Laccase Biosensor on Monolayer‐Modified Gold Electrode

Laccase enzymes from two different sources, namely, tree laccase from Rhus vernicifera and fungal laccase from Coriolus hirsutus were used for the development of biosensor for catechol. Laccase was immobilized onto the amine terminated thiol monolayers on gold surface by glutaraldehyde coupling. From the different thiol monolayers investigated, cystamine was found to be optimal with respect to sensitivity, stability, reproducibility, and other electrochemical properties of the enzyme electrode. Linear calibration in the range between 1 and 400 μM for catechol was obtained for fungal laccase covalently coupled on the electrode surface. The kinetic parameters determined using the Lineweaver‐Burk and Eadie‐Hofstee plots were K_m=0.65 mM and V_max=24.5 μA for fungal laccase compared to K_m=5.4 mM and V_max=6.6 μA for tree laccase on cystamine monolayer. The electrode showed good stability for 1 month without loosing appreciable activity when stored dry in a refrigerator at ?20 °C.     

Self‐Assembled Monolayer of L‐Cysteine on a Gold Electrode as a Support for Fatty Acid. Application to the Electroanalytical Determination of Unsaturated Fatty Acid

This article describes the formation of a SAM with chemisorbed cysteine to a gold surface by the thiol group to obtain a surface electrode with an amino and an acid group free for later reaction and accumulation with other molecules on the electrode surface. We explore the accumulation of unsaturated fatty acid and the electrochemical response of the electrode after modification with cysteine. The electrochemical study confirmed the accumulation of linoleic acid on the modified electrode. The optimum conditions for the determination of linoleic acid by differential pulse voltammetry of linoleic acid are studied a detection limit (3σ) of 0.03 μg mL^?1 and a determination limit (10σ) of 0.10 μg mL^?1 were obtained and applied to determination in olive oil and ham from Iberian breed hams.     

Voltammetric Determination of Epinephrine with a 3‐Mercaptopropionic Acid Self‐Assembled Monolayer Modified Gold Electrode

Epinephrine (EP) could exhibit an anodic peak at a bare gold electrode, but it was very insensitive. However, when the bare gold electrode was modified with 3‐mercaptopropionic acid (3MPA) self‐assembled monolayer (3MPA SAM), the peaks of EP became more reversible and sensitive due to the accumulation and mediate efficiency of 3MPA SAM. Conditions such as solution pH, concentration of supporting electrolyte and accumulation time were optimized. Under the selected conditions (i.e., 0.02 M pH 6.8 sodium phosphate buffer, accumulation time: 2 min under open‐ circuit.), the height of the anodic peak at about 0.18 V (vs. SCE) was linear to EP concentration in the range of 2×10^?7 ?1×10^?6 M and 1×10^?6?5×10^?4 M with correlation coefficient of 0.995 and 0.999, respectively. When the 3MPA/Au was further modified with cysteamine, the interference of H₂O₂ and BrO₃^? was eliminated. But the resulting electrode still suffered from the interference of ascorbic acid. This method was used to determine the content of EP in adrenaline hydrochloride injections, and the recovery was in the range of 97.0% to 105.1%.     

FIA Determination of Paracetamol in Pharmaceutical Drugs by Using Gold Electrodes Modified with a 3‐Mercaptopropionic Acid Monolayer

A flow injection analysis (FIA) method for the determination of paracetamol in pharmaceutical drugs using a gold electrode modified with a self‐assembled monolayer (SAM) of 3‐mercaptopropionic acid is described. At optimized experimental conditions the dynamic concentration range was 0.15 to 15.0 mg L^?1 with a detection limit of 0.2 μg mL^?1 (S/N=3). The repeatability of current responses for injections of 10 μmol L^?1 paracetamol was evaluated to be 3.2% (n=30) and the analytical frequency was 180 h^?1. The lifetime of the modified electrode was found to be 15 days. The results obtained by using the proposed amperometric method for paracetamol determination in four different drug samples compared well with those found by spectrophotometry.     

Voltammetric Study of Methylene Blue at Thiol SAMs‐Modified Gold Electrodes

The voltammetric behavior of methylene blue (MB) at thiol self‐assembled monolayers modified gold electrodes (SAMs/Au) has been investigated. MB exhibited a redox peak at about ?0.35 V (vs.SCE) in alkaline solution at bare gold electrodes. When the gold electrodes were modified with thiol SAMs, the peak grew due to the accumulation of MB at SAMs. With the solution pH rising, more MB was accumulated, hence the peak height increased, which differed from that at bare gold electrodes. The electrode process at SAMs/Au featured the characteristics of adsorption and/or electrode reaction controlled. The enhancing action of glutathione monolayer (GSH SAM), 3‐mercaptopropionic acid monolayer (3MPA SAM) and other thiol SAMs was compared. Among these, GSH SAM made the MB peak increase more. At GSH SAM/Au, the peak height varied linearly with MB concentration over the range of 2 μM to 400 μM. So this can be developed for the determination of MB and studies concerned. The accumulation behavior caused by GSH SAM and native fish sperm dsDNA was compared. The interaction between DNA and MB was also discussed under this condition.     

Electrochemical Behavior and Determination of L‐Tyrosine at Single‐walled Carbon Nanotubes Modified Glassy Carbon Electrode

Based on single‐walled carbon nanotubes (SWCNTs) modified glassy carbon electrode (GCE/SWCNTs), a novel method was presented for the determination of L ‐tyrosine. The GCE/SWCNTs exhibited remarkable catalytic and enhanced effects on the oxidation of L ‐tyrosine. In 0.10 mol/L citric acid‐sodium citrate buffer solution, the oxidation potential of L ‐tyrosine shifted negatively from +1.23 V at bare GCE to +0.76 V at GCE/SWCNTs. Under the optimized experimental conditions, the linear range of the modified electrode to the concentration of L ‐tyrosine was 5.0×10^?6–2.0×10^?5 mol/L (R₁=0.9952) and 2.7×10^?5–2.6×10^?4 mol/L (R₂=0.9998) with a detection limit of 9.3×10^?8 mol/L. The kinetic parameters such as α (charge transfer coefficient) and D (diffusion coefficient) were evaluated to be 0.66, 9.82×10^?5 cm² s^?1, respectively. And the electrochemical mechanism of L ‐tyrosine was also discussed.     

Electrochemical Studies of Chemically Modified Nanometer‐Sized Electrodes

Self‐assembled monolayers (SAMs) of 4‐aminothiophenol (4‐ATP) has been successfully deposited onto nanometer‐sized gold (Au) electrodes. The cyclic voltammograms obtained on a 4‐ATP SAMs modified electrode show peaks and the peak height is proportional to the scan rate, which is similar to that on an electroactive SAMs modified macro electrode. The electrochemical behavior and mechanism of outer‐sphere electron transfer reaction on the 4‐ATP SAMs modified nanometer‐sized electrode has also been studied. The 4‐ATP SAMs modified electrode is further modified with platinum (Pt) nanoparticles. Electrochemical behaviors show that there is electrical communication between Pt nanoparticles and Au metal on the Pt nanoparticles/4‐ATP SAMs/Au electrode. However, scanning electron microscopic image shows that the Pt nanoparticles are not evenly covered the electrode.     

Gold Electrode Modified with Self‐Assembled Monolayer of Cysteamine‐Functionalized MWCNT and Its Application in Simultaneous Determination of Dopamine and Uric Acid

The voltammetric behavior of dopamine (DA) and uric acid (UA) on a gold electrode modified with self‐assembled monolayer (SAM) of cysteamine (CA) conjugated with functionalized multiwalled carbon nanotubes (MWCNTs) was investigated. The film modifier of functionalized SAM was characterized by means of scanning electron microscopy (SEM) and also, electrochemical impedance spectroscopy (EIS) using para‐hydroquinone (PHQ) as a redox probe. For the binary mixture of DA and UA, the voltammetric signals of these two compounds can be well separated from each other, allowing simultaneous determination of DA and UA. The effect of various experimental parameters on the voltammetric responses of DA and UA was investigated. The detection limit in differential pulse voltammetric determinations was obtained as 0.02 µM and 0.1 µM for DA and UA, respectively. The prepared modified electrode indicated a stable behavior and the presence of surface COOH groups of the functionalized MWCNT avoided the passivation of the electrode surface during the electrode processes. The proposed method was successfully applied for the determination of DA and UA in urine samples with satisfactory results. The response of the gold electrode modified with MWCNT‐functionalized SAM method toward DA, UA, and ascorbic acid (AA) oxidation was compared with the response of the modified electrode prepared by the direct casting of MWCNT.     

4-巯基吡啶自组装修饰金电极的电化学性质及对抗坏血酸的测定

研究了 4 巯基吡啶自组装膜 (SAM)修饰金电极的制备及其电化学性质 ,并用于抗坏血酸 (AA)的测定。在pH 3.0盐酸 邻苯二甲酸氢钾缓冲溶液中 ,AA在SAM修饰金电极上产生一灵敏的氧化峰 ,峰电流与AA浓度在 4 .0× 10 - 6 ～ 1.0× 10 - 3mol L范围内呈良好的线性关系 ,检出限为 2 .7× 10 - 6 mol L ,相关系数为0 9978。该电极对多巴胺 (DA)有排斥作用 ,重现性良好 ,可用于AA的灵敏测定。     

Highly‐Sensitive Determination of 6‐Mercaptopurine and Its Metabolites by Electrochemical Reductive Desorption Measurements

The electrochemical detection of 6‐mercaputopurine (6‐MP), which is one of the most well‐known drugs of antimetabolites, and its analogues was studied. The presented method is based on chemisorption (accumulation)/desorption processes of drugs onto/from the Au electrode surface. Using this method, we succeeded in detecting 6‐MP even in very diluted solution (concentration of 6‐MP: 1×10^?8 M). The lower detection limit of 6‐MP was considerably improved compared with the previous HPLC studies by using our proposed system. Desorptive potentials of 6‐MP‐metabolites and its analogues were different from those of 6‐MP. This provides the possibility of simultaneous detection of 6‐MP and its typical metabolites.     

Impact of Polycyclic Aromatic Hydrocarbons on the Electrochemical Responses of a Ferricyanide Probe at Template‐Modified Self‐Assembled Monolayers on Gold Electrodes

The impact of polycyclic aromatic hydrocarbons (PAHs) on the electrochemical responses of a ferricyanide probe using gold electrodes coated with template‐containing self‐assembled monolayers (SAMs) was investigated using cyclic voltammetry and square‐wave voltammetry. The thiolated compounds that were used to form SAMs included 1‐hexadecanethiol, 11‐mercapto‐undecanoic acid, 11‐mercaptoundecanol, and (3‐mercaptopropyl) trimethoxysilane (MPTS). When the SAMs were formed from 1‐hexadecanethiol or 11‐mercapto‐undecanoic acid in the absence of pyrene, the SAM‐modified electrodes prohibited access of the ferricyanide probe and no impact of pyrene was observed. SAM‐modified electrodes (all except for MPTS) that were formed in the presence of pyrene then washed free of pyrene showed an increase in accessibility of the probe ferricyanide upon the addition of pyrene to the electrolyte solution. When electrodes were modified with MPTS to form stabilized SAMs in the presence of pyrene, however, a reduced redox current for the ferricyanide probe was observed with increased pyrene or naphthalene in the electrolyte solution. A degree of selectivity was noted in that this current response was not observed for addition of benzo[a]pyrene.     

Development of an Amperometric Immunosensor for the Quantification of Staphylococcus aureus Using Self‐Assembled Monolayer‐Modified Electrodes as Immobilization Platforms

Amperometric immunosensors for the detection and quantification of S. aureus using MPA self‐assembled monolayer modified electrodes for the immobilization of the immunoreagents are reported. Two different immunosensor configurations were compared. A competitive mode, in which protein A‐bearing S. aureus cells and antiRbIgG labeled with horseradish peroxidase (HRP) compete for the binding sites of RbIgG immobilized onto the 3‐mercaptopropionic acid (MPA) modified electrode, was evaluated. Moreover, a sandwich configuration in which S. aureus cells were immobilized onto the MPA SAM, and RbIgG and antiRbIgG labeled with HRP were further linked to the electrode surface, was also tested. In both cases, TTF was used as the redox mediator of the HRP reaction with H₂O₂, and it was co‐immobilized onto the MPA‐modified gold electrode. After optimization of the working variables for both configurations, the analytical performance of the amperometric measurements carried out at 0.00 V (vs. Ag/AgCl) showed that the competitive immunosensor exhibited a lower limit of detection (1.6×10⁵ S. aureus cells mL^?1), as well as a better repeatability and reproducibility of the measurements.     

Selective Detection of Ascorbic Acid Using Octacyanomolybdate‐Doped‐Glutaraldehyde‐Cross‐Linked Poly‐L‐Lysine Film Modified Glassy Carbon Electrode

The present work describes oxidation of ascorbic acid (AA) at octacyanomolybdate‐doped‐glutaraldehyde‐cross‐linked poly‐L ‐lysine (PLL‐GA‐Mo(CN) film modified glassy carbon electrode in 0.1 M H₂SO₄. The modified electrode has been successfully prepared by means of electrostatically trapping Mo(CN) mediator in the cationic film of glutaraldehyde‐cross‐linked poly‐L ‐lysine. The dependence of peak current of modified electrode in pure supporting indicates that the charge transfer in the film was a mixed process at low scan rates (5 to 200 mV s^?1), and kinetically restrained at higher scan rates (200 to 1000 mV s^?1). Cyclic voltammetry and rotating disk electrode (RDE) techniques are used to investigate the electrocatalytic oxidation of ascorbic acid and compared with its oxidation at bare and undoped PLL‐GA film coated electrodes. The rate constant of catalytic reaction k obtained from RDE analysis was found to be 9.5×10⁵ cm³ mol^?1 s^?1. The analytical determination of ascorbic acid has been carried out using RDE technique over the physiological interest of ascorbic acid concentrations with a sensitivity of 75 μA mM^?1. Amperometric estimation of AA in stirred solution shows a sensitivity of 15 μA mM^?1 over the linear concentration range between 50 and 1200 μM. Interestingly, PLL‐GA‐Mo(CN) modified electrode facilitated the oxidation of ascorbic acid but not responded to other electroactive biomolecules such as dopamine, uric acid, NADH, glucose. This unique feature of PLL‐GA‐Mo(CN) modified electrode allowed for the development of a highly selective method for the determination of ascorbic acid in the presence of interferents.     

胆碱与谷氨酸共价单层混合修饰玻碳电极的制作与表征及其测定亚硝酸根的分析应用

A choline and L-glutamic acid mixed monolayer covalently modified glassy carbon electrode (Ch-Glu/GCE) was fabricated and characterized by X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). It provided an excellent example of mixed covalent monolayer modification of carbon electrodes with alkanol and amino acid, and also a facile means for altering the interfacial architecture. The Ch-Glu/GCE displayed good catalytic activity toward the oxidation of nitrite anions. Differential pulse voltammetry was used for determination of nitrite at the Ch-Glu/GCE. The Ch-Glu/GCE showed higher capability for restraint of pollutions than a simple Ch modified electrode or a simple Glu modified electrode.     

Self‐Assembled Redox System for Bioelectrocatalytic Assay of L‐Ascorbylphosphate and Alkaline Phosphatase Activity

Ferrocene‐terminated self‐assembled monolayer (Fc‐SAM) on gold was used as an electron‐transfer mediator in the electrochemical assay of L ‐ascorbic acid 2‐phosphate (AAP). The assay is based on the enzymatic action of alkaline phosphatase (ALP), which triggers the release of vitamin C (L ‐ascorbic acid, AA) from AAP. The latter is easily oxidized on the Fc‐SAM under the diffusion limiting conditions that favors quantitative measurement of the AA concentration on a rotating disk electrode. We demonstrate the utility of the electrochemically active Fc‐SAM to probe the mechanism and to determine the kinetic parameters of an enzymatic reaction. The electrochemical technique was compared to a conventional spectrophotometric method of ALP activity detection using p‐nitrophenylphosphate (p‐NPP) as a substrate. We demonstrate that our new technique is also suitable for the analytical determination of ALP activity. The detection limits for both AAP and ALP were found to be 13 μM and 2 pM, respectively.     

Electrochemical Characterization of In Situ Functionalized Gold Cysteamine Self‐Assembled Monolayer with 4‐Formylphenylboronic Acid for Detection of Dopamine

Functionalization of gold cysteamine (Au? CA) self‐assembled monolayer with 4‐formylphenylboronic acid (BA) via Schiff's base formation, through in situ method to fabricate Au‐CA‐BA electrode is presented and described. The fabricated electrode was used as a novel sensor for accumulation and determination of dopamine (DA). The accumulation of DA as a diol on the topside of Au‐CA‐BA as a Lewis acid, was performed via esterification (Au? CA? BA? DA), and followed for determination of DA. Functionalization, characterization, and determination steps were probed by electrochemical methods like cyclic voltammetry and electrochemical impedance spectroscopy. The data will be presented and discussed from which a new sensor for DA is introduced.     

Electrochemical Determination of Cu(II) Ions by 4‐Formylphenylboronic Acid Modified Gold Electrode

A self‐assembled monolayer (SAM) modified by 4‐formylphenylboronic acid was formed on the gold electrode, which was applied for the determination of trace concentrations of Cu(II). The formation of advanced SAM on the gold electrode was evidenced by electrochemical impedance spectroscopy, atomic force microscopy and contact angle measurements. Electrochemical determination of Cu(II) ions was performed by square wave voltammetry. Some mutual interferences caused by Cd(II), Co(II), Fe(II), Ni(II) and Pb(II) ions were investigated and it was demonstrated how the negative effects of these interfering ions could be eliminated by adjustment of proper parameters of square wave voltammetry.     

Electrocatalytic Nitrite Determination Using Iron Phthalocyanine Modified Gold Nanoparticles

Electrochemical detection of nitrite was achieved via electrodeposition of gold nanoparticles (AuNPs) onto glassy carbon electrodes, followed by 3‐mercaptopropionic acid (MPA) self‐assembly, enabling attachment of an iron(III) monoamino‐phthalocyanine (FeMAPc) catalyst via amide bond formation. The use of scanning electron microscopy, energy dispersive X‐ray spectroscopy and ultraviolet‐visible spectroscopy realised surface characterisation while cyclic voltammetry and electrochemical impedance spectroscopy techniques were applied for electrochemical interrogation. The electrochemical behaviour of nitrite at the bare (GCE), AuNPs/GCE, FeMAPc/GCE and FeMAPc‐MPA/AuNPs/GCE was further scrutinised using differential pulse voltammetry in phosphate buffer solution (0.1 M PBS, pH 5.8). Overall the FeMAPc‐MPA/AuNPs/GCE resulted in sensitivity 14.5 nA/µM, which was double that of AuNPs/GCE, 2.4 times FeMAPc/GCE and 3.5 times the response at a bare GCE, with linear range 1.9 µM–2.04 mM (PBS, pH 5.8) and LOD 0.21 µM. An interference study revealed that the proposed sensor (FeMAPc‐MPA/AuNPs/GCE) exhibited a selective response in the presence of interfering anions and the analytical capability of the sensor was demonstrated via nitrite ion determination in real water samples.     

Electrocatalytic Characteristics of Ferrocenecarboxylic Acid Modified Carbon Paste Electrode in the Oxidation and Determination of L‐Cysteine

The electrochemical behavior of L ‐cysteine studied at the surface of ferrocenecarboxylic acid modified carbon paste electrode (FCMCPE) in aqueous media using cyclic voltammetry and double step potential chronoamperometry. It has been found that under optimum condition (pH 7.00) in cyclic voltammetry, the oxidation of L ‐cysteine is occurs at a potential about 580 mV less positive than that an unmodified carbon paste electrode. The kinetic parameters such as electron transfer coefficient, α and catalytic reaction rate constant, K′_h were also determined using electrochemical approaches. The electrocatalytic oxidation peak current of L ‐cysteine showed a linear dependent on the L ‐cysteine concentration and linear calibration curves were obtained in the ranges of 10^?5 M–10^?3 M and 4.1×10^?8 M–3.7×10^?5 M of L ‐cysteine concentration with cyclic voltammetry (CV) and differential pulse voltammetry (DPV) methods respectively. The detection limits (2δ) were determined as 2.4×10^?6 M and 2.5×10^?8 M by CV and DPV methods. This method was also examined for determination of L ‐cysteine in some samples, such as Soya protein powder, serum of human blood by using recovery and standard addition methods.     

Preparation and Electrochemical Behaviour of Silver Pentacyanonitrosylferrate Film Modified Glassy Carbon Electrode and Its Electrocatalytic Oxidation to L‐cysteine

A glassy carbon (GC) electrode modified with silver pentacyanonitrosylferrate (AgPCNF) film as a redox mediator was fabricated. Cyclic voltammetry was used to study the redox property of AgPCNF modified electrode. The modified electrode showed a well‐defined redox couple due to [Ag^IFe^III/II(CN)₅NO]^1‐/2‐system. The effects of scan rates, supporting electrolytes and solution pHs were studied on the electrochemical behavior of the modified electrode. The feasibility of using the AgPCNF modified electrode to measure L ‐cysteine was investigated. It showed an excellent electrocatalytic activity towards the oxidation of L ‐cysteine and the anodic currents were proportional to the L ‐cysteine concentration in the range of 0.1 μM to 20 μM, the linear regression equation is I_pa(μA) = ‐68.58 ‐ 5.78C_{L ‐cysteine} (μM), with a correlation coefficient 0.998 for N = 23. The detection limit was down to 3.5 × 10^‐8 M (three times the ratio of signal to noise).