector. It is capable of sweeping the frequency while the magnetic field is held constant enabling the use of small permanent magnets. This drastically reduces the experimental complexity of EPR. Hence, the EPRoC fulfils the requirements for an SOC sensor. We, therefore, evaluate the potential for utilisation of an EPRoC dipstick spectrometer as an operando and continuously online monitor for the SOC of VRFBs. Herein, we present quantitative proof-of-principle submersible EPRoC experiments on variably charged vanadium electrolyte solutions. EPR data obtained with a commercial EPR spectrometer are in good agreement with the EPRoC data

Microwave field mapping for EPR-on-a-chip experiments

Electron paramagnetic resonance–on-a-chip (EPRoC) devices use small voltage-controlled oscillators (VCOs) for both the excitation and detection of the EPR signal, allowing access to unique sample environments by lifting the restrictions imposed by resonator-based EPR techniques. EPRoC devices have been successfully used at multiple frequencies (7 to 360 gigahertz) and have demonstrated their utility in producing high-resolution spectra in a variety of spin centers. To enable quantitative measurements using EPRoC devices, the spatial distribution of the B1 field produced by the VCOs must be known. As an example, the field distribution of a 12-coil VCO array EPRoC operating at 14 gigahertz is described in this study. The frequency modulation–recorded EPR spectra of a “point”-like and a thin-film sample were investigated while varying the position of both samples in three directions. The results were compared to COMSOL simulations of the B1-field intensity. The EPRoC array sensitive volume was determined to be ~19 nanoliters. Implications for possible EPR applications are discussed

A Mitochondrial Polymorphism Alters Immune Cell Metabolism and Protects Mice from Skin Inflammation

Several genetic variants in the mitochondrial genome (mtDNA), including ancient polymorphisms, are associated with chronic inflammatory conditions, but investigating the functional consequences of such mtDNA polymorphisms in humans is challenging due to the influence of many other polymorphisms in both mtDNA and the nuclear genome (nDNA). Here, using the conplastic mouse strain B6-mtFVB, we show that in mice, a maternally inherited natural mutation (m.7778G > T) in the mitochondrially encoded gene ATP synthase 8 (mt-Atp8) of complex V impacts on the cellular metabolic profile and effector functions of CD4+ T cells and induces mild changes in oxidative phosphorylation (OXPHOS) complex activities. These changes culminated in significantly lower disease susceptibility in two models of inflammatory skin disease. Our findings provide experimental evidence that a natural variation in mtDNA influences chronic inflammatory conditions through alterations in cellular metabolism and the systemic metabolic profile without causing major dysfunction in the OXPHOS system

Quantitative Elektronenspinresonanzspektroskopie in rauen wässrigen Lösungen

This thesis presents the advancement of electron paramagnetic resonance (EPR) spectroscopy for quantitative analysis of paramagnetic species in harsh aqueous environments. EPR is the method of choice for investigating and quantifying paramagnetic species in many applications in materials science, biology, and chemistry. Of particular interest is the in situ monitoring of paramagnetic states in solution generated by chemical reactions. However, their investigation is limited by the concept of commercial EPR spectrometers, being based on the use of microwave (MW) resonators. To perform such in situ experiments, either the entire process must be confined to the resonator, or the sample solution must flow through the resonator by means of tubing, limiting the use of EPR to dedicated laboratories. Consequently, a redesign of EPR spectrometers is required for the more widespread use of this method. The EPR-on-a-Chip (EPRoC) device circumvents these limitations by integrating the entire spectrometer, except for the magnet, onto a single microchip. Instead of an MW resonator, the planar coil of a voltage-controlled oscillator (VCO) with a diameter of a few hundred micrometers is used simultaneously as the MW source and detector. Frequency-swept EPR spectra can thus be recorded owing to the use of the VCO, which enables the use of permanent magnets. Covering the EPRoC with a protective coating enables it to be submerged directly in the sample solution, leading to a dipstick-type EPR spectrometer, thereby expanding accessible environments. To acquire quantitative information of the sample, the effect of the inherently inhomogeneous MW magnetic field of the planar coil on the recorded signal amplitude is investigated. It is shown that the simulations are in good agreement with the experimental results. The sensitivity of EPR, especially for samples with long relaxation times, may be improved by means of rapid-scan EPR. This method is implemented for EPRoC, improving the sensitivity per unit time by almost two orders of magnitude compared to the standard continuous wave operation. An example of application for the EPRoC dipstick is the state of charge monitoring of a vanadium redox flow battery. Quantitative EPR measurements show that the EPRoC can be used as a monitoring device. In addition, these experiments serve as proof of principle for a quantitative EPRoC dipstick operating in a harsh environment. In combination with a small permanent magnet, the EPRoC dipstick may find its way beyond the laboratory as a quantification tool for paramagnetic species in solution.In dieser Arbeit wird der Einsatz der Elektronenspinresonanz-Spektroskopie (ESR, EPR) für die quantitative Analyse paramagnetischer Spezies in chemisch aggressiven Lösungen beschrieben. EPR eignet sich hervorragend zur Untersuchung und Quantifizierung paramagnetischer Spezies in vielen Anwendungen der Materialwissenschaften, Biologie und Chemie. Von besonderem Interesse ist die in situ-Messung paramagnetischer Zustände in Lösungen, die durch chemische Reaktionen erzeugt werden. Solche Untersuchungen werden jedoch durch die Konstruktion üblicher EPR-Spektrometer eingeschränkt, die auf Mikrowellen-Resonatoren (MW) basieren. Um solche Experimente durchzuführen, muss entweder der gesamte Prozess auf den Resonator beschränkt werden oder die Lösung muss durch diesen durchgeleitet werden. Zur Vereinfachung dieser Experimente ist folglich eine Neugestaltung der Spektrometer erforderlich. EPR-on-a-Chip (EPRoC) umgeht diese Einschränkungen, indem das gesamte Spektrometer, außer des Magneten, auf einen Mikrochip integriert wird. Anstelle eines Resonators wird die planare Spule eines spannungsgesteuerten Oszillators mit einem Durchmesser von einigen hundert Mikrometern verwendet, die gleichzeitig als Mikrowellenquelle und -detektor dient. Dies erlaubt, frequenzvariable Spektren aufzuzeichnen, was wiederum den Einsatz von Permanentmagneten ermöglicht. Durch eine Schutzbeschichtung kann der EPRoC-Dipstick direkt in die Probenlösung eingetaucht werden, wodurch die zugänglichen Probenumgebungen erweitert werden. Um quantitative Informationen über die Probe zu erhalten, wird der Einfluss des inhärent inhomogenen MW-Feldes der planaren Spule auf das Messsignal untersucht, was mit Simulationen gut übereinstimmt. Die Rapid-Scan-Methode ist eine hervorragende Möglichkeit um die Empfindlichkeit der EPR zu verbessern. Diese wird in den EPRoC implementiert und getestet, wobei die Empfindlichkeit um fast zwei Größenordnungen gegenüber dem Dauerstrichbetrieb verbessert wird. Als Beispiel einer Anwendungsmöglichkeit wird der Ladezustandeines Vanadium-Redox-Flussakkumulators untersucht. Quantitative Messungen zeigen, dass EPRoC als Überwachungsmethode verwendet werden kann. Darüber hinaus zeigen die in dieser Arbeit vorgestellten Experimente, dass mit dem EPRoC-Dipstick quantitative Messungen in chemisch aggressiven Lösungen durchgeführt werden können. Der EPRoC-Dipstick verfügt als innovative Methode über die besten Voraussetzungen, um in Zukunft über das Labor hinaus als Quantifizierungswerkzeug für paramagnetische Spezies in Lösung eingesetzt zu werden