ESR micro-imaging of LiNc-BuO crystals in PDMS: spatial and spectral grain distribution

Harnessing Electron Spin Hyperpolarization in Chromophore-Radical Spin Probes for Subcellular Resolution in Electron Paramagnetic Resonance Imaging: Concept and Feasibility

Obtaining a subcellular resolution for biological samples doped with stable radicals at room temperature (RT) is a long-sought goal in electron paramagnetic resonance imaging (EPRI). The spatial resolution in current EPRI methods is constrained either because of low electron spin polarization at RT or the experimental limitations associated with the field gradients and the radical linewidth. Inspired by the recent demonstration of a large electron spin hyperpolarization in chromophore-nitroxyl spin probe molecules, the present work proposes a novel optically hyperpolarized EPR imaging (OH-EPRI) method, which combines the optical method of two-photon confocal microscopy for hyperpolarization generation and the rapid scan (RS) EPR method for signal detection. An important aspect of OH-EPRI is that it is not limited by the abovementioned restrictions of conventional EPRI since the large hyperpolarization in the spin probes overcomes the poor thermal spin polarization at RT, and the use of two-photon optical excitation of the chromophore naturally generates the required spatial resolution, without the need for any magnetic field gradient. Simulations based on time-dependent Bloch equations, which took into account both the RS field modulation and the hyperpolarization generation by optical means, were performed to examine the feasibility of OH-EPRI. The simulation results revealed that a spatial resolution of up to 2 fL can be achieved in OH-EPRI at RT under in vitro conditions. Notably, the majority of the requirements for an OH-EPRI experiment can be fulfilled by the currently available technologies, thereby paving the way for its easy implementation. Thus, the proposed method could potentially bridge the sensitivity gap between the optical and magnetic imaging techniques.

Electron paramagnetic resonance imaging for real-time monitoring of Li-ion batteries

Batteries for electrical storage are central to any future alternative energy paradigm. The ability to probe the redox mechanisms occurring at electrodes during their operation is essential to improve battery performances. Here we present the first report on Electron Paramagnetic Resonance operando spectroscopy and in situ imaging of a Li-ion battery using Li₂Ru_0.75Sn_0.25O₃, a high-capacity (>270 mAh g⁻¹) Li-rich layered oxide, as positive electrode. By monitoring operando the electron paramagnetic resonance signals of Ru⁵⁺ and paramagnetic oxygen species, we unambiguously prove the formation of reversible (O₂)ⁿ⁻ species that contribute to their high capacity. In addition, we visualize by imaging with micrometric resolution the plating/stripping of Li at the negative electrode and highlight the zones of nucleation and growth of Ru⁵⁺/oxygen species at the positive electrode. This efficient way to locate ‘electron’-related phenomena opens a new area in the field of battery characterization that should enable future breakthroughs in battery research.

It is important as well as challenging to in situ probe redox mechanisms occurring at battery electrodes. Here, the authors develop an in situ electron paramagnetic resonance imaging technique and provide measurements on the nucleation growth of the anionic and cationic redox species at a battery electrode.

Statistical reconstruction algorithms for continuous wave electron spin resonance imaging

Electron spin resonance imaging (ESRI) is an important branch of ESR that deals with heterogeneous samples ranging from semiconductor materials to small live animals and even humans. ESRI can produce either spatial images (providing information about the spatially dependent radical concentration) or spectral-spatial images, where an extra dimension is added to describe the absorption spectrum of the sample (which can also be spatially dependent). The mapping of oxygen in biological samples, often referred to as oximetry, is a prime example of an ESRI application. ESRI suffers frequently from a low signal-to-noise ratio (SNR), which results in long acquisition times and poor image quality. A broader use of ESRI is hampered by this slow acquisition, which can also be an obstacle for many biological applications where conditions may change relatively quickly over time. The objective of this work is to develop an image reconstruction scheme for continuous wave (CW) ESRI that would make it possible to reduce the data acquisition time without degrading the reconstruction quality. This is achieved by adapting the so-called "statistical reconstruction" method, recently developed for other medical imaging modalities, to the specific case of CW ESRI. Our new algorithm accounts for unique ESRI aspects such as field modulation, spectral-spatial imaging, and possible limitation on the gradient magnitude (the so-called "limited angle" problem). The reconstruction method shows improved SNR and contrast recovery vs. commonly used back-projection-based methods, for a variety of simulated synthetic samples as well as in actual CW ESRI experiments.

The world as viewed by and with unpaired electrons

Recent advances in electron paramagnetic resonance (EPR) include capabilities for applications to areas as diverse as archeology, beer shelf life, biological structure, dosimetry, in vivo imaging, molecular magnets, and quantum computing. Enabling technologies include multifrequency continuous wave, pulsed, and rapid scan EPR. Interpretation is enhanced by increasingly powerful computational models.

Cryogenic electron spin resonance microimaging probe

A new probe for acquiring ESR images with microscopic resolution and high spin sensitivity, at a temperature range of ~4.2-300 K, is presented. Details of the probe design, as well as its principle of operation, are provided. The probe incorporates a unique surface loop-gap microresonator. Experimental results demonstrate the system's capability to acquire two - as well as three-dimensional images with a flat test sample of phosphorus-doped silicon. The imaging results also allow verifying the resonator's resonance mode - they show its B(1) distribution, which also makes it possible to estimate the number of spins measured in the sample.

Detection and imaging of superoxide in roots by an electron spin resonance spin-probe method

The detection, quantification, and imaging of short-lived reactive oxygen species, such as superoxide, in live biological specimens have always been challenging and controversial. Fluorescence-based methods are nonspecific, and electron spin resonance (ESR) spin-trapping methods require high probe concentrations and lack the capability for sufficient image resolution. In this work, a novel (to our knowledge), sensitive, small ESR imaging resonator was used together with a stable spin probe that specifically reacts with superoxide with a high reaction rate constant. This ESR spin-probe-based methodology was used to examine superoxide generated in a plant root as a result of an apical leaf injury. The results show that the spin probe rapidly permeated the plant's extracellular space. Upon injury of the plant tissue, superoxide was produced and the ESR signal decreased rapidly in the injured parts as well as in the distal part of the root. This is attributed to superoxide production and thus provides a means of quantifying the level of superoxide in the plant. The spin probe's narrow single-line ESR spectrum, together with the sensitive imaging resonator, facilitates the quantitative measurement of superoxide in small biological samples, such as the plant's root, as well as one-dimensional imaging along the length of the root. This type of methodology can be used to resolve many questions involving the production of apoplastic superoxide in plant biology.

High-sensitivity Q-band electron spin resonance imaging system with submicron resolution

A pulsed electron spin resonance (ESR) microimaging system operating at the Q-band frequency range is presented. The system includes a pulsed ESR spectrometer, gradient drivers, and a unique high-sensitivity imaging probe. The pulsed gradient drivers are capable of producing peak currents ranging from ∼9 A for short 150 ns pulses up to more than 94 A for long 1400 ns gradient pulses. Under optimal conditions, the imaging probe provides spin sensitivity of ∼1.6 × 10(8) spins∕√Hz or ∼2.7 × 10(6) spins for 1 h of acquisition. This combination of high gradients and high spin sensitivity enables the acquisition of ESR images with a resolution down to ∼440 nm for a high spin concentration solid sample (∼10(8) spins∕μm(3)) and ∼6.7 μm for a low spin concentration liquid sample (∼6 × 10(5) spins/μm(3)). Potential applications of this system range from the imaging of point defects in crystals and semiconductors to measurements of oxygen concentration in biological samples.

A molecular paramagnetic spin-doped biopolymeric oxygen sensor

Electron paramagnetic resonance (EPR) oximetry is a powerful technique capable of providing accurate, reliable, and repeated measurements of tissue oxygenation, which is crucial to the diagnosis and treatment of several pathophysiological conditions. Measurement of tissue pO(2) by EPR involves the use of paramagnetic, oxygen-sensitive probes, which can be either soluble (molecular) in nature or insoluble paramagnetic materials. Development of innovative strategies to enhance the biocompatibility and in vivo application of these oxygen-sensing probes is crucial to the growth and clinical applicability of EPR oximetry. Recent research efforts have aimed at encapsulating particulate probes in bioinert polymers for the development of biocompatible EPR probes. In this study, we have developed novel EPR oximetry probes, called perchlorotriphenylmethyl triester (PTM-TE):polydimethyl siloxane (PDMS) chips, by dissolving and incorporating the soluble (molecular) EPR probe, PTM-TE, in an oxygen-permeable polymer matrix, PDMS. We demonstrate that such incorporation (doping) of PTM-TE in PDMS enhanced its oxygen sensitivity several fold. The cast-molding method of fabricating chips enabled them to be made with increasing amounts of PTM-TE (spin density). Characterization of the spin distribution within the PDMS matrix, using EPR micro-imaging, revealed potential inhomogeneties, albeit with no adverse effect on the oxygen-sensing characteristics of PTM-TE:PDMS. The chips were resistant to autoclaving or in vitro oxidoreductant treatment, thus exhibiting excellent in vitro biostability. Our results establish PTM-TE:PDMS as a viable probe for biological oxygen-sensing, and also validate the incorporation of soluble probes in polymer matrices as an innovative approach to the development of novel probes for EPR oximetry.