NMR Characterization of Phosphonic Acid Capped SnO₂ Nanoparticles

Phosphonic acid capped SnO 2 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 SnO 2 surface, producing (i) water soluble SnO 2 nanoparticles capped with 2-carboxyethanephosphonic acid (CEPA) and (ii) insoluble SnO 2 nanoparticles capped with phenylphosphonic acid (PPA). Multiple surface environments were observed with 31 P solution and solid-state MAS NMR for both capping agents. The 31 P resonances of derivatized SnO 2 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 SnO 2 surface. 1 H 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. 1 H → 31 P 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 1 H− 31 P 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 SnO 2 surface, in both CEPA and PPA capped nanoparticles.