Preparation,Characterization, and Electrochemical Behaviors of Self-Assembled Monolayer on Gold Electrode Surface with Mercapto-Derivatized Cobaltous Porphyrin

Abstract

5-[4-(3-Mercaptopropyloxy)phenyl]-10,15,20-tris (2-chlorphenyl) porphyrin was utilized for fabricating a self-assembled monolayer (SAM) on a gold electrode. Then cobalt(II) was inserted into the the monolayer by refluxing the cobalt(II) solution in which the pre-assembled porphyrin-modified electrode was immersed. The monolayers were characterized by cyclic voltammetry (CV). Oxidation and catalytic mechanisms of dopamine on the modified electrode were also investigated and elucited respectively. Catalytic currents increased linearly with dopamine (DA) concentration in the range of 1.0 × 10^?9 to 2.0 × 10^?3 mol/L with the correlation coefficient of 0.9989. The modified electrode can be help develop a simple, quick, sensitive, and accurate method for the determination of the biomolecules such as dopamine.     

Joint voltammetric determination of dopamine and uric acid

Electrocatalytic activities of gold particles deposited onto an electrode unmodified and modified with a self-organized monolayer of cystamine in oxidation of dopamine and uric acid were compared. The possibility of joint determination of dopamine and uric acid on an electrode modified with self-organized monolayer of cystamine with gold nanoparticles was examined.     

Electroanalytical applications of cationic self-assembled monolayers: square-wave voltammetric determination of dopamine and ascorbate

Gold electrodes modified with cationic self-assembled monolayers (SAMs) of 2,2'-dithiobisethaneamine (CYST) and 6,6'-dithiobishexaneamine (DTH) were used for the simultaneous determination of dopamine (DA) and ascorbate (AA). The cationic SAM modified electrodes have several advantages over the bare electrode for the oxidation of AA. A very large (approximately 450 mV) decrease in the overpotential for the oxidation of AA when compared with the bare electrode has been observed at the cationic monolayer-modified electrode. The electrostatic interaction of negatively charged AA with the monolayer shift the oxidation peak potential of AA to less positive potential and enhances the peak current. On the other hand, the positively charged DA is repelled from the monolayer and the oxidation potential shifts to more positive potential when compared to the bare electrode. The electrochemical oxidation of AA at the mixed monolayer of CYST and diethyl disulfide (DEDS) supports the influence of cationic terminal group of the monolayer on the oxidation of AA. Since the oxidation of AA occurs well before the oxidation potential of DA is reached, the homogeneous catalytic oxidation of AA by the oxidized DA has been advantageously eliminated at the monolayer-modified electrode. The cationic self-assembled monolayers successfully detect DA in the presence of high concentration of AA. The sensitivity of the electrode modified with CYST monolayer was found to be 0.036 and 0.021 microA/microM towards AA and DA, respectively.     

多巴胺在2-氨基-5-巯基-[1,3,4]三氮唑自组装膜电极上的准可逆行为研究及分析应用

利用新合成的巯基试剂2-氨基-5-巯基-[1，3，4]三氮唑在金电极表面进行了首次自组装，用电化学法和扫描电子显徽镜对自组装膜电极进行了表征。研究了多巴胺在该自组装膜电极上的电化学行为，发现该自组装膜能有效促进多巴胺在电极与溶液之间的电子传递．表现为二电子传递的准可逆行为，电极反应速率常数为0．105cm／s。该自组装膜电极用于多巴胺注射液含量的测定，结果满意。     

A catechol-terminated self-assembled monolayer at the surface of a gold electrode and its application for the electrocatalytic determination of dopamine

An electroactive self-assembled monolayer was fabricated by covalent attachment of protocatechuic acid at a gold surface and its electrochemical behavior was investigated using cyclic voltammetry. The modification involves a three-step method: (i) preparation of a cysteamine self-assembled monolayer, (ii) activation of the carboxylic groups of protocatechuic acid in solution and (iii) modification of cysteamine self-assembled monolayers by activated protocatechuic acid to functionalize the self-assembled monolayer by catechol-terminated groups. This resulting thin film modified electrode was tested successfully for the electrocatalytic determination of dopamine in aqueous solution.     

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.     

Dopamine Oxidation at Per(6‐deoxy‐6‐thio)‐α‐Cyclodextrin Monolayer Modified Gold Electrodes

The gold electrode is functionalized by sequential self‐assembly of a monolayer of the title thiolated cyclodextrin with and without dopamine included in the cavities. The structure of α‐cyclodextrin modified gold electrode is carefully characterized using STM and AFM. Surface complexation of dopamine is examined and its association constant is evaluated. Chemical reactions accompanying the electrode process of dopamine, which interfere in the electrochemical dopamine determination, are described and the conditions to avoid them are proposed. Dopamine incorporated in the α‐cyclodextrin sites anchored to the electrode surface was found to provide electrochemical contact of the electrode with the solution‐resident dopamine. Dopamine present in the α‐cyclodextrin cavities has different properties compared to dopamine in the bulk buffer solution and can act as a mediator for the dopamine molecules diffusing to the electrode. This unique mediation effect leads to improvement of the sensitivity of dopamine determination using the α‐cyclodextrin modified electrode and a procedure for the determination of dopamine in large excess of ascorbate is proposed.     

Electroanalysis of dopamine at a gold electrode modified with N-acetylcysteine self-assembled monolayer

Voltammetric behavior of dopamine (DA) on a gold electrode modified with the self-assembled monolayer (SAM) of N-acetylcysteine has been investigated, and one pair of well-defined redox peaks of dopamine is obtained at the SAM modified gold electrode. The oxidation peak current increases linearly with the concentration of dopamine in the range of  mol l⁻¹. The detection limit is 8.0×10⁻⁷ mol l⁻¹. This method will be applicable to the determination of dopamine in injection of dopamine hydrochloride, and the good recovery of dopamine is obtained. Furthermore, The SAM modified gold electrode can resolve well the voltammetric responses of dopamine and ascorbic acid (AA), so it can also be applied to the determination of dopamine in the presence of ascorbic acid.     

电容滴定法测定自组装膜表面酸度

利用本实验室自制的电容法时测量装置对巯基丙酸自组装膜的表面酸度进行电容滴定研究，相对于先前的测量和滴定技术。本方法可同时对界面电容值和溶液的PH值进行实时测定。同时对影响电容测定和自组装膜表面pKa值的各种因素进行了详细的讨论。 滴定结果和文献报道相近，证明了测定方法和仪器的可靠性实用性。     

Potential driven deposition of poly(diallyldimethylammonium chloride) onto the surface of 3-mercaptopropionic acid monolayers assembled on gold

Electrochemical impedance spectroscopy (EIS) and quartz crystal microbalance (QCM) measurements are used to examine the ability of applied potential to drive the ionic self-assembly of poly(diallyldimethylammonium) chloride (PDDA) onto a substrate modified with a monolayer of 3-mercaptopropionic acid (3-MPA). The potential of zero charge (PZC) of the gold electrode modified with a monolayer of 3-MPA was found by differential capacitance measurements to be -0.12 (+/-0.01) V versus Ag-AgCl. Changing the substrate potential to values positive (-0.01 V vs Ag-AgCl) of the PZC induces interfacial conditions that are favorable for the electrostatic deposition of cationic polymers onto the surface of 3-MPA monolayers. This result is also consistent with experimental observations obtained when the 3-MPA-modified substrate is exposed to 0.10 mol L (-1) NaOH solutions. When potentials equal or negative to the PZC are applied to the substrate, no significant accumulation of the PDDA is found by either QCM or EIS measurement. This result is consistent with results obtained when the 3-MPA modified substrate is exposed to 0.10 mol L (-1) HCl solutions where no PDDA adsorption is expected because the monolayer is neutral under these conditions. Changes in the impedance and quartz crystal frequency obtained after potential is applied to the substrate are interpreted in terms of the applied potential creating interfacial conditions that are favorable for the deprotonation of the terminal carboxylic acid groups and the subsequent electrostatic assembly of the polycation onto the negatively charged monolayer.     

基于有序生物膜的电容型层胶粘蛋白新型免疫传感器

免疫传感器以其高度特异性抗原抗体结合反应的特点 ,可直接、快速而灵敏地测定抗原或抗体分子而被广泛应用于临床医学以及环境污染物检测等领域 [1] .目前 ,检测抗原抗体结合的无标志方法主要有石英晶体微天平 [2 ] 、表面等离子共振 [3 ] 、界面电容 [4 ] 及原子力显微镜 [5]     

Langmuir–Blodgett monolayer modified electrodes based on thiol-derived bifunctional alkanes

The well-known strong affinity of the OH terminal group towards water and specific interaction of the thiol terminal group with gold lead us to propose the use of bifunctional α,ω-hydroxyalkanethiol for the controlled deposition of monolayers on electrodes by the modified Langmuir–Blodgett method. The procedure allows us to cover the metal substrates with a well-ordered and blocking monolayer of bifunctional α,ω-hydroxyalkanethiol. Cyclic voltammetry was used to probe the Langmuir–Blodgett monolayer modified electrode.     

金电极上L-半胱氨酸短链分子自组装单层的电化学性能和拉曼光谱研究

用循环伏安法分别测定了金电极表面L-半胱氨酸(L-Cys)和十二硫醇自组装单分子层的电化学行为, 实验发现虽然单层结构排列致密, 但并不能有效地阻碍铁氰化钾与电极间异相电子转移过程, 同时观察到十二烷基硫醇自组装层能较好地阻碍电子转移作用. 运用表面增强拉曼散射光谱技术, 以十二烷基硫醇作为缺陷探针, 从分子水平上证实了L-半胱氨酸自组装单层的稳定性和致密性.     

Light-harvesting and photocurrent generation by gold electrodes modified with mixed self-assembled monolayers of boron-dipyrrin and ferrocene-porphyrin-fullerene triad

Three different kinds of mixed self-assembled monolayers have been prepared to mimic photosynthetic energy and electron transfer on a gold surface. Pyrene and boron-dipyrrin were chosen as a light-harvesting model. The mixed self-assembled monolayers of pyrene (or boron-dipyrrin) and porphyrin (energy acceptor model) reveal photoinduced singlet-singlet energy transfer from the pyrene (or boron-dipyrrin) to the porphyrin on the gold surface. The boron-dipyrrin has also been combined with a reaction center model, ferrocene-porphyrin-fullerene triad, to construct integrated artificial photosynthetic assemblies on a gold electrode using mixed monolayers of the respective self-assembled unit. The mixed self-assembled monolayers on the gold electrode have established a cascade of photoinduced energy transfer and multistep electron transfer, leading to the production of photocurrent output with the highest quantum yield (50 +/- 8%, based on the adsorbed photons) ever reported for photocurrent generation at monolayer-modified metal electrodes and across artificial membranes using donor-acceptor linked molecules. The incident photon-to-current efficiency (IPCE) of the photoelectrochemical cell at 510 and 430 nm was determined as 0.6% and 1.6%, respectively. Thus, the present system provides the first example of an artificial photosynthetic system, which not only mimics light-harvesting and charge separation processes in photosynthesis but also acts as an efficient light-to-current converter in molecular devices.     

Effect of gramicidin on phospholipid-modified monolayers and on ion transfer at a liquid-liquid interface.

The interaction of hybrid lipid/gramicidin A (gA) monolayers with dextran sulfate (DS) and the effect of this interaction on ion transfer at a liquid-liquid interface is reported. The interfacial and physicochemical properties are studied with Langmuir-Blodgett (LB) and electrochemical techniques. The results obtained from compression isotherms demonstrate that the interactions between the different species in the hybrid monolayer vary according to the chemical nature of the lipid (hydrocarbon region and charge of the head group). Interfacial capacitance measured with AC voltammetry indicates that the DS chains form a rather flat and compact layer when adsorbed to either zwitterionic or negatively charged phospholipid monolayers, and that calcium, even at low concentrations, interacts with the monolayers. These results are successfully described by a model based on the solution of the Poisson-Boltzmann equation in the interfacial region. Ion transfer and interactions with the lipid/gA/DS-modified monolayers were also studied with electrochemical techniques. Admittance data show that although the studied ions are not using gA channels for the transfer through the lipid membranes, the incorporation of gA in the lipid domain and the adsorption of DS at the interface have a significant effect on ion transfer across the monolayers. This effect can be explained as a consequence of the modified surface charge and of the compactness of the lipid domain due to its interaction with gA and to calcium and DS adsorption at the interface. The ion-transfer rate, therefore, depends on the composition of the monolayer and the chemical nature of the ion.     

Electroactive dipyrromethene-Cu(II) self-assembled monolayers: complexation reaction on the surface of gold electrodes

In the work presented, thiol- and COOH-terminated dipyrromethene derivatives have been applied for gold electrode modification. Dipyrromethene deposited onto a solid support, after binding Cu2+, can act as a redox active monolayer. The complexation of Cu(II) ions has been performed on the surface of gold electrodes modified with dipyrromethene. The characterization of dipyrromethene-Cu(II) self-assembled monolayers (SAMs) has been done by cyclic voltammetry (CV), wettability contact angle measurements, and atomic force microscopy (AFM). The new electroactive monolayer could be applied for the immobilization of proteins and ssDNA or for electrochemical anion sensing without redox markers in the solution.     

Study of Factors Affecting the Performance of Voltammetric Copper Sensors Based on Gly‐Gly‐His Modified Glassy Carbon and Gold Electrodes

This paper reports a study of the factors affecting the analytical performance of gold and glassy carbon electrodes modified with the tripeptide Gly‐Gly‐His for the detection of copper ions. Gly‐Gly‐His is attached to a glassy carbon (GC) surface modified with 4‐carboxyphenyl moieties or a gold surface modified with 3‐mercaptopropionic acid by the reaction of the N‐terminal amine group of the peptide with the carboxylic acid groups of the monolayer via carbodiimide activation. X‐ray photoelectron spectroscopy was used to characterize the steps in the biosensor fabrication. It was found that the analytical performance of a sensor prepared with Gly‐Gly‐His on a GC electrode was similar to that on a gold electrode under the same conditions. The performance was greatly enhanced at higher temperature, no added salt during copper accumulation and longer accumulation time within a pH range of 7–9. Interference studies and investigations of stability of the Gly‐Gly‐His sensor are reported. Analysis of natural water samples show that the sensors measure only copper ions that can complex at the sensor surface. Strongly complexed copper in natural water is not measured. Despite greater stability of diazonium salt derived monolayers on carbon surfaces compared with alkanethiols self‐assembled monolayers on gold, the stability of the sensors was essentially the same regardless of the modification procedure.     

Electrochemical behavior of dopamine at a quercetin-SAM-modified gold electrode and analytical application

A stable quercetin–thioglycolic acid-modified gold electrode (Qu–TCA/Au) was prepared as a self-assembled monolayer (SAM) and its electrochemical behavior was investigated by electrochemical methods. In 0.05-M phosphate buffer solution (pH 7.0) quercetin exhibits quasi-reversible signals at the Qu–TCA/Au electrode. The stability of the quercetin-modified gold electrode is very good. The quercetin self-assembled monolayer is an effective mediator for the oxidation of dopamine, which was investigated by cyclic voltammetry and differential pulse voltammetry. Ascorbic acid does not interfere with determination of dopamine at an electrode modified with a mixture of quercetin–thioglycolic acid and quercetin–11-mercaptoundecanoic acid. This modification allows dopamine to be determined in the presence of ascorbic acid in the range from 3×10^–5 to 3×10^–4 M. The detection limit is 1×10^–6 M. Scanning electrochemical microscopy (SECM) was employed to study the electrochemical performances of the modified gold electrode indicating different feedback modes at differently modified surfaces.     

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.