Carbon Microelectrodes Research Articles

Indirect Voltammetry Detection of Non-Electroactive Neurotransmitters Using Glassy Carbon Microelectrodes: The Case of Glutamate

Glassy carbon (GC) microelectrodes have been successfully used for the detection of electroactive neurotransmitters such as dopamine and serotonin through voltammetry. However, non-electroactive neurotransmitters such as glutamate, lactate, and gamma-aminobutyric acid (GABA) are inherently unsuitable for detection through voltammetry techniques without functionalizing the surface of the microelectrodes. To this end, we present here the immobilization of the L-glutamate oxidase (GluOx) enzyme on the surface of GC microelectrodes to enable the catalysis of a chemical reaction between L-glutamate, oxygen, and water to produce H2O2, an electroactive byproduct that is readily detectable through voltammetry. This immobilization of GluOx on the surface of bare GC microelectrodes and the subsequent catalytic reduction in H2O2 through fast-scan cyclic voltammetry (FSCV) helped demonstrate the indirect in vitro detection of glutamate, a non-electroactive molecule, at concentrations as low as 10 nM. The functionalized microelectrodes formed part of a four-channel array of microelectrodes (30 μm × 60 μm) on a 1.6 cm long neural probe that was supported on a flexible polymer, with potential for in vivo applications. The types and strengths of the bond between the GC microelectrode surface and its functional groups, on one hand, and glutamate and the immobilized functionalization matrix, on the other hand, were investigated through molecular dynamic (MD) modeling and Fourier transform infrared spectroscopy (FTIR). Both MD modeling and FTIR demonstrated the presence of several covalent bonds in the form of C-O (carbon–oxygen polar covalent bond), C=O (carbonyl), C-H (alkenyl), N-H (hydrogen bond), C-N (carbon–nitrogen single bond), and C≡N (triple carbon–nitrogen bond). Further, penetration tests on an agarose hydrogel model confirmed that the probes are mechanically robust, with their penetrating forces being much lower than the fracture force of the probe material.

Batch Fabrication of Microelectrode Arrays with Glassy Carbon Microelectrodes and Interconnections for Neurochemical Sensing: Promises and Challenges.

Flexible multielectrode arrays with glassy carbon (GC) electrodes and metal interconnection (hybrid MEAs) have shown promising performance in multi-channel neurochemical sensing. A primary challenge faced by hybrid MEAs fabrication is the adhesion of the metal traces with the GC electrodes, as prolonged electrical and mechanical stimulation can lead to adhesion failure. Previous devices with GC electrodes and interconnects made of a homogeneous material (all GC) demonstrated exceptional electrochemical stability but required miniaturization for enhanced tissue integration and chronic electrochemical sensing. In this study, we used two different methods for the fabrication of all GC-MEAs on thin flexible substrates with miniaturized features. The first method, like that previously reported, involves a double pattern-transfer photolithographic process, including transfer-bonding on temporary polymeric support. The second method requires a double-etching process, which uses a 2 µm-thick low stress silicon nitride coating of the Si wafer as the bottom insulator layer for the MEAs, bypassing the pattern-transfer and demonstrating a novel technique with potential advantages. We confirmed the feasibility of the two fabrication processes by verifying the practical conductivity of 3 µm-wide 2 µm-thick GC traces, the GC microelectrode functionality, and their sensing capability for the detection of serotonin using fast scan cyclic voltammetry. Through the exchange and discussion of insights regarding the strengths and limitations of these microfabrication methods, our goal is to propel the advancement of GC-based MEAs for the next generation of neural interface devices.

Integration of Ultrathin Bubble Walls and Electrochemistry: Innovation in Microsensing for Forensic Nitrite Detection and Microscale Metallic Film Deposition.

We present a strategy for electrochemical measurements using a durable minute bubble wall with a thickness of 27 μm (D = 1.8 cm) as an innovative electrochemical medium. The composition, thickness, and volume of the tiny bubble film were investigated and estimated using the spectroscopic method and the Beer-Lambert law. A carbon microelectrode (D = 10 μm) was then employed as the working electrode, inserted through the bubble wall to function as the solution interface. First, the potential of this method for microelectrodeposition of metallic Ag and Pd films in a tiny bubble was investigated. Interestingly, microscopic images of the deposited film clearly demonstrated that the bubble thickness determines and confines the electrochemical deposition zone. In other words, innovative template-free microelectrodeposition was achieved. In the second phase of our work, microelectroanalysis of trace levels of nitrite ions was performed within the bubble wall and on a foam-covered hand, between the fingers directly, with a low limit of detection of 28 μM. This technique holds significance in criminal investigations, as the presence of NO2- ions on the hand indicates the potential presence of gunshot residue and aids in identifying suspects. In comparison to current methods, this approach is rapid, simple, cost-effective, and amenable to on-site applications, eliminating the need for sample treatment. Ultimately, the utilization of a bubble wall as a novel electrochemical microreactor can open new ways in microelectrochemical analysis, presenting novel opportunities and applications in the field of electrochemical sensors.

Beyond two dimensions: Exploring 3D dielectrophoresis for microparticle control using carbon electrodes.

This study explores the frontiers of microparticle manipulation by introducing an actuator platform for the three-dimensional positioning of microparticles using dielectrophoresis (DEP), a technique known for its selectivity and ease of integration with microtechnology. Leveraging advancements in carbon-based devices due to their biocompatibility and electrochemical stability, our work extends the application of DEP from two-dimensional constraints to precise 3D positioning within microvolumes, employing a photolithography-based fabrication process known as Carbon-MEMS technology (C-MEMS). We present the design, finite element simulation, fabrication, and testing of this platform, which utilizes a unique combination of planar and 3D carbon microelectrodes individually addressable on a transparent substrate. This setup enables the application of DEP forces, allowing for high-throughput manipulation of multiple microparticles simultaneously, as well as displacement of individual microparticles in any desired direction. Demonstrated with spherical 1μm and 10μm diameter polystyrene microparticles, this platform features straightforward fabrication and is suitable for batch industrial production. The study concludes with a discussion of the platform's advantages and limitations, marking a significant step toward a valuable tool for studying complex biological systems.

Mechanistic Insights into Tyrosine Oxidation Revealed by Fast-Scan Cyclic Voltammetry

A precise understanding of opioid dynamics in the brain is lacking, largely because few analytical tools are available that are sufficiently sensitive and selective for directly monitoring these important neurochemicals in situ. Tyrosine is a key part of the opioid neuropeptides, and its redox activity can be exploited in their detection. However, a solid understanding of tyrosine electrochemistry on carbon electrodes is first required. It is known that, upon oxidation, tyrosine quickly undergoes chemical transformation to products that foul the electrode, compromising the electrochemical information. This issue can be avoided by using fast-scan cyclic voltammetry (FSCV) which enables collection of well-defined voltammograms on a millisecond timescale before appreciable accumulation of byproducts. In this work, the tyrosine oxidation reaction is characterized and electrode fouling is assessed when tyrosine is incorporated into short synthetic peptides containing other amino acids that are part of the opioid neuropeptides: Gly, Phe, Met, Leu, and Arg. Voltammetric data were obtained in phosphate buffered solution on carbon microelectrodes at scan rates up to 1 kV/s. The results demonstrate the dependence of the peak oxidation potential on solution pH, and identify possible reaction pathways and byproducts that foul the electrode. Peak oxidation potential, peak shape, and ultimately, sensitivity to tyrosine are dependent on the peptide sequence. NMR spectroscopy is used to relate the proximity of proton acceptor/donor functional groups to the electrochemical results in the context of proton-coupled electron transfer (PCET) theory. These fundamental studies are important for the electrochemical detection of important neuropeptides in the complex in vivo environment. Furthermore, a better understanding of the tunability of PCET reaction rates in electrochemical systems has broad implications for technology in a range of areas including artificial photosynthesis, solar energy conversion, and energy storage.

Nanoengineered Carbon Surfaces for Ultrasensitive Neurochemical Detection

Hypothesis-driven methods to tailor the functionality, topology, carbon orientation, and three-dimensional (3D) structure of carbon microelectrodes, for specific classes of neurochemicals, would significantly improve analyte specificity and lower the limits of detection for fast-scan cyclic voltammetry (FSCV) detection in the brain and beyond. FSCV at carbon-fiber microelectrodes is an established technique to study the signaling dynamics of many different neurochemicals in the brain. Despite its widespread applicability to many analytes, most in vivo studies are conducted on bare, unmodified carbon-fiber microelectrodes and many of the fundamental studies of new electrode materials focus on dopamine detection. Manipulations of the carbon surface can have profound effects on electrochemical detection and carefully controlled experiments to manipulate the analyte-electrode interface could provide exquisite information for future design of electrode materials. Here, we will discuss our latest work on synthesizing and manipulating the surfaces of carbon-based materials including graphene-based fibers and porous PAN-derived carbon-fiber, to exploit specific neurochemical-electrode interactions for improved detection. This presentation will demonstrate the importance of understanding the analyte-structure-electrode-surface interface for designing optimal electrodes.

Electrode surfaces based on multiwall carbon nanotubes-chitosan composites validated in the detection of homocysteine biomarkers for cardiovascular disease risk monitoring

Abstract This study aimed to modify screen-printed carbon micro-electrode surfaces by coating them with multiwall carbon-based nanotubes conjugated with chitosan and then validated the formed multiwall carbon-based nanotubes-chitosan coated screen printed carbon micro-electrode for the detection of homocysteine, a biomarker analyte known as a risk indicator in cardiovascular disease. The microstructure surface and crystallographic structure stability of the formed multiwall carbon-based nanotubes-chitosan obtained at formed multiwall carbon-based nanotubes per chitosan ratios of 1:1, 2:1, 3:1, and 4:1 were examined via field emission scanning electron microscopy, X-ray radiation, Raman spectroscopy, surface area and pore size, and thermogravimetric analyses. Homocysteine solutions at 30–100 µM were measured by cyclic voltammetry using the different formed multiwall carbon-based nanotubes-chitosan compositions as sensor electrodes. That with an optimal formed multiwall carbon-based nanotubes per chitosan ratio of 4:1 showed the highest crystallinity and electrical conductivity and gave a high coefficient of determination (R2 = 0.9036) between the homocysteine concentration and the oxidation current detection over an operating range of 30–100 µM. This new composite microelectrode for detecting homocysteine concentration makes it a promising candidate for clinical applications.

Detection, quantification, and characterization of polystyrene microplastics and adsorbed bisphenol A contaminant using electroanalytical techniques

The potential applications of electroanalytical techniques for the quantification and size characterization of nonelectroactive polystyrene microplastics is reported, in addition to characterizing the kinetics of adsorption of bisphenol A on these polystyrene microparticles. The individual adsorption events of very diluted polystyrene microparticles dispersions on glassy-carbon microelectrodes produce the blocking of the charge transfer of a mediator (ferrocene-methanol) thus decreasing the current of the recorded chronoamperogram in a stepwise manner. The magnitude of the current steps are in the order of pA values and can be related to the diameter of the plastic microparticles in the size range 0.1 to 10 µm. The frequency of the current steps in the domain time used (120 s) allows to quantify the number concentration of these microparticles in the range 0.005 to 0.500 pM. Electrochemical impedance spectroscopy confirms the adsorption of the polystyrene microplastics on carbon microelectrodes (and to a lesser extent on platinum microelectrodes) under the same experimental conditions as above. On the other hand, the adsorbed microplastics become concentrators of other pollutants found in the environment. The sensitive differential-pulse voltammetry determination of bisphenol A (linear range 0.80–15.00 µM; detection limit 0.24 µM) was used together with a simple separation procedure for studying the adsorption of bisphenol A on polystyrene microparticles. The adsorption capacity (mg of bisphenol A retained per g of the polystyrene microplastics) decreased from approximately 5.7 to 0.8 mg g−1 with increasing dosages of polystyrene microparticles from 0.2 to 1.6 g l−1. The adsorption isotherms were modeled resulting in a monolayer of bisphenol A adsorbed on the microplastics (i.e., best fitted to a Langmuir model).Graphical abstract

Characterizing the removal of Pb2+ and Zn2+ from an acidic smelting wastewater using electrocatalytic internal Fe0/C micro-electrolysis

The discharge of acid non-ferrous metal smelting wastewater has received unprecedented attention with increasing threat to ecosystems and human health. A combined process of electrocatalytic internal Fe0/C micro-electrolysis (ECIME) fluidized bed was developed at lab scale (2.0L) to investigate the co-removal performance, optimization conditions and reaction characteristics differences of Pb2+ and Zn2+ from acid smelting wastewater. The results of cyclic voltammogram (CV) and XPS reveal that Pb2+ and Zn2+ can be electrodeposited on the carbon microelectrodes surface in ECIME system, and the positive shift degree of the reductive peak potentials under cathodic polarized is positively correlated with the voltage intensity. Metal ions reaction flow direction of ECIME system and SEM, EDS, XPS characterizations indicated that the removal of Pb2+ and Zn2+ were mainly attribute to electroreduction deposition and in-situ iron polymer hydroxyl flocculation, and including four reaction processes of electroreduction on surface of external electrode, iron-carbon particles microelectrode, self-flocculation and secondary flocculation. But the removal of Pb2+ and Zn2+ show different dominant reaction path, Pb2+ removal mainly depended on the electroreduction deposition (64.11%), whereas Zn2+ dominated by iron-based flocculation (62.93%). Response surface optimization based on the average efficiency indicated that the co-removal of Pb2+ and Zn2+ mainly depended on polarization voltage, reaction time, initial pH and electrolyte concentration with optimal conditions of 11.0 V, 58.65 min, 5.0, and 0.08 mol/L, and its optimal average removal efficiency reached 98.52% (initial concentration 100 mg/L), and the residual concentrations were 0.081 mg/L and 1.890 mg/L, respectively. These findings provide insights and theoretical for developing electrochemical platform to realize heavy metal wastewater zero discharge.

Stable in-vivo electrochemical sensing of tonic serotonin levels using PEDOT/CNT-coated glassy carbon flexible microelectrode arrays.

Chronic sampling of tonic serotonin (5-hydroxytryptamine, 5-HT) concentrations in the brain is critical for tracking neurological disease development and the time course of pharmacological treatments. Despite their value, in vivo chronic multi-site measurements of tonic 5-HT have not been reported. To fill this technological gap, we batch-fabricated implantable glassy carbon (GC) microelectrode arrays (MEAs) onto a flexible SU-8 substrate to provide an electrochemically stable and biocompatible device/tissue interface. To achieve detection of tonic 5-HT concentrations, we applied a poly(3,4-ethylenedioxythiophene)/carbon nanotube (PEDOT/CNT) electrode coating and optimized a square wave voltammetry (SWV) waveform for selective 5-HT measurement. In vitro, the PEDOT/CNT-coated GC microelectrodes achieved high sensitivity to 5-HT, good fouling resistance, and excellent selectivity against the most common neurochemical interferents. In vivo, our PEDOT/CNT-coated GC MEAs successfully detected basal 5-HT concentrations at different locations within the CA2 region of the hippocampus of both anesthetized and awake mice. Furthermore, the PEDOT/CNT-coated MEAs were able to detect tonic 5-HT in the mouse hippocampus for one week after implantation. Histology reveals that the flexible GC MEA implants caused less tissue damage and reduced inflammatory response in the hippocampus compared to commercially available stiff silicon probes. To the best of our knowledge, this PEDOT/CNT-coated GC MEA is the first implantable, flexible sensor capable of chronic in vivo multi-site sensing of tonic 5-HT.

Printed Silk Microelectrode Arrays for Electrophysiological Recording and Controlled Drug Delivery.

The use of soft and flexible bioelectronic interfaces can enhance the quality for recording cells' electrical activity by ensuring a continuous and intimate contact with the smooth, curving surfaces found in the physiological environment. This work develops soft microelectrode arrays (MEAs) made of silk fibroin (SF) films for recording interfaces that can also serve as a drug delivery system. Inkjet printing is used as a tool to deposit the substrate, conductive electrode, and insulator, as well as a drug-delivery nanocomposite film. This approach is highly versatile, as shown in the fabrication of carbon microelectrodes, sandwiched between a silk substrate and a silk insulator. The technique permits the development of thin-film devices that can be employed for in vitro extracellular recordings of HL-1 cell action potentials. The tuning of SF by applying an electrical stimulus to produce a permeable layer that can be used in on-demand drug delivery systems is also demonstrated. The multifunctional MEA developed here can pave the way for in vitro drug screening by applying time-resolved and localized chemical stimuli.

Design of optoelectrodes for the remote imaging of cells and in situ electrochemical detection of neurosecretory events

Optical fibers have opened avenues for remote imaging, bioanalyses and recently optogenetics. Besides, miniaturized electrochemical sensors have offered new opportunities in sensing directly redox neurotransmitters. The combination of both optical and electrochemical approaches was usually performed on the platform of microscopes or within microsystems. In this work, we developed optoelectrodes which features merge the advantages of both optical fibers and microelectrodes. Optical fiber bundles were modified at one of their extremity by a transparent ITO deposit. The electrochemical responses of these ITO-modified bundles were characterized for the detection of dopamine, epinephrine and norepinephrine. The analytical performances of the optoelectrodes were equivalent to the ones reported for carbon microelectrodes. The remote imaging of model neurosecretory PC12 cells by optoelectrodes was performed upon cell-staining with common fluorescent dyes: acridine orange and calcein-AM. An optoelectrode placed by micromanipulation at a few micrometers-distance from the cells offered remote images with single cell resolution. Finally, in situ electrochemical sensing was demonstrated by additions of K+-secretagogue solutions near PC12 cells under observation, leading to exocytotic events detected as amperometric spikes at the ITO surface. Such dual sensors should pave the way for in vivo remote imaging, optogenetic stimulation, and simultaneous detection of neurosecretory activities.

Asymmetric enhancement of persulfate activation by N-doped carbon microelectrode: Electro-adsorption and activation pathway regulation

N-doped activated carbon as particle electrode was applied to form a three-dimensional persulfate electro-activation system (ACN/PS/EC) for p-chlorophenol (4CP) removal. The synergy between electro-catalysis (EC) and ACN/PS not only greatly accelerated the PS activation and 4CP degradation processes, but also markedly changed the activation mechanism. Basing on the results of radicals scavenging and electro-adsorption experiments, it is demonstrated that the radical pathway rather than the non-radical pathway was significantly strengthened by using EC owing to the asymmetric electro-adsorption of persulfate ions and 4CP. By quantitative structure-activity relationship analysis, the vital function of graphitic N was unveiled, which exerted action as an active center for PS activation. The effects of operation parameters (including PS concentration, current density, pH, etc.) on the persulfate activation process were also investigated, and the results suggest that the ACN/PS/EC system is a flexible and efficient oxidative system for pollutant degradation, even in complicated wastewaters.

Evaluating carbon-electrode dielectrophoresis under the ASSURED criteria.

Extreme point-of-care refers to medical testing in unfavorable conditions characterized by a lack of primary resources or infrastructure. As witnessed in the recent past, considerable interest in developing devices and technologies exists for extreme point-of-care applications, for which the World Health Organization has introduced a set of encouraging and regulating guidelines. These are referred to as the ASSURED criteria, an acronym for Affordable (A), Sensitive (S), Specific (S), User friendly (U), Rapid and Robust (R), Equipment-free (E), and Delivered (D). However, the current extreme point of care devices may require an intermediate sample preparation step for performing complex biomedical analysis, including the diagnosis of rare-cell diseases and early-stage detection of sepsis. This article assesses the potential of carbon-electrode dielectrophoresis (CarbonDEP) for sample preparation competent in extreme point-of-care, following the ASSURED criteria. We first discuss the theory and utility of dielectrophoresis (DEP) and the advantages of using carbon microelectrodes for this purpose. We then critically review the literature relevant to the use of CarbonDEP for bioparticle manipulation under the scope of the ASSURED criteria. Lastly, we offer a perspective on the roadmap needed to strengthen the use of CarbonDEP in extreme point-of-care applications.

Application of electrocatalytic effects of a newly synthesized monomer and graphene quantum dots to modify glassy carbon microelectrode as a sensor for determination of riluzole

Application of electrocatalytic effects of a newly synthesized monomer and graphene quantum dots to modify glassy carbon microelectrode as a sensor for determination of riluzole

(Digital Presentation) Fabrication and Characterization of C-MEMS Based 3-Dimensional Microelectrode Arrays for Cardiac Electrophysiological Studies

In-vitro examination of the electrical conduction characteristics and the beating of cardiomyocytes can be done using Microelectrode arrays(MEAs). The microelectrodes continuously monitor the electrical activity of the active cardiac cells on a microsecond timescale and presents a precise data based on that. Since the introduction of MEAs, many efforts have been made by scientists to evolve this technology by means of microfabrication. Previously, planar metallic electrodes were mostly used for this study but they could not penetrate the dead cells and reach the active cell layers. To overcome this, protruding 3-D microelectrodes were introduced that could puncture the dead tissues and reach the active cells for effective recording of cardiac electrical potentials.The characteristics of an electrode are defined by the characteristics of the material used to develop it. In this paper, we focus on the fabrication of cylindrical 3-dimensional Glassy Carbon microelectrode arrays (MEAs) for analyzing cell preparations from cardiac tissues. In order to improve the electrical properties of the commercially available electrodes, we have used a highly conductive material for fabrication. Glassy Carbon microelectrodes were fabricated by the pyrolysis of SU-8 microelectrodes which were developed by a standard lithographic process on Silicon wafer. The ohmic nature and high conductivity of Glassy Carbon makes it a suitable material to be implemented in electrophysiological studies. Further in our work, we have performed the theoretical and structural analysis using Comsol Multiphysics 5.2 considering different geometries of microelectrodes. The skin insertion force is calculated to be 0.012N, which is much lesser than the comprehensive force (15.38N), the maximum bending force (0.885N) and the maximum buckling force (3.28N) per cylindrical microelectrode. The maximum Von Mises stress (as shown in the figure) acting at the base where it is attached to the substrate is found out to be 6.56x106 N/m2 for each cylindrical microneedle. Among all the considered geometries, it can be concluded that the fabricated cylindrical microelectrodes penetrate successfully without breaking or bending and causing minimal damage to the cardiac tissues. The slanted tip microelectrode was also favored as it’s design makes it able to easily pierce the cardiac cell layers. However, the Maximum Von Mises stress distribution showed that the stress at the tip was much higher than the Yield strength of Glassy Carbon (4Gpa) making it evident that the tip might break while penetrating. With the theoretical and simulation analysis performed we were thus able to optimize the geometry and establish that Glassy Carbon proves to be a good material for fabrication of 3D microelectrode arrays. Figure 1

Structure and Dynamics of Adsorbed Dopamine on Solvated Carbon Nanotubes and in a CNT Groove.

Advanced carbon microelectrodes, including many carbon-nanotube (CNT)-based electrodes, are being developed for the in vivo detection of neurotransmitters such as dopamine (DA). Our prior simulations of DA and dopamine-o-quinone (DOQ) on pristine, flat graphene showed rapid surface diffusion for all adsorbed species, but it is not known how CNT surfaces affect dopamine adsorption and surface diffusivity. In this work, we use molecular dynamics simulations to investigate the adsorbed structures and surface diffusion dynamics of DA and DOQ on CNTs of varying curvature and helicity. In addition, we study DA dynamics in a groove between two aligned CNTs to model the spatial constraints at the junctions within CNT assemblies. We find that the adsorbate diffusion on a solvated CNT surface depends upon curvature. However, this effect cannot be attributed to changes in the surface energy roughness because the lateral distributions of the molecular adsorbates are similar across curvatures, diffusivities on zigzag and armchair CNTs are indistinguishable, and the curvature dependence disappears in the absence of solvent. Instead, adsorbate diffusivities correlate with the vertical placement of the adsorbate’s moieties, its tilt angle, its orientation along the CNT axis, and the number of waters in its first hydration shell, all of which will influence its effective hydrodynamic radius. Finally, DA diffuses into and remains in the groove between a pair of aligned and solvated CNTs, enhancing diffusivity along the CNT axis. These first studies of surface diffusion on a CNT electrode surface are important for understanding the changes in diffusion dynamics of dopamine on nanostructured carbon electrode surfaces.

Low-temperature and high-voltage planar micro-supercapacitors based on anti-freezing hybrid gel electrolyte

Micro-supercapacitors (MSCs) are considered as highly competitive power sources for miniaturized electronics. However, narrow voltage window and poor anti-freezing properties of MSCs in conventional aqueous electrolytes lead to low energy density and limited environmental adaption. Herein, we report the construction of low-temperature and high-energy-density MSCs based on anti-freezing hybrid gel electrolytes (HGE) through introducing ethylene glycol (EG) additives into aqueous LiCl electrolyte. Since EG partially destroys hydrogen bond network among water molecules, the HGE exhibits maximum electrochemical stability window of 2.7 V and superior anti-freezing features with a glass transition temperature of −62.8 °C. Further, the optimized MSCs using activated carbon microelectrodes possess impressive volumetric capacitance of 28.9 F cm−3 and energy density of 10.3 mWh cm−3 in the voltage of 1.6 V, 2.6 times higher than MSCs tested in 1.2 V. Importantly, the MSCs display 68.3% capacitance retention even at −30 °C compared to the value at 25 °C, and ultra-long cyclability with 85.7% of initial capacitance after 15,000 times, indicating extraordinary low-temperature performance. Besides, our devices offer favorable flexibility and modular integration. Therefore, this work provides a general strategy of realizing flexible, safe and anti-freezing microscale power sources, holding great potential towards subzero-temperature microelectronic applications.

Selective Passivation of Three-Dimensional Carbon Microelectrodes by Polydopamine Electrodeposition and Local Laser Ablation

In this article, a novel approach for selective passivation of three-dimensional pyrolytic carbon microelectrodes via a facile electrochemical polymerization of a non-conductive polymer (polydopamine, PDA) onto the surface of carbon electrodes, followed by a selective laser ablation is elaborated. The 3D carbon electrodes consisting of 284 micropillars on a circular 2D carbon base layer were fabricated by pyrolysis of lithographically patterned negative photoresist SU-8. As a second step, dopamine was electropolymerized onto the electrode by cyclic voltammetry (CV) to provide an insulating layer at its surface. The CV parameters, such as the scan rate and the number of cycles, were investigated and optimized to achieve a reliable and uniform non-conductive coating on the surface of the 3D pyrolytic carbon electrode. Finally, the polydopamine was selectively removed only from the tips of the pillars, by using localized laser ablation. The selectively passivated electrodes were characterized by scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy methods. Due to the surface being composed of highly biocompatible materials, such as pyrolytic carbon and polydopamine, these 3D electrodes are particularly suited for biological application, such as electrochemical monitoring of cells or retinal implants, where highly localized electrical stimulation of nerve cells is beneficial.

Atomistic Simulations of Dopamine Diffusion Dynamics on a Pristine Graphene Surface.

Carbon microelectrodes enable in vivo detection of neurotransmitters, and new electrodes aim to optimize the carbon surface. However, atomistic detail on the diffusion and orientation of neurotransmitters near these surfaces is lacking. Here, we employ molecular dynamics simulations to investigate the surface diffusion of dopamine (DA), its oxidation product dopamine-o-quinone (DOQ), and their protonated forms on the pristine basal plane of flat graphene. We find that all DA species rapidly adsorb to the surface and remain adsorbed, even without a holding potential or graphene surface defects. We also find that the diffusivities of the adsorbed and the fully solvated DA are similar and that the protonated species diffuse more slowly on the surface than their corresponding neutral forms, while the oxidized species diffuse more rapidly. Structurally, we find that the underlying graphene lattice has little influence over the molecular adsorbate's lateral position, and the vertical placement of the amine group on dopamine is highly dependent upon its charge. Finally, we find that solvation has a large effect on surface diffusivities. These first results from molecular dynamics simulations of dopamine at the aqueous-graphene interface show that dopamine diffuses rapidly on the surface, even without an applied potential, and provide a basis for future simulations of neurotransmitter structure and dynamics on advanced carbon materials electrodes.