Plasmonic Enhanced Emissions from Cubic Nayf4:Yb: Er/Tm

ARTICLE
pubs.acs.org/cm

Plasmonic Enhanced Emissions from Cubic NaYF4:Yb:Er/Tm
Nanophosphors
L. Sudheendra,† Volkan Ortalan,‡ Sanchita Dey,‡ Nigel D. Browning,‡ and Ian M. Kennedy*,†
†

Mechanical and Aerospace Engineering Department and ‡Department of Chemical Engineering and Materials Science,
University of California, Davis 95616, USA

S
b Supporting Information

ABSTRACT: A metal shell was used in this study to provide significant enhancement of the
up-converted emission from cubic NaYF4 nanoparticles, creating a valuable composite
material for labeling in biology and other applications; use of the cubic form of the material
obviates the need to undertake a high temperature transformation to the naturally more
efficient hexagonal phase. The NaYF4 matrix contained ytterbium sensitizer and an Erbium
(Er) or Thulium (Tm) activator. The particle sizes of the as-synthesized nanoparticles were
in the range of 20À40 nm with a gold shell thickness of 4À8 nm. The gold shell was
macroscopically amorphous. The synthesis method was based on a citrate chelation. In this
approach, we exploited the ability of the citrate ion to act as a reductant and stabilizer.
Confining the citrate ion reductant on the nanophosphor surface rather than in the solution
was critical to the gold shell formation. The plasmonic shell enhanced the up-conversion
emission of Tm from visible and near-infrared regions by up to a factor of 8, in addition to
imparting a visible color arising from the plasmon absorption of the gold shell. The upconversion enhancements observed with Tm and Er were different for similar gold coverages, with local crystal field changes as a
possible route to enhance up-conversion emission from high symmetry structural hosts. These novel up-converting nanophosphor
particles combine the phosphor and features of a gold shell, providing a unique platform for many biological imaging and labeling
applications.
KEYWORDS: cubic-NaYF4, up-converting...