Various embodiments of the invention describe the synthesis of upconverting nanoparticles (UCNPs), lanthanide-doped hexagonal .beta.-phase sodium yttrium fluoride NaYF.sub.4:Er.sup.3+/Yb.sup.3 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 fluorescent proteins. We use Advanced single-particle characterization and theoretical modeling is demonstrated to find that surface effects become critical at diameters under 20 nm, and that the fluences used in single-molecule imaging change the dominant determinants of nanocrystal brightness. These results demonstrate that factors known to increase brightness in bulk experiments lose importance at higher excitation powers, and that, paradoxically, the brightest probes under single-molecule excitation are barely luminescent at the ensemble level.
STATEMENT OF GOVERNMENTAL SUPPORT
 The invention described and claimed herein was made in part utilizing funds supplied by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 between the U.S. Department of Energy and the Regents of the University of California for the management and operation of the Lawrence Berkeley National Laboratory. The government has certain rights in this invention.