Stability and Reversible Oxidation of Sub-Nanometric Cu5 Metal Clusters: Integrated Experimental Study and Theoretical Modeling.

Sub-nanometer metal clusters have special physical and chemical properties, significantly different from those of nanoparticles. However, there is a major concern about their thermal stability and susceptibility to oxidation. In situ X-ray Absorption spectroscopy and Near Ambient Pressure X-ray Photoelectron spectroscopy results reveal that supported Cu5 clusters are resistant to irreversible oxidation at least up to 773 K, even in the presence of 0.15 mbar of oxygen. These experimental findings can be formally described by a theoretical model which combines dispersion-corrected DFT and first principles thermochemistry revealing that most of the adsorbed O2 molecules are transformed into superoxo and peroxo species by an interplay of collective charge transfer within the network of Cu atoms and large amplitude "breathing" motions. A chemical phase diagram for Cu oxidation states of the Cu5 -oxygen system is presented, clearly different from the already known bulk and nano-structured chemistry of Cu.

Synthesis of photocatalytic cysteine-capped Cu≈10 clusters using Cu5 clusters as catalysts.

We report an easily scalable synthesis method for the preparation of cysteine-capped Cu≈10 clusters through the reduction of Cu(II) ions with NaBH4, using Cu5 clusters as catalysts. The presence of such catalytic clusters allows controlling the formation of the larger Cu≈10 clusters and prevents the production of copper oxides or Cu(I)-cysteine complexes, which are formed when Cu5 is absent or at lower concentrations, respectively. These results indicate that small catalytic clusters could be involved, as precursor species before the reduction step, in the different methods developed for the synthesis of clusters. The visible light-absorbing Cu≈10 clusters, obtained by the cluster-catalysed method, display high photocatalytic activities for the decomposition of methyl orange with visible light.

Ag5 nanoclusters with dual catalytic antiradical activities.

Silver nanoclusters of five atoms (Ag5) display outstanding catalytic activities for the deactivation of radicals. Using 2,2-diphenyl-1­picrylhydrazyl (DPPH) radical as a model system, we observed a fast radical reduction to DPPH anions using only [Ag5] 3 to 4 orders of magnitude less than [DPPH]. Moreover, nanoclusters remain stable at the end of the reaction, and can deactivate again DPPH radicals at the same rate, indicating that they act as anti-radical catalysts. The radical scavenger catalytic activity of Ag5 proceeds selectively through the oxidation of methanol (used to dissolve the radical) to formaldehyde, which is supported by DFT calculations. The obtained catalytic rate constants are almost 2 orders of magnitude higher than oxidases, and more than 4 orders of magnitude larger than graphene quantum dots. We also show that Ag5 not only catalyze the reduction of radicals but also their oxidation, promoting the inhibition of the autoxidation mechanisms of hydrocarbon polymers, which are very sensitive to the presence of radicals. For this purpose, thin films of two industrially relevant polymers (polyisoprene and acrylonitrile-butadienestyrene copolymer), were exposed to standard simulated photo-ageing conditions in the presence of Ag5. Using Attenuated Total Reflection-FTIR and DFT modeling we observed that, although Ag5 nanoclusters, with ≈ 15% surface coverage, do not totally inhibit the oxidation, they favour a decomposition that yields inactive products, in contrast with the more detrimental ketone formation pathway. These results not only open new possibilities for developing a post-process inhibition of polymer degradation, for which nowadays there are no efficient procedures, but also, they could be used as very efficient dual-redox catalytic radical scavengers for different industrial or biomedical purposes.

A Simple Entropic-Driving Separation Procedure of Low-Size Silver Clusters, Through Interaction with DNA.

Synthesis and purification of metal clusters without strong binding agents by wet chemical methods are very attractive for their potential applications in many research areas. However, especially challenging is the separation of uncharged clusters with only a few number of atoms, which renders the usual techniques very difficult to apply. Herein, we report the first efficient separation of Ag2 and Ag3 clusters using the different entropic driving forces when such clusters interact with DNA, into which Ag3 selectively intercalates. After sequential dialysis of the samples and denaturalizing the DNA-Ag3 complex, pure Ag2 can be found in the dialysate after extensive dialysis. Free Ag3 is recovered after DNA denaturation.

Tailored surface composition of Au/Pt nanocatalysts synthesized in microemulsions: a simulation study.

Au/Pt nanoparticles show an optimized catalytic activity when compared with Pt nanoparticles because Pt activity is improved by the presence of Au on the surface. It was checked whether a controllable surface composition can be achieved by the simple strategy of varying the Au : Pt ratio. We present an in-depth kinetic simulation study on the influence of Au : Pt ratio on the formation of Au/Pt nanoparticles synthesized in microemulsions. This study is able to explain the resulting nanoarrangement as a function of kinetic parameters such as Au : Pt ratio and intermicellar exchange rate. The role of the micelles as a dosing pump of the Au precursor explains that a higher Au amount results in a Au reduction which takes place over a longer period of time. It implies that Au is deposited until longer stages of the synthesis, so Au is present at the nanoparticle surface. Micelles as reaction media produce a minor impact on Pt due to its slower reduction. These different kinetic behaviours of Au and Pt give rise to a surface composition which can be tailored by tuning the Au : Pt ratio. Numerical results on surface composition successfully reproduce experimental data and further support the outcomes of the degree of atomic mixing under different Au : Pt ratios.

Increasing the optical response of TiO2 and extending it into the visible region through surface activation with highly stable Cu5 clusters.

The decoration of semiconductors with subnanometer-sized clusters of metal atoms can have a strong impact on the optical properties of the support. The changes induced differ greatly from effects known for their well-studied, metallic counterparts in the nanometer range. In this work, we study the deposition of Cu5 clusters on a TiO2 surface and investigate their influence on the photon-absorption properties of TiO2 nanoparticles via the computational modeling of a decorated rutile TiO2 (110) surface. Our findings are further supported by selected experiments using diffuse reflectance and X-ray absorption spectroscopy. The Cu5 cluster donates an electron to TiO2, leading to the formation of a small polaron Ti3+ 3d1 state and depopulation of Cu(3d) orbitals, successfully explaining the absorption spectroscopy measurements at the K-edge of copper. A monolayer of highly stable and well fixated Cu5 clusters is formed, which not only enhances the overall absorption, but also extends the absorption profile into the visible region of the solar spectrum via direct photo-induced electron transfer and formation of a charge-separated state.

On the Stability of Cu5 Catalysts in Air Using Multireference Perturbation Theory.

An ab initio study of the interaction of O2, the most abundant radical and oxidant species in the atmosphere, with a Cu5 cluster, a new generation atomic metal catalyst, is presented. The open-shell nature of the reactant species is properly accounted for by using the multireference perturbation theory, allowing the experimentally confirmed resistivity of Cu5 clusters toward oxidation to be investigated. Approximate reaction pathways for the transition from physisorption to chemisorption are calculated for the interaction of O2 with quasi-iso-energetic trapezoidal planar and trigonal bipyramidal structures. Within the multireference approach, the transition barrier for O2 activation can be interpreted as an avoided crossing between adiabatic states (neutral and ionic), which provides new insights into the charge-transfer process and gives better estimates for this hard to localize and therefore often neglected first intermediate state. For Cu5 arranged in a bipyramidal structure, the O-O bond cleavage is confirmed as the rate-determining step. However, for planar Cu5, the high energy barrier for O2 activation, related to a very pronounced avoided crossing when going from physisorption to chemisorption, determines the reactivity in this case.

Silver Atomic Quantum Clusters of Three Atoms for Cancer Therapy: Targeting Chromatin Compaction to Increase the Therapeutic Index of Chemotherapy.

Nanomaterials with very low atomicity deserve consideration as potential pharmacological agents owing to their very small size and to their properties that can be precisely tuned with minor modifications to their size. Here, it is shown that silver clusters of three atoms (Ag3 -AQCs)-developed by an ad hoc method-augment chromatin accessibility. This effect only occurs during DNA replication. Coadministration of Ag3 -AQCs increases the cytotoxic effect of DNA-acting drugs on human lung carcinoma cells. In mice with orthotopic lung tumors, the coadministration of Ag3 -AQCs increases the amount of cisplatin (CDDP) bound to the tumor DNA by fivefold without modifying CDDP levels in normal tissues. As a result, CDDP coadministered with Ag3 -AQCs more strongly reduces the tumor burden. Evidence of the significance of targeting chromatin compaction to increase the therapeutic index of chemotherapy is now provided.

Synthesis of nickel entities: From highly stable zerovalent nanoclusters to nanowires. Growth control and catalytic behavior.

Non-noble metal nanoclusters synthesis is receiving increased attention due to their unique catalytic properties and lower cost. Herein, the synthesis of ligand-free Ni nanoclusters with an average diameter of 0.7 nm corresponding to a structure of 13 atoms is presented; they exhibit a zero-valence state and a high stability toward oxidation and thermal treatment. The nanoclusters formation method consists in the electroreduction of nickel ions inside an ordered mesoporous alumina; also, by increasing the current density, other structures can be obtained reaching to nanowires of 10 nm diameter. A seed-mediated mechanism is proposed to explain the growth to nanowires inside these mesoporous cavities. The size dependence on the catalytic behavior of these entities is illustrated by studying the reduction of methylene blue where the nanoclusters show an outstanding performance.

Bimetallic nanoparticles synthesized in microemulsions: A computer simulation study on relationship between kinetics and metal segregation.

Computer simulations were carried out to study the origin of the different metal segregation showed by bimetallic nanoparticles synthesized in microemulsions. Our hypothesis is that the kinetics of nanoparticle formation in microemulsions has to be considered on terms of two potentially limiting factors, chemical reaction itself and the rate of reactants exchange between micelles. From the kinetic study it is deduced that chemical reduction in microemulsions is a pseudo first-order process, but not from the beginning. At the initial stage of the synthesis, redistribution of reactants between micelles is controlled by the intermicellar exchange rate, meanwhile the core and middle layers are being built. This exchange control has a different impact depending on the reduction rate of the particular metal in relation to the intermicellar exchange rate. For the case of Au/Pt nanoparticles, the kinetic constant of Au (fast reduction) is strongly dependent on intermicellar exchange rate and reactant concentration. On the contrary, the kinetic constant of Pt (slower reduction) remains constant. Therefore, the fact that the reaction takes place in a microemulsion affects more or less depending on the reduction rate of the metals. As a consequence, the final nanostructure not only depends on difference between the reduction rates of both metals, but also on the reduction rate of each metal in relation to the intermicellar exchange rate.

Gold nanorod synthesis catalysed by Au clusters.

Gold nanorods have been successfully synthesized by the seed mediated method using Au clusters. This synthesis does not require silver ions to obtain large amounts of Au nanorods and has good control over their aspect ratio. Au clusters are produced with the same recipe as for Au seeds, but using shorter reaction times. This very simple scheme confirms the important catalytic influence of clusters in the anisotropic growth control.

Understanding the Metal Distribution in Core-Shell Nanoparticles Prepared in Micellar Media.

The factors that govern the reaction rate of Au/Pt bimetallic nanoparticles prepared in microemulsions by a one-pot method are examined in the light of a simulation model. Kinetic analysis proves that the intermicellar exchange has a strong effect on the reaction rates of the metal precursors. Relating to Au, reaction rate is controlled by the intermicellar exchange rate whenever concentration is high enough. With respect to Pt, the combination of a slower reduction rate and the confinement of the reactants inside micelles gives rise to an increase of local Pt salt concentration. Two main consequences must be emphasized: On one hand, Pt reduction may continue independently whether or not a new intermicellar exchange takes place. On the other hand, the accumulation of Pt reactants accelerates the reaction. As the reactant accumulation is larger when the exchange rate is faster, the resulting Pt rate increases. This results in a minor difference in the reduction rate of both metals. This difference is reflected in the metal distribution of the bimetallic nanoparticle, which shows a greater degree of mixture as the intermicellar exchange rate is faster.

Photostability of gold nanoparticles with different shapes: the role of Ag clusters.

Anisotropic gold nanostructures prepared by the seed method in the presence of Ag ions have been used to study their photostability to low-power UV irradiation (254 nm) at room temperature. It has been observed that, whereas spheres are very stable to photoirradiation, rods and prisms suffer from photocorrosion and finally dissolve completely with the production of Au(III) ions. Interpretation of these differences is based on the presence of semiconductor-like Ag clusters, adsorbed onto rods and prisms, able to photocorrode the Au nanoparticles, which are absent in the case of Au spheres. We further show direct evidence of the presence of Ag clusters in Au nanorods by XANES. These results confirm a previous hypothesis (J. Am. Chem. Soc., 2014, 136, 1182-1185) about the major influence of very stable small Ag clusters, not only on the anisotropic formation of nanostructures but also on their photostability.

Controlling Bimetallic Nanostructures by the Microemulsion Method with Subnanometer Resolution Using a Prediction Model.

We present a theoretical model to predict the atomic structure of Au/Pt nanoparticles synthesized in microemulsions. Excellent concordance with the experimental results shows that the structure of the nanoparticles can be controlled at subnanometer resolution simply by changing the reactant concentration. The results of this study not only offer a better understanding of the complex mechanisms governing reactions in microemulsions, but open up a simple new way to synthesize bimetallic nanoparticles with ad hoc controlled nanostructures.

Ag2 and Ag3 clusters: synthesis, characterization, and interaction with DNA.

Subnanometric samples, containing exclusively Ag2 and Ag3 clusters, were synthesized for the first time by kinetic control using an electrochemical technique without the use of surfactants or capping agents. By combination of thermodynamic and kinetic measurements and theoretical calculations, we show herein that Ag3 clusters interact with DNA through intercalation, inducing significant structural distortion to the DNA. The lifetime of Ag3 clusters in the intercalated position is two to three orders of magnitude longer than for classical organic intercalators, such as ethidium bromide or proflavine.

Large stability and high catalytic activities of sub-nm metal (0) clusters: implications into the nucleation and growth theory.

Clusters are stable catalytic species, which are produced during the synthesis of nanoparticles (NPs). Their existence contradicts the thermodynamic principles used to explain the formation of NPs by the classical nucleation and growth theories (NGTs). Using chemical and electrochemical methods we will show that depending on the experimental conditions one can produce either Ag clusters or Ag NPs. Moreover, using already prepared Ag clusters one can observe the disappearance of the usual induction period observed for the kinetics of NP formation, indicating that clusters catalyze the formation of NPs. Taking these data together with some previous examples of cluster-catalyzed anisotropic growth, we derived a qualitative approach to include the catalytic activities of clusters into the formation of NPs, which is incorporated into the NGT. Some qualitative conclusions about the main experimental parameters, which affect the formation of clusters versus NPs, as well as the catalytic mechanism versus the non-catalytic one, are also described.

Synthesis of ultra-small cysteine-capped gold nanoparticles by pH switching of the Au(I)-cysteine polymer.

We report a synthetic approach for the production of ultra-small (0.6 nm) gold nanoparticles soluble in water with a precise control of the nanoparticle size. Our synthetic approach utilizes a pH-depending Au-cysteine polymer as a quencher for the AuNPs grown. The method extends the synthetic capabilities of nanoparticles with sizes down to 1 nm. In addition to the strict pH control, the existence of free -SH groups present in the mixture of reaction has been observed as a key requirement for the synthesis of small nanoparticles in mild conditions. UV-Vis, SAXS, XANES, EXAFS and HR-TEM, has been used to determinate the particle size, characterization of the gold precursor and gold-cysteine interaction.

Structure-directing and high-efficiency photocatalytic hydrogen production by Ag clusters.

H2 production by water splitting is hindered mainly by the lack of low-cost and efficient photocatalysts. Here we show that sub-nanometric silver clusters can catalyze the anisotropic growth of gold nanostructures by preferential adsorption at certain crystal planes of Au seeds, with the result that the final nanostructure can be tuned via the cluster/seed ratio. Such semiconducting Ag clusters are extremely stable and retain their electronic structure even after adsorption at the tips of Au nanorods, enabling various photocatalytic experiments, such as oxygen evolution from basic solutions. In the absence of electron scavengers, UV irradiation generates photoelectrons, which are stored within the nanorods, increasing their Au Fermi level up to the redox pinning limit at 0 V (RHE), where hydrogen evolution occurs with an estimated high efficiency of 10%. This illustrates the considerable potential of very small zerovalent, nonmetallic clusters as novel atomic-level photocatalysts.

Exceptional oxidation activity with size-controlled supported gold clusters of low atomicity.

The catalytic activity of gold depends on particle size, with the reactivity increasing as the particle diameter decreases. However, investigations into behaviour in the subnanometre regime (where gold exists as small clusters of a few atoms) began only recently with advances in synthesis and characterization techniques. Here we report an easy method to prepare isolated gold atoms supported on functionalized carbon nanotubes and their performance in the oxidation of thiophenol with O2. We show that single gold atoms are not active, but they aggregate under reaction conditions into gold clusters of low atomicity that exhibit a catalytic activity comparable to that of sulfhydryl oxidase enzymes. When clusters grow into larger nanoparticles, catalyst activity drops to zero. Theoretical calculations show that gold clusters are able to activate thiophenol and O2 simultaneously, and larger nanoparticles are passivated by strongly adsorbed thiolates. The combination of both reactants activation and facile product desorption makes gold clusters excellent catalysts.