We report a sensor for mercuric ion based on polyaniline modified with cryptand-222. The transduction process is deprotonation and reprotonation of polyaniline following the protonation and deprotonation, respectively, of cryptand-222 on binding with Hg2+ ion. The sensor response has been investigated under various conditions, like the amount of receptor, pH, and the electrochemical state of the polymer. The device has sensitive segments in the range of 10(-12)-10(-8) M in various conditions, which is an important region as per World Health Organization guideline for drinking water. The interference on the sensor response with other metal ions has been investigated. (c) 2007 Elsevier B.V. All rights reserved.