Mar 16, 2010
SERS aggregates patterned by laser trapping
Gold and silver nanoparticles are important for chemical sensing, non-invasive cancer treatment therapy, and even carbon nanotube and nanowire growth, due to their strong surface plasmon resonance occurring at wavelengths around 550 nm. This resonance condition is affected by the size and shape of the nanoparticles, as well as their spatial arrangement. For nearly touching nanoparticles, the resonant enhancement can increase the local electric fields by several orders of magnitude. Recently, various techniques have been used to create different assembles of the plasmonic nanoparticles, including electron-beam lithography, block copolymer lithography and DNA linking.
In a recent study, scientists in the US have demonstrated a simple method for depositing plasmonic aggregations of gold nanoparticles. First, gold nanoparticles with an average diameter of around 20 nm are mixed in aqueous suspension with a citrate stabilizer to prevent precipitation. Next, the sample is irradiated with a 532 nm wavelength laser focused in the suspension through an optical microscope objective lens.
The gold nanoparticles aggregate under the force generated by the optical intensity gradient, forming a strongly plasmonic region. With the laser continuously on, a bubble filled with water vapour is formed due to the heat generated in the plasmonic region. As a result, the gold nanoparticles are pushed away to form a ring shape.
The temperature in the region of the focused laser spot before the bubble formation can be determined precisely by measuring the Raman peak of the hydroxyl bond of water. After the bubble formation, the temperature can be estimated by measuring the change in the size of the vapour bubble.
Measurements performed by the group show that the temperature increased to the boiling point of water in about 15 seconds during the gold nanoparticle aggregation, and then increased to about 400 °C after the bubble formation. As a control experiment, the team also focused the laser in water without gold nanoparticles, which showed no bubble formation.
This experiment presents a novel route to create plasmonic assemblies of metallic nanoparticles and also demonstrates their strong plasmonic effect. For applications where it is desirable to avoid the bubble formation, lower incident laser intensities or higher boiling-point solvents can be used.
More information can be found in the journal Nanotechnology.
About the author
Steve Cronin is a professor in the electrical engineering department of the University of Southern California, US.