Iron　plays　an　important　role　in　various　physiological　processes.　However,　the　detailed　biological　functions　of　iron　have　not　been　sufficiently　explored　because　of　a　lack　of　effective　methods　to　monitoring　iron,　especially　the　labile　ferrous　ion　(Fe2+).　In　the　current　study,　a　novel　turn-on　phosphorescent　probe　for　Fe2+　quantification　and　visualization　has　been　proposed　based　on　the　hybrid　nanocomposite　of　manganese　dioxide　and　gemini　iridium　complex　(MnO2-GM-Ir).　The　surfactant-like　GM-Ir　with　positive　charges　was　beneficial　to　combine　with　the　negatively　charged　manganese　dioxide　(MnO2)　nanosheets,　and　thus　endowing　the　MnO2-GM-Ir　nanocomposite　excellent　dispersion　ability　in　the　water　as　well　as　efficiently　avoiding　the　interference　to　the　detection　caused　by　the　agglomeration　of　nanocomposite.　Phosphorescence　of　GM-Ir　was　effectively　quenched　by　MnO2　nanosheets　through　fluorescence　resonance　energy　transfer　(FRET)　and　the　inner　filter　effect　(IFE),　while　the　phosphorescence　could　be　significantly　recovered　in　the　presence　of　Fe2+　via　a　selective　Fe2+-mediated　reduction　of　MnO2　nanosheets,　indicating　a　highly-specific　selectivity　towards　Fe2+　with　a　low　detection　limit　(80　nM).　The　drug　test　assay　and　in　vitro　imaging　studies　further　proved　that　the　MnO2-GM-Ir　nanocomposite　could　be　employed　as　a　promising　probe　for　the　quantitative　detection　of　exogenous　Fe2+　in　drug　and　in　vitro　imaging　of　living　cells.　©　2020