Here we describe, a simple solution and solid-state sensor for the ultrasensitive detection of Hg(II) ions in both standardized and environmental samples through changes of fluorescence intensity of a zinc-based metal-organic framework (MOF). The MOF is highly water stable in a wide pH range from 4 to 11. The lower detection limit of Hg2+ is 10(-11) M with a very high binding constant of 1.011 X 10(9) M-1 s(-1). It also exhibits a high selectivity toward mercury ion in the presence of other interfering metal ions. The MOF is found to retain its efficiency in detecting mercury ion in spiked environmental water samples.