Nucleation and Island Growth of Alkanethiolate Ligand Domains on Gold Nanoparticles

The metal oxide cluster alpha-AIW(11)O(39)(9-) (1), 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 1 on 13.8 +/- 0.9 nm diameter gold nanoparticles are displaced from the gold surface by mercaptoundecacarboxylate, HS(CH2)(10)CO2- (11-MU). During this process, no aggregation is observed by UV-vis spectroscopy, and the intermediate ligand-shell organizations of 1 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 1 by thiol, reveal that the 330 +/- 30 molecules of 1 initially present on each gold nanoparticie are eventually replaced by 2800 +/- 30 molecules of 11-MU. UV-vis kinetic data for 11-MU-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 11-MU ligands irreversibly displace 1 from the Au-NP surface via an associative mechanism, with k(nudeation) = (6.1 +/- 0.4) x 10(2) M-1 s(-1), and 19 +/- 8 nuclei, each comprised of ca. 8 alkanethiolates, appear on the gold-nanoparticle surface before rapid growth becomes kinetically dominant Island growth is also first-order in [11-MU], and its larger rate constant, k(grawth), (2.3 +/- 0.2) x 10(4) M-1 s(-1), is consistent with destabilization of molecules of 1 at the boundaries between the hydrophobic (alkanethiolate) and the electrostatically stabilized (inorganic) domains.