Imaging at the single-molecule level reveals heterogeneities that are lost in ensemble
imaging experiments, but an ongoing challenge is the development of luminescent probes
with the photostability, brightness and continuous emission necessary for single-molecule
microscopy,,,,,. Lanthanide-doped upconverting nanoparticles overcome problems of
photostability and continuous emission,,,,, and their upconverted emission can be excited
with near-infrared light at powers orders of magnitude lower than those required for …

Various embodiments of the invention describe the synthesis of upconverting nanoparticles
(UCNPs), lanthanide-doped hexagonal B-phase sodium yttrium fluoride NaYF: Er"/Yb
nanocrystals, less than 10 nanometers in diameter that are over an order of magnitude
brighter under single-particle imaging conditions than existing compositions, allowing
visualization of single UCNPs as small (d= 4.8 nm) as fluo rescent proteins. We use
Advanced single-particle character ization and theoretical modeling is demonstrated to find …