Dye-loaded microcapsules that change colour with pH can be used as indicator paper for cells.

Wolfgang Parak, from the Ludwig Maximilians University of Munich, and his colleagues have embedded a pH-sensitive dye in a polymer capsule that can transport the dye into living cells. A colour change in the dye's fluorescence indicates when it has entered a cell and this can be followed by fluorescence microscopy. The change depends on pH: capsules in the alkaline medium outside cells emit in red, capsules ingested by the cells emit in green due to the acidic pH.

According to the German and UK researchers, the capsules could be used in cell analysis, acting as artificial cell components. 'For any kind of cellular diagnostics it would be helpful to have a non-invasive reporter inside cells that constantly measures the concentration of important molecules and sends this information to a detector outside the cell,' said Parak. 'This is similar to the idea of having a miniature diagnosis submarine inside the body, as has been featured in some science fiction movies.'

The polymer capsules turn from red to green as the pH is lowered

Parak says that the capsules need many modifications before they can be used for in vivo studies. Eventually though, he envisions them being used to develop a system that detects concentrations of several important molecules inside cells in real time and non-invasively. 'This could be an important tool for diagnostics,' he said.

Elinor Richards