New method detects single molecule of explosives, contaminants

Scientists have developed a new technique using Raman spectroscopy that can detect single molecules of contaminants, explosives or disease-related compounds from gaseous, liquid or solid samples.
The technique will open new applications in analytical chemistry, molecular diagnostics, environmental monitoring and national security, researchers said.
The technique called SLIPSERS is a combination of slippery liquid-infused porous surfaces (SLIPS), which is a biologically inspired surface based on the Asian pitcher plant, and surface enhanced Raman scattering (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," said Tak-Sing Wong, from Pennsylvania State University in US.

Raman spectroscopy is a method of analysing 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.
SLIPS is composed of a surface coated with regular arrays of nanoscale posts infused with a liquid lubricant that does not mix with other liquids.
The small spacing of the nanoposts traps the liquid between the posts and the result is a slippery surface that nothing adheres to.

"The problem is that trying to find a few molecules in a liquid medium is like trying to find a needle in a haystack," Wong said.
"But if we can develop a process to gradually shrink the size of this liquid volume, we can get a better signal," he said.
"To do that we need a surface that allows the liquid to evaporate uniformly until it gets to the micro or nanoscale. Other surfaces can't do that, and that is where SLIPS comes in," he said.
If a droplet of liquid is placed on any normal surface, it will begin to shrink from the top down. When the liquid evaporates, the target molecules are left in random configurations with weak signals.

However, if all the molecules can be clustered among gold nanoparticles, they will produce a strong Raman signal.
"First we need to use noble metal nanoparticles, like gold. And then we have to assemble them so they make nanoscale gaps between the particles, called SERS 'hot spots'," said Shikuan Yang, post-doctoral fellow at Penn State.
"If we have a laser with the right wavelength, the electrons will oscillate and a strong magnetic field will form in the gap area. This gives us very strong SERS signals of the molecules located within the gaps," Yang said.
The research was published in the journal PNAS.