Electroassisted Incorporation of Ferrocene within Sol-Gel Silica Films to Enhance Electron Transfer.

The sol-gel method is a straightforward technique that allows electrode modification with silica thin films. Furthermore, the silica pores could be functionalized to enhance the electrical conductivity and reactivity of the silica films. In this context, silica thin films were functionalized with ferrocene species. This functionalization was performed by electroassisted accumulation, generating a micro-structured composite electrode (Fc@SiO2 electrode). These modified electrodes were characterized by electrochemical and spectroelectrochemical methods, pointing out that ferrocene species were confined with high stability within the microporous silica thin film, demonstrating the good adsorption capacity of the silica. While the spectroelectrochemical characterization indicates that only a fraction of the confined species within the silica films were electroactive, the electrochemical results demonstrate that the Fc@SiO2 film enhances the electrochemical response of cytochrome c in a solution, which gives rise to further applications of these films for redox-controlled release and electrochemical detection of other redox-active proteins.

Potential-Modulated Ion Distributions in the Back-to-Back Electrical Double Layers at a Polarised Liquid|Liquid Interface Regulate the Kinetics of Interfacial Electron Transfer.

Biphasic interfacial electron transfer (IET) reactions at polarisable liquid|liquid (L|L) interfaces underpin new approaches to electrosynthesis, redox electrocatalysis, bioelectrochemistry and artificial photosynthesis. Herein, using cyclic and alternating current voltammetry, we demonstrate that under certain experimental conditions, the biphasic 2-electron O2 reduction reaction can proceed by single-step IET between a reductant in the organic phase, decamethylferrocene, and interfacial protons in the presence of O2. Using this biphasic system, we demonstrate that the applied interfacial Galvani potential difference Δ o w φ provides no direct driving force to realise a thermodynamically uphill biphasic IET reaction in the mixed solvent region. We show that the onset potential for a biphasic single-step IET reaction does not correlate with the thermodynamically predicted standard Galvani IET potential and is instead closely correlated with the potential of zero charge at a polarised L|L interface. We outline that the applied Δ o w φ required to modulate the interfacial ion distributions, and thus kinetics of IET, must be optimised to ensure that the aqueous and organic redox species are present in substantial concentrations at the L|L interface simultaneously in order to react.

Electrosynthesis of Biocompatible Free-Standing PEDOT Thin Films at a Polarized Liquid|Liquid Interface.

Conducting polymers (CPs) find applications in energy conversion and storage, sensors, and biomedical technologies once processed into thin films. Hydrophobic CPs, like poly(3,4-ethylenedioxythiophene) (PEDOT), typically require surfactant additives, such as poly(styrenesulfonate) (PSS), to aid their aqueous processability as thin films. However, excess PSS diminishes CP electrochemical performance, biocompatibility, and device stability. Here, we report the electrosynthesis of PEDOT thin films at a polarized liquid|liquid interface, a method nonreliant on conductive solid substrates that produces free-standing, additive-free, biocompatible, easily transferrable, and scalable 2D PEDOT thin films of any shape or size in a single step at ambient conditions. Electrochemical control of thin film nucleation and growth at the polarized liquid|liquid interface allows control over the morphology, transitioning from 2D (flat on both sides with a thickness of <50 nm) to "Janus" 3D (with flat and rough sides, each showing distinct physical properties, and a thickness of >850 nm) films. The PEDOT thin films were p-doped (approaching the theoretical limit), showed high π-π conjugation, were processed directly as thin films without insulating PSS and were thus highly conductive without post-processing. This work demonstrates that interfacial electrosynthesis directly produces PEDOT thin films with distinctive molecular architectures inaccessible in bulk solution or at solid electrode-electrolyte interfaces and emergent properties that facilitate technological advances. In this regard, we demonstrate the PEDOT thin film's superior biocompatibility as scaffolds for cellular growth, opening immediate applications in organic electrochemical transistor (OECT) devices for monitoring cell behavior over extended time periods, bioscaffolds, and medical devices, without needing physiologically unstable and poorly biocompatible PSS.

On the origin of chaotrope-modulated electrocatalytic activity of cytochrome c at electrified aqueous|organic interfaces.

Electrochemical, spectroscopic and computational methods are used to demonstrate that electrified aqueous|organic interfaces are a suitable bio-mimetic platform to study and contrast the accelerated electrocatalytic activity of cytochrome c towards the production of reactive oxygen species (ROS) in the presence of denaturing agents such as guanidinium chloride and urea.

Modulating the pro-apoptotic activity of cytochrome c at a biomimetic electrified interface.

Programmed cell death via apoptosis is a natural defence against excessive cell division, crucial for fetal development to maintenance of homeostasis and elimination of precancerous and senescent cells. Here, we demonstrate an electrified liquid biointerface that replicates the molecular machinery of the inner mitochondrial membrane at the onset of apoptosis. By mimicking in vivo cytochrome c (Cyt c) interactions with cell membranes, our platform allows us to modulate the conformational plasticity of the protein by simply varying the electrochemical environment at an aqueous-organic interface. We observe interfacial electron transfer between an organic electron donor decamethylferrocene and O2, electrocatalyzed by Cyt c. This interfacial reaction requires partial Cyt c unfolding, mimicking Cyt c in vivo peroxidase activity. As proof of concept, we use our electrified liquid biointerface to identify drug molecules, such as bifonazole, that can potentially down-regulate Cyt c and protect against uncontrolled neuronal cell death in neurodegenerative disorders.

A soft on/off switch based on the electrochemically reversible H-J interconversion of a floating porphyrin membrane.

Soft molecular assemblies that respond reversibly to external stimuli are attractive materials as on/off switches, in optoelectronic, memory and sensor technologies. In this Edge Article, we present the reversible structural rearrangement of a soft porphyrin membrane under an electrical potential stimulus in the absence of solid-state architectures. The free-floating porphyrin membrane lies at the interface between immiscible aqueous and organic electrolyte solutions and is formed through interfacial self-assembly of zinc(ii) meso-tetrakis(4-carboxyphenyl)porphyrins (ZnPor). A potential difference between the two immiscible electrolyte solutions induces the intercalation of bis(triphenylphosphoranylidene)ammonium cations from the organic electrolyte that exchange with protons in the porphyrin membrane. In situ UV/vis absorbance spectroscopy shows that this ionic intercalation and exchange induces a structural interconversion of the individual porphyrin molecules in the membrane from an H- to a J-type molecular configuration. These structural rearrangements are reversible over 30 potential cycles. In situ polarisation-modulation fluorescence spectroscopy further provides clear evidence of structural interconversion of the porphyrin membrane, as intercalation of the organic electrolyte cations significantly affects the latter's emissive properties. By adjusting the pH of the aqueous phase, additional control of the electrochemically reversible structural interconversion can be achieved, with total suppression at pH 3.

Electrogeneration of a Free-Standing Cytochrome c-Silica Matrix at a Soft Electrified Interface.

Interactions of a protein with a solid-liquid or a liquid-liquid interface may destabilize its conformation and hence result in a loss of biological activity. We propose here a method for the immobilization of proteins at an electrified liquid-liquid interface. Cytochrome c (Cyt c) is encapsulated in a silica matrix through an electrochemical process at an electrified liquid-liquid interface. Silica condensation is triggered by the interfacial transfer of cationic surfactant, cetyltrimethylammonium, at the lower end of the interfacial potential window. Cyt c is then adsorbed on the previously electrodeposited silica layer, when the interfacial potential, Δowϕ, is at the positive end of the potential window. By cycling of the potential window back and forth, silica electrodeposition and Cyt c adsorption occur sequentially as demonstrated by in situ UV-vis absorbance spectroscopy. After collection from the liquid-liquid interface, the Cyt c-silica matrix is characterized ex situ by UV-vis diffuse reflectance spectroscopy, confocal Raman microscopy, and fluorescence microscopy, showing that the protein maintained its tertiary structure during the encapsulation process. The absence of denaturation is further confirmed in situ by the absence of electrocatalytic activity toward O2 (observed in the case of Cyt c denaturation). This method of protein encapsulation may be used for other proteins (e.g., Fe-S cluster oxidoreductases, copper-containing reductases, pyrroloquinoline quinone-containing enzymes, or flavoproteins) in the development of biphasic bioelectrosynthesis or bioelectrocatalysis applications.

Signal amplification by electro-oligomerisation for improved isoproturon detection.

A novel concept is introduced for signal amplification in electrochemical sensing: the electro-oligomerisation stripping voltammetry, which has been applied here to the improved detection of the isoproturon herbicide in spring waters as a proof-of-principle. It involves a potentiostatic accumulation step onto a glassy carbon electrode (at +1.5 V vs Ag/AgCl reference electrode for 300 s) leading to the formation of an oligomeric film, which is then detected by cathodic stripping square wave voltammetry (SWV). The presence and composition of the film are confirmed by confocal Raman spectroscopy. Its characterisation by cyclic voltammetry demonstrates the reversible nature of the electrodeposited material, confirming its interest for sensitive detection by SWV. Adding a mesoporous silica membrane with vertically oriented nanochannels further enhances the sensitivity of the sensor, exhibiting a linear response in the 10-100 µM concentration range. This effect was even more interesting for real media analysis thanks to the permselective properties of such nanoporous coating in rejecting interferences and/or surface fouling agents. The method should be applicable to other analytes that are usually not detectable by conventional accumulation/stripping voltammetry.

Closed bipolar electrochemistry in a four-electrode configuration.

Closed bipolar electrochemistry in a 4-electrode configuration is a highly versatile, but under-utilized, technique with major potential to emerge as a powerful methodology impacting areas as diverse as spectro-electroanalysis, energy storage, electrocatalysis and electrodeposition. In this perspective, we provide the thermodynamic framework for understanding all such future applications of closed bipolar electrochemistry in a 4-electrode configuration. We distinguish the differences between open and closed bipolar electrochemical cells. In particular, the use of the 4-electrode configuration in both open and closed bipolar electrochemical cells with immiscible aqueous-organic solutions is outlined. A comprehensive overview of the influence of external bias on the thermodynamics underpinning electron transfer from an organic redox couple to an aqueous redox couple, or vice versa, by electrons flowing along a conducting bipolar electrode serving as an electronic bridge is provided. Fermi level equilibration between redox species at opposite poles of a bipolar electrode under external bias is discussed. The concept of the Line of Zero Overpotential (LZO) on the bipolar electrode at steady-state conditions under an external bias is introduced. The influence of a series of experimental variables (redox potential of each redox couple, rate constant of electron transfer at each pole, an excess bulk concentration of one redox couple over the other, and areas of the poles of the bipolar electrode in contact with each electrolyte solution) on the final position of the LZO on the bipolar electrode is highlighted. A cyclic voltammogram obtained using a closed bipolar electrochemical cell in a 4-electrode configuration with immiscible aqueous-organic electrolyte solutions is explained using the thermodynamic theory detailed throughout the perspective. The theory presented herein is equally applicable to a closed bipolar electrochemical cell in a 4-electrode configuration with aqueous electrolyte solutions, each containing redox active species, in both compartments connected by a bipolar electrode.

Investigation of modified nanopore arrays using FIB/SEM tomography.

The investigation of electrochemical processes at the interface of two immiscible electrolyte solutions (ITIES) is of great interest for sensing applications, and serves as a surrogate to the study of biological transport phenomena, e.g. ion channels. Alongside e-beam lithography, focused ion beam (FIB) milling is an attractive method to prototype and fabricate nanopore arrays that support nanoITIES. Within this contribution, we explore the capability of FIB/scanning electron microscopy (SEM) tomography to visualize the actual pore structure and interfaces at silica-modified nanoporous membranes. The nanopores were also characterized by atomic force microscopy (AFM) using ultra-sharp AFM probes to determine the pore diameter, and using scanning transmission electron microscopy (STEM) and energy dispersive X-ray (EDX) spectroscopy, providing additional information on the elemental composition of deposits within the pores. Si-rich particles could be identified within the pores as well as at the orifice that had faced the organic electrolyte solution during electrochemical deposition. The prospects of the used techniques for investigating the interface at or within FIB-milled nanopores will be discussed.

Vertically Aligned and Ordered One-Dimensional Mesoscale Polyaniline.

The growth of vertically aligned and ordered polyaniline nanofilaments is controlled by potentiostatic polymerization through hexagonally packed and oriented mesoporous silica films. In such small pore template (2 nm in diameter), quasi-single PANI chains are likely to be produced. From chronoamperometric experiments and using films of various thicknesses (100-200 nm) it is possible to evidence the electropolymerization transients, wherein each stage of polymerization (induction period, growth, and overgrowth of polyaniline on mesoporous silica films) is clearly identified. The advantageous effect of mesostructured silica thin films as hard templates for the generation of isolated polyaniline nanofilaments is demonstrated from enhancement of the reversibility between the conductive and the nonconductive states of polyaniline and the higher electroactive surface areas displayed for all mesoporous silica/PANI composites. The possibility to control and tailor the growth of conducting polymer nanofilaments offers numerous opportunities for applications in various fields including energy, sensors and biosensors, photovoltaics, nanophotonics, or nanoelectronics.

Visualization of Diffusion within Nanoarrays.

The direct experimental characterization of diffusion processes at nanoscale remains a challenge that could help elucidate processes in biology, medicine and technology. In this report, two experimental approaches were employed to visualize ion diffusion profiles at the orifices of nanopores (radius (ra) of 86 ± 6 nm) in array format: (1) electrochemically assisted formation of silica deposits based on surfactant ion transfer across nanointerfaces between two immiscible electrolyte solutions (nanoITIES); (2) combined atomic force - scanning electrochemical microscopy (AFM-SECM) imaging of topography and redox species diffusion through the nanopores. The nature of the diffusion zones formed around the pores is directly related to the interpore distance within the array. Nanopore arrays with different ratios of pore center-to-center separation (rc) to pore radius (ra) were fabricated by focused ion beam (FIB) milling of silicon nitride (SiN) membranes, with 100 pores in a hexagonal arrangement. The ion diffusion profiles determined by the two visualization methods indicated the formation of overlapped or independent diffusion profiles at nanopore arrays with rc/ra ratios of 21 ± 2 and 91 ± 7, respectively. In particular, the silica deposition method resulted in formation of a single deposit encompassing the complete array with closer nanopore arrangement, whereas individual silica deposits were formed around each nanopore within the more widely spaced array. The methods reveal direct experimental evidence of diffusion zones at nanopore arrays and provide practical illustration that the pore-pore separation within such arrays has a significant impact on diffusional transport as the pore size is reduced to the nanoscale. These approaches to nanoscale diffusion zone visualization open up possibilities for better understanding of molecular transport processes within miniaturized systems.

Modulation of the silica sol-gel composition for the promotion of direct electron transfer to encapsulated cytochrome c.

The direct electron transfer between indium-tin oxide electrodes (ITO) and cytochrome c encapsulated in different sol-gel silica networks was studied. Cyt c@silica modified electrodes were synthesized by a two-step encapsulation method mixing a phosphate buffer solution with dissolved cytochrome c and a silica sol prepared by the alcohol-free sol-gel route. These modified electrodes were characterized by cyclic voltammetry, UV-vis spectroscopy, and in situ UV-vis spectroelectrochemistry. The electrochemical response of encapsulated protein is influenced by the terminal groups of the silica pores. Cyt c does not present electrochemical response in conventional silica (hydroxyl terminated) or phenyl terminated silica. Direct electron transfer to encapsulated cytochrome c and ITO electrodes only takes place when the protein is encapsulated in methyl modified silica networks.