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

Abstract

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.