Plasmonic nanoparticles

Plasmons are free electrons on the surface of metals that become excited by the input of energy, typically from light. Moving plasmons can transform optical energy into heat. 
Plasmonic nanoparticles are particles whose electron density can couple with electromagnetic radiation of wavelengths that are far larger than the particle. This is due to the nature of the dielectric-metal interface between the medium and the particles unlike in a pure metal where there is a maximum limit on what size wavelength can be effectively coupled based on the material size. Plasmonic nanoparticles also exhibit interesting scattering, absorbance, and coupling properties based on their geometries and relative positions. These unique properties have made them a focus of research in many applications including solar cells, spectroscopy, signal enhancement for imaging, and cancer treatment.
Plasmonic gold nanoparticles
Gold nanoparticles can be used for efficiently converting energy because of their optical absorbance is about a million times higher than any other molecules in nature. Rice University scientists have shown that common gold nanoparticles, known as gold colloids, heat up at near-infrared wavelengths as narrow as a few nanometers when hit by very short pulses of laser light.
The effect reported appears to be related to nonstationary optical excitation of plasmonic nanoparticles. Plasmonic gold nanoparticles make pinpoint heating on demand possible. Researchers have found a way to selectively heat diverse nanoparticles that could advance their use in medicine and industry.
The above 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. According to researchers 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 and hence only one type of nanoparticle can be used at a time.
The discovery allowed researchers to use controlled laser pulses to tune the absorbance spectrum of plain gold colloids. The Rice lab showed basic colloidal gold nanoparticles could be efficiently activated by a short laser pulse at 780 nanometers, with an 88-fold amplification of the photothermal effect seen with a continuous laser.


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