Analysis of the State of the Art: Raman Spectroscopy

In honor of Spectroscopy's celebration of 30 years covering the latest developments in materials analysis, we asked a panel of experts to assess the current state of the art of Raman spectroscopy and to try to predict how the technology will develop in the future.

Sir C.V. Raman discovered Raman spectroscopy in the late 1920s, which is where the nomenclature comes from (1). In the eight decades since that discovery, Raman spectroscopy has had plenty of time to mature, yet users continue to find new application areas for the technique. In celebration of this 30th year of Spectroscopy, we asked a panel of Raman experts about the current state of Raman spectroscopy-including the various advanced subsets of Raman such as stimulated Raman scattering (SRS), coherent anti-Stokes Raman scattering (CARS), and tip enhanced Raman scattering (TERS)-including recent advances, ongoing challenges, and possible future developments. This article is part of a special group of six articles covering the state of the art of key techniques, also including infrared (IR) spectroscopy, near-infrared (NIR) spectroscopy, inductively coupled plasma–mass spectrometry (ICP-MS), laser-induced breakdown spectroscopy (LIBS), and X-ray fluorescence spectroscopy.

Recent Advances

We started by asking our experts what they considered the most important development in Raman spectroscopy or its instrumentation in the last 5–10 years. The answers covered a variety of advances and specialized application areas, making it clear that Raman continues to break through its limitations and find new ways to support the fields of research and industries where it is used.

Dennis J. Walls, a senior research associate at DuPont, is excited about the progress in SRS and CARS that has made Raman imaging more practical in terms of speed and applicability. “Developments in coherent Raman scattering microscopy approaches such as SRS and CARS microscopy offer exciting potential in biological imaging applications,” Walls said. He also mentioned the importance of TERS. “I also am excited about the potential for sub-diffraction limited microscopic approaches like TERS, which are now crossing boundaries of spatial resolution that have previously not been accessible with normal Raman microscopy.”

Zachary D. Schultz, an associate professor of chemistry and biochemistry at the University of Notre Dame, agrees that TERS is an important development. “The understanding of the relationship between plasmon resonances and Raman signal enhancements has rapidly advanced in the last decade,” Schultz said. “This is evident in the increased interest in surface-enhanced Raman scattering (SERS) for applications and the emergence of TERS for sub-diffraction imaging.” Schultz explained that recent work has taken advantage of this understanding, demonstrating the reproducibility and quantitative capabilities of SERS and TERS. “It is really becoming possible to talk about using Raman to investigate individual molecules,” Schultz concluded.

Heinz W. Siesler, a professor at the University of Duisburg-Essen, said that handheld Raman was the most important recent advance. Gary Johnson, a research Chemist at Intertek, agreed. “I am very interested in the development of compact, portable instruments that have the capability to obtain spectra equivalent to the performance of benchtop systems,” Johnson said. This is valuable, he said, because there are a lot of potential applications of Raman spectroscopy for which it is impractical to bring samples into a laboratory.

John A. Reffner, a professor at John Jay College of Criminal Justice in New York, said that solid-state lasers and the development and application of optical filters for Rayleigh scattering rejection have had the greatest impact on Raman instrumentation.

From a different perspective, Juergen Popp, a professor of physical chemistry at Friedrich-Schiller University Jena and the Leibniz Institute of Photonic Technology Jena, said that the most important recent advances have been developments in instrumentation that have pushed Raman spectroscopy further into life sciences and biomedicine. “This development was certainly triggered by the technical progress made in spectrometer, detector (charge-coupled device [CCD]), and filter (notch filter) technology,” said Popp. “These technological advancements made Raman spectroscopy accessible for a broad circle of users because of the availability of commercially available and fairly easy-to-use Raman setups, in particular, confocal Raman microscopes.” The availability of confocal Raman microscopes allowed for label-free molecular characterization of biological cells and tissue with a spatial resolution in the submicrometer range, he said. “This paved the way for medical Raman spectroscopy, that is, Raman spectroscopic clinical diagnosis,” he said.

These advances wouldn’t have been possible, he noted, without the simultaneous achievements in statistical Raman data analysis-that is, the development of innovative chemometric strategies to analyze Raman spectra of biological specimens. “In other words, I consider the development of innovative chemometric approaches to qualitatively and quantitatively analyze Raman images with respect to utilizing the Raman signatures for diagnostic purposes as one of the most important aspects in biomedical Raman spectroscopy,” he said.

Fluorescence Background Response

Despite these important advances, however, fluorescence background response still limits the use of Raman spectroscopy for many applications of interest. In light of this, we asked our panel the following questions: How much of a problem is this in your work? What tools do you have to eliminate it? Are there any additional developments anticipated that could lead to significant improvements in dealing with this problem in the near future?

Common Problem

All of our panelists agreed that fluorescence is a common problem in their work. Walls said that fluorescence is one of the “major factors that limits applications for Raman spectroscopy” to the varied analyses that come through his laboratory.

Reffner agreed. “Fluorescence creates an uncertainty that limits the reliability of using Raman for analysis of some samples,” he said.

Johnson said he runs into this problem frequently, particularly with industrial samples and materials that contain natural products. Schultz agreed that background signals are still a problem. “In SERS and TERS, the metals often provide some fluorescence quenching, but molecules distant from the nanostructures can still cause problems,” said Schultz.

Tools to Eliminate Fluorescence

Most of our panelists said they use excitation wavelength selection to mitigate the problem of fluorescence.

Popp’s main focus lies in the application of Raman spectroscopic approaches for biomedical diagnosis, so he primarily uses excitation wavelengths in the NIR region (mostly 785 nm) to avoid the excitation of fluorescence. Siesler agreed that NIR excitation is the best approach. “The recently patented sequentially shifted excitation (SSE) technique (2) might possibly help to suppress fluorescence, but I have not had the opportunity to test it yet,” he said.

Walls said that he also uses NIR excitation at 785 nm to avoid fluorescence, but even then, many classes of samples remain outside of the scope of application for Raman spectroscopy because of fluorescence background issues. “Alternatively, excitation at 1064 nm can address some of these samples,” he said.

Another approach that Popp’s laboratory is pursuing is the use of excitation wavelengths in the deep ultraviolet (UV) at 244 nm. “The application of deep UV wavelengths also avoids fluorescence since most fluorophores do not show fluorescence below 250 nm,” he said. He noted that this deep UV approach is especially powerful for mineralogical studies. “Furthermore, I think shifted excitation Raman difference spectroscopy is a powerful approach to eliminate fluorescence.”

Johnson mentioned photobleaching as an alternative that is sometimes effective, but otherwise he agreed that 785-nm or near-infrared 1064-nm lasers were the best tools to eliminate the fluorescence problem.

Finally, Reffner said that SERS is showing “real progress in reducing the fluorescence problem while amplifying the Raman scattering.”

Potential Future Solutions

A few of our panelists suggested some future developments that might help to solve the fluorescence interference problem. Schultz explained one possible approach: rapidly switching the laser frequency over a small wavelength range. This technique would change the Raman signal but not the fluorescence. “Incorporating this or similar ideas into commercial instruments may resolve the fluorescence background problem at some future time,” he said. He added that the advent of NIR cameras and array detectors makes Raman possible with excitation wavelengths that do not cause fluorescence. “These new technologies are helping to resolve this problem,” Schultz concluded.

Walls said that continued advances in the incorporation of the 1064-nm excitation source in dispersive instruments would allow practical data collection (from the standpoints of time and laser power) and would be an important advance.

Popp said that a technical improvement to avoid fluorescence interference could be the development of fast detectors to separate fluorescence and Raman scattering in time. “One example of this is superconducting nanowire single-photon detectors” he said.

Biomedical Applications

Recent reports of the successful use of Raman spectroscopy for important biomedical applications are quite exciting. These applications include imaging for disease diagnosis, including significant improvements for endoscopic probes, and identification of microorganisms. We asked our panelists for their opinions about the practicality of implementing these technologies in the clinical environment.

Siesler said that the implementation of these technologies as routine tools would likely be very slow. “The promises in various ads are too optimistic,” he said.

Popp had a more positive outlook on the work being done in this area, which is the focus of his research group. He has observed an enormous increase in the development and application of Raman-based approaches to address biomedical questions. “The ability to obtain specific chemical information label-free makes Raman spectroscopy attractive for many applications in clinical diagnostics of bodily fluids, pathogens, cells, and tissue biopsies,” he said. “I am absolutely convinced that Raman spectroscopy might be the solution for many clinical challenges that have unmet medical needs, for example, in the early detection of cancer.”

As another example, Popp mentioned the significant progress made toward the application of Raman spectroscopy as a point-of-care test for the fast identification of pathogens (such as sepsis-causing pathogens) and the determination of their antibiotic resistances. “Overall, I am sure that a few years from now we will see the first Raman approaches as standard diagnostic or therapeutic tools in daily clinical practice,” he said.

Schultz agrees that nonlinear Raman techniques have a lot of potential in the biomedical imaging community. “The label-free, video-rate images that are obtainable may have a tremendous impact on identifying cancerous tissue or other abnormalities,” he said.

“In respect to identifying microorganisms, rapid assays based on SERS are really starting to make an impact,” Schultz said. He explained that applications identifying tuberculosis, flu viruses, and other pathogens have all been demonstrated already. “Handheld Raman instrumentation may make these assays highly portable and inexpensive,” he said. Schultz also said that work with high-throughput detection with moving fluids, such as chromatography, flow injection analysis, and microfluidic approaches, is another important advance for clinical environments. “I think Raman has an important future in the biomedical community,” he concluded.

Johnson added a caution, however, noting that the primary challenge in applying Raman to clinical use lies in data analysis and interpretation. “Considerable care is needed to avoid misdiagnosis, especially when using chemometric methods that don’t rely on well-defined spectral features,” he said.

TERS

Many problems at the frontiers of materials science require spatial resolution that is outside of the diffraction-limited regime. Sub-diffraction-limited approaches like TERS seem to offer the potential to bridge this gap, but currently the approach does not appear to be practical because of the difficulty in preparing tips. So we asked our panelists if they thought TERS-like approaches to Raman imaging would someday become more practical.

“This remains an important opportunity for Raman spectroscopy that could be put to work today with practical instrumentation offerings and a ready source of appropriate tips,” said Walls. “In addition to biological systems, the increasing importance of structural organization at the nanoscale in materials science calls for characterization approaches that can operate in this size regime.”

Schultz said he was not sure if the difficulty is really preparing tips. “I think the biggest obstacle to TERS is false expectations,” he said. “Many TERS tips perform exactly as expected; however, the observed signal enhancement from a single nanostructure on a single or small number of molecules, or in complex samples, is not always that large.”

Schultz added that the advances in commercial TERS instruments is helping practicality, but also increasing frustration when people cannot get the expected result. “I think TERS has significant potential and will be an important technique, but it will also have its limitations,” Schultz said. “As the technique matures, people will better appreciate what it can and cannot do.”

Popp mentioned several examples that show how TERS-like approaches are becoming more practical, such as in the characterization of single viruses and in protein analysis. “TERS has made a lot of progress within the last few years, especially with respect to practical life science or biomedical applications,” he said. “I am convinced that the difficulties in preparing reproducible TERS tips will be overcome soon.”

SERS

SERS approaches offer the potential to increase the sensitivity of Raman spectroscopy in a wide range of systems. We asked our panel if SERS-based solutions have become easier for nonexperts to apply, and whether SERS is likely to become more universally applicable in the near future.

All agreed that the lack of reproducible substrates-and having them commercially available-is the biggest obstacle to more widespread use of SERS.

“One commercial supplier recently discontinued a SERS substrate product,” lamented Schultz. “A few other products have emerged as possibilities, but the development of a robust high-enhancement substrate would facilitate use by nonexperts,” he said.

Siesler agreed. “Once commercial substrates are available, SERS will become more universally applicable.”

Several panelists noted that there are many efforts underway to try to address this challenge. “Recent advances in producing colloids with reproducible size and morphology are helping to address this problem,” said Reffner.

Popp agreed. “One of the most promising approaches in my opinion is the usage of colloidal silver or gold nanoparticles, which are easy to prepare together with microfluidic chips,” he said.

Despite the reproducibility problem, Popp thinks that lab-on-a-chip SERS concepts are very promising, particularly for drug and metabolite monitoring. “The routine analytical application of SERS comes down to the availability of reproducible SERS substrates,” he said. “Once such SERS substrates are available, SERS-based solutions will be easy to apply-even for nonexperts-making them universally applicable.”

Johnson is less optimistic. “I see SERS as a method that works well for very specific types of samples, but not so much as a general analytical technique,” he said.

New Areas for Raman: Applications and Challenges

Most people consider Raman spectroscopy a mature technique, but it is still being applied to new areas-in addition to fields like clinical diagnoses and medical imaging discussed above. So we asked our panel what new applications are emerging for Raman and what challenges are involved with those applications.

New Application Areas

New application areas for Raman spectroscopy range from art conservation, forensics, and homeland security, to nutrition, health, and agriculture.

Reffner said that a growing segment of Raman applications resides in field-portable Raman spectrometers, which are currently being used in homeland security and forensic applications.

Schultz also mentioned the increased use of Raman in the forensics field. “I think Raman will continue to see increased use in applications where the samples are sensitive to chemical modification,” he said. “The art community has seen increasing use of Raman for restoration, conservation, and forgery detection.”

Schultz also explained that advances in Raman instrumentation are making it a highly affordable method. “Mass spectrometry (MS) may still provide better absolute molecular identification, but Raman is holding its own and seems to provide complementary information where MS has trouble,” he said.

Walls would like to see the role of Raman spectroscopy expand for applications in high growth areas like industrial biosciences, nutrition, and health. “One example is our interest in expanding the role of Raman microscopy in the analysis of agricultural plant systems to support the development of crop protection chemicals,” he said. “A key attribute needed will be the sensitivity to detect low levels of the analytes of interest in complex (and potentially fluorescent) backgrounds. Ideally, these measurements would be performed without disturbing the viability of the plant.”

Popp believes that Raman is one of the most important spectroscopic methods. “In my view, it is not exaggerated to say that Raman spectroscopy has become an indispensable analytical method for an extremely broad area of applications,” he said.

Application Challenges and How to Overcome Them

Even though Popp considers Raman a rather mature technique, he still believes it faces challenges. “The great challenge faced in using linear Raman spectroscopy for almost all applications is its intrinsically low scattering cross section-that is, low sensitivity very often leading to rather long acquisition times,” he said.

Techniques like SERS, resonance Raman, and nonlinear Raman effects like CARS or SRS try to address this problem by enhancing the Raman signal. “These nonlinear Raman approaches have seen a renaissance mainly because of the recent progress made in the development of high intensive ultrashort laser sources,” said Popp. “The implementation of these nonlinear Raman phenomena in laser scanning microscopes has resulted in powerful biomedical imaging approaches that show great potential to complement established clinical pathological diagnostic tools.”

Schutlz believes that biggest challenge for Raman lies in creating large databases of reference spectra that can be searched to identify molecules, as can be done today with gas chromatography (GC)–MS. “The challenge here is still the variances between spectra with different SERS strategies, and there is currently some debate about TERS signals,” he said. “If this can be resolved, I think Raman could compete with GC–MS characterization.”

The challenges that Walls foresees in areas like industrial biosciences, nutrition, and health applications include fluorescence, sensitivity, and the ability to differentiate a meaningful analyte signal from a complex background response.

Automation or Spectral Processing

Next we asked our experts what types of new automation or spectral processing could help in their work. The majority of answers addressed software improvements.

Johnson would like to see the integration of image analysis routines into Raman instrument software that is used for mapping applications. “Particle count and size distribution as well as more sophisticated three-dimensional (3D) imaging applications would be beneficial too,” he said.

Walls touched upon the need for libraries, similar to what Schultz cited as a main challenge earlier. “For those of us who use Raman spectroscopy for the identification of unknown species, the availability of larger, more-comprehensive electronic libraries would significantly impact the effectiveness of these efforts,” he said. Most vendors are starting to incorporate chemometrics data reduction approaches within their software, he said, and the continued expansion of these efforts will be helpful in his work.

Looking to the Future

Finally, we asked our experts what future developments in Raman spectroscopy or its instrumentation can be expected in the coming years.

According to Schultz, SRS is an area that should be watched closely in the next few years, because the sensitivity associated with this nonlinear technique is impressive. There are still instrumentation challenges with this technique, however. “In SRS, you have to measure the difference in the laser intensity at the Raman frequency of interest before and after the sample, which typically requires expensive lock-in amplifiers and stable laser sources,” he said. “This also often limits the spectral range and resolution.” A potential solution, he suggested, would be a lock-in array detector. “If we had lock-in arrays with 1024 channels like modern array detectors used for Raman, SRS may provide some additional advances,” he concluded.

Popp thinks that the greatest potential for Raman lies in medical diagnostics. A key next step, he explained, is transferring Raman spectroscopy from ex vivo to in vivo applications by implementing Raman spectroscopy into endoscopic setups. “In this context, within the next few years we will see the development of various other novel Raman fiber probes for in vivo tissue screening of internal organs like the colon, stomach, or aorta,” he said. “These developments will utilize advances in fiber technology (for example, fiber Bragg gratings, waveguide optics, and imaging fiber arrays) and easy-to-operate fiber laser approaches.”

Reffner said that the advantage of Raman spectroscopy to record low energy rotational and vibration spectra is a promising area to develop.

The future development of capable, less expensive, and even portable instrumentation piques Walls’s interest. With continued refinement and technology advances, he said, these developments will foster growth into new applications across a wide range of application areas. “These developments, along with more powerful ‘expert user’ driven software interfaces, will catalyze the growth of Raman technology into the hands of nonspecialists in spectroscopy in an increasingly diverse range of technical applications,” he predicted.

Conclusion

Raman spectroscopy is an invaluable technique for a wide array of application areas. It’s safe to say that this technique has a strong foothold on the future, no matter what new application areas come its way.

Acknowledgments

I would like to extend a special thank you to Fran Adar of Horiba and Dennis J. Walls of Dupont for their assistance with the questions and panelist recommendations.

References

(1) http://en.wikipedia.org/wiki/Raman_spectroscopy

(2) J. Cooper, M.F. Abdelkader, and K. Wise, US patent 8,570,507 B1, 29 October 2013.

Megan L’Heureux is the managing editor of Spectroscopy magazine.