New NMR technique can probe functional groups on nanomaterials

"By examining the role of various experimental conditions, our NMR techniques can give scientists the mechanistic insight they need to guide the synthesis of MSNs in a more controlled way."Takeshi Kobayashi, Ames Laboratory

Advanced nuclear magnetic resonance (NMR) techniques at the US Department of Energy's Ames Laboratory have revealed surprising details about the structure of a key group of nanomaterials known as mesoporous silica nanoparticles (MSNs) and about the placement of their active chemical sites. These details are reported in a paper in ACS Catalysis.

MSNs are honeycombed with tiny (2–15nm wide), three-dimensionally ordered tunnels or pores, and can serve as supports for organic functional groups tailored to a wide range of needs. With possible applications in catalysis, chemical separations, biosensing and drug delivery, MSNs are the focus of intense scientific research.

"Since the development of MSNs, people have been trying to control the way they function," said Takeshi Kobayashi, an NMR scientist with the Division of Chemical and Biological Sciences at Ames Laboratory. "Research has explored doing this through modifying particle size and shape, pore size, and by deploying various organic functional groups on their surfaces to accomplish the desired chemical tasks. However, understanding of the results of these synthetic efforts can be very challenging."

Ames Laboratory scientist Marek Pruski explained that despite the existence of different techniques for functionalizing MSNs, no one knew exactly how they were different. In particular, an atomic-scale description of how the organic groups were distributed on the surface of MSNs had been lacking.

"It is one thing to detect and quantify these functional groups, or even determine their structure," said Pruski. "But elucidating their spatial arrangement poses additional challenges. Do they reside on the surfaces or are they partly embedded in the silica walls? Are they uniformly distributed on surfaces? If there are multiple types of functionalities, are they randomly mixed or do they form domains? Conventional NMR, as well as other analytical techniques, have struggled to provide satisfactory answers to these important questions."

Kobayashi, Pruski and other researchers used a technique known as DNP-NMR to obtain a much clearer picture of the structures of functionalized MSNs. DNP, which stands for ‘dynamic nuclear polarization’, uses microwaves to excite unpaired electrons in radicals and transfer their high spin polarization to the nuclei in the sample being analyzed. It offers drastically higher sensitivity than conventional NMR, often by two orders of magnitude, and even larger savings of experimental time.

Conventional NMR, which measures the responses of the nuclei of atoms placed in a magnetic field to direct radio-frequency excitation, lacks the sensitivity needed to identify the internuclear interactions between different sites and functionalities on surfaces. When paired with DNP, as well as fast magic angle spinning (MAS), NMR can be used to detect such interactions with unprecedented sensitivity.

Not only did the DNP-NMR methods elicit the atomic-scale location and distribution of the functional groups, but the results disproved some of the existing notions of how MSNs are made and how the different synthetic strategies influence the dispersion of functional groups throughout the silica pores.

"By examining the role of various experimental conditions, our NMR techniques can give scientists the mechanistic insight they need to guide the synthesis of MSNs in a more controlled way," said Kobayashi.

This story is adapted from material from Ames Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.