A cationic porphyrin-based self-assembled film for mercury ion detection

A cationic 5,15-(p-(9,9-bis(6-trimethylammoniumhexyl)fluorenylethynyl)phenyl)porphyrin tetrabromide was synthesized and the self-assembled films were used for Hg²⁺ detection in aqueous media. The detection response is based on fluorescence quenching of the porphyrin molecule upon coordination with Hg²⁺. The detection shows high selectivity for Hg²⁺ over Cu²⁺, Zn²⁺, Pb²⁺, Cd²⁺, Mn²⁺, Ni²⁺, Co²⁺ and Ca²⁺. A linear response toward Hg²⁺ in a concentration range of 1 × 10⁻¹⁰-1 × 10⁻⁶ M was observed for the film with a detection limit of 0.1 nM. The cationic porphyrin film shows higher stability and significant improvement in detection sensitivity, as compared to other porphyrin-based sensors. The amphiphilic cationic nature of the porphyrin synthesized also allows for the direct deposition of a porphyrin layer on a bare glass surface through self-assembly.