The 2008 Nobel Prize for chemistry was awarded in recognition of the discovery of a green fluorescent protein widely used to image cells in fluorescent microscopes. Ultraviolet radiation is used to excite the protein within cells, which then emits a characteristic lower-energy green radiation. However, as this hard UV light can damage cells, alternatives are being investigated. A promising solution has now been developed by researchers from the National University of Singapore.1

Fig. 1: Fluorescence imaging of cells using NaYF4/silica core-shell nanocrystals.

The new method is based on fluorescence upconversion, where two or more low-energy near-infrared photons are converted into visible emission, avoiding the use of damaging light (Fig. 1). The infrared light used in the upconversion process is only weakly absorbed by the cells, reducing local heating. Furthermore, unlike related processes such as two-photon absorption, it is not necessary for the two photons to arrive simultaneously. This significantly improves the upconversion efficiency.

The material enabling this upconversion-based bioimaging scheme is NaYF4 nanocrystals doped with upconverting lanthanide ions. However, these small crystals are not sufficiently biocompatible. Therefore, the researchers developed a new synthesis process that allows these nanocrystals to be coated with a thin layer of biocompatible silica.

Furthermore, such core-shell nanoparticles only allow for a limited number of possible fluorescence colors and are therefore not suitable for multiplex biodetection. “To produce upconversion with multi-color emission, we incorporated organic dyes and small quantum dots into the silica layer,” says Yong Zhang from the research team. In these multicolor core-shell nanoparticles, the excited electrons are transferred from the NaYF4 core to either organic dyes or small quantum dots in the silica sphere, where light emission at almost any desirable color takes place.

These nanoparticles show promise for a broad range of bioimaging applications and in vivo and in vitro studies have started. ”We believe there is a big commercial potential in this approach, as many people are doing fluorescence imaging in their studies,” says Zhang. Possible applications include the imaging of cells for example in cancer detection, but also more demanding applications such as single-molecule detection. The latter is feasible owing to a high signal-to-noise ratio in the light emission from the nanoparticles.