The Open Circuit Voltage in Biofuel Cells: Nernstian Shift in Pseudocapacitive Electrodes

In the development of biofuel cells great effort is dedicated to achieving outstanding figures of merit, such as high stability, maximum power output, and a large open circuit voltage. Biofuel cells with immobilized redox mediators, such as redox polymers with integrated enzymes, show experimentally a substantially higher open circuit voltage than the thermodynamically expected value. Although this phenomenon is widely reported in the literature, there is no comprehensive understanding of the potential shift, the high open circuit voltages have not been discussed in detail, and hence they are only accepted as an inherent property of the investigated systems. We demonstrate that this effect is the result of a Nernstian shift of the electrode potential when catalytic conversion takes place in the absence or at very low current flow. Experimental evidence confirms that the immobilization of redox centers on the electrode surface results in the assembled biofuel cell delivering a higher power output because of charge storage upon catalytic conversion. Our findings have direct implications for the design and evaluation of (bio)fuel cells with pseudocapacitive elements.     

Electrochemically aided solid phase microextraction: conducting polymer film material applicable for cationic analytes

The cation uptake and release properties of a poly(pyrrole-sulfated β-cyclodextrin) (PPy-SβCD) film electrode have been investigated under both open circuit and controlled potential conditions for prospective applications in electrochemically aided solid-phase microextraction (EA SPME). The EDAX and ion chromatography results show that the K⁺ and Na⁺ cation uptake is enhanced if a small negative potential is applied to the electrode in the range where PPy is in its neutral form. These cations are released rapidly from the film if the applied potential is switched to the value at which PPy is converted to its positively charged form, i.e., oxidized state. The cation ingress and egress mechanism is affected both by the cation exchange at the negative sulfate moiety on the cyclodextrin sites and electrostatic interactions generated by the applied potential. The electrochemical "switching" capability increases the speed of the cation uptake and release, presumably due to electro migration, as compared to the open circuit ion exchange which is controlled solely by diffusion. Our preliminary fundamental results show that the PPy-SβCD film is suitable for the future design of EA SPME devices. Electronic Publication     

分流式湿度发生器温度控制系统的设计

采用控制加热的方法对分流式湿度发生器的增湿饱和器进行精密的温度控制。介绍了增湿饱和器的结构及温度控制电路、温度控制软件的设计要点。通过精密控制增湿饱和器的温度，减少了环境温度变化对分流式湿度发生器输出湿度的影响，提高了湿度校准的准确度。     

Design of pressure-driven microfluidic networks using electric circuit analogy

This article reviews the application of electric circuit methods for the analysis of pressure-driven microfluidic networks with an emphasis on concentration- and flow-dependent systems. The application of circuit methods to microfluidics is based on the analogous behaviour of hydraulic and electric circuits with correlations of pressure to voltage, volumetric flow rate to current, and hydraulic to electric resistance. Circuit analysis enables rapid predictions of pressure-driven laminar flow in microchannels and is very useful for designing complex microfluidic networks in advance of fabrication. This article provides a comprehensive overview of the physics of pressure-driven laminar flow, the formal analogy between electric and hydraulic circuits, applications of circuit theory to microfluidic network-based devices, recent development and applications of concentration- and flow-dependent microfluidic networks, and promising future applications. The lab-on-a-chip (LOC) and microfluidics community will gain insightful ideas and practical design strategies for developing unique microfluidic network-based devices to address a broad range of biological, chemical, pharmaceutical, and other scientific and technical challenges.     

Computerized electroanalysis: Part I. Instrumentation and programming

The electronic circuit of a computerized electroanalytical instrument is described. The computer is programmed in a high-level language. The instrument can generate linear potential scans of any conceivable form within a wide range of scan rates. Pulses of varying height and duration can be superimposed on the linear scans. Stirrer motors, inert gas flow and addition of standard reagent are under computer control, thus permitting completely automatic analytical procedures. Data are displayed on a graphical screen. Results from anodic stripping experiments are given.     

Design of a fixed-frequency impedance matching network and measurement of plasma impedance in an inductively coupled plasma for atomic emission spectroscopy

The design of a fixed frequency impedance matching network for inductively coupled plasma-atomic emission spectroscopy (ICP-AES) is developed based upon an accurate model. Plasma impedance is determined by a substitution method in conjunction with the matching network model. The model is precise enough to allow the determination of electrical stresses on components of the circuit for design purposes. Variations of the plasma impedance caused by gas flow rate changes or by sample introduction are measured.Practical considerations in the design are made to ease ignition, to improve stability of the power transferred to the plasma, and to increase the efficiency of this power transfer. This has resulted in the production of a commercial matching network with power transfer efficiency of 86%.     

Control of d.c. bias current in radio-frequency glow discharge source and its emission characteristics

A novel technique to control r.f.-powered Grimm-style glow discharge plasmas is described. A new channel of d.c. current driven by the self-bias potential is opened by using a low-pass filter circuit and a load resistor; as the result, a large number of electrons can flow along the d.c. circuit channel including the plasma body from the grounded electrode to the sample electrode. This phenomenon is effective for improvement of the detection sensitivity in the optical emission spectrometry. Atomic emission lines having lower excitation energies are predominantly enhanced by a factor of 10–20. The conduction of the d.c. bias current could promote these excitations.     

Directional flow induced by synchronized longitudinal and zeta-potential controlling AC-electrical fields

Electroosmotic flow (EOF) in a microchannel can be controlled by electronic control of the surface charge using an electrode embedded in the wall of the channel. By setting a voltage to the electrode, the zeta-potential at the wall can be changed locally. Thus, the electrode acts as a "gate" for liquid flow, in analogy with a gate in a field-effect transistor. In this paper we will show three aspects of a Field Effect Flow Control (FEFC) structure. We demonstrate the induction of directional flow by the synchronized switching of the gate potential with the channel axial potential. The advantage of this procedure is that potential gas formation by electrolysis at the electrodes that provide the axial electric field is suppressed at sufficiently large switching frequencies, while the direction and magnitude of the EOF can be maintained. Furthermore we will give an analysis of the time constants involved in the charging of the insulator, and thus the switching of the zeta potential, in order to predict the maximum operating frequency. For this purpose an equivalent electrical circuit is presented and analyzed. It is shown that in order to accurately describe the charging dynamics and pH dependency the traditionally used three capacitor model should be expanded with an element describing the buffer capacitance of the silica wall surface.     

两个不可逆电对共存体系的流动注射双安培分析法

流动注射分析已有电位法[1]、单安培法[2]和双安培法[3～8]等电化学检测方法.安培法比电位法灵敏,颇受重视.单安培法由于在控制工作电位范围内受其它可氧化还原物质的干扰,选择性不高.双安培法仪器装置较简单,但应用范围仅限于I2/I-,Fe3+/Fe2+等少数几种可逆电对体系.迄今为止,尚未见到将双安培法应用于不可逆体系的报道.本文讨论两个独立的不可逆电对共存的流动注射双安培检测法,以拓展双安培法的应用范围,并选用溶解氧分别与抗坏血酸、羟胺和联氨构成的3个体系进行了考察验证.1　实验部分1.1　仪器和试剂　流动注射双安培检测系统由IFI…     

Quantifying the effect of membrane potential in chemical osmosis across bentonite membranes by virtual short-circuiting

Clay liners are charged membranes and show semipermeable behavior regarding the flow of fluids, electrical charge, chemicals and heat. At zero gradients of temperature and hydrostatic pressure, a salt concentration gradient across a compacted clay sample induces not only an osmotic flux of water and diffusion of salt across the membrane but also an electrical potential gradient, defined as membrane potential. Laboratory experiments were performed on commercially available bentonite samples in a rigid-wall permeameter connected to two electrically insulated fluid reservoirs filled with NaCl solutions of different concentrations and equipped with Ag/AgCl electrodes to measure the electrical potential gradient. The effect of membrane potential could be cancelled out by short-circuiting the clay with the so-called virtual shortcut. The potential gradient across the sample is brought to zero with a negative feedback circuit. It was observed that the water flux and the diffusion of Cl- were hindered by the occurrence of a membrane potential, indicating that an electroosmotic counterflow is induced. Flow parameters were calculated with modified coupled flow equations of irreversible thermodynamics. They were in excellent agreement with values reported in the literature. Comparing the method of short-circuiting with a study elsewhere, where the electrodes were physically short-circuited, it was shown that the virtual shortcut is more appropriate because physically short-circuiting induces additional effects that are attributed to the fluxes.     

Multipumping flow systems devoid of computer control for process and environmental monitoring

In this paper we describe for the first time the use of astable multivibrator circuits for computer-less control of solenoid micropumps and application for analytical flow techniques. Triggering and powering were accomplished using the NE555 integrated circuit. The activation and deactivation time intervals were adjusted using 10-turn trimmer potentiometers. The potential and characteristics of the instrumentation were studied on a two-channel flow system injecting an indigo carmine indicator solution. Subsequently, a three-channel flow system was assembled and successfully applied to the determination of nitrite in well waters. One circuit was used to control the activation time intervals of three further circuits used for the control of the flow rates or pulsation of solenoid micropumps. These were used for driving carrier, reagent, and sample in an analytical flow system. In the present work, the utility of the circuits for the construction of simple multipumping flow systems was demonstrated. A main feature to be highlighted was the simplicity and very low costs of the controlling circuits, favouring economic and miniaturised flow analysers. Second, no expenses or knowledge are required for the usual software control of the solenoid micropumps. This allows working with every existing detector without considering any problems of software and control compatibility. Third, owing to these features, the proposed assembly is especially suited for simple monitoring analysers, sample provision from an environmental or industrial process, or chemical education.     

Acoustofluidics 1: Governing equations in microfluidics

In Part 1 of the thematic tutorial series "Acoustofluidics--exploiting ultrasonic standing waves forces and acoustic streaming in microfluidic systems for cell and particle manipulation", we establish the governing equations in microfluidics. Examples of basic flow solutions are presented, and equivalent circuit modeling for determining flow rates in microfluidic networks is introduced.     

Double interdigitated array microelectrode-based impedance biosensor for detection of viable Escherichia coli O157:H7 in growth medium

Double interdigitated array microelectrodes (IAM)-based flow cell was developed for an impedance biosensor to detect viable Escherichia coli O157:H7 cells after enrichment in a growth medium. This study was aimed at the design of a simple flow cell with embedded IAM which does not require complex microfabrication techniques and can be used repeatedly with a simple assembly/disassembly step. The flow cell was also unique in having two IAM chips on both top and bottom surfaces of the flow cell, which enhances the sensitivity of the impedance measurement. E. coli O157:H7 cells were grown in a low conductivity yeast-peptone-lactose-TMAO (YPLT) medium outside the flow cell. After bacterial growth, impedance was measured inside the flow cell. Equivalent circuit analysis indicated that the impedance change caused by bacterial growth was due to double layer capacitance and bulk medium resistance. Both parameters were a function of ionic concentration in the medium, which increased during bacterial growth due to the conversion of weakly charged substances present in the medium into highly charged ions. The impedance biosensor successfully detected E. coli O157:H7 in a range from 8.0 to 8.2x10(8)CFUmL(-1) after an enrichment growth of 14.7 and 0.8h, respectively. A logarithmic linear relationship between detection time (T(D)) in h and initial cell concentration (N(0)) in CFUmL(-1) was T(D)=-1.73logN(0)+14.62, with R(2)=0.93. Double IAM-based flow cell was more sensitive than single IAM-based flow cell in the detection of E. coli O157:H7 with 37-61% more impedance change for the frequency from 10Hz to 1MHz. The double IAM-based flow cell can be used to design a simple impedance biosensor for the sensitive detection of bacterial growth and their metabolites.     

苯胺自催化聚合反应的混合电位

在苯胺自催化聚合反应的开路电位实验中,Pt、Pd和聚苯胺膜具有远正于其它金属的开路电位.该实验结果可以根据混合电位理论来解释.由于苯胺自催化聚合反应具有电化学机理,因此该反应的基底材料将具有同时对应于阳极半反应和阴极半反应的混合电极电位.混合电位在实验上表现为开路电位,其数值大小可以看作是苯胺自催化聚合反应能否进行的标志.有关开路电位的实验结果进一步证明了苯胺自催化聚合反应的电化学机理.     

Design,Synthesis, and Photovoltaic Characterization of a Small Molecular Acceptor with an Ultra‐Narrow Band Gap

The design of narrow band gap (NBG) donors or acceptors and their application in organic solar cells (OSCs) are of great importance in the conversion of solar photons to electrons. Limited by the inevitable energy loss from the optical band gap of the photovoltaic material to the open‐circuit voltage of the OSC device, the improvement of the power conversion efficiency (PCE) of NBG‐based OSCs faces great challenges. A novel acceptor–donor–acceptor structured non‐fullerene acceptor is reported with an ultra‐narrow band gap of 1.24 eV, which was achieved by an enhanced intramolecular charge transfer (ICT) effect. In the OSC device, despite a low energy loss of 0.509 eV, an impressive short‐circuit current density of 25.3 mA cm⁻² is still recorded, which is the highest value for all OSC devices. The high 10.9 % PCE of the NBG‐based OSC demonstrates that the design and application of ultra‐narrow materials have the potential to further improve the PCE of OSC devices.     

Design,Synthesis, and Photovoltaic Characterization of a Small Molecular Acceptor with an Ultra‐Narrow Band Gap

The design of narrow band gap (NBG) donors or acceptors and their application in organic solar cells (OSCs) are of great importance in the conversion of solar photons to electrons. Limited by the inevitable energy loss from the optical band gap of the photovoltaic material to the open‐circuit voltage of the OSC device, the improvement of the power conversion efficiency (PCE) of NBG‐based OSCs faces great challenges. A novel acceptor–donor–acceptor structured non‐fullerene acceptor is reported with an ultra‐narrow band gap of 1.24 eV, which was achieved by an enhanced intramolecular charge transfer (ICT) effect. In the OSC device, despite a low energy loss of 0.509 eV, an impressive short‐circuit current density of 25.3 mA cm⁻² is still recorded, which is the highest value for all OSC devices. The high 10.9 % PCE of the NBG‐based OSC demonstrates that the design and application of ultra‐narrow materials have the potential to further improve the PCE of OSC devices.     

Substrate cross-conduction effect on the performance of serially connected microbial fuel cell stack

This study presented a new design of scalable, air-cathode microbial fuel cell (MFC) stack that shared a common fuel feed passage. As two individual cells were electrically connected in series by metal wires and hydraulically joined by conductive substrate flow, the performance degradation phenomenon was observed. The open circuit voltage (OCV) and low current behavior of stacked MFC were lower than should be expected. This energy loss was proposed to be a consequence of parasitic current flow due to the substrate cross-conduction effect and can be likely minimized through controlling the distance between the anode electrodes or/and the cross-sectional area of substrate flow. The unique and simple water distribution system of the tubular MFC stack would contribute to the further scale-up and implementation of MFC technologies, especially for wastewater treatment.     

Numerical study of the vortex-induced electroosmotic mixing of non-Newtonian biofluids in a nonuniformly charged wavy microchannel: Effect of finite ion size

We propose a micromixer for obtaining better efficiency of vortex induced electroosmotic mixing of non-Newtonian bio-fluids at a relatively higher flow rate, which finds relevance in many biomedical and biological applications. To represent the rheology of non-Newtonian fluid, we consider the Carreau model in this study, while the applied electric field drives the constituent components in the micromixer. We show that the spatial variation of the applied field, triggered by the topological change of the bounding surfaces, upon interacting with the non-uniform surface potential gives rise to efficient mixing as realized by the formation of vortices in the proposed micromixer. Also, we show that the phase-lag between surface potential leads to the formation of asymmetric vortices. This behavior offers better mixing performance following the appearance of undulation on the flow pattern. Finally, we establish that the assumption of a point charge in the paradigm of electroosmotic mixing, which is not realistic as well, under-predicts the mixing efficiency at higher amplitude of the non-uniform zeta potential. The inferences of the present analysis may guide as a design tool for micromixer where rheological properties of the fluid and flow actuation parameters can be simultaneously tuned to obtain phenomenal enhancement in mixing efficiency.     

Microfluidic Wheatstone bridge for rapid sample analysis

We developed a microfluidic analogue of the classic Wheatstone bridge circuit for automated, real-time sampling of solutions in a flow-through device format. We demonstrate precise control of flow rate and flow direction in the "bridge" microchannel using an on-chip membrane valve, which functions as an integrated "variable resistor". We implement an automated feedback control mechanism in order to dynamically adjust valve opening, thereby manipulating the pressure drop across the bridge and precisely controlling fluid flow in the bridge channel. At a critical valve opening, the flow in the bridge channel can be completely stopped by balancing the flow resistances in the Wheatstone bridge device, which facilitates rapid, on-demand fluid sampling in the bridge channel. In this article, we present the underlying mechanism for device operation and report key design parameters that determine device performance. Overall, the microfluidic Wheatstone bridge represents a new and versatile method for on-chip flow control and sample manipulation.     

Role of streaming potential on pulsating mass flow rate control in combined electroosmotic and pressure-driven microfluidic devices

In the present study, we investigate the implications of streaming potential on the mass flow rate control in a microfluidic device actuated by the combined application of a pulsating pressure gradient and a pulsating, externally applied, electric field. We demonstrate that the temporal dynamics due to streaming potential effects may lead to interesting non-trivial aspects of the resultant transport characteristics. Our results highlight the importance of an adequate accounting of the streaming potential effects for temporally tunable mass flow rate control strategies, which may act as a useful design artifice to augment mass flow rates in practical scenarios.