Resolving the Structure of Ligands Bound to the Surface of Superparamagnetic Iron Oxide Nanoparticles by High-Resolution Magic-Angle Spinning NMR Spectroscopy

Abstract

A major challenge in magnetic nanoparticle synthesis and (bio)functionalization concerns the precise characterization of the nanoparticle surface ligands. We report the first analytical NMR investigation of organic ligands stably anchored on the surface of superparamagnetic nanoparticles (MNPs) through the development of a new experimental application of high-resolution magic-angle spinning (HRMAS). The conceptual advance here is that the HRMAS technique, already being used for MAS NMR analysis of gels and semisolid matrixes, enables the fine-structure-resolved characterization of even complex organic molecules bound to paramagnetic nanocrystals, such as nanosized iron oxides, by strongly decreasing the effects of paramagnetic disturbances. This method led to detail-rich, well-resolved (1)H NMR spectra, often with highly structured first-order couplings, essential in the interpretation of the data. This HRMAS application was first evaluated and optimized using simple ligands widely used as surfactants in MNP synthesis and conjugation. Next, the methodology was assessed through the structure determination of complex molecular architectures, such as those involved in MNP3 and MNP4. The comparison with conventional probes evidences that HRMAS makes it possible to work with considerably higher concentrations, thus avoiding the loss of structural information. Consistent 2D homonuclear (1)H- (1)H and (1)H- (13)C heteronuclear single-quantum coherence correlation spectra were also obtained, providing reliable elements on proton signal assignments and carbon characterization and opening the way to (13)C NMR determination. Notably, combining the experimental evidence from HRMAS (1)H NMR and diffusion-ordered spectroscopy performed on the hybrid nanoparticle dispersion confirmed that the ligands were tightly bound to the particle surface when they were dispersed in a ligand-free solvent, while they rapidly exchanged when an excess of free ligand was present in solution. In addition to HRMAS NMR, matrix-assisted laser desorption ionization time-of-flight MS analysis of modified MNPs proved very valuable in ligand mass identification, thus giving a sound support to NMR characterization achievements.

HR-MAS NMR Spectroscopy in Material Science

“…[56] HR-MAS NMR has also been used to identify the binding motif of peptides on the Au NP surface. [57] While HR-MAS NMR is typically employed to overcome anisotropy of magnetic susceptibility or residual dipolar interactions, Poito and coworkers [58,59] have reported an interesting set of experiments that demonstrated HR-MAS can be employed to overcome paramagnetic effects present in iron oxide NPs. Through a careful analysis of these iron oxide systems, the surface structure and ligand binding (chelation) configuration was determined, and in several cases were quite different from the standard single point attachment proposed by others.…”

“…Through a careful analysis of these iron oxide systems, the surface structure and ligand binding (chelation) configuration was determined, and in several cases were quite different from the standard single point attachment proposed by others. [58] 1 H HR-MAS NMR has also been used to look at the surface modification in polymer based NPs, including the monitoring of multiple synthetic steps employed during the peptidesurface modification of poly(vinylidene fluoride) (PVDF) nanoparticles, [60] or the surface modification of Dendron based NPs. [61] Polystyrene embedded silver clusters produced by a thermolysis reactions have also been characterized using HR-MAS NMR.…”

“…These results were able to distinguish between NP-bound and free ligands in these materials. [58] These types of PFG HR-MAS NMR experiments should prove useful in understanding the surface-ligand dynamics present in modified NPs.…”

Perfil químico de cultivares de feijão (phaseolus vulgaris) pela técnica de high resolution magic angle spinning (HR-MAS)

“…4 A aplicação dessa técnica tem apresentado ainda excelentes resultados na caracterização de géis, 5 moléculas adsorvidas em zeólitas 6 e ligadas a resinas, 7 proteínas nanocristalinas, 8 micro-organismos intactos 9 e, recentemente, aplicada a nanopartículas de óxido de ferro -contendo ferro paramagnético, onde demonstrou ser possível a obtenção de espectros com sinais finos e bem resolvidos. 10 Na quimiotaxonomia de liquens, essa técnica demonstrou ser uma relevante ferramenta na discriminação entre famílias, gêneros e espécies, onde a identificação, baseada somente na análise morfológica, é comumente complexa e pouco conclusiva. 11 Vale destacar que a técnica HR-MAS é muito sensível e requer poucos minutos para o registro de sinais de RMN de 1 H utilizando alguns miligramas do analito.…”

Nanoparticle and poly(methyl methacrylate) co-dispersion in anisole

“…The most effective approach is nanoparticle surface functionalization to provide hydrophobic functional groups that can mix homogeneously with hydrophobic liquid media, such as organic solvents, and thus readily integrate with the dissolved polymer chains [25][26][27]. Recent work with hybrid materials highlights the importance of organic dispersants as the anchoring and binding species for the nanoparticles and polymers [26,[28][29][30].…”