Phosphorescence decay time measurements using intensity correlation spectroscopy

In this paper, we report on phosphorescence measurements for oxygen dynamics in cells by means of a correlation method, which is an expansion of the fluorescence correlation spectroscopy. The intensity correlation function of the emission excited by a pulsed light source was measured. With changing the pulse timing, both the fluorescence correlation function and the decay time of phosphorescence could be analyzed. This method was applied for the analysis of the oxygen dynamics in HeLa cells stained by Pd(II)-porphine. The decay function consisted of two exponential components, which might be attributed to free and protein-bound forms of Pd(II)-porphine in the cell, respectively. The relative change of the oxygen concentration under normal and uncoupled respiration conditions was also measured. The simplicity of this method is a great advantage in the biological applications. Although the current system we used was limited in the temporal resolution, the method is in principle applicable to faster decay time measurements down to the nano-second range of the fluorescence decay times.