SYDNEY & ADELAIDE, Australia, Sept. 3, 2013 — The combination of a special kind of nanocrystal known as a SuperDot™ and a unique optical fiber that enables light to interact with nanoscale volumes of liquid has allowed the detection of a single nanoparticle from a distance using light. These superbright, photostable and background-free nanocrystals — three orders of magnitude brighter than quantum dots — enable a new approach to highly advanced sensing technologies using optical fibers.

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The supersensitive nanocrystals enable the microstructured optical fiber to detect and track the movement of a single nanocrystal remotely. Courtesy of Dr. Mathieu Juan. 

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"Up until now, measuring a single nanoparticle would have required placing it inside a very bulky and expensive microscope," said professor Tanya Monro, director of the University of Adelaide's Institute for Photonics and Advanced Sensing (IPAS) and a member of the team that made the discovery with colleagues from Macquarie University in Sydney and Peking University in China. "For the first time, we've been able to detect a single nanoparticle at one end of an optical fiber from the other end. That opens up all sorts of possibilities in sensing."

For applications such as biodetection, bioimaging, solar cells and 3-D display technologies, nanocrystals can be doped with sensitizer ions, called dopants, that absorb infrared radiation, then transfer their excitation to activator ions that then emit visible light. The more dopants added, the higher the emission brightness — up to a point. At a certain relatively low threshhold, activator ions are maxed out, and brightness begins to diminish.

In investigating ways to overcome this limitation, the team found that high levels of infrared radiation combined with higher activator concentrations in excess of the threshhold led to significantly enhanced luminescence signals by up to a factor of 70. This represents a sensitivity improvement of three orders of magnitude over benchmark nanocrystals such as quantum dots, the researchers said in their paper.

These single nanocrystals were bright and sensitive enough to be detected remotely using an optic fiber, or seen with the naked eye through an automated scanning microscope, which provided high-contrast biolabels to track individual cells, or sense single molecules.

The special optical fiber engineered at IPAS also proved useful in understanding the properties of nanoparticles.

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The microstructured optical fiber has been employed as a nanoliter-volume spectroscope to analyze the optical properties of nanocrystals. Courtesy of Matthew Henderson.

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"Material scientists have faced a huge challenge in increasing the brightness of nanocrystals," said Dr. Dayong Jin, Australian Research Council Fellow at Macquarie's Advanced Cytometry Laboratories. "Using these optical fibers, however, we have been given unprecedented insight into the light emissions. Now, thousands of emitters can be incorporated into a single SuperDot™ - creating a far brighter and more easily detectable nanocrystal."

"Using optical fibers, we can get to many places, such as inside the living human brain, next to a developing embryo, or within an artery — locations that are inaccessible to conventional measurement tools," Monro said. "This advance ultimately paves the way to breakthroughs in medical treatment. For example, measuring a cell's reaction in real time to a cancer drug means doctors could tell at the time treatment is being delivered whether or not a person is responding to the therapy."

Under infrared illumination, SuperDots™ selectively produce bright blue, red and infrared light with 1000 times more sensitivity than existing materials.

"Neither the glass of the optical fiber nor other background biological molecules respond to infrared, so that removed the background signal issue. By exciting these SuperDots™, we were able to lower the detection limit to the ultimate level — a single nanoparticle," Jin said.

Macquarie is working with industry partners Minomic International Ltd. and Patrys Ltd. to develop uses for SuperDots™ in cancer diagnostic kits, "detecting incredibly low numbers of biomarkers within conditions like prostate and multiple myeloma cancer," Jin said. The university is now seeking other industrial partners with the capacity to jointly develop solutions outside of these fields.

The work appears in Nature Nanotechnology (doi: 10.1038/nnano.2013.171)   

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