The influence of gold nanoparticle modified electrode on the structure of mercaptopropionic acid self-assembly monolayer

The comparison of assembly structure and property of mercaptopropionic acid (MPA) self-assembled monolayers (SAMs) on gold nanoparticle modified electrode (nanogold electrode) with that on planar gold electrode was studied by cyclic voltammetry (CV). The electron transfer of through the MPA SAMs and Cu underpotential deposition (UPD) on MPA-covered electrode indicated that MPA molecules assemblied on the planar gold electrode could form a very compact layer, which could surpass the electron transfer K₃Fe(CN)₆ greatly, whereas on the surface on the nanogold, which curvature make the compact packing loose. The reductive desorption in 0.5 M KOH and oxidative desorption in phosphate buffer solution (PBS) (pH 6.8) showed that gold nanoparticles could enhance the Au-S bond and stabilize the MPA molecules.     

Comparison of electrochemical oxidation of epinephrine in the presence of interfering ascorbic and uric acids on gold electrodes modified with S-functionalized compounds and gold nanoparticles

Mercaptopropionic acid (MPA), gold nanoparticles (Au-NPs) and cystamine (CA) modified gold bare electrodes have been applied in voltammetric sensors for simultaneous detection of epinephrine (EP), ascorbic (AA) and uric (UA) acids. Modification of the electrode surface by self-assembled layers (SAMs) improves the reactivity of a gold electrode for EP oxidation remarkably. A linear relationship between the epinephrine concentration and the current response is obtained in the range of 0.1–700 μM with the detection limit ≥0.042 μM for the electrodes modified at 2D template and in the range of 0.1–800 μM with the detection limit ≥0.040 μM for the electrodes modified at 3D template. The results have shown that the overlapping voltammetric response of epinephrine, ascorbic and uric acids is well resolved at modified electrodes. The modified SAMs electrodes show high selectivity, sensitivity, reproducibility and stability.     

Electrochemical oxidation of alcoholamines at gold

Gold is a good electrocatalyst for alcoholamine oxidation in basic media. In this work the effect of alcoholamine concentration, electrolyte pH and potential scan rate on electrooxidation was studied. The adsorption of alcoholamines on a layer plays a significant role in the oxidation mechanism. The rate determining step of the process was found to be heterogeneous dehydrogenation of the alcoholamine molecule, involving electron transfer to the gold electrode and the formation of water molecule. The catalytic effect of the gold electrode on alcoholamine oxidation is higher than that observed both for aliphatic alcohol and amines.     

Study of the deposition and characterisation of a 5,10,15,20-tetrakis-(4-sulphonatophenyl)porphyrin Co(II) layer at gold surfaces in alkaline solution

In this paper, the electrodeposition of Co(II)TSPor at gold electrodes is described. It was found that this deposition is initially controlled by kinetic parameters leading to a nearly 100% coverage of the electrode surface. However, once formed a reorganisation of the layer occurs. Experimental evidence that Co(II)TSPor is deposited was provided by using Raman spectroscopy.     

Self-assembled gold nanoparticles modified ITO electrodes: The monolayer binder molecule effect

The fabrication of gold attached organosilane-coated indium tin oxide Au_NPs-MPTMS/ITO and Au_NPs-APTES/ITO electrodes [MPTMS = 3-(mercaptopropyl)-trimethoxysilane, APTES = 3-(aminopropyl)-triethoxysilane, ITO = indium tin oxide] was carried out making use of a well-known two-step procedure and the role played by the -SH and -NH₂ functional groups in the two electrodes has been examined and compared using different techniques. Information about particle coverage and inter-particle spacing has been obtained using trasmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM) whereas, bulk surface properties have been probed with UV-vis spectroscopy, CV and electrochemical impedance spectroscopy (EIS). The catalytic activity of the two electrodes has been evaluated studying the electrooxidation of methanol in alkaline conditions. The results obtained show that the NH₂ functionality in the APTES binder molecule favours the formation of isle-like Au nanoparticle aggregates that lead to both a higher electron transfer and electrocatalytic activity.     

Porphyrin-functionalized gold nanoparticles for selective electrochemical detection of peroxyacetic acid

Two layers of cationic iron(III) meso-tetrakis (N-methylpyridinum-4-yl)porphyrin (FeTMPyP) and anionic gold nanoparticles (GNPs) were alternately assembled on a poly(diallyldimethylammonium chloride)-wrapped carbon nanotube (PDDA-CNT)-modified electrode via electrostatic interactions. The porphyrin-functionalized gold nanoparticles were characterized by scanning electron microscopy and UV–vis absorption spectrometry. The (FeTMPyP–GNP)₂/PDDA-CNT modified electrode showed two stable and well-defined peaks at −0.112 V and −0.154 V, which were attributed to the GNP-accelerated redox process of Fe(III)TMPyP/Fe(II)TMPyP. The modified electrode possessed excellent electrocatalytic behavior for the reduction of peroxyacetic acid (PAA). The resulting biosensor exhibited a fast amperometric response to PAA (∼3 s), with a wide linear range from 2.5 × 10⁻⁶ M to 1.05 × 10⁻³ M and a detection limit of 0.5 μM at a signal-to-noise ratio of 3. More importantly, H₂O₂ did not interfere with the detection. Thus, this biosensor enabled highly sensitive detection of PAA without removing H₂O₂ and showed a promising potential in practical applications.     

Electrochemical behaviour of phenylethanolamine at gold in aqueous solution

The electrooxidation of phenylethanolamine (2-amino-1-phenylethanol) at a gold electrode in alkaline electrolyte has been studied. Measurement of the differential capacitance of the electric double layer versus the electrode potential has shown that the adsorption of phenylethanolamine at the gold-solution interface plays a significant role in the oxidation mechanism. The effect of amine concentration, electrolyte pH and potential scan rate on the electrooxidation is analysed.     

Activity of gold towards methylamine electrooxidation

The kinetics of methylamine oxidation at a gold electrode in contact with an alkaline electrolyte solution was studied. The adsorptive behaviour of substrate molecules was determined by changes in the differential capacitance of the double layer at the electrode/solution interface. The electrooxidation of methylamine occurs only in the potential range of gold oxide layer formation. The general reaction pathways of methylamine oxidation on the gold electrode is proposed.     

A nano-structured Ni(II)–ACDA modified gold nanoparticle self-assembled electrode for electrocatalytic oxidation and determination of tryptophan

A nano-structured Ni (II)/ACDA (2-amino-1-cyclopentene-1-dithiocarboxylic acid) film was electrodeposited on a gold nanoparticle–cysteine–gold electrode. The formation of Ni (II)/ACDA film and electrocatalytic oxidation of tryptophan on the surface of the modified electrode were investigated with cyclic voltammetric and chronoamperometric techniques. The hydrodynamic amperometry at rotating modified electrode was used for determination of tryptophan in the range of 0.085–43.0 μmol l⁻¹. The detection limit was found to be 23 nmol l⁻¹. The rate constant, transfer coefficient for the catalytic reaction and the diffusion coefficient of tryptophan in the solution were found to be 9.1 × 10² M⁻¹ S⁻¹, 0.52 and 1.09 × 10⁻⁵ cm² s⁻¹ respectively. It is worth noting that the as formed matrix in our work possesses a 3D porous network structure with a large effective surface area and high catalytic activity and behaves like microelectrode ensembles. The modified electrode indicated reproducible behavior and a high level stability during the experiments, making it particularly suitable for the analytical purposes.     

Electrochemical behaviour of benzylamine, 2-phenylethylamine and 4-hydroxyphenylethylamine at gold. A comparative study

The electrooxidation of benzylamine, 2-phenylethylamine and 4-hydroxyphenylethylamine (tyramine) at a gold electrode in contact with an alkaline electrolyte solution was studied. The effect of amine concentration, electrolyte pH and potential scan rate on the electrooxidation was analysed. The adsorption of amines on the gold-solution interface was found to play a significant role in the oxidation mechanism. The rate determining step was the heterogeneous dehydrogenation of the amine molecule, involving electron transfer to the gold electrode and the formation of a water molecule. The catalytic effect of the gold electrode on both benzylamine and 2-phenylethylamine oxidation is higher than that for tyramine.     

Protoporphyrin-modified gold surfaces for the selective monitoring of catecholamines

Protoporphyrin IX (PPIX) was immobilized on Au surfaces for the electrochemical oxidation of dopamine (DA) and epinephrine (EP). Two procedures for the immobilization were employed. One class of modified electrodes was prepared by immobilizing the PPIX into polypyrrole (PPy) films synthesized by anodic electropolymerization. The other procedure involved the immobilization of PPIX on an electrode modified by a 11-mercaptoundecanoic acid (MUA) self-assembled monolayer (SAM). The modified surfaces were characterized by cyclic voltammetry (CV).The efficiency of these modified surfaces for the electrochemical oxidation of DA and EP was investigated and compared. The peak potential for the oxidation of each analyte at the different electrode surfaces was determined by square-wave voltammetry (SWV).The dependence of the sensitivity of the electrodes for the detection of the catecholamines on the applied potential was also measured. The SAM-based electrodes showed better selectivity for detecting DA in the potential range between 0.2 and 0.4 V versus Pt (QRE). However, higher sensitivity and linearity were observed for the film-based electrodes.     

Electrooxidation of methanol on polycrystalline and single crystal gold electrodes

Oxidation of methanol has been investigated on polycrystalline and single crystal gold electrodes: Au(poly), Au(1 1 1) and Au(2 1 0), in acidic, neutral and alkaline solutions. As expected, catalytic activity of gold towards methanol oxidation increases with increasing pH of the solution. It has been found that in all studied solutions methanol is oxidised in two potential regions, prior to gold surface oxide monolayer formation and in more positive potentials, on gold surface oxide after so called “turn over”. Surface structure of the electrode has little influence on the oxidation current, however potentials at which oxidation is observed depends on the crystallographic orientation. The mechanism of electro-oxidation of methanol on gold is discussed.     

Microbial deposition of gold nanoparticles by the metal-reducing bacterium Shewanella algae

Microbial reduction and deposition of gold nanoparticles was achieved at 25 °C over the pH range 2.0-7.0 using the mesophilic bacterium Shewanella algae in the presence of H₂ as the electron donor. The reductive deposition of gold by the resting cells of S. algae was a fast process: 1 mM AuCl₄⁻ ions were completely reduced to elemental gold within 30 min. At a solution pH of 7, gold nanoparticles 10-20 nm in size were deposited in the periplasmic space of S. algae cells. At pH 2.8, gold nanoparticles 15-200 nm in size were deposited on the bacterial cells, and the biogenic nanoparticles exhibited a variety of shapes that included nanotriangles: in particular, single crystalline gold nanotriangles 100-200 nm in size were microbially deposited. At a solution pH of 2.0, gold nanoparticles about 20 nm in size were deposited intracellularly, and larger gold particles approximately 350 nm in size were deposited extracellularly. The solution pH was an important factor in controlling the morphology of the biogenic gold particles and the location of gold deposition. Microbial deposition of gold nanoparticles is potentially attractive as an environmentally friendly alternative to conventional methods.     

Anomalous Helmholtz-capacitance on stepped surfaces of silver and gold

We have studied the capacitance of the solid/electrolyte interface on Ag(11n) and Au(11n) surfaces in KClO₄ and HClO₄ as function of the electrolyte concentration and the step density. We find that the inner layer capacitance (Helmholtz-capacitance) at the potential of (total) zero charge is dramatically reduced on stepped surfaces. Standard theories which describe the Helmholtz-capacitance by properties of the liquid, a hard wall boundary condition and the polarizability of the electron gas at the metal surface fail to describe this behavior. We propose that the different polarizability of water bonded to the surface need be taken into account and attribute the reduced capacitance at steps to the lower polarizability of water molecules bonded to step edges.     

Electrocatalytic oxidation of chlorophenols by electropolymerised nickel(II) tetrakis benzylmercapto and dodecylmercapto metallophthalocyanines complexes on gold electrodes

This work reports on the use of nickel(II) tetrakis benzylmercapto (NiTBMPc) and dodecylmercapto (NiTDMPc) metallophthalocyanine complexes films on gold electrodes for the electrochemical oxidation of 4-chlorophenol (4-CP) and 2,4,5-trichlorophenol (TCP). Both NiTBMPc and NiTDMPc complexes were successfully deposited on gold electrodes by electropolymerisation. The films were electro-transformed in aqueous 0.1 M NaOH solution to the ‘O-Ni-O oxo’ bridged form. For both complexes, films with different thickness were prepared and characterised by electrochemical impedance and UV-vis (on indium tin oxide) spectroscopies and the results showed typical behaviour for modified electrodes with increasing charge transfer resistance values (R_p) with polymer thickness. The poly-Ni(OH)NiPcs showed better catalytic activity than their poly-NiPcs counterparts.     

Immobilization of amines at carbon fiber surfaces

The reaction between amines acting as nucleophiles and the C=C bonds on the carbon fiber surface acting as electrophilic vinyl groups has not yet been explored. In this contribution it is demonstrated that both thermal reactions and electrochemical oxidation of amines at carbon fibers allow the covalent bonding of these molecules directly to the carbon fiber surface, presumably via nucleophilic attack of the amine at electrophilic C=C sites at the surface and subsequent formation of C–N bonds between the surface and the amine. A novel strategy for a quantitative assay of the number of amines attached to the surface is developed in which Fe(CN)₆³⁻ is electrostatically bound to the protonated, cationic amine sites, followed by electrochemical determination of the amount of bound Fe(CN)₆³⁻ as a function of its concentration in solution. Analysis of the isotherm for this electrostatic binding process then provides a measure of the number of interfacially immobilized amines. The composition of the amine layer is also probed using X-ray photoelectron spectroscopy (XPS). Mechanisms are discussed by which attachment of amines at the electrophilic vinyl groups of the carbon fibers can occur. The likely influence that this type of reaction has on the interfacial shear strength in carbon fiber/epoxy composite materials is also discussed.     

Electrocatalytic oxidation of ascorbic acid using a single layer of gold nanoparticles immobilized on 1,6-hexanedithiol modified gold electrode

This paper describes the electrocatalytic oxidation of ascorbic acid (AA) in phosphate buffer solution by the immobilized citrate capped gold nanoparticles (AuNPs) on 1,6-hexanedithiol (HDT) modified Au electrode. X-ray photoelectron spectrum (XPS) of HDT suggests that it forms a monolayer on Au surface through one of the two SH groups and the other SH group is pointing away from the electrode surface. The free SH groups of HDT were used to covalently attach colloidal AuNPs. The covalent attachment of AuNPs on HDT monolayer was confirmed from the observed characteristic carboxylate ion stretching modes of citrate attached with AuNPs in the infra-red reflection absorption spectrum (IRRAS) in addition to a higher reductive desorption charges obtained for AuNPs immobilized on HDT modified Au (Au/HDT/AuNPs) electrode in 0.1 M KOH when compared to HDT modified Au (Au/HDT) electrode. The electron transfer reaction of [Fe(CN)₆]^4−/3− was markedly hindered at the HDT modified Au (Au/HDT) electrode while it was restored with a peak separation of 74 mV after the immobilization of AuNPs on Au/HDT (Au/HDT/AuNPs) electrode indicating a good electronic communication between the immobilized AuNPs and the underlying bulk Au electrode through a HDT monolayer. The Cottrell slope obtained from the potential-step chronoamperometric measurements for the reduction of ferricyanide at Au/HDT/AuNPs was higher than that of bare Au electrode indicating the increased effective surface area of AuNPs modified electrode. The Au/HDT/AuNPs electrode exhibits excellent electrocatalytic activity towards the oxidation of ascorbic acid (AA) by enhancing the oxidation peak current to more than two times with a 210 mV negative shift in the oxidation potential when compared to a bare Au electrode. The standard heterogeneous electron transfer rate constant (k_s) calculated for AA oxidation at Au/HDT/AuNPs electrode was 5.4 × 10⁻³ cm s⁻¹. The oxidation peak of AA at Au/HDT/AuNPs electrode was highly stable upon repeated potential cycling. Linear calibration plot was obtained for AA over the concentration range of 1–110 μM with a correlation coefficient of 0.9950. The detection limit of AA was found to be 1 μM. The common physiological interferents such as glucose, oxalate ions and urea do not show any interference within the detection limit of AA. The selectivity of the AuNPs modified electrode was illustrated by the determination of AA in the presence of uric acid.     

A hydrogen peroxide sensor based on the peroxidase activity of hemoglobin immobilized on gold nanoparticles-modified ITO electrode

A novel ITO electrode surface modified with spherical and rod-shaped gold nanoparticles was prepared by a surfactant-assisted seeding growth approach, which provided a biocompatible matrix for the immobilization of hemoglobin (Hb). By electrochemical impedance measurements, gold nanoparticles modification and Hb immobilization on the electrode surfaces were characterized using [Fe(CN)₆]³⁻/[Fe(CN)₆]⁴⁻ redox probe. Owing to the promoted electron transfer of Hb by gold nanoparticles, the Hb immobilized gold nanoparticles-modified ITO (Hb/Au/ITO) electrode exhibited an effective catalytic response to the reduction of H₂O₂ with good reproducibility and stability. The linear relationship existed between the catalytic current and the H₂O₂ concentration in the range of 1 × 10⁻⁵ to 7 × 10⁻³ M. The detection limit (S/N = 3) was 4.5 × 10⁻⁶ M.     

Hydrodynamic voltammetric studies of the oxygen reduction at gold nanoparticles-electrodeposited gold electrodes

The electrocatalytic reduction of oxygen at Au nanoparticles-electrodeposited Au electrodes has been studied using rotating disk electrode (RDE) voltammetry in 0.5 M H₂SO₄. Upon analyzing and comparison of the limiting currents data obtained at various rotation speeds of this RDE with those obtained at the bulk Au electrode, an effective value of the number of electrons, n, involved in the electrochemical reduction of O₂ was estimated to be ca. 4 for the former electrode and ca. 3 for the bulk Au electrode at the same potential of −350 mV versus Ag/AgCl/KCl(sat.). This indicates the higher possibility of further reduction and decomposition of H₂O₂ at Au nanoparticles-electrodeposited Au electrode in this acidic medium. The reductive desorption of the self-assembled monolayer of cysteine, which was formed on the Au nanoparticles-electrodeposited Au electrode, was used to monitor the change of the specific activity of the bulk Au electrode upon the electrodeposition of the Au nanoparticles.     

Electrocatalysis of gold nanoparticles/layered double hydroxides nanocomposites toward methanol electro-oxidation in alkaline medium

A nanocomposite based on layered double hydroxides (LDHs) and gold nanoparticles (AuNPs) was prepared via hydrothermal treatment followed by a reduction procedure. The AuNPs were obtained in Mg-Al LDHs, and they maintained good stability. The electrocatalytic activities of AuNPs/LDH-modified glassy carbon electrodes for methanol oxidation in alkaline medium were investigated in detail. Under the same conditions, the modified electrode exhibited higher electrocatalytic activity than both the pure AuNPs-modified electrode and LDH-modified electrode. The role of the AuNPs and LDHs in this composite system was explored by cyclic voltammetry and chronoamperometry, respectively. Further studies demonstrated that the promoting effect of LDHs could be due to its strong adsorption and partly to the discharge of OH⁻ during methanol oxidation. This work indicates that LDHs is expected to be a good supporting material in the development of methanol anode catalysts.