Electrochemical oxidative formation of ordered monolayers of thiol molecules on Au(111) surface

Electrochemical oxidative formation of thiolate monolayers on a Au(111) surface in KOH ethanol solutions of various thiol concentrations is described. The formation process was investigated by electrochemistry, in situ scanning tunneling microscopy (STM), and surface X‐ray diffraction (SXRD). The reductive charge in the linear sweep voltammogram after keeping the potential at +0.1 V increased with holding time and reached the saturated value of 103 µC cm^?2, corresponding to the full monolayer coverage of the ( ) structure. The desorption peak shifted negatively with holding time even after the monolayer was formed, suggesting that ordering of the monolayer requires a much longer time than full coverage adsorption. The herringbone structure, corresponding to the ( × 23) structure, was observed on the Au(111) surface in KOH ethanol solution by in situ STM, which shows that a clean surface was exposed. When hexanethiol ethanol solution was added into the ethanol solution at ?450 mV so that the final thiol concentration was higher than ca. 5 µM, generation of vacancy islands (VIs) was observed, which shows the potentiostatic monolayer formation. When the potential was scanned positively from ?950 mV where a clean reconstructed Au(111) surface was exposed, generation of VIs was observed accompanied by anodic current flow. During both oxidative adsorption and reductive desorption of the monolayer, the shape of the steps of the gold surface changed drastically, which suggests that the gold atoms on the surface are extremely mobile during the monolayer formation. SXRD measurement confirmed the surface reconstruction lifting upon monolayer formation. © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 9: 199–209; 2009: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.200900002     

Electrochemical metal deposition on top of an organic monolayer

Electrochemical deposition of metals (platinum or gold) only on top of an organothiolate, 1,4-benzenedimethanethiol (BDMT) or hexanedithiol (HDT), self-assembled monolayer (SAM) on a Au(111) substrate was achieved by electrochemical reduction of PtCl(4)(2-) or AuCl(4)(-) ion, which was preadsorbed on one free thiol end group of the dithiol SAM formed on a Au surface, in a metal-ion-free sulfuric acid solution at potentials more negative than the reduction potential of the metal ion. Angle-resolved X-ray photoelectron spectroscopy (AR-XPS) measurement after the reduction of preadsorbed PtCl(4)(2-) ion on BDMT/Au(111) electrode showed the presence of Pt not underneath but on top of the BDMT SAM. After a negative potential scan of the Pt/BDMT/Au(111) electrode to -1.30 V in 0.1 M KOH solution, a typical cyclic voltammogram of a clean Au(111) electrode was obtained, showing that the BDMT SAM with a Pt layer was reductively desorbed. These results proved that a Pt-BDMT SAM-Au substrate sandwich structure without a short circuit between the two metals was successfully constructed by this technique. Furthermore, a decanethiol (DT) monolayer was constructed on a Au layer, which was formed by the reduction of preadsorbed AuCl(4)(-) ion on HDT/Au(111) electrode. The formation of DT/Au/HDT/Au(111) structure was confirmed as two cathodic peaks corresponding to reductive desorption of DT from Au on top of the HDT/Au(111) at -0.97 V and that of Au/ HDT from Au(111) at -1.12 V were observed when potential was scanned negatively to -1.35 V.     

New method to measure packing densities of self-assembled thiolipid monolayers

For a monolayer of 2,3-di-phytanyl-sn-glycerol-1-tetraethylene glycol-D,L-a-lipoic acid ester lipid (DPTL) self-assembled (SAM) at a gold electrode surface we propose a new method to determine the charge number per adsorbed molecule and the packing density (area per molecule) in the monolayer. The method relies on chronocoulometry to measure the charge density at the SAM covered gold electrode surface. Two series of measurements have to be performed. In the first series, charge densities are measured for a monolayer transferred from the air-solution to the metal-solution interface using the Langmuir-Blodgett (LB) technique. This series of measurements allows one to determine charge numbers per adsorbed DPTL molecule. The second series is performed using a gold electrode covered with a self-assembled monolayer. The charge densities obtained in this series are then used to calculate the packing density with the help of charge numbers per adsorbed DPTL determined in the first series. The area per adsorbed molecule determined by the new method was compared to the area per molecule determined by the popular reductive desorption method. The molecular area determined with the new method is about 20% larger than the area calculated from the van der Waals model, which is a physically reasonable result. In contrast, the popular reductive desorption method gives an area per molecule 20% lower than the minimum estimated based on a van der Waals model. This is a physically unreasonable result. It is also shown that the charge numbers per adsorbed molecule depend on the electrode potential and may assume values smaller than the number of electrons participating in the reductive desorption step. An explanation of the origin of the "partial charge numbers" is provided. We recommend the new method be used in future studies of thiol adsorption at metal surfaces.     

Electrochemical study and detection of perphenazine using a gold electrode modified with decanethiol SAM

A novel method for the determination of perphenazine has been developed. The method is based on the accumulation of perphenazine at a gold electrode modified with decanethiol (DEC) self-assembled monolayer (SAM) and its oxidation at about 0.6 V (vs. saturated calomel electrode (SCE)). Because some coexistent electroactives were blocked and perphenazine was selectively accumulated by the SAM, the electrode exhibited good selectivity and sensitivity. Various conditions were optimized for practical application. Under the selected conditions (i.e. 0.05 M pH 10 sodium borate buffer, accumulation time: 120 s, accumulation potential: −0.4 V, scan rate: 100 mV s⁻¹), the anodic stripping peak current was linear to perphenazine concentration in the ranges of 6×10⁻⁹–5×l0⁻⁷ and 5×10⁻⁷–5×10⁻⁶ M with correlation coefficients of 0.998 and 0.995, respectively. For a 1.0×10⁻⁶ M perphenazine solution, the relative standard deviation of peak height was 2.3% (n=8). This method was applied to the determination of perphenazine in some drugs and the recovery was 92–101%. In addition, it was found that in the presence of perphenazine, the SAM structure changed a little and more needle holes appeared. However, the SAM could recover the original form when perphenazine and its redox product were removed from the monolayer by repeatedly cycling the electrode in a blank solution for a minute. The modified electrode was characterized by alternating current impedance and electrochemical probe.     

Comparative electrochemical study of self-assembly of octanethiol from aqueous and aqueous ethanol solutions on a gold electrode

The self-assembly of octanethiol (OT) on the surface of a polycrystalline gold electrode in aqueous and aqueous ethanol thiol-containing (1 × 10^–4 М) 0.1 М NaClO₄ solutions was studied. The blocking properties and electrochemical stability of monolayer OT films were studied by chronopotentiometry during OT adsorption under the open circuit conditions (chronoamperometry at a fixed potential) combined with cyclic voltammetry for modified Au/OT electrodes. It was found from the change in the rate of electrochemical reactions in the range of monolayer stability potentials that in aqueous media, compact insulating OT monolayer films formed at a open circuit potential within ~100 s, and the shift of the adsorption potential toward negative values (to–0.6 V) allowed a considerable decrease in the monolayer self-assembly time. The potential shift toward higher negative values (–0.9 V) leads to a removal of OT from the electrode surface during the reductive desorption, with a multipeak current signal recorded on the voltammograms. A transition from aqueous to aqueous ethanol solutions accelerated the formation of an insulating OT monolayer (≈6 s) and led to a change in the shape of the desorption current peak, whose value was almost independent of the ОТ accumulation time and potential.     

Adsorptive Stripping Voltammetric Determination of Phenol at an Electrochemically Pretreated Carbon-Paste Electrode with Solid Paraffin as a Binder

An adsorptive stripping voltammetric method for determination of phenol at an electrochemically pretreated carbon-paste electrode has been developed. Solid paraffin was used as the binder of the carbon-paste electrode. The carbon-paste electrode was pretreated in the solution of 0.001 mol L⁻¹NaOH by holding it at +1.8 V (versus an Ag/AgCl electrode) for 5 min. On the pretreated electrode, the adsorption of phenol was greatly enhanced. Phenol was accumulated in NH₃–NH₄Cl (pH 9.25) medium at the potential of +0.1 V (versus Ag/AgCl electrode) for a certain time and then determined by second order differential anodic stripping voltammetry. An oxidative peak was observed at about +0.66 V. The relationship between second order peak current and phenol concentration was linear in the range of 2.5 × 10⁻⁷–5.0 × 10⁻⁶mol L⁻¹phenol, and the detection limit was 5.0 × 10⁻⁸mol L⁻¹. The method has been applied to the determination of phenol in tap water and waste water. The relative standard deviation (six determinations) was less than 3.5%.     

Electrochemical desorption of n-alkylthiol SAMs on polycrystalline gold: studies using a ferrocenylalkylthiol probe

The reductive voltammetric desorption of n-alkylthiol self-assembled monolayers (SAMs) was studied using ferrocenyldodecanethiol (FcC12SH) as a probe. A short (10 s) incubation in a 2 mM FcC12SH solution labels the pre-existing defect sites present in a tetradecanethiol (C14S-Au) SAM. Additional defects in a C14S-Au SAM are then created by voltammetric cycling to reductive potentials. The effects of changing the desorption potential and the hold time at the desorption potential were investigated by monitoring the change in the two characteristic FcC12S-Au peaks. These two peaks are associated with FcC12S-Au filling individual single site defects (peak I at 260 mV) and pinhole defects (peak II at 380 mV). The reductive desorption potential applied to a binary (FcC12S-/C14S-Au) SAM results in a partial desorption and a potential "remixing" of the phase-separated state.     

3,4-二羟基苯甲酸在自组装结构中的电化学行为

电极与有确定取向的电活性基团之间的电子传递是电化学领域的研究热点．利用二维有序薄膜固定电活性官能团是一个成熟的方法［１］,主要包括ＬＢ技术和自组装技术,这两者都存在着样品合成困难的问题．近年来,通过表面逐层反应来固定电活性官能团已有研究,但是反应过程...     

Fluorescence imaging of the oxidative desorption of a BODIPY-alkyl-thiol monolayer coated Au bead

The reductive and oxidative desorption of a BODIPY labeled alkylthiol self-assembled monolayer (SAM) on Au was studied using electrochemical methods coupled with fluorescence microscopy and image analysis procedures to monitor the removal of the adsorbed layer. Two SAMs were formed using two lengths of the alkyl chain (C10 and C16). The BODIPY fluorescent moiety used is known to form dimers which through donor-acceptor energy transfer results in red-shifted fluorescence. Fluorescence from the monomer and dimer were used to study the nature of the desorbed molecules during cyclic step changes in potential. The reductive desorption was observed to occur over a small potential window (0.15 V) signified by an increase in capacitance and in fluorescence. Oxidative readsorption was also observed through a decrease in capacitance and a lack of total removal of the fluorescent layer. Removal by oxidative desorption occurred at positive potentials over a broad potential range near the oxidation of the bare Au. The resulting fluorescence showed that the desorbed molecules remained near the electrode surface and were not dispersed over the 20 s waiting time. The rate of change of the fluorescence for oxidative desorption was much slower than the reductive desorption. Comparing monomer and dimer fluorescence intensities indicated that the dimer was formed on the Au surface and desorbed as a dimer, rather than forming from desorbed monomers near the electrode surface. The dimer fluorescence can only be observed through energy transfer from the excited monomer suggesting that the monomers and dimers must be in close proximity in aggregates near the electrode. The fluorescence yield for longer alkyl chain was always lower presumably due to its decreased solubility in the interfacial region resulting in a more efficient fluorescence quenching. The oxidative desorption process results in a significantly etched or roughened electrode surface suggesting the coupling of thiol oxidative removal and Au oxide formation which results in the removal of Au from the electrode.     

Alkanethiol monolayers at reduced and oxidized zinc surfaces with corrosion protection: a sum frequency generation and electrochemistry investigation

In this work, octadecanethiol (ODT) was demonstrated to form ordered monolayers at either electrochemically reduced or oxidized Zn surfaces, by means of sum frequency generation (SFG) spectroscopy, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The SFG spectra of ODT-modified Zn electrodes featured three methyl group resonances in the C-H vibrational region (2800-3100 cm(-1)). A significant decrease in interfacial capacitance and an increase in charge-transfer resistance were observed in EIS measurement after ODT modification. The alkane chain tilt angle of ODT within a monolayer at the Zn surface was estimated as 0 degrees with respect to the surface normal by interfacial capacitance measurement via EIS. CV and SFG investigation revealed that ODT monolayers undergo reductive desorption from the Zn electrode in 0.5 M NaOH at -1.66 V (vs SCE) and in 0.5 M NaClO4 at -1.62 V. The integrated charge consumed to the desorption of ODT is determined as 87 mC/cm2 from the reductive peak on CV curve, resulting in a coverage of 9.0 x 10(-10) mol/cm2 (5.4 x 10(14) molecules/cm2) if assuming the reduction follows a one-electron process. ODT monolayers show corrosion protection to underlying zinc at the early immersion stage in base, salt, and acid media. However, the protection efficiency was reduced with immersion time due to the presence of defects within the monolayers.     

Voltammetric Study of Methylene Blue at Thiol SAMs‐Modified Gold Electrodes

The voltammetric behavior of methylene blue (MB) at thiol self‐assembled monolayers modified gold electrodes (SAMs/Au) has been investigated. MB exhibited a redox peak at about ?0.35 V (vs.SCE) in alkaline solution at bare gold electrodes. When the gold electrodes were modified with thiol SAMs, the peak grew due to the accumulation of MB at SAMs. With the solution pH rising, more MB was accumulated, hence the peak height increased, which differed from that at bare gold electrodes. The electrode process at SAMs/Au featured the characteristics of adsorption and/or electrode reaction controlled. The enhancing action of glutathione monolayer (GSH SAM), 3‐mercaptopropionic acid monolayer (3MPA SAM) and other thiol SAMs was compared. Among these, GSH SAM made the MB peak increase more. At GSH SAM/Au, the peak height varied linearly with MB concentration over the range of 2 μM to 400 μM. So this can be developed for the determination of MB and studies concerned. The accumulation behavior caused by GSH SAM and native fish sperm dsDNA was compared. The interaction between DNA and MB was also discussed under this condition.     

Surface‐Confined Electropolymerization of Methylene Blue on Gold Electrodes

We report on the electrochemical behaviour and electropolymerization of self‐assembled monolayers (SAMs) of methylene blue (MB) on gold electrodes. The SAMs of MB on gold electrodes were prepared by immersing the substrates into a solution of 1.0 mM MB in absolute ethanol for different times at room temperature. Cyclic voltammetry experiments exhibited that reductive desorption of MB monolayer takes place at three different potentials on polycrystalline gold electrodes, while reductive desorption of MB monolayer consists of only one peak on single crystal Au(111) substrates. Calculated charge densities for different immersion times indicated that optimal immersion time for self‐assembly of MB is 96 h. Electropolymerization of SAMs of MB on gold electrode was achieved by applying 0.95 V for 1 s in 0.1 M borate buffer solution (pH: 9.0). It was observed that poly(MB) monolayers are highly stable in acidic media. ATR‐FTIR and UV‐vis spectra exhibited differences between monomer and polymer monolayers, which are attributed to surface‐confined electropolymerization. STM image of poly(MB) monolayer on Au(111) substrate revealed a surface that is covered by well‐ordered, collateral nanowires with an average size of 3 nm.     

Electrochemical surface plasmon resonance investigation of dodecyl sulfate adsorption to electroactive self-assembled monolayers via ion-pairing interactions

The redox-induced assembly of amphiphilic molecules and macromolecules at electrode surfaces is a potentially attractive means of electrochemically modulating the organization of materials and nanostructures on solid substrates via ion-pairing interactions or charge-transfer complexation. In this regard, we have investigated the potential-induced adsorption and aggregation of dodecyl sulfate, a common anionic surfactant, at a ferrocenylundecanethiolate (FcC11SAu) self-assembled monolayer (SAM)/aqueous solution interface by electrochemical surface plasmon resonance (ESPR) spectroscopy. The surfactant anions adsorb onto the electroactive SAM by specific ion-pairing interactions with the oxidized ferricinium species. The ferricinium charge density (QFc+) obtained by cyclic voltammetry and surface coverage measured by SPR indicate that the dodecyl sulfate forms an interdigitated monolayer, where half of the surfactant molecules have their sulfate headgroups paired to the surface and half have their headgroups exposed to the aqueous solution. The surface coverage of dodecyl sulfate was found to depend on both the ferricinium surface concentration and the surfactant aggregation state in solution. A maximum coverage of dodecyl sulfate on the ferricinium surface is obtained below the critical micelle concentration (cmc), in contrast to dodecyl sulfate adsorption to SAM surfaces of static positive charge. This marked difference in adsorption behavior is attributed to the dynamic generation of ferricinium by potential cycling and the specific nature of the ion-pairing interactions versus pure electrostatic ones. The results presented point to a new way of organizing molecules via electrical stimulus.     

Oxygen reduction at silver monolayer islands deposited on gold substrate

Underpotential deposition (UPD) of Ag on an Au(1 1 1) coated with a self-assembled monolayer (SAM) of decanethiol gives Ag islands having monatomic height. After reductive desorption of the SAM, Ag monolayer islands were exposed and their catalytic activities for oxygen reduction were examined by voltammetry and hanging meniscus rotating disk (HMRD) measurements. Comparison of electrochemical properties and STM images of electrodes revealed that an island with less than 750 Ag-atoms showed two-electron reduction and that an island with over than 4000 Ag-atoms is necessary to show the original ability of four-electron reduction.     

Selective reductive desorption of a SAM-coated gold electrode revealed using fluorescence microscopy

The reductive desorption of a self-assembled monolayer (SAM) of a fluorescent thiol molecule (BodipyC10SH) from Au was characterized using electrochemistry and epi-fluorescence microscopy. Molecular luminescence is quenched near a metal surface, so fluorescence was only observed for molecules reductively desorbed and then separated from the electrode surface. Fluorescence imaging showed that reductive desorption was selective, with desorption occurring from different regions of the Au electrode depending on the extent of the negative potential excursion. When desorbed, the molecules were sufficiently mobile, diffusing away from the electrode surface, thereby preventing oxidative readsorption. At sufficiently negative desorption potentials, all of the thiol was desorbed from the electrode surface, resulting in fluorescence at the air/solution interface. The selective removal of the thiol monolayer from distinct regions was correlated to features on the electrode surface and was explained through potential-dependent interfacial energies. This in situ electrofluorescence microscopy technique may be useful in sensor development.     

Immobilization of liposomes onto quartz crystal microbalance to detect interaction between liposomes and proteins

To study the interaction between liposomes and proteins, intact liposomes were immobilized on a metal planar support by chemical binding and/or bioaffinity using a quartz crystal microbalance (QCM). A large decrease in the resonance frequency of quartz crystal was observed when the QCM, modified by a self-assembled monolayer (SAM) of carboxythiol, was added to liposome solutions. The stable chemical immobilization of intact liposomes onto SAM was judged according to the degree with which adsorbed mass depended on the prepared size of liposomes, as well as on the activation time of SAMs when amino-coupling was introduced, where the liposome coverage of electrodes was 69+/-8% in optimal conditions. When avidin-biotin binding was used on amino-coupling liposome layers, liposome immobilization finally reached 168% coverage of the electrode surface. Denatured protein was also successfully detected according to the change in the frequency of the liposome-immobilized QCM. The adsorbed mass of denatured carbonic anhydrase from bovine onto immobilized liposomes showed a characteristic peak at a concentration of guanidine hydrochloride that corresponded to a molten globule-like state of the protein, although the mass adsorbed onto deactivated SAM increased monotonously.     

Electrochemical quartz crystal microbalance study of azurin adsorption onto an alkanethiol self-assembled monolayer on gold

A quartz crystal microbalance coupled with electrochemistry was used to examine the adsorption of azurin on a gold electrode modified with a self-assembled monolayer of octanethiol. Azurin adsorbed irreversibly to form a densely packed monolayer. The rate of azurin adsorption was related to the bulk concentration of azurin in solution within the concentration range studied. At a high azurin concentration (2.75 muM), adsorption was rapid with a stable adsorption maximum attained in 2-3 min. At a lower azurin solution concentration (0.35 muM), the time to reach a stable adsorption maximum was approximately 30 min. Interestingly, the maximum surface concentration attained for all solution concentrations studied by the QCM method was 25 +/- 1 pmol cm-2, close to that predicted for monolayer coverage. The dissipation was monitored during adsorption, and only small changes were detected, implying a rigid adsorption model, as needed when using the Sauerbrey equation. Cyclic voltammetric data were consistent with a one-electron, surface-confined CuII/CuI azurin process with fast electron-transfer kinetics. The electroactive surface concentration calculated using voltammetry was 7 +/- 1 pmol cm-2. The differences between the QCM and voltammetrically determined surface coverage values reflect, predominantly, the different measurement methods but imply that all surface-confined azurin is not electrochemically active on the time scale of cyclic voltammetry.     

Adsorption behaviors of poly(amido amine) dendrimers with an azacrown core and long alkyl chain spacers on solid substrates

Adsorption behaviors of functional poly(amido amine) dendrimers with an azacrown core and long alkyl chain spacers were investigated on gold and self-assembled monolayer (SAM) by means of time course attenuated total reflection-surface enhanced infrared absorption and surface plasmon resonance spectroscopies. While 1.5th and 2.5th generation (G1.5 and G2.5) ester-terminated dendrimers were slightly adsorbed on all substrates examined, the adsorption of G2 amine-terminated dendrimer increased in the order dodecanethiol SAM相似文献   

Adsorptive voltammetry of 2-(5-bromo-2-pyridyl)azo-5-[N-n-propyl-N-(3-sulfopropyl)amino]phenol on a carbon paste electrode in the presence of organic cations and polycation.

The adsorption of ion-association complexes on a carbon paste electrode (CPE) was investigated by cyclic voltammetry using an electroactive hydrophobic anion probe. The redox reactions of 2-(5-bromo-2-pyridyl)azo-5-[N-n-propyl-N-(3-sulfopropyl)amino]phenol (5-Br-PAPS), the analytical probe, were irreversible. The reduction of the azo group and the oxidation of the phenol were observed at -0.1 V and 0.9 V vs. SCE, respectively, in a 0.1 mol L(-1) H(2)SO(4) solution. The peak currents for the redox reaction increased with the concentration of the cationic surfactant and the accumulation time. The increase in the ratio of the peak current to the concentration of cationic surfactants was proportional to the hydrophobicity. The peak current for 5-Br-PAPS also increased when a polycation, polyhexamethylene biguanide hydrochloride, was added and was strongly dependent on the ionic strength and pH, in contrast to cationic surfactants.     

Adsorption and release behavior of bare and DNA-wrapped-carbon nanotubes on self-assembled monolayer surface

The adsorption and release behavior of single-stranded DNA-wrapped single-walled carbon nanotubes (ssDNA-w-SWCNTs) on alkylthiol self-assembled monolayer (SAM) surface was systematically characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Fast electron transfer between bare Au electrode and redox species blocked by the alkylthiol SAM can be restored by SWCNTs or ssDNA-w-SWCNTs. The release of ssDNA-w-SWCNTs is carried out by positive or negative desorption potential. SWCNTs/SAM or ssDNA-w-SWCNTs/SAM is completely removed from Au surface at +0.90 V or -1.40 V (vs. 3.0 M KCl|Ag|AgCl). The controlled release of SWCNTs/SAM and ssDNA-w-SWCNTs/SAM holds great promise for gene delivering.