Response characteristics of thin film oxygen sensors, Pt and Pd octaethylporphyrins in polymer films

A series of six thin film optical oxygen sensors covering a wide range of sensitivities have been prepared and characterised. The sensor films, made by encapsulation of Pt and Pd octaethylporphyrins in one of three polymers: ethyl cellulose (EC), cellulose acetate butyrate (CAB), and polyvinylchloride (PVC), show a 600-fold variation in sensitivity to oxygen. The variation in sensitivity with lumophore and polymer can be understood in terms of the difference in unquenched lifetime (τ0) for the two porphyrins, together with the differences in oxygen permeability of the polymers used. In all cases non-linear Stern–Volmer plots are observed with a slight downward curvature, indicating some heterogeneity in the sensor films. The sensor response characteristics can be very well modelled using a power law identical to that of the Freundlich isotherm, (I0/I)−1=α(pO2)β, which can be used to give simple and precise calibration data. β gives a measure of deviation from linearity of the Stern–Volmer plot and film heterogeneity, and varies little from sensor to sensor, with βave=0.90±0.03, while α is the Stern–Volmer constant measured between 0 and 1 Torr pO2. The small variation in β from sensor to sensor suggests that the nature of the inhomogeneity within the films is general, and not strongly dependent upon the lumophore or polymer. Examination of sensor response with respect to lumophore τ0 and the oxygen permeabilities of the polymers, PO2, suggests a generalised modified Stern–Volmer equation in terms of a variable which includes all factors influencing the luminescence response, i.e. PO2τ0pO2. This, together with the Freundlich isotherm power law, results in a general equation for the oxygen response of all sensors prepared of the form, (I0/I)−1=Cβ(PO2τ0pO2)β where C is a constant for all films studied. Theory suggests that C=(fL4π(rO2+rL)) where f is a quenching efficiency factor (usually taken to be between 1/9 and 1), L is Avogadro’s number, and (rO2+rL) is the sum of the collision radii for oxygen and lumophore. The experimental value of C=398×1013 mol−1 m compares well with the theoretical value of 416×1013 mol−1 m calculated for f=1. Laser kinetic studies show there to be no significant distribution in τ0 for any of the sensor films prepared, and therefore, the cause of inhomogeneity lies in a distribution of PO2.