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 mu s. 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.