Electrochemical impedance study of self-assembled layer-by-layer iron–silicotungstate/poly(ethylenimine) modified electrodes

Electrochemical impedance spectroscopy (EIS) has been used to study multilayer films containing anionic iron-substituted silicotungstate [SiW₁₁Fe^III(H₂O)O₃₉]⁵⁻ (SiW₁₁Fe) and positively charged poly(ethylenimine) self-assembled by the layer-by-layer method on glassy carbon and indium tin oxide electrodes. The effect of the charge of the outermost layer of the multilayer assembly on the electron transfer of soluble species was studied using the redox probes [Fe(CN)₆]³⁻ and [Ru(NH₃)₆]³⁺; cyclic voltammetry indicating that the surface charge has a significant effect on the process. EIS demonstrated that the electrostatic attraction or repulsion between the surface and the redox probes plays a significant role. Analysis of the impedance spectra showed that the charge transfer resistance increases with an increasing number of bilayers for both redox probes and that the porosity of the multilayer film, which varies with the electrode substrate, also has a significant effect on the electrochemical response.     

Comparative study of layer-by-layer deposition techniques for poly(sodium phosphate) and poly(allylamine hydrochloride)

An inorganic short chain polymer, poly(sodium phosphate), PSP, together with poly(allylamine hydrochloride), PAH, is used to fabricate layer-by-layer (LbL) films. The thickness, roughness, contact angle, and optical transmittance of these films are studied depending on three parameters: the precursor solution concentrations (10^-3 and 10^-4 M), the number of bilayers deposited (20, 40, 60, 80, and 100 bilayers), and the specific technique used for the LbL fabrication (dipping or spraying). In most cases of this experimental study, the roughness of the nanofilms increases with the number of bilayers. This contradicts the basic observations made in standard LbL assemblies where the roughness decreases for thicker coatings. In fact, a wide range of thickness and roughness was achieved by means of adjusting the three parameters mentioned above. For instance, a roughness of 1.23 or 205 nm root mean square was measured for 100 bilayer coatings. Contact angles close to 0 were observed. Moreover, high optical transmittance is also reported, above 90%, for 80 bilayer films fabricated with the 10^-4 M solutions. Therefore, these multilayer structures can be used to obtain transparent superhydrophilic surfaces.     

Fabrication and stability of porous poly(allylamine) hydrochloride (PAH)/poly(acrylic acid) (PAA) multilayered films via a cleavable-polycation template

Porous films were fabricated from nonporous layer-by-layer multilayers composed of a blend of positively charged disulfide-containing polyamidoamine and poly(allylamine hydrochoride), and negatively charged poly(acrylic acid), followed by removal of cleavable disulfide-containing polycation after incubation in 1 mM DTT solution. The thickness of original multilayered films decreased with the increase of incubation time in DTT solution. Atomic force microscopy (AFM) measurements and electrochemical analysis demonstrated the formation of nanopores with sizes ranging from 50 to 120 nm. The formed porous films were stable in buffer solution at pHs ranging from 7.4 to 1.6, whereas they showed slight changes in pore number and pore size when incubated in PBS buffer at a pH of 10.0. This research might provide a universal method for the fabrication of noncrosslinked porous multilayered films.     

High conducting multilayer films from poly(sodium styrenesulfonate) and graphite nanoplatelets by layer-by-layer self-assembly

Exfoliated graphite (G) nanoplatelet was modified with hexadecyltrimethylammonium bromide (C₁₆TAB) and was constructed as multilayer films by electrostatic self-assembly. A [poly(sodium styrenesulfonate)/graphite]_n (PSS/G)_n multilayer film was self-assembled by alternate adsorption of polyanionic PSS and cationic graphite nanoplatelets G. An uniform deposition process was detected by UV-vis absorption spectra. The (PSS/G)_n multilayer film exhibits an excellent electrical conductivity in the range of 50-200 S cm⁻¹, when bilayer number (n) exceeds a threshold value four, the conductivity of the multilayer film increases dramatically. Cyclic voltammogram measurement reveals that the (PSS/G)_n film with more bilayer has small charge-transfer resistance and high electrocatalytic activity.     

The electrochemical synthesis of poly(aniline-co-o-anisidine) on copper and their corrosion performances

The copolymer films PANI-co-POA and poly(aniline-co-o-anisidine) were carried out on copper (Cu) electrode, by applying two different scan rates (20 and 50 mV s⁻¹) and using two different thicknesses at high scan rate. Synthesizes were achieved under cyclic voltammetric conditions from 0.075 M aniline and 0.075 M o-anisidine containing sodium oxalate solutions. The synthesized copolymer films were strongly adherent and homogeneous in both cases. AC impedance spectroscopy (EIS), anodic polarization plots and open circuit potential–time curves were used to evaluate the corrosion performance of copolymer coated and uncoated electrodes in 3.5% NaCl. It was shown that the copolymer film coated at low scan rate exhibited a better property initially when compared with the copolymer film produced at high scan rate. However, it could not resist the attack of corrosion products, in longer time and meanwhile its barrier property significantly diminished. It was found that the thin copolymer film produced at high scan rate by its catalysing effect led to the formation of highly protective copper oxides on the surface whereby providing a better protection for long exposure times. It also emerged that the corrosion resistances of thin copolymer film produced at high scan rate and copolymer film synthesized at low scan rate were almost same and relatively higher for much longer periods when compared with the one observed for bare copper electrode.     

Template polymerization of sodium methacrylate onto poly(allylamine) hydrochloride

The radical polymerization in aqueous solution of sodium methacrylate in the presence of poly(allylamine) hydrochloride as a template was studied using dilatometry. The properties of the polyelectrolyte complexes resulting from these template polymerizations were investigated and compared with those of the poly(sodium methacrylate)/poly(allylamine) hydrochloride complexes obtained by simple mixing of the preformed polymers. The kinetic study provides evidence for the presence of a strong template effect and indicates that the polymerization occurs by a zip mechanism. The results of the different characterization analyses have shown that the complexes obtained by template polymerization have a more ordered structure than the complexes prepared by mixing the two polymers.     

Influence of surface inhomogeneities of boron doped CVD-diamond electrodes on reversible charge transfer reactions

The electrochemical behaviour of reversible charge transfer reactions on boron doped diamond (BDD) was studied by cyclic voltammetry and electrochemical impedance spectroscopy using rotating disc electrodes under defined convection. Diamond films of 5 m thickness with doping levels of 200, 3000 and 6000 ppm were prepared by hot filament chemical vapour deposition on niobium substrate. The electrochemical measurements were carried out on BDD electrodes in deaerated 0.5 M Na₂SO₄ + 5 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] solution at rotation frequencies 0 < f _rot < 4000 rpm. Comparative measurements were carried out on an active Pt electrode. The BDD electrodes exhibit distinct irreversibilities indicated by a larger peak potential difference in the cyclic voltammograms, lower diffusion limiting current densities and an additional impedance element at high frequencies. Mechanical polishing with carbon fleece and SiC paper strongly affects the irreversible behaviour of the BDD electrodes. The experimental results are explained by a partial blocking of the diamond surface with reversible charge transfer at active sites. The impedance spectra are analysed quantitatively using a transport impedance model for reversible reactions on partially blocked electrode surfaces.     

A comparison of charge transport behavior in functionalized and non-functionalized poly 3,4-(ethylenedioxythiophene) films

Poly 3,4-(ethylenedioxythiophene) (PEDOT) films electropolymerized from an aqueous micellar solution containing sodium dioctyl sulfosuccinate and the monomer were functionalized with 1-fluoro-2-nitro-4-azidobenzene (FNAB) molecules by a photochemical nitrene insertion reaction. The variation in redox activity and the changes in the charge transfer and diffusion (through bulk) behavior of the functionalized and the non-functionalized PEDOT films have been followed by electrochemical impedance spectroscopy and cyclic voltammetry. While the functionalized film allows a reversible insertion and extraction of guest cations and anions, the non-functionalized film is capable of exchanging only anions. The higher level of oxidation attained in the functionalized film is also reflected in the longer diffusion length (l_D) observed for the ions in this film. In both films the barrier to charge transfer is resistive rather than capacitive. Both charge transfer and diffusion resistance (R_CT and R_D) are lower for the functionalized film, a consequence of a higher surface roughness and a more nodular morphology and therefore higher optical contrast and faster color-bleach kinetics are achieved in this film. For the functionalized and the non-functionalized films, both R_CT and R_D are greatly enhanced during reduction than for oxidation. In particular, in the low frequency regime, the hindered diffusion-controlled extraction of anions from the bulk of the film is also evident from the larger R_D as compared to R_CT and the difference in their magnitudes is more pronounced for the functionalized film thus confirming that functionalization is a useful method for controlling the redox response of conducting polymer films.     

Electrochemical synthesis of poly(5-nitroindole) on 316L-stainless steel in LiClO4-acetonitrile solution and its corrosion performance

The preparation of poly(5-nitroindole) (P5NI) coating was achieved on 316L-stainless steel (SS). Poly(5-nitroindole) was deposited via anodic oxidation of the corresponding monomer in acetonitrile (ACN) solution containing LiClO₄. The influence of P5NI coating against SS corrosion was studied in 3.5% NaCl solution by electrochemical impedance spectroscopy (EIS), anodic polarization curves and the open circuit potential–time (E_ocp–t) diagrams. The results obtained suggest that P5NI coating forms a sacrificial layer but the efficiency against corrosion is limited with increasing time.     

Influence of folate conjugation on the cellular uptake degree of poly(allylamine hydrochloride) microcapsules

The microcapsules in drug delivery systems can prevent degradation of drugs and help to control the release rate. To enhance the targeted delivery effect of the microcapsules to cancer cells, some specific ligands such as folic acid (FA) are necessarily further conjugated. Herein, covalent poly(allylamine hydrochloride) (PAH) multilayers were fabricated on CaCO₃ microparticles under the cross‐linking of glutaraldehyde, which were further immobilized with different amount of FA molecules via the spacer of diamino terminated poly(ethylene glycol) (PEG). As a comparison study, four types of microcapsules, i.e., the PAH capsules, the PAH capsules grafted with PEG, and the PAH capsules conjugated with two different amount of FA via the PEG spacer were prepared. Their chemical and physical structures were confirmed by infrared spectroscopy, UV–vis spectroscopy and scanning electron microscopy. In vitro cell culture found that the cellular uptake of the PAH capsules grafted with PEG was reduced significantly compared with that of the pure PAH capsules. The FA‐modified microcapsules could be selectively delivered into HepG2 tumor cells which overexpress FA receptors but not into the endothelial cells. The number of HepG2 cells which ingested the FA‐conjugated capsules showed a positive correlation with FA amount. The results indicate that these FA conjugated capsules have a high selectivity to be delivered to tumor cells, endowing them with a larger opportunity functioning as targeted delivery vehicle for anticancer drugs. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

An electrochemical DNA biosensor developed on novel multinuclear nickel(II) salicylaldimine metallodendrimer platform

An electrochemical DNA biosensor (EDB) was prepared using an oligonucleotide of 21 bases with sequence NH₂-5′-GAGGAGTTGGGGGAGCACATT-3′ (probe DNA) immobilized on a novel multinuclear nickel(II) salicylaldimine metallodendrimer on glassy carbon electrode (GCE). The metallodendrimer was synthesized from amino functionalized polypropylene imine dendrimer, DAB-(NH₂)₈. The EDB was prepared by depositing probe DNA on a dendrimer-modified GCE surface and left to immobilize for 1 h. Voltammetric and electrochemical impedance spectroscopic (EIS) studies were carried out to characterize the novel metallodendrimer, the EDB and its hybridization response in PBS using [Fe(CN)₆]^3−/4− as a redox probe at pH 7.2. The metallodendrimer was electroactive in PBS with two reversible redox couples at E°′ = +200 mV and E°′ = +434 mV; catalytic by reducing the E_pa of [Fe(CN)₆]^3−/4− by 22 mV; conducting and has diffusion coefficient of 8.597 × 10⁻⁸ cm² s⁻¹. From the EIS circuit fitting results, the EDB responded to 5 nM target DNA by exhibiting a decrease in charge transfer resistance (R_ct) in PBS and increase in R_ct in [Fe(CN)₆]^3−/4− redox probe; while in voltammetry, increase in peak anodic current was observed in PBS after hybridization, thus giving the EDB a dual probe advantage.     

Mediated electron transfer reactions at electrodes coated with poly(viologen)s

Electron and ion transport processes in microdomains of pendant-type poly(viologen)s were analysed by means of rotating disc voltammetry in aqueous media. Stability of poly(viologen)s on the electrode was improved by introducing long alkyl chains. Penetration or the diffusion of ions and the electron-propagation through the polymer layers were greatly affected by the type of supporting electrolyte and chain length of side alkyl groups. Indeed, almost no penetration of large anionic ions was observed in the case of pendant-type poly(viologen) possessing cetyl groups in a perchlorate supporting electrolyte. The electron-propagation through the polymer layers by the self-exchange reactions among redox-active viologen units seemed also to be slow, as the migration of the counter anion of poly(viologen) was limited inside the cationic domain. Similar behaviour was observed in the case of poly(viologen) coatings with ethyl pendant groups, but the electron-propagation was much slower in comparison with the former case. This difference could be attributed to the nature of cationic domains, since the polycations with long alkyl side chains might form a polymer micelle structure with a segregated hydrophobic domain and cationic channel for the electron-propagation.     

Anodic polymerization of 2,5-di-(2-thienyl)-furan in ethanol

Poly(2,5-di-(2-thienyl)-furan) (PSOS) was synthesized via anodic polymerization of 2,5-di-(2-thienyl)-furan (SOS) in ethanol solution containing 0.2 M LiClO₄ as supporting electrolyte. The electrochemical and spectroelectrochemical properties were investigated using electroanalytical and UV–vis spectroscopic techniques, respectively. The band gap of the polymer film was found as 2.22 eV and the film was successfully switched between black oxidized state and orange neutral state. Fluorescence and electrochemical impedance spectroscopy (EIS) studies were also performed.     

Optimization of electrochemical polymerization parameters of polypyrrole on Mg–Al alloy (AZ91D) electrodes and corrosion performance

We report on the optimum electropolymerization conditions of polypyrrole (PPy) coatings on Mg alloy AZ91D electrodes from aqueous electrolytes of sodium salicylate via cyclic voltammetry (CV). Results show that initial and end potential values during the electrochemical coating procedure play an important role on the adhesion and corrosion performance of PPy films. Corrosion tests of AZ91D electrodes coated with PPy under optimized conditions show a good corrosion performance during 10 days in Na₂SO_4, without peeling off of these thin films.     

Preparation of gold nanoparticles on poly(methyl methacrylate) nanospheres with surface-grafted poly(allylamine)

A facile method for in situ anchoring of gold nanoparticles onto the surface of polymer nanospheres was successfully developed in this study. As polymer nanospheres, amphiphilic poly(methyl methacrylate) (PMMA)/poly(allylamine) (PAA) nanospheres were prepared by graft copolymerization of methyl methacrylate from PAA. The gold nanoparticles anchored were spherically symmetric and the average sizes were ∼12 nm for all samples. It was found that surface-grafted PAA effectively anchored and stabilized gold nanoparticles for a long period of time.     

Electrochemical behavior of (Cr,W, Al,Ti, Si)N multilayer coating on nitrided AISI 316L steel in natural seawater

AISI 316L steel is often used in materials applied toward nuclear power but are subjected to pitting corrosion in a marine environment. In this study, (Cr, W, Al, Ti, Si)N multilayer coatings were deposited using multi-arc ion plating on the surface of non-nitrided and nitrided AISI 316L steel. The microstructure and corrosion resistance of four different systems were investigated, namely, (i) untreated AISI 316L steel, (ii) plasma nitrided (PN), (iii) coated on an untreated matrix (coating) only, and (iv) coated on nitrided (hybrid) specimens. The phase structures, morphologies, and compositions of the different specimens were characterized using X-ray diffraction, transmission electron microscope, Atomic Force Microscope, scanning electron microscope, X-ray photoelectron spectroscopy, and energy dispersive x-ray spectroscopy. The results show that a thin CrWAlTiSiN multilayer coating, approximately 2.3 μm in thickness, is deposited on the surface of an ~12 μm nitrided layer. Potentio-dynamic polarization and electrochemical impedance spectroscopy were used to evaluate the assessment of the electrochemical behavior in the natural seawater of China's Yellow Sea. The hybrid specimens exhibited excellent corrosion resistance compared to both the nitrided and coated specimens.     

Chemisorption of oxygen on carbon produced by electrochemical reduction of poly(tetrafluoroethylene)

The electrochemical reduction of poly(tetrafluoroethylene) with lithium amalgam and washing the product with water yields a very reactive and pure carbonaceous substance which is capable of the chemisorption of oxygen from the gas phase or from aqueous solution at room temperature. The product was studied by infrared spectroscopy and electron paramagnetic resonance. Chemisorption of oxygen leads to the formation of IR-active C-O and C = O surface groups. The concentration of paramagnetic sites does not change after the chemisorption of oxygen; this is manifested only by a broadening of the EPR line.     

Electrochemical impedance spectroscopy of fully hydrated Nafion membranes at high and low hydrogen partial pressures

The proton transport mechanism in fully hydrated Nafion 117 membranes was examined via electrochemical impedance spectroscopy (EIS) and steady-state current–potential measurements both in a symmetric H₂, Pt|Nafion|Pt, H₂ cell and in a H₂, Pt|Nafion|Pt, air PEM fuel cell with hydrogen partial pressure values, PH₂, varied between 0.5 kPa and 100 kPa. In agreement with recent studies it is found that for low PH₂ values the steady-state current–potential curves exhibit bistability and regions of positive slope. In these regions the Nyquist plots are found to exhibit negative real part impedance with a large imaginary component, while the Bode plots show a pronounced negative phase shift. These observations are consistent with the mechanism involving two parallel routes of proton conduction in fully hydrated Nafion membranes, one due to proton migration in the aqueous phase, the other due to proton transfer, probably involving tunneling, between adjacent sulfonate groups in narrow pores. The former mechanism dominates at high PH₂ values and the latter dominates in the low PH₂ region where the real part of the impedance is negative.     

Commentary on ‘The interface between Au(1 1 1) and an ionic liquid’

Recently, Gnahm et al. [Electrochim. Acta 55 (2010) 6212] analyzed and discussed capacitive processes at the interface between the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate and a single-crystalline Au(1 1 1) electrode. In order to derive interfacial capacitance spectra, they subtracted the bulk resistance R_S from the experimental impedance spectra and used the following expression . The subtraction procedure was based on the assumption that . In this commentary we argue that this assumption neglects the high-frequency capacitance C_s acting in parallel to R_S. In a three-electrode setup, the high-frequency capacitance is often governed by the input capacitance of the reference electrode. The parallel action of C_S and R_S results in . Consequently, the subtraction of R_S leads, in general, to negative −C^″sub(ν) values at high frequencies which have no physical meaning. Moreover, the subtraction procedure causes an artificial distortion of the double layer semicircle in the complex capacitance plane.     

Electrochemical behavior of organic and inorganic complexes of Zn(II) as corrosion inhibitors for mild steel: Solution phase study

This work intends to study inhibitive performance of organic and inorganic complexes of Zn(II) using electrochemical techniques along with surface analysis. In this regard, inorganic zinc aluminum polyphosphate pigment as modified zinc phosphate and zinc acetylacetonate and benzimidazole mixture representing organic replacement of zinc phosphate were employed. Through taking advantage of electrochemical impedance spectroscopy and DC polarization, two mentioned approaches were indicated to be efficient. Charge transfer resistance and corrosion current density values exhibited superiority of zinc aluminum polyphosphate and mixture of zinc acetylacetonate and benzimidazole compared to zinc phosphate and also zinc acetylacetonate and benzimidazole as individual inhibitors. Corrosion inhibition efficiencies calculated based on charge transfer resistance in consistent with those calculated from corrosion current density showed the following sequence; zinc aluminum polyphosphate > mixture of zinc acetylacetonate and benzimidazole > zinc acetylacetonate > zinc phosphate > benzimidazole. Showing film formation, surface analysis SEM/EDX confirmed the results obtained by electrochemical methods.