Room Temperature High Resolution Electron Spin Resonance
Most molecules in nature contain even number of electrons. It is known that each electron behaves like a tiny permanent magnet and normally these electrons come in pairs in which the magnetic field of each electron cancels out the field of its counterpart.
Some molecules, however, contain odd number of electrons, or have even number of electrons but with “partial pairing”. These molecules are called “paramagnetic” and they can be characterized through their magnetic behavior. When paramagnetic molecules are placed in a strong static magnetic field, their tiny magnets can be aligned either parallel or anti-parallel to this field. It was found that when by applying these samples, in addition to the static field, with an alternating magnetic field, one can induce transitions between these two states. The transition from state to state is most effective when the frequency, v, of the alternating field matched the energy difference between the parallel and anti-parallel potions (trough the relation E=hv, where h Planck const).
The notion that the transition frequency can be tuned by varying the static magnetic field led people to realize that by applying static magnetic field gradient, one can spatially encode the sample and obtain an image. In other words, an inhomogeneous sample that is subjected to a range of static magnetic fields (position dependant) , will exhibit many transition frequencies where each frequency can be traced back to a specific location in the sample. This principle is the basis for the well known clinical Magnetic Resonance Imaging (MRI), which makes use of the tiny proton magnets rather than the electron magnets.
Today, commercial systems that produce MRI images (based on protons) with a resolution of ~ 10 micron resolution are widely available. Commercial imaging system for paramagnetic samples has ~ 30-50 micron resolution. These resolution figures are not good enough for modern biological research of live cells and related phenomenon, which is mainly done with optical microscopes having ~ 0.2 micron resolution.
Our aim in this project is to significantly increase the resolution of electron spin resonance imaging, performed at ambient conditions, down to the sub-micron level. This capability will lead to a variety of applications in single cell and in-vitro samples imaging such as O2 concentration mapping, imaging of free radicals generation, imaging of viscosity and other parameters that are not accessible by optical methods.
The project involves the development of state-of-the-art pulsed and continuous wave electron spin resonance imaging spectrometer that can transmit and receive microwave signals in the 6-18, 35 and 60 GHz range and produce ultra-strong magnetic field gradients. In addition we develop a micro-imaging probe that includes sensitive resonator and efficient gradient coils, along with unique sample contraries that enable easy sample preparation, maximize the signal from the sample, and facilitate the use of combined electron spin resonance and optical microscopy, which is part of this project.
Images and Results
- High-resolution ESR micro-images of LiPc crystals in two test samples. (a) Optical image of the crystals in the first sample. (b) Optical image of the crystals in the second sample. (c) 2D ESR image of the first sample with in plane resolution of ~ 0.951.1 mm. (d) Same as (c) but for the second sample. (e) 2D slice out of a 3D ESR image of the second sample with in plane resolution of ~0.951.1 mm and slice thickness of ~ 12.5 mm. (f) The same as (e) but for a different 2D slice. The color scale represents the voxel signal intensity in a scale that is normalized to the strongest voxel signal in each image.
- Optical and ESR images of N@C60:C60 powder inserted into photolithography-prepared letters. (a) Optical image showing the powder inside the patterns (the lithography thickness is 120 mm). (b) ESR 2D image of the pattern. Here the gradient X-axis of the imaging probe is along the image y-axis (the sample was inserted to the probe with the words written along the Y gradient axis). (c) 1D cut of the ESR image along in the image x-axis (at y = 225 microns), showing the level of sharpness of the patterns imaged.
Amplitude and T2 images of a heterogeneous sample made of four different types of N@C60:C60 powder with N@C60 enrichment levels of 0.1, 0.4, 0.8 and 1.6%. (a) Optical image showing the powder grains dispersed in a specially prepared 4 quadrant pattern (by photolithography). (b) ESR amplitude image. (c) ESR T2 image. (d) Histogram showing the distribution of T2 vales in the ESR T2 image.