An apparatus for sensing a chemical of interest in a fluid of interest includes: a tube permeable to the chemical of interest, having a polymer, and configured to be disposed into the fluid of interest; and a reactant disposed in a hollow portion of the tube and configured to react with the chemical of interest causing a change to transmissiveness of light. The apparatus further includes: a light source configured to illuminate the reactant; a photodetector configured to detect light traversing the reactant; a processor coupled to the photodetector and configured to determine a rate of change of detected light in response to the chemical of interest reacting with the reactant in order to sense the chemical of interest; and a reactant purging system in fluid communication with the hollow portion and configured to purge out pre-existing reactant present in the hollow portion with new unreacted reactant.