Photostability of oxygen-sensitive core-shell nanofibers

摘要： Core-shell electrospun nanofibers constitute a vehicle enabling novel uses for phosphorescent oxygen-sensitive molecules. In this work, we investigate compositional parameters and other factors governing fiber design appropriate to long-term sensor applications. These nanofibers contain an oxygen-sensitive palladium (II) porphyrin species within a polysulfone core; an outer polycaprolactone shell protects the core from the surrounding environment. The presence of a core-shell architecture was confirmed using advanced time-of-flight secondary ion mass spectrometry (ToF-SIMS). Although these sensors exhibit excellent photostability in the short-term, long-term exposures (up to 1,000 hr) are shown to detrimentally impact performance via photobleaching phenomenon. We also investigated higher porphyrin concentrations to determine whether the resulting sensors are predisposed to photobleaching. High porphyrin loadings (up to 10 wt%) could be successfully incorporated into the core. While increased probe content should logically enhance emission intensity, we sought to investigate the balance between a stronger signal and the potential for decreased photostability. The resulting behavior is rationalized by (1) establishing the spacing associated with a ‘perfect’ molecular dispersion and (2) invoking five distinct populations of porphyrin agglomeration. Populations that are not molecularly dispersed experience varying degrees of agglomeration-catalyzed photobleaching. The photobleaching rate increases with the extent of agglomeration and, therefore, the initial porphyrin content. Although it is demonstrated that low porphyrin loadings exhibit improved photobleaching resistance, the initial brightness is inadequate. In contrast, enhanced brightness via increased porphyrin content also leads to diminishing returns due to the apparent emergence of a self-quenched population. For a given application, selection of an appropriate porphyrin concentration is critical to achieving a compromise between initial brightness and photobleaching resistance. Detector sensitivity and anticipated duration of use must be considered when weighing the relative benefits of these attributes and selecting a specific porphyrin content.