A water-soluble, non-sulfur, 2,4-dimethyl-7-amino-1,8-naphthyridine (Z 1 ) chemosensor was synthesized to fluorescently sense Hg 2+ in water. The single crystal structure of the sensor confirmed the synthesis results. The detection limit of the sensor towards Hg 2+ was 8.859 Â 10 À8 M, which indicates its high detection sensitivity. The immediate response of Z 1 to Hg 2+ provided a real-time detection method.Mercury is currently used in industrial chemicals, electrical apparatus, dental amalgam, and batteries. The highly poisonous nature of mercury and its oxidized species poses a severe risk to human health and the environment. 1-4 To detect and monitor mercury species in various circumstances, efficient detection methods are required. Among various detection methods for mercury species, uorescent probes have received much attention due to their sensitive and easy-to-operate features.Many uorescent probes for Hg 2+ have been developed so far. 5-9 Most of these traditional chemosensors contain a sulfur moiety and the sensing process involves an irreversible coordination of Hg 2+ to the S atom. 10 The mechanism involved in traditional sensors includes an affinity irreversible organic reaction driven by the extremely strong Hg-S affinity. (1) Intramolecular cyclic guanylation of thiourea derivatives, (2) ring opening of spirocyclic systems (rhodamine and uorescein, etc.), and (3) conversion of thiocarbonyl compounds into their carbonyl analogues or a sequential desulfurizationlactonization reaction; this thiophilic approach will suffer from potential interference in sulfur-rich environments where mercury is abundant 11 (Scheme 1). Correspondingly, most of the traditional chemosensors, which contain a sulfur moiety and a sensing process involving an irreversible coordination of Hg 2+ to the S atom, are not recyclable. It is therefore desirable to create recyclable sensors for Hg 2+ .On the other hand, in biological and environmental systems, Hg 2+ sensor interactions commonly occur in aqueous solution, therefore, much attention has been paid to developing Hg 2+ sensors that work in the aqueous phase. However, water soluble Hg 2+ sensors are still very scarce.In view of this requirement and as part of our research effort devoted to ion recognition, 12 we herein report a water-soluble, non-sulfur, and efficient uorescent sensor Z 1 which can sense Hg 2+ with specic selectivity and high sensitivity in water. Z 1 was prepared in a reasonable yield (41% overall) by the synthetic route outlined in Scheme S1 (see ESI †). 13 C NMR (ESI, Fig. S1 †) spectra conrm the structure of the sensor. We also investigated the single-crystal structures of the sensor Z 1 (ESI, Fig. S2 †). This molecule combines naphthyridine as the signaling subunits, while the presence of two nitrogen atoms and amino groups, which can form multiple hydrogen bonds in water, enhance the water solubility of the sensor. In addition, the presence of nitrogen atoms that easily bind with the Hg 2+ Scheme 1 Tactics for the sensing of Hg 2+ .