Neurotransmitters are important chemicals in human physiological systems for initiating neuronal signaling pathways and in various critical health illnesses. However, concentration of neurotransmitters in the human body is very low (nM or pM level) and it is extremely difficult to detect the fluctuation of their concentrations in patients using existing electrochemical biosensors. In this work, we report the performance of highly densified carbon nanotubes fiber (HD-CNTf) crosssections called rods (diameter ∼ 69 μm, and length ∼ 40 μm) as an ultrasensitive platform for detection of common neurotransmitters. HD-CNTf rods microelectrodes have open-ended CNTs exposed at the interface with electrolytes and cells and display a low impedance value, i.e., 1050 Ω. Their fabrication starts with dry spun CNT fibers that are encapsulated in an insulating polymer to preserve their structure and alignment. Arrays of HD-CNTf rods microelectrodes were applied to detect neurotransmitters, i.e., dopamine (DA), serotonin (5-HT), epinephrine (Epn), and norepinephrine (Norepn), using square wave voltammetry (SWV) and cyclic voltammetry (CV). They demonstrate good linearity in a broad linear range (1 nM to 100 μM) with an excellent limit of detection, i.e., 32 pM, 31 pM, 64 pM, and 9 pM for DA, 5-HT, Epn, and Norepn, respectively. To demonstrate practical application of HD-CNTf rod arrays, detection of DA in human biological fluids and real time monitoring of DA release from living pheochromocytoma (PC12) cells were performed.
Recordings and stimulations of neuronal electrical activity are topics of great interest in neuroscience. Many recording techniques, and even treatment of neurological disorders, can benefit from a microelectrode that is flexible, chemically inert, and electrically conducting and preferentially transfers electrons via capacitive charge injection. Commercial electrodes that currently exist and other electrodes that are being tested with the purpose of facilitating and improving the electron transport between solid materials and biological tissues still have some limitations. This paper discusses carbon nanotube (CNT)-based microelectrodes to record and stimulate neurons and compares their electron transport capabilities to noble metals such as Au and Ag. The recording ability of electrodes is tested through electroretinography on Sarcophaga bullata fly eyes by using Au and Ag wires and CNT fibers as electrodes. Stimulation is demonstrated through the implantation of Au wire and CNT fibers into the antennas of the Madagascar hissing cockroach (Gromphadorhina portentosa) to control their locomotion. Our results demonstrate that a particular property of the CNT fiber is its high rate of electron transfer, leading to an order of magnitude lower impedance compared to Au and Ag and an impressive 15.09 charge injection capacity. We also established that this carbon nanomaterial assembly performs well for in vivo electrophysiology, rendering it a promising prospect for neurophysiological applications.
The exceptional physical properties of carbon nanotubes (CNTs) have the potential to transform materials science and various industrial applications. However, to exploit their unique properties in carbon-based electronics, CNTs regularly need to be chemically interfaced with metals. Although CNTs can be directly synthesized on metal substrates, this process typically requires temperatures above 350 °C, which is not compatible for many applications. Additionally, the CNTs employed here were highly densified, making them suitable as interconnecting materials for electronic applications. This paper reports a method for the chemical bonding of vertically aligned CNTs onto metal substrates that avoids the need for high temperatures and can be performed at temperatures as low as 80 °C. Open-ended CNTs were directly bonded onto Cu and Pt substrates that had been functionalized using diazonium radical reactive species, thus allowing bond formation with the open-ended CNTs. Careful control during grafting of the organic species onto the metal substrates resulted in functional group uniformity, as demonstrated by FT-IR analysis. Scanning electron microscopy images confirmed the formation of direct connections between the vertically aligned CNTs and the metal substrates. Furthermore, electrochemical characterization and application as a sensor revealed the nature of the bonding between the CNTs and the metal substrates.
Implantable neural electrodes are generally used to record the electrical activity of neurons and to stimulate neurons in the nervous system. Biofouling triggered by inflammatory responses can dramatically affect the performance of neural electrodes, resulting in decreased signal sensitivity and consistency over time. Thus, long-term clinical applications require electrically conducting electrode materials with reduced dimensions, high flexibility, and antibiofouling properties that can reduce the degree of inflammatory reactions and increase the lifetime of neural electrodes. Carbon nanotubes (CNTs) are well known to form flexible assemblies such as CNT fibers. Herein, we report the covalent functionalization of predefined CNT fiber and film surfaces with hydrophilic, antibiofouling phosphorylcholine (PC) molecules. The electrochemical and spectroscopic characteristics, impedance properties, hydrophilicity, and in vitro antifouling nature of the functionalized CNT surfaces were evaluated. The hydrophilicity of the functionalized CNT films was demonstrated by a decrease in the static contact angle from 134.4°± 3.9°before to 15.7°± 1.5°after one and fully wetting after three functionalization cycles, respectively. In addition, the extent of protein absorption on the functionalized CNT films was significantly lower than that on the nonfunctionalized CNT film. Surprisingly, the faradic charge-transfer properties and impedance of the CNT assemblies were preserved after functionalization with PC molecules. These functionalized CNT assemblies are promising for the development of low-impedance neural electrodes with higher hydrophilicity and protein-fouling resistance to inhibit inflammatory responses.
Assembling carbon nanotubes (CNTs) into macroscopic materials allows multiple applications that takes advantage of their physical properties. Specifically, the synthesis, fiber assembly, polymer coating and application of CNTs as microelectrodes has allowed us explore physiological applications as well as sensor development. This talk will summarize our research in CNT microelectrode development, neural recording and stimulation, and neurotransmitter detection and quantification through voltammetric techniques. Recordings and stimulations of neuronal electrical activity is a topic of great interest in neuroscience. Current commercial electrodes for transferring charges from solid electrode materials to and from biological tissue still have some limitations. A microelectrode that is flexible, chemically inert, electrically conducting, and preferentially transfers electrons via capacitive charge-injection is needed to treat multiple neurological disorders. This talk will compare charge transport capabilities from carbon nanotube fibers, noble metals Au and Ag wire microelectrodes to biological tissue. The recording ability of microelectrodes is demonstrated through Electroretinography (ERG) on Sarcophaga bullata fly eyes, where light stimulation generates potential changes that were detected by our microelectrodes. Stimulation is demonstrated through Au wire and carbon nanotube fiber implants in Madagascar hissing cockroach (Gromphadorhina portentosa) antennas to control their locomotion directions. In addition, CNT microelectrodes has been found to detect neurotransmitters dopamine (DA), serotonin (5-HT), epinephrine (Epn) and nor-epinephrine (Nor-epn) using voltammetric techniques with excellent limits of detection in the range of ~10-11 M. For practical applicability, the electrodes can detect in real time DA release from culture rat pheochromocytoma (PC12) cells upon concentrate K+stimulation.
Assembling carbon nanotubes (CNTs) into macroscopic materials allows multiple applications that take advantage of their unique physical properties. Specifically, the synthesis, fiber assembly, polymer coating and application of CNTs as microelectrodes in electrochemical detection and quantification of heavy metals and neurotransmitters will be presented. CNT microelectrodes has been found to detect subnanomolar concentrations of Pb, Hg, Cd and Cu simultaneously. Excellent limits of detection of neurotransmitters (in the range of ~10-11M) such as dopamine (DA), serotonin (5-HT), epinephrine (Epn) and nor-epinephrine (Nor-epn) have been observed when using voltammetric techniques. As a proof of practical application, microelectrodes have been tested in real time DA release from culture rat pheochromocytoma (PC12) cells upon concentrate K+stimulation. In addition, the microelectrodes have been tested for recordings and stimulations of neuron electrical activity that is a topic of great interest in neuroscience. Multiple neurological disorders can benefit from a microelectrode that is flexible, chemically inert, electrically conducting, and preferentially transfers electrons via capacitive charge-injection. We compare the electron transfer capabilities among CNT fiber, noble metals Au and Ag wire microelectrodes when connected biological tissue. The recording ability of microelectrodes is demonstrated through Electroretinography (ERG) on Sarcophaga bullata fly eyes (Fig 1, left), where light stimulation generates potential changes that were detected by our microelectrodes. Stimulation is demonstrated through Au wire and carbon nanotube fiber implants in Madagascar hissing cockroach (Gromphadorhina portentosa) antennas (Fig 1, right)to control their locomotion directions. Interface Impedance, charge injection capacity and electrical conductivity of CNT microelectrodes demonstrates their potential in neuroscience applications.
Electrochemistry at open ends and sidewalls of carbon nanotubes (CNTs) has been under debate, with opposing viewpoints as to which sites are more electrochemically active. A particular challenge in this field has been the ability to conduct electrochemical studies selectively at the open-ends of CNTs, without measuring contributions from the sidewalls. This talk will discuss the synthesis and assembly of CNTs into electrochemical sensor where open-ended CNTs were employed for electrochemical measurements. The assembly employs drawable CNTs that minimize sample handling and contamination, in the attempt to preserve the pristine nature of CNTs. Highly densified multiwalled carbon nanotube (HD-CNT) fibers were embedded within a polymer matrix protecting the sidewalls and limiting the reactions to the tips of the CNTs. Cyclic voltammetry was employed to examine the electrochemical properties of open-ended CNTs using a conventional bulk electrochemical cell and scanning electrochemical cell microscopy (SECCM). This assemblies have shown to detect extremely low concentrations of Pb2+ in water, neurotransmitters, NADH, furosemide (diuretic drug), and the electrodes were tested with multiple electrochemical techniques.
Carbon nanotubes (CNTs) are widely used in neuroscience research for its excellent properties such as high conductivity, electrochemical stability, biocompatibility, mechanical compatibility with tissues, tunable mechanical and chemical properties. CNTs are investigating for its performance as neuronal scaffolds to support neural tissues, neuron growth and differentiation as well as neural stimulating and recording electrodes to control and monitor neural activity in both vitro and in vivo environments. Surprisingly neurons show a better interaction with CNTs when neurons are grown on CNT substrates. Previous research has shown neuronal differentiation on pristine CNT and CNT composites and the alignment of neurites on CNT substrates. Current research focus on interaction of neurons at the interface of pristine CNT films with aligned and cross hatch patterned CNT layers prepared from multi-walled CNT arrays. CNTs have nano scale surface roughness and contribution of alignment of CNTs can retain and adhere neurons without any adhesive proteins. As an initial step to study the interaction of neurons with CNT films, the effect of different CNT alignment on neuron adhesion and neurite development was chosen. PC12 cell line was used to study neuron adhesion and neurite alignment on CNT substrates. It is a commonly used neuronal cell model due to its ability to attain sympathetic neurons characteristics in the presence of nerve growth factor (NGF) in low serum medium. For the study, CNT films were prepared as aligned and in cross hatch pattern on PET sheets. After sterilization with UV irradiation, PC12 cells were seeded and cultured at a density of 20 000 cells/mL on CNT films and allowed to differentiate in NGF medium. For comparison poly l lysine (PLL) coated CNT films were used. Fig 1(Left) shows SEM image of a differentiating PC12 cell on PLL coated film of aligned CNTs and Fig 1(Right) shows SEM image of a differentiating PC12 cell on a crosshatch patterned CNT film with no adhesive coating. While neurites of PC12 cells align with the CNTs on aligned CNT film as states in the literature, pristine CNTs in crosshatch pattern were also able to hold the cells without any surface adhesive coating and allow differentiation of cells. Also PC12 cells tend to show random alignment on crosshatch patterned CNTs and adhere well due to higher degree of porosity and surface roughness. As pristine CNT films were able to hold PC12 cells, the interface properties will be further evaluated by impedance spectroscopy. Figure 1
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