The fluorophore derived from stilbene with two cyano groups in a single aromatic ring shows reasonably large two-photon absorption cross-section and emits strong two-photon fluorescence. A water-soluble two-photon sensor composed of the fluorophore containing bis[2-(2-hydroxyethylsulfanyl)ethyl]amino group (ionophore) has a very high sensitivity and selectivity for Hg 2+ in water. It can be used to detect trace Hg 2+ in near neutral aqueous solution (pH>6.5), and its binding constant (logK) for Hg 2+ is as high as 5.72±0.03. Nitrogen atom property and the sulfur atom number on the ionophore are believed to be responsible for the unique selectivity of probe 1 for Hg 2+ .