Carbon-centered radicals as oxygen sensors for in vivo electron paramagnetic resonance: screening for an optimal probe among commercially available charcoals

It is known that some charcoals possess paramagnetic centers with an electron paramagnetic resonance (EPR) linewidth which can be broadened by oxygen. In order to identify potential candidates as sensors for in vivo EPR oximetry, we carried out a systematic study among commercially available charcoals. A total of 34 charcoals were tested. The steps used for the screening were: (1) to check the presence of paramagnetic centers in the material; (2) to measure the EPR linewidth in nitrogen and in air on the dry material and on a aqueous suspension of particles; (3) to calibrate the oxygen sensitive materials (EPR linewidth vs. pO2); (4) to test the sensitivity and stability of the response to changes of pO2 in a simple model of hypoxia induced in mice. Seventeen charcoals contained paramagnetic centers detectable by low-frequency EPR (1.1 GHz). The EPR spectrum consist of one single line which is typical of carbon-centered radicals (g-factor approximately 2). Eight charcoals presented sufficient interesting EPR properties (linewidth in nitrogen 0.15 mT) to be further characterized in vivo. Only three charcoals presented a stable, reproducible, and sensitive response to pO2 for more than 2 months. These three coals should be considered as good candidates to be used as oxygen sensor using in vivo EPR spectroscopy