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 Au 144 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−Huckel 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. ■ INTRODUCTIONNanoparticles (NPs, size range 1−100 nm) have many interesting properties, as they bridge the gap between bulk materials and atomic or molecular structures.1,2 Typically, the physical properties of bulk materials do not depend on the size of the sample, while at the nanoscale size-dependent properties are frequently encountered. Two contributing factors for the size dependence are (a) number of surface atoms whose percentage reduces as the NP size increases toward the bulk limit and (b) quantum confinement effects at the smallest length scales (<10 nm) where the electronic structure plays a significant role in determining the composition, stability, structure, and function of NPs. 3,4 Nanoparticles often display fascinating optical properties because of quantum effects, and, e.g., gold nanoparticles (AuNPs) appear from yellow to deep red and black in solution depending on their size. 5 In photovoltaic cells, absorption of solar radiation is much higher for semiconductor materials comprised of NPs than for continuous sheets of thin films (e.g., CdTe, ZnO).6 For phase-change materials used in optical data storage and nonvolatile computer ...
Despite being chemically inert as a bulk material, nanoscale gold can pose harmful side effects to living organisms. In particular, cationic Au nanoparticles (AuNP +) of 2 nm diameter or less permeate readily through plasma membranes and induce cell death. We report atomistic simulations of cationic Au nanoparticles interacting with realistic membranes and explicit solvent using a model system that comprises two cellular compartments, extracellular and cytosolic, divided by two asymmetric lipid bilayers. The membrane−AuNP + binding and membrane reorganization processes are discovered to be governed by cooperative effects where AuNP + , counterions, water, and the two membrane leaflets all contribute. On the extracellular side, we find that the nanoparticle has to cross a free energy barrier of about 5 k B T prior forming a stable contact with the membrane. This results in a rearrangement of the zwitterionic lipids and nanoparticle side groups in the contact area, giving rise to the initial stage of pore formation on the membrane surface. Such behavior is not seen on the cytosolic side, where AuNP + is spontaneously captured by the negatively charged phosphatidylserine lipids that diffuse to enrich the membrane leaflet underneath AuNP + , further pointing to AuNP + accumulation on the inner leaflet of a plasma membrane. The results suggest AuNP + permeation to take place through the formation of a pore together with partial nanoparticle neutralization/deprotonation, leading to membrane disruption at higher nanoparticle concentrations. The data also suggest a potential mechanism for cytotoxicity as AuNP + binding to the extracellular leaflet may trigger apoptosis through translocation of phosphatidylserine.
Experimental observations indicate that the interaction between nanoparticles and lipid membranes varies according to the nanoparticle charge and the chemical nature of their protecting side groups. We report atomistic simulations of an anionic Au nanoparticle (AuNP(-)) interacting with membranes whose lipid composition and transmembrane distribution are to a large extent consistent with real plasma membranes of eukaryotic cells. To this end, we use a model system which comprises two cellular compartments, extracellular and cytosolic, divided by two asymmetric lipid bilayers. The simulations clearly show that AuNP(-) attaches to the extracellular membrane surface within a few tens of nanoseconds, while it avoids contact with the membrane on the cytosolic side. This behavior stems from several factors. In essence, when the nanoparticle interacts with lipids in the extracellular compartment, it forms relatively weak contacts with the zwitterionic head groups (in particular choline) of the phosphatidylcholine lipids. Consequently, AuNP(-) does not immerse deeply in the leaflet, enabling, e.g., lateral diffusion of the nanoparticle along the surface. On the cytosolic side, AuNP(-) remains in the water phase due to Coulomb repulsion that arises from negatively charged phosphatidylserine lipids interacting with AuNP(-). A number of structural and dynamical features resulting from these basic phenomena are discussed. We close the article with a brief discussion of potential implications.
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