Diverse types of ordinary nanoparticles can be selectively heated on demand by short laser pulses at near-infrared wavelengths. The technique could advance the use of these particles in medical and industrial applications. The Rice University discovery allows controlled laser pulses to tune the absorbance spectrum of common gold nanoparticles, known since the 19th century as gold colloids. “This novel approach is counter to the established paradigm that assumes optical properties of nanoparticles are pre-set during their fabrication and stay constant during their optical excitation,” said Dmitri Lapotko. “The key idea with gold nanoparticles and plasmonics in general is to convert energy. There are two aspects to this: One is how efficiently you can convert energy, and here gold nanoparticles are world champions. Their optical absorbance is about a million times higher than any other molecules in nature. Rice University researchers found that pulsed (or nonstationary) lasers could narrow the response spectra of 60-nm-wide gold nanoshells to a very narrow spectral band (red peak), as opposed to continuous (stationary) excitation by laser (green peak). The discovery opens new possibilities for using metallic nanoparticles in medical and electronic applications. Images courtesy of Lapotko Group/Rice University. “The second aspect is how precisely one can use laser radiation to make this photothermal conversion happen,” he said. Particles traditionally respond to wide spectra of light, and not much of it is in the valuable near-infrared region. Near-infrared light is invisible to water and, more critically for biological applications, to tissue. “All nanoparticles, beginning with solid gold colloids and moving to more sophisticated, engineered gold nanoshells, nanorods, cages and stars, have very wide spectra, typically about 100 nanometers, which means we were allowed to use only one type of nanoparticle at a time. If we tried to use different types, their spectra overlapped and we did not benefit from the high tenability of lasers.” The strong response of plasmonic gold nanoparticles to pulsed (nonstationary) lasers rather than continous (stationary) excitation by lasers seems to result from the influence of nanobubbles on the particles, according to researchers at Rice University. Lapotko and colleague Ekaterina Lukianova-Hleb showed basic colloidal gold nanoparticles could be efficiently activated by a short laser pulse at 780 nm, with an 88-fold amplification of the photothermal effect seen with a continuous laser. They repeated their experiment with nanoparticle clusters in water, in living cancer cells and in animals, with the same or better results: They showed spectral peaks 2 nm wide. Such narrow photothermal spectra had never been seen for metal nanoparticles, either singularly or in clusters. The effect seems to depend on vapor nanobubbles that form when the particles heat liquid in their immediate environment. The bubbles grow and burst in an instant. “Instead of using the nanoparticle as a heat sink with a continous, stationary laser, we’re creating a transient, nonstationary situation in which the particle interacts with the incident laser in a totally different way,” Lapotko said. The effect is repeatable and works with laser pulses shorter than 100 ps, he said. Different types of nanoparticles — in this case, shells, rods and solid spheres — mixed together can be activated individually with pulsed laser light at different wavelengths, according to researchers at Rice University. The tuned particles’ plasmonic response, enhanced by nanobubbles that form at the surface, can be narrowed to a few nanometers under a spectroscope and are easily distinguishable from each other. When the researchers carried out an experiment with mixed nanorods and nanoshells, they discovered that the nanoparticles responded to laser pulses with strong, distinct signals at wavelengths 10 nm apart. This means that two or more types of nanoparticles in the same location can be selectively activated on demand. "The nanoparticles we used were nothing fancy; they were used in the 19th century by Michael Faraday, and it was believed they could do nothing in the near-infrared," he said. "That was the major motivation for people to invent nanorods, nanoshells and the other shapes. Here, we prove these inexpensive particles can behave quite well in near-infrared." The discovery opens the possibility that many metal nanoparticles could be used in biomedical and industrial applications where spectral selectivity and tuning would provide exceptional precision. "This is still more a phenomenon rather than a firmly established mechanism, with a nice theoretical basis," Lapotko said. "But when fully clarified, it could become a universal tool." The findings were reported in Advanced Materials (doi: 10.1002/adma.201204083). For more information, visit: www.rice.edu