Eric Vieil

Electronically conductive polymer grafted with oligonucleotides as electrosensors of DNA: Preliminary study of real time monitoring by in situ techniques

Polypyrrole bearing covalently linked oligonucleotide (ODN) probes has been used for electrode modification through one-step electrosynthesis. The biofilms obtained were employed for the direct and real-time detection of DNA hybridisation by using photocurrent spectroscopy and a quartz crystal microbalance (QCM). The photoelectrochemical properties of polypyrrole appear to be influenced by the presence of ODN allowing the transduction of the hybridisation event into a measurable photocurrent. The operational parameters of the biosensor (film thickness, ODN load, …) were assessed to optimise the response to DNA hybridisation. In addition, QCM detection, which is based on the monitoring of the biofilm mass uptake generated by molecular recognition, has been used as a validation technique. The sensors obtained have been demonstrated to be fully reusable. They have also shown a suitable sensitivity and a good reproducibility in terms of kinetic behaviour and intensity of the measured signal. Finally, this study considers the pertinence of both techniques regarding DNA hybridisation detection.

Ion and solvent transfer discrimination at a nickel hydroxide film exposed to LiOH by combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) techniques

Abstract A combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) instrument was used to monitor the mobile species transfers associated with the redox processes of thin ( Γ ≅100–150 nmol cm −2 ) α- and β-nickel hydroxide films exposed to aqueous LiOH solution. A comparison of the measured PBD signal with the predicted PBD profiles, calculated by temporal convolution analysis of the current and mass responses, enabled the contributions to redox switching of anion (OH − ) and solvent (H 2 O) transfers to be discriminated quantitatively. The responses from the combined instrument are reconciled in terms of H + deintercalation/intercalation within the nickel hydroxide structure as OH − ions enter/exit the film. Hydroxide ion movement is associated with a counterflux of water. Thin nickel hydroxide films show a gradual α→β phase transformation with continuous voltammetric cycling, especially when the films are exposed to high concentrations of electrolyte. α-Films are characterised by OH − transfers that dominate the H + and H 2 O movements; β-films are characterised by an increased participation of water and protons to the exchange dynamics.

Multi-component diffusion approach to transport across electroactive-polymer films with two mobile charge carriers

Abstract This paper aims to apply general relations between the thermodynamical forces (gradients of electrochemical potentials) and resulting fluxes of the species to transport phenomena in a uniform film of the electroactive polymer in contact with some other conducting media, metal(s) and/or solution(s), in the case of a low-amplitude perturbation imposed. Two kinds of mobile charged species are assumed to be present inside the film, the “electronic” and “ionic” ones. The coefficients in the above relations (friction coefficients) are expressed through the experimentally measurable macroscopic transport parameters, the total high-frequency conductivity, migration transference numbers, binary diffusion coefficient and differential redox capacitance of the film. The non-stationary diffusion equation is found to be valid for several local characteristics of the film, in particular for electron or ion charge density, or for the low-frequency current density. This equation has been solved analytically for three usual geometries of the system, metal/film/metal, metal/film/solution and solution/film/solution, upon a sinusoidal variation of the electrode potential. The final expressions for complex impedance contain contributions of the bulk film, interfacial charge-transfer resistances and (in contact with solution(s)) bulk solution. The functional form of their frequency dependence as well as the shape of complex-impedance plots has turned out to be highly simple for all geometries, being in accordance with those derived earlier within the framework of the Nernst-Planck-Einstein equations. However, the parameters of those dependences have a form different with respect to the previous expectations, leading to a modification of the procedure to interpret experimental data.

Influence of ionic size on the mechanism of electrochemical doping of polypyrrole films studied by cyclic voltammetry

Charging/discharging process of a polypyrrole film has been studied in contact with a large-size anion (tetraphenylborate) solution in acetonitrile with a comparison to that in perchlorate solutions. The overall redox activity in the former case is significantly reduced. Nevertheless, we have been able to give a clear cyclic-voltammetry characterization of the film at various bulk-electrolyte concentrations for different sweeping rates. Those data reveal two well separated waves in each potential scan direction located in the 0.1 M NaBPh4 solution at −0.38 V and −0.1 V (vs Ag/AgBPh4 0.1 M in CH3CN) in the anodic branch. The first oxidation wave obeys an ultra-thin layer mass transfer mode (temporal mode), while the second one is diffusional for sufficiently high scan rates but it approaches the same quasi-equilibrium charging regime at lower sweeping rates. Decrease of the bulk electrolyte concentration leads to a shift of both peaks in opposite directions from the merging point, keeping their shapes and intensities unchanged, with the positive or negative 60 mV slope in the semi-logarithmic coordinates. Both peaks have been attributed to a single redox transition with participation of single-charged electronic species but with different mechanisms of the charge compensation by ions. Splitting the curve into two peaks is assumed to be due to the existence of two different forms of ionic species inside the polymer phase, “free” and “bound” ones. Presence of some amount of bound anions in the reduced state is accompanied by the corresponding concentration of cations which are removed from the file during the first step of the charging process. The second redox wave is related to the free-anion insertion into the film. This approach is able to reproduce properly all features of experimental data.

Mass transfer and convolution: Part II. In situ optical beam deflection study of ionic exchanges between polyphenylene films and a 1:1 electrolyte

Abstract The optical beam deflection technique (mirage effect) shows unambiguously that the ionic compensation of positive charges created in polyphenylene films on an electrode is affected only by the anion during the whole oxidation process. Use of a new technique of convolution by a frequency transfer function featuring diffusion in the electrolyte allows quantitative determination of the amount of exchanged species during cyclic voltammetry. The theoretical relationship between the mirage deflection and electrical current is derived for a symmetrical binary electrolyte exchanging only one ion with the interface, and when both diffusion and migration feature mass transfer. Measurement of the diffusion coefficient is possible during any electrochemical experiment by scanning the diffusion layer at various locations. Correction of the propagation delay between the electrode and the laser probe is possible with this convolutive approach. This allows the use of in situ optical beam deflection as a quantitative technique for studying dynamic mass exchange in complicated systems.

Ionic exchange of the polypyrrole film with the PC lithium perchlorate solution during the charging process

Abstract Two theoretical approaches to the interpretation of the EQCM data for the charging-discharging process in electron-conducting polymer film coated electrodes have been proposed, with their application to systems without attached charged groups, or fixed charges. The first model treats cations and anions inside the polymer phase as “free” mobile species which exchange with the solution may be retarded due to low values of the interfacial exchange constants. Depending on its value for the anion transfer, three limiting regimes of the charging process have been found, 1. (1) complete ionic equilibrium with the solution, 2. (2) “irreversible anion transfer”, 3. (3) “cation compensation”, the electronic charge in the two former regimes being dominantly balanced by counter-ions whereas the latter regime being the co-ion process. Theoretical EQCM curves upon cyclic variation of the potential may possess a complicated shape, with a pronounced “first cycle” and “relaxation” effects as well as with a considerable hysteresis, the mass at the anodic scan being greater than that at the cathodic branch, for the anion exchange constants between regimes 2 and 3. Another model proposes existence of two different forms of ions inside the film, “free” and “bound”, with possible kinetic limitations for their mutual transformation but a complete equilibrium between the “free” ions and the solution. Corresponding theoretical EQCM plots demonstrate a great variety of shapes depending on the maximum amount and thermodynamical stability of “bound” species as well as on the kinetic exchange parameters. This model has enabled one to reproduce most of the features of experimental stabilized EQCM curves for PPy: non-monotonic variation of the film mass (“co-ion” type at low charges and “counter-ion” one at higher charges), with a drastic change of the mass vs. charge slope at the cathodic scan and a flat extremum in the anodic branch, as well as a lower mass at the cathodic scan at very low charges and a crossing point of the branches.

Simultaneous voltammetric and in situ conductivity studies of n-doping of polythiophene films with tetraalkylammonium, alkali, and alkaline-earth cations

Polythiophene (PTh) films, both p- and n-doped, have been fully characterized in a solution of TEABF 4 (tetraethylammonium tetrafluoroborate), in propylene carbonate (PC) by a simultaneous application of cyclic voltammetry and in situ conductometry. We observed a considerable charge-trapping effect during n-doping of PTh with TEA + cations. The release of the trapped, negatively charged polarons becomes possible only at the onset of the subsequent p-doping of this polymer, accompanied by an increase in conductivity. The interaction between the trapped, negatively charged polarons and mobile, positively charged polarons resulted in their neutralization. This effect was accompanied by the simultaneous exaltation of the voltammetric current and the transient decrease in conductivity. Correction for the trapped charge allowed us to estimate the mobility of the free negatively charged polarons: μ = 2.4 X 10 -3 cm 2 /V s (doping level with TEA + , cations, X = 0.022). At the same doping level with BF 4 - -anions, positively charged polarons exhibited similar mobility, μ = 2.8 × 10 -3 cm 2 /V s. This similarity is in good agreement with the reported electron spin resonance characteristics of PTh. We have studied the influence of a low content of cations (with different ionic radii) in the mother TEABF 4 solution on the deterioration of the n-type redox-capacity and conductivity increasing in the sequence: TEA + < K + << Li + < Ba 2+ . This remarkable effect is discussed in terms of competition between the solvation of small cations in PC and the hard-soft pinning of negatively charged polarons. This pinning seems to be partially reversible, as revealed by the subsequent n-redoping in a clean TEABF 4 solution.

Electropolymerization as a versatile route for immobilizing biological species onto surfaces

Biosensors based on electronic conducting polymers appear particularly well suited to the requirements of modern biological analysis—multiparametric assays, high information density, and miniaturization. We describe a new methodology for the preparation of addressed DNA matrices. The process includes an electrochemically directed copolymerization of pyrrole and oligonucleotides bearing on their 5′ end a pyrrole moiety. The resulting polymer film deposited on the addressed electrode consists of pyrrole chains bearing covalently linked oligonucleotides (ODN). An oligonucleotide array was constructed on a silicon device bearing a matrix of 48 addressable 50 × 50 µm gold microelectrodes. This technology was successfully applied to the genotyping of hepatitis C virus in blood samples. Fluorescence detection results show good sensitivity and a high degree of spatial resolution. In addition, gravimetric studies carried out by the quartz crystal microbalance technique provide quantitative data on the amount of surface-immobilized species. In the case of ODN, it allows discrimination between hybridization and nonspecific adsorption. The need for versatile processes for the immobilization of biological species on surfaces led us to extend our methodology. A biotinylated surface was obtained by coelectropolymerization of pyrrole and biotin-pyrrole monomers. The efficiency for recognition (and consequently immobilization) of R-phycoerythrin-avidin was demonstrated by fluorescence detection. Copolymerization of decreasing ratios of pyrrole-biotin over pyrrole allowed us to obtain a decreasing scale of fluorescence.

A combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) study of a poly(o-toluidine) modified electrode in perchloric acid solution

A combined electrochemical quartz crystal microbalance (EQCM) and probe beam deflection (PBD) instrument was used to monitor the effects of film history and redox state of an unhydrated poly(o-toluidine) modified electrode immersed in aqueous perchloric acid (pH=0) solution. The degree of film hydration affected the relative contributions of protons and anions exchanged during the first redox process and the potential of the first anodic voltammetric peak. A distinct proton contribution to the ion exchange during the first redox process was detected during the second and subsequent cycles. Electroneutrality during the second charge transfer step was maintained predominantly by proton expulsion. The determination of the diffusion coefficient of the exchanged species in the first redox process by the time dependence of laser beam deflection delay and by temporal convolution of the current signal are compared. The concomitant changes in the gravimetric and optical signals during film hydration demonstrate the participation of both proton and solvent during redox cycling of poly(o-toluidine).

Study of DNA hybridization on polypyrrole grafted with oligonucleotides by photocurrent spectroscopy

Recognition of DNA sequences by biochemical sensor is generally performed by analysis after completion of hybridization. Using a technique able to directly translate the biological event into an electrical signal allows the in situ monitoring of the hybridization kinetics. In this aim, the photoelectrochemical behavior of one electroactive polymeric sensor based on a copolymer of polypyrrole and polypyrrole-oligonucleotide has been investigated in aqueous solution. This sensor has been studied as such (i) and in two other situations: (ii) when the copolymer is in presence of non-complementary oligonucleotides; and (iii) when the copolymer is in presence of complementary oligonucleotides. From the photocurrent spectra obtained at -0.6 V/SCE versus incident energy the allowed direct and indirect transitions for each polymer have been evidenced. The photocurrent evolution during hybridization and adsorption processes has been recorded in real time and the hybridization kinetics has revealed to be comparable with mass variations obtained by quartz crystal microbalance under the same experimental conditions.