Synthetic inorganic nanoparticles are often stabilized by an organic ligand shell that defines much of their chemical and biological properties, but little is known about the morphology and dynamics of these ligands. Here, we report an NMR-based analysis of (16-mercaptohexadecyl)trimethylammonium bromide (MTAB) functionalized gold nanospheres (AuNSs) with sizes from 1.5 nm to 40 nm and gold nanorods (AuNRs) with aspect ratios from 1.3 to 3.8. Broadened and shifted peaks in 1H NMR spectra suggest MTAB ligands are bound to the nanoparticles. Aging experiments show that MTAB ligands desorb over time as MTAB-disulfide. The dissociation equilibrium is achieved over 20 days with less than 1% of the ligands staying free in the solution. Quantitative NMR shows that the ligand density of MTAB-AuNSs is size-dependent, ranging from 2.5 to 6 molecules per nm2. The chemical shift analysis of the proton signals from the solvent-exposed head groups suggests an increasing degree of hydrocarbon chain packing until the sizes of the nanoparticles increase to 12 nm. The ligand environment of MTAB-AuNSs larger than 12 nm resembles to that of a planar surface. The anisotropic functionality of cetyltrimethylammonium bromide (CTAB) capped AuNRs at the ends and sides have been observed experimentally in other reports. However, T2 relaxation curves for MTAB-AuNRs of different aspect ratios all decay mono-exponentially, similar to that of MTAB-AuNSs. The ligand density of MTAB-AuNRs is 2.5 molecules per nm2, which is independent of the aspect ratios and similar to that of MTAB-AuNSs with sizes larger than 12 nm. These results suggest that no differences in MTAB ligands on the sides and ends of the AuNRs have been observed, and that the MTAB ligands on the ends and sides both resembles those on a planar surface. The loss in ligand anisotropy in MTAB-AuNRs can be explained by the differences in ligand binding modes: CTAB binds to the gold surface by the bigger head groups whereas MTAB binds by the smaller thiol groups.

NMR analysis of ligand environments on gold nanoparticles: The effect of surface curvature and ligand binding modes

256th National Meeting (2018)

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