L.M. Abrantes

On the Mechanism of Electroless Ni‐P Plating

The electroless deposition mechanism of nickel, in a chloride‐based solution, using hypophosphite ion as reducing agent was examined using potentiodynamic method and open‐circuit potential measurements. The results indicate that the mixed potential theory is unable to describe this system. This is due to the nature of the involved mechanism. It is assumed that the process is initialized by adsorption of hypophosphite ions, followed by the homolysis of primary reductant species, resulting in the formation of atomic and ionic radicals which promote nickel and hypophosphorous acid reduction to originate the Ni‐P deposit. The initial deposition stages of Ni‐P were observed, revealing that the substrate induction, which leads to an increase in the catalytic activity, is not related to a galvanic displacement process, but rather to the establishment of the electrochemical conditions which allow the adsorption/breakdown of the hypophosphite ions. The experimental results are in good agreement with the proposed mechanism.

Electronically conductive polymer grafted with oligonucleotides as electrosensors of DNA: Preliminary study of real time monitoring by in situ techniques

Polypyrrole bearing covalently linked oligonucleotide (ODN) probes has been used for electrode modification through one-step electrosynthesis. The biofilms obtained were employed for the direct and real-time detection of DNA hybridisation by using photocurrent spectroscopy and a quartz crystal microbalance (QCM). The photoelectrochemical properties of polypyrrole appear to be influenced by the presence of ODN allowing the transduction of the hybridisation event into a measurable photocurrent. The operational parameters of the biosensor (film thickness, ODN load, …) were assessed to optimise the response to DNA hybridisation. In addition, QCM detection, which is based on the monitoring of the biofilm mass uptake generated by molecular recognition, has been used as a validation technique. The sensors obtained have been demonstrated to be fully reusable. They have also shown a suitable sensitivity and a good reproducibility in terms of kinetic behaviour and intensity of the measured signal. Finally, this study considers the pertinence of both techniques regarding DNA hybridisation detection.

Electrochemical synthesis and redox behaviour of polypyrrole coatings on copper in salicylate aqueous solution

Abstract In this work, polypyrrole (PPy) coatings have been successfully electrodeposited on copper substrates from aqueous salicylate solutions. Homogenous and uniform films can be grown under galvanostatic or potentiostatic control without noticeable substrate dissolution. The redox behaviour obtained for the films is similar to that displayed by PPy grown under the same conditions on noble metals. Strong adherence to copper is observed as far as PPy is in its oxidised state but it is lost when the system is subjected to potentials lower than −0.5 V versus saturated calomel electrode (SCE).

Redox induced orientational changes in a series of short chain ferrocenyl alkyl thiols self-assembled on gold(111) electrodes

Abstract A series of short chain alkyl thiols (n=3–10) with a ferrocene terminal group were allowed to form organised monolayers at gold(111) surfaces. The redox activity of the monolayers was monitored using cyclic voltammetry and subtractively normalised interfacial Fourier transform infrared spectroscopy (SNIFTIRS). The in situ infrared spectra show that the oxidation of the ferrocene groups in the monolayers studied is accompanied by a rotation of the ferrocene groups towards a position where the plane of the cyclopentadienyl rings moves to a position normal to the surface of the electrode. Furthermore, electrochemical and spectroscopic data indicate that monolayers with longer alkyl chains are more ordered than the shorter analogues. A trend was observed in which the rate of adsorption and the surface coverage vary between the length of the alkyl chains.

Characterization of a natural and an electro-oxidized arsenopyrite: a study on electrochemical and X-ray photoelectron spectroscopy

This paper discusses the results of a detailed study on the electrochemistry of an arsenopyrite mineral and a concentrate as well as other mineral species contained in it in a chloride medium (NaCl 1.9 M+HCl 0.1 M) using cyclic voltammetry. The surface modification promoted by the anodic oxidation of arsenopyrite mineral samples (+0.8 V for 1 h) is also analysed. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy have been used to provide information on the chemical state of natural and electrooxidized surfaces. The results showed that layers of elemental sulphur are produced on the oxidized surface of arsenopyrite by the formation of an intermediate metal-deficient sulphide Fe1 � xAs1 � yS. Other surface oxidation products such as iron oxides, arsenic oxides and oxysulphur species have also been detected, confirming the interpretation of the voltammetric studies. The identified surface products show that arsenopyrite oxidation occurs by the diffusion of the metal atoms from the bulk to the interface region and by their interaction with air forming metal oxide layers (mainly iron and arsenic oxides) and leaving a predominant elemental sulphur layer.

On the initiation and growth of polymer films onto electrode surfaces

Abstract The probe beam deflection technique (PBD) has been successfully used to detect in situ the influence of electrochemical mode and conditions on the initial stages of poly-3-methylthiophene formation. The interpretation of the results is coherent with the previously reported oligomer formation in solution prior to deposition. The effect of the initial polymerization steps in the subsequent stages of the polymer growth has also been investigated, analysing the redox behaviours along with the correspondent deflectograms obtained for thin films synthesized under different deposition charges. The data indicate that up to a given thickness the polymer responses reflect the nature of the first deposited layer. The electropolymerization behaviour of methylthiophene has also been contrasted to that of pyrrole under the same experimental conditions (solvent, monomer concentration, anionic and cationic components of the electrolyte). The results clearly reveal an earlier deposition process and that oligomer formation, if any, is not determinant for polypyrrole deposition.

Self-assembled monolayer of an iron(III) porphyrin disulphide derivative on gold

A novel iron(III) porphyrin disulphide derivative have been successfully immobilised on gold surfaces by self-assembly. The redox response of the modified electrodes was compared with the obtained for a similar iron porphyrin in solution, confirming the immobilisation of the metalloporphyrin. The gravimetric data obtained by electrochemical quartz crystal microbalance (EQCM) during adsorption allowed an estimation of the electrode coverage, providing further evidence for the formation of the porphyrin SAM. The modified electrodes were also measured by conventional and imaging ellipsometry. The electrocatalytic activity of the two modified electrodes was tested for the reduction of the molecular oxygen.

EQCM study of polypyrrole modified electrodes doped with Keggin-type heteropolyanion for cation detection

The incorporation of a Keggin-type heteropolyanion, the phosphotungstate ([PW12O40]3−), into polypyrrole has been achieved during the electrochemical preparation of the polymer films in aqueous solution. The redox behaviour of these modified electrodes is described by using cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM). The data allowed to estimate the doping level that indicates the extent of the heteropolyanion insertion. It is found that the polymer films, in aqueous solution, exhibit sensibility to cations, namely to Na+.

Study of DNA hybridization on polypyrrole grafted with oligonucleotides by photocurrent spectroscopy

Recognition of DNA sequences by biochemical sensor is generally performed by analysis after completion of hybridization. Using a technique able to directly translate the biological event into an electrical signal allows the in situ monitoring of the hybridization kinetics. In this aim, the photoelectrochemical behavior of one electroactive polymeric sensor based on a copolymer of polypyrrole and polypyrrole-oligonucleotide has been investigated in aqueous solution. This sensor has been studied as such (i) and in two other situations: (ii) when the copolymer is in presence of non-complementary oligonucleotides; and (iii) when the copolymer is in presence of complementary oligonucleotides. From the photocurrent spectra obtained at -0.6 V/SCE versus incident energy the allowed direct and indirect transitions for each polymer have been evidenced. The photocurrent evolution during hybridization and adsorption processes has been recorded in real time and the hybridization kinetics has revealed to be comparable with mass variations obtained by quartz crystal microbalance under the same experimental conditions.

Polypyrrole incorporating electroless nickel

Abstract The electroless deposition of Ni–P (phosphorus content about 20 at.%) into polypyrrole (PPy) film is described and conditions for achieving good dispersions of small particles discussed. The influence of the loading level/particle size on the electrocatalytic properties of these new electrodes is illustrated by the study of the hydrogen evolution reaction (HER). The usefulness of electrochemical treatments, not only in overcoming the loss of catalyst at anodic potentials but also in promoting the formation of a catalytic nickel oxide phase is demonstrated. The performance towards nitrite ion oxidation reveals that electroless deposition on conductive polymers can be envisaged as an attractive method for the fabrication of good electrocatalytic materials with application in a wide range of potentials.