A Biocompatible Core–Shell Nanoparticle Encapsulating Cyclometalated Iridium(III) Complexes and Ultrasmall Gold Nanoclusters for Ratiometric Imaging of Intracellular Oxygen

Maintaining optimal oxygen levels is crucial in biological systems where deviations can signal hypoxia or toxicity. Luminescent oxygen probes, particularly phosphorescent transition-metal complexes and fluorescent organic dyes, enable real-time monitoring but face challenges in sensitivity, temporal resolution, and biocompatibility. Ultrasmall gold nanoparticles (UAuNPs) offer promising alternatives due to their tunable emission, photostability, and low toxicity, while cationic Ir(III) complexes provide bright, long-lived phosphorescence and target-specific design. Here, we integrate these advantages by self-assembling Ir(III) complexes with UAuNPs and developing two bimetallic core-shell nanosensors (Ir-1@Au and Ir-2@Au) using an amphipathic copolymer. The hydrophilic Ir(III) complexes embed in the polyethylene glycol (PEG)-rich shell, while thiolated UAuNPs anchor to the poly(propylene glycol) (PPG) core. Ir-2@Au with a benzoxazole ligand outperforms Ir-1@Au and exhibits superior membrane permeability and ratiometric oxygen sensing, with dual emission arising from Ir(III) coordination disruption and UAuNP core etching in response to O₂. The microsecond-scale phosphorescence of Ir-2@Au, together with fast acquisition of lifetime images (temporal resolution of ∼200 ms), enables time-resolved ratiometric imaging of intracellular dissolved O₂ as low as ∼0.1 mg L⁻¹ and effectively mitigates autofluorescence. The hypoxia-responsive lifetime and intensity profiles, combined with therapeutic potential, highlight this nanoprobe's utility for live-cell imaging and cancer theranostics, offering a robust platform for studying oxygen dynamics.