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.     

An electrochemical quartz crystal impedance study on cystine precipitation onto an Au electrode surface during cysteine oxidation in aqueous solution

Electrochemical quartz crystal impedance studies on l-cysteine oxidation in aqueous solutions were conducted. The precipitation of electrogenerated species on the Au electrode surface was found in acetic and phosphate buffers. Changes in the equivalent circuit parameters, resonant frequencies and half peak width of the conductance spectra (Δf_G1/2) obtained simultaneously during electrochemical experiments were discussed. Significant changes in the motional resistance and Δf_G1/2 revealed that the adhering precipitate acted as a non-rigid mass loading to the piezoelectric quartz crystal resonance. The resonant frequency shift found in solution was approximately twice that in air. Lots of loose and separate precipitate agglomerates formed and stood like islands on the electrode surface, and for a frequency shift of ?10?000 Hz in the acetic buffer, the height of the precipitate agglomerate could be greater than 5 μm, as found by SEM. In addition, a white precipitate and similar responses of the equivalent circuit parameters were obtained during ferricyanide titration of l-cysteine in acetic, phosphate and ammonia buffers. IR analysis revealed that the white precipitate is cystine. Electrode collection efficiencies of cystine were evaluated. Moreover, the frequency-point-selection fitting routine has been discussed and taken for analyses of the admittance data, and an equation of Δf_G1/2 as a function of the equivalent circuit parameters was given.     

The role of the potential distribution at the electrode interface in determining the electron transfer rate constant

Simultaneous electrochemical impedance (EI) and electroreflectance (ER) measurements were performed on ‘bare’ gold, N-acetyl-l-cysteine(NAC) modified gold, and cytochrome c (cyt c) adsorbed on a NAC modified gold electrode. The electrical impedance of the electrode interface was modeled by a series connection of a constant phase element (CPE), a capacitor, and a resistor. The analysis of the combined EI and ER data yielded the heterogeneous electron transfer (ET) rate constant at each frequency of the modulating potential. This is in contrast to previous techniques which used the frequency dependence of the EI or ER response to obtain a value of the rate constant. Assuming that the Faradaic potential was equal to the potential at the nodes of the three element impedance model yielded frequency dependent rate constants. A small modification of the three element impedance model was needed to obtain a frequency independent rate constant of 850±80 s^?1. This suggests that the distribution of the potential at the electrode interface (reflected in the choice of the electrical impedance model) is an important factor in the determination of the ET rate constant.     

An investigation of the hydrogen absorption reaction into,and the hydrogen evolution reaction from,a Pd foil electrode

The hydrogen absorption reaction (har) into, and the hydrogen evolution reaction (her) from, Pd foil electrode have been investigated in 0.1 M NaOH solution by using electrochemical impedance spectroscopy. The generalised Faradaic admittance for the indirect har into, and her from, metal foil electrodes under the impermeable boundary conditions has been derived on the basis of Lim and Pyun's kinetic approach to har and her. The measured impedance spectra were analysed by using a complex non-linear least squares data-fitting method applied to the derived Faradaic admittance. Also, impedance spectra were theoretically calculated with different kinetic parameters for the har and her having physical significance in order to characterise the influence of each kinetic parameter on the change of the impedance spectra in the Pd foil electrode.     

Electrodissolution of Ti and p-Si in acidic fluoride media: formation ratio of oxide layers from electrochemical impedance spectroscopy

The electrodissolution of Ti and p-Si electrodes in acidic fluoride solutions has been investigated by electrochemical impedance spectroscopy as a function of potential E, fluoride concentration, pH and angular speed of the electrode, with the aim of characterising the oxide layers covering both materials. The impedance diagrams have been analysed to obtain the low frequency capacitance C_lf, the high frequency capacitance C_hf and the product R_hfI (where R_hf is the high frequency resistance and I the steady-state current). In the potential domain where the oxide thickness x is proportional to E, the formation ratio dx/dE has been computed from C_lf, C_hf and R_hfI. The calculated values have been found to be in good mutual agreement and close to the ones found in the literature.     

Theory of the electrochemical impedance of anomalous diffusion

This paper addresses the electrochemical impedance of diffusion in a spatially restricted layer. A physically grounded framework is provided for the behavior Z(iω)∝(iω)^?β/2 (0<β<2), thus generalising the Warburg impedance (β=1). The analysis starts from the notion of anomalous diffusion, which is characterized by a mean squared displacement of the diffusing particles that has a power law dependence on time $\text{r}{}^2\text{∝t}{}^{\mathrm{\beta }}$. Using a theoretical approach to anomalous diffusion that employs fractional calculus, several models are presented. In the first model, the continuity equation is generalised to a situation in which the number of diffusing particles is not conserved. In the second model the constitutive equation is derived from the stochastic scheme of a continuous time random walk. And in the third, the generalised constitutive equation can be interpreted within a non-local transport theory as establishing a relationship of the flux to the previous history of the concentration through a power-law behaving memory kernel. This third model is also related to diffusion in a fractal geometry. The electrochemical impedance is studied for each of these models, and the representation in terms of transmission lines is established. The main finding is that, while models with quite different non-trivial diffusion mechanisms behave similarly in a semi-infinite situation, the consideration of the effect of the boundaries gives rise to neatly different impedance spectra.     

Langmuir–Blodgett films of azocrown ethers on electrodes — voltammetric recognition of isomers

Monolayers of amphiphilic derivatives of crown ethers bearing electroactive azo groups in the macrocycle were transferred from the air ∣ water interface onto indium–tin oxide (ITO) or thin mercury film electrodes (TMFE) using the Langmuir–Blodgett technique. Differences in the electrochemical reversibility of the systems observed on these two electrodes were explained by different orientation of the azocrown molecules on the hydrophobic (TMFE) and hydrophilic (ITO) electrode surfaces. The electrochemical studies of the monolayer modified electrodes proved that the organization of the monolayer and the redox properties of the azocrown molecules depend on the geometry around the NN moiety. Voltammetry allowed Z- and E-isomers of the azocrowns to be recognized and their isomerization processes in monolayers to be followed. The effects of pH and of the alkali metal cations on the stability of the isomers were studied. Isomerization to the more stable E-isomer was hindered in alkaline solutions and under these conditions well separated reduction peaks of the E- and Z-isomers were obtained.     

Effect of pH and the extent of micellization on the redox behavior of non-ionic surfactants containing an anthraquinone group

Cyclic voltammetry has been employed for the study of aqueous electrochemistry of the surfactants, α-(anthraquinonyloxyhexyl)-ω-hydroxy-oligo(ethylene oxide) (ACPEG) and α-anthraquinonyl-ω-hydroxy-oilgo(ethylene oxide) (APEG), which have wide differences in surface activity. Potential–pH diagrams have been constructed and the various features of the diagrams have been analyzed in the light of the change in solution equilibria and the difference in the extent of micellization. The redox potentials of the surfactants have been found to exhibit strong pH dependence. The electrode reaction involves two-electron reduction of anthraquinone (AQ) to its dianion (AQ^2?), which is highly sensitive to the pH of the solution. At controlled pH, potential–pH plots allow the establishment of the values of the ionization constants for dihydroanthraquinone (AQH₂) and its monoanion (AQH^?) as pK_a¹=7.83 and pK_a²=11.38, respectively. Under unbuffered conditions, the effective pH close to the electrode surface controls the potential of the electrode process. The changeover from the H⁺-available to the H⁺-depleted electrode process gives rise to a sudden jump in potential. In highly alkaline solutions, AQ forms an adduct with hydroxyl ion, which causes a linear decrease in the potentials with increase in pH. The different extent of micellization results in a difference in the peak current and the half wave potentials (E_1/2) for ACPEG and APEG but causes no significant change in the shapes of the E_1/2–pH diagrams. This has been explained in terms of the disruption reaction of the micelles, preceding the electrochemical reaction.     

Orthognathic model surgery by using of a passive Robot Arm

Purpose

The possibility of using a passive Robot Arm (3D method) in model surgery and comparing with manual technique model surgery.

Patients and methods

Seventeen patients undergoing orthognathic surgery gave consent for this study. Model surgery was performed by using a manual technique and using the Robot Arm. The model surgery that was performed by using the manual technique named group A and the one performed by the Robot Arm named group B. Patients’ maxillary casts were measured before and after model surgery, and results were compared with those for the original treatment plan in the horizontal (X-axis), vertical (Y-axis), and transverse (Z-axis) planes.

Results

Statistical analysis using Mann–Whitney U test for X- and Y-axis and independent sample t test for Z-axis have shown significant differences between both groups in X-axis (P = .026) and Y-axis (P = .021) but not in Z-axis (P = .762).

Conclusions

Model surgery performed with a Robot Arm is more accurate in all dimensions X, Y, and Z than the manual model surgery.     

Electrochemical investigations and morphology of poly(4,9-dihydro-o-benzenonaphtho[2,3-c]pyrrole) and poly(acenaphtho[1,2-c]pyrrole)

The electrochemical characteristics and morphology of poly(4,9-dihydro-o-benzenonaphtho[2,3-c]pyrrole) (PDBNP) and poly(acenaphtho[1,2-c]pyrrole) (PANP) films prepared by controlled potential oxidation in acetonitrile containing 0.002 M monomer and 0.1 M tetrabutylammonium perchlorate (TBAP) have been studied. The impedance of PDBNP and PANP films coated onto glassy carbon electrodes measured in 0.1 M TBAP shows that the ionic conductivities of these two films increase with increasing electrode potential (oxidation level) as ClO₄^? ions are incorporated. It is concluded that anion transport is primarily responsible for the ionic conductivities. PANP has a 45° Warburg region at all electrode potentials. However, for PDBNP, the 45° Warburg region is seen only at low electrode potentials. The difference in the mode of charge transport shows that the value of electronic resistance of PDBNP at higher doping levels is similar to the ionic resistance, but for PANP, the electronic resistance is much smaller than the ionic resistance at all doping levels. The apparent electrochemical reversibility was seen to be higher for PDBNP than for PANP from cyclic voltammetry. Evidence for this interpretation is that the ionic conductivities increase dramatically with electrode potentials for PDBNP, indicating that the counterion ClO₄^? moves more easily in PDBNP than PANP. The higher low-frequency capacitance obtained from impedance spectroscopy for PANP is discussed in the light of in situ atomic force microscopy (AFM) observation of the film structure morphology.     

Mechanical resonance effects in electroactive polycarbazole films

We describe crystal impedance data acquired dynamically during the electropolymerization of carbazole, to produce polycarbazole films on the electrode surface. Data were obtained at the fundamental and third harmonic modes of a 10 MHz thickness shear mode resonator. At a critical thickness, the system exhibits mechanical resonance, a special condition in which the mechanical shear deformation across the polymer film corresponds to one quarter of the acoustic wavelength. This situation has not been reported previously for electroactive polymer films exposed to a liquid phase. At this point, the resonant frequency and admittance data show dramatic changes with polymer coverage. Prior to mechanical resonance, the impedance spectra can be fitted readily to a modified Butterworth–Van Dyke model, recognizing the presence of an ideal mass layer, a finite viscoelastic layer and a semi-infinite Newtonian fluid (the electrolyte). The data indicate spatial variation of shear modulus.     

Kinetics of CH3OH oxidation on PtRu/C studied by impedance and CO stripping voltammetry

The kinetics of the CH₃OH oxidation reaction at 60 °C on well-alloyed platinum–ruthenium supported on carbon (Pt₅₀Ru₅₀/C) was studied by electrochemical impedance spectroscopy and compared with carbon-supported platinum (Pt/C). The reaction rate of the overall CH₃OH oxidation increased with increasing electrode potential for both Pt/C and PtRu/C. In the case of Pt/C, when the electrode potential was E ? 450 mV vs. RHE only a capacitive behavior was observed. Resistive and pseudo-inductive types of behavior were evident above 500 and 600 mV vs. RHE. In the case of PtRu/C, a similar change in behavior was observed, except that the two types of behavior were observed at 200 mV lower electrode potentials than for Pt/C. Correlation of the impedance data with pre-adsorbed carbon monoxide (CO_ad) stripping voltammetry allowed the understanding of the methanol oxidation reaction. The change in the reaction rate of the oxidation of CH₃OH to CO_ad as a function of the electrode potential as well as the promotional effect of Ru was evident from a change in the frequency where the frequency deviated from the ～90° phase angle. The change in the reaction rate of the oxidation of CO_ad to CO₂ as a function of the electrode potential as well as the alloying with Ru was evident from a change in the frequency where the phase angle approached zero.     

Analysis of the impedance spectra of Pt|poly(o-phenylenediamine) electrodes – hydrogen adsorption and the brush model of the polymer films

Electrochemical impedance spectra obtained for poly(o-phenylenediamine) films on platinum in contact with 0.5 M H₂SO₄ solution were analyzed on the basis of the brush model elaborated for conducting polymer films on electrodes. It is assumed that the polymer film consists of short and long polymer chains forming bundles in such a way that only a small part of the metal substrate is covered by the polymeric material. The analysis of the impedance spectra taken at several potentials in the region of hydrogen adsorption on platinum supports the validity of the model. The model has been tested also by varying the film thickness and the roughness of the platinum substrate.     

Monitoring bacterial-demineralization of human dentine by electrochemical impedance spectroscopy

Objective

The purpose of this study was two-fold: (1) to monitor bacterial biofilm formation and bacteria-induced demineralization of dentine in situ by using electrochemical impedance spectrum (EIS); (2) to examine the relationship between EIS findings and changes in the chemical composition and ultrastructure of dentine during bacteria-induced demineralization.

Methods

In this study, dentine demineralization was induced by Streptococcusmutans (ATCC 25175) in the presence of sucrose in culture medium and was monitored using two EIS measurement systems (Type A with a working electrode and Type B without a working electrode). Scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), energy dispersive X-ray (EDX) and X-ray diffraction (XRD) were employed to examine the morphology, element contents and crystallinity of hydroxyapatite (HAP) on the dentine surface. Transverse microradiography (TMR) was used to characterize the lesion depth and degree of mineral loss during demineralization.

Results

The resistance of the bulk dentine (R_d) and the apparent resistance of dentine (R_a) measured from the Type A and Type B EIS systems, respectively, decreased gradually with demineralization. The resistance of the biofilm formed on dentine surface was determined by fitting the EIS data with equivalent circuits. The presence of biofilm slightly increased R_a of dentine before demineralization. However, the electrochemical behavior of biofilm did not affect the decreasing impedance of dentine with demineralization. The SEM, EDX, XRD and TMR results demonstrated that the surface and bulk dentine gradually became more porous due to the loss of minerals during demineralization, which in turn resulted in the decrease in R_d and R_a values obtained from EIS systems.

Conclusions

This investigation highlighted EIS as a potential technique to monitor biofilm formation and bacterial-induced demineralization in situ.     

Adsorption of human serum albumin on electrochemical titanium dioxide electrodes: Protein–oxide surface interaction effects studied by electrochemical techniques

The adsorption of Human Serum Albumin (HSA) on a semiconductor TiO₂ electrochemical oxide was investigated using Cyclic Voltammetry (CV), Capacity–Potential curves (C–E) and time-resolved techniques as a function of electrode potential and protein concentration. The presence of HSA adsorbed on the electrode surface modifies the voltamperometric behavior of the hydrogen evolution reaction (her) and also produces a major modification in the diffusional layer thickness of the H⁺ ions. The adsorbed amounts of HSA were analyzed throughout different adsorption isotherms. The experimental data were modeled with a modified Langmuir type isotherm, considering a weak chemisorption on a surface with heterogeneity in site-energy distribution with some degree of attractive lateral interactions between the adsorbed protein molecules. The effect of the adsorption potential (E_ads) was investigated polarizing the electrode at −0.70, −0.50 and −0.08 V vs. SCE. The capacity response obtained from the different impedance experiments was determined by the space charge region in the semiconductor. It was possible to correlate processes produced by the protein adsorption on the surface (occurring in the electrolyte side of the interface) with changes in the semiconductor properties of the TiO₂ (in the oxide side of the interface). The adsorption of HSA produces an increase in donor concentration (N_D) of the semiconductor and a shift of the E_fb to more negative values. These effects are more pronounced with an increase in the protein concentration. The relative change in N_D is lower and the change in E_fb is higher when the adsorption occurs at less negative applied potentials. Adsorption kinetics and thermodynamic parameters were calculated for a wide protein concentration range.     

Electrochemical characterization of polyethylene glycols as solid polymer electrolytes

Polymer electrolytes prepared from polyethylene glycol (PEG)-LiClO₄ complexes have been characterized at a stainless steel electrode using cyclic voltammetry, chronoamperometry, and ac impedance techniques. The charge transfer process appeared to be affected by the oxide film on the stainless steel electrode surface in the early stages of redox processes. With the electrode surface cleaned off by reduction of oxide films with electrogenerated lithium, very well defined, chemically reversible voltammograms were obtained for Li⁺ reduction. Different diffusion properties were observed in three different time zones in potentiostatic experiments. An exchange current density has been determined from the Tafel plot for the Li⁺ reduction using impedance data. The conductivity of the PEG-10000 electrolyte has been determined to be 4.7×10^?5 S cm^?1. This value is higher than that of the corresponding polyethylene oxide electrolyte by about two orders of magnitude, which is attributed to the anionic end groups increasing the polarity of the matrix.     

A strategy for immobilisation of carbon nanotubes homogenised in room temperature ionic liquids on carbon electrodes

Functionalised multi-walled carbon nanotubes (MWCNT) were homogenised in the room temperature ionic liquids (RTILs) 1-butyl-3-methylimidazolium bis(trifluoromethane)sulfonimide (BmimNTF₂), 1-butyl-1-methylpyrrolidinium bis(trifluoromethane)sulfonimide (BmpyNTF₂) and 1-butyl-3-methylimidazolium nitrate, and the composite obtained was applied on the top of glassy carbon or carbon film electrode substrates. The modified electrodes were characterised electrochemically in aqueous electrolytes and ionic liquids using cyclic voltammetry and electrochemical impedance spectroscopy and the model redox couples potassium hexacyanoferrate and ferrocene were used for electrode characterisation in aqueous and RTIL media, respectively. It was found that the combination of MWCNT with BmimNTF₂ gave the best composite which augurs well for application in sensors and biosensors.     

The influence of the PANI structure on the conductive mechanism and on the electrical equivalent circuit analysis

The influence of the PANI structural changes during the oxidation of PANI on conductive mechanism and on the electrochemical impedance analysis was investigated. The results obtained by the electron spin resonance (ESR), Raman spectroscopy, ac electrogravimetry and probe beam deflection were used to explain the influence of the PANI structural changes on conductive mechanism. It was explained that the ingress of water molecules that occurs in the potential region of the current peak A, has an important influence on the PANI structure i.e., the PANI chain stretching process. The water molecules increase the distance between the positive charge (polaron) and the negative charge (counter-ion), which facilitates the potentially dependent PANI chain stretching process. The increase in opposite charge distance was explained to be crucial for the process of PANI chain stretching i.e., the chain is transformed from coil type to expanded coil type chain. The influence of the stretching process to the ESR signal properties was commented. It was explained that a decrease in the interaction between polarons, which was promoted by the stretching process, leads to the narrow ESR signal. The influence of structure and/or PANI chain ends on polaron to bipolaron transition was clarified. It was found that the polaron to bipolaron transition is hindered in the condition when the thicker PANI layer is investigated, which leads to greater structural disorder and/or the higher population of the chain ends. The calculated experimental impedance response of the different thickness PANI layers was successfully used in the EIS analysis. The correlation between the PANI structure and physical meaning of EEC parameters was effectively applied to approve or disprove the application of the proposed electrical equivalent circuit (EEC). The structural PANI changes and the EEC analysis of impedance data were used to test the proposed method that examines the interaction of the χ²-value and obtained EEC parameters, which are related to the PANI structure properties. The proposed method discarded EEC, which resulted in the lowest χ²-value, from the further analysis due to different trends of experimental and simulated data presented in the low frequency region of Nyquist spectra. In addition, the proposed method rejected EEC from further analysis if its parameters have not shown the correlation with the changes in the PANI structure. The presentations of data in Bode and admittance spectra were found to be especially useful when investigating systems that have pronounced capacitive behavior in the low frequency region.     

The electrogenerated chemiluminescence kinetics at a microdisc electrode

In this paper we demonstrate the ADI method in combination with Amatore transformation of R and Z co-ordinates and an exponential expanding grid in the time direction that gives a fast and efficient solution of the two-dimensional (2D) non-steady state diffusion electrogenerated chemiluminescence (ECL) problem at a microdisc electrode.     

Impedance voltammetry of electro-dimerization mechanisms: Application to the reduction of the methyl viologen di-cation at mercury electrodes and aqueous solutions

The faradaic impedance for an electrode mechanism with a reversible homogeneous dimerization reaction following the electron transfer step is derived. The chemical reaction shows up in the frequency dependence of the faradaic impedance and admittance in a similar way as deduced by Sluyters-Rehbach and Sluyters (J. Electroanal. Chem. 23 (1989) 457; J. Electroanal. Chem. 26 (1990) 237) for a homogeneous first-order chemical reaction. Two limiting cases can be distinguished in which the general expression reduces to the simpler Randles or pseudo-Randles expression. Under those conditions, the presence of the dimerization reaction can be inferred from the potential dependence of the impedance parameters. The theory is applied to the reduction of the methyl viologen di-cation at mercury electrodes in aqueous solution. The rate and the equilibrium constants for the dimerization reaction and the standard potential for the electron transfer step are obtained from the Warburg coefficient, while the potential dependence of the irreversibility coefficient allows the calculation of the standard rate constant and the transfer coefficient for the electron transfer step.