Functionalization of gold cysteamine self-assembled monolayer with ethylenediaminetetraacetic acid as a novel nanosensor

Electrochemical characterization of gold cysteamine self-assembled monolayer, in situ functionalized with ethylenediaminetetraacetic acid (Au-CA-EDTA SAM), is described by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and Osteryoung square wave voltammetry (OSWV). The results obtained by EIS and CV, in the presence of [Fe(CN)₆]^3−/4− redox probe, show that EDTA is successfully grafted to the surface of Au-CA electrode. Reproducible and reversible variation of the R_ct and ΔEp as a function of solution pH show that Au-CA-EDTA SAM is stable in a wide range of pH and potentials. Accumulation of the Pb²⁺ and Cu²⁺ ions on the Au-CA-EDTA SAM electrode is investigated using faradaic currents or impedimetric effects measured by OSWV and EIS, respectively. These results reveal the presence of active complexing functional groups of EDTA on the surface, and thus, the formation of Au-CA-EDTA SAM electrode. The new sensor responds to the Pb²⁺ and Cu²⁺ separately and simultaneously in a wide linear range of concentrations.     

Self-assembled monolayers on Au microelectrodes

Long chain alkanethiols self-assembled monolayers (SAMs) formed on Au microelectrodes showed higher sensitivity towards defects than the same monolayers on macroelectrodes. The analysis of cyclic voltammetry and electrochemical impedance spectroscopy (EIS) experiments performed on covered microelectrodes were consistent with the formation of pinholes of about 10 nm in diameter. Moreover, the EIS data exhibited a specific behavior that was interpreted invoking the short circuiting of the pinholes impedance by the surrounding surface of the microelectrode in the high frequency domain, whereas in the low frequencies, the surface covered by the SAM was assume to act as an insulator.     

Molecularly Imprinted Impedimetric Sensor for Determination of Mycotoxin Zearalenone

The mycotoxin zearalenone (ZEA) prompts reproductive toxicity due to its strong estrogenic effects. In this work, an electrochemical sensor for determination of ZEA was developed by electropolymerization of a molecularly imprinted poly (o‐phenylenediamine) (PPD) film on screen‐printed gold electrode (SPGE) surface. The sensor was examined by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) using K₃[Fe(CN)₆]/K₄[Fe(CN)₆] as redox probe. The molecularly imprinted polymer (MIP) sensor showed a wide determination range from 2.50 to 200.00 ngmL^?1 for ZEA. The Limit of detection (LOD) was calculated to be 0.20 ngmL^?1, based on the signal to noise (S/N) ratio equal to 3.0. The sensor displayed good repeatability, with RSD values≤4.6 %, and maintained 93.2 % of its initial response after storage for 10 days in air at room temperature. The developed method was successfully applied for the determination of ZEA in corn flakes with mean recoveries ranged from 96.2 % to 103.8 % and RSDs within the interval of 2.1 % to 3.8 %.     

Characterization of gold-thiol-8-hydroxyquinoline self-assembled monolayers for selective recognition of aluminum ion using voltammetry and electrochemical impedance spectroscopy

Gold electrode surface is modified via covalent attachment of a synthesized thiol functionalized with 8-hydroxyquinoline, p-((8-hydroxyquinoline)azo) benzenethiol (SHQ), for the first time. The behavior of the nanostructured electrode surface (Au–SHQ) is characterized by electrochemical techniques including cyclic and differential pulse voltammetry (CV and DPV), and electrochemical impedance spectroscopy (EIS). The modified surface is stable in a wide range of potentials and pHs. A surface pK_a of 6.0 ± 0.1 is obtained for Au–SHQ electrode using surface acid/base titration curves constructed by CV and EIS measurements as a function of pH. These results helped to determine the charge state of the surface as a function of pH. The gold modified electrode surface showed good affinity for sensing the Al(III) ion at pH 5.5. The sensing process is based on (i) accumulation and complex formation between Al(III) from the solution phase and 8HQ function on the Au electrode surface (recognition step) and (ii) monitoring the impedance of the Au–SHQ–Al(III) complex against redox reaction rate of parabenzoquinone (PBQ) (signal transduction step). The PBQ is found to be a more suitable probe for this purpose, after testing several others. Thus, the sensor was tested for quantitative determination of Al(III) from the solution phase. At the optimized conditions, a linear response, from 1.0 × 10⁻¹¹ to 1.2 × 10⁻⁵ M Al(III) in semi-logarithmic scale, with a detection limit of 8.32 × 10⁻¹² M and mean relative standard deviation of 3.2% for n = 3 at 1.0 × 10⁻⁷ M Al(III) is obtained. Possible interferences from coexisting cations and anions are also studied. The results show that many ions do not interfere significantly with the sensor response for Al(III). Validity of the method and applicability of the sensor are successfully tested by determination of Al(III) in human blood serum samples.     

Label-free detection of DNA molecules on the dendron based self-assembled monolayer by electrochemical impedance spectroscopy

We report preparation of a novel platform for effective DNA hybridization and its application to the detection of single mismatched DNA. Cone-shaped dendrimer molecules have been immobilized on the gold surface at equidistance, 3.1 nm, from each other with a probe DNA molecule attached to the top of each dendrimer so that enough space would be secured for effective hybridization. This arrangement allows each probe DNA molecule to form a natural DNA double helix upon hybridization with a target DNA molecule. The single nucleotide polymorphism at either the central or end position of the 25-mer target DNA has been shown to be effectively discriminated against on this platform from each other as well as from a complementary DNA by electrochemical impedance measurements. We also report adverse effects exerted by probe ions, Fe(CN)₆^3−/4−, on DNA hybridization reactions. The significance of the results for the use in DNA analysis is discussed.     

Self‐assembly of p‐Aminothiophenol on Gold Surface: Application for Impedimetric and Potentiometric Sensing of Cobalt (II) Ions – A Comparative Study

Cobalt, despite an essential biological element, imposes threat to humans when exposed to high concentration or even to low concentration for long term which demands the development of highly sensitive and selective analytical methods for its trace analysis. In the present work, self‐assembly of p‐aminothiophenol (p‐ATP) on gold surface (Au?ATP SAM) was carried out and for the first time, applied as a platform for impedimetric and potentiometric sensing of Co²⁺. Au?ATP SAM was characterized using electrochemical techniques: cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), in the presence of two redox probes: [Fe(CN)₆]^3?/4? and [Ru(NH₃)₆]^2+/3+ to evaluate associated passivating behaviour. Au?ATP SAM completely blocked [Fe(CN)₆]^3?/4? as compared to [Ru(NH₃)₆]^2+/3+ which may be attributed to inner‐sphere and outer‐sphere ET mechanisms, respectively. Au?ATP SAM was found to exhibit excellent sensitivity towards Co²⁺ in a wider concentration range from 1.0×10^?12 M to 1.0×10^?5 M (r²=0.963) at pH 5.5 with a detection limit of 6.0×10^?13 M and superior selectivity. Further, carbon paste electrode (CPE) was prepared by incorporating p‐ATP bound gold nanoparticles and explored for potentiometric sensing of Co²⁺ which exhibited Nernstian slope of 29.2±0.2 mV/dec in linear concentration range of 1.0×10^?6 M–1.0×10^?1 M (r²=0.971) with a detection limit of 8.0×10^?7 M. The proposed sensors were successfully applied for estimation of Co²⁺ content in water samples.     

Fractional coverage of defects in self-assembled thiol monolayers on gold

Au electrodes are alkylated by self-assembled organic monolayers of octadecanethiol from alcohol solution. The electron tunnelling resistance of a monolayer-coated gold electrode has been investigated by ac impedance. The relation between the fractional coverage of different defects and the corresponding film thickness at these ‘collapsed’ sites has been deduced from electron tunnelling theory. By using the concepts of average film thickness at defect (d_a) and average fractional coverage of defect (θ_a), we have obtained the θ_ad_a plot. The influence of the apparent standard rate constant on the shape of the θ_ad_a plot has been discussed. In our experiments, Fe(CN)₆^3−/4− is used as a redox probe to study the θ_ad_a plot of an octadecanethiol monolayer. The θ_a versus d_a plot indicates that the defects with d_a<6 methylene groups and θ_a<0.1 can increase the apparent standard rate constant from 1.9×10⁻¹⁰ cm s⁻¹, which is the theoretical value calculated from electron tunnelling theory, to 2.9×10⁻⁷ cm s⁻¹. The average thickness of the whole monolayer (ATWM), which is obtained from the θ_a versus d_a plot and which can indicate the blocking property of the monolayer, is 11 methylene groups.     

Sensitive determination of iron(III) by gold electrode modified with 2-mercaptosuccinic acid self-assembled monolayer

Preparation and application of gold 2-mercaptosuccinic acid self-assembled monolayer (Au-MSA SAM) electrode for determination of iron(III) in the presence of iron(II) is described by cyclic voltammetry, electrochemical impedance spectroscopy, and Osteryoung square wave voltammetry. The square wave voltammograms showed a sharp peak around positive potentials +0.250 V that was used for construction of the calibration curve. Parameters influencing the method were optimized. A linear range calibration curve from 1.0 × 10⁻¹⁰ to 6.0 × 10⁻⁹ M iron(III) with a detection limit of 3.0 × 10⁻¹¹ M and relative standard deviation (R.S.D.) of 6.5% for n = 8 at 1.0 × 10⁻⁹ M iron(III) was observed in the best conditions. Possible interferences from the coexisting ions were also investigated. The results demonstrated that sensor could be used for determination of iron(III) in the presence of various ions. The validity of the method and applicability of the sensor were successfully tested by determining of iron(III) in natural waters (tap and mineral waters) and in a pharmaceutical sample (Venofer^® ampoule) without interference from sample matrix. The experimental data are presented and discussed from which the new sensor is characterized.     

Voltammetry of immobilized cytochrome c on novel binary self-assembled monolayers of thioctic acid and thioctic amide modified gold electrodes

Horse heart cytochrome c (cyt c) was adsorbed on the binary self-assembled monolayers (SAMs) composed of thioctic acid (T-COOH) and thioctic amide (T-NH₂) at gold electrodes via electrostatic interaction. The cyt c adsorbed on the modified gold electrode exhibited well-defined reversible electrochemical behavior in 10 mM phosphate buffer solution (PBS, pH 7.0). The surface concentration (Γ) of electroactive species, cyt c, on the binary SAMs was higher than that in single-component SAMs of T-COOH, and reached a maximum value of 9.2 × 10⁻¹² mol cm⁻² when the ratio of T-COOH to T-NH₂ in adsorption solution was of 3:2, and the formal potential (E^0′=(E_pa+E_pc)/2) of cyt c was −0.032 V (vs. Ag|AgCl (3 M NaCl)) in a 10 mM PBS. The interaction between cyt c and the binary SAMs made the E^0′ shift negatively when compared with that of cyt c in solution (+0.258 V vs. NHE, i.e., +0.058 V vs. Ag|AgCl (3 M NaCl)). The fractional coverage of bound cyt c was a 0.64 theoretical monolayer. The standard electron transfer rate constant of cyt c immobilized on the binary SAMs was also higher than that on single-component SAMs of T-COOH, and the maximum value of 15.8 ± 0.6 s⁻¹ was obtained when the ratio of T-COOH to T-NH₂ in adsorption solution was at 3:2. The results suggest that the electrode modified with the binary SAMs functions better than the electrode modified with single-component SAMs of T-COOH.     

Impedimetric DNA Sensor Based on Poly(proflavine) for Determination of Anthracycline Drugs

Electropolymerization is often used as a tool for immobilization of charged biopolymers and their electric wiring in the biosensor assembly. In this work, electropolymerization of proflavine has been for the first time used for the physical adsorption of DNA and measurement of anthracycline drugs (doxorubicin and daunorubicin) able to intercalate DNA. Redox properties of the proflavine polymers on the glassy carbon electrode and DNA deposition were characterized with cyclic voltammetry, scanning electron microscopy and electrochemical impedance spectroscopy. It was shown that DNA decreased the peak currents on voltammogram but increased the charge transfer resistance. The latter effect is pronounced with the following drug application. The impedimetric response regularly increased with the drugs concentration in the range 1 nM to 0.1 μM for doxorubicin and 1 pM–10 nM for daunorubicin (limit of detections 0.3 and 0.001 nM, respectively). The biosensor was tested on pharmaceutical preparations and spiked solution simulating the plasma electrolytes and possible interference of serum proteins.     

Detection of Prunus Necrotic Ringspot Virus in Plant Extracts with Impedimetric Immunosensor based on Glassy Carbon Electrode

This paper concerns the development of an immunosensor for detection of Prunus necrotic ringspot virus (PNRSV) in plant extracts. The immunosensor fabrication consists of successive modification steps of glassy carbon electrodes: (i) creation of COOH groups, (ii) covalent immobilization of protein A with EDC/NHS coupling reaction, (iii) immobilization of anti‐PNRSV IgG polyclonal antibody, (iiii) filling free spaces with BSA. Each step was controlled with cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The immunosensor was applied for the determination of the PNRSV virus in extracts from cucumber leaves using EIS technique. It was capable of discriminating between samples consisting of extracts from healthy plants and consisting of leaf extracts from infected plants diluted 10 000 times with extract from healthy plants.     

温度对石墨电极性能的影响

运用电化学阻抗谱(EIS)并结合循环伏安法(CV)研究了石墨电极25和60 ℃时在1 mol·L-1 LiPF6-EC(碳酸乙烯酯):DEC(碳酸二乙酯):DMC(碳酸二甲酯)电解液中, 以及60 ℃时在1 mol·L-1 LiPF6-EC:DEC:DMC+5%VC(碳酸亚乙烯酯)电解液中的首次阴极极化过程. 发现高温下(60 ℃)石墨电极在1 mol·L-1 LiPF6-EC:DEC:DMC电解液中可逆循环容量衰减的主要原因在于其表面无法形成稳定的固体电解质相界面(SEI)膜. 实验结果显示, VC添加剂能够增进高温下石墨电极表面SEI膜的稳定性, 进而改进石墨电极的循环性能.     

A reusable capacitive immunosensor for detection of Salmonella spp. based on grafted ethylene diamine and self-assembled gold nanoparticle monolayers

Fabrication of a novel capacitive immunosensor based on grafted ethylene diamine and self-assembled gold nanoparticle monolayer on glassy carbon electrode for the detection of Salmonella spp. is described for the first time. In the present study, the Salmonella spp. monoclonal antibodies (denoted as McAbs) was immobilized on gold nanoparticles. Interaction of McAbs and Salmonella spp. was detected directly using the electrochemical impedance spectroscopy (EIS) technique. The experimental results showed that the concentration of antigen was measured through the relative change in capacitance in the corresponding specific binding of Salmonella spp. and McAbs. Under the optimized conditions, the relative changes in capacitance were proportional to the logarithmic values of Salmonella spp. concentrations in the range of 1.0 × 10² to 1.0 × 10⁵ CFU mL⁻¹ (r = 0.991) with the detection limit of 1.0 × 10² CFU mL⁻¹. The stability of proposed immunosensor could be estimated by determining the relative change in capacitance, which remained almost the same in two months and decreased gradually to 85.3% of initial value after four months’ storage. The used immunosensor could be regenerated repeatedly by immersing in glycine-HCl buffer solution (pH 2.8). Finally, the proposed immunosensor was successfully used for the detection of Salmonella spp. in lab-processed commercial pork samples.     

辅酶I在Co/CFUE上的电化学行为及应用

辅酶I(NAD )在0.005moL/LTris 0.01moL/LNaCl溶液(pH7.0)中,于钴离子注入修饰碳纤维电极上出现一个S形的还原波,半波电位为-1.45V(vs.SCE)。峰电流与NAD 的浓度在2.38×10-5～4.76×10-4mol/L(r=0 9992)和4.76×10-4～1.78×10-3mol/L(r=0.9982)之间成线性关系,检出限为1.19×10-5mol/L,回收率在96.7%～103.4%之间。用线性扫描、循环伏安法研究了钴离子注入修饰碳纤维电极上NAD 的电化学行为。电极反应机理为:NAD e NAD·;NAD· e H NADH;2NAD·→NAD2。另外,钴离子注入修饰碳纤维电极对NAD 具有电催化作用。     

辅酶Ⅰ在Co／CFUE上的电化学行为及应用

辅酶I(NAD+)在0.005moL/LTris 0.01moL/LNaCl溶液(pH7.0)中,于钴离子注入修饰碳纤维电极上出现一个S形的还原波,半波电位为-1.45V(vs.SCE)。峰电流与NAD+的浓度在2.38×10-5～4.76×10-4mol/L(r=0 9992)和4.76×10-4～1.78×10-3mol/L(r=0.9982)之间成线性关系,检出限为1.19×10-5mol/L,回收率在96.7%～103.4%之间。用线性扫描、循环伏安法研究了钴离子注入修饰碳纤维电极上NAD+的电化学行为。电极反应机理为:NAD++e NAD·;NAD·+e+H+ NADH;2NAD·→NAD2。另外,钴离子注入修饰碳纤维电极对NAD+具有电催化作用。     

Electroanalytical method for determination of shikonin based on the enhancement effect of cyclodextrin functionalized carbon nanotubes

A simple and sensitive electroanalytical method for determination of shikonin,a widely used antitumoral agent,using β-cyclodextrin-functionalized multiwalled carbon nanotubes composite modified glassy carbon electrodes(MWCNTs/β-CD/GCE) was presented.CDs are water-soluble and environmentally friendly and can improve the dispersibility of MWCNTs/β-CD functional materials,which was confirmed by SEM.The electrochemical behaviors of shikonin on different electrodes were investigated by cyclic voltammetry(CV) and differential pulse voltammograms(DPVs).The results demonstrated that the redox peak currents of shikonin obtained at MWCNTs/β-CD/GCE were much higher than those at theβ-CD/GCE and MWCNTs/GCE,which can be attributed to the combination of the excellent electrocatalytic properties of MWCNTs and the molecular recognition ability of β-CD.At MWCNTs/β-CD/GCE,the response current exhibits a linear range from 5.0 nmol/L to 10.0 μmol/L with a detection limit of 1.0 nmol/L(S/N = 3).As a practical application,the proposed method was applied to quantitatively determine shikoninin urine samples with satisfying results.     

Selective electrochemical sensing of calcium dobesilate based on the nano-Pd/CNTs modified pyrolytic graphite electrode

A new palladium nanoparticle functionalized multi-wall carbon nanotubes (nano-Pd/CNTs) modified pyrolytic graphite electrode (PGE) has been fabricated for electrochemical sensing of calcium dobesilate (CD) in pharmaceutical capsules. The nano-Pd/CNTs were characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The nano-Pd/CNTs composite showed a strong electrocatalytic property for CD. The anodic peak current is 6-fold than that obtained in bare PGE and the oxidation potential has an obvious shift to negative. The anodic peak current is proportional to the concentration of CD in the range of 1.0 × 10⁻⁷ to 7.0 × 10⁻⁴ mol L⁻¹, with a linear relative coefficient r = 0.999 and a detection limit 4.0 × 10⁻⁸ mol L⁻¹ (S/N = 3). This kind of electrode shows good stability, sensitivity, reproducibility, large linear range and low detection limit towards electrochemical determination of CD. The proposed method provides a selective and sensitive electrochemical sensor of calcium dobesilate.     

Coated wire linear alkylbenzenesulfonate sensor based on polypyrrole and improvement of the selectivity behavior

The potentiometric behavior of coated wire electrodes based on dodecylbenzenesulfonate-doped polypyrrole (PPy-DBS) and hyamine as ion exchanger was investigated. The PPy-DBS was prepared electrochemically by anodic polymerization of pyrrole in the presence of DBS⁻ ions in aqueous solution and used as ionophore for construction of the sensor. Two types of coated wire electrodes made of PVC-PPy-DBS and PVC-Hyamine-DBS, plasticized with ortho-nitrophenyloctylether (o-NPOE) showed the Nernstian behavior (with respective calibration slopes of about 58 and 60 mV per decade) over the DBS⁻ concentration range of 3.0×10⁻⁶ to 1.1×10⁻³ M and 5.0×10⁻⁶ to 1.3×10⁻³ M, respectively. The influence of membrane composition, type of plasticizer, and pH of test solution on the potentiometric responses of the two electrodes was investigated. The potentiometric response was independent of the pH of test solution in the range 3-10. The response time of electrodes was fast (10 s for both types of electrode), and they can be used for at least 3 months without any significant change in potential. The proposed electrodes revealed very good selectivity for DBS⁻ ion over diverse inorganic and organic anions. The potentiometric selectivity coefficients for the PPy-DBS based electrode revealed a significant improvement as compared to the electrode made by conventional Hyamine-DBS (Hya-DBS) anion exchanger. The proposed electrode was used for determination of DBS⁻ ion in some commercial detergents. The results of the potentiometric determinations were in satisfactory agreement with those obtained by a standard method (two-phase titration).     

Development of electrochemical biosensors based on sol-gel enzyme encapsulation and protective polymer membranes

Protective polymer coatings have been used to enhance the retention of enzymes in sol-gel films as immobilisation phases in electrochemical biosensors. Carbon film electrodes were electrochemically modified with poly(neutral red) (PNR). These electrodes were coated with oxysilane sol-gels incorporating glucose oxidase and an outer coating of carboxylated PVC (CPVC) or polyurethane (PU), with and without Aliquat-336 or isopropyl myristate (IPM) plasticizer, was applied. The biosensors were characterised electrochemically using cyclic voltammetry and amperometry, electrochemical impedance spectroscopy and scanning electron microscopy. Impedance spectra showed that the electrode surface is most active when the sol-gel–GOx layer is not covered with a membrane. However, membranes without plasticizer extend the lifetime of the biosensor to more than 2 months when PU is used as an outer membrane. The linear range of the biosensors was found to be 0.05–0.50 mM of glucose and the biosensor with PU outer membrane exhibited higher sensitivity (ca.117 nA mM⁻¹) in the region of linear response than that with CPVC. The biosensors were applied to glucose measurement in natural samples of commercial orange juice.     

Label-free capacitive immunosensor based on quartz crystal Au electrode for rapid and sensitive detection of Escherichia coli O157:H7

A label-free capacitive immunosensor based on quartz crystal Au electrode was developed for rapid and sensitive detection of Escherichia coli O157:H7. The immunosensor was fabricated by immobilizing affinity-purified anti-E. coli O157:H7 antibodies onto self-assembled monolayers (SAMs) of 3-mercaptopropionic acid (MPA) on the surface of a quartz crystal Au electrode. Bacteria suspended in solution became attached to the immobilized antibodies when the immunosensor was tested in liquid samples. The change in capacitance caused by the bacteria was directly measured by an electrochemical detector. An equivalent circuit was introduced to simulate the capacitive immunosensor. The immunosensor was evaluated for E. coli O157:H7 detection in pure culture and inoculated food samples. The experimental results indicated that the capacitance change was linearly correlated with the cell concentration of E. coli O157:H7. The immunosensor was able to discriminate between cellular concentrations of 10²–10⁵ cfu mL⁻¹ and has applications in detecting pathogens in food samples. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were also employed to characterize the stepwise assembly of the immunosensor.