Polyaniline nanowire arrays generated through oriented mesoporous silica films: effect of pore size and spectroelectrochemical response.

Indium-tin oxide electrodes modified with vertically aligned silica nanochannel membranes have been produced by electrochemically assisted self-assembly of cationic surfactants (cetyl- or octadecyl-trimethylammonium bromide) and concomitant polycondensation of the silica precursors (tetraethoxysilane). They exhibited pore diameters in the 2-3 nm range depending on the surfactant used. After surfactant removal, the bottom of mesopores was derivatized with aminophenyl groups via electrografting (i.e., electrochemical reduction of in situ generated aminophenyl monodiazonium salt). These species covalently bonded to the ITO substrate were then exploited to grow polyaniline nanofilaments by electropolymerization of aniline through the nanochannels. Under potentiostatic conditions, the length of polyaniline wires is controllable by tuning the electropolymerization time. From cyclic voltammetry characterization performed either before or after dissolution of the silica template, it appeared that both the polyaniline/silica composite and the free polyaniline nanowire arrays were electroactive, yet with much larger peak currents in the latter case as a result of larger effective surface area offered to the electrolyte solution. At identical electropolymerization time, the amount of deposited polyaniline was larger when using the silica membrane with larger pore diameter. All polyaniline deposits exhibited electrochromic properties. However, the spectroelectrochemical data indicated more complete interconversion between the coloured oxidized form and colourless reduced polyaniline for the arrays of nanofilaments in comparison to bulky films. In addition, the template-free nanowire arrays (i.e., after silica dissolution) were characterized by faster electrochromic behaviour than the polyaniline/silica hybrid, confirming the potential interest of such polyaniline nano-brushes for practical applications.

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.

Voltammetric study of cefotaxime at the macroscopic and miniaturized interface between two immiscible electrolyte solutions.

The electrochemical behavior of cefotaxime (CTX+) was investigated at the polarized macro- and micro-interface between two immiscible electrolyte solutions (ITIES) by cyclic voltammetry and alternating current voltammetry. Miniaturization was achieved with fused silica microcapillary tubing entrapped in a polymeric casing. Scanning electron microscopy (SEM) was employed for the fabricated LLI support characterization. Voltammetric investigation of CTX+ at macro- and µ-ITIES allowed the determination of many physicochemical parameters, such as formal Galvani potential of the ion transfer reaction ([Formula: see text]), diffusion coefficients (D), formal free Gibbs energy of the ion transfer reaction (∆G'aq → org), and water-1,2-dichloroethane partition coefficient ([Formula: see text]). Additionally, based on the results obtained the analytical parameters including voltammetric sensitivity, limits of detection and the limits of quantification (in micromolar range) were calculated. The applicability of the developed procedures was verified in spiked still mineral and tap water samples.

Electrochemical Filter To Remove Oxygen Interference Locally, Rapidly, and Temporarily for Sensing Applications.

An electrochemical oxygen filter is described that removes efficiently dissolved oxygen from the surface of an electrochemical sensor. Simulations show that 99% of oxygen can be removed in less than 60 s if an electrochemical filter made of a porous electrode is positioned at less than 200 µm from the sensor surface. For an experimental demonstration, the metallic filter was made with either a stainless steel or a platinum grid separated from the sensor by a porous separator. It was combined with a sensor for analysis of paraquat, an herbicide widely used over the world. In aerated solutions, paraquat signal was not distinguished due to the strong interference of oxygen. When using the oxygen filter, paraquat was clearly detected with a better-defined response than the one obtained under a N2 atmosphere that requires a longer time period before analysis.

Electrochemically Modulated Liquid-Liquid Extraction for Sample Enrichment.

A new sample preparation method is proposed for the extraction of pharmaceutical compounds (Metformin, Phenyl biguanide, and Phenformin) of varied hydrophilicity, dissolved in an aqueous sample. When in contact with an organic phase, an interfacial potential is imposed by the presence of an ion, tetramethylammonium (TMA+), common to each phase. The interfacial potential difference drives the transfer of ionic analytes across the interface and allows it to reach up to nearly 100% extraction efficiency and a 60-fold enrichment factor in optimized extraction conditions as determined by HPLC analysis.

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.

Electrografting of 3-Aminopropyltriethoxysilane on a Glassy Carbon Electrode for the Improved Adhesion of Vertically Oriented Mesoporous Silica Thin Films.

Vertically oriented mesoporous silica has proven to be of interest for applications in a variety of fields (e.g., electroanalysis, energy, and nanotechnology). Although glassy carbon is widely used as an electrode material, the adherence of silica deposits is rather poor, causing mechanical instability. A solution to improve the adhesion of mesoporous silica films onto glassy carbon electrodes without compromising the vertical orientation and the order of the mesopores will greatly contribute to the use of this kind of modified carbon electrode. We propose here the electrografting of 3-aminopropyltriethoxysilane on glassy carbon as a molecular glue to improve the mechanical stability of the silica film on the electrode surface without disturbing the vertical orientation and the order of the mesoporous silica obtained by electrochemically assisted self-assembly. These findings are supported by a series of surface chemistry techniques such as X-ray photoelectron spectroscopy, scanning and transmission electron microscopy, and cyclic voltammetry. Finally, methylviologen was used as a model redox probe to investigate the cathodic potential region of both glassy carbon and indium tin oxide electrodes modified with mesoporous silica in order to demonstrate further the interest in the approach developed here.

Recent developments in electrochemistry at the interface between two immiscible electrolyte solutions for ion sensing.

Ion transfer at the interface between two immiscible electrolyte solutions allows the non-redox electrochemical detection of ions ranging from protons to macromolecules such as proteins. New electrochemical methods and analytical procedures have been developed in recent years to achieve limits of detection of from µM down to tens of pM for ion sensing in biomedical diagnostics and in environmental monitoring. This article reviews the developments of the period 2010-2015.

Combined Raman microspectrometer and shearforce regulated SECM for corrosion and self-healing analysis.

Shearforce regulated scanning electrochemical microscopy (SECM) has been associated with Raman microspectrometry in order to perform combined electrochemical and spectrochemical analysis on reactive interfaces. The interest of the method was evaluated by analyzing local corrosion phenomena in damaged Zn(Mg, Al) self-healing coatings deposited on steel. Despite the high aspect ratio of the analyzed sample displaying here more than a 50 µm depth profile, the optimized setup allowed (1) precise electrode positioning with the help of shearforce detection, (2) electrochemical measurement at a constant distance from the sample surface, and (3) local chemical analysis of the solid surface by confocal Raman microspectroscopy performed at a constant focal distance from the sample. All in all, this new setup allows one to approach the detailed reactivity involved in defective metal samples.

Electrochemically assisted generation of silica deposits using a surfactant template at liquid/liquid microinterfaces.

The electrochemically assisted generation of mesoporous silica deposits at arrays of microscopic liquid/liquid interfaces was investigated. Ion transfer voltammetry was used in order to initiate the formation of silica material by electrochemical transfer of template species (cetyltrimethylammonium, CTA(+)), initially present in the organic phase, to the aqueous phase containing the hydrolyzed silica precursors (tetraethoxysilane, TEOS). The deposition mechanism was investigated using cyclic voltammetry, based on the analysis of diffusion layer profiles of CTA(+) species from the organic side of the interface. The morphology of the deposits varied from hemispherical to almost flat with the potential scan rate, the spacing factor of the microinterfaces array supporting the liquid/liquid interfaces, or the initial CTA(+) and TEOS concentrations, as evidenced by scanning electron microscopy and profilometry analyses. The amount of deposited material can be related to the amount of CTA(+) species passing across the liquid/liquid interfaces. Confocal Raman spectroscopy was used to confirm the presence of surfactant-templated silica deposits and to analyze the effectiveness of calcination in removing the organic molecules filling the interior of the pores. After template removal, the mesoporous network became accessible to external reagents, as checked by interfacial alkylammonium cation transfer, suggesting a possible analytical interest of such modified micro-liquid/liquid interfaces.

Interfacial processes studied by coupling electrochemistry at the polarised liquid-liquid interface with in situ confocal Raman spectroscopy.

Interfacial processes controlled by ion transfer voltammetry at the interface between two immiscible electrolyte solutions were studied by in situ Raman spectroscopy. Raman spectra of the interface between a 5 mM NaCl aqueous solution and 10 mM bis(triphenyl-phosphoranydieneammonium) tetrakis(4-chlorophenyl)borate in 1,2-dichloroethane were recorded at open circuit potential and at various interfacial potential differences. At open-circuit potential, Raman peaks assigned to vibrational modes of 1,2-dichloroethane are clearly visible and peaks of weak intensity are measured for the organic electrolyte ions. When a negative interfacial potential difference is applied, the intensity of the peaks of the cation of the organic electrolyte increases, confirming its transfer induced by the interfacial potential difference applied. The electrochemically assisted generation of mesoporous silica deposits was then followed by in situ confocal Raman spectroscopy. The condensation of mesoporous silica was controlled by the transfer of cetyltrimethylammonium (CTA(+)) ions to an aqueous phase containing hydrolysed silanes. The transfer of CTA(+) at the interface was monitored in situ by confocal Raman spectroscopy, and formation of silica was observed.

Potential-Modulated Surface-Enhanced Raman Spectroscopy of Tolmetin at Gold Nanoparticle Film Functionalized Polarizable Liquid-Liquid Interfaces.

An aqueous colloidal suspension of gold nanoparticles (AuNPs) may be condensed into a thin fractal film at the polarizable liquid-liquid interface formed between two immiscible electrolyte solutions upon injection of millimolar concentrations of sodium chloride to the aqueous phase. By adjusting the interfacial polarization conditions (negative, intermediate, and positive open-circuit potentials), the morphology of the film is modified, resulting in unique surface plasmon properties of the film, which enable in situ surface-enhanced Raman spectroscopy (SERS). Intense SERS signals are observed at the polarizable liquid-liquid interface when micromolar concentrations of tolmetin, a nonsteroidal anti-inflammatory drug, are entrapped in the AuNP fractal film. The change in the signal intensity, averaged over multiple spectra, with respect to the concentration of tolmetin, depends on the polarization conditions and suggests the presence of chemical-induced damping effects on the surface plasmons of the gold film.

Impact of surface nano-textured stainless steel prepared by focused ion beam on endothelial cell growth.

The modification of stent surfaces with nano-structures has the potential for limiting late stent restenosis. We report here the patterning of 316L austentitic stainless steel with arrays of nano-pits of two nominal diameters: 120 and 180 nm. These nano-textured surfaces were prepared by focused ion beam milling. The influence of the ion beam current on the nano-features was investigated by scanning electron and atomic force microscopies. The optimum ion beam currents were 280 pA for 120 nm nano-pits and 920 pA for 180 nm nano-pits. The depths of the nano-pits formed were (65 +/- 24) nm (120 nm) and (84 +/- 36) nm (180 nm). This wide distribution of the depths is due to the polycrystalline nature of 316 L stainless steel, which has a strong influence on the milling rates. Endothelial cells were grown in vitro on these substrates for 1, 3 and 5 days. The cells were viable for the duration of the cell culture on the nano-textured substrates. There was no significant difference in the adhesion and the proliferation based on the nano-pit diameter.

Electroanalytical behavior of poly-L-lysine dendrigrafts at the interface between two immiscible electrolyte solutions.

In this work, the electrochemical behavior of nonredox-active poly-L-lysine dendrigraft molecules of four different generations was investigated at the interface between two immiscible electrolyte solutions (ITIES). The influence of the dendrigraft generation on the electrochemical response, sensitivity of the calibration curves, and limit of detection was studied. Cyclic voltammetry at the ITIES revealed that the sensitivity increased (1840 to 25 800 nA µM(-1)) and the limit of detection decreased (11.10 to 0.65 µM) as the dendrigraft generation increased from generation G2 through to generation G5, respectively. The results are compared to those for protein voltammetry at the ITIES. Our studies suggest that the sensitivity expected for a synthetic ionized macromolecule can be predicted on the basis of its net charge and its diffusion coefficient. However, electrochemistry at the ITIES demonstrates a greater sensitivity toward proteins, which is attributed to their tertiary structure.

Amperometric Sensor for Selective On-Site Analysis of Free Sulfite in Wines.

Accurate and rapid on-site analysis of free SO2 content is crucial in the process of winemaking from a producer and consumer perspective. Herein, we present an amperometric sensor based on commercially available screen-printed electrodes coupled with an electrochemical oxygen filter. The developed amperometric method gave a linear response in a concentration range up to 200 mg L-1 with a limit of quantification of 7.5 mg L-1. The applicability of the developed sensor was successfully tested on 27 white and red wine samples and compared to the Ripper method (iodometry) that is a standard procedure for free SO2 determination. The sensor exhibits similar precision and accuracy but shows no interference from oxidizable species such as ascorbic acid, which is a major advantage over iodometric titration. The performance of the sensor was in addition positively evaluated during on-site analysis in a winery.

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.

Single nanoskived nanowires for electrochemical applications.

In this work, we fabricate gold nanowires with well controlled critical dimensions using a recently demonstrated facile approach termed nanoskiving. Nanowires are fabricated with lengths of several hundreds of micrometers and are easily electrically contacted using overlay electrodes. Following fabrication, nanowire device performance is assessed using both electrical and electrochemical characterization techniques. We observe low electrical resistances with typical linear Ohmic responses from fully packaged nanowire devices. Steady-state cyclic voltammograms in ferrocenemonocarboxylic acid demonstrate scan rate independence up to 1000 mV s(-1). Electrochemical responses are excellently described by classical Butler-Volmer kinetics, displaying a fast, heterogeneous electron transfer kinetics, k(0) = 2.27 ± 0.02 cm s(-1), α = 0.4 ± 0.01. Direct reduction of hydrogen peroxide is observed at nanowires across the 110 pM to 1 mM concentration range, without the need for chemical modification, demonstrating the potential of these devices for electrochemical applications.

Electroanalytical metal sensor with built-in oxygen filter.

A rapid and reliable oxygen elimination system was evaluated here for the electroanalytical study of metals. Dissolved oxygen was removed locally in the vicinity of a sensor by the means of electrochemical oxygen filter constructed from platinum grids. Three metals (Cd, Pb, and Zn) were determined by stripping chronopotentiometry (SCP) at a mercury film screen-printed electrode. Limits of detection of metals were in the nanomolar range under optimized experimental conditions. The electrochemical device was also tested for metal quantification in simple electrolyte solutions and in a natural water matrix. The proposed combination of oxygen elimination system with the metal sensor completely removes the need to purge the sample before SCP measurement. This makes the determination of metals by SCP faster, portable and more suited for on-field applications.