Methods for preserving catalytic activity of a PSA polymer membrane in a humid environment by immobilizing in the membrane an organic acid having a pKa greater than the pKa of the PSA polymer membrane optical sensors based on the PSA membranes further including an immobilized organic reagent capable of reacting with a target compound in a humid environment to produce a detectable color shifted product and non-invasive methods for estimating blood glucose concentration by utilizing an optical sensor to detect concentration of acetone in exhaled human breath and correlating it to blood glucose concentration.
