Effect of humidity on the response characteristics of luminescent PtOEP thin film optical oxygen sensors

Abstract

Steady-state and time-resolved studies have shown that humidity has a significant effect on the sensitivity of luminescent platinum octaethylporphyrin (PtOEP) thin film oxygen sensors when hydrophilic polymers are used as a matrix. When using ethyl cellulose (EC) as the polymer matrix sensitivity is almost halved at 85% relative humidity (RH) as compared to 0% RH. For cellulose acetate butyrate sensitivity is reduced by 14% at 85% RH, while for sensors using silicones, polystyrene (PS) and polyvinylchloride as polymer matrix humidity is not an important factor in sensor response characteristics. The degree of influence humidity has on the sensitivity of films made using hydrophilic polymers can be altered by plasticization, when using EC the humidity dependence can be removed by use of tributylphosphate as plasticizer at a concentration of 40 parts per hundred resin. In the absence of oxygen luminescence decay kinetics can be well fitted to a double exponential curve with 95% of the total luminescence originating from a state with lifetime of 95 μs (similar to that for PtOEP in fluid solution) and 5% from a state with lifetime of 20 μs. However, time-resolved studies of oxygen quenching give data which are inconsistent with a simple dual site model for oxygen quenching of the PtOEP lumophore. The similarity of decay curves under conditions of similar quenching efficiencies at 0 and 85% RH suggest that the effect of humidity lies in changes to the oxygen permeability rather than change in the distribution of lumophore sites in the polymer.