Solid-State NMR Characterization of Mixed Phosphonic Acid Ligand Binding and Organization on Silica Nanoparticles

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• 作者：Stephen K. Davidowski ; Gregory P. Holland
• 刊名：Langmuir
• 出版年：2016
• 出版时间：April 5, 2016
• 年：2016
• 卷：32
• 期：13
• 页码：3253-3261
• 全文大小：475K
• ISSN：1520-5827

文摘

As ligand functionalization of nanomaterials becomes more complex, methods to characterize the organization of multiple ligands on surfaces is required. In an effort to further the understanding of ligand–surface interactions, a combination of multinuclear (¹H, ²⁹Si, ³¹P) and multidimensional solid-state nuclear magnetic resonance (NMR) techniques was utilized to characterize the phosphonic acid functionalization of fumed silica nanoparticles using methylphosphonic acid (MPA) and phenylphosphonic acid (PPA). ¹H → ²⁹Si cross-polarization (CP)–magic angle spinning (MAS) solid-state NMR was used to selectively detect silicon atoms near hydrogen atoms (primarily surface species); these results indicate that geminal silanols are preferentially depleted during the functionalization with phosphonic acids. ¹H → ³¹P CP-MAS solid-state NMR measurements on the functionalized silica nanoparticles show three distinct resonances shifted upfield (lower ppm) and broadened compared to the resonances of the crystalline ligands. Quantitative ³¹P MAS solid-state NMR measurements indicate that ligands favor a monodentate binding mode. When fumed silica nanoparticles were functionalized with an equal molar ratio of MPA and PPA, the MPA bound the nanoparticle surface preferentially. Cross-peaks apparent in the 2D ¹H exchange spectroscopy (EXSY) NMR measurements of the multiligand sample at short mixing times indicate that the MPA and PPA are spatially close (≤5 Å) on the surface of the nanostructure. Furthermore, ¹H–¹H double quantum–single quantum (DQ-SQ) back-to-back (BABA) 2D NMR spectra further confirmed that MPA and PPA are strongly dipolar coupled with observation of DQ intermolecular contacts between the ligands. DQ experimental buildup curves and simulations indicate that the average distance between MPA and PPA is no further than 4.2 ± 0.2 Å.