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

## TL;DR

This paper introduces a new optical method for detecting potassium ions using a combination of light-based techniques, enabling precise sensing in biological conditions.

## Contribution

The study demonstrates a novel integration of TTA-UC and PET for K+ sensing with excitation-dependent ion binding.

## Key findings

- The anthracene-crown ether sensor combines TTA-UC and PET for K+ detection at biologically relevant concentrations.
- Ion binding constants differ in ground and excited states of the anthracene unit.
- A microfluidic device enables scattering-free upconversion signals for in-flow ion sensing.

## Abstract

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.