Phan Quang Gia Chuong (left) and Associate Professor Ling Xing Yi (right) demonstrating their gas identification technology.
Researchers at Nanyang Technological University, Singapore (NTU Singapore) have invented a way to identify a wide range of airborne chemicals instantly and at a distance. The technology can be used to identify chemical hazards during natural disasters or chemical spills, or to monitor industrial air pollution.
The invention was developed in the laboratory of Associate Professor Ling Xing Yi at NTU's School of Physical and Mathematical Sciences, together with researchers from the Agency for Science, Technology and Research (A*STAR) and the optics company TechnoSpex. A paper describing the new technology was accepted for publication in the journal ACS Nano in September 2019.
Currently, the identification of gas samples uses a laboratory technique called GC-MS (gas chromatography - mass spectrometry). Though reliable, GC-MS requires tedious sample collection and takes between a few hours and a few days to produce results. This is problematic during events such as chemical disasters, when emergency responders require immediate information about the chemicals present in the air.
The new technology uses a small patch made of a porous and metallic nanomaterial that absorbs and concentrates molecules in the air. A technique called Raman spectroscopy is then used to determine what chemicals are present. A laser is shone on the patch, and the light re-emitted by the trapped molecules acts as a ‘chemical fingerprint’, which can be identified using a digital reference library. The entire process takes only about 10 seconds to complete.
The technology uses a patch made of metallic nanomaterial (left), which allows the trapped molecules to be identified via laser spectroscopy (right).
Previously, the use of Raman spectroscopy has mainly been limited to solid and liquid samples, because gaseous chemicals are too dilute to produce a detectable spectroscopic fingerprint.
To overcome this limitation, Assoc. Prof. Ling and her PhD student Mr Phan Quang Gia Chuong developed a nanostructure containing a highly porous material known as a metal-organic framework, which actively absorbs and ‘cages’ airborne molecules. The structure also contains metal nanoparticles, which boost the intensity of the light surrounding the molecules to provide a million-fold enhancement in the strength of the spectroscopic signal.
According to Assoc. Prof. Ling, the invention was sparked by a 2017 incident in which a strong unidentified smell was detected over many parts of Singapore. The cause was only determined a few days later, and traced to volatile organic compounds released by factories outside of Singapore.
Together with her husband Dr Phang In-Yee, a project leader and scientist at the Institute of Materials Research and Engineering (part of A*STAR), Assoc. Prof. Ling conceptualised the idea of identifying gases instantly and from a distance.
“Our device can work remotely, as the laser camera and analyser can be operated at a distance from the sample. This is especially useful if one is unsure whether the gases involved are hazardous to human health,” explains Assoc. Prof. Ling, who is also the Head of the Division of Chemistry and Biological Chemistry at NTU.
The team has demonstrated that their method can reliably identify airborne polyaromatic hydrocarbons, including naphthalene and derivatives of benzene, at parts-per-billion (ppb) concentrations. It can also perform continuous monitoring of the concentration of carbon dioxide (CO2), which could be a useful application in industrial settings.
These tests were performed using laser powers of 50 miliwatts, more than seven times weaker than in other applications of Raman spectroscopy. Lower laser powers make the system safer and cheaper to operate. The prototype has been shown to work at a distance of 10 meters, and the team believes that it can work at further distances with further refinement. Another variant of the technology involves capturing gas samples with the nanomaterial patch for subsequent analysis in a laboratory.
Through NTUitive, NTU’s innovation and enterprise company, the team has filed a patent and is now pursuing commercialization for use in pollution monitoring, chemical disaster response, and other industrial applications.
G. C. Phan-Quang, N. Yang, H. K. Lee, H. Y. F. Sim, C. S. L. Koh, Y.-C. Kao, Z. C. Wong, E. K. M. Tan, Y.-E. Miao, W. Fan, T. Liu, I. Y. Phang, and X. Y. Ling, Tracking Airborne Molecules from Afar: Three-Dimensional Metal–Organic Framework-Surface-Enhanced Raman Scattering Platform for Stand-Off and Real-Time Atmospheric Monitoring, ACS Nano (2019)