Structural modification of rhodamine-based sensors toward highly selective mercury detection in mixed organic/aqueous media

Abstract

In virtue of the sulfurphilic nature of Hg²⁺, three new sensors RN1, RN2 and RST1 that combine a thiophene group and one or two rhodamine choromophores, or a thiospirolactam rhodamine chromophore, were designed and prepared for the selective detection of Hg²⁺ in aqueous media, respectively. These sensors all displayed good brightness and fluorescence enhancement following Hg²⁺ coordination with limits of detection for Hg²⁺ at the ppb level. Thus, they have the potential for distinguishing between safe and toxic levels of inorganic mercury in drinking water. RN1 exhibited chromogenic and fluorogenic selectivity over alkali, alkaline earth metals, divalent first-row transition metal ions as well as heavy metals, but the presence of Cu²⁺ had a small but significant influence on the absorption detection of Hg²⁺. Compared to RN1, the introduction of sufficient sulfur atoms could increase the binding capability of RST1 towards Hg²⁺ relative to the sensor RN1, but decrease its Hg specific ability. The existence of some heavy and transition metal ions, such as Pb²⁺, Ag⁺, Cu²⁺ enhance the silent absorption spectra of RST1. Spectral evidence and X-ray structural investigations of the mercury complex revealed a possible 1:2 complexation behaviour between the Hg²⁺ ion and the sensor RN1 or RST1. Sensor RN2 which contains two rhodamine carboxhydrazone arms exhibited better selectivity, compared to those of RN1 and RST1. The addition of Cu²⁺ only caused a small interference for the absorption detection of Hg²⁺ under the same conditions, demonstrating the efficiency of the robust bis-chelating mode with regard to the selectivity for Hg²⁺.