Turn-off and -on fluorescence switching of a self-assembled sensor for mercury(II) induced by anionic micelles

The self-assembling of the 10-methylacridinium perchlorate fluorophore and azathiocrown ether derivative receptor through sodium dodecyl sulfate (SDS) or sodium 4-octylbenzenesulfonate (SOBS) aggregates in water has allowed the realization of a new effective sensing system for Hg2+ ions. In the absence of metal ions, occurrence of the fluorophore-receptor interaction was signaled through the fluorescence quenching of acridinium. Upon addition of increasing quantities of Hg2+, fluorescence experiments pointed out an unusual further quenching of the acridinium fluorescence, followed by an emission enhancement with a switching point around 1 × 10−7 and 1 × 10−8 M in SDS and SOBS, respectively. Such a behavior was attributed to the formation of mercury-receptor complexes with different coordination number and accordingly with distinct redox activity. The stoichiometry of the mercury complexes was assessed by spectrophotometric titration, while insights into their structures were obtained with density functional theory (DFT) calculation. Hg2+ ions can be selectively detected in SDS solutions, and its concentration in the nanomolar range can be linearly determined. Altogether, this modular approach allows quick variation of the components, which makes the optimal tuning of the process up to its maximum efficiency level very easy.