Proteinaceous microspheres filled with nitroxides dissolved in an organic liquid have been synthesized for the first time using high intensity ultrasound; these were used to measure oxygen concentrations in living biological systems. The microspheres have an average size of 2.5 microns, and the proteinaceous shell is permeable to oxygen. Encapsulation of the nitroxides into the microsphere greatly increased the sensitivity of the electron paramagnetic resonance signal line width to oxygen because of the higher solubility of oxygen in organic solvents. The encapsulation also protected the nitroxide from bioreduction. No decrease in intensity of the electron paramagnetic resonance signal was observed during 70 min after intravenous injection of the microspheres into a mouse. Measurement of the changes in oxygen concentration in vivo by means of restriction of blood flow, anesthesia, and change of oxygen content in the respired gas were made using these microspheres.
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