Tip Enhanced Raman Spectroscopy: Transforming Analytical Sciences

“Seeing is believing” is a phrase commonly used in scientific corridors. Microscopy, therefore, is a magnificent branch that equips researchers with the power to visualize objects in a manner previously thought impossible. Despite the sophistication involved in interpreting images, scientists and the general audience alike are consistently captivated by visual evidence detailing the intricacies of a cell, molecule, or nanoparticle.

The advent of electron-based microscopy techniques, such as Scanning Tunneling Microscopy (STM), has further empowered researchers to delve into the world of atoms. In other words, these techniques have made it possible to observe molecules with atomic resolution.

The charm of an image detailing the components of a complicated system — say, a DNA molecule — is enduring. However, the flip side is that spectroscopic characterization is essential to understand events occurring at the microscopic or nanoscopic scale. Spectroscopy and microscopy should, therefore, go hand in hand. As early as 1985, scientists suggested combining scanning probe techniques with methods like Raman or Infrared spectroscopy to obtain simultaneous information regarding topography and chemical nature.

Tip-Enhanced Raman Spectroscopy, popularly abbreviated as TERS, is one such method that combines Atomic Force Microscopy (AFM) or STM with Raman Spectroscopy. This unique combination gives the analytical scientist the ability to examine a surface in optical, topographical, and chemical detail. Previously, it was thought impossible to view an object with dimensions less than half the wavelength of light — a constraint known as the Abbe limit. TERS, however, breaks this limit, making molecular-level resolution possible with the added advantage of chemical information.

In TERS technology, a nanometric, locally enhanced field is generated in the immediate vicinity of a metallized tip apex under laser illumination. This acts as a local excitation source on the nanometer scale. It is used to excite optical signals from the specimen, producing Raman scattering solely from the nano-volume in close proximity to the tip. The tip apex is the key component in TERS; acting as a local scatterer, it enhances and scatters the localized evanescent field to the far-field to be detected. This ensures high enhancement and resolution well beyond the diffraction limit.

In essence, TERS is capable of recording sub-molecular resolution images that contain the spectral signature of each spot probed by the tip. Prof. Volker Deckert and co-workers (Friedrich Schiller University Jena, Germany) have used TERS to sequence an RNA molecule. The application of TERS to nucleic acids has been a revelation, as it allows for the resolution of closely spaced individual nucleobases. Another group at ETH Zurich, led by Prof. Renato Zenobi, has extensively characterized the Raman spectra of DNA. These studies could potentially lead to a deeper understanding of how DNA interacts with proteins and anticancer drugs.

In another report, a TERS study of Amyloid β — a critical factor in the “synaptic failure” observed in Alzheimer’s disease — was carried out, showcasing the immense applicability of TERS to unexplored domains of biology. Despite the challenges involved in making TERS a routine analytical technique, its future in biological studies looks brighter than ever. To conclude, the proverb “A picture is worth a thousand words” seems apt for a TERS image, with one slight modification: “A TERS image is worth a thousand spectra loaded with priceless chemical information.”