Phase-sensitive Ac Voltammetry Research Articles

Electrochemistry of Tetrathiafulvalene Ligands Assembled on the Surface of Gold Nanoparticles

The synthesis of a tetrathiafulvalene (TTF) derivative, S-[4-({4-[(2,2'-bi-1,3-dithiol-4-ylmethoxy)methyl] phenyl}ethynyl)phenyl] ethanethioate, suitable for the modification of gold nanoparticles (AuNPs), is described in this article. The TTF ligand was self-assembled on the AuNP surface through ligand exchange, starting from dodecanethiol-stabilized AuNPs. The resulting modified AuNPs were characterized by TEM, UV-Vis spectroscopy, and electrochemistry. The most suitable electrochemical method was the phase-sensitive AC voltammetry at very low frequencies of the sine-wave perturbation. The results indicate a diminishing electronic communication between the two equivalent redox centers of TTF and also intermolecular donor-acceptor interactions manifested by an additional oxidation wave upon attachment of the ligand to AuNPs.

Formation and reduction of anodic film on polycrystalline Bi electrode in pure methanol solutions

The processes of film formation and reduction of bismuth in pure methanol are phenomenologically studied by means of cyclic, ac voltammetric and electrochemical impedance spectroscopic methods. Film formation takes place under low electrode potentials within the potential range from -0.1 to about 0.2 V vs. Ag|AgCl resulting in the development of Bi(CH3O)ads layer. The scan rate effect on the anodic current profile is interpreted in terms of a gradual variation of uncompensated resistance, accompanying the processes of film formation and reduction. Phase sensitive ac voltammetry measurements suggest leaky insulating character of a thin anodic film in agreement with the results of electrochemical impedance experiments.

Mercury-Supported Biomimetic Membranes for the Investigation of Antimicrobial Peptides

Tethered bilayer lipid membranes (tBLMs) consist of a lipid bilayer interposed between an aqueous solution and a hydrophilic “spacer” anchored to a gold or mercury electrode. There is great potential for application of these biomimetic membranes for the elucidation of structure-function relationships of membrane peptides and proteins. A drawback in the use of mercury-supported tBLMs with respect to gold-supported ones is represented by the difficulty in applying surface sensitive, spectroscopic and scanning probe microscopic techniques to gather information on the architecture of these biomimetic membranes. Nonetheless, mercury-supported tBLMs are definitely superior to gold-supported biomimetic membranes for the investigation of the function of membrane peptides and proteins, thanks to a fluidity and lipid lateral mobility comparable with those of bilayer lipid membranes interposed between two aqueous phases (BLMs), but with a much higher robustness and resistance to electric fields. The different features of mercury-supported tBLMs reconstituted with functionally active membrane proteins and peptides of bacteriological or pharmacological interest may be disclosed by a judicious choice of the most appropriate electrochemical techniques. We will describe the way in which electrochemical impedance spectroscopy, potential-step chronocoulometry, cyclic voltammetry and phase-sensitive AC voltammetry are conveniently employed to investigate the structure of mercury-supported tBLMs and the mode of interaction of antimicrobial peptides reconstituted into them.

Electrochemical Study of the EuIII/EuII Redox Properties of Complexes with Potential MRI Ligands

AbstractProperties of the EuIII/EuII redox couple, in the presence of one of a group of different ligands of ligands, were investigated using several electrochemical methods. Ligands used in this study included cyclodextrins, polyazamacrocycles and poly(aminocarboxylates). The poly(aminocarboxylate) containing ligands currently used as MRI contrast agents (DTPA, DOTA) as well as newly developed ligands [1,4‐DOTA(GAC12)2, 1,7‐DOTA(GAC12)2] with potential use as MRI contrast agents were studied. Cyclic voltammetry, phase sensitive AC voltammetry, DC polarography and highly effective electrochemical impedance spectroscopy were utilised to elucidate the mechanism of the reduction/oxidation of the Eu ion in the presence of these compounds. Stability constants of the EuIII and/or EuII complexes and heterogeneous rate constants of the charge transfer were determined. Theoretical calculations of molecular volumes and diffusion coefficients were performed as well.

Characterization of atmospheric surfactants in the bulk precipitation by electrochemical tools

Characterization of atmospheric surfactants in bulk precipitations was carried out by adsorption study at the mercury electrode/solution interface using phase sensitive AC voltammetry. The structure and permeability of the adsorbed organic films at the mercury electrode was tested using the redox processes of lead and cadmium ions as electrochemical tools. The effects of organic matter from the bulk precipitations (rainwater and melted snow) were compared to those obtained for aqueous solutions of model substances: aromatic hydrocarbon (naphtalene), anionic surfactant (sodium dodecylbenzene sulfonate, NDBS), protein (albumine) and selected model substances suggested to be representative of water soluble organic compounds (WSOC) in atmospheric water (monocarboxylic, dicarboxylic, and polyacidic compounds and levoglucosan). It was found that substances like humic acid and 3-hydroxybutanoic acid may play an important role in atmospheric heterogeneous chemistry because of their surface active potential.

Speciation of trace metals in natural waters: The influence of an adsorbed layer of natural organic matter (NOM) on voltammetric behaviour of copper

The influence of an adsorbed layer of the natural organic matter (NOM) on voltammetric behaviour of copper on a mercury drop electrode in natural water samples was studied. The adsorption of NOM strongly affects the differential pulse anodic stripping voltammogram (DPASV) of copper, leading to its distortion. Phase sensitive ac voltammetry confirmed that desorption of adsorbed NOM occurs in general at accumulation potentials more negative than −1.4 V. Accordingly, an application of negative potential (−1.6 V) for a very short time at the end of the accumulation time (1% of total accumulation time) to remove the adsorbed NOM was introduced in the measuring procedure. Using this protocol, a well-resolved peak without interferences was obtained. It was shown that stripping chronopotentiogram of copper (SCP) in the depletive mode is influenced by the adsorbed layer in the same manner as DPASV. The influence of the adsorbed NOM on pseudopolarographic measurements of copper and on determination of copper complexing capacity (CuCC) was demonstrated. A shift of the peak potential and the change of the half-peak width on the accumulation potential (for pseudopolarography) and on copper concentration in solution (for CuCC) were observed. By applying a desorption step these effects vanished, yielding different final results.

Electrochemical characterization of a mixed self-assembled monolayer of 6-thioguanine and guanine on a mercury electrode

A monolayer constituted by chemisorbed 6-thioguanine and physisorbed guanine molecules was formed on a hanging mercury drop electrode from deoxygenated solutions of 6-thioguanine (6TG) and guanine (G). The mixed monolayer was characterized by chronoamperometry, cyclic voltammetry, and phase-sensitive ac voltammetry under in situ conditions, and the experimental data revealed a compact packing of molecules in the film. Ex situ measurements in the presence of dissolved oxygen proved that the mixed monolayer is stable in solutions lacking 6TG and G.

Electrochemical characterization of a mixed self-assembled monolayer of 6-thioguanine and guanine on a mercury electrode

A monolayer constituted by chemisorbed 6-thioguanine and physisorbed guanine molecules was formed on a hanging mercury drop electrode from deoxygenated solutions of 6-thioguanine (6TG) and guanine (G). The mixed monolayer was characterized by chronoamperometry, cyclic voltammetry, and phase-sensitive ac voltammetry under in situ conditions, and the experimental data revealed a compact packing of molecules in the film. Ex situ measurements in the presence of dissolved oxygen proved that the mixed monolayer is stable in solutions lacking 6TG and G.

O1-[6-(Methylselanyl)hexanoyl]glycerol as an Anchor for Self-Assembly of Biological Compounds at the Gold Surface

Adsorption ofO1-[6-(methylselanyl)hexanoyl]glycerol (SeOG) on the gold surface was investigated by cyclic voltammetry, phase-sensitive AC voltammetry, electrochemical impedance spectroscopy and piezoelectric microgravimetry. SeOG adsorption results in a stable and compact surface layer with the coverage degree close to unity for an adsorption time of 30 to 80 min and 4.6 mM SeOG acetonitrile solution. Such a layer displays minute defects (pinholes) with the radius of ca. 1-3 μm, separated by 6-50 μm intervals (depending on the adsorption time). The adsorbed compound undergoes anodic desorption in the gold oxide region and also undergoes a cathodic process leading to the removal of the surface layer. Both these processes are similar to those demonstrated by short-chain alkanethiols and have been interpreted as a indication for the conversion of the selena to selenol function as a result of a dissociative adsorption process. Apparently, the main component of the surface layer isO1-(6-selanylhexanoyl)glycerol that results by the cleavage of the C6-Se bond in SeOG. The two free hydroxy groups in SeOG allow to use it as a bridge for binding other compounds to the gold surface. This possibility was illustrated by building up surface layers of a carotenoid derivative (O1-(8'-apo-β-apo-caroten-8'-oyl)-O2-[6-(methylselanyl)hexanoyl]glycerol,II) or carotenoid- and phosphocholine-derivatized SeOG (O1-(8'-apo-β-caroten-8'-oyl)-O2-[6-(methylselanyl)hexanoyl]-O3-{[2-(trimethylammonio)ethoxy]phosphoryl}glycerol,III). The compoundIIIgenerates a less densely packed layer due to the constraints induced by the phosphocholine substituent. Each of these compounds undergoes anodic reactions that are typical of carotenoids in the adsorbed state. However, the polar and hydrophilic phosphocholine residue inIIIshifts the anodic peak to a less pozitive potential. SeOG allows therefore to tune the molecular environment of a surface attached compound by means of a suitable co-substituent.

Fractal and Voltammetric Study of Linoleic Acid Adsorption at the Mercury/Electrolyte Solution Interface

A new method, size scaling of the hanging mercury drop electrode, is introduced and used to determine the fractal properties of the adsorbed layer and its dependence on fractional electrode coverage. Simultaneously, the adsorption process of linoleic acid at a mercury electrode surface has been studied by means of phase-sensitive ac voltammetry in 0.5 M NaCl electrolyte solution, pH 8.3, simulating seawater conditions. Three concentration domains that exhibit different fractal behavior characterize the adsorption process. In the first domain, corresponding to the fractional electrode coverage theta < 0.5, the fractal dimension of the adsorbed layer D 2. Further increase in surface concentration (electrode coverage) is followed by a swift change in fractal dimension, reaching the peak value D approximate to 2.4. This indicates that, at fractional electrode coverage of 0.7-0.8, depending on adsorption conditions, a second-order phase change in the adsorbed layer of linoleic acid takes place. In the third domain, the fractal dimension rapidly decreases to D approximate to 2.0 and remains stable regardless of bulk concentration increase, reflecting formation of a rather uniform and homogeneous layer of perpendicularly oriented molecules of linoleic acid. The observed changes of fractal dimension are a result of restructuring of the adsorbed layer and a change in the orientation of adsorbed molecules due to molecular interactions. The influence of electrode polarity on the adsorption process of linoleic acid and fractal properties of the adsorbed layer is discussed. Comparison with the apparent isotherm, obtained in a classical voltammetric way, shows that the analysis of fractal properties of the adsorbed layer provides a useful tool for studies of the adsorption process and a sensitive method for detection of phase changes.

Interaction of Polysaccharides with Lipid Monolayers

The interactions of model neutral lipids such as lecithin and dipalmitoyl phosphatidylcholine (DPPC), and of the positively charged lipid dioctadecyl dimethyl ammonium bromide (DOMA) with neutral (dextrans) and acidic polysaccharides (carrageenans and xanthan), representing the hydrophilic solutes occurring in natural aquatic systems, have been monitored electrochemically and by monolayer techniques. Surface pressure-area (π-A) and surface potential-area (ΔV-A) isotherms have been measured. The values for compressibility of mixed films and the dipol moments (effective dipole moment μT and polar head group dipole moment μα) have been calculated from π-A and Δ V-A isotherms, respectively. In electrochemical experiments lipid-coated mercury electrodes have been used. The influence of polysaccharides on the monolayers structure has been studied by capacitance measurements and by using an electrochemical probe (redox processes of cadmium) with phase-sensitive ac voltammetry. The results presented here are part of more general studies of the interaction of organic solutes naturally occurring in the aquatic systems with hydrophobic lipids at different phase boundaries. These investigations are of great importance for the interfacial properties of mixed monolayers and for understanding the influence of hydrophilic and hydrophobic substances on the physicochemical properties of natural surface films.

Study of Oleate Adsorption at the Mercury/Electrolyte Solution Interface as a Function of Electrode Potential and Time

The adsorption of sodium oleate from electrolyte solutions on a polarized mercury electrode can be studied by monitoring the differential capacitance of the electrode double layer. Its variation with potential and time can provide qualitative information on the state of the oleate adsorbate. A new experimental methodology, three-dimensional phase-sensitive ac voltammetry, is used to provide safer information on the equilibrium transition characteristics and also to follow the time evolution of the phenomenon. The steady-state differential capacitance vs electrode potential curve is interpreted by means of recently developed thermodynamic criteria for phase transitions of surfactants at charged interfaces. At concentrations below the bulk critical micelle concentration monolayer adsorption takes place at moderate and negative polarizations, whereas there is some evidence of multilayer adsorption at more positive potentials. At higher concentrations, multilayer adsorption occurs throughout the potential range studied. In both cases, the formation of a close-packed structure is likely to be restricted to the first layer and the phase transitions of the latter to be realized via aggregates formed by its interaction with the outer layers. In addition, a close-packed polylayer is also observed at extreme positive potentials for high bulk oleate concentrations. Analysis of the capacitance transients based on simple diffusion, adsorption, or Avrami formalisms cannot describe successfully the time dependence of oleate adsorption on mercury, indicating that the latter is under mixed control or/and a complex process.

Surface activity and orientation of inosine at a mercury/solution interface

A systematic investigation on the adsorption and association of inosine at a hanging mercury drop electrode by phase-sensitive ac voltammetry has been performed. The adsorption equilibrium bas been determined as a function of various parameters such aspH, adsorption potential, adsorption time, and bulk concentration of inosine. It was found that the association of the adsorbed species depends predominantly on the stacking interactions of neutral inosine molecules. Over a wide potential range, inosine exhibits a dilute adsorption characteristic where it is adsorbed with the base flat on the electrode surface. Above a certain threshold value, inosine appears to undergo a surface reorientation and adopts a perpendicular position. The nucleation and growth mechanism is analyzed applying theAvrami equation. Characteristic properties and adsorption parameters of dilute and compact layers of inosine were evaluated from the two-stepFrumkin isotherm and from the potential dependence of adsorption.

The effect of the removal of trace impurities on the adsorption behavior of octanoic acid at mercury–solution and at air–solution interfaces

AbstractIn this article we explore the kinetics of adsorption of octanoic acid at the mercury‐water interface using phase sensitive AC voltammetry. These measurements were performed before and after stepwise purification of the octanoic acid solution, in order to distinguish between the adsorption behavior of the octanoic acid and its impurities. The adsorption of both octanoic acid and the surface active impurities decrease the capacitance with time. Removing the latter during the purification procedure results in a faster achievement of the equilibrium state and in higher capacitance values. The adsorption was also studied at the air‐water interface using dynamic surface tension measurements. Comparable results are obtained for both interfaces.

The Interfacial Behaviour of Mono-, Di-, and Triphosphate of Inosine at the Charged Interface

The interfacial behaviour and adsorption equilibria of mono-, di-, and triphosphate of inosine (IMP, IDP, and ITP) were carried out in different buffer solutions by phase-sensitive ac voltammetry at HMDE. The characteristic properties and adsorption parameters of dilute and compact layers were evaluated from the obtained Frumkin isotherm at different pH values. The effect of some divalent metal ions on the adsorption stage and association of the investigated compounds has been studied.

Structural and Thermodynamic Aspects of Phase Transitions in Uracil Adlayers. A Chronocoulometric Study

The phase formation of six alkyl derivatives of 2,4-dihydroxypyrimidine on a Au(111) single crystal electrode from aqueous 0.05 M KClO4 has been investigated using chronocoulometry, cyclic voltammetry and phase-sensitive ac voltammetry. As a function of the applied electrode potential and/or the adsorbate concentration uracil (U), 5-methyluracil (5-MU) and 5,6-dimethyluracil (5,6-DMU) assume four different adsorption regions. These are assigned to a “dilute” phase (I), a two-dimensional condensed physisorbed film (II), a partial charge transfer/deprotonation region (III), and a chemisorbed film (IV). Substitution of the N(1)- and/or the N(3)-ring nitrogen with methyl groups prevents the formation of the ordered physisorbed film II. In the case of 1-methyluracil (1-MU) and 3-methyluracil (3-MU) region I is followed immediately by regions III and IV. No indications for the latter two regions were found with 1,3-dimethyluracil, where both ring nitrogen atoms are blocked simultaneously with methyl groups. Quantitative thermodynamic adsorption parameters such as film pressure, Gibbs surface excess, Gibbs energy of adsorption, and electrosorption valency were determined on the basis of chronocoulometric measurements for the “dilute” and the two-dimensional condensed physisorbed film. All molecules studied are planar oriented to the electrode surface in both ordered regions. The corresponding values of the Gibbs energies of adsorption ΔGI° and ΔGII° of the six uracil derivatives on Au(111) indicate weak chemisorption. On the basis of structural and energetic data the formation of the 2D-condensed physisorbed film of U, 5-MU, and 5,6-DMU is interpreted as a replacement of residual interfacial water molecules accompanied by the creation of a 2D network of planar oriented and, via hydrogen bonds, interconnected molecules. The adsorption and phase formation parameters obtained for the six uracil derivatives on Au(111) in regions I and II will be complementary analyzed with selected results on Ag(111) and mercury electrodes. No quantitative thermodynamic data were extracted in regions III and IV.

Adsorption behavior of bis (2-ethylhexyl) sodium sulfosuccinate (AOT) at the mercury-electrolyte solution interface as a function of electrode potential and time

The dependence of the differential capacitance (C) of the electrode double layer of a hanging mercury drop electrode in bis (2-ethylhexyl) sodium sulfosuccinate (AOT) solutions on electrode potential (E) and time is measured using three-dimensional phase sensitive ac voltammetry. This methodology, possessing a very wide time window that permits a detailed study of the adsorption phenomena, is based on the reconstruction of C vs E curves, sampled after many phase-sensitive ac chronoamperometric experiments. The shape of these curves allows an estimation of the structure of the layer of AOT molecules absorbed at the electrode surface. AOT molecules form micelles in bulk solutions and they also associate in the charged interface under the strong influence of the electric field into surface aggregates which depend on their concentration and applied potential. The presence of AOT micelles in the bulk solution can be linked with the appearance of a surface film at potentials more negative than those corresponding to a condensed film linked with a capacitance value slightly higher than that normally attributed to a compact layer. The whole phenomenon is proved to be very dependant upon time.

Voltammetric studies of the adsorption and conformation of polyribocytidylic acid at the mercury electrode surface

The adsorption and interfacial behaviour of polyribocytidylic acid (poly-C) was studied by phase sensitive ac voltammetry and single-sweep voltammetry. The two helical conformations of poly-C were investigated as a function of various parameters such as pH, adsorption potential, temperature and the nature of the anions of supporting electrolyte. The adsorbed acid form of poly-C undergoes an opening of the double helix because the electric field at the charged interface renders some of cytosine moieties accessible for reduction. The limiting value of the surface concentration of poly-C indicates that the adsorption occurs via the base residues attached to the nucleotide chain. The results are compared with the similar adsorption behaviour of polyriboadenylic acid (poly-A) and DNA.

Three-dimensional phase sensitive ac voltammetric study of the film development of phosphonium cations at a stationary mercury electrode

The interfacial behaviour of two phosphonium cations, tetraphenylphosphonium chloride (TPPCl) and cetyltributylphosphonium bromide (CTBPBr), is studied in the polarized mercury electrode/aqueous electrolyte solution interface by means of differential capacitance measurements. Both phosphonium cations are extremely surface active compounds and in a concentration as low as 5 × 10 −6 mol/l their adsorption is controlled only by diffusion. CTBPBr molecules, which are known to form micelles in bulk, are also associated into surface aggregates at the Hg/electrolyte solution charged interface. This is extracted from the shape of the differential capacitance ( C) vs. applied potential ( E) curves. The application of a new methodology, known as three-dimensional phase sensitive ac voltammetry, based on the reconstruction of C vs. E curves from isochronous C data, sampled after many chronocapacitometric experiments, gives the opportunity to produce a picture of the transition curves that cannot be obtained by the classic ac voltammetry. This methodology provides the equilibrium C vs. E curves for long time periods and also gives information about the time evolution of adsorption phenomena.

Adsorption of pyridine at the Au(210)—solution interface

Adsorption of pyridine at an Au(210) single-crystal electrode has been investigated using chronocoulometry, cyclic voltammetry and phase-sensitive ac voltammetry. The adsorption parameters, such as the relative Gibbs surface excesses, the Gibbs energies of adsorption, the electrosorption valencies and the potential drop across the inner layer, have been calculated. These results suggest that pyridine molecules assume a vertical, N-bonded, orientation at the Au(210) surface. The full coverage Gibbs energy of adsorption at the potential of maximum adsorption is approximately equal to −63 kJ mol −1, which is a value typical of weak chemisorption. The adsorption of pyridine at the (210) surface was compared with adsorption at the (311), (110), (100) and (111) planes of gold.