Commercially available as instrumentation designed for macro-size sampling, Raman spectroscopy drew interest for providing information similar but complementary to infrared (FTIR) spectroscopy for chemical identification. In addition to chemical fingerprinting, the technique could provide molecular backbone information, materials morphology, sensitivity to symmetric bonds and the ability to analyze inorganic samples and components. Raman is a technique that is insensitive to water, can analyze samples through glass and, because it uses visible wavelengths, it can be used as a microscopy technique for performing full spectroscopic analysis on micron and sub-micron size samples.
Since its inception, the applications of Raman spectroscopy and the types of instrumentation available have evolved rapidly. Now seen primarily in three main formats—microscopes, spectrometers and portable systems for field use—these instruments are commonly used in academic research, industry, law enforcement and security. New applications continue to increase, and they include analysis for pharmaceuticals, materials science, mineralogy and explosives identification. Not only is this instrumentation sought for laboratory purposes, it has found its way to the streets and military fields as a quality tool in manufacturing processes.
The growing number of applications is being fed by the technical progress of Raman products designed to fit specific purposes and user types. The fundamental design common across the wide range of commercial products is based on a few key optical components: detectors, lasers and filters.
“Many companies specialize in these components, and they are evolving as well. They continuously enable new experimental techniques and uses for Raman” says Scot Ellis, Marketing Manager for Raman Spectroscopy, Thermo Fisher Scientific, Madison, Wisc. “These advances in sampling and applications often start with researchers in universities rather than instrument companies, where scientists build their own system and try different components to generate new techniques.” But these advances were typically the domain of Raman experts until instrument companies created designs that brought the technique to users focused on their area of research. It is these researchers ultimately who assess whether those techniques are suitable for their given research application, and then the users share their techniques with instrument companies to evolve into more feasible products that other people can access. This iterative process has brought us the advances we see in Raman today.
Some instruments are designed for technical experts as toolboxes to advance the technique. Others are focusing their efforts on packing high-end Raman performance into systems that are more user-friendly and do not require extensive Raman expertise. The Thermo Scientific DXR Raman microscope is one such system in which high-end Raman performance was combined with technologies used in FTIR to make the technique reliable and accessible by experts and non-experts alike, allowing more users to benefit from the technique. And in basic and applied research, the concept of hyphenating techniques has increased. Combining Raman with atomic force microscopy (AFM) not only integrates the two techniques, but provides a different capability with AFM, allowing Raman to achieve spatial resolution hundreds of times higher than it previously could as only an optical microscope, says Ellis.
Raman unlocks carbon nanotubes potential
In late September, Stanford Univ. researchers unveiled the first working computer built entirely from carbon nanotube transistors. The computer contains 178 transistors formed from several tens of thousands of single-walled carbon nanotubes. The researchers assembled 985 of the computers on a single chip wafer, using standard chip-fabrication techniques and design tools. And, while still primitive, the invention proved that transistors made from single-walled carbon nanotubes can be assembled into a general-purpose computer that can run a basic operating system, perform calculations and switch between different processes running at the same time.
Carbon nanotubes are obtained typically through an arc discharge process. Freshly produced carbon nanotubes will be a mixture of residual catalysts, amorphous carbon, single-walled and multi-walled carbon nanotubes. “In the Stanford project, the team used single-walled carbon nanotubes,” says Mark Wall, applications specialist in Raman products at Thermo Fisher Scientific. “So they must isolate those single-walled carbon nanotubes from multi-walled carbon nanotubes, while also removing residual catalysts and any amorphous carbon.” Raman spectroscopy can be used to ascertain whether the purification steps have isolated the single-walled carbon nanotubes from the freshly prepared carbon nanotubes mixture.
“Additionally, from the process of synthesizing these materials, there are some nuances to the single-wall carbon nanotubes that are actually obtained,” says Wall. There are two fractions: one fraction that is metallic in character, and the other fraction that has semiconducting properties, which typically represent two-thirds of the single-walled carbon nanotubes. These fractions can be identified using Raman spectroscopy or microscopy. And in the case of using these materials for computing devices, it is of utmost importance to isolate the semiconducting from the metallic carbon nanotubes, since the presence of any metallic carbon nanotubes could cause short-circuiting. For use in electronics and computing devices, carbon nanotubes need to exhibit a 99.999% semiconducting purity, says Wall.
To this end, highly dispersed solutions of purified carbon nanotubes can be casted as thin films which allow researchers the ability to test a whole collection of carbon nanotubes. Raman microscopes equipped with automated stages can be used to make high-spatial resolution Raman chemical images of these nanotubes dispersed on a surface. The chemical image can then be analyzed to assess whether or not there is any contamination or the presence of the unwanted nanotubes in the semiconducting fraction.
At Thermo Fisher Scientific, the DXR Raman microscope achieves this goal. Built as a user-friendly, high-performance instrument for applied areas, the DXR Raman microscope allows researchers involved in carbon nanomaterials and graphene applications the ability to obtain reliable answers and conduct quality research without having to be experts at Raman techniques. “The system is accessible, yielding useable information, has the right configurability and has the right selection of laser wavelengths for these studies,” says Ellis.
The search for illicit drugs, the perfect pharmaceutical solution
Not only does Raman have applications in materials science, it also is used often in forensics investigations where interest in this technique is driven by the portability and field benefits of new technology.
Ahura Scientific, acquired by Thermo Fisher Scientific in 2009, was the first company to innovate portable and handheld Raman instruments as a true solution for users in the law enforcement and security arena. “Before Ahura’s innovation, Raman wasn’t well suited for use in the field,” says Ellis. The products are extremely good at positively identifying so many of the illicit drugs encountered on the street. In the hands of law enforcement and protection personnel, these products allow investigators to benefit from Raman technology without having any expertise in spectroscopy. It has now become a widely used and accepted tool in the field.
“In the laboratory, Raman microscopy is used for forensic applications like fibers and explosives, where you might be dealing with a trace amount of material and the spatial resolution in a Raman microscope can provide a positive chemical identification on very small amounts of materials,” says Ellis. “There may be inorganic components in the sample where Raman is a capable of analyzing these elements that other forms of micro-spectroscopy cannot. Infrared spectroscopy lacks this ability.”
For field use, the Thermo Scientific TruNarc Raman analyzer, a 2013 R&D 100 Award winner, is suitable for forensics applications for identifying illicit drugs, and finds itself an important part of the handheld Raman evolution. This product provides a unique advancement in the field identification of heroin. One of the main issues with heroin identification using Raman spectroscopy in the past is that it exhibits fluorescence, an overriding signal that tends to eclipse the standard Raman spectral signature. “Heroin is one of those materials that is particularly tricky with fluorescence,” says Ellis. “It makes it difficult to analyze with Raman and obtain a positive identification.” Thermo Fisher Scientific was able to develop a sampling technology to overcome fluorescence and make rapid positive identification with handheld Raman possible.
The TruNarc analyzer, by employing surface-enhanced Raman spectroscopy and sampling sticks, boosts the Raman signal for heroin so high that the signal comes up over the fluorescence signal interference, allowing definitive identification of the substance. Before the TruNarc analyzer, many street drugs would require quarantine and analysis back at a laboratory or would require law enforcement to use colorimetric kits that weren’t accurate and were more time consuming to use. However, with the advent of technologies such as the TruNarc analyzer, a better and faster analysis can be achieved, which leads to fast arrests for law enforcement teams.
For years in the laboratory, Raman has been used for drug analysis, because the materials tend to have strong chemical fingerprints that Raman spectroscopy can pin-point rather accurately. Raman spectroscopy in pharmaceutical applications is best known for its ability to distinguish polymorphs and to verify incoming raw materials, and do so non-destructively without any sample preparation. In the field, a variant of handheld Raman, called standoff Raman, is typically used for explosive detection, as this technology can analyze materials around 20 feet away and provide an accurate chemical identification.
The future of Raman
What does the future hold for Raman techniques? Raman is far from mature, and as camera, laser, and other photonic product companies continue to develop new component technology for Raman, new applications will be developed with instruments increasingly tailored to them. There continues to be interest from researchers developing Raman techniques to explore new ways of measuring and assessing different sample types from organic LEDs to biological sample.
“I think the technology will continue to develop rapidly in the hands of capable spectroscopy researchers,” says Ellis. “But we will see more and more new technologies that are eventually designed back into more approachable instruments that more people can benefit from.” More instruments will evolve to address specific applications in a more turnkey approach to enhance spectral libraries and spectral interpretation with intuitive software built into these systems.