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中国新闻技联六届四次理事会在渝召开 谢胜和理事长致辞
Abstract
Advanced medical applications of gold nanoparticles can use amino acids as surface modifiers. For this reason, understanding the adsorption of amino acids on gold is crucial to improve these applications. Here, the adsorption of glutamate (Glu) on Au(1 0 0) and Au(1 1 0) electrodes has been studied using a combination of electrochemical experiments, and DFT calculations. The adsorption properties have been examined in two different regions, the double layer region, and the OH adsorption/oxide formation region. In the first one, the combined results from the electrochemical experiments and the DFT calculations indicate that Glu is adsorbed in acidic solutions by the two terminal carboxylate groups, each one in a bidentate configuration, and exchanges two electrons upon adsorption. The comparison with the results obtained for the Au(1 1 1) electrode and other molecules containing carboxylic groups confirms this adsorption mode. Glu adsorption also affects the reconstruction process of the Au(1 1 0) and Au(1 0 0) surfaces. On the other hand, in alkaline solutions, glutamate is not adsorbed because the negative charge of the surface prevents its adsorption. In the OH adsorption/oxide formation region, Glu is oxidized when OH is adsorbed, and the results indicate that OH is consumed in this oxidation process. The formation of gold surface oxides inhibits the Glu oxidation reaction.
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The immobilization of redox proteins on electrode surfaces has been crucial for understanding the fundamentals of electron transfer in biological systems and has led to the development of biosensors and other bioelectronic devices. Novel materials, such as carbon nanotubes, gold and other metallic nanoparticles, carbon nanofibre and mesoporous materials have been widely used in the construction of these bioelectrodes, and have been shown to greatly improve the efficiency of electron transfer between the electrode and the redox centre of the protein. The use of these materials has spawned a diversity of covalent and non-covalent techniques for protein immobilization that offer different advantages and disadvantages to the performance of the bioelectrode. This review covers the important properties of these novel electrode materials relevant to the bioconjugation of proteins, and discusses the various methods of attachment from recent examples in the literature.
Proteins have been the subject of electrochemical studies. It is possible to apply electrochemical techniques to obtain information about their structure due to the presence of five electroactive amino acids that can be oriented to the outside of the peptidic chain. These amino acids are L-Tryptophan (L-Trp)?, L-Tyrosine (L-Tyr)?, L-Histidine (L-His)?, L-Methionine (L-Met)? and L-Cysteine (L-Cys)?; their electrochemical behavior being subject of extensive research, but it is still controversial. No spectroscopic investigations have been reported on L-Trp, and due to the short life time of the intermediates, ex situ techniques cannot be employed, leading to a never-ending discussion about possible intermediates. In the L-Tyr and L-His cases, spectroelectrochemical studies were performed and different intermediates were observed, suggesting that some intermediates may be observed under specific conditions, as proposed for L-Cys. This amino acid is the most interesting among the electroactive ones because of the presence of a thiol moiety at its side chain, leading to a wide range of oxidation states. It can adsorb onto surfaces of different crystallographic orientation in stereoselective conformation, modifying the surface for different applications.as a surface engineering tool since it plays the role of as an anchor for the growing of nanocrystals inside proteic templates.
Due to the numerous possible oxidation states of sulfur, L-cysteine and L-cystine are very important amino acids that are utilized to understand the adsorption and redox activity of proteins and peptides. Therefore, the study of the adsorption/oxidation of these molecules can be used as a model system to understand protein behaviour. The aim of the present communication is to study this adsorption/oxidation process by infrared spectroscopy and DFT/PBE-D3/def2-SVP/COSMO (water) calculations. A bridge geometry (S bonded to two Pt atoms) was found to be the chemical adsorption link formed, and experimental data corroborate the DFT predictions. For both species, the surface–carboxyl group interaction is stronger at lower pH values, and the conformations of the adsorbed species were found to be dependent on the pH, with the amino group getting closer to the surface along with increases in the electrolyte alkalinity. The oxidation study was also performed at different pH values, and a Kolbe decarboxylation was found to be a side reaction at every pH, being more important in strong acidic solutions. Sulfur-containing adsorbed oxidized species were inferred based on the optimized structures of the adsorbed/desorbed species. A complex oxidation mechanism is proposed here, and species, such as sulfoxide, adsorbed sulfoxide, sulfenic and sulfinic acids and adsorbed sulfone, are suggested as intermediates in the oxidation of L-cysteine to sulfonic acid.
Significant progress has been done in the electrochemical (EC) analysis of practically all proteins, based on the electroactivity of amino acid (aa) residues in proteins. From a transducer point of view, it can be anticipated that many different ways of how nanomaterials can be integrated into the EC detection platform of detection will be developed. This can be done by direct modification of electroactive surfaces by nanomaterials or by advanced patterning protocol and by using nanomaterials as amplification tags, helping to produce lectin biosensors/biochips working in an ultrasensitive and selective way. Further, it can be anticipated that EC-based biosensors will compete in a future with instrumental techniques or lectin microarrays only if such devices are integrated into a biochip format offering multiplexed glycan measurements. Moreover, it has been shown that some glucosamine-containing poly- and oligosaccharides are electroactive under conditions close to physiological and that most polysaccharides and glycans can be transformed into electrochemically active substances by a simple chemical modification. Usually 4-10 biomarkers have to be detected to obtain good specificity and selectivity of detection. EC detection appears particularly advantageous for the preparation of low-density chips with this number of biomarkers.
The specific adsorption of OH? anion and the changes in the surface atomic structures of (100), (311) and (111) gold faces with charge density are discussed on the basis of cyclic voltammograms and differential capacity—potential curves obtained, in dilute NaOH solutions.
We present a method for calculating the stability of reaction intermediates of electrochemical processes on the basis of electronic structure calculations. We used that method in combination with detailed density functional calculations to develop a detailed description of the free-energy landscape of the electrochemical oxygen reduction reaction over Pt(111) as a function of applied bias. This allowed us to identify the origin of the overpotential found for this reaction. Adsorbed oxygen and hydroxyl are found to be very stable intermediates at potentials close to equilibrium, and the calculated rate constant for the activated proton/ electron transfer to adsorbed oxygen or hydroxyl can account quantitatively for the observed kinetics. On the basis of a database of calculated oxygen and hydroxyl adsorption energies, the trends in the oxygen reduction rate for a large number of different transition and noble metals can be accounted for. Alternative reaction mechanisms involving proton/electron transfer to adsorbed molecular oxygen were also considered, and this peroxide mechanism was found to dominate for the most noble metals. The model suggests ways to improve the electrocatalytic properties of fuel-cell cathodes.
We present a parameter-free method for an accurate determination of long-range van der Waals interactions from mean-field electronic structure calculations. Our method relies on the summation of interatomic C6 coefficients, derived from the electron density of a molecule or solid and accurate reference data for the free atoms. The mean absolute error in the C6 coefficients is 5.5% when compared to accurate experimental values for 1225 intermolecular pairs, irrespective of the employed exchange-correlation functional. We show that the effective atomic C6 coefficients depend strongly on the bonding environment of an atom in a molecule. Finally, we analyze the van der Waals radii and the damping function in the C6R(-6) correction method for density-functional theory calculations.
Adsorbed amino acids can modulate the behavior of metal nanoparticles in advanced applications. Using a combination of electrochemical experiments, FTIR spectroscopy, and DFT calculations, glutamate species interacting with the Au(111) surface in solution are here investigated. Electrochemical results indicate that the adsorption behavior depends on the solution pH (which controls the glutamate ionization) and on the charge of the surface. Glutamate adsorption starts at potentials slightly negative to the potential of zero charge. The thermodynamic analysis of these results indicates that two electrons are exchanged per molecule, implying that both carboxylic groups become deprotonated upon adsorption. The FTIR spectra reveal that carboxylate groups are bonded to the surface in the bidentate configuration (with both oxygen atoms attached to the surface). Plausible adsorbed configurations, consistent with the whole of these insights, were found using DFT. -Additionally, it was observed that glutamate oxidation only takes place when the surface is oxidized, which suggests that this oxidation process involves the transfer of an oxygen group to the molecule, though, according to the FTIR spectra, the main chain remains intact.
Advanced applications of colloidal nanoparticles (NPs) become to depend on their specific shape, which is controlled by the adsorption behavior of the capping agent involved in their synthesis. To understand the way in which citric acid determines the shape of gold NPs, the adsorption behavior of citrate on gold under the synthesis conditions is here investigated from electrochemical experiments on well-defined surfaces. Gibb excesses and charge numbers for the citrate adlayers deposited on the Au (111), Au(100) and Au(110) electrodes when a potential is applied were estimated at pHs 1 and 3. From these results, FTIR spectra and DFT calculations, it is concluded that solvated citrate can become simultaneously adsorbed through three dehydrogenated carboxylic groups in bidentate configuration on Au(111), but only through two on Au(100) and Au(110). As a result of this behavior, citrate can become more strongly adsorbed on Au(111) than on the other two basal planes of gold under the synthesis conditions, which would explain why tetrahedral and octahedral colloidal gold NPs are preferentially shaped when citric acid is used as the capping agent in water. This conclusion coincides with the previously one obtained on platinum, suggesting that the mechanism here described would operate also on other metals having fcc structure.
Oxidative damage to peptides, proteins and DNA is considered to be one of the major causes of cancer and age-related diseases. The interaction of biomolecules, peptides, proteins, nucleic acids and pharmaceuticals with solid electrode surfaces is not only a fundamental phenomenon but also a key to important and novel analytical sensing applications in biosensors, biotechnology, medical devices and drug-delivery schemes. Electrochemical methods can provide insight into the redox mechanisms and the electron-transfer reactions of a variety of fundamental biological processes.
The performance of many advanced catalytic systems depends not only on the size and composition but also on the specific shape of the metal nanoparticles (NPs) from which they are assembled. In turn, the shape of colloidal NPs depends on the specific capping agent involved in their synthesis, though the mechanism is still poorly understood. Here, supported by electrochemical experiments, FTIR spectra and DFT calculations, on well-defined surfaces, we show how a specific capping agent determines the shape of colloidal NPs. Solvated citrate can become simultaneously adsorbed on the Pt(111) surface through three dehydrogenated carboxylic groups, each one of them in bidentate configuration. On the other basal planes, citrate is adsorbed through only two of them. For this reason, under the synthesis conditions, citrate is more favorably adsorbed on the Pt(111) than on the other two basal planes of platinum. This adsorption behavior explains why colloidal platinum NPs of tetrahedral and octahedral shape are produced when citrate is used as the capping agent in water. The mechanism for citrate would also operate determining the shape of other pure fcc metals and can inspire the engineering of future capping agents.
Oxidative stress-dependent inflammatory diseases represent a major concern for the population's health worldwide. Biocompatible nanomaterials with enzymatic properties could play a crucial role in the treatment of such pathologies. In this respect, platinum nanoparticles (PtNPs) are promising candidates, showing remarkable catalytic activity, able to reduce the intracellular reactive oxygen species (ROS) levels and impair the downstream pathways leading to inflammation. This review reports a critical overview of the growing evidence revealing the anti-inflammatory ability of PtNPs and their potential applications in nanomedicine. It provides a detailed description of the wide variety of synthetic methods recently developed, with particular attention to the aspects influencing biocompatibility. Special attention has been paid to the studies describing the toxicological profile of PtNPs with an attempt to draw critical conclusions. The emerging picture suggests that the material per se is not causing cytotoxicity, while other physicochemical features related to the synthesis and surface functionalization may play a crucial role in determining the observed impairment of cellular functions. The enzymatic activity of PtNPs is also summarized, analyzing their action against ROS produced by pathological conditions within the cells. In particular, we extensively discuss the potential of these properties in nanomedicine to down-regulate inflammatory pathways or to be employed as diagnostic tools with colorimetric readout. A brief overview of other biomedical applications of nanoplatinum is also presented.
This chapter presents some results that provide insight on the effect of the interactions with the metal surface, a water molecule and an external electric field, on the geometry and vibrational frequencies of adsorbed zwitterions of three amino acids: glycine, L‐alanine, and L‐serine. It reports a comparative spectroelectrochemical study of the behavior of the three amino acids with the simpler structures. The chapter focuses on the electrochemical behavior of L‐alanine and L‐serine, presenting their adsorption on gold thin‐film electrodes in acidic media. The results reported here, conclude that, for the interpretation of the experimental spectra of some adsorbed species, in particular for the assignment of bands to modes, the situation can become nontrivial, and the use of density functional theory (DFT) calculations of optimized geometries and vibrational frequencies show great help, in particular if the relevant ingredients are included in the model used for such calculations.
The electrochemical oxidation of serine and alanine was performed at Pt(100) and Pt(110) electrodes and analysed by means of FTIR spectroscopy and cyclic voltammetry. The open circuit adsorption of both amino acids was also investigated. The oxidation of serine gives rise to the formation of two types of strongly adsorbed species, COads coming from the CH2OH group and CNads, which appears as a consequence of the dehydrogenation of the amino group. These species can be isolated at the surface of both electrodes during open-circuit adsorption experiments. The oxidation of alanine also yields adsorbed cyanide adlayers on Pt(100) and Pt(110) electrodes. However, the amount of adsorbed cyanide formed from this amino acid is much lower than that formed in the case of serine. Thus, the presence of the methyl group bonded to the alpha carbon atom seems to stabilise the amino acid against its oxidation on both Pt(100) and Pt(110) electrodes.
A spectroelectrochemical study of the adsorption and oxidation of glycine on a Pt(111) electrode has been carried out in 0.1 M HClO4 solution. Two types of adsorbate were identified. On the one hand, strongly adsorbed cyanide which comes from the oxidative adsorption of the molecule. This adsorbate has been isolated and characterized. On the other hand reversibly adsorbed glycinate anions, which adsorb at potentials above 0.3 V, have been detected. As in the case of acetate anions, glycinate is two-fold coordinated to the Pt(111) surface through the carboxylate group. The analysis of the potential dependent band centre frequencies and integrated band intensities suggests that lateral interactions between adsorbed glycinate anions are at the origin of the small frequency shift observed with increasing potential.
The electrochemical behaviour of glycine on well-defined Pt(100) and Pt(110) electrodes has been studied in an acid medium by means of cyclic voltammetry and in situ FTIR spectroscopy. The results presented in this paper and those reported previously for the Pt(111)|glycine system have shown that glycine oxidation is a surface structure-sensitive reaction. Adsorbed cyanide (band around 2100 cm?1) and adsorbed nitric oxide (band at 1610 cm?1) were the main poisoning species formed during the electro-oxidation of this amino acid on Pt(100). Adsorbed cyanate (2165 cm?1) has been also identified as an intermediate species during such oxidation. The formation of a carbon monoxide ad-layer when the Pt(100) electrode was polarised below 0.4 V in a glycine-containing solution has been attributed to a cyanide surface reaction to produce COads and, probably, NH4+. On Pt(110), glycine electro-oxidation produces adsorbed cyanide as a unique poisoning species. The adsorbed cyanide can be removed from the surface at potentials below 0.1 V. Finally, the presence of reversibly adsorbed glycinate anions on both Pt(100) and Pt(110) surfaces has been shown. Glycinate anions are 2-fold coordinated to the electrode surface through their carboxylate groups.
The adsorption and oxidation of l-alanine and l-serine at Au(1 1 1) and Au(1 0 0) single crystal and evaporated thin-film electrodes with preferential (1 1 1) orientation was studied in perchloric acid solutions. For this purpose, cyclic voltammetry experiments were combined with external reflection infrared spectroscopy (gold single crystals) and surface-enhanced infrared reflection–absorption spectroscopy under attenuated total reflection conditions (ATR-SEIRAS) (gold thin films). In addition, theoretical harmonic vibrational frequencies, obtained from B3LYP/LANL2DZ, 6-31+G(d) calculations for the zwitterionic species adsorbed on Au clusters with (1 1 1) orientation, were used to interpret the experimental spectra. The optimized geometry obtained from DFT calculations for the corresponding zwitterion plus a water molecule, under the application of an external electric field of 0.01 a.u. corresponds to a bidentate asymmetrical bridge adsorption configuration. The absence of an adsorbate band for the asymmetric OCO stretching in the experimental infrared spectra confirms the bidentate bonding of the adsorbed zwitterion through the oxygen atoms of the carboxylate group irrespective of the crystallographic orientation of the electrode surface, the adsorbate coverage and the electrode potential. In addition to typical interfacial water bands associated to perchlorate anions, which are co-adsorbed in order to compensate the positive charge of the ammonium group, the ATR-SEIRA spectra also show bands around 2950 cm?1 that can be related to the formation of hydrogen bonds between interfacial water and the ammonium group of the adsorbed zwitterion. The voltammetric experiments have shown that, as in the case of platinum electrodes, l-serine oxidizes at lower potentials than l-alanine. Under these conditions, the in situ infrared experiments show the formation of carbon dioxide and adsorbed cyanide as oxidation products of l-serine. In the case of l-alanine, only carbon dioxide was detected.
α-Amino acids are highly functional small molecules whose side chain groups are like prototypes of metal coordination and weak interactions in proteins. This perspective focuses on non-covalent or weak interactions of the side chain groups of amino acids, such as the charged groups of arginine and aspartic acid and the aromatic rings of tyrosine and tryptophan, in metal complexes in solution and in the solid state. The structure and function of small complexes exhibiting weak interactions and their biological relevance are described.
The Au(110) surface has been investigated by scanning tunneling microscopy. The reconstructed surface consists of long ribbons of narrow (111) facets along the [10] direction with maximum three free rows. Two-row facets give rise to the 1 × 2 reconstruction of the missing-row type, while three-row facets account for the 1 × 3 reconstruction. Disorder arises from locally random sequences of the two facets.
The DMol COSMO method is revisited and generalized for infinite polymer and surface models with periodic boundary conditions. The procedure works also for three dimensionally periodic solid models with internal surfaces. A new solvent accessible surface grid construction is presented, where the grid points and weights are a continuous function for all atomic geometries. The calculated solvation energy is also continuous by consequence, which is useful for all calculations which involve geometry changes of the atomic framework. The new method is tested with a few examples.
Clean and well-ordered Au(111) and Pt(111) electrodes were prepared in a UHV chamber, transferred to an electrochemical cell by a closed-transfer system and immersed in 0.1 M HClO4 at various potentials. From the charge flowing during contact with the electrolyte under potential control, the potential of zero charge (pzc) was derived. Au(111) was used as a test system and very good agreement was obtained between the pzc values determined by in-situ capacity measurements in dilute solutions and by the immersion technique. For Pt(111) the pzc was found to be very positive, within the oxide region, and hence not directly accessible by experiment. However, from charge measurements in the double layer region, the pzc of Pt(111) could be estimated to lie around 0.9 V vs. Ag/AgCl.
The adsorption behaviour of serine was studied in acid medium (1 mol dm?3) by a direct radiotracer method using 14 C labelled serine and by means of an indirect method studying the competitive adsorption between non-labelled serine and 36Cl-labelled Cl? ions. A strong irreversible chemisorption was found in a wide potential range (200–500 mV on the RHE scale). It follows from a comparison of the adsorption behaviour of alanine and serine that the strong chemisorption observed for the latter species should be ascribed to the presence of the -CH2-OH entity in the molecule. Study of the behaviour of the chemisorbed molecules confirms this assumption.
Monocrystalline gold surfaces having five different crystallographic orientations have been characterized by means of cyclic voltammetry in aqueous 0.11 M NaOH in order to examine the electrochemistry of oxide formation and removal in comparison with the well-characterized behavior in acidic media. For the higher-index faces Au(210) and (311), these voltammetric features in 0.11 M NaOH are closely similar to those observed in acidic electrolytes, although the occurrence of substantial OH? adsorption is signaled by a broad reversible current-potential feature at less positive potentials. For the low-index faces Au(100) and (111), however, the voltammograms in 0.11 M NaOH differ substantially from those in acid. These differences are ascribed to the influence of potential-dependent OH? adsorption, which overlaps with oxide formation and especially reduction, and attendant potential-induced surface reconstruction. The behavior of Au(110) is intermediate between that of these other two low-index and the higher-index faces. As for the voltammetric features in acidic electrolytes, such crystal face-dependent voltammetry in 0.11 M NaOH can be employed as a sensitive (albeit empirical) check of the chemical and physical state of the interface prior to other electrochemical measurements.
A detailed experimental and mechanistic analysis of the elementary surface-chemical steps involved in the beginning stages of electrooxidation of Au is given with respect to the processes that occur on the (111) plane. At Au, specific adsorption of anions, even oxyanions such as ClO?4 and SO2?4 or HSO?4, plays a major role in the initial stages of oxidation of the metal. In the case of the (111) plane investigated here, the symmetry of the surface lattice has the strongest effect on the specific adsorption of the above anions compared with behaviour at other planes. This is connected with the matching symmetry of the tetrahedral ions with the trigonal symmetry of an unreconstructed (111) plane.The results indicate that the hydrated anions, which are found to be partially or fully discharged on the metal surface, form overlay-lattice arrays. This network of hydrated anions blocks the surface, preventing the formation of sublattices of OH discharged on the free surface of the metal but allows partial discharge of water molecules of the hydration shells of the ions in two energy states in a fast upd reaction. This provides the path for the initial stages of 2-dimensional oxidation of the surface. The resulting network of anion-OH(1?γ)? complexes stabilizes the surface, preventing the turn-over process which is possible only after, or coupled with, the desorption of the anions in an anion replacement, MOH turn-over process. This is possible when a potential is reached at which partly discharged OH(1?γ)? in complexes with anions can become fully discharged, forming free MOH's which are no longer part of the complex. In this way, the network is destroyed and the stabilization of the anions by the H-bonds of hydration H2O or partially discharged OH no longer remains, allowing the anions to be desorbed and processes of phase-oxide development to begin.
Among all the gold faces studied to date, the (210) face has the most negative pzc and therefore should be less affected by preoxidation than the other faces. But the (210) gold face-sodium fluoride solution interface is ideally polarizable only in a short range of potential. Nevertheless, differential capacity-potential curves have been tentatively exploited. There seems to be adsorption of the fluoride ions on the (210) face of gold. The inner-layer capacity-charge curves are discussed.
Tin adlayers formed on well defined gold electrodes upon irreversible adsorption from Sn-II solutions have been characterized by means of cyclic voltammetry in sulfuric and perchloric acid solutions. The surface redox reactions undergone by the tin adatom have been found to be structure sensitive. From the analysis of the voltammetric charges involved in these processes it can be suggested that adsorbed tin is oxidized to oxygenated Sn-II species, which are stable at the electrode surface as long as the electrode potential is kept below 0.25 V. At higher potentials, soluble Sn-IV species are formed and the adlayer is removed from the electrode surface. The stability of the adlayer is strongly dependent on the solution pH. On Au(111), adsorbed tin is directly oxidized to Sn-IV species.
A new type of pseudopotentials for local orbital methods is presented. Hardness conserving semilocal pseudopotentials have been generated for all elements from H to Am. The construction is based on a minimization of errors with the norm conservation conditions for 2–3 relevant ionic configurations of the atom. Besides the transferability between atomic states, the portability among density functionals is also of interest. This paper explores if the norm-conservation errors can be kept reasonably small when minimized for two functionals, e.g., the generalized gradient approximation (GGA) and local density approximation, simultaneously. It is found that the errors can be kept at roughly the same low level as for a single functional. Since these pseudopotentials are mainly designed for use with local orbital methods, semicore functions may be treated as valence functions, helping to increase the accuracy and portability. Therefore the name density functional semicore pseudopotential or DSPP is suggested. To further improve portability and, importantly, also aid numerical stability with GGA’s, a core density (nonlinear core correction) is used. As with other pseudopotentials, scalar relativistic corrections to atomic scattering properties can easily be incorporated into this PP. Finally performance DSPP’s versus all electron DSPP’s with the same method, will be shown for an extensive set of test calculations. It is found that the DSPP is a very well behaved pseudo-potential.
A method for accurate and efficient local density functional calculations (LDF) on molecules is described and presented with results. The method, Dmol for short, uses fast convergent three‐dimensional numerical integrations to calculate the matrix elements occurring in the Ritz variation method. The flexibility of the integration technique opens the way to use the most efficient variational basis sets. A practical choice of numerical basis sets is shown with a built‐in capability to reach the LDF dissociation limit exactly. Dmol includes also an efficient, exact approach for calculating the electrostatic potential. Results on small molecules illustrate present accuracy and error properties of the method. Computational effort for this method grows to leading order with the cube of the molecule size. Except for the solution of an algebraic eigenvalue problem the method can be refined to quadratic growth for large molecules.
Recent extensions of the DMol3 local orbital density functional method for band structure calculations of insulating and metallic solids are described. Furthermore the method for calculating semilocal pseudopotential matrix elements and basis functions are detailed together with other unpublished parts of the methodology pertaining to gradient functionals and local orbital basis sets. The method is applied to calculations of the enthalpy of formation of a set of molecules and solids. We find that the present numerical localized basis sets yield improved results as compared to previous results for the same functionals. Enthalpies for the formation of H, N, O, F, Cl, and C, Si, S atoms from the thermodynamic reference states are calculated at the same level of theory. It is found that the performance in predicting molecular enthalpies of formation is markedly improved for the Perdew–Burke–Ernzerhof [Phys. Rev. Lett. 77, 3865 (1996)] functional. ? 2000 American Institute of Physics.
Detailed surface atomic rearrangements at the ordered Au(111)‐aqueous interface triggered by negative electrode charges are examined by in situ scanning tunneling microscopy (STM). Not only the (√3 × 22) atomic reconstruction, but also the longer‐range superstructures, formed at this electrochemical interface are seen to be remarkably similar to those observed previously by STM on clean Au(111) in ultrahigh vacuum.
Acetate adsorption at gold electrodes is studied in perchloric acid solutions by cyclic voltammetry and in-situ infrared spectroscopy. External reflection measurements, performed with gold single crystal electrodes, are combined with Surface Enhanced Infrared Reflection Absorption Spectroscopy experiments under attenuated total reflection conditions (ATR-SEIRAS) carried out with sputtered gold thin-film electrodes. Theoretical harmonic IR frequencies of acetate species adsorbed with different geometries on Au clusters with (1 1 1), (1 0 0) and (1 1 0) orientations have been obtained from B3LYP/LANL2DZ, 6–31 + G* calculations. The theoretical and experimental results confirm that, irrespective of the surface crystallographic orientation, bonding of acetate to the surface involves the two oxygen atoms of the carboxylate group, with the OCO plane perpendicular to the metal surface. DFT calculations reveal also that the total charge of the metal cluster–acetate supermolecule has small effect on the vib
Au(100, Au(111) and Au(110) electrodes with reconstructed surfaces of the type (5 × 20), (1 × 23) and (1 × 2), respectively, have been prepared by the so-called flame treatment and their properties investigated in various electrolytes by electrochemical and optical methods. The reconstructed (100) and (111) surfaces are found to be stable only in a potential range where no specific adsorption occurs. The Au(100)-(5 × 20) surface has optical properties which are distinctly different from those of the unreconstructed Au(100)-(1 × 1). This difference was used to monitor by in situ spectroscopy the adsorbate-induced (5 × 20) → (1 × 1) transition, in order to obtain information on the transition kinetics. For a certain fraction of the surface, electrochemically induced reconstruction, (1 × 1) → (5 × 20) and (1 × 1) → (1 × 23), has been observed for Au(100) and Au(111). The potentials of zero charge of the reconstructed surfaces have been determined and are compared with those of the unreconstructed ones.
In order to gain further understanding of the capacity-potential curves observed at the single crystal gold-aqueous solution interphases, the five faces (100), (110), (111), (311) and (210) which should have extreme electrochemical behaviour are discussed. Correlations are established between hysteresis, of C(E) curves and current peaks. A surface reconstruction exists for the three faces of lowest indices, as is observed in ultra-high vacuum (UHV) for 5d metals. The behaviour of the (311) and (210) faces is analysed. The results obtained for each face form a sequence dependent on the adsorbability of the anions. The explanation given in this paper for secondary phenomena observed on C(E) curves for gold faces, coordinates the results obtained.
Reconstructed Au(100)-(5×20) electrodes have been prepared by flame annealing and their electrochemical and optical behaviour investigated in aqueous electrolytes. The (5×20) surface is found to be stable only in the absence of specific adsorption. The potential zero charge as well as the overall electroreflectance effect are very similar to Au(111). The empty surface states of the unreconstructed Au(100) were not detected. Specific adsorption removes the reconstruction, and the (5×20) surface can no longer be restored in-situ. Monitoring optical transitions into empty surface states of the Au(100)-(1×1) surface allowed the study of the kinetics for the electrochemically induced (5×20) → (1×1) transition.
A spectroelectrochemical study of the adsorption and oxidation of serine and alanine on Pt(111) electrodes has been carried out in 0.1 M HClO4 solutions. From the voltammetric and FTIR spectroscopic results, we concluded that adsorbed cyanide is formed at the Pt(111) surface as a product of serine and alanine oxidation. In the case of serine, adsorbed cyanide is detected at potentials above 0.4 V, whereas a higher potential is needed in the case of alanine. Linear and multibonded CO have also been detected as poisoning species during serine oxidation. These adsorbed species have been isolated by irreversible adsorption experiments of amino acids. Dissolved CO2 has been found by FTIR spectroscopy as an oxidation product of both amino acids. The role played by the R group in the electrochemical behaviour of these molecules is significant. Reversibly adsorbed serinate and alaninate anions have been also detected. As in the case of the anions present in glycine solutions, the serinate and alaninate anions are two-fold coordinated to the Pt(111) surface through the car?ylate group.
The utilization of infrared reflection–absorption spectroscopy (IRAS) and scanning tunneling microscopy (STM) for extracting atomic‐resolution information for ordered metal–solution interfaces in a related (and relatable) fashion to metal‐ultrahigh vacuum (UHV) surfaces is illustrated by means of some recent results from our laboratory. Two specific topics are addressed. The first involves the potential‐dependent properties of saturated CO adlayers on low‐index platinum and rhodium electrodes in aqueous and nonaqueous media. The central role of the surface potential in controlling the CO adlayer structure is discussed on the basis of in situ IRAS data, especially in comparison with the properties of corresponding metal‐UHV interfaces. The application of in situ atomic‐resolution STM in tandem with IRAS for elucidating real‐space adsorbate structures is noted for saturated CO adlayers on Rh(111) and Rh(110) electrodes. The second topic concerns the application of in situ STM to probe potential‐induced reconstruction at gold‐aqueous interfaces. All three low‐index gold surfaces are seen to undergo reconstruction at potentials corresponding to small (～10–15 μC?cm-2) negative electrode charges. The subtle surface relaxation observed for Au(111) is essentially identical to that observed recently by atomic‐resolution STM in UHV. The (5×27) and (1×n) (n=2,3) symmetries observed for reconstructed Au(100) and (110) electrodes, respectively, are compatible with the structures deduced for the UHV systems by diffraction methods, although the STM data afford greater real‐space detail.
Single-crystal planes of Au electrodes in the (111), (100) and (110) orientation were studied by the potentiodynamic sweep method in solutions of anions which adsorb weakly (HClO4) and strongly (H2SO4) on Au. It is shown how the specific adsorption of anions which appear to be fully discharged on the (111) and (110) planes, but very little or not at all on the (100) planes, depends on the symmetry of the arrangement of surface atoms in relation to the three-fold geometry of the tetrahedral anions. This in turn determines the type and extent of a coordinative deposition (UPD) of OH in between adsorbed anions, which seems to occur with only partial charge transfer (γ = 0.5).The resolution of the processes of OH deposition in sub-lattices amongst the adsorbed anions is found to be specific for the three low-index planes examined and is interpreted in terms of the states of adsorption of the anions in relation to lattice geometry and charge transfer.The adsorbed anions, by having a stabilizing effect on the MOH(1?γ)-species on the surface, influence also the kinetics of the process of replacement of adsorbed anions by deposited OH on the surface and the concerted M/OH turn-over process which constitutes the beginning stage of formation of bulk-phase oxide material on the electrode.
The structures of the reconstructed Ir(100), Pt(100) and Au(100) surfaces have been investigated. Low energy electron diffraction (LEED) patterns are analyzed and LEED intensity versus energy data are measured. A variety of structures is observed by LEED: Ir(100) exhibits a relatively simple (1 × 5) pattern; Pt(100) shows a series of closely related patterns, a typical representative of which has a structure; Au(100) usually exhibits a c(26 × 68) pattern, often inaccurately described in the literature as a (20 × 5) pattern. The reconstruction of Au(111) is also considered for comparison. Various plausible structural models are discussed, while laser simulation is used to lessen the number of these models. The analysis is completed in a companion paper where LEED intensity calculations are reported to determine the atomic locations.
A simple formulation of a generalized gradient approximation for the exchange and correlation energy of electrons has been proposed by Perdew, Burke, and Ernzerhof (PBE) [Phys. Rev. Lett. 77, 3865 (1996)]. Subsequently Zhang and Yang [Phys. Rev. Lett. 80, 890 (1998)] have shown that a slight revision of the PBE functional systematically improves the atomization energies for a large database of small molecules. In the present work, we show that the Zhang and Yang functional (revPBE) also improves the chemisorption energetics of atoms and molecules on transition-metal surfaces. Our test systems comprise atomic and molecular adsorption of oxygen, CO, and NO on Ni(100), Ni(111), Rh(100), Pd(100), and Pd(111) surfaces. As the revPBE functional may locally violate the Lieb-Oxford criterion, we further develop an alternative revision of the PBE functional, RPBE, which gives the same improvement of the chemisorption energies as the revPBE functional at the same time as it fulfills the Lieb-Oxford criterion loc
Mutagenesis and immobilization are usually considered to be unrelated techniques with potential applications to improve protein properties. However, there are several reports showing that the use of site-directed mutagenesis to improve enzyme properties directly, but also how enzymes are immobilized on a support, can be a powerful tool to improve the properties of immobilized biomolecules for use as biosensors or biocatalysts. Standard immobilizations are not fully random processes, but the protein orientation may be difficult to alter. Initially, most efforts using this idea were addressed towards controlling the orientation of the enzyme on the immobilization support, in many cases to facilitate electron transfer from the support to the enzyme in redox biosensors. Usually, Cys residues are used to directly immobilize the protein on a support that contains disulfide groups or that is made from gold. There are also some examples using His in the target areas of the protein and using supports modified with immobilized metal chelates and other tags (e.g., using immobilized antibodies). Furthermore, site-directed mutagenesis to control immobilization is useful for improving the activity, the stability and even the selectivity of the immobilized protein, for example, via site-directed rigidification of selected areas of the protein. Initially, only Cys and disulfide supports were employed, but other supports with higher potential to give multipoint covalent attachment are being employed (e.g., glyoxyl or epoxy-disulfide supports). The advances in support design and the deeper knowledge of the mechanisms of enzyme-support interactions have permitted exploration of the possibilities of the coupled use of site-directed mutagenesis and immobilization in a new way. This paper intends to review some of the advances and possibilities that these coupled strategies permit.
Gold nanoparticles have been used in biomedical applications since their first colloidal syntheses more than three centuries ago. However, over the past two decades, their beautiful colors and unique electronic properties have also attracted tremendous attention due to their historical applications in art and ancient medicine and current applications in enhanced optoelectronics and photovoltaics. In spite of their modest alchemical beginnings, gold nanoparticles exhibit physical properties that are truly different from both small molecules and bulk materials, as well as from other nanoscale particles. Their unique combination of properties is just beginning to be fully realized in range of medical diagnostic and therapeutic applications. This critical review will provide insights into the design, synthesis, functionalization, and applications of these artificial molecules in biomedicine and discuss their tailored interactions with biological systems to achieve improved patient health. Further, we provide a survey of the rapidly expanding body of literature on this topic and argue that gold nanotechnology-enabled biomedicine is not simply an act of 'gilding the (nanomedicinal) lily', but that a new 'Golden Age' of biomedical nanotechnology is truly upon us. Moving forward, the most challenging nanoscience ahead of us will be to find new chemical and physical methods of functionalizing gold nanoparticles with compounds that can promote efficient binding, clearance, and biocompatibility and to assess their safety to other biological systems and their long-term term effects on human health and reproduction (472 references).
We present total-energy, force, and electronic-structure calculations for Na and K adsorbed in various geometries on an Al(111) surface. The calculations apply density-functional theory together with the local-density approximation and the ab initio pseudopotential formalism. Two adsorbate meshes, namely, (&surd;3 × &surd;3 )R30° and (2×2), are considered and for each of them the geometry of the adlayer relative to the substrate is varied over a wide range of possibilities. By total-energy minimization we determine stable and metastable geometries. For Na we find for both adsorbate meshes that the ordering of the calculated binding energies per adatom is such that the substitutional geometry, where each Na atom replaces a surface Al atom, is most favorable and the on-top position is most unfavorable. The (&surd;3 × &surd;3 )R30° structure has a lower energy than the (2×2) structure. This is shown to be a substrate effect and not an effect of the adsorbate-adsorbate interaction. In contrast to the results for Na, we find for the (&surd;3 × &surd;3 )R30° K adsorption that the calculated adsorption energies for the on-top, threefold hollow, and substitutional sites are equal within the accuracy of our calculation, which is +/-0.03 eV. The similarity of the energies of the on-surface adsorption sites is explained as a consequence of the bigger size of K which implies that the adatom experiences a rather small substrate electron-density corrugation. Therefore for potassium the on-top and hollow sites are close in energy already for the unrelaxed Al(111) substrate. Because the relaxation energy of the on-top site is larger than that of the threefold hollow site both sites receive practically the same adsorption energy. The unexpected possibility of surface-substitutional sites is explained as a consequence of the ionic nature of the bonding which, at higher coverages, can develop strongest when the adatom can dive into the substrate as deep as possible. The interesting result of the studied systems is that the difference in bond strengths between the ``normal'' and substitutional geometries is sufficiently large to kick out a surface Al atom.
Gold colloids functionalized with amino acids provide a scaffold for effective DNA binding with subsequent condensation. Particles with lysine and lysine dendron functionality formed particularly compact complexes and provided highly efficient gene delivery without any observed cytotoxicity. Nanoparticles functionalized with first generation lysine dendrons (NP-LysG1) were approximately 28-fold superior to polylysine in reporter gene expression. These amino acid-based nanoparticles were responsive to intracellular glutathione levels, providing a tool for controlled release and concomitant expression of DNA.
Atomic-resolution scanning-tunneling-microscopy images of ordered Au(110) in aqueous 0.1M HClO4, reported as a function of electrode potential, provide an unusually detailed picture of surface reconstruction. Lowering the potential of a freshly annealed surface to -0.3 versus saturated calomel electrode (SCE) yield images consisting primarily of domains having (1×2) symmetry. While the (1×2) structure exhibits an atomic density commensurate with the usual ``missing-row'' model, the images suggest that significant relaxation of both top- and second-layer atoms occurss. Three-atom-wide ribbons, lying along the [11ˉ0] direction, are seen to provide the basic building blocks of the reconstruction; these units also yield ``added-row'' domains of (1×n) symmetry, with n=3 or higher. The reconstruction is lifted, yielding the (1×1) Au(110) surface, rapidly (within ~2 s) upon altering the potential to 0 V vs SCE, yet reappears immediately upon returning to -0.3 V.
Detailed atomic-resolution images of ordered Au(100) surfaces in aqueous 0.1M HClO4 as obtained by in situ scanning tunneling microscopy (STM) are reported as a function of electrode potential. Below about -0.25 V (versus saturated calomel electrode), the (1×1) surface reconstructs to form corrugated quasihexagonal domains. Multiple distinct, yet related, structures are formed that resemble those postulated from diffraction measurements. The reconstruction can be lifted by returning to 0.2 V. The results demonstrate the promise of atomic-resolution STM for examining reconstruction at ordered electrochemical surfaces.
Macrophages are one of the principal immune effector cells that play essential roles as secretory, phagocytic, and antigen-presenting cells in the immune system. In this study, we address the issue of cytotoxicity and immunogenic effects of gold nanoparticles on RAW264.7 macrophage cells. The cytotoxicity of gold nanoparticles has been correlated with a detailed study of their endocytotic uptake using various microscopy tools such as atomic force microscopy (AFM), confocal-laser-scanning microscopy (CFLSM), and transmission electron microscopy (TEM). Our findings suggest that Au(0) nanoparticles are not cytotoxic, reduce the production of reactive oxygen and nitrite species, and do not elicit secretion of proinflammatory cytokines TNF-alpha and IL1-beta, making them suitable candidates for nanomedicine. AFM measurements suggest that gold nanoparticles are internalized inside the cell via a mechanism involving pinocytosis, while CFLSM and TEM studies indicate their internalization in lysosomal bodies arranged in perinuclear fashion. Our studies thus underline the noncytotoxic, nonimmunogenic, and biocompatible properties of gold nanoparticles with the potential for application in nanoimmunology, nanomedicine, and nanobiotechnology.
Tailoring molecular specificity toward a crystal facet: A lesson from biorecognition toward pt{111}
- Ruan
Interphases in systems of conducting phases
- Trasatti
