# Efficient Singlet Oxygen Monitoring in Aqueous Media Comprising a Polymer‐embedded Eu3+‐Complex

## TL;DR

This paper introduces a new method to detect singlet oxygen using a europium complex embedded in polymer beads, which could improve cancer and antimicrobial therapies.

## Contribution

A novel Eu3+-based sensor with a 500-fold luminescence boost for singlet oxygen detection in aqueous environments.

## Key findings

- The Eu3+ complex in polymer beads shows a 500-fold increase in luminescence intensity when exposed to singlet oxygen.
- The sensor exhibits long decay times and stability in acidic and basic conditions.
- The beads can stain bacterial surfaces, enabling microscopic detection of singlet oxygen.

## Abstract

Singlet dioxygen (1O2) plays a pivotal role as the active agent in photodynamic therapy (PDT) for cancer treatment, as well as in the photo‐inactivation of antibiotic‐resistant microbes (antimicrobial photodynamic therapy, aPDT). The ability to sensitively monitor the production and behavior of ¹O₂ following its photo‐catalytic generation is crucial for developing effective therapeutic strategies. Optical sensor molecules that respond to ¹O₂ through changes in absorption or, more sensitively, fluorescence, are suitable choices. While most monitors report 1O2 via altered absorption spectra, only few compounds respond by the onset of fluorescence, even fewer based on lanthanide luminescence. By embedding a novel lanthanide complex (Eu3+) into polystyrene nanoparticles (beads), we achieved close to a 500‐fold emission intensity boost in the presence of 1O2, very long decay times of up to 879 µs and unprecedented stability in acidic and basic media. Furthermore, the beads present a high‐surface charge (>+30 mV), yielding stable aqueous dispersions, which we exploited in a preliminary “proof of principle” staining experiment of (negatively charged) bacterial surfaces. The straightforward synthesis circumvents intricate preparative steps and restrictive costs. The decay characteristics furthermore pave the road to time‐gated measurements, that is, to the suppression of interfering autofluorescence from biological samples.

A singlet dioxygen (1O2) sensitive Eu(III) complex embedded in polymer nanoparticles efficiently detects 1O2 in aqueous media by a remarkable increase in Eu3+ luminescence. These particles also adhere to bacteria surfaces, where the cells appear brightly stained in the presence of 1O2 enabling spectroscopic evaluation and easy detection under the microscope.