Highly Hg2+-sensitive and selective fluorescent sensors in aqueous solution and sensors-encapsulated polymeric membrane

Abstract

Two Hg²⁺ chemosensors, rhodamine B hydrazide (RBH) and rhodamine 6G hydrazide (R6GH), were synthesized by a single step. In the presence of Hg²⁺, both of the sensors RBH and R6GH exhibited highly sensitive OFF–ON fluorescence enhancement. Importantly, the sensors showed a selective binding to Hg²⁺ over other common metal ions such as K⁺, Na⁺, Fe³⁺, Ca²⁺, Cu²⁺, Ag⁺ and Pb²⁺ in aqueous solutions. The OFF–ON fluorescence enhancement upon Hg²⁺ binding could be ascribed to conformational change of the spirolactam moiety of the rhodamine fluorophore through a spirolactam ring opening process. Furthermore, encapsulation of the sensors by polymeric membranes (PMMA) provided high selectivity and high sensitivity, especially for the sensors-encapsulated polymeric membrane RBH that showed an extremely low detection limit (0.2 ppb), which was 245 times lower than the sensor RBH in aqueous solution. In addition to fluorescence enhancement, the presence of Hg²⁺ also induced a noticeable color change from colorless to pink for the sensors dissolved in aqueous solution (10% v/v MeOH/water). The detection limits of sensor RBH and R6GH in both formats were in the range of 10⁻⁹–10⁻⁷ M of Hg²⁺ and were sufficient for on-site Hg²⁺ detection in the environmental and biological systems such as ground water, drinking water and edible fish. In particular, the extremely high sensitivity of the novel sensors-encapsulated polymeric membrane RBH could pave the way for development of a real time Hg²⁺ detection portable device.