Tracking Particles with Raman Holography

Tracking of live-cell SERS individual particles. The tracks of each of the particles are color coded to show the respective z-positions within the volume. (Source: ICFO / URV)

Raman spectro­scopy is widely used in analytical sciences to identify molecules via their structural fingerprint. In the biological context the Raman response provides a valuable label-free specific contrast that allows distin­­guishing different cellular and tissue contents. Unfor­tunately, spontaneous Raman scattering is very weak, over ten orders of magnitude weaker than fluorescence. Unsurprisingly, fluorescence microscopy is often the preferred choice for appli­cations such as live cell imaging. Luckily, Raman can be enhanced drama­tically on metal surfaces or in metallic nanogaps and this surface enhanced Raman scattering (SERS) can even overcome the fluorescence response. Nanometric SERS probes are thus promising candidates for biological sensing appli­cations, preserving the intrinsic molecular speci­ficity. Still, the effectiveness of SERS probes depends criti­cally on the particle size, stability and brightness, and, so far, SERS-probe based imaging is rarely applied.

Now, ICFO researchers Matz Liebel and Nicolas Pazos-Perez have developed holo­graphic Raman microscopy. First, they synthesized plasmonic super­clusters from small nano­particle building blocks, to generate very strong electric fields in a restricted cluster size. These extremely bright SERS nanoprobes require very low illu­mination light exposure in the near-infrared, thus reducing potential photo-damage of live cells to a minimum, and allow wide-field Raman imaging. Second, they took advan­tage of the bright SERS probes to realize 3D holographic imaging, using the scheme for incoherent holo­graphic micro­scopy developed by Liebel and his team. Remarkably, the incoherent Raman scat­tering is made to self-interfere to achieve Raman holo­graphy for the first time.

Liebel and Pazos-Perez demons­trated Fourier transform Raman spectro­scopy of the wide-field Raman images and were able to localize single-SERS-particles in 3D volumes from one single-shot. The researchers  then used these capabilities to identify and track single SERS nano­particles inside living cells in three dimensions. The results represent an important step towards multiplexed single-shot three-dimen­sional concen­tration mapping in many different scenarios, including live cell and tissue interrogation and possibly anti-counter­feiting appli­cations. (Source: ICFO)

Reference: M. Liebel et al.: Surface-enhanced Raman scattering holography, Nat. Nanotech. 15, 1005 (2020); DOI: 10.1038/s41565-020-0771-9

Link: Molecular Nanophotonics, ICFO Institut de Ciencies Fotoniques, Barcelona Institute of Science and Technology, Barcelona, Spain

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