Solution NMR Analysis of Ligand Environment in Quaternary Ammonium-Terminated Self-Assembled Monolayers on Gold Nanoparticles: The Effect of Surface Curvature and Ligand Structure

Abstract

We report a solution NMR-based analysis of (16-mercaptohexadecyl)­trimethylammonium bromide (MTAB) self-assembled monolayers on colloidal gold nanospheres (AuNSs) with diameters from 1.2 to 25 nm and gold nanorods (AuNRs) with aspect ratios from 1.4 to 3.9. The chemical shift analysis of the proton signals from the solvent-exposed headgroups of bound ligands suggests that the headgroups are saturated on the ligand shell as the sizes of the nanoparticles increase beyond ∼10 nm. Quantitative NMR shows that the ligand density of MTAB-AuNSs is size-dependent. Ligand density ranges from ∼3 molecules per nm2 for 25 nm particles to up to 5−6 molecules per nm2 in ∼10 nm and smaller particles for in situ measurements of bound ligands; after I2/I– treatment to etch away the gold cores, ligand density ranges from ∼2 molecules per nm2 for 25 nm particles to up to 4−5 molecules per nm2 in ∼10 nm and smaller particles. T 2 relaxation analysis shows greater hydrocarbon chain ordering and less headgroup motion as the diameter of the particles increases from 1.2 nm to ∼13 nm. Molecular dynamics simulations of 4, 6, and 8 nm (11-mercaptoundecyl)­trimethylammonium bromide-capped AuNSs confirm greater hydrophobic chain packing order and saturation of charged headgroups within the same spherical ligand shell at larger nanoparticle sizes and higher ligand densities. Combining the NMR studies and MD simulations, we suggest that the headgroup packing limits the ligand density, rather than the sulfur packing on the nanoparticle surface, for ∼10 nm and larger particles. For MTAB-AuNRs, no chemical shift data nor ligand density data suggest that two populations of ligands that might correspond to side-ligands and end-ligands exist; yet T 2 relaxation dynamics data suggest that headgroup mobility depends on aspect ratio and absolute nanoparticle dimensions.

Covalently functionalized uniform amino-silica nanoparticles. Synthesis and validation of amine group accessibility and stability

“…It should also be noted that for the T 2 NMR experiments only the terminal protons are shown, H25 in Fig. 7b, because the protons farthest from the nanoparticle surface have been shown 60,62 to have the longest T 2 due to increase mobility in solution. Fig.…”

Protein Interactions with Nanoparticle Surfaces: Highlighting Solution NMR Techniques

“…Meng et al . investigated the potential difference between MTAB ligands at the ends and the sides of gold nanorods (AuNRs) using chemical shift perturbation and T 2 relaxation . As compared to the spherical AuNPs, MTAB‐AuNRs have two distinctly different chemical environments (elongated sides and spherical ends) due to the rod‐like nature of the nanoparticle.…”

“…As compared to the spherical AuNPs, MTAB‐AuNRs have two distinctly different chemical environments (elongated sides and spherical ends) due to the rod‐like nature of the nanoparticle. Both and relaxation times were compared for different aspect ratios of MTAB‐AuNRs . When the aspect ratio increased, the difference between and also increased, and from this data, the authors concluded that the MTAB headgroup mobility depends on the nanorod aspect ratio.…”

Oberflächenstabilisatoren und ihre Rolle bei der formkontrollierten Synthese von kolloidalen Metall‐Nanokristallen

“…Um weitere Erkenntnisse über die Verteilung der im Oberflächenstabilisator vorhandenen Elemente zu gewinnen, wurde die energiedispersive Röntgenspektroskopie (EDX) implementiert, während die Röntgen‐Photoelektronenspektroskopie (XPS) verwendet wurde, um das Vorhandensein bestimmter Elemente sowie deren chemische Berschaffenheit zu untersuchen. Auch die Kernspinresonanz (NMR)‐Spektroskopie wurde verwendet, um den Bindungsmodus eines Oberflächenstabilisators auf der Metalloberfläche zu untersuchen. Die thermogravimetrische Analyse (TGA) kann anhand der Thermostabilität das Vorhandensein eines Oberflächenstabilisators nachweisen.…”