Effect of PpIX photoproducts formation on pO2 measurement by time-resolved delayed fluorescence spectroscopy of PpIX in solution and in vivo

- Photoproducts disturbs pO2 measurement based on PpIX delayed fluorescence in vivo.
- pO2 measurement must be performed between 620 and 640 nm to avoid photoproduct bias.
- Low doses as a fraction of J/cm2 at 405 nm can be used to excite PpIX.

The measurement of Protoporphyrin IX delayed fluorescence lifetime is a minimally invasive method for monitoring the levels of oxygen in cells and tissues. The excitation of Protoporphyrin IX during this measurement can lead to the formation of photoproducts in vitro and in vivo. The influence of their luminescence on the measured Protoporphyrin IX delayed fluorescence lifetimes was studied in solution and in vivo on the Chick's chorioallantoic membrane (CAM) model under various oxygen enriched air conditions (0 mm Hg, 37 mm Hg and 155 mm Hg). The presence of photoproducts disturbs such measurements since the delayed fluorescence emission of some of them spectrally overlaps with that of Protoporphyrin IX. One possible way to avoid this obstacle is to detect Protoporphyrin IX's delayed fluorescence lifetime in a very specific spectral range (620-640 nm). Another possibility is to excite Protoporphyrin IX with light doses much lower than 10 J/cm2, quite possibly as low as a fraction 1 J/cm2 at 405 nm. This leads to an increased accuracy of pO2 detection. Furthermore, this method allows combination of diagnosis and therapy in one step. This helps to improve detection systems and real-time identification of tissue respiration, which is tuned for the detection of PpIX luminescence and not its photoproducts.

Graphical Abstract