Controlling the sensitivity of optical oxygen sensors

Abstract

Most optical oxygen sensors work on the principle of luminescence quenching by oxygen. The sensitivity of such sensors depends upon the factors commonly associated with the Stern–Volmer constant, K SV, i.e. the natural lifetime, τ o, of the luminescent electronically excited state and the solubility of oxygen, K H, and quenching rate constant, k Q, in the encapsulating medium. The natural lifetime of the luminescent excited state varies considerably from lumophore to lumophore; thus, the choice of lumophore is an important first decision when creating an optical oxygen sensor for use around a defined oxygen level. The natural lifetimes of most lumophores are not very sensitive to the nature of the encapsulating medium, nor is the solubility of oxygen. However, the rate constant for quenching the luminescent excited state by oxygen appears often to be diffusion controlled, or nearly so. As a consequence, k Q is very sensitive to the oxygen diffusion coefficient, D, for the encapsulating medium. This dependence is so pronounced that by using the same oxygen-quenchable lumophore in a range of different polymer-encapsulating media of different oxygen permeabilities (permeability is directly related to the product of D and K H) a range of oxygen sensors of markedly different oxygen sensitivities can be generated. If a plasticiser is used to improve the oxygen permeability in an oxygen sensor polymer film, the effect is most dramatic when both polymer and plasticizer are highly compatible.