Studies on interaction of porphyrin and its complexes with DNA at interface on gold electrode modified by thiol-porphyrin self-assembled monolayer

The interaction of 5-[p-(mercaptopropyloxy)-phenyl]-10, 15, 20-triphenylporphyrin (H₂MPTPP) and its metalloporphyrin (Co, Ni-MPTPP) with calf thymus deoxyribonucleic acid (DNA) has been studied on gold electrode modified by thiol-porphyrin self-assembled monolayer (SAM). The mode and characteristics of their interaction with DNA have been studied by cyclic voltammetry, scanning electrochemical microscope (SECM), and alternating current (AC) impedance. Some electrochemical parameters have been determined, i.e., apparent heterogeneous reaction rate constant (k _eff from SECM and k _f from AC impedance) and the hindrance (B) of electrode. K₃[Fe(CN)₆] was used as probe to obtain some electrochemical information of electrode interface. SECM images obtained from interface on SAM interacted with DNA showed very good resolution with different topography. Based on a comparison with the results from experiments, a reasonable agreement between SECM and AC impedance can be obtained, which means a conjunction of them. It is proposed to be electrostatic interaction of H₂MPTPP, Co-MPTPP and Ni-MPTPP with DNA, and the attractive force between porphyrins and DNA follows the order Ni-MPTPP > Co-MPTPP > H₂MPTPP.     

Electrochemical characterization of self-assembled thiol-porphyrin monolayers on gold electrodes by SECM.

Herein, the scanning electrochemical microscopy (SECM) approach is applied to study the formation of thiol-porphyrin self-assembled monolayer (SAMs). Using cyclic voltammetry (CV), the formation process is characterized adopting different probe molecules. The observed phenomena indicate that the formation process is affected by solution properties and the molecular structure of the probe molecules. In K(3)Fe(CN)(6) , the SAMs show a strong electron-transfer (ET) blocking effect on a pure porphyrin-modified electrode. However, addition of metal ions to the porphyrin molecules leads to ET. A consistent tendency is observed throughout the modification process using CV and SECM methods. Furthermore, k(eff) values, the apparent heterogeneous rate constants, obtained for different modification periods affirm the validity of these results. SECM images are used to collect surface information in the course of the modification process when the substrate potential is 0.5 V versus Ag/AgCl. The effect of the substrate potential indicates that the oxidation of the porphyrin molecules is supported by more positive potentials because of the similar bimolecular reaction of the porphyrin ring with positive charge and the probe molecules with negative charge.     

三维有序金掺杂纳米TiO_2修饰电极用于抗肿瘤药物与DNA相互作用的研究

利用模板法在氧化铟锡(ITO)电极表面制备了三维有序多孔结构的金掺杂纳米Ti O2薄膜修饰电极(3DOM GTD/ITO),并在此修饰电极上成功固定小牛胸腺DNA(ct DNA),从而构建了一种新型的DNA生物传感器(DNA/3DOM GTD/ITO),并通过透射电镜(TEM)、扫描电镜(SEM)对修饰电极的表面形貌进行表征。采用电化学交流阻抗(EIS)法研究了ct DNA在3DOM GTD/ITO修饰电极表面的固定情况,结果表明,ct DNA已被成功地固定在3DOM GTD/ITO修饰电极表面。采用循环伏安法、微分脉冲伏安法等电化学方法研究了抗肿瘤药物槲皮素(Qu)在3DOM GTD/ITO修饰电极表面的电化学性质及与ct DNA的相互作用。结果表明,Qu在3DOM GTD/ITO修饰电极表面有1对准可逆的氧化还原峰,其氧化还原反应为2电子和2质子的转移过程。Qu可与固定在修饰电极上的ct DNA发生较强的结合作用,其结合常数(K)为3.61×106L/mol。循环伏安实验、紫外-可见吸收光谱、分子荧光光谱、圆二色性光谱均表明Qu与ct DNA之间的相互作用模式为嵌插作用。Qu与ct DNA的碱基结合具有序列选择性,对Qu与聚(d G-d C)及聚(d A-d T)的结合常数进行计算,得到结合常数比K(d G-d C)/K(d A-d T)=3.5,表明Qu与ct DNA发生嵌插作用时更倾向于结合在GC富集区域。     

Scanning electrochemical microscopy study of ion transfer process across water/2-nitrophenyloctylether interface supported by hydrophobic carbon ceramic electrode

A carbon ceramic electrode (CCE) modified with the redox probe—decamethylferrocene solution in hydrophobic organic solvent—2-nitrophenyloctyl ether and immersed into an aqueous solution was studied by scanning electrochemical microscopy (SECM). After the electrochemical oxidation of decamethylferrocene, its cations were detected near the electrode surface in the aqueous phase. This indicates that some fraction of the redox-active cations electrochemically produced in the organic phase is transferred across the liquid/liquid interface. They are reduced at the SECM tip and form a solid deposit. The amount of deposited decamethylferrocene was estimated by the anodic reaction at the tip. It is affected by the substrate–tip distance, deposition time, and electrolyte concentration. The SECM images of unmodified and modified CCEs are consistent with their heterogeneous structure.     

Disposable Amperometric A-fetoprotein Immunosensor Based on the Biocompatible Silk Protein Membranes-Modified Indium Tin Oxide Electrodes

A simple and disposable electrochemical immunosensor for detection of 68 kDa alpha-fetoprotein (AFP) was fabricated based on films of silk fibroin protein membrane (SFPM)/Prussian blue (PB)/deposition of gold nanoparticles (DpAu). First, DpAu and PB were electrochemically deposited successively on the surface of indium tin oxide (ITO) electrode. Then, SFPM with excellent biocompatibility was modified on the surface of PB/DpAu/ITO. The SFPM could form a stable matrix on the electrode surface for the deposition of immunoactive agents. More importantly, the SFPM could prevent the possible leakage of electron mediator and enhance the stability of immunosensor. Scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were used to characterize the assembly process of the modified electrode. The linear range of the proposed immunosensor extended from 1.0 to 200.0 ng/mL for detection of AFP with a detection limit of 0.6 ng/mL. Moreover, the CV test demonstrated the immunosensor exhibited acceptable reproducibility and stability. This composite membrane could be applied for the detection of different biomarkers, diagnosis, and monitoring of carcinoma.

[Supplementary materials are available for this article. Go to the publisher's online edition of Analytical Letters for the following free supplemental resources: additional figures.]     

Chlorpromazine Electro‐oxidation at BDD Electrode Modified with nZVI Nanoparticles Impregnated NiAl LDH

Nanocomposite of nanoscale zero‐valent iron (nZVI) and layered double hydroxide (LDH) was used as modifier for boron‐doped diamond electrode in determination of anti‐psychotic drug chlorpromazine (CPZ). nZVI nanoparticles were prepared by liquid phase reduction of ferric chloride with sodium borohydride on the surface of NiAl LDH matrix owing to the strong exchange and confinement efficiency of LDH. The structure, binding and surface properties of the nZVI@LDH nanocomposite were monitored using powder X‐ray diffractometry, FT‐IR spectroscopy, scanning and transmission microscopy and BET techniques. The electrochemical properties of the modified electrode were investigated by CV and EIS, performed in a phosphate buffer containing ferro/ferricyanide as redox probe. The modified electrode exhibited excellent electrochemical performance compared with unmodified electrode. As regard potential application of the nanocomposite surface to the CPZ detection, square‐wave voltammetric signals showed a good linear correlation over CPZ concentrations in a broad range from 0.1 to 8.0 μM with low detection limit of 0.005 μM. Nevertheless, these results suggest that the proposed nanocomposite modifier surface provides exceptional synergy and significant enhancement effect to the voltammetric response of CPZ and thus could be applied as highly efficient and stable platform of sensors in clinical analysis.     

恒电位氧化改性石墨毡及其氧还原电极的电化学性能

分别采用循环伏安改性法和恒电位氧化法对石墨毡进行改性处理,并采用循环伏安法对其电化学性能进行研究,实验结果表明,恒电位氧化改性较循环伏安改性的石墨毡有较好的氧还原活性。通过XRD、FTIR、接触角和CV针对恒电位氧化处理石墨毡进行了进一步的测试。测试结果显示,随恒电位氧化时间的增加,石墨毡表面亲水性含氧官能团增加,润湿性增强。恒电位氧化改性处理25 min的石墨毡氧还原峰电位及电流密度分别为~-0.43 V和~0.003 4 mA·cm^-2,显示出很好的电化学催化性能。基于以上结果,恒电位氧化法改性处理能够极大提高石墨毡的氧阴极活性。     

恒电位氧化改性石墨毡及其氧还原电极的电化学性能

分别采用循环伏安改性法和恒电位氧化法对石墨毡进行改性处理,并采用循环伏安法对其电化学性能进行研究,实验结果表明,恒电位氧化改性较循环伏安改性的石墨毡有较好的氧还原活性。通过XRD、FTIR、接触角和CV针对恒电位氧化处理石墨毡进行了进一步的测试。测试结果显示,随恒电位氧化时间的增加,石墨毡表面亲水性含氧官能团增加,润湿性增强。恒电位氧化改性处理25 min的石墨毡氧还原峰电位及电流密度分别为~-0.43 V和~0.003 4 mA·cm^-2,显示出很好的电化学催化性能。基于以上结果,恒电位氧化法改性处理能够极大提高石墨毡的氧阴极活性。     

Visualization of the Local Catalytic Activity of Electrodeposited Pt–Ag Catalysts for Oxygen Reduction by means of SECM

Pt–Ag nanoparticle co‐deposits with different Pt–Ag ratios were prepared on a glassy carbon (GC) surface by pulsed electrodeposition and investigated for their catalytic activity in electrocatalytic oxygen reduction by using cyclic voltammetry (CV), rotating disc electrode (RDE) and scanning electrochemical microscopy (SECM) in 0.1 M phosphate buffer (pH 7.0). The atomic composition of the Pt–Ag co‐deposits was studied by means of energy‐dispersive X‐ray analysis (EDAX). In combination with X‐ray diffraction (XRD), the presence of partly alloyed Pt and Ag on the GC surface was confirmed. Scanning electron microscopy (SEM) images indicate that the prepared Pt–Ag catalyst particles are homogenously dispersed over the GC surface. Their size and morphology depend on their composition. The electrocatalytic activity of Pt–Ag deposits with high Pt content was the highest, exceeding even that of electrodeposited Pt as evaluated by quantitative RDE analysis. The redox competition mode of scanning electrochemical microscopy (RC‐SECM) was successfully used to visualize the local catalytic activity of the deposited Pt–Ag particles. Semi‐quantitative assessment of the SECM results confirmed the same order of activity of the different catalysts as the RDE investigations.     

Scanning Electrochemical Microscopy for Electrochemical Detection of Single‐base Mismatches by Tagging Ferrocenecarboxylic Acid as a Redox Probe to DNA

Scanning electrochemical microscopy (SECM) was employed for sensitive detection of single base mismatches (SBMs) in a sandwiched dsDNA. Ferrocenecarboxylic acid (Fc), covalently conjugated to the dsDNA, was oxidized to Fc⁺ via the DNA‐mediated charge transfer from the underlying gold substrate, and reduced back to Fc by SECM tip generated ferrocyanide. The electrocatalytic oxidation of SECM tip‐generated ferrocyanide was sensitive to presence, as well as the type of SBMs. Apparent standard rate constants (k⁰_app) values for different SBMs, both near the electrode surface and far from it, were evaluated by SECM. The method can detect SBMs independent of their position in dsDNA.     

Investigation of Active and Inactivated Yeast Cells by Scanning Electrochemical Impedance Microscopy

Scanning electrochemical microscopy (SECM), electrochemical impedance spectroscopy (EIS) and scanning electrochemical impedance microscopy (SEIM) were used to investigate electrochemical activity of active and inactivated yeast Saccharomyces cerevisiae cells. SEIM experiment was performed using a unique electrochemical impedance spectrometer with a fast Fourier transform (FFT‐EIS) function, which enabled simultaneously perturb/evaluate electrochemical system at 50 frequencies. This allowed very quick observing the differences between impedance spectra, which were taken every few seconds. Therefore, we were able to apply SEIM for relatively fast determination of electrochemical impedance dependence on the distance between ultramicroelectrode (UME) and surface modified by immobilized yeast cells. It was determined that electrochemical activity and ‘breathing’ (a consumption of dissolved oxygen) of yeast can be electrochemically observed when the distance between UME and surface of yeast cells is in the range from 0 μm to 25 μm. Therefore, 25 μm is the maximum distance suitable for efficient investigation of yeast cell activity when experiments are performed in FFT‐SEIM mode. Charge transfer resistance of active and inactivated yeast cells was determined using EIS. It was calculated that charge transfer resistance of active yeast cells is 1.5 times lower than that of inactivated yeast cells. Lipophilic vitamin K₃ (Vit‐K₃) and hydrophilic vitamin K₁ (Vit‐K₁) were mixtured and used as redox mediators for charge transfer from yeast cells.     

Local Activities of Hydroxide and Water Determine the Operation of Silver‐Based Oxygen Depolarized Cathodes

Local ion activity changes in close proximity to the surface of an oxygen depolarized cathode (ODC) were measured by scanning electrochemical microscopy (SECM). While the operating ODC produces OH^? ions and consumes O₂ and H₂O through the electrocatalytic oxygen reduction reaction (ORR), local changes in the activity of OH^? ions and H₂O are detected by means of a positioned Pt microelectrode serving as an SECM tip. Sensing at the Pt tip is based on the pH‐dependent reduction of PtO and obviates the need for prior electrode modification steps. It can be used to evaluate the coordination numbers of OH^? ions and H₂O, and the method was exploited as a novel approach of catalyst activity assessment. We show that the electrochemical reaction on highly active catalysts can have a drastic influence on the reaction environment.     

Cyclic voltammetric study on the catalysis of electrodeposited manganese oxide on the electrochemical oxygen reduction reaction (ORR) in room temperature ionic liquids (RTILs) of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF<Subscript>4</Subscript>) on glassy carbon clectrode

For the first time, the electrochemical oxygen reduction reaction (ORR), was investigated using cyclic voltammetry (CV) on the electrodeposited manganese oxide (MnO _x )-modified glassy carbon (MnO _x -GC) electrode in the room temperature ionic liquids (RTILs) of EMIBF₄, i.e. 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF₄). The results demonstrated that, after being modified by MnO _x on a GC electrode, the reduction peak current of oxygen was increased to some extent, while the oxidation peak current, corresponding to the oxidation of superoxide anion, i.e., O₂⁻ was attenuated in some degree, suggesting that MnO _x could catalyze ORR in RTILs of EMIBF₄, which is consistent with the results obtained in aqueous solution. To accelerate the electron transfer rate, multi-walled carbon nanotubes (MWCNTs) was modified the GC electrode, and then MnO _x was electrodeposited onto the MWCNTs-modified GC electrode to give rise to a MnO _x /MWCNTs-modified GC electrode, consequently, the improved standard rate constant, k_s, originated from the modified MWCNTs, along with the modification of electrodeposited MnO _x , showed us a satisfactory electrocatalysis for ORR in RTILs of EMIBF₄. Published in Russian in Elektrokhimiya, 2009, Vol. 45, No. 3, pp. 340–345. The article is published in the original.     

Cyclic Voltammetrically‐Prepared MnO2 Coated on an ITO Glass Substrate

For the first time, nanostructured manganese dioxide was successfully electrodeposited onto an ITO (indium tin oxide) glass substrate by cyclic voltammetry (CV) method from an aqueous solution of 0.1 M Na₂SO₄ containing 5 × 10⁻³ M MnSO₄. The obtained manganese dioxide‐modified ITO glass substrates were characterized by energy dispersive spectrometry (EDS), Fourier transform infrared spectrometry (FTIR) and scanning electron microscopy (SEM), respectively. All results not only proved the existence of MnO₂ on an ITO glass substrate but also demonstrated that the morphology of the obtained MnO₂ was greatly affected by the electrodeposition conditions. Also, this MnO₂‐modified ITO electrode was systematically investigated by cyclic voltammetry (CV), chronopotentiometry and electrochemical impedance spectroscopy (EIS) in an aqueous electrolyte of 0.1 M Na₂SO₄. The results obtained from electrochemical measurement indicated that this developed MnO₂‐modified ITO electrode has a satisfied specific capacitance value of 264 F·g⁻¹ and exhibits excellent electrochemical stability and reversibility.     

Study on Electrocatalytic Activity of Platinum for Hydrogen Oxidation in Acidic Solution by Scanning Electrochemical Microscopy

The electrocatalytic activity of platinum for hydrogen oxidation in 0.01 M H₂SO₄ + 0.1 MNa₂SO₄ solution has been investigated by scanning electrochemical microscopy (SECM) technique. The cyclic voltammogram (CV), approach curve, area scan imaging and chronoamperometric methods have been used. The results indicate that the imaging capability of the SECM feedback mode can be used more efficiently to visually identify materials' electrocatalytic activity, compared with the approach curve method for identification of the conductive or insulating nature of a surface. The SECM imaging method has demonstrated the effects of Pt substrate potential on the electrocatalytic oxidation of hydrogen under a constant tip potential. It is found that the more positive the Pt substrate potential, the lower the electrocatalytic activity of the Pt. Furthermore, the chronoamperometric results support the variation of the electrocatalytic activity with the Pt substrate potential as well.     

A novel nonenzymatic biosensor for evaluation of oxidative stress based on nanocomposites of graphene blended with CuI

A high-sensitive nonenzymatic hydrogen peroxide (H₂O₂) biosensor based on cuprous iodide and graphene (CuI/Gr) composites has been explored for the detection of H₂O₂ released by living cells and monitoring the oxidative stress of cells under excellular stimulation. The biosensor properties were evaluated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), amperometric i-t curve, and the redox-competition mode of scanning electrochemical microscopy (SECM). Our observations demonstrate that the CuI/Gr nanocomposites modified glassy carbon electrode (GCE) exhibits excellent catalytic activity for H₂O₂ with relatively low detection limit and a wide linear range from 0.5 μM to 3 mM. Moreover, the redox-competition mode of SECM imaging study further illustrates the improved electrochemical catalytic capability for H₂O₂ reduction with CuI/Gr nanocomposites deposited on graphite electrode. Hence, the as-prepared nonenzymatic H₂O₂ biosensor could be used to detect H₂O₂ release from different kinds of living cells under stimulation while eliminating the interference of ascorbic acid.     

Non‐destructive Patterning of Carbon Electrodes by Using the Direct Mode of Scanning Electrochemical Microscopy

Patterning of glassy carbon surfaces grafted with a layer of nitrophenyl moieties was achieved by using the direct mode of scanning electrochemical microscopy (SECM) to locally reduce the nitro groups to hydroxylamine and amino functionalities. SECM and atomic force microscopy (AFM) revealed that potentiostatic pulses applied to the working electrode lead to local destruction of the glassy carbon surface, most likely caused by etchants generated at the positioned SECM tip used as the counter electrode. By applying galvanostatic pulses, and thus, limiting the current during structuring, corrosion of the carbon surface was substantially suppressed. After galvanostatic patterning, unambiguous proof of the formation of the anticipated amino moieties was possible by modulation of the pH value during the feedback mode of SECM imaging. This patterning strategy is suitable for the further bio‐modification of microstructured surfaces. Alkaline phosphatase, as a model enzyme, was locally bound to the modified areas, thus showing that the technique can be used for the development of protein microarrays.     

Electrocatalytic Reduction of Oxygen at Multi‐Walled Carbon Nanotubes and Cobalt Porphyrin Modified Glassy Carbon Electrode

The multi‐walled carbon nanotubes (MWNTs) modified glassy carbon electrode exhibited electrocatalytic activity to the reduction of oxygen in 0.1 M HAc‐NaAc (pH 3.8) buffer solution. Further modification with cobalt porphyrin film on the MWNTs by adsorption, the resulted modified electrode showed more efficient catalytic activity to O₂ reduction. The reduction peak potential of O₂ is shifted much more positively to 0.12 V (vs. Ag/AgCl), and the peak current is increased greatly. Cyclic voltammetry (CV), transmission electron microscopy (TEM), X‐ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM), were used to characterize the material and the modified film on electrode surface. Electrochemical experiments gave the total number of electron transfer for oxygen reduction as about 3, which indicated a co‐exist process of 2 electrons and 4 electrons for reduction of oxygen at this modified electrode. Meanwhile, the catalytic activities of the multilayer film (MWNTs/CoTMPyP)_n prepared by layer‐by‐layer method were investigated, and the results showed that the peak current of O₂ reduction increased and the peak potential shifted to a positive direction with the increase of layer numbers.     

Electrocatalysis of Cyclic Voltammetrically Prepared Co‐MnO2 Composite towards Oxygen Reduction Reaction (ORR)

For the first time, cobalt particles were electrodeposited on the surface of manganese oxides by cyclic voltammetry (CV) from an aqueous solution of 0.1 M Na₂SO₄ containing 5 mM CoSO₄, and then the samples obtained were characterized by scanning electron microscopy (SEM) and energy dispersive X‐ray analysis (EDAX), respectively. And then, the as‐prepared Co/MnO₂‐coated graphite electrode was employed to the oxygen reduction reaction (ORR). Interestingly, the reduction peak potential of ORR on a Co/MnO₂‐modified graphite electrode was positively shifted for about 100 mV as compared with that on a MnO₂‐modified graphite electrode, indicating that the electrocatalysis of Co/MnO₂ composite towards ORR is superior to that of pure MnO₂.     

Scanning Droplet Cell for Chemoselective Patterning through Local Electroactivation of Protected Quinone Monolayers

A reagentless strategy for template‐free patterning of uniformly inert surfaces is suggested. A layer of p‐hydroquinone (HQ) protected by the tert‐butyldimethylsilyl (TBDMS) group is electrografted onto glassy carbon electrodes. Chemoselective activation is performed through electrochemically controlled cleavage of the TBDMS group, which yields the redox‐active surface‐confined quinone moieties. The latter are shown to undergo electrochemically induced Michael addition, which serves for subsequent functionalization of the electrode surface. Patterning of the TBDMS–quinone‐modified surface is accomplished by using selective localized cleavage of the protecting group. State‐of‐the‐art direct‐mode scanning electrochemical microscopy (SECM) patterning fails to yield the anticipated interfacial reaction; however, the electrochemical scanning droplet cell (SDC) is capable of conducting the localized chemoselective reaction. In a small area, dictated by the dimensions of the droplet, electrochemically induced cleavage of the protecting group can be performed locally to give rise to arrays of active quinone spots. Upon deprotection, the redox signals, attributed to the hydroquinone/benzoquinone couple, provide the first direct evidence for chemoselective electrochemical patterning of sensitive functionalities. Subsequent SECM studies of the resulting modified areas demonstrate spatial control of the proposed patterning technique.