Probing Biomolecular Interactions at Conductive and Semiconductive Surfaces by Impedance Spectroscopy: Routes to Impedimetric Immunosensors,DNA‐Sensors,and Enzyme Biosensors

Impedance spectroscopy is a rapidly developing electrochemical technique for the characterization of biomaterial‐functionalized electrodes and biocatalytic transformations at electrode surfaces, and specifically for the transduction of biosensing events at electrodes or field‐effect transistor devices. The immobilization of biomaterials, e.g., enzymes, antigens/antibodies or DNA on electrodes or semiconductor surfaces alters the capacitance and interfacial electron transfer resistance of the conductive or semiconductive electrodes. Impedance spectroscopy allows analysis of interfacial changes originating from biorecognition events at electrode surfaces. Kinetics and mechanisms of electron transfer processes corresponding to biocatalytic reactions occurring at modified electrodes can be also derived from Faradaic impedance spectroscopy. Different immunosensors that use impedance measurements for the transduction of antigen‐antibody complex formation on electronic transducers were developed. Similarly, DNA biosensors using impedance measurements as readout signals were developed. Amplified detection of the analyte DNA using Faradaic impedance spectroscopy was accomplished by the coupling of functionalized liposomes or by the association of biocatalytic conjugates to the sensing interface providing biocatalyzed precipitation of an insoluble product on the electrodes. The amplified detections of viral DNA and single‐base mismatches in DNA were accomplished by similar methods. The changes of interfacial features of gate surfaces of field‐effect transistors (FET) upon the formation of antigen‐antibody complexes or assembly of protein arrays were probed by impedance measurements and specifically by transconductance measurements. Impedance spectroscopy was also applied to characterize enzyme‐based biosensors. The reconstitution of apo‐enzymes on cofactor‐functionalized electrodes and the formation of cofactor‐enzyme affinity complexes on electrodes were probed by Faradaic impedance spectroscopy. Also biocatalyzed reactions occurring on electrode surfaces were analyzed by impedance spectroscopy. The theoretical background of the different methods and their practical applications in analytical procedures were outlined in this article.     

Electrical probing of endothelial cell behaviour on a fibronectin/polystyrene/thiol/gold electrode by Faradaic electrochemical impedance spectroscopy (EIS)

The electrochemical impedance spectroscopy (EIS) technique has been shown to be an effective tool for monitoring endothelial cell behaviour on a multilayer functionalised gold electrode. Polystyrene, a reproducible model substrate, is deposited as a thin layer on a thiol functionalised gold electrode. Fibronectin, a protein promoting endothelial cell adhesion, is then adsorbed on the polystyrene surface. The different steps of this multilayer assembly are characterized by Faradaic impedance. The charge transfer resistance and the capacitance for the total layer are modified at each step according to the electrical properties of each layer. This gives the endothelial cells' electrical state in terms of its resistive and capacitive properties. In this study, the endothelial cell layer presents a specific charge transfer resistance equal to 1.55 kOmega cm(2) with no large defects in the cell layer, and a specific capacitance equal to few microF cm(-2) explained by the existence of pseudopods. These electrical properties are correlated to the endothelial cell viability, adhesion and cytoskeleton organization.     

Studies of structural,dielectric relaxor and electrical characteristics of lead-free double Perovskite:Gd2NiMnO6

In the current communication, structural, microstructural, dielectric, relaxor, impedance, AC conductivity, and electrical modulus characteristics of double perovskite Gd₂NiMnO₆ (synthesized by a solid state reaction route) as a function of temperature (25–125 °C) and frequency (1 kHz–1MHz) have mainly been reported. From preliminary X-ray structural analysis, it is found that the crystal structure of the material is monoclinic. In temperature dependence of dielectric constant analysis, relaxor behaviour of the material was observed. Such type of behaviour is described by modified Curie–Weiss law and a Vogel–Fulcher law. From Nyquist plots, the existence of grain and grain boundary effect in the material is observed. The non–Debye type of relaxation is confirmed from the complex impedance spectroscopy. From the impedance data, the determined grain resistance reduces with increment of temperature showing negative temperature co-efficient of resistance (NTCR)-type nature of the material which also confirmed from conductivity analysis. Again, non-Debye type of relaxation phenomena is observed from the analysis of modulus spectroscopy which is also proved by complex impedance plot. From these result it may be concluded that this material may be used for different high temperature applications.     

Modified Electrodes with Switchable Selectivity for Cationic and Anionic Redox Species

The modified electrode functionalized with a mixed‐polymer brush composed of poly(2‐vinylpyridine) and polyacrylic acid tethered to the surface demonstrated switchable interfacial properties discriminating negatively and positively charged redox species. The switchable electrochemical process was characterized by differential pulse voltammetry and Faradaic impedance spectroscopy. The electrochemical system was discussed as a model of an electrochemical multiplexer with two chemical redox inputs, the pH input operating as the selecting signal and one electronic output signal readable by the impedance spectroscopy in the form of the interfacial resistance. The modified electrode represents a novel component for integration with biocatalytic and biocomputing systems aiming at biochemically and electronically controlled actuators.     

Deviations of capacitive and inductive loops in the electrochemical impedance of a dissolving iron electrode.

The electrochemical impedance of an iron electrode often shows the capacitive and inductive loops on the complex plane. The capacitive loop originates from the time constant of the charge transfer resistance and the electric double layer capacitance. The inductive loop is explained by Faradaic processes involving the reaction intermediate. In some cases, these loops deviate from a true semicircle. In this paper, the origins and curve-fitting methods for the deviated loops of electrochemical impedance are discussed. The constant phase element (CPE) was used to present the deviation of the capacitive loop instead of electric double layer capacitance. The reaction rate constants, which are a function of the frequency, are proposed for the Faradaic impedance to present the deviated inductive loop.     

Hybridization-induced interfacial changes detected by non-Faradaic impedimetric measurements compared to Faradaic approach

A biosensor for direct label-free DNA detection based on a polythiophene matrix is investigated by electrochemical impedance spectroscopy (EIS). Impedimetric experiments are performed with and without redox probe in solution. The non-Faradaic impedance measurements reveal two relaxation processes located at 50 Hz and 5 kHz, respectively. The first relaxation process, located at low frequencies, allows to detect biorecognition events by measuring the phase angle decrease, in accordance with a hindrance of the polaronic conduction. The second relaxation process, located at 5 kHz and originating from DNA modification, seems to increase with the length of the target sequence. These results suggest that this loaded support provides a platform for impedimetric detection of hybridization at high frequencies, leading to less time-consuming detection procedure. For a better understanding, results obtained in non-Faradaic mode are compared with Faradaic approach.     

A new procedure for separation of the enamel and dentin of human teeth

A new method for the separation of enamel and dentin in various types of human teeth is proposed. The separated enamel and dentin samples have been analysed for thirteen elements by inductively-coupled plasma emission-spectroscopy and also for fluoride.     

A study of the electrical behaviour of isolated tomato cuticular membranes and cutin by impedance spectroscopy measurements

Different isolated tomato fruit cuticular membranes (ripe and green tomato cuticles and the cutin of these membranes) were studied by impedance spectroscopy measurements when the membranes were in contact with NaCl solutions at different concentrations. Remarkable differences in the impedance plots and the equivalent circuits associated to each membrane sample were obtained: the ripe tomato cuticle and the cutin, only present a relaxation process, but for the green tomato cuticle two relaxation processes were obtained. Using the equivalent circuits as models, electrical and electrochemical parameters for each membrane were determined. These results permit us to assign the relaxation processes to the different components of the tomato membrane (polyester matrix, carbohydrates and pigments), obtaining in this way a detailed picture of the different environments of the plant interface. Variation with NaCl concentration for the different electrical parameters was also studied, and the electrical resistance of the biopolymer matrix was obtained.     

Structural features of human tooth tissues affected by high dose of external ionizing radiation after nuclear catastrophe of Chernobyl plant

The application of micro-Raman spectroscopy is discussed for the analysis of structural features of human tooth tissues affected by high doses of external ionizing radiation (0.5–1.7 Gy) after the nuclear plant catastrophe in Chernobyl in 1986. The results have shown significant changes in the mineral matrix of dental enamel that lead to the decrease of tooth enamel hardness. Destruction of the collagen chain of the organic matrix has been observed for dentin and cementum.     

Possibility of Human Gender Recognition Using Raman Spectra of Teeth

Gender determination of the human remains can be very challenging, especially in the case of incomplete ones. Herein, we report a proof-of-concept experiment where the possibility of gender recognition using Raman spectroscopy of teeth is investigated. Raman spectra were recorded from male and female molars and premolars on two distinct sites, tooth apex and anatomical neck. Recorded spectra were sorted into suitable datasets and initially analyzed with principal component analysis, which showed a distinction between spectra of male and female teeth. Then, reduced datasets with scores of the first 20 principal components were formed and two classification algorithms, support vector machine and artificial neural networks, were applied to form classification models for gender recognition. The obtained results showed that gender recognition with Raman spectra of teeth is possible but strongly depends both on the tooth type and spectrum recording site. The difference in classification accuracy between different tooth types and recording sites are discussed in terms of the molecular structure difference caused by the influence of masticatory loading or gender-dependent life events.     

Overcoming Diffusion Limitation of Faradaic Processes: Property-Performance Relationships of 2D Conductive Metal-Organic Framework Cu3(HHTP)2 for Reversible Lithium-Ion Storage

Faradaic reactions including charge transfer are often accompanied with diffusion limitation inside the bulk. Conductive two-dimensional frameworks (2D MOFs) with a fast ion transport can combine both—charge transfer and fast diffusion inside their porous structure. To study remaining diffusion limitations caused by particle morphology, different synthesis routes of Cu-2,3,6,7,10,11-hexahydroxytriphenylene (Cu₃(HHTP)₂), a copper-based 2D MOF, are used to obtain flake- and rod-like MOF particles. Both morphologies are systematically characterized and evaluated for redox-active Li⁺ ion storage. The redox mechanism is investigated by means of X-ray absorption spectroscopy, FTIR spectroscopy and in situ XRD. Both types are compared regarding kinetic properties for Li⁺ ion storage via cyclic voltammetry and impedance spectroscopy. A significant influence of particle morphology for 2D MOFs on kinetic aspects of electrochemical Li⁺ ion storage can be observed. This study opens the path for optimization of redox active porous structures to overcome diffusion limitations of Faradaic processes.     

交流阻抗法在细胞分析应用中的研究进展

交流阻抗法作为一种新型的无标记、全程动态、实时分析方法已在细胞研究中得到了广泛应用。本文综述了基于交流阻抗法进行细胞分析的研究新进展,重点对非法拉第阻抗谱法和法拉第阻抗谱法用于细胞分析的原理及应用进行了总结,主要包括交流阻抗法在细胞形态、细胞生长、细胞增殖、细胞凋亡以及作用于细胞的药效和毒性研究中的应用,并对其发展趋势进行了展望。     

Influence of gas atmosphere and temperature on the conductivity and the photoconductivity of a TiO2 single crystal in the surface region

The electrical photoconductivity and conductivity at (and near) the surface of a TiO(2) single crystal (rutile) was studied in a range of temperatures between 300 and 573 K and under different ambient gases (oxygen and nitrogen) by means of impedance spectroscopy. The long times required (many hours) to reach steady state photoconductivity can be explained by the reduction of the material upon illumination. At about 475 K a maximum is observed in the equilibrium photoconductivity and a minimum in the rate constants of the rise and decay after switching on and off, respectively, the light. After switching off the light a fast decay takes place during the first milliseconds followed by a slow exponential decay. The first one is related to recombination through defects, while the latter is due to re-oxidation processes of the material. The results are correlated with measurements of photocatalytic activity.     

Elemental comparison in sound and carious human teeth by instrumental neutron activation analysis

Instrumental neutron activation analysis (INAA) was applied to determine Ca, Cl, Mg, Mn, Na, Sr and Zn in human dentin and enamel tissues from carious and sound teeth. Comparisons were made between the results obtained in carious and sound dental tissues for dentin and enamel. The findings obtained were also compared with the published data. Accuracy and precision of the results were evaluated by analyzing certified reference material.     

Aggregational processes of low molecular weight polyethylene-b-polyethylene oxide diblock copolymer probed by electrical impedance spectroscopy

In this paper, we analyze the use of electrical impedance spectroscopy (EIS) to investigate the aggregational processes of low molecular weight polyethylene-b-polyethylene oxide copolymer (PE-b-PEO (50% PEO)) induced by concentration. The real and imaginary part of impedance have been compared to theoretical results based on equivalent circuits, providing information pertaining to the properties of the bulk and contributions due to the interactions at the electrolyte/electrode interface. The bulk resistance is used as a convenient tool for identifying structures such as spheres (S), hexagonally packed cylinders (HPC) and lamellar phases. To characterize the transition mechanisms involved in micellization we applied the Phillips criteria, associating the maximum change in the gradient of bulk resistance with the formation of micelles. The transition from micelles to HPC structures is characterized from the minimum in the bulk resistance, while the electrical signature relative to the formation of lamellar structures is characterized by the decrease in the diffusion (reduction in the polarization effects) and the inversion in the slope of bulk resistance as a function of copolymer concentration. Thus, the EIS may be defined as a convenient tool for characterizing the conformation and phase transitions in amphiphilic block copolymers.     

Synthesis and characterization of cancrinite-type zeolite, and its ionic conductivity study by AC impedance analysis

The synthesis of cancrinite in the system NaOH-SiO₂-Al₂O₃-NaHCO₃-H₂O was performed, according to methods described in the literature, in an autoclave under hydrothermal conditions at T = 473 K. The electrical properties of cancrinite-type zeolite pellets were investigated by complex impedance spectroscopy in the temperature range 465?C800°C. The effect of temperature on impedance parameters was studied using an impedance analyzer in a wide frequency range (1 Hz to 13 MHz). The real and imaginary parts of complex impedance trace semicircles in the complex plane are plotted. The bulk resistance of the material decreases with rise in temperature. This exhibits a typical negative temperature coefficient of resistance (NTCR) behavior of the material. The results of bulk electrical conductivity and its activation energy are presented. The modulus analysis suggests that the electrical transport processes in the material are very likely to be of electronic nature. Relaxation frequencies follow an Arrhenius behavior with activation energy values not comparable to those found for the electrical conductivity.     

Performance of a Faradaic impedimetric immunosensor for blood group antigen A

The performance is described of a label-free Faradaic impedimetric immunosensor based on immobilized monoclonal IgM antibodies to blood group antigen A (anti–A) for blood typing. Anti–A was directly immobilized onto gold electrodes modified with an amine-reactive self assembled monolayer of dithiobis(succinimidylundecanoate). The alteration of the interfacial features of the electrodes due to different modification or recognition steps was probed by Faradaic impedance spectroscopy and cyclic voltammetry in the presence of a hexacyanoferrate(II)/(III) redox couple. Various optimization studies were undertaken with respect to the construction and potential use of the immunosensors as a diagnostic tool for blood typing.     

Insights into the electro-oxidation of hydrazine at single-walled carbon-nanotube-modified edge-plane pyrolytic graphite electrodes electro-decorated with metal and metal oxide films

Electrochemistry of edge-plane pyrolytic graphite electrodes (EPPGEs) modified with Aldrich single-walled carbon nanotubes (SWCNTs) electro-decorated with metal (Ni, Fe and Co) and their oxides have been studied. The morphology and identity of the metallic dispersions were examined by scanning electron microscopy and energy-dispersive spectroscopy. We show that SWCNTs serve as efficient conducting carbon material for electronic communication between metal films and the underlying carbon electrode. By using cyclic voltammetry and electrochemical impedance spectroscopy (EIS) techniques, it is proved that both EPPGE-SWCNT-Ni and EPPGE-SWCNT-Fe exhibit comparable electrochemical response in buffered aqueous solution (pH 7.0) and towards electro-oxidation of hydrazine in Na₂SO₄ solution. The impedance spectra of these SWCNT-metal hybrids were complicated and follow electrical equivalent circuit model typical of adsorption-controlled charge transfer kinetics. Hydrazine impedance spectra exhibited inductive loop, characteristic of Faradaic current being governed by the occupation of an intermediate state. On the other hand, the EIS data obtained in a simple redox probe, [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻, showed that EPPGE-SWCNT and EPPGE-SWCNT-Ni followed electrical equivalent circuit models typical of partial charge transfer or adsorption-controlled kinetics with some resemblance to the behaviour of electrolyte–insulator–semiconductor sensors.     

Dielectrophoresis: a model to transport drugs directly into teeth

The article describes an innovative delivery system based on the principles of dielectrophoresis to transport drugs directly into site-specific intraoral targets. The hypothesis that a drug can be driven into tooth enamel during the application of an applied electrical potential difference was tested by the authors in in vitro studies comparing dielectrophoresis to diffusion to transport carbamide peroxide and fluoride. The studies showed that these agents can be transported directly into teeth using an alternating current (AC) electric field more effectively than diffusion. It was found that a 20-min bleaching treatment on human teeth with dielectrophoresis increased carbamide peroxide absorption by 104% and, on average, improved the change in shade guide unit 14 times from 0.6 SGU to 9 SGU. After applying a 1.23% acidulated phosphate fluoride gel to bovine incisors for 20 min by dielectrophoresis or diffusion, analysis with wavelength dispersive spectrometry determined that dielectrophoresis doubled fluoride uptake in the superficial layers compared to diffusion, and drove the fluoride significantly deeper into enamel with an uptake 600% higher than diffusion at 50 μm depth. Finally, dielectrophoresis promises to be a viable model that can potentially be used clinically to deliver other targeted drugs of variable molecular weight and structure.     

Synthesis and electrochemical characterization of carbon nanotubes decorated with nickel nanoparticles for use as an electrochemical capacitor

Attachment of nickel nanoparticles on multiwalled carbon nanotubes (MWCNTs) was conducted to explore the influence of Ni loading on the electrochemical capacitance of MWCNT electrodes. A chemical impregnation leaded to homogeneously disperse Ni particles onto the surface of MWCNTs, and the Ni particles were found to be an average size of 30–50 nm. The capacitive behavior of the MWCNT electrodes was investigated in 6 M KOH, by using cyclic voltammetry (CV), charge–discharge cycling, and ac electrochemical impedance spectroscopy. CV measurements showed that the Faradaic current was found to increase with the Ni coverage, indicating that the presence of Ni would enhance the pseudocapacitance through the redox process. Equivalent circuit analysis indicated that both of electrical connection and charge transfer resistances accounted for the major proportion of the overall resistance and were found to decrease with the amount of nickel. A linearity relationship between the total capacitance and the Ni population reflected that each Ni particle exhibits an identical electrochemical activity in enhancing the electrochemical capacitance. The overall electrochemical capacitance (including double layer capacitance and pseudocapacitance) of Ni-MWCNT electrode can reach a maximum of 210 F/g over 500 cycles.