pH-Responsive Luminescent Lanthanide-Functionalized Gold Nanoparticles with "On-Off" Ytterbium Switchable Near-Infrared Emission

A pH indicator: Near-infrared emitting lanthanide-functionalized gold nanoparticles have been prepared through self-assembly at the gold surface between a ytterbium(III)-cyclen complex and xylenol orange. Excitation of the xylenol orange unit with visible light, up to 600?nm, results in the sensitization of the Yb(III) -centered near-infrared emission that can be reversibly switched "on-off" as a function of the pH value.     

"Black swan events" in organic synthesis

When a research area "goes viral", the event typically occurs in conjunction with a major change in "conventional wisdom". In retrospect, the literature often contains earlier hints that the original judgment was not correct. These antecedents are referred to as "Black Swan" events. The picture shows research on homogeneous gold catalysis "going viral".     

2D "soap"-assembly of nanoparticles via colloid-induced condensation of mixed Langmuir monolayers of fatty surfactants

We describe a new type of colloidal 2D gels formed in mixed Langmuir monolayers of stearic acid and octadecylamine on a surface of gold hydrosol. The adsorption of gold nanoparticles on the mixed monolayer led to an increase of interactions between oppositely charged surfactants giving a "soap" of mixed fatty salt. The observed effect is equivalent to a virtual "cooling" of floating monolayer, which undergoes rapid condensation on a surface of aqueous colloid. The consequent shrinking and rearrangement of the monolayer resulted in aggregation of nanoparticles into colloidal 2D "soap"-gels, which represented arrested colloidal phases within nonadsorbing organic medium. When sequentially deposited onto solids by Langmuir-Blodgett technique, the 2D "soap"-gels separated into organic and colloidal phases and gave dendrite-like bilateral organic crystallites coated with gold nanoparticles. The reported colloidal "soap"-assembly can offer a new opportunity to design 2D colloidal systems of widely variable chemistry and structures.     

DNA hybridization "turns on" electro-catalysis at gold electrodes

The efficiency of electro-catalysis occurring at DNA-modified gold electrodes is highly dependently on the density of DNA monolayers, as a result, DNA hybridization can "turn on" electro-catalysis by increasing the DNA surface density.     

Azide-derivatized gold nanorods: functional materials for "click" chemistry

We describe herein the synthesis of functional gold nanorods suitable for carrying out "click" chemistry reactions. Gold nanorods modified with a copolymer containing sulfonate and maleic acid groups have been conjugated to a bifunctional azide molecule (amine-PEG-azide). The maleic acid molecules in the copolymer participate in carbodiimide-mediated amide bond formation with amine groups of the azide linker, whereas the sulfonate groups prevent nanorod aggregation in water. Spectroscopic and zeta-potential measurements have been used to confirm the successful surface modification of the gold nanorods. These azide-functionalized nanorods can carry out chemical reactions based on click chemistry. As a case study, we have demonstrated the "clicking" of azide-nanorods to an acetylene-functionalized enzyme, trypsin, by a copper-catalyzed 1,3-dipolar cycloaddition reaction. The enzyme is not only stable after bioconjugation but is also biologically active, as demonstrated by its digestion of the protein casein. For comparison, the biological activity of trypsin conjugated to gold nanorods by two other commonly used methods (carbodiimide-mediated covalent attachment via amide bond formation and simple electrostatic adsorption) has been studied. The enzyme conjugated by click chemistry demonstrates improved biological activity compared with other forms of bioconjugation. This general and simple approach is easy, specific with higher yields, environmentally benign, and applicable to a wide range of analytes and biomolecules.     

pH-driven conformational switch of "i-motif" DNA for the reversible assembly of gold nanoparticles

Herein we demonstrate that gold nanoparticles conjugated to "i-motif" DNA behave like a pH dependent switch that undergoes reversible aggregations which can be easily visualized by the naked eye.     

The "staple" motif: a key to stability of thiolate-protected gold nanoclusters

Recently obtained single-crystal structure of a thiolate-protected gold cluster shows that all thiolate groups form "staple" motifs on the cluster surface. To find out the driving force for such a formation, we use first-principles density functional theory simulations to model formation of "staple" motifs on an Au38 cluster from zero to full coverage. By geometry optimization, molecular dynamics, and simulated annealing, we show that formation of "staples" is strongly preferred on a cluster surface and helps stabilize the cluster by pinning the surface Au atoms and increasing the HOMO-LUMO gap. We devise a method to generate initial structural models for thiolate-protected gold clusters by adding "staples" to the cluster surface. Using this method, we obtain a staple-covered, low-energy structure for Au38(SCH3)24, a much studied cluster whose structure is not yet known. Optical band-edge energy computed from time-dependent DFT for our Au38(SCH3)24 structure shows good agreement with experiment.     

Hybridization-based unquenching of DNA hairpins on au surfaces: prototypical "molecular beacon" biosensors

There is a keen interest in developing techniques for rapid genetic analysis that do not require labeling of an analyte. Here we demonstrate that fluorophore-tagged DNA hairpins attached to gold films can function as immobilized "molecular beacons". Two DNA hairpins incorporating portions of the Staphlococcus aureus FemA and mecR methicillin-resistance genes were attached to a gold substrate. Upon exposure to the complement, a approximately 26-fold increase in fluorescence intensity was measured corresponding to a 96 +/- 5% quenching efficiency. Studies with nonspecific DNA indicate that DNA hairpins immobilized on a gold surface retain their ability to bind complementary DNA sequences selectively.     

Detection of human serum albumin through surface-enhanced Raman scattering using gold "pearl necklace" nanomaterials as substrates

We describe a surface-enhanced Raman scattering (SERS)-based sensor for the detection of human serum albumin (HSA) using gold "pearl necklace" nanomaterials (Au PNNs) as the substrate and AB 580 as the reporter.     

In situ Raman monitoring triazole formation from self-assembled monolayers of 1,4-diethynylbenzene on Ag and Au surfaces via "click" cyclization

We prepared acetylenyl-terminated aromatic self-assembled monolayers (SAMs) of 1,4-diethynylbenzene on silver and gold. After the fabrication of pendent acetylenyl SAMs, the formation of triazoles was performed via Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition "click" chemistry. A density functional theory (DFT) calculation of Raman frequencies showed good agreement with our experimental data to provide evidence of the formation of the triazole molecule. Our results indicated that "click" chemistry could be successfully applied to simple aromatic SAMs proximate (<1 nm) to roughened gold surfaces. The reaction process could be monitored in real time by measuring intensity changes of the nu(CC)(free) band in surface-enhanced Raman scattering (SERS) spectra.     

"Belt and braces": a peptide-based linker system of de novo design

A new self-assembling peptide-based linker is described. The system comprises three leucine-zipper sequences of de novo design: one peptide, "the belt", templates the co-assembly of the other two-half-sized peptides, "the braces". These basic features were confirmed by circular dichroism spectroscopy and analytical ultracentrifugation: when mixed, the three peptides reversibly formed a predominantly helical and stable 1:1:1 ternary complex. Surface plasmon resonance experiments demonstrated assembly of the complex on gold surfaces, while the ability of the system to bring together peptide-bound cargo was demonstrated using colloidal gold nanoparticles. In the latter experiments, the nanoparticles were derivatized with the brace peptides prior to the addition of the belt. Transmission electron microscopy images of the resulting networks revealed regular approximately 7 nm separations between adjacent particles, consistent with the 42-amino acid helical design of the belt and braces. To our knowledge, belt and braces is a novel concept in leucine-zipper assembly and the first example of employing peptides to guide nanoparticle assembly.     

Similarities and differences between the "relativistic" triad gold, platinum, and mercury in catalysis

Relativistic effects in the valence shell of the elements reach a maximum in the triad Pt-Au-Hg and determine their catalytic activity in organic reactions. In this Review we examine the catalytic activity of Pt, Au, and Hg compounds for some representative reactions, and discuss the respective benefits and disadvantages along with other relevant properties, such as toxicity, price, and availability. For the reactions considered, gold catalysts are generally more active than mercury or platinum catalysts.     

Size evolution study of "molecular" and "atom-in-cluster" polarizabilities of medium-size gold clusters

A study on static polarizabilities for a family of gold clusters (Au(n), n = 6, 12, 20, 34, 54) is presented. For each cluster, a density functional theory perturbation theory calculation was performed to compute the cluster polarizability and the polarizability of each atom in the cluster using Bader's "quantum theory of atoms in molecules" formalism. The cluster polarizability tensor, α(cluster), is expressed as a sum of the atom-in-molecule tensors, α(cluster)=∑(Ω)α(Ω). A strong quadratic correlation (R(2) = 0.98) in the isotropic polarizability of atoms in the cluster and their distance to the cluster center of mass was observed. The cluster polarizabilities are in agreement with previous calculations.     

Amphiphilic Janus gold nanoparticles via combining "solid-state grafting-to" and "grafting-from" methods

Despite the great efforts that have been made toward obtaining Janus architectures, synthesizing sub-10 nanometer Janus nanoparticles (NPs) modified with different types of polymers remains a challenging task. In this Communication, "solid-state grafting-to" and "grafting-from" methods were combined to obtain Janus gold NPs (AuNPs) modified with two types of polymer chains on the opposite sides of the NP. We used functionalized polymer single crystals as the solid substrates to immobilize AuNPs. We then used atom transfer radical polymerization to grow polymer chains on the "free" side of the AuNPs. Amphiphilic polyethylene oxide (PEO)-Au-poly(methyl methacrylate), PEO-Au-poly(tert-butyl acrylate) and hydrophilic PEO-Au-poly(acrylic acid) were synthesized. The Janus nature was demonstrated using a platinum-nanoparticle-decoration method. Using polymer single crystals as the reaction substrates is advantageous because they afford higher throughput compared with self-assembled monolayers. Dissolution of the single crystal also leads to NPs with defined polymer patches. We anticipate that our approach could serve as a generic method for synthesizing polymer-functionalized, sub-10 nm Janus NPs. This unique system holds promises for achieving controlled assembly and tunable optic and electronic properties of NPs.     

Self-induced "electroclick" immobilization of a copper complex onto self-assembled monolayers on a gold electrode

We report the self-induced "electroclick" immobilization of the [Cu(II)(6-ethynyl-TMPA)(H(2)O)](2+) complex, by its simple electro-reduction, onto a mixed azidoundodecane-/decane-thiol modified gold electrode. The redox response of the grafted [Cu(II/I)(TMPA)] at the modified electrode is fully reversible indicating no Cu coordination change and a fast electron transfer.     

"Silver effect" in gold(I) catalysis: an overlooked important factor

Clear experimental evidence from X-ray photoelectron spectroscopy and (31)P NMR spectroscopy has been obtained for the first time to confirm that the combination of Ag(+) cation with [L-Au](+) results in the formation of different complexes in solution. Re-evaluation of literature-reported gold-catalyzed reactions revealed a significant difference in the reactivities with and without silver. In extreme cases (more than "rare"), the conventional [L-Au](+) catalysts could not promote the reaction without the presence of silver. This investigation has therefore revealed a long-overlooked "silver effect" in gold catalysis and should lead to revision of the actual mechanism.     

Discovery of the DNA "genetic code" for abiological gold nanoparticle morphologies

DNA is in control: Different combinations of DNA nucleotides can control the shape and surface roughness of gold nanoparticles during their synthesis. These nanoparticles were synthesized in the presence of either homogenous oligonucleotides or mixed-base oligonucleotides using gold nanoprisms as seeds. The effect of the individual DNA bases and their combinations on shape control are shown in the figure.     

Electro-oxidation of d-mannose on platinum, gold and nickel electrodes in aqueous medium

The oxidation of d-mannose is studied on platinum, gold and nickel in alkaline medium. The electro-reactivity of this compound and the rate determining step of the reactions were determined by cyclic voltammetry. Lead adatoms have important effect on the oxidation of d-mannose on platinum electrode. Five folder of increase in current densities was observed after the addition of 10⁻⁶ M Pb²⁺. Electrolyses carried out on upd-lead modified platinum, and gold electrodes show good conversion yields (75% and 80%, respectively) and reasonable selectivities towards mannonic acid. High amount of cleavage products were detected at the end of the electrolysis at nickel electrode. The reactivity-functional group relationship has been discussed considering the results of the electrolyses.     

Self-associated, "distillable" ionic media

Liquid or low melting association products of carbon dioxide and a secondary amine, both neutral molecules that may be gaseous, are recognised as "distillable" ionic media.     

Gold catalysts for pure hydrogen production in the water-gas shift reaction: activity, structure and reaction mechanism

The production of hydrogen containing very low levels of carbon monoxide for use in polymer electrolyte fuel cells requires the development of catalysts that show very high activity at low temperatures where the equilibrium for the removal of carbon monoxide using the water-gas shift reaction is favourable. It has been claimed that oxide-supported gold catalysts have the required high activity but there is considerable uncertainty in the literature about the feasibility of using these catalysts under real conditions. By comparing the activity of gold catalysts with that of platinum catalysts it is shown that well-prepared gold catalysts are significantly more active than the corresponding platinum catalysts. However, the method of preparation and pre-treatment of the gold catalysts is critical and activity variations of several orders of magnitude can be observed depending on the methods chosen. It is shown that an intimate contact between gold and the oxide support is important and any preparative procedure that does not generate such an interaction, or any subsequent treatment that can destroy such an interaction, may result in catalysts with low activity. The oxidation state and structure of active gold catalysts for the water-gas shift reaction is shown to comprise gold primarily in a zerovalent metallic state but in intimate contact with the support. This close contact between small metallic gold particles and the support may result in the "atoms" at the point of contact having a net charge (most probably cationic) but the high activity is associated with the presence of metallic gold. Both in situ XPS and XANES appear unequivocal on this point and this conclusion is consistent with similar measurements on gold catalysts even when used for CO oxidation. In situ EXAFS measurements under water gas shift conditions show that the active form of gold is a small gold cluster in intimate contact with the oxide support. The importance of the gold/oxide interface is indicated but the possible role of special sites (e.g., edge sites) on the gold clusters cannot be excluded. These may be important for CO oxidation but the fact that water has to be activated in the water gas shift reaction may point towards a more dominant role for the interfacial sites. The mechanism of the water gas shift reaction on gold and other low temperature catalysts has been widely investigated but little agreement exists. However, it is shown that a single "universal" model is consistent with much of the experimental literature. In this, it is proposed that the dominant surface intermediate is a function of reaction conditions. For example, as the temperature is increased the dominant species changes from a carbonate or carboxylate species, to a formate species and eventually at high temperatures to a mechanism that is characteristic of a redox process. Similar changes in the dominant intermediate are observed with changes in the gas composition. Overall, it is shown that reported variations in the kinetics, structure and reaction mechanism for the water gas shift reaction on gold catalysts can now be understood and rationalised.