Researchers at the US Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have discovered surprising new rules for creating ultra-bright light-emitting crystals that are less than ten nanometers in diameter.

These ultra-tiny but ultra-bright nanoprobes should be a big asset for biological imaging, especially deep-tissue optical imaging of neurons in the brain.

A multidisciplinary team of researchers led by James Schuck and Bruce Cohen of Berkeley Lab’s Materials Sciences Division used advanced single-particle characterisation and theoretical modelling to study what are known as ‘upconverting nanoparticles’ or UCNPs.

Upconversion is the process by which a molecule absorbs two or more photons at a lower energy and emits them at higher energies.

The research team determined that the rules governing the design of UCNP probes for ensembles of molecules do not apply to UCNP probes designed for single-molecules.

"The widely accepted conventional wisdom for designing bright UCNPs has been that you want to use a high concentration of sensitiser ions and a relatively small concentration of emitter ions, since too many emitters will result in self-quenching that leads to lower brightness," said Schuck. "Our results show that under the higher excitation powers used for imaging single particles, emitter concentrations should be as high as possible without compromising the structure of the nanocrystal, while sensitiser content can potentially be eliminated."