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Gastrin‐releasing peptide receptor (GRPr) plays proliferative and inflammatory roles in living systems. Here, we report a highly selective GRPr antagonist (JMV594)‐tethered iridium(III) complex for probing GRPr in living cancer cells and immune cells. This probe exhibited desirable photophysical properties and also displayed negligible cytotoxicity, overcoming the inherent toxicity of the iridium(III) complex. Its long emission lifetime enabled its luminescence signal to be readily distinguished from the interfering fluorescence of organic dyes by using a time‐resolved technique. This probe selectively visualized living cancer cells via specific binding to GRPr, while it also modulated the function of GRPr on TNF‐α secretion in immune cells. To our knowledge, this is the first peptide‐conjugated iridium(III) complex developed as a GRPr bioimaging probe and modulator of GRPr activity. This theranostic agent shows great potential at unmasking the diverse roles of GRPr in living systems. [ABSTRACT FROM AUTHOR]

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]