Preparation and application of l-cysteine-doped Keggin polyoxometalate microtubes

l-cysteine-doped tungstosilicate (Lcys-SiW12) microtubes are prepared, and the amount of l-cysteine doped in the microtubes can be tuned to some extent. The as-prepared Lcys-SiW12 microtubes are sensitive to ammonia gas exhibited through the distinct color change of the microtubes from light purple to dark blue after exposing to ammonia gas. A possible mechanism of the coloration is that the adsorbed ammonia molecules increase the basicity of the Lcys-SiW12 microtubes and promote the redox reaction between l-cysteine and polyoxometalate. This is a pH-dependent solid–solid redox reaction, which is triggered by proton capture agent. The Lcys-SiW12 microtubes show application in chemical sensors for alkaline gases.