Fluorescent dyes incorporating transition metals offer significant advantages over existing dyes in cell imaging, say UK scientists. Mike Coogan at Cardiff University and his colleagues have prepared a series of rhenium bipyridine dyes and demonstrated their potential in fluorescence microscopy cell imaging.

Fluorescence microscopy is commonly used for imaging biological systems that have been pre-loaded with dyes. However, most current dyes are organic molecules, similar to those found in the biological samples. This can make it difficult to distinguish light produced by a dye from the background fluorescence of the biological material. Coogan's dyes avoid this problem because transition metal complexes fluoresce by a different mechanism - known as metal to ligand charge transfer (MLCT). This gives very different emission properties, so a clear image is obtained as dye emission is easily differentiated from the background.

According to Coogan, 'there has always been reluctance to apply transition metals in cell imaging due to worries about heavy metal toxicity, and the traditional feeling that organometallics are incompatible with water and oxygen. This study shows that our complexes at least are non-toxic and biocompatible, giving very clear images. It indicates a bright future for transition metals in fluorescence imaging.'

But there are still challenges to overcome, said Coogan. Ultraviolet radiation, which is often used to excite MLCT, is damaging to cells and the blue end of the visible spectrum has poor penetration through tissue. If researchers can shift the photochemistry of the dyes to lower energy by changing the wavelength of excitation and emission to visible or near infrared regions, Coogan explained, the procedure will be less damaging and allow greater penetration. This could open the path to imaging larger organisms, he said.

Kathryn Lees