The Interaction of Amines with Gold Nanoparticles

Understanding the interactions between amines and the surface of gold nanoparticles is important because of their role in the stabilization of the nanosystems, in the formation of the protein corona, and in the preparation of semisynthetic nanozymes. By using fluorescence spectroscopy, electrochemistry, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and molecular simulation, a detailed picture of these interactions is obtained. Herein, it is shown that amines interact with surface Au(0) atoms of the nanoparticles with their lone electron pair with a strength linearly correlating with their basicity corrected for steric hindrance. The kinetics of binding depends on the position of the gold atoms (flat surfaces or edges) while the mode of binding involves a single Au(0) with nitrogen sitting on top of it. A small fraction of surface Au(I) atoms, still present, is reduced by the amines yielding a much stronger Au(0)-RN.+ (RN. , after the loss of a proton) interaction. In this case, the mode of binding involves two Au(0) atoms with a bridging nitrogen placed between them. Stable Au nanoparticles, as those required for robust semisynthetic nanozymes preparation, are better obtained when the protein is involved (at least in part) in the reduction of the gold ions.

SERS microsensors for pH measurements in the lumen and ECM of stem cell derived human airway organoids

Patient derived organoids have the potential to improve the physiological relevance of in vitro disease models. However, the 3D architecture of these self-assembled cellular structures makes probing their biochemistry more complex than in traditional 2D culture. We explore the application of surface enhanced Raman scattering microsensors (SERS-MS) to probe local pH gradients within patient derived airway organoid cultures. SERS-MS consist of solid polymer cores decorated with surface immobilised gold nanoparticles which are functionalised with pH sensitive reporter molecule 4-mercaptobenzoic acid (MBA). We demonstrate that by mixing SERS-MS into the extracellular matrix (ECM) of airway organoid cultures the probes can be engulfed by expanding organoids and report on local pH in the organoid lumen and ECM.

Development of Classical Force Fields for Interfaces between Single Molecules and Au

Interfaces between metals and organic materials play an essential role in molecular surface science, photovoltaics, or molecular electronics. Modeling the evolution of interface geometry over sufficiently long timescales requires an accurate parameterization of the relevant metal-molecule interactions. Here, we describe a method for calculating interface parameters from reference density functional theory calculations of small metal-molecule complexes. We apply this method to develop a parameter set for a series of metal-molecule-metal junctions. We study the dynamics of short oligophenyls with amine, methyl-sulfide, or direct Au-C links, which are bonded to Au(111) via small adatom structures. Nanosecond classical molecular dynamics simulations using the generated parameter set reveal insight into molecular degrees of freedom not accessible from ab initio molecular dynamics simulations.

Surfactant Induced Synthesis of LiAlH4 and NaAlH4 Nanoparticles for Hydrogen Storage

LiAlH4 and NaAlH4 are considered to be promising hydrogen storage materials due to their high hydrogen density. However, their practical use is hampered by the lack of hydrogen reversibility along with poor kinetics. Nanosizing is an effective strategy to enable hydrogen reversibility under practical conditions. However, this has remained elusive as the synthesis of alanate nanoparticles has not been explored. Herein, a simple solvent evaporation method is demonstrated to assemble alanate nanoparticles with the use of surfactants as a stabilizer. More importantly, the roles of the surfactants in enabling control over particle size and morphology was determined. Surfactants with long linear carbon chains and matching the hard character of alanates are more prone to lead to the formation of small particles of ~10 nm due to steric hindrance. This can result in significant shifts in the temperature for hydrogen release.

Nitrogen Donor Protection for Atomically Precise Metal Nanoclusters

Surface organic ligands are critical in dictating the structures and properties of atomically precise metal nanoclusters. In contrast to the conventionally used thiolate, phosphine and alkynyl ligands, nitrogen donor ligands have not been used in the protection for well-defined metal nanoclusters until recently. This review focuses on recent developments in atomically precise metal nanoclusters stabilized by different types of nitrogen donor ligands, in which the synthesis, total structure determination and various properties are covered. We hope that this review will provide insights into the rational design of N donor-protected metal nanoclusters in terms of structural and functional modulation.

Detection of a Double-Stranded MGMT Gene Using Electrochemically Reduced Graphene Oxide (ErGO) Electrodes Decorated with AuNPs and Peptide Nucleic Acids (PNA)

The ability to detect double-stranded DNA (dsDNA) as a biomarker without denaturing it to single-stranded DNA (ss-DNA) continues to be a major challenge. In this work, we report a sandwich biosensor for the detection of the ds-methylated MGMT gene, a potential biomarker for brain tumors and breast cancer. The purpose of this biosensor is to achieve simultaneous recognition of the gene sequence, as well as the presence of methylation. The biosensor is based on reduced graphene oxide (rGO) electrodes decorated with gold nanoparticles (AuNPs) and uses Peptide Nucleic Acid (PNA) that binds to the ds-MGMT gene. The reduction of GO was performed in two ways: electrochemically (ErGO) and thermally (TrGO). XPS and Raman spectroscopy, as well as voltammetry techniques, showed that the ErGO was more efficiently reduced, had a higher C/O ratio, showed a smaller crystallite size of the sp2 lattice, and was more stable during measurement. It was also revealed that the electro-deposition of the AuNPs was more successful on the ErGO surface due to the higher At% of Au on the ErGO electrode. Therefore, the ErGO/AuNPs electrode was used to develop biosensors to detect the ds-MGMT gene. PNA, which acts as a bio-recognition element, was used to form a self-assembled monolayer (SAM) on the ErGO/AuNPs surface via the amine-AuNPs interaction, recognizing the ds-MGMT gene sequence by its invasion of the double-stranded DNA and the formation of a triple helix. The methylation was then detected using biotinylated-anti-5mC, which was then measured using the amperometric technique. The selectivity study showed that the proposed biosensor was able to distinguish between blank, non-methylated, non-complementary, and target dsDNA spiked in mouse plasma. The LOD was calculated to be 0.86 pM with a wide linear range of 1 pM to 50 µM. To the best of our knowledge, this is the first report on using PNA to detect ds-methylated DNA. This sandwich design can be modified to detect other methylated genes, making it a promising platform to detect ds-methylated biomarkers.

Glucosamine Phosphate Induces AuNPs Aggregation and Fusion into Easily Functionalizable Nanowires

The challenge to obtain plasmonic nanosystems absorbing light in the near infrared is always open because of the interest that such systems pose in applications such as nanotherapy or nanodiagnostics. Here we describe the synthesis in an aqueous solution devoid of any surfactant of Au-nanowires of controlled length and reasonably narrow dimensional distribution starting from Au-nanoparticles by taking advantage of the properties of glucosamine phosphate under aerobic conditions and substoichiometric nanoparticle passivation. Oxygen is required to enable the process where glucosamine phosphate is oxidized to glucosaminic acid phosphate and H2O2 is produced. The process leading to the nanosystems comprises nanoparticles growth, their aggregation into necklace-like aggregates, and final fusion into nanowires. The fusion requires the consumption of H2O2. The nanowires can be passivated with an organic thiol, lyophilized, and resuspended in water without losing their dimensional and optical properties. The position of the broad surface plasmon band of the nanowires can be tuned from 630 to >1350 nm.

Probing the Sulfur-Modified Capping Layer of Gold Nanoparticles Using Surface Enhanced Raman Spectroscopy (SERS) Effects

Gold nanoparticles (AuNP) exhibit particular plasmonic properties when stimulated by visible light, which makes them a promising tool to many applications in sensor technology and biomedical applications, especially when associated to sulfur-based compounds. Sulfur species form a great variety of self-assembled structures that cap AuNP and this interaction rules the optical and plasmonic properties of the system. Here, we report the behavior of citrate-stabilized gold nanospheres in two distinct sulfur colloidal solutions, namely, thiocyanate and sulfide ionic solutions. Citrate-capped gold nanospheres were characterized using ultraviolet-visible (UV-Vis) absorption, transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and atomic force microscopy (AFM). In the presence of sulfur species, we have observed the formation of NP clusters and chain-like structures, giving rise to surface-enhanced effects. Surface-enhanced Raman spectroscopy (SERS) pointed to a modification in citrate vibrational modes, which suggests substitution of citrate by either thiocyanate or sulfide ions with distinct dynamics, as showed by in situ fluorescence. Moreover, we report the emergence of surface-enhanced infrared absorption (SEIRA) effect, which corroborates SERS conclusions. Further, SEIRA shows a great potential as a tool for specification of sulfur compounds in colloidal solutions, which is particularly useful when dealing with sensor technology.

The van der Waals Interactions of n-Alkanethiol-Covered Surfaces: From Planar to Curved Surfaces

The van der Waals (vdW) interactions of n‐alkanethiols (ATs) adsorbed on planar Au(111) and Au(100) surfaces and curved Au nanoparticles of different diameters are reported. By means of electrochemical measurements and molecular dynamic calculations, the increase in the average geometrical curvature of the surface influences the global interactions, that is, decreasing vdW interactions between neighboring molecules. Small NPs do not present the same electrochemical behavior as planar surfaces. The transition between nanoparticle to flat surface electrochemical response is estimated to occur at a circa 13–20 nm diameter range.

Plasmon-driven sub-picosecond breathing of metal nanoparticles

We present the first real-time atomistic simulation on the quantum dynamics of icosahedral silver nanoparticles under strong laser pulses, using time dependent density functional theory (TDDFT) molecular dynamics. We identify the emergence of sub-picosecond breathing-like radial oscillations starting immediately after laser pulse excitation, with increasing amplitude as the field intensity increases. The ultrafast dynamic response of nanoparticles to laser excitation points to a new mechanism other than equilibrium electron-phonon scattering previously assumed, which takes a much longer timescale. A sharp weakening of all bonds during laser excitation is observed, thanks to plasmon damping into excited electrons in anti-bonding states. This sudden weakening of bonds leads to a uniform expansion of the nanoparticles and launches coherent breathing oscillations.

Coating agent-induced mechanical behavior of 3D self-assembled nanocrystals

The Young's modulus of three-dimensional self-assembled Ag nanocrystals, as so-called supracrystals, is correlated with the type of coating agent as well as the nanocrystal morphology.

Trapping of Charged Gold Adatoms by Dimethyl Sulfoxide on a Gold Surface

We report the formation of dimethyl sulfoxide (DMSO) molecular complexes on Au(111) enabled by native gold adatoms unusually linking the molecules via a bonding of ionic nature, yielding a mutual stabilization between molecules and adatom(s). DMSO is a widely used polar, aprotic solvent whose interaction with metal surfaces is not fully understood. By combining X-ray photoelectron spectroscopy, low temperature scanning tunneling microscopy, and density functional theory (DFT) calculations, we show that DMSO molecules form complexes made by up to four molecules arranged with adjacent oxygen terminations. DFT calculations reveal that most of the observed structures are accurately reproduced if, and only if, the negatively charged oxygen terminations are linked by one or two positively charged Au adatoms. A similar behavior was previously observed only in nonstoichiometric organic salt layers, fabricated using linkage alkali atoms and strongly electronegative molecules. These findings suggest a motif for anchoring organic adlayers of polar molecules on metal substrates and also provide nanoscale insight into the interaction of DMSO with gold.