Polito Laura, Colombo Miriam, Monti Diego, Melato Sergio, Caneva Enrico, Prosperi Davide
Department of Organic and Industrial Chemistry and Centro Interdipartimentale Grandi Apparecchiature, University of Milan, Institute of Molecular Science and Technology, CNR, Milan, Italy.
J Am Chem Soc. 2008 Sep 24;130(38):12712-24. doi: 10.1021/ja802479n. Epub 2008 Aug 30.
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.
磁性纳米颗粒合成及（生物）功能化过程中的一个主要挑战涉及纳米颗粒表面配体的精确表征。我们通过开发高分辨率魔角旋转（HRMAS）的一种新实验应用，首次报道了对稳定锚定在超顺磁性纳米颗粒（MNP）表面的有机配体进行的分析核磁共振研究。这里的概念性进展在于，HRMAS技术已用于凝胶和半固体基质的MAS NMR分析，通过大幅降低顺磁干扰的影响，能够对与顺磁性纳米晶体（如纳米级氧化铁）结合的甚至复杂的有机分子进行精细结构分辨表征。该方法产生了细节丰富、分辨率良好的¹H NMR谱，通常具有高度结构化的一级耦合，这对数据解释至关重要。此HRMAS应用首先使用在MNP合成和缀合中广泛用作表面活性剂的简单配体进行评估和优化。接下来，通过确定复杂分子结构（如MNP3和MNP4中涉及的结构）来评估该方法。与传统探针的比较表明，HRMAS能够在相当高的浓度下工作，从而避免结构信息的丢失。还获得了一致的二维同核¹H-¹H和¹H-¹³C异核单量子相干相关谱，为质子信号归属和碳表征提供了可靠依据，并为¹³C NMR测定开辟了道路。值得注意的是，结合对混合纳米颗粒分散体进行的HRMAS¹H NMR和扩散有序光谱的实验证据证实，当配体分散在无配体溶剂中时，它们紧密结合在颗粒表面，而当溶液中存在过量游离配体时，它们会快速交换。除了HRMAS NMR外，对改性MNP的基质辅助激光解吸电离飞行时间质谱分析在配体质量鉴定方面证明非常有价值，从而为NMR表征成果提供了有力支持。
