Researchers from the University of Houston (UH) have reported a technique to determine the chemical composition of materials using near-infrared light. Substances that can’t be accurately measured by sensors operating on the infrared spectrum can now be studied in far more detail using the near-infrared spectrum.

The work could have a number of potential applications, including improving downhole drilling analysis in the oil and gas industry and broadening the spectrum of solar light that can be harvested and converted to electricity.The sensing technique could simplify downhole fluid analysis by reducing the sample size required for laboratory examination. Image credit: Pixabay.The sensing technique could simplify downhole fluid analysis by reducing the sample size required for laboratory examination. Image credit: Pixabay.

“From a scientific point of view, it’s quite a novel discovery to excite plasmonic resonance at near-infrared and make it work for us,” says Wei-Chuan Shih, associate professor of electrical and computer engineering at the University of Houston and lead author of a paper published in the journal Nano Letters describing the discovery.

Spectroscopy using the infrared spectrum—an analytical technique utilizing infrared light to scan and identify the chemical composition of organic, polymeric and some inorganic materials—is an important tool, but it has limitations. Infrared light is absorbed by water, so the technique doesn’t work with water-based samples.

Near-infrared light scanning is compatible with water, but current techniques are less sensitive than those using other wavelengths. To overcome these barriers, UH researchers developed a technique to simultaneously obtain chemical and refractive index sensing in the 1-2.5 µm near-infrared wavelength range on nanoporous gold disks, which feature high-density plasmonic hot spots of localized electric field enhancement.

“For the first time, surface-enhanced near-infrared absorption (SENIRA) spectroscopy has been demonstrated for high-sensitivity chemical detection,” the researchers wrote.

According to Shih, working with near-infrared light is usually a "double-edged sword,” as it can be used with water-based samples but doesn’t provide the needed detail. “We showed water is not an issue, but we can also increase the sensitivity of what we want to measure by 10,000 times,” he says.

He and members of his lab have worked with nanoporous gold disks since discovering the structure in 2013. For this project, Shih says they “tuned,” or designed, the nanodisks to react when exposed to specific wavelengths, making it possible to develop a sensing technique with the advantages of both infrared and near-infrared scanning.

Shih says the technique, which was tested with various crude oil and other hydrocarbon samples, could be helpful in downhole fluid analysis, which uses near-infrared spectroscopy to analyze material found deep in a well. The new sensing technology could simplify that process because it requires a smaller sample for analysis—an obvious advantage in laboratory characterization.

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