Biohybrid microarrays – Impedimetric biosensors with 3D in vitro tissues for toxicological and biomedical screening

To investigate the effectiveness of potential anti-cancer therapeutics or therapies, efficient screening methods are required. On the one hand, multicellular 3D aggregates (spheroids) are a powerful in vitro model for simulating the in vivo situation and on the other hand, planar electrode structures are generally highly suitable for automation and parallel testing. Here, the detection of the effect of active substances on spheroids positioned on electrodes of substrate integrated electrode arrays is exemplarily investigated. As a 3D tissue model a reaggregation system of T47D clone 11 tumor cells is used. The effect of cytotoxins (DMSO, Triton X-100) on spheroids can be detected by recording the effective impedance of planar electrodes covered by spheroids. The equivalent circuit model parameter of electrodes covered by cytotoxin treated spheroids are determined from recorded impedance spectra and compared to the parameter of electrodes covered by control spheroids as well as not covered electrodes. Spheroids on electrodes mainly influence the electrode impedance in the frequency range of 10 kHz to 1 MHz. The results are discussed in view of an optimal electrode/spheroid-interface for sensing the effects of therapeutics with high sensitivity.     

Biohybrid microarrays--impedimetric biosensors with 3D in vitro tissues for toxicological and biomedical screening

To investigate the effectiveness of potential anticancer therapeutics or therapies, efficient screening methods are required. On the one hand, multicellular 3D aggregates (spheroids) are a powerful in vitro model for simulating the in vivo situation and on the other hand, planar electrode structures are generally highly suitable for automation and parallel testing. Here, the detection of the effect of active substances on spheroids positioned on electrodes of substrate integrated electrode arrays is exemplarily investigated. As a 3D tissue model a reaggregation system of T47D clone 11 tumor cells is used. The effect of cytotoxins (DMSO, Triton X-100) on spheroids can be detected by recording the effective impedance of planar electrodes covered by spheroids. The equivalent circuit model parameter of electrodes covered by cytotoxin treated spheroids are determined from recorded impedance spectra and compared to the parameter of electrodes covered by control spheroids as well as not covered electrodes. Spheroids on electrodes mainly influence the electrode impedance in the frequency range of 10 kHz to 1 MHz. The results are discussed in view of an optimal electrode/spheroid-interface for sensing the effects of therapeutics with high sensitivity.     

A contactless conductivity detector for capillary electrophoresis: effects of the detection cell geometry on the detector performance

The effect of the gap between the electrodes and of their width on the behavior of a capacitively wired contactless conductivity detector was studied. The results obtained have indicated that the detector response can be qualitatively described by a model based on the concept of the effective electrode width which is a complex parameter determined by the gap between the electrodes, the frequency of the input signal and the conductivity of the test solution. The detector sensitivity and the effect on the separation efficiency depend on the difference between the effective and geometric electrode widths. Higher detection sensitivities have been attained for detectors with wide electrodes operating at lower frequencies, however, better separation efficiencies have been achieved using detectors with narrow electrodes and higher operational frequencies. The noise increases with decreasing gap between the electrodes and increasing frequency, especially with detectors employing narrow electrodes.     

Development and experimental verification of a mathematical model of lithium ion battery

A model of the lithium ion battery is developed which takes into account intercalation and extraction of lithium ions in the active mass of negative and positive electrodes, the dependences of equilibrium electrode potentials on the concentration of intercalated lithium, the ion transfer in pores of electrodes and the separator, the kinetics of electrode reactions, and the electric double layer charging. As the active material for the negative electrode, UAMS graphite material is used. Lithium-nickel-cobalt oxide serves as the positive electrode. The porous structure of electrodes is studied by the method of standard contact porosimetry. Sufficiently high porosity values found for both electrodes (50% for anode and 27% for cathode) made it possible to consider the interface as regards the internal pore surface found from porosimetry data rather than as regards their external surface as in the previous studies. A comparison of calculated and experimental discharge curves demonstrates their closeness, which points to the correctness of the model. By the fitting procedure, the coefficients of solid-state diffusion of lithium ions and the rate constants for reactions on both electrodes are found.     

On the interpretation of the potentiometric response of the inert solid electrodes in the monitoring of the oscillatory processes involving hydrogen peroxide

Solid inert electrodes are frequently used in potentiometry. However, potentiometric responses may significantly depend on the inert electrode material, a fact which may manifest itself particularly distinctly for the dynamical chemical systems—oscillating processes. We found that for the homogeneous oscillators involving hydrogen peroxide and either thiocyanates or thiosulfates, the periodic variations of the platinum and palladium indicator electrode potential are both not in phase with the variations of the potential of the gold and glassy carbon electrodes, the latter two exhibiting in turn concordant, in-phase responses. Potentiometric responses were compared with the impedance characteristics of the electrodes during the oscillations. In spite of high mechanistic complexity of the studied homogeneous oscillatory systems, we explain different responses of inert electrodes in terms of the concept of the mixed electrode potential, i.e., determined by more than one redox couple of different kinetic characteristics (exchange current densities). In our model explanation, two coupled Ox₁/Red₁ and Ox₂/Red₂ redox systems are considered. It is suggested that for Au or glassy carbon electrodes, the mixed potential is largely determined by the Ox₁/Red₁ couple. For Pt or Pd electrodes, due to the catalytic effect of their surfaces on the Ox₂/Red₂ couple, its exchange current largely controls the measured mixed potential. Our concept is supported by numerical calculations involving the classical Brusselator as the model generator of chemical oscillations. The proper interpretation of potentiometric kinetic data is crucial for the diagnosis of the correct reaction mechanism.     

Flow-injection approach for the determination of the dynamic response characteristics of ion-selective electrodes. Part 1. Theoretical considerations

A single-line flow-injection system with a straight tube reactor is proposed for investigating the dynamic response behaviour of ion-selective electrodes. The principle of the method is based on the fact that the concentration—time curve at the electrode surface can be described theoretically in the flow-injection system under certain practically realizable conditions. The response of the ion-selective electrode to that input signal can be measured experimentally. Thus, knowing the input and the output signal of an ion-selective electrode, an appropriate model describing its dynamic behaviour can be selected among the relevant models existing in the literature. Theoretical expressions for predicting the transient response of ion-selective electrodes in the flow system when the rate-determining step is an ion-transport process through a diffusion layer or a kinetic process were elaborated.     

Specific Features of the Structure of the Electrical Double Layer on Intermetallic Compound Ag3Sn in Aqueous Surface-Inactive Electrolytes

The behavior of a polycrystalline electrode of intermetallic compound Ag₃Sn in aqueous NaF solutions is studied using an impedance method. At potentials of –1.3 to –0.6 V (SCE), the electrode may be viewed as ideally polarizable. The Gouy–Chapman–Stern–Grahame model quite satisfactorily describes capacitance curves obtained at various electrolyte concentrations. Experimental capacitance curves for an Ag₃Sn electrode are compared with those for polycrystalline Ag and Sn electrodes renewed by cutting. Characteristics of an Ag₃Sn electrode are unique and its capacitance curves cannot be simulated from the data for Ag and Sn electrodes within the known phenomenological approaches.     

Analytical performance characteristics of thin and thick film amperometric microcells

The analytical performance of amperometric microcells with different electrode geometries is compared for enzyme activity measurements. The microcells were fabricated with thin film photolithography or thick film screen-printing in four different designs. The cells made with the thin film process used flexible substrate with microelectrode array or a circular, disk-shaped working electrode. The screen-printed working electrodes had semicircle or disk shape on ceramic chips. Putrescine oxidase (PUO) activity measurement was used as a model. The determination of PUO activity is important in the clinical diagnosis of premature rupture of the amniotic membrane. An electropolymerized m-phenylenediamine size-exclusion layer was used to eliminate common interferences. The size exclusion layer revealed also to be advantageous in protecting the electrodes from fouling by putrescine (enzyme substrate). The electrode fouling of bare electrodes was insignificant for screen-printed electrodes, but very severe for electroplated platinum working electrodes. The microelectrode array electrodes demonstrated smaller RSD and higher normalized sensitivities for hydrogen peroxide and PUO activity. All the other electrodes were demonstrating comparable analytical performances.     

Analytical performance characteristics of thin and thick film amperometric microcells

The analytical performance of amperometric microcells with different electrode geometries is compared for enzyme activity measurements. The microcells were fabricated with thin film photolithography or thick film screen-printing in four different designs. The cells made with the thin film process used flexible substrate with microelectrode array or a circular, disk-shaped working electrode. The screen-printed working electrodes had semicircle or disk shape on ceramic chips. Putrescine oxidase (PUO) activity measurement was used as a model. The determination of PUO activity is important in the clinical diagnosis of premature rupture of the amniotic membrane. An electropolymerized m-phenylenediamine size-exclusion layer was used to eliminate common interferences. The size exclusion layer revealed also to be advantageous in protecting the electrodes from fouling by putrescine (enzyme substrate). The electrode fouling of bare electrodes was insignificant for screen-printed electrodes, but very severe for electroplated platinum working electrodes. The microelectrode array electrodes demonstrated smaller RSD and higher normalized sensitivities for hydrogen peroxide and PUO activity. All the other electrodes were demonstrating comparable analytical performances.     

Flow-injection approach for the determination of the dynamic response characteristics of ion-selective electrodes. Part 2. Application to tubular solid-state iodide electrode

The flow-injection approach proposed in Part 1 for investigating the dynamic behaviour of flow-through detectors was applied to the study of a tubular iodide electrode prepared from a pressed precipitate of silver iodide. It was found that the electrode response cannot be described by either of the models developed in Part 1 on the basis of the concepts in the literature for the transient response mechanism of ion-selective electrodes. Scratches and microcavities in the electrode bore were observed under the microscope, most probably resulting from bore drilling. These surface non-idealities make the accessibility of the electrode surface to the analyte variable. A mathematical model developed on the basis of the assumption that part of the electrode surface is directly exposed to the flow while the remainder of the surface incorporates all microcavities in which ideal mixing exists, describes fairly successfully the experimentally obtained potential—time transients. Moreover, this study shows that the rate of response of solid-state ion-selective electrodes is in principle comparable to that of other electrochemical sensors.     

Electrode commutation sequence for honeycomb arrangement of electrodes in electrochemotherapy and corresponding electric field distribution

Electrochemotherapy is a treatment based on combination of chemotherapeutic drug and electroporation. It is used in clinics for treatment of solid tumours. For electrochemotherapy of larger tumours multiple needle electrodes were already suggested. We developed and tested electrode commutation circuit, which controls up to 19 electrodes independently. Each electrode can be in one of three possible states: on positive or negative potential or in the state of high impedance. In addition, we tested a pulse sequence using seven electrodes for which we also calculated electric field distribution in tumour tissue by means of finite-elements method. Electrochemotherapy, performed by multiple needle electrodes and tested pulse sequence on large subcutaneous murine tumour model resulted in tumour growth delay and 57% complete responses, thus demonstrating that the tested electrode commutation sequence is efficient.     

Electrochemical insertion of lithium in thin tin films

The behavior of model electrodes made of lithiated thin films of tin during their cycling is studied. The electrodes show high values of useful specific capacity for the extension of several tens of operation cycles. Dependences of the diffusion coefficient for lithium into tin on the initial electrode potential, temperature, and direction of the electrode process are determined by a chronopotentiometric method. The dependences have a complex character, which is connected with the phase composition of the lithium-tin alloy.     

Facilitated electrochemical oxidation of NADH and its model compound at gold electrode modified with terminally substituted electroinactive self-assembled monolayers

The electrochemical oxidation of NADH and its model compound, N-benzyl-1,4-dihydronicotinamide (DHN), has been studied at gold electrode modified with self-assembled monolayer of terminally substituted thiols/disulfide, i.e., cystamine (CYST), mercaptopropionic acid (MPA) and mercaptoethanol (ME). A substantial decrease in the overpotential (approximately 250 mV) when compared to the bare electrode has been observed for the oxidation of NADH at the monolayer-modified electrodes, containing no so-called redox mediator. The bare electrode shows an ill-defined voltammetric peak for the oxidation of DHN, whereas the monolayer-modified electrodes showed a well-defined voltammetric peak. The monolayer assembly on the gold electrode prevents the fouling of electrode surface by the oxidation products, which favors the oxidation at the less positive potential. The square-wave voltammograms showed a sharp voltammetric signal for the oxidation of NADH at all the monolayer-modified electrodes. All the monolayer-modified electrodes showed a linear current response to change in the NADH concentration in its range of 25-300 microM and their sensitivities were found to be 0.005+/-0.0003, 0.0063+/-0.0002 and 0.0052+/-0.0003 microA/microM for CYST-Au, ME-Au and MPA-Au electrodes, respectively. The hydrodynamic voltammograms obtained at the rotating CYST-Au electrode for the oxidation of NADH and DHN were used to estimate the diffusion coefficient of DHN, and the number of electrons involved in the oxidation process of NADH.     

电解煤浆制氢钛基阳极铂铁合金催化剂的制备及电催化活性研究

将预处理后的钛片作为电极基体, 采用恒电流法沉积Pt和Fe, 通过高温热处理得到Ti/Pt-Fe电极, 通过扫描电镜(SEM)、X射线衍射(XRD)、电子能谱(EDS)以及等离子发射光谱(ICP)等方法对所制备电极表面形貌、组分的合金化程度、催化层成分组成以及电极寿命等进行了表征; 在煤浆电解过程中, 采用两电极体系, 对所制备电极的电催化活性进行了测试. 结果表明: 所制备的电极表面呈层状结构, 且有大量峰形突起, 催化层形成了PtFe合金, 合金化程度较高, 与同面积的铂片电极、Ti/TiO₂Pt-Ru电极相比, 大大提高了电催化活性, 降低了电极成本.     

Voltammetry at partially covered electrodes: Part I. Chronopotentiometry and chronoamperometry at model electrodes

Equations for chronopotentiometry and chronoamperometry at partially covered electrodes have been derived using a model of hexagonal array of cylidrical spaces terminated, at the electrode surface, by concentric active and inactive regions. The boundary value problem was shown to be analogous to that for a charge transfer preceded by a chemical reaction. Experiments with the reduction of ferricyanide on gold model electrodes partially covered with photoresist layer showed excellent agreement with the theory. Application of the equations to estimation of coverage and size of active sites distributed on a electrode surface is discussed.     

Comparative study of the thermodynamics and kinetics of the ion transfer across the liquid/liquid interface by means of three-phase electrodes

A comparative study of the behavior of different sorts of three-phase electrodes applied for assessing the thermodynamics and kinetics of the ion transfer across the liquid/liquid (L/L) interface is presented. Two types of three-phase electrodes are compared, that is, a paraffin-impregnated graphite electrode at the surface of which a macroscopic droplet of an organic solvent is attached and an edge pyrolytic graphite electrode partly covered with a very thin film of the organic solvent. The organic solvent contains either decamethylferrocene or lutetium bis(tetra-tert-butylphthalocyaninato) as a redox probe. The role of the redox probe, the type of the electrode material, the mass transfer regime, and the effect of the uncompensated resistance are discussed. The overall electrochemical process at both three-phase electrodes proceeds as a coupled electron-ion transfer reaction. The ion transfer across the L/L interface, driven by the electrode reaction of the redox compound at the electrode/organic solvent interface, is independent of the type of redox probe. The ion transfer proceeds without involving any chemical coupling between the transferring ion and the redox probe. Both types of three-phase electrodes provide consistent results when applied for measuring the energy of the ion transfer. Under conditions of square-wave voltammetry, the coupled electron-ion transfer at the three-phase electrode is a quasireversible process, exhibiting the property known as "quasireversible maximum". The overall electron-ion transfer process at the three-phase electrode is controlled by the rate of the ion transfer. It is demonstrated for the first time that the three-phase electrode in combination with the quasireversible maximum is a new tool for assessing the kinetics of the ion transfer across the L/L interface.     

The Influence of Electrode Porosity on Diffusional Cyclic Voltammetry

A simple generic model to predict the influence of electrode porosity on the cyclic voltammetric response of an electrode is presented. The conditions under which deviation from the behavior of a perfectly flat, planar electrode can be expected are predicted. The scope for misinterpretation when conventional flat electrode theory is applied to porous electrodes is highlighted, especially in respect to the extraction of electrode kinetic parameters and the influence of ‘electrocatalysis’.     

Harmonic Analysis of Pulsed Photovoltaic Response of Titanium Dioxide Films under Local Illumination

A change in the photoelectrochemical behavior of a semiconductor–electrolyte system after transition from overall to local illumination of the electrode surface is studied. The proposed model accounts for charge interactions between illuminated and dark electrode portions and describes the frequency spectrum of photopotential for an electrode locally illuminated by a periodic sequence of light pulses a few nanoseconds in length. As shown with polycrystalline thin-film TiO₂electrodes, local values of concentrations of ionized donors and flat-band potentials of semiconductor electrodes may be determined with a harmonic analysis of frequency spectra of photovoltaic responses. The possibility of using the proposed approach in photoelectrochemical microscopy is discussed.