A technique that combines the ultrasensitivity of surface-enhanced Raman scattering (SERS) with a slippery surface invented by Penn State researchers will make it feasible to detect single molecules of a number of chemical and biological species from gaseous, liquid or solid samples. This combination of slippery surface and laser-based spectroscopy will open new applications in analytical chemistry, molecular diagnostics, environmental monitoring and national security.
The researchers, led by Tak-Sing Wong, assistant professor of mechanical engineering and the Wormley Family Early Career Professor in Engineering, call their invention SLIPSERS, which is a combination of Wong's slippery liquid-infused porous surfaces (SLIPS), a biologically inspired surface based on the Asian pitcher plant, and SERS.
"We have been trying to develop a sensor platform that allows us to detect chemicals or biomolecules at a single-molecule level whether they are dispersed in air, liquid phase, or bound to a solid," Wong said. "Being able to identify a single molecule is already pretty difficult. Being able to detect those molecules in all three phases, that is really challenging."
Wong needed the help of postdoctoral fellow Shikuan Yang to combine SERS and SLIPS into a single process. Yang was trained in Raman spectroscopy in the characterization laboratory of Penn State's Materials Research Institute. His expertise in the SERS technique and Wong's knowledge of SLIPS enabled them to develop the SLIPSERS technology. Their work appeared online on December 31, 2015 in the Proceedings of the National Academy of Sciences.
Raman spectroscopy is a well-known method of analyzing materials in a liquid form using a laser to interact with the vibrating molecules in the sample. The molecule's unique vibration shifts the frequency of the photons in the laser light beam up or down in a way that is characteristic of only that type of molecule. Typically, the Raman signal is very weak and has to be enhanced in some way for detection. In the 1970s, researchers found that chemically roughening the surface of a silver substrate concentrated the Raman signal of the material adsorbed on the silver, and SERS was born.