Determination of singlet oxygen quantum yield based on the behavior of solvent dimethyl sulfoxide oxidation by singlet oxygen.

Background: Photodynamic therapy (PDT) is emerging as a promising cancer treatment. The PDT efficacy is primarily attributed to the generation of singlet oxygen (O), stemming from the integrated effects of the photosensitizer, oxygen, and light. The singlet oxygen quantum yield (Φ) serves as a bridge that links these parameters to the overall efficacy of PDT. The near-infrared luminescence of O provides a direct way for determining Φ, but suffers from a poor signal-to-noise ratio. While the chemical trap probe method is detection-friendly, but it has a strict requirement for the excitation wavelength. Therefore, the existing methods for Φ measurement are insufficient.

Results: In this work, we developed an approach to determine Φ of a broader range of photosensitizers using only the commonly used solvent dimethyl sulfoxide (DMSO), which can be oxidized by O to dimethyl sulfone. This method establishes the relationship between O production and changes in DMSO absorption spectra, eliminating the need for additional chemical probes. This method was validated by measuring the Φ of rose bengal (RB) through systematic changes in absorption spectrum of DMSO under various RB concentrations and different excitation light power densities. Moreover, the Φ of hematoporphyrin monomethyl ether (HMME), as determined by this method, is consistent with measurements obtained using the 1,3-diphenylisobenzofuran (DPBF) trapping probe. This consistency further validates the reliability of this method.

Significance And Novelty: This work presents a direct, probe-free method to determine Φ, reducing potential interference and expanding the range of useable excitation wavelengths. Its ability to measure Φ using only DMSO enhances the accuracy of photosensitizer measurement, and broadens the applicability of the method to a wide range of samples, thereby advancing research on the properties of photosensitizers and further promoting the development of PDT.