Fluorescence Lifetime Imaging of Triplet–Triplet Annihilation Upconversion for Quantitative Mapping of Environmental Stimuli

Photon upconversion via triplet-triplet annihilation (TTA-UC) is a promising technology for environmentally responsive sensing, characterized by delayed fluorescence and anti-Stokes shifts. However, it faces quantification challenges due to intensity-based detection limitations, such as fluorophore concentration, excitation source instability, and environmental scattering. To address these issues, we report a time-resolved fluorescence strategy that exploits the intrinsic delayed fluorescence lifetime of TTA-UC systems as a robust, concentration-independent parameter for the quantitative spatiotemporal mapping of environmental stimuli. Using a TTA-UC platform comprising a lutetium(III) porphyrin photosensitizer and a 9,10-bis(2-phenylethynyl)anthracene (BPEA) annihilator, we demonstrate that the TTA-UC lifetime acts as a universal reporter for diverse physicochemical parameters, including temperature, viscosity, analyte concentration, and pH. As a practical demonstration, we engineered nanoparticles encapsulating the pH-responsive Lu(III) porphyrin/BPEA system, which exhibited a linear lifetime-pH correlation (20.5-34.0 μs vs pH 4.0-8.0, R2 > 0.99) enabling real-time and in situ pH monitoring in beverages, spoiling milk, and living cells. Fluorescence lifetime imaging microscopy integration achieved long-lived emission, exceptional photostability, and high environmental contrast, establishing a versatile platform for quantitative sensing in real-world scenarios. This work bridges the gap between TTA-UC photophysics and practical sensing applications, offering a generalizable platform for stimuli-responsive materials design.