Activity of AC electrokinetically immobilized horseradish peroxidase

Dielectrophoresis (DEP) is an AC electrokinetic effect mainly used to manipulate cells. Smaller particles, like virions, antibodies, enzymes, and even dye molecules can be immobilized by DEP as well. In principle, it was shown that enzymes are active after immobilization by DEP, but no quantification of the retained activity was reported so far. In this study, the activity of the enzyme horseradish peroxidase (HRP) is quantified after immobilization by DEP. For this, HRP is immobilized on regular arrays of titanium nitride ring electrodes of 500 nm diameter and 20 nm widths. The activity of HRP on the electrode chip is measured with a limit of detection of 60 fg HRP by observing the enzymatic turnover of Amplex Red and H₂O₂ to fluorescent resorufin by fluorescence microscopy. The initial activity of the permanently immobilized HRP equals up to 45% of the activity that can be expected for an ideal monolayer of HRP molecules on all electrodes of the array. Localization of the immobilizate on the electrodes is accomplished by staining with the fluorescent product of the enzyme reaction. The high residual activity of enzymes after AC field induced immobilization shows the method's suitability for biosensing and research applications.     

Bi-directional flow induced by an AC electroosmotic micropump with DC voltage bias

This paper discusses the principle of biased alternating current electroosmosis (ACEO) and its application to move the bulk fluid in a microchannel, as an alternative to mechanical pumping methods. Previous EO-driven flow research has looked at the effect of electrode asymmetry and transverse traveling wave forms on the performance of electroosmotic pumps. This paper presents an analysis that was conducted to assess the effect of combining an AC signal with a DC (direct current) bias when generating the electric field needed to impart electroosmosis (EO) within a microchannel. The results presented here are numerical and experimental. The numerical results were generated through simulations performed using COMSOL 3.5a. Currently available theoretical models for EO flows were embedded in the software and solved numerically to evaluate the effects of channel geometry, frequency of excitation, electrode array geometry, and AC signal with a DC bias on the flow imparted on an electrically conducting fluid. Simulations of the ACEO flow driven by a constant magnitude of AC voltage over symmetric electrodes did not indicate relevant net flows. However, superimposing a DC signal over the AC signal on the same symmetric electrode array leads to a noticeable net forward flow. Moreover, changing the polarity of electrical signal creates a bi-directional flow on symmetrical electrode array. Experimental flow measurements were performed on several electrode array configurations. The mismatch between the numerical and experimental results revealed the limitations of the currently available models for the biased EO. However, they confirm that using a symmetric electrode array excited by an AC signal with a DC bias leads to a significant improvement in flow rates in comparison to the flow rates obtained in an asymmetric electrode array configuration excited just with an AC signal.     

铜镍和铜钴合金电极在碱性介质中的光电化学

用动电位伏安法对含镍量10％、30％和50％的铜镍合金以及含钴量5.1％、9.7％、15 ％、25％和40％的铜钴合金电极在硼砂-硼酸缓冲溶液（pH 8.5）中的光电化学行为进行了 研究.铜镍合金和铜钴合金均显示p-型光响应,铜镍合金的光响应来自Cu2O,铜钴合金的光 响应来自Cu2O和氧化钴.含镍量10％和30％的铜镍合金电极以及含钴量5.1％铜钴合金电极的 最大光电流iph,max均大于纯铜电极,含钴量15％、25％和40％的铜钴合金电极以及含镍量 50％的铜镍合金电极由于电极表面相当一部分面积分别被氧化钴和氧化镍所占有,iph,max 小于纯铜电极.铜镍合金电极的φv值（电位负向扫描过程中电极表面完全还原为Cu时的电位 ）负于纯铜电极,而铜钴合金电极的φv值与纯铜电极大致相等, NiO的存在致使铜镍合金 表面Cu2O膜具有更大的稳定性.从光电化学角度通过φv和iph,max反映铜合金的耐腐蚀性能 与交流阻抗法测得的结果相符.     

多壁纳米碳管空气电极的交流阻抗研究

研究了多壁纳米碳管、活性炭和石墨等空气电极的交流阻抗特性.结果表明,纳米碳管空气电极的阻抗谱由两个半圆组成,高频区半圆对应欧姆极化阻抗,低频区半圆对应电化学极化阻抗.催化剂Pt以纳米颗粒的形式沉积在碳管的外表面,明显减小了电极的欧姆阻抗和电化学极化阻抗,提高了氧还原反应的电催化活性.活性炭电极除存在电化学阻抗外,还存在薄液膜扩散阻抗(Nernst扩散),石墨电极形成的薄液膜反应区域较小,电极反应呈Warburg扩散阻抗特征,相应的电催化活性较低.采用交流阻抗等效电路分析方法,对拟合的动力学数据进行了解释.     

Microband electrodes of ideal and nonideal geometries: AC impedance spectroscopy

The AC impedance behavior of microband electrode geometries which deviate from the ideal is derived via numerical modelling of the chronoamperometric response under diffusion-only conditions. Specifically attention is given to four electrode shapes in addition to the ideal microband geometry: elevated microband electrodes (with conducting supporting sides), recessed microband electrodes (with insulating pit walls), platform electrodes (with insulating supporting sides) and, for the purposes of comparison, a hypothetical line electrode without any support which permits diffusional mass transport to both sides of the infinitesimally thin electrode. Simple analytical expressions are established for the frequency dependence of the AC impedance in each case.     

硫酸介质中Pb电极氧化物成长过程的研究

采用实时交流阻抗测量与激光扫描微区光电流技术相结合的现场方法,对铅电极在硫酸溶液中ＰｂＯ和ＰｂＯ＿２的生长过程进行了研究．发现电极在ＰｂＯ的形成区中氧化５ｍｉｎ后,电极欧姆阻抗和ｔ￣（１／２）呈一直线关系,这表明ＰｂＯ是均匀分布在电极表面而成长的．激光扫描微区光电流的实验进一步证实了这一结论．实验中还发现ＰｂＯ＿２在ＰｂＯ层中是局部发生与发展的．     

Investigation of pumping mechanism for non‐Newtonian blood flow with AC electrothermal forces in a microchannel by hybrid boundary element method and immersed boundary‐lattice Boltzmann method

Efficient pumping of blood flow in a microfluidic device is essential for rapid detection of bacterial bloodstream infections (BSI) using alternating current (AC) electrokinetics. Compared with AC electro‐osmosis (ACEO) phenomenon, the advantage of AC electrothermal (ACET) mechanism is its capability of pumping biofluids with high electrical conductivities at a relatively high AC voltage frequency. In the current work, the microfluidic pumping of non‐Newtonian blood flow using ACET forces is investigated in detail by modeling its multi‐physics process with hybrid boundary element method (BEM) and immersed boundary‐lattice Boltzmann method (IB‐LBM). The Carreau–Yasuda model is used to simulate the realistic rheological behavior of blood flow. The ACET pumping efficiency of blood flow is studied in terms of different AC voltage magnitudes and frequencies, thermal boundary conditions of electrodes, electrode configurations, channel height, and the channel length per electrode pair. Besides, the effect of rheological behavior on the blood flow velocity is theoretically analyzed by comparing with the Newtonian fluid flow using scaling law analysis under the same physical conditions. The results indicate that the rheological behavior of blood flow and its frequency‐dependent dielectric property make the pumping phenomenon of blood flow different from that of the common Newtonian aqueous solutions. It is also demonstrated that using a thermally insulated electrode could enhance the pumping efficiency dramatically. Besides, the results conclude that increasing the AC voltage magnitude is a more economical pumping approach than adding the number of electrodes with the same energy consumption when the Joule heating effect is acceptable.     

Bi‐directional ACET micropump for on‐chip biological applications

The ability to control and pump high ionic strength fluids inside microchannels forms a major advantage for clinical diagnostics and drug screening processes, where high conductive biological and physiological buffers are used. Despite the known potential of AC electro‐thermal (ACET) effect in different biomedical applications, comparatively little is known about controlling the velocity and direction of fluid inside the chip. Here, we proposed to discretize the conventional electrodes to form various asymmetric electrode structures in order to control the fluid direction by simple switching the appropriate electric potential applied to the discretized electrodes. The ACET pumping effect was numerically studied by solving electrical, thermal and hydrodynamic multi‐physic coupled equations to optimize the geometrical dimensions of the discretized system. PBS solutions with different ionic strength were seeded with 1 μm sized fluorescent particles and electrothermally driven fluid motion was observed inside the channel for different electrode structures. Experimental analyses confirm that the proposed micropump is efficient for a conductivity range between 0.1 and 1 S/m and the efficiency improves by increasing the voltage amplitude. Behavior of the proposed electrode–electrolyte system is discussed by lumped circuit model. Frequency response of system illustrated that the optimal frequency range increases by increasing the conductivity of medium. For 0.18 S/m PBS solution, the constant pumping effect was observed at frequency range between 100 kHz and 1 MHz, while frequency range of 100 kHz to 5 MHZ was observed for 0.42 S/m. The characteristics of experimental results were in good agreement with the theoretical model.     

活性炭表面性质对氧气扩散电极电催化性能的影响

 分别采用硝酸和空气氧化处理制得具有不同表面性质的粉末活性炭,并以此为催化层材料制成炭基氧气扩散电极,测定了不同电极的极化曲线和电化学阻抗谱. N2物理吸附和He程序升温脱附(He-TPD)研究表明,硝酸处理对活性炭孔结构的影响较小,但可使其表面含氧基团明显增加; 而空气氧化处理则会导致活性炭的中孔面积和孔容显著增大,但对表面含氧基团的影响较小. 极化曲线和电化学阻抗谱研究发现,当活性炭的孔结构相近时,电极的催化性能随着表面含氧基团的增多而急剧下降; 当活性炭表面含氧基团的量相近时,中孔孔容增大将导致电极催化性能的恶化. 与活性炭表面含氧基团相比,孔结构对氧气扩散电极的电化学性能具有更显著的影响.     

Limitation of linear response in flow-injection systems with ion-selective electrodes

The lower limit of linear response in flow-injection systems employing membrane and second-kind ion-selective electrodes as detectors depends on the dispersion in the system and is usually much worse than in batch measurements. Below a certain value of the peak height, linearity of the electrode response is not maintained. The limiting value depends on the process causing the loss of linear response (solubility of the electrode material, contamination, or adsorption at the electrode surface), and varies from 15 to 70 mV. Chloride- iodide-, fluoride- and copper(II)-selective electrodes are discussed.     

Exterior‐electrode electrically driven microconcentrator

This study uses negative dielectrophoresis and AC electroosmosis as a driving mechanism and presents an electrically driven microconcentrator that concentrates the sample in the region exterior to the electrodes (termed as exterior‐electrode electrically driven microconcentrator in this paper). The proposed microconcentrator uses a 3‐D face‐to‐face electrode pair; the top electrode is a relatively large planar electrode, and the bottom electrode is formed with three to six long and thin electrodes connected into an open ring. The sample is brought to the vicinity of the open electrode at the bottom by electroosmotic flow; then, negative dielectrophoresis is used to push the sample away from the electrode and concentrate it in the region surrounded by the open ring electrode. Concentration using an exterior‐electrode electrically driven microconcentrator offers promise for convenient use in conjunction with relevant detection systems. The results indicate that for the proposed exterior‐electrode electrically driven microconcentrator, the optimal frequency is 100 kHz and the optimal voltage is 13 V_p‐p. The corner concentration process at the corners of the bottom open electrodes enables the multi‐corner electrodes to exhibit better concentration results than that exhibited by semicircular‐shaped electrodes. The concentration performance is most favorable when the shape of the open electrode at the bottom is a five‐vertex electrode, enabling a concentration enhancement factor of 55 times for a latex particle solution and 11 times for E. coli. The experimental results also demonstrate that the concentration phenomenon in this study is not induced by non‐specific adsorption and can be repeated multiple times.     

Preparation and electrochemical properties of nanofiber poly(2,5-dihydroxyaniline)/activated carbon composite electrode for supercapacitor

In this study, poly(2,5-dihydroxyaniline) (PDHA) was successfully prepared by electrochemical method on the surface of active carbon (AC) electrodes. The physical and electrochemistry properties of PDHA/AC composite electrode compared with pure AC electrode were investigated by scanning electronic microscope (SEM), cyclic voltammetry (CV), electrochemical impedance spectroscopy, cycle life test. From SEM, PDHA presents nanofiber network morphology. The diameter of the nanofiber PDHA is about 200–300 nm. PDHA/AC composite electrode shows redox peaks in CV curve and voltage plateaus in galvanostatic charge–discharge curve, and all these indicate that PDHA/AC composite electrode has more advantages. The maintenance of the capacitance compared to initial cycle capacitance of composite electrode is about 90% during the charge–discharge cycles. In conclusion, The PDHA/AC composite electrode shows much higher specific capacitance (958 F g⁻¹), better power characteristics, longer cycle life. Therefore, PDHA/AC composite electrodes were more promising for application in capacitor. This can be attributed to the introduction of nanofiber PDHA. The effect and role of PDHA in the composite electrodes were also discussed in detail.     

Molecular-level functionalization of electrode surfaces. An overview

Electrochemical reactions occur at electrode/electrolyte interfaces. Hence, manipulation and design of electrochemical interfaces accompanied by surface modifications have assumed vital importance. Molecular level modification, either at the monolayer or multilayer level of electrode surfaces and leading to functionalization of electrodes, is being actively pursued by researchers. Modification based on the self-assembled monolayer approach has enabled electrodes to acquire molecular recognition and molecular electronic characteristics. Functionalization of electrode surfaces using polymeric materials and enzymes has facilitated electrodes in exhibiting properties like catalysis, molecular recognition, electrochromism and birefringence. The results of such molecular level functionalization studies of electrode surfaces carried out recently in our laboratories are presented in this overview. Besides, some representative results reported from elsewhere are also included.     

Recycling Chocolate Aluminum Wrapping Foil as to Create Electrochemical Metal Strip Electrodes

The development of low-cost electrode devices from conductive materials has recently attracted considerable attention as a sustainable means to replace the existing commercially available electrodes. In this study, two different electrode surfaces (surfaces 1 and 2, denoted as S1 and S2) were fabricated from chocolate wrapping aluminum foils. Energy dispersive X-Ray (EDX) and field emission scanning electron microscopy (FESEM) were used to investigate the elemental composition and surface morphology of the prepared electrodes. Meanwhile, cyclic voltammetry (CV), chronoamperometry, electrochemical impedance spectroscopy (EIS), and differential pulse voltammetry (DPV) were used to assess the electrical conductivities and the electrochemical activities of the prepared electrodes. It was found that the fabricated electrode strips, particularly the S1 electrode, showed good electrochemical responses and conductivity properties in phosphate buffer (PB) solutions. Interestingly, both of the electrodes can respond to the ruthenium hexamine (Ruhex) redox species. The fundamental results presented from this study indicate that this electrode material can be an inexpensive alternative for the electrode substrate. Overall, our findings indicate that electrodes made from chocolate wrapping materials have promise as electrochemical sensors and can be utilized in various applications.     

Catalytic activity of glucose oxidase after dielectrophoretic immobilization on nanoelectrodes

Dielectrophoresis (DEP) is an AC electrokinetic effect that is proven to be effective for the immobilization of not only cells, but also of macromolecules, for example, antibodies and enzyme molecules. In our previous work, we have already demonstrated the high catalytic activity of immobilized horseradish peroxidase after DEP. To evaluate the suitability of the immobilization method for sensing or research in general, we want to test it for other enzymes, too. In this study, glucose oxidase (GOX) from Aspergillus niger was immobilized on TiN nanoelectrode arrays by DEP. Fluorescence microscopy showed the intrinsic fluorescence of the immobilized enzymes flavin cofactor on the electrodes. The catalytic activity of immobilized GOX was detectable, but a fraction of less than 1.3% of the maximum activity that was expected for a full monolayer of immobilized enzymes on all electrodes was stable for multiple measurement cycles. Therefore, the effect of DEP immobilization on the catalytic activity strongly depends on the used enzyme.     

Redox functionalization of carbon electrodes of electrochemical capacitors

It is shown that the liquid oxidative treatment of microporous active carbon (AC) of the Norit DLC Supra 30 grade by nitric acid in the presence of carbamide results in an increase in the content of hydroxy groups on the AC surface at the practically unchanged content of carboxyl groups. Redox functionalization and appearance of pseudocapacity result in an increase in the carbon electrode capacity by 26%. The surface state of the carbon material is characterized using the infrared spectroscopy and Boehm titrimetry techniques, while the electrochemical characteristics are studied using the method of cyclic voltammetry in 3 M sulfuric acid solution. Studies of degradation of the electrodes of the initial and modified active carbons show that capacity decreases by 3 and 8%, accordingly, after 1 thousand charging–discharge cycles.     

Electrocatalytic properties of Pt(111), Pt(332), Pt(331) and Pt(110) single crystal electrodes towards ethylene glycol oxidation in sulphuric acid solutions

In the present paper four platinum single crystal electrodes, two basal planes of Pt(111) and Pt(110) and two stepped surfaces of Pt(332) and Pt(331), were prepared and used in the study of electro-oxidation of ethylene glycol (EG). All of these Pt single crystal electrodes belong to the [1 0] zone of crystallography, and exhibit on their surface (111) symmetry sites or certain combinations of terraces of (111) symmetry with steps of (111) symmetry type. It has been found that as a result of a favourable steric matching of surface sites the Pt(110) electrode manifested a higher activity both for EG dissociative adsorption and oxidation than that of the Pt(111) electrode. The stepped surfaces of Pt(332) and Pt(331) operated with certain combinations of characteristics of Pt(111) and Pt(110). The best electrocatalytic properties have been obtained with a Pt(331) electrode, and this is attributed both to the configuration of the atomic arrangement and to the stability of this surface.In summary, the above results show that the performance of a given Pt single crystal electrode in EG oxidation at a potential below 1.0 V may be evaluated by three factors.

1. (1) The ability to resist self-poisoning (AB) which describes the difficulty of EG dissociative adsorption on the electrode surface.

2. (2) The activity for EG oxidation (AC). In considering that the threshold potential for EG oxidation on all electrodes is at 0.3 V and that the self-poisoning is encountered in PGPS, the activity for EG oxidation may be reasonably characterized by the intensity of the peak current acquired in NGPS near 0.6 V, which corresponds to the maximum current of EG oxidation on an activated (non-poisoned) surface of the electrode.

3. (3) The stability of activity during potential cycling (SA) between 0.05 and 1.0 V, which describes the resistance to the decrease of intensity of the EG oxidation current during voltammetric cycling.

For the two basal planes studied, the AB and SA of Pt(111) are higher than those of Pt(110), but its AC is much lower than that of Pt(110). These differences are clearly related to the surface atomic arrangement of the two electrodes. As has been discussed above, the surface of Pt(111) is atomically smooth and stable during voltammetric cycling. The surface of Pt(110) presents, however, atomic steps and is reconstructed under experimental conditions, i.e. certain steric configurations are encountered on the Pt(110) surface. The high AC and the low AB may be assigned to a favourite stereographic matching during EG adsorption and oxidation on Pt(110).The two electrodes with stepped surfaces, Pt(332) and Pt(331), contain different densities of (110) sites, which are formed on the border between terrace and step, as shown in Fig. 8. The AB of these two electrodes has been observed at a moderate range between that of Pt(111) and the AB of Pt(110). With a majority of (111) sites on its surface, the electrode of Pt(332) operates at a relatively higher AC than Pt(111) does, and its SA is not as good as that of Pt(111) but is much better than the SA of a Pt(110) electrode. In all cases the highest AC and SA are obtained with a Pt(331) electrode. It may be seen from the profile of a (331) plane (shown by the cross-section of A-A in Fig. 8) that all atoms on the top of the surface participated in forming (110) sites, and the atom on the step has two functions — one is to form a (110) site with an atom located in the terrace of second layer and the other is to form a (111) site in the terrace of the same layer. It has been mentioned in the above discussions that the Pt(110) electrode keeps a higher AC due to favourite stereographic matching in EG adsorption and oxidation, but its SA is the worst, due to the instability of the surface. The highest AC and SA obtained with Pt(331) may be ascribed not only to the high density of (110) sites existing on the surface, but also to the stabilization of these (110) sites, and moreover, the synergy generated by the atomic arrangement of the Pt(331) surface may also contribute to the performance of the Pt(331) electrode.     

Electrochemical determination of dopamine in the presence of ascorbic acid and uric acid using the synergistic effect of gold nanoflowers and L-cysteamine monolayer at the surface of a gold electrode

This paper describes a facile and effective method to synthesize gold nanoflowers (AuNFs) by a controllable electrodeposition method induced by a L-cysteamine (L-Cys) monolayer self-assembled on the surface of a gold electrode. The AuNFs/L-Cys/Au electrodes were characterized by field emission scanning electron microscopy (FE-SEM), cyclic voltammetry, and AC impedance spectroscopy methods. This obtained AuNFs/L-Cys/Au electrode exhibits excellent electrocatalytic activity towards the oxidation of dopamine (DA) due to the synergistic effect of AuNFs and a L-Cys monolayer. Differential pulse voltammetry (DPV) experiment results show that the oxidation peak of DA is separated from the oxidation peaks of ascorbic acid (AA) and uric acid (UA), which can be used to detect DA in the presence of AA and UA, and the results are satisfactory.     

Scalable activated carbon/graphene based supercapacitors with improved capacitance retention at high current densities

Scalable, highly stable supercapacitor electrodes were developed from the mixture of a tea factory waste based activated carbon (AC) and a low-cost electrochemical exfoliated graphene (EEG). The hybrid electrodes showed notably enhanced stability at high current densities. The AC sample was prepared by chemical method and exposed to a further heat treatment to enhance electrochemical performance. Graphene used in the preparation of hybrid electrodes was obtained by direct electrochemical exfoliation of graphite in an aqueous solution. Detailed structural characterization of AC, EEG, and hybrid material was performed. The original electrochemical performances of AC and EEG were examined in button size cells using an aqueous electrolyte. The hybrid materials were prepared by mixing AC and EEG at different mass percentage ratios, and tested as supercapacitor electrodes under the same conditions. Capacitance stability of the electrodes developed from AC:EEG (70:30) at high currents increased by about 45% compared to the original AC. The highest gravimetric capacitance (110 F/g) was achieved by this hybrid electrode. The hybrid electrode was scaled up to the pouch size and tested using an organic electrolyte. The organic electrolyte was preferred for scaling up due to its wider voltage ranges. The pouch cell had a gravimetric capacitance of 85 F/g and exhibited as good performance as the coin cell in the organic electrolyte.     

Optical manipulation of charged microparticles in polar fluids

In this study, we report a systematic study of the response of a charged microparticle confined in an optical trap and driven by electric fields. The particle is embedded in a polar fluid, hence, the role of ions and counterions forming a double layer around the electrodes and the particle surface itself has been taken into account. We analyze two different cases: (i) electrodes energized by a step‐wise voltage (DC mode) and (ii) electrodes driven by a sinusoidal voltage (AC mode). The experimental outcomes are analyzed in terms of a model that combines the electric response of the electrolytic cell and the motion of the trapped particle. In particular, for the DC mode we analyze the transient particle motion and correlate it with the electric current flowing in the cell. For the AC mode, the stochastic and deterministic motion of the trapped particle is analyzed either in the frequency domain (power spectral density, PSD) or in the time domain (autocorrelation function). Moreover, we will show how these different approaches (DC and AC modes) allow us, assuming predictable the applied electric field (here generated by plane parallel electrodes), to provide accurate estimation (3%) of the net charge carried by the microparticle. Vice versa, we also demonstrate how, once predetermined the charge, the trapped particle acts as a sensitive probe to reveal locally electric fields generated by arbitrary electrode geometries (in this work, wire‐tip geometry).