Electrochemical Characteristics of Poly(<Emphasis Type="Italic">o</Emphasis>-Aminobenzoic Acid) Modified Glassy-Carbon Electrode and Its Electrocatalytic Activity towards Oxidation of Epinephrine

Poly(o-aminobenzoic acid) (o-ABA) film is deposited on glassy-carbon electrode (GCE) by electropolymerization in pH 7.0 phosphate buffer solution (PBS). Electrochemical behavior of modified electrode is investigated by electrochemical impedance spectroscopy (EIS), different pulse voltammetry (DPV), and cyclic voltammetry (CV). The results indicate that there is a greater resistance during the electron transfer process in poly(o-ABA) film than in bare GCE for the redox of [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻. Further research indicates that epinephrine (EP) can be strongly absorbed on the surface of the poly(o-ABA) film-modified electrode. The modified electrode shows an excellent electrocatalytical activity on EP oxidation. The EP cathodic peak potential shifts negatively with a slope of −53.5 mV/pH, indicating that equal amounts of proton and electron are involved in the electrode reaction process. In pH 7.0 PBS, the peak current of EP and the concentration has a linear relationship from 0 to 65 μM by amperometric current-time curve. __________ From Elektrokhimiya, Vol. 41, No. 9, 2005, pp. 1059–1065. Original English Text Copyright ? 2005 by Cheng, Jin, Zhang. The text was submitted by the authors in English.     

Molecularly Imprinted Poly-o-phenylenediamine Electrochemical Sensor for Entacapone

We have developed a molecularly imprinted polymer (MIP) electrochemical sensor for entacapone (ETC) based on an electropolymerised polyphenylenediamine (Po-PD) on a glassy carbon electrode (GCE) surface. The direct electropolymerisation of the o-phenylenediamine monomer (o-PD) was carried out with ETC as a template. The steps of electropolymerization process, template removal and binding of the analyte were tested by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) using [Fe(CN)₆]³⁻/[Fe(CN)₆]^{4 −} as a redox probe. The operation of the sensor has been investigated by differential pulse voltammetry (DPV). Under optimal experimental conditions, the response of the DPV was linearly proportional to the ETC concentration between 1.0×10⁻⁷ and 5.0×10⁻⁶ M ETC with a limit of detection (LOD) of 5.0×10⁻⁸ M. The developed sensor had excellent selectivity without detectable cross-reactivity for levodopa and carbidopa. The MIP sensor was successfully used to detect ETC in spiked human serum samples.     

Layer-by-layer thin film-coated electrodes for electrocatalytic determination of ascorbic acid

Multilayer thin films composed of poly(allylamine hydrochloride) (PAH) and carboxymethyl cellulose (CMC) have been prepared on the surface of a gold (Au) disk electrode by a layer-by-layer deposition of PAH and CMC and ferricyanide ions ([Fe(CN)₆]³⁻) were confined in the film. [Fe(CN)₆]³⁻ ions can be successfully confined in the films from weakly acidic or neutral [Fe(CN)₆]³⁻ solutions, while, in basic solution, [Fe(CN)₆]³⁻ ion was not confined. The [Fe(CN)₆]³⁻ ion-confined Au electrode showed clear redox peaks in the cyclic voltammogram around 0.35 V versus Ag/AgCl. The amounts of [Fe(CN)₆]³⁻ ions confined in the films depended on the thickness of the films or the number of layers in the LbL films. The [Fe(CN)₆]³⁻ ion-confined Au electrode was used for electrocatalytic determination of ascorbic acid in the concentration range of 1-50 mM.     

A new sensor architecture based on carbon Printex 6L to the electrochemical determination of ranitidine

A glassy carbon electrode (GCE) modified with carbon Printex 6L (Printex6L/GCE) as a novel sensor is proposed. A morphological study was carried out using scanning electron microscopy, and an electrochemical characterization of the proposed electrode was performed by cyclic voltammetry (CV) using [Fe(CN)₆]^4? as a redox probe. With the incorporation of the carbon Printex 6L film onto the GCE surface, the [Fe(CN)₆]^4? analytical signal was substantially increased and the difference between the oxidation and reduction potentials (ΔE _p) decreased, a characteristic of the electrocatalytic effect. Furthermore, the use of carbon Printex 6L film resulted in an 84 % increase in the oxidation current and a 123 % increase in the reduction current. Faster charge transfer was observed at the proposed electrode/electrolyte interface during CV when compared with GCE. The Printex6L/GCE was tested for ranitidine (RNT) sensing and showed a decrease in the working potential and an increase in the analytical signal, when compared with GCE, again demonstrating an electrocatalytic effect. Under optimized experimental conditions, the developed square-wave adsorptive anodic stripping voltammetry (SWAdASV) method presented an analytical curve that was linear in RNT concentration range from 1.98 × 10^?6 to 2.88 × 10^?5 mol L^?1 with a detection limit of 2.44 × 10^?7 mol L^?1. The developed Printex6L/GCE was successfully applied to the determination of RNT concentrations in human body fluid samples (urine and serum).     

Investigating Electrochemical Stability and Reliability of Gold Electrode‐electrolyte Systems to Develop Bioelectronic Nose Using Insect Olfactory Receptor

This article aims to demonstrate an electrochemically stable and reliable gold electrode‐electrolyte system to develop an insect odorant receptor (Drosophila melanogaster Or35a) based bioelectronic nose. Cyclic voltammograms (CVs) and electrochemical impedance spectroscopy (EIS) of bare gold electrodes, after modification with the self‐assembled monolayer (SAM) of 6‐mercaptohexanoic acid (MHA) and after immobilization with Or35a integrated into the lipid bilayers of liposomes were conducted in the presence of four different redox probes. Potassium ferri/ferrocyanide [Fe(CN)₆]^3?/[Fe (CN)₆]^4? and hydroquinone (H₂Q) redox probes revealed variable and irreversible signals at the time scale of our measurements, with atomic force microscopy (AFM) images and x‐ray photoelectron spectroscopy (XPS) results suggesting gold surface etching due to the presence of CN^? ions in case of [Fe(CN)₆]^3?/[Fe (CN)₆]^4?. Although the hexaammineruthenium complex showed stable electrochemical behaviour at all stages of biosensor development, changes in CV and EIS readings after each surface modifications were insignificant. PBS buffer as a non‐Faradaic medium, was found to provide reliable systems for electrochemical probing of modified gold electrodes with Or35a/liposomes in aqueous media. Using this system, we have shown that this novel biosensor can detect its known odorant E2‐hexenal selectively compared to methyl salicylate down to femtomolar concentration.     

A Polyaniline-Deposited Nonplatinum Metal as a Potentiometric Sensor—An Inexpensive Alternate to Conventional Platinum for Some Potentiometric Redox Reactions

For the purpose of employing an inexpensive alternative to conventional platinum for use by upper-division as well as graduate students, polyaniline (PANI)-deposited stainless steel (SS) and mild steel (MS) electrodes are described as indicator electrodes for potentiometry and potentiometric titrations of some redox reactions. PANI is deposited on the nonplatinum metal by electrochemical polymerization of aniline using cyclic voltammetric technique. Alternate methods to produce the PANI electrodes are also suggested. The electrodes respond to concentration changes of hydroquinone (H₂O), Fe²⁺/Fe³⁺, and [Fe(CN)₆]^4–/[Fe(CN)₆]^3– in HCL electrolytes, and the potential variation with concentration follows the Nernst relationship. Under identical experimental conditions, the response time of the PANI/SS, PANI/MS, and Pt electrodes for a change in concentration of Fe³⁺ in a mixed electrolyte of Fe²⁺ and Fe³⁺ is found to be about 20 s. Neutralization reaction of HC1 versus NaOH, redox reaction of Fe²⁺ and Ce⁴⁺, and redox reaction of Fe²⁺ and KMnO₄ in several concentrations in the range from 1 mM to 100 mM are carried out using the PANI/SS, PANI/MS, and Pt indicator electrodes. The performance of the PANI/SS and PANI/MS electrodes is as good as that of the Pt at all concentration levels of the titrations. The electrodes can be reused for several titrations by storing them in an acid electrolyte for a long period of time. Thus, the conventional inert Pt or Au can be substituted for by using a PANI-deposited nonplatinum reactive metal as a potentiometric sensor for redox titrations.     

A Novel Three‐Electrode System Fabricated on Polymethyl Methacrylate for On‐Chip Electrochemical Detection

A new strategy of three‐electrode system fabrication in polymer‐based microfluidic systems is described here. Standard lithography, hot embossing and UV‐assisted thermal bonding were employed for fabrication and assembly of the microfluidic chip. For the electrode design the gold working (WE) and counter electrodes (CE) are placed inside a main channel through which the sample solution passes. A silver reference electrode (RE) is embedded in a small side channel containing KCl solution that is continuously pushed into the main channel. In the present work, the overall electrochemical set up and its microfabrication is described. Conditions including silver ion concentration, cyclic voltammetry (CV) settings, and the flow rate of KCl solution in the RE channel were optimized. The electrochemical performance of the three‐electrode system was evaluated by CV and also by amperometric oxidation of ferro hexacyanide ([Fe(CN)₆]^4?) and ruthenium bipyridyl ([Ru(bipy)₃]²⁺) at 400 mV and 1200 mV, respectively. CV analysis using ferri/ferro hexacyanide showed a stable, quasi‐reversible redox reaction at the electrodes with 96 mV peak separation and an anodic/cathodic peak ratio of 1. Amperometric analysis of the electrochemical species resulted in linear correlation between analyte concentration and current response in the range of 0.5–15 µM for [Fe(CN)₆]^4?, and 0–1000 µM for [Ru(bipy)₃]²⁺. Upon the given experimental conditions, the limit of detection was found to be 3.15 µM and 24.83 µM for [Fe(CN)₆]^4? and [Ru(bipy)₃]²⁺, respectively. As a fully integrated three‐electrode system that is fabricated on polymer substrates, it has great applications in microfluidic‐based systems requiring stable electrochemical detection.     

扫描电化学显微镜沉积六氰合铁酸盐微阵及其催化活性成像

采用扫描电化学显微技术在玻碳电极表面沉积出K₂Cu[Fe(CN)₆]和K₂Fe[Fe(CN)₆]微阵, 并对所得的微阵结构进行了可视化表征. 铜微电极和镀铁铂微电极阳极化产生金属离子, 然后与玻碳电极(基底电极)上还原产生的[Fe(CN)₆]^4－在微区生成六氰合铁酸盐沉淀, 操纵探针以跳跃沉积方式可以得到沉淀的点阵结构. 通过改变K₃[Fe(CN)₆]的浓度和沉积时间可以调整沉淀斑的直径和厚度. 扫描电化学显微镜成像表明微阵结构对多巴胺的氧化和过氧化氢的还原有明显的电催化作用.     

Redox Properties of Ferricyanide Ion on Layer‐by‐Layer Thin Film‐Coated Gold Electrodes; Effects of Type of Polycationic Components in the Film

The surface of a gold (Au) electrode was coated with layer‐by‐layer (LbL) thin films composed of poly(vinyl sulfate) (PVS) and different type of poly(amine)s including poly(allylamine) (PAH), poly(ethyleneimine) (PEI) and poly(diallyldimethylammonium chloride) (PDDA) and redox properties of ferricyanide ion ([Fe(CN)₆]^3?) on the LbL film‐coated Au electrodes were studied. The LbL film‐coated electrodes exhibited redox response to [Fe(CN)₆]^3? ion when the outermost surface of the LbL film was covered with the cationic poly(amine)s while virtually no response was observed on the LbL film‐coated electrodes whose outermost surface was covered with PVS due to an electrostatic repulsion between [Fe(CN)₆]^3? ion and the negatively‐charged PVS layer. The redox properties of [Fe(CN)₆]^3? ion on the LbL film‐coated electrodes significantly depended on the type of polycationic materials in the LbL film. The LbL film‐coated electrodes which had been immersed in the [Fe(CN)₆]^3? solution for 15 min exhibited redox response even in a [Fe(CN)₆]^3? ion‐free buffer solution, suggesting that [Fe(CN)₆]^3? ion is confined in the films. In the buffer solution, redox peaks were observed between +0.1 and 0.4 V depending on the type of polycations in the film. Thus, [Fe(CN)₆]^3? ion can be confined in the film and the redox potential is polycation‐dependent.     

Fabrication of Fe(CN)63− Functionalized PDDA/CdTe Layer‐by‐layer Film with Good Electron Transfer Ability

(PDDA/CdTe)_n layer‐by‐layer (LBL) film immobilized with Fe(CN)₆^3? was fabricated on the gold electrode. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used to investigate the electrochemical properties of this film. The peak current of the immobilized Fe(CN)₆^3? increased as the number of the bilayers increased and was proportional to the scan rate. Compared with pure (PDDA/CdTe)_n and (PDDA/PSS)_n LBL film, Fe(CN)₆^3? immobilized (PDDA/CdTe)_n LBL film had good electron transfer ability. The immobility of Fe(CN)₆^3? into the film was attributed to its interaction with Cd²⁺ on the surface of CdTe QDs. Fe(CN)₆^3? also can interact with other metal ions, which would make Fe(CN)₆^3? release from the film. The concentrations of metal ions will affect the CV response of Fe(CN)₆^3? immobilized LBL film. It has provided a novel prototype of device or sensor for quantitative detection of metal ions.     

A sensitive non-enzymatic glucose sensor in alkaline media based on Cu/MnO2-modified glassy carbon electrode

A novel and simple glucose sensor based on layer-by-layer (LBL) assembly of Cu and MnO₂ nanoparticles on the glassy carbon electrode (Cu/MnO₂/GCE) was constructed. The morphology and composition of the Cu and Cu/MnO₂ on the electrode surface were observed by scanning electron microscopy and X-ray diffraction. Electrochemical experiments showed that the proposed Cu/MnO₂/GCE exhibited excellent electrocatalytic properties to glucose. The oxidation peak currents of glucose on the Cu/MnO₂/GCE were linearly related to glucose concentration in a wider linearity range from 0.25???M to 1.02?mM with a correlation coefficient of 0.9977. The sensitivity and detection limit was 26.96???A?mM^?1 and 0.1???M (S/N?=?3), respectively. The Cu/MnO₂ nanocomposite-modified electrode presented attractive features such as high sensitivity, stability, reproducibility, and interference-free property. The applicability of the proposed method to the determination of glucose in serum samples was demonstrated with satisfactory results.     

Impedimetric DNA‐Based Biosensor for Silver Ions Detection with Hemin/G‐Quadruplex Nanowire as Enhancer

A DNA‐based biosensor was reported for detection of silver ions (Ag⁺) by electrochemical impedance spectroscopy (EIS) with [Fe(CN)₆]^4?/3? as redox probe and hybridization chain reaction (HCR) induced hemin/G‐quadruplex nanowire as enhanced label. In the present of target Ag⁺, Ag⁺ interacted with cytosine‐cytosine (C? C) mismatch to form the stable C? Ag⁺? C complex with the aim of immobilizing the primer DNA on electrode, which thus triggered the HCR to form inert hemin/G‐quadruplex nanowire with an amplified EIS signal. As a result, the DNA biosensor showed a high sensitivity with the concentration range spanning from 0.1 nM to 100 µM and a detection limit of 0.05 nM.     

Selective DNA detection at Zeptomole level based on coulometric measurement of gold nanoparticle-mediated electron transfer across a self-assembled monolayer

A selective DNA sensing with zeptomole detection level is developed based on coulometric measurement of gold nanoparticle (AuNPs)-mediated electron transfer (ET) across a self-assembled monolayer on the gold electrode. After immobilization of a thiolated hairpin-structured DNA probe, an alkanethiol monolayer was self-assembled on the resultant electrode to block [Fe(CN)6 ]-3-/4in a solution from accessing the electrode. In the presence of DNA target, hybridization between the DNA probe and the DNA target breaks the stem duplex of DNA probe. Consequently, stem moiety at the 3′-end of the DNA probes was removed from the electrode surface and made available for hybridization with the reporter DNA-AuNPs conjugates (reporter DNA-AuNPs). The thiolated reporter DNA matches the stem moiety at the 3′-end of the DNA probe. AuNPs were then enlarged by immersing the electrode in a growth solution containing HAuCl 4 and H2O2 after the reporter DNA-AuNPs bound onto the electrode surface. The enlarged AuNPs on the electrode restored the ET between the electrode and the [Fe(CN)6]3 -/4- , as a result, amplified signals were achieved for DNA target detection using the coulometric measurement of Fe(CN)6 3- electro-reduction by prolonging the electrolysis time. The quantities of ET on the DNA sensor increased with the increase in DNA target concentration through a linear range of 3.0 fM to 1.0 pM when electrolysis time was set to 300 s, and the detection limit was 1.0 fM. Correspondingly, thousands of DNA (zeptomole) copies were detected in 10L samples. Furthermore, the DNA sensor showed excellent differentiation ability for single-base mismatch.     

Fabrication of Chitosan‐Multiwall Carbon Nanotube Nanocomposite Containing Ferri/Ferrocyanide: Application for Simultaneous Detection of D‐Penicillamine and Tryptophan

We report a simple and effective strategy for fabrication of the nanocomposite containing chitosan (CS) and multiwall carbon nanotube (MWNT) coated on a glassy carbon electrode (GCE). The characterization of the modified electrode (CS‐MWNT/GC) was carried out using scanning electron microscopy (SEM) and UV–vis absorption spectroscopy. The electrochemical behavior of CS‐MWNT/GC electrode was investigated and compared with the electrochemical behavior of chitosan modified GC (CS/GC), multiwalled carbon nanotube modified GC (MWNT/GC) and unmodified GC using cyclic voltammetry (CV) and electron impedance spectroscopy (EIS). The chitosan films are electrochemically inactive; similar background charging currents are observed at bare GC. The chitosan films are permeable to anionic Fe(CN)₆^3?/4? (FC) redox couple. Electrochemical parameters, including apparent diffusion coefficient for the Fe(CN)₆^3?/4? redox probe at FC/CS‐MWNT/GC electrode is comparable to values reported for cast chitosan films. This modified electrode also showed electrocatalytic effect for the simultaneous determination of D‐penicillamine (D‐PA) and tryptophan (Trp). The detection limit of 0.9 μM and 4.0 μM for D‐PA and Trp, respectively, makes this nanocomposite very suitable for determination of them with good sensitivity.     

Immunosensors Based on Layer‐by‐Layer Self‐Assembled Au Colloidal Electrode for the Electrochemical Detection of Antigen

Colloidal Au particles have been deposited on the gold electrode through layer‐by‐layer self‐assembly using cysteamine as cross‐linkers. Self‐assembly of colloidal Au on the gold electrode resulted in an easier attachment of antibody, larger electrode surface and ideal electrode behavior. The redox reactions of [Fe(CN)₆]^4?/[Fe(CN)₆]^3? on the gold surface were blocked due to antibody immobilization, which were investigated by cyclic voltammetry and impedance spectroscopy. The interaction of antigen with grafted antibody recognition layers was carried out by soaking the modified electrode into a phosphate buffer at pH 7.0 with various concentrations of antigen at 37 °C for 30 min. Further, an amplification strategy to use biotin conjugated antibody was introduced for improving the sensitivity of impedance measurements. Thus, the sensor based on this immobilization method exhibits a large linear dynamic range, from 5–400 μg/L for detection of Human IgG. The detection limit is about 0.5 μg/L.     

Using an Electrochemical MIP Sensor for Selective Determination of 1-Naphthol in Oilfield Produced Water

A molecularly imprinted polymer (MIP) sensor was successfully constructed on glassy carbon electrode for the determination of 1-naphthol (1-Nph). The sensor was constructed by electropolymerization on bare GCE in the presence of the target molecule. The recognition of 1-Nph was conducted indirectly using [Fe(CN)₆]^3−/4− as redox probe. The MIP sensor presented wide linear working range and limit of detection of 1.5×10⁻⁹ mol L⁻¹. The MIP sensor was applied for the determination of 1-Nph in oilfield produced water. The results obtained showed good selectivity and sensitivity of the proposed sensor in terms of 1-Nph quantification.     

Amperometric detection of dopamine based on tyrosinase-SWNTs-Ppy composite electrode

A novel 3-dimensional single wall carbon nanotubes (SWNTs)-polypyrrole (Ppy) composite was prepared as an electrode by chemically polymerizing polypyrrole onto SWNTs using a LiClO₄ oxidant. This composite electrode was characterized by scanning electron microscopy (SEM) and cyclic voltammetry with 1 mM [Fe(CN)₆]⁻³/[Fe(CN)₆]⁻⁴. The SWNTs were thickly coated with chemically polymerized polypyrrole and the composite had many surface pores and crevices which could enhance mass transfer. The SWNTs-Ppy composite electrode showed a large specific surface area (30 m²/g) and a good reproducible current response, at about 100 times the peak current of a glassy carbon electrode (GCE). The diffusion coefficient was calculated to be 4.81 × 10⁻⁶ cm²/s. As a biosensor application, tyrosinase was immobilized on the functionalized SWNTs and tyrosinase-SWNTs-Ppy composite was prepared in the same manner. This tyrosinase-SWNT-Ppy composite electrode was used for amperometric detection of dopamine in the presence of ascorbic acid and showed high sensitivity (467 mA/M cm²) and lower detection limit (5 μM) compared to previous reports.     

Copper hexacyanoferrate multilayer films on glassy carbon electrode modified with 4-aminobenzoic acid in aqueous solution

4-Aminobenzoic acid (4-ABA) was covalently grafted on a glassy carbon electrode (GCE) by amine cation radical formation during the electrooxidation process in 0.1 M KCl aqueous solution. X-ray photoelectron spectroscopy (XPS) measurement proves the presence of 4-carboxylphenylamine on the GCE. Electron transfer processes of Fe(CN)₆³⁻ in solutions of various pHs at the modified electrode are studied by both cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Changing the solution pH would result in the variation of the terminal group's charge state, based on which the surface pK_a values were estimated. The copper hexacyanoferrate (CuHCF) multilayer films were formed on 4-ABA/GCE prepared in aqueous solution, and which exhibit good electrochemical behavior with high stability.     

A novel electrochemical sensor based on molecularly imprinted polymers for caffeine recognition and detection

A sensitive and selective electrochemical sensor based on molecularly imprinted polymers (MIPs) was developed for caffeine (CAF) recognition and detection. The sensor was constructed through the following steps: multiwalled carbon nanotubes and gold nanoparticles were first modified onto the glassy carbon electrode surface by potentiostatic deposition method successively. Subsequently, o-aminothiophenol (ATP) was assembled on the surface of the above electrode through Au–S bond before electropolymerization. During the assembled and electropolymerization processes, CAF was embedded into the poly(o-aminothiophenol) film through hydrogen bonding interaction between CAF and ATP, forming an MIP electrochemical sensor. The morphologies and properties of the sensor were characterized by scanning electron microscopy, cyclic voltammetry, and differential pulse voltammetry. The recognition and determination of the sensor were observed by measuring the changes of amperometric response of the oxidation-reduction probe, [Fe(CN)₆]^3?/[Fe(CN)₆]^4?, on modified electrode. The results demonstrated that the prepared sensor had excellent selectivity and high sensitivity for CAF, and the linear range was 5.0?×?10^?10?~?1.6?×?10^?7?mol?L^?1 with a detection limit of 9.0?×?10^?11?mol?L^?1 (S/N?=?3). The sensor was also successfully employed to detect CAF in tea samples.     

Improving the electrochemical performances of active carbon-based supercapacitors through the combination of introducing functional groups and using redox additive electrolyte

High-performance and low-cost electrochemical capacitors (ECs) are essential for large-scale applications in energy storage. In this work, the specific capacitance of active carbon (AC) electrode was significantly improved through the combination of introducing functional groups on the surface of AC and adding redox-active molecules (K₃Fe(CN)₆) into 2?M KOH aqueous electrolytes. The surface-oxygen functionalized AC (FAC) was synthesized using HNO₃ echoed as the electrode and 2?M KOH with 0.1?M K₃Fe(CN)₆ as the electrolyte. The surface functional groups of the AC not only contribute to the pseudocapacitance but also increase the active sites of the electrode/electrolyte interface, which enhances the electrochemical activity of the Fe(CN)₆^3?/Fe(CN)₆^4? redox pair, thus leading to high capacitance. In the redox electrolyte, the specific capacitance was much higher in 229.17?F?g^?1 (1?A?g^?1) achieved for those FAC than in raw AC (only 147.06?F?g^?1). Similarly, the FAC electrode suggested high energy density and extended cycling stability in the KOH?+?K₃Fe(CN)₆ electrolyte.