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

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.

Freeze Drying Improves the Shelf-Life of Conductive Polymer Modified Neural Electrodes.

Coating microelectrodes with conductive polymer is widely recognized to decrease impedance and improve performance of implantable neural devices during recording and stimulation. A concern for wide-spread use of this approach is shelf-life, i.e., the electrochemical stability of the coated microelectrodes prior to use. In this work, we investigated the possibility of using the freeze-drying process in order to retain the native low impedance state and, thereby, improve the shelf-life of conductive polymer poly(3,4-ethylenedioxythiophene) (PEDOT)-PSS modified neural electrodes. Control PEDOT-PSS coated microelectrodes demonstrated a significant increase in impedance at 1 kHz after 41-50 days of room temperature storage. Based on equivalent circuit modeling derived from electrochemical impedance spectroscopy, this increase in impedance could be largely attributed to a decrease in the interfacial capacitance consistent with a collapse and closing of the porous structure of the polymeric coating. Time-dependent electrochemical impedance measurements revealed higher stability of the freeze-dried coated microelectrodes compared to the controls, such that impedance values after 41-50 days appeared to be indistinguishable from the initial levels. This suggests that freeze drying PEDOT-PSS coated microelectrodes correlates with enhanced electrochemical stability during shelf storage.

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.

Effects of carbon nanotube and conducting polymer coated microelectrodes on single-unit recordings in vitro.

Neuronal networks cultured on microelectrode arrays (MEAs) have been utilized as biosensors that can detect all or nothing extracellular action potentials, or spikes. Coating the microelectrodes with carbon nanotubes (CNTs), either pristine or conjugated with a conductive polymer, has been previously reported to improve extracellular recordings presumably via reduction in microelectrode impedance. The goal of this work was to examine the basis of such improvement in vitro. Every other microelectrode of in vitro MEAs was electrochemically modified with either conducting polymer, poly-3,4-ethylenedioxythiophene (PEDOT) or a blend of CNT and PEDOT. Mouse cortical tissue was dissociated and cultured on the MEAs to form functional neuronal networks. The performance of the modified and unmodified microelectrodes was evaluated by activity measures such as spike rate, spike amplitude, burst duration and burst rate. We observed that the yield, defined as percentage of microelectrodes with neuronal activity, was significantly higher by 55% for modified microelectrodes compared to the unmodified sites. However, the spike rate and burst parameters were similar for modified and unmodified microelectrodes suggesting that neuronal networks were not physiologically altered by presence of PEDOT or PEDOT-CNT. Our observations from immunocytochemistry indicated that neuronal cells were more abundant in proximity to modified microelectrodes.

Carbon nanotube compared with carbon black: effects on bacterial survival against grazing by ciliates and antimicrobial treatments.

The ingestion and digestion of Escherichia coli by the ciliated protozoan, Tetrahymena thermophila, was investigated after an initial exposure to either water-soluble single-walled carbon nanotubes (SWNT) or to carbon black (CB). Both SWNT and CB were internalised and visible in food vacuoles of ciliates. When presented with E. coli expressing green-fluorescent protein (GFP), these ciliates internalised bacteria as well. However, ciliates that had first internalised SWNT but not CB subsequently externalised or egested vesicle-like structures with fluorescent bacteria inside. These egested bacteria were viable and less susceptible than planktonic E. coli to killing either by the antibiotic, chloramphenicol or the disinfectant, glutaraldehyde. These results suggest that SWNT can alter the intracellular trafficking of vesicles within ciliates, leading to bacterial prey being packaged externally and protected for a time from environmental killing, which could have implications for sewage treatment and for public health.

Enhanced Sensitivity of Delocalized Plasmonic Nanostructures.

This work reports on the observation of a delocalized surface plasmon resonance (DSPR) phenomenon in linear chains of square-shaped silver nanoparticles (NP) as a function of the chain length and the distance between the nanoparticles in the chain. Transmission spectra of the silver nanoparticle chains reveal the emergence of new, red-shifted extinction peaks that depend strongly on the spacing between the nanoparticles and the polarization of the exciting light with respect to the chain axis. As the spacing between the nanoparticles in the linear chain decreases and the number of nanoparticles in the linear chain increases, the strength of the new extinction features increase strongly. These changes can be described by a tight-binding model for the coupled chain, which indicates that the origin of the phenomenon is consistent with an increased coupling between the nanoparticles. FDTD calculations reveal that the electric field is strongly enhanced between the nanoparticles in the chain. The DSPR response is found to be much more sensitive to dielectric changes than the localized surface plasmon resonance (LSPR).

Carbon nanotube thin film biosensors for sensitive and reproducible whole virus detection.

Here, we report the label-free, sensitive, and real-time electrical detection of whole viruses using carbon nanotube thin film (CNT-TF) field effect devices. Selective detection of approximately 550 model viruses, M13-bacteriophage, is demonstrated using a simple two-terminal (no gate electrode) configuration. Chemical gating through specific antibody-virus binding on CNT surface is proposed to be the sensing mechanism. Compared to electrical impedance sensors with identical microelectrode dimensions (no CNT), the CNT-TF sensors exhibit sensitivity 5 orders higher. We believe the reported approach could lead to a reproducible and cost-effective solution for rapid viral identification.

Study of amyloid ß-peptide (Aß12-28-Cys) interactions with Congo red and ß-sheet breaker peptides using electrochemical impedance spectroscopy.

A surface-based approach is presented to study the interactions of Aß12-28-Cys assembled on gold surfaces with Congo red (CR) and a ß-sheet breaker (BSB) peptide. The various aspects of the peptide film have been examined using different electrochemical and surface analytical techniques. Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) results using redox probes [Fe(CN)(6)](3-/4-) show that Aß12-28-Cys on gold forms a stable and reproducible blocking film. EIS analysis shows that CR and BSB have different effects on the electrochemical properties of Aß12-28-Cys films, presumably due to changes in the interactions between the film and CR and BSB. EIS results indicate that in the case of CR film resistance decreases significantly presumably due to better penetration of the solution-based redox probe into the film, whereas in the case of BSB, the film resistance increases. We interpret this difference to BSB being able to interact with the Aß12-28-Cys on the surface and presumably forming a film that presents a higher resistance for electron transfer from the redox probe to the solution.

The Influence of Molecular Dipole Moment on the Redox-Induced Reorganization of α-Helical Peptide Self-Assembled Monolayers: An Electrochemical SPR Investigation.

Self-assembled monolayers (SAMs) of ferrocene-labeled α-helical peptides were prepared on gold surfaces and studied using electrochemical surface plasmon resonance (EC-SPR). The leucine-rich peptides were synthesized with a cysteine sulfhydryl group either at the C- or N-terminus, enabling their immobilization onto gold surfaces with control of the direction of the molecular dipole moment. Two electroactive SAMs were studied, one in which all of the peptide dipole moments are oriented in the same direction (SAM1), and the other in which the peptide dipole moment of one peptide is aligned in the opposite direction to that of its surrounding peptide molecules (SAM2). Cyclic voltammetry combined with SPR measurements revealed that SAM reorientations concomitant with the oxidation of the ferrocene label were more significant in SAM2 than in SAM1. The substantially greater change in the peptide film thickness in the case of SAM2 is attributed to the electrostatic repulsion between the electrogenerated ferrocinium moiety and the positively charged gold surface. The greater permeability of SAM1 to electrolyte anions, on the other hand, appears to effectively neutralize this electrostatic repulsion. The film thickness change in SAM2 was estimated to be 0.25 ± 0.05 nm using numerical simulation. The timescale of the redox-induced SPR changes was established by chronoamperometry and time-resolved SPR measurements, followed by fitting of the SPR response to a stretched exponential function. The time constants measured for the anodic process were 16 and 6 ms for SAM1 and SAM2 respectively, indicating that the SAM thickness changes are notably fast.

Impact of carbon nanotubes on the ingestion and digestion of bacteria by ciliated protozoa.

Research on the toxicity of carbon nanotubes has focused on human health risks, and little is known about their impact on natural ecosystems. The ciliated protozoan Tetrahymena thermophila has been widely studied by ecotoxicologists because of its role in the regulation of microbial populations through the ingestion and digestion of bacteria, and because it is an important organism in wastewater treatment and an indicator of sewage effluent quality. Here we show that single-walled carbon nanotubes are internalized by T. thermophila, possibly allowing the nanotubes to move up the food chain. The internalization also causes the protozoa to aggregate, which impedes their ability to ingest and digest their prey bacteria species, although it might also be possible to use nanotubes to improve the efficiency of wastewater treatment.

Study of electron transfer in ferrocene-labeled collagen-like peptides.

This study describes the electron transfer (ET) phenomenon through a series of (Pro-Hyp-Gly) repeat units containing collagen mimics. The peptides contain redox-active ferrocene (Fc) and thiol-functionalized cystein (Cys) at the N- and C-terminals, respectively. Peptide films were prepared on gold surfaces and characterized by X-ray photoelectron spectroscopy (XPS), ellipsometry, and Fourier transform-reflection absorption infrared spectroscopy (FT-RAIRS). Electrochemical investigations of the films showed a linear but weakly distance-dependent ET. The importance of H-bonding was realized, and the possibility of a conformationally gated ET mechanism has been discussed.

Investigation of laser induced photocurrent generation experiments.

Current signals produced by the laser-illumination of bare and non-chromophore containing peptide modified gold electrodes were investigated, and we suggest that these current signals which are due to the interfacial potential drop induced by laser heating, may have been mistakenly assigned to molecular-based photocurrents in several recent publications.