Amino group position dependent orientation of self-assembled monomolecular films of tetraaminophthalocyanatocobalt(II) on Au surfaces

Self-assembled monomolecular films of 1,8,15,22-tetraaminophthalocyanatocobalt(II) (4α-CoIITAPc) and 2,9,16,23-tetraaminophthalocyanatocobalt(II) (4β-CoIITAPc) on Au surfaces were prepared by spontaneous adsorption from solution. These films were characterized by cyclic voltammetry and Raman spectroscopy. Both the surface coverage (Γ) and intensity of the in-plane stretching bands obtained from Raman studies vary for these monomolecular films, indicating different orientations adopted by them on Au surfaces. The 4α-CoIITAPc-modified electrode exhibits an E1/2 of 0.35 V, while the 4β-CoIITAPc-modified electrode exhibits an E1/2 of 0.19 V, corresponding to the CoII/CoIII redox couple in 0.1 M H2SO4. The Γ estimated from the charge associated with the oxidation of Co(II) gives (2.62 ± 0.10) × 10-11 mol cm-2 for 4α-CoIITAPc and (3.43 ± 0.14) × 10-10 mol cm-2 for 4β-CoIITAPc. In Raman spectral studies, the intensity ratio between in-plane phthalocyanine (Pc) stretching and the Au−N stretching was found to be 6.6 for 4β-CoIITAPc, while it was 1.6 for 4α-CoIITAPc. The obtained lower Γ and intensity ratio values suggest that 4α-CoIITAPc adopts nearly a parallel orientation on the Au surface, while the higher Γ and intensity ratio values suggest that 4β-CoIITAPc adopts a perpendicular orientation. The electrochemical reduction of dioxygen was carried out using these differently oriented Pc's in phosphate buffer solution (pH 7.2). Both the Pc's catalyze the reduction of dioxygen; however, the 4α-CoIITAPc-modified electrode greatly reduces the dioxygen reduction overpotential compared to 4β-CoIITAPc-modified and bare Au electrodes.

Phase diagram of the system Ca-Ti-O at 1200 K

Phase relations in the system Ca-Ti-O have been established by equilibration of several samples at 1200 K for prolonged periods and identification of phases in quenched samples by optical and scanning electron microscopy, XRD and EDS. Samples representing 20 compositions in the ternary system were analyzed. There was negligible solid solubility of Ca in the phases along the binary Ti-O, and of Ti in CaO. Four ternary oxides were identified: CaTiO3, Ca4Ti3O10 and Ca3Ti2O7 containing tetravalent titanium, and CaTi2O4 containing trivalent titanium. Tie-lines link calcium titanite (CaTi2O4) with the three calcium titanates (CaTiO3, Ca4Ti3O10 and Ca3Ti2O7), CaO, oxygen excess TiO1+delta and stoichiometric TiO. Tie-lines connect CaTiO3 with TiO2-x, Magneli phases TinO2n-1 (28 >= n >= 4), Ti3O5, Ti2O3 and TiO1+delta. CaO was found to coexist with TiO, and Ti-O solid solutions alpha and beta. The phase diagram is useful for understanding the mechanisms and kinetics of direct calciothermic reduction of TiO2 to metal and electrochemical reduction of TiO2 using graphite anode and molten CaCl2 electrolyte.

Inter And Intramolecular Interactions Of Porphyrins With 5,5′-Dithiobis(2-Nitrobenzoic Acid)

meso-Tetraphenylporphyrin and its metal [zinc(II) and copper(II)] derivatives form both inter and intramolecular complexes with 5,5′-dithiobis(2-nitrobenzoic acid) (DTNB). The nature of interaction is predominantly charge transfer (CT) in origin, with the porphyrin functioning as a II-donor and DTNB as an acceptor. Among the covalently linked intramolecular systems, the magnitude of CT interaction varies with the position (of one of the aryl groups of the porphyrin) to which DTNB is attached as ortho meta > para. Steady-state and time-resolved fluorescence studies revealed electron transfer to be the dominant pathway for the fluorescence quenching in these systems. Steady-state photolysis experiments probed using EPR and optical absorption studies have shown that electron transfer (from the excited singlet state of the porphyrin) to DTNB results in the formation of thiyl radical and production of free thiolate anion. It is found that the products of electrochemical reduction of covalently linked porphyrin-DTNB systems are different from those observed for the photochemical studies.

Direct Electrochemistry of Glucose Oxidase and Biosensing for Glucose Based on Graphene

We first reported that polyvinylpyrrolidone-protected graphene was dispersed well in water and had good electrochemical reduction toward O-2 and H2O2. With glucose oxidase (GOD) as an enzyme model, we constructed a novel polyvinylpyrrolidone-proteeted graphene/polyethylenimine-ftmctionalized ionic liquid/GOD electrochemical biosensor, which achieved the direct electron transfer of GOD, maintained its bioactivity and showed potential application for the fabrication of novel glucose biosensors with linear glucose response up to 14 mM.

Antioxidant Sensors Based on Iron Diethylenetriaminepentaacetic Acid, Hematin, and Hemoglobin Modified TiO2 Nanoparticle Printed Electrodes

Antioxidant amperometric sensors based on iron-containing complexes and protein modified electrodes were developed. Indium tin oxide glass was printed with TiO2 nanoparticles, onto which iron-containing compounds and protein were adsorbed. When applied with negative potentials, the dissolved oxygen is reduced to H2O2 at the electrode surface, and the H2O2 generated in situ oxidizes Fe-II to Fe-III, and then electrochemical reduction of Fe-III therefore gives rise to a catalytic current. In the presence of antioxidants, H2O2 was scavenged, the catalytic current was reduced, and the decreased current signal was proportional to the quantity of existing antioxidants. A kinetic model was proposed to quantify the H2O2 scavenging capacities of the antioxidants. With the use of the sensor developed here, antioxidant measurements can be done quite simply: put the sensor into the sample solutions (in aerobic atmosphere), perform a cathodic polarization scan, and then read the antioxidant activity values. The present work can be complementary to the previous studies of antioxidant sensor techniques based on OH radicals and superoxide ions scavenging methods, but the sensor developed here is much easier to fabricate and use.

Direct electrochemistry and electrocatalysis of horseradish peroxidase immobilized in Nafion-RTIL composite film

The composite film based on Nafion and hydrophobic room-temperature ionic liquid (RTIL) 1-butyl-3-methyl-imidazolium hexafluorophosphate ([bmim] PF6) was explored. Here, Nafion was used as a binder to form Nafion-ionic liquids composite film and help [bmim] PF6 effectively adhered on glassy carbon (GC) electrode. X-ray photoelectron spectroscopy (XPS), cyclic voltammtery (CV) and electrochemical impedance spectroscopy (EIS) were used to characterize this composite film, showing that the composite film can effectively adhere on the GC electrode surface through Nafion interacting with [bmim] PF6 and GC electrode. Meanwhile, doping [bmim] PF6 in Nafion can also effectively reduce the electron transfer resistance of Nafion. The composite film can be readily used as an immobilization matrix to entrap horseradish peroxidase (HRP). A pair of well-defined redox peaks of HRP was obtained at the HRP/Nafion[bmim] PF6 composite film-modified GC electrode through direct electron transfer between the protein and the underlying electrode. HRP can still retain its biological activity and enhance electrochemical reduction towards O-2 and H2O2. It is expected that this composite film may find more potential applications in biosensors and biocatalysis.

Self-assembled silver nanoparticle monolayer on glassy carbon: an approach to SERS substrate

In this paper, the fabrication of an active surf ace-enhanced Raman scattering (SERS) substrate by self-assembled silver nanoparticles on a monolayer of 4-aminophenyl-group-modified glassy carbon (GC) is reported. Silver nanoparticles are attached to the substrate through the electrostatic force between the negatively charged silver nanoparticles and the positively charged 4-aminophenyl groups on GC. The active SERS substrate has been characterized by means of tapping-mode atomic force microscopy (AFM), indicating that large quantities of silver nanoparticles are uniformly coated on the substrate. Rhodamine 6G (R6G) and p-aminothiophenol (p-ATP) are used as the probe molecules for SERS, resulting in high sensitivity to the SERS response, with the detection limit reaching as low as 10(-9) m. This approach is easily controlled and reproducible, and more importantly, can extend the range of usable substrates to carbon-based materials for SERS with high sensitivity.

Electropolymerization and catalysis of well-dispersed polyaniline/carbon nanotube/gold composite

Polyaniline/multi-walled carbon nanotube/gold (PANI/MWNT/Au) composite film was synthesized via a two-step electrochemical process. First the mixture of aniline and MWNT was heated at refluxing and was electropolymerized. Then, the An nanoparticles were dispersed into the film of PANI/MWNT by electrochemical reduction of HAuCl4. The morphology of sample was analyzed by scanning electron microscopy (SEM). Raman measurement indicates a well electrochemical deposition of PANI on MWNT, and XPS result confirms the formation of Au-0 nanoparticles. Further, cyclic voltammograms show that the film exhibits a good electrochemical activity and electrocatalysis towards ascorbic acid. Based on these investigations, a formation mechanism of the PANI/MWNT composite film was proposed.

Immobilization and direct electrochemistry of copper-containing enzymes on active carbon

Two typical and important copper-containing enzymes, laccase (Lac) and tyrosinase (Tyr), have been immobilized on the surface of active carbon with simple adsorption method. The cyclic voltammetric results indicated that the active carbon could promote the direct electron transfer of both Lac and Tyr and a pair of well-defined and nearly symmetric redox peaks appeared on the cyclic voltammograms of Lac or Tyr with the formal potential, E-0', independent on the scan rate. The further experimental results showed that the immobilized copper-containing oxidase displayed an excellent electrocatalytic activity to the electrochemical reduction of O-2. The immobilization method presented here has several advantages, such as simplicity, easy to operation and keeping good activity of enzyme etc., and could be further used to study the direct electrochemistry of other redox proteins and enzymes and fabricate the catalysts for biofuel cell.

Copper hexacyanoferrate multilayer films on glassy carbon electrode modified with 4-aminobenzoic acid in aqueous solution

4-Aminobenzoic acid (4-ABA) was covalently grafted on a glassy carbon electrode (GCE) by amine cation radical formation during the electrooxidation process in 0.1 M KCl aqueous Solution. X-ray photoelectron spectroscopy (XPS) measurement proves the presence of 4-carboxylphenylamine on the GCE. Electron transfer processes of Fe(CN)(6)(3-) in solutions of various pHs at the modified electrode are studied by both cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Changing the solution pH would result in the variation of the terminal group's charge state, based on which the surface pK(a) values were estimated. The copper hexacyanoferrate (CuHCF) multilayer films were formed on 4-ABA/GCE prepared in aqueous solution, and which exhibit good electrochemical behavior with high stability.

Effect of alkyl group length on the conductivity in the 2,3,7,8,12,13,17,18-octakis(alkyl-thio)tetraazaporphyrins and redox properties of the metallooctakis(hexyl-thio)tetraazaporphyrins

A series of 2,3,7,8,12,13,17,18-octakis(alkyl-thio)tetraazaporphyrins (H(2)OATTAP) with different alkyl chain lengths have been synthesized. Cyclic voltammetry and differential pulse voltammetry have been used to investigate the effect of the controlled lengths of the eight peripheral thioether tails on the redox behavior of the molecules. The electrochemical reduction of octakis(hexyl-thio)tetraazaporphyrins, MOHTTAP (where M = Cu, Ni), was studied in 1,2-dichloroethane at a platinum electrode. The Cu derivative was oxidized in one single-electron-transfer step to yield a pi-cation radical and reduced in three single-electron-transfer steps to yield a pi-anion radical, dianion and trianion, respectively. For the Ni derivative, electron transfer reactions involving both the central metal atom and the macrocyclic ring were observed. Electron transfer pathways are proposed based upon voltammetric and in situ spectroelectrochemical results.

Direct electrochemistry of hemoglobin in egg-phosphatidylcholine films and its catalysis to H2O2

Direct electrochemistry of hemoglobin was observed in stable thin film composed of a natural lipid (egg-phosphatidylcholine) and hemoglobin on pyrolytic graphite (PG) electrode. Hemoglobin in lipid films shows thin layer electrochemistry behavior. The formal potential Edegrees' of hemoglobin in the lipid film was linearly varied with pH in the range from 3.5 to 7.0 with a slope of -46.4 mV pH(-1) Hemoglobin in the lipid film exhibited elegant catalytic activity for electrochemical reduction of H202, based which a unmediated biosensor for H2O2 was developed.

Fabrication and characterization of ordered oligoaniline film

highly organized phenyl-capped teraniline (PC-teraniline) film at the molecular level was fabricated on carbon surfaces by electrochemical reduction of diazonium salts. Cyclic voltammetry (CV). scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) were employed for the characterization of the film.

Oriented polyoxometalate-polycation multilayers on a carbon substrate

In this paper, a new method of fabricating multilayers on a carbon substrate is presented. First, a uniformly charged carbon surface was prepared through molecular design. Then an ultrathin film consisting of layer-pairs of oppositely charged polymeric cationic poly(diallyldimethylammonium chloride) (PDDA) and silicotungstate, SiW12O404- (SiW12), was grown layer-by-layer onto the grafted carbon substrate using a molecular self-assembly technique and an electrochemical method. The technique allows one to prepare highly adherent, dense and smooth films of polyoxometalates with special properties. By combining cyclic voltammetry (CV) and X-ray (XR) reflectometry, it was determined that the average surface density of SiW12 was 2.10 x 10(-10) mol cm(-2), and the thickness increase per adsorption of PDDA-SiW12 was 1.7 +/- 0.2 nm, indicating that the amount of SiW12 anion per one layer adsorption corresponded to a monolayer coverage. Atomic force microscopy (AFM) was also used to examine the surface morphology and determine the grain size distribution and roughness for multilayer films. An increase in root-mean-square (RMS) surface roughness from 7 to 9 Angstrom was observed as the number of layer-pairs in the film increased from 2 to 6. FTIR results showed that the good stability of the multilayer films was due to Coulomb interactions between the SiW12 anion and the polymeric cations PDDA. Moreover, the multilayer films, in acidic aqueous solution, showed good electrocatalytic activity toward the reduction of NO2-, and the catalytic currents increased with increasing the layer numbers of SiW12 adsorption. These characteristics of the multilayer films might find potential applications in the field of sensors and microelectronics devices.

Conformational transition of DNA in electroreduction studied by in situ UV and CD thin layer spectroelectrochemistry

Electrochemically induced three conformational transitions of calf thymus DNA from B-10.4 to Z(10.2)-DNA and from B-10.2 to B-10.4 and to C-DNA in 10 mM phosphate buffer solution (pH 7.21) at glassy carbon electrode are found and studied by in situ circular dichroism (CD) thin layer spectroelectrochemistry with singular value decomposition least square (SVDLS) analysis. It indicates that the so-called B-10.2 form and the C-form of DNA may be composed of B-10.4 and left-A DNA and of B-10.4 and right-A DNA, respectively. The irreversible electrochemical reduction of adenine and cytosine groups in the DNA molecule is studied by UV-Vis spectroelectrochemistry. Some electrochemical parameters alphan = 0.17, E-0' = -0.70 V (vs. Ag/AgCl), and the standard heterogeneous electron transfer rate constant, k(0) = 1.8 x 10(-5) cm s(-1) are obtained by double logarithmic analysis and non-linear regression. (C) 2000 Elsevier Science B.V. All rights reserved.