Upconverted emission from core-shell nanoparticles can take on different colors, by doping the shells with different activator elements. Image: 2011 NPG

The
light that a luminescent particle emits is usually less energetic than
the light that it absorbs. Some applications require the emitted light
to be more energetic, but this so-called upconversion process has been
observed in only a small handful of materials. Xiaogang Liu at the
A*STAR Institute of Materials Research and Engineering and co-workers
have now succeeded in expanding the list of upconversion materials,
easing the path to new applications.

Traditional
upconversion particles are distinguished by their evenly-spaced or
‘ladder-like’ energy levels which their internal electrons can take on.
The even spacings allow an electron to be promoted up in energy many
times consecutively, by absorbing many photons of the same color. When
an electron that has been promoted to a high energy finally relaxes back
to the lowest-energy state, it emits a photon which is more energetic
than the photons that excited it to begin with.            

Nanoparticles
doped with elements from the lanthanide group of the periodic table are
capable of upconversion, and are useful for biological imaging because
their high-energy emission can be clearly distinguished from background
noise. However, only three elements from the lanthanide series are
efficient at upconversion: erbium, thulium, and holmium. This list is so
short because of the simultaneous requirements that an upconversion
particle exhibit a ladder-like electronic energy structure, and also
efficient emission.

Liu
and colleagues solved this problem by using different lanthanides to
perform different stages of the upconversion process. Sensitizer
elements absorb incident light, and transfer the absorbed energy to
nearby accumulators, whose electrons rise to high energy levels. Then,
the energy stored in accumulators transfers by hopping through many
migrators, until an activator is reached. Finally, the activator
releases a high-energy photon.

By
assigning different elements to each of these four functions, the
researchers were able to ease the requirements on any individual
element. In addition, unwanted interactions among different elements
were avoided by separating them spatially inside a single spherical
nanoparticle that has sensitizers and accumulators in the core,
activators in the shell and migrators in both the core and the shell.

This
design allowed Liu and his team to observe a spectrum of colors from
the upconverted emission of europium, terbium, dysprosium and samarium
(see image). The same approach may also allow other elements to emit
efficiently.

“Our
results may lead to advances in ultrasensitive biodetection,” says Liu,
“and should inspire more researchers to work in this field.”

Tuning upconversion through energy migration in core–shell nanoparticles

Source:  A*STAR