a-method-for-systematic-electrochemical-electrophysiological

A Method for Systematic Electrochemical and Electrophysiological Evaluation of Neural Recording Electrodes

New materials and designs for neural implants are typically tested separately, with a demonstration of performance but without reference to other implant ...

La Trobe University

ARC Centre of Excellence for Electromaterials Science

The electrochemical properties of porphyrin-appended dendrimers containing 2-, 4-, 8-, 16-, 32- and 64-porphyrin macrocycles in their free-base and zinc(II) forms have been investigated. Both series gave diffusional based voltammetric responses in dichloromethane. There was minimal effect of dendrimer generation on the redox potentials. Multiple pi-cation and anion radicals as well as dications and dianions were formed on the surface of the dendrimers on oxidation or reduction as appropriate, with each cyclic voltammetric wave representing electron transfer to or from multiple non-interacting porphyrin sites. Electrostatic interactions in the higher generation dendrimers result in kinetic effects being observed for the highly charged species generated when each porphyrin unit is doubly or triply oxidised. The number of electrons transferred on reduction or oxidation of the dendrimers was evaluated using steady-state microelectrode voltammetry. For the lower generations of species a good correlation was observed between numbers of electrons transferred and number of porphyrin entities per molecule; for the dendrimers containing 32 and 64 units, however, slight negative deviations were observed, possibly due to electrostatic interactions as the porphyrins become closer packed.

Electrochemical studies of porphyrin-appended dendrimers.

The electrochemical reduction of TCNQ to TCNQ*- in acetonitrile in the presence of [Cu(MeCN)4]+ has been undertaken at boron-doped diamond (BDD) and indium tin oxide (ITO) electrodes. The nucleation and growth process at BDD is similar to that reported previously at metal electrodes. At an ITO electrode, the electrocrystallization of more strongly adhered, larger, branched, needle-shaped phase I CuTCNQ crystals is detected under potential step conditions and also when the potential is cycled over the potential range of 0.7 to -0.1 V versus Ag/AgCl (3 M KCl). Video imaging can be used at optically transparent ITO electrodes to monitor the growth stage of the very large branched crystals formed during the course of electrochemical experiments. Both in situ video imaging and ex situ X-ray diffraction and scanning electron microscopy (SEM) data are consistent with the nucleation of CuTCNQ taking place at a discrete number of preferred sites on the ITO surface. At BDD electrodes, ex situ optical images show that the preferential growth of CuTCNQ occurs at the more highly conducting boron-rich areas of the electrode, within which there are preferred sites for CuTCNQ formation.

Electrocrystallization of Phase I, CuTCNQ (TCNQ = 7,7,8,8-Tetracyanoquinodimethane), on indium tin oxide and boron-doped diamond electrodes.

Electrocrystallization of single nanowires and/or crystalline thin films of the semiconducting and magnetic Co[TCNQ]2(H2O)2 (TCNQ=tetracyanoquinodimethane) charge-transfer complex onto glassy carbon, indium tin oxide, or metallic electrodes occurs when TCNQ is reduced in acetonitrile (0.1 M [NBu4][ClO4]) in the presence of hydrated cobalt(II) salts. The morphology of the deposited solid is potential dependent. Other factors influencing the electrocrystallization process include deposition time, concentration, and identity of the Co2+(MeCN) counteranion. Mechanistic details have been elucidated by use of cyclic voltammetry, chronoamperometry, electrochemical quartz crystal microbalance, and galvanostatic methods together with spectroscopic and microscopic techniques. The results provide direct evidence that electrocrystallization takes place through two distinctly different, potential-dependent mechanisms, with progressive nucleation and 3-D growth being controlled by the generation of [TCNQ]*- at the electrode and the diffusion of Co2+(MeCN) from the bulk solution. Images obtained by scanning electron microscopy reveal that electrocrystallization of Co[TCNQ]2(H2O)2 at potentials in the range of 0.1-0 V vs Ag/AgCl, corresponding to the [TCNQ]0/*- diffusion-controlled regime, gives rise to arrays of well-separated, needle-shaped nanowires via the overall reaction 2[TCNQ]*-(MeCN)+Co2+(MeCN)+2H2O right harpoon over left harpoon {Co[TCNQ]2(H2O)2}(s). In this potential region, nucleation and growth occur at randomly separated defect sites on the electrode surface. In contrast, at more negative potentials, a compact film of densely packed, uniformly oriented, hexagonal-shaped nanorods is formed. This is achieved at a substantially increased number of nucleation sites created by direct reduction of a thin film of what is proposed to be cobalt-stabilized {(Co2+)([TCNQ2]*-)2} dimeric anion. Despite the potential-dependent morphology of the electrocrystallized Co[TCNQ]2(H2O)2 and the markedly different nucleation-growth mechanisms, IR, Raman, elemental, and thermogravimetric analyses, together with X-ray diffraction, all confirmed the formation of a highly pure and crystalline phase of Co[TCNQ]2(H2O)2 on the electrode surface. Thus, differences in the electrodeposited material are confined to morphology and not to phase or composition differences. This study highlights the importance of the electrocrystallization approach in constructing and precisely controlling the morphology and stoichiometry of Co[TCNQ]2-based materials.

Tuning the electrocrystallization parameters of semiconducting Co[TCNQ]2-based materials to yield either single nanowires or crystalline thin films.

A general theory has been developed for voltammetric ion sensing of cations and anions based on the use of an electrode coated with a membrane containing an electroactive species, an ionophore, and a supporting electrolyte dissolved in a plasticizer. In experimental studies, a membrane coated electrode is fabricated by the drop coating method. In one configuration, a glassy carbon electrode is coated with a poly(vinyl chloride) based membrane, which contains the electroactive species, ionophore, plasticizer and supporting electrolyte. In the case of a cation sensor, ionophore facilitated transfer of the target cation from the aqueous solution to the membrane phase occurs during the course of the reduction of the electroactive species present in the membrane in order to maintain charge neutrality. The formal potential is calculated from the cyclic voltammogram as the average of the reduction and oxidation peak potentials and depends on the identity and concentration of the ion present in the aqueous solution phase. A plot of the formal potential versus the logarithm of the concentration exhibits a close to Nernstian slope of RT/F millivolts per decade change in concentration when the concentration of K(+) and Na(+) is varied over the concentration range of 0.1 mM to 1 M when K(+) or Na(+) ionophores are used in the membrane. The slope is close to RT/2F millivolts for a Ca(2+) voltammetric ion-selective electrode fabricated using a Ca(2+) ionophore. The sensor measurement time is only a few seconds. Voltammetric sensors for K(+), Na(+), and Ca(2+) constructed in this manner exhibit the sensitivity and selectivity required for determination of these ions in environmentally and biologically important matrixes. Analogous principles apply to the fabrication of anion voltammetric sensors.

Voltammetric ion-selective electrodes for the selective determination of cations and anions.

Neural recording electrodes suffer from poor signal to noise ratio, charge density, biostability and biocompatibility. This paper investigates the ability of conducting polymer coated electrodes to record acute neural response in a systematic manner, allowing in depth comparison of electrochemical and electrophysiological response.

Alexander R. Harris

School of Psychological Science

La Trobe University

A Method for Systematic Electrochemical and Electrophysiological Evaluation of Neural Recording Electrodes

Alexander R. Harris

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La Trobe University

A Method for Systematic Electrochemical and Electrophysiological Evaluation of Neural Recording Electrodes

School of Psychological Science