Electrochemical impedance spectroscopy for study of aptamer-thrombin interfacial interactions

A simple and highly sensitive electrochemical impedance spectroscopy (EIS) biosensor based on a thrombin-binding aptamer as molecular recognition element was developed for the determination of thrombin. The signal enhancement was achieved by using gold nanoparticles (GNPs), which was electrodeposited onto a glassy carbon electrode (GCE), as a platform for the immobilization of the thiolated aptamer. In the measurement of thrombin, the change in interfacial electron transfer resistance of the biosensor using a redox couple of [Fe(CN)₆]^3−/4− as the probe was monitored. The increase of the electron transfer resistance of the biosensor is linear with the concentration of thrombin in the range from 0.12 nM to 30 nM. The association and dissociation rate constants of the immobilized aptamer–thrombin complex were 6.7 × 10³ M⁻¹ s⁻¹ and 1.0 × 10⁻⁴ s⁻¹, respectively. The association and dissociation constants of three different immobilized aptamers binding with thrombin were measured and the difference of the dissociation constants obtained was discussed. This work demonstrates that GNPs electrodeposited on GCE used as a platform for the immobilization of the thiolated aptamer can improve the sensitivity of an EIS biosensor for the determination of protein. This work also demonstrates that EIS method is an efficient method for the determination of association and dissociation constants on GNPs modified GCE.     

Selective detection of uric acid in the presence of ascorbic acid at physiological pH by using a beta-cyclodextrin modified copolymer of sulfanilic acid and N-acetylaniline

A beta-cyclodextrin (CD) modified copolymer membrane of sulfanilic acid (p-ASA) and N-acetylaniline (SPNAANI) on glassy carbon electrode (GCE) was prepared and used to determine uric acid (UA) in the presence of a large excess of ascorbic acid (AA) by differential pulse voltammetry (DPV). The properties of the copolymer were characterized by X-ray photoelectron spectra (XPS) and Raman spectroscopy. The oxidation peaks of AA and UA were well separated at the composite membrane modified electrode in phosphate buffer solution (PBS, pH 7.4). A linear relationship between the peak current and the concentration of UA was obtained in the range from 1.0 x 10(-5) to 3.5 x 10(-4)mol L(-1), and the detection limit was 2.7 x 10(-6)mol L(-1) at a signal-to-noise ratio of 3. Two hundred and fifty-fold excess of AA did not interfere with the determination of UA. The application of the prepared electrode was demonstrated by measuring UA in human serum samples without any pretreatment, and the results were comparatively in agreement with the spectrometric clinical assay method.     

Real-time determination of kinetics of adsorption of lead(II) onto palm shell-based activated carbon using ion selective electrode

In this study, the kinetics of adsorption of Pb(II) from aqueous solution onto palm shell-based activated carbon (PSAC) were investigated by employing ion selective electrode (ISE) for real-time Pb(II) and pH monitoring. Usage of ISE was very appropriate for real-time adsorption kinetics data collection as it facilitated recording of adsorption data at very specific and short time intervals as well as provided consistent kinetics data. Parameters studied were initial Pb(II) concentration and agitation speed. It was found that increases in initial Pb(II) concentration and agitation speed resulted in higher initial rate of adsorption. Pseudo first-order, pseudo second-order, Elovich, intraparticle diffusion and liquid film diffusion models were used to fit the adsorption kinetics data. It was suggested that chemisorption was the rate-controlling step for adsorption of Pb(II) onto PSAC since the adsorption kinetics data fitted both the pseudo second-order and Elovich models well.     

Electrochemical behaviors of amino acids at multiwall carbon nanotubes and Cu2O modified carbon paste electrode

A carbon paste electrode modified with multiwall carbon nanotubes and copper(I) oxide (MWCNT-Cu₂O CPME) was fabricated, and the electrochemical behaviors of 19 kinds of natural amino acids at this modified electrode were studied. The experimental results showed that the various kinds of amino acids without any derivatization displayed obvious oxidation current responses at the modified electrode. It was also found that the current response values of amino acids were dependent mainly on pH values of buffer solutions. The phenomenon could be explained by the fact that the amino acids suffered complexation or electrocatalytic oxidation processes under different pH values. Six kinds of amino acids (arginine, tryptophan, histidine, threonine, serine, and tyrosine), which performed high-oxidation current responses in alkaline buffers, were selected to be detected simultaneously by capillary zone electrophoresis coupled with amperometric detection (CZE-AD). These amino acids could be perfectly separated within 20 min, and their detection limits were as low as 10⁻⁷ or 10⁻⁸ mol L⁻¹ magnitude (signal/noise ratio = 3). The above results demonstrated that MWCNT-Cu₂O CPME could be successfully employed as an electrochemical sensor for amino acids with some advantages of convenient preparation, high sensitivity, and good repeatability.     

Enzyme microgels in packed-bed bioreactors with downstream amperometric detection using microfabricated interdigitated microsensor electrode arrays.

In this article, we describe the use of pH- responsive hydrogels as matrices for the immobilization of two enzymes, glucose oxidase (GOx) and glutamate oxidase (GlutOx). Spherical hydrogel beads were prepared by inverse suspension polymerization and the enzymes were immobilized by either physical entrapment or covalent immobilization within or on the hydrogel surface. Packed-bed bioreactors were prepared containing the bioactive hydrogels and these incorporated into flow injection (FI) systems for the quantitation of glucose and monosodium glutamate (MSG) respectively. The FI amperometric detector comprised a microfabricated interdigitated array within a thin-layer flow cell. For the FI manifold incorporating immobilized GOx, glucose response curves were found to be linear over the concentration range 1.8-280 mg dL(-1) (0.1-15.5 mM) with a detection limit of 1.4 mg dL(-1) (0.08 mM). Up to 20 samples can be manually analyzed per hour, with the hydrogel-GOx bioreactor exhibiting good within-day (0.19%) precision. The optimized FI manifold for MSG quantitation yielded a linear response range of up to 135 mg dL(-1) (8 mM) with a detection limit of 3.38 mg dL(-1) (0.2 mM) and a throughput of 30 samples h(-1). Analysis of commercially produced soup samples gave a within-day precision of 3.6%. Bioreactors containing these two physically entrapped enzymes retained > 60% of their initial activities after a storage period of up to 1 year.     

A Study of Protein Electrochemistry on a Supported Membrane Electrode

Protein electrochemistry offers a direct method to identify and characterize biological electron transfer processes, potentially leading to commercial applications such as biosensors and diagnostic tools. However, establishing a biocompatible electrode interface that maintains the native state of the redox protein involves several challenges. In general, membrane proteins require the presence of a phospholipid bilayer to maintain their biological activity. Synthetic `biomimetic’ membranes are widely used to characterize membrane proteins, however they have seldom been applied to measurements of protein redox activity in electrochemical cells due to their inherent insulating property. In this study we demonstrate the use of the phospholipids: PC, PC/PG and PC/PG/cholesterol membrane mixtures on chemically modified (supported) gold electrode surfaces for direct protein electrochemistry. We compare the electrochemical activity of a relatively small, redox active “test protein”, cytochrome c, in the presence and absence of phospholipid on a gold electrode modified with thiol self assembled monolayers, to explore the effect of chain length and composition of the thiol on the charge coupling. Three thiols were investigated as self assembled monolayers on a gold electrode: octanethiol, mercaptopropionic and mercaptoundecanoic acid. We demonstrate here that the charge transfer efficiency of cytochrome c is better in the presence of the membrane and in addition, a superior redox response is obtained with surfaces modified with a thiol functionalised with a carboxylic acid.On leave from: Research Group on Laser Physics of the Hungarian Academy of Sciences, University of Szeged, Szeged, Hungary.Australian Peptide Conference Issue.     

Influence of chlorophyll concentrations on the reliability of measurements with an oxygen electrode when studying photosynthetic efficiency in bioassays using microalgae

Euglena gracilis and Chlamydomonas reinhardtii were used as model organisms to establish the best conditions for studying photosynthetic efficiency using the Light Pipette – experimental system, which enables sensitive detection of changes in oxygen evolution/consumption and an easy collection and digitalisation of data. Chlorophyll concentrations of 0.005, 0.025, 0.050 and 0.075 mgmL^-1 were investigated using different light regimes. Cultures of E. gracilis at the same chlorophyll concentration absorbed more light(measured at 580 μmol m^-2 s^-1) than those of C. reinhardtii. Cell density had a considerable effect on the reliability of measurements. Chlorophyll concentrations between 0.025 mg_Chl ml^-1 and 0.050 mg_Chlml^-1 can be recommended when applying the Light Pipette system in bioassays using microalgae. This revised version was published online in August 2006 with corrections to the Cover Date.     

Root exudates from sunflower (Helianthus annuus L.) show a strong adsorption ability toward Cd(II)

Abstract

Cd(II) adsorption of root exudates from sunflower (Helianthus annuus L.) seedling was investigated by Cd ion-selective electrode, Fourier Transform Infrared spectroscopy, and fluorescence spectroscopy. Root exudates from Helianthus annuus L. had strong adsorption ability toward Cd(II). The adsorption process was pH-dependent and the maximum adsorption capacity, 150.8 mg g^?1, was observed at pH 7.0. Root exudates had pK _a1 at 4.7 for carboxyl and pK _a2 at 9.2 for phenolic, and amino groups. The aliphatic and aromatic (C?H) groups, amide III group, and the C (=O)?O and sulfonate groups were responsible for Cd(II) adsorption. The excitation emission matrix fluorescence spectroscopy showed protein-like substances participated in Cd adsorption and formed strong complexes, with conditional stability constants of 4.70 and 4.32, which is a little lower than that determined by potentiometric methods, 5.13. The strong Cd complexing ability of root exudates implies that root exudates may significantly affect mobility, toxicity, and phytoavailability of Cd. Cd binding of root exudates may be attributed to its interaction with the proteins, polysaccharides, and phenolic compounds in root exudates.     

A Material Perspective of Rechargeable Metallic Lithium Anodes

Rechargeable lithium‐based batteries are long considered as the most promising candidates for application in various electronic devices, electric vehicles, and even electrical grids owing to their ultrahigh energy densities. However, to date, metallic lithium‐based batteries are still far from practical applications due to the low Coulombic efficiency and fast capacity decay of lithium anodes. The poor electrochemical performances of metallic lithium anodes are inherently related to random growth of lithium dendrites and infinite volume charge of lithium anodes. In this review, the failure mechanisms of metallic lithium anodes are summarized and ascribed to the unstable and inhomogeneous solid electrolyte interphase, uneven distributions of electric field, and lithium‐ion flux during the lithium plating processes. Correspondingly, efficient strategies for mitigating these problems, including surficial engineering, electric field, and lithium‐ion flux regulation are discussed from the perspective of anode materials. Finally, an outlook is proposed for the design and fabrication of next‐generation rechargeable metallic lithium anodes that aims to address the intrinsic problems of metallic lithium anodes.     

Mechanical Property Evolution of Silicon Composite Electrodes Studied by Environmental Nanoindentation

Mechanical degradation is largely responsible for the short cycle life of silicon (Si)‐based electrodes for future lithium‐ion batteries. An improved fundamental understanding of the mechanical behavior of Si electrodes, which evolves, as demonstrated in this paper, with the state of charge (SOC) and the cycle number, is a prerequisite for overcoming mechanical degradation and designing high capacity and durable Si‐based electrodes. In this study, Young's modulus (E) and hardness (H) of Si composite electrodes at different SOCs and after different cycle numbers are measured by nanoindentation under both dry and wet (liquid electrolyte) conditions. Unlike electrodes made of Si alone, E and H values of Si composite electrodes increase with increasing Li concentration. The composite electrodes under wet conditions are softer than that under dry conditions. Both E and H decrease with the cycle number. These findings highlight the effects of porosity, liquid environment, and degradation on the mechanical behavior of composite electrodes. The methods and results of this study on the mechanical property evolution of Si/polyvinylidene fluoride electrodes form a basis for exploring more effective binders for Si‐based electrodes. Furthermore, the evolving nature of the mechanical behavior of composite electrodes should be taken into consideration in future modeling efforts of porous composite electrodes.