BUFFALO, N.Y. -- An international research team has created
unique photoluminescent nanoparticles that shine clearly through
more than 3 centimeters of biological tissue -- a depth that makes
them a promising tool for deep-tissue optical bioimaging.
Though optical imaging is a robust and inexpensive technique
commonly used in biomedical applications, current technologies lack
the ability to look deep into tissue, the researchers said.
This creates a demand for the development of new approaches that
provide high-resolution, high-contrast optical bioimaging that
doctors and scientists could use to identify tumors or other
anomalies deep beneath the skin.
The newly created nanoparticles consist of a nanocrystalline
core containing thulium, sodium, ytterbium and fluorine, all
encased inside a square, calcium-fluoride shell.
The particles are special for several reasons. First, they
absorb and emit near-infrared light, with the emitted light having
a much shorter wavelength than the absorbed light. This is
different from how molecules in biological tissues absorb and emit
light, which means that scientists can use the particles to obtain
deeper, higher-contrast imaging than traditional fluorescence-based
Second, the material for the nanoparticles' shell --calcium
fluoride -- is a substance found in bone and tooth mineral. This
makes the particles compatible with human biology, reducing the
risk of adverse effects. The shell is also found to significantly
increase the photoluminescence efficiency.
To emit light, the particles employ a process called
near-infrared-to-near-infrared up-conversion, or "NIR-to-NIR."
Through this process, the particles absorb pairs of photons and
combine these into single, higher-energy photons that are then
One reason NIR-to-NIR is ideal for optical imaging is that the
particles absorb and emit light in the near-infrared region of the
electromagnetic spectrum, which helps reduce background
interference. This region of the spectrum is known as the "window
of optical transparency" for biological tissue, since the
biological tissue absorbs and scatters light the least in this
The scientists tested the particles in experiments that included
imaging them injected in mice, and imaging a capsule full of the
particles through a slice of pork more than 3 centimeters thick. In
each case, the researchers were able to obtain vibrant,
high-contrast images of the particles shining through tissue.
The results of the study appeared online on Aug. 28 in the ACS
Nano journal. The international collaboration included researchers
from the University at Buffalo and other institutions in the U.S.,
China, South Korea and Sweden. It was co-led by Paras N. Prasad, a
SUNY Distinguished Professor and executive director of UB's
Institute for Lasers, Photonics and Biophotonics (ILPB), and Gang
Han, an assistant professor at University of Massachusetts Medical
"We expect that the unprecendented properties in the core/shell
nanocrystals we designed will bridge numermous disconnections
between in vitro and in vivo studies, and eventully lead to new
discoveries in the fields of biology and medicine," said Han,
expressing his excitement about the research findings.
Study co-author Tymish Y. Ohulchanskyy, a deputy director of
ILPB, believes the 3-centimeter optical imaging depth is
unprecedented for nanoparticles that provide such high-contrast
"Medical imaging is an emerging area, and optical imaging is an
important technique in this area," said Ohulchanskyy. "Developing
this new nanoplatform is a real step forward for deeper tissue
The paper's first authors were Guanying Chen, research assistant
professor at ILPB and scientist at China's Harbin Institute of
Technology and Sweden's Royal Institute of Technology and Jie Shen
of the University of Massachusetts Medical School. Other
institutions that contributed included Roswell Park Cancer
Institute, the University of North Carolina at Chapel Hill and
Korea University at Seoul.
The next step in the research is to explore ways of targeting
the nanoparticles to cancer cells and other biological targets that
could be imaged. Chen, Shen and Ohulchanskyy said the hope is for
the nanoparticles to become a platform for multimodal