Preparation and characterization of novel nanometer-scale platinum electrodes

Observation of the oxidation–reduction processes occurring at the nanoelectrode–solution interface demonstrates how electrochemical behavior depends upon nanoelectrode size. The use of a modified form of pulsed laser ablation as an improved method to synthesize nanometer-scaled electrode materials easily and consistently is reported. This method of fabrication enables platinum metal nanoparticles averaging 3 nm in diameter and approximately 5.0 × 10¹¹ particles/cm² to be deposited onto silicon substrates using optimum ablation parameters. A platinum silicide phase exists at the interface of the platinum and silicon as a result of the ablation process. Electrochemical results demonstrate the presence of a large number of isolated platinum particles (1.1 × 10⁷ particles/cm²), separated by an average edge to edge distance of 14 nm, which are electrochemically active nanoelectrodes.     

Diffusion regimes at nanoelectrode ensembles in different ionic liquids

The electrochemical and diffusion behaviour of different redox probes in different ionic liquids is studied at gold nanoelectrode ensembles (NEEs) in comparison with millimetre sized gold (Au-macro) and glassy carbon (GC) disk electrodes. The redox probes are neutral ferrocene (Fc), the ferrocenylmethyltrimetylammonium cation (FA⁺) and the ferrocenylmonocarboxylate anion (FcCOO⁻). The ILs are the dicyanamide, [N(CN)₂] or bis(trifluoromethylsulfonyl)amide), [N(Tf)₂] salts of the following cations: 1-butyl-3-methylimidazolium, [BMIm], 1-butyl-3-methylpyrrolidonium, [BMPy], or tris(n-hexyl)tetradecylphosphonium [P_14,666]. These ILs are characterized by different viscosities, ranging from 32 to 277 cP. The cyclic voltammetric behaviour of the redox probes is reversible and diffusion controlled at GC electrodes. Diffusion coefficients (D) calculated by the Randles-Sevcik equation scales inversely with the IL viscosity, ranging from 2 × 10⁻⁸ to 3 × 10⁻⁷ cm² s⁻¹. Ionic solutes, namely FA⁺ and FcCOO⁻, present slightly lower D values than neutral Fc. At the Au-macro the electrochemical behaviour of the redox probes is diffusion controlled in the ILs containing the [N(Tf)₂] anion, while it involves relevant adsorption processes in the [N(CN)₂] containing electrolyte. For this reason the diffusion at gold NEEs is studied only in the former ILs.The CVs of the redox probes at the NEEs are peak shaped at low scan rate (v), while they are sigmoidally shaped at high v, but with some shift between forward and backward patterns. This is indicative of the occurrence of a total overlap (TO) diffusion condition when v is low which becomes a mixed diffusion layers (MDL) regime, with only a partial overlapping of individual diffusion layers, at high v values. In the most viscous IL, namely [P_14,666] [N(Tf)₂], at v higher than 0.8 V s⁻¹, a plateau current independent on the scan rate is achieved, indicating the tendency to reach the pure radial regime in this IL. The v values at which the transition between TO and MDL is observed scales directly with D and inversely with the IL viscosity. This behaviour is interpreted on the basis of the dependence of individual diffusion layers at each nanoelectrode on redox probe/IL interaction which fits with existing theoretical models very recently developed for nanoelectrode arrays.     

The voltammetric responses of nanometer-sized electrodes in weakly supported electrolyte: A theoretical study

The effect of the supporting electrolyte concentration on the interfacial profiles and voltammetric responses of nanometer-sized disk electrodes have been investigated theoretically by combining the Poisson-Nernst-Planck (PNP) theory and Butler-Volmer (BV) equation. The PNP-theory is used to treat the nonlinear couplings of electric field, concentration field and dielectric field at electrochemical interface without the electroneutrality assumption that has been long adopted in various voltammetric theories for macro/microelectrodes. The BV equation is modified by using the Frumkin correction to account for the effect of the diffuse double layer potential on interfacial electron-transfer (ET) rate and by including a distance-dependent ET probability in the expression of rate constant to describe the radial heterogeneity of the ET rate constant at nanometer-sized disk electrodes. The computed voltammetric responses for disk electrodes larger than 200 nm in radii in the absence of the excess of the supporting electrolyte using the present theoretical scheme show reasonable agreements with the predications of the conventional microelectrode voltammetric theory which uses the combined Nernst-Planck equation and electroneutrality equation to describe the mixed electromigration-diffusion mass transport without including the possible effects of the diffuse double layer (Amatore et al. [25]). For electrodes smaller than 200 nm, however, the voltammetric responses predicated by the present theory exhibit significant deviation from the microelectrode theory. It is shown that the deviations are mainly resulted from the overlap between the diffuse double layer and the concentration depletion layer (CDL) at nanoscale electrochemical interfaces in weakly supported media, which will result in the invalidation of the electroneutrality condition in CDL, and from the radial inhomogeneity of ET probability at nanometer-sized disk electrodes.     

Modification of nanoelectrode ensembles by thiols and disulfides to prevent non specific adsorption of proteins

The possibility to functionalize selectively with thiols or disulfides the surface of the gold nanoelectrodes of polycarbonate templated nanoelectrode ensembles (NEEs) is studied. It is shown that the Au nanoelectrodes can be coated by a self assembled monolayer (SAM) of thioctic acid (TA) or 2-mercaptoethanesulfonic (MES) acid. The study of the electrochemical behavior of SAM-modified NEEs by cyclic voltammetry (CV) at different solution pH, using ferrocenecarboxylate as an anionic redox probe (FcCOO⁻) and (ferrocenylmethyl)trimethylammonium (FA⁺) as a cationic redox probe, demonstrate that the SAM-modified nanoelectrodes are permselective, in that only cationic or neutral probes can access the SAM-coated nanoelectrode surface. CV, AFM and FTIR-ATR data indicate that proteins such as casein or bovine serum albumin, which are polyanionic at pH 7, adsorb on the surface of NEEs untreated with thiols, tending to block the electron transfer of the ferrocenyl redox probes. On the contrary, the pre-treatment of the NEE with an anionic SAM protects the nanoelectrodes from protein fouling, allowing the detection of well shaped voltammetric patterns for the redox probe. Experimental results indicate that, in the case of MES treated NEEs, the protein is bound only onto the polycarbonate surface which surrounds the nanoelectrodes, while the tips of the gold nanoelectrodes remain protein free.     

Focused ion beam production of nanoelectrode arrays

We present a method for the production of nanoelectrodes using focussed ion beam techniques (FIB). The electrodes utilise nanometric holes milled in a silicon nitride based pasivation layer, followed by wet etching of a silicon oxide based pasivation layer, to expose an underlying gold electrode. After functionalisation using a surface assembled monolayer and an electrochemically grown polypyrrole, these gold nanoelectrodes have been tested, via cyclic voltammetry, in the detection of [Fe(CN)₆]^4−/3− ions. The nanoelectrodes will be used to investigate the electrical properties of nanometric biological specimen.     

Enhanced thermal stability of organic solar cells on nano structured electrode by simple acid etching

The long-term thermal stability of organic photovoltaic device has been emerged as a crucial characteristic to satisfy the demand of industry requiring lifespan of 20 year. In this paper, etched indium–tin-oxide (ITO) nanoelectrodes are employed to enhance the thermal stability and power conversion efficiency in poly(3hexylthiophene):methanofullerene bulk-heterojunction solar cells. A simple etching process for the ITO electrode was carried out using a hydrochloric acid solvent in order to significantly increase the roughness of the ITO surface. This nanostructured ITO not only induced efficient light harvesting, but also acted as a barrier that suppressed the PCBM phase separation, resulting in the lifespan increased from 33 h to 77 h during thermal annealing.     

Micro and nano-platforms for biological cell analysis

In this paper some technological platforms developed for biological cell analysis will be presented and compared to existing systems. In brief, we present a novel micro cell culture chamber based on diffusion feeding of cells, into which cells can be introduced and extracted after culturing using normal pipettes, thus making it readily usable for clinical laboratories. To enhance the functionality of such a chamber we have been investigating the use of active or passive 3D surface modifications. Active modifications involve miniature electrodes able to record electrical or electrochemical signals from the cells, while passive modifications involve the presence of a peptide nanotube based scaffold for the cell culturing that mimics the in vivo environment. Two applications involving fluorescent in situ hybridization (FISH) analysis and cancer cell sorting are presented, as examples of further analysis that can be done after cell culturing. A platform able to automate the entire process from cell culturing to cell analysis by means of simple plug and play of various self-contained, individually fabricated modules is finally described.     

Nanostructures from directionally solidified NiAl-W eutectic alloys

A directionally solidified eutectic NiAl-W alloy was employed as a source for NiAl nanopore arrays, W-nanowire arrays and W-nanowires. The NiAl-W eutectic alloy containing 1.5 at.% W. A growth rate of 30 mm h⁻¹ was used at a temperature gradient of 40 K cm⁻¹ in a Bridgman-type directional solidification furnace. A combined stability diagram was derived from the Pourbaix diagrams of the three elements involved (Ni, Al, W). It allowed predicting proper conditions for the selective dissolution of either of the phases. Etching in a mixture of HCl:H₂O₂ released parallel aligned W-nanowires with a wire diameter of ∼200 nm. The aspect ratio observed in these measurements was over 400. Selective electrodissolution of the W minor phase was possible in a neutral buffer at 0.5 V versus SHE. This procedure yielded regular nanopore arrays with pore diameters of ∼200 nm. The pore density obtained was 8 × 10¹⁰ m⁻². The relative coverage of the wires or the corresponding pores in a cross section was 0.3%. For the material employed in this study a 12 μm² area corresponds to a single nanowire electrode. The potential of this material and the processing developed is discussed in terms of the production of nanofilters, nanoelectrode sensors, nanowire arrays and quantitative amounts of single crystalline tungsten nanowires.     

Nanostructured Materials for Electrochemical Energy Conversion and Storage Devices

One of the greatest challenges for our society is providing powerful electrochemical energy conversion and storage devices. Rechargeable lithium‐ion batteries and fuel cells are amongst the most promising candidates in terms of energy densities and power densities. Nanostructured materials are currently of interest for such devices because of their high surface area, novel size effects, significantly enhanced kinetics, and so on. This Progress Report describes some recent developments in nanostructured anode and cathode materials for lithium‐ion batteries, addressing the benefits of nanometer‐size effects, the disadvantages of ‘nano’, and strategies to solve these issues such as nano/micro hierarchical structures and surface coatings, as well as developments in the discovery of nanostructured Pt‐based electrocatalysts for direct methanol fuel cells (DMFCs). Approaches to lowering the cost of Pt catalysts include the use of i) novel nanostructures of Pt, ii)new cost‐effective synthesis routes, iii) binary or multiple catalysts, and iv) new catalyst supports.