Rice develops nanosensor for precision chemical analysis

Nanoshell sensor opens door for new methods to exam single molecules

Nanotechnology researchers at Rice University have demonstrated the ability to precisely control the electromagnetic field around nanoparticles, opening the door for chemical screening techniques that could allow doctors, life scientists and chemists to routinely analyze samples as small as a single molecule.

The research is detailed in the current issue of Applied Physics Letters. It builds upon a widely used method of molecular analysis called Raman spectroscopy and capitalizes on the tunable optical properties of metal nanoshells, a novel type of nanoparticle invented at Rice.

“This result is extremely important because it is the first time that anyone has actually designed and engineered a nanosensor specifically for obtaining chemical information,” said nanoshell inventor Naomi Halas, the Stanley C. Moore Professor of Electrical and Computer Engineering. “There are widespread applications for this technology in environmental science, chemistry and biosensing, and it may have very important applications in the early detection of cancer.”

Scientists commonly use spectroscopy to discern detailed information about everything from distant galaxies to individual molecules. By studying the spectrum of light that an object emits, scientists can decipher which elements are present in the sample, and in some cases, how those elements relate to one another. Raman spectroscopy, in particular, allows scientists to observe the vibrational states of molecules, giving clues about where and how much molecules bend, for example, and serves as a “fingerprint” for the identification of specific molecules that may be of interest, such as environmental contaminants or chemical or biological toxins.

Scientists have long known that they could boost the Raman light emissions from a sample by a million times or more by placing the sample next to small particles of metal called colloids. Scientists have even observed single molecules with this method, but they have never been able to precisely control the electromagnetic state of the metal colloids, so results and interpretations of such studies vary widely.

Rice’s research offers scientists a chance to precisely control “surface enhanced Raman scattering,” or SERS. In the Rice experiments, Halas’s group was able to dramatically enhance the SERS effect, making it up to a billion times more powerful in some cases.

Similar in structure to a hard-shelled chocolate candy, nanoshells are layered colloids that consist of a core of non-conducting material covered by a thin metallic shell. By varying the thickness of the conducting shell, researchers in Halas’ group can precisely tune the electric and optical properties of nanoshells.

Nanoshells are so useful for enhancing SERS and for other applications because of their size and precise structure. Nanoshells are just slightly larger than the size of molecules, measuring just a few tens of nanometers, or billionths of a meter, in diameter. Tuning the properties of nanoshells gives Halas’ group the ability to exert new forms of precision control at the molecular level.

The SERS research is described in the Jan. 13 issue of Applied Physics Letters in a paper titled “Controlling the Surface Enhanced Raman Effect via the Nanoshell Geometry,” by J.B. Jackson, S.L. Westcott, L.R. Hirsch, J.L. West and N.J. Halas. The paper is available online at http://ojps.aip.org/aplo/.

The research was funded by the National Science Foundation, the Robert A. Welch Foundation and the Army Research Office’s Multidisciplinary University Research Initiative.