A. Jiménez-Morales

A new silver-ion selective sensor based on a polythiacrown-ether entrapped by sol–gel

Abstract We report a new electrode based on a selective complexing agent for Ag + ions, the 6-oxa-3,9-dithiabicicle[9.3.1]pentadeca-1(15),11,13-triene (MAO), incorporate into an organopolysiloxane matrix prepared via sol–gel from a mixture of γ-methacryloxypropyltrimetoxysilane (MAPTMS) and tetramethoxysilane (TMOS). The modified electrode is prepared by coating the surface of graphite electrodes with a thin film of the nanocomposite material. SEM and EDX were used to determine the continuous coverage and composition of the films constituting the active phase of the electrodes. The application of electrochemical impedance spectroscopy (EIS) and potentiometric techniques, allows establishing the selective response of the electrode towards the Ag + ions. The selectivity over other cations as Cd 2+ , Pb 2+ , NH 4 + , Na + , etc. is explained by the molecular recognition character of the electrode introduced by the presence of the MAO ionophere into the xerogel film. These new hybrid organic–inorganic systems constitute an alternative to classical PVC-matrices for preparation ion-selective electrodes showing also additional advantages such as higher reproducibility, lifetime, detection limits and adhesion with the electrode.

Improvement of rheological properties of Inconel 718 MIM feedstock using tailored particle size distributions

Abstract One of the most important stages in the metal injection moulding (MIM) process is feedstock fabrication, since the rheological properties of the feedstock are of crucial importance in the injection moulding step. In the present work, the possibility of replacing a conventional MIM superalloy feedstock (particle size <20 μm) by a mixture of powders with different particle size ranges, to tailor the flow properties, was investigated. Five Inconel 718 feedstocks were prepared with the same binder (HDPE–paraffin wax–stearic acid) and different mixes of four spherical gas atomised powder fractions (<22; 16–45; 45–63; 65–212 μm). The rheological characteristics and processability of each feedstock were evaluated with a capillary rheometer to determine flow index, yield stress and activation energy.

Proton conductivity in Al-montmorillonite pillared clays

Abstract Pillared clays (PILCs) are materials derived from smectite clay minerals (2:1 phyllosilicates) in which an interlayer formation of metal oxides, acting as pillars, produces a permanent separation between the silicate layers. It is assumed that protons are generated during the thermal treatment necessary for the pillar oxide formation, as it is the case for Al-montmorillonite PILCs. We have applied the electrochemical impedance technique with the aim of studying the proton conductivity of these pillared materials. Enhancement of electrical conductivity has been observed upon adsorption of oxyethylene compounds (crown-ethers and PEO) into the PILC galleries.

Experimental and theoretical methods for optimal solids loading calculation in MIM feedstocks fabricated from powders with different particle characteristics

Abstract The aim of this work is to investigate the influence of the particle characteristics of powder to the mixing stage of the powder injection moulding process to identify the best procedure with which to evaluate the optimal solids loading for feedstock fabrication. To perform the present study, powder blends that are made from three bronze and four Inconel 718 powders, with different particle size distributions and morphologies, are mixed with a binder system that is based on polyethylene and wax. Powder–binder blends are prepared by varying the powder content to study how the powder characteristics affect the optimal solids loading. In addition, a new method is presented for determining the optimal solids loading in powder injection moulding feedstocks through the activation energy calculation of powder–binder blends that are prepared with different powder contents. This novel method is compared with the optimal loading values that have been determined by other ways as torque rheometry, determination of the tap-real density ratio and through rheological models.

Nanocomposite materials based on organopolysiloxane/macrocyle systems for electrochemical sensors

Abstract Hybrid inorganic–organic nanocomposite materials are prepared via sol–gel from γ-metacryloxypropyltrimetoxysilane (MAPTMS) and tetramethoxysilane (TMOS) precursors of a organopolysiloxane matrix in which ionophore ligands (crown-ethers: 12-crown-4, 15-crown-5 and 18-crown-6) are incorporated under controlled experimental conditions. Chemical analysis, FTIR and solid-state NMR ( 13 C and 29 Si ) spectroscopies, TG and DTA thermal analyses and specific surface area determinations indicate that the ionophore remains homogeneously distributed in the organopolysiloxane network. Ion-selective electrodes are prepared by coating metal substrates with gels to form homogeneous and continuous thin xerogel films. Impedance and potentiometric techniques are applied for electrochemical characterisation of the sensors. The resulting electrodes are very sensitive towards alkaline metal ions giving linear potentiometric responses with near-Nernstian slopes in wide cation concentration ranges. Additional advantages of this new type of sensors are better adhesion, sensibility, reproducibility, and detection limit compared to “conventional” (i.e., with internal solutions or PVC) electrodes.

Composite membranes based on macrocycle/polysiloxanes: preparation, characterization and electrochemical behaviour

Composite membranes based on complexing macrocyclic compounds (crown ethers and cryptands) have been studied. These compounds were included into a polyorganosiloxanic matrix in an attempt to obtain new ion-selective materials. This macrocycle/polyorganosiloxane constitutes the active phase of the membrane which was prepared via the sol–gel method and then used to fill a fibrous support of borosilicate. Characterization, by chemical and thermal analysis, FTIR and NMR spectroscopies and scanning electron microscopy (SEM), reveals a homogeneous distribution of the entrapped macrocycles in the polyorganosiloxane network. Electrochemical impedance spectroscopy studies of the composite membranes have been carried out in order to evaluate their ionic transport properties when they are in contact with aqueous salt solutions. From the impedance spectra, the ionic resistance values of the membranes are obtained which vary according to the nature and concentration of the involved macrocyclic compound. The presence of these macrocycles produces a significant decrease in the ionic resistance compared with membranes without macrocyclic compounds. The ionic resistance of these systems is also strongly related to the nature of the electrolyte and to its concentration. Thus, cations exhibiting greater hydration energies show a higher ionic resistance. As an example, for a membrane doped with 12C4 crown ether the Ri values are 213, 128, 109 and 99 kΩ when the electrolyte consists of 10–2 mol I–1 solutions of LiCl, NaCl, KCl and CsCl, respectively. The selectivity of the membranes towards different ions, as well as the reversibility of the electrochemical response when the nature and concentration of the ions is changed, highlights the potential of these materials for sensor applications.

Electrochemical study of the corrosion behaviour of copper surfaces modified by nitrogen ion implantation

Electrochemical impedance spectroscopy (EIS) and d.c polarization resistance measurements (Rp) were used to study the corrosion resistance of surface layers produced by nitrogen ion implantation into copper substrates. Ion implantation was carried out using a Wickham ion beam generator, applying an acceleration voltage of 100keV, a mean current of 0.40 mA and a nitrogen dosage of 4 × 1017 ions cm−2. Surface analyses were made by Auger electron spectroscopy (AES). Electrochemical measurements (EIS and Rp) performed in a 0.6m sodium chloride solution show nitrogen-implanted specimens have greater a.c. and d.c. apparent polarization resistance than nonimplanted specimens. The results obtained with electrochemical measurements indicate that nitrogen ion implantation in copper forms a protective surface layer which improves the corrosion resistance of the pristine material, a feature of great interest for the design of new contact materials for the electricity and electronic industries.

Enhancing in vitro biocompatibility and corrosion protection of organic–inorganic hybrid sol–gel films with nanocrystalline hydroxyapatite

Application of novel organic-inorganic hybrid sol-gel coatings containing dispersed hydroxyapatite (HAp) particles improves the biocompatibility, normal human osteoblast (NHOst) response in terms of osteoblast viability and adhesion of a Ti6Al4V alloy routinely used in medical implants. The incorporation of HAp particles additionally results in more effective barrier proprieties and improved corrosion protection of the Ti6Al4V alloy through higher degree of cross-linking in the organopolysiloxane matrix and enhanced film thickness.

Hybrid Organic-Inorganic Electrode-Membranes Based on Organo-Polysiloxane/Macrocycle Systems

Specific complexing agents for alkaline ions such as some crown-ethers of different intramolecular cavity size (12-crown-4, 15-crown-5 and 18-crown-6) are incorporated into a organo-polyorganosiloxane network generated via the sol-gel process. The resulting xerogels embody macrocycle compounds with different ion-selectivity. These xerogels are deposited onto porous supports (borosilicate, polyacrylonitrile,…) to obtain new composite electrode- membranes. The electrochemical characterization of the membranes and the electrode- membranes by electrochemical impedance spectroscopy (EIS) shows information about the reversible behavior and the ion resistance of the membranes, which are a function of both, nature and content of the entrapped macrocycle as well as the salt solution concentration. The electrochemical response of these systems acting as electrode-membranes shows their sensitivity towards different metal ions at variable concentrations.

The design and characterisation of sol–gel coatings for the controlled-release of active molecules

The controlled release of active agents from a matrix has become increasingly important for oral, transdermal or implantable therapeutic systems, due to the advantages of safety, efficacy and patient convenience. Controlled-release hybrid (organic–inorganic) sol–gel coating synthesis has been performed to create a sol with an active molecule included (procaine). Synthesis procedures included acid-catalysed hydrolysis, sol preparation, the addition of a procaine solution to the sol, and the subsequent gelation and drying. The alkoxide precursors used were triethoxyvinylsilane and tetraethyl-orthosilicate (TEOS) in molar ratios of 1:0, 9:1, 8:2 and 7:3. After the determination of the optimal synthesis parameters, the material was physicochemically characterised by silicon-29 nuclear magnetic resonance (29Si-NMR) and Fourier transform infrared spectroscopy, contact angle analysis and electrochemical impedance spectroscopy tests. Finally, the materials were assayed in vitro for their ability to degrade by hydrolysis and to release procaine in a controlled manner. The sustained release of procaine over a 3-day period was demonstrated. A close correlation between release and degradation rates suggests that film degradation is the main mechanism underlying the control of release. Electrochemical analysis reveals the formation of pores and water uptake during the degradation. The quantity of TEOS is one of the principal parameters used to determine the kinetics of degradation and procaine release.