Stable Particulate Paramagnetic Materials as Oxygen Sensors in EPR Oximetry

Abstract

Particulate materials with a strong, exchange-narrowed electron paramagnetic resonance (EPR) absorption line, can be used as oxygen sensors in EPR oximetry. Advantages of this kind of sensor include a single, nearly Lorentzian line shape which depends on T_2e, size flexibility (from less than a micron to several millimeters), very low toxicity, and good chemical stability in biological environments. Two basic types of particulate sensors have been developed: carbon-based sensors derived from coal or synthesized from wood, and lithium phthalocyanine (LiPc). Both types of material report on local oxygen concentration through a variation in EPR line width with [O₂], an approach sometimes called T₂ oximetry. Interaction between oxygen molecules and unpaired electrons in the sensor is thought to proceed via Heisenberg spin exchange. Microscopy reveals very different morphology for LiPc and chars, the former being a crystalline structure and the latter being a complex network of channels and pores. These structural differences manifest themselves in the different behaviors of the two classes of sensor materials. Models of coal and char behavior, involving electron spin exchange, can explain the line shape behavior of these materials under various oxygen concentrations. Magnetic resonance and dynamic nuclear polarization can be employed along with other physical methods to develop a detailed picture of char surface area, porosity, and the interaction of the surface with water.