Use of silver/octadecanethiol coating and a reference-gas correction algorithm to minimize the water effect in determining oxygen with a light emission based optical sensor

Humidity and water condensation may influence the response of light intensity based oxygen optical sensors by changing the nature of the gas–membrane interface. Their effect were reduced and compensated by using an active membrane protective coating and a suitable mathematical algorithm. The coating was made of an octadecanethiol self-assembly monolayer deposited on a silver film directly in contact with the polysulfone/platinum porphyrin sensing membrane. The composite protective layer reduced the interface influence on the signal and produced a two-fold enhancement of the signal without increasing significantly the sensor response time. Bare membranes exhibited a 7.7% irreversible decrease of the Stern–Volmer constant after calibrations with water-saturated gas mixtures. On the other hand, coated membranes were less affected by humidity (with a Stern–Volmer constant irreversible decrease lower than 2%) and were not affected by water condensation on the membrane surface. In these conditions it was possible to apply a reference gas based algorithm to fully correct the negative effect. In particular, mean accuracies in determining the oxygen percentage over 15 calibrations in wet and dry gas were −0.02 (0.11) for naked membrane in the absence of water condense and −0.04 (0.14) for coated one.