A sprinkling of gold nanoparticles coated with a thin oxide shell allows surface-enhanced Raman scattering (SERS) to probe a wider variety of samples, including those with irregular morphology.
SERS usually involves placing a sample on a roughened metal surface, which amplifies the Raman signal from the sample. Zhong Qun Tian of Xiamen University, in China; Zhong Lin Wang of Georgia Tech; and coworkers turn this usual configuration upside down.
The researchers sprinkle the sample with silica- or alumina-coated gold nanoparticles and then collect Raman spectra (Nature 2010, 464, 392). They call their new method shell-isolated nanoparticle-enhanced Raman spectroscopy, or SHINERS.
The “smart dust” consists of 55-nm gold nanoparticles encased in a shell of silica or alumina. The shell, which is about 2 nm thick, isolates the particles from the sample and from each other but still allows enhancement of the Raman signal. “Each particle is like an independent probe, but the layer is thin enough to allow the gold to effectively enhance the Raman signal of the surface molecule to be detected,” Wang says.
“The dielectric coating is a good idea, because it renders the particles inert,” says Renato Zenobi of the Swiss Federal Institute of Technology, Zurich, the inventor of a related method called tip-enhanced Raman spectroscopy (TERS).
The dust improves the sensitivity of SHINERS relative to SERS and TERS. Compared with TERS, SHINERS has less spatial resolution but higher signal intensity from the greater number of nanoparticles on the sample, Zenobi says.
Using SHINERS, Tian, Wang, and coworkers analyzed a wide range of samples. They measured hydrogen adsorption on single-crystal flat surfaces made of platinum and silicon. They also obtained spectra of proteins in yeast cell walls. They were even able to detect pesticide residues on fresh fruit.
Wang envisions using the technique in handheld portable devices that could “make Raman go from a laboratory to people’s daily lives.”