Fluorescent Metal Nanoshells: Lifetime-Tunable Molecular Probes in Fluorescent Cell Imaging

We reported the preparation of lifetime-tunable fluorescent metal nanoshells and used them as lifetime imaging agents for the potential detection of multiple target molecules by a single cell imaging scan. These metal nanoshells were generated to have 40 nm silica cores and 10 nm silver shells. Three kinds of metal-ligand complexes, tris(5-amino-1,10-phenanthroline)ruthenium(II) (Ru(NH2-Phen)(3)(2+)), tris(2,2'-bipyridine) ruthenium(II) (Ru(bpy)(3)(2+)), and tris(2,3-bis(2-pyridyl)pyrazine))ruthenium(II) (Ru(dpp)(3)(2+)), that have similar excitation and emission wavelengths but different lifetimes were respectively encapsulated in the cores of metal nanoshells for the purpose of fluorescence. Compared with the metal-free silica spheres, these metal nanoshells were found to display enhanced emission intensities and shortered lifetimes due to near-field interactions of Ru(II) complexes with the metal shells. The shortened lifetimes of these metal nanoshells were definitely unique relevant to the Ru(II) complexes. 10 ns for the Ru(Phen-NH2)(3)(2+)-Ag nanoshells, 45 ns for the Ru(bpy)(3)(2+)-Ag nanoshells, and 200 ns for the Ru(dpp)32+-Ag nanoshells. These lifetime were longer than the lifetime of cellular autofluorescence (2-5 ns); therefore the emission signals of these metal nanoshells could be distinctly isolated from the cellular background on the lifetime cell images. Moreover, these lifetimes were also different from one another, resulting in the emission signals of three metal nanoshells being distinguished from one another on the cell images. This feature may often an opportunity to detect multiple target molecules in a single cell imaging scan when the metal nanoshells are bound with various targets in the cells.