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.     

银纳米修饰电极的制备及电化学行为

金属纳米粒子由于其小的体积和大的比表面积而具有独特的电子、光学和异相催化特性,是目前表面纳米工程及功能化纳米结构制备的一种理想研究对象[1]。银纳米粒子可广泛应用于催化剂材料、电池的电极材料、低温导热材料和导电材料等,成为近年来人们研究的热点[2,3]。在电化学方面,银纳米粒子具有比其他纳米粒子更为优异的导电性能和电催化性能。因此,研究银纳米粒子修饰电极有重要的应用价值和前景[4]。1实验部分1.1仪器CHI660电化学工作站(USA);TU-1901型双光束紫外可见分光光度计(北京普析通用仪器公司);KQ-100型超声清洗器(昆山市超声…     

Approaches to separation interfaces: Electrostatic interaction and local structures of ions at zwitterionic interface

The interaction and separation of ions with zwitterionic layers are reviewed principally based on a series of the author's work. An electrostatic model has allowed us to discuss the chromatographic retention of ions on the zwitterionic stationary phase, and has revealed the ionic interaction occurring at the zwitterionic interface. Similar consideration is applicable to the ionic partition into zwitterionic micelles having the spherical dimension. In the electrostatic models, ion association and solvation changes of ions have been assumed to explain the selectivity in ion recognition. Both assumptions are applicable to polarizable large ions, whereas the former cannot account for the results obtained for small and well-hydrated ions (Cl⁻ and Br⁻). A special X-ray absorption finestructure (XAFS) measurement, which allows selective access to ions interacting with surface monolayers, has been developed, and applied to ions attracted by a zwitterionic monolayer. The X-ray absorption spectra suggest that Zn²⁺ attracted by the zwitterionic monolayer is still hydrated. In contrast, the direct interaction of Br⁻ has been confirmed, indicating that the electrostatic model involving either ion association or the solvation change of an ion does not properly explain the observed phenomena but both effects should be taken into consideration.     

Enhanced Electrocatalytic Oxidation of Paracetamol at DNA Modified Gold Electrode

Cysteine monolayer has been assembled onto bare gold electrode (SAM/Au), and subsequently deoxyribonucleic acid (DNA) has been successfully immobilized at the SAM/Au electrode. The thus modified electrode is assigned DNA/SAM/Au. Modification steps of the electrode were followed electrochemically using K₄[Fe(CN₆)] electrochemical marker. Also, the build‐up of the modified electrode composition is followed using EDX and the crystallographic orientation is inspected using XRD. The electrochemical behavior of paracetamol (PC) at DNA/SAM/Au electrode is investigated. Interestingly, the sluggish irreversible behavior of PC at the bare gold electrode is converted to a quasi‐reversible one at DNA/SAM/Au electrode pointing to some interaction between the immobilized DNA and PC. The enhanced electrochemical behavior of PC at modified DNA/SAM/Au electrode is successfully used for a sensitive electrochemical determination of PC. Square wave voltammetry (SWV) was used for this purpose. The concentration of PC was in linear relation with the peak current at the optimum conditions within the range 10.0–110.0 μg mL^?1 with correlation coefficient (R²) of 0.998. Also, the standard deviation (SD) and relative standard deviation (RSD) were calculated and found to be 0.817 and 1.52, respectively, indicating the significance of the present method.     

Electrochemical strategy for sensing DNA methylation and DNA methyltransferase activity

The present work demonstrates a novel signal-off electrochemical method for the determination of DNA methylation and the assay of methyltransferase activity using the electroactive complex [Ru(NH₃)₆]³⁺ (RuHex) as a signal transducer. The assay exploits the electrostatic interactions between RuHex and DNA strands. Thiolated single strand DNA1 was firstly self-assembled on a gold electrode via Au–S bonding, followed by hybridization with single strand DNA2 to form double strand DNA containing specific recognition sequence of DNA adenine methylation MTase and methylation-responsive restriction endonuclease Dpn I. The double strand DNA may adsorb lots of electrochemical species ([Ru(NH₃)₆]³⁺) via the electrostatic interaction, thus resulting in a high electrochemical signal. In the presence of DNA adenine methylation methyltransferase and S-adenosyl-l-methionine, the formed double strand DNA was methylated by DNA adenine methylation methyltransferase, then the double strand DNA can be cleaved by methylation-responsive restriction endonuclease Dpn I, leading to the dissociation of a large amount of signaling probes from the electrode. As a result, the adsorption amount of RuHex reduced, resulting in a decrease in electrochemical signal. Thus, a sensitive electrochemical method for detection of DNA methylation is proposed. The proposed method yielded a linear response to concentration of Dam MTase ranging from 0.25 to 10 U mL⁻¹ with a detection limit of 0.18 U mL⁻¹ (S/N = 3), which might promise this method as a good candidate for monitoring DNA methylation in the future.     

Electrocatalysis of the hydrogen evolution reaction by nanocomposites of poly(amidoamine)-encapsulated platinum nanoparticles and phosphotungstic acid

Poly(amidoamine) dendrimer (Generation-4) encapsulated platinum nanoparticles (PtNP-PAMAM) were prepared and used to fabricate nanocomposites with Keggin-type phosphotungstic acid (PW₁₂O₄₀³⁻) using a layer by layer electrostatic assembly technique. Indium tin oxide (ITO) electrodes, which were first modified with a monolayer of 3-aminopropyl triethoxysilane (3-APTES), were used as substrates for assembly of the PW₁₂O₄₀³⁻ monolayer. Nanocomposites were then fabricated by depositing PtNP-PAMAM on the monolayer of PW₁₂O₄₀³⁻. The amount of PtNP-PAMAM deposited was controlled by using different concentrations of PtNP-PAMAM diluted in 0.1 M H₂SO₄ solution. The hydrogen evolution reaction (HER) was used to test electrocatalytic activities of these nanocomposite modified electrodes. Modification of ITO|3-APTES with PW₁₂O₄₀³⁻ |PtNP-PAMAM showed significantly higher electrocatalytic activities toward the HER than electrodes modified with either PW₁₂O₄₀³⁻ or PtNP-PAMAM alone. The electrocatalytic activities were found to depend on the composition of PtNP-PAMAM and PW₁₂O₄₀³⁻ on electrode surfaces, which was attributed to an interaction between these species. Heat treatment of ITO|3-APTES|PW₁₂O₄₀³⁻ |PtNP-PAMAM electrodes at 200 °C produced significantly higher electrocatalytic activities, which supported the suggestion of an interaction. Presented at the 4th Baltic conference on Electrochemistry, Griefswald, March 13.−16., 2005.     

Directly Imaging and Regulating the Nanoscale Inhomogeneity of S-Vacancies in Molybdenum Disulfide Monolayer during Electrocatalytic Hydrogen Evolution

The study of electron transfer event on two-dimensional (2D) layered transition metal dichalcogenides has attracted tremendous attentions attributing to their promising applications in electrochemical devices. Herein, we demonstrate an opto-electrochemical strategy to directly map and regulate electron transfer event on molybdenum disulfide (MoS₂) monolayer by combining bright field (BF) imaging technique with electrochemical modulation. The heterogeneity of electrochemical activity on MoS₂ monolayer down to nanoscale is resolved spatiotemporally. The thermodynamics of MoS₂ monolayer is measured during electrocatalytic hydrogen evolution, and the Arrhenius correlations are obtained. We validate that the defect generation engineered by oxygen plasma bombardment dramatically enhances the local electrochemical activity of MoS₂ monolayer, which can be attributed to point defects of S-vacancies as evidenced. Furthermore, by comparing the difference of electron transfer event on MoS₂ with various layers, the interlayer coupling effect is uncovered. This study represents a facile method to image the heterogeneity of electrochemical properties for nanomaterials with atomic thickness and regulate the local activity within the plane by extrinsic factors. It also has potential applications in the design and evaluation of high-performance layered electrochemical systems down to nanoscale.     

Electrochemical properties of niosomes modified Au electrode and DNA recognition

Non-ionic surfactant vesicles (NSVs), also referred to as niosomes, have been studied as an alternative to conventional liposomes. In this paper, electrochemical inspection of the interaction between Herring sperm DNA and niosomes has been investigated after a simple and novel method for the formation of niosomes on Au electrode. Each step of electrode modification has been confirmed with cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The assembly of octadecanethiol (ODT) layer on the electrode surface generates a packed film that introduces a barrier to the interfacial electron transfer (R_et), and the subsequent immobilization of niosomes onto the self-assembled monolayer (SAM) layer results in a further increase of R_et, due to the formed bilayer almost blocked the redox probe to the electrode surface. When Herring sperm DNA was added, the R_et value decreased, indicating that the barrier of the redox probe to the surface was disrupted. The addition of DNA caused the formation of some transmembrane channels for the redox probe across the niosomes. A good linear relationship between R_et value and DNA concentration was found over the 0–0.05 mg mL⁻¹ concentration range.     

Voltammetric study of the interaction of the ofloxacin–copper complex with DNA, and its analytical application

The electrochemical behavior of the ofloxacin–copper complex, Cu(II)L₂, at a mercury electrode, and the interaction of DNA with the complex have been investigated. The experiments indicate that the electrode reaction of Cu(II)L₂ is an irreversible surface electrochemical reaction and that the reactant is of adsorbed character. In the presence of DNA, the formation of the electrochemically non-active complexes Cu(II)L₂-DNA, results in the decrease of the peak current of Cu(II)L₂. Based on the electrochemical behavior of the Cu(II)L₂ with DNA, binding by electrostatic interaction is suggested and a new method for determining nucleic acid is proposed. Under the optimum conditions, the decrease of the peak current is in proportional to the concentration of nucleic acids in the range from 3 × 10⁻⁸ to 3 × 10⁻⁶ g · mL⁻¹ for calf thymus DNA, from 1.6 × 10⁻⁸ to 9.0 × 10⁻⁷ g · mL⁻¹ for fish sperm DNA, and from 3.3 × 10⁻⁸ to 5.5 × 10⁻⁷ g · mL⁻¹ for yeast RNA. The detection limits are 3.3 × 10⁻⁹, 6.7 × 10⁻⁹ and 8.0 × 10⁻⁹ g · mL⁻¹, respectively. The method exhibits good recovery and high sensitivity in synthetic samples and in real samples.     

Electrochemical Studies of Thiol Self‐Assembled Monolayers at a Heated Gold‐Wire Microelectrode

Mercaptoundecanoic acid (MUA) and glutathione (GSH) self‐assembled monolayers were prepared on gold‐ wire microelectrode. Cyclic voltammetry was used to investigate the influence of temperature on electrochemical behaviors of Fe(CN)₆^3?/4? and Ru(NH₃)₆^3+/2+ at these SAMs modified electrodes in aqueous solution. It is found that temperature shows great influence on electron transfer (ET) and mass transport (MT) for the two SAMs modified electrodes and the influence of temperature depends on the charge properties of the redox couples and terminal groups of SAMs and the structure of the monolayer on gold surface. The temperature can greatly increase MT rate of Fe(CN)₆^3?/4? at both MUA and GSH modified electrodes. However, the increased MT rate doesn't have any effect on the CV's for Fe(CN)₆^3?/4? /MUA system. For Ru(NH₃)₆^3+/2+ , temperature can greatly improve the electrochemical reaction in both MUA and GSH modified electrodes, which is ascribed to temperature‐induced diffusion and convection and the electrostatic interaction between Ru(NH₃)₆^3+/2+ and negatively charged carboxyl groups on the electrode surface.     

Square-wave voltammetry and square-wave voltacoulommetry applied to the study of the electrocatalytic behaviour of surface confined myoglobin

Analytical previous results corresponding to square-wave voltammetry (SWV) and square-wave voltacoulommetry (SWVC) have been applied to the electrochemical characterization of myoglobin immobilized at a self-assembled monolayer of l-cystine on a gold electrode and its electrocatalytic activity towards the oxidation of sodium ascorbate. The obtained results have been compared with those previously reported in Langmuir (2011) 27:2052–2057 by using cyclic voltammetry. It has been shown that double layer effects can be easily removed in SWV and SWVC techniques. Accurate values for the thermodynamic and electrochemical kinetic parameters have been obtained by assuming dispersion in the formal potential of the redox center, and a value k _c ′?=?1.37?×?10⁵?M^?1?s^?1 has been found for the catalytic constant.     

Theoretical studies on the structure of M+BF−4 ion pairs M = Li+, NH+4: the role of electrostatics and electron correlation

A systematic investigation of the M⁺BF₄ ⁻ (M = Li or NH₄) ion-pair conformers has been carried out using an electrostatic docking model based on the molecular electrostatic potential topography of the free anion. This method provides a guideline for the subsequent ab initio molecular orbital calculations at the Hartree-Fock (HF) and second-order M?ller-Plesset perturbation theory (MP2) levels. It has been demonstrated that the model presented here yields more than 75% of the HF interaction energy when Li⁺ is the cation involved and more than 90% for the case of NH₄ ⁺. Inclusion of MP2 correlation in the HF-optimized geometries leads to stationary point geometries with different numbers of imaginary frequencies and in some places where the energies of two adjacent conformers are very close, the energy rank order is altered. The HF lowest-energy minima for the Li⁺BF₄ ⁻ and NH₄ ⁺BF₄ ⁻ show a bidentate and tridentate coordinating cation, respectively, whereas at the MP2 level, this ordering is reversed. Received: 9 September 1997 / Accepted: 5 November 1997     

Effect of electrostatic interaction on hydrolytic activity of phospholipase A2 at the polarized oil-water interface

The hydrolytic activity of phospholipase A₂ (PLA₂) against the dipalmitoylphosphatidylcholine monolayer formed at the nitrobenzene-water interface has been studied under the control of the potential drop across the monolayer. The activities of both porcine pancreatic and Naja naja PLA₂S was the highest when the potential of the nitrobenzene phase was 60 mV negative with respect to that of the aqueous phase. The local electrostatic interaction between the positively charged domain, the recognition site, of PLA₂ molecules with the negatively charged substrate side of the interface, where zwitterionic substrate molecules and negatively charged product molecules were adsorbed, is an important factor in determining the interfacial enzymatic activity. Irreversible adsorption of PLA₂ molecules on the substrate monolayer is confirmed, giving unequivocal evidence for the scooting mode of hydrolysis by PLA₂.     

Electrochemical properties of a gold electrode modified with a mixed monolayer

The electrochemical properties of a gold electrode modified with a mixed thiol monolayer containing both a polar and a non-polar head group have been investigated in aqueous Fe(CN)₆⁴⁻, flavin adenine dinucleotide (FAD) and ubiquinone-0 (2,3-dimethoxy-5-methyl-1,4-benzoquinone, UQ₀) solutions. The cyclic voltammetric current-potential (i-E) response of Fe(CN)₆⁴⁻ was found to be affected considerably by the polarity of the head group contained in the mixed monolayer assembly, as compared with those of FAD and UQ₀. It was also found that in the cases of UQ₀ and FAD the i-E responses for the modified electrode were affected by their own molecular size rather than the polarity of the mixed monolayer head group. Furthermore, compared with Fe(CN)₆⁴⁻ ion, these biologically related molecules are able to permeate readily into the well-organized and hydrophobic alkyl chains of the monolayer assembly. The voltammetric profile of UQ₀ was improved by the modification of aminoethanethiol, as compared with those of bare gold and the electrode modified with other polar thiols. Further, two different permeation paths of the electrode species into the mixed monolayer are suggested from the variation of the i-E response with the cycle of the potential scan.     

Electrochemical Nucleic Acid Biosensors for the Detection of Interaction Between Peroxynitrite and DNA

We studied the reactivity of peroxynitrite and different nucleic acid molecules using DNA electrochemical biosensors. SIN‐1 (3‐morpholinosydnonimine) has been used for the simultaneous generation of NO?and superoxide, i.e., as a peroxynitrite (ONOO^?) donor. Double strand DNA (dsDNA), single strand DNA (ssDNA) and 15 guanine bases oligonucleotide (Oligo(dG)₁₅) were immobilized on a carbon paste electrode to generate the biosensor and DPV was selected as the electroanalytical technique. Results showed that electrochemical biosensors were very sensitive for detecting interaction between ONOO^? and DNA. A down/up effect was observed, i.e., at low ONOO^? concentrations the guanine oxidation signal decreased while at high ONOO^? concentrations the guanine oxidation current increased. Oligo(dG)₁₅ exhibited greater interaction at low ONOO^? concentrations than the other DNA molecules. The reactivity between ONOO^? and DNA was also evaluated in solution phase, showing the same down/up effect. Finally, the capacity of DNA to hybridize was prevented after interaction with ONOO^?.     

In situ electrochemical SERS studies on electrodeposition of aniline on 4-ATP/Au surface

The electrochemical polymerization of 0.01 M aniline in 1 M H₂SO₄ aqueous solution on roughened Au surface modified with a self-assembled monolayer (SAM) of 4-aminothiophenol (4-ATP) has been investigated by in situ electrochemical surface-enhanced Raman scattering spectroscopy (SERS). The repeat units and possible structures of the electrodeposited polyaniline (PANI) film were proposed; i.e., aniline monomer is coupled in head-to-tail predominately at the C₄ of aniline and amine of 4-ATP, and the thin PANI film is orientated vertically to substrate surface. Simultaneous Raman spectra during potential scanning indicate clearly that the ultrathin PANI film (in initial growth of the film) consists of semiquinone radical cation (IP⁺), para-disubstituted benzene (IP and IP⁺) and quinine diimine (NP) while it is oxidized, and without quinine diimine and semiquinone radical cation while reduced. Meanwhile, the results confirm that 4-ATP monolayer shows a strong promotion on the electrodeposition of aniline monomer, and a possible polymerization mechanism was proposed.     

Electronic structural studies of pyrrolidinium-based ionic liquids for electrochemical application

Electrochemical stability and noncovalent interactions escorting the cyclic ammonium-based ionic liquids composed of N-alkyl-substituted N-methyl pyrrolidinium (P_yr1R) (R = methyl, ethyl, propyl, butyl, pentyl, hexyl) cations and four anions hexafluorophosphate (PF₆), tetrafluoroborate (BF₄), bis(trifluoromethylsulfonyl-imide (TFSI), and trifluoromethane sulfonate (TFO) have been analyzed using the density functional theory. Electronic structures, electrochemical window, frontier orbital energy difference (HOMO-LUMO gap), binding energies, vibrational spectra of these ion pairs were characterized. It has been established that ion pair formation is largely reigned by C H⋯F interactions between anionic fluorine for BF₄⁻ and PF₆⁻ anions and C H⋯O interactions between anionic oxygen for TFSI and TFO anions and pyrrolidinic proton, methyl, or alkyl group protons of the cations. The effect of alkyl chain length and pairing anions of the alkyl substituted N-methyl pyrrolidinium-based ionic liquids on the electrochemical window was investigated. The results revealed that the HOMO energy of pairing anions is the key factor to decide the electrochemical window. Further quantification of noncovalent interactions in terms of electrostatic and hydrogen bonding interactions has been brought out employing a novel method with the aid of Mulliken and Merz-Singh-Kollman charges, prevailed in pyrrolidinium-based ionic liquids.     

Effect of Anionic Electrolytes and Precursor Concentrations on the Electrodeposited Pt Structures

Electrodeposition of functional metal surfaces has received great attention because of their useful applications. Recently, interesting electrodeposition behavior of Pt at −0.8 V (vs. Ag/AgCl) was reported, where underpotential deposited H (H_upd) layers played a unique role in the electrodeposition. Here, we report the effect of anionic electrolytes and precursor concentrations on the electrochemical deposition behavior of Pt. Depending on these two experimental parameters, two distinct Pt structures, monolayer Pt films and Pt spheres, were electrodeposited at −0.8 V. In addition to the underpotential deposited H (H_upd) layers formed at −0.8 V, the adsorption of Cl⁻ also plays a significant role in determining the electrodeposited Pt structures. When the PtCl₄²⁻ concentration was low and the Cl⁻ concentration was high enough for the adsorption of PtCl₄²⁻ to be blocked by the H_upd and Cl⁻ layers, monolayer Pt films were electrodeposited. Otherwise, further electrodeposition of Pt spheres over the monolayer Pt films occurred. The effect of other halide ion adsorption and the controlled growth of Pt spheres during the Pt electrodeposition were also investigated. The electrochemical deposition behavior of Pt demonstrated in this work provides insight into the fabrication of functional Pt surfaces.     

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.     

Synthesis,structure, and DNA-binding of a new binuclear copper(II) complex: [Cu2(heap)(H2O)2](pic)2 · 2H2O

In this study, a new μ-oxamido-bridged binuclear copper(II) complex [Cu₂(heap)(H₂O)₂](pic)₂ ? 2H₂O, where heap and pic stand for the anion of N,N′-bis(N-hydroxyethylaminopropyl)oxamide and 2,4,6-trinitrophenol, respectively, has been synthesized and characterized by elemental analyses, IR, UV, molar conductance, and single-crystal X-ray diffraction. The complex has an embedded inversion center at the middle of the C–C bond of the oxamido group. Each copper(II) is in a square-pyramidal coordination geometry. The bridging ligand (heap) adopts a bis-tetradentate trans conformation and the oxamido group is in an imidic acid configuration. Hydrogen bonds contribute to a 3-D supramolecular structure in the crystal. The interaction of the binuclear complex with herring sperm deoxyribonucleic acid (HS-DNA) has been investigated by using absorption and emission spectra, electrochemical techniques, and viscometry. The results suggest that the binuclear copper(II) complex interacts with HS-DNA by electrostatic interaction with intrinsic binding constant of 2.67 × 10⁴ (mol L^?1)^?1. The influence of anions on the structure and the interaction of the binuclear complex with HS-DNA were preliminarily discussed.