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

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.