THE RESPONSE-TIME OF OPTICAL SENSORS BASED ON LUMINESCENCE QUENCHING

The response time of optical sensors based on dynamic quenching usuall y shows an asymmetry. The response is faster when the signal decreases than when it increases. This has been adequately explained by Opitz a nd Lubbers for a sensing film (SF) obeying strictly the Stern-Volmer r elationship 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 SE For downwar d curved or linear Stern-Volmer (SV) plots, the response time is alway s shorter when the signal decreases than when it increases; whereas fo r upward curved SV plots the asymmetry of the response depends on the definition of the response time, i.e. the fraction of the final respon se that must be reached (usually 90 or 99%). In the second part, vario us computer simulations were performed with a view to test the effect of gas mixing during transport from the cylinders to the measuring cel l, 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 p olystyrene films of different thicknesses. It is found that gas mixing in the inlet tubing has a marginal effect on the response time, and t hat a quencher gradient in the film affects the response time. A by-pr oduct of the computations is an approximate value of the O-2 diffusion coefficient in polystyrene which is found to be in reasonable agreeme nt with that recently measured by Ogilby: D-O2 (computed)=7.7 x 10(-7) cm(2) s(-1); D-O2 (experimental)=2.3 x 10(-7) cm(2) s(-1) at 25 degre es C. (C) 1998 Elsevier Science B.V.