Core-shell type dually fluorescent polymer nanoparticles for ratiometric pH-sensing

The synthesis and pH-sensing properties of fluorescent polymer nanoparticles (NE's) in the 20 nm diameter range with a sensitive dye covalently attached to the particle surface and a reference dye entrapped within the particle core are presented. Fluorescein-functionalized NPs were readily obtained by conjugation of fluorescein isothiocyanate (FITC) to amine-coated crosslinked polystyrene-based nanoparticles prepared by microemulsion polymerization followed by postfunctionalization. This all water-based method gave access to stable aqueous suspensions of pH-sensing fluorescent NPs. The encapsulation of the insensitive reference fluorescent dye (1,9-diphenylanthracene, DPA) was then conveniently achieved by soaking leading to dual fluorescent NE's containing about 20 DPA and 55 fluorescein, as deduced from spectroscopic analyses. This core-shell type architecture maximizes the interactions of the sensing dye with the medium while protecting the reference dye. The variations of the ratio of the fluorescence emission intensities of the sensitive dye (fluorescein) to the reference dye (DPA) with pH show that the dual fluorescent NPs act as a ratiometric pH sensor with a measuring range between pH 4 and pH 8. This pH nanosensor was found to be fast, fully reversible, and robust without any leaching of dye over a long period of time. (C) 2008 Wiley Periodicals, Inc.