Cyclometalated Iridium(III) Bipyridine-Phenylboronic Acid Complexes as Bioimaging Reagents and Luminescent Probes for Sialic Acids

Sialic acids play important roles in mammalian development, cell-cell attachment, and signaling. As cancer cells utilize their overexpressed sialylated antigens to propagate metastases, the development of probes for sialic acids is of high importance. Herein, we report three luminescent cyclometalated iridium(III) bipyridine complexes bearing a phenylboronic acid (PBA) moiety. Spectrophotometric titrations revealed that the PBA complexes displayed higher binding affinity for the most common sialic acid N-acetylneuraminic acid (Neu5Ac) compared with simple sugars that are commonly found on glycoproteins. Notably, cellular imaging and uptake experiments showed that the PBA complexes were able to recognize cellular sialic acid residues, resulting in more efficient uptake than the boronic acid-free analogs. Additionally, one of the PBA complexes was shown to discriminate between cancerous and noncancerous cells.