The response time of optical sensors based on luminescence quenching

Abstract

The response time of optical sensors based on dynamic quenching usually shows an asymmetry. The response is faster when the signal decreases than when it increases. This has been adequately explained by Opitz and Lübbers for a sensing film (SF) obeying strictly the Stern–Volmer relationship and for a homogeneous quencher concentration. In the first part of this paper, we extend this treatment to any response function and to a non-homogeneous quencher concentration in the SF. For downward curved or linear Stern–Volmer (SV) plots, the response time is always shorter when the signal decreases than when it increases; whereas for upward curved SV plots the asymmetry of the response depends on the definition of the response time, i.e. the fraction of the final response that must be reached (usually 90 or 99%). In the second part, various computer simulations were performed with a view to test the effect of gas mixing during transport from the cylinders to the measuring cell, the relative importance of the rate constants for crossing the gas/polymer phase interface and the rate constant for gas diffusion inside the SF. The calculated data are compared with those obtained on two polystyrene films of different thicknesses. It is found that gas mixing in the inlet tubing has a marginal effect on the response time, and that a quencher gradient in the film affects the response time. A by-product of the computations is an approximate value of the O 2 diffusion coefficient in polystyrene which is found to be in reasonable agreement with that recently measured by Ogilby: D O 2 (computed)=7.7×10 −7 cm 2 s −1; D O 2 (experimental)=2.3×10 −7 cm 2 s −1 at 25°C.