Molecular diffusion in porous media by PGSE ESR.

Diffusion in porous media is a general subject that involves many fields of research, such as chemistry (e.g. porous catalytic pallets), biology (e.g. porous cellular organelles), and materials science (e.g. porous polymer matrixes for controlled-release and gas-storage materials). Pulsed-gradient spin-echo nuclear magnetic resonance (PGSE NMR) is a powerful technique that is often employed to characterize complex diffusion patterns inside porous media. Typically it measures the motion of at least approximately 10(15) molecules occurring in the milliseconds-to-seconds time scale, which can be used to characterize diffusion in porous media with features of approximately 2-3 mum and above (in common aqueous environments). Electron Spin Resonance (ESR), which operates in the nanoseconds-to-microseconds time scale with much better spin sensitivity, can in principle be employed to measure complex diffusion patterns in porous media with much finer features (down to approximately 10 nm). However, up to now, severe technical constraints precluded the adaptation of PGSE ESR to porous media research. In this work we demonstrate for the first time the use of PGSE ESR in the characterization of molecular restricted diffusion in common liquid solutions embedded in a model system for porous media made of sub-micron glass spheres. A unique ESR resonator, efficient gradient coils and fast gradient current drivers enable these measurements. This work can be further extended in the future to many applications that involve dynamical processes occurring in porous media with features in the deep sub-micron range down to true nanometric length scales.

Ex situ endorectal MRI probe for prostate imaging.

A unique ex situ MRI probe, which examines samples external to its geometry, is presented. The probe is intended to be used for imaging the prostate gland via an endorectal pathway. It has a semicylindrical shape with a length of 6 cm and typical diameter of approximately 3 cm. The probe's imaging field of view spans almost along its entire length and up to a distance of 2 cm away from its surface, with an angular sector of approximately 90 degrees . The detailed design of the probe is presented, followed by a set of representative results obtained by the current bench prototype of this system.

Earth field NMR with chemical shift spectral resolution: theory and proof of concept.

A new method for obtaining an NMR signal in the Earth's magnetic field (EF) is presented. The method makes use of a simple pulse sequence with only DC fields which is much less demanding than previous approaches in terms of the pulses' rise and fall times. Furthermore, it offers the possibility of obtaining NMR data with enough spectral resolution to allow retrieving high resolution molecular chemical shift (CS) information - a capability that was not considered possible in EF NMR until now. Details of the pulse sequence, the experimental system, and our specially tailored EF NMR probe are provided. The experimental results demonstrate the capability to differentiate between three types of samples made of common fluorine compounds, based on their CS data.

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.

Electron spin resonance micro-imaging of live species for oxygen mapping.

This protocol describes an electron spin resonance (ESR) micro-imaging method for three-dimensional mapping of oxygen levels in the immediate environment of live cells with micron-scale resolution(1). Oxygen is one of the most important molecules in the cycle of life. It serves as the terminal electron acceptor of oxidative phosphorylation in the mitochondria and is used in the production of reactive oxygen species. Measurements of oxygen are important for the study of mitochondrial and metabolic functions, signaling pathways, effects of various stimuli, membrane permeability, and disease differentiation. Oxygen consumption is therefore an informative marker of cellular metabolism, which is broadly applicable to various biological systems from mitochondria to cells to whole organisms. Due to its importance, many methods have been developed for the measurements of oxygen in live systems. Current attempts to provide high-resolution oxygen imaging are based mainly on optical fluorescence and phosphorescence methods that fail to provide satisfactory results as they employ probes with high photo-toxicity and low oxygen sensitivity. ESR, which measures the signal from exogenous paramagnetic probes in the sample, is known to provide very accurate measurements of oxygen concentration. In a typical case, ESR measurements map the probe's lineshape broadening and/or relaxation-time shortening that are linked directly to the local oxygen concentration. (Oxygen is paramagnetic; therefore, when colliding with the exogenous paramagnetic probe, it shortness its relaxation times.) Traditionally, these types of experiments are carried out with low resolution, millimeter-scale ESR for small animals imaging. Here we show how ESR imaging can also be carried out in the micron-scale for the examination of small live samples. ESR micro-imaging is a relatively new methodology that enables the acquisition of spatially-resolved ESR signals with a resolution approaching 1 micron at room temperature(2). The main aim of this protocol-paper is to show how this new method, along with newly developed oxygen-sensitive probes, can be applied to the mapping of oxygen levels in small live samples. A spatial resolution of ~30 x 30 x 100 µm is demonstrated, with near-micromolar oxygen concentration sensitivity and sub-femtomole absolute oxygen sensitivity per voxel. The use of ESR micro-imaging for oxygen mapping near cells complements the currently available techniques based on micro-electrodes or fluorescence/phosphorescence. Furthermore, with the proper paramagnetic probe, it will also be readily applicable for intracellular oxygen micro-imaging, a capability which other methods find very difficult to achieve.

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.

ESR imaging in solid phase down to sub-micron resolution: methodology and applications.

Electron spin resonance microcopy (ESRM) is an imaging method aimed at the observation of paramagnetic species in small samples with micron-scale spatial resolution. At present, this technique is pursued mainly for biological applications at room temperature and in relatively low static magnetic fields. This work is focused on the use of ESRM for the measurement of solid samples. First, a brief comparison of various electron spin resonance (ESR) detection techniques is provided, with an emphasis on conventional "induction detection". Following that, some methodological details are provided along with experimental examples carried out at room temperature and in a static field of approximately 0.5 T. These examples show for the first time the imaging of solid samples measured by "induction detection" ESR with a resolution better than 1 mum. Based on these experimental examples and capabilities, an outlook for the future prospects of this methodology in terms of spin sensitivity and resolution is provided. It is estimated that single-spin sensitivity could be achieved for some samples at liquid-helium temperatures and static fields of approximately 2 T. Furthermore, under these conditions, spatial resolution could reach the nanometer scale. Finally, a description of possible applications of this new methodology is provided.