Redox Polymer Films Research Articles

Synthesis, structural characterisations, electrochemical behavior and antibacterial activities of a chromium(III) resorcinolate complex

The integration and implementation of transition metal complexes as working electrode for surface modifications has become an area of research interest now a days. Based on this, a novel anionic complex having a formula of Na3[Cr(HR)4Cl2] (HR- = resorcinolate) with octahedral geometry, was prepared and characterized by elemental analysis, molar conductance, vibrational spectra. Electropolymerization of the ligand, salt, and complex was done on glassy carbon electrodes with effective surface area 0.15, 0.17, and 0.2 cm2 for poly(NaHR)/GCE, Cr(0)/GCE, and poly(Na3[Cr(HR)4(Cl2)]/GCE respectively. The presence of a redox polymer film on the surface of the electrode was indicated by the experimental changes observed for a solution containing Na3[Cr(HR)4Cl2]. Specifically, the anodic peak currents, which represent the oxidation process, showed significant increases at different potential values compared to the ligand and salt. These potential values were approximately + 114 mV and + 573 mV. Additionally, the cathodic peak current, which is corresponding to the reduction process, exhibited changes at − 230 mV as the scan cycles were increased. These variations strongly suggested the formation of a film composed of a redox polymer on the electrode's surface. Further, the resistance to charge transfer (Rct) of the compound is in the lower voltage compared to the free ligand, CrCl3•6H2O, and the unmodified GCE. This indicates that the modified compound exhibits improved conductivity due to its reduced charge transfer resistance. The antibacterial activity assessed against two Gram-positive bacteria strains (Staphylococcus aureus and Streptococcus epidermis) and two Gram-negative bacteria strains (Escherichia coli and Klebsiella pneumoniae), indicate that the Cr(III) complex exhibit higher antibacterial activity compared to the ligand and the metal salt but lower than the reference. The synthesized is a highly conductive material suitable for potential applications in sensors and catalysis, especially in situations that require efficient and rapid electron transfer process

Electrosynthesis of toluidine blue on Fe3O4 nanoparticles modified electrodes in a deep eutectic solvent for the electrochemical determination of neurotransmitter epinephrine

The electrochemical methodology is important and challenging for rapid, simple, low-cost, sensitive, selective and reliable determination of biomolecules to diagnose and treat diseases. Our work presents the combination of the catalytic ability of a toluidine blue (TB) redox polymer and the important physical and chemical properties of Fe3O4 nanoparticles (Fe3O4NPs) to develop a new disposable sensor with advanced analytical performancecharacteristics for the determination of EP. Fe3O4NPs combined with poly(toluidine blue) (PTB) redox polymer film on a PGE support have been developed involving electropolymerization of TB in deep eutectic solvents (DES), which is a green solvent. Scanning electron microscopy (SEM) was used to evaluate the effect of the electrode composition on the surface morphology of the nanostructured PTB films, being correlated with the electrochemical characteristics observed by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and differential pulse voltammetry (DPV). It has been shown that combining Fe3O4NPs with PTB synthesized in DES improves the analytical properties of the sensor more than using each modifier individually due to synergistic effects. The analytical performance of PTBethaline-HCl/Fe3O4/PGE was evaluated for epinephrine (EP) reduction in the presence of an excess dopamine (DA) by DPV. The proposed sensor was used to determine EP using the reduction peak as a voltammetric signal instead of the oxidation peak usually employed to minimize possible interfering effects in biological samples. The developed sensor showed good response with high sensitivity (9.26 µA µM−1 cm−2), low detection limit (0.05 µM), wide linear range (0.19–20 µM), and excellent reproducibility (RSD 1.7 %) and repeatability (RSD 2.9 %). The practical applicability of the proposed sensor has been successfully tested for the quantification of EP in human serum and injection ampoule samples. This is a very promising analytical method for monitoring the level of EP in real samples.

Tools to Simulate Resonance Raman Spectra for in-Situ Studies of Electron Transport Mediators and Bioelectrocatalysts

Raman spectroscopy is widely applied as an in-situ probe of electrochemical processes. When the excitation light is resonant with an electronic transition, the detected signal can be enhanced by more than 1,000-fold. Resonance Raman (RR) spectroscopy has broad application in the study of common organic electron transport mediators (i.e., viologens, phenazines, photosynthetic pigments) and active sites of redox proteins (i.e., laccases, peroxidases, flavin-containing enzymes). To support investigations of bioelectrocatalytic membranes, we have been adapting RR spectroscopy to gain insights into the properties of immobilized redox mediators and biocatalyst active sites.1,2 Computational methods can be useful for guiding spectral interpretation. This poster reports on applications of recently advanced time-dependent density functional theory (TDDFT) codes that enable the simulation of RR spectra for small electron transport mediators (e.g., methyl viologen radical anion) and redox protein active site models (e.g., laccase T1 site mimics). The ORCA program suite3 was adapted. A series of Cu-centered compounds that model the T1 active site in laccases was studied along with the methyl viologen radical cation. The T1 site in laccases facilitates direct electron transport to an electrode and is a target for in-situ RR measurements. The reported work demonstrates careful selection of the functional (B3-LYP versus the long-range corrected hybrid exchange functional, ωB97X-D3) results in TDDFT-calculated electronic absorption and RR spectra that have features in excellent agreement with experiment. The results provide a foundation to build from in advancing models of small redox mediators and laccase catalytic active sites in support of sustainable technologies. Ozuguzel, U.; Aquino, A.J.A.; Nieman, R.; Minteer, S.D.; Korzeniewski, C. “Calculation of Resonance Raman Spectra and Excited State Properties for Blue Copper Protein Model Complexes” ACS Sustainable Chem. Eng. 2022, 10, 14614. Xu, J.; Koh, M.; Minteer, S. D.; Korzeniewski, C. “In Situ Confocal Raman Microscopy of Redox Polymer Films on Bulk Electrode Supports” ACS Measurement Science Au 2023 (DOI: 10.1021/acsmeasuresciau.2c00064). Neese, F.; Wennmohs, F.; Becker, U.; Riplinger, C. “ORCA Quantum Chemistry Program Package” J. Chem. Phys. 2020, 152, 224108.

In Situ Confocal Raman Microscopy of Redox Polymer Films on Bulk Electrode Supports.

A spectroelectrochemical cell is described that enables confocal Raman microscopy studies of electrode-supported films. The confocal probe volume (∼1 μm3) was treated as a fixed-volume reservoir for the observation of potential-induced changes in chemical composition at microscopic locations within an ∼20 μm thickness layer of a redox polymer cast onto a 3 mm diameter carbon disk electrode. Using a Raman system with high collection efficiency and wavelength reproducibility, spectral subtraction achieved excellent rejection of background interferences, opening opportunities for measuring within micrometer-scale thickness redox films on widely available, low-cost, and conventional carbon disk electrodes. The cell performance and spectral difference technique are demonstrated in experiments that detect transformations of redox-active molecules exchanged into electrode-supported ionomer membranes. The in situ measurements were sensitive to changes in the film oxidation state and swelling/deswelling of the polymer framework in response to the uptake and discharge of charge-compensating electrolyte ions. The studies lay a foundation for confocal Raman microscopy as a quantitative in situ probe of processes within electrode-immobilized redox polymers under development for a range of applications, including electrosynthesis, energy conversion, and chemical sensing.

Theoretical Understanding of the Penetration of O2 in Enzymatic Redox Polymer Films: The Case of Unidirectional Catalysis and Irreversible Inactivation in a Film of Arbitrary Thickness

AbstractRedox catalysts, including hydrogenases, can be embedded into films made of redox polymers, whose side chains mediate electrons between the catalyst and an electrode. These films can be used as bioanodes in H2‐based biofuel cells, because they protect the catalyst from O2‐induced inactivation: self‐protection occurs because a fraction of the incoming H2 is used in the outer region of the film to catalytically produce electrons that reduce the O2 molecules that penetrate the film. Here, we focus on the case of unidirectional catalysis (e. g. H2 oxidation) by an enzyme that is irreversibly inactivated by O2, embedded in a film of arbitrary thickness. We analytically solve the reaction/diffusion system to fully describe the time evolution of the penetration of O2 and we discuss the amount of H2 consumed by the protection mechanism. We establish the relations between film thickness, electron conduction, catalyst use and life time. This provides the theoretical framework required to optimize the design of these systems.

Reversible H2 oxidation and evolution by hydrogenase embedded in a redox polymer film

Efficient electrocatalytic energy conversion requires the devices to function reversibly, i.e. deliver a significant current at minimal overpotential. Redox-active films can effectively embed and stabilise molecular electrocatalysts, but mediated electron transfer through the film typically makes the catalytic response irreversible. Here, we describe a redox-active film for bidirectional (oxidation or reduction) and reversible hydrogen conversion, consisting of [FeFe] hydrogenase embedded in a low-potential, 2,2’-viologen modified hydrogel. When this catalytic film served as the anode material in a H2/O2 biofuel cell, an open circuit voltage of 1.16 V was obtained - a benchmark value near the thermodynamic limit. The same film also acted as a highly energy efficient cathode material for H2 evolution. We explained the catalytic properties using a kinetic model, which shows that reversibility can be achieved despite intermolecular electron transfer being slower than catalysis. This understanding of reversibility simplifies the design principles of highly efficient and stable bioelectrocatalytic films, advancing their implementation in energy conversion.

Artificial maturation of [FeFe] hydrogenase in a redox polymer film.

We demonstrate that the insertion of the dinuclear active site of [FeFe] hydrogenase into the apo-enzyme can occur when the enzyme is embedded in a film of redox polymer, under conditions of mediated electron transfer. The maturation can be monitored by electrochemistry, and is as fast as under conditions of direct electron transfer. This new approach further clears the way to the implementation of hydrogenases in large scale real life processes.

Potentiodynamic fabrication and characterization of poly(4-amino-3-hydroxynaphthalene sulfonic acid) modified glassy carbon electrode

Electropolymerization technique is an effective to control the thickness of the conducting polymer film. Poly (4-amino-3-hydroxynaphthalene sulfonic acid), as polymer of aniline derivative, synthesized potentiodynamically on surface of glassy carbon electrode, experienced catalytic property towards several electroactive alkaloids and pesticides in our previous reports. However, controversial arguments on whether the reported catalytic property is due to the deposited polymer film, activation of the surface during scanning the electrode in acidic medium, or the monomer itself have not been addressed. Thus, this paper describes properties of the polymer film modified electrode as compared to unmodified glassy carbon electrode, activated glassy carbon electrode, pretreated glassy carbon electrode, and monomer casted glassy carbon electrode. The effective surface area, film thickness, elemental composition, circuit elements including charge transfer resistance & double layer capacitance, and morphology determined using cyclic voltammetry, electrochemical impedance spectroscopy, scanning electron microscopy- equipped with an energy dispersive spectrometer, atomic force microscopic, and x-ray photoelectron spectroscopic techniques as applicable confirmed the modification of the surface of the glassy carbon electrode with a conductive, electroactive, nano-sized (40-60 nm) polymer film that increased the effective surface area of the modified electrode (PGCE). The authors are now confident to conclude that the observed catalytic property of the polymer modified glassy carbon electrode towards the analytes in our reported works is thus solely due to the synergetic effects of properties of the deposited redox polymer film on the electrode surface.

Metal-centered redox activity in a polymeric Cobalt(II) complex of a sterically hindered salen type ligand

This paper reports on the peculiarities of the metal-centered redox activity in a polymeric cobalt(II) complex of a sterically hindered tetradentate N2O2 Schiff base (salen type) ligand. A Co(III)/Co(II) redox process in the polymer film during its formation on the electrode and subsequent electrochemical switching in a monomer-free electrolyte has been characterized by cyclic voltammetry, electrochemical quartz crystal microbalance, and electrochemical impedance spectroscopy. The obtained results indicate that cobalt ions in the polymer can effectively function as independent redox sites. The metal-centered redox activity largely governed by the unique features of a sterically hindered salen type ligand is time-dependent and sensitive to the presence of coordinating species in the electrolyte. In relatively fast potentiodynamic scans, the Co(III)/Co(II) redox switching in an acetonitrile solution exhibits a high degree of chemical and electrochemical reversibility and stability and can be considered as a quasi-equilibrium process. A novel analytical solution for modeling quasi-equilibrium voltammetric curves accounting for the existence of short-range interactions within redox polymer films is proposed and used to estimate the attraction constant in the cobalt-salen type polymer. The study findings contribute to a better understanding of the prospects of using electrodes modified with the investigated polymeric complex in energy storage and catalytic applications.

Towards highly conducting bicarbazole redox polymer films with plateau-like conductivities

Chemical doping of bicarbazole redox polymer films leads to plateau-conductivities up to 2 × 10−2 S cm−1. The stability due to crosslinking and the transparency make them e.g. suitable as hole-transport layers in organic opto-electronic devices.

(Keynote) Electrochemistry for Studying and Using Biological and Synthetic Catalysts of Hydrogen Oxidation and Production

Direct electrochemistry has proved invaluable for studying natural and synthetic catalysts of redox reactions that are of interest in the context of solar fuels production, including H2 oxidation and production. With the catalyst either attached to the electrode or free to diffuse in solution, the current response to changing the electrochemical driving force, the so-called voltammetric signature, can be interpreted to learn about various aspects of the mechanism that are difficult to probe using more conventional methods: long-range electron transfer, diffusion along gas channels, redox-driven (in)activations, active site chemistry and photoreactivity under conditions of turnover. It can also be used to define the sequence of events in the catalytic cycle and to benchmark the performances of the catalyst in terms of rate, directionality and reversibility. This approach contrasts with and complements the usual strategy which aims at stabilizing species that are presumed to be catalytic intermediates, and determining their structure using spectroscopic or structural methods. Embedding the enzyme in a film of redox polymer provides further opportunities for stabilising the enzyme-electrode and tuning its apparent properties, particularly the resistance to oxygen, making it possible to achieve long-term aerobic hydrogen oxidation using enzymes that are notoriously oxygen sensitive. Exemples will be given that illustrate electrochemical studies and applications of hydrogenase enzymes and synthetic catalysts. [1] M. Del Barrio, Melisa and M. Sensi and C. Orain and C. Baffert and S. Dementin and V. Fourmond and C. Leger, “Electrochemical investigations of hydrogenases and other enzymes that produce and use solar fuels” Acc. Chem. Res. 51 769 (2018) [2] M Sensi, M del Barrio, C Baffert, V Fourmond, C Leger “New perspectives in hydrogenase direct electrochemistry”, Curr. Op. Electrochem 1:5 135 (2017) [3] V Fourmond and C. Leger, “Modelling the voltammetry of adsorbed enzymes and molecular catalysts”, Curr. Op. Electrochem 1:1, 110 (2017) [4] Vincent Fourmond, Eric S. Wiedner, Wendy J. Shaw and Christophe Léger, “On the design of molecular catalysts of reversible and bidirectional H2 oxidation/production”, submitted. [5] Huaiguang Li, Darren Buesen, Christophe Léger, Vincent Fourmond and Nicolas Plumeré, in preparation. Figure 1

Polymer/Iron Oxide Nanoparticle Modified Glassy Carbon Electrodes for the Enhanced Detection of Epinephrine

AbstractNovel electrochemical sensors for epinephrine (EP) based on a glassy carbon electrode (GCE) modified with a redox polymer film and iron (III) oxide nanoparticles (Fe2O3NP) have been developed. Two redox polymers‐poly(brilliant cresyl blue) (PBCB) and poly(Nile blue) (PNB), and two different architectures‐polymer/Fe2O3/GCE and Fe2O3/polymer/GCE were investigated. The electrochemical oxidation of epinephrine at the modified electrodes was performed by differential pulse voltammetry (DPV), in pH 7 electrolyte, and the analytical parameters were determined. The results show enhanced performance, more sensitive responses and lower detection limits at the modified electrodes, compared to other electrochemical epinephrine sensors reported in the literature. The best voltammetric response with the lowest detection limit was obtained for the determination of epinephrine at PBCB/Fe2O3/GCE. The novel sensors are reusable, with good reproducibility and stability, and were successfully applied to the determination of epinephrine in commercial injectable adrenaline samples.

Electrochemical Diagnosis of Chemical Switch: Impact of Structural Changes on Charge Transport Mechanism of “Redox Anion Bound Polysilsesquioxane” Film

AbstractHerein, we report the structurally dependent charge transport mechanism of ferrocyanide ions immobilized in a protonated amine functionalized polysilsesquioxane (PSQ) film (Redox polymer film). This redox polymer film shows anion actuated reversible “active to inactive” electrochemical interfaces when the electrolyte anion is changed from chloride to perchlorate as a result of swelling and hydrophobic collapse of the PSQ film, respectively. This concept was successfully applied for switchable electrocatalytic oxidation of ascorbic acid by a [Fe(CN)6]4−‐PSQ film. In addition, it was shown in cyclic voltammetric (CV) studies that the presence of HPO42− and CO32− is leading to strong irreversible structural collapse of the PSQ film, whereas, the presence of SO42− and S2O82− is leading to reversible structural changes of the PSQ film within chloride ion medium. Reversible structural transition between swelling and aggregation (voltammetric response reversibly changing from peak to sigmoidal shape) described in this work is new and provides mechanistic insight into redox polymer electrochemistry and allows differentiating electron hopping from physical motion of redox polymers. Moreover, ion‐exchange between bound redox anions and divalent anions in the electrolyte at fixed positive surface sites of PSQ film was explained in terms of Jones‐Dole B‐coefficient. Electrochemical impedance spectroscopy, Atomic force microscopy, Contact angle measurement and UV‐visible spectroscopy analysis of above phenomenon were consistent with CV results.

Tuned Amperometric Detection of Reduced β-Nicotinamide Adenine Dinucleotide by Allosteric Modulation of the Reductase Component of the p-Hydroxyphenylacetate Hydroxylase Immobilized within a Redox Polymer

We report the fabrication of an amperometric NADH biosensor system that employs an allosterically modulated bacterial reductase in an adapted osmium(III)-complex-modified redox polymer film for analyte quantification. Chains of complexed Os(III) centers along matrix polymer strings make electrical connection between the immobilized redox protein and a graphite electrode disc, transducing enzymatic oxidation of NADH into a biosensor current. Sustainable anodic signaling required (1) a redox polymer with a formal potential that matched the redox switch of the embedded reductase and avoided interfering redox interactions and (2) formation of a cross-linked enzyme/polymer film for stable biocatalyst entrapment. The activity of the chosen reductase is enhanced upon binding of an effector, i.e. p-hydroxy-phenylacetic acid ( p-HPA), allowing the acceleration of the substrate conversion rate on the sensor surface by in situ addition or preincubation with p-HPA. Acceleration of NADH oxidation amplified the response of the biosensor, with a 1.5-fold increase in the sensitivity of analyte detection, compared to operation without the allosteric modulator. Repetitive quantitative testing of solutions of known NADH concentration verified the performance in terms of reliability and analyte recovery. We herewith established the use of allosteric enzyme modulation and redox polymer-based enzyme electrode wiring for substrate biosensing, a concept that may be applicable to other allosteric enzymes.

Reversible Charge Trapping in Bis-Carbazole-Diimide Redox Polymers with Complete Luminescence Quenching Enabling Nondestructive Read-Out by Resonance Raman Spectroscopy.

The coupling of substituted carbazole compounds through carbon–carbon bond formation upon one-electron oxidation is shown to be a highly versatile approach to the formation of redox polymer films. Although the polymerization of single carbazole units has been proposed earlier, we show that by tethering pairs of carbazoles double sequential dimerization allows for facile formation of redox polymer films with fine control over film thickness. We show that the design of the monomers and in particular the bridging units is key to polymer formation, with the diaminobenzene motif proving advantageous, in terms of the matching to the redox potentials of the monomer and polymer film and thereby avoiding limitations in film thickness (autoinsulation), but introduces unacceptable instability due to the intrinsic redox activity of this moiety. The use of a diimide protecting group both avoids complications due to p-diamino-benzene redox chemistry and provides for a redox polymer in which the photoluminescence of the bis-carbazole moiety can be switched reversibly (on/off) with redox control. The monomer design approach is versatile enabling facile incorporation of additional functional units, such as naphthalene. Here we show that a multicomponent carbazole/naphthalene containing monomer (APCNDI) can form redox polymer films showing both p- and n- conductivity under ambient conditions and allows access to five distinct redox states, and a complex electrochromic response covering the whole of the UV/vis–NIR spectral region. The highly effective quenching of the photoluminescence of both components in poly-APCNDI enables detailed characterization of the redox polymer films. The poly-APCNDI films show extensive charge trapping, which can be read out spectroscopically in the case of films and is characterized as kinetic rather than chemical in origin on the basis of UV/vis–NIR absorption and resonance Raman spectroscopic analyses. The strong resonantly enhanced Raman scattering for the various oxidized and reduced states of APCNDI enables nondestructive “read-out” of the state of the polymer, including that in which charges are trapped kinetically at the surface, making poly-APCNDI highly suitable for application as a component in organic nonvolatile memory devices.

Double Layer Structural Effects in Cyclic Voltammetry Curves Complicated with Non-Equilibrium Injection of Charge Carriers into Redox Polymer Films

An expressed asymmetry of cyclic voltammetry curves of electrodes modified with redox and conducting polymers is often considered as a consequence of slow injection of charge carriers into/out polymer films. The Butler-Volmer equation is usually applied to quantitative treatment of the rates of such injection processes. It is generally considered that the above equation does not take into account the influence of the double layer structure on the rates of electrode processes and, hence, one can assume that its use might lead only to a qualitative explanation of the observed asymmetry effect. Using different models of film double layers (at the film/electrode substrate and film/adjacent electrolyte interfaces) and the proper kinetic equations that include the so-called Frumkin correction, we have calculated the corresponding voltammetric curves in case of electrodes modified with redox polymer films. As expected, a good agreement between the data calculated and the results followed from the Butler-Volmer equation takes place only for the Helmholtz model of the both interfacial regions, while significant quantitative differences in such curves appear with using the Stern and Gouy models of film double layers.

Fabrication and electrochemical characterization of polydopamine redox polymer modified screen-printed carbon electrode for the detection of guanine

A modified screen-printed carbon electrode (SPCE) was fabricated by electrodeposition of dopamine to create a suitable polydopamine (PDA) redox polymer film for sensing applications. The fabricated sensor, designated as PDA-SPCE, was characterized by electrochemical impedance spectroscopy, contact angle measurements and chronoamperometry. The surface coverage of redox species on the polymer film was optimized and found to be ∼6.1×10−6molcm−2. Using square wave voltammetry, the PDA-SPCE was used to facilitate and observe the electrocatalytic oxidation of guanine at different concentrations. A calibration curve for guanine was constructed, with a sensitivity of 90.5μAμM−1cm−2 and a limit of detection (based on 3× the baseline noise) of 15.8μM.

(Invited) Determination of Solvation Populations and Dynamics in Electroactive Polymer Films By Modelling Time-Resolved Neutron Reflectivity

Characterization of electroactive polymer films and their application in practical devices (electronic, chemical, sensing, energy storage, optical, actuator) tends to focus on the transfer of charged species (electrons and counter ions), since these are the entities most directly associated with the response to the electrochemical control function. However, transport rates of ions (“dopant”, in the context of conducting polymer systems) are dictated by solvent content of the film, which governs the local viscosity of the medium through which mobile species move. Further, the presence of solvent (and other small molecules) creates free volume that facilitates the dynamics of polymer motion, macroscopically seen as a plasticising effect on film viscoelasticity. In the light of the above, it is of value to determine the solvent content of electroactive polymer films; this is the focus of the present work. Experimentally, the challenge is that the solvent is (by choice) electrochemically “silent” and optically transparent, and its selective observation within a film exposed to a vast reservoir of bulk solution is not trivial. Nanogravimetric measurements made using the EQCM have provided innumerable insights into changes in film solvent population, e.g. in response to redox state changes, but the technique cannot provide absolute solvent population in the film. Spectroscopic techniques generally have difficulty in distinguishing solvent in a film from that in bulk solution, but reflectivity techniques do possess surface selectivity. Of these, ellipsometry has the advantage of rapid data acquisition, i.e. good time resolution, but modelling the data can present ambiguities. Here we use neutron reflectivity (NR) which, through the penetrating nature of neutrons, allows access to "buried" interfaces. Traditionally, the negative consequence of this weak interaction with matter is long data acquisition times; addressing this is the central feature of this presentation. Since neutrons interact with the nuclei in the system (cf. photons interacting with the electrons in ellipsometry), NR has the huge advantage of isotopic selectivity. This is commonly referred to as “contrast variation” and has the critical characteristic of allowing one to vary the visibility of a selected chemical component without (to a first approximation) altering the (electro)chemistry of the system. Most frequently, this is exploited by deuteration, since the neutron scattering lengths of H and D are very different. Correlation of the h- and d-system NR data sets removes ambiguities in modelling the data. We have previously used NR with solvent contrast variation to determine solvent volume fraction depth profiles, ΦS(z), in redox polymer films at equilibrium (fixed potential) and on long timescales during slow cycling. We now present data under dynamic conditions more relevant to fabrication or operation of a working device. This necessitates an order of magnitude improvement in time resolution. Instrumentally, the challenge can be met using more sophisticated detection systems, including event mode data capture, in which every incident/reflected neutron is detected individually. The dramatic increase in the size of data sets presents challenges in modelling the data, but the facility to (re-)select signal averaging time post experiment permits optimisation of competing aspirations of time resolution and signal-to-noise. This is key for single shot experiments, such as film deposition or evolution of film behaviour. Here we present neutron reflectivity data for aniline-, pyrrole- and thiophene-based polymer films. For these systems, we have the opportunity to exploit h- and d- variants of both solvent and monomer. Four significant insights are revealed. First, the data reveal that film deposition protocol (i.e. potentiostatic, potentiodynamic or galvanostatic control function) influences film solvation state and spatial distribution; while anecdotally suspected, this provides unambiguous quantitative evidence. Second, there are substantial spatial variations in film solvent content; this is a feature commonly not accommodated in models of modified electrodes. Third, for some systems, film solvent population and spatial disribution are "frozen" into the system during fabrication, so deposition protocol determines film characteristics in the longer term. Fourth, in selected cases the structure evolves with electrochemical manipulation, such that differently prepared films subsequently exposed to the same environment and history evolve to a common solvation profile. The implications of these phenomena for film design and handling are discussed for particular types of application.

Stabilizing redox polymer films by electrochemically induced crosslinking

Electrochemically induced crosslinking is suggested to stabilize electrodeposition polymer/enzyme films selectively on an electrode surface. 4 different protected diamine or dithiol based bi-functional crosslinkers have been synthesized, which can be activated by a pH-shift invoked by electrochemical water oxidation or proton reduction. Deprotection occurs either simultaneously or sequentially to the deposition of specifically designed redox electrodeposition polymers. The stability of the resulting polymer films was substantially enhanced as evaluated using continuous potentiodynamic cycling alternated by difference pulse voltammetry. Electrochemically induced crosslinking is compatible with biological recognition elements using Trametes hirsuta laccase or glucose oxidase entrapped within specifically adapted Os-complex modified or phenothiazine-modified redox polymers.

Charge Transport through Geobacter sulfurreducens Biofilms Grown on Graphite Rods

Biofilms of the electroactive bacterium Geobacter sulfurreducens were induced to grow on graphite-rod electrodes under a potential of 0 V (vs Ag/AgCl) in the presence of acetate as an electron donor. Increased anodic currents for bioelectrocatalytic oxidation of acetate were obtained when the electrodes were incubated for longer periods with periodic electron-donor feeding. The maximum current density for acetate oxidation increased 2.8-fold, and the biofilm thickness increased by 4.25-fold, over a time period of 83-147 h. Cyclic voltammetry in the presence of acetate supports a model of heterogeneous electron transfer, one electron at time, from biofilm to electrode through a dominant redox species centered at -0.41 V vs Ag/AgCl. Voltammetry performed under nonturnover conditions provided an estimate of the surface coverage of the redox species of 25 nmol/cm(2). This value was used to estimate a redox species concentration of 7.3 mM within the 34-μm-thick biofilm and a charge-transport diffusion coefficient of 3.6 × 10(-7) cm(2)/s. This value of diffusion coefficient is greater than that observed in traditional thin-film voltammetric studies with redox polymer films containing much higher surface concentrations of redox species and might be associated with proton transport to ensure electroneutrality within the biofilm upon electrolysis.