A correlation study between the conformation of the 1,4-dithiane SAM on gold and its performance to assess the heterogeneous electron-transfer reactions

A gold electrode spontaneously modified by 1,4-dithiane (1,4-dt) organosulfur species has been characterized by cyclic voltammetry of [Fe(CN)₆]^3?/4? and cytochrome c (cyt c) probe molecules, SERS (surface-enhanced Raman scattering), and EIS (electrochemical impedance spectroscopy), using [Fe(CN)₆]^3?/4? as the redox-active molecule in solution. The cyclic voltammograms showed a decrease in the faradaic current response with an increase of the immersion time of the gold substrate into the 1,4-dt modifier solution, suggesting that the heterogeneous electron-transfer (hET) process of the [Fe(CN)₆]^3?/4? probe molecule occurs through the surface pinholes. This observation is an indication that the 1,4-dt chemisorption mechanism must involve a slow step that is responsible for the final configuration of the modifier molecules on the surface. The SERS spectra acquired for the surface as a function of immersion time of the gold electrode in the modifier solution presented an intensity enhancement of the bands assigned to the trans and gauche conformation of the 1,4-dt on the surface. The apparent rate constant, k_app, estimated by the EIS results, decreases with increase of the immersion time indicating that the pinholes on the surface act as a microarray. This result is consistent with those observed by cyclic voltammetry of the probe molecules, i.e., the pinhole density decreases with increase of the immersion time of the gold electrode in the 1,4-dt modifier solution, and a strong attenuation of the probe molecules interfacial electron-transfer response is observed.     

Electrooxidative coupling of a toluidine blue O terminated self-assembled monolayer studied by electrochemistry and surface enhanced Raman spectroscopy

A toluidine blue O (TBO) terminated self-assembled monolayer (SAM-I) was fabricated on a gold disk electrode by nucleophilic attack of the amino group in the reduced TBO on an immobilized carbonyl moiety of terephthaloyl chloride with cyclic scanning. The TBO units in the SAM-I were observed by surface enhanced Raman spectroscopy (SERS). The average TBO surface coverage of SAM-I was (2.5±0.5)×10^?10 mol cm^?2. Meanwhile, another TBO derived SAM (SAM-II) was prepared for comparison by activating the free carboxyl group of bound terephthaloyl chloride and then reacting it with the amino group of TBO. The average TBO surface coverage of SAM-II was only (6.8±0.1)×10^?11 mol cm^?2. Both SAMs possessed good electroactivity and showed identical properties. With an electrooxidative coupling of TBO units in SAM-I, a novel electroactive polymerized TBO monolayer (Film-I) was constructed, which displayed an improved stability and obvious electrochemical blocking effect. As a comparison, a poly(TBO) film (Film-II) was obtained by the electrochemical polymerization of TBO in solution on SAM-I for remedying the defectiveness of the knitted structure of Film-I. The SERS displayed more TBO units in Film-II than in Film-I. Film-II provided a much better barrier to the redox probe of K₃[Fe(CN)₆] due to the formation of the perfect structure.     

Characterization of defects in the formation process of self-assembled thiol monolayers by electrochemical impedance spectroscopy

Self-assembled monolayers of octadecanethiol (ODT) on gold have been studied by electrochemical impedance spectroscopy (EIS). The fractional coverage, pinhole size and separation have been examined as a function of immersion time of Au in the ODT deposition solution. The fractional coverage of ODT monolayer increases sharply from zero to more than 99% of its maximum within the first minute. However, it takes hours or a day for the fractional coverage to approach its final value. The pinhole separation increases from ca. 1.8 μm to 53 μm as the assembling time increases from 5 s to 24 h, indicating that the number of pinholes decreases. The pinhole radius increases slowly from ca. 0.3 μm to 1.6 μm in the same time interval. The slight trend of increasing pinhole size with adsorption time is attributed to the increasing influence of collapsed sites in the ODT monolayer.     

Simultaneous determination of epinephrine and uric acid at a gold electrode modified by a 2-(2,3-dihydroxy phenyl)-1, 3-dithiane self-assembled monolayer

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 155 mV 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 500.0 μM and a detection limit (3σ) of 0.51 μM. 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.     

Studies of structural disorder of self-assembled thiol monolayers on gold by cyclic voltammetry and ac impedance

Self-assembly of octadecyl mercaptan on gold has been investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). CV experiments show that well-assembled thiol monolayers on gold are essentially free of pinhole defects. Support for the existence of ‘collapsed’ sites in thiol monolayers is obtained by comparing the apparent electron transfer rate constant, which is obtained from CV experiments at a film covered Au electrode, with the theoretical one, which is calculated by assuming electrons tunnel across the collapsed-site-free monolayer. The empirical relationship Z=R_s+1/[σ(jω)^α] is used to describe the film coated Au in inert electrolyte and the degree of disorder of the monolayer structure is estimated from the α value. It is demonstrated that the admittance plane plot of an ideally polarizable electrode with constant phase element behavior is a circular arc which can be used to determine α. Results of EIS experiments show that α increases sharply after the initial adsorption time and approaches its final value slowly. This is attributed to the rapid adsorption step followed by a slow crystallization process in the kinetics of alkanethiol adsorption onto the Au electrode. For a well-assembled thiol monolayer on gold, α can approach 0.99, which means that the number of collapsed sites is not large and the surface of the monolayer Au electrode is rather smooth.     

Electrochemical and spectral studies of self-assembled monolayer of 1,8,15,22-tetraaminophthalocyanatocobalt(II) on indium tin oxide surface

Self-assembled monolayer (SAM) of 1,8,15,22-tetraaminophthalocyanatocobalt(II) (4α-Co^IITAPc) was prepared on indium tin oxide (ITO) electrode by spontaneous adsorption from dimethylformamide (DMF) solution containing 4α-Co^IITAPc. The SAM of 4α-Co^IITAPc formed on ITO electrode was characterized by cyclic voltammetry, Raman and UV–visible spectroscopic techniques. The cyclic voltammogram (CV) of 4α-Co^IITAPc SAM shows two pairs of well-defined redox peaks corresponding to Co^III/Co^II and Co^IIIPc⁻¹/Co^IIIPc⁻². The surface coverage (Γ) was calculated by integrating the charge under the anodic wave corresponding to Co^II oxidation and it was found to be 2.25 × 10⁻¹⁰ mol cm⁻². Raman spectrum obtained for the SAM of 4α-Co^IITAPc on ITO surface shows strong stretching and breathing bands of Pc macrocycle, pyrrole ring and isoindole ring. Further, the –NH₂ bending mode of vibration was absent for the SAM of 4α-Co^IITAPc on ITO surface which indirectly confirmed that all the amino groups of 4α-Co^IITAPc are involved in bonding with ITO surface. UV–visible spectrum for the SAM of 4α-Co^IITAPc on ITO surface shows an intense B-band, Q-band and n–π^∗ transition with slight broadening when compared to that of 4α-Co^IITAPc in DMF.     

Exchange of self-assembled thiol monolayers on gold: characterization by FT-IR external reflection spectroscopy

The exchange behavior of various thiol monolayers self-assembled on a gold surface is described. FT-IR external reflection spectroscopy was used for monitoring of the self-exchange of unsubstituted alkanethiol with the aid of deuterated thiol. Intensity changes of the absorption peaks for a specific functionality were used for the quantization of thiols in the monolayers. Exchange of compact monolayers such as eicosanethiol with the same type of alkanethiol in ethanol solution was found to be very slow. We used n-eicosanethiol [CH₃(CH₂)₁₉SH] and partially deuterated eicosanethiol [CD₃(CH₂)₁₉SH] in order to avoid the chain length difference effect of adsorption and maintain exactly the same chain length of molecules. The exchange of the non-compact ferrocene terminated alkanethiol monolayer with dodecanethiol was faster than the self-exchange of the compact unsubstituted alkanethiol monolayer. The result shows that the compactness of the monolayer is critical to the exchange kinetics of self-assembled monolayers.     

The influence of surfactants on the electron-transfer reaction at self-assembled thiol monolayers modifying a gold electrode

Monolayer films of thiols with different terminal groups (CH₃, COOH and NH₂) were prepared on gold electrodes using a self-assembling procedure. Cyclic voltammetry was used to investigate the electron-transfer reaction of potassium ferricyanide at the thiol modified gold electrode in the presence of surfactants. It was found that surfactants show some effects on the electron-transfer reaction at the thiol modified electrode. Cationic surfactants can improve the reversibility of a redox reaction at the thiol modified gold electrodes, while anionic surfactants cause a little inhibition of electron transfer of the redox at both methyl and carboxyl terminal thiol modified gold electrodes. Surfactants can interact with thiol monolayers in different ways, changing the structure and properties of the monolayer film, and can further affect electron transfer at the modified gold electrode.     

Electrogenerated chemiluminescence of lucigenin enhanced by the modifications of electrodes with self-assembled monolayers and of solutions with surfactants

Electrogenerated chemiluminescence (ECL) of lucigenin (Luc²⁺·2NO^?₃, N,N′-dimethyl-9,9′-biacridinium dinitrate) in dioxygen-saturated alkaline aqueous solutions of pH 10 has been examined utilizing modifications of electrodes (i.e. self-assembled monolayer (SAM)-modified gold electrodes) and of solutions (i.e. micellar solutions containing a nonionic surfactant, Triton X-100) for the first time. In both cases of the modifications, enhanced ECL was observed, and the ECL was considered to be derived from the decomposition of a dioxetane-type intermediate formed by the radical–radical coupling reaction between an electrogenerated superoxide ion (O₂^?) and a one-electron reduced form of Luc²⁺ (i.e. a radical cation, Luc⁺). The surface modification of gold electrodes was achieved with dimercaptosuccinic acid (DMSA) and dl-thioctic acid (dl-TA) having carboxyl end groups. The amount of ECL at dl-TA-SAM-modified electrodes was about five times as great as that at the bare electrode. The enhancement of ECL at the SAM-modified electrodes would be due to the concentration effect of positively charged Luc²⁺ ions, the prevention of adsorption of the electrogenerated chemiluminescent product (i.e. N-methylacridone (NMA)) and the two-electron reduced form of Luc²⁺ (Luc⁰) on the electrode surfaces, and the effective generation of O₂^?. On the other hand, the amount of ECL in the micellar solution increased by about six times in comparison with that in the solution containing no surfactant. The enhanced ECL in micellar solutions would result mainly from the inhibition of adsorption of NMA and Luc⁰ insoluble in water on electrode surfaces.     

Cyanide adsorption on gold electrodes: a combined surface enhanced Raman spectroscopy and density functional theory study

A combined SERS and DFT study of cyanide adsorption on a gold electrode is presented. From our analysis, the high-frequency mode at ～2100 cm^?1 is ascribed to the C–N stretching frequency at (1 0 0) and (1 1 0) sites. The lower frequency modes at ～370 and ～300 cm^?1 are ascribed to the Au–CN stretching and bending modes, respectively. The Stark tuning slopes of these modes agree well with the DFT computations. The bending mode at 300 cm^?1 has a very small Stark tuning slope of ～4 cm^?1 V^?1, experimentally, compared to ～1 cm^?1 V^?1, computationally. The Au–CN stretching frequency has a Stark tuning slope of ～15 cm^?1 V^?1, experimentally, compared to 16–22 cm^?1 V^?1, computationally. The positive Stark tuning slopes suggest that cyanide adsorbs as an anion, with a bond polarity between that of the Au–Cl and Au–Br surface bonds. The anionic character of the Au–CN bond is also confirmed by different charge analyses based on the DFT computations. The ordering in binding strength on the three different surfaces is the same as the ordering in bond ionicity. The C–N stretching frequency has two different Stark tuning slopes in the SERS experiments: ～14 cm^?1 V^?1 in the more negative potential region corresponding to cyanide adsorption, and ～35 cm^?1 V^?1 in the more positive potential region corresponding to gold dissolution. The computational result of ～8 cm^?1 V^?1 agrees well with the lower Stark tuning slope. The higher Stark tuning slope is presumably related to the formation of gold–cyanide complexes in the double layer.     

Ion-channel-mimetic sensor for trivalent cations based on self-assembled monolayers of thiol-derivatized 4-acyl-5-pyrazolones on gold

Self-assembled monolayers (SAMs) of thiol-derivatized 4-acyl-5-pyrazolone on gold electrodes were used for ion-channel-mimetic sensing of inorganic cations. The SAMs on gold electrodes were characterized with reductive-desorption and capacitance measurements. The pH dependence of the cyclic voltammograms (CVs) of [Fe(CN)₆]^3? and [Ru(NH₃)₆]³⁺ as electroactive markers suggested the protonation not only of the β-diketone part but also of the nitrogen moiety in the pyrazolone. With regard to the voltammetric responses to metal cations, an increase and a decrease in redox current of [Fe(CN)₆]^3?/4? and [Ru(NH₃)₆]^3+/2+, respectively, were observed with increasing concentrations of the trivalent cations examined (La³⁺, Gd³⁺, Yb³⁺ or Al³⁺) from 10^?6–10^?4 up to 10^?2 M at pH 5.5, at which the chelating group is present as its deprotonated form. In contrast, such responses were negligible in the presence of up to 10^?2 M divalent cation (Mg²⁺, Ca²⁺, Sr²⁺ or Ba²⁺), Li⁺ or Na⁺. The order of the magnitudes of the responses was Al³⁺>Yb³⁺≈Gd³⁺>La³⁺?divalent cations, which is quite similar to that of the stability of 1:1 and 1:2 complexes between a β-diketonate-type chelate and the metal ions. The highly selective responses to trivalent cations seem to reflect the selectivity of the chelating group as well as the large change in the surface charge induced by the complexation.     

One-impedance for one-concentration impedimetry as an electrochemical method for determination of the trace zirconium ion

A new method is established for the electrochemical detection of zirconium ion (Zr(IV)) based on electrochemical impedance spectroscopy transduction method, and hydroxamated gold surface recognition system. The method successfully served for the detection of Zr(IV) with a wide dynamic range of 1.0 × 10⁻⁹ to 5.0 × 10⁻⁵ mol L⁻¹, and a detection limit in nmol L⁻¹ scale. Construction of the calibration curve by “one-impedance for one-concentration” method developed here helped us to save the experimental time by saving the data acquisition time by a factor of ∼20, and further, eliminating data approximation and parameter extraction times.     

Preparation of a platinum layer-modified aluminum electrode by electrochemical and electroless cementations and its use for the electrooxidation of methanol

The deposition of platinum on the aluminum electrode by two methods involving electroplating by single step potential from an aqueous buffered H₂PtCl₆ solution (pH 7) and electroless cementation from an aqueous solution of H₂PtCl₆ saturated with NaF is described. The electrochemical characteristic of the platinum layer-modified aluminum electrode (Pt/Al) in aqueous solution with various pHs was investigated. The Pt/Al layers were characterized by imaging with scanning electron microscopy and optimum conditions for the preparation of the Pt/Al modified electrode was chosen. The modified electrode was used as electrocatalyst for the electrooxidation of methanol in 0.1 M H₂SO₄ solution. After characterization of some features of the electrochemical system on the stationary and rotating electrode, an oxidation pathway is suggested. The current density of the methanol oxidation at Pt/Al electrode measured in optimized conditions and compared with that obtained on the smooth Pt, Pt/Pt, and Pt/PoPD/Pt electrocatalysts. The stability of the electrode was followed over the course of the time and its stability and reproducibility was confirmed.     

Redox switching hysteresis in polyaniline–acetate systems: a search of molecular factors important for the dynamics of the polymer reaction

The CV redox hysteresis of three polyaniline–aqueous trihalogenoacetic acid systems has been analyzed. The polymer phases studied involve monocarboxylate monoanion bases characterized by similar pK_a values in water to narrow the differences in molecular interactions. The hysteresis diminishes by 70 mV with size of the anions – from trifluoroacetate, through trichloroacetate, to tribromoacetate. The diffusion coefficients of the trihalogenoacetate anions in the oxidized polyaniline do not differ significantly. Their values are of the order of 10^?10 cm²/s, which is one order of magnitude lower than for the chloride anion in polyaniline. Ex situ 1750–1050 cm^?1 Raman spectra of polyaniline–trihalogenoacetates show small noticeable systematic changes that correlate with the hysteresis behavior. Electrochemical impedance spectroscopy provides evidence for the chemical rate process involved in the redox switching of the polyaniline–tribromoacetate system.     

Interpretation of the ultra-fast electropolymerization of pyrrole in aqueous media on zinc in a one-step process: The specific role of the salicylate salt investigated by X-ray photoelectron spectroscopy (XPS) and by electrochemical quartz crystal microbalance (EQCM)

The use of sodium salicylate as an electrolyte makes it possible to deposit polypyrrole (PPy) films on oxidizable metals such as zinc by the electrochemical oxidation of pyrrole. In spite of the very large difference between the pyrrole and zinc oxidation potentials, which thermodynamically should lead to metal dissolution and not polymer formation, PPy films are formed as easily as on a platinum electrode. X-ray photoelectron spectroscopy (XPS) and in situ electrochemical quartz crystal microbalance (EQCM) experiments reveal that a very thin composite passivating zinc salicylate layer is formed prior to pyrrole electropolymerization and prevents zinc dissolution without inhibiting polymer formation. Ex situ XPS analysis of the surface at different potentials (?1 to 0.7 V) at a grazing and normal incidence of the photoelectrons shows that pyrrole is adsorbed on zinc at an early stage of the polarization and remains on the metal surface up to the beginning of electropolymerization. In situ EQCM measurements indicate that this passivation layer is very thin (about 5 nm), and does not desorb at the potential where pyrrole is oxidized. The rapid oxidation of pyrrole through this passivating layer is explained by the presence of conductive pyrrolic paths inside the salicylate layer which make this composite layer as active as a noble metal.