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.     

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.     

Electrochemical Studies of Ochratoxin A Mycotoxin at Gold Electrodes Modified with Cysteamine Self‐Assembled Monolayers. Its Ultrasensitive Quantification in Red Wine Samples

The quantification of ochratoxin A is studied at cysteamine self‐assembled monolayer modified gold electrodes in red wine samples by square wave voltammetry. Detection and quantification limits of 0.004 µg L^?1 and 0.012 µg L^?1, respectively, were determined. The recovery percentages were in the range from 146 % to 94.0 % at spiking levels ranging from 0.02 to 5 µg L^?1. The variation coefficients for within‐laboratory repeatability varied from 31.4 to 11.5 % for spiked level from 0.02 to 2.0 µg L^?1. The developed electrochemical method is efficient, reproducible, and ultrasensitive for the quantification of OTA in red wine samples.     

Electron Transfer Behavior through Densely Packed Self‐Assembled Monolayers of a Novel Heteroaromatic Thiol Derivative onto the Gold Surface

This article is describing the electrical characteristics of the self‐assembled monolayers (SAMs) formed during spontaneous chemical adsorption of a recently synthesized heteroaromatic thiol 2‐(2‐mercaptophenylnitrilomethylidyne)‐phenol ( L ). Some surfactants were used to regulate the electron transfer through the resulting SAMs, as investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results revealed that the surface structure is approximately complete and fractional coverage is very close to unity. The use of surfactants clearly improved the electron transfer properties. Furthermore, complementary experiments were carried out to investigate the electron transfer from modified surface to cytochrome C (cyt‐C), as a biological iron containing protein, which exists in living cells with important life roles. It was found that cyt‐C is able to interact with the modified surface so that it can be used as a scaffold to study the electrochemical characteristics of sensitive biological compounds like proteins. The voltammetric behavior of the redox protein on the resulting SAMs was found to be highly reproducible, emphasizing the fact that the protein denaturation is greatly suppressed.     

Efficient Electrocatalytic Detection of Epinephrine at Gold Electrodes Modified with Self‐Assembled Metallo‐Octacarboxyphthalocyanine Complexes

Electrocatalysis of epinephrine at gold electrode pre‐modified with the self‐assembled monolayer of cysteamine and subsequently integrated with novel metallo‐octacarboxyphthalocyanine (MOCPc where M=Fe, Co and Mn) complexes (Au‐Cys‐MOCPc) was investigated. The electrodes showed response to the presence of epinephrine. The oxidation peak potential (E_p/V vs. Ag|AgCl, sat'd KCl) and charge transfer resistance (R_ct (kΩ)) in epinephrine solution depend markedly on the central metal of the phthalocyanine cores: Au‐Cys‐FeOCPck_ch=4.1×10⁷ M^?1 s^?1) which is higher than that of the Au‐cys‐CoOCPc or Au‐cys‐MnOCPc electrode. Mechanism, recognizing the mediation of the electrocatalytic process by the central M(II)/M(III) redox processes was proposed. Epinephrine electro‐oxidation at the Au‐cys‐FeOCPc electrode was studied in more details for the response characteristics. The diffusion coefficient of epinephrine was evaluated as (2.62±0.23)×10^?9 cm² s^?1. It was established that Au‐Cys‐FeOCPc is suitable for sensitive determination of epinephrine in physiological pH (7.40) conditions showing linear concentration range of up to 300 nM, with excellent sensitivity (0.53±0.01 nA nM^?1), and very low limits of detection (13.8 nM) and quantification (45.8 nM). The peak separation between ascorbic acid and epinephrine is large enough (190 mV) to permit simultaneous determination of both epinephrine and ascorbic acid in physiological pH 7.4 conditions using the Au‐cys‐FeOCPc electrode. Au‐cys‐FeOCPc electrode was successfully used for the determination of epinephrine in epinephrine hydrochloric acid injection with recovery of ca. 98.4%.     

Studies of Adsorption Kinetics and Defects of Self‐Assembled Thiol Monolayers on Gold by Capacitance Plane Plot

The capacitive property of an electrode/electrolyte interface can be described by complex capacitance. The capacitance plane plots (CPPs) of ideal polarized and kinetic controlled electrodes are derived based on the concept of complex capacitance. By using CPPs, the capacitance of electrode/electrolyte interface can be conveniently determined. In this work, CPPs obtained in ac impedance experiments are employed for the first time in studying the kinetics of adsorption process of the thiol monolayer. The coverage of octadecanethiol (ODT) monolayer on gold is examined as a function of adsorption time. The adsorption process of ODT molecules on gold exhibits two distinct phases: an initial rapid step followed by a slow one. The simple Langmuir model best explains our experimental data in the initial adsorption stage. CPPs and cyclic voltammetry (CV) indicate that, in the initial adsorption step, the ODT monolayer contains defects whose number decreases with the increasing of adsorption time.     

Electrochemical Study of Schiff Bases by Means of Self‐Assembled Monolayers

In this paper, Schiffbases were investigated using cyclic voltammetry (CV) and impedance electrochemical spectroscopy (EIS) techniques by means of self‐assembled monolayers for the first time, where a 0.1 M KCl solution and the redox couple of Fe(CN)₆^3?/Fe(CN)₆^4?were used as the electrolyte and probing‐pin, respectively. The monolayers formed by the employed Schiff base were proved to be relatively stable, and its electrochemical response in the studied system with different pH values was also de scribed clearly with CV and EIS plots. The results show that the monolayer of Schiff bases could exist in the solution with pH value from 2 to 10. In the EIS measurement in the concentration range from 10^?5 M to 5× 10^?4 M, a nearly linear relation ship between the charge transfer resistance (R_ct) and the logarithm concentration of Cu²+was observed, suggesting that Cu²+ could be titrated with the EIS method quasi‐quantitatively. The phenomenon agreed with the former report very well. Using the self‐assembled monolayers to study Schiff bases with the electrochemical method is the major contribution of our work.     

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 Characterization of Self‐Assembled Monolayer of a Novel Manganese Tetrabenzylthio‐Substituted Phthalocyanine and Its Use in Nitrite Oxidation

Manganese phthalocyanine MnPc(SPh)₄ has been synthesized and used to form self assembled monolayers on gold electrodes. The well packed SAM monolayer was characterized by analyzing the blocking of a number of Faradic processes by cyclic voltammetry, evaluating the electrical characteristics of the modified electrode by electrochemical impedance and imaging the modified surface by electrochemical scanning microscopy. Finally, MnPc(SPh)₄‐SAM modified electrode displayed an electrocatalytic behavior toward the oxidation of nitrite.     

Microstructures by Selective Desorption of Self‐Assembled Monolayer from Polycrystalline Gold Electrodes

Potential‐controlled partial reductive desorption of a self‐assembled monolayer of mercaptopropionic acid (MPA) formed on polycrystalline gold electrodes is used to expose subdomains of bare gold to the electrolyte solution. Two sets of cathodic waves are observed in the reduction scan with MPA self‐assembled on a polycrystalline gold electrode. The origin of the two waves is ambiguous but there are indications that the waves are correlated with reductive desorption from the (111) and then simultaneously (100) and (110) index faces of a polycrystalline gold electrode. Consecutive reduction scans with reversing the potential direction after the first peak (but before the onset of the second wave) results in disappearance of the first wave. The exposed domains are then blocked by an assembling process of longer chain alkanethiols to create a mixed self‐assembled monolayer on polycrystalline gold electrodes. Desorption of the remaining MPA creates a partially blocked electrode and the blocking behavior towards hexacyanoferrate(III) is analyzed using the theory of partially blocked electrodes and indicates an array of interacting centers. The approach of partial reductive desorption may be exploited for use in biosensing applications where the exposed gold domains could be used for anchoring of DNA probes.     

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%.     

Intermolecular Interactions in Mixed Self‐Assembled Monolayers of Ferrocene

Electrochemical characterization of mixed self‐assembled monolayers (SAMs) of 6‐ferrocenyl‐1‐hexanethiol (FcH) and mercaptoundecanoic acid tyrosinamide (MUATyr) on gold is reported. Single‐component SAMs of FcH presented repulsive intermolecular interactions (vGθ_T=?1.12), while mixed SAMs of FcH/MUATyr (1 : 1) exhibited attractive interactions (vGθ_T=+0.20), with a homogeneous distribution of both components. Electrochemical kinetic determinations on mixed SAMs of FcH/MUATyr, indicated a secondary electron transfer pathway between the redox centers of both components. Higher amounts of FcH in the mixed SAMs lowered the observed rate of electron transfer of MUATyr. The oxidation of FcH caused an anodic shift of 160 mV in the voltammetric wave of MUATyr.     

Electrochemical Characterization of Horse Spleen Ferritin Covalently Immobilized on Self‐Assembled Monolayer Modified Gold Electrodes

Horse spleen ferritin was covalently attached to SAM‐modified gold electrodes using cross‐linking agents. Reduction of ferritin occurs at negative potentials and is electrochemically irreversible. The voltammetry reveals the presence of a new electrochemical couple that has been determined to be a dissolved iron species released upon the reduction of ferritin. Covalently attached ferritin retains its ability to release iron as evidenced by the absence of the dissolved couple peaks when ferritin is reduced in the presence of nitrilotriacetate. As the SAM chain length increases, the reduction potential becomes more negative, suggesting a tunneling mechanism is involved in the electron transfer.     

Polycrystalline Gold Electrodes: A Comparative Study of Pretreatment Procedures Used for Cleaning and Thiol Self‐Assembly Monolayer Formation

The influence of different surface pretreatment procedures on the electrochemical response of a polycrystalline gold electrode was evaluated. Mechanical polishing with slurry alumina (M), chemical oxidation with H₂SO₄/H₂O₂ (C), electrochemical polishing (potential cycling between ?0.1 V and 1.2 V vs. SCE) (E), chemical reduction with ethanol, and combinations among these treatments were employed to change the surface electrode characteristics. The efficiency of the proposed pretreatments was evaluated by electrochemical responses towards the redox couple ferri(II/III)‐ammonium sulfate and by the formation of a self‐assembly monolayer of 3‐mercaptopropionic acid (3 MPA SAM) on gold electrodes. The procedure (C) allowed important gold surfaces activation. Using procedures (C) and (E) the roughness of polycrystalline gold surfaces was significantly minimized and more reproducible surfaces could be obtained. From the profile of reductive desorption of 3 MPA SAM it was possible to verify that reduced gold surfaces generated better packed monolayers than oxidized ones and a comparative study using CV and DPV techniques showed that between the two desorption peaks, the one localized at more negative potential values corresponds to the cleavage of Au‐S bond from the chemisorbed thiol. In general, the improvement in the studied electrochemical responses could not only be attributed to an increase in the real surface area of the electrode, but to the chemical surface states set off by the pretreatment procedure.     

A Lactulose Bienzyme Biosensor Based on Self‐Assembled Monolayer Modified Electrodes

A bienzyme biosensor in which the enzymes β‐galactosidase (β‐Gal), fructose dehydrogenase (FDH), and the mediator tetrathiafulvalene (TTF) were coimmobilized by cross‐linking with glutaraldehyde atop a 3‐mercaptopropionic acid (MPA) self‐assembled monolayer on a gold disk electrode, is reported. The working conditions selected were E_app=+0.10 V and (25±1) °C. The useful lifetime of one single TTF‐β‐Gal‐FDH‐MPA‐AuE was surprisingly long, 81 days. A linear calibration plot was obtained for lactulose over the 3.0×10^?5–1.0×10^?3 mol L^?1 concentration range, with a limit of detection of 9.6×10^?6 mol L^?1. The effect of potential interferents (lactose, glucose, galactose, sucrose, and ascorbic acid) on the biosensor response was evaluated. The behavior of the SAM‐based biosensor in flow‐injection systems in connection with amperometric detection was tested. The analytical usefulness of the biosensor was evaluated by determining lactulose in a pharmaceutical preparation containing a high lactulose concentration, and in different types of milk. Finally, the analytical characteristics of the TTF‐β‐Gal‐FDH‐MPA‐AuE are critically compared with those reported for other recent enzymatic determinations of lactulose.     

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.     

Attachment of Amine‐ and Maleimide‐Containing Ferrocene Derivatives onto Self‐Assembled Alkanethiol and Alkanedithiol Monolayers: Voltammetric Evaluation of Cross‐Linking Efficiencies and Surface Coverage of Electroactive Groups

Ferrocene derivatives containing primary amines and maleimide groups were attached covalently onto N‐hydrosuccinimidyl (NHS)‐terminated alkanethiol self‐assembled monolayers (SAMs) and SAMs of alkanedithiol. The surface coverage and efficiencies of the two cross‐linking reactions were evaluated with cyclic voltammetry. All the ferrocene derivatives attached onto the alkanethiol or alkanedithiol SAMs exhibit reversible redox waves. The surface coverage of the aminated ferrocene groups was compared to that of N‐hydrosuccinimidyl (NHS)‐terminated alkanethiol SAM. The covalent attachment of β‐ferrocenylethylamine onto a 11,11′‐dithio‐bis(succinimidylundecanoate) SAM yielded an efficiency as high as 63.1%. The cross‐linking efficiency of this reaction was found to increase with the nucleophilicity of the amino groups. SAMs of longer alkyl chains favor the attachment of a greater number of ferrocene derivatives. As for the Michael‐type electrophilic addition between the sulfhydryl groups of the alkanedithiol SAMs and the ferrocenyl maleimide, the cross‐linking efficiencies were found to range from 6.5% to 25.7%, depending on the alkanedithiol chain length. The difference in the efficiencies between the two types of cross‐linking reactions might be partially attributable to the steric hindrance imposed by the SAMs and the relative sizes of the functional groups.     

Molecular Suction Pads: Self‐Assembled Monolayers of Subphthalocyaninatoboron Complexes on Gold

Subphthalocyaninatoboron complexes with six long‐chain alkylthio substituents in their periphery are applicable for the formation of self‐assembled monolayers (SAMs) on gold. Such films are prepared from solution with the axially chlorido‐substituted derivatives and characterised by near‐edge X‐ray absorption fine structure (NEXAFS) spectroscopy, X‐ray photoelectron spectroscopy (XPS) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS). The results are in accord with the formation of SAMs assembled by the chemisorption of both covalently bound thiolate‐type as well as coordinatively bound thioether units. The adsorbate molecules adopt an essentially flat adsorption geometry on the substrate, resembling a suction pad on a surface.     

Self‐assembled Azobenzenethiol Monolayer on Electrode Surfaces: Effect of Photo‐switching on the Surface and Electrical Property

Azobenzenethiol molecules carrying different para‐substituents were used to form mixed monolayers with n‐alkanethiol molecules on Au and Ag surfaces. UV‐ and visible light irradiation of the surfaces resulted in reversible alternation of contact angle and characteristic infrared absorption peak intensities, as well as the work function of the metal surfaces. The alternations can be correlated with the cis‐trans isomerization of the azobenzene moieties at the surface. Electron transport from the metal electrode to a redox center in a contacting solution was measured and analyzed based on the change in the work function of the electrode as well as the monolayer film structure upon isomerization.     

Electrochemical Characterization of Gold 6‐Amino‐2‐mercaptobenzothiazole Self‐Assembled Monolayer for Dopamine Detection in Pharmaceutical Samples

A new sensor, gold‐6‐amino‐2‐mercaptobenzothiazole (6A2MBT), was fabricated via a self‐assembly procedure. Electrochemical properties of the monolayer were investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The modified electrode showed excellent antifouling property against the oxidation products of DA, allowed us to construct a dynamic calibration curve with two linear parts, 1.00×10^?6 to 3.72×10^?4 and 3.72×10^?4 to 6.42×10^?4 M DA, with correlation coefficients of 0.997 and 0.992 and a detection limit of 1.57×10^?7 M DA by using differential pulse voltammetry (DPV), respectively. Finally, the performance of the Au‐6A2MBT modified electrode was successfully tested for electrochemical detection of DA in a pharmaceutical sample.