Data analysis and aging in phosphorescent oxygen-based sensors

The stretched exponential analysis of the photoluminescence (PL) decay curves of the oxygen-sensitive dye Pt octaethylporphyrin (PtOEP) embedded in a polystyrene (PS) film and used in gas-phase oxygen, dissolved oxygen (DO), glucose, and lactate sensors is discussed. Light emitting diodes (LEDs) and organic LEDs (OLEDs) served as the pulsed excitation sources for the PL. Typically, the stretched exponential analysis resulted in excellent fits of the oxygen-quenched PL decay curves, superior to the single exponential analysis (including an offset) and other models, in particular at the higher oxygen levels. While some previous studies of gas-phase oxygen sensors analyzed the decay curves with a single value of the stretching factor β (which was not suitable in this work), and other studies used the product of a single exponential and a stretched exponential with a fixed β, in this study only the stretched exponential term was used with β as a variable. As a result, β was found to decrease with increasing O2 concentration ([O2]), from β = 1, i.e., a simple exponential decay, at gas-phase [O2] = 0 and [DO] = 0. The effect of doping the PtOEP:PS films with 360 nm titania particles (which enhance the PL) on the data analysis was also examined. In general, the TiO2 increased the PL decay time and β. The results indicate that a distribution of O2:dye collision rates, induced by the microheterogeneity of the sensor films, is responsible for the non-exponential decay kinetics. The [O2]-dependent β is possibly associated with shallow multiple quencher trapping sites in the PS matrix that affect the frequency of dye:O2 collisions. The TiO2 particles may possibly increase O2 trapping at their surface, reducing the effective concentration of O2 molecules that collide with the dye, and thus increasing the PL decay time and β. Additionally, the long-term stability, data analysis, and detection sensitivity of the DO sensor during and following one-year aging, with the sensing film constantly immersed in water, are described. The findings impact commercial PL-based DO sensors.