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The luminous nanocages around
the living cell appear like stars;
© Purdue University/Ji-Xin Cheng
The system works by shining near-infrared laser pulses through the skin to detect hollow nanocages and solid nanoparticles - made of an alloy of gold and silver - that are injected into the bloodstream. The laser pulses 80 million times per second to illuminate tissues and organs after nanocages have been injected. The researchers tested the method in mice and took images of the tiny structures in tissue samples from organs such as the liver and spleen.
Unlike previous approaches, the new technique does not cause heat damage to tissue being imaged. Another advantage is that it does not produce a background "auto fluorescent" glow of surrounding tissues, which interferes with the imaging and reduces contrast and brightness, said Ji-Xin Cheng, an associate professor of biomedical engineering and chemistry at Purdue University. "This lack of background fluorescence makes the images much more clear," he said.
The gold-silver structures yielded images ten times brighter than other experimental imaging research using gold nanospheres and nanorods, the scientists report. The improved performance could make the early detection and treatment of cancer possible. The gold-silver cages also might be used to deliver time-released anticancer drugs to diseased tissue, the researchers say.
The new imaging approach uses a phenomenon called "three-photon luminescence," which provides higher contrast and brighter images than conventional fluorescence imaging methods. Normally, three-photon luminescence is too dim to be used for imaging. However, the presence of gold and silver nanoparticles enhances the brightness, overcoming this obstacle. The ultrafast laser also is thought to possibly play a role by causing "third harmonic generation," which increases the brightness.
Previous research to develop the imaging system has required the use of "plasmons," or clouds of electrons moving in unison, to enhance brightness and contrast. However, using plasmons generates tissue-damaging heat. The new technique does not use plasmon enhancement, eliminating this heating, Cheng said. The three-photon effect might enable scientists to develop advanced "non-linear optical techniques" that provide better contrast than conventional technologies.
MEDICA.de; Source: Purdue University