Microscale Electrodes Research Articles

Numerical Analysis of Real-Scale Reverse Electrodialysis with Microscale Structures and Electrode Segmentation

Numerical Analysis of Real-Scale Reverse Electrodialysis with Microscale Structures and Electrode Segmentation

Faradaic-free electrokinetic nucleic acid amplification (E-NAAMP) using localized on-chip high frequency Joule heating.

We present a novel Faradaic reaction-free nucleic acid amplification (NAA) method for use with microscale liquid samples. Unlike previous Joule heating methods where the electrodes produce electrolysis gaseous by-products and require both the electrodes be isolated from a sample and the venting of produced electrolysis gas, our electrokinetic Nucleic Acid Amplification (E-NAAMP) method alleviates these issues using a radio frequency (RF) alternating current electric field. In this approach, a pair of microscale thin film gold electrodes are placed directly in contact with a nucleic acid reaction mixture. A high frequency (10-40 MHz) RF potential is then applied across the electrode pair to induce a local Ohmic current within the sample and drive the sample temperature to increase by Joule heating. The temperature increase is sustainable in that it can be generated for several hours of constant use without generating any pH change to the buffer or any microscopically observable gaseous electrolysis by-products. Using this RF Joule heating approach, we demonstrate successful direct thermal amplification using two popular NAA biochemical reactions: loop-mediated isothermal amplification and polymerase chain reaction. Our results demonstrate that a simple microscale electrode structure can be used for thermal regulation for NAA reactions without observable electrolytic reactions, minimal enzyme activity loss and sustained (>50 h use per device) continuous operations without electrode delamination. As such, E-NAAMP offers substantial miniaturization of the heating elements for use in microfluidic or miniaturized NAA reaction systems.

Highly hydroresponsive nacre-like oligo proanthocyanidin-intercalated Ca–Al-Layered double hydroxides/graphene oxide/polyvinyl alcohol as a potential neural implant material

Neural implantation with a microscale electrode permits a real-time neuromoduration to enhance the treatment of neural disorders. However, the penetration of a neural microprobe into soft brain tissues always resulted in the invasive tissue damage and mechanical tissue-device friction, which further induced the extensive chronic inflammation that limited the device lifetime. To develop a biocompatible and biocompliant neural implant, this study presented a smart substrate with bioinspired nacre-like structure that can exhibit ultra-hydroresponsive mechanical strength transition, sustained anti-inflammatory drug delivery, and anti-biofouling ability. An organic-inorganic composite, called LOG-P, was synthesized with oligo proanthocyanidin (OPC)-intercalated Ca–Al layered double hydroxide (Ca–Al-LDH) and graphene oxide (GO), followed by crosslinked with linear structural polyvinyl alcohol (PVA) to form a brick-and-mortar architecture. LOG-P exhibited strong mechanical strength of ~ 5 GPa in Young's modulus at dehydrated state, while showing ~99% decrease of Young's modulus when getting hydrated. In addition, the intercalation of OPC in LDH led to zero-order release with duration up to 80 days. LOG-P with negatively charged surfaces inhibited the adhesion of astrocytes, while the released OPC led to robust neuronal cell viability. Finally, integrated with Ag circuits by aerosol jet printing, the fabricated LOG-P-based microelectrode arrays showed stable electroactivity, also permitting a complete penetration into brain-tissue phantoms without bending. Such organic-inorganic nanocomposite-based neural interface with physical, chemical, and biological compatibility was expected to be a revolutionary platform for development of next-generation neural implants.

Microscale Silicon-Based Anodes: Fundamental Understanding and Industrial Prospects for Practical High-Energy Lithium-Ion Batteries.

To accelerate the commercial implementation of high-energy batteries, recent research thrusts have turned to the practicality of Si-based electrodes. Although numerous nanostructured Si-based materials with exceptional performance have been reported in the past 20 years, the practical development of high-energy Si-based batteries has been beset by the bias between industrial application with gravimetrical energy shortages and scientific research with volumetric limits. In this context, the microscale design of Si-based anodes with densified microstructure has been deemed as an impactful solution to tackle these critical issues. However, their large-scale application is plagued by inadequate cycling stability. In this review, we present the challenges in Si-based materials design and draw a realistic picture regarding practical electrode engineering. Critical appraisals of recent advances in microscale design of stable Si-based materials are presented, including interfacial tailoring of Si microscale electrode, surface modification of SiOx microscale electrode, and structural engineering of hierarchical microscale electrode. Thereafter, other practical metrics beyond active material are also explored, such as robust binder design, electrolyte exploration, prelithiation technology, and thick-electrode engineering. Finally, we provide a roadmap starting with material design and ending with the remaining challenges and integrated improvement strategies toward Si-based full cells.

Conductive Writing with High Precision by Laser‐Induced Point‐to‐Line Carbonization Strategy for Flexible Supercapacitors

AbstractFlexible electronic technologies have broad development prospects. Fabrication of micro devices on flexible carbon precursors via laser carbonizing has the advantages of involving only one step, requiring no mask, and wide adaptability. It has attracted significant attention in recent years to fabricate micro devices on polyimide (PI) via laser carbonization. However, the precision of laser carbonization is low because large areas of the material are easily carbonized simultaneously. Thus, the fabrication of micro devices with high precision and narrow line widths via laser carbonization remains challenging. Herein, a laser‐induced carbonization method with point‐to‐line carbonization strategy is proposed and used to fabricate supercapacitors on PI. The carbon line width decreases from over 150 to below 10 µm using this method. High‐precision conductive writing is realized on PI. Carbon lines written by this method have low resistance and porous surface. Hence, these carbon structures are suitable as electrodes for supercapacitors. Supercapacitors with microscale electrode widths are fabricated using this method. This work demonstrates that the point‐to‐line strategy improves the precision of laser carbonization and provides an effective method for fabricating flexible electronics on polymers with high precision.

A Dynamic Multi-Scale Model for Solid Oxide Cells Validated on Local Current Measurements: Impact of Global Cell Operation on the Electrodes Reaction Mechanisms

A dynamic physically based model has been developed to unravel the relationships between the overall Solid Oxide Cells response and the reaction mechanisms taking place in the electrodes. This tool combines modules at three different length-scales: electrode microstructures, active functional layers, and single repeat unit. It allows computing the cell polarization together with the distribution of the local current densities, overpotentials, and gas composition. It has been shown that the synthetic electrode model reproduces accurately the real microstructure, which has been checked by comparison with 3D reconstructions. The microscale electrode models have been validated using polarization curves and impedance spectra measured in symmetrical cells. Finally, tests were conducted at different temperatures and inlet flow rates to validate the integrated multiscale approach. For this purpose, a special setup was developed to measure the local current densities in standard cells. A good agreement was found between the experimental and simulated data.

In Vivo Evaluation of Fractal Microelectrodes Towards a More Targeted and Energy‐Efficient Vagus Nerve Stimulation

Vagus nerve stimulation (VNS) is a moderately effective treatment option for intractable epilepsy, depression, and pain, with promising applications in the treatment of gastrointestinal motility disorders like gastroparesis. The wide-ranging potential indications of VNS suggest an opportunity for a better understanding of underlying neural circuits that can be targeted using microscale electrodes. However, microscale Platinum (Pt) electrodes often have limited charge injection capacity due to their high impedance. Recently, we experimentally demonstrated that microscale Pt electrodes with Vicsek fractal geometry can facilitate electrochemical charge transfer more efficiently with 57% higher cathodic charge storage capacity than conventional circular microelectrodes. Here we are investigating whether the fractal electrode may be more effective in eliciting small fibers in the vagus nerve with less energy consumption due to its enhanced charge injection capability. To evaluate the in vivo performance of these novel microelectrode designs and to determine whether these electrodes are capable of stimulating small, unmyelinated C fibers that supply the stomach, we designed a cuff electrode with an array with four pairs of fractal and circular microelectrode to explore the possibility of radial selective stimulation of vagus nerve. We determined the dimension of the cuff to be 760 μm in diameter to wrap around a rat's vagus nerve so that it can withstand the swelling of the nerve from chronic implantation. We patterned fractal microelectrode to have a geometrical surface area equivalent to that of a circle with 100 μm of diameter. Using a rat model, we have stimulated the right cervical vagus nerve with fractal/circular electrode and recorded the compound action potential (CAP) distal from the stimulation site with 5-7 mm of conduction distance or the dorsal trunk of the abdominal vagus (conduction distance: 70 mm). We used an autonomous neural control (ANC) system, which can stimulate and record from the nerve with randomized stimulus parameter combinations (pulse amplitude: 0-0.4 mA, pulse width: 0.02-1 ms). We adapted a cathode-first, alternating monophasic stimulation for the reliable suppression of stimulus artifact (pulse repetition frequency: 5 Hz). Results from right cervical VNS experiments in anesthetized rats showed that both the fractal and the circular electrodes can recruit small, myelinated Ad and B fibers to a similar degree (pulse amplitude: 0-0.4 mA, pulse width: 0.1 ms) (Figure 1.), but fractal electrodes produce a more uniform and consistent fiber recruitment profile (Figure 2.). We observed volleys within the slow conduction velocity range (< 3 m/s), which we suspect to be signals from the C-fibers. Our next plan is to obtain more experimental data to statistically identify the efficiency of various electrode designs in stimulating small fibers and to investigate the electrical potential profile of these electrodes in a biophysical model.

Enhanced Electrochemical Performance of Micro-Supercapacitors Via Laser-Scribed Cobalt/Reduced Graphene Oxide Hybrids.

The evolution of "smart life," which connects all internet-of-things (IoT) microdevices and microsensors under wireless communication grids, requires microscale energy storage devices with high power and energy density and long-term cyclability to integrate them with sustainable power generators. Instead of Li-ion batteries with a short lifetime, pseudocapacitors with longer or infinite cyclability and high-power density have been considered as efficient energy storage devices for IoT. However, the design and fabrication of microscale pseudocapacitors have difficulties in patterning microscale electrodes when loading active materials at specific points of the electrodes using conventional microfabrication methods. Here, we developed a facile, one-step fabrication method of micro-supercapacitors (MSCs) through the in situ formation of Co metals and the reduced graphene oxides (rGOs) in a one-pot laser scribing process. The prepared Co/rGO MSC thus exhibited four times higher capacitance than the rGO MSC, due to the Faradaic charge capacitance behavior of the Co/rGO composites.

Microdevice With Planar Corrugated Electrodes for Dielectrophoretic Focusing of Micro-Particles

This paper conceptualizes and mathematically models a dielectrophoretic microdevice with planar corrugated electrodes for focusing micro-particles at any lateral location along the width of the micro-scale flow passage; two of these electrodes are placed on the top and bottom surfaces of the micro-scale flow passage with the electrodes on the top and bottom aligned with each other to form a pair. The mathematical model includes equations of motion, Navier-Stokes equations, and equations of electric voltage and field and considers the influence of several phenomena, including inertia, sedimentation, drag, virtual mass and dielectrophoresis on the focusing of micro-particles. The mathematical model is solved using the finite difference method. The mathematical model is used for parametric study, thereby revealing that the performance metrics related to focusing depend on the geometric (micro-scale flow passage and electrode dimensions) and operating (applied electric voltages and volumetric flow rate) parameters of the microdevice. The mathematical model allows for determining the operating and geometric parameters for achieving the desired performance metrics based on constraints. The mathematical model is validated using experimental data from the literature.

(Invited) Relationship between Protective Film on Sulfide Inclusions and Pitting Corrosion Resistance on Type 304 Stainless Steel Under Applied Stress

Stainless steels are widely used due to their superior corrosion resistance. In actual applications, stainless steels tend to suffer from localized corrosion such as pitting, crevice corrosion, and stress corrosion cracking (SCC) in chloride environments. Among others, SCC is dangerous because its growth rate is often rapid and SCC readily causes material failure due to a combination of a material and a corrosive environment under tensile stress. Pitting corrosion is well known to be the initiation sites for SCC.1 Previous research shows that the chemical compositions of sulfide inclusions play an important role in preventing pitting corrosion on Type 304 stainless steels. 2, 3 Muto et.al. reported that CrS and Ti4C2S2 inclusions have higher pitting corrosion resistance than MnS inclusion without stress. 2, 3 However, the pitting corrosion resistance of the sulfide inclusions under applied stress remains unclear. In this research, electrochemical measurements were conducted for stainless steels with different sulfide inclusions with applied stress, and the pitting corrosion resistance of the sulfide inclusions under applied stress was analyzed.Three types of Fe-18Cr-8Ni austenitic stainless steels (Type 304) were used in this study. Each stainless steel has MnS, CrS, and Ti4C2S2 inclusions, respectively. A solution treatment was conducted at 1373 K for 0.5 h, and then water-quenched. After the heat-treatment, the surfaces of the specimen was mechanically ground with 1500-grit SiC paper and polished down to 1 μm with a diamond paste. The surfaces of the specimen, except for the electrode area, was covered with a resin. Tensile tests were performed using a Beben microtest 5 kN module that was a horizontal type tensile test machine. In the air, the tensile stress increased to 180 MPa (=75% of the 0.2% proof stress of the steels) and then was kept constant. After that, a small acrylic cell was attached to the gauge section of the specimen surface with a micro-scale electrode area. Micro-scale electrochemical measurements and immersion tests were carried out in 0.1 M MgCl2.To reveal the relationship between the inclusion compositions and electrochemical behavior of the inclusions, potentiodynamic polarization was conducted for Type 304 stainless steels with different sulfide inclusions with and without applied stress. The pitting potential of the CrS and the Ti4C2S2 inclusions was higher than that of the MnS inclusion. The polarization was repeated three times for the stainless steel with the Ti4C2S2 inclusion, no pitting was observed for two times until 1.4 V vs. Ag/AgCl (3.33 M KCl) even under applied stress. After polarization, the specimen surfaces were observed by SEM, and sulfide inclusions were found inside the pits for all the specimens. The MnS, CrS, and Ti4C2S2 inclusions were thought to act as the pit initiation sites. Moreover, the oxide films were formed on the Ti4C2S2 inclusions after polarization. The potential-pH diagrams were calculated to ascertain the possibility of oxide film formation on the inclusion surfaces and compare the thermodynamic stability of MnS, CrS, and Ti4C2S2 inclusions. Ti4C2S2 is stable at lower potentials in the all pH range, and the experimental conditions of the polarization corresponded to the TiO2 region. The high pitting corrosion resistance of the Ti4C2S2 inclusion is attributed to the formation of TiO2 on the Ti4C2S2 inclusions. The pitting corrosion behavior at sulfide inclusion such as CrS and Ti4C2S2 under applied stress was discussed. Reference H. Masuda, Corros. Sci., 49, 120–129 (2007).I. Muto, S. Kurokawa, and N. Hara, J. Electrochem. Soc., 156, C395-C399 (2009). 3. N. Shimahashi, I. Muto, Y. Sugawara, and N. Hara, J. Electrochem. Soc., 160, C262-C269 (2013).

A Scalable and Low Stress Post-CMOS Processing Technique for Implantable Microsensors

Implantable active electronic microchips are being developed as multinode in-body sensors and actuators. There is a need to develop high throughput microfabrication techniques applicable to complementary metal–oxide–semiconductor (CMOS)-based silicon electronics in order to process bare dies from a foundry to physiologically compatible implant ensembles. Post-processing of a miniature CMOS chip by usual methods is challenging as the typically sub-mm size small dies are hard to handle and not readily compatible with the standard microfabrication, e.g., photolithography. Here, we present a soft material-based, low chemical and mechanical stress, scalable microchip post-CMOS processing method that enables photolithography and electron-beam deposition on hundreds of micrometers scale dies. The technique builds on the use of a polydimethylsiloxane (PDMS) carrier substrate, in which the CMOS chips were embedded and precisely aligned, thereby enabling batch post-processing without complication from additional micromachining or chip treatments. We have demonstrated our technique with 650 μm × 650 μm and 280 μm × 280 μm chips, designed for electrophysiological neural recording and microstimulation implants by monolithic integration of patterned gold and PEDOT:PSS electrodes on the chips and assessed their electrical properties. The functionality of the post-processed chips was verified in saline, and ex vivo experiments using wireless power and data link, to demonstrate the recording and stimulation performance of the microscale electrode interfaces.

Microscale Biosensor Array Based on Flexible Polymeric Platform toward Lab-on-a-Needle: Real-Time Multiparameter Biomedical Assays on Curved Needle Surfaces.

In vivo sensing of various physical/chemical parameters is gaining increased attention for early prediction and management of various diseases. However, there are major limitations on the fabrication method of multiparameter needle-based in vivo sensing devices, particularly concerning the uniformity between sensors. To address these challenges, we developed a microscale biosensor array for the measurement of electrical conductivity, pH, glucose, and lactate concentrations on a flexible polymeric polyimide platform with electrodeposited electrochemically active layers. The biosensor array was then transferred to a medical needle toward multiparametric in vivo sensing. The flexibility of the sensor platform allowed an easy integration to the curved surface (φ = 1.2 mm) of the needle. Furthermore, the electrodeposition process was used to localize various active materials for corresponding electrochemical sensors on the microscale electrodes with a high precision (patterning area = 150 μm × 2 mm). The biosensor array-modified needle was aimed to discriminate cancer from normal tissues by providing real-time discrimination of glucose, lactate concentration, pH, and electrical conductivity changes associated with the cancer-specific metabolic processes. The sensor performance was thus evaluated using solution samples, covering the physiological concentrations for cancer discrimination. Finally, the possibility of in vivo electrochemical biosensing during needle insertion was confirmed by utilizing the needle in a hydrogel phantom that mimicked the normal and cancer microenvironments.

Pitting Corrosion Behavior at Sulfide Inclusions on Type 304 Stainless Steel with Applied Stress

Stainless steels are widely used due to their excellent corrosion resistance. In actual applications, stainless steels tend to suffer from localized corrosion such as pitting, crevice corrosion, and stress corrosion cracking (SCC) in chloride environments. Among others, SCC is dangerous because its growth rate is often rapid and SCC readily causes material failure due to a combination of a material and a corrosive environment under tensile stress. Pitting corrosion is well known to be the initiation sites for SCC.1 To ascertain the initiation mechanism of SCC, elucidation of the effect of applied stress on pitting corrosion is necessary. In this research, electrochemical measurements were conducted under applied stress, and the role of applied stress in pitting corrosion of stainless steels was analyzed. AISI 304 (18Cr-8Ni) austenitic stainless steels were used in this study. A solution treatment was conducted at 1373 K for 0.5 h and then sensitized at 923 K for 2 h. After the heat-treatment, the surfaces of the specimen was mechanically ground with 1500-grit SiC paper and polished down to 1 μm with a diamond paste. The surfaces of the specimen, except for the electrode area, was covered with a resin. Tensile tests were performed using a Beben microtest 5 kN module that was a horizontal type tensile test machine. In air, the tensile stress increased to 180 MPa (=75% of the 0.2% proof stress of the steels) and then was kept constant. After that, a small acrylic cell was attached to the gauge section of the specimen surface with a micro-scale electrode area. Micro-scale electrochemical measurements and immersion tests were carried out in 4 M MgCl2 and 0.1 M Na2SO4. The pH value of 4 M MgCl2 and 0.1 M Na2SO4 was 4.3 and 5.6, respectively. To elucidate the role of applied stress in the pit initiation, the distribution density of pit initiation sites was investigated in the immersion tests in 4 M MgCl2 with and without applied stress. The size of the electrode area was changed from ca. 4.3 × 103 µm2 (65 µm × 65 µm) to ca. 1.0 × 106 µm2 (1 mm × 1 mm). In the absence of applied stress, the occurrence of pitting decreased with the size of the exposed area, and the threshold between pitting and no pitting was approximately 5.1 × 104 µm2 (226 µm × 226 µm). Under applied stress, pitting corrosion occurred regardless of the size of the exposed area. After the immersion tests, the pits were found to occur at the sensitized grain boundary by electrolytic etching. Additionally, the residual of the MnS inclusion was observed inside the pit. It is thought that applied stress likely promoted the pit initiation at the MnS inclusion at the sensitized grain boundary. In situ observation during micro-scale polarization was conducted at the electrode area containing a single MnS inclusion to reveal the dissolution behavior of MnS inclusion under applied stress. Before polarization, the color of the MnS surface was gray. Without applied stress, almost no change on the appearance of the inclusion surface was detected during the polarization although the electrode potential increased. On the other hand, the surface color of MnS inclusion changed from gray to black at open-circuit potential under applied stress. Moreover, many round products were observed during polarization, and the number of them increased with electrode potential. The round products were thought to be gas bubbles originated from the MnS dissolution as a by-product. This is evidence that MnS dissolution was accelerated under applied stress. The promotion of pit initiation on Type 304 stainless steels under applied stress contributed to the acceleration of the dissolution of the MnS inclusions. To improve the pitting corrosion resistance of Type 304 stainless steels at sulfide inclusions, the relationship between the inclusion compositions and electrochemical behavior of the inclusions were investigated. It has been reported that CrS, TiS, and Ti-carbosulfide are resistant to pitting in the absence of applied stress.2, 3 Pitting corrosion behavior at sulfide inclusion such as CrS and Ti-carbosulfide under applied stress is also discussed. Reference H. Masuda, Corros. Sci., 49, 120–129 (2007).I. Muto, S. Kurokawa, and N. Hara, J. Electrochem. Soc., 156, C395-C399 (2009).N. Shimahashi, I. Muto, Y. Sugawara, and N. Hara, J. Electrochem. Soc., 160, C262-C269 (2013).

Assorted micro-scale interdigitated aluminium electrode fabrication for insensitive electrolyte evaluation: zeolite nanoparticle-mediated micro- to nano-scaled electrodes

Biosensor with insensitive electrolyte variation is one of the basic norms to be investigated for developing a high-performance disease diagnostic with a non-biofouling. On the other hand, the optimum dimension of biosensor plays a significant factor to be considered for generating a biosensensing system with a least influence by the buffering agents. In this research, interdigitated aluminium electrode (AlIDE) with four different microscale gap sizes, 400, 100, 60 and 45 µm were fabricated through a photolithography technique on the silicon substrate. The accuracy of the device fabrication was justified by the morphological examination using field emission scanning electron microscopy, high-power microscopy and 3D-nano profilometer. The current amplified by four devices under bare condition was 2.55E−3 A at 2 V explains the accuracy of the device fabrication and the uniformity in the electric flow. Electrolyte scouting was executed on the fabricated AlIDEs with the pH ranges between 1 and 12, the results interpret small gap sized AlIDEs (60 and 45 µm) show the variation in the current with different pH solutions due to the ionic flux. The insensitivity of zeolite nanoparticle (~ 10 nm size) attached AlIDE with 400 µm and 100 µm at different pH solutions were analyzed. Linear current curves with least variations were attained from pH 1 to 11. At 2 V voltage supply, the amount of current generated by AlIDE with and without zeolite attachment during pH scouting were same (1E−4 A), with the same microscale gap sizes. However, there was no current fluctuation observed with zeolite attached AlIDE during pH scouting analysis. Further, the chemically functionalized zeolite nanoparticle behaves better than unmodified one and gold nanoparticle with the similar modifications. The results obtained in the present research denotes that microscale gap size was altered to nanoscale gap size in the presence of nanomaterials on the AlIDE sensing surface, enhances the sensitivity of the device and improves the insensitivity of device to the electrolytes. Thus, optimum microscale gap sized AlIDE is easier to fabricate, could be utilized for medical diagnostics with insensitive pH variation through the advanced physiochemical properties of nanomaterials.

Li Buffer Interest in Silicon Based Anode to Improve Li-Ion Battery Cycle Life

Silicon is a promising material at the negative electrode due to its huge specific capacity and its low potential. However, this high capacity reflects huge volume changes of around 270% which cause at nanoscale solid electrolyte interphase (SEI) fracture associated with irreversible Li consumption and at microscale electrode cracks with active material loss. To achieve a suitable cycle life, many studies reports the use of silicon particles incorporated in graphite material composite. This approach is consistent with the lowest volume capacity at the positive electrode. Considering the energy density at cell level, no significant improvement is obtained with higher silicon content at the negative electrode. However the huge specific capacity of the silicon is not only a way to improve the energy density at cell. It is also a way to implement a lithium buffer at the negative electrode without sacrificing the energy density. In a standard Li-ion cell, the active lithium content is stored within the cathode. During the first cycle a certain amount of lithium is lost by SEI formation so that the remaining lithium is reduced. Many strategies are reported to incorporate extra lithium in the anode to compensate this loss [1]. By adding much more lithium, we were able to evaluate the interest of a lithium buffer which can a priori compensate the lithium loss at the first cycle and all along the cycle life. Usingsilicon alloy blended with graphite material, several electrodes were coated with various silicon content, loadings and porosity by keeping constant the electrode thickness. Different prelithiation levels were then studied in pouch cell using NMC622 positive electrodes. Results show that it is possible to have 90% of capacity retention at cell level after 1000 cycles at C/2, 100% DOD and 25°C. [1]: F. Holtstiege, P. Bärmann, R. Nölle, M. Winter, T. Packle, Batteries, 2018, 4, 4.

(Invited) Brief Historical Overview and Current Status of Electrochemical Neurotransmitter Detection

Abnormal neurotransmitter concentrations are associated with serious disorders including traumatic brain injury, Parkinson’s and Alzheimer’s diseases, epilepsy, and chronic pain. Understanding the causes and potential cures of these disorders requires highly selective, sensitive, and responsive neurotransmitter detection. From its inception in the early 1970s, electrochemical neurotransmitter detection has become a vibrant research field that is addressing the need for sensitive, real-time quantitative neurotransmitter analysis. Electrochemical techniques can detect both electroactive and nonelectroactive species but different materials and methods are used depending on how easily the species of interest can be electrolyzed and distinguished from other analytes. Electrode materials and design have evolved from being macroscale devices for sensing one analyte at a single electrode to the point that there now exist microscale electrode arrays, some of which are capable of simultaneously monitoring multiple neurotransmitters. In addition, some electrode arrays are flexible in order to more amenably be integrated into soft, living tissue for real-time in vivo monitoring. After a brief historical overview, this presentation summarizes the current state of electrochemical neurotransmitter detection by highlighting notable examples, applications, challenges and future directions. Figure 1

Effect of Applied Stress on Pit Initiation of Sensitized Type 304 Stainless Steel in Chloride Solution

Stainless steels are widely used due to their superior corrosion resistance. In the actual applications of stainless steels, sensitization is sometimes caused by welding. When Cr-depleted zones are formed, intergranular stress corrosion cracking (IGSCC) readily occurs at heat affected zone in chloride environments under applied stress. Pitting corrosion is well known to be the initiation site for SCC 1. Elucidation of the effect of applied stress on pitting corrosion is necessary to ascertain the initiation mechanism of IGSCC. In this research, electrochemical measurements were conducted under applied stress, and the effect of applied stress on pitting corrosion of sensitized stainless steels was analyzed. Two types of commercial AISI 304 (18Cr-8Ni) austenitic stainless steel sheets were used in this study (low S and re-sulfurized steels). The chemical compositions are shown in Table 1 (mass %). After solution treatment (1373 K, 30 min, W.Q.), the specimens were subjected to sensitization treatment (923 K, 2 h, W.Q.). After these heat-treatments, the surfaces of the specimens were successively polished with a diamond paste down to 1 μm. The surfaces of the specimen, except for the electrode area, were covered with resin. Tensile tests were performed using a Beben microtest 5 kN module that was a horizontal type tensile test machine. In air, the tensile stress was increased to 180 MPa (=80% of the 0.2% proof stress of the steels) with a crosshead speed of 0.02 mm min−1, and then kept constant. After that, in order to preserve the electrolyte, a small acrylic cell was attached to the gauge section of the specimen surface with a micro-scale electrode area. Electrochemical measurements were performed at 180 MPa. To elucidate the effect of applied stress on pit initiation of sensitized Type 304 (low S), immersion tests in 4 M MgCl2 (pH was adjusted at 5.0) were conducted under applied stress for 24 h. Electrode areas were changed from 100 to 500 square µm. In the absence of applied stress, the critical electrode area in which pitting corrosion was observed was ca. 230 square µm. Under applied stress, pitting corrosion was observed even when the electrode area was ca. 100 square µm. The critical electrode area required for pit initiation decreased by applied stress, indicating applied stress promotes pit initiation. Previous research suggested that the pit initiation mechanism can be divided into three steps: acidification around sulfide inclusion; depassivation of stainless steel; and local active dissolution of stainless steel2. In situ observations were conducted during potentiodynamic polarization in 0.1 M Na2SO4 to elucidate the dissolution behavior of the sulfide inclusion of Type 304 (re-sulfurized). Around the corrosion potential, the dissolution of the sulfide inclusion was promoted under applied stress. The depassivation pH values in naturally aerated in 1 M MgCl2 were measured with and without stress. The pH of the solution was lowered stepwise with HCl. Depassivation pH without and with stress were 0.6 and 0.8, respectively. It is thought that the stainless steel was readily depassivated under applied stress. To investigate the rate of active dissolution, potentiodynamic polarization was carried out in 4 M MgCl2 at pH 0.0. However, no difference in current densities was observed without and with applied stress. It is supposed that applied stress affects depassivation pH of the steel matrix and accelerates the MnS dissolution, and hence pit initiation is seemed to be promoted by applied stress. References H. Masuda, Corros. Sci., 49, 120–129 (2007).A. Chiba, I. Muto, Y. Sugawara, and N. Hara, J. Electrochem. Soc., 160, 511–520 (2013). Figure 1

(Invited) Electrochemical Biosensors Based on CMOS LSI Chips

Electrochemical biosensors are the sensors for bio-related substances/phenomena based on electrochemical principles. While optical and colorimetric biosensors are widely used for clinical and research purposes, electrochemical biosensors offer various advantages such as compact/affordable instrumentation and high throughput. As a platform of such electrochemical biosensors, a complementary metal-oxide-semiconductor (CMOS) large-scale integrated (LSI) circuit chip can be an ideal platform for its microscale structures and electronic circuit integration. In this paper, general overview and some examples of such CMOS-based electrochemical sensing experiments/simulations are presented. The electrochemical methods can be categorized into three types, that is, (i) amperometoric and voltammetric method, (ii) potentiometric method, and (iii) impedance method. In the amperometric method, electrochemical current is measured as a function of time under constant voltage application, while voltammetric method often means that the voltage is swept and current vs. voltage is plotted. On the other hand, in potentiometric method, no voltage is applied and therefore no current will be induced. Instead, open-circuit potential of a floating electrode in contact with sample solution is measured. Finally, in impedance method, the relation between sinusoidal voltage applied to the sample solution and resulting sinusoidal current is measured as a complex valued impedance. We have been working on various CMOS-based electrochemical biosensing experiments and simulations. Experimental demonstrations of amperometric and voltammetric methods can been seen in [1, 2], where carbon-ink electrodes are formed on CMOS chip and used to measure current under constant/swept voltages. Computer simulation of amperometric measurement using CMOS on-chip electrode has also been presented in [3]. As for the potentiometric method, we have demonstrated a potentiometric glucose sensor with carbon electrode and enzyme-functionalized chromatography paper on CMOS chip [4]. Computer simulation of ion-sensitive field-effect transistors (ISFET) for DNA sensing [5, 6] and lipid bilayer (cell membrane) detection [7] have also been demonstrated. In terms of impedance measurement using CMOS, we have developed a computationally efficient three-dimensional model of electrolytic solution and living cells for finite-element method (FEM) simulation. Using this, we have demonstrated computer simulation of impedance measurement of single mammalian cell or bacterial cell using microscale electrode on CMOS chip [8, 9]. Compact formulae for equivalent circuit model of such cell/electrolyte/electrode system have demonstrated a good accuracy [9]. Our current goal is the experimental implementation of such single cell monitoring by impedance measurement using CMOS on-chip microelectrodes. In summary, we have been developing both experimental and simulation techniques of the major electrochemical biosensing methods using CMOS on-chip microelectrodes. Such effort would lead to a comprehensive understanding of multi-modal electrochemical biosensing where CMOS LSI chips are used as a platform. This work was partially supported by JSPS KAKENHI Grant Numbers 25220906 and 26289111, as well as VLSI Design and Education Center (VDEC), the University of Tokyo in collaboration with Synopsys, Cadence Design Systems, and Mentor Graphics. [1] M. Miki, S. Iwahara, and S. Uno, “A hybrid system of carbon ink electrodes and chromatography paper on a CMOS chip and its application to Enzymatic glucose sensor”, Jpn. J. Appl. Phys., vol. 53, p. 04EL06 (2014). [2] J. Eguchi, K. Yamaoka, and S. Uno, “Simultaneous Electrochemical Measurement using Paper Fluidic Channel on CMOS Chip”, TELKOMNIKA, vol. 15, no. 2, pp. 847-852 (2017). [3] J. Hasegawa, S. Uno, and K. Nakazato, “Amperometric Electrochemical Sensor Array for On-Chip Simultaneous Imaging: Circuit and Microelectrode Design Considerations”, Jpn. J. Appl. Phys., vol. 50, p. 04DL03 (2011). [4] K. Yamaoka, J. Eguchi, and S. Uno, “Potentiometric Glucose Detection by Paper-based Electrochemical Sensor on CMOS Chip”, TELKOMNIKA, vol. 15, no. 2, pp. 836-841 (2017). [5] Y. Nishio, S. Uno, and K. Nakazato, “Three-Dimensional Simulation of DNA Sensing by Ion-Sensitive Field-Effect Transistor: Optimization of DNA Position and Orientation”, Jpn. J. Appl. Phys., vol. 52, p. 04CL01 (2013). [6] S. Uno, M. Iio, H. Ozawa, and K. Nakazato, “Full Three-dimensional Simulation of ISFET Flatband Voltage Shift due to DNA Immobilization and Hybridization”, Jpn. J. Appl. Phys., vol. 49, no. 1, p. 01AG07 (2010). [7] S. Uno, “Computer simulation of lipid bilayer detection using ion-sensitive field-effect transistors” SPIE BIOS, San Francisco, USA (January 26, 2012) 8231-28. [8] S. Uno, “Simulation of Microelectrode Electrochemical Impedance Sensor for Single-cell Monitoring”, The 11th International Symposium on Electrochemical Micro & Nanosystem Technologies (EMNT2016), Brussel, Belgium (August 17-19, 2016), P-18. [9] S. Uno, and K. Nakazato “Numerical and Equivalent Circuit Analysis of Electrochemical Impedance Spectroscopy of Single Bacterial Cell Detection using CMOS LSI Chip”, 11th International Symposium on Electrochemical Impedance Analysis 2017, Camogli, Italy (November 7-11, 2017).

(Invited) Electrochemical Biosensors Based on CMOS LSI Chips

We have been working on electrochemical biosensing using CMOS chip as a platform. Three major electrochemical methods, amperometric (voltammetric), potentiometric, and impedance methods, are aimed to be done by electrodes on a CMOS chip. In amperometric measurement, carbon-ink is used to form electrochemically-stable electrodes instead of expensive gold electrode formation by photolithography. Enzymes and mediator (redox) molecules are immobilized in pieces of chromatography paper, and layers of such paper are placed directly on CMOS chip. Both amperometric and potentiometric detection of glucose concentration have been demonstrated. We have also been working on computer simulation and modeling of the three electrochemical measurements done by microscale electrodes on CMOS chip. Examples include 3D amperometric current simulation of 2D microelectrode array on a CMOS chip, potentiometric measurement of pH and DNA on an ion-sensitive field-effect transistor, as well as living cell monitoring based on impedance method. We also design CMOS integrated circuits for on-chip biosensing based on the electrochemical methods.

An introductory study using impedance spectroscopy technique with polarizable microelectrode for amino acids characterization

Portable, low power, yet ultra-sensitive life detection instrumentations are vital to future astrobiology flight programs at NASA. In this study, initial attempts to characterize amino acids in an aqueous environment by electrochemical impedance spectroscopy (EIS) using polarizable (blocking) electrodes in order to establish a means of detection via their electrical properties. Seven amino acids were chosen due to their scientific importance in demonstrating sensitivity levels in the range of part per billion concentration. Albeit more challenging in real systems of analyst mixtures, we found individual amino acids in aqueous environment do exhibit some degree of chemical and physical uniqueness to warrant characterization by EIS. The polar amino acids (Asp, Glu, and His) exhibited higher electrochemical activity than the non-polar amino acids (Ala, Gly, Val, and Leu). The non-polar amino acids (Gly and Ala) also exhibited unique electrical properties which appeared to be more dependent on physical characteristics such as molecular weight and structure. At concentrations above 1 mM where the amino acids play a more dominant transport role within the water, the conductivity was found to be more sensitive to concentrations. At lower concentrations <1 mM, however, the polar amino acid solution conductivity remained constant, suggesting poor chemical activity with water. As revealed by equivalent circuit modeling, the relaxation times showed a 1-2 order of magnitude difference between polar and non-polar amino acids. The pseudo-capacitance from EIS measurements on sample mixtures containing salt water and individual amino acids revealed the possibility for improvement in amino acid selectivity using gold nanoporous surface enhanced electrodes. This work establishes important methodologies for characterizing amino acids using EIS combined with microscale electrodes, supporting the case for instrumentation development for life detection and origin of life programs.