We present colorimetric sensory membranes for detecting Hg(II) in aqueous media. The colour response of the sensory materials can be tuned for detection with the naked eye, such as the maximum contaminant level of Hg(II) that is set by the United States Environmental Protection Agency (EPA) for drinking water. Furthermore, the concentration of Hg(II) can be monitored using digital pictures of the membranes taken with conventional cameras. Thus, nanomolar concentration of Hg(II) could be detected by the naked eye due to colour changes of membranes, and the concentration of Hg(II) could be quantified, within the millimolar to nanomolar range, by means of analysing the digital information of pictures taken of the membranes after dipping them in water containing this environmentally poisonous cation.