Ligand-modulated interactions between charged monolayer-protected Au144(SR)60 gold nanoparticles in physiological saline

In order to determine how functionalized gold nanoparticles (AuNPs) interact in a near-physiological environment, we performed all-atom molecular dynamics simulations on the icosahedral Au144 nanoparticles each coated with a homogeneous set of 60 thiolates selected from one of these five (5) types: 11-mercapto-1-undecanesulfonate –SC11H22(SO3−), 5-mercapto-1-pentanesulfonate –SC5H10(SO3−), 5-mercapto-1-pentaneamine –SC5H10(NH3+), 4-mercapto-benzoate –SPh(COO−), or 4-mercapto-benzamide –SPh(CONH3+). These thiolates were selected to elucidate how the aggregation behavior of AuNPs depends on ligand parameters, including the charge of the terminal group (anionic vs. cationic), and its length and conformational flexibility. For this purpose, each functionalized AuNP was paired with a copy of itself, placed in an aqueous cell, neutralized by 120 Na+/Cl− counter-ions and salinated with a 150 mM concentration of NaCl, to form five (5) systems of like-charged AuNPs pairs in a saline. We computed the potential of mean force (the reversible work of separation) as a function of the intra-pair distance and, based on which, the aggregation affinities. We found that the AuNPs coated with negatively charged, short ligands have very high affinities. Structurally, a significant number of Na+ counter-ions reside on a plane between the AuNPs, mediating the interaction. Each such ion forms a “salt bridge” (or “ionic bonds”) to both of the AuNPs when they are separated by its diameter plus 0.2–0.3 nm. The positively charged AuNPs have much weaker affinities, as Cl− counter-ions form fewer and weaker salt bridges between the AuNPs. In the case of Au144(SC11H22(SO3−))60 pair, the flexible ligands fluctuate much more than the other four cases. The large fluctuations disfavor the forming of salt bridges between two AuNPs, but enable hydrophobic contact between the exposed hydrocarbon chains of the two AuNPs, which are subject to an effective attraction at a separation much greater than the AuNP diameter and involve a higher concentration of counter ions in the inter-pair space.