Analysis of Singlet Oxygen Luminescence Generated By Protoporphyrin IX.

Abstract

The effectiveness of photodynamic therapy (PDT) for cancer treatment relies on the generation of cytotoxic singlet oxygen (¹O₂) in type II PDT. Hence, monitoring of ¹O₂ generation during PDT enables optimal treatment delivery to the tumor target with reduced off-target effects. Direct ¹O₂ observation by measuring its luminescence at 1270 nm remains challenging due to the very weak signal. This study presents ¹O₂ luminescence measurements using a time-resolved singlet oxygen luminescence detection system (TSOLD) applied to protoporphyrin IX (PpIX) in different solvents (ethanol and acetone) and biological media (bovine serum albumin and agarose-based solid phantom). The compact experimental setup includes a nanosecond diode laser with a function generator, a cuvette with photosensitizer solution, optical filtering and mirrors, an InGaAs single-photon avalanche diode detector, and time-tagger electronics. Increasing the concentration of PpIX in these media from 1 to 10 µg/g resulted in a 3-5 × increase in the ¹O₂ luminescence signal. Furthermore, increasing light scattering in the sample using Intralipid from 0.1 to 1% led to a decrease in the ¹O₂ luminescence signal and lifetime. These results confirm the marked effect of the microenvironment on the ¹O₂ signal and, hence, on the photodynamic efficacy.