Nucleation and Island Growth of Alkanethiolate Ligand Domains on Gold Nanoparticles

The metal oxide cluster α-AlW₁₁O₃₉^9– (<b>1</b>), readily imaged by cryogenic transmission electron microscopy (cryo-TEM), is used as a diagnostic protecting anion to investigate the self-assembly of alkanethiolate monolayers on electrostatically stabilized gold nanoparticles in water. Monolayers of <b>1</b> on 13.8 ± 0.9 nm diameter gold nanoparticles are displaced from the gold surface by mercaptoundecacarboxylate, HS(CH₂)₁₀CO₂^– (<b>11-MU</b>). During this process, no aggregation is observed by UV–vis spectroscopy, and the intermediate ligand-shell organizations of <b>1</b> in cryo-TEM images indicate the presence of growing hydrophobic domains, or “islands”, of alkanethiolates. UV–vis spectroscopic “titrations”, based on changes in the surface plasmon resonance upon exchange of <b>1</b> by thiol, reveal that the 330 ± 30 molecules of <b>1</b> initially present on each gold nanoparticle are eventually replaced by 2800 ± 30 molecules of <b>11-MU</b>. UV–vis kinetic data for <b>11-MU</b>-monolayer formation reveal a slow phase, followed by rapid self-assembly. The Johnson, Mehl, Avrami, and Kolmogorov model gives an Avrami parameter of 2.9, indicating continuous nucleation and two-dimensional island growth. During nucleation, incoming <b>11-MU</b> ligands irreversibly displace <b>1</b> from the Au-NP surface <i>via</i> an associative mechanism, with <i>k</i>_nucleation = (6.1 ± 0.4) × 10² M^–1 s^–1, and 19 ± 8 nuclei, each comprised of <i>ca</i>. 8 alkanethiolates, appear on the gold-nanoparticle surface before rapid growth becomes kinetically dominant. Island growth is also first-order in [<b>11-MU</b>], and its larger rate constant, <i>k</i>_growth, (2.3 ± 0.2) × 10⁴ M^–1 s^–1, is consistent with destabilization of molecules of <b>1</b> at the boundaries between the hydrophobic (alkanethiolate) and the electrostatically stabilized (inorganic) domains

Nucleation and Island Growth of Alkanethiolate Ligand Domains on Gold Nanoparticles

The metal oxide cluster α-AlW₁₁O₃₉^9– (<b>1</b>), readily imaged by cryogenic transmission electron microscopy (cryo-TEM), is used as a diagnostic protecting anion to investigate the self-assembly of alkanethiolate monolayers on electrostatically stabilized gold nanoparticles in water. Monolayers of <b>1</b> on 13.8 ± 0.9 nm diameter gold nanoparticles are displaced from the gold surface by mercaptoundecacarboxylate, HS(CH₂)₁₀CO₂^– (<b>11-MU</b>). During this process, no aggregation is observed by UV–vis spectroscopy, and the intermediate ligand-shell organizations of <b>1</b> in cryo-TEM images indicate the presence of growing hydrophobic domains, or “islands”, of alkanethiolates. UV–vis spectroscopic “titrations”, based on changes in the surface plasmon resonance upon exchange of <b>1</b> by thiol, reveal that the 330 ± 30 molecules of <b>1</b> initially present on each gold nanoparticle are eventually replaced by 2800 ± 30 molecules of <b>11-MU</b>. UV–vis kinetic data for <b>11-MU</b>-monolayer formation reveal a slow phase, followed by rapid self-assembly. The Johnson, Mehl, Avrami, and Kolmogorov model gives an Avrami parameter of 2.9, indicating continuous nucleation and two-dimensional island growth. During nucleation, incoming <b>11-MU</b> ligands irreversibly displace <b>1</b> from the Au-NP surface <i>via</i> an associative mechanism, with <i>k</i>_nucleation = (6.1 ± 0.4) × 10² M^–1 s^–1, and 19 ± 8 nuclei, each comprised of <i>ca</i>. 8 alkanethiolates, appear on the gold-nanoparticle surface before rapid growth becomes kinetically dominant. Island growth is also first-order in [<b>11-MU</b>], and its larger rate constant, <i>k</i>_growth, (2.3 ± 0.2) × 10⁴ M^–1 s^–1, is consistent with destabilization of molecules of <b>1</b> at the boundaries between the hydrophobic (alkanethiolate) and the electrostatically stabilized (inorganic) domains