Hydrogen peroxide is a dynamic signaling molecule in biological systems. We report herein a versatile double emulsion sensor that can detect femtomolar quantities of aqueous hydrogen peroxide. The mechanism responsible for this sensitivity is a peroxide induced change in double emulsion structure, which results in a modified directional emission from dyes dissolved in the high index organic phase. The morphology (structure) of the double emulsion is controlled via interfacial tensions and a methyltrioxorhenium catalyzed sulfide oxidation results in an enhancement of the surfactant effectiveness. The incipient polar sulfoxide induced decrease of the interfacial tension at the organic-water (O-W) interface results in an increased interfacial area between the organic phase and water and a diminished emission perpendicular to the supporting substrate. The modularity of our sensory system is demonstrated through cascade catalysis between methyltrioxorhenium and oxidase enzymes, with the latter producing hydrogen peroxide as a byproduct to enable for the selective and sensitive detection of molecular and ionic enzymatic substrates.