Particles are triggered by infrared light and could be embedded deep in the body.

Fluorescent nanometre-scale particles can be programmed to emit either red or green light — and thereby control the rhythmic beating of heart-muscle cells.

Fluorescent materials give off photons of light after being excited by photons from an outside source. Usually, the emitted light is of lower energy than the excitatory light, but substances called upconverting fluorescent materials emit photons more energetic than the ones that excite them. However, such materials have proved difficult to make.

Yong Zhang at the National University of Singapore and his colleagues invented upconverting nanoparticles that emit either red or green light, depending on the wavelength of the infrared radiation used to excite them.

The team’s nanoparticles, which are simple to manufacture, are made of an inner core rich in erbium that is surrounded by layers of materials rich in ytterbium and neodymium. The outer layers harvest incoming energy and transmit it to the erbium-rich core. The incoming light’s wavelength dictates the energy’s precise path, which determines whether the erbium emits red or green light.

The team used the particles to control the beating of heart-muscle cells that were modified to respond to each colour: red slowed them down, and green sped them up.