Multicolor polystyrene nanospheres tagged with up-conversion fluorescent nanocrystals

Compared to conventional down-conversion fluorescent materials, NIR-to-visible up-conversion fluorescent materials which emit visible light upon near-infrared (NIR) excitation are better suited for biodetection/bioimaging due to their advantages such as minimum photo-damage to living organisms, weak background fluorescence, low signal-to-noise ratio and high detection sensitivity. Uniform hexagonal NaYF(4) (β-NaYF(4)) nanocrystals with up-conversion fluorescence were synthesized. Monodisperse polystyrene nanospheres with an average size of 400 nm in diameter, tagged with different color β-NaYF(4) nanocrystals, were prepared using a miniemulsion polymerization method. More than 20 β-NaYF(4) nanocrystals were encapsulated in a single polystyrene nanosphere. The nanospheres emit multicolor NIR-to-visible up-conversion fluorescence upon excitation at a wavelength of 980 nm. The nanospheres could be used for a variety of biological applications which require high-sensitivity detection of biomolecules.