OXYGEN EFFECTS ON THE EPR SIGNALS FROM WOOD CHARCOALS: RESULTS AND NEW MODEL

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The quantitative measurement of pO2 in living tissue has important clinical and research applications, and EPR oximetry has considerable potential for these measurements. This technique requires a material with EPR-detectable paramagnetic centers and involves concentration dependent perturbation of the EPR spectral properties by oxygen. A number of molecular radicals (e.g. nitroxides, trityls) and materials with paramagnetic centers, including certain carbon blacks, coals and charcoals and crystals of lithium phthalocyanine (LiPc), have been studied for their potential as probes for EPR oximetry. The EPR spectra of nitroxides or trityls, dissolved in aqueous solution typically exhibits ~0.5 mG/micromole broadening by dissolved oxygen. However, certain charcoals synthesized from tropical hardwoods by means of controlled pyrolysis contain EPR-detectable species that can exhibit 50-100 mG/micromole broadening by dissolved oxygen, and this response is even larger in the gas phase. Successful application of in vivo EPR oximetry requires the development of oxygen-sensitive paramagnetic materials with optimized properties. Currently, LiPc and related compounds are widely used, but there are several probe criteria (sensitivity, stability, reversibility, selectivity, etc.) that need to be optimized for different applications of EPR oximetry. In particular, there is a need for materials with increased sensitivity to oxygen at the low levels found in tumors and at the higher levels found in the brain. To rationally develop these materials, however, requires an understanding of the physical basis and mechanism of oxygen effects on the char paramagnetic centers. Further, this understanding may guide the improvement of in vivo data analysis by incorporating physical principles into the spectral model. Charcoals have certain physical properties that are relevant to their interaction with gases. They are highly porous materials with a surface area that is inversely proportional to the pore diameter. Charcoal pores are generally classified in three sizes based on their diameter, macropores (>50 nm), mesopores (50-2 nm) and micropores (<2 nm). In addition, they are hydrophobic materials with poor water penetration, particularly into smaller pores. Finally, chars are known for their ability to adsorb gases on their large surface area. We have studied the effect of oxygen on the EPR spectral properties of wood charcoals. These include quantifying the concentration of spins, and determining the temperature-dependence and frequency-dependence of the EPR spectrum. Three components are required to fit the O2-broadened EPR spectrum of the chars, and the oxygen dependence of the line width, intensity and resonance position of the three components have been measured.