A major challenge in magnetic nanoparticle synthesis and (bio)functionalization concerns the precise characterization of their 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 HRMAS technique, already being used for MAS NMR analysis of gels and semi-solid matrices, 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 1H 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 1H-1H (COSY) and 1H-13C heteronuclear single quantum coherence (HSQC) correlation spectra were also obtained, providing reliable elements on proton signal assignments and carbon characterization, and opening the way to the 13C NMR determination. Notably, combining the experimental evidences from HRMAS 1H NMR and diffusion-ordered spectroscopy (DOSY) 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, MALDI-TOF MS analysis of modified MNPs proved very valuable in ligand mass identification, thus giving a sound support to NMR characterization achievements.
Polito, L., Colombo, M., Monti, D., Melato, S., Caneva, E., Prosperi, D. (2008). Resolving the structure of ligands bound to the surface of superparamagnetic iron oxide nanoparticles by hrmas nmr spectroscopy. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 130(38), 12712-12724 [10.1021/ja802479n].
Resolving the structure of ligands bound to the surface of superparamagnetic iron oxide nanoparticles by hrmas nmr spectroscopy
COLOMBO, MIRIAM;PROSPERI, DAVIDE
2008
Abstract
A major challenge in magnetic nanoparticle synthesis and (bio)functionalization concerns the precise characterization of their 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 HRMAS technique, already being used for MAS NMR analysis of gels and semi-solid matrices, 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 1H 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 1H-1H (COSY) and 1H-13C heteronuclear single quantum coherence (HSQC) correlation spectra were also obtained, providing reliable elements on proton signal assignments and carbon characterization, and opening the way to the 13C NMR determination. Notably, combining the experimental evidences from HRMAS 1H NMR and diffusion-ordered spectroscopy (DOSY) 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, MALDI-TOF MS analysis of modified MNPs proved very valuable in ligand mass identification, thus giving a sound support to NMR characterization achievements.
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