Morphology-dependent electrochemistry and electrocatalytical activity of cytochrome c

The morphology-dependent electrochemistry and electrocatalytical activity of cytochrome c (cyt. c) were investigated at pyramidal, rodlike, and spherical gold nanostructures directly electrodeposited onto sputtered gold surfaces. Direct, reversible electron transfer of cyt. c, for the first time, was realized at nanorod-like and nanopyramidal gold surfaces without any mediators or promoters, while no redox reaction was observed at the nanospherical gold electrode. The electrochemical properties of cyt. c vary with the shape of gold nanostructures with respect to the reversibility of electrode reactions, kinetic parameters, the formal potentials (E0'), and charge-transport resistance (Rct), suggesting shape-dependent mechanisms for the electrode reactions of cyt. c. The experimental results manifest that cyt. c was stably immobilized on the nanostructured gold electrodes with different conformational changes of the heme microenvironment. Consequently, not only the electroactivity, but also the inherent biological activity of the immobilized cyt. c strongly depended on the shape of the electrode surfaces. The facilitated electron transfer combined with the intrinsic catalytical activity of cyt. c substantially constructed a third-generation H2O2 biosensor with high selectivity, quick response time, large linear range, and good sensitivity. The electrocatalytical activity of the immobilized cyt. c toward H2O2 was also found to be morphology dependent, and the linear range of H2O2 detection could be tuned by means of employing the nanostructured gold surfaces with different shapes.     

Electrochemical growth of two-dimensional gold nanostructures on a thin polypyrrole film modified ITO electrode

Two-dimensional gold nanostructures have been fabricated by electrochemical deposition of gold nanoparticles onto indium tin oxide (ITO) glass substrate modified with thin polypyrrole film. By controlling the electrodeposition conditions, gold nanoparticles with dendritic rod, sheet, flower-like (consisting of staggered nanosheets), and pinecone-like structures were generated. The flower-like gold nanoparticles showed high catalytic activity on electrochemical reduction of oxygen, and its activity was measured to be approximately 25 times that of gold pinecones and 10(4) times that of gold nanosheets in terms of gold weight. The pinecone-like nanoparticles can form a compact film with nano-/microscale binary structure like a lotus leaf surface. After modification with n-dodecanethiol, the surface showed superhydrophobic properties with a water contact angle of 153.4 degrees and a tilt angle of 4.4 degrees (5 microL droplet).     

Enhancing Hydrogen Evolution Activity of Au(111) in Alkaline Media through Molecular Engineering of a 2D Polymer

The electrochemical splitting of water holds promise for the storage of energy produced intermittently by renewable energy sources. The evolution of hydrogen currently relies on the use of platinum as a catalyst—which is scarce and expensive—and ongoing research is focused towards finding cheaper alternatives. In this context, 2D polymers grown as single layers on surfaces have emerged as porous materials with tunable chemical and electronic structures that can be used for improving the catalytic activity of metal surfaces. Here, we use designed organic molecules to fabricate covalent 2D architectures by an Ullmann‐type coupling reaction on Au(111). The polymer‐patterned gold electrode exhibits a hydrogen evolution reaction activity up to three times higher than that of bare gold. Through rational design of the polymer on the molecular level we engineered hydrogen evolution activity by an approach that can be easily extended to other electrocatalytic reactions.     

Organic dye molecules as reducing agent for the synthesis of electroactive gold nanoplates

Highly crystalline, hexagonal and triangular nanoplates of gold are synthesized in high yield by a new wet chemical method using multifunctional molecules, Bismarck brown R (BBR) and Bismarck brown Y (BBY). This method involves a simple approach by keeping a mixture of aqueous HAuCl4 solution and BBR/BBY solution in presence of poly(vinyl pyrrolidone) for 24 h. These nanostructures show unprecedented electrochemical properties exhibiting surface confinement effect. The UV-visible (UV-vis) spectrum shows certain distinct features with absorptions at 300, 400, and 650 nm extending up to the near infrared region. Selected area electron diffraction patterns of these nanoparticles show highly oriented (111) crystal facets. X-ray diffraction analysis also confirms the predominant orientation in the (111) crystal planes with lattice constant approximately 4.07 angstroms of face-centered-cubic (fcc) gold. X-ray photoelectron (XP) and Fourier transform infrared (FTIR) spectroscopic analysis shows the presence of a fraction of reducing molecules as surface passivating agent either in the unreacted molecular state or as a mixture of reacted and unreacted product, which probably undergoes charge transfer with gold nanocrystals giving absorption at approximately 300 nm.     

Lightweight ITO Electrodes Decorated with Gold Nanostructures for Electrochemical Applications

Gold nanostructures were fabricated on a transparent indium tin oxide (ITO) coated PET substrate by an electrodeposition technique from a potassium gold (III) chloride solution for two different types of applications. It was found that the optical transparency of lightweight ITO electrodes could be maintained by depositing isolated gold nanostructures while opening up the use of these electrodes for inner sphere electron reactions, such as hydroquinone oxidation, which are not possible at ITO electrodes. For practical applications the adhesion of gold to the ITO electrode was improved by modifying the ITO surface with 3‐mercaptopropyl‐trimethoxysilane (MPS). Compared to Au/ITO, the Au/MPS/ITO electrode showed vastly improved electrochemical activity toward various electron transfer reactions when subjected to mechanical stress. The biosensing properties of the Au/MPS/ITO electrode was also investigated by studying the detection of immobilized DNA on the Au/MPS/ITO electrode via electrochemical impedance spectroscopy (EIS).     

Pyramidal, rodlike, spherical gold nanostructures for direct electron transfer of copper, zinc-superoxide dismutase: application to superoxide anion biosensors

It is the first time that direct electron transfer of copper, zinc-superoxide dismutase (Cu, Zn-SOD) is realized at nanospherical, nanorodlike, and nanopyramidal gold nanostructures, without any mediators or promoters. Thermodynamic and kinetic parameters of the electron transfer vary with the morphology of the electrodeposited gold nanostructures, suggesting the morphology-dependent electrochemistry of SOD. Experimental results reveal that SOD is strongly confined onto the nanostructured gold surfaces and processes its inherent enzymatic activity after being adsorbed on all three kinds of gold nanostructures, which also enable the direct electron transfer of SOD itself. A combination of the facilitated direct electron transfer and the bifunctional enzymatic catalytic activities of the SOD substantially offers a dual electrochemical approach to determination of O2(*-), in which O2(*-) could be detected both anodically and cathodically. In both the oxidation and reduction regions, the present O2(*-) biosensors display excellent analytical performance, such as wide linear range, low detection limit, quick response time, and good stability and reproducibility, while not being limited by interferences, for instance, uric acid, ascorbic acid, and hydrogen peroxide.     

Facile Synthesis of Multi-Branched Gold Nanostructures through a TBAB-Assisted Route in Aqueous Solution and Their SERS Property

Facile synthesis of multi‐branched gold nanostructures by using the tetrabutyl ammonium bromide (TBAB) as a capping agent is described. The reaction is carried out in a one‐step process at mild temperature. Gold nanostructures with more than six sharp branches ranging from 70 to 130 nm in length are synthesized in high yield. It is proposed that the relative weak adsorption capacity of TBAB leads to the incompletely covered gold surface and the growth of nanoparticles occurs on the uncovered gold surface, and therefore short branches appear consequently. Then positively charged TBAB layers on the gold surfaces prevent the branches from aggregating with each other which stimulates the branch growth. The prepared branched gold nanoparticles show efficient surface‐enhanced Raman scattering (SERS) properties. Low temperature (4°C) is unfavorable to the formation of multi‐branched gold nanostructures, and only thin small irregular plate‐like nanoparticles are produced. The addition of SDS in TBAB aqueous solution results in forming SDS micelles at much lower concentration of SDS (0.4 mmol/L) as compared to that in pure water, and short branched gold nanoparticles are obtained in the SDS‐TBAB system.     

Electrochemical synthesis of gold nanocrystals and their 1D and 2D organization

Size-controlled gold nanocrystals were conveniently synthesized through direct electroreduction of bulk AuCl(4)(-) ions in the presence of poly(N-vinylpyrrolidone) (PVP). PVP greatly enhanced the gold particle formation process and also significantly retarded the gold electrodeposition process, allowing the electrochemical synthesis of gold nanocrystals to be carried out in the form of simple electroreduction. This novel electrochemical method may be extended to synthesis of other noble metal nanoparticles with controllable size on a large scale. The PVPK90-protected gold nanocrystals spontaneously self-assembled into nearly ordered 2D close-packed arrays and interesting 1D nanostructures. The aggregation of unstable PVPK17-protected gold nanocrystals resulted in the formation of ultrathin single-crystalline films. PVP plays multifunctional roles in controlling the size and shape of gold nanocrystals and in inducing individual gold nanocrystals to construct 1D nanostructures. The nanoparticle self-assembling technique based on PVP offers a simple, but effective, path to organize individual gold nanoparticles into various 1D and 2D nanostructured materials.     

Passivation layer–dependent catalysis of zinc oxide nanostructures

Electrochemical and photoelectrochemical catalysis of surface-passivated zinc oxide (ZnO) nanostructures with three different metal oxides were investigated. Initially, vertically aligned ZnO nanorods structures were developed over conductive substrates by a two-step approach and then passivated with an ultrathin zinc hydroxide, that is, Zn(OH)₂, cobalt oxide, that is, CoO, and Zn(OH)₂/CoO as bilayer, by electrochemical deposition. Compared with the pristine ZnO structures, the surface-passivated nanostructures possess slightly rough surfaces, whereas their crystal structure remains unchanged. From electrochemical catalysis studies under dark and illumination, it is noticed that vertically aligned ZnO nanostructures passivated with narrow band-gap CoO layers have a predominant water oxidation performance than that of the structures passivated with other oxide materials. It is mainly attributed to the eradication of surface states present on ZnO nanorods. Interestingly, the structures passivated with bilayers, that is, Zn(OH)₂/CoO, showed significant stability and durability (～103% retention in current density@60^th min) with a continuous oxygen evolution reaction process for long durations.     

Single-crystalline gold microplates: synthesis, characterization, and thermal stability

Single-crystalline gold microplates of several 10 microm in lateral size, characterized by hexagonal, truncated triangular, and triangular shapes with (111) planes as two basal surfaces, have been synthesized in large quantities through a solution phase process. Significantly, such anisotropic Au nanostructures exhibit remarkable optical properties, in which the dipole plasmon resonance shifting in the NIR region and the quadrupole plasmon resonance at approximately 820 nm were observed. Fragmentation of Au microplates is found when the temperature is higher than 450 degrees C, indicating they are not thermodynamically stable structure at high temperature. Investigations on the Au microplates upon heating suggest that the melting and collapsing start mainly at the edges that should be Au (110) facets. This work is valuable for Au nanostructures applied at elevated temperatures.     

Analytical Application of Pyramidal,Rodlike, and Spherical Gold Nanostructures: Simultaneous Detection of Ascorbic Acid and Uric Acid

Simultaneous detection of ascorbic (AA) and uric acid (UA) is developed at pyramidal (NP), rodlike (NR), and spherical (NS) gold nanostructures, due to their high electrocatalytic activities toward the oxidation of AA and UA. Unlike at bare gold electrode, the fouling resulted from the oxidized product of AA is eliminated at the nanostructured gold electrode. The voltammetric signals of AA and UA are completely separated with a potential difference of 216 mV, 158 mV and 195 mV, respectively, at the pyramidal, rodlike, and spherical gold surfaces. The experimental results reveal that solution pH effects the peak separation of AA and UA, acidic solution is more favorable for the simultaneous determination of AA and UA than neutral one, than alkaline one. The coexistence of a large excess of AA does not interfere with the voltammetric sensing of UA, vice versa. All the three kinds of nanostructured gold electrodes show excellent sensitivity, stability, selectivity, low detection limit, quick response and wide linear range in the repeated detection of AA and UA. The practical utility of the present nanostructured gold electrodes is demonstrating by determining the concentration of AA in fruit juice and UA in urine sample.     

Contrasting effect of gold nanoparticles and nanorods with different surface modifications on the structure and activity of bovine serum albumin

Nanoparticles exposed to biofluids become coated with proteins, thus making protein-nanoparticle interactions of particular interest. The consequence on protein conformation and activity depends upon the extent of protein adsorption on the nanoparticle surface. We report the interaction of bovine serum albumin (BSA) with gold nanostructures, particularly gold nanoparticles (GNP) and gold nanorods (GNR). The difference in the geometry and surface properties of nanoparticles is manifested during complexation in terms of different binding modes, structural changes, thermodynamic parameters, and the activity of proteins. BSA is found to retain native-like structure and properties upon enthalpy-driven BSA-GNP complexation. On the contrary, the entropically favored BSA-GNR complexation leads to substantial loss in protein secondary and tertiary structures with the release of a large amount of bound water, as indicated by isothermal calorimetry (ITC), circular dichroism (CD), and Fourier transform infrared (FTIR) and fluorescence spectroscopies. The esterase activity assay demonstrated a greater loss in BSA activity after complexation with GNR, whereas the original activity is retained in the presence of GNP. The formation of large assemblies (aggregates) and reduced average lifetime, as evidenced from dynamic light scattering and fluorescence decay measurements, respectively, suggest that GNR induces protein unfolding at its surface. The effect of temperature on the CD spectra of BSA-GNP was found to be similar to that of pristine BSA, whereas BSA-GNR shows distortion in CD spectra at lower wavelengths, strengthening the perception of protein unfolding. High binding constant and entropy change for BSA-GNR complexation determined by ITC are consistent with large surfacial interaction that may lead to protein unfolding. The present work highlights the differential response of a protein depending on the nature of the nanostructure and its surface chemistry, which need to be modulated for controlling the biological responses of nanostructures for their potential biomedical applications.     

Density functional theory study of the adsorption of alkanethiols on Cu(111), Ag(111), and Au(111) in the low and high coverage regimes

The structure, the surface bonding, and the energetics of alkanethiols adsorbed on Cu(111), Ag(111), and Au(111) surfaces were studied under low and high coverages. The potential energy surfaces (PES) for the thiol/metal interaction were investigated in the absence and presence of externally applied electric fields in order to simulate the effect of the electrode potential on the surface bonding. The electric field affects the corrugation of the PES which decreases for negative fields and increases for positive fields. In the structural investigation, we considered the relaxation of the adsorbate and the surface. The highest relaxation in a direction perpendicular to the surface was observed for gold atoms, whereas silver atoms presented the highest relaxation in a plane parallel to the surface. The surface relaxation is more important in the low coverage limit. The surface bonding was investigated by means of the total and projected density of states analysis. The highest ionic character was observed on the copper surface whereas the highest covalent character occurs on gold. This leads to a strong dependence of the PES with the tilt angle of the adsorbate on Au(111) whereas this dependence is less pronounced on the other metals. Thus, the adsorbate-relaxation and the metal-relaxation contributions to the binding energy are more important on gold. The adsorption of thiols on gold was investigated on the 111 surface as well as on a surface with gold adatoms in order to elucidate the effect of thiols on the surface diffusion of gold. The CH(3)CH(2)S radical adsorbs ontop of the gold adatom. The diffusional barrier of the CH(3)CH(2)SAu species is lower than that for a bare gold adatom and is also lower than that for the bare thiol radical. The adsorption of the molecular species CH(3)SH and CH(3)CH(2)SH was also investigated on Au(111). They adsorb via the sulfur atom ontop of a gold atom. On the other hand, the adsorption of the alkanethiol radicals on the perfect 111 surfaces occurs on the face centered cubic (fcc)-bridge site in the low coverage limit for all metals and shifts toward the fcc site at high coverage on copper and silver.     

In-situ infrared study of the adsorption and oxidation of oxalic acid at single-crystal and thin-film gold electrodes: a combined external reflection infrared and ATR-SEIRAS approach

The adsorption and oxidation of oxalic acid at gold electrodes were studied by in-situ infrared spectroscopy. External reflection experiments carried out with gold single-crystal electrodes were combined with internal reflection (ATR-SEIRAS) experiments with gold thin-film electrodes. These gold thin films, with a typical thickness of ca. 35 nm, were deposited on silicon substrates by argon sputtering. As previously reported for evaporated gold films, the voltammetric curves obtained in sulfuric acid solutions after electrochemical annealing show typical features related to the presence of wide bidimensional (111) domains with long-range order. The in-situ infrared data collected for solutions of pH 1 confirmed the potential-dependent adsorption of either oxalate (Au(100)) or a mixture of bioxalate and oxalate (Au(111), Au(110), and gold thin films) anions in a bidentate configuration. The better signal-to-noise ratio associated with the SEIRA effect in the case of the gold thin-film electrodes allows the observation of the carbonyl band for adsorbed bioxalate that was not detected in the external reflection experiments. Besides, additional bands are observed between 2000 and 3000 cm(-)(1) that can be tentatively related to the formation of hydrogen bonds between neighboring bioxalate anions. The intensities of these bands decrease with increasing solution pH values, disappearing for pH 3 solutions in which adsorbed oxalate anions are the predominant species. The analysis of the intensities of the nu(s)(O-C-O) and nu(C-OH) + delta(C-O-H) bands for adsorbed oxalate and bioxalate, respectively, suggests that the pK(a) for the surface equilibrium between these species is significantly lower than that for the solution equilibrium.     

偶氮苯衍生物-β-环糊精包合物的自组装行为

研究了4-N（2’-巯基-乙基）羧基酰胺偶氮苯（Azo）与β-环糊精（β-CD）形成的包合物在金表面上的自组装行为．X射线光电子能谱（XPS）结果证实,Azo和Azo与β-CD形成的包合物均可在金表面上自组装形成单分子层膜．在包合物形成的自组装膜中,Azo与β-CD的摩尔比约为1：1．Azo自组装膜的电化学反应表现速率常数（Kobs）随组装时间的延长而明显减小,反映出自组装膜的排列随时间延长而趋于更加致密,从而抑制了偶氮苯基团的电化学诱导构型转化,降低了其电活性．而Azo与β-CD包合物自组装膜的Kobs值随组装时间变化不大,在组装76h以后,包合物自组装膜的Kobs比单纯偶氮苯自组装膜的Kobs高2个数量级以上．表明环糊精能够将偶氮苯分子隔开,从而抑制了偶氮苯在自组装膜中的聚集作用,有效地提高其电化学活性．     

Silver residues as a possible key to a remarkable oxidative catalytic activity of nanoporous gold

Recently, several forms of unsupported gold were shown to display a remarkable activity to catalyze oxidation reactions. Experimental evidence points to the crucial role of residual silver present in very small concentrations in these novel catalysts. We focus on the catalytic properties of nanoporous gold (np-Au) foams probed via CO and oxygen adsorption/co-adsorption. Experimental results are analyzed using theoretical models represented by the flat Au(111) and the kinked Au(321) slabs with Ag impurities. We show that Ag atoms incorporated into gold surfaces can facilitate the adsorption and dissociation of molecular oxygen on them. CO adsorbed on top of 6-fold coordinated Au atoms can in turn be stabilized by co-adsorbed atomic oxygen by up to 0.2 eV with respect to the clean unsubstituted gold surface. Our experiments suggest a linking of that most strongly bound CO adsorption state to the catalytic activity of np-Au. Thus, our results shed light on the role of silver admixtures in the striking catalytic activity of unsupported gold nanostructures.     

Hydrothermal Synthesis of a rGO Nanosheet Enwrapped NiFe Nanoalloy for Superior Electrocatalytic Oxygen Evolution Reactions

Graphene‐based hybrid nanostructures possess many advantages in the field of electrochemical energy applications. In this work, a facile and efficient hydrothermal approach has been developed for the preparation of NiFe alloy nanoparticles/rGO hybrid nanostructures, in which the nanoparticles are well combined with rGO nanosheets and the size of the nanoparticles is about 100 nm. Moreover, the electrochemical oxygen evolution reaction (OER) tests confirmed that the obtained NiFe/rGO hybrid nanostructures possess notably higher activity than both the rGO‐free NiFe nanoparticles and pure Ni/rGO hybrids, and the optimal NiFe ratio is 2:1. The OER overpotential at 20 mA cm^?1?2 with Ni₂Fe/rGO is as low as 0.285 V, which is 96 mV lower than that of pure Ni/rGO hybrids. Meanwhile, the Ni₂Fe/rGO catalyst has excellent stability. Therefore, this work contributes a facile and efficient method to prepare a NiFe alloy nanoparticles/rGO hybrid structure for potential applications in the field of electrochemical energy devices, such as electrochemical water splitting cells, rechargeable metal/air batteries, etc.     

Preoxidation of CO on Os-modified Pt(111): a comparison with Ru-modified Pt(111)

The variation in CO adsorption structures during the preoxidation of CO on Os-modified Pt(111) (Pt(111)/Os) was investigated using cyclic voltammetry and electrochemical scanning tunneling microscopy. The spontaneous deposition of Os on Pt(111) resulted in randomly scattered islands with a coverage range of 0.13-0.54. During preoxidation on Pt(111)/Os, a phase transition from (2 × 2)-α to (√19 × √19) via the transient structures of (2 × 2)-β and (1 × 1) took place as on unmodified Pt(111). As the amount of Os increased, however, the transient structures of (2 × 2)-β and (1 × 1) appeared at lower potentials with higher populations. When the population of the transient structures was greater than 50%, an oxidative CO stripping process took place to the structure of (√19 × √19), completing the preoxidation. These observations strongly support the idea that the presence of Os increases the mobility of adsorbed CO by electronic modification of the Pt(111) surface (electronic effect). In addition, the results obtained with Pt(111)/Os were compared with those of Pt(111)/Ru.     

Self-organization of a self-assembled supramolecular rectangle, square, and three-dimensional cage on Au111 surfaces

The structure and conformation of three self-assembled supramolecular species, a rectangle, a square, and a three-dimensional cage, on Au111 surfaces were investigated by scanning tunneling microscopy. These supramolecular assemblies adsorb on Au111 surfaces and self-organize to form highly ordered adlayers with distinct conformations that are consistent with their chemical structures. The faces of the supramolecular rectangle and square lie flat on the surface, preserving their rectangle and square conformations, respectively. The three-dimensional cage also forms well-ordered adlayers on the gold surface, forming regular molecular rows of assemblies. When the rectangle and cage were mixed together, the assemblies separated into individual domains, and no mixed adlayers were observed. These results provide direct evidence of the noncrystalline solid-state structures of these assemblies and information about how they self-organize on Au111 surfaces, which is of importance in the potential manufacturing of functional nanostructures and devices.     

铈基模型催化剂的纳米结构:有序CeO2(111)薄膜上的Pt-Co纳米颗粒

制备并表征了原子分散的模型体系:氧化铈负载的Pt-Co核壳催化剂.采用超高真空物理气相沉积法制备了有序CeO2(111)膜上的Pt@Co和Co@Pt核壳纳米结构,并用同步辐射光电子能谱和共振光发射光谱对其进行了研究.在低Co覆盖率(0.5 ML)下Co在CeO2(111)上沉积生成Co-CeO2(111)固溶体,然后在更高Co覆盖率下生长为金属Co纳米粒子.Pt@Co和Co@Pt两种模型结构在300-500 K温度范围内都能稳定地抗烧结.在500 K退火后, Pt@Co纳米结构含有接近纯的钴壳,而Co@Pt中的铂壳部分被金属钴覆盖.在550 K以上,在Pt@Co和Co@Pt纳米结构中近表面区域的重新排序中产生了次表层的Pt Co合金和富铂外壳.对于Co@Pt纳米粒子,近表面区域的化学有序性取决于沉积铂壳的初始厚度.无论初始铂壳的厚度如何,在有氧存在下对Co@Pt纳米结构进行退火,都会导致Pt-Co合金的分解以及Co的氧化.Co的逐步氧化与吸附质诱导的Co偏析共同导致在负载的Co@Pt纳米结构表面形成厚的Co O层.这一过程伴随着CeO2(111)薄膜的裂解,以及在550K以上氧气中退火后CeO2包裹氧化的Co@Pt纳米结构.很明显,于不同温度下在氧气和氢气的氧化-还原循环过程中,无论铂的初始厚度是多少,负载的Co@Pt纳米颗粒的结构和化学成分的变化主要是由氧化所致,而还原处理的影响则很小.