Scanning Raman picoscopy: A new methodology for determining molecular chemical structure

January 21, 2020

Precise determination of the chemical structure of a molecule is of vital importance to any molecular related field and is the key to a deep understanding of its chemical, physical, and biological functions. Scanning tunneling microscope and atomic force microscope have outstanding abilities to image molecular skeletons in real space, but these techniques usually lack chemical information necessary to accurately determine molecular structures. Raman scattering spectra contain abundant structural information of molecular vibrations. Different molecules and chemical groups exhibit distinct spectral features in Raman spectra, which can be used as the "fingerprints" of molecules and chemical groups. Therefore, the above mentioned deficiency can in principle be overcome by a combination of scanning probe microscopy with Raman spectroscopy, as demonstrated by the tip enhanced Raman spectroscopy (TERS), which opens up opportunities to determine the chemical structure of a single molecule.

In 2013, a research group led by Zhenchao Dong and Jianguo Hou at University of Science and Technology of China (USTC) demonstrated sub-nanometer resolved single-molecule Raman mapping for the first time [Nature 498, 82 (2013)], driving the spatial resolution with chemical identification capability down to ~5 Å. Since then, researchers around the world have been keeping on developing such single-molecule Raman imaging technique to explore what is the ultimate limit of the spatial resolution and how this technique can be best utilized.

Recently, the USTC group published a research paper in National Science Review (NSR) entitled "Visually Constructing the Chemical Structure of a Single Molecule by Scanning Raman Picoscopy", pushing the spatial resolution to a new limit and proposing an important new application for the state-of-art technique. In this work, by developing cryogenic ultrahigh-vacuum TERS system at liquid-helium temperatures and fine-tuning the highly localized plasmon field at the sharp tip apex, they further drive the spatial resolution down to 1.5 Å on the single-chemical-bond level, which enables them to achieve full spatial mapping of various intrinsic vibrational modes of a molecule and discover distinctive interference effects in symmetric and antisymmetric vibrational modes. More importantly, based on the Ångström-level resolution achieved and the new physical effect discovered, by combining with Raman fingerprint database of chemical groups, they further propose a new methodology, coined as Scanning Raman Picoscopy (SRP), to visually construct the chemical structure of a single molecule. This methodology highlights the remarkable ability of Raman-based scanning technology via an atomistically sharp tip to reveal the molecular chemical structure in real space, just by "looking" at a single molecule optically, as schematically shown in Figure (a).

By applying the SRP methodology to a single magnesium porphyrin model molecule, the researchers at USTC obtained a set of real-space imaging patterns for different Raman peaks, and found that these patterns show different spatial distributions for different vibrational modes. Taking the typical C-H bond stretching vibration on the pyrrole ring as an example, for the antisymmetric vibration (3072 cm-1) of two C-H bonds, the phase relation of their local polarization responses is opposite. When the tip is located right above the center between two bonds, the contributions from both bonds to the Raman signals cancel out, giving rise to the "eight-spot" feature in the Raman map for the whole molecule, with the best spatial resolution down to 1.5 Å. These "eight spots" have good spatial correspondence with the eight C-H bonds on the four pyrrole rings of a magnesium porphyrin molecule, indicating that the detection sensitivity and spatial resolution have reached the single-chemical-bond level. Raman imaging patterns of other vibrational peaks also show good correspondence to relevant chemical groups in terms of characteristic peak positions and spatial distributions [as shown in Figures (b) and (c)]. The correspondence provided by the simultaneous spatially and energy-resolved Raman imaging allows them to correlate local vibrations with constituent chemical groups and to visually assemble various chemical groups in a "Lego-like" manner into a whole molecule, thus realizing the construction of the chemical structure of a molecule.

The scanning Raman picoscopy (SRP) is the first optical microscopy that has the ability to visualize the vibrational modes of a molecule and to directly construct the structure of a molecule in real space. The protocol established in this proof-of-principle demonstration can be generalized to identify other molecular systems, and can become a more powerful tool with the aid of imaging recognition and machine learning techniques. The ability of such Ångström-resolved scanning Raman picoscopy to determine the chemical structure of unknown molecules will undoubtedly arouse extensive interests of researchers in the fields of chemistry, physics, materials, biology and so on, and is expected to stimulate active research in the fields as SRP is developed into a mature and universal technology.
-end-
This work received funding from the National Key R&D Program of China, the National Natural Science Foundation of China, the Chinese Academy of Sciences, and Anhui Initiative in Quantum Information Technologies.

See the article:

Yao Zhang†, Ben Yang†, Atif Ghafoor†, Yang Zhang, Yu-Fan Zhang, Rui-Pu Wang, Jin-Long Yang, Yi Luo*, Zhen-Chao Dong*, Jian Guo Hou*
Visually Constructing the Chemical Structure of a Single Molecule by Scanning Raman Picoscopy
National Science Review, nwz180,
https://doi.org/10.1093/nsr/nwz180

Science China Press

Related Technology Articles from Brightsurf:

October issue SLAS Technology now available
The October issue of SLAS Technology features the cover article, 'Role of Digital Microfl-uidics in Enabling Access to Laboratory Automation and Making Biology Programmable' by Varun B.

Robot technology for everyone or only for the average person?
Robot technology is being used more and more in health rehabilitation and in working life.

Novel biomarker technology for cancer diagnostics
A new way of identifying cancer biomarkers has been developed by researchers at Lund University in Sweden.

Technology innovation for neurology
TU Graz researcher Francesco Greco has developed ultra-light tattoo electrodes that are hardly noticeable on the skin and make long-term measurements of brain activity cheaper and easier.

April's SLAS Technology is now available
April's Edition of SLAS Technology Features Cover Article, 'CURATE.AI: Optimizing Personalized Medicine with Artificial Intelligence'.

Technology in higher education: learning with it instead of from it
Technology has shifted the way that professors teach students in higher education.

Post-lithium technology
Next-generation batteries will probably see the replacement of lithium ions by more abundant and environmentally benign alkali metal or multivalent ions.

Rethinking the role of technology in the classroom
Introducing tablets and laptops to the classroom has certain educational virtues, according to Annahita Ball, an assistant professor in the University at Buffalo School of Social Work, but her research suggests that tech has its limitations as well.

The science and technology of FAST
The Five hundred-meter Aperture Spherical radio Telescope (FAST), located in a radio quiet zone, with the targets (e.g., radio pulsars and neutron stars, galactic and extragalactic 21-cm HI emission).

AI technology could help protect water supplies
Progress on new artificial intelligence (AI) technology could make monitoring at water treatment plants cheaper and easier and help safeguard public health.

Read More: Technology News and Technology Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.