PORTLAND, Ore. — Hot on the trail of sensors that can sniff out even a single molecule of a hazardous substance, scientists recently discovered that specially grown carbon nanotubes can greatly boost the sensitivity of a Raman spectroscope.
By boosting sensitivity by as much as 100,000-times, carbon nanotube enhanced Raman spectroscopes are approaching the goal of single-molecule detection in an inexpensive portable electronic nose (e-nose), according to scientists at the Eidgenössische Technische Hochschule (ETH, Zurich) and Lawrence Livermore National Laboratory (LLNL, Livermore, Calif.)
Officially called surface-enhanced Raman spectroscopy (SERS) professor Hyung Gyu Park at ETH in collaboration with research scientist Tiziana Bond at LLNL, claim to have found an easy-to-manufacture way to enhance SERS for e-nose applications.
"Our SERS sensor is much improved in detection sensitivity compared to many existing sensors -- by 10,000-to-100,000 times," Park told EETimes. "The important point or breakthrough of our result is that we achieved a facile and cheap way of making this ultrahigh-sensitivity sensor -- one that allows reliable and repeatable detection of chemical species."
New high-sensitivity sensor uses bundles of carbon nanotubes having curved tips that let through Raman scattered light to enable ultra-sensitive chemical detection.
SOURCE: H.G. Park / ETH Zurich)
Raman spectroscopy has many advantages over traditional detection methods. It works by illuminating molecules with a single frequency of light -- usually from a laser -- then observing the resulting scattered light whose frequency pattern can be used as a fingerprint to identify the substance. But to identify just a few individual molecules, these frequencies must be greatly amplified, usually achieved by placing the surface of the detector very close to the sample thus prompting the name "surfaced-enhanced" Raman spectroscopy.
The researchers specifically set out to discover a new way of more reliably detecting even the faintest signals in Raman-scattered light. They found that densely packed bundles (caespitose) of carbon nanotubes could be used to create "hot spots" for the e-nose where sensitivity was greatly enhanced.
The technique works by curving the ends of the nanotube bundles to enable the Raman light to reach any molecules that are mingled among the nanotubes at the business end of the e-nose (see image above). To enhance the effect further, and make detection more reliable, the team coated the end of the nanotubes first with an oxide (hafnium dioxide), then with gold, creating thousands of high-density nanometer-sized hot spots within a three-dimensional structure.
"The wide distribution of nanoscale crevices in the tip of the sensor not only greatly enhances its sensitivity, but also makes its results much more reliable and reproducible," said Bond.
Next the researchers will begin optimizing the process for commercialization, as well as begin working on a portable device for using their e-nose in the field to detect trace amounts of substances, from environmental pollutants to pharmaceutical residues to chemical weapons.
Doctoral candidate at the ETH Institute of Energy Technology, Ali Altun, also contributed to the work.