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.
A transmission electron microscopy image of nanoparticles designed for deep-tissue imaging. Each particle consists of a core encased inside a square, calcium-fluoride shell. Photo credit: Zhipeng Li
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 techniques.
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 emitted.
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 range.
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 School.
"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 visualization.
"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 optical
bioimaging."
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 bioimaging.
Source: Buffalo University.