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 (¹O₂), stemming from the integrated effects of the photosensitizer, oxygen, and light. The singlet oxygen quantum yield (Φ_Δ) serves as a bridge that links these parameters to the overall efficacy of PDT. The near-infrared luminescence of ¹O₂ provides a direct way for determining Φ_Δ, 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_Δ measurement are insufficient.

Results: In this work, we developed an approach to determine Φ_Δ of a broader range of photosensitizers using only the commonly used solvent dimethyl sulfoxide (DMSO), which can be oxidized by ¹O₂ to dimethyl sulfone. This method establishes the relationship between ¹O₂ production and changes in DMSO absorption spectra, eliminating the need for additional chemical probes. This method was validated by measuring the Φ_Δ 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_Δ 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_Δ, reducing potential interference and expanding the range of useable excitation wavelengths. Its ability to measure Φ_Δ 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.