Boosting Near-Infrared Sensitivity for Chemical Detection

In absorption spectroscopy, near-infrared (NIR) wavelengths—from 1 to 2.5 μm—have several advantages over longer wavelengths: broader features, better spatial resolution and less water absorption. Weak signal strength and low detector sensitivity, however, have remained the downsides to relying on this band.
 
Researchers at the University of Houston (U.S.A.) have exploited the plasmonic properties of a precious metal to analyze the spectra of hydrocarbons (Nano Lett., doi:10.1021/acs.nanolett.6b01959). The scientists sent NIR light through an array of nanopatterned gold disks sandwiched with a liquid sample   to detect the sample's refractive index as well as its extinction spectra, a technique they dubbed surface-enhanced near-infrared absorption (SENIRA) spectroscopy.
 
The team, led by Houston electrical engineering professor Wei-Chuan Shih, fabricated the nanoporous gold disks to be smaller than the wavelength of NIR light: diameters of 350 and 600 nm, with a pore size of roughly 10 nm. These disks have “plasmonic hot-spots” where the local electromagnetic field is highly enhanced. Shih and his colleagues have been studying the properties of these nanostructured disks for the past three years.
 
To demonstrate the concept, the researchers first acquired NIR extinction spectra of plain water and showed that the localized surface plasmon resonance of the disks red-shifted when immersed in the water. They also performed SENIRA sensing on several different single hydrocarbons, whose C—H and O—H vibrational bands fall in the NIR range. Finally, the group examined the spectra of a polycyclic aromatic hydrocarbon (PAH) and a crude oil sample to show that SENIRA could be used to detect PAH pollutants, which can be carcinogenic, in petroleum products.
 
According to the researchers, the sensing technique can also be applied to a wide variety of molecular species, not just hydrocarbons.