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Surface Interactions between Gold Nanoparticles and Biochar.

ABSTRACT: Engineered nanomaterials are directly applied to the agricultural soils as a part of pesticide/fertilize formulations or sludge/manure amendments. No prior reports are available to understand the surface interactions between gold nanoparticles (nAu) and soil components, including the charcoal black carbon (biochar). Retention of citrate-capped nAu on 300-700 °C pecan shell biochars occurred rapidly and irreversibly even at neutral pH where retention was less favorable. Uniform organic (primarily citrate ligands) layer on nAu was observable by TEM, and was preserved after the retention by biochar, which resulted in the aggregation or alignment along the edges of multisheets composing biochar. Retention of nAu was (i) greater on biochars than a sandy loam soil, (ii) greater at higher ionic strength and lower pH, and (iii) pyrolysis temperature-dependent: 500 < 700 ≪ 300 °C at pH 3. Collectively, carboxyl-enriched 300 °C biochar likely formed strong hydrogen bonds with the citrate layer of nAu. The charge transfer between the conduction band of nAu and π* continuum of polyaromatic sheets is likely to dominate on 700 °C biochar. Surface area-normalized retention of nAu on biochars was several orders of magnitude higher than negatively charged hydroxyl-bearing environmental surfaces, indicating the importance of black carbon in the environmental fate of engineered nanomaterials.

SUBMITTER: Uchimiya M

PROVIDER: S-EPMC5503990 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description: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.

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Surface Interactions between Gold Nanoparticles and Biochar.

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