High resolution electron spin resonance microscopy

Recent advances in microresonators and supporting instrumentation for electron paramagnetic resonance spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy characterizes the magnetic properties of paramagnetic materials at the atomic and molecular levels. Resonators are an enabling technology of EPR spectroscopy. Microresonators, which are miniaturized versions of resonators, have advanced inductive-detection EPR spectroscopy of mass-limited samples. Here, we provide our perspective of the benefits and challenges associated with microresonator use for EPR spectroscopy. To begin, we classify the application space for microresonators and present the conceptual foundation for analysis of resonator sensitivity. We summarize previous work and provide insight into the design and fabrication of microresonators as well as detail the requirements and challenges that arise in incorporating microresonators into EPR spectrometer systems. Finally, we provide our perspective on current challenges and prospective fruitful directions.

Development of a scanning tunneling microscope for variable temperature electron spin resonance

Recent advances in improving the spectroscopic energy resolution in scanning tunneling microscopy (STM) have been achieved by integrating electron spin resonance (ESR) with STM. Here, we demonstrate the design and performance of a homebuilt STM capable of ESR at temperatures ranging from 1 to 10 K. The STM is incorporated with a homebuilt Joule-Thomson refrigerator and a two-axis vector magnet. Our STM design allows for the deposition of atoms and molecules directly into the cold STM, eliminating the need to extract the sample for deposition. In addition, we adopt two methods to apply radio-frequency (RF) voltages to the tunnel junction: the early design of wiring to the STM tip directly and a more recent idea to use an RF antenna. Direct comparisons of ESR results measured using the two methods and simulations of electric field distribution around the tunnel junction show that, despite their different designs and capacitive coupling to the tunnel junction, there is no discernible difference in the driving and detection of ESR. Furthermore, at a magnetic field of ∼1.6 T, we observe ESR signals (near 40 GHz) sustained up to 10 K, which is the highest temperature for ESR-STM measurement reported to date, to the best of our knowledge. Although the ESR intensity exponentially decreases with increasing temperature, our ESR-STM system with low noise at the tunnel junction allows us to measure weak ESR signals with intensities of a few fA. Our new design of an ESR-STM system, which is operational in a large frequency and temperature range, can broaden the use of ESR spectroscopy in STM and enable the simple modification of existing STM systems, which will hopefully accelerate a generalized use of ESR-STM.

Electron-Spin-Resonance Dipstick

Electron spin resonance (ESR) is a powerful analytical technique used for the detection, quantification, and characterization of paramagnetic species ranging from stable organic free radicals and defects in crystals to gaseous oxygen. Traditionally, ESR requires the use of complex instrumentation, including a large magnet and a microwave resonator in which the sample is placed. Here, we present an alternative to the existing approach by inverting the typical measurement topology, namely placing the ESR magnet and resonator inside the sample rather than the other way around. This new development relies on a novel self-contained ESR sensor with a diameter of just 2 mm and length of 3.6 mm, which includes both a small permanent magnet assembly and a tiny (∼1 mm in size) resonator for spin excitation and detection at a frequency of ∼2.6 GHz. The spin sensitivity of the sensor has been measured to be ∼10¹¹ spins/√Hz, and its concentration sensitivity is ∼0.1 mM, using reference samples with a measured volume of just ∼10 nL. Our new approach can be applied for monitoring the partial pressure of oxygen in vitro and in vivo through its paramagnetic interaction with another stable radical, as well as for simple online quantitative inspection of free radicals generated in reaction vessels and electrochemical cells via chemical processes.

Recent trends in high spin sensitivity magnetic resonance

Magnetic resonance is a very powerful methodology that has been employed successfully in many applications for about 70years now, resulting in a wealth of scientific, technological, and diagnostic data. Despite its many advantages, one major drawback of magnetic resonance is its relatively poor sensitivity and, as a consequence, its bad spatial resolution when examining heterogeneous samples. Contemporary science and technology often make use of very small amounts of material and examine heterogeneity on a very small length scale, both of which are well beyond the current capabilities of conventional magnetic resonance. It is therefore very important to significantly improve both the sensitivity and the spatial resolution of magnetic resonance techniques. The quest for higher sensitivity led in recent years to the development of many alternative detection techniques that seem to rival and challenge the conventional "old-fashioned" induction-detection approach. The aim of this manuscript is to briefly review recent advances in the field, and to provide a quantitative as well as qualitative comparison between various detection methods with an eye to future potential advances and developments. We first offer a common definition of sensitivity in magnetic resonance to enable proper quantitative comparisons between various detection methods. Following that, up-to-date information about the sensitivity capabilities of the leading recently-developed detection approaches in magnetic resonance is provided, accompanied by a critical comparison between them and induction detection. Our conclusion from this comparison is that induction detection is still indispensable, and as such, it is very important to look for ways to significantly improve it. To do so, we provide expressions for the sensitivity of induction-detection, derived from both classical and quantum mechanics, that identify its main limiting factors. Examples from current literature, as well as a description of new ideas, show how these limiting factors can be mitigated to significantly improve the sensitivity of induction detection. Finally, we outline some directions for the possible applications of high-sensitivity induction detection in the field of electron spin resonance.

Electron Paramagnetic Resonance of a Single NV Nanodiamond Attached to an Individual Biomolecule

Electron paramagnetic resonance (EPR), an established and powerful methodology for studying atomic-scale biomolecular structure and dynamics, typically requires in excess of 10(12) labeled biomolecules. Single-molecule measurements provide improved insights into heterogeneous behaviors that can be masked in ensemble measurements and are often essential for illuminating the molecular mechanisms behind the function of a biomolecule. Here, we report EPR measurements of a single labeled biomolecule. We selectively label an individual double-stranded DNA molecule with a single nanodiamond containing nitrogen-vacancy centers, and optically detect the paramagnetic resonance of nitrogen-vacancy spins in the nanodiamond probe. Analysis of the spectrum reveals that the nanodiamond probe has complete rotational freedom and that the characteristic timescale for reorientation of the nanodiamond probe is slow compared with the transverse spin relaxation time. This demonstration of EPR spectroscopy of a single nanodiamond-labeled DNA provides the foundation for the development of single-molecule magnetic resonance studies of complex biomolecular systems.

Atomic-like spin noise in solid-state demonstrated with manganese in cadmium telluride

Spin noise spectroscopy is an optical technique which can probe spin resonances non-perturbatively. First applied to atomic vapours, it revealed detailed information about nuclear magnetism and the hyperfine interaction. In solids, this approach has been limited to carriers in semiconductor heterostructures. Here we show that atomic-like spin fluctuations of Mn ions diluted in CdTe (bulk and quantum wells) can be detected through the Kerr rotation associated to excitonic transitions. Zeeman transitions within and between hyperfine multiplets are clearly observed in zero and small magnetic fields and reveal the local symmetry because of crystal field and strain. The linewidths of these resonances are close to the dipolar limit. The sensitivity is high enough to open the way towards the detection of a few spins in systems where the decoherence due to nuclear spins can be suppressed by isotopic enrichment, and towards spin resonance microscopy with important applications in biology and materials science.

Coupling an atomic vapour to the polarization of light has enabled the creation of spin squeezed and entangled atomic states. Here, the authors realize a solid-state approach by adapting spin noise spectroscopy to probe the spin of manganese ions in cadmium telluride.

A magnetic resonance probehead for evaluating the level of ionizing radiation absorbed in human teeth

A miniature electron spin resonance (ESR) probehead that includes a static field source and a microwave resonator for in vivo measurement of paramagnetic defects in tooth enamel was developed. These defects are known to be a good marker for quantifying the ionizing radiation dose absorbed in teeth. The probehead has a typical length of just 30 mm and total weight of 220 g. The patient "bites" into the probehead while the measurement procedure is being carried out. The probehead operates in pulsed mode at a frequency of ∼ 11.2 GHz and supplies a static magnetic field of ∼ 400 mT. A detailed design of the probehead is provided together with its specifications in terms of measurement volume and signal-to-noise ratio for a typical sample. A specially developed simulation program was used to predict the spatial distribution of the acquired signal under conditions of grossly inhomogeneous static and RF fields. Experimental results with irradiated incisor teeth validated the probehead's sensitivity, being able to detect signals in tooth irradiated by only 2 Gy. Subject to additional improvements and tests, this type of probehead can potentially have significant clinical applications ranging from mass triage following major nuclear events to routine occupational evaluation of ionizing radiation absorbed over long periods of time.

Echo-based Single Point Imaging (ESPI): a novel pulsed EPR imaging modality for high spatial resolution and quantitative oximetry

A novel time-domain spectroscopic EPR imaging approach, that is a unique combination of already known techniques, is described. The first one is multi-gradient Single Point Imaging involving pure phase-encoding where the oximetry is based on T(2)(∗). Line width derived from T(2)(∗) is subject to susceptibility effects and therefore needs system-dependent line width calibrations. The second approach utilizes the conventional 90°-τ-180° Spin-Echo pulse sequence where the images are obtained by the filtered back-projection after FT of the echoes collected under frequency-encoding gradients. The spatially resolved oximetry information is derived from a set of T(2)-weighted images. The back-projection images suffer susceptibility artifacts with resolution determined by T(2)(∗), but the oximetry based on T(2) is quite reliable. The current approach combines Single Point Imaging and the Spin-Echo procedure to take advantage the enhanced spatial resolution associated with the former and the T(2) dependent contrast of the latter. Pairs of images are derived choosing two time points located at identical time intervals on either side of the 180° pulse. The refocusing pulse being exactly in the middle of the two points ensures that artifacts associated with susceptibility and field inhomogeneities are eliminated. In addition, the net phase accumulated by the two time points being identical results in identical field of views, thus avoiding the zoom-in effect as a function delay in regular SPI and the associated interpolation requirements employed in T(2)(∗)-weighted oximetry. The end result is superior image resolution and reliable oximetry. In spite of the fact that projection-reconstruction methods require less number of measurements compared to SPI, the enormous advantage in SNR of the SPI procedure makes the echo-based SPI equally efficient in terms of measurement time. The Fourier reconstruction, line width independent resolution and the true T(2)-weighting make this novel procedure very attractive for in vivo EPR imaging of tissue oxygen quantitatively.

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.

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.

Microimaging of oxygen concentration near live photosynthetic cells by electron spin resonance

We present what is, to our knowledge, a new methodology for high-resolution three-dimensional imaging of oxygen concentration near live cells. The cells are placed in the buffer solution of a stable paramagnetic probe, and electron spin-resonance microimaging is employed to map out the probe's spin-spin relaxation time (T(2)). This information is directly linked to the concentration of the oxygen molecule. The method is demonstrated with a test sample and with a small amount of live photosynthetic cells (cyanobacteria), under conditions of darkness and light. Spatial resolution of approximately 30 x 30 x 100 microm is demonstrated, with approximately microM oxygen concentration sensitivity and sub-fmol absolute oxygen sensitivity per voxel. The use of electron spin-resonance microimaging for oxygen mapping near cells complements the currently available techniques based on microelectrodes or fluorescence/phosphorescence. Furthermore, with the proper paramagnetic probe, it will also be readily applicable for intracellular oxygen microimaging, a capability which other methods find very difficult to achieve.

Sensitive surface loop-gap microresonators for electron spin resonance

This work presents the design, construction, and experimental testing of unique sensitive surface loop-gap microresonators for electron spin resonance (ESR) measurements. These resonators are made of "U"-shaped gold structures with typical sizes of 50 and 150 μm that are deposited on a thin (220 μm) rutile substrate and fed from the rear by a microstrip line. This allows accommodating a large flat sample above the resonator in addition to having variable coupling properties. Such resonators have a very small volume which, compared to previous designs, improves their absolute spin sensitivity by a factor of more than 2 (based on experimental results). They also have a very high microwave field-power conversion ratio of up to 86 gauss/√Hz. This could facilitate the use of very short excitation pulses with relatively low microwave power. Following the presentation and the discussion of the experimental results, ways to further increase sensitivity significantly are outlined.

Force-detected nuclear magnetic resonance: recent advances and future challenges

We review recent efforts to detect small numbers of nuclear spins using magnetic resonance force microscopy. Magnetic resonance force microscopy (MRFM) is a scanning probe technique that relies on the mechanical measurement of the weak magnetic force between a microscopic magnet and the magnetic moments in a sample. Spurred by the recent progress in fabricating ultrasensitive force detectors, MRFM has rapidly improved its capability over the last decade. Today it boasts a spin sensitivity that surpasses conventional, inductive nuclear magnetic resonance detectors by about eight orders of magnitude. In this review we touch on the origins of this technique and focus on its recent application to nanoscale nuclear spin ensembles, in particular on the imaging of nanoscale objects with a three-dimensional (3D) spatial resolution better than 10 nm. We consider the experimental advances driving this work and highlight the underlying physical principles and limitations of the method. Finally, we discuss the challenges that must be met in order to advance the technique towards single nuclear spin sensitivity-and perhaps-to 3D microscopy of molecules with atomic resolution.

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

Microcrystals of lithium octa-n-butoxynaphthalocyanine (LiNc-BuO) in a bio-compatible and oxygen-permeable polymer matrix of poly-dimethyl-siloxane (PDMS) can be used for repetitive non-invasive imaging of oxygen in live specimens by means of mm-scale electron spin resonance (ESR) imaging. This probe denoted as "oxychip" was characterized by high-resolution mum-scale ESR microcopy to reveal the fine details of its spatial and spectral properties. The ESR micro-images of a typical oxychip device showed that while the spatial distribution of the microcrystals in the polymer is fairly homogenous (as revealed by optical microscopy), the ESR signal originates only from a very few dominant crystals. Furthermore, spectral-spatial analysis in a microcrystal and a sub-microcrystal spatial resolution reveals that each crystal has a slightly different g-factor and also exhibits variations in linewidth, possibly due to the slightly different individual crystallization process.

Deducing 1D concentration profiles from EPR imaging: a new approach based on the concept of virtual components and optimization with the genetic algorithm

Application of the genetic algorithm (GA) in conjunction with the concept of virtual components (VC) to determine 1D concentration profiles from EPRI spectra (images) is described. In this approach the concentration profile is expressed as the superposition of virtual components described by analytical functions of the Gaussian and Boltzmann type. The method was implemented in the computer program ACon, which allows for fully automated profile extraction via the nonlinear least-squares fitting of experimental images. The parametric sensitivity of the GA internal parameters such as population size, probabilities of the crossover, mutation and elitist retention to the search space was investigated in detail in order to find their optimal settings. The customized genetic algorithm was evaluated using simulated and experimental test data sets and its performance was compared with the Monte Carlo approach.

Current density imaging by pulsed conduction electron spin resonance

In analogy with Nuclear MRI, the ESR signal phase shift of conduction electrons moving in electrical currents along controlled magnetic field gradients can be used to generate spatial electronic current density maps. First two-dimensional images of the current density distribution in quasi-one-dimensional organic conductors are presented.

Stray field magnetic resonance tomography using ferromagnetic spheres

The methodology for obtaining two- and three-dimensional magnetic resonance images by using azimuthally symmetric dipolar magnetic fields from ferromagnetic spheres is described. We utilize the symmetric property of a geometric sphere in the presence of a large externally applied magnetic field to demonstrate that a complete two- or three-dimensional structured rendering of a sample can be obtained without the motion of the sample relative to the sphere. Sequential positioning of the integrated sample-sphere system in an external magnetic field at various angular orientations provides all the required imaging slices for successful computerized tomographic image reconstruction. The elimination of the requirement to scan the sample relative to the ferromagnetic tip in this imaging protocol is a potentially valuable simplification compared to previous scanning probe magnetic resonance imaging proposals.

Electron spin resonance microscopy applied to the study of controlled drug release

We describe our recent developments towards 3D micron-scale imaging capability, based on electron spin resonance (ESR), and its application to the study of controlled release. The method, termed ESR microscopy (ESRM), is an extension of the conventional "millimeter-scale" ESR imaging technique. It employs paramagnetic molecules (such as stable radicals or spin-labeled drugs) and may enable one to obtain accurate 3D spatially resolved information about the drug concentration, its self-diffusion tensor, rotational correlation time and the pH in the release matrix. Theoretical calculations, along with initial experimental results, suggest that a 3D resolution of approximately 1 microm is feasible with this method. Here we were able to image successfully a high spin concentration sample with a resolution of approximately 3 x 3 x 8 microm and subsequently study a single approximately 120 microm biodegradable microsphere, internalized with a dilute solution of trityl radical, with a resolution of approximately 12.7 x 13.2 x 26 microm. Analysis of the microsphere ESR imaging data revealed a likely increase in the viscosity inside the sphere and/or binding of the radical molecule to the sphere matrix. Future directions for progress are also discussed.

Conduction-electron drift velocity measurement via electron spin resonance

In analogy with NMR, motion induced phase shift of pulsed ESR signals enables in principle the direct detection of electron drift velocity or electronic current, respectively. Overcoming the difficulties with additional magnetic field gradients induced by the current itself, we succeeded in demonstrating the detection of electron flow via ESR. Measuring the electron drift velocity in the organic conductor (fluoranthene)2PF6 the microscopic Ohmic law could be observed in a current range of more than +/-0.25 A.

Nanomagnetic planar magnetic resonance microscopy "lens"

The achievement of three-dimensional atomic resolution magnetic resonance microscopy remains one of the main challenges in the visualization of biological molecules. The prospects for single spin microscopy have come tantalizingly close due to the recent developments in sensitive instrumentation. Despite the single spin detection capability in systems of spatially well-isolated spins, the challenge that remains is the creation of conditions in space where only a single spin is resonant and detected in the presence of other spins in its natural dense spin environment. We present a nanomagnetic planar design where a localized Angstrom-scale point in three-dimensional space is created above the nanostructure with a nonzero minimum of the magnetic field magnitude. The design thereby represents a magnetic resonance microscopy "lens" where potentially only a single spin located in the "focus" spot of the structure is resonant. Despite the presence of other spins in the Angstrom-scale vicinity of the resonant spin, the high gradient magnetic field of the "lens" renders those spins inactive in the detection process.

The solution conformation of triarylmethyl radicals

Hyperfine coupling tensors to 1H, 2H, and natural abundance 13C were measured using X-band pulsed electron nuclear double resonance (ENDOR) spectroscopy for two triarylmethyl (trityl) radicals used in electron paramagnetic resonance imaging and oximetry: methyl tris(8-carboxy-2,2,6,6-tetramethyl-benzo[1,2d:4,5-d']bis(1,3)dithiol-4-yl) and methyl tris(8-carboxy-2,2,6,6-tetramethyl(-d3)-benzo[1,2d:4,5-d']bis(1,3)dithiol-4-yl). Quantum chemical calculations using density functional theory predict a structure that reproduces the experimentally determined hyperfine tensors. The radicals are propeller-shaped with the three aryl rings nearly mutually orthogonal. The central carbon atom carrying most of the unpaired electron spin density is surrounded by the sulfur atoms in the radical and is completely shielded from solvent. This structure explains features of the electron spin relaxation of these radicals and suggests ways in which the radicals can be chemically modified to improve their characteristics for imaging and oximetry.

Functional and Highly Spatially Resolved ESR Imaging

Electron spin resonance imaging is a technique using similar principles to the established nuclear magnetic resonance imaging; however, the very fast electron-spin relaxation time constants of the former are an experimental challenge. In order to obtain T2-weighted as well as functional images spatially resolved in the micrometer range, we use temperature-dependent pulsed X-band ESR and the back-projection reconstruction method. We present here recent results of our imaging project focussing on quasi one-dimensional organic conductors, where even pulsed conduction electron-spin resonance is possible.

Composite nanowire-based probes for magnetic resonance force microscopy

We present a nanowire-based methodology for the fabrication of ultrahigh sensitivity and resolution probes for atomic resolution magnetic resonance force microscopy (MRFM). The fabrication technique combines electrochemical deposition of multifunctional metals into nanoporous polycarbonate membranes and chemically selective electroless deposition of optical nanoreflector onto the nanowire. The completed composite nanowire structure contains all the required elements for an ultrahigh sensitivity and resolution MRFM sensor with (a) a magnetic nanowire segment providing atomic resolution magnetic field imaging gradients as well as large force gradients for high sensitivity, (b) a noble metal enhanced nanowire segment providing efficient scattering cross-section from a sub-wavelength source for optical readout of nanowire vibration, and (c) a nonmagnetic/nonplasmonic nanowire segment providing the cantilever structure for mechanical detection of magnetic resonance.

Single spin detection by magnetic resonance force microscopy

Magnetic resonance imaging (MRI) is well known as a powerful technique for visualizing subsurface structures with three-dimensional spatial resolution. Pushing the resolution below 1 micro m remains a major challenge, however, owing to the sensitivity limitations of conventional inductive detection techniques. Currently, the smallest volume elements in an image must contain at least 10(12) nuclear spins for MRI-based microscopy, or 10(7) electron spins for electron spin resonance microscopy. Magnetic resonance force microscopy (MRFM) was proposed as a means to improve detection sensitivity to the single-spin level, and thus enable three-dimensional imaging of macromolecules (for example, proteins) with atomic resolution. MRFM has also been proposed as a qubit readout device for spin-based quantum computers. Here we report the detection of an individual electron spin by MRFM. A spatial resolution of 25 nm in one dimension was obtained for an unpaired spin in silicon dioxide. The measured signal is consistent with a model in which the spin is aligned parallel or anti-parallel to the effective field, with a rotating-frame relaxation time of 760 ms. The long relaxation time suggests that the state of an individual spin can be monitored for extended periods of time, even while subjected to a complex set of manipulations that are part of the MRFM measurement protocol.