Raman spectroscopy is an indispensable tool in chemical, biological and biomedical analysis. It provides a high density of information regarding the chemical composition and molecular structure of the samples tested. Until just a few years ago, prohibitively high cost of Raman spectrometers meant that these instruments were found only in the research laboratories of large companies and research facilities.  

Thanks to rapid developments in the field of diode lasers and CCD camera technology in recent years, subassemblies are now available that enable a more economically priced construction of Raman spectrometers. As a result, Raman spectrometers are finding their way into market segments where their use was simply not feasible until today. In particular in the field of process control in the chemical, pharmaceutical and food industries, as well as in small analytical and research labs, completely new areas of applications for this analytical tool are opening up.

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Tailored by LLG

The Göttingen Laser Laboratorium (LLG) develops customized Raman spectrometers and Raman microscopes for applications such a those mentioned above. To be able to respond as flexibly as possible to customer's requests, special emphasis is placed on the modular design of the system. Ideally this can be implemented through the use of optical assemblies made by LINOS, from the Microbench system to the compact NANO 250-785-100-RAMAN-1N Raman laser module.

Modular Design

Figure 1 shows a fluorescence microscope with an integrated Raman spectrometer, made at LLG in their Photonic Sensor Technology department. The housing has been removed to allow a detailed view of the components. This modular system offers the considerable advantage that a single instrument can be used for both fluorescence microscopy and Raman spectroscopy. Figure 2 shows the prototype of a miniaturized system, specifically designed for process control applications, which can also be used as a lightweight, portable system in the chemical industry, for example in incoming inspection and material identification. Their modular structure makes these instruments easy to adapt for special customer requirements, while at the same time achieving an acceptable compromise between spectral resolution, sensitivity and overall cost.

Principles of Raman spectroscopy

The interaction of electromagnetic radiation with matter produces various types of scattering. A distinction is made between elastic scattering, where no energy is transferred between electromagnetic field and matter, and inelastic scattering, where energy is transferred from the radiation field to the matter or vice versa. This interchange can only take place between defined energy states of the molecules involved, as predicted by quantum mechanics, so that the inelastically scattered light delivers a wealth of information about the type and inner structure of the material in question. The spectrum of Raman scattering provides a kind of fingerprint that serves both for identification (qualitative analysis) and for determining the concentration of substances (quantitative analysis). Spontaneous Raman scattering has a low scattering cross section and thus is typically very weak, which can cause problems, in particular in the detection of low concentrations in samples. Still, there is strong interest in analyzing complex chemical compounds, especially in environmental diagnostics, as well as in chemical, biological and biomedical analysis, in some cases with very low concentrations. One solution is provided by a physical effect discovered back in 1974: surface enhanced Raman scattering (SERS). This is a near-surface process in direct proximity to nanostructured precious metal surfaces, and it amplifies the Raman scattering by several orders of magnitude. The irradiation of the electromagnetic field excites the surface plasmons in the metallic nanostructure, which – with some local limitation – can intensify the incident light and the Raman scattering by several orders of magnitude. Depending on the surface structure, amplification factors of 105 to 108 can be attained. 

Preparing SERS-active substrates

For commercial use of nanostructured SERS-active substrates, a high amplification factor, with as constant a value as possible over the entire substrate, is prerequisite. Of the SERS surfaces published to date, only very few meet this requirement. Researchers at LLG have now succeeded in making SERS substrates that provide higher amplification factors than those available commercially. These substrates were presented to the public for the first time at the "Laser World of Photonics 2009" trade show, held 15 to 18 June at the fairgrounds in Munich.  The atomic force microscope (AFM) image in Figure 3 shows the nanostructure made by LLG.

Application examples

There are currently several research projects in progress in the Photonic Sensor Technology department at the LLG, developing Raman spectroscopy techniques and applying them to address specific scientific questions. One project involves the detection of airborne explosives, in particular TNT and TATP. In this context, cryogenic enrichment methods are combined with surface-enhanced Raman spectroscopy to achieve the high sensitivity required by this detection system. This procedure is also useful for the detection of chemical warfare agents, combustion markers and other potentially dangerous materials. Other projects examine complex aqueous solutions, for example to identify and quantify trace elements in a chemically complex fluid matrix (e.g. waste water or, in the field of biomedicine, any of a number of bodily fluids, such as blood, urine or saliva). The innovative feature here is the coupling of electrophoretic separation and enrichment processes with the highly sensitive SERS detection technique. Figure 4 shows the effectiveness of surface-enhanced Raman spectroscopy in testing insulin and insulin solutions.

Summary 

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In the research projects currently in progress, scientific expertise at LLG is bundled for application-specific production of SERS substrates, separation and enrichment processes (cryogenic and electrophoretic) and the construction of high-sensitivity Raman spectrometers.

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Please find more information here. 

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The authors

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Dr. V. Beushausen, Dr. H. Wackerbarth, K. Christou, A. Göhmann, W. Hüttner - Laser-Laboratorium Göttingen