The future is bright in the world of biological imaging with the recent creation of ultrabright, ultrasmall light-emitting crystals that can image single proteins. A team from the US Department of Energy’s Lawrence Berkeley National Laboratory developed the nanoprobes, which are less than 10 nm in diameter and could be especially effective for deep-tissue optical imaging of neurons in the brain. Using advanced single-particle characterization and theoretical modeling, the team has studied upconverting nanoparticles (UCNPs). Upconverted nanoparticles, created with nanocrystals of sodium yttrium fluoride doped with ytterbium and erbium, have been found to safely image single proteins in a cell without disrupting its activity. Courtesy of Berkeley Lab. Up-conversion is a process by which a molecule absorbs two or more photons at a lower energy and emits them at higher energies. Rules for the design of UCNP probes for groups of molecules are not the same as those for probes for single molecules, the team found. Bruce Cohen, of Berkeley Lab’s Materials and Sciences Division, noted that to make the UCNPs more compatible with cellular imaging, the team had to come up with new synthetic methods to shrink them. “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 sensitizer content can potentially be eliminated,” said James Schuck, also at Berkeley Lab, and director of the Molecular Foundry’s Imaging and Manipulation of Nanostructure facility. Previous work by the team involved synthesizing and imaging single UCNPs made from nanocrystals of sodium yttrium fluoride doped with trace amounts of the lanthanide elements ytterbium for the sensitizer ions, and erbium for the emitter ions. These UCNPs upconverted near-IR photons into green or red visible light. They featured photostability and could be excited with NIR light, which is less damaging to cells than visible or UV light. “The widely accepted conventional wisdom for designing bright UCNPs has been that you want to use a high concentration of sensitizer ions and a relatively small concentration of emitter ions, since too many emitters will result in self-quenching that leads to lower brightness,” Schuck said. The original lanthanide-doped UCNPs contained 20 percent ytterbium and 2 percent erbium, which were originally thought to be optimal concentrations for brightness in bulk and nanocrystals. However, studies have shown that in UCNPs smaller than 10 nm, the erbium concentration could be raised to 20 percent, and the ytterbium concentration could be reduced to 2 percent or eliminated when approaching 5 nm. “UCNPs heavily doped with erbium start slowly out of the gate, being incredibly dim at low powers, but by the time the laser intensity is cranked up to high power, they have passed up the conventionally doped UCNPs that are the highfliers at low powers,” said researcher Emory Chan. In past studies of proteins and cell operation, finding light-emitting probes that are bright and small enough not to disrupt those functions has been challenging. The researchers’ recent discovery helps clear this hurdle. The work was supported by the DOE Office of Science. The research is published in Nature Nanotechnology. For more information, visit: www.lbl.gov.