Water-Soluble Chitosan–Europium Hybrid Sensor for Singlet Oxygen Detection

The ability to effectively monitor singlet oxygen (¹O₂) with fluorescence probes in biological systems is severely restricted mainly by the background autofluorescence of these systems. Though the application of lanthanide complexes as ¹O₂ monitors successfully resolves this problem with time-gated luminescence measurements, the insolubility of these complexes in an aqueous medium heavily limits their application in biological systems. Here, we present a water-soluble ¹O₂ sensor based on a chitosan-europium hybrid material. A procedure for the modification of chitosan to expand its solubility to neutral and basic pH, while maintaining its free active amine groups, is described. These are then coupled covalently to a europium-based probe for the detection of ¹O₂. The resulting hybrid sensor is readily soluble in water across the pH scale and efficiently signals the presence of ¹O₂ at physiological pH, with the characteristic Eu³⁺ emission at 611 nm yielding up to a 15-fold increase in emission intensity and a decay time of 332 μs. Being of particular interest for time-gated measurements, this long decay time, coupled with the biocompatibility of chitosan, describes a material with potential biological applications, where ¹O₂ plays a vital role.