Toward Brighter Hybrid Magnetic-Luminescent Nanoparticles: Luminosity Dependence on the Excited State Properties of Embedded Dyes

A series of hybrid, magnetic, and luminescent silica nanoparticles (SNP) have been synthesized and their photophysical properties characterized. The nanoparticles contain, on average, one iron oxide nanoparticle as a magnetic core. The embedded dyes include fluorescein, Alexa Fluor 546, tetramethylrhodamine (TMR), and 4,4-difluoro-5-(2-pyrrolyl)-4-bora-3a,4a-diaza-s-indacene (Bodipy), which are known to have a singlet excited state, and 5-(dimethylamino)naphthalene-1-sulfonyl (dansyl), 7-(diethylamino)-coumarin-3-carboxy (coumarin), and tris(bipyridine)ruthenium(II) dichloride (Rubipy) derivatives, which have a charge transfer excited state. In general, the photophysical properties of the magnetic-core dye-doped silica nanoparticles, SNPdye, where dye is one of the seven dyes studied in this project, are dictated by homogeneous energy transfer between dye molecules and by dye aggregation. Molecules with a small Stokes shift and affinity for aggregation upon silica encapsulation (TMR and Bodipy derivatives) can be only sparsely loaded (<50-60 molecules per a 60 nm in diameter nanoparticle) into the nanoparticle matrix before strong fluorescence quenching takes effect. These dyes yield the least bright nanoparticles whose luminosity strongly decreases as the intraparticle dye concentration increases above the rather low maximum brightness loading. Other common fluorophores (Alexa Fluor 546 and fluorescein) can be loaded into the silica matrix with little to no fluorescence quenching until the intermolecular separation becomes less than similar to 5 nm (similar to 100-200 fluorophores per a 60 nm in diameter nanoparticle). Finally, significantly more (similar to 200-4600) dye molecules with a charge transfer lowest excited state and large Stokes shift (Rubipy, dansyl, coumarin) can be loaded into the 60 nm in dameter nanoparticle with no indication of luminescence quenching. The results of this study suggest that the luminosity of a hybrid nanoparticle is highest when the embedded dye has a large Stokes shift and is not susceptible to aggregation, which both guarantee no or little intrananoparticle luminescence quenching. No luminescence quenching by the iron oxide magnetic core has been observed for any SNPdye series.