Electrochemical Characterization of the Self-Assembled Monolayer of 8-Mercaptoquinoline on Polycrystalline Gold Electrode

Self-assembled monolayer of 8-mercaptoquinoline (MQ) on the surface of gold from MQ dilute ethanolic solutions is investigated by electrochemical methods. Some aqueous redox probes, such as ferrocene carboxylic acid and Fe(CN)₆ ^4–/3– can sufficiently diffuse into the monolayer because significant diffusion-limited current peaks are observed when the redox reactions take place, showing that the monolayer is very loosely packed or dominated by defects. However, the study on the electron transfer of other aqueous probes, such as Cu²⁺ and Ru(NH₃)₆ ^3+/2+, confirm that the monolayer can block the electron transfer on the gold electrode surface rather effectively for its low ratio of pinhole defects. These studies show that the MQ monolayer on the electrode can provide an excellent barrier for penetration of some probes but cannot resist the penetration of other probes effectively. The unusual properties of the self-assembled monolayers are attributed to the entity of the very large heterocyclic moiety.     

Electroreduction of oxygen on octadecylmercaptan self-assembled monolayers

The reduction of oxygen has been studied on octadecylmercaptan self-assembled monolayers adsorbed on gold substrates in borate buffer solutions with a rotating disc electrode. A great inhibition of the oxygen reduction and other electrochemical reactions by these monolayers has been found. However, after polarisation at –0.80 V_SHE the protecting properties of the film against electron transfer reactions are lost, and a behaviour similar to bare gold is observed. Ex situ XPS indicates that the thiol monolayer has not been desorbed to a large extent during oxygen reduction. Disorders of the monolayer structure and desorption of thiol molecules are proposed as the main reasons for the accessibility of electrochemical reactions to the surface.     

Potential-Dependent Rectified Electron Transfer at a Meldola′s Blue Incorporated Decanethiol-Modified Electrode as a Model of Biological Membrane

Meldola′s blue was immobilized into a self-assembled decanethiol monolayer modified on a gold electrode to provide a biological membrane model for electron transport having a molecular gate. Cyclic voltammograms of ferricyanide at this modified electrode showed only its reduction current at potentials where Meldola′s blue was reduced, but not at the redox potential of ferricyanide itself and no reaction was observed for ferrocyanide, indicating the direction of electron flow was controlled through the functionalized monolayer. Similar electrochemical responses were also observed for both octacyanotungustate and octacyanomolybdate. The cathodic peak currents observed in metal cyanide solutions at the modified electrode decreased in the order of Fe(CN)₆³⁻ > W(CN)₈³⁻ > Mo(CN)₈³⁻ at a given pH. From the analysis of the voltammograms using the microelectrode assembly model, the potential-dependent rectified electron flow was explained in terms of a gate function of Meldola′s blue in the monolayer, and the apparent electron transfer rate constant, k⁰_app, and the apparent diffusion coefficient, D_app, of metal cyanide ions in the monolayer were also estimated.     

Electronic donor-acceptor interactions of anthracene with a dehydroxylated surface of silica gel and aerosil

The ability of a dehydroxylated surface of silica gel and aerosil to enter into donor-acceptor interactions with poly-acenes was demonstrated by methods of steadystate and kinetic spectrofluorometry. The anthracene molecule forms EDA complexes with electron-acceptor sites of silica gel and aerosil in the ground state (charge-transfer complex, CTC) and the excited state (exciplex). At a degree of coverage of the silica gel and aerosil surfaces by polyacene molecules of 10^–1–10^–3% of a monolayer, energetic and structural inhomogeneity of the OH groups of silica gel appears. Silica gel possesses stronger electron acceptor sites; the electron-acceptor sites of aerosil are more accessible to EDA interactions and more monotypic. Vacancy defects of the SiO₂ matrix are suggested as accepting sites.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 6, pp. 693–698, November–December, 1986.     

The light-dependent oxygen reduction by monolayers of hydrated chlorophyll a oligomer

The phenomenon of light-dependent O₂ uptake by monolayers of hydrated chlorophyll a oligomer deposited by the Langmuir-Blodgett technique on an SnO₂ optically transparent electrode has been observed. Spectra of cathodic photocurrents coincided with the absorption spectrum of hydrated oligomer of chlorophyll a. In the presence of an artificial electron donor, hydroquinone, and an oxygen electron acceptor, both the cathodic and the anodic photocurrent caused by dry and wet chlorophyll a molecules of monolayer were measured under illumination in the range 400–800 nm. The effects of electrode potentials and redox reagents on the magnitude of solar energy conversion by the chlorophyll a monolayer at the optically transparent electrode are discussed. ESR and circular dichroism spectra show that hydrated oligomer of chlorophyll a consists of six molecules of chlorophyll a bonded with water molecules.     

Coadsorption of Tetraalkylammonium Cations and Halide Anions on the Uncharged Surface of Mercury Electrode

The data on coadsorption of tetraethylammonium (Et₄N⁺), tetrapropylammonium (Pr₄N⁺), and tetrabutylammonium (Bu₄N⁺) cations with Cl^–, Br^–, and I^– anions on an uncharged mercury electrode are compared with the models of coadsorption in a common monolayer and two parallel layers. The second model is shown to be in best agreement with experimental isotherms. However, the least discrepancy between calculations and experimental results is obtained when coadsorption of mentioned cations and anions is described by the Frumkin isotherm for neutral molecules with certain effective adsorption parameters.     

Cyclic voltammetric and computational study of a 4-bromophenyl monolayer on a glassy carbon electrode

A glassy carbon (GC) surface modified with monolayer of 4-bromophenyl was examined as voltammetric electrode for some redox systems. The modified electrode exhibited very slow electron transfer in comparison to the unmodified surface by factors which varied with the redox systems. However, after scanning the modified electrode in 0.1 M tetrabutylammonium tetrafluoroborate (TBABF₄) in acetonitrile from 0.4 to −1.1 V vs. Ag/AgCl for 20–25 cycles, the modified electrode showed much faster electron transfer kinetics, e.g., the results for Fe(CN)₆ ^3−/4− were approaching those observed with unmodified surfaces. The effect is attributed to an apparently irreversible structural change in the 4-bromophenyl monolayer, which increases the rate of electron tunneling. The transition to the conducting state is associated with electron injection into the monolayer and causes a significant decrease in the calculated HOMO-LUMO gap for the monolayer molecule. Once the monolayer is switched to the conducting state, it supports rapid electron exchange with the redox system, but not with dopamine, which requires adsorption to the electrode surface. A conductive surface modified electrode may have useful properties for electroanalytical applications and possibly in electrocatalysis. Correspondence: Abbas A. Rostami, Department of Chemistry, Faculty Basic of Science, University of Mazandaran, Babolsar, Iran.     

Neutral Nickel(II) and Copper(II) Tetraazamacrocyclic Complexes as Molecular Rods Attached to Gold Electrodes

New dithiolated derivatives of neutral Cu^II and Ni^II tetraazamacrocyclic complexes have been synthesized and characterized by spectroscopic and diffractional methods. These rod‐shaped molecules were assembled in monocomponent and mixed monolayers on gold electrodes. In the mixed monolayers, the active molecules were embedded in a hexanethiol matrix. The dithiolated complexes are oriented perpendicularly to the electrode, and reveal faster kinetics of electron transfer than those assembled in a single‐component monolayer. They appear as protrusions, which are easily addressed by using the STM method. In the presence of a suitable electron acceptor in the solution, the donor properties of the anchored Cu complex were weakened, which revealed donor–acceptor interactions with the monolayer. The peak position in the voltammogram indicates a stronger interaction of the solution‐based acceptor with the reduced Cu^II form than with the Cu^III complex. This suggests the possibility of switching the association on or off by applying an appropriate potential.     

The blocking and structural properties of a Schiff base self-assembled monolayer on the surface of Au(111)

Electrochemistry and in situ electrochemical scanning tunneling microscopy (STM) were used to study the blocking and structural properties of Shiff base V-ape-V self-assembled monolayers (SAMs) on the surface of Au(111) in perchloric acid solution. The complex-plane impedance plots for the SAM covered Au(111) electrodes, with the redox couple of Fe(CN)₆^4–/3– present in solution, exhibit arc shapes, revealing that the electrochemical kinetics were controlled by the electron-transfer step. For bare Au(111), the electrode process was mass transport limited. The molecules adsorb on Au(111) with a flat-lying orientation and form a long-range well-defined adlayer. A new structure of was observed in the double-layer potential region. A structural model is proposed to interpret the molecular registry with Au(111) substrate.     

Electrochemistry of redox-active self-assembled monolayers

Redox-active self-assembled monolayers (SAMs) provide an excellent platform for investigating electron transfer kinetics. Using a well-defined bridge, a redox center can be positioned at a fixed distance from the electrode and electron transfer kinetics probed using a variety of electrochemical techniques. Cyclic voltammetry, AC voltammetry, electrochemical impedance spectroscopy, and chronoamperometry are most commonly used to determine the rate of electron transfer of redox-activated SAMs. A variety of redox species have been attached to SAMs, and include transition metal complexes (e.g., ferrocene, ruthenium pentaammine, osmium bisbipyridine, metal clusters) and organic molecules (e.g., galvinol, C₆₀). SAMs offer an ideal environment to study the outer-sphere interactions of redox species. The composition and integrity of the monolayer and the electrode material influence the electron transfer kinetics and can be investigated using electrochemical methods. Theoretical models have been developed for investigating SAM structure. This review discusses methods and monolayer compositions for electrochemical measurements of redox-active SAMs.     

Electrochemical behavior of dopamine at a quercetin-SAM-modified gold electrode and analytical application

A stable quercetin–thioglycolic acid-modified gold electrode (Qu–TCA/Au) was prepared as a self-assembled monolayer (SAM) and its electrochemical behavior was investigated by electrochemical methods. In 0.05-M phosphate buffer solution (pH 7.0) quercetin exhibits quasi-reversible signals at the Qu–TCA/Au electrode. The stability of the quercetin-modified gold electrode is very good. The quercetin self-assembled monolayer is an effective mediator for the oxidation of dopamine, which was investigated by cyclic voltammetry and differential pulse voltammetry. Ascorbic acid does not interfere with determination of dopamine at an electrode modified with a mixture of quercetin–thioglycolic acid and quercetin–11-mercaptoundecanoic acid. This modification allows dopamine to be determined in the presence of ascorbic acid in the range from 3×10^–5 to 3×10^–4 M. The detection limit is 1×10^–6 M. Scanning electrochemical microscopy (SECM) was employed to study the electrochemical performances of the modified gold electrode indicating different feedback modes at differently modified surfaces.     

Properties of monolayers of crown ethers bearing an azo and azoxy group in the macrocycle

Two methods are described for preparing monomolecular layers of crown ethers with an azo or azoxy group in the macrocycle. When the molecules used to build the monolayer are soluble in aqueous solutions, adsorptive preconcentration on mercury electrodes was used to prepare the monolayer coating. The monolayer was electroactive due to the presence of the azo or azoxy unit in the molecules. Monolayers of crown ethers bearing an azo group in the macrocycle were shown to recognize alkali metal cations present in the solution. Changes of the parameters of the voltammetric reduction peaks - peak potential and peak width, served as an indication of specific interactions of the monolayer of 13-membered and 16-membered azocrown ethers with Na⁺ and K⁺ cations, respectively.The monolayers capable of recognizing cations have also been prepared on the aqueous solution-air interface, using the Langmuir technique. In this approach, amphiphilic derivatives of the azocrowns were synthesized and the monolayer has been assembled on the subphase containing metal cations. Binding of the cation by the macrocycle has a stabilizing effect on the monolayer and higher collapse pressures are achieved than on the pure water subphase. The monolayer was transferred from the air-water interface on the solid substrate using the Langmuir-Blodgett technique. Thin mercury film electrodes on the Ag substrate, or An films evaporated on glass slides were employed as the electrode substrates. The former gave monolayer modified electrodes of higher stability.Presented at the Sixth International Seminar on Inclusion Compounds, Istanbul, Turkey, 27–31 August, 1995.     

Fractional coverage of defects in self-assembled thiol monolayers on gold

Au electrodes are alkylated by self-assembled organic monolayers of octadecanethiol from alcohol solution. The electron tunnelling resistance of a monolayer-coated gold electrode has been investigated by ac impedance. The relation between the fractional coverage of different defects and the corresponding film thickness at these ‘collapsed’ sites has been deduced from electron tunnelling theory. By using the concepts of average film thickness at defect (d_a) and average fractional coverage of defect (θ_a), we have obtained the θ_ad_a plot. The influence of the apparent standard rate constant on the shape of the θ_ad_a plot has been discussed. In our experiments, Fe(CN)₆^3−/4− is used as a redox probe to study the θ_ad_a plot of an octadecanethiol monolayer. The θ_a versus d_a plot indicates that the defects with d_a<6 methylene groups and θ_a<0.1 can increase the apparent standard rate constant from 1.9×10⁻¹⁰ cm s⁻¹, which is the theoretical value calculated from electron tunnelling theory, to 2.9×10⁻⁷ cm s⁻¹. The average thickness of the whole monolayer (ATWM), which is obtained from the θ_a versus d_a plot and which can indicate the blocking property of the monolayer, is 11 methylene groups.     

Surface and Subsurface Oxygen on Platinum in a Copper Perchlorate Solution

The formation of an adatom layer on polycrystalline platinum and the three-dimensional nucleation of copper in a copper perchlorate solution are studied by cyclic voltammetry at 0.1 V s^–1 while varying potential ranges and by recording potentiostatic current transients. About 0.6 monolayers of copper adatoms are deposited when cycling with anodic limit E _a = 1.35 V, the process is slower than that in an acid sulfate solution. Decreasing E _a accelerates the process (nearly one monolayer forms for E _a = 0.80–0.95 V in a cathodic scan) due to an increased number of active centers (metastable copper oxides) and, probably, to a change in the platinum surface microstructure. Oxygen for copper oxides is presumably supplied by water molecules adsorbed on a monolayer of copper adsorbed atoms and by subsurface oxygen (O_ss), which appears on the platinum surface after the destruction of complexes O_ss–Pt _n –ClO₄. Both the copper nucleation and the deposit growth accelerate at higher concentrations of copper oxides, which form at low E _a. High cathodic overvoltages decrease the number of active crystallization centers due to reduction or removal of copper oxides.     

Electrochemical reduction and flow detection of iodate on (Bu<Subscript>4</Subscript>N)<Subscript>2</Subscript>Mo<Subscript>6</Subscript>O<Subscript>19</Subscript> self-assembled monolayer

A stable monolayer of the inorganic–organic hybrid polyoxometalate (Bu₄N)₂Mo₆O₁₉, denoted as Mo₆O₁₉, was formed on a sodium-3-mercapto-1-propanesulfonate (MPPS)-covered gold electrode surface, interlaced with an anionic poly(dimethyldiallylammonium chloride) (PDDA) binding layer based on the electrostatic self-assembled (ESA) technique. Electrochemical characterization of the Mo₆O₁₉ self-assembled thin films on the solid surface by cyclic voltammetry and AC impedance spectroscopy revealed a stable and sensitive electrocatalytic response to the reduction of iodate. Iodate was determined amperometrically through a flow injection cell at the modified electrode in the concentration range of 1.0×10^–6 to 1.0×10^–1 M with a detection limit of 8×10^–8 M (signal-to-noise ratio = 3). Performance was improved to meet practical needs compared with previously reported analogues.     

Thick-film electrodes for measurement of superoxide and hydrogen peroxide based on direct protein–electrode contacts

Cytochrome c was immobilized on screen-printed thick-film gold electrodes by a self-assembly approach using mixed monolayers of mercaptoundecanoic acid and mercaptoundecanol. Cyclic voltammetry revealed quasi-reversible electrochemical behavior of the covalently fixed protein with a formal potential of +10 mV vs. Ag/AgCl. Polarized at +150 mV vs. Ag/AgCl the electrode was found to be sensitive to superoxide radicals in the range 300–1200 nmol L^–1. Compared with metal needle electrodes sensitivity and reproducibility could be improved and combined with the easiness of preparation. This allows the fabrication of disposable sensors for nanomolar superoxide concentrations. By changing the electrode potential the sensor can be switched from response to superoxide radicals to hydrogen peroxide—another reactive oxygen species. H₂O₂ sensitivity can be provided in the range 10–1000 mol L^–1 which makes the electrode suitable for oxidative stress studies.     

Study of the interaction and activation of lipase from Pseudomonas fluorescens in surfactant monolayers and precipitates

The activation of lipase from Pseudomonas fluorescens (PFL) upon its immobilization in surfactant coprecipitates (hexadecane-1,2-diol (HDD), cetyl alcohol (CetOH), N-cetylacetamide (CetAA), and cetylamine (CetNH₂)) organized in monolayers at the interface were studied by the Langmuir—Blodgett monolayer technique. Incorporation of the enzyme into surfactant monolayers at the surface pressure = 10 mN m^–1 results in an apparent increase in the area per molecule. In the series of noncharged surfactants CetOH—HDD—CetAA, this effect increases in proportion to the amount of the enzyme incorporated in the monolayer. The catalytic activity of the lipase—surfactant coprecipitates in an organic solvent as regards esterification increases in the same sequence, indicating similarity of the interaction of lipase with surfactant monolayers and coprecipitates. For = 10 mN m^–1, the CetNH₂ monolayer with liquid-expanded state incorporates the largest amount of the enzyme (PFL : CetNH₂ = 1 : 290); the CetOH monolayer, which exists in the condensed state under the same conditions, incorporates the smallest amount (PFL : CetOH = 1 : 1700). The hydrolytic activity of PFL in mixed monolayers with surfactants increases 1.5—11-fold; the esterification activity in surfactant coprecipitates, 1.6—9-fold. The lipase activation effects are explained by facilitated transport of substrates into mixed monolayers and surfactant—enzyme precipitates in aqueous and organic media, respectively.     

Electrochemical Investigation of Cytochrome c Immobilized onto Self‐Assembled Monolayer of Captopril

The electrochemical behavior of cytochrome c (cyt‐c) that was electrostatically immobilized onto a self‐assembled monolayer (SAM) of captopril (capt) on a gold electrode has been investigated. Cyclic voltammetry, scanning electrochemical microscopy (SECM) and electrochemical impedance spectroscopy were employed to evaluate the blocking property of the capt SAM. SECM was used to measure the bimolecular electron transfer (ET) kinetics (k_BI) between a solution‐based redox probe and the immobilized protein. In addition, the tunneling ET between the immobilized protein and the underlying gold electrode was calculated. A k_BI value of (5.0±0.6)×10⁸ mol^?1 cm³ s^?1 for the bimolecular ET and a standard tunneling rate constant (k⁰) of 46.4±0.2 s^?1 for the tunneling ET have been obtained.     

A novel electrochemical method to detect mercury (II) ions

A novel and sensitive electrochemical method for determination of mercury (II) ions (Hg²⁺) based on the formation of thymine–Hg²⁺–thymine complexes and gold nanoparticle-mediated signal amplification is reported. Two 5′ end thiolated complementary oligonucleotides containing six strategically placed thymine–thymine mistakes were introduced in this work. One of the two oligonucleotides was immobilized on a gold electrode and the other one on gold nanoparticles (AuNPs). Due to six thymine–thymine mistakes the two oligonucleotides were not able to be hybridized, so AuNPs could not be immobilized onto the electrode surface after the electrode was immersed in the DNA–AuNPs solution. However, if Hg²⁺ existed, T–Hg²⁺–T complexes could be formed and AuNPs could be immobilized onto the electrode surface. Meanwhile, large numbers of [Ru(NH₃)₆]³⁺ molecules as electrochemical species could be localized onto the electrode surface. The Hg²⁺ detection limit of this assay could be as low as 10 nM, which is the US Environmental Protection Agency (EPA) limit of Hg²⁺ for drinkable water. This method is proven to be simple, convenient, high sensitive and selective.