A new and an easy-to-make colorimetric azo-pyridine, 1, and its recyclable mesoporous silica-immobilized nanoparticles for nerve-agent detection are synthesized. The binding site, comprising an azo-pyridine moiety, is capable of selectively sensing diethylchlorophosphate (DCP), one of the nerve-agent mimics of chemical-warfare agents, over a series of other phosphate compounds. Compound 1 shows ratiometric changes in absorption spectroscopy to the extent of a 60 nm red-shift upon the addition of DCP, mainly due to a change in the intramolecular charge transfer (ICT) in 1. The color change of receptor 1 from yellow to red in the concentration region ≈1.0 × 10−6M is sufficient for the selective detection of the DCP nerve-agent mimic by the naked eye. With regards to solid-phase application, mesoporous silica nanoparticles using 1 (MSIAP) are also prepared using a sol-gel grafting reaction. The color of the MSIAP also changes from red to yellow when dipped into an aqueous DCP solution, and turns back to red when treated with NaOH solution, with nontoxic diethylphosphoric acid being given off. The absorption changes of MSIAP in the presence of DCP are consistent within the 3–9 pH range.