Impact of scattering medium on singlet oxygen luminescence generation in photodynamic therapy

Investigating singlet oxygen (¹O₂) generation in a heterogeneous medium such as biological tissue is important in quantifying tumor treatment efficacy during photodynamic therapy (PDT). In this study, we measured the variation in ¹O₂ generation in different liquid phantom media using an in-house technique called time-resolved singlet oxygen luminescence detection (TSOLD). The TSOLD system includes a supercontinuum laser with single-mode optical fiber, customized optical filtering, mirrors, a cuvette holder, a cuvette containing the photosensitizer solution, a single-photon avalanche diode (SPAD) detector with multimode fiber, and time-tagger electronics. The liquid phantoms were prepared by mixing various concentrations of intralipid solution in different solvents such as deionized water, ethanol, acetone, and deuterated water. Rose Bengal (RB) powder at 10μg/g was dissolved in the phantom to generate ¹O₂ under light exposure. Increasing the intralipid concentration from 0 to 1% led to a corresponding decrease in the ¹O₂ luminescence signal of ~7.5-fold, and ~1.5-fold for RB-embedded liquid phantoms prepared with deionized water, and ethanol respectively. Moreover, phantoms prepared using deuterated water (D2O), exhibit a ~4-fold reduction in ¹O₂ luminescence when the intralipid concentration was increased within the range of 0 to 0.1%. The lifetime of ¹O₂ for RB is significantly influenced by the solvent, with values of ~4μs in deionized water, ~ 14μs in ethanol, ~48μs in acetone, and ~59μs in D₂O. Also, a significant impact of the intralipid concentration on the ¹O₂ lifetime was observed. These results showed that the scattering of the medium can significantly reduce ¹O₂ generation.