Phosphorescent imaging of living cells using a cyclometalated iridium (III) complex

Abstract

A cell permeable cyclometalated iridium(III) complex has been developed as a phosphorescent probe for cell imaging. The iridium(III) solvato complex [Ir(phq)2(H2O]2)] preferentially stains the cytoplasm of both live and dead cells with a bright luminescence.

Figure 1. Chemical structures of iridium(III) solvato complexes 1–3 bearing different C^∧N ligands.

Figure 2. Luminescence photographs (upper panel) of (left) [Ir(phq)₂(H₂O)₂)]OTf (1), (middle) [Ir(ppy)₂(H₂O)₂]OTf (3), and (right) [Ir(bzq)₂(H₂O)₂]OTf (2) at 1 mM concentration in DMSO solution under UV-transillumination.

Emission spectra (lower panel) of complex 1 (50 µM) in 20 mM Tris buffer (pH 7.4) and DMSO.

Figure 3. Emission spectra of complex 1 (50 µM) in 20 mM Tris buffer (pH 7.4) with various natural amino acids (200 µM).

Group 1: L-alanine, L-arginine, L-asparagine, L-glutamine, L-threonine; Group 2: L-glycine, L-isoleucine, L-lysine, L-phenylalanine, L-proline, L-serine; Group 3: L-tryptophan, L-tyrosine, L-valine, L-glutamic acid, L-cysteine, L-methionine.

Figure 4. Photograph image of complex 1 (50 µM) in absence (left) or presence of BSA (50 µM, middle) or ct DNA (50 µM, right) under UV-transillumination.

Figure 5. Luminescence intensity changes of complex 1 (50 µM) in 20 mM Tris buffer (pH 7.4) with various amounts of BSA or histidine (0, 12.5, 25, 50 and 100 µM).

Figure 6. Brightfield images of live HeLa cells (top left).

Luminescence images of cells stained with complex 1 (10 µM) in DMSO/PBS (pH 7.4, 1∶99 v/v) for 10 min at 37°C (top right) and then with Hoechst 33258 for a further 20 min (bottom left). Overlay of images in (b) and (c) (bottom right).

Figure 7. Cytotoxicity of complex 1 (concentration of 1 = 10 µM; incubation time  = 10 min).

Figure 8. Brightfield images of fixed HeLa cells (top left).

Luminescence images of cells stained with complex 1 (10 µM) in DMSO/PBS (pH 7.4, 1∶99 v/v) for 10 min at 37°C (top right) and then with Hoechst 33258 for a further 20 min (bottom left). Overlay of images in (b) and (c) (bottom right).