The chemistry of the sulfur-gold interface: in search of a unified model

Thiol-Stabilized Gold Nanoparticles: New Ways To Displace Thiol Layers Using Yttrium or Lanthanide Chlorides

We use the aurophilic interactions shown by lanthanides to overcome the sulfur-gold interaction. UV-vis and X-ray photoelectron spectroscopy confirm that yttrium or lanthanide chlorides easily displace sulfur ligands from the surface of thiol-stabilized gold nanoparticles.

Complex molecular surfaces and interfaces: concluding remarks

This paper is derived from our concluding remarks presentation and the ensuing conversations at the Faraday Discussions meeting on Complex Molecular Surfaces and Interfaces, Sheffield, UK, 24th-26th July 2017. This meeting was comprised of sessions on understanding the interaction of molecules with surfaces and their subsequent organisation, reactivity or properties from both experimental and theoretical perspectives. This paper attempts to put these presentations in the wider context and focuses on topics that were debated during the meeting and where we feel that opportunities lie for the future development of this interdisciplinary research area.

Improving gold nanorod delivery to the central nervous system by conjugation to the shuttle Angiopep-2

AIM

To improve the in vivo delivery of gold nanorods (GNRs) to the central nervous system of rats, these gold nanoparticles were conjugated to Angiopep-2, a shuttle peptide that can cross the blood-brain barrier.

MATERIALS & METHODS

GNRs were synthesized and modified using polyethylene glycol and Angiopep-2 (GNR-PEG-Angiopep-2). The physicochemical properties, in vitro cytotoxicity and ex vivo biodistribution of the conjugate were examined.

RESULTS

GNR-PEG-Angiopep-2 was stable over the following days, and the different concentrations that were tested did not affect the viability of microvascular endothelial cells. The conjugation of Angiopep-2 to GNRs enhanced the endocytosis of these particles (in vitro) and the accumulation in brains (in vivo), when compared with GNRs modified only with PEG.

CONCLUSION

This study provides evidence that Angiopep-2 improves the delivery of GNRs to the brain parenchyma. This property is highly relevant for future applications of GNRs as platforms for photothermal and theranostic purposes.

Some Theoretical and Experimental Insights on the Mechanistic Routes Leading to the Spontaneous Grafting of Gold Surfaces by Diazonium Salts

The spontaneous grafting of diazonium salts on gold may involve the carbocation obtained by heterolytic dediazonation and not necessarily the radical, as usually observed on reducing surfaces. The mechanism is addressed on the basis of DFT calculations and experiments carried out under conditions where the carbocation and the radical are produced selectively. The calculations indicate that the driving force of the reaction leading from a gold cluster, used as a gold model surface, and the carbocation to the modified cluster is higher than that of the analogous reaction starting from the radical. The experiments performed under conditions of heterolytic dediazonation show the formation of thin films on the surface of gold. The grafting of a carbocation is therefore possible, but a mechanism where the cleavage of the Ar-N bond is catalyzed by the surface of gold cannot be excluded.

Structural Characterization of Biofunctionalized Gold Nanoparticles by Ultrahigh-Resolution Mass Spectrometry

Biofunctionalized gold nanoparticles (AuNPs) enable innovative translational research and development in biomedicine. Biomolecules such as peptides, proteins, lipids, and carbohydrates can be assembled onto AuNPs to yield nanomaterials with unique properties for applications in imaging, photothermal therapy, vaccination strategies, and drug delivery. The characterization of functionalized AuNPs still remains an analytical challenge that normally requires the combination of multiple techniques. Laser desorption/ionization (LDI) and matrix-assisted LDI (MALDI) have been applied successfully in combination with time-of-flight (TOF) mass spectrometry (MS) for the analysis of the surface chemistry of AuNPs functionalized with synthetic ligands, however only for ligands with a molecular mass limited to 1000 Da. TOF-MS-based approaches in addition exhibit limited performance in terms of mass resolution and MS/MS possibilities. To overcome these limitations, we designed an approach for the analysis of AuNPs based on ultrahigh resolution Fourier transform ion cyclotron resonance (FTICR) MS and a combination of LDI and MALDI. To illustrate the performance of the method, we present a comprehensive characterization of the surface chemistry of AuNPs conjugated via a thiol-ending linker to either the ovalbumin peptide (OVA 323-339), the Lewis X antigen (Galβ1-4[Fucα1-3]GlcNAcβ1) trisaccharide, the tetramannoside Manα1-2Manα1-2Manα1-3Manα1, or a mixture of both carbohydrates. Collision-induced dissociation (CID) was used to characterize the structure of pseudomolecular ions generated by LDI/MALDI in-depth. These included [M + H]⁺ and [M + Na]⁺, and importantly also [M + Au]⁺ and [M + 2Au-H]⁺ ions. This first observation of gold-containing pseudomolecular ions provides direct evidence for the Au-conjugation of ligands. In addition, we show the applicability of the method to monitor proteolytic cleavage of peptides that are conjugated to the AuNP surface. The presented LDI/MALDI-FTICR-MS and MS/MS approach will be applicable to the characterization of a wide range of functionalized AuNPs.

The Role of Thermal Activation and Molecular Structure on the Reaction of Molecular Surfaces

Though surface modifications of organic thin films dramatically improve optoelectronic device performance, chemistry at organic surfaces presents new challenges that are not seen in conventional inorganic surfaces. This work demonstrates that the subsurface of pentacene remains highly accessible, even to large adsorbates, and that three distinct reaction regimes (surface, subsurface, and bulk) are accessed within the narrow thermal range of 30-75 °C. Progression of this transition is quantitatively measured via polarization modulation infrared reflection absorption spectroscopy, and atomic force microscopy is used to measure the thin-film morphology. Together, they reveal the close relationship between the extent of the reaction and the morphology changes. Finally, the reaction kinetics of the pentacene thin film is measured with a series of adsorbates that have different reactivity and diffusivity in the thin film. The results suggest that reaction kinetics in the thin film is controlled by both the reactivity and the adsorbate diffusivity in the thin-film lattice, which is very different than the traditional solution kinetics that is dominated by the chemical activation barriers. Combined, these experiments guide efforts toward rationally functionalizing the surfaces of organic semiconductors to enable the next generation of flexible devices.

Nanoscale Surface Curvature Effects on Ligand-Nanoparticle Interactions: A Plasmon-Enhanced Spectroscopic Study of Thiolated Ligand Adsorption, Desorption, and Exchange on Gold Nanoparticles

The interfacial adsorption, desorption, and exchange behaviors of thiolated ligands on nanotextured Au nanoparticle surfaces exhibit phenomenal site-to-site variations essentially dictated by the local surface curvatures, resulting in heterogeneous thermodynamic and kinetic profiles remarkably more sophisticated than those associated with the self-assembly of organothiol ligand monolayers on atomically flat Au surfaces. Here we use plasmon-enhanced Raman scattering as a spectroscopic tool combining time-resolving and molecular fingerprinting capabilities to quantitatively correlate the ligand dynamics with detailed molecular structures in real time under a diverse set of ligand adsorption, desorption, and exchange conditions at both equilibrium and nonequilibrium states, which enables us to delineate the effects of nanoscale surface curvature on the binding affinity, cooperativity, structural ordering, and the adsorption/desorption/exchange kinetics of organothiol ligands on colloidal Au nanoparticles. This work provides mechanistic insights on the key thermodynamic, kinetic, and geometric factors underpinning the surface curvature-dependent interfacial ligand behaviors, which serve as a central knowledge framework guiding the site-selective incorporation of desired surface functionalities into individual metallic nanoparticles for specific applications.

Post-printing surface modification and functionalization of 3D-printed biomedical device

3D printing is a technology well-suited for biomedical applications due to its ability to create highly complex and arbitrary structures from personalized designs with a fast turnaround. However, due to a limited selection of 3D-printable materials, the biofunctionality of many 3D-printed components has not been paid enough attention. In this perspective, we point out that post-3D printing modification is the solution that could close the gap between 3D printing technology and desired biomedical functions. We identify architectural reconfiguration and surface functionalization as the two main post-3D printing modification processes and discuss potential techniques for post-3D printing modification to achieve desired biofunctionality.

Silanization of Sapphire Surfaces for Optical Sensing Applications

Well-characterized silane layers are essential for optimized attachment of (bio)molecules enabling reliable chem/biosensor performance. Herein, binding properties and orientation of 3-mercaptopropyltrimethoxysilane layers at crystalline sapphire (0001) surfaces were determined by water contact angle measurements, infrared reflection absorption spectroscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. Infrared reflection absorption spectroscopy measurements suggest an almost perpendicular arrangement of the MPTMS molecules to the substrate surface. Adhesion force studies between a silicon nitride AFM tip and modified sapphire, gold, and silicon dioxide substrates were investigated by peak force tapping atomic force microscopy and used to define the silane binding properties on these surfaces. As expected, the Al-O-Si bond was determined to be responsible for the layer formation at the sapphire substrate surface.

Size specific emission in peptide capped gold quantum clusters with tunable photoswitching behavior

Three different types of fluorescent gold clusters (namely blue, green and red emitting) have been prepared from a gold precursor (chloroauric acid) under moderate conditions in aqueous medium. A cysteine containing dipeptide has been used for the formation of these quantum clusters as this peptide molecule contains a thiol group in the side chain to cap these nascently formed clusters and the free amino and carboxylic moieties assist in water solubility. Thus, the clusters are also environmentally friendly as the capped peptide is made up of only naturally occurring protein amino acids. These clusters have been well characterized by using UV-visible, fluorescence, X-ray photoelectron spectroscopy (XPS)spectroscopy, matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) and ultrahigh resolution field emission gun-transmission electron microscopy (UHR-FEG-TEM). Arrangements of gold atoms and their interaction with the corresponding ligands in three different fluorescent clusters have been predicted computationally. The excited state behavior of three different clusters has also been studied using time dependent density functional theory (TD-DFT). Time correlated single photon counting (TCSPC) and computational studies suggest intersystem crossing (S₁ → T₁) in the case of red-emitting Au₂₃ clusters. Interestingly, these gold clusters exhibit semiconducting and photoswitching properties (I_on/I_off), which are shown to be controlled by varying the size of these clusters. This holds future promise of using these gold cluster based nanomaterials for optoelectronic applications.

Towards an all-in fiber photodetector by directly bonding few-layer molybdenum disulfide to a fiber facet

Although photodetectors based on two dimensional (2D) materials have been intensively studied, there are few reports of optical fiber compatible devices. Herein we successfully fabricated an all-in fiber photodetector (FPD) based on an end-face bonded with few-layer molybdenum disulfide (MoS₂). Our FPD has a considerably high photo-responsivity of ∼0.6 A W^-1 at a bias voltage of 4 V and 0.01 A W^-1 under the bias-free conditions. We believe that the proposed platform may provide a new strategy for the integration of 2D materials in fibers and realization of optoelectronic and sensing applications.

Self-Assembled Gold Nanoparticle-Fluorescent Protein Conjugates as Platforms for Sensing Thiolate Compounds via Modulation of Energy Transfer Quenching

The ability of Au and other metal nanostructures to strongly quench the fluorescence of proximal fluorophores (dyes and fluorescent proteins) has made AuNP conjugates attractive for use as platforms for sensor development based on energy transfer interactions. In this study, we first characterize the energy transfer quenching of mCherry fluorescent proteins immobilized on AuNPs via metal-histidine coordination, where parameters such as NP size and number of attached proteins are varied. Using steady-state and time-resolved fluorescence measurements, we recorded very high mCherry quenching, with efficiency reaching ∼95-97%, independent of the NP size or number of bound fluorophores (i.e., conjugate valence). We further exploited these findings to develop a solution phase sensing platform targeting thiolate compounds. Energy transfer (ET) was employed as a transduction mechanism to monitor the competitive displacement of mCherry from the Au surface upon the introduction of varying amounts of thiolates with different size and coordination numbers. Our results show that the competitive displacement of mCherry depends on the thiolate concentration, time of reaction, and type of thiol derivatives used. Further analysis of the PL recovery data provides a measure for the equilibrium dissociation constant (Kd-1) for these compounds. These findings combined indicate that the AuNP-fluorescent protein conjugates may offer a potentially useful platform for thiol sensing both in solution and in cell cultures.

Controllable gold-capped nanoporous anodic alumina chip for label-free, specific detection of bacterial cells

A gold-capped nanostructured PAA sensor that uses aptamers detected bacterial cells in a quantitative manner with high specificities on a single chip.

The role of the crystalline face in the ordering of 6-mercaptopurine self-assembled monolayers on gold

Well-ordered molecular films play an important role in nanotechnology, from device fabrication to surface patterning. Self-assembled monolayers (SAMs) of 6-mercaptopurine (6MP) on the Au(100)-(1 × 1) and Au(111)-(1 × 1) have been used to understand the interplay of molecule-substrate interactions for heterocyclic thiols capable of binding to the surface by two anchors, which spontaneously form a highly disordered film on Au(111). Our results reveal that for the same surface coverage the simple change of the substrate from Au(111)-(1 × 1) to Au(100)-(1 × 1) eliminates molecular disorder and yields well-ordered SAMs. We discuss these findings in terms of differences in the surface mobility of 6MP species on these surfaces, the energetics of the adsorption sites, and the number of degrees of freedom of these substrates for a molecule with reduced surface mobility resulting from its two surface anchors. These results reveal the presence of subtle molecule-substrate interactions involving the heteroatom that drastically alter SAM properties and therefore strongly impact on our ability to control physical properties and to build devices at the nanoscale.

Determining the composition of gold nanoparticles: a compilation of shapes, sizes, and calculations using geometric considerations

ABSTRACT

Size, shape, overall composition, and surface functionality largely determine the properties and applications of metal nanoparticles. Aside from well-defined metal clusters, their composition is often estimated assuming a quasi-spherical shape of the nanoparticle core. With decreasing diameter of the assumed circumscribed sphere, particularly in the range of only a few nanometers, the estimated nanoparticle composition increasingly deviates from the real composition, leading to significant discrepancies between anticipated and experimentally observed composition, properties, and characteristics. We here assembled a compendium of tables, models, and equations for thiol-protected gold nanoparticles that will allow experimental scientists to more accurately estimate the composition of their gold nanoparticles using TEM image analysis data. The estimates obtained from following the routines described here will then serve as a guide for further analytical characterization of as-synthesized gold nanoparticles by other bulk (thermal, structural, chemical, and compositional) and surface characterization techniques. While the tables, models, and equations are dedicated to gold nanoparticles, the composition of other metal nanoparticle cores with face-centered cubic lattices can easily be estimated simply by substituting the value for the radius of the metal atom of interest.

GRAPHICAL ABSTRACT

Reversible Trapping of Functional Molecules at Interfaces Using Diazonium Salts Chemistry

Developing thin polymeric films for trapping, releasing, delivering, and sensing molecules is important for many applications in chemistry, biotechnology, and environment. Hence, a facile and scalable technique for loading specific molecules on surfaces would rapidly translate into applications. This work presents a novel method for the trapping of functional molecules at interfaces by exploiting diazonium salt chemistry. We demonstrate the efficiency of this approach by trapping two different molecules, 4-nitrobenzophenone and paracetamol, within polycarboxyphenyl layers grafted on gold and glassy carbon (GC) and by releasing them in acidic medium. The former molecule was chosen as a proof of concept for its electrochemical and spectroscopic properties, and the latter one was selected as an example of a pharmaceutical molecule. Advantages of the present approach rely on the simplicity, rapidity, and efficiency of the procedure for the reversible, on demand, trapping and release of functional molecules.

Methacrylate Polymer Monoliths for Separation Applications

This review summarizes the development of methacrylate-based polymer monoliths for separation science applications. An introduction to monoliths is presented, followed by the preparation methods and characteristics specific to methacrylate monoliths. Both traditional chemical based syntheses and emerging additive manufacturing methods are presented along with an analysis of the different types of functional groups, which have been utilized with methacrylate monoliths. The role of methacrylate based porous materials in separation science in industrially important chemical and biological separations are discussed, with particular attention given to the most recent developments and challenges associated with these materials. While these monoliths have been shown to be useful for a wide variety of applications, there is still scope for exerting better control over the porous architectures and chemistries obtained from the different fabrication routes. Conclusions regarding this previous work are drawn and an outlook towards future challenges and potential developments in this vibrant research area are presented. Discussed in particular are the potential of additive manufacturing for the preparation of monolithic structures with pre-defined multi-scale porous morphologies and for the optimization of surface reactive chemistries.

The bright side of plasmonic gold nanoparticles; activation of Nrf2, the cellular protective pathway

Plasmonic gold nanoparticles (AuNPs) are widely investigated for cancer therapy, due to their ability to strongly absorb light and convert it to heat and thus selectively destroy tumor cells. In this study we shed light on a new aspect of AuNPs and their plasmonic excitation, wherein they can provide anti-oxidant and anti-inflammatory protection by stimulating the cellular protective Nrf2 pathway. Our study was carried out on cells of the immune system, macrophages, and on skin cells, keratinocytes. A different response to AuNPs was noted in the two types of cells, explained by their distinct uptake profiles. In keratinocytes, the exposure to AuNPs, even at low concentrations, was sufficient to activate the Nrf2 pathway, without any irradiation, due to the presence of free AuNPs inside the cytosol. In contrast, in macrophages, the plasmonic excitation of the AuNPs by a low, non-lethal irradiation dose was required for their release from the constraining vesicles. The mechanism by which AuNPs activate the Nrf2 pathway was studied. Direct and indirect activation were suggested, based on the inherent ability of the AuNPs to react with thiol groups and to generate reactive oxygen species, in particular, under plasmonic excitation. The ability of AuNPs to directly activate the Nrf2 pathway renders them good candidates for treatment of disorders in which the up-regulation of Nrf2 is beneficial, specifically for topical treatment of inflammatory skin diseases.

AFM Imaging of Mercaptobenzoic Acid on Au(110): Submolecular Contrast with Metal Tips

A self-assembled monolayer of mercaptobenzoic acid (MBA) on Au(110) is investigated with scanning tunneling and atomic force microscopy (STM and AFM) and density functional calculations. High-resolution AFM images obtained with metallic tips show clear contrasts between oxygen atoms and phenyl moieties. The contrast above the oxygen atoms is due to attractive covalent interactions, which is different than previously reported high-resolution images, where Pauli repulsion dominated the image contrast. We show that the bonding of MBA to the substrate occurs mainly through dispersion interactions, whereas the thiol-Au bond contributes only a quarter of the adsorption energy. No indication of Au adatoms mediating the thiol-Au interaction was found in contrast to other thiol-bonded systems. However, MBA lifts the Au(110)-(2 × 1) reconstruction.

Gold-Mediated Exfoliation of Ultralarge Optoelectronically-Perfect Monolayers

Gold-mediated exfoliation of ultralarge optoelectronically perfect monolayers with lateral dimensions up to ≈500 μm is reported. Electrical, optical, and X-ray photo-electron spectroscopy characterization show that the quality of the gold-exfoliated flakes is similar to that of tape-exfoliated flakes. Large-area flakes allow manufacturing of large-area mono-layer transition metal dichalcogenide electronics.

Instrument-Free Synthesizable Fabrication of Label-Free Optical Biosensing Paper Strips for the Early Detection of Infectious Keratoconjunctivitides

We introduce a surface-enhanced Raman scattering (SERS)-functionalized, gold nanoparticle (GNP)-deposited paper strip capable of label-free biofluid sensing for the early detection of infectious eye diseases. The GNP biosensing paper strip was fabricated by the direct synthesis and deposition of GNPs on wax-divided hydrophilic areas of a permeable porous substrate through a facile, power-free synthesizable, and highly reproducible successive ionic layer absorption and reaction (SILAR) technique. To maximize localized surface plasmon resonance-generated SERS activity, the concentration of the reactive solution and number of SILAR cycles were optimized by controlling the size and gap distance of GNPs and verified by computational modeling with geometrical hypotheses of Gaussian-estimated metallic nanoparticles. The responses of our SERS-functionalized GNP paper strip to Raman intensities exhibited an enhancement factor of 7.8 × 10(8), high reproducibility (relative standard deviation of 7.5%), and 1 pM 2-naphthalenethiol highly sensitive detection limit with a correlation coefficient of 0.99, achieved by optimized SILAR conditions including a 10/10 mM/mM HAuCl4/NaBH4 concentration and six SILAR cycles. The SERS-functionalized GNP paper is supported by a multivariate statistics-preprocessed machine learning-judged bioclassification system to provide excellent label-free chemical structure sensitivity for identifying infectious keratoconjunctivitis. The power-free synthesizable fabrication, label-free, rapid analysis, and high sensitivity feature of the SILAR-fabricated SERS-functionalized GNP biosensing paper strip makes it an excellent alternative in point-of-care applications for the early detection of various infectious diseases.

Controlling the Interaction and Non-Close-Packed Arrangement of Nanoparticles on Large Areas

In light of the importance of nanostructured surfaces for a variety of technological applications, the quest for simple and reliable preparation methods of ordered, nanometer ranged structures is ongoing. Herein, a versatile method to prepare ordered, non-close-packed arrangements of nanoparticles on centimeter sized surfaces by self-assembly is described using monodisperse (118-162 nm Ø), amino-functionalized silica nanoparticles as an exploratory example. It is shown that the arrangement of the particles is governed by the interplay between the electrostatic repulsion between the particles and the interaction between particles and surfaces. The latter is tuned by the properties of the particles such as their surface roughness as well as the chemistry of the linkage. Weak dispersive interactions between amino groups and gold surfaces are compared to a covalent amide linkage of the amino groups with carboxylic acid functionalized self-assembled monolayers. It was shown that the order of the former systems may suffer from capillary forces between particles during the drying process, while the covalently bonded systems do not. In turn, covalently bonded systems can be dried quickly, while the van der Waals bonded systems require a slow drying process to minimize aggregation. These highly ordered structures can be used as templates for the formation of a second, ordered, non-close-packed layer of nanoparticles exemplified for larger polystyrene particles (Ø 368 ± 14 nm), which highlights the prospect of this approach as a simple preparation method for ordered arrays of nanoparticles with tunable properties.

Semiconductor-Metal Nanofloret Hybrid Structures by Self-Processing Synthesis

We present a synthetic strategy that takes advantage of the inherent asymmetry exhibited by semiconductor nanowires prepared by Au-catalyzed chemical vapor deposition (CVD). The metal-semiconductor junction is used for activating etch, deposition, and modification steps localized to the tip area using a wet-chemistry approach. The hybrid nanostructures obtained for the coinage metals Cu, Ag, and Au resemble the morphology of grass flowers, termed here Nanofloret hybrid nanostructures consisting of a high aspect ratio SiGe nanowire (NW) with a metallic nanoshell cap. The synthetic method is used to prepare hybrid nanostructures in one step by triggering a programmable cascade of events that is autonomously executed, termed self-processing synthesis. The synthesis progression was monitored by ex situ transmission electron microscopy (TEM), in situ scanning transmission electron microscopy (STEM) and inductively coupled plasma mass spectrometry (ICP-MS) analyses to study the mechanistic reaction details of the various processes taking place during the synthesis. Our results indicate that the synthesis involves distinct processing steps including localized oxide etch, metal deposition, and process termination. Control over the deposition and etching processes is demonstrated by several parameters: (i) etchant concentration (water), (ii) SiGe alloy composition, (iii) reducing agent, (iv) metal redox potential, and (v) addition of surfactants for controlling the deposited metal grain size. The NF structures exhibit broad plasmonic absorption that is utilized for demonstrating surface-enhanced Raman scattering (SERS) of thiophenol monolayer. The new type of nanostructures feature a metallic nanoshell directly coupled to the crystalline semiconductor NW showing broad plasmonic absorption.

Ultrastable polyethyleneimine-stabilized gold nanoparticles modified with polyethylene glycol for blood pool, lymph node and tumor CT imaging

Development of new long-circulating contrast agents for computed tomography (CT) imaging of different biological systems still remains a great challenge. Here, we report the design and synthesis of branched polyethyleneimine (PEI)-stabilized gold nanoparticles (Au PSNPs) modified with polyethylene glycol (PEG) for blood pool, lymph node, and tumor CT imaging. In this study, thiolated PEI was first synthesized and used as a stabilizing agent to form AuNPs. The formed Au PSNPs were then grafted with PEG monomethyl ether via PEI amine-enabled conjugation chemistry, followed by acetylation of the remaining PEI surface amines. The formed PEGylated Au PSNPs were characterized via different methods. We show that the PEGylated Au PSNPs with an Au core size of 5.1 nm have a relatively long half-decay time (7.8 h), and display a better X-ray attenuation property than conventionally used iodine-based CT contrast agents (e.g., Omnipaque), and are hemocompatible and cytocompatible in a given concentration range. These properties of the Au PSNPs afford their uses as a contrast agent for effective CT imaging of the blood pool and major organs of rats, lymph node of rabbits, and the xenografted tumor model of mice. Importantly, the PEGylated Au PSNPs could be excreted out of the body with time and also showed excellent in vivo stability. These findings suggest that the formed PEGylated Au PSNPs may be used as a promising contrast agent for CT imaging of different biological systems.

Surface-site reactivity in small-molecule adsorption: A theoretical study of thiol binding on multi-coordinated gold clusters

BACKGROUND

The adsorption of organic molecules on metal surfaces has a broad array of applications, from device engineering to medical diagnosis. The most extensively investigated class of metal-molecule complexes is the adsorption of thiols on gold.

RESULTS

In the present manuscript, we investigate the dependence of methylthiol adsorption structures and energies on the degree of unsaturation at the metal binding site. We designed an Au20 cluster with a broad range of metal site coordination numbers, from 3 to 9, and examined the binding conditions of methylthiol at the various sites.

CONCLUSION

We found that despite the small molecular size, the dispersive interactions of the backbone are a determining factor in the molecular affinity for various sites. Kink sites were preferred binding locations due to the availability of multiple surface atoms for dispersive interactions with the methyl groups, whereas tip sites experienced low affinity, despite having low coordination numbers.

Theoretical exploration of seleno and tellurophenols as promising alternatives to sulfur ligands for anchoring to gold (111) materials

Selenium and tellurium ligands are proposed as improved alternatives to sulfur ligands as a new strategy to anchor functional molecules to gold based materials.

Influence of steric hindrance on the molecular packing and the anchoring of quinonoid zwitterions on gold surfaces

The N-substituent on quinonoid zwitterions influences the molecules packing and impacts their anchoring on gold surfaces.

Analyte-concentrating 3D hybrid plasmonic nanostructures for use in highly sensitive chemical sensors

This paper characterizes the analyte-concentrating effects of 3D porous Ag hybrid nanostructures to develop ultrasensitive SERS-based toxin sensors.

Properties of the gold-sulphur interface: from self-assembled monolayers to clusters

The gold-sulphur interface of self-assembled monolayers (SAMs) was extensively studied some time ago. More recently tremendous progress has been made in the preparation and characterization of thiolate-protected gold clusters. In this feature article we address different properties of the two systems such as their structure, the mobility of the thiolates on the surface and other dynamical aspects, the chirality of the structures and characteristics related to it and their vibrational properties. SAMs and clusters are in the focus of different communities that typically use different experimental approaches to study the respective systems. However, it seems that the nature of the Au-S interfaces in the two cases is quite similar. Recent single crystal X-ray structures of thiolate-protected gold clusters reveal staple motifs characterized by gold ad-atoms sandwiched between two sulphur atoms. This finding contradicts older work on SAMs. However, newer studies on SAMs also reveal ad-atoms. Whether this finding can be generalized remains to be shown. In any case, more and more studies highlight the dynamic nature of the Au-S interface, both on flat surfaces and in clusters. At temperatures slightly above ambient thiolates migrate on the gold surface and on clusters. Evidence for desorption of thiolates at room temperature, at least under certain conditions, has been demonstrated for both systems. The adsorbed thiolate can lead to chirality at different lengths scales, which has been shown both on surfaces and for clusters. Chirality emerges from the organization of the thiolates as well as locally at the molecular level. Chirality can also be transferred from a chiral surface to an adsorbate, as evidenced by vibrational spectroscopy.

Subnanometre ligand-shell asymmetry leads to Janus-like nanoparticle membranes

Self-assembly of nanoparticles at fluid interfaces has emerged as a simple yet efficient way to create two-dimensional membranes with tunable properties. In these membranes, inorganic nanoparticles are coated with a shell of organic ligands that interlock as spacers and provide tensile strength. Although curvature due to gradients in lipid-bilayer composition and protein scaffolding is a key feature of many biological membranes, creating gradients in nanoparticle membranes has been difficult. Here, we show by X-ray scattering that nanoparticle membranes formed at air/water interfaces exhibit a small but significant ∼6 Å difference in average ligand-shell thickness between their two sides. This affects surface-enhanced Raman scattering and can be used to fold detached free-standing membranes into tubes by exposure to electron beams. Molecular dynamics simulations elucidate the roles of ligand coverage and mobility in producing and maintaining this asymmetry. Understanding this Janus-like membrane asymmetry opens up new avenues for designing nanoparticle superstructures.

SCC-DFTB parameters for simulating hybrid gold-thiolates compounds

We present a parametrization of a self-consistent charge density functional-based tight-binding scheme (SCC-DFTB) to describe gold-organic hybrid systems by adding new Au-X (X = Au, H, C, S, N, O) parameters to a previous set designed for organic molecules. With the aim of describing gold-thiolates systems within the DFTB framework, the resulting parameters are successively compared with density functional theory (DFT) data for the description of Au bulk, Aun gold clusters (n = 2, 4, 8, 20), and Aun SCH3 (n = 3 and 25) molecular-sized models. The geometrical, energetic, and electronic parameters obtained at the SCC-DFTB level for the small Au3 SCH3 gold-thiolate compound compare very well with DFT results, and prove that the different binding situations of the sulfur atom on gold are correctly described with the current parameters. For a larger gold-thiolate model, Au25 SCH3 , the electronic density of states and the potential energy surfaces resulting from the chemisorption of the molecule on the gold aggregate obtained with the new SCC-DFTB parameters are also in good agreement with DFT results.

The influence of thiolate readsorption on the quality of mixed monolayers formed through an electrochemcial method

Lateral Force Microscopy (LFM) was used to probe the quality of binary mixed monolayers formed on planar polycrystalline gold through an electrochemical method. In the approach, portions of a self-assembled monolayer (SAM) composed of 2-aminoethanethiol (AET) were removed from the Au(111) surface facets by selective reductive desorption which maintained undisrupted regions of AET elsewhere on the polycrystalline surface. Monolayer voids created by this method were backfilled with 11-mercaptoundecanoic acid (MUA) and the interface characterized with ex situ LFM. This produced images with domains of high and low friction corresponding to isolated zones of MUA and AET respectively. Reverse sequence mixed monolayers were also prepared with MUA as the starting layer and rendered LFM images that mirrored the AET based layers. This demonstrates flexibility of the electrochemical method to produce heterogeneous binary SAMs, and to further probe the quality of mixed monolayers, a number of experimental conditions including desorption time, electrode configuration, and initial incubation period were studied. AET/MUA layers that produced the most enhanced LFM images were formed on a planar electrode that was vertically submerged into the electrolyte while maintaining a selective desorption potential for 5 min before backfilling with MUA. This condition allowed for the effective diffusion of AET away from the interface and created well-defined monolayer voids for backfilling. At desorption times lower than 1 min, some of the AET molecules that remained near the interface would readsorb onto the surface and interfere with the backfilling process thereby creating lower contrast LFM images. Structural features of these layers were independent of initial incubation time (10 min and 16 h); however, the contrast between domains was improved when using AET layers formed over a longer incubation period. Interestingly, the contrast was significantly reduced when mixed layers were created on electrodes set in a hanging meniscus with the electrolyte. Here, electrochemical evidence pointed to prolonged readsorption of thiolates creating less well-defined voids for backfilling, and the event was most pronounced for MUA based layers.

Nanomechanics of bidentate thiolate ligands on gold surfaces

The effect of the chain length separating sulfur atoms in bidentate thiols attached to defective gold surfaces on the rupture of the respective molecule-gold junctions has been studied computationally. Thermal desorption always yields cyclic disulfides. In contrast, mechanochemical desorption leads to cyclic gold complexes, where metal atoms are extracted from the surface and kept in tweezer-like arrangements by the sulfur atoms. This phenomenon is rationalized in terms of directional mechanical manipulation of Au-Au bonds and Au-S coordination numbers. Moreover, the flexibility of the chain is shown to crucially impact on the mechanical strength of the junction.