**2.6. Soil and humic materials**

292 Advanced Aspects of Spectroscopy

of the NPs.

characterization of the surface-attached ligands being the primary objective. For NPs the solvent plays the role of suspending or dispersing the material, in contrast to the role of swelling that the solvent plays in analysis of resins. For HR-MAS NMR studies of NPs, the optimal solvent provides both high solubility, and good dispersion (prevents aggregation)

**Figure 7.** The 2D 1H HR-MAS NOESY NMR correlation spectra for the ionic liquid [MBPyrr]+[TFSI] adsorbed into an inorganic aluminum oxide membrane (pore size 200 nm). The spectrum in A) shows numerous through space correlation between the protons of the IL, while B) shows the spectral expansion for the water and surface associate-water species and corresponding NOE correlations.

HR-MAS NMR studies of surface modified NPs include the 1D and 2D 1H NMR investigation of modified gold (Au) NPs using a wide range of high resolution correlation experiments: COSY, TOCSY and HMQC.[55] This revealed that the relative signal sensitivity (intensity) depends on the distance between the detected 1H and the surface of the Au NP. This distance correlation is either a function of reduced local dynamics, or spin-spin T2 relaxation effects governed by the Au surface. Another study involved the active molecular compounds Aloin A and Aloesin extracted from the leaf of Cape Aloe, and was able to show how these compounds selectively stabilize Au NPs preferentially through the glucose component.[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.[58]

While standard solid state MAS NMR techniques have become common in the investigations of humic, soil and coal materials, the implementation of HR-MAS techniques for these materials has been more limited. The ability of HR-MAS to identify different motional regimes has proven powerful in the elucidation of the chemistry at water/soil interfaces.[74-79] These NMR studies show that at this interface fatty acids, aliphatic esters and alcohols are the prominent species, and that aromatic functional groups are protected by hydrophobic regions and are not directly accessible to the penetrating water,[74] while surface polymethylene groups may control the sorption properties of organo-clay complexes.[76, 80] These types of studies have been extended to three-dimensional (3D) HMQC-TOCSY to further increase the resolution of the highly overlapping spectra from humic materials.[81]
