Electroactive Surfaces Research Articles

Photocorrosion of ZnO Single Crystals during Electrochemical Water Splitting.

Degradation and dissolution of transparent semiconducting oxides is central to various areas, including design of catalysts and catalysis conditions, as well as passivation of metal surfaces. In particular, photocorrosion can be significant and plays a central role during photoelectrochemical activity of transparent semiconducting oxides. Here, we utilize an electrochemical flow cell combined with an inductively coupled plasma mass spectrometer (ICP-MS) to enable the in situ study of the time-resolved release of zinc into solution under simultaneous radiation of UV-light. With this system we study the dissolution of zinc oxide single crystals with (0001) and (101̅0) orientations. At acidic and alkaline pH, we characterized potential dependent dissolution rates into both the oxygen and the hydrogen evolving conditions. A significant influence of the UV radiation and the pH of the electrolyte was observed. The observed dissolution behavior agrees well with the surface chemistry and stabilization mechanism of ZnO surfaces. In particular, polar ZnO(0001) shows ideal stability at low potentials and under hydrogen evolution conditions. Whereas ZnO(101̅0) sustains higher dissolution rates, while it is inactive for water splitting. Our data demonstrates that surface design and fundamental understanding of surface chemistry provides an effective path to rendering electroactive surfaces stable under operating conditions.

Photovoltage generation in enzymatic bio-hybrid architectures

Most of the photochemical activity of bacterial photosynthetic apparatuses occurs in the reaction center, a transmembrane protein complex which converts photons into chargeseparated states across the membrane with a quantum yield close to unity, fuelling the metabolism of the organism. Integrating the reaction center from the bacterium Rhodobacter sphaeroides onto electroactive surfaces, it is possible to technologically exploit the efficiency of this natural machinery to generate a photovoltage upon Near Infra-Red illumination, which can be used in electronic architectures working in the electrolytic environment such as electrolyte-gated organic transistors and bio-photonic power cells. Here, photovoltage generation in reaction center-based bio-hybrid architectures is investigated by means of chronopotentiometry, isolating the contribution of the functionalisation layers and defining novel surface functionalization strategies for photovoltage tuning.

Enhancing the sensitivity of point-of-use electrochemical microfluidic sensors by ion concentration polarisation – A case study on arsenic

Point of use (POU) sensors are extremely relevant, being capable of providing fast and reliable analysis in remote and resource-limited settings. Of all the diverse techniques utilised for POU sensors, a combination of electrochemistry and microfluidics may have the greatest potential towards quantitative assessment of heavy metal ions. The major challenge in combining these for sensing applications lies in the complexity of fabricating integrated devices and the insufficient quantity of analytes in the sample volume. To address these issues, we have developed a radial microfluidic device capable of electrokinetic preconcentration by ion concentration polarization (ICP) and integrated it with electroactive surfaces. The proposed sensor is the first demonstration of concentration of heavy metal ions by ICP and its quantitative assessment by voltammetry. Utilising the integrated sensor, we have shown the sensing of As3+ down to 1 ppb by linear sweep voltammetry with ∼40 μL of sample. The sensor was also tested successfully for sensing As3+ in a field sample from an arsenic affected region of India. The sensor was also tested for the detection of other metal ions too. This work would facilitate the development of highly sensitive POU hand-held sensors for water quality monitoring in resource-limited areas.

Laser-induced graphene and carbon nanotubes as conductive carbon-based materials in environmental technology

Abstract Nanotechnology and nanomaterials have attracted interest due to their potential in mitigating contemporary environmental challenges, such as the stressors imposed by increased industrial and agricultural activities, and the deterioration of air, soil and water quality. In particular, advanced technologies that harness carbon-based nanomaterials are poised to emerge as tools that provide new solutions for the global water crises. These tools include, electrically conductive membrane processes, which uniquely combine a separation process with a functional surface. In this respect, laser-induced graphene (LIG) and carbon nanotubes (CNTs) are electrically conductive carbon nanomaterials that hold great utility in a multitude of environmental applications, including the development of fouling-resistant systems for desalination and water treatment, enhanced separation methods, and innovative pollutant sensing and electrocatalytic platforms. Consequently, this review article describes and compares some important recent advances in LIG- and CNT-based electroactive surfaces. The discussion of LIG as an emerging carbon material set in context with CNTs is intended to shed light on future directions and development possibilities to meet the growing global challenges in terms of water treatment applications of both materials as well as other electrically conductive carbon-based nanomaterials exhibiting exceptional performance and versatility.

Stokes-layer formation under absence of moving parts—A novel oscillatory plasma actuator design for turbulent drag reduction

A novel plasma actuator concept is proposed to mimic the effect of spanwise wall oscillations without mechanically moving parts, where four groups of electrodes and three independently operated high-voltage power supplies maintain a pulsatile dielectric barrier discharge (DBD) array. Time-resolved planar velocity fields are obtained with high-speed particle image velocimetry (PIV) in proximity of the discharge zones for quiescent ambient conditions. Resulting flow topologies and wall-normal velocity profiles indicate the Stokes-layer-like flow formation, which is elevated above the wall due to the no-slip condition. The underlying body forces are derived from the PIV data to provide further insight into cause-effect relations between pulsatile discharge and oscillatory flow. The momentum transfer domain is found to be only interrupted with the width of the exposed electrode, which is an important step toward homogeneous virtual wall oscillations. A comparison with earlier studies by Gatti et al. [“Experimental assessment of spanwise-oscillating dielectric electroactive surfaces for turbulent drag reduction in an air channel flow,” Exp. Fluids 56, 110 (2015)] leads to the hypothesis that DBD-based turbulent drag reduction might be a competing alternative to conventional active and passive shear-layer formation strategies, where the adjustability of both oscillation frequency and velocity amplitude might cover a wide range of Reynolds numbers.

Electroactive polyglycine coatings for nanobiosensing applications: Label-free DNA hybridization, DNA-Antitumor agent interaction and antitumor agent determination

Bioinspired materials have attracted great attention due to their great functionality, bioactivity and biocompability. In particular, electroactive polyamino acid surfaces endow preparation of robust coatings for various applications. In this research article, preparation of carbon nanotubes doped polyglycine coated electrodes and their applications in the biomedical field such as DNA hybridization, DNA-antitumor agent interaction and antitumor agent determination were described. This biosensing platform was created using a simple, reproducible and fast in situ and one-pot electropolymerization procedure onto graphite surfaces by cyclic voltammetry. The coated electrodes were characterized with cyclic voltammetry and electrochemical impedance spectroscopy (EIS). After the characterization studies, bioapplications of the proposed electrode were demonstrated. The new electrode led to significant improvement for the investigation of the electrochemical behavior of double-stranded DNA (dsDNA). 6-fold and 5-fold improvements were obtained for oxidation of the electroactive DNA bases, guanine (G) and adenine (A), respectively over the bare electrode. For these steps, each electrode was characterized with scanning electron microscopy (SEM) and energy dispersive X-ray (EDX). Then, DNA hybridization studies were performed in the light of these results. The proposed electrode allowed the quantification of specific target DNA down to 11.2 fM in serum samples (n = 3). In addition, it constituted a sensitive biosensing platform for electrochemical monitoring of the interaction between dsDNA and a commonly used antitumor agent, Mitomycin C. Mitomycin C determination was also carried out using the inhibition effect occurred at the guanine oxidation signal. The detection limit of this antitumor agent was found as 1.78 mg L−1 in untreated serum samples (n = 3).

Coupling orientation and mediation strategies for efficient electron transfer in hybrid biofuel cells

Enzymes are promising electrocatalysts for electron transfer (ET) in many biological processes. Strategies to enhance ET between enzymes and electroactive surfaces include orientation and immobilization of the enzymes and electron mediation. Here, we develop a strategy to couple orientation and electron mediation on electrodes based on carbon nanotubes. This is achieved by the synthesis of a redox mediator that contains an enzyme-orientation site (pyrene), an electron-carrier redox mediator (2,2′-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS)) and an electropolymerizable monomer (pyrrole). The coupling of an enzymatic orientation and a mediated ET in the same chemical structure (pyrrole–ABTS–pyrene (pyrr–ABTS–pyr)) provides a much-improved performance in the bioelectrocatalysis. We demonstrate two fuel cells for the synthesized redox mediator. In a proton-exchange membrane hydrogen/air fuel cell and in a membraneless fuel cell, the pyrr–ABTS–pyr biocathode provides a power density of 1.07 mW cm−2 and 7.9 mW cm−2, respectively. The principle of coupling an enzyme orientation and a redox mediator allows a great variety of mediators to be engineered and provides vast possibilities for the development of fuel cells.

Gold nanoparticles as markers for fluorinated surfaces containing embedded amide groups

Indium tin oxide (ITO) substrates were functionalized with fluoroalkylsilanes (FAS) having formula RFC(O)N(R)(CH2)3Si(OMe)3 (1, R = H, RF = C5F11; 2, R = CH3, RF = C5F11;3, R = H, RF = C3F7) and containing embedded amide moieties between the perfluoroalkyl chain and the syloxanic moiety. Subsequently, Au nanoparticle deposition (AuNP) onto the ITO-FAS functionalized surfaces was carried out by immersion into a solution of citrate-stabilized AuNP. The ITO-FAS and AuNP/ITO-FAS modified systems were characterized by various complementary techniques and compared with AuNP/ITO modified with RF(CH2)2Si(OEt)3 (4, RF = C6F13), free from functional groups between the fluorinated tail and the syloxanic moiety. The results showed that only ITO glasses modified with 1, 2 and 3 displayed an oleophobic, as well as hydrophobic, behaviour and that the AuNP Surface Coverage (SC %) directly depended on the fluoroalkylsilane nature with the following trend: 60% ITO-2 > 16% ITO-3 > 9% ITO-1 > 3% ITO-4. The obtained results revealed that, in organosilane 2, the presence of a methyl group on the amide nitrogen increases the steric hindrance in the rotation around the NCO bond, resulting in the co-presence of two stable conformers in comparable amounts. Their co-presence in solution, combined with the lack of intermolecular NH⋯OCN hydrogen bonds among the anchored molecules, has dramatic influences on the functionalized ITO, yielding a disorderedly packed coating able to accommodate a large quantity of AuNP. These results indicate that AuNP can act as excellent probes to evaluate the coating layer quality but, at the same time, it is possible to tune the gold loading on electroactive surfaces depending on the chemical structure of the used fluorinated silane.

Surface Electrolyte Interphase Control on Magnetite, Fe3O4, Electrodes: Impact on Electrochemistry

ABSTRACTIn battery systems, a solid electrolyte interphase (SEI) is formed through electrolyte reaction on an electrode surface. The formation of SEI can have both positive and negative effects on electrochemistry. The initial formation of the layer protects the electrode from further reactivity, which can improve both shelf and cycle life. However, if the layer continues to form, it can impede charge transfer, which increases cell resistance and limits cycle life. The role of SEI is particularly important when studying conversion electrodes, since phase transformations which unveil new electroactive surfaces during reduction/oxidation can facilitate electrolyte decomposition. This manuscript highlights recent developments in the understanding and control of SEI formation for magnetite (Fe3O4) conversion electrodes through electrolyte and electrode modification.

Uncovering the real active sites of ruthenium oxide for the carbon monoxide electro-oxidation reaction on platinum: The catalyst acts as a co-catalyst

The poisoning of platinum electroactive surfaces by the adsorption of CO is a critical hindrance for the development of fuel cells. Combining Pt with (hydrous) ruthenium oxide has been shown to enhance the CO electro-oxidation reaction on the electrocatalyst. However, how the co-catalyst surface provides oxygenated species to remove CO adsorbed on platinum is still an undiscussed matter. Here we show that an indispensable intermediate form of RuO2, catalyzed by Pt, is responsible for the removal of CO on Pt at low potential. In order to unravel the synergetic effect of ruthenium oxide and platinum towards CO electro-oxidation, ruthenium oxide nanosheets (RuO2(ns)) with monoatomic thickness were deposited onto the surface of polycrystalline Pt, glassy carbon and polycrystalline Au rotating disk electrodes. Combined with the well-defined electrochemical behavior of RuO2(ns), these model electrodes enabled to investigate both (i) the synergetic effect of ruthenium oxide towards the electro-oxidation of CO on Pt and (ii) the influence of the metal substrate (Pt or Au) towards the electrochemical behavior of RuO2. When deposited on the smooth Pt surface, RuO2(ns) were found to fully interact as a co-catalyst for the electro-oxidation of (monolayer adsorbed and bulk) CO on Pt, without blocking the electroactive surface of Pt. The addition of RuO2(ns) lead to two distinct synergetic effects for the bulk electro-oxidation of CO on Pt. Upgrading the general understanding, an interdependency between RuO2 and Pt is observed for the ‘low potential’ electro-oxidation of bulk CO (onset at 0.65V vs. RHE).

Switchable presentation of cytokines on electroactive polypyrrole surfaces for hematopoietic stem and progenitor cells

Hematopoietic stem cells are used in transplantations for patients with hematologic malignancies. Scarce sources require efficient strategies of expansion, including polymeric biomaterials mimicking architectures of bone marrow tissue. Tissue microenvironment and mode of cytokine presentation strongly influence cell fate. Although several cytokines with different functions as soluble or membrane-bound mediators have already been identified, their precise roles have not yet been clarified. A need exists for in vitro systems that mimic the in vivo situation to enable such studies. One way is to establish surfaces mimicking physiological presentation using protein-immobilization onto polymer films. However these films merely provide a static presentation of the immobilized proteins. It would be advantageous to also dynamically change protein presentation and functionality to better reflect the in vivo conditions. The electroactive polymer polypyrrole shows excellent biocompatibility and electrochemically alters its surface properties, becoming an interesting choice for such setups. Here, we present an in vitro system for switchable presentation of membrane-bound cytokines. We use interleukin IL-3, known to affect hematopoiesis, and show that when immobilized on polypyrrole films, IL-3 is bioavailable for the bone marrow-derived FDC-P1 progenitor cell line. Moreover, IL-3 presentation can be successfully altered by changing the redox state of the film, in turn influencing FDC-P1 cell viability. This novel in vitro system provides a valuable tool for stimuli-responsive switchable protein presentation allowing the dissection of relevant mediators in stem and progenitor cell behavior.

Tailoring electroactive surfaces by non-template molecular assembly. Towards electrooxidation of L-cysteine

We have prepared a nanoelectrode ensemble containing vertically aligned single walled carbon nanotubes (SWCNTs) using a non-template molecular self-assembling strategy. We used a bottom-up construction approach to assemble amino functionalized SWCNTs (af-SWCNTs) in a well-defined architecture. These af-SWCNTs were linked and vertically aligned to pre-formed self-assembled monolayers of 4-MBA. A Cobalt(II) tetracarboxyphthalocyanine (Co(COOH)4Pc) complex was covalently bonded to external portion of af-SWCNTs to complete the final nanoelectrode ensemble. X-ray photoelectron spectroscopy (XPS) and Atomic Force Microcopy (AFM) confirmed the effectiveness of the assembling steps on the gold surface starting from the Au/MBA SAMs. The system Au/4-MBA/af-SWCNTs shows an interface with large ordered array, which exhibits a high activity for the electrooxidation of L-cysteine (L-cys). Theoretical calculations suggest that the incorporation of the af-SWCNTs increased the activity of the assembly to electronic transfer and it was observed that the electrooxidation reaction is energetically favorable.

Dopant macroinitiator for electropolymerisation and functionalisation of conducting polymer thin films

Grafting of polymer brushes from conducting polymer (CP) thin films by controlled radical polymerisation provides a versatile route for the synthesis of functional, electroactive surfaces, with applications in diverse fields. However, drawbacks of this approach include difficulty in upscaling synthesis due to the need for specialised CP precursor monomers functionalised with initiation sites. We herein describe an alternative approach to the synthesis of CP-based polymer brushes whereby ATRP initiation sites are attached to a macrodopant incorporated into CP films during electropolymerisation. The facile electropolymerisation of commonly studied CPs (PEDOT, PPy, and PTTh) with an initiator-functionalised macrodopant - poly(styrene sulfonate-co-(2-bromopropionyloxy ethyl methacrylate)) (PeesBro) – is demonstrated. The composite polymer films thus synthesised were used as substrates for grafting of hydrophilic polymer brushes. Although PSS is commonly used as a macrodopant in CP films, its initiator-functionalised derivatives have not previously been utilised in this manner. Despite the elegance of this approach, the authors’ knowledge, there have been no previous examples reported of utilising macromolecular dopants as initiators for subsequent grafting of polymer brushes.

Redox-active conducting polymers modulate Salmonella biofilm formation by controlling availability of electron acceptors

Biofouling is a major problem caused by bacteria colonizing abiotic surfaces, such as medical devices. Biofilms are formed as the bacterial metabolism adapts to an attached growth state. We studied whether bacterial metabolism, hence biofilm formation, can be modulated in electrochemically active surfaces using the conducting conjugated polymer poly(3,4-ethylenedioxythiophene) (PEDOT). We fabricated composites of PEDOT doped with either heparin, dodecyl benzene sulfonate or chloride, and identified the fabrication parameters so that the electrochemical redox state is the main distinct factor influencing biofilm growth. PEDOT surfaces fitted into a custom-designed culturing device allowed for redox switching in Salmonella cultures, leading to oxidized or reduced electrodes. Similarly large biofilm growth was found on the oxidized anodes and on conventional polyester. In contrast, biofilm was significantly decreased (52–58%) on the reduced cathodes. Quantification of electrochromism in unswitched conducting polymer surfaces revealed a bacteria-driven electrochemical reduction of PEDOT. As a result, unswitched PEDOT acquired an analogous electrochemical state to the externally reduced cathode, explaining the similarly decreased biofilm growth on reduced cathodes and unswitched surfaces. Collectively, our findings reveal two opposing effects affecting biofilm formation. While the oxidized PEDOT anode constitutes a renewable electron sink that promotes biofilm growth, reduction of PEDOT by a power source or by bacteria largely suppresses biofilm formation. Modulating bacterial metabolism using the redox state of electroactive surfaces constitutes an unexplored method with applications spanning from antifouling coatings and microbial fuel cells to the study of the role of bacterial respiration during infection.

Characterization of Hydroporphyrins Covalently Attached to Si(100)

Attachment of synthetic analogs of natural tetrapyrroles to electroactive surfaces enables physicochemical interrogation and may provide material for use in catalysis, diagnostics, and energy conversion. Six synthetic zinc chlorins and one free base bacteriochlorin, tailored analogs of chlorophyll and bacteriochlorophyll, respectively, have been attached to Si(100) via a high-temperature (400°C) baking method. The hydroporphyrins bear diverse functional groups that enable surface attachment (vinyl, acetyl, triisopropylsilylethynyl, pentafluorophenyl, and hydroxymethylphenyl) and a geminal dimethyl group in each reduced ring for stabilization toward adventitious dehydrogenation. The films were examined by cyclic voltammetry, FTIR spectroscopy, X-ray photoelectron spectroscopy, and ellipsometry. Monofunctionalized and difunctionalized hydroporphyrins gave monolayer and multilayer films, respectively, indicating robustness of the hydroporphyrin molecules, but in each case the film was more heterogeneous than observed with comparable porphyrins. The data suggest that some amount of unattached molecules remain intercalated with surface-attached molecules. Additional molecular designs will need to be examined to develop a deep understanding of the structure-activity relationship for formation of homogeneous monolayers and multilayers of synthetic hydroporphyrins.

Electrochemical detection of nitrite and ascorbic acid at glassy carbon electrodes modified with carbon nano-onions bearing electroactive moieties

In this work, we evaluate two novel surfaces for the simultaneous detection of nitrite and ascorbic acid in aqueous media. Glassy carbon electrodes were first modified with small carbon nano-onions (GCE/CNO) and further functionalized either by electrografting ortho-aminophenol (oAP) groups or by physical adsorption of thionine to give electroactive surfaces that were characterized by cyclic voltammetry. The modified surfaces exhibited a fast electrochemical response and were able to detect the presence of nitrite and ascorbic acid by amperometry. The presence of both CNOs and the electroactive moiety on the electrode surface enhanced the sensitivity and lowered the limit of detection of the determinations as compared to GCE modified only with oAP or thionine. The CNO-modified electrodes showed good reproducibility and stability with time and were applied in the detection of ascorbic acid in orange juice.

Probing Ionic Liquid/Electrode Interfaces by Hyperspectral Imaging

Understanding the interface between ionic liquids (ILs) and electroactive surfaces is of great importance for a wide range of applications including batteries, capacitors, and sensors. In this work, we discuss analyses and characterization of the structure and dynamics of the electric double layer (EDL) by surface-enhanced infrared absorption spectroscopy (SEIRA). In particular, we are interested in developing a high throughput imaging version of the SEIRA consisting of a quantum cascade laser coupled to a hyperspectral imaging camera detector. Rather than generating single spectra at a time for single experiments, hyperspectral imaging techniques can be utilized to both increase signal to noise for capturing rapid dynamics while multiplexing experiments simultaneously. We are particularly interested in IL systems which exhibit hysteresis at interfaces. That is, systems for which interfacial structure is dependent on past events. Here, we discuss several cases studies and some preliminary findings.

Preparation of Co‐cultured Cell Sheets Using Electroactive Surfaces

Preparation of Co‐cultured Cell Sheets Using Electroactive Surfaces

Sonoelectrodeposition of poly(pyrrole) films: Electrochemical and morphological effects caused by the ultrasonic amplitude

In this manuscript we present the interfacial characterization of poly(pyrrole) films doped with dodecylbenzenesulphonate anions (DBS) electrosynthesized under the influence of ultrasonic radiation. The modified electrodes were studied by means of scanning electron microscopy and electrochemical experiments. It was shown that the ultrasonic radiation changes the morphology of the deposits producing smoother films, with distinct voltammetric behavior. A detailed study employing the electrochemical impedance spectroscopy was done where important features such as the charge-transfer resistance, intercalated charge distribution and double-layer capacitance were discussed. The results showed herein are of great importance on the development and characterization of electroactive surfaces where modified electrodes can be easily obtained by using the ultrasonic radiation and the studies of electrochemical based devices such as sensors, biosensors and supercapacitors can be boosted.

A Comparative Study on Surface Treatments in the Immobilization Improvement of Hexahistidine-tagged Protein on the Indium Tin Oxide Surface

Biosensors are becoming increasingly important in various sectors. However, choosing transducer type and surface treatment still faces severe limitations. Indium-tin oxide is widely used in electrochemical sensing, though surface cleaning and functionalization are not always straightforward and often poorly controlled. Here, we have covalently immobilized a hexahistidine-tagged model protein on three different treated ITO surfaces using a Ni2+ chelator moiety. The study shows that compared to two other treatments (H2O/H2O2/NH4OH and KOH treatments), the ITO Piranha treatment gives satisfactory results in term of yields of EC12 protein immobilized on the surface. The study in detail of treatments effect was realized by applying a combination of local and global techniques such X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), Attenuated total reflectance Fourier transform infrared spectroscopy (FTIR-ATR), contact angle measurements, and electrochemical impedance spectroscopy (EIS). The biofunctionalization allowed proteins to move freely around their anchoring bond. As the polyhistidine tags are widely used in protein engineering, such substrates offer a large panel of applications. This covalent and oriented immobilization process of recombinant proteins applied to ITO could provide transparent electroactive surfaces of high quality for electrochemical detection of soluble compounds, as well as cells.