M.Sc Thesis

M.Sc StudentTalmon Yael
SubjectElectron Spin Resonance Microscopy Applied to Observation
of Diffusion Processes
DepartmentDepartment of Chemistry
Supervisor PROF. Aharon Blank
Full Thesis textFull thesis text - English Version


The critical role of diffusion in the mechanism of various biological and chemical processes has led researches to an ongoing search for methods to measure its main characteristic, the diffusion coefficient.  The measurements of diffusion occurring over relatively long distances (of at least a few µm) and during relatively long times (of a few milliseconds), can be carried out with techniques such as nuclear magnetic resonance (NMR) or fluorescence recovery after photo-bleaching (FRAP), and are quite established.  However, methods for the direct measurements of diffusion occurring over short distances (of ~10-6- ~10-8 meter) and during short times (of a few µsec) have not yet been developed.  Our first goal in this work was to establish experimental methods for the direct measurements of such fast motion, both restricted and unrestricted by electron spin resonance (ESR).  We showed that by utilizing a well known NMR pulse sequence (the pulse gradient spin echo - PGSE), in ESR experiments, one can measure the diffusion coefficients over short range distances of few hundred nm occurring during 10-100µsec.  In order to adapt PGSE to ESR we had to develop high sensitivity micro-resonators and a capability to generate very intense and short gradient pulses of ~1 micosecond in length and ~100 T/m in magnitude.  Our preliminary work included the measurements of the unrestricted diffusion process of three types of radical solutions: trityl radical in water, N@C60 in 1-chloronaphthalene and N@C60 in CS2.  Following these experiments we have measured restricted diffusion in porous media of N@C60 in CS2 and of trityl in water.  The experimental results were compared to the theoretical expectations for both cases and revealed an excellent agreement.  Our second goal was to find a way to measure slower diffusion, in the time scales of hours, days or even weeks occurring in a controlled release process. Specifically we want to look at the release of a drug out of polymeric microspheres.  In that aspect we have established a protocol to investigate this slow diffusion by methods of ESR micro imaging.  Here we measured the reciprocal process of trityl radical diffusion into the polymeric microsphere.  In the future this or similar radicals could be coupled to a drug molecule and thus be used to examine the diffusion process of the drug out of the sphere by imaging.  Furthermore the drug motion inside the polymeric porous environment could be measured by the PGSE ESR experiment.