The creation of self-contained cellular lasers is reported online this week in Nature Photonics. The approach offers a new way of labelling biological cells and monitoring their health.

Luminescent probes, such as fluorescent dyes and proteins, are useful tools for biochemical sensing applications. However, these probes have relatively broad emission spectra, which makes them difficult to distinguish from the broad background emission of molecules in biological tissue.

Seok Yun and Matjaz Humar demonstrate that ordinary cells can be transformed into miniature lasers by injecting small droplets of oil or natural lipids mixed with a fluorescent dye into the cell. The droplet acts as a tiny spherical laser cavity that confines light, and when the cell is excited by short pulses of light, it lases. Importantly, the exact wavelength of the laser light depends on the level of mechanical stress within the cell, offering a very sensitive means for detecting internal cytoplasmic stress. In addition, by using fluorescent polystyrene beads of varying sizes instead of droplets, the authors found that it is possible to vary the colour of the lasing and thus uniquely identify or label a cell. In principle, the approach can be scaled to individually tag thousands of cells.