Gold nanoparticles enhance laser heating

26 Apr 2006

Ice melts at lower incident laser intensities when the sample contains gold nanoparticles, say researchers in the US.

Researchers at Ohio University, US, have used a laser to heat gold nanoparticles embedded in ice. The nanoparticles enabled the laser to melt the ice at lower light intensities (Nano Letters 6 783).

Nanoparticle heating

"The motivation is using gold nanoparticles as site-directed nano heaters that can be actuated with optical light," Hugh Richardson of Ohio University told nanotechweb.org. "The applications are broadly based - photothermal therapy, lithography, chemical reaction initiation at specific sites - and the impact is large."

The team used a Raman/near-field scanning optical microscope to look at 50 nm diameter gold nanoparticles embedded in ice at -20°C. They illuminated the particles with a 532 nm laser at various powers up to 50 mW.

In the absence of nanoparticles, a laser intensity of 50 mW was not enough to melt the ice. The researchers estimated that a single 50 nm gold nanoparticle gave a heat flux intensity of around 9.6 µW for a melting light power of 4 mW.

"The amount of heat generated from gold nanoparticles depends upon the number, geometry and density of the particles," said Richardson. Several large agglomerations of nanoparticles were present. "Each different complex has its own threshold laser power that would cause melting," he explained.

According to the scientists, the most obvious application for their work is in the field of nanomedicine.

"Gold nanoparticles can be complexed with a wide assortment of different functional groups that will allow the gold nanoparticles to preferentially bind to a target membrane in a biological host," said Richardson. "Once the gold nanoparticles are in place then they can be excited with light. Light absorption is orders of magnitude higher for the gold nanoparticles compared to the surrounding tissue. The light energy is converted by the gold nanoparticle into heat and local heating of the membrane actuates a change that is desirable."

The team is currently using layer-by-layer polyelectrolyte techniques to assemble gold nanoparticle complexes, characterizing the complexes with atomic force microscopy (AFM), transmission electron microscopy (TEM) and photoluminescence, and determining the heat generation from the complexes.