HARNESSING ELECTRON SPIN LABELS FOR SINGLE MOLECULE MAGNETIC RESONANCE IMAGING

Abstract

Having a universal platform for subsurface, three-dimensional, nanoscale imaging of individual biomolecules or biomolecular complexes would be an enabling advance. By detecting magnetic resonance mechanically, as a force or force-gradient, on an ultrasensitive microcantilever, magnetic resonance force microscopy (MRFM) couples the sensitivity of scanning probe microscopy with the isotopic specificity of magnetic resonance. While MRFM offers the depth of view and sensitivity required for magnetic resonance imaging with nanometer resolution, imaging experiments to date have been largely limited by long signal acquisition times and low signal-to-noise imparted by imaging small spin ensembles. In the limit of detecting nuclear magnetic moments or electron spin labels from a single biomolecule, the magnetic resonance signal is largely dominated by statistical polarization fluctuations --- so-called `spin noise' --- rather than the well-defined Curie-law magnetization detected in macroscale magnetic resonance experiments. In this dissertation, we discuss the development of a microscope with the sensitivity required to perform three-dimensional nuclear magnetic resonance imaging with nanometer resolution and imaging of nitroxide spin radicals at the single spin level. We propose and demonstrate protocols to overcome the spin-noise limit by generating hyperthermal proton spin magnetization via microwave-assisted dynamic nuclear polarization for the first time in a MRFM experiment. The modest 10 to 20 times enhancement in nuclear spin magnetization reported in this work is sufficient to push a spin ensemble containing ~$10^5$ proton spins, the approximate size of a protein, out of the spin-noise limit and into the regime where thermal polarization is dominant. Furthermore, we discuss microscope developments and sample preparation protocols to reduce the cantilever frequency noise experienced by a nanomagnet tipped cantilever operating near a sample surface by nearly two orders of magnitude. With these methods implemented, we propose a spin modulation protocol capable of detecting and imaging individual electron spin probes affixed to a biological sample. Promising preliminary results show the sensitivity required for imaging a single electron spin with an attonewton sensitivity cantilever. The magnetic resonance force microscope experiments described here offer new paths to achieving high-resolution, single molecule magnetic resonance imaging --- an extremely exciting prospect.