Revealing Surface Siloxane Structure With Silicon-29 NMR

Silica surfaces functionalized with siloxanes are potentially useful in many applications—including biomedicine, coatings, and fuel cells—but they are challenging to characterize. Solid-state, two-dimensional ²⁹Si-²⁹Si NMR can reveal the chemical architecture but only after months of data collection. Adding dynamic nuclear polarization to enhance NMR signals enables those experiments to be completed in hours, reports a team led by Gaël De Paëpe of the French Alternative Energies & Atomic Energy Commission (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja506688m). The researchers would like to use organosiloxane-functionalized silica nanoparticles in proton exchange membrane fuel cells to reduce degrading species. Silanizing the nanoparticles, however, can yield Si monomers and Si–O–Si polymers bound to the surface in various ways. Without a good analytical technique, it is difficult to determine the best conditions to produce the desired material, which for the fuel cells is polymers with each Si bound simultaneously to the surface and other Si groups. The new NMR approach involves mixing the nanoparticles with a biradical polarizing agent and then irradiating the sample with microwaves to transfer the polarization of the biradical electrons’ spins to the Si spins. The approach can distinguish among various surface-bound siloxane structures and yield Si-Si distances.