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

Meng Wu, Ariane M. Vartanian, Gene Chong, Arun Kumar Pandiakumar, Robert J. Hamers, Rigoberto Hernandez, Catherine Jones Murphy

Research output: Contribution to journalArticle

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. T2 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 T2 relaxation dynamics data suggest that headgroup mobility depends on aspect ratio and absolute nanoparticle dimensions.

Original languageEnglish (US)
Pages (from-to)4316-4327
Number of pages12
JournalJournal of the American Chemical Society
Volume141
Issue number10
DOIs
StatePublished - Mar 13 2019

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Self assembled monolayers
Ammonium Compounds
Gold
Nanoparticles
ligand
curvature
nuclear magnetic resonance
ammonium
gold
Ligands
Nuclear magnetic resonance
Bromides
bromide
Molecules
Chemical shift
analysis
effect
nanoparticle
Aspect ratio
shell

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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Solution NMR Analysis of Ligand Environment in Quaternary Ammonium-Terminated Self-Assembled Monolayers on Gold Nanoparticles : The Effect of Surface Curvature and Ligand Structure. / Wu, Meng; Vartanian, Ariane M.; Chong, Gene; Pandiakumar, Arun Kumar; Hamers, Robert J.; Hernandez, Rigoberto; Murphy, Catherine Jones.

In: Journal of the American Chemical Society, Vol. 141, No. 10, 13.03.2019, p. 4316-4327.

Research output: Contribution to journalArticle

Wu, Meng ; Vartanian, Ariane M. ; Chong, Gene ; Pandiakumar, Arun Kumar ; Hamers, Robert J. ; Hernandez, Rigoberto ; Murphy, Catherine Jones. / Solution NMR Analysis of Ligand Environment in Quaternary Ammonium-Terminated Self-Assembled Monolayers on Gold Nanoparticles : The Effect of Surface Curvature and Ligand Structure. In: Journal of the American Chemical Society. 2019 ; Vol. 141, No. 10. pp. 4316-4327.
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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. T2 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 T2 relaxation dynamics data suggest that headgroup mobility depends on aspect ratio and absolute nanoparticle dimensions.",
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