The reaction between biologically green synthesized silver nanoparticles (Ag NPs) and mercury (II) ions was introduced as a new and high potential calorimetric sensor for the selective recognition and monitoring of mercuric ions in aqueous samples. The green synthesized silver nanoparticles were characterized with surface plasmon resonance (SPR) ultraviolet spectroscopy (UV-vis), SEM and X-ray diffraction analysis (XRD) techniques. The fresh biologically synthesized silver nanoparticles are yellowish-brown in color due to the intense SPR absorption band. In the presence of Hg2+, the yellow Ag NPs solution was turned to colorless, accompanying the broadening and blue shifting of SPR band. The sensitivity and selectivity of green prepared Ag NPs toward other representative transition-metal ions, alkali metal ions and alkaline earth metal ions were studied. Also the effect of the concentration of Hg2+ to the Ag NPs was considered and the LOD for mercury (II) ion was 2.2 x 10(-6) mol L-1. The proposed method has been successfully used for the determination of mercury (II) ions in various water samples. (C) 2011 Elsevier B.V. All rights reserved.