Chitosan-cross-linked osmium polymer composites as an efficient platform for electrochemical biosensors.

A new family of chitosan-cross-linked osmium polymer composites was prepared and its electrochemical properties were examined. The composites were prepared by quaternization of the poly(4-vinylpyridine) osmium bipyridyl polymer (PVP-Os) which was then cross-linked with chitosan, yielding PVP-Os/chitosan. Films made of the composites showed improved mass and electron transport owing to the porous and hydrophilic structure which is derived from the cross-links between the Os polymer and chitosan. The rate for glucose oxidation was enhanced four times when glucose oxidase (GOx) was immobilized on PVP-Os/chitosan compared immobilization on PVP-Os.

The Photoactive Photosynthetic Reaction Center of a Rhodobacter sphaeroides Mutant Lacking 3-Vinyl (Bacterio)Chlorophyllide a Hydratase Contains 3-Vinyl Bacteriochlorophyll a.

Rhodobacter sphaeroides mutant BF-lacking 3-vinyl (bacterio)chlorophyllide a hydratase (BchF)-accumulates chlorophyllide a (Chlide a) and 3-vinyl bacteriochlorophyllide a (3V-Bchlide a). BF synthesizes 3-vinyl bacteriochlorophyll a (3V-Bchl a) through prenylation of 3V-Bchlide a and assembles a novel reaction center (V-RC) using 3V-Bchl a and Mg-free 3-vinyl bacteriopheophytin a (3V-Bpheo a) at a molar ratio of 2:1. We aimed to verify whether a bchF-deleted R. sphaeroides mutant produces a photochemically active RC that facilitates photoheterotrophic growth. The mutant grew photoheterotrophically-implying a functional V-RC-as confirmed by the emergence of growth-competent suppressors of bchC-deleted mutant (BC) under irradiation. Suppressor mutations in BC were localized to bchF, which diminished BchF activity and caused 3V-Bchlide a accumulation. bchF expression carrying the suppressor mutations in trans resulted in the coproduction of V-RC and wild-type RC (WT-RC) in BF. The V-RC had a time constant (τ) for electron transfer from the primary electron donor P (a dimer of 3V-Bchl a) to the A-side containing 3V-Bpheo a (HA) similar to that of the WT-RC and a 60% higher τ for electron transfer from HA to quinone A (QA). Thus, the electron transfer from HA to QA in the V-RC should be slower than that in the WT-RC. Furthermore, the midpoint redox potential of P/P+ of the V-RC was 33 mV more positive than that of the WT-RC. R. sphaeroides, thus, synthesizes the V-RC when 3V-Bchlide a accumulates. The V-RC can support photoheterotrophic growth; however, its photochemical activity is inferior to that of the WT-RC. IMPORTANCE 3V-Bchlide a is an intermediate in the bacteriochlorophyll a (Bchl a)-specific biosynthetic branch and prenylated by bacteriochlorophyll synthase. R. sphaeroides synthesizes V-RC that absorbs light at short wavelengths. The V-RC was not previously discovered because 3V-Bchlide a does not accumulate during the growth of WT cells synthesizing Bchl a. The levels of reactive oxygen species increased with the onset of photoheterotrophic growth in BF, resulting in a long lag period. Although the inhibitor of BchF is unknown, the V-RC may act as a substitute for the WT-RC when BchF is completely inhibited. Alternatively, it may act synergistically with WT-RC at low levels of BchF activity. The V-RC may broaden the absorption spectra of R. sphaeroides and supplement its photosynthetic ability at various wavelengths of visible light to a greater extent than that by the WT-RC alone.

A miniature, nongassing electroosmotic pump operating at 0.5 V.

Electroosmotic pumps are arguably the simplest of all pumps, consisting merely of two flow-through electrodes separated by a porous membrane. Most use platinum electrodes and operate at high voltages, electrolyzing water. Because evolved gas bubbles adhere and block parts of the electrodes and the membrane, steady pumping rates are difficult to sustain. Here we show that when the platinum electrodes are replaced by consumed Ag/Ag(2)O electrodes, the pumps operate well below 1.23 V, the thermodynamic threshold for electrolysis of water at 25 °C, where neither H(2) nor O(2) is produced. The pumping of water is efficient: 13 000 water molecules are pumped per reacted electron and 4.8 mL of water are pumped per joule at a flow rate of 0.13 mL min(-1) V(-1) cm(-2), and a flow rate per unit of power is 290 mL min(-1) W(-1). The water is driven by protons produced in the anode reaction 2Ag(s) + H(2)O → Ag(2)O(s) + 2H(+) + 2e(-), traveling through the porous membrane, consumed by hydroxide ions generated in the cathode reaction Ag(2)O(s) + 2 H(2)O + 2e(-) → 2Ag(s) + 2 OH(-). A pump of 2 mm thickness and 0.3 cm(2) cross-sectional area produces flow of 5-30 µL min(-1) when operating at 0.2-0.8 V and 0.04-0.2 mA. Its flow rate can be either voltage or current controlled. The flow rate suffices for the delivery of drugs, such as a meal-associated boli of insulin.

Ligand topology effect on the reactivity of a mononuclear nonheme iron(IV)-oxo complex in oxygenation reactions.

Mononuclear nonheme iron(IV)-oxo complexes with two different topologies, cis-α-[Fe(IV)(O)(BQCN)](2+) and cis-ß-[Fe(IV)(O)(BQCN)](2+), were synthesized and characterized with various spectroscopic methods. The effect of ligand topology on the reactivities of nonheme iron(IV)-oxo complexes was investigated in C-H bond activation and oxygen atom-transfer reactions; cis-α-[Fe(IV)(O)(BQCN)](2+) was more reactive than cis-ß-[Fe(IV)(O)(BQCN)](2+) in the oxidation reactions. The reactivity difference between the cis-α and cis-ß isomers of [Fe(IV)(O)(BQCN)](2+) was rationalized with the Fe(IV/III) redox potentials of the iron(IV)-oxo complexes: the Fe(IV/III) redox potential of the cis-α isomer was 0.11 V higher than that of the cis-ß isomer.

Nafion-coating of the electrodes improves the flow-stability of the Ag/SiO2/Ag2O electroosmotic pump.

When a current or a voltage is applied across the ceramic membrane of the nongassing Ag/Ag(2)O-SiO(2)-Ag/Ag(2)O pump, protons produced in the anodic reaction 2Ag(s) + H(2)O → Ag(2)O(s) + 2H(+) + 2e(-) are driven to the cathode, where they are consumed by the reaction Ag(2)O(s) + H(2)O + 2e(-) → 2Ag(s) + 2 OH(-). The flow of water is induced by momentum transfer from the electric field-driven proton-sheet at the surface of the ceramic membrane. About 10(4) water molecules flowed per reacted electron. Because dissolved ions decrease the field at the membrane surface, the flow decreases upon increasing the ionic strength. For this reason Ag(+) ions introduced through the anodic reaction and by dissolution of Ag(2)O decrease the flow. Their accumulation is reduced by applying Nafion-films to the electrodes. The 20 µL min(-1) flow rate of 6 mm i.d. pumps with Nafion coated electrodes operate daily for 5 min at 1 V for 1 month, for 70 h when the pump is pulsed for 30 s every 30 min, and for 2 h when operating continuously.

Dioxygen activation by a non-heme iron(II) complex: formation of an iron(IV)-oxo complex via C-H activation by a putative iron(III)-superoxo species.

Iron(III)-superoxo intermediates are believed to play key roles in oxygenation reactions by non-heme iron enzymes. We now report that a non-heme iron(II) complex activates O(2) and generates its corresponding iron(IV)-oxo complex in the presence of substrates with weak C-H bonds (e.g., olefins and alkylaromatic compounds). We propose that a putative iron(III)-superoxo intermediate initiates the O(2)-activation chemistry by abstracting a H atom from the substrate, with subsequent generation of a high-valent iron(IV)-oxo intermediate from the resulting iron(III)-hydroperoxo species.

Axial ligand tuning of a nonheme iron(IV)-oxo unit for hydrogen atom abstraction.

The reactivities of mononuclear nonheme iron(IV)-oxo complexes bearing different axial ligands, [Fe(IV)(O)(TMC)(X)](n+) [where TMC is 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane and X is NCCH(3) (1-NCCH(3)), CF(3)COO(-) (1-OOCCF(3)), or N(3)(-) (1-N(3))], and [Fe(IV)(O)(TMCS)](+) (1'-SR) (where TMCS is 1-mercaptoethyl-4,8,11-trimethyl-1,4,8,11-tetraazacyclotetradecane), have been investigated with respect to oxo-transfer to PPh(3) and hydrogen atom abstraction from phenol O H and alkylaromatic C H bonds. These reactivities were significantly affected by the identity of the axial ligands, but the reactivity trends differed markedly. In the oxidation of PPh(3), the reactivity order of 1-NCCH(3) > 1-OOCCF(3) > 1-N(3) > 1'-SR was observed, reflecting a decrease in the electrophilicity of iron(IV)-oxo unit upon replacement of CH(3)CN with an anionic axial ligand. Surprisingly, the reactivity order was inverted in the oxidation of alkylaromatic C H and phenol O H bonds, i.e., 1'-SR > 1-N(3) > 1-OOCCF(3) > 1-NCCH(3). Furthermore, a good correlation was observed between the reactivities of iron(IV)-oxo species in H atom abstraction reactions and their reduction potentials, E(p,c), with the most reactive 1'-SR complex exhibiting the lowest potential. In other words, the more electron-donating the axial ligand is, the more reactive the iron(IV)-oxo species becomes in H atom abstraction. Quantum mechanical calculations show that a two-state reactivity model applies to this series of complexes, in which a triplet ground state and a nearby quintet excited-state both contribute to the reactivity of the complexes. The inverted reactivity order in H atom abstraction can be rationalized by a decreased triplet-quintet gap with the more electron-donating axial ligand, which increases the contribution of the much more reactive quintet state and enhances the overall reactivity.

A nitrite biosensor based on co-immobilization of nitrite reductase and viologen-modified chitosan on a glassy carbon electrode.

An electrochemical nitrite biosensor based on co-immobilization of copper-containing nitrite reductase (Cu-NiR, from Rhodopseudomonas sphaeroides forma sp. denitrificans) and viologen-modified chitosan (CHIT-V) on a glassy carbon electrode (GCE) is presented. Electron transfer (ET) between a conventional GCE and immobilized Cu-NiR was mediated by the co-immobilized CHIT-V. Redox-active viologen was covalently linked to a chitosan backbone, and the thus produced CHIT-V was co-immobilized with Cu-NiR on the GCE surface by drop-coating of hydrophilic polyurethane (HPU). The electrode responded to nitrite with a limit of detection (LOD) of 40 nM (S/N = 3). The sensitivity, linear response range, and response time (t(90%)) were 14.9 nA/µM, 0.04-11 µM (r(2) = 0.999) and 15 s, respectively. The corresponding Lineweaver-Burk plot showed that the apparent Michaelis-Menten constant (K(M) (app)) was 65 µM. Storage stability of the biosensor (retaining 80% of initial activity) was 65 days under ambient air and room temperature storage conditions. Reproducibility of the sensor showed a relative standard deviation (RSD) of 2.8% (n = 5) for detection of 1 µM of nitrite. An interference study showed that anions commonly found in water samples such as chlorate, chloride, sulfate and sulfite did not interfere with the nitrite detection. However, nitrate interfered with a relative sensitivity of 64% and this interference effect was due to the intrinsic character of the NiR employed in this study.

Dioxygen activation by mononuclear nonheme iron(II) complexes generates iron-oxygen intermediates in the presence of an NADH analogue and proton.

One primary goal in biomimetic research is to understand mechanisms of dioxygen activation, structures of reactive intermediates, and reactivities of the intermediates involved in catalytic oxidation reactions by metalloenzymes, such as heme and nonheme iron oxygenases. In this communication, we have reported the first example of generating nonheme iron(III)-hydroperoxo and iron(IV)-oxo complexes by activating O(2) with a biologically important electron donor, an NADH analogue, and an acid. The formation of iron(III)-hydroperoxo and iron(IV)-oxo complexes was found to depend on the supporting ligands. We have also demonstrated that high-spin nonheme iron(II) complexes with a low oxidation potential are able to bind and activate O(2) to generate the iron-oxygen intermediates.

Electrocatalytic Conversion of Carbon Dioxide and Nitrate Ions to Urea by a Titania-Nafion Composite Electrode.

CO2 and nitrate ions were successfully converted to urea by a TiO2 -Nafion nanocomposite electrode under ambient conditions. The composite electrode was constructed by dropcasting the mixture of P25 titania and Nafion solution on an indium-doped tin oxide (ITO) electrode. When the electrode was electrolyzed in CO2 -saturated 0.1 m KNO3 (pH 4.5) solution at -0.98 V versus Ag/AgCl, urea was formed with a Faradaic efficiency of 40 %. The other reduced products obtained were NH3 , CO, and H2 .

A miniature biofuel cell operating at 0.78 V.

We report the highest voltage miniature biofuel cell to date, a membrane-less cell operating at 37 degrees C in pH 5 buffer at 0.78 V.

Hydroquinone modified chitosan/carbon film electrode for the selective detection of ascorbic acid.

A redox active polymer, hydroquinone modified chitosan (Q-chitosan) was synthesized and characterized by IR and (1)H NMR spectroscopy. The nanocomposite of Q-chitosan with carbon was prepared and used to construct a stable conductive film on the electrode surface. The SEM studies confirms that the Q-chitosan/C composite covers the electrode surface with polymer embedded 50 nm size carbon particles. The formal redox potential, E(0) of the Q-chitosan/C composite modified electrode is evaluated to be 0.09 V vs. Ag/AgCl at pH 7 and the composite electrode shows an excellent electrocatalytic activity toward the ascorbic acid (AA) at 0.0 V vs. Ag/AgCl. It is more negative potential than the similar AA biosensors and the lower potential oxidation of AA enabled the selective detection of AA over major interferences such as dopamine and uric acid. Using amperometric method, the linear range for ascorbic acid is estimated to be 10 µM to 5 mM with the detection limit of 3 µM and the sensitivity is 0.07 6µA µM(-1)cm(-2).

Reduction of CO2 to CO at low overpotential in neutral aqueous solution by a Ni(cyclam) complex attached to poly(allylamine).

Less is more: Nickel cyclam complexes are known as efficient catalysts for the electrochemical reduction of CO(2) to CO, despite having overvoltages of more than 0.6 V. Incorporating [Ni(cyclam)](2+) into poly(allylamine) through axial coordination of pyridine enables the electrochemical reduction of CO(2) to CO near the thermodynamic potential of -0.78 V (vs. Ag/AgCl) at pH 8 with a current efficiency of 90 %.

A miniature, single use, skin-adhered, low-voltage, electroosmotic pumping-based subcutaneous infusion system.

A programmable, skin-attached, 36 × 30 × 8 mm system for subcutaneous infusion of 1.2 mL of a drug solution is described. The system is intended to be replaced daily. It comprises a 20 × 14 × 8 mm electronic controller and power source, an 8 mm diameter 2 mm thick electroosmotic pump, a two-compartment reservoir for a pumped water and a drug solution, an adhesive tape for attachment to the skin, and a 6 mm long 27 gauge needle. Its removable electronic controller programs the dose rate and dose and is re-used. The electroosmotic pump consists of a porous ceramic membrane sandwiched between a pair of Ag/Ag2O plated carbon paper electrodes. It operates below 1.23 V, the thermodynamic threshold for water electrolysis without gassing. The flow rate can be adjusted between 4 and 30 µL min(-1) by setting either by the voltage (0.2-0.8 V) or the current (30-200 µA). For average flow rates below 4 µL min(-1), the pump is turned on and off intermittently. For example, a flow rate of 160 µL day(-1), i.e., 0.13 µL min(-1) for basal insulin infusion in type 1 diabetes management, is obtained when 10 s pulses of 75 µA is applied every 15 min. High flow rates of 10-30 µL min(-1), required for prandial insulin administration, are obtained when the pump operates at 50-200 µA. To prevent fouling by the drug, only pure water passes the pump; the water pushes a drop of oil, which, in turn, pushes the drug solution.

Facile preparation of Fe2O3 thin film with photoelectrochemical properties.

We report a simple approach for preparing undoped and Pt-doped Fe(2)O(3) thin films with excellent photoactivity via facile hydrothermal growth. The photocurrent densities of undoped and Pt-doped Fe(2)O(3) thin films were recorded up to 1.2 and 1.38 mA cm(-2) at 0.23 V Ag/AgCl under 1 sun illumination, respectively.

Ionic conduction in Zn3(PO4)2.4H2O enables efficient discharge of the zinc anode in serum.

Batteries contain corrosive or reactive components necessitating containment in a case, setting a limit to their miniaturization. Miniature, small-capacity batteries could power medical sensors in the body, for example sensing glucose for diabetes management. A miniature, case-less Zn-Ag/AgCl battery would consist merely of a zinc anode and a bioinert gel-coated Ag/AgCl cathode, if both operated efficiently in the interstitial fluid. Such a battery has not previously been built, primarily because of rapid corrosion of Zn. We show that the corrosion of zinc is significantly reduced by growth of Zn2+-ion-conducting, O2-impermeable, hopeite [Zn3(PO4)2.4H2O] lamellae on the Nafion-coated Zn anode. The hopeite lamellae allow discharge of the Zn anodes over three weeks at 86% current efficiency in physiological buffer and at 60% efficiency over two weeks in serum. The Zn|physiological buffer|Ag/AgCl cell operates at 1.00 V at 13 muA cm-2 and at 0.94 V at 0.2 mA cm-2 anodic current density.

Oxygen is electroreduced to water on a "wired" enzyme electrode at a lesser overpotential than on platinum.

The first enzyme-based catalyst that is superior to platinum in the four-electron electroreduction of oxygen to water is reported. The smooth Pt cathode reached half and 90% of the mass transport-limited current density at respective overpotentials of -0.4 and -0.58 V in 0.5 M sulfuric acid, and only at even higher overpotentials in pH 7.2 phosphate buffer. In contrast, the smooth "wired" bilirubin oxidase cathode reached half and 90% of the mass transport-limited current density at respective overpotentials as low as -0.2 and -0.25 V. The mass transport-limited current density for the smooth "wired" enzyme cathode in PBS was twice that with smooth Pt in 0.5 M sulfuric acid. Under 1 atm O2 pressure, O2 was electroreduced to water on a polished carbon cathode, coated with the "wired" BOD film, in pH 7.2 saline buffer (PBS) at an overpotential of -0.31 V at a current density of 9.5 mA cm-2. At the same overpotential, the current density of the polished platinum cathode in 0.5 M H2SO4 was 16-fold lower, only 0.6 mA cm-2.