Synthesis and properties of three novel rhodamine-based fluorescent sensors for Hg^2

Three novel rhodamine-based Hg^2＋ fluorescent sensors were designed and synthesized. The sensors could work in semi-aqueous solutions with nearly neutral p H and showed high selectivity and sensitivity to Hg^2＋ with remarkable fluorescence enhancement. For these three sensors, the linear working range broadened（0–80, 0–100 and 0–140 μmol/L, respectively） and the sensitivity increased（7.7, 15.5 and 17.6 folds of the fluorescence enhancement and 512, 66.2 and 37.6 ppb of the detection limit） with the rising of the thiourea-unit numbers. Furthermore the sensors exhibited excellent interference immunity to multiple environmentally and biologically relevant metal ions. Pond and tap water assay showed good practicability of the sensors. The number of the bound Hg~（2＋） equaling to that of the thiourea units and the irreversible recognition process implied a new interaction way between Hg~（2＋） and the sensor.