Development of novel one-dimensional (1D) solid sensors for the highly selective and sensitive detection of 2,4,6-trinitrophenol (TNP) in aqueous media has attracted considerable attention. Herein, we describe how we fabricated a novel polydiacetylene (PDA) microtube and demonstrated that a single PDA microtube optical waveguide could serve as a novel optical sensor with fast response and high selectivity for TNP probing. The use of a single PDA microtube optical waveguide not only bestows the sensor with a small footprint, fast response and high sensitivity, but also enables its integration into the chip for real-time remote detection in the near future.