NOVELTY - Preservation of catalytic activity of solid sulfonic acid catalyst polymer involves immobilizing an organic acid in the solid sulfonic acid catalyst polymer. The pKa value of immobilized acid is higher than the pKa value of solid sulfonic acid catalyst polymer.    USE - Preservation of catalytic activity of solid sulfonic acid catalyst polymer used for forming perfluorosulfonic acid polymer membrane e.g. Nafion (RTM: sulfonated tetrafluoroethylene-based fluoropolymer-copolymer) membrane used in optical sensor used for portable optical sensing device for detecting target compound e.g. formaldehyde in fuel cell effluent and volatile organic compound such as acetone in exhaled human breath, and estimating blood glucose concentration for treating diabetes (all claimed).    ADVANTAGE - The method enables efficient preservation of catalytic activity of solid sulfonic acid catalyst polymer, which provides portable optical sensing device with excellent reliability in measurement of concentration of volatile organic compounds in high-humidity and/or high-salt environment in medical diagnostics and determination of fuel cell efficiency, by simple and economical process.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) optical sensor, which consists of perfluorosulfonic acid polymer membrane comprising immobilized organic acid and immobilized organic reagent capable of reacting with a target compound for producing a color shifted product, where exposure of optical sensor to the target compound in a humid environment produces a detectable color shift on the optical sensor;    (2) real-time non-invasive method for estimating blood glucose concentration in human based on calculated concentration of acetone in exhaled breath of human, which involves providing perfluorosulfonic acid polymer membrane, exposing the perfluorosulfonic acid polymer membrane to volume of exhaled breath, measuring concentration of product of reaction based on detected color shift, calculating the concentration of acetone in the volume of exhaled breath from the concentration of product measured, and correlating the concentration of acetone to a standard blood glucose concentration;    (3) method for treating diabetic patient, which involves monitoring blood glucose level in patient across a time frame by estimating blood glucose concentration, and adjusting the patient's insulin levels; and    (4) manufacture of optical sensor, which involves immersing perfluorosulfonic acid polymer membrane in a solution comprising organic reagent and a suitable solvent for period of time, rinsing and drying the membrane, soaking dried membrane in a solution comprising organic acid dissolved in an imbibing solvent comprising mineral oil until rate of solution uptake by membrane reaches zero, and rinsing and drying the membrane.