Real-Time, Minimally Invasive Oxygen Sensing via Luminescence Loss in Singlet Oxygen-Resistant Triplet-Triplet Annihilation Upconversion Nanocapsules.

Oxygen concentration in the extracellular microenvironment plays a critical role in regulating metabolism and disease progression, particularly under hypoxic conditions. Here, we introduce a radically new optical sensing strategy for real-time, minimally invasive monitoring of dissolved oxygen in the extracellular aqueous microenvironment. Instead of relying on the oxygen-dependent quenching of phosphorescence or delayed fluorescence lifetimes, which is commonly used in existing sensors, we demonstrate that the local cumulative loss of upconverted fluorescence intensity in triplet-triplet annihilation upconversion (TTA-UC) nanocapsules provides a highly sensitive, one-to-one correspondence with the local oxygen concentration. This method enables an oxygen sensing dynamic range from normoxia (160 mmHg) to deep hypoxia (1.5 mmHg), achieving a detection limit of 45 nM. The sensing platform combines nanoconfined TTA-UC chromophores with sacrificial singlet oxygen scavengers (SSOS) within a hydrophobic nanocapsule core, ensuring complete chemical sequestration of photogenerated singlet oxygen. Furthermore, the use of a hydrophobic surfactant n-octadecyl trimethylammonium chloride (OTAC) minimizes free surfactant in the aqueous phase, drastically reducing cytotoxicity. With excitation intensities as low as 6.6 W cm-2 (λ = 631 nm), radiation stress is kept negligible. Crucially, the method requires no signal averaging, enabling real-time, high-fidelity oxygen monitoring without artifacts from dynamic equilibria. This approach offers a transformative tool for studying oxygen dynamics in living cells and tissues.