Advanced nuclear magnetic resonance (NMR) techniques at the U.S. Department of Energy’sAmes Laboratory have revealed surprising details about the structure of a key group ofmaterials in nanotechology, mesoporous silica nanoparticles (MSNs), and the placement of their active chemical sites.
MSNs are honeycombed with tiny (about 2-15 nm wide) three-dimensionally ordered tunnels orpores, and 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 BiologicalSciences at Ames Laboratory. “Research has explored doing this through modifying particlesize and shape, pore size, and by deploying various organic functional groups on their surfacesto accomplish the desired chemical tasks. However, understanding of the results of thesesynthetic efforts can be very challenging.”
Ames Laboratory scientist Marek Pruski explained that despite the existence of differenttechniques for MSNs’ functionalization, no one knew exactly how they were different. Inparticular, atomic-scale description of how the organic groups were distributed on the surfacewas lacking until recently.
“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 theyreside on the surfaces or are they partly embedded in the silica walls? Are they uniformlydistributed on surfaces? If there are multiple types of functionalities, are they randomly mixed ordo they form domains? Conventional NMR, as well as other analytical techniques, havestruggled to provide satisfactory answers to these important questions.”
Kobayashi, Pruski, and other researchers used DNP-NMR to obtain a much clearer picture ofthe structures of functionalized MSNs. “DNP” stands for “dynamic nuclear polarization,” amethod which uses microwaves to excite unpaired electrons in radicals and transfer their highspin polarization to the nuclei in the sample being analyzed, offering drastically highersensitivity, 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 magneticfield to direct radio-frequency excitation, lacks the sensitivity needed to identify the internuclearinteractions 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 withunprecedented sensitivity.
Not only did the DNP-NMR methods elicit the atomic-scale location and distribution of thefunctional groups, but the results disproved some of the existing notions of how MSNs are madeand how the different synthetic strategies influenced the dispersion of functional groupsthroughout the silica pores.
“By examining the role of various experimental conditions, our NMR techniques can givescientists the mechanistic insight they need to guide the synthesis of MSNs in a more controlledway” said Kobayashi.
The research is further discussed in “Spatial distribution of silica-bound catalytic organicfunctional groups can now be revealed by conventional and DNP-enhanced solid-state NMRmethods,” authored by T. Kobayashi and M. Pruski; and published in ACS Catalysis.
Ames Laboratory is a U.S. Department of Energy Office of Science national laboratory operated by Iowa State University. Ames Laboratory creates innovative materials, technologies andenergy solutions. We use our expertise, unique capabilities and interdisciplinary collaborationsto solve global problems.
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