Critical Sequence Dependence in Multicomponent Ligand Binding to Gold Nanoparticles

Multicomponent ligand interactions are involved in essentially all practical nanoparticle (NP) applications. Presented herein is the finding that multicomponent ligand binding to gold nanoparticles (AuNPs) can be highly dependent on the sequence of ligand mixing with AuNPs. Quantitative study revealed that the competitive adenine and glutathione (GSH) adsorption onto both as-synthesized and pegylated AuNPs is predominantly kinetically controlled, and adenine that binds only nonspecifically to AuNP adsorbs faster than GSH. This raises concerns about the validity of the popular practice in current NP research of using the Langmuir isotherm or its variants to model multicomponent ligand adsorption on NPs. Mechanistically, this sequence dependency is due to the fact that there is no spontaneous ligand desorption even for the model protein and small molecules that can bind only nonspecifically to AuNPs. The insights and experimental methods provided in this work should be important for molecular-level understanding of nanoparticle interfacial interactions

NaHS Induces Complete Nondestructive Ligand Displacement from Aggregated Gold Nanoparticles

Ligand displacement from gold is important for a series of gold nanoparticle (AuNP) applications. Complete nondestructive removal of organothiols from aggregated AuNPs is challenging due to the strong Au–S binding, the steric hindrance imposed by ligand overlayer on AuNPs, and the narrow junctions between the neighboring AuNPs. Presented herein is finding that monohydrogen sulfide (HS^–), an anionic thiol, induces complete and nondestructive removal of ligands from aggregated AuNPs. The model ligands include aliphatic (ethanethiol­(ET)) and aromatic monothiols, methylbenzenethiol (MBT), organodithiol (benzenedithiol (BDT)), thioamides (mercaptobenzimidazole (MBI) and thioguanine (TG)), and nonspecific ligand adenine. The threshold HS^– concentration to induce complete ligand displacement varies from 105 μM for MBI and TG to 60 mM for BDT. Unlike using HS^–, complete ligand displacement does not occur when mercaptoethanol, the smallest water-soluble organothiol, is used as the incoming ligand. Mechanistically, HS^– binding leads to the formation of sulfur monolayer on AuNPs that is characterized with S–S bonds and S–Au bonds, but with no detectable S–H spectral features. The empirical HS^– saturation packing density and Langmuir binding constant on AuNPs are 960 ± 60 pmol/cm² and (5.5 ± 0.8) × 10⁶ M^–1, respectively. The successful identification of an effective ligand capable of inducing complete and nondestructive removal of ligands from AuNPs should pave the way for using AuNP for capture-and-release enrichment of biomolecules that have high affinity to AuNP surfaces

Iodide-Induced Organothiol Desorption and Photochemical Reaction, Gold Nanoparticle (AuNP) Fusion, and SERS Signal Reduction in Organothiol-Containing AuNP Aggregates

Gold nanoparticles (AuNPs) have been used extensively as surface-enhanced Raman spectroscopic (SERS) substrates for their large SERS enhancements and widely believed chemical stability. Presented is the finding that iodide can rapidly reduce the SERS intensity of the ligands, including organothiols adsorbed on plasmonic AuNPs through both iodide-induced ligand desorption and AuNP fusion. The organothiols trapped inside the fused AuNPs have negligible SERS activities. Multiple photochemical processes were involved when organothiol-containing AuNP aggregates were treated with KI under photoillumination. The photocatalytically produced I₃^– reacts with both organothiol and AuNPs. Chloride and bromide also induce partial organothiol displacement and the fusion of the as-synthesized AuNPs, but neither of the two halides has detectable effects on the morphology and Raman signals of the organothiol-containing AuNP aggregates. The insight provided in this work should be important for the understanding of interfacial interactions of plasmonic AuNPs and their SERS applications

M-Chem: a modular software package for molecular simulation that spans scientific domains

We present a new software package called M-Chem that is designed from scratch in C++ and parallelised on shared-memory multi-core architectures to facilitate efficient molecular simulations. Currently, M-Chem is a fast molecular dynamics (MD) engine that supports the evaluation of energies and forces from two-body to many-body all-atom potentials, reactive force fields, coarse-grained models, combined quantum mechanics molecular mechanics (QM/MM) models, and external force drivers from machine learning, augmented by algorithms that are focused on gains in computational simulation times. M-Chem also includes a range of standard simulation capabilities including thermostats, barostats, multi-timestepping, and periodic cells, as well as newer methods such as fast extended Lagrangians and high quality electrostatic potential generation. At present M-Chem is a developer friendly environment in which we encourage new software contributors from diverse fields to build their algorithms, models, and methods in our modular framework. The long-term objective of M-Chem is to create an interdisciplinary platform for computational methods with applications ranging from biomolecular simulations, reactive chemistry, to materials research.</p