Lighting Up the Shadowy Fringes orne out of the stellar atomic sources of astronomy, and brought down to terrestrial systems by Robert Bunsen and Gustav Kirchoff, spectroscopy is a widely used analytical tool. Exploitive of material interactions with photons, this near ubiquity of opportunity makes it notoriously prone to interference in samples containing a significant matrix wrapped about the target analyte(s). Although the background’s influence on the foreground is arguably the definition of “environmental” chemistry, its signaldeadening ways has required additional methodological savvy for spectroscopic techniques. Environmental Science & Technology has played host to the development of such methods and sees their regular application both at the bench and in the field. Herein, Hunt et al.’s (Environ. Sci. Technol. DOI 10.1021/es100698m) field detection of nitrate and Luo et al.’s (Environ. Sci. Technol. DOI 10.1021/es1024433) interrogating the influence of soil-based metals on dissolved organic carbon, provide immediate examples. Spectroscopy’s deployment in physical chemistry realms has developed its ability to hone in on molecular dances to literally elucidate chemical happenings. Passive tracking of kinetics has led to an improved understanding of how to exploit oxidizing species like peroxides to clean wastewater, as demonstrated by Popov et al. (Environ. Sci. Technol. DOI 10.1021/es101959y). Similarly, the quantum mechanical origins of spectroscopy permit its incredible utility to turning kinetic studies into mechanistic modeling as shown by Maddigapu et al. (Environ. Sci. Technol. DOI 10.1021/es102458n). These subtle “fringes” are informationally rich thanks in no small part to the advent of the “LASER” half a century ago. Providing both the surgical precision of single wavelength photons and a blindingly intense source thereof, lasers have taken analysis into the fuzzier portions of spectra. For example, the faint redshift of solar photons first observed in the 1920s has become a viable benchtop and later in situ method eponymously coined Raman spectroscopy thanks to laser source light. The signal is due to a few photons inelastically scattering off molecular vibrational modes, often sapping their energy (red- or Stokes shift) and sometimes boosting it (blue- or anti-Stokes shift). Most of the delivered light is of course elastically scattered (Rayleigh). The laser is required to deliver a detectable number of photons and provide a sharp Rayleigh line that when filtered out puts the Raman signal in high relief. For example, Zhang et al. show Raman’s utility to optimizing mineralogical
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7748 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / October 15, 2010
carbon sequestration approaches (Environ. Sci. Technol. DOI 10.1021/es1019788). The selection rules underlying Raman spectroscopy make it complementary to the more familiar and easily accessible vibrational spectroscopy: infrared (IR). Therefore strong IR absorbers are essentially invisible to Raman. Because H2O is notoriously IR-opaque, this makes Raman a compelling possibility for aqueous systems such as those found throughout the environment. Yet while lasers do permit several orders of magnitude boosting of latent Raman signals, the quivering complexity of organic matter has required an additional level of methodological savvy for environmental Raman spectroscopy. Through the 1980s and 1990s, chemists learned how to exploit (metal) surfaces’ ability to enhance Raman signals to near trillion-fold amounts. As applied surface chemistry and nanoparticle synthesis have evolved in lockstep, the reproducible tethering of molecular systems has brought surface-enhanced Raman spectroscopy (SERS) usefully into environmental realms. In this issue’s cover Feature, Halvorson and Vikesland overview the theory and applications of environmental SERS (Environ. Sci. Technol. DOI 10.1021/es101228z). As application of pure science begets instrumentation to understand natural chemistry, so too can it help mitigate anthropogenic influence. Fluorescent dyes in consumer products such as detergents enhance their whitening (extra apparent in sunlight thanks some to Raman in addition to fluorescent emission). The widespread use of these bath- and laundry-room chemicals has them whisked into wastewater; that they are nearly impervious to biodegradation means they provide a fluorescent marker to spectroscopically track the propagation of wastewater in the natural environment. Yamaji et al. exploit this fact to determine the photochemical fate of wastewater in lakes (Environ. Sci. Technol. DOI 10.1021/es100465v). Our ability to use photons can thus help to create molecules and photondriven technologies that can better break down molecules harmlessly. ES&T continues to welcome science and engineering developments of this ilk, lighting a path to a cleaner and brighter future.
Darcy J. Gentleman Managing Editor*
[email protected] 10.1021/es103144m
2010 American Chemical Society
Published on Web 10/13/2010
