Surface-enhanced Raman technique improves atmospheric monitoring

22 Oct 2019

Nanyang Technological University platform offers long-distance analysis of the environment.

A new portable device able to rapidly identify a wide range of airborne gases and chemicals could assist the monitoring of air quality, improve the detection of gas leaks, and provide valuable public health data.

Developed at Singapore's Nanyang Technological University (NTU), it employs a surface-enhanced Raman technique specifically designed to tackle the detection of airborne species, rather than solids and liquids. The work was reported in ACS Nano.

Current identification of gases in the air often involves forms of gas chromatography-mass spectrometry, a technique which can require hours or days to obtain results from air samples.

The new device uses instead a small patch of porous metallic nanomaterial to trap gas molecules, and then carries out analysis through surface-enhanced Raman spectroscopy (SERS), an established way to enhance Raman scattering of molecules adsorbed onto metal surfaces.

The spectroscopic readout from the sample is referenced against a digital library of spectral fingerprints, allowing the chemicals present on the patch to be identified in about 10 seconds according to the researchers.

"Our device can work remotely, so the operation of the laser camera and analysis of chemicals can be done safely at a distance," said Ling Xing Yi of NTU. "This is especially useful when it is not known if the gases are hazardous to human health."

According to the project's published paper, variants of stand-off Raman spectroscopy, where the instrumentation is physically separated from the sample under investigation and the spectra are recorded a safe distance away, are usually restricted to the detection of pure solids or liquids, and until now not widely applicable for dispersed molecules in air.

Environmental conservation and homeland defense

The NTU team tackled the problem by integrating a long-range optic system with a SERS platform based around a 3D metal-organic framework.

"Our 3D plasmonic architecture exhibits micrometer-thick SERS hotspots, to allow active sorption and rapid detection of aerosols, gas, and volatile organic compounds down to parts-per-billion levels, notably at a distance up to 10 meters away," commented the project team.

The new nanomaterial builds on NUT's previous research into SERS platforms, which has tackled the challenge of detecting molecules which have no specific affinity to plasmonic surfaces. This has led to the development of design strategies intended to capture and confine analytes near SERS-active surfaces, strategies that can then be integrated with established plasmonic hotspot designs for an overall boost to SERS sensitivities.

In trials of the new system, it proved able to carry out temporal monitoring of gaseous carbon dioxide as well as perform remote and multiplex quantification of polycyclic aromatic hydrocarbon mixtures. The platform detected parts-per-billion concentrations of naphthalene and derivatives of benzene, pollutants known to be carcinogenic, in real time under outdoor daylight.

The laser used in the device has an energy intensity of 50 miliwatts, said by NTU to be more than seven times weaker than in other applications of Raman spectroscopy. This makes the system safer to operate and more energy efficient.

"By overcoming core challenges in current remote Raman spectroscopy, our strategy creates an opportunity in the long-distance and sensitive monitoring of air and gaseous environments at the molecular level," said Ling Xing Yi. "This is especially important in environmental conservation, disaster prevention, and homeland defense."