Ratiometric oxygen sensing using the tunable ratio of phosphorescence to fluorescence emissions from gadolinium porphyrin and porphyrin

Abstract

The monitoring of photosensitizers (PSs) and tissue oxygen are both important in photodynamic therapy. PSs can be monitored by the magnetic resonance imaging of Gd-porphyrins. Dissolved oxygen measurements based on hematoporphyrin monomethyl ether coordinated to trivalent gadolinium ion (Gd-HMME) and free base HMME were studied for determining the PSs and tissue oxygen concentrations simultaneously. Fluorescence of HMME is independent of oxygen, while phosphorescence of Gd-HMME is observed to be sensitive to oxygen. Ratiometric oxygen sensing based on Gd-HMME phosphorescence and HMME fluorescence was proposed, and the relationship between the ratio of phosphorescence to fluorescence and oxygen concentration was found to be linear. The relative uncertainty of the measured values for oxygen concentration reaches its minimum when the fluorescence intensity equals the phosphorescence intensity because of the obtainment of high signal-to-noise ratio for them. The uncertainty of the measured values for 62.5μM oxygen was 1.2μM, which indicates that the measurement range and precision of the oxygen measurement system we proposed can reach 0–300μM and 1.9%, respectively. For detection of oxygen at different concentrations, the measurement uncertainty can be decreased by using HMME and Gd-HMME with suitable concentration ratio to equalize the fluorescence and phosphorescence intensities. Our results indicate that the ratiometric oxygen detection method can fulfill the requirement for tissue oxygen evaluation.