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Highlights • Complex 1 is the first long-lived probe to detect IO 4 − in living cells. • Complex 1 has a detection limit of 0.077 μM for IO4− in aqueous buffer, more sensitive than reported fluorescence probes. • The long lifetime of Complex 1 enables its luminescence response to IO4− to be detected in the presence of organic dyes. Abstract Periodate (IO 4 −) is a widely used reagent in organic synthesis and for biomolecular modification. However, highly sensitive fluorescence detection methods for IO 4 − are lacking. In this work, we designed and synthesized iridium(III) complexes for the detection of IO 4 − by the incorporation of a methylthio group to serve as a luminescence trigger. The results showed that complex 1 could detect IO 4 − in a sensitive and selective manner with a detection limit of 0.077 μM, rendering it more sensitive than reported fluorescence methods. Meanwhile, its inherently long emission lifetime enables its luminescence response to IO 4 − to be detected even in the presence of organic dyes by the use of time-resolved emission spectroscopy (TRES). This probe was further demonstrated to detect IO 4 − in living cells. To our knowledge, complex 1 is first long-lived iridium(III)-based probe for the sensitive detection of IO 4 −, and is also one of few existing imaging probes for IO 4 −. [ABSTRACT FROM AUTHOR]