"We are really proud that we have shown one nanoparticle can be used for catalysing the reaction and providing a very large SERS [surface enhanced Raman sensing] enhancement," says Hui Wang, an assistant professor in the Department of Chemistry and Biochemistry at the University of South Carolina, and principle investigator of this latest research.

SERS, which uses local enhancements to the electromagnetic field to boost signals, is a powerful tool for identifying reaction intermediates and products to track reaction dynamics. However, only nanoparticles of around 100 nm in size have tunable optical resonances that can be harnessed for SERS. In contrast, to act as good catalysts the nanoparticles must be no bigger than 5 nm.

"If you use a SERS substrate for the enhancement and a separate particle for the catalysis you can’t track the reaction because the reaction occurs at the catalytic particles and the SERS enhances other molecules," says Nicolas Large, a postdoctoral researcher in Peter Nordlander’s group at Rice University in Texas, and also a collaborator in the work.

The secret to the dual-action particles achieved by Wang and colleagues was to etch several facets onto nanoparticles that were large enough to be optically tuned for enhanced signal detection. These tailored facets contain a high density of atomic-scale kinks, steps and ledges, which act as active sites for breaking chemical bonds so that the nanoparticles also perform well as catalysts.

Combining catalysis and sensing

Wang and the University of South Carolina group teamed up with Large and colleagues at Rice University and Nantong University, China. The researchers produced cuboid-shaped nanorods of gold coated in silver and etched them into "nanorice" and dumb-bell shapes. Silver has very strong and highly tuneable resonances but is chemically unstable so the team added a gold core to provide support.

To demonstrate their dual action the researchers used the nanoparticles to catalyse the reduction of p-nitrothiophenol by NaBH4. The same nanoparticles enhanced SERS signals to track the reaction.

The dumb-bell-shaped nanoparticles were the best catalysts. They increased the reaction rate 10 times more than the nanorice-shaped particles and about 100 times more than the nanocuboids that had flat surfaces. "For the flat nanoparticles the reactions were really slow – there was a really strong contrast," explains Wang.

Different facets

Key to increasing SERS signals are "plasmon resonances" – collective oscillations of electrons in response to light that enhance the electromagnetic field nearby. While nanoparticles of noble metals such as silver and gold have strong, highly tunable plasmon resonances, their large surface energy makes fabricating high-index-faceted nanoparticles tricky. To achieve this, the researchers at the University of Carolina identified the right balance of chemical reactants to control etching.

During the etching process, the team took spectra and scanning electron microscope images of the nanoparticles, as well as calculating their expected spectral properties with a numerical algorithm – the finite difference time domain (FDTD). "There was a very nice quantitative agreement between FDTD and experiment both in the shift and the evolution of the peak intensities, which means we understand the evolution of these nanoparticles," says Large.

Next the researchers plan to look at other catalytic materials that can be engineered for tracking additional chemical reactions.

The researchers report full details in Nano Letters.