Immobilizing redox enzymes at mesoporous and nanostructured electrodes

Three key challenges are stimulating intensive research in the development of productive direct electron transfer mode enzyme electrodes: proper enzyme orientation, high enzyme loading, and full retention of enzyme activity. In this review, we summarize some significant advances that have been reported in the last years on the design of mesoporous and nanostructured electrodes as enzyme scaffolds and of innovative methodologies for wiring enzymes to electrodes. Particular attention is given to investigations on physical factors that determine a favorable enzyme immobilization, to provide rational guidelines for the design of productive enzymatic electrodes. Finally, some emerging trends focused on the spatial organization of either single enzymes or enzyme cascades are also briefly addressed.     

Using magnetic coupling to eliminate right half-plane zeros in boost converters

A dynamic analysis of the boost converter with an output filter reveals that magnetic coupling between inductors allows transfer of the zeros to the left half-plane of the control-to-output transfer function. Similar results requiring smaller magnetic components are obtained by combining magnetic coupling with damping of the output filter. The analysis is based on the application of the Routh-Hurtwitz's criterion to the numerator of the transfer function in order to derive the design conditions for the converter parameters. A design example illustrates the procedure, and experimental results verify the theoretical predictions. The application of these techniques will allow the design of high efficiency voltage boost-based regulators with dynamic behavior similar to buck-derived structures.     

Intermolecular interactions in electroactive thiol monolayers probed by linear scan voltammetry

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Large-signal modeling and simulation of switching DC-DC converters

A general nonlinear continuous formulation procedure for large-signal analysis of switching DC-DC converters is presented. The method can be applied in either of the two conduction modes, and it is easily programmed for computer-aided analysis with small simulation time. A boost regulator operating in constant-frequency current-programmed mode is used to illustrate the application of the method. A stability graph is subsequently developed to facilitate the design of DC-DC switching regulators for large-signal applications. The graph provides an estimation of the values of input voltage and load resistance leading to a stable regulator behavior     

Prediction of subharmonic oscillation in switching regulators: from a slope to a ripple standpoint

New exact critical conditions for predicting subharmonic instability in switching regulators are approximated by simple design-oriented expressions valid under practical conditions. These simplified expressions contain the ripple and slope information of the feedback control signal. Depending on the converter topology, the controller used and values of parasitic parameters, either the slope or the ripple can be dominant in predicting instability. A discussion on the validity of this interpretation is illustrated through six different examples of switching regulators using the concept of the spectral radius and the relative degree of the system loop. Using this approach, the boundary between the desired stable region and the subharmonic instability can be easily obtained. The theoretical results are validated by means of numerical simulations.     

Ionic strength effects in binary aqueous mixtures: Study of the reaction between Co(en)2(2-pzCO2)2+ and Fe(CN)5H2O3−

The reaction between Co(en)₂(2?pzCO₂)²⁺ (bis-ethylenediamine (2-pyrazinecarboxylato)cobalt(III)) and Fe(CN)₅H₂O^3? (aquopentacyanoferrate(II)) to form the binuclear complex [(en)₂Co( μ-pzCO₂)Fe(CN)₅]^? has been studied in several isodielectric binary mixtures at 298.2 K (cosolvents: methanol, ethanol, tertbutyl alcohol, ethyleneglycol, and glycerol). Results were rationalized by using a free energy relationship. The importance of correcting the rate constants obtained in the different mixtures from the ionic strength influence has been shown. © 1995 John Wiley & Sons, Inc.     

Experimental study of the interplay between long-range electron transfer and redox probe permeation at self-assembled monolayers: evidence for potential-induced ion gating

Evidence for the competition between long-range electron transfer across self-assembled monolayers (SAMs) and incorporation of the redox probe into the film is reported for the electroreduction of Ru(NH(3)) at hydroxyl- and carboxylic-acid-terminated SAMs on a mercury electrode, by using electrochemical techniques that operate at distinct time scales. Two limiting voltammetric behaviors are observed, consistent with a diffusion control of the redox process at mercaptophenol-coated electrodes and a kinetically controlled electron transfer reaction in the presence of neutral HS-(CH(2))(10)-COOH and HS-(CH(2))(n)()-CH(2)OH (n = 3, 5, and 10) SAMs. The monolayer thickness dependence of the standard heterogeneous electron transfer rate constant shows that the electron transfer plane for the reduction of Ru(NH(3)) at hydroxyl-terminated SAMs is located outside the film | solution interface at short times. However, long time scale experiments provide evidence for the occurrence of potential-induced gating of the adsorbed structure in some of the monolayers studied, which takes the form of a chronoamperometric spike. Redox probe permeation is shown to be a kinetically slow process, whose activation strongly depends on redox probe concentration, applied potential, and chemical composition of the intervening medium. The obtained results reveal that self-assembled monolayers made of mercaptobutanol and mercaptophenol preserve their electronic barrier properties up to the reductive desorption potential of a fully grown SAM, whereas those of mercaptohexanol, mercaptoundecanol, and mercaptoundecanoic acid undergo an order/disorder transition below a critical potential, which facilitates the approach of the redox probe toward the electrode surface.     

Chemical reactivity and electrochemistry of metal-metal-bonded zincocenes

Attempts to prepare mixed-ligand zinc-zinc-bonded compounds that contain bulky C(5)Me(5) and terphenyl groups, [Zn(2)(C(5)Me(5))(Ar')], lead to disproportionation. The resulting half-sandwich Zn(II) complexes [(η(5)-C(5)Me(5))ZnAr'] (Ar' = 2,6-(2,6-(i)Pr(2)C(6)H(3))(2)-C(6)H(3), 2; 2,6-(2,6-Me(2)C(6)H(3))(2)-C(6)H(3), 3) can also be obtained from the reaction of [Zn(C(5)Me(5))(2)] with the corresponding LiAr'. In the presence of pyr-py (4-pyrrolidinopyridine) or DBU (1,8-diazabicyclo[5.4.0]undec-7-ene), [Zn(2)(η(5)-C(5)Me(5))(2)] reacts with C(5)Me(5)OH to afford the tetrametallic complexes [Zn(2)(η(5)-C(5)Me(5))L(μ-OC(5)Me(5))](2) (L = pyr-py, 6; DBU, 8), respectively. The bulkier terphenyloxide Ar(Mes)O(-) group (Ar(Mes) = 2,6-(2,4,6-Me(3)C(6)H(2))(2)-C(6)H(3)) gives instead the dimetallic compound [Zn(2)(η(5)-C(5)Me(5))(OAr(Mes))(pyr-py)(2)], 7, that features a terminal Zn-OAr(Mes) bond. DFT calculations on models of 6-8 and also on the Zn-Zn-bonded complexes [Zn(2)(η(5)-C(5)H(5))(OC(5)H(5))(py)(2)] and [(η(5)-C(5)H(5))ZnZn(py)(3)](+) have been performed and reveal the nonsymmetric nature of the Zn-Zn bond with lower charge and higher participation of the s orbital of the zinc atom coordinated to the cyclopentadienyl ligand with respect to the metal within the pseudo-ZnL(3) fragment. Cyclic voltammetric studies on [Zn(2)(η(5)-C(5)Me(5))(2)] have been also carried out and the results compared with the behavior of [Zn(C(5)Me(5))(2)] and related magnesium and calcium metallocenes.