Enhanced nanoscale imaging in liquids

The nanoscale is a huge playground for scientists to explore, and as a young group leader at the Laboratory of Physics of Interfaces and Thin Films (LPICM*), Aleix Güell is one of them. During his PhD in chemistry, between the university of Barcelona, his hometown, and the university of California at Irvine, he synthesized nanomaterials. His research is now focused on developing tools to characterize those materials at such a small scale.

 A widely used tool in nanoscience are scanning probe microscopes. “Unlike conventional microscopes that form a direct image of the sample through an optical system, these instruments operate such as a blind person would: it uses a probe, a tiny and sharp rod of metal for instance, to “touch” the surface of the sample” explains Aleix Güell. The interaction between the probe and the surface, whatever its nature (electrical, mechanical or electromagnetic for example), is related to the distance between them. By scanning all over the sample with the probe one can construct a topographical map at the nanoscale. This map can be complemented with chemical information, such as the structure of the sample or its composition, using jointly another technique called Raman spectroscopy. In this case, light is shone onto the materials and some of the photons are backscattered, carrying the chemical information. These photons are collected and the information extracted.

Raman spectroscopy is based on a low efficiency phenomenon, especially at the nanoscale.  However, adding metallic nanoparticles can act as an antenna and amplify the signal. Physicists call it a plasmonic effect. Putting one nanoparticle at the end of a probe-tip lead to “Tip-enhanced Raman spectroscopy” (TERS), a technique that combine scanning probe microscopy and Raman spectroscopy. This technique has already been developed to study samples in an air or vacuum environment.

Light coming from a laser is scatterd when focused on the probe of a TERS system

“But the new direction to go is liquid” explains Aleix Güell. Since 2017, this young researcher leads an ERC-funded project “AQUARAMAN” appropriately named after the superhero Aquaman, whose poster hangs on the wall of Aleix Güell's laboratory. “The potential of this technique to study systems where liquids are present is huge. It can be living cells in a physiological liquid, or battery materials in an electrolyte”. His team is developing TERS systems to study such environments and manufacturing probes that include one single lab-synthezised gold nanoparticles on top. “Having the lab located in a scientific hub of excellence such as Institut Polytechnique de Paris is key to progress in the project and expand its applications”, he said whilst explaining his collaborations with the Laboratory LuMIn at Université Paris-Saclay and Optics and Biosciences Laboratory (LOB*) at Ecole Polytechnique on biological studies. “The E4C interdisciplinary center is also an opportunity to impact in the field of materials for batteries”.  

For years applying TERS in liquid has been difficult, with results highly sensitive to experimental conditions and are therefore rather qualitative than quantitative. His current project is to design new probes with an enhanced reliability that would allow quantitative comparison between experiments performed under different conditions. ERC has just funded Aleix Güell with a new “Proof of Concept” grant in order to further investigate the probes’ practical viability and their possible commercial perspectives. “Hopefully, I’ll be able to hire one more person in my team and expand the network of labs that could use these probes”, says Aleix. “It also could lead to the creation of a start-up company.”

*LPICM: a joint research unit CNRS, École Polytechnique - Institut Polytechnique de Paris

LOB: a joint research unit CNRS, École Polytechnique - Institut Polytechnique de Paris, Inserm