The remarkable world of Raman imaging

At first glance, cement, cancer cells, interstellar dust, two-dimensional materials, billion-year-old microfossils, emulsions and the Kramers-Heisenberg-Dirac formula appear to have little if anything in common. Yet they were all among the topics recently discussed by some 50 biologists, physicists, pharmaceutical researchers and chemists within the context of Raman imaging.

For the 12th time, WITec, the German manufacturer of Raman imaging systems, invited scientists from all over the world to an interdisciplinary symposium in Ulm (Germany) at the end of September. The constituent sessions were grouped conventionally in areas of interest such as materials science, life science, 2D materials and geosciences, with Raman imaging as the common investigative technique. An example was the analysis of everyday materials whose features and production processes are still not fully understood at the molecular level.

While two billion tonnes of cement are used every year worldwide, the associated chemical reactions and their kinetics during the production of clinker from limestone and silicate materials remain partially obscure. Using Raman imaging, Mika Lindén from the University of Ulm identified and located various phases during the hydration of clinker, one of the steps in the production of cement.

Production processes in glass fabrication also continue to be investigated, as Ralf Seuwen from Schott in Mainz (Germany) explained. He uses Raman spectroscopy to find the source of bubbles in glasses. As the composition of a bubble’s gas indicates its origin, Raman spectroscopy applied in the production line can reduce waste and improve the manufacturing process.

The food production industry also takes advantage of Raman imaging. Maria Sovago from Unilever in Vlaardingen (the Netherlands) analysed the molecular composition of emulsions and showed how monoglycerides at the interface between oil drops and the aqueous phase, as well as crystalline lipids, stabilise emulsions.

Martin Hilchenbach and Frédéric Foucher are both undertaking exploratory work that will ultimately lead to Raman analyses being conducted on materials in outer space. The ESA Rosetta spacecraft carries a secondary ion mass spectrometer (SIMS) to investigate dust from the comet 67P/Churyumov-Gerasimenko. On Earth, Hilchenbach, of the Max Planck Institute for Solar System Research in Göttingen (Germany), evaluates reference materials from meteorites with Raman imaging and SIMS. Raman has been shown capable of detecting contaminations with great sensitivity. Using these results, he intends to calibrate the SIMS on the spacecraft.

Foucher, from the Center for Molecular Biophysics in Orléans (France), searches for traces of microbial remains in Martian rocks, one of the aims of future Mars missions. In preparation, he analysed fossilised, 800 Ma old microbes from the Draken Formation in Svalbard. Though Raman spectroscopy is very sensitive to both organic and mineral compounds, it is difficult to distinguish them in a fossilised state. Foucher, however, succeeded in identifying specific signatures associated with biotic microfossils by performing Raman mapping instead of single spot analysis. Exactly how a space-qualified Raman imaging system can be miniaturised for a journey to Mars remains an outstanding challenge.

Back in terrestrial exploration, surprising data with the potential for technological applications were presented by José Fernández from the Institute for Ceramics and Glass in Madrid (Spain). He observed by confocal Raman microscopy that changing the polarisation of the Raman laser can move ferroelectric domain walls of BaTiO₃ single crystals. With the AFM mounted on the Raman microscope, he correlated the local changes with the topography there. As BaTiO₃ is a perovskite, applications of this effect in random access memories or piezoelectric actuators are conceivable.

While Raman imaging has long been used in materials sciences and geosciences, it only recently gained a foothold in life sciences. The majority of the posters this year dealt with biological, medical and pharmaceutical research, ranging from the detection of nanoplastics in the Baltic Sea to the imaging of living cells. Two of the contributed talks also emerged from life science: Carmen Lawatschek from Humboldt University in Berlin (Germany) showed how Raman imaging can accelerate the screening of peptide sequences for drug binding. Samir El-Mashtoly from the University of Bochum (Germany) described how this technology can be used to study cellular responses of specific tyrosine kinase inhibitors that bind to extracellular receptors known to play important roles in cancer development. His results indicate that non-invasive Raman microscopy could be a useful tool in studying the action and pharmacokinetics of drugs.

These contributions were preceded by an overview from Halina Abramczyk of the Laboratory of Laser Molecular Spectroscopy in Lodz (Poland) on Raman investigations of breast cancer. In addition to structural studies, she identified carotenoids, mammaglobin and specific fatty acids as Raman diagnostic markers for breast cancer prognosis. Dominique Lunter from the University of Tübingen (Germany) presented a confocal Raman microscopy-based methodology for investigating the drug contents and their distribution in an ex vivo pig skin assay.

Interdisciplinary conferences such as our Raman symposium thrive on research being communicated across fields, accessible to those whose expertise lies elsewhere. There were, however, no constraints on physicists characteristically challenging their audience. Sebastian Schlücker from the University of Duisburg-Essen (Germany) not only reviewed the history of Raman spectroscopy up through its latest imaging variants, he also presented the fundamentals of resonance Raman scattering, molecular vibrations and symmetry using the example of the water molecule. Glen Birdwell from the US Army Research Laboratory in Adelphi (USA) then cast light on the subtle interactions between layers of bifold graphene, whose stacking configurations exhibit different properties. He presented data that allows for a comparison of the positions of the superlattice-related Raman modes with existing theory.

Finally, the poster award was given to the physicist Kishan Thodkar from the University of Basel (Switzerland). He analysed the shifts in the position of graphene’s 2D peaks as a marker of how temperature influences the formation of nano-gaps in CVD graphene. Using large Raman imaging scans of graphene’s D, G and 2D peaks, he also documented the effects of solvent cleaning on graphene field-effect transistors.

This article offers only a glimpse of the topics presented at the 12th Symposium on Confocal Raman Imaging, which provided a format for the exchange of ideas and developments in Raman imaging and their applications in science and industry. The 13th Confocal Raman Imaging Symposium will be held again in Ulm from 26–28 September 2016.