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. (c) 2013 Elsevier B. V. All rights reserved.