An improved optical sensor and methods for measuring the presence of various materials or constituents in a fluid sample uses one or more reactive materials in a fluid environment. The reactive materials have optical properties that change in the presence of one or more target materials that may be present in the environment. One or more optical emitters generate light that is directed to the reactive material(s), and one or more optical detectors receive light from the reactive material(s), and the presence or absence of the target material is determined based on the light received at the optical detector(s). The reactive material(s), emitter(s), and detector(s) are selected based on the desired target material to be sensed.