Atomistic Simulations of Functional Au 144 (SR) 60 Gold Nanoparticles in Aqueous Environment

  • Heikkilä E
  • Gurtovenko A
  • Martinez-Seara H
 et al. 
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Abstract

Charged monolayer-protected gold nanoparticles (AuNPs) have been studied in aqueous solution by performing atomistic molecular dynamics simulations at physiological temperature (310 K). Particular attention has been paid to electrostatic properties that modulate the formation of a complex comprised of the nanoparticle together with surrounding ions and water. We focus on Au144 nanoparticles that comprise a nearly spherical Au core (diameter ?2 nm), a passivating Au?S interface, and functionalized alkanethiol chains. Cationic and anionic AuNPs have been modeled with amine and carboxyl terminal groups and Cl?/Na+ counterions, respectively. The radial distribution functions show that the side chains and terminal groups show significant flexibility. The orientation of water is distinct in the first solvation shell, and AuNPs cause a long-range effect in the solvent structure. The radial electrostatic potential displays a minimum for AuNP? at 1.9 nm from the center of the nanoparticle, marking a preferable location for Na+, while the AuNP+ potential (affecting the distribution of Cl?) rises almost monotonically with a local maximum. Comparison to Debye?Hückel theory shows very good agreement for radial ion distribution, as expected, with a Debye screening length of about 0.2?0.3 nm. Considerations of zeta potential predict that both anionic and cationic AuNPs avoid coagulation. The results highlight the importance of long-range electrostatic interactions in determining nanoparticle properties in aqueous solutions. They suggest that electrostatics is one of the central factors in complexation of AuNPs with other nanomaterials and biological systems, and that effects of electrostatics as water-mediated interactions are relatively long-ranged, which likely plays a role in, e.g., the interplay between nanoparticles and lipid membranes that surround cells. Charged monolayer-protected gold nanoparticles (AuNPs) have been studied in aqueous solution by performing atomistic molecular dynamics simulations at physiological temperature (310 K). Particular attention has been paid to electrostatic properties that modulate the formation of a complex comprised of the nanoparticle together with surrounding ions and water. We focus on Au144 nanoparticles that comprise a nearly spherical Au core (diameter ?2 nm), a passivating Au?S interface, and functionalized alkanethiol chains. Cationic and anionic AuNPs have been modeled with amine and carboxyl terminal groups and Cl?/Na+ counterions, respectively. The radial distribution functions show that the side chains and terminal groups show significant flexibility. The orientation of water is distinct in the first solvation shell, and AuNPs cause a long-range effect in the solvent structure. The radial electrostatic potential displays a minimum for AuNP? at 1.9 nm from the center of the nanoparticle, marking a preferable location for Na+, while the AuNP+ potential (affecting the distribution of Cl?) rises almost monotonically with a local maximum. Comparison to Debye?Hückel theory shows very good agreement for radial ion distribution, as expected, with a Debye screening length of about 0.2?0.3 nm. Considerations of zeta potential predict that both anionic and cationic AuNPs avoid coagulation. The results highlight the importance of long-range electrostatic interactions in determining nanoparticle properties in aqueous solutions. They suggest that electrostatics is one of the central factors in complexation of AuNPs with other nanomaterials and biological systems, and that effects of electrostatics as water-mediated interactions are relatively long-ranged, which likely plays a role in, e.g., the interplay between nanoparticles and lipid membranes that surround cells.

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Authors

  • Elena Heikkilä

  • Andrey a. Gurtovenko

  • Hector Martinez-Seara

  • Hannu Häkkinen

  • Ilpo Vattulainen

  • Jaakko Akola

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