Enhanced oxygen sensing sensitivity by eliminating the protection of triplet phosphorescence.

High oxygen sensitivity (the slope of the Stern-Volmer plot reaches 0.73/μM) is achieved with a phosphorescence indicator, gadolinium-hematoporphyrin monomethyl ether (Gd-HMME), by decreasing the extent of its protection. In air-saturated solution, the phosphorescence quantum efficiency (QE) of Gd-HMME in a non-rigid microenvironment is lower than that in a rigid microenvironment. In contrast, when oxygen is removed, the QE of Gd-HMME in the non-rigid microenvironment was found to be same as that of Gd-HMME in the rigid microenvironment. This indicates that Gd-HMME is much more sensitive to oxygen in the non-rigid microenvironment. The oxygen sensitivity of Gd-HMME was found to increase as the rigidity of its microenvironment decreases. The oxygen response of Gd-HMME without any protection reaches 240 (0-374 μM oxygen), whereas that in the rigid microenvironment is only 3 in this range. The measurement precision of Gd-HMME without any protection is lower than that in the rigid microenvironment. These results indicate that the measurement of oxygen in different concentration ranges would require the rigidity of the microenvironment to be varied. Gd-HMME without any protection can be applied to detect oxygen as low as 0.1 μM. The detection limit of oxygen was evaluated to be as low as 20 nM based on Gd-HMME without any protection.