Intracellular oxygen measurement in vivo by smartphone readout of endogenous Protoporphyrin IX delayed fluorescence

Direct measurement of mitochondrial oxygen tension in vivo provides direct information on tissue metabolism and could facilitate new approaches in disease detection, function monitoring, and treatment efficacy assessment with cellular level data, with far superior sensitivity to oxygen changes than blood saturation measures. Here, a platform system was engineered to quantify fast sampling of oxygen partial pressure (pO₂) inside tissue by utilizing the inherent rolling shutter readout of a smartphone CMOS camera detector for measuring the oxygen-sensitive time-resolved delayed fluorescence (DF) signal from Protoporphyrin IX (PpIX) which naturally occur in mitochondria for most tissues. The CMOS rolling shutter readout produces a microsecond-level time difference in the pixels row-by-row detection of light, here utilized as a time-gated shutter to sample the time distributed DF intensity. This novel technique eliminated the necessity of high-speed intensified camera with excitation isolation facility as well as advanced precise time synchronization system as required in the conventional time-resolved fluorescence lifetime measurement platforms for quantifying time-dependent very low intensity DF distribution of PpIX conjugated with prompt fluorescence. Both steady state and dynamic performance of the instrument were validated in tissue phantoms at different PpIX concentrations (0.5-10 μM) for wide range of pO₂ detection (0-160 mmHg) with tunable fast response time (1-3.5 s) which is substantially faster than electrode-based systems that measure over 10's of seconds. Finally, it was tested in vivo to assess the impact of dynamic inhaled oxygen concentration variation on skin tissue pO₂ under the conditions of normoxia, hyperoxia and hypoxia.