Search

Abstract 3‑Hydroxy‑N‑(5‑mercapto‑1,3,4‑thiadiazol‑2‑yl) benzamide (HMTB) was synthesized and self-assembled on the surface of gold electrode. The self-assembled monolayer of HMTB (Au-HMTB SAM) so formed was electrochemically characterized by using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in the presence of K 3 [Fe(CN) 6 ]/K 4 [Fe(CN) 6 ] as a redox couple and found to be well-assembled free from defects and pinholes. Atomic force microscopy (AFM) was used to examine the morphology of Au-HMTB-SAM and confirmed that the molecules of HMTB form dense and well-packed SAM on gold electrode. The Au-HMTB SAM was applied as a platform for impedimetric sensing of various metal ions by measuring R ct values from the fit of Randles circuit. EIS results indicated that Au-HMTB SAM electrode exhibited a selective impedimetric response towards Co2+ over a wide linear concentration range of 1.0 × 10−10 M to 1.0 × 10−4 M (r2 = 0.991 ± 0.054) at pH = 6 with a detection limit of 2.5 × 10−11 M. The impedimetric sensor selectively respond to Co2+ in the presence of diverse interfering ions such as Na+, K+, Mg2+, Ca2+, Ni2+, Cd2+, Cu2+, Hg2+, Zn2+, Pb2+, Ag+, Fe3+ and Cr3+. The sensor has been successfully applied for estimation of cobalt content in different environmental samples such as water, soil and food crops (wheat and rice). The impedimetric cobalt sensor based on Au-HMTB SAM could serve as a simple, reliable and sensitive method for the electrochemical detection of cobalt(II) in other environmental and biological samples. Graphical abstract Unlabelled Image Highlights • HMTB was synthesized and self-assembled on the surface of gold electrode. • Au-HMTB SAM was characterized by using CV, EIS and AFM. • Au-HMTB SAM was successfully applied as a platform for impedimetric sensing of Co2+ • The sensor could sense Co2+ in a wide concentration range of 1 × 10−10 M to 1 × 10−4 M. [ABSTRACT FROM AUTHOR]

Analytical expressions describing the electrochemical impedance spectrum of a metallic electrode modified with an acid thiol monolayer, that undergoes a potential-induced proton transfer, have been derived from an electrostatic model of the interface. The frequency dispersion of the electrode impedance is described by an equivalent circuit that bears some similarities with that associated with a surface redox process. Good agreement is found between the theoretical predictions and the impedance of a Au(111) electrode modified with a 11-mercaptoundecanoic monolayer recorded as a function of ac frequency, dc potential and surface concentration of electrochemically active carboxylic groups. The same thermodynamic and kinetic parameter values are obtained from independent analysis of the voltammetric and impedimetric responses, except in the case of the reorganization energy for proton transfer, which seems to be affected by a slow potential-induced reorientation and dissociation of the external acid groups of the monolayer. [ABSTRACT FROM AUTHOR]

A comprehensive experimental investigation of the influence of the chain length on the electric field induced proton transfer at α,ω-mercaptoalkanecarboxylic acids self-assembled monolayers (SAMs) is conducted by means of voltammetric and electrochemical impedance spectroscopic measurements. The chain length has a significant impact on the total protonation/deprotonation charge and on the kinetics of the phenomenon. The results are discussed in light of existing theoretical models describing this electric field driven protonation/deprotonation process. Characteristic proton transfer frequencies were independently determined by impedance spectroscopy and by scan rate dependent voltammetric analyses. The two types of measurements are in mutual agreement and show that the protonation/deprotonation slows down as the chain length increases. [ABSTRACT FROM AUTHOR]

The incorporation and effective containment of gold nanostructures on a substrate electrode applied to electrochemical sensors for detecting neurotransmitters is studied. Gold nanoparticles (AuNPs) and gold nanorods (AuNRs) were synthesized, and these nanostructures were incorporated into a metal substrate mediated by self-assembled monolayers (SAMs) of 4-mercaptopyridine (4MPy), which allows the effective confinement and communication of the nanostructure, in addition to evaluating the anisotropic effect for the oxidation of neurotransmitters. Characterizations were made to corroborate the constructed system. First, the nanostructures synthesized in solution were characterized by UV–visible spectroscopy and transmission electron microscopy (TEM). Second, the systems were characterized both chemically and morphologically. Third, an electrochemical characterization was performed as working electrodes. Finally, it was studied by cyclic voltammetry detection of neurotransmitters, for the device formed with AuNRs obtained a better electrochemical response than the system consisting of the AuNPs. Additionally, a shift of the peak oxidation potential of the neurotransmitters used took place depending on the nanostructure observed. Therefore, the oxidation potential of the neurotransmitter depends on the anisotropy of the nanostructure. [ABSTRACT FROM AUTHOR]

Redox behaviors of electrochemically active self-assembled monolayers (SAMs) prepared on gold surfaces from ferrocene (Fc)-terminated alkanethiols were investigated in more depth in response to the authors' previous study where ionic liquids (ILs) with bis[(trifluoromethyl)sulfonyl]amide (Tf 2 N − ) anions were found to restrict the oxidation of Fc moieties probably due to a steric hindrance of large-sized anions. The redox behaviors in ILs were compared with those in aqueous solutions containing only Tf 2 N − as anions. Cyclic voltammograms (CVs) suggested that the use of Tf 2 N − anions lower the amount of electrochemically responsive Fc-groups by about 20% regardless of whether the electrolyte is aqueous or IL-base. Residual water that is sometimes contained in ILs had no effect on the amount of responsive Fc-groups. We tried to directly monitor the adsorption/desorption of anions associated with the redox cycles of Fc-groups by an electrochemical QCM equipped with a resonator using a separately-excited oscillation method. The QCM exhibited periodic frequency changes, which were quantitatively in agreement with the response amounts determined by the CVs, indicating that synchronous but restricted adsorption/desorption of Tf 2 N − anions takes place. [ABSTRACT FROM AUTHOR]

We report the potential-dependent interactions of trimesic acid with Cu surfaces in EtOH. CV experiments and electrochemical surface-enhanced Raman spectroscopy show the presence of an adsorbed trimesic acid layer on Cu at potentials lower than 0 V vs Cu. The BTC coverage increases as the potential increases, reaching a maximum at 0 V. Based on molecular dynamics simulations, we report adsorption geometries and possible structures of the organic adlayer. We find that, depending on the crystal facet, trimesic acid adsorbs either flat or with one or two of the carboxyl groups facing the metal surface. At higher coverages, a multi-layer forms that is composed mostly of flat-lying trimesic acid molecules. Increasing the potential beyond 0 V activates the Cu-adsorbate interface in such a way that under oxidation of Cu to Cu 2 + , a 3-D metal-organic framework forms directly on the electrode surface. [ABSTRACT FROM AUTHOR]

The electro-oxidation of fisetin (FIS) at gold electrodes modified with self-assembled monolayers (SAM) is studied in 15% dimethylsulfoxide + 85% phosphate buffer solutions of different pH using cyclic and square wave voltammetries. Gold electrodes were modified with 2-mercaptoethanesulfonic acid (2-MES) and 4-mercaptophenol (4-MP). Both modified electrodes were characterized by reductive desorption, electrochemical impedance spectroscopy, contact angle measurements, and by the determination of the apparent surface pKa. The FIS first oxidation peak showed an electrode process controlled by diffusion at 2-MES modified gold electrodes, while the electrode process showed a diffusion/adsorption mixed control at 4-MP modified gold electrodes. Square wave voltammetry was used to perform the FIS quantitative determination at both modified electrodes using the commercial reagent. Calibration curves were linear in the concentration range from 1 × 10 − 7 to 1 × 10 − 4 M and from 1 × 10 − 6 to 1 × 10 − 5 M for 2-MES and 4-MP SAM modified electrodes, respectively. Limits of detection for a 3:1 signal to noise ratio were 5 × 10 − 8 M (14.3 ppb) and 5 × 10 − 7 M (143 ppb) for 2-MES and 4-MP modified electrodes, respectively. The reproducibility and the repeatability were 2.2% and 0.5% for 2-MES SAM modified electrodes, and 1.6% and 1.3% for 4-MP SAM modified electrodes, respectively. [ABSTRACT FROM AUTHOR]

Electrodes modified with organic layers and nanoparticles are promising tools for analytical devices. A major challenge however is how to make electrodes that resist the nonspecific protein adsorption from biological media? Herein, we report gold electrode–organic layer-gold nanoparticle constructs as a platform for electrochemical biosensing. This study focuses on the fabrication of antifouling electrodes with the organic layers formed by two different approaches; self-assembled monolayer (SAM) procedure and electrochemical reduction of aryl diazonium salts, followed by the attachment of gold nanoparticle (AuNP) to allow the electrochemistry to proceed. The results show that the electrodes modified with ethylene oxide-based SAM enable a better antifouling behaviour with less adsorption of human serum albumin (HSA) as compared to the electrodes modified with ethylene oxide-based aryl diazonium-derived layer. Furthermore, findings suggest such distinction could be due to the surface packing and/or conformation of organic film on Au electrodes. [ABSTRACT FROM AUTHOR]

The graphene quantum dots (GQDs) were synthesized by a convenient method and immobilized to glassy carbon electrode (GCE) through covalent self-assembled method. The GQDs and the obtained (GQDs–NHCH 2 CH 2 NH)/GCE were characterized by transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) spectroscopy and atomic force microscopy (AFM), etc. Its electrochemical property of the (GQDs–NHCH 2 CH 2 NH)/GCE was explored carefully by electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and differential pulse voltammetry (DPV). It was demonstrated that the modified electrode presents good electrical conductivity and has favorable electrochemical response to dopamine (DA). The (GQDs–NHCH 2 CH 2 NH)/GCE displays a linear range from 1 to 150 μM and a detection limit of 0.115 μM (S/N = 3) to DA, which illustrates that the modified electrode is highly sensitive towards the determination of DA. The (GQDs–NHCH 2 CH 2 NH)/GCE also possesses good anti-interference capability and well stability. Furthermore, the sensor displays satisfactory results in real samples determination which demonstrate its great potential in practical applications. [ABSTRACT FROM AUTHOR]

Highlights: [•] We describe electrochemical analysis of interfacial organic reactions. [•] We demonstrated that the interfacial reactions were strongly affected by the ionization status of background molecules. [•] The direct interaction between the reactant and the background molecules is suggested as a governing mechanistic factor. [ABSTRACT FROM AUTHOR]

[Display omitted] • Initiator DTBU on gold electrode (SAM) enhances electrochemical mediation ATRP. • CV, EIS, CA, XPS and IRRAS methods are used for DTBU monolayer characterization. • Well organized DTBU SAM with polar head and non-polar inside. • EIS, CA and IRRAS prove preferential presence of 6MP in mixed SAM with DTBU. • Mixed monolayers with 6MP provide optimal conditions for adaptation and use in e-ATRP. Bis[2-(2-bromoisobutyryloxy) undecyl] disulphide (DTBU) is an initiator for surface-initiated atom transfer radical polymerization (SI-ATRP) able to functionalize metal surfaces through formation of self-assembled monolayers (SAM). The goal of this work is the making monolayers with the DTBU initiator on gold polycrystalline electrodes and their study by electrochemical methods as cyclic voltammetry (CV), capacity-potential curves (C-E), electrochemical impedance spectroscopy (EIS), and angle contact measurements for their characterization. Information of the integrity and permeability of the monolayers has been obtained by capacitance-potential measurements and impedance spectra. Also, the blocking effect of the physical barrier that SAM can produce to different redox probes is explored by using voltammetry and impedance. DTBU forms a compact monolayer whose main features are related with heterogeneous chain structures, having a non-polar hydrocarbon internal part and a polar head group. This is confirmed by X-ray photoelectron spectroscopy (XPS) which shows the presence of the elements of the DTBU adsorbate of SAM. Moreover, mixed monolayers of DTBU and 6-mercaptopurine (6MP) have been prepared and characterized, to check if the thiol facilitates electron exchange through monolayers. On this approach, different DTBU:6MP ratios have been employed and characterized studying IRRAS spectra, electrochemical properties, and contact angle on the modified surface. These mixed monolayers provide optimal conditions for adaptation and use in e-ATRP. [ABSTRACT FROM AUTHOR]

Highlights: [•] 2-Mercaptonicotinic acid monolayers were investigated at Au(111) surfaces. [•] The Fe2+/3+ redox probe penetrates the SAM films at pinholes for electron transfer reaction. [•] Monolayer properties were obtained by reductive desorption and SERS. [•] Surface pK a characterization of SAMs at different pH. [•] Different adsorbed structures with pH are found. [ABSTRACT FROM AUTHOR]

Highlights: [•] Dehaloperoxidase adsorption takes place on COOH/OH self-assembled monolayers. [•] Voltammetrically determined surface concentration is time dependent. [•] Dynamic reorientation of latent adsorbates is consistent with voltammetry. [•] A model based on distributions of orientation and electroactivity is proposed. [•] Dehaloperoxidase structure and surface properties are compatible with the model. [Copyright &y& Elsevier]

Abstract: Surface enhanced Raman scattering (SERS) technique was used to observe the adsorption structure of serotonin (5-hydroxytryptamine, 5-HT) monolayers formed on silver electrode surface at pH 11 and also by the aid of density functional theory (DFT). The SERS spectrum implied that monolayers of 5-HT could be self-assembled on the silver surface through O10 and N6 atoms and the molecular plane should be tilted on the surface. In situ electrochemical SERS experiment was performed in 0.1molL−1 KCl solution to investigate the stability of the 5-HT monolayers on the silver electrode surface. It could be found that 5-HT monolayers on silver surface experienced the changes of adsorption fashions as the potential shifting to a more negative direction prior to complete detachment from the silver surface. [Copyright &y& Elsevier]

Abstract: Protein monolayer electrochemistry (PME), a strategy using synthetic platforms to study the electron transfer (ET) properties of adsorbed proteins, has been successfully applied to proteins adsorbed at monolayer-protected gold cluster (MPCs) assembled films, an adsorption interface shown to be an effective alternative, compared to traditional self-assembled monolayer (SAM) films, for the immobilization and study of ET proteins. Within PME studies, cyclic voltammetry (CV) remains the most commonly applied electrochemical technique in spite of several limitations that occur when the sweep technique is used at either platform. In particular, CV for PME at MPC films results in analysis complications stemming from the increased charging current inherent to electrochemical interfaces incorporating MPCs with capacitive properties. In this study, multiple electroanalytical techniques, involving step (chronocoulometry, CC), pulse (square wave voltammetry, SWV), and frequency-based impedance (electrochemical impedance spectroscopy, EIS) measurements, are applied to monolayers of adsorbed Pseudomonas aeruginosa azurin and horse heart cytochrome c at both MPC film assemblies as well as traditional SAMs. Electrochemical parameters (formal potential, electroactive surface coverage, double-layer capacitance, and ET rate constant) measured from these various techniques are directly compared and offer insight into the performance and reliability of each technique’s effectiveness in PME. While certain techniques result in measurements indistinguishable from CV, others offer distinct differences. Moreover, the application of alternative techniques reveals systemic limitations and complications within the electrochemical analysis that we further explore, including strategies for applying fast scanning techniques like SWV as well as the construction of MPC platforms with controlled levels of charging current that enable successful impedance analysis. The application of more advanced electrochemical techniques to developing electrochemical interfaces such as MPC film assemblies allows for a greater understanding of not only PME but also the applicability and effectiveness of these techniques to optimize the measurement of specific electrochemical parameters. [Copyright &y& Elsevier]

Abstract: The redox behavior and potential dependent adsorption structure of heptyl viologen (1,1′-Diheptyl-4,4′-bipyridinium dichloride, DHV2+) on a Cu(100) electrode have been investigated by Cyclic Voltammetry (CV), in situ Electrochemical Scanning Tunneling Microscopy (EC-STM) and Infrared Reflection Absorption Spectroscopy (IRRAS). Current peaks in CV measurements of the redox-active DHV molecule are ascribed to two one-electron transfer steps and phase transformation processes. A highly ordered 2D dot-array structure emerges on the c(2×2)-Cl modified Cu(100) electrode surface between +200 and −100mV in STM images. One-electron reduction of the dications DHV2+ around −150mV causes a phase transition from a dot to a stripe pattern which has a bi-layer structure in the STM images. With a further decrease of the applied potential, the structure of the adlayer undergoes a further change from the bi-layer phase to a close-packed monolayer of stripes. An IRRAS analysis on the orientation of the bipyridine moieties reveals a slight tilt of the long molecular axis with respect to the surface in the dicationic phase, but a parallel orientation in the monocationic striped phases. Additionally a large fraction of the hydrocarbon chains are in gauche conformation with low order among them. Only in the bi-layer stripe phase evidence for a marginal order between the alkyl chains was found. Calculation of the average angle of the alkyl chains with respect to the surface normal reveals an orientation along the surface within the dot structure but an outward tilt within the closed packed monolayer due to space limitation between the stripes. [Copyright &y& Elsevier]

Abstract: Self-assembled adlayers of alkanedithiols (DTs) on Au(111) are studied with electrochemical techniques (reductive desorption experiments, redox couples cyclic voltamperometry and electrochemical impedance spectroscopy) in order to characterize adlayer structures. The formation of intralayer and interlayer disulphide bonds (S–S) strongly affects the structure of the layers. We considered different variables which influence the amount of S–S bonds: the length of the alkyl chain (from 2 to 9 C atoms), the polarity of the solvent of the forming solution (ethanol, n-hexane and toluene), the O2 level and the use of a post-deposition treatment with mild reducing agents. Optimal experimental conditions for the formation of DT monolayers with controlled amount of S–S bonds for both short and long alkyl chains are precisely identified. According to our results, short chain DTs form multilayer structures (interlayer S–S bonds) under typical procedures; whereas long chain DTs present intralayer S–S bonds. The use of low polarity solvents during the layer formation as well as the use of reducing agents after deposition favours the formation of good quality DT monolayers. [Copyright &y& Elsevier]

Abstract: We investigated structural aspects of electron transfer (ET) in tunneling junctions (Au(111)∣FcN∣solution gap∣Au STM tip) with four different redox-active N-thioalk(ano)ylferrocenes (FcN) embedded. The investigated molecules consist of a redox-active ferrocene (Fc) moiety connected via alkyl spacers with N =4,6,8 and 11 carbon atoms to a thiol anchoring group. We found that for short FcNs (N =4,6,8) the redox-mediated ET response increases with the increase of the alkyl chain length, while no enhancement of the ET was observed for Fc11. The model of two-step ET with partial vibrational relaxation by Kuznetsov and Ulstrup was used to rationalize these results. The theoretical ET steps were assigned to two processes: (1) electron tunneling from the Fc group to the Au tip through the electrolyte layer and (2) electron transport from the Au(111) substrate to the Fc group through the organic adlayer. We argue that for the three short FcNs, the first process represents the rate-limiting step. The increase of the length of the alkyl chain leads to an approach of the Fc group to the STM tip, and consequently accelerates the first ET step. In case of the Fc11 junctions the rather high thickness of the organic layer leads to a decrease of the rate of the second ET step. In consequence, the contribution of the redox-mediated current enhancement to the total tunneling current appears to be insignificant. Our work demonstrates the importance of combined structural and transport approaches for the understanding of ET processes in electrochemical nanosystems. [Copyright &y& Elsevier]

Abstract: Electrochemical desorption (ECD) of self-assembled monolayers (SAMs) is an important process that can dynamically modify surface properties. ECD of SAMs has found applications in biochemical assays and the precise control of cell adhesion. As cell adhesion is controlled by the chemistry of underlying substrate, ECD provides a means to achieve spatiotemporal control of cell behaviors by electrically addressing the substrate. In this minireview, we first introduce the principles of ECD, followed by its applications in dynamically modifying chemistry and biochemistry at the intersurface. Finally, we highlight its applications in cell biology. [Copyright &y& Elsevier]

Abstract: In this paper, two new thiols, [4-(9H-fluoren-9-ylmethyl)-phenyl]-methanethiol (6a) and [4-(2-nitro-9H-fluoren-9-ylmethyl)-phenyl]-methanethiol (6b), were synthesized, and self-assembled monolayers (SAMs) of these thiols were formed on gold electrodes. The structure and surface properties of molecular films were investigated by contact angle measurements and attenuated total reflectance infrared spectroscopy (ATR-FTIR). The blocking behavior of Au-6a and Au-6b SAMs was examined with cyclic voltammetry in the presence of redox probes such as K3Fe(CN)6, Ru(NH3)6Cl3 and ferrocene. Electrochemical measurements revealed that the voltammetric behavior of the redox probes was dependent on the nature of the probe molecules, the electrolytic solution composition and the monolayer structure. The optical properties of the SAMs were studied by steady-state and time-resolved fluorescence spectroscopy. It was obtained that the fluorescence emission bands of Au-6a and Au-6b monolayers were red-shifted and broadened compared to those of free thiols in solution as well as significant reduction at their emission intensities. The fluorescence decay profiles of 6a and 6b monolayers were described by a monoexponential function. In order to determine the possible deactivation mechanism between the metal support and photoexcited molecules, spectroelectrochemical steady-state fluorescence and lifetime measurements on the Au-fluorophore electrodes were also performed as a function of the applied potential. These results indicate that the fluorescence quenching occurs via an energy transfer mechanism from the excited molecules to the gold substrate. [Copyright &y& Elsevier]

Abstract: Electrochemical properties of a 1-hydroxy-anthraquinone derivative (1-hydroxyAQ) self-assembled monolayer (SAM) were studied using reflectance infrared, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results of the electrochemical investigation into the proton-coupled electron transfer (PCET) reaction at various pHs and temperatures demonstrate a strong pH-charge transfer rate dependence. The charge transfer rates are faster in both acidic and basic solutions than near physiological pH. Interestingly, the activation enthalpy for the charge transfer reactions at physiological pH is low at 23kJmol−1 compared to 67kJmol−1 at pH 1.5. Variable temperature studies showed the entropic cost associated with the charge transfer reaction at pH 7.5 causes the slow charge transfer rate and suggests that pre-organizing proton donor solvents could lead to very fast charge transfer reactions at the hydrophobic|hydrophilic interface. [Copyright &y& Elsevier]

Abstract: In the present paper, the use of a gold electrode modified by 2-(2,3-dihydroxy phenyl)-1,3-dithiane self-assembled monolayer (DPDSAM) for the determination of epinephrine (EP) and uric acid (UA) was described. Initially, cyclic voltammetry was used to investigate the redox properties of this modified electrode at various scan rates. The apparent charge transfer rate constant, k s, and transfer coefficient, α, were calculated. Next, the mediated oxidation of EP at the modified electrode was described. At the optimum pH of 8.0, the oxidation of EP occurs at a potential about 155mV less positive than that of an unmodified gold electrode. The values of electron transfer coefficients (α =0.356), catalytic rate constant (k =1.624×104 M−1 s−1) and diffusion coefficient (D =1.04×10−6 cm2 s−1) were calculated for EP, using electrochemical approaches. Based on differential pulse voltammetry, the oxidation of EP exhibited a dynamic range between 0.7 and 500.0μM and a detection limit (3σ) of 0.51μM. Furthermore, simultaneous determination of EP and UA at the modified electrode was described. Finally, this method was used for the determination of EP in EP ampoule. [ABSTRACT FROM AUTHOR]

Abstract: l-cysteine modification of palladium (Pd) electrodes have been characterized using various electrochemical and surface analytical techniques such as cyclic voltammetry (CV), reductive/oxidative desorption, electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS), and specular reflectance Fourier transform infrared (FT-IR) spectroscopy. The electron transfer reaction of l-cysteine modified Pd electrode was probed by employing redox couple using cyclic voltammetry and electrochemical impedance. The CVs results revealed a quasi-reversible voltammograms for the Pd modified electrode, demonstrating that the electron transfer is not blocked due to surface modification. EIS results for the Pd modified electrode showed a decrease in the resistance in comparison with the bare Pd electrode. The surface coverage was 6.4±0.4×10−10 mol/cm2. The XPS survey study for the l-cysteine modified Pd electrode exhibited the presence of sulfur on the surface. High energy resolution XPS and FT-IR indicated that the l-cysteine molecules are sulfur bonded to the Pd surface. These results suggest the modification of palladium surface by the l-cysteine amino acids. [ABSTRACT FROM AUTHOR]

Abstract: The effect of aliphatic alcohols (1-pentanol, 1-hexanol, 2-hexanol, and 1-heptanol) on the rates of electrochemical electron tunneling across monolayers of n-alkanethiols (CH3(CH2) n –SH, n =8,9,11) self-assembled on an expandable hanging mercury drop electrode (HMDE) is described. The electron transfer between the solution dissolved and mercury electrode across the n-alkanethiol monolayer occurs via super-exchange through-bond tunneling (ET) with a tunneling coefficient decay constant β =1.1/CH2. Gradual additions of aliphatic alcohols to the solution result in a decrease of the long-range electron transfer rate, by a maximum of 7-fold, dependent upon the alcohol concentration, but independent of the thickness of the n-alkanethiol monolayer (SAM) and the alcohol’s chain length. The mechanism involving competitive adsorption of the alcohol on the SAM surface is proposed to interpret the kinetic data. The previously reported, liquid-like nature of n-alkanethiol monolayers on mercury, allows stepwise expansion of the mercury drop electrodes coated with n-decanethiolate monolayer to ca. 125% of the initial surface area, resulting in an increase of the interfacial capacitance consistent with the overall decrease of the film thickness due to hydrocarbon tilt. The rate of ET, in the absence of alcohols, increases upon the hydrocarbon tilt consistent with the 2-pathway electron tunneling model. In the presence of aliphatic alcohols, the alkanethiol-coated Hg drop can be expanded up to ca. 150%, resulting in electron transfer rates about 10-fold smaller than expected by the 2-pathway model. This result is interpreted in terms of alcohol partitioning into the n-alkanethiol monolayer leading to partly disorganized films. [ABSTRACT FROM AUTHOR]

Abstract: Redox behaviors of binary self-assembled monolayers (SAMs) consisting of ferrocenyl- and methyl-terminated alkanethiols on gold were investigated in three different ionic liquids (ILs) and were compared with the behaviors in an aqueous HClO4 solution. A set of cyclic voltammograms showed that, when the surface density of ferrocenyl-groups was relatively high, the shape of redox waves (e.g. peak area and peak broadening towards positive potential) for the ferrocenyl-groups was strongly affected by the anion size of the ILs, suggesting that the compensation of surface positive charge emerged when oxidized to ferrocenium state is sterically-hindered in the case of large anions like bis[(trifluoromethyl)sulfonyl]amide (TFSA−). This is supported by the fact that the anion-size effect disappeared if the density of ferrocenyl-groups was diluted. A potential-step chronocoulometry was concomitantly employed to estimate the net faradaic charge of the redox. The redox behaviors in mixed ILs containing two different sized anions also gave clear evidence that smaller anions, e.g. ions, have a greater tendency to make ion pairs with ferrocenium moieties. [Copyright &y& Elsevier]

Abstract: The dynamic response of low-density self-assembled monolayers (SAMs) on gold has been investigated with electrochemical impedance spectroscopy (EIS) under non-faradaic conditions and contact angle measurements. The impedance of low-density 16-mercaptohexadecanoic acid (LD-MHA) SAMs is compared with that of densely-packed MHA and tetraethylammonium–MHA (TEA–MHA) ion-pair monolayers in 0.1M KCl (pH 8.4). The films composed of ion-pairs and loosely-packed MHA have higher capacitance and are more permeable to ions than fully-packed MHA SAMs. The impedance phase angle of the LD-MHA SAM at 0.1Hz decreases reversibly by 10° upon scanning the applied potential from −0.1V to +0.3V (vs. SCE). Fitting the impedance data to an equivalent circuit correlates the decrease in phase angle with an order-of-magnitude drop in monolayer resistance (i.e. increase in the ionic permeability of the monolayer). The ion-pair and dense MHA SAMs do not exhibit an increase in permeability upon scanning the applied potential from −0.1V to +0.3V (vs. SCE). The capacitances of the LD-MHA and TEA–MHA films exhibit a small, but significant (15%), change with applied potential between −0.1V and +0.3V (vs. SCE). The impedance of the LD-MHA SAM is found to be independent of the electrolyte concentration (0.01–1M KCl), and displays hysteresis with the direction of the potential scan. Receding contact angle measurements performed at −0.1V and +0.3V using the same electrolyte show that the higher ionic permeability of the LD-MHA SAM at +0.3V is caused by a voltage-induced structural change (molecular re-organization) within the film. The agreement between EIS measurements performed at pH 3.3 and pH 8.4 shows that electrostatic attraction between a negatively-charged thiol tailgroup and oppositely-charged gold electrode is not required for this structural change to occur. [Copyright &y& Elsevier]

Abstract: The stability of alkanethiol self-assembled monolayers (SAMs) on metallic substrates, in particular copper, is an important aspect for their application. Some technologies, such as lithographic patterning, can potentially benefit from the use of a transient easily removal protective coatings. Other applications like corrosion resistance, and thin-film lubrication request rather robust and stable films. The aim of this work consists in a comparative investigation of three organothiols (n-tetradecanethiol or RSH, 2-dodecylpropane-1,3-dithiol or R(SH)2 and n-tetradecanedithiocarboxylic or RS2H) on their ability to form SAMs on electrochemically reduced polycrystalline copper substrates and their respective stability. Characterizations of the SAMs are carried out using contact angle goniometry, XPS, PM-IRRAS and electrochemical studies (CV, LSV and cathodic desorption). R(SH)2 leads to monolayers with lower molecular organization than in the case of RSH and RS2H molecules. R(SH)2 monolayers are found the more stable when subjected to both anodic and cathodic desorption tests. [Copyright &y& Elsevier]

Abstract: We investigate the dielectric properties of self-assembled monolayers (SAMs) of OH and terminated alkyl-thiols on polycrystalline gold films by electrochemical impedance spectroscopy (EIS) in a non-electroactive electrolyte. We show that impedance spectroscopy, coupled to surface potential monitoring allows a fine inspection of the stability and homogeneity of these layers. In particular, we compare two preparation modes for these layers, respectively, pure ethanol and mixed ethanol–phosphate buffer. Using a simple electric model combining a constant phase element CPE with a highly resistive element , both in series with a bulk resistor R, to fit the experimental EIS curves, we propose a direct survey of the SAM homogeneity during the gold film potential scan. The topological imperfections of polycrystalline gold films and self-assembled monolayers are interpreted by a constant phase element, i.e. an interfacial capacity behaving as a power law of frequency. We show that the type of medium chosen for the SAM preparation is not so critical, when the alkyl-thiol is not charged (OH), and these SAMs are very stable, independently of the gold surface potential. However, the preparation of charged SAMs, such as those obtained from terminated alkyl-thiols requires the preparation medium to be similar to the type of solution in which it will be used afterwards, for instance if the SAM has to be used for aqueous medium applications, its preparation should be made in a mixed ethanol (organic solvent)–buffer solution. We also show that for both charged and uncharged SAMs, the periodic cycling of the gold potential promotes the homogenization of the SAM. [Copyright &y& Elsevier]

Abstract: The desorption mechanism for electrochemically-formed self-assembled monolayers from cysteine and cystine on a Au(111) electrode was investigated. The monolayer desorption in 0.1M KClO4 is found to occur in two steps with differing potentials. At potentials more positive than the desorption potential (circa −0.65V vs. SCE), a quasi-reversible process, assigned to protonation/deprotonation, is observed. This process is found to occur at very discreet potentials when cysteine monolayers are probed by impedance spectroscopy. Monolayers derived from the disulfide (cystine) are found to protonate/deprotonate over a wider potential range. This suggests that electrochemically-formed monolayers obtained from disulfides do not afford the same level of organization as those derived from thiol species. The value of the double layer capacitance increases from 15 to 30μFcm−2 after deprotonation. This suggests that the adsorption of cysteine weakens following the deprotonation. As expected, the protonation/deprotonation step is pH dependent and it is not observed in strong basic medium (0.1M NaOH). [Copyright &y& Elsevier]

Abstract: This paper examines the role of reactant mass transfer in the preparation of monolayers derived by the chemisorption of thiols on gold and related systems. As a starting point, equations are derived that relate the total impingement of thiol molecules, specifically n-hexanethiol, onto a gold surface for two substrate/deposition vessel geometries. The first model is based on a construct that delivers the reactant to the surface by linear diffusion. The second geometry mimics the case in which hemispherical mass transport is a critical component in the diffusional delivery of reactants. The relationships describe the effects of thiol concentration and immersion time solely on the impingement of the reactant on the gold surface with respect to both deposition geometries. With these predictive equations as guides, adlayer formation is followed by using infrared reflection-absorption spectroscopy, contact angle measurements, and assessments of heterogeneous electron-transfer kinetics. This work demonstrates that, in addition to pretreatment protocols of the substrate surface, the mode of reactant mass transport must be carefully considered and controlled in order to repetitively yield adlayers with comparable structures and, therefore, interfacial properties. Both the total impingement and the rate of impingement of the reactant are found to be major contributors, which implicates the role of nucleation and growth in ultimately dictating subtle, but important differences in the interfacial properties of the adlayer. [Copyright &y& Elsevier]

Ruthenocene is a promising electrochemical agent and a potential one-electron reservoir with a stable and irreversible redox system. In this respect, two new molecules, Rc—C≡C—C6H4—C6H4—SAc (REBTA) and Rc—C≡C—C6H4—C6H4SH (REBTH) (Rc = (η5-C5H5)Ru(η5-C5H4); Ac = C(O)Me) were synthesized. The single crystal structure of REBTA was determined, showing that this compound forms a 3D-network structure set-up by π–π interactions of C6H4/C6H4 and C6H4/C5H5 units. Cyclic voltammetric measurements were carried out with REBTA, REBTH, and Rc—C≡C—CH for comparison. In view of possible electrochemical applications, REBTH and REBTH were used to prepare self-assembled monolayers (SAMs) on Au(1 1 1) substrates. The resulting SAMs were characterized by high-resolution X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. The films were found to be well-ordered and densely packed, even though with a lower density than the analogous films formed from similar ferrocene-substituted molecules, which can be explained by the somewhat larger volume of the ruthenocene moiety, hindering the molecular packing. The presence of the acetyl protection group did not affect the quality of the resulting films significantly, even though some differences between the individual SAMs were found, dependent on the deprotection agent or its absence in the primary solution. The protection group was completely removed in all cases, even when no deprotection agent was used. [Copyright &y& Elsevier]

Abstract: We have combined electrochemical reductive desorption with microdisplacement printing to create patterned self-assembled monolayers inaccessible by other methods. The resulting structures were characterized with field emission scanning electron microscopy, scanning tunneling microscopy, atomic force microscopy, and cyclic voltammetry. Additionally, we have demonstrated that the chemical patterns produced by microdisplacement printing on Au{111} can function as resists for patterned metallic films where 1-adamantanethiolate regions can be selectively desorbed, followed by metal deposition into regions of exposed gold substrates or metal films can be deposited directly through the 1-adamantanethiolate regions, because the 1-adamantanethiolate monolayers allow electron transfer from the supporting electrolyte to the electrode surface (like an unfunctionalized electrode). This is in contrast to typical alkanethiolate monolayers on Au{111}, which block the same electrochemical process. These selective electrochemical desorption and deposition processes can be coupled to hybrid soft-lithography strategies to enable fabrication of novel hybrid metallic and organic structures. [Copyright &y& Elsevier]

Abstract: 4-Amino-TEMPO is attached to a self-assembled monolayers of HS(CH2)10COOH on gold electrodes and the kinetics of the TEMPO /TEMPO+ redox couple are measured over a wide range of scan rates in 1M H2SO4. Analysis of the rate constants yields the standard rate constant (1.2s−1) and the reorganization energies for TEMPO (1.5eV) and TEMPO+ (1.3eV). These values are the first experimental measurements of reorganization energies of TEMPO in aqueous solution. [Copyright &y& Elsevier]

Abstract: We present the dynamics of electrochemical desorption of self-assembled monolayers of HS-(CH2) n -fluorescein (n =3 and 12; C3FAM and C12FAM) and HS-(CH2)6-poly(dT)18-fluorescein (ssDNAFAM) as monitored in-situ by fluorescence spectroscopy. Surface coverage and fluorescence intensity were also correlated, as sense during electrochemical desorption of the SAM. Changes in potential scan rate did not impact the fluorescence emission. Co-adsorption of mercaptohexanol (MCH) could be detected electrochemically and by in-situ fluorescence spectroscopy. Based on these results, energy transfer from an excited dye to the metal can be used to study interfacial dynamics. The transition of a single stranded DNA self-assembled monolayer from a condensed to an elongated state was monitored by in-situ fluorescence. Its electrochemical desorption from the electrode surface was measured as the electrochemical potential was applied above −1.0V vs. Ag/AgCl. The DNA transition from condensed to elongated state can be exploited to study DNA hybridization process and other possible recognition events at the interface. [Copyright &y& Elsevier]

Abstract: A combined structural and electrochemical study of self-assembled monolayers of the azobenzene-containing alkane thiol N-(2-mercaptoethyl)-4-phenylazobenzamide on Au(111) by scanning tunneling microscopy, synchrotron X-ray reflectivity, and cyclic voltammetry is presented. Densely packed monolayers are observed, in which the molecules are arranged in chains forming an ordered structure with a coverage of 2/7ML. The molecules are tilted by 40° relative to the surface normal, resulting in a monolayer thickness of 13.7Å. A total charge of 65.2±4.3μC/cm2 corresponding to 1e− per molecule and apparent electron transfer coefficients of ≈0.5 are found. [Copyright &y& Elsevier]

Abstract: Different immobilization procedures for horseradish peroxidase (HRP) were investigated using, as support matrices, self-assembled monolayers (SAM) formed on gold electrodes. The influences of these immobilization processes in the biosensor performance were also evaluated. For this, monolayers were prepared from thiols with different structures, carbon chain sizes and terminal groups. It was shown that the thiol carbon chain size influences especially in monolayer coverage and, consequently, in the biomolecule immobilization efficiency. From the studies carried out for SAM characterization on the electrode surface it was verified that thiols with shorter chains (n <9) tend to form monolayers with a considerable amount of defects on the gold surface that lead to a lower coverage. However, thiols with a longer carbon chain present a higher coverage degree, which are not suited as substrate to the development of electrochemical biosensors, because they passivate the transduction interface, making difficult the electron transfer and, consequently, reducing electrode sensitivity. In relation to the enzyme immobilization on gold electrodes, it was verified, using different techniques, that monolayers that possess –NH2 terminal groups provided the best results, probably due to the use of glutaraldeyde as ligand during the immobilization process. Analyzing biosensor performances for hydrogen peroxide, it was verified that SAM formed by cysteamine is the most adequate for HRP immobilization, because it provides better efficiency for enzyme immobilization associated to high sensitivity for H2O2. [Copyright &y& Elsevier]

Abstract: In this report we present electrochemical studies of the immobilization of disulfide-modified single-stranded DNA (ssDNA) on gold. The effect of immobilization time on surface coverage was probed by cyclic voltammetry and electrochemical impedance spectroscopy (EIS). Additionally, linear sweep voltammetry was used to perform reductive electrochemical desorption of the ssDNA molecules, and the variations in surface coverage, charge transfer resistance (R ct) and capacitance (C) with desorption cycles were monitored by EIS. The results show surface coverages greater than 0.9 for immobilization times of 15 hours or more. The coverage and R ct increased, while the C decreased for higher immobilization times. The opposite trend was observed for an increasing number of desorption cycles. These results suggest that a combination of variations in immobilization time and number of desorption cycles represents an alternative to optimize the density of immobilized DNA strands, which is desirable for the subsequent optimization of DNA hybridization and the development of DNA biosensors. [Copyright &y& Elsevier]

Abstract: An electroactive self-assembled monolayer (SAM) was fabricated by covalent attachment of a novel hydroquinone-terminated dodecanethiol onto the gold surface and its electrochemical behavior was investigated using cyclic voltammetry and electrochemical impedance spectroscopy. The capability of the designed SAM in immobilization of organic molecules onto the gold surface was studied utilizing the Michael-addition as a model reaction. The results obtained from cyclic voltammetry, electrochemical impedance and grazing incidence Fourier transform infrared (GI-FTIR) spectroscopy revealed that, upon applying an anodic potential to the Au-SAM electrode system in the presence of glutathione, the electrochemically generated p-quinone participated in a Michael-addition reaction with glutathione and the corresponding Michael adduct was formed at the solid–liquid interface. The kinetic parameters were then derived for this interfacial Michael-addition reaction. [Copyright &y& Elsevier]

Abstract: We report a study of the complexes formed by the non-covalent functionalization of carbon nanotubes (CNTs) with disulfide-modified-single-stranded DNA, and the subsequent immobilization of these DNA–CNT complexes by the self-assembly technique on Au surfaces. This technique is a straightforward route for the attachment of CNTs to solid substrates, which is simpler than conventional methods, such as chemical vapor deposition. Self-assembled monolayers (SAMs) of the DNA–CNT hybrids were characterized with atomic force microscopy (AFM), cyclic voltammetry (CV), and linear sweep voltammetry (LSV). SAMs containing aggregates of DNA–CNT hybrids with surface coverage up to 10% were obtained. The DNA–CNT complexes have a small length distribution (from 40 to 100nm) and were attached to the surface at an ∼20° angle from the surface normal. These results suggest that the DNA–CNT complexes might be promising materials for the design of novel nanodevices and sensors. [Copyright &y& Elsevier]

Abstract: The following study presents the analysis of electrochemically induced conformational changes of self-assembled single-stranded DNA (mercapto-n-hexyl-poly(dT)18-fluorescein, 18TFAM) on polished polycrystalline Au electrodes. This was compared to a self-assembled monolayer of 6-mercapto-1-hexanol (MCH). Double layer capacitance (C dl) was determined by electrochemical impedance spectroscopy (EIS) with electrochemical perturbation from 0.0V to 0.7V (vs. Ag/AgCl) for bare Au, Au/mercaptohexanol (Au/MCH), and Au/S-(CH2)6-poly(dT)18-fluorescein (Au/18TFAM) self-assembled monolayers. The EIS study was done with two electrolyte systems, KH2PO4 and NH4H2PO4 solutions at pH 9.9 and 10.05, respectively. The results were used to determine the ion barrier factor for MCH and 18TFAM for each phosphate solution. Moreover, detailed experimental analysis carried out by repetitively switching the electrochemical potential between 0.2 and 0.5V (vs. Ag/AgCl) strongly suggests the stretching/condensing of the specifically attached 18TFAM on the Au surface and the progressive desorption of non-specifically adsorbed 18TFAM. [Copyright &y& Elsevier]

Abstract: The adsorptive accumulation of ATX-I, a mycotoxin produced by the fungi the Alternaria alternate genus, onto polycrystalline gold electrodes modified by a self-assembled monolayer (SAM) of 1-dodecanethiol (DDT) from unstirred solutions of the pure commercial reagent in 20% acetone (Ac)+80% pH 7 phosphate buffer solutions (PBS) has been studied by cyclic (CV) and square wave voltammetries (SWV). The influence of different experimental conditions on the adsorptive process has been analyzed. The specific interaction of ATX-I with these modified gold electrodes was adequately described by a Freundlich adsorption isotherm. Electrochemical impedance spectroscopy (EIS) measurements were used to assess the state of the adsorbed thiol molecules, demonstrating that optimum conditions for analytical detection require the presence of defects in the monolayer, where analyte accumulation and charge transfer rate are enhanced with respect to well ordered monolayers. SWV was employed to generate the calibration curves. For a signal-to-noise ratio of 3:1, a detection limit of 4×10−8 moldm−3 (14ppb) was calculated at a frequency (f) of 40Hz. [Copyright &y& Elsevier]

Abstract: We have thoroughly compared cyclic voltammetry and chronocoulometry, the two commonly used electrochemical techniques in quantitative determination of DNA surface density on gold electrodes. The electrostatic binding of redox cations, such as [Ru(NH3)6]3+, to the anionic DNA phosphate backbone allows the determination of the number of phosphate residues and thereby the surface density of the oligonucleotide strands. The results obtained by the two methods agreed well with each other for thiol-terminated DNA immobilized on gold electrodes via Au–S linkages and for unmodified DNA physically adsorbed on the surface. An improved procedure has been developed for the preparation of DNA-modified gold chips with a relatively high surface coverage. The density of thiol-terminated single-stranded DNA probes was determined to be (2.2±0.3)×1013 molecules/cm2 upon reaching adsorption equilibrium, which was in good agreement with that obtained by phosphor imaging of 32P-radiolabeled DNA. [Copyright &y& Elsevier]

Abstract: The kinetic of the assembly and adsorption processes of mono(6-deoxy-6-mercapto)-β-cyclodextrin (βCDSH) on gold surfaces in situ and in real time have been investigated by surface plasmon resonance technique (SPR). The film thickness, dielectric constant and its coverage capability were determined by SPR with one-wavelength approach. To unambiguously determine the film parameters avoiding multi-layer adsorption as well as an underestimation of the binding constants of the adsorption process, the influence of the mass transport of βCDSH was investigated and controlled. Under flow regime the SPR angle was monitored in several concentrations of thiol and the data were fitted by using a two-step adsorption model based on a fast adsorption of the thiol molecules in a first step followed by its slow rearrangement on the gold surface. Finally, the selectivity of the host–guest property of the βCDSH monolayer was investigated using [Fe(CN)6]3−/4− and ferrocenemonocarboxylic acid as redox probes. [Copyright &y& Elsevier]

Abstract: The modification of gold electrodes with 4-carboxyphenyl diazonium salts to form stable layers for sensing applications is reported. Electrochemical reduction of 4-carboxyphenyl diazonium salts on gold electrodes yielded more stable layers than alkanethiol self-assembled monolayers in terms of extremes of electrode potential, sonication and with time. The application of the 4-carboxyphenyl modified electrodes for electrochemical sensing, which typically requires short chain alkanethiols on gold electrodes, is demonstrated via the covalent attachment of oligopeptides for the selective detection of Cu2+, Cd2+ and Pb2+. The diazonium salt/peptide modified gold electrodes not only had greater stability but also performed with lowest detected concentration to alkanethiol/peptide modified electrodes and with far greater sensitivity than the metal ion sensors when diazonium salt/peptide modified similar glassy carbon electrodes were employed. [Copyright &y& Elsevier]

Abstract: Redox recycling systems enable marked improvement of electrochemical detection capabilities. Enhanced sensitivity is achieved by employing a sacrificial redox species that recycles the analyte back to its original oxidation state through a catalytic homogeneous electron transfer. Repetition of the cycle leads to multiple heterogeneous electron transfer events for each analyte molecule, serving to enhance the transduced signal. The success of redox recycling is intimately linked to the selectivity of heterogeneous electron transfer: the analyte should undergo a fast reaction while the sacrificial species should ideally be excluded from contributing directly to the current. This requirement stems from the relationship between selectivity and detection limit in that the direct heterogeneous electrolysis of the sacrificial additive can increase the background current which can degrade detection capabilities. Earlier work has shown that electrodes with suitable selectivity can be constructed using alkanethiolate monomolecular films on gold, with various ferrocenes (FcX) serving as a model analyte and ferrocyanide acting as the recycling agent [A.J. Bergren, M.D. Porter, J. Electroanal. Chem. 585 (2005) 172, A.J. Bergren, M.D. Porter, J. Electroanal. Chem. 591 (2006) 189, P.T. Radford, M. French, S. E. Creager, Anal. Chem. 71 (1999) 5101, P.T. Radford, S.E. Creager, Anal. Chim. Acta 449 (2001) 199, S.E. Creager, P.T. Radford, J. Electroanal. Chem. 500 (2001) 21]. The work herein investigates the origins of the selectivity for this system by analysis of the different pathways (e.g., electron transfer kinetics, size-exclusion, and partitioning) that can suppress the heterogeneous electrolysis of , while maintaining that for FcX at a rapid level. Comparisons of experimental data to expectations derived from model assessments are used to evaluate the relative importance of each possibility. The properties (i.e., size and hydrophobicity) of several FcX molecules and are also examined to provide additional insight into the processes that are important in creating a potent redox recycling system. These results show how the inherent differences in the heterogeneous electron transfer reaction rates can dictate the kinetic selectivity of the system. These findings indicate that partitioning augments the kinetic selection for the system, leading to the high level of observed selectivity. [Copyright &y& Elsevier]

Abstract: Long chain n-alkanethiol self-assembled monolayers (SAMs) on a hanging mercury drop electrode were studied in detail for the first time by impedance spectroscopy (IS) and potentiometry. IS reveals the detailed behavior of the monolayers in the absence and presence of two one-electron redox couple: hexaamineruthenium(III), and tris(4,4′-bipyridine)cobalt(III), . The pinholes-free layers behave as almost ideal capacitors and are permeable to some extent to redox species, depending on their hydrophobicity. Interestingly, that exhibits sluggish kinetics on a bare Hg electrode reveals a more facile kinetics than on an n-octadecanethiol/Hg interface. Potentiometric measurements recorded upon immersing the bare and SAM modified Hg electrode into a solution containing a mixture of the oxidized and reduced forms of the redox couples provide invaluable information on the charge transfer across the monolayer and its ohmic resistance. While cannot freely penetrate into the monolayer and therefore establishes a potential difference across the monolayer; penetration of into the film causes the Fermi level of the Hg surface to attain the Nernst potential of the solution. Finally, we find that increasing the length of the alkane chain of the thiols increases linearly the ohmic resistance of the layer. [Copyright &y& Elsevier]

Abstract: 1-(β-Apo-8′-carotenoyl)-2-(7-selenaoctanoyl)-glycerol was attached to the gold electrode surface by chemisorption from an acetonitrile solution and the surface layer was investigated by electrochemical methods and piezoelectric nanogravimetry in aqueous solutions. The adsorbate binds to the gold surface by the selenium end group and adopts a roughly perpendicular orientation to the electrode surface. The carotenoid moiety in the adsorbate undergoes an anodic reaction (peak potential, 0.78V vs. Ag∣AgCl (1M KCl) electrode) which occurs via an electron transfer followed by the uptake of two hydroxyls. Hydrogen radicals also form and initiate a polymerization process which leads to a more compact and rigid structure of the surface layer. At more positive potentials, the product of these reactions undergoes a slow oxidative desorption which is accompanied by gold oxidation and partial oxidation of organic selenium to . Redox probe investigations revealed that the anodic reaction at 0.78V results in major changes in the film structure and morphology. At a moderate coverage degree, electrochemically generated O2H radicals cause the fragmentation of the conjugated system by addition to the double bonds but do not induce a cross-linking reaction. No cathodic desorption of the CSeOG generated layer occurred in a 0.5M KOH solution. [Copyright &y& Elsevier]

Abstract: 2-Methyl-3-(ω-mercaptoalkyl)-1,4-naphthoquinones, containing 5, 7, and 9 carbon atoms in the alkyl chain, were synthesized, and the structural and electrochemical parameters of their self-assembled monolayers (SAMs) on gold were studied by in situ Fourier-transform surface-enhanced Raman spectroscopy (FT-SERS) and cyclic voltammetry (CV). We have demonstrated that the alkyl chains of SAM prepared from the concentrated (1mM) substance solution contain a number of gauche defects. Nevertheless, it has been found that the 2-methyl-1,4-naphthoquinone (2-MeNQ) group does not interact directly with gold. In contrast, spectral changes for surface layers prepared from diluted (0.05μM) solutions have indicated flat ring orientation and direct involvement of 2-MeNQ group in the interaction with Au. It has been found that the Au–S stretching band at 291cm−1 considerably increases in intensity and blue-shifts by ∼10cm−1 in the surface spectra of monolayers prepared from low concentration coating solution indicating different Au–S bonding geometry and coordination of sulphur atom with Au. Up-shift in the Com 1654 to 1660cm−1 has been detected for the mixed monolayer prepared using shorter alkyl chain diluent, heptanethiol, and indicates absence of stacking interaction between the 2-MeNQ rings as well as involvement of the Cn the hydrogen bonding with interfacial water. The electrochemical response of 2-MeNQ group buried in the long-chain thiols containing mixed monolayers was found to be controlled by the nature of the diluent terminal group. Thus, replacement of the CH3 group in dodecanethiol by COOH has induced electrochemical activity of 2-MeNQ group in the mixed SAM. [Copyright &y& Elsevier]

Abstract: Mixed self-assembled monolayers of ferrocene, triarylamine or phenothiazine terminated alkanethiols were prepared on gold-ball electrodes. Also, a ferrocene terminated biphenylthiol and anthracenethiol were investigated. The heterogeneous electron transfer rate constant was measured in all cases by impedance spectroscopy between 1MHz and 0.1Hz. For equal spacer lengths, we obtained significantly higher rate constants for the triarylamine species compared to the ferrocene compounds, which is attributed to the much lower reorganisation energy of triarylamines. [Copyright &y& Elsevier]

Abstract: The structure and electrochemical properties of pure and mixed monolayers of 3-mercaptopropionic acid (MPA) and 11-mercaptoundecanoic acid (MUA) self-assembled on a gold disk electrode (AuE) have been investigated by electrochemical impedance spectroscopy (EIS) in electrolytes containing a one-electron redox couple. These results are compared with those obtained by cyclic voltammetry (CV). The monolayer surface coverage was determined using impedance measurements through the charge-transfer resistance, and by the pore size model. Cyclic voltammetry considering gold oxide formation/removal, as well as the oxidation/reduction process of a redox probe, was also employed to calculate the surface coverage. Taking into account the stability of the studied monolayers during the electrochemical experiment, the comparison of the barrier to charge transfer at both, the modified and the unmodified electrodes, resulted to be an appropriate method for estimating the surface coverage percentage given by each of the considered monolayers. The obtained results demonstrated that the MPA-monolayer-coated electrode behaves like a microelectrode array, with pinholes acting as the microelectrodes. The average size and the separation of the pinholes, estimated by the pore size model, are 1.9 and 21μm, respectively. However, MUA forms a closely packed monolayer that acts as an ionic insulator and is virtually defect-free. The adsorption kinetics of MPA and MUA SAMs on the gold electrode (AuE) was evaluated. As expected, mixed monolayers of MPA and MUA show characteristics of both, long and short-chain alkanethiol monolayers. [Copyright &y& Elsevier]