A new chemosensor RF1 that combines a ferrocene unit and a rhodamine block via the linkage of a carbohydrazone binding unit was designed and prepared for the highly selective detection of Hg2+ in natural water. This chemosensor displays great brightness and fluorescence enhancement following Hg2+ coordination within the limit of detection for Hg2+ at 1 parts per billion (ppb). The fluorescence intensities are nearly proportional to the amount of Hg2+ at the ppb level. It is capable of distinguishing between the safe and the toxic levels of inorganic mercury in drinking water. Hg2+-binding also arouses the absorption of the rhodamine moiety in RF1 significantly with the chromogenic detection limit for Hg2+ at 50 ppb. The conventional UV-vis spectroscopic method thus has the potential to provide the critical information about the mercury hazard assessment for industrial wastewater discharging. The obvious and characteristic color change of the titration solution from colorless to pink upon the addition of Hg2+ demonstrates that RF1 can be used for "naked-eye" detection of Hg2+ in water. The Hg2+ complexation also causes a significant shift of the redox potential about the ferrocene/ferrocenium couple. The electrochemical responses provide the possibility to quantitative analysis of Hg2+ at the parts per million (ppm) level. Preliminary investigations in natural water samples including seawater and freshwater indicate that RF1 offers a direct and immediate Hg2+ detection in complex media, pointing out its potential utility in environment monitoring and assessment. The responses of RF1 are Hg2+ specific, and the chemosensor exhibits high selectivity toward Hg2+ over other Group 12 metals, alkali, alkaline earth metals, and most of the divalent first-row transition metals. The RF1-Hg2+ complex is successfully isolated and the Hg2+-binding is reversible. The crystal structure and spectral properties of its congener RF2 that contains one ferrocene group and two rhodamine 6G moieties were also investigated for a comparison.