Monolayer-protected Gold Nanoclusters Research Articles

A theoretical study of the monolayer-protected gold cluster Au317(SR)110.

Significant efforts have been made to uncover the structures of monolayer-protected gold nanoclusters. However, the synthesis, crystallization, and structural analysis of gold nanoclusters with over 300 metal atoms is a grand challenge. In this work, a new gold nanocluster containing 317 gold atoms and 110 thiolate (SH) ligands (referred to as Au317(SH)110) is theoretically studied, which is larger in size than the formerly reported Au279(SR)84 cluster. The stability of the Au317(SH)110 cluster is studied based on calculations of the averaged cluster formation energy (Eave), indicating that Au317(SH)110 has good structural stability and that the SPhCOOH (p-MBA) ligand is a good candidate for stabilizing the cluster. The calculation of density of state and the time-dependent density functional theory (TD-DFT) calculations of the optical absorption properties show that Au317(SH)110 is in a metallic state.

A new strategy to construct gold nanocluster-based optical probes using luminescence resonance energy transfer.

Monolayer-protected metal nanoclusters (MPCs) are emerging as intriguing luminescent materials, but the construction of MPC-based optical probes is still scarce because of both the limited photoluminescence efficiency of MPCs and the lack of recognition mechanism. We herein propose a luminescence resonance energy transfer-based strategy to circumvent these problems.

NanoModeler: A Webserver for Molecular Simulations and Engineering of Nanoparticles.

Functionalized nanoparticles (NPs) are at the frontier of nanoscience. They hold the promise of innovative applications for human health and technology. In this context, molecular dynamics (MD) simulations of NPs are increasingly employed to understand the fundamental structural and dynamical features of NPs. While informative, such simulations demand a laborious two-step process for their setup. In-house scripts are required to (i) construct complex 3D models of the inner metal core and outer layer of organic ligands, and (ii) correctly assign force-field parameters to these composite systems. Here, we present NanoModeler ( www.nanomodeler.it ), the first Webserver designed to automatically generate and parametrize model systems of monolayer-protected gold NPs and gold nanoclusters. The only required input is a structure file of one or two ligand(s) to be grafted onto the gold core, with the option of specifying homogeneous or heterogeneous NP morphologies. NanoModeler then generates 3D models of the nanosystem and the associated topology files. These files are ready for use with the Gromacs MD engine, and they are compatible with the AMBER family of force fields. We illustrate NanoModeler's capabilities with MD simulations of selected representative NP model systems. NanoModeler is the first platform to automate and standardize the construction and parametrization of realistic models for atomistic simulations of gold NPs and gold nanoclusters.

Structural characterization of site-modified nanocapsid with monodispersed gold clusters

Hepatitis E Virus-like particles self-assemble in to noninfectious nanocapsids that are resistant to proteolytic/acidic mucosal delivery conditions. Previously, the nanocapsid was engineered to specifically bind and enter breast cancer cells, where successful tumor targeting was demonstrated in animal models. In the present study, the nanocapsid surface was modified with a solvent-exposed cysteine to conjugate monolayer protected gold nanoclusters (AuNC). Unlike commercially available gold nanoparticles, AuNCs monodisperse in water and are composed of a discrete number of gold atoms, forming a crystalline gold core. Au102pMBA44 (Au102) was an ideal conjugate given its small 2.5 nm size and detectability in cryoEM. Au102 was bound directly to nanocapsid surface cysteines via direct ligand exchange. In addition, Au102 was functionalized with a maleimide linker (Au102_C6MI) for maleimide-thiol conjugation to nanocapsid cysteines. The AuNC-bound nanocapsid constructs were conjugated in various conditions. We found Au102_C6MI to bind nanocapsid more efficiently, while Au102 remained more soluble over time. Nanocapsids conjugated to Au102_C6MI were imaged in cryoEM for single particle reconstruction to localize AuNC position on the nanocapsid surface. We resolved five unique high intensity volumes that formed a ring-shaped density at the 5-fold symmetry center. This finding was further supported by independent rigid modeling.

Synthesis, Structural and Photophysical Properties of Pentacene Alkanethiolate Monolayer-Protected Gold Nanoclusters and Nanorods: Supramolecular Intercalation and Photoinduced Electron Transfer with C60

6,13-Bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene: TP) alkanethiolate monolayer-protected gold nanoclusters (TP-Cn-X-MPCs: X stands for small (S) and large (L) nanocluter sizes) and nanorods (TP-Cn-MPRs) with different alkyl chain lengths (n = 7, 11) were synthesized to examine the structural and photophysical properties as well as intercalation trends with C60. The syntheses of TP-Cn-X-MPCs and TP-Cn-MPRs were successfully performed using two different precursors: TP disulfides and TP alkanethiols. The detailed structural properties were confirmed by 1H NMR, elemental analyses, and transmission electron micrograph (TEM). In the spectroscopic absorption and fluorescence excitation measurements, spectral shapes of TP units on the gold surface were clearly observed, whereas fluorescence intensities of TP units were strongly quenched as compared to the corresponding reference monomer (TP-Ref). Then, fluorescence quenching titration experiments to determine the association constants (Kapp) between C60 and TP assemblies (TP-Cn-X-MPCs and TP-Cn-MPRs) were performed by adding C60 in toluene. The Kapp values were largely dependent on the sizes of nanoclusters and alkyl chain lengths in TP-Cn-X-MPC. For example, the Kapp value of TP-C7-S-MPC (73 800 M–1) was much larger than those of TP-C11-S-MPC (37 800 M–1) and TP-C7-L-MPC (5350 M–1). This trend is in sharp contrast with the similar Kapp values (∼66 000 M–1) in TP-Cn-MPR (n = 7, 11). These results suggest that the intercalation behaviors are dependent on the surface structures (nanocluster vs nanorod). Such fluorescence quenching processes by photoinduced electron transfer (PET) in the complex between TP-C7-S-MPC and C60 were directly observed by femtosecond transient absorption measurements, monitoring the TP radical cation and C60 radical anion.

Simple and Efficient Synthesis of Gold Nanoclusters and Their Performance as Solid Contact of Ion Selective Electrode

Gold nanoclusters are ideal materials as solid transducer in all-solid-state ion-selective electrodes (ISEs) due to their high hydrophobicity, excellent ion-to-electron transduction, high specific capacitance and outstanding chemical stability. In this paper, monolayer-protected gold nanoclusters (MPCs) were synthesized by a simple one-phase method requiring a relatively short reaction time with high yield (60%). Transmittance electronic microscopy (TEM) and atomic-force microscopy (AFM) were used to characterize the morphology of MPCs. The composition of MPCs was studied by UV–vis spectrometry, Matrix-assisted laser desorption ionization (MALDI) mass spectrometry, X-ray diffraction (XRD) measurement and X-ray photoelectron spectroscopy (XPS) measurements. The performance of MPCs based ISEs was thoroughly characterized by electrochemical impedance, electromotive force and chronopotentiometry measurement. Comparison with the performance of ISE with other signal transducing materials such as conductive polymer, carbon nanotube, graphene and fullerene, the MPCs based ISEs have higher potential st ability and reproducibility, lower detection limit, longer operating life (one year) and higher anti-interference ability.

Assessment of the basis set effect on the structural and electronic properties of organic-protected gold nanoclusters

We have investigated the structural and optical properties of five monolayer-protected gold nanoclusters with a combination of exchange–correlation functionals, namely B-PBE for the geometry relaxation and CAM-B3LYP for the time-dependent calculations. We have tested the accuracy of five different basis sets in reproducing the experimental structures of these nanoclusters, and we have found that even a rather small basis set (single zeta) can outperform a significantly larger one (double zeta) if some selected atoms are treated with polarization functions. Namely, the sulfur and phosphorous atoms of the capping thiols and phosphines usually are hypervalent when bonded to the gold inner core; therefore, polarization functions allow them significantly more structural flexibility. With the two best performing basis sets, we carried out optical calculations and found that the resulting UV–Vis profiles are largely similar, in particular the energy and orbital contributions of the optical gaps are very close. The results support the use of the small basis set proposed here to investigate larger nanoclusters with general hybrid and range-corrected hybrid functionals.

The Shell Structure Effect on the Vapor Selectivity of Monolayer-Protected Gold Nanoparticle Sensors

Four types of monolayer-protected gold nanoclusters (MPCs) were synthesized and characterized as active layers of vapor sensors. An interdigitated microelectrode (IDE) and quartz crystal microbalance (QCM) were used to measure the electrical resistance and mass loading changes of MPC films during vapor sorption. The vapor sensing selectivity was influenced by the ligand structure of the monolayer on the surface of gold nanoparticles. The responses of MPC-coated QCM were mainly determined according to the affinity between the vapors and surface ligands of MPCs. The responses to the resistance changes of the MPC films were due to the effectiveness of the swelling when vapor was absorbed. It was observed that resistive sensitivity to polar organics could be greatly enhanced when the MPC contained ligands that contain interior polar functional groups with exterior nonpolar groups. This finding reveals that reducing interparticle attraction by using non-polar exterior groups could increase effective swelling and therefore enhance the sensitivity of MPC-coated chemiresistors.

Birth of the Localized Surface Plasmon Resonance in Monolayer-Protected Gold Nanoclusters

Gold nanoclusters protected by a thiolate monolayer (MPC) are widely studied for their potential applications in site-specific bioconjugate labeling, sensing, drug delivery, and molecular electronics. Several MPCs with 1-2 nm metal cores are currently known to have a well-defined molecular structure, and they serve as an important link between molecularly dispersed gold and colloidal gold to understand the size-dependent electronic and optical properties. Here, we show by using an ab initio method together with atomistic models for experimentally observed thiolate-stabilized gold clusters how collective electronic excitations change when the gold core of the MPC grows from 1.5 to 2.0 nm. A strong localized surface plasmon resonance (LSPR) develops at 540 nm (2.3 eV) in a cluster with a 2.0 nm metal core. The protecting molecular layer enhances the LSPR, while in a smaller cluster with 1.5 nm gold core, the plasmon-like resonance at 540 nm is confined in the metal core by the molecular layer. Our results demonstrate a threshold size for the emergence of LSPR in these systems and help to develop understanding of the effect of the molecular overlayer on plasmonic properties of MPCs enabling engineering of their properties for plasmonic applications.

Intramolecular Charge Transfer and Solvation Dynamics of Thiolate-Protected Au20(SR)16 Clusters Studied by Ultrafast Measurement

It is accepted that the monolayer ligand shell in monolayer-protected gold nanoclusters (MPCs) plays an important role in stabilizing the metal core structure. Previous reports have shown that the core and shell do not interact chemically, and very few studies investigating the intramolecular charge transfer (ICT) between the core and ligand shell in clusters have been reported. The underlying excited state relaxation mechanisms about the influence of solvents, the optically excited vibration, and the roles of the core and shell in charge transfer remain unknown to a large extent. Here we report a femtosecond transient absorption study of a Au20(SR)16 (R = CH2CH2Ph) cluster in toluene and tetrahydrofuran. The ICT from the outside shell to the inside core upon excitation in Au20(SR)16 is identified. The observed solvation-dependent oscillations in different solvents further confirm the photoinduced ICT formation in Au20(SR)16. The results provide a fundamental understanding of the structure-property relationships about the solvation-dependent core-shell interaction in Au MPCs.

Effect of chloride anion on the electrochemical charging of gold nanoparticle films

Reactivity of halide anion (Cl−) with monolayer-protected gold nanoclusters (MPCs) of 1.8 nm in diameter has been studied. Typically, thin films of MPCs were prepared on an electrode surface and immersed in aqueous solutions containing Cl−. It was observed that Cl− inevitably resulted in the destruction of electrochemical charging of MPC films, which was studied and analyzed in details by cyclic voltammetry, electrochemical quartz crystal microbalance, and X-ray photoelectron spectroscopy measurements. The destruction is most likely due to the strong affinity of Cl− for the surface of MPCs, leading to a significant variation of the surface structure and thereby quenching the electrochemical charging property.

Structural and Theoretical Basis for Ligand Exchange on Thiolate Monolayer Protected Gold Nanoclusters

Ligand exchange reactions are widely used for imparting new functionality on or integrating nanoparticles into devices. Thiolate-for-thiolate ligand exchange in monolayer protected gold nanoclusters has been used for over a decade; however, a firm structural basis of this reaction has been lacking. Herein, we present the first single-crystal X-ray structure of a partially exchanged Au(102)(p-MBA)(40)(p-BBT)(4) (p-MBA = para-mercaptobenzoic acid, p-BBT = para-bromobenzene thiol) with p-BBT as the incoming ligand. The crystal structure shows that 2 of the 22 symmetry-unique p-MBA ligand sites are partially exchanged to p-BBT under the initial fast kinetics in a 5 min timescale exchange reaction. Each of these ligand-binding sites is bonded to a different solvent-exposed Au atom, suggesting an associative mechanism for the initial ligand exchange. Density functional theory calculations modeling both thiol and thiolate incoming ligands postulate a mechanistic pathway for thiol-based ligand exchange. The discrete modification of a small set of ligand binding sites suggests Au(102)(p-MBA)(44) as a powerful platform for surface chemical engineering.

Utilizing diversified properties of monolayer protected gold nano-clusters to construct a hybrid sensor array for organic vapor detection

The present study demonstrated improvement in the vapor recognition rate of a sensor array employing monolayer-protected gold nano-clusters (MPCs) by combining two different MPC transduction properties. The number of sensors was increased by using the same set of MPC materials on different platforms and combined them as a hybrid array. Resistive changes in MPC film due to the insufficient swelling of inter-particle distance, which resulted from nature of absorbed vapor and surface thiolates, provided information in addition to the mass increases during vapor sorption. Quartz crystal microbalance (QCM) was used as the mass-change transducer and MEMS-fabricated interdigit electrode was used as the resistive transducer. Four different thiolate ligands were used to cap the gold core for diversification of the chemical selectivity. Fifteen organic vapors with various functional groups were tested for QCM and chemiresistor (CR) presence. Cluster analysis indicated that the arrays of QCMs and CRs show different selectivity for these 15 VOCs. Linear discriminant analyses (LDA) showed that single-compound identification rates within the response data library of 15 VOCs were 87.7% for the 4-QCM array, 82.1% for the 4-CR array, and 96.1% when a hybrid array of 8-sensors was used. This result proves that different properties of the same MPC can be used as different sensors to increase sensor numbers, and, therefore, to improve recognition rates via a hybrid platform.

A micro GC detector array based on chemiresistors employing various surface functionalized monolayer-protected gold nanoparticles

Aspects of the design, fabrication, and characterization of a chemiresistor type of microdetector for use in conjunction with gas chromatograph are described. The detector was manufactured on silicon chips using microelectromechanical systems (MEMS) technology. Detection was based on measuring changes in resistance across a film comprised of monolayer-protected gold nanoclusters (MPCs). When chromatographic separated molecules entered the detector cell, the MPC film absorbed vapor and undergoes swelling, then the resistance changes accordingly. Thiolates were used as ligand shells to encapsulate the nano-gold core and to manipulate the selectivity of the detector array. The dimensions of the μ-detector array were 14(L)×3.9(W)×1.2(H)mm. Mixtures of eight volatile organic compounds with different functional groups and volatility were tested to characterize the selectivity of the μ-detector array. The detector responses were rapid, reversible, and linear for all of the tested compounds. The detection limits ranged from 2 to 111ng, and were related to both the compound volatility and the selectivity of the surface ligands on the gold nanoparticles. Design and operation parameters such as flow rate, detector temperature, and width of the micro-fluidic channel were investigated. Reduction of the detector temperature resulted in improved sensitivity due to increased absorption. When a wider flow channel was used, the signal-to-noise ratio was improved due to the larger sensing area. The extremely low power consumption and small size makes this μ-detector array potentially useful for the development of integrated μ-GC.

Discrete reductive charging of immobilized gold nanoclusters in aqueous media

Multilayer films consisting of monolayer-protected gold nanoclusters (MPCs) coated on electrode surfaces and immersed in aqueous media were investigated to elucidate the suppression of the reductive charging. On the use of water-soluble electrolytes featuring a lipophilic cation, discrete reductive charging of MPC films was unambiguously observed. It was proved to be tightly concomitant with the cation transfer at the film/solution interface by the cation concentration dependence and electrochemical quartz crystal microbalance measurements. From this study, it was rationalized that the inadequate lipophilic cations used in previous works accounted for the suppression of reductive charging current.

Establishing linear solvation energy relationships between VOCs and monolayer-protected gold nanoclusters using quartz crystal microbalance

Linear solvation energy relationships (LSERs) have been recognized as a useful model for investigating the chemical forces behind the partition coefficients between vapor molecules and absorbents. This study is the first to determine the solvation properties of monolayer-protected gold nanoclusters (MPCs) with different surface ligands. The ratio of partition coefficients/MPC density (K/ρ) of 18 volatile organic compounds (VOCs) for four different MPCs obtained through quartz crystal microbalance (QCM) experiments were used for the LSER model calculations. LSER modeling results indicate that all MPC surfaces showed a statistically significant (p<0.05) preference to hydrogen-bond acidic molecules. Through dipole–dipole attraction, 4-methoxythiophenol-capped MPCs can also interact with polar organics (s=1.04). Showing a unique preference for the hydrogen bond basicity of vapors (b=1.11), 2-benzothiazolethiol-capped MPCs provide evidence of an intra-molecular, proton-shift mechanism on surface of nano-gold.

Synthesis, Characterization, and Double Layer Capacitance Charging of Nanoclusters Protected by 6-Mercaptopurine

The synthesis of gold nanoparticles by using the one-phase method of Brust has been carried out in this work to obtain 6-mercaptopurine monolayer protected gold nanoclusters (6MP-MPCs). The 6MP-MPCs are somewhat polar clusters that are soluble in polar solvents such as DMF and DMSO and in neutral and alkaline aqueous solutions. The average size of the clusters is 2.4 ± 0.5 nm and a stoichiometry of Au459(6MP)62 has been established by taking into account high resolution transmission electron microscopy, X-ray diffraction, and thermogravimetric analysis results of the 6MP-MPCs. The mode of binding of the 6MP molecules to the gold surface has been studied by FT-infrared and X-photoelectronic spectroscopies that evidence the existence of the S−Au interaction. Moreover, the different tautomers of the 6MP molecules in homogeneous solution suffer the transformation into the thiol N(9)H tautomer in the adsorbed state. Finally, taking together the size of these MPCs with the low effective dielectric constant of the 6MP monolayer in contact with the DMF solution makes the observation of the quantized charging of the double layer of these 6MP-MPCs possible. A capacitance value of 1.06 aF consistent with the core dimension and protective monolayer dielectric properties has been determined for the individual 6MP-MPCs.

Optically Active Gold Nanoclusters

In recent years, increasing attention has been paid for the construction of optically active nanomaterials. In particular, monolayer- protected gold nanoclusters are attractive for such systems. To date, optically active gold nanoclusters have been prepared by using chiral adsorbates or surface modifiers. In general, two major explanations have been proposed to interpret the optical activity: The first assumes a chiral core as a result of lattice distortion caused by the adsorbate. The other involves an achiral core with chirality induced by a chiral adsorption pattern or by dissymmetric fields (vicinal effect) from the chiral adsorbates. This mini-review gives an outline of the syntheses and chiroptical properties of the chiral monolayer-protected gold nanoclusters on the basis of our recent work as well as the other groups studies. Keywords: Monolayer-protected gold nanoclusters, Optically active, chiral core, dissymmetric field

Electrochemical Investigations of 3-(3-Thienyl) Acrylic Acid Protected Nanoclusters and Planar Gold Surfaces

Formation of self assembled monolayers on gold surface by thiols and disulphides is a well known phenomenon and extensive research work has been carried out in this area with envisaged applications in the area of sensors, molecular electronics, lithography, device fabrication using bottom-up approach, etc. Recently, it has been established that thiophene molecules can self assemble on gold surface due to Au-S interactions. 3-(3-thienyl) acrylic acid, a bifunctional ligand is used in this work to form self-assembled monolayers on planar gold surfaces (two dimensional assemblies) and to prepare monolayer protected gold nano clusters (three-dimensional assemblies). The electron transfer blocking properties of the two-dimensional monolayers were evaluated by using standard redox probes like ferrocyanide anions and Ruthenium hexamine cations. The functionalisation of the two-dimensional and three-dimensional assemblies has been carried out with ferrocene carboxylic acid and the functionalised monolayers were characterized by Cyclic voltammetry. The formation of thienyl acrylic acid protected nanoclusters has been verified by TEM and surface plasmon resonance absorption. It has been observed that when thiophene based ligands are used as stabilizers for the formation of metal nanoparticles, they tend to aggregate as a result of pi-pi interactions between adjacent thiophene ligands. In this case it is found that aggregation is prevented. The substituent at the thiophene ring hinders pi-pi interactions. The quantised nature of electrochemical charging of these nanoparticles has been demonstrated by differential pulse voltammetry (DPV), which exhibit peak like features (coulomb's staircase). This work also explores the possibility of using 3-(3-thienyl) acrylic acid as building blocks or spacers on planar and colloidal gold surfaces for potential applications in the field of sensors and devices.

Electrospray Mass Spectrometry Study of Tiopronin Monolayer-Protected Gold Nanoclusters

Electrospray ionization time-of-flight mass spectrometry (ESI-TOF MS) and gel permeation chromatography (GPC) were used to study the synthesis of a series of tiopronin monolayer-protected gold nanoclusters (MPCs) and to monitor their postsynthesis peptide ligand place-exchange reactions. All mass spectra identified the presence of cyclic gold(I)-thiolates with a strong preference for tetrameric species. During the synthesis of pre-monolayer-protected nanoclusters (pre-MPCs), esterified gold(I)-thiolate tetramers were initially observed in minor abundance (with respect to disulfide bridged tiopronin species) before dramatically increasing in abundance and precipitating from solution. After conversion of pre-MPCs to MPCs, ESI-TOF mass spectra demonstrated an overall predominance of tetrameric species with conversion from ester-terminated end groups to carboxyl-terminated end groups. Further modifications were performed through postsynthesis ligand place-exchange reactions to validate the existence of the tetramers. This work suggests that monolayer protection is accomplished by cyclized gold(I)-thiolate tetramers on the gold core surface, and/or that gold(I)-thiolates are a basic building block within the nanoparticles.