Polycrystalline Gold Surface Research Articles

Electrosynthesis of cadmium selenide films on a selenium-modified gold surface

A two-step method for the growth of CdSe films is described that is based on the initial chemical modification of a polycrystalline gold surface with a selenium overlayer. In the second step, this overlayer is cathodically stripped as Se 2− in a Se(IV)-free electrolyte medium (0.1 M Na 2SO 4) that is dosed with the requisite amount of Cd 2+ ions. Unlike the classical cathodic route, this new approach does not suffer from problems with excess Se admixed with CdSe. The two-step approach is validated using a combination of voltammetry, microgravimetry, and photoelectrochemical experiments.

A biosensor approach to probe the structure and function of the adsorbed proteins: fibrinogen at the gold surface

The kinetics of adsorption and surface structure of adsorbed layers of the human fibrinogen on the gold surface, determined by Surface Plasmon Resonance (SPR) and Atomic Force Microscopy (AFM) analysis, was employed to probe the lateral distribution and preferred orientation of protein molecules within the monolayer. In this study, special sets of immunoassays are presented for fibrinogen adsorption/conformation analysis. The results show that kinetic parameters of antigen-antibody interac- tions are directly related to the interfacial conformation of fibrinogen molecules. Various interfacial structures of adsorbed fibrinogen aggregates, namely single, bi- and three- molecular aggregates, were obtained using a combination of AFM imaging and SPR analysis. Adsorption of fibrinogen onto the surface of polycrystalline gold is a complex process including surface-induced unfolding, local self- assembly and adsorption, occurring concurrently with n and on the time scale of n each other. This result confirmed the utility of the proposed approach for detecting the spatial distribution and biofunctional properties of specific proteins adsorbed from biological liquids in biosensors.

Electroreduction of oxygen on a (100)-like polycrystalline gold surface in an alkaline solution containing Pb(II)

In a previous work from this laboratory, it was shown that it is possible to electrochemically modify a polycrystalline surface of gold so that it behaves towards O 2 reduction like the (100) gold plane. To check whether this similarity in behavior is due to the presence on the (100)-like surface of sufficiently large (100) spots, the process of the upd of Pb(II) was used in this work as a probe of the surface crystallographic orientations both in deaerated and in O 2−saturated solutions. The observed cyclic voltammograms (CV) and polarization curves were entirely different from those reported in the literature for a typical polycrystalline gold surface. The CV data were analyzed in an attempt to gain insight on the crystallography of the (100)-like surface. The influence of adsorbed lead on the electroreduction of oxygen in 1 M NaOH on the (100)-like surface was also scrutinised. It was found that the reduction is differently affected by adsorbed Pb(II) ions and by underpotential deposited Pb adatoms. Adsorbed Pb(II) do not change the reduction mechanism, it only alters the rate constant for HO 2 − disproportionation, whereas the surface becomes able to reduce O 2 to OH − without desorption of HO 2 − to the solution bulk when modified by Pb adatoms. The details of the electroreduction of O 2 and of HO 2 − are thoroughly discussed in relation to the surface processes underneath. Together with the CV data, they show that there are no significant (100) spots on the (100)-like surface and that this surface is structurally different from the gold (100) plane.

Formation of Microcrystalline Zirconia Using the Functionalized Interface of a Self-Assembled Monolayer of Dithiol on Polycrystalline Gold at Room Temperature

A self-assembled monolayer of dithiol, namely 1,4-benzenedimethanethiol (BDT), on a polycrystalline gold surface was effectively used to carry out a two-dimensional reaction between pendent −SH and zirconium ion in solution to produce oriented monoclinic zirconia at room temperature on potentiodynamic cycling. Cyclic voltammetry (CV), X-ray diffraction (XRD), and scanning electron microscopy (SEM) were used to follow the monolayer formation, Zr-attachment on the SAM surface, and the subsequent formation of ZrO2 phase. The XRD analysis shows the growth of the crystal as monoclinic form with a size ranging from 5 to 15 μm. A tentative reaction scheme and also a mechanism are proposed to understand the role of SAM in changing the size and habit during the crystallization of ZrO2 at room temperature.

Fluorescence probes for chemical reactivity at the interface of a self-assembled monolayer

New methods for reversible photoimaging of spatially defined surfaces represent attractive technological applications of thin organized films. Surface immobilization of photoresponsive derivatives of alkanethiols on gold can influence attainable excited state partitioning among competing reaction pathways. Surface fluorescence measurements on such reflective metal surfaces can be used to monitor surface composition when highly emissive molecules are attached to a self-assembled monolayer. These measurements can also be used as a probe for in situ chemical modification of a self-assembled monolayer and for photoreactivity of coumarin- and anthracene-derivatives attached at the end of a variable length tether anchored through an alkyl sulfide onto a smooth polycrystalline gold surface. The observed changes in the fluorescence can be related to local structure and to the unusual environment encountered by the probe molecule within the self-assembled monolayer.

Infra-red reflection absorption spectroscopic study on adsorption structures of acrolein on polycrystalline gold and Au(111) surfaces under ultra-high vacuum conditions

Infra-red reflection absorption (IRA) spectra were measured at 80 K under ultra-high vacuum conditions for acrolein adsorbed on two kinds of gold films; Au(111) and polycrystalline gold surfaces. Upon increasing the amount of exposure from 0.02 to 200 L (1 L=1×10 −6 Torr·s), the adsorbate at Au(111) gave rise to a series of sharp IRA bands due to a CH 2 out-of-plane wagging vibration [ ω(CH 2)] successively, indicating discrete adsorption states, i.e. 964 (type 1)→978(type 1′)→991(type 2)→1003 cm −1(type 3). All these states have the molecular plane parallel to the surface; type 1 is in an isolated state, and type 2 is in an associated state with a two-dimensional arrangement, whereas type 3 forms an ordered multilayered structure. Type 1′ was tentatively assigned either to a trapped state at step sites or to an associated state forming small oligomers at the surface. Only type 3 gives rise to IRA bands due to ν(CO), which appears at 1677 cm −1 as a singlet at relatively small exposure levels and splits into doublets, giving the 1686 and 1672 cm −1 components at 2.0 L. The doublets were explained as being due to a crystal field splitting, which conforms to the fact that the adsorbate forms an ordered three-dimensional arrangement. The IRA spectrum of type 3 is readily converted to that of a more stable polycrystalline state upon increasing the temperature from 80 to 100 K. Thus, type 3 is a thermodynamically metastable state. Acrolein adsorbed on a polycrystalline gold film assumes an amorphous state in the exposure level of 0.06–4.8 L, giving broad IRA bands due to ν(CO) and ω(CH 2) in the 1686–1699 and 974–991 cm −1 regions, respectively. The IRA spectra of acrolein adsorbed on Ag(111) were also measured, which indicated that the adsorbates exist in a less ordered state than those on Au(111), although a multilayered structure gives IRA features that are almost identical with those of type 3.

Impedance analysis of self-assembled naphthalene disulfide monolayer on gold using external redox probes

A self-assembled monolayer (SAM) of naphthalene disulfide (NDS) has been formed on a polycrystalline gold surface. Techniques like cyclic voltammetry and impedance measurement with an external redox probe along with scanning tunnelling microscopy (STM) are used to investigate the monolayer structure and ion permeation through it. Impedance analysis shows that the rate constant of a fast redox couple like Fe(CN)64−/3− has decreased from 2.3×10−2 to 9.4×10−4 cm s−1 due to the barrier provided by the monolayer. The STM image clearly shows the presence of defects (as pinholes) on the monolayer surface in agreement with the residual current observed in the voltammograms.

Fluorescence and Redox Activity of Probes Anchored through an Aminotrithiol to Polycrystalline Gold

A self-assembled monolayer of tris[3-(mercaptopropyl)methyl)]-N-(aminoacetyl)amine (1) covalently anchored onto a polycrystalline gold surface acts as an effective linking agent for binding surface probes that can be activated by light or by an applied potential. Fluorescence from two appended chromophores was observed when scattering of incident light from the gold support was minimized. Monolayers of 1 coupled to an end-terminated ferrocenyl group showed blocking electrochemical behavior by cyclic voltammetry, while retaining the characteristic redox activity of the attached group. These results indicate high yields of in situ surface coupling and significant local order within the two-dimensional array.

Kinetics of Formation of Long-Chain n-Alkanethiolate Monolayers on Polycrystalline Gold

The effect of immersion time on the ordering properties of self-assembled docosanethiolate (C22) monolayers has been studied by reflection/absorption infrared spectroscopy (RAIRS). Starting with a chemically cleaned polycrystalline gold surface, we have found that an immersion time of 45 s in micromolar thiol solutions is sufficient to produce ordered “monolayer” films. These films have RAIR spectral features (peak position, line width, and integrated intensity) similar to those obtained with much longer immersion times. Decreasing the immersion time below 45 s gradually increases the disorder. The temperature dependence of the CH2 antisymmetric stretching (d-) peak position of thiol films obtained with an immersion time of 120 s is identical to ones obtained with very long immersion time (0.5 h to several days). In contrast, films obtained with a 5 s immersion time disorder at a lower temperature. Apart from immersion time and temperature, a large solvent effect has been observed based on real-time RAIRS measurements of the continuing film ordering during aging in vacuum. Finally, a new assignment of the shoulder appearing around 2931 cm-1 is proposed. We present arguments suggesting that the generally accepted Fermi resonance assignment of this shoulder should be modified to include a contribution from another vibrational mode.

Measurement of Electron Transfer Rates between Adsorbed Azurin and a Gold Electrode Modified with a Hexanethiol Layer

In this work we report the application of electroreflectance (ER) spectroscopy for the study of the redox reaction of the nonheme protein azurin. Azurin was irreversibly adsorbed on a polycrystalline gold surface modified with a hexanethiol layer. Surface-enhanced resonant Raman spectroscopy (SERRS) demonstrated that the immobilized azurin's copper site remained intact upon adsorption and that it underwent reversible reduction–oxidation. Cyclic voltammetry (CV) data showed that the peak current of the oxidation (or reduction) wave depended linearly on the sweep rate as appropriate for an immobilized species. The formal potential of the adsorbed azurin was 95 ± 8 mV vs Ag/AgCl electrode, nearly identical to the earlier observed value of the native azurin in solution. All measurements were analyzed in the context of Marcus theory of electron transfer. The separation of the reduction–oxidation peaks in the CV data gave an estimate of the electron transfer (ET) rate in the range from 4 to 12 s−1. ER, with light of wavelength of 640 nm, yielded a strong signal at the same potential as the midpotential determined in CV. The frequency dependence of the ER response was consistent with an ET rate of 150 to 200 s−1. Electrochemical impedance measurements indicated an ET rate of the order of 300 s−1. The disparity between the ET rates measured with CV and ER suggests that ET may not be a single-step process. The results also point to the importance of hydrophobic interactions in adsorption and redox transformations of azurin.

Three-probe voltammetric characterisation of octadecanethiol self-assembled monolayer integrity on gold electrodes

Abstract The passivating behaviour of self-assembled octadecanethiol monolayers on polycrystalline gold electrodes was tested by means of the cyclic voltammetry technique in the presence of three different redox probe molecules. The three compounds chosen for this purpose, ruthenium hexamine chloride [Ru(NH 3 )Cl 3 ], potassium ferrocyanide [K 4 Fe(CN) 6 ] and benzoquinone, were selected on the basis of the differences in their heterogeneous electron transfer rate constants and their hydrophobic/hydrophilic character. Our studies show that passivating properties of octadecanethiol monolayers against these probes are different. The results obtained are discussed in terms of Tafel approach and monolayer integrity. It appears that the screening of self-assembled films with different probes is necessary in order to properly characterise the film integrity and passivating behaviour. Additionally, the interaction of octadecanethiol molecules with a polycrystalline gold surface was evaluated by means of potentiostatic pulse measurements.

Electrochemical Oxidation of Glucose on Polycrystalline Gold Surface by Treating with Square Wave Potential Signal

AbstractThis study concerns with treatment of polycrystalline gold surface with square wave potential program to enhance the signal of electrochemical oxidation of glucose. After such perturbation the glucose oxidation wave is increased almost ten times. Consequently, gold is a useful electrode for sensor applications.

Self-assembly of short and long-chain n-alkyl thiols onto gold surfaces: A real-time study using surface plasmon resonance techniques

Gold surfaces coated with monolayers of alkyl thiols are significant in the field of biosensors and chromatography. There is a general concern about the quality of coatings in terms of surface density and voids. The self-assembly of the short-chain hexane (C6) thiol and long-chain dodecane (C12) and hexadecane (C16) thiols to polycrystalline gold surfaces has been investigated in situ and in real time using surface plasmon resonance (SPR) spectroscopy and surface plasmon microscopy (SPM). The self-assembly was followed based on observed changes in reflectivity at a fixed angle of incidence as a function of time. Our results indicate that the data for hexane, dodecane, and hexadecane thiol adsorption to gold surfaces were best fit by a two-step process rather than a one-step process. The mechanism may involve fast adsorption to the surface to give 80% (C12, C16) or 50% (C6) coverage followed by a slow (100-fold slower) "rearrangement" of the adsorbed thiol. SPM shows these surfaces to be smooth and homogenous at a 4 μm scale. An understanding of the process of rearrangement could lead to control of the quality of coatings for analytical applications. Key words: surface plasmon, alkyl thiols, gold, monolayer, kinetics, ellipsometry, microscopy.

Negative surface ionization of hydrogen atoms and molecules

The total yield of H− ions, Y(Ein), produced in backscattering of low-energy H+ and H+2 ions from polycrystalline gold, tungsten, and molybdenum converter surfaces was measured at normal incidence in the energy range Ein=2–30 eV per nucleus. The yield per nucleus is independent of the ion mass. This indicates that the molecular ions are dissociated before colliding with the converter surface. A universal expression for Y(Ein) was developed by combining the electron tunneling theory with atomic scattering theory. This expression agrees well with measurements. The yield is completely characterized by two parameters, Eth/RE and RNη0, which can be determined experimentally: Y=0 for Ein=Eth/RE, and Y approaches the maximum yield Rη0 as Ein increases. These parameters were determined from measured H− yields in ion beam backscattering experiments, as well as for backscattering of thermal distributions of hydrogen atoms. For beam experiments, the maximum yield of 0.3 per nucleus was obtained for Mo/Cs converters with 1.5 eV work function. A higher maximum yield of 0.42 was obtained from experiments on backscattering thermal distributions of H atoms. This is attributed to high extraction fields. The universal yield formula made it possible to compare the results of the two different types of experiments.

Electrochemical Oxidation of Glucose on Single Crystal and Polycrystalline Gold Surfaces in Phosphate Buffer

The electrochemical oxidation of glucose has been studied in phosphate buffer (pH = 7.4) on single crystal and polycrystalline gold electrodes using electrochemical techniques, ex situ NMR, in situ FTIR spectroscopy, and isotope labeling. Under these conditions, the results indicate that the rate determining step for the electro‐oxidation of glucose is bond breaking between the hydrogen atom and the carbon atom. Gluconolactone appears to be an intermediate and sodium gluconate is the reaction product. First order kinetics with respect to glucose were also found. Gold would be a useful electrode for sensor applications, if the inhibition of the glucose oxidation by the adsorption of could be avoided.

Bulk copper electrodeposition on gold imaged by in situ STM: morphology and influence of tip potential

Electrochemical measurements were carried out simultaneously with acquisition of in situ STM images of copper electrodeposition at low cathodic overpotentials and subsequent dissolution from the underlying polycrystalline gold surfaces. The morphologies of the copper deposits were examined for correlation with features of the current-voltage diagram. Copper growth is by nucleation and formation of 3D islands. During the initial stages of bulk copper growth the potentials were fixed at selected values and a balance observed between formation of polycrystalline copper nuclei and of copper crystals. After the first cycle of copper deposition and dissolution the morphology of the polycrystalline gold surface had apparently changed into a recrystallized phase of a copper-gold alloy. At a given stage of the cycle the potential of the electrode was found to depend linearly on the tip potential. In a wide range of tip potentials the onset of copper deposition and end of dissolution showed a potential separation of 59 ± 5 mV indicating a single electron process.

Underpotentially Deposited Metal Layers of Silver Provide Enhanced Stability to Self-Assembled Alkanethiol Monolayers on Gold

This letter details a straightforward electrochemical method for increasing the thermal stability of alkanethiol-based self-assembled monolayers (SAMs) on polycrystalline gold surfaces. Ex situ ell...

Electrochemical deposition and stripping of copper, nickel and copper nickel alloy thin films at a polycrystalline gold surface: a combined voltammetry-coulometry-electrochemical quartz crystal microgravimetry study

Cyclic voltammetry, anodic stripping voltammetry (ASV) and coulometry were used in conjunction with electrochemical quartz crystal microgravimetry (EQCM) and ex situ X-ray photoelectron spectroscopy (XPS) for a study of the electrodeposition and stripping of Cu, Ni and CuNi alloy thin films at a polycrystalline Au surface. A deaerated Watts bath was employed for the electrodeposition of Ni and CuNi thin films, the bath containing Cu 2+ ions (Ni:Cu = 200:1 mole ratio) in the latter case. These films showed the most satisfactory stripping in either 0.1 M HCl or 0.1 M H 2SO 4. Coulometry was used in conjunction with EQCM data for mapping the (potentiostatic) plating efficiency as a function of deposition potential in the Cu and Ni systems. For the CuNi alloy, similar data were acquired by additionally utilizing XPS compositional information and assuming current additivity in the (non-interactive) co deposition of Cu and Ni. Finally, the feasibility of employing the combined ASV-EQCM technique for analysis of CuNi/Ni bilayers is presented.

A study of surface diffusion on gold with an atomic force microscope

Surface self-diffusion on a polycrystalline gold surface has been measured in the temperature range from 300 K up to 773 K using an atomic force microscope (AFM). The so called single surface scratch method was used for this study. A technique was developed to produce reproducible surface scratches with the AFM. The analysis of these scratches was made by AFM, too. So, with these methods the activation energy (0.4 eV) of the surface diffusion process was deduced by time and temperature dependent scratch profile measurements.

Bulk crystalline copper electrodeposited on polycrystalline gold surfaces observed by in-situ scanning tunnelling microscopy

Bulk copper electrodeposition onto technical gold surfaces in electrolytes of 0.05 M H 2SO 4 and 1 mM CuSO 4 was investigated by in-situ scanning tunnelling microscopy at fixed overpotentials. At potentials between -60 and -30 mV the growth of bulk copper proceeds in cycles of nucleation, agglomeration and crystallization. Crystalline copper is seen as involving an intermediate stage in the progress of growth. The final stage in the growth involves an equilibrium of copper electrochemically dissolving and precipitating. The drift velocity was measured for a gold surface subjected to flame annealing and subsequently installed in the cell compartment. It was found that the drift velocity decays with time in an exponential-like manner, and a 70 min waiting time before experiments with atomic resolution is recommended. Atomic resolution on Au(111) has been obtained, and an apparent surface reconstruction was observed. It is suggested that in reality no reconstruction took place, and that the observation was due to a distortion of the image caused by a constant drift velocity. A mathematical expression which relates the observed surface structure to the drift velocity is presented.