Local molecular spectroscopy using the evanescent microwave probe technique: electron spin resonance

A method based on evanescent microwave microscopy (EMM) was developed to spatially resolve small number of electron spin resonant (ESR) transitions in single crystal ruby (Cr³⁺). The EMM probe operates at a resonance frequency of 3.77 GHz (S-band ESR) in a modulated electromagnetic field in the range of 0.02 to 0.6 Tesla. The current EMM probe with a magnetic dipole loop of around 1 mm radius was capable of resolving 20,000 spin transitions compared to standard ESRs with ~10⁷ detection limit. We are currently improving the spatial resolution of our probe to enable detection of 2000 spin transitions with the goal of reducing it to a few spins by using a microwave probe integrated with an atomic force microscope. Even with its relatively low current resolution, the probe is unique and can be used in microfluidic channels and biological tissues to detect free radicals. We are pursuing applications of the probe in Alzheimer and other tissue studies where mapping the spatial distribution of free radicals may shed some light on the cause and diagnosis of disease