Excitation-Dependent K⁺ Sensing by Combining Photoinduced Electron Transfer and Triplet-Triplet Annihilation

Triplet-triplet annihilation photon upconversion (TTA-UC) combines the energy of two photons to provide one of higher energy. Detecting such high energy photons can be more selective than conventional fluorescence, because artifacts like scattering and autofluorescence do not contribute to the signal. Ions play crucial roles in biology, and quantitative in-flow sensing of ions using an all-optical readout is therefore of significant importance. Here, we assess the applicability of an anthracene-crown ether based ion sensor, which incorporates TTA-UC in combination with photoinduced electron transfer (PET). We find that these two mechanisms are compatible with each other in one functional molecule, enabling the detection of K⁺ at biologically relevant concentrations. We further find that ion binding constants differ in the electronic ground and excited states of the anthracene unit. As triplet lifetimes are on the same time scale as ion dissociation constants of crown ethers, the measured equilibrium constant depends on excitation conditions, which therefore must be taken into account in the analysis. Lastly, we built a microfluidic device in order to demonstrate how in-flow ion sensing could be conducted and achieve scattering free upconversion signals and predictable binding constants. This work examines TTA-UC-based ion sensing from a mechanistic to an application perspective and provides a step toward quantitative all-optical sensing of biologically relevant ions in flow.