Abstract: Certain embodiments of the present invention relate to the preparation of microbeads that exhibit a “turn on” fluorescence response within seconds of exposure to an analyte vapor (e.g., a chemical warfare agent or a reactive stimulant). This sensing approach is modeled after the mechanism for inhibition of acetylcholinesterase enzyme activity, and utilizes a specific and irreversible reaction between phosphonyl halides and a fluorescent indicator. The present invention also relates to a sensor and a method for sensing an analyte through detection of changes in the fluorescing properties of the inventive microbeads.