By studying the thermal effects taking place in SERS, a technique for reporting on surface phenomena, scientists may be able to drastically alter the sensitivity of their system and provide vital information for the design of new SERS probes.
Surface-enhanced Raman spectroscopy (SERS) is a widely exploited effect in both materials science and in biology. The technique uses a distribution of nanoparticles on a surface to cause an enhancement of scattered light signals when a laser is directed at the surface. Much work has been done to understand the origins of the SERS effect and to develop new nanoparticle probes that will disrupt the studied surface less, and enhance the signal still more.
A team of researchers based at Sogang and Korea Universities, S. Korea, and University of California at Berkeley, USA, has come together to investigate the causes of a less-studied aspect of SERS; the photothermal effect. Here, the light beam can cause temperature gradients on the surface, which then cause movement of the particles being used to report on the surface information. The particles will aggregate in certain points around the laser hotspot, and tend away from other points. This shifting of particles caused by heating means that particles become more concentrated in some places relative to the laser than in others, and consequently enhance the signal more in those areas than in others.
The researchers also found that the movement of the particles will also cause the SERS signal to reduce in intensity over time. They made this prediction using modelling and tested it on a SERS-labelled DNA molecule.
Luis Liz-Marzán, who works on metal nanoparticles and their use in SERS at University of Vigo, Spain, says “I like the idea of considering what happens to the molecules when they get heated.” However he believes that there may be problems in this calculation when thicker films are used than those the team have studied. This may be the basis of further investigation.
The team believe that this piece of fundamental research on SERS could help to maximize SERS signals in complex systems in the future, as well as providing clues to the best design of new SERS probes, such that they will naturally target themselves around the hotspot. Their findings may also have further-reaching implications for anyone working with plasmonic-based techniques, as similar effects could be seen in these.