Redox-active Films Research Articles

Investigating Ion-Coupled Electron Transfer of Conducting Polymers

After years of basic and applied studies of electron transfer in redox active films, the fundamentals of ion transfer during the exchange remain to be understood. Empirically, the processes are ion-transport limited. Using a conducting polymer as a model system, we aim to study the relationship between electron and ion transfer during redox switching.We work on electrochromic conducting polymers because of their observable and detectable color changes in addition to the current during the redox switching. Tracing optical signals can enhance the understanding of electrical signals from redox reactions. When a potential is applied to the electroactive polymer, its color will change from lighter to darker or vice versa. Keeping in mind that absorbances are additive, the ratio of the contents that make up the overall color of the polymer changes. This ratio change is directly proportional to the amount of matter under inter-conversion during the redox reaction.This study aims to explore the ion-electron mechanism in conductive polymers using the simultaneous Spectro-CV method. To achieve this, we conducted cyclic voltammetry at exceptionally high scanning rates, leveraging the conductive polymer's color-changing property. At these high rates, ion movement was restricted due to limited time for forward and backward motion, potentially reaching a point of stagnation. The investigation focused on whether the polymer would continue changing color, providing insights into ion-electron transfer. In addition, we are conducting EIS measurements alongside simultaneous Spectro-CV measurements and attempting to fit the impedance response of the polymer with an appropriate model. We aim to track the time constants provided by the polymer at different voltage values and understand the corresponding physical processes associated with these time constants. Additionally, we aim to use our EIS model to validate the desired redox current information obtained from optical data. We mathematically reach the faradaic portion of the total current with EIS and compare the results obtained from optical data.

The effect of different immobilization strategies on the electrochemical performance of enzymatic carbonaceous electrodes developed using carbonized biomass sources

In this study, typha tassel (TT) and pussy willow (PW) as biomass sources were used to synthesize carbonaceous materials for enzymatic electrode modification. Carbonization was performed at high temperatures of up to 1000 °C under an inert atmosphere and the resulting carbonized TT (CTT) and PW (CPW) were dispersed in dimethylformamide. CTT and CPW were then characterized by SEM/EDX, MAPPING, FTIR, and XRD to confirm the carbonaceous structures. Four different immobilization strategies were used to demonstrate the use of CTT and CPW together with a glucose oxidizing enzyme (glucose oxidase, GOx), an electron transfer mediator (ferrocene, Fc), and a protective polymer coating (Chitosan, Chit). The effect of the Chit layer was first investigated on the performance of the prepared enzymatic electrodes and it was shown that Chit could help to preserve the GOx activity. The effect of the electron transfer mediator whether in a solution or co-immobilized with GOx was also investigated using a mixture of Fc and CTT or CPW and Fc-only redox active film approaches. The results indicate that when Fc co-immobilized with GOx, a better performance was achieved. The prepared electrodes showed promising results for glucose biosensing with a limit of detection and limit of quantification values of 0.97 mM and 3.2 mM, respectively, operating up to 10 mM glucose. This study presents a comprehensive investigation of different immobilization strategies of GOx on carbonaceous electrodes and provides insight into the possible use of such materials as biomass to bioelectronics approaches.

Non-covalent small molecule partnership for redox-active films: Beyond polydopamine technology

HypothesisThe possibility to use hexamethylenediamine (HMDA) to impart film forming ability to natural polymers including eumelanins and plant polyphenols endowed with biological activity and functional properties has been recently explored with the aim to broaden the potential of polydopamine (PDA)-based films overcoming their inherent limitations. 5,6-dihydroxyindole-2-carboxylic acid, its methyl ester (MeDHICA) and eumelanins thereof were shown to exhibit potent reducing activity. ExperimentsMeDHICA and HMDA were reacted in aqueous buffer, pH 9.0 in the presence of different substrates to assess the film forming ability. The effect of different reaction parameters (pH, diamine chain length) on film formation was investigated. Voltammetric and AFM /SEM methods were applied for analysis of the film redox activity and morphology. HPLC, MALDI-MS and 1HNMR were used for chemical characterization. The film reducing activity was evaluated in comparison with PDA by chemical assays and using UV stressed human immortalized keratinocytes (HaCat) cells model. FindingsRegular and homogeneous yellowish films were obtained with moderately hydrophobic properties. Film deposition was optimal at pH 9, and specifically induced by HMDA. The film consisted of HMDA and monomeric MeDHICA accompanied by dimers/small oligomers, but no detectable MeDHICA/HMDA covalent conjugation products. Spontaneous assembly of self-organized networks held together mainly by electrostatic interactions of MeDHICA in the anion form and HMDA as the dication is proposed as film deposition mechanism. The film displayed potent reducing properties and exerted significant protective effects from oxidative stress on HaCaT.

Modelling electrochemical modulation of ion release in thin-layer samples

In this work, we present a model based on the finite element approach to describe the electrochemically controlled release of ions from a redox-active film into a sample confined to a thin-layer spatial domain. The model includes the effect of interfacial charge transfer kinetics and 1D-diffusion treatment for an electron transfer-ion transfer (ET–IT) coupled reaction. More in detail, the oxidation of the redox-active film (ET) involves an ion release to an aqueous phase (IT). The dynamic concentration of the released ion is calculated when the ET–IT reaction proceeds under potentiostatic control, and the effect of the thickness of each phase (i.e., film or aqueous) on the diffusion profile is analyzed. The model is experimentally validated for the particular case in which oxidation of a thin film of polyaniline (PANI, 10 μm in thickness) is linked to the release of protons from the film into an electrolyte solution. The proton release produces certain pH changes in the electrolyte that are monitored by a pH sensor located at 330 μm from the PANI film. The charge associated with the proton release is related to the dynamic concentration of protons in the electrolyte through pH-coulograms that agree with the theoretical predictions. Overall, the model can reproduce the general behavior of the experimental proton pump and provides key insights into the functioning mechanism of electrochemical systems where redox and ion transfer reactions are coupled.

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.

Spectroelectrochemical and electrochromic behavior of poly(methylene blue) and poly(thionine)-modified multi-walled carbon nanotubes

The increasing efforts devoted to fabricating electrochromic (EC) devices have motivated a lot of studies to develop new EC materials. Herein, we introduce two poly(phenothiazine)-type redox active polymer films as new EC materials. Poly(methylene blue) (PMB) and poly(thionine) (PTH) films were electrochemically deposited onto the transparent fluorine-doped tin oxide (FTO) electrode modified with multi-walled carbon nanotubes (MWCNTs), which are named as FTO/MWCNTs/PMB and FTO/MWCNTs/PTH, respectively. The as-prepared polymer films were characterized using field emission scanning electron microscopy (FESEM), Fourier transform infrared (FT-IR), cyclic voltammetry (CV), and spectroelectrochemical techniques. Both polymer films exhibited EC behavior at approximately 600 nm. PMB and PTH films are blue and pale purple, respectively, in the oxidized state and change their color to colorless, if reduced at − 0.6 V. In particular, FTO/MWCNTs/PMB showed a satisfactory ΔT (24%) at 600 nm. The obtained results demonstrate that PMB and PTH, as derivatives of phenothiazine dyes, can be introduced as novel EC materials.

Atmospheric plasma deposition of bioinspired catechol-rich polymers: a promising route for the simple construction of redox-active thin films

The atmospheric aerosol assisted plasma polymerization of 4-vinyl-catechol allows the facile deposition of robust redox active catechol-rich films with promising properties as organic cathode materials for lithium ion battery.

Oxidative Spin-Spray-Assembled Coordinative Multilayers as Platforms for Capacitive Films.

The spin-spray-assisted layer-by-layer (LbL) assembly technique was used to prepare coordinative oxidative multilayers from Ce(IV), inorganic polyphosphate (PP), and graphene oxide (GO). The films consist of successive tetralayers and have a general structure (PP/Ce/GO/Ce)n. Such oxidative multilayers have been shown to be a general platform for the electrodeless generation of conducting polymer and melanin-type films. Although the incorporation of GO enhances the film growth, the conventional dip LbL method is very time consuming. We show that the spin-spray method reduces the time required to grow thick multilayers by the order of magnitude and the film growth is linear from the beginning, which implies a stratified structure. We have deposited poly(3,4-ethylenedioxothiophene), PEDOT, on the oxidative multilayers and studied these redox-active films as models for melanin-type capacitive layers for supercapacitors to be used in biodegradable electronics, both before and after the electrochemical reduction of GO to rGO. The amount of oxidant and PEDOT scales linearly with the film thickness, and the charge transfer kinetics is not mass transfer-limited, especially after the reduction of GO. The areal capacitance of the films grows linearly with the film thickness, reaching a value of ca. 1.6 mF cm–2 with 20 tetralayers, and the specific volumetric (per film volume) and mass (per mass of PEDOT) capacitances are ca. 130 F cm–3 and 65 F g–1, respectively. 5,6-Dihydroxyindole can also be polymerized to a redox-active melanin-type film on these oxidative multilayers, with even higher areal capacitance values.

Suppressing hydrogen peroxide generation to achieve oxygen-insensitivity of a [NiFe] hydrogenase in redox active films

Redox-active films were proposed as protective matrices for preventing oxidative deactivation of oxygen-sensitive catalysts such as hydrogenases for their use in fuel cells. However, the theoretical models predict quasi-infinite protection from oxygen and the aerobic half-life for hydrogenase-catalyzed hydrogen oxidation within redox films lasts only about a day. Here, we employ operando confocal microscopy to elucidate the deactivation processes. The hydrogen peroxide generated from incomplete reduction of oxygen induces the decomposition of the redox matrix rather than deactivation of the biocatalyst. We show that efficient dismutation of hydrogen peroxide by iodide extends the aerobic half-life of the catalytic film containing an oxygen-sensitive [NiFe] hydrogenase to over one week, approaching the experimental anaerobic half-life. Altogether, our data support the theory that redox films make the hydrogenases immune against the direct deactivation by oxygen and highlight the importance of suppressing hydrogen peroxide production in order to reach complete protection from oxidative stress.

The electron as a probe to measure the thickness distributions of electroactive films.

Electron conducting films are ubiquitous in applications such as energy conversion, and their ability to fulfill their catalytic function can be greatly limited by inhomogeneities in their thickness or breaks within the film. Knowing the electroactive film thickness distribution would greatly facilitate optimization efforts, but techniques to measure this are lacking. Here, we present an electroanalytical method that provides the thickness distribution of the electrochemically accessible fraction of redox-active films in which the transfer of electrons is diffusional, i.e. by electron hopping. In this method, as the time scale of the experiment (the scan rate) is changed, the location of the diffusion layer boundary relative to the film roughness features is varied, allowing for the extraction of the film thickness distribution. In addition to being conveniently carried out in the solvated state, which is often the operational state of these conductive films, this approach is highly complementary to classical microscopy methods since it samples the entire modified electrode and is specific to the electroactive portions of the film. Therefore, this approach provides information on film morphology that is truly relevant for the catalytic processes being optimized, and thus can guide the optimization of catalyst integration in films towards macroscale cohesion and thickness homogeneity which are essential for optimal performances.

Redox active films of salicylic acid-based molecules as pH and ion sensors for monitoring ionophore activity in supported lipid deposits

5-aminosalicylic acid and salicylic acid have been used to form redox active films onto glassy carbon electrodes through recurrent cyclic voltammetry. The variation of the formal potential of the film obtained from 5-aminosalicylic acid as a function of pH is linear over the entire pH range studied (pH 2 to 10) with a slope of −80 mV per pH unit. Salicylic acid-based redox active films permit the detection of sodium and potassium ions (with a slope of −10 mV per 1 mM of cation) and chloride (with a slope of +11 mV per 1 mM of chloride). A lipid deposit of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) onto these modified electrodes allowed the integration of ionophores (valinomycin and nigericin) and the monitoring of the pH and potassium ion concentration variation at the modified electrode/lipid deposit interface.

A kinetic model for redox-active film based biophotoelectrodes.

Redox-active films are advantageous matrices for the immobilization of photosynthetic proteins, due to their ability to mediate electron transfer as well as to achieve high catalyst loading on an electrode for efficient generation of electricity or solar fuels. A general challenge arises from various charge recombination pathways along the light-induced electron transfer chain from the electrode to the charge carriers for electricity production or to the final electron acceptors for solar fuel formation. Experimental methods based on current measurement or product quantification are often unable to discern between the contributions from the photocatalytic process and the detrimental effect of the short-circuiting reactions. Here we report on a general electrochemical model of the reaction-diffusion processes to identify and quantify the "bottlenecks" present in the fuel or current generation. The model is able to predict photocurrent-time curves including deconvolution of the recombination contributions, and to visualize the corresponding time dependent concentration profiles of the product. Dimensionless groups are developed for straightforward identification of the limiting processes. The importance of the model for quantitative understanding of biophotoelectrochemical processes is highlighted with an example of simulation results predicting the effect of the diffusion coefficient of the charge carrier on photocurrent generation for different charge recombination kinetics.

Charge percolation in redox-active thin membrane hybrids of mesoporous silica and poly(viologens)

This work reports the fabrication of redox-active films of oligomeric and molecular viologens and mesoporous silica via the infiltration method. Pore-ellipsometry and UV-vis confirm that low-molecular-weight poly(viologens) in solution are able to enter the mesoporous structure, in contrast to high-molecular weight polymers that adsorb only on top of the film. Cyclic voltammetry shows that viologens are able to reach the bottom of the pores and access the electrode/film interface. However, the number of viologen sites that can be accessed by cyclic voltammetry at 50 mV s-1 is only a tenth of the total viologen population determined by UV-vis and pore-ellipsometry. The effect is ascribed to the very small apparent diffusion coefficient for charge transport within the film (Dapp < 10-12 cm2 s-1). A theoretical model is put forward to describe charge transport via the electron-hopping mechanism for redox sites randomly adsorbed on the inner walls of the pores. Our model predicts that the threshold of charge percolation occurs for viologen surface coverages close to those observed in our experiments; therefore, the low fraction of electrochemically addressable viologens is ascribed to inefficient charge percolation via the electron-hopping mechanism.

Chitosan-gold collapse gel/poly (bromophenol blue) redox-active film. A perspective for selective electrochemical sensing of flutamide

Chitosan-gold collapse gel (CS-Au CG) was prepared by reducing chloroauric acid (HAuCl4) with a polysaccharide, chitosan (CS), in the absence of chemical and physical agents. CS-Au CG was used for the first time as a suitable nano-biocomposite sensing film for efficient one-step electrochemical deposition of poly (bromophenol blue) (PBPB) redox mediator through amino-hydroxyl reaction to prepare a novel anti-androgen drug flutamide (FLU) sensor using glassy carbon electrode (GCE). The effect of electropolymerization cycle, scan rate, pH, and concentration of CS-Au CG/PBPB film on electrochemical behavior of FLU molecules was investigated. The excellent synergetic effect of CS-Au CG/PBPB film showed substantially enhanced electrocatalytic activity for FLU due to the halogen-nitro synthon molecular recognition processes. The selectivity of CS-Au CG/PBPB film sensor for FLU was discussed in detail. The fabricated electrochemical sensor exhibited good linearity in the ranges of 0.01–1245 μM. And also superior sensitivity (0.63 μAμM−1 cm−2) along with low limit of detection (4.8 nM) was obtained for FLU determination. The CS-Au CG/PBPB film showed an excellent selectivity, good reproducibility, and stability. In addition, the proposed sensor was successfully used to analysis of FLU drug in human urine and human blood serum samples with satisfactory results.

Variation of Carbon Based Materials on the Electropolymerization of Tyramine

AbstractThe electropolymerization of tyramine has been investigated using glassy carbon electrodes modified with six classes of carbon materials, namely carbon black, graphitized carbon, graphite, graphene oxide, chemical vapour deposition based multi‐walled carbon nanotubes and arc discharged based multi‐walled carbon nanotubes. These materials were characterized before and after electrodeposition of polytyramine (PT) by Raman spectroscopy, inductively coupled plasma optical emission spectrometry, cyclic voltammetry and amperometry. Previously, other groups have established that impurities present in carbon materials, can play a critical role in electrocatalytic activity. In this study, the presence of graphitic basal plane carbon, rather than metallic impurities, is believed to initiate the formation of a redox active PT film that mediates the oxidation of NADH. The importance of graphitic basal plane carbon was supported by examining the impact of the graphitization of carbon black over the range of 500 to 2000 °C. Graphitic basal plane carbon impurity was demonstrated to be highly active and important with respect to the electrodeposition of a PT film that possesses significant catalytic activity towards the oxidation of NADH.

Biomimetically modified chitosan for electrophoretic deposition of composites

A one-step cathodic electrophoretic deposition (EPD) method has been developed for the deposition of chitosan (CHIT) films modified with catechol. Our approach was based on the use of 3,4-dihydroxybenzaldehyde (DHBA) molecules, which contain a catechol moiety for CHIT modification using a Schiff base reaction. We discussed the deposition mechanism as well as advantages of our one-step EPD method, compared to other techniques. It was found that the EPD method allowed the fabrication of redox-active films. A conceptually new strategy has been developed based on the use of CHIT-DHBA as a reducing, capping, dispersing, charging and film forming agent for the in-situ reduction of Ag+ ions, followed by catecholate type bonding of CHIT-DHBA to Ag particles, their electrosteric dispersion and EPD of composite CHIT-DHBA-Ag films. Building on the processing advantages offered by one-step cathodic deposition and remarkable bonding properties of catechol we performed EPD of CHIT-DHBA-hydroxyapatite (HA) and CHIT-DHBA-TiO2 films. Another major finding was the use of CHIT-DHBA for electrosteric co-dispersion and charging of HA and TiO2, followed by EPD of composite CHIT-DHBA-TiO2-HA films. Comprehensive electrochemical and electron microscopy testing was used to characterize the microstructure of the films, as well as the electrochemical properties. The results of this investigation pave the way for the synthesis and agglomerate-free processing of various functional inorganic materials and fabrication of organic-inorganic composites for biomedical, sensor and water purification applications.

Electrochemical nanoarchitectonics through polyaminobenzylamine-dodecyl phosphate complexes: redox activity and mesoscopic organization in self-assembled nanofilms.

Molecular design and preparation of redox active films displaying mesoscopic levels of organization represents one of the most actively pursued research areas in nanochemistry. These mesostructured materials are not only of great interest at the fundamental level because of their unique properties but they can also be employed for a wide range of applications such as electrocatalysts, electronic devices, and electrochemical energy conversion and storage. Herein, we introduce a simple and straightforward strategy to chemically modify electrode surfaces with self-assembled electroactive polyelectrolyte-surfactant complexes. These assemblies are composed of amino-appended polyaniline and monododecyl phosphate. The complexes were deposited by spin-coating and the films were characterized by spectroscopic and X-ray-based techniques: XRR, GISAXS, WAXS, and XPS. The films presented a well-defined lamellar structure, directed by the strong interaction between the phosphate groups and the positively charged amine groups in the polyelectrolyte. These films also displayed intrinsic electroactivity in both acidic and neutral solutions, showing that the polymer remains electroactive and ionic transport is still possible through the stratified and hydrophobic coatings. The stability and enhanced electroactivity in neutral solutions make these assembled films promising building blocks for the construction of nanostructured electrochemical platforms.

Voltammetric Chloride Sensing Based on Trace-Level Mercury Impregnation Into Amine-Functionalized Carbon Nanoparticle Films

A voltammetric sensor for the determination of chloride ions is proposed. At trace level, Hg(II) ions (ca. 15 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> mol or only approximately 3 Hg <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> cations per carbon nanoparticle) are adsorbed at the surface of amine-functionalized carbon nanoparticles in a film supported on a glassy carbon electrode. With this redox-active film, voltammetric chloride determination is possible without loss of mercury/signal in repeatable measurements and over a very wide chloride concentration range. The sensor mechanism is based on the shift of the voltammetric peak potential for the Hg/Hg <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> redox transformation in the presence of chloride anions. Only trace-level mercury is employed, so that problems associated with traditional bulk mercury electrodes are not encountered. Differential pulse voltammograms were recorded over a wide range of chloride concentrations resulting in two regimes (from 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> to 1 M and from 1 to 3 M) with linear peak potential shift with a chloride concentration of -63.2 ± 0.09 mV/pCl and -109.1 ± 0.4 mV/pCl, respectively (at 25 °C). The sensor performance is promising in real samples, such as lake water, sea water, and table salt.

Electrochemical Synthesis of Films Based on Polybithiophene and Fullerene Derivatives with Potential Use in Bulk Heterojunction Photovoltaic Devices

Two electrochemical methods, cyclic voltammetry and constant potential, were used for the electrodeposition of redox active composite film containing polybithiophene-fullerene (E-(pBTh/C60 derivative)) and fullerene derivatives (PC60BM and IC60BA) on ITO substrates. Preparation was carried out in one step synthesis directly from a solution containing the monomer and the fullerene derivative. The incorporation of the fullerene compound was characterized by UV-Vis and NIR, as well as conductance measurements. The prepared thin films were also electrochemically analyzed, determining that those generated by cyclic voltammetry have slightly greater reversibility: (Qa/Qc > 80% after 50 cycles of charge-discharge). Morphological analysis (AFM) showed that voltammetric deposition generates rougher and thicker films (roughness greater than 100 nm and thickness higher than 200 nm) than constant potential films, whose roughness is in the range from 20 to 60 nm and the thickness is 150 nm. For these reasons, films prepared by constant potential were used for manufacturing the active layer of a simple photovoltaic device Glass/ITO/E-(pBTh:PC60BM)/Field’s metal, which showed charge separation with Voc of 0.28 V and Jsc of 42 μA/cm2 respectively.

Two-Component Polymeric Materials of Fullerenes and the Transition Metal Complexes: A Bridge between Metal-Organic Frameworks and Conducting Polymers.

In this review, we examined the interactions of metal complexes and metal surfaces with fullerenes. That information has been related to the formation of redox-active materials produced by electrochemical reduction of solutions of various transition metal complexes and fullerene or fullerene adducts. These redox-active polymers are strongly bound to electrode surfaces and display electrochemical activity in solutions containing only supporting electrolyte. Extensive studies of the electrochemical behavior of these films have been used to characterize their properties and structure. The process that produces these poly-Pd(n)C60 and poly-Pt(n)C60 films can also produce composite materials that consist of metal nanoparticles interspersed with the poly-Pd(n)C60 and poly-Pt(n)C60 materials. The relationship between these redox-active films and conducting metal organic framework materials has been examined. These insoluble, redox-active polymers have potential utility for the adsorption of various gases, for the construction of capacitors, for sensing, for the preparation of metal-containing heterofullerenes, and for catalysis.