Emerging specific selectivity towards mercury(II) cations in water through supramolecular assembly at interfaces

This work deals with the development of ultra-thin film nanosensors for selective and sensitive detection of toxic Hg2+ cations in aqueous media. 1,8-Bis[(2-aminoethyl)amino]anthraquinone D0 was functionalized by alkoxy groups of different length to prepare ligands D3 and D12 with linear arrangement of two polyamine receptors and two lipophilic alkyl chains. Chemosensor D0 only binds copper(II) and mercury(II) cations in water/methanol (1:1 v/v) solutions with no interference by other metal cations. The introduction of alkoxy groups to the anthraquinone scaffold, which are required to form Langmuir monolayers and ultra-thin films on solid supports, does not change the sensing properties of the anthraquinone D0. In contrast to systems without specific molecular order (i.e. solutions and drop-cast films), Langmuir monolayers and ultra-thin Langmuir-Blodgett films of alkoxy-substituted ligands selectively bind only mercury(II) cations, even in the presence of copper(II) ions and 10 other interfering cations. Selective binding of mercury(II) cations was confirmed by the UV–vis absorption and X-ray fluorescence spectroscopies. Thus, ultra-thin films, in which chemosensor molecules are assembled in highly ordered supramolecular systems, display a higher selectivity as compared to that of disordered molecular systems. Ultra-thin sensory film allows for the selective detection of Hg2+ ions in water when their concentration is exceeded 0.01 μM, which corresponds to the action level for Hg2+ ions in drinking water recommended by the U.S. Environmental Protection Agency. Chemosensor D12 is also suitable for the fabrication of electrochemical sensors for mercury(II) cations that show similar sensitivity.