Electroanalysis of Norepinephrine at Bare Gold Electrode Pure and Modified with Gold Nanoparticles and S‐Functionalized Self‐Assembled Layers in Aqueous Solution. Part II

Gold electrodes modified with S‐containing compounds and gold were used for determination of norepinephrine (NEP) in aqueous solution. A linear relationship between norepinephrine concentration and current response was obtained in the range of 0.1 µM to 600 µM with the detection limit ≤0.090 µM for the electrodes modified at 2D template and in the range of 0.1 µM to 700 µM with the detection limit ≤0.075 µM for the electrodes modified at 3D template. The results have shown that modified electrodes could clearly resolve the oxidation peaks of norepinephrine, ascorbic (AA) and uric acid (UA) with peak‐to‐peak separation enabling determination of NEP, AA and UA in the presence of each other.     

Epinephrine Oxidation in the Presence of Interfering Molecules on Gold and Gold Electrodes modified with Gold Nanoparticles and Thiodipropionic Acid in Aqueous Solution. A Comparative Study

Thiodipropionoc acid (TDPA), cysteamine (CA) and gold nanoparticles (Au‐NPs) modified gold pure electrodes have been applied in voltammetric sensors for simultaneous detection of epinephrine (EP), ascorbic (AA) and uric (UA) acids. Modified electrodes with self assembled layers (SAMs) show high selectivity, sensitivity, reproducibility and stability. A linear relationship between the epinephrine concentration and the current response is obtained in the range of 0.1 μM to 0.65 μM with the detection limit ≤0.065 μM for the electrodes modified at 2D surface and in the range of 0.1 μM to 0.75 μM with the detection limit ≤0.082 μM for the electrodes modified at the 3D surface.     

Determination of Norepinephrine Alone and in the Presence of Ascorbic and Uric Acids Using a Gold Electrode Modified with Gold Nanoparticles and Self‐Assembled Layers of meso‐2,3‐Dimercaptosuccinic Acid

This paper reports on the modification of gold electrodes with self‐assembled layers (SAMs) composed of meso‐2,3‐dimercaptosuccinic acid, cysteamine and gold nanoparticles, respectively and their application to quantitative determination of norepinephrine alone and in the presence of ascorbic and uric acids in solution at pH 7. The modification was carried out on two kinds of templates: a bare gold electrode (2D electrode) and a gold electrode coated in the first step with gold nanoparticles (3D electrode). Cyclic voltammograms reveal an enhancement of the norepinephrine electrooxidation in comparison to a bare, (non‐modified) gold electrode. The oxidation peaks for norepinephrine, ascorbic acid and uric acid have a peak‐to‐peak separation that enables their selective determination even in a complex mixture.     

Gold and Nanogold Electrodes Modified with Gold Nanoparticles and meso‐2,3‐Dimercaptosuccinic Acid for the Simultaneous,Sensitive and Selective Determination of Dopamine and Its Biogenic Interferents

Gold and nanogold electrodes modified with self‐assembled layers composed of gold nanoparticles and organic sulfur compounds were applied for quantitative determination of dopamine and its biogenic interferents like: ascorbic and uric acids. For the novel sensor a linear relationship between the current response of dopamine at the potential of peak maximum and the concentration was found over a wide analyte concentration range in solution with a very good detection sensitivity and detection limit. It was proved that current peaks of dopamine and both ascorbic and uric acids were clearly separated from each other enabling selective detection of these compounds coexisting in a mixture in solution of pH 7.     

Electrocatalytic Application of an Overoxidized Dopamine Film Prepared on a Gold Electrode Surface to Selective Epinephrine Sensing

A dopamine polymer film was prepared ex situ on a bare gold template from a 10 mM dopamine solution in phosphate buffer, pH 7 followed by an overoxidation in 500 mM NaOH. The modified electrode was used for quantitative determination of epinephrine. A linear relationship between epinephrine concentration and current response was obtained in the range between 2 μM and 800 μM with the detection limit of 0.3 μM. The results have shown that using the overoxidized dopamine film it is possible to perform electrochemical analysis of epinephrine without interference of ascorbic and uric acids.     

Electrochemical Sensor for the Determination of Dopamine in Presence of High Concentration of Ascorbic Acid Using a Fullerene‐C60 Coated Gold Electrode

A fullerene‐C₆₀‐modified gold electrode is employed for the determination of dopamine in the excess of ascorbic acid using square‐wave voltammetry. Based on its strong catalytic function towards the oxidation of dopamine and ascorbic acid, the overlapping voltammetric response of both the biomolecules at the bare electrode is resolved into two well‐defined voltammetric peaks with lowered oxidation potential and enhanced oxidation currents. Linear calibration curves for dopamine are obtained using square‐wave voltammetry over the concentration range 1 nM–5.0 μM in 0.1 M phosphate buffer solution at pH 7.2 with a correlation coefficient of 0.9931 and the detection limit (3σ) is estimated to be 0.26×10^?9 M. The interference studies showed that the presence of physiologically common interferents (i.e. uric acid, citric acid, tartaric acid, glucose and sodium chloride) negligibly affects the response of dopamine. The practical analytical utility of the method is illustrated by quantitative determination of dopamine in commercially available pharmaceutical formulation and human body fluids, viz. urine and blood plasma, without any preliminary treatment.     

Cathodized Gold Nanoparticle‐Modified Graphite Pencil Electrode for Non‐Enzymatic Sensitive Voltammetric Detection of Glucose

A highly sensitive enzymeless electrochemical glucose sensor has been developed based on the simply prepared cathodized gold nanoparticle‐modified graphite pencil electrode (AuNP‐GPE). Cyclic voltammetry (CV) experiments show that AuNP‐GPE is able to oxidize glucose partially at low potential (around −0.27) whereas the bare GPE cannot oxidize glucose in the entire tested potential windows. Besides, fructose and sucrose cannot be oxidized at potential lower than +0.1 V at AuNP‐GPE. As a result, the glucose oxidation peak at around −0.27 V is suitable enough for selective detection of glucose in the presence of fructose and sucrose. Cathodization of AuNP‐GPE under optimum condition (‐1.0 V for 30 s) in the same glucose solution before voltammetric measurement enhanced glucose oxidation peak current around −0.27 V to achieve an efficient electrochemical sensor for glucose with a detection limit of 12 μM and dynamic range between 0.05 to 5.0 mM with a good linearity (R²= 0.999). Almost no interference effect was observed for sensing of glucose in the presence of ascorbic acid, alanine, phenylalanine, fructose, sucrose, and NaCl.     

1,4‐Michael Addition – The Analytical Way for Quantitative Sensing of Neurotransmitter at Bare Gold Electrode in Physiological Solution in the Presence of Interfering Biogenic Compounds

Electrooxidation of norepinephrine in the presence a nucleophile was investigated on a bare gold electrode. Electrochemically produced norepinephrinequinone undergoes an attack by morpholine as nucleophile via 1,4‐Michael addition. The reaction products were identified by electrospray ionization mass spectrometry. The procedure used was suitable for quantitative norepinephrine determination in the concentration range from 1×10^?9 M to 8×10^?4 M with a detection limit of 8.7×10^?10 M in a samples containing an excess of ascorbic and uric acids. The proposed method is simple, green which means that it does not require the use of toxic compounds and solvents and thus is promising for detection of norepinephrine in physiological environment.     

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 Study of Gold Microelectrodes Modified with PEDOT to Quantify Uric Acid in Milk Samples

A gold microelectrode (10 μm diameter) with an electropolymerized layer of poly(3,4‐ethylenedioxythiophene) (PEDOT) was used to quantify uric acid and investigate the antioxidant profile of milk and flavored milks. Comparisons were made with a bare gold microelectrode and a PEDOT‐glassy carbon macroelectrode (3 mm diameter). Two different electropolymerization processes were undertaken in an aqueous and an organic solution, and superior polymer growth was observed for PEDOT polymerized in lithium perchlorate/propylene carbonate. In the presence of a ferri/ferrocyanide redox couple, diffusion‐controlled redox peaks were observed with the PEDOT‐gold microelectrode rather than the plateau current typical of a bare microelectrode. Likewise, an anodic peak for uric acid was observed at the high surface‐area PEDOT‐gold microelectrode, with evidence for pre‐adsorption of uric acid at the electrode. The linear concentration range for uric acid standards was from 6 to 200 μM, and the limit of detection, limit of quantification, and sensitivity were determined to be 7 μM, 24 μM, and 397 μAμM^?1cm², respectively. Cyclic voltammograms of chocolate and espresso flavored milks exhibited significant contributions from the phenolic compounds present. Peak separation was more clearly defined using the PEDOT‐microelectrode compared to a PEDOT‐glassy carbon macroelectrode.     

Amperometric Determination of Ascorbic Acid at an Electrodeposited Redox Polymer Film Modified Gold Electrode

A sensitive and selective electrochemical method for the determination of ascorbic acid was developed. It was shown that a hydrated osmium complex‐containing redox polymer film can be electrodeposited at the gold electrode and it exhibits excellent electrocatalytic activity towards the oxidation of ascorbic acid. In contrast to a bare gold electrode, the oxidation current of ascorbic acid increased greatly and the oxidation peak potential shifted negatively to about 0.01 V (vs. SCE) at the modified electrode. Amperometric measurements were performed at an applied potential of 0.01 V and a linear response was obtained in the range of 2–400 μM with a limit of detection (LOD) of 0.6 μM (S/N=3). The interference studies showed that the modified electrode exhibits excellent selectivity in the presence of large excess of uric acid and dopamine. The proposed procedure was successfully applied to the determination of ascorbic acid in human urine samples.     

Flow Injection Amperometric Detection at Enzyme‐Modified Gold Nanoelectrodes

Gold nanotubular electrode ensembles were prepared by using electroless deposition of the metal within the pores of polycarbonate particle track‐etched membranes. Glucose oxidase (Gox), used as a model enzyme, has been immobilized onto preformed self‐assembled monolayers (mercaptoethylamine or mercaptopropionic acid) on electroless gold via cross‐linking with glutaraldehyde or covalent attachment by carbodiimide coupling. Flow‐injection analysis systems in flow‐through or wall‐jet configurations using these Gox‐modified nanoelectrodes are described. The influence of different experimental parameters (i.e., applied potential, flow rate, interferents…?) on the analytical response of the sensor to glucose has been evaluated. Under optimized conditions, very reproducible results (standard deviations <4%, n=38) were obtained, linear calibration was achieved in the 2×10^?4 M to 3×10^?2 M concentration range and the detection limit was 2×10^?4 M. Moreover, no significant interferences from species like ascorbic and uric acids were observed at a potential of +0.9 V.     

Gold Surface Functionalization with S‐containing Organic Compounds and Gold Nanoparticles for Ethylene Glycol Electrooxidation

In this work a gold electrode modified with self‐assembled layers (SAMs) composed with organic S‐containing compound and gold nanoparticles was prepared. The electrode with SAMs endowed with gold nanoparticles gave the high catalytic effect for ethylene glycol (EG) electrooxidation in solution at pH 7. For this novel sensor a linear relationship between the current response of EG at the potential of peak maximum (j_p) and the concentration of this compound in solution (c_EG) was found over the range 0.1 µM to 0.7 M with the detection sensitivity j_p/c_EG equal to about 5 A cm^?2 mol^?1 dm³ (at v=0.1 V s^?1) and the detection limit of 0.046 µM.     

Electrocatalytic Oxidation of Dithionite at a Cobalt(II)tetrasulfonated Phthalocyanine and 5,10,15,20‐Tetrakis‐(4‐sulfonatophenyl)porphyrin Cobalt(II) Modified Gold Electrode in Alkaline Solution

In this paper it is shown that the charge transfer kinetics of the oxidation of sodium dithionite at a gold electrode in alkaline solution can be increased by modifying this electrode with cobalt(II)tetrasulfonated phthalocyanine, sodium salt (CoTSPc) or 5,10,15,20‐tetrakis‐(4‐sulfonatophenyl)porphyrin cobalt(II), tetrasodium salt (CoTSPor). Electrodeposition of these catalysts is discussed as well as the oxidation of sodium dithionite at these immobilized catalysts. Despite an observed acceleration in the charge transfer kinetics, sodium dithionite is still oxidized irreversibly. However, the improvement in kinetics has beneficial consequences in the field of electroanalysis of sodium dithionite. Comparison of the oxidation at bare gold and modified gold electrodes showed that with improved charge transfer kinetics at the modified electrodes a higher sensitivity and selectivity and a much lower detection limit is obtained.     

Superior Performance of a MoS2‐RGO Composite and a Borocarbonitride in the Electrochemical Detection of Dopamine,Uric Acid and Adenine

A MoS₂‐RGO composite and borocarbonitride (BC₅N) have been used as electrodes to selectively detect dopamine and uric acid in the presence of ascorbic acid. Both the electrodes show excellent eletrocatalytic activity towards the detection of dopamine, the detection limits being 0.55 μM and 2.1 μM in the case of MoS₂‐RGO and BCN respectively. MoS₂‐RGO shows a linear range of current over the 1–110 μM concentrations of dopamine, while BCN shows over the 2.3–20 μM range. BCN also exhibits satisfactory performance in the oxidation of uric acid with a detection limit of 3.8 μM and the linear range from 4 to 40 μM. The MoS₂‐RGO has also been used to detect adenine as well.     

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.     

Simultaneous Determination of Ascorbic Acid,Dopamine and Uric Acid Based on Gold Nanoparticles‐PTCA‐Cys Composites Modified Electrodes

In this study, a nanocomposite of 3, 4, 9, 10‐perylenetetracarboxylic acid and L‐cysteine (PTCA‐Cys) with satisfactory water‐solubility and film‐forming ability was prepared and worked as substrate for modifying the glassy carbon electrode. Then, gold nanoparticles (AuNPs) were immobilized to achieve a PTCA‐Cys‐AuNPs modified electrode which provided more reaction positions on the sensor. Scanning electron microscopy, transmission electron microscopy, cyclic voltammetry and different pulse voltammetry were employed to characterize the assembly process of the sensor. The constructed sensor displayed desirable sensitivity, selectivity and stability towards the simultaneous detection of ascorbic acid (AA), dopamine (DA) and uric acid (UA). Under the optimal experimental conditions, the oxidation peaks of AA, DA and UA appeared at 64, 240 and 376 mV, respectively. The corresponding linear response ranges were 3.2–435, 0.04–100 and 0.80–297 μM, and the detection limits were 1.1, 0.010 and 0.27 μM (S/N=3), respectively.     

Electrochemical Formation of Nanostructured Gold Surfaces on Glassy Carbon for the Determination of Dopamine

Nanostructured gold surfaces were prepared by potentiostatic, potentiodynamic or galvanostatic Au electrodeposition on glassy carbon electrodes. The nanostructured gold electrodes (nsAu/GC) were used for the determination of dopamine (DA) in aqueous media. A directly proportional relationship was found between the peak current for DA (obtained by square wave voltammetry, SWV) and its concentration for all cases. However, the best performance for DA determination was attained with potentiodynamically electrodeposited surfaces. The SWV peak current was linearly dependent on DA concentration up to 10 μM, with a detection limit (3σ) of 0.57 μM, and a correlation coefficient (r) of 0.9966. A study on the effect of common interfering species such as ascorbic acid (AA) and uric acid (UA) on DA determination was also carried out. The use of a nanostructured surface gives rise to peaks for AA and UA that appear at 0.15–0.20 V above the peak potential for DA. The detection limit obtained for dopamine is below 1 μM in the presence of 0.1 mM AA and 0.1 mM UA. Thus, nanostructuring of glassy carbon surfaces with gold conveniently and easily improves the detection of DA in the presence of their principal interfering species.     

Analytical Application of Pyramidal,Rodlike, and Spherical Gold Nanostructures: Simultaneous Detection of Ascorbic Acid and Uric Acid

Simultaneous detection of ascorbic (AA) and uric acid (UA) is developed at pyramidal (NP), rodlike (NR), and spherical (NS) gold nanostructures, due to their high electrocatalytic activities toward the oxidation of AA and UA. Unlike at bare gold electrode, the fouling resulted from the oxidized product of AA is eliminated at the nanostructured gold electrode. The voltammetric signals of AA and UA are completely separated with a potential difference of 216 mV, 158 mV and 195 mV, respectively, at the pyramidal, rodlike, and spherical gold surfaces. The experimental results reveal that solution pH effects the peak separation of AA and UA, acidic solution is more favorable for the simultaneous determination of AA and UA than neutral one, than alkaline one. The coexistence of a large excess of AA does not interfere with the voltammetric sensing of UA, vice versa. All the three kinds of nanostructured gold electrodes show excellent sensitivity, stability, selectivity, low detection limit, quick response and wide linear range in the repeated detection of AA and UA. The practical utility of the present nanostructured gold electrodes is demonstrating by determining the concentration of AA in fruit juice and UA in urine sample.     

Gold and Silver Micro‐Wire Electrodes for Trace Analysis of Metals

Micro‐wire electrodes were made from gold and silver wires (diameter: 25 μm; length: 3–21 mm) and sealed in a polyethylene holder; micro‐disk electrodes were made from the same wires and polished. The gold electrodes were electrochemically coated with mercury before use; the silver wires were used without coating. Comparative measurements demonstrated that the micro‐wire electrodes had much higher sensitivity, and a much (10–100×) lower limit of detection, than micro‐disk electrodes, and the sensitivity increased linearly with the area and length of the electrodes. Using a gold micro‐wire electrode of 21 mm and a deposition time of 300 s the limit of detection was 0.07 nM Pb in seawater of natural pH, compared to a limit of detection of 10 nM Pb (more than 100×greater) using a gold micro‐disk electrode of the same diameter. Using the silver micro‐wire electrode the limit of detection of lead was improved by a factor of 10 to 0.2 nM in acidified seawater. It is expected that the improved sensitivity of micro‐wire electrodes will lead to successful in situ detection of metals in natural waters.