NMR characterization of phosphonic acid capped SnO₂ nanoparticles

Phosphonic acid capped SnO₂ nanoparticles with diameters less than 5 nm were synthesized and characterized with multinuclear solution and solid-state magic angle spinning (MAS) NMR. Two types of phosphonic acid ligands were used to derivatize the SnO₂ surface, producing (i) water soluble SnO₂ nanoparticles capped with 2-carboxyethanephosphonic acid (CEPA) and (ii) insoluble SnO₂ nanoparticles capped with phenylphosphonic acid (PPA). Multiple surface environments were observed with ³¹P solution and solid-state MAS NMR for both capping agents. The ³¹P resonances of derivatized SnO₂ nanoparticles display isotropic chemical shifts that are more shielded compared to the native phosphonic acids. This observation is indicative of a strong interaction between the phosphonic acid group and the SnO₂ surface. ¹H MAS NMR spectra display a complete absence of the acidic protons of the phosphonic acid groups, strongly supporting the formation of P-O-Sn linkages. ¹H → ³¹P cross polarization (CP) build-up behavior confirms the absence of the vast majority of phosphonic acid protons. Some of the build-up curves displayed oscillations that could be fit to extract the magnitude of the ¹H-³¹P dipolar coupling constant. The dipolar coupling can then be used to calculate the distance between phosphorus and the close proximity protons. The results presented herein indicate primarily bi- and tridentate phosphonic acid bonding configuration at the SnO₂ surface, in both CEPA and PPA capped nanoparticles.