Optical alternative: light-based technique to complement NMR

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Published: Dec 15, 2010
Author: David Bradley
Channels: NMR Knowledge Base

Optical alternative

New research published in The Journal of Chemical Physics introduces an alternative path to the nuclear information delivered by NMR by using light to observe the electrons in their orbitals around the nuclei and infer information about those nuclei indirectly.

Nuclear magnetic resonance (NMR) spectroscopy has, in recent years, become one of the most widely used and powerful tools for getting to the heart of molecular structure and for investigating molecular dynamics. NMR turns on the fact that atomic nuclei behave differently depending on their exact chemical environment, whether a hydrogen or carbon atom in different lengths of hydrocarbon chain or the atoms from which a protein's amino acids are built. But the radio waves and magnetic fields of NMR are not the only way to access nuclear information, according to Carlos Meriles of the City University of New York and colleagues.

Shedding light on atomic nuclei

Writing in The Journal of Chemical Physics, Meriles and colleagues, Daniela Pagliero, Wei Dong and Dimitris Sakellariou hope to shed light on molecular structure and dynamics by observing atomic nuclei indirectly through the lens of the electrons that orbit those nuclei.

"We are not looking at a way to replace the conventional technique but there are a number of applications in which optical detection could provide complementary information," explains Meriles. The new technique is based on optical Faraday rotation (OFR).

OFR was discovered by Michael Faraday in 1845 and is a magneto-optical phenomenon in which light and a magnetic field interact in a transparent dielectric medium. The plane of polarisation of light incident on the medium is rotated to a degree that is proportional to the intensity of the component of the applied magnetic field in the direction of the beam of light. Meriles and his team are now exploiting the fact that nuclei that are sufficiently polarized generate their own additional magnetic field and this affects the electrons in a sample adding to the Faraday rotation. The precise changes in rotation are due to the interaction between the electrons and the nuclei, which in turn depends on the local molecular structure, so that this OFR-based approach to spectroscopy can acquire complementary information to conventional magnetic resonance detection.

The team has demonstrated how they use OFR to monitor the nuclear-spin signal in a set of model fluids rich in either fluorine-19 or hydrogen-1. "Our approach integrates optical detection with high-field, pulsed NMR so as to record the time-resolved evolution of nuclear-spins after radio frequency excitation," the team explains. They can compare the chemical-shift-resolved resonances and so are able to apply a set order-of-magnitude constraints on the relative amplitudes of hyperfine coupling constants for different bonding geometries.

Sensitive evaluation

"When evaluated against coil induction, the present detection modality suffers from poorer sensitivity, but improvement could be attained via multipass schemes," the team says. Intriguingly, however, and a point not lost on the researchers, is that unlike conventional NMR, the signal strength is proportional to the length of the sample, the distance the incident light must travel, rather than the sample volume as with NMR. This means that a good spectrum can be obtained from a very small volume of sample held in a long thin sample tube, whereas obtaining an NMR spectrum from the same small volume might not be possible.

"Although we have not yet demonstrated it, our calculations show that we could magnify the signal by creating a very long optical path in a short, thin tube," Meriles explains. The signal might be magnified using a simple physical method, of placing mirrors at both ends of a channel in a microfluidics device to reflect the incident laser light repeatedly through the sample, increasing the signal amplitude with each pass.

The team adds that, "Because illumination is off-resonant i.e., the medium is optically transparent, this methodology could find extensions in a broad class of fluids and soft condensed matter systems."

Resonance and peaks

"From a purely spectroscopic point of view, it is important to note that the amplitude of OFR is proportional to the hyperfine coupling (between nuclei and electrons), and also to the Verdet constant of the material (which grows with the inverse square of the difference between the illumination and material transition energies), Meriles told SpectroscopyNOW. In a typical NMR spectrum, the relative amplitudes of the different 'chemical-shift'-resolved resonances is proportional to the number of nuclei of a given species, he points out. "By contrast, for an OFR-detected spectrum, we expect additional spectral contrast: on the one hand, one will have hyperfine contrast between the NMR resonances of a single compound (nuclei within a molecule with a stronger hyperfine coupling will yield stronger OFR signal)." However, on the other hand, if one sets the illumination wavelength closer to the optical transition of one particular compound (in a multicompound sample), the corresponding peaks will be preferentially highlighted. This could open new, intriguing ways of spectral editing and solvent suppression (which could be very useful in solution NMR). "We are presently conducting experiments in this direction and have found some very promising results," he adds.

The second area of application is the detection of small amounts of fluids. "Our technique integrates very well with optofluidic technology and therefore can capitalize on the big current drive in this direction," says Meriles. "Chip-integrated Fabry-Perot (or linear) resonators are one option that, we think, may ultimately attain superior sensitivity than micro-coil NMR."

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.