Perhaps one of the key analytical methods that have benefited the most from the development of monochromatic coherent light sources, advanced optical filters and multichannel optical detectors is Raman spectroscopy. Raman spectroscopy reveals the vibrational signatures of molecular species that comprise a sample under interrogation. Unfortunately, the Raman process is rather inefficient, with roughly only one out of a million scattered photons contributing to the Raman signal. Advances in laser sources, CCD detectors and optical filters have helped to make Raman spectroscopy more sensitive and cost effective and to produce instrumentation that is portable. This in turn has opened up a broad range of applications including identification and structural characterization of novel materials, quality control in pharmaceutical manufacturing process, development of new diagnostic tools for monitoring diseases and authentication of artifacts and legal documentation. However, it is the application of one class of materials, known as the plasmonic nanostructures, that has enabled the surface enhanced Raman scattering (SERS) phenomenon that provides the most significant boost in Raman detection sensitivity yet realized.
Development of a robust, rapid and sensitive technology for detecting chemical warfare agents released into the air (vapour or aerosol) provides a critical personal protection advantage for our first responders. The ideal analytical platform for the detection of airborne chemical agents should be optimized in speed, sensitivity, accuracy, specificity, cost and portability. Surface enhanced Raman spectroscopy (SERS) is an attractive alternative to many of the conventional detection methods.
SERS is the quintessential poster-child of nanotechnology. Although measured as an optical phenomenon, it is a result of resonant interaction of light with the collective oscillation of free electrons near the surface of metal nanostructures that is important. This interaction of light with free electrons in a gold or silver nanostructure gives rise to localized surface plasmon resonance (LSPR). The LSPR provide a powerful means of confining light to metal/dielectric interfaces, thereby generating intense and highly focused electromagnetic fields that significantly amplify the Raman scattering process. It is this field focusing effect from silver and gold nanostructures that enables the enhanced Raman scattering strength and enhanced detection sensitivity of SERS.
The LSPR are also responsible for the extraordinarily intense and brilliant colours of colloidal silver and gold nanoparticles, whether viewed with the unaided eye in a liquid suspension or under the darkfield microscope as seen in the figure above. While an individual silver or gold nanostructure is capable of supporting LSPR and hence SERS effects, the most prominent electromagnetic field enhancement (and thus the greatest SERS effect) comes if two or more nanostructure moieties lies in close proximity and are tightly coupled. This has been demonstrated experimentally as well as through computational simulations as shown in the Figure and references below. The fruits of this modelling provide important guides for the design of a SERS sensor suitable for high sensitivity detection, and especially for trace chemical detection.
Similar to traditional Raman spectroscopy, SERS provides rich chemical species information based on a molecule’s vibrational fingerprint. Matching of SERS spectra to analyte molecules can be made instantly through comparison with entries from the large spectral library databases that are available commercially. This information can be critical for frontline security personnel and first responders in their decision making process. In a continuing effort with our collaborative partners in defence organizations, the National Research Council of Canada (NRC) has fabricated and evaluated SERS sensors capable of rapid identification of hazardous aerosols. Colloidal silver and gold nanoparticles are deposited on a variety of porous substrates to enable the uptake of vapor or aerosol from the ambient environment. Spectral measurements and identification are achieved with a field portable handheld Raman spectrometer ensuring its applicability in real-world conditions.
Our team has also developed new measurement capabilities to transfer reliably this originally laboratory-based optical detection technology to the field. Rather than relying on the popular SERS enhancement factor measurements that are based on the field enhancement of the nanostructure substrate, our measurement protocols rely on statistical analyses and calibration to provide a high degree of confidence in the detection of a particular level of hazard present. Working together with our partners, we continue to refine our engineered nanostructure substrates and to push the detection limit of our SERS sensors.
L.-L. Tay, J. Hulse, D. Kennedy and J.P. Pezacki, J. Phys. Chem. C, 114, 7356, 2010
L. L. Tay, J. Hulse, J. Modern Optics, 60 (14), 1107, 2013
Li-Lin Tay is a senior research officer at the National Research Council of Canada, Measurement Science and Standards portfolio in the Photometry, Radiometry and Thermometry group. Her primary research effort is focused on the measurement of optical properties of nanostructures, surface enhanced Raman and infrared spectroscopy and the development of Raman spectroscopy standards. She is best known for her work on the correlation between surface enhanced Raman spectroscopy and morphology of nanoclusters and for the application of nanoparticles chemical- and bio-sensors.