Iridium-Coumarin Ratiometric Oxygen Sensors

Ratiometric sensing is the simplest and most accurate method for measuring O2. Our sensors use an oxygen insensitive fluorescence signal and an oxygen attenuated phosphorescence signal. A ratio made from these two signals allows for the definitive measurement of oxygen concentrations. Oxygen measurements taken with ratiometric oxygen sensors remove the calibration inaccuracies resulting from oxygen sensor measurements in heterogeneous environments. We produced a new ratiometric oxygen sensor for hypoxic environments (pO2 ≤ 160 mmHg) with a blue-fluorescent part attached to the phosphorescent iridium. The color of phosphorescence observed is dependent on the ligands appended to the iridium. Adjustment of the spacer between the two modulates energy transfer to ensure both components' simultaneous luminescence. The most recent candidate shows improved signal resolution between the oxygen-insensitive blue fluorescence and the oxygen-sensitive red phosphorescence over a physiologically relevant range of concentrations. This candidate has the desired dual luminescence due to the increased intramolecular spacing between the coumarin and iridium, which inhibits Dexter energy transfer. The effect of this spacer alteration highlights one of the challenges in developing molecules composed of parts both capable of spin-forbidden and spin-allowed luminescence. A few applications for ratiometric oxygen sensors include investigating tumor growth rates and informing ground control of oxygen deprivation. We have reasonably minimized the overlapping of fluorescence and phosphorescence in sensor 2 and continue to improve our designs.