Erika Scavetta

Electrochemical characterisation of Ni/AlX hydrotalcites and their electrocatalytic behaviour

Abstract Some ‘conductive’ hydrotalcites containing nickel as divalent cation and Cl − , SO 4 2− or CO 3 2− as interlayer anion, have been synthesised and membranised with different types of polymeric matrices to modify glassy carbon electrodes. The mechanism responsible of the conductive properties has been deeply investigated by voltammetric techniques using both a stationary and a rotating disk electrode, demonstrating that the interlayer anion does not affect significantly the electrochemical behaviour of the material. The electrocatalytic properties have been also studied pointing out the key role of the steric hindrance of the oxidisable substrate. In particular, mono- and polyhydric compounds have been taken into account. As to the electrocatalytic efficiency, the nature of the interlayer anion and, hence, the dimension of the interlayer spacing, is important in determining the sensitivity of the measurement since it can affect the analyte diffusion inside the hydrotalcite structure.

AC impedance study of a synthetic hydrotalcite-like compound modified electrode in aqueous solution

This paper deals with the electrochemical characterisation of Ni/Al/Cl hydrotalcite modified electrodes. The electrochemical impedance spectroscopy technique has been used in order to study the electronic and ionic conduction, both inside and on the surface of the material. The electronic and ionic contributions have been separated and the behaviour of the respective parameters has been studied as a function of the potential. In order to determine the kinetic limiting step of the overall electrochemical process we performed experiments at different temperatures, and calculated the activation energies of the electron hopping process and ion transport process. In addition we studied the behaviour of the system at different OH concentrations (pH 9.7/12.8) with the aim of clarifying the role of OH ions in the electrochemical process. # 2003 Elsevier Science Ltd. All rights reserved.

[Ni/AlCl]-based hydrotalcite electrodes as amperometric sensors: preparation and electrochemical study

Abstract A study has been carried out to develop electrochemical devices based on the hydrotalcite (HT) [Ni/Alue5f8Cl] suitable for the amperometric determination of oxidisable species. The amperometric sensors have been tested on methanol, mannitol and glucose substrates. The measured signal is an electrocatalytic current due to the oxidation of Ni(II) centres present in the brucite layers of HT to Ni(III) or Ni(IV). Different kinds of electrodes have been studied in terms of chemical and mechanical stability, lifetime, and simple regenerability of the surface. Cyclic voltammetry has been used to evaluate the electrochemical responses. The best performances have been achieved when HT particles were entrapped into an epoxy resin and the electrical contact was guaranteed by a carbon-based material.

Physical and Electrochemical Properties of PEDOT:PSS as a Tool for Controlling Cell Growth

UNLABELLEDnConducting polymers are promising materials for tissue engineering applications, since they can both provide a biocompatible scaffold for physical support of living cells, and transmit electrical and mechanical stimuli thanks to their electrical conductivity and reversible doping. In this work, thin films of one of the most promising materials for bioelectronics applications, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (nnnPEDOTnPSS), are prepared using two different techniques, spin coating and electrochemical polymerization, and their oxidation state is subsequently changed electrochemically with the application of an external bias. The electrochemical properties of these different types ofnnnPEDOTnPSS are studied through cyclic voltammetry and spectrophotometry to assess the effectiveness of the oxidation process and its stability over time. Their surface physical properties and their dependence on the redox state ofnnnPEDOTnPSS are investigated using atomic force microscopy (AFM), water contact angle goniometry and sheet resistance measurements. Finally, human glioblastoma multiforme cells (T98G) and primary human dermal fibroblasts (hDF) are cultured onnnnPEDOTnPSS films with different oxidation states, finding that the effect of the substrate on the cell growth rate is strongly cell-dependent: T98G growth is enhanced by the reduced samples, while hDF growth is more effective only on the oxidized substrates that show a strong chemical interaction with the cell culture medium.

Hydrotalcite-like compounds as ionophores for the development of anion potentiometric sensors

Abstract A double layered hydroxide anionic clay containing chloride or nitrate as the interlayer anion has been used to prepare ion-selective electrodes (ISE) for potentiometric sensing of the intercalated anionic species. The electrodes exhibited a linear range for the potential response to logarithm of concentration: from 5.0×10 −5 to 2.0×10 −1 M with an average slope equal to −54.2 mV decade −1 for chloride and from 2.0×10 −4 to 1.0 M with an average slope equal to −48.3 mV decade −1 for nitrate. The response time was from 1 to 6 min, depending on the analyte concentration; the stability of the signal was quite satisfactory. The effect of interfering ions has been investigated and the values of the relevant selectivity constants have been determined. The lifetime of the sensors was at least 2 weeks.

Selective detection of dopamine with an all PEDOT:PSS Organic Electrochemical Transistor.

An all PEDOT:PSS Organic Electrochemical Transistor (OECT) has been developed and used for the selective detection of dopamine (DA) in the presence of interfering compounds (ascorbic acid, AA and uric acid, UA). The selective response has been implemented using a potentiodynamic approach, by varying the operating gate voltage and the scan rate. The trans-conductance curves allow to obtain a linear calibration plot for AA, UA and DA and to separate the redox waves associated to each compound; for this purpose, the scan rate is an important parameter to achieve a good resolution. The sensitivities and limits of detection obtained with the OECT have been compared with those obtained by potential step amperometric techniques (cyclic voltammetry and differential pulse voltammetry), employing a PEDOT:PSS working electrode: our results prove that the all-PEDOT:PSS OECT sensitivities and limits of detection are comparable or even better than those obtained by DPV, a technique that employs a sophisticate potential wave and read-out system in order to maximize the performance of electrochemical sensors and that can hardly be considered a viable readout method in practical applications.

A simple all-PEDOT:PSS electrochemical transistor for ascorbic acid sensing

An ascorbic acid (AA) sensor was developed by employing an organic electrochemical transistor (OECT) based only on PEDOT:PSS as a conductive material. The device was prepared by spin coating using the CLEVIOS™ PH 1000 suspension (PEDOT:PSS) masking the gate and the channel areas with tape. The device was electrically characterized while the doping level of the PEDOT:PSS in the channel was controlled using both the gate electrode and the potentiostat. It was demonstrated that the current that flows in channel (Id) is controlled by the concentration of oxidized sites in the examined potential range. AA reacts with the conductive polymer leading to the extraction of charge carriers from the channel, and thus resulting in a decrease of the absolute value of Id. It was observed that Id linearly depends on the logarithm of the AA concentration between 10-6 and 10-3 M. The OECT response to AA was studied by varying the gate voltage or the PEDOT:PSS thickness. The performance of the device for optimized conditions shows a limit of detection equal to 10-8 M and a sensitivity of 4.5 ± 0.1 × 10-6 A decade-1.

Dopamine amperometric detection at a ferrocene clicked PEDOT:PSS coated electrode

Chemically modified electrodes are widely employed in electroanalytical chemistry and an important goal is to strongly anchor redox mediators on the electrode surface. In this work, indium tin oxide (ITO) electrodes have been coated with PEDOT:PSS that has been ferrocene-functionalized, by a two-step procedure consisting of the electrodeposition of PEDOT-N3 followed by copper-catalyzed azide-alkyne cycloaddition of ethynylferrocene. The coated electrodes have been characterized by XPS, showing successful ferrocene immobilization, by AFM, and by cyclic voltammetry (CV), which is dominated by the stable and highly reversible response of ferrocene. The electrocatalytical performance of the device is assessed by analyzing 3,4-dihydroxyphenyl ethylamine, also commonly known as dopamine (DA). The sensor presents a linear range between 0.01 and 0.9 mM, a mean sensitivity of 196 mA M-1 cm-2 and a limit of detection (LoD) of 1 µM.

Role of Fe in the oxidation of methanol electrocatalyzed by Ni based layered double hydroxides: X-ray spectroscopic and electrochemical studies

Ni/Al and Ni/Fe layered double hydroxides (LDHs) were electrosynthesized on Pt electrodes to be used as catalysts for the development of methanol fuel cells. The electrochemical characterization and the electrocatalytic activity of the two LDHs towards methanol electro-oxidation in alkaline conditions were investigated. Furthermore, the role of Fe on the electronic and structural properties of the LDHs was investigated performing X-ray Absorption Spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS). By the means of these techniques the materials were studied just electrosynthesized and after potentiostatic oxidation. The percentage of Ni active sites in the Ni/Fe LDH was higher than that of Ni/Al LDH, leading to a more efficient catalytic effect towards methanol oxidation in terms of current recorded at any potential value. Furthermore, methanol oxidation occurred at a lower potential for the same current density in the case of Ni/Fe LDH. The electrocatalytic performance displayed by the Ni/Fe LDH suggested the occurrence of a synergic effect between Ni and Fe sites, even if Fe is not directly involved in the redox process, and this evidence was confirmed by XAS and XPS experiments.

PEDOT: Dye-Based, Flexible Organic Electrochemical Transistor for Highly Sensitive pH Monitoring

Organic electrochemical transistors (OECTs) are bioelectronic devices able to bridge electronic and biological domains with especially high amplification and configurational versatility and thus stand out as promising platforms for healthcare applications and portable sensing technologies. Here, we have optimized the synthesis of two pH-sensitive composites of PEDOT (poly(3,4-ethylenedioxythiophene)) doped with pH dyes (BTB and MO, i.e., Bromothymol Blue and Methyl Orange, respectively), showing their ability to successfully convert the pH into an electrical signal. The PEDOT:BTB composite, which exhibited the best performance, was used as the gate electrode to develop an OECT sensor for pH monitoring that can reliably operate in a two-fold transduction mode with super-Nernstian sensitivity. When the OECT transconductance is employed as analytical signal, a sensitivity of 93 ± 8 mV pH unit-1 is achieved by successive sampling in aqueous electrolytes. When the detection is carried out by dynamically changing the pH of the same medium, the offset gate voltage of the OECT shifts by (1.1 ± 0.3) × 102 mV pH unit-1. As a further step, the optimized configuration was realized on a PET substrate, and the performance of the resulting flexible OECT was assessed in artificial sweat within a medically relevant pH range.