Electroactive Complex Research Articles

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 10UmL−1 with a detection limit of 0.18UmL−1 (S/N=3), which might promise this method as a good candidate for monitoring DNA methylation in the future.

Measurement of iron chemical speciation in seawater at 4 °C: The use of competitive ligand exchange–adsorptive cathodic stripping voltammetry

Iron is mostly bound to poorly characterised organic ligands; thus, organic ligands are paramount in defining Fe biogeochemical cycling and its control on oceanic primary productivity. Since 1994, Fe chemical speciation has been determined by Competitive Ligand Exchange–Adsorptive Cathodic Stripping Voltammetry (CLE–AdCSV) at room temperature. However, chemical speciation is strongly dependent on temperature and some organic ligands can be temperature sensitive. Here, we compare the use of the CLE–AdCSV at room temperature and at 4°C—a temperature closer to that found in the Southern Ocean, one of the largest iron-limited regions. For both temperatures, similar detection limits and total Fe concentrations were found. However, at 4°C the analytical detection window (αFe(TAC)2) was shifted by 1.4-fold towards the detection of weaker ligands, resulting in up to 2-fold lower ligand concentrations as well as a 2- to 5-fold and 10- to70-fold lower conditional stability constants with inorganic Fe (Fe′) and Fe(III), respectively. As a result, the Fe′ concentration at 4°C was 2-fold greater, resulting in direct implication for Fe bioavailability. Results show that difference in Fe chemical speciation at 4°C was not solely explained by temperature effect on thermodynamics with the exchange ligands or the diffusion of the electroactive complex towards the Hg drop. Lowering analytical window during analysis at room temperature is proposed as a first estimate of temperature effect on iron chemical speciation.

45 Electrochemical investigation of the interaction between G-quadruplex and some ligands using MWCNT modified carbon paste electrode

The G-quadruplex is an unusual DNA secondary structure based on the Hoogesteen G–G base pairing, which stabilizes in the presence of certain metal cations, mainly alkali ions such as K+ (Elahi, Bathaie, Mousavi, Hoshyar, & Ghasemi, 2012). In continuation of our previous studies on the interaction between DNA and small molecules (Ghanbari, Bathaie, Mousavi, 2008; Heli, Bathaie, & Mousavi, 2004), more recently, interaction of G-DNA with polyamines and ethidium bromide was studied (Elahi et al., 2012). In the present study, we investigated the interaction of carotenoids (crocin and crocetin) and monoterpene aldehydes (safranal) of saffron with G-DNA. The MWCNT was ultrasonically treated in 35% solution of nitric acid for 6 h to be functionalized with carboxylic acid groups. The carbon paste electrode was prepared by mixing graphite powder with paraffin at the ratio 65:35. The resulting paste was packed into Teflon tube. The outer layer of the electrode was made of carbon paste modified by the addition of modified MWCNT at the concentration of 10% (w/w). G-DNA was covalently immobilized onto the CPE modified with MWCNT (MWCNT/CPE) in the presence of EDC/NHS. Interaction of G-DNA with the named ligands was investigated by cyclic voltammetry and differential pulse voltammetry using the electroactive complex [Ru(NH3)6]3+ (RuHex). The results showed that crocetin has more affinity than crocin and safranal for binding to G-DNA. Binding constants for these interactions are 4.4 × 104, 1.4 × 104, and 1.6 × 104 M−1 for crocetin, crocin and safranal, respectively.

Tetrathiafulvalene-Triazine-Dipyridylamines as Multifunctional Ligands for Electroactive Complexes: Synthesis, Structures, and Theoretical Study

The electroactive ligands (2,4-bis-tetrathiafulvalene[6-(dipyridin-2'-ylamino)]-1,3,5-triazine) TTF(2)-tz-dpa (1) and (2-tetrathiafulvalene[4,6-bis-(dipyridin-2'-ylamino)]-1,3,5-triazine) TTF-tz-dpa(2) (2) have been synthesized by palladium cross-coupling catalysis, and the single crystal X-ray structure for 1 was determined. In the solid state the TTF and triazine units are practically coplanar and short intermolecular S···S contacts are established. Two neutral and one tetracationic Zn(II) complexes, formulated as (TTF(2)-tz-dpa)ZnCl(2) (3), [ZnCl(2)(TTF-tz-dpa(2))Zn(H(2)O)Cl(2)] (4), and {[(H(2)O)(2)Zn(TTF-tz-dpa(2))](ClO(4))(2)}(2) (5) have been crystallized and analyzed by single crystal X-ray analysis. A peculiar feature is the evidence for anion-π interactions, as shown by the short Cl···triazine and O(perchlorate)···triazine distances of 3.52 and 3.00 Å, respectively. A complex set of intermolecular π···π, S···S, and hydrogen bonding interactions sustain the supramolecular organizations of the complexes in the solid state. Electronic absorption spectra provide evidence for the intramolecular charge transfer from TTF to triazine, also supported by time-dependent density functional theory (TD DFT) calculations.

Chemometric Analysis of Voltammetric Data on Metal Ion Binding by Selenocystine

The behavior of selenocystine (SeCyst) alone or in the presence of various metal ions (Bi(3+), Cd(2+), Co(2+), Cu(2+), Cr(3+), Ni(2+), Pb(2+), and Zn(2+)) was studied using differential pulse voltammetry (DPV) over a wide pH range. Voltammetric data matrices were analyzed using chemometric tools recently developed for nonlinear data: pHfit and Gaussian Peak Adjustment (GPA). Under the experimental conditions tested, no evidence was found for the formation of metal complexes with Bi(3+), Cu(2+), Cr(3+), and Pb(2+). In contrast, SeCyst formed electroinactive complexes with Co(2+) and Ni(2+) and kinetically inert but electroactive complexes with Cd(2+) and Zn(2+). Titrations with Cd(2+), Co(2+), Ni(2+), and Zn(2+) produced data that were reasonably consistent with the formation of stable 1:1 M(SeCyst) complexes.

Hg 2+ detection by measuring thiol groups with a highly sensitive screen-printed electrode modified with a nanostructured carbon black film

A sensor based on a screen-printed electrode (SPE) modified with a stable dispersion of commercially available carbon black (CB) N220 was optimised and challenged with several thiol-containing compounds. This probe showed a significantly enhanced electrochemical activity respect to a bare SPE when tested with thiocholine, cysteine, glutathione and cysteamine. When challenged in amperometric batch mode, the response was stable and showed a linear dependence up to 1 × 10 −5 mol l −1 for thiocholine and cysteine. The very high sensitivity towards these thiols (299 mA mol −1 l cm −2 for thiocholine and 441 mA mol −1 l cm −2 for cysteine) was then used as the basis for developing an analytical method for mercury ion detection since a non electroactive complex (thiol–Hg) is formed in the presence of the metal. By selecting an appropriate concentration of thiocholine, a concentration of mercury as low as 5 × 10 −9 mol l −1 (1 ppb) was detected. Satisfactory recovery was obtained when the system was tested on drinking water samples.

In situ immobilization of cobalt phthalocyanine on the mesoporous carbon ceramic SiO 2/C prepared by the sol–gel process. Evaluation as an electrochemical sensor for oxalic acid

The mesoporous carbon ceramics SiO 2/20 wt% C ( S BET = 160 m 2 g −1) and SiO 2/50 wt% C ( S BET = 170 m 2 g −1), where C is graphite, were prepared by the sol–gel method. Scanning electron microscopy images and the respective element mapping showed that, within the magnification used, no phase segregation was detectable. The materials containing 20 and 50 wt% of C presented electric conductivities of 9.2 × 10 −5 and 0.49 S cm −1, respectively. These materials were used as matrices to support cobalt phthalocyanine (CoPc), prepared in situ on their surfaces, to assure homogeneous dispersion of the electroactive complex in the pores of both matrices. The surface densities of cobalt phthalocyanine on both matrix surfaces were 0.014 mol cm −2 and 0.015 mol cm −2 for materials containing 20 and 50 wt% of C, respectively. Pressed disk electrodes made with SiO 2/50 wt% C/CoPc and SiO 2/20 wt% C/CoPc were tested as sensors for oxalic acid. The electrode was chemically very stable and presented very high sensitivity for this analyte, with a limit of detection, LOD = 5.8 × 10 −7 mol L −1.

Electrocatalysis of the first and second kind: Theoretical and experimental study in conditions of square-wave voltammetry

A comparative analysis of two common electrocatalytic mechanisms in conditions of square-wave voltammetry (SWV) is presented from both theoretical and experimental points of view. The catalytic mechanism of the first kind is represented by a simple regenerative EC′ reaction scheme, where C′ is a follow-up irreversible chemical reaction through which the substrate is transformed and the catalyst regenerated. The second mechanism, known as electrocatalysis of the second kind, assumes the formation of an electroactive complex YS between the catalyst Y and the electroinactive substrate S in a chemically reversible reaction, followed by an irreversible electrochemical reaction of YS that yields the catalyst Y and the final electroinactive product P. The latter mechanism is mathematically modelled based on the semi-infinite diffusion model, deriving a general solution in the form of an integral equation that may be used for any voltammetric technique. A specific recurrent formula is derived based on the step-function method, enabling the simulation of SW voltammetric experiments with a wide range of experimental parameters. The properties of the response are thoroughly analyzed in terms of the kinetics and thermodynamics of the preceding chemical reaction, as well as the kinetics of the electrode reaction. The comparative analysis of the two reaction schemes reveals that both mechanisms are rather similar, exhibiting comparable sensitivity toward the kinetics of the catalytic reaction. Theoretical analysis is experimentally illustrated with the electrocatalytic reduction of bromate and nitrite ions catalyzed by Fe(II) (catalysis of the first kind) and Co(II) ions (catalysis of the second kind), respectively.

Label-free electrochemical selection of G-quadruplex-binding ligands based on structure switching

G-quadruplexes are higher-order DNA and RNA structures formed from G-rich sequences. Ligands that bind and stabilize G-quadruplex DNA structures are of significant interest because of their potential to inhibit telomerase and halt tumor cell proliferation. This paper demonstrated a label-free electrochemical method for the selection of quadruplex-binding ligands using the electroactive complex [Ru(NH 3) 6] 3+ (RuHex) as signal transducer. The assay exploits the electrostatic interactions between RuHex and anionic phosphate backbones of DNA strands. In the presence of quadruplex-binding ligand, the oligonucleotide of the human telomeric sequence folded into a G-quadruplex structure, as a result, the adsorption amount of RuHex reduced and the peak current decreased. Six traditional Chinese medicine monomers were investigated as potential ligands using proposed method. To further study the interaction of GDNA with drugs, the competition between daidzein and complementary DNA of GDNA has been studied. The combined data from CD spectroscopy, melting curve and electrochemical measurements indicated that ligands selected by this electrochemical method could induce the GDNA folding into G-quadruplex. So this electrochemistry method offers a simple and effective approach to identify ligands with potential anticancer activity.

Electroactive complex in thermally treated Ge-Si 〈Cu, Al〉 crystals

It is shown by Hall measurements that quenching complexly doped Ge1 − x Si x 〈Cu, Al〉 (0 ≤ x ≤ 0.20) crystals from 1050–1080 K leads to the formation of additional electroactive acceptor centers in them. The activation energy of these centers increases linearly with an increase in the silicon content in the crystal and is described by the relation E k x = (52 + 320x) meV. Annealing these crystals at 550–570 K removes the additional acceptor levels. It is established that the most likely model for the additional electroactive centers is a pair composed of substituent copper and aluminum atoms (Cu s Al s ) or interstitial copper and substituent aluminum atoms (Cu i Al s ). It is shown that the generation of additional deep acceptor levels must be taken into account when using the method of precise doping of Ge1 − x Si x 〈Al〉 crystals with copper.

Electrochemical sensor for melamine based on its copper complex

A reliable and highly sensitive electrochemical sensor was first developed for analysis of non-electroactive melamine (Mel) based on its conversion to an electroactive complex by coordination of copper salt to Mel. This provides a simple and easy approach to the detection of Mel in milk products.

Bistable electrical switching and electronic memory effect in a solution-processable graphene oxide-donor polymer complex

A solution-processable and electroactive complex of poly(N-vinylcarbazole)-derivatized graphene oxide (GO-PVK) was prepared via amidation of end-functionalized PVK, from reversible addition fragmentation chain transfer polymerization, with tolylene-2,5-diisocyanate-functionalized graphene oxide. The Al/GO-PVK/ITO device exhibits bistable electrical conductivity switching and nonvolatile rewritable memory effects. Both the OFF and ON states of the memory device are stable under a constant voltage stress of −1 V for up to 3 h, or under a pulse voltage stress of −1 V for up to 108 read cycles, with an ON/OFF state current ratio in excess of 103.

Kinetics of the labeling reactions of thymine, cytosine and uracil with osmium tetroxide bipyridine

The electroactive complex osmium tetroxide bipyridine holds great promise as a covalent label for biosensor applications regarding nucleic acids and protein detection. Labeling can easily be performed in the laboratory. Until now, almost only DNA species have been investigated using this label. Thymine (which occurs exclusively in DNA) is known to react much faster than cytosine and uracil. In order to explore the possibilities to modify and detect also RNA species in a timely fashion, we have investigated the kinetics of reactions of osmium tetroxide bipyridine with the pyrimidine bases in the micromolar concentration range at different temperatures by means of spectrophotometry. Results were confirmed using voltammetric detection for the determination of labeled oligonucleotides. The modification reaction can be easily completed at room temperature within 7 h, even in case of cytosine and uracil. At 60 °C, 3 h are sufficient for complete modification of all pyrimidine bases that are found in natural nucleic acids. These findings will be important for future biosensor applications with RNA species as target molecules.

Preparation and study of cellulose acetate membranes modified with linear polymers covalently bonded to Starburst polyamidoamine dendrimers

AbstractNovel ion‐selective membranes were prepared by means of the noncovalent modification of a cellulose acetate (CA) polymer with either poly(ethylene‐alt‐maleic anhydride) or poly(allylamine hydrochloride) chains covalently linked to Starburst amine‐terminated polyamidoamine (PAMAM) dendrimers generations 4 and 3.5, respectively. Linear polymer incorporation within the porous CA membrane was performed with mechanical forces, which resulted in modified substrates susceptible to covalent adsorption of the relevant dendritic materials via the formation of amide bonds with a carbodiimide activation agent. The membranes thus prepared were characterized by chemical, physical, and spectroscopic measurements, and the results indicate that the dendrimer peripheral functional groups were the species that participated in the ion‐exchange events. The prepared materials were also evaluated for their ion‐exchange permeability with sampled current voltammetry experiments involving cationic and anionic species {[Ru(NH3)6]3+ and [Fe(CN6)]3−, respectively} as redox probe molecules under different pH conditions. As expected, although permeability was favored by opposite charges between the dendrimer and the electroactive probe, a clear blocking effect took place when the charge in the dendritic polymer and the electroactive complex was the same. Electrochemical impedance spectroscopy measurements, on the other hand, showed that the PAMAM‐modified membranes were characterized by good selectivity and low resistance values for multivalent ions compared to a couple of commercial ion‐exchange membranes. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Studies on the Electrochemical Behavior of the Pilocarpine Complex and its Application Using a Flow-through Polarographic Sensor

Pilocarpine (2(3H)-Furanone, -ethyldihydro-4-(1-methyl-1H-imidazol-5-yl)methyl]-) reacts with cobalt (II) in an acid medium and forms an electroactive complex with two sensitive reduction waves that are measurable using cyclic and differential pulse voltammetry on mercury-coated gold electrodes. We studied the electrochemical behaviors of this complex and its use for detecting pilocarpine in pharmaceuticals. We used carbon fiber with a surface deposit of mercury as a sensor in liquid chromatography-electrochemical detection (LCEC) to increase the sensitivity and selectivity of analytical methods for the determining trace concentrations of the cobalt (II)-pilocarpine complex. Keywords: Metal-pilocarpine, Polarographic sensor

Mechanism of metals’ electrodeposition from complex electrolytes allowing for the anions’ adsorption: Effect of the position of ligands and complex ions in the compact layer on the reaction orders

Effect of the relative position in the compact layer of specifically adsorbed electroactive complex ion and complexing ligand on the magnitude of current is considered. Effect of parameters that characterize the adsorption of such ions and the electrical double layer structure on the dependence of current on the solution composition and effective reaction orders is analyzed.

Charge transport in two-component monolayers of lipid matrix and adsorbed electro-active component

Monolayers of lipid matrix and electro-active metal bipyridine (bpy) complexes were formed by spreading the lipid on 0.1 M aqueous solutions of Me(bpy) 3Cl 2 (Me = Ru, Co, Os) in the presence of 10 mM KCl. The complexes bind to the head groups of the lipids octadecyl-malonic acid (OMA) and dihexadecyl phosphate (DHP), respectively, as evidenced by surface pressure–area and surface potential–area isotherms as well as by reflection spectroscopy and Brewster angle microscopy (BAM). Cyclic voltammetry was done using line electrodes, and by subtracting the voltammograms obtained in the absence of the monolayer from those obtained in the presence of the monolayer, difference cyclic voltammograms were determined. Apparent diffusion constants D app of the electro-active complexes were obtained from the peak currents. D app of Co(bpy) 3Cl 2 adsorbed to a monolayer of OMA is 100 times smaller than in bulk solution. For Os(bpy) 3Cl 2 adsorbed to OMA a larger D app is obtained due to electron hopping as charge transport mechanism in addition to molecular diffusion along the interface. In the case of DHP, D app of Os(bpy) 3Cl 2 increases abruptly upon monolayer compression indicating percolation.

Cobalt Complexes of Tripodal Hexadentate Ligands: Electrochemically Driven Rearrangements

The CoIII complexes of the hexadentate tripodal ligands HOsen (3-(2´-aminoethylamino)-2,2-bis((2´´-aminoethylamino)methyl)propan-1-ol) and HOten (3-(2´-aminoethylthia)-2,2-bis((2´´-aminoethylthia)methyl)propan-1-ol) have been synthesized and fully characterized. The crystal structures of [Co(HOsen)]Cl3∙H2O and [Co(HOten)](ClO4)Cl2 are reported and in both cases the ligands coordinate as tripodal hexadentate N6 and N3S3 donors, respectively. Cyclic voltammetry of the N3S3 coordinated complex [Co(HOten)]3+ is complicated and electrode dependent. On a Pt working electrode an irreversible CoIII/II couple (formal potential –157 mV versus Ag|AgCl) is seen, which is indicative of dissociation of the divalent complex formed at the electrode. The free HOten released by the dissociation of [Co(HOten)]2+ can be recaptured by Hg as shown by cyclic voltammetry experiments on a static Hg drop electrode (or in the presence of Hg2+ ions), which leads to the formation of an electroactive HgII complex of the N3S3 ligand (formal potential +60 mV versus Ag|AgCl). This behaviour is in contrast to the facile and totally reversible voltammetry of the hexaamine complex [Co(HOsen)]3+ (formal potential (CoIII/II) –519 mV versus Ag|AgCl), which is uncomplicated by any coupled chemical reactions. A kinetic and thermodynamic analysis of the [Co(HOten)]2+/[Hg(HOten)]2+ system is presented on the basis of digital simulation of the experimental voltammetric data.

Stripping chronopotentiometry at scanned deposition potential (SSCP). Part 6: Features of irreversible complex systems

The features of SSCP waves for complex species involving a nonreversible electron transfer process are described. For quasi-reversible systems, with electron transfer rate constants, k 0, between O(10 −4) to O(10 −6) m s −1, the shape of the SSCP wave is dependent on the value of k 0, the deposition time and the electrode size. Under these conditions, the stability of a metal complex can be determined from the shift in half-wave deposition potential, E d,1/2, as compared to the metal-only case. This is true even when the system is not fully labile, so long as the shape of the SSCP wave is not altered in the presence of the complexant, i.e., k 0 remains the same. The experimental parameters, notably deposition time and electrode size, can be optimised to achieve this condition. For irreversible systems, k 0 < O(10 −7) m s −1, the SSCP wave shape is independent of k 0, deposition time, and electrode size. Further decreasing k 0 only leads to additional shift in the wave from E 0. In such cases the contribution to the E d,1/2 from the stability constant cannot be a priori deconvoluted from that due to k 0 if its value changes due to the presence of ligand. In irreversible systems, direct reduction of electroactive complexes is not uncommon, nor is its appearance at potentials more positive, i.e., more reversibly, than that for the metal itself.

Synthesis of Conducting Polyelectrolyte Complexes of Polyaniline and Poly(2-acrylamido-3-methyl-1-propanesulfonic acid) Catalyzed by pH-Stable Palm Tree Peroxidase

Comparison of the stability of five plant peroxidases (horseradish, royal palm tree leaf, soybean, and cationic and anionic peanut peroxidases) was carried out under acidic conditions favorable for synthesis of polyelectrolyte complexes of polyaniline (PANI). It demonstrates that palm tree peroxidase has the highest stability. Using this peroxidase as a catalyst, the enzymatic synthesis of polyelectrolyte complexes of PANI and poly(2-acrylamido-3-methyl-1-propanesulfonic acid) (PAMPS) was developed. The template polymerization of aniline was carried out in aqueous buffer at pH 2.8. Varying the concentrations of aniline, PAMPS, and hydrogen peroxide as reagents, favorable conditions for production of PANI were determined. UV-vis-NIR absorption and EPR demonstrated that PAMPS and PANI formed the electroactive complex similar to PANI doped traditionally using low molecular weight sulfonic acids. The effect of pH on conformational variability of the complex was evaluated by UV-vis spectroscopy. Atomic force microscopy showed that a size of the particles of the PANI-PAMPS complexes varied between 10 and 25 nm, depending on a concentration of PAMPS in the complex. The dc conductivity of the complexes depends also on the content of PAMPS, the higher conductivity being for the complexes containing the lower content of the polymeric template.