Judith F. Rubinson

Electrochemical control of solid phase micro-extraction using unique conducting polymer coated fibers

The use of a solid phase micro-extraction (SPME) method with poly(3-methylthiophene) coated platinum micro-fiber electrodes to extract arsenate ions from aqueous solutions without derivatization is described. The fibers were fabricated by cycling the working electrode between –0.20 and +1.7 V (vs. Ag/AgCl) in an acetonitrile solution containing 50 mM 3-methylthiophene monomer and 75 mM tetrabutylammonium tetrafluoroborate (TBATFB) electrolyte. All electrochemical procedures (extraction and expulsion) were conducted in a three-electrode system. After fabrication, the conducting polymer film was immersed in the sample solution and converted to its oxidized, positively charged form by applying a constant potential of +1.2 V with respect to Ag/AgCl reference electrode. Arsenate ions migrated into the film to maintain electroneutrality. Upon subsequent reversal of the potential to –0.60 V vs. Ag/AgCl, the polymer film was converted to its reduced, neutral form and the arsenate ions were expelled into a smaller volume (200 µL) of de-ionized water for analysis using flow injection with inductively coupled plasma mass spectrometric (ICP-MS) detection.

Improving the performance of poly(3,4-ethylenedioxythiophene) for brain–machine interface applications

Conducting polymers, especially poly(3,4-ethylenedioxythiophene) (PEDOT) based materials, are important for developing highly sensitive and microscale neural probes. In the present work, we show that the conductivity and stability of PEDOT can be significantly increased by switching the widely used counter anion poly(styrenesulfonate) (PSS) to the smaller tetrafluoroborate (TFB) anion during the electrodeposition of the polymer. Time-dependent impedance measurements of polymer modified implantable microwires were conducted in physiological buffer solutions under accelerated aging conditions and the relative stability of PEDOT:PSS and PEDOT:TFB modified microwires was compared over time. This study was also extended to carbon nanotube (CNT) incorporated PEDOT:PSS which, according to some reports, is claimed to enhance the stability and electrical performance of the polymer. However, no noticeable difference was observed between PEDOT:PSS and CNT:PEDOT:PSS in our measurements. At the biologically relevant frequency of 1kHz, PEDOT:TFB modified microwires exhibit approximately one order of magnitude higher conductivity and demonstrate enhanced stability over both PEDOT:PSS and CNT:PEDOT:PSS modified microwires. In addition, PEDOT:TFB is not neurotoxic and we show the proof-of-concept for both in vitro and in vivo neuronal recordings using PEDOT:TFB modified microelectrode arrays and chronic electrodes, respectively. Our findings suggest that PEDOT:TFB is a promising conductive polymer coating for the recording of neural activities.

Voltammetric studies of the oxidation of reduced nicotinamide adenine dinucleotide at a conducting polymer electrode

Voltammetric studies of the redox behaviour of NADH at a poly(3-methylthiophene) conducting polymer electrode showed a large electrocatalytic effect without the use of electron-transfer mediators.

Histocompatibility and in vivo Signal Throughput for PEDOT, PEDOP, P3MT and Polycarbazole Electrodes

Stimulation and recording of the in vivo electrical activity of neurons are critical functions in contemporary biomedical research and in treatment of patients with neurological disorders. The electrodes presently in use tend to exhibit short effective lifespans due to degradation of signal transmission resulting from the tissue response at the electrode-brain interface, with signal throughput suffering most at the low frequencies relevant for biosignals. To overcome these limitations, new electrode designs to minimize tissue responses, including conducting polymers (CPs) have been explored. Here, we report the short-term histocompatibility and signal throughput results comparing platinum and CP-modified platinum electrodes in a Sprague-Dawley rat model. Two of the polymers tested elicited significantly decreased astrocyte responses relative to platinum. These polymers also showed improved signal throughput at low frequencies and comparable signal-to-noise ratios during targeted intracranial electroencephalograms. These results suggest that CP electrodes may present viable alternatives to the metal electrodes that are currently in use.

Chronic intracortical neural recordings using microelectrode arrays coated with PEDOT-TFB.

UNLABELLED Microelectrode arrays have been extensively utilized to record extracellular neuronal activity for brain-machine interface applications. Modifying the microelectrodes with conductive polymers such as poly(3,4-ethylenedioxythiophene) (PEDOT) has been reported to be advantageous because it increases the effective surface area of the microelectrodes, thereby decreasing impedance and enhancing charge transfer capacity. However, the long term stability and integrity of such coatings for chronic recordings remains unclear. Previously, our group has demonstrated that use of the smaller counter ion tetrafluoroborate (TFB) during electrodeposition increased the stability of the PEDOT coatings in vitro compared to the commonly used counter ion poly(styrenesulfonate) (PSS). In the current work, we examined the long-term in vivo performance of PEDOT-TFB coated microelectrodes. To do so, we selectively modified half of the microelectrodes on NeuroNexus single shank probes with PEDOT-TFB while the other half of the microelectrodes were modified with gold as a control. The modified probes were then implanted into the primary motor cortex of rats. Single unit recordings were observed on both PEDOT-TFB and gold control microelectrodes for more than 12 weeks. Compared to the gold-coated microelectrodes, the PEDOT-TFB coated microelectrodes exhibited an overall significantly lower impedance and higher number of units per microelectrode specifically for the first four weeks. The majority of PEDOT-TFB microelectrodes with activity had an impedance magnitude lower than 400 kΩ at 1 kHz. Our equivalent circuit modeling of the impedance data suggests stability in the polymer-related parameters for the duration of the study. In addition, when comparing PEDOT-TFB microelectrodes with and without long-term activity, we observed a distinction in certain circuit parameters for these microelectrodes derived from equivalent circuit modeling prior to implantation. This observation may prove useful in qualifying PEDOT-TFB microelectrodes with a greater likelihood of registering long-term activity. Overall, our findings confirm that PEDOT-TFB is a chronically stable coating for microelectrodes to enable neural recording. STATEMENT OF SIGNIFICANCE Microelectrode arrays have been extensively utilized to record extracellular neuronal activity for brain-machine interface applications. Poly(3,4-ethylenedioxythiophene) (PEDOT) has gained interest because of its unique electrochemical characteristics and its excellent intrinsic electrical conductivity. However, the long-term stability of the PEDOT film, especially for chronic neural applications, is unclear. In this manuscript, we report for the first time the use of highly stable PEDOT doped with tetrafluoroborate (TFB) for long-term neural recordings. We show that PEDOT-TFB coated microelectrodes on average register more units compared to control gold microelectrodes for at least first four weeks post implantation. We collected the in vivo impedance data over a wide frequency spectrum and developed an equivalent circuit model which helped us determine certain parameters to distinguish between PEDOT-TFB microelectrodes with and without long-term activity. Our findings suggest that PEDOT-TFB is a chronically stable coating for neural recording microelectrodes. As such, PEDOT-TFB could facilitate chronic recordings with ultra-small and high-density neural arrays.

Improved Poly(3,4-Ethylenedioxythiophene) (PEDOT) for Neural Stimulation

This study compares the stability of three variations of the conductive polymer poly(3,4‐ethylenedioxythiophene) or PEDOT for neural micro‐stimulation under both in vitro and in vivo conditions. We examined PEDOT films deposited with counter‐ions tetrafluoroborate (TFB) and poly(styrenesulfonate) (PSS), and PEDOT:PSS combined with carbon nanotubes (CNTs).

Catalysis of the reduction of acetylene at poly-3-methylthiophene electrodes

Abstract Widespread interest has been evoked in recent years in conducting polymers due to their possibilities in the areas of battery/fuel cell research and electroanalysis. This study is part of a long-term effort to explore the potential applications of poly-3-methylthiophene (P3MT) electrodes, deposited both on noble metal electrodes and on active metals such as nickel, copper, iron and stainless steel, molybdenum (these are designated here as M/P3MT electrodes). Reported here are the results for the reduction of acetylene at M/P3MT electrodes, where M=Pt, glassy carbon, glassy carbon and Mo. We have found that it is possible to shift the product ratio for the reduction toward ethylene (as opposed to ethane) to a varying extent in these electrodes, especially when molybdenum is incorporated into the P3MT film.

Near-ohmic behavior for conducting polymers: extension beyond PEDOT on gold-plated platinum to other polymer-counterion/substrate combinations.

Conducting polymers constitute a class of materials for which electrochemical and electron transport properties are a function not only of their chemical identity but also of their complex morphology. In this paper, we investigate and compare the frequency dependence behavior of the impedance of poly(3,4-ethylenedioxythiophene), or PEDOT, and that of poly(3,4-ethylenedioxypyrrole), or PEDOP, which are doped with a series of polyatomic anions during electrodeposition. We also contrast the behavior of PEDOT on Pt|Au, Pt, glassy carbon, and gold. Initial results for polycarbazole, PCz, electrodes are, in addition, included. Deposition parameters were adjusted to produce morphologically similar films for PEDOT, PEDOP, and PCz. In doing so, we have been successful in producing frequency-independent impedance behavior similar to that previously reported for PEDOT on Pt|Au. Although the impedance behavior of these polymers appears to be primarily determined by morphological features, the impact of counterion identity (beyond ionic charge transport) is also discussed. These studies suggest that choice of polymer/dopant combination and electrodeposition parameters can be manipulated to tune the impedance characteristics of electrodes, thereby optimizing them for capacitive or faradaic charge injection, or some combination of the two.

Preparation of Microfiber and Smooth Film Conducting Polymer Electrodes

As part of a long-range effort involving the design and fabrication of electrodes for in vivo use, we describe herein a method for the production of conducting polymer fibers with diameters on the order of a few micrometers. This method has been shown to be applicable to several different combinations of monomers [pyrrole, N-methylpyrrole, 3-methylthiophene, poly(3,4-ethylene dioxythiophene)], deposition substrates (Pt, stainless steel), and dopant ions (dodecyl benzene sulfonate, perchlorate, chloride, polymethylmethacrylate). Cyclic voltammetric characterization shows that the background current is minimal, and normal behavior is seen for the ferri/ferrocyanide couple with the exception of a slightly increased peak separation. An ancillary benefit of these studies is the production of extremely smooth films of these polymers, even in cases where the films are up to 20 μm thick.