Editorial Feature

Raman Spectroscopy: A Tool for Conservation

Raman spectroscopy is one of the most powerful analytical techniques for identifying the molecular structure of matter. This capability is vital in environmental monitoring and conservation.

Raman Spectroscopy: A Tool for Conservation

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What is Raman Spectroscopy?

Raman spectroscopy is a spectroscopic technique used to observe a system's low-frequency modes, including rotational and vibrational modes.1 The Raman effect was initially identified in 1928 in India by Sir Chandrasekhara Venkata Raman, who would go on to win the Nobel Prize.

In the Raman method, incident light is scattered on a molecule. Most of this scattered light has no discernible information and is at the same wavelength as the source. However, a small quantity of light disperses at different wavelength shifts depending on the molecule's internal composition.

This phenomenon, known as the Raman shift, is instrumental in revealing details about the molecule.

Rise of Raman Spectroscopy as an Analytical Tool

Before the 1990s, Raman spectroscopy was largely confined to research laboratories due to the bulky setups it required, which included expensive equipment such as powerful lasers and delicate detectors. These factors made it impractical for wider use.2

The telecom boom changed the game. The widespread adoption of optical components in telecommunications drastically reduced their cost. This affordability trickled down to Raman spectroscopy, making the systems more accessible and leading to the development of portable spectrometers.

As a purely optical technique, Raman spectroscopy offers a significant advantage: it works even in liquids. This versatility, coupled with portability, has propelled Raman spectroscopy to the forefront of two key applications:

  • Molecular Fingerprinting: Raman spectroscopy can uniquely identify molecules, creating a "fingerprint" for easy recognition.
  • Structural Analysis: By studying the Raman spectrum, scientists can gain valuable insights into the structure of molecules. 

In terms of sample pre-processing and measurement runtime, Raman Spectroscopy is significantly more efficient than mass spectrometry (MS), another widely employed technique. Raman spectroscopy also allows measurements to be made in the field, which is typically not feasible with mass spectrometry.

Most importantly, when combined with advanced techniques like Surface Enhancement Raman Spectroscopy (SERS), Raman spectroscopy can match the sensitivity of mass spectrometry.3

SERS: The Basics

SERS is a spectroscopic method that increases the signal and, consequently, the detection of target molecules. This is accomplished through selective chemical and electrical interactions between the spectrometer's excitation source, the target analyte, and a specific substrate.

Colloidal metal nanoparticle systems, often based on gold or silver, are the most widely used substrates for SERS. These nanoparticles are not only efficient and simple to prepare, but they also allow for chemically controlled variation of the nanoparticle size and type to tune analytical sensitivity.

In theory, SERS can identify single molecules and increase signal detection by more than ten billion times compared to conventional Raman spectroscopy.4

While single-molecule detection is exceedingly difficult outside of very specific laboratory setups, SERS continues to gain popularity due to its high sensitivity, rapid analysis capabilities, and versatility in detecting a wide range of target compounds, including pollutants.

These attributes have made SERS a valuable tool for environmental monitoring and conservation.

The Impact of SERS in Environmental Monitoring

SERS can significantly reduce the costs and time associated with environmental detection while enabling large-scale, remote, automated source sampling. Below are some of the major impacts of SERS in water quality monitoring and conservation:

  • Polycyclic Aromatic Hydrocarbons (PAHs): Composed of two or more condensed aromatic rings, PAHs are a class of chemical compounds recognized as carcinogenic pollutants. Since PAHs pose significant cancer and mutation risks, their presence in saltwater has been a persistent issue. SERS, using a substrate treated with calixarene, effectively identifies PAHs.
  • Benzene Detection: Benzene, which is present in industrial chemicals, can severely damage human bone marrow and the central nervous system. Numerous studies have demonstrated the effectiveness of Raman spectroscopy in identifying benzene.
  • Antibiotics in Aquatic Environments: Antibiotics in aquatic environments often originate from veterinary, medical, and industrial wastewater. A significant proportion of antibiotics found in aquaculture are also derived from this source. High-sensitivity silver sol films have proven extremely useful for the quick, on-site trace detection of antibiotics in aquatic environments using SERS.
  • Pesticide Detection: Misuse of pesticides in agriculture can impact aquatic organisms and their environments. To generate a milieu conducive to pesticide assembly and SERS detection, particular absorbers were employed as bifunctional linkers during the functionalization of metal nanoparticles. The findings verified that SERS has a high sensitivity for detecting organochlorine pesticides, with a detection limit of up to 100 million.

Beyond organic pollutants, SERS is also a premier tool for monitoring and identifying inorganic materials. Detectable substances include perchlorates, nitrates, and sulfates.

Heavy metals, such as mercury, cadmium, lead, chromium, and copper, are widely distributed in the environment and primarily originate from industrial waste discharges. These substances are also identifiable through SERS.

The Future of SERS in Conservation

Raman spectroscopy techniques have been extensively used to identify various contaminants in aquatic settings, such as organic, inorganic, and biological pollutants.

This technique has evolved into a quick and precise method for the quantitative and qualitative detection of substances that negatively impact water quality and the environment, especially when combined with cutting-edge Raman technologies and instrumentation.

However, improvements in instrumentation are required. These include the development of more appropriate light sources for excitation, more stable and accurate detectors, precise and quick data processing systems, and more portable spectrometers.

Combining techniques such as SERS, Resonance Raman Spectroscopy (RRS), micro-Raman spectroscopy, near-infrared Raman spectroscopy, and nonlinear Raman spectroscopy could lead to breakthroughs in the rapid, online, and quantitative detection of environmental pollutants.

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References and Further Reading

  1. Heller, EJ., Sundberg, R., Tannor, D. (1982). Simple aspects of Raman scattering. J. Phys.Chem. doi.org/10.1021/j100207a018
  2. Ong, TTX., Blanch, EW., Jones, OAH. Surface Enhanced Raman Spectroscopy in environmental analysis, monitoring, and assessment. Science of The Total Environment. doi.org/10.1016/j.scitotenv.2020.137601.
  3. Zacharioudaki, DE., Fitilis, I., Kotti, M. (2022). Review of Fluorescence Spectroscopy in Environmental Quality Applications. Molecules. doi.org/10.3390/molecules27154801
  4. Terry, LR., Sanders, S., Potoff, RH., Kruel, JW., Jain, M., Guo, H. (2022). Applications of surface-enhanced Raman spectroscopy in environmental detection. Anal Sci Adv. doi.org/10.1002/ansa.202200003

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Written by

Ilamaran Sivarajah

Ilamaran Sivarajah is an experimental atomic/molecular/optical physicist by training who works at the interface of quantum technology and business development.

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