Correction: Quantum dot-imprinted polymers with size and shell-selective recognition properties.

Correction for 'Quantum dot-imprinted polymers with size and shell-selective recognition properties' by S. Gam-Derouich et al., Chem. Commun., 2015, DOI: 10.1039/c5cc05203c.

Quantum dot-imprinted polymers with size and shell-selective recognition properties.

The emergence of nanotechnology has stimulated a great deal of research to detect engineered nanoparticles spread out in the environment. We address this issue here by designing quantum dot-imprinted polymers for the speciation of nanoparticles based on their size, shape and surface chemistry.

Rate constants in two dimensions of electron transfer between pyruvate oxidase, a membrane enzyme, and ubiquinone (coenzyme Q8), its water-insoluble electron carrier.

The functionality of the membrane-bound, ubiquinone-dependent pyruvate oxidase from the respiratory chain of Escherichia coli was reconstituted with a supported lipidic structure. The artificial structure was especially designed to allow the electrochemical control of the quinone pool through the lateral mobility of the ubiquinone (Q(8)) molecules. The kinetic coupling of the enzyme bound to the lipid structure with the quinone pool was ensured by the regeneration of the oxidized form of ubiquinone at the electrochemical interface. Such an experimental approach enabled us to carry out an unprecedented determination of the kinetic parameters controlling the reaction between the enzyme bound and the electron carrier under conditions taking rigorously into account the fact that the freedom of motion is restricted to two dimensions. The kinetic constants we found show that the activated enzyme can be efficiently regulated by the oxidation level of the quinone pool in natural membranes.

Simple formal kinetics for the reversible uptake of molecular hydrogen by [Ni-Fe] hydrogenase from Desulfovibrio gigas.

Enzymatic electrocatalysis, triggered and monitored by means of cyclic voltammetry, enabled us to achieve quantitative analysis of the kinetics of the hydrogenase catalyzed process, in the 7.8-10.0 pH range, in the presence of an electrochemically generated redox mediator. The quantitative analysis can be carried out by use of a quite simple SRC model. The simplicity of the SRC model is compatible with the existence of multiple redox microstates, which can be combined in a potential adjustable triangular mechanism consisting of three catalytic cycles, which are formally identical from the kinetic point of view. The steps involved in the kinetic control of the reversible process are H2 uptake or production at the Ni-Fe catalytic site and the intermolecular electron transfer between the mediator and the distal [4Fe-4S] cluster. The related rate constants have been determined. For the two accompanying intramolecular electron transfers which proceed at equilibrium, the equilibrium constants were found to be in very good agreement with previously published data.

Construction of multicomponent catalytic films based on avidin-biotin technology for the electroenzymatic oxidation of molecular hydrogen.

Two methods based on the avidin-biotin technology were developed for the multimonolayer immobilization of Desulfovibrio gigas hydrogenase on glassy carbon or gold electrodes. In both methods the molecular structure of the modified interface was the result of a step-by-step process. The first method alternates monolayers of avidin and biotinylated hydrogenase, the mediator (methyl viologen) being free to diffuse in the structure. In the second method, the avidin monolayers were used to immobilize both the biotinylated enzyme and a long-chain biotinylated viologen derivative. The viologen head of this hydrophilic arm shuttles the electrons between the electrode and the enzyme. The modified electrodes were evaluated for the electroenzymatic oxidation of molecular hydrogen, which has interest for the development of enzymatic fuel cells. The parameters that affect the current density of mediated oxidation of H(2) at the modified electrodes was studied. The second structure, which has given typical catalytic currents of 25 microA per cm(2) for 10 monolayers, was found clearly less efficient than the first structure (500 microA per cm(2) for 10 monolayers). In both methods the catalytic currents increased linearly with the number of monolayers of hydrogenase immobilized, which indicates that the multilayer structures are spatially ordered.

Electrochemical approach to the dynamics of molecular recognition of redox enzyme sites by artificial cosubstrates in solution and in integrated systems.

Application of antigen-antibody technology allows the attachment to an electrode surface of an enzyme monolayer structure to which both the enzyme and the mediator are bound. As illustrated with the example of glucose oxidase and a ferrocene mediator, the enzyme preserves its full activity in such structures, which may be easily reproduced. In spite of their fixation to the structure, the mobility of the ferrocene heads is sufficient to ensure that its transport to the enzyme prosthetic group is not rate determining. The reaction is rather controlled by the prior formation of a complex between the ferrocenium ion and the flavin required for electron transfer to occur. The efficiency of this step is affected by steric hindrance and the various observations made with free-moving and attached ferrocene-ended poly(ethylene glycol) chains may be rationalized by the interplay of factors controlling their distribution and shape. Analyzing the dynamics of this system, in comparison with previous systems, was thus an occasion to shed further light on the recognition phenomenon. The enzyme monolayer integrated system is a good starting point for the step-by-step construction of spatially ordered multilayered assemblies with strong catalytic efficiencies. Fast responding systems are expected both in terms of electron transport and electron transfer between the mediator and the enzyme. The spatial order resulting from the step-by-step construction should allow a much more precise analysis of electron transport and electron transfer than in conventional assemblies of redox centers. Mastering both the construction and the functioning of such systems should help the design of more complex systems, integrating additional functionalities electrically controlled by means of their electron transport/electron transfer connection to the electrode surface.

Can the combination of electrochemical regeneration of NAD+, selectivity of L-alpha-amino-acid dehydrogenase, and reductive amination of alpha-keto-acid be applied to the inversion of configuration of a L-alpha-amino-acid?

The inversion of configuration of L-alanine can be carried out by combining its selective oxidation in the presence of NAD+ and L-alanine dehydrogenase, electrochemical regeneration of the NAD+ at a carbon felt anode, and reductive amination of pyruvate, i.e., reduction of its imino derivative at a mercury cathode, the reaction mixture being buffered with concentrated ammonium/ammonia (1.28M / 1. 28M). The dehydrogenase exhibits astonishing activity and stability under such extreme conditions of pH and ionic strength. The main drawback of the process is its slowness. At best, the complete inversion of a 10 mM solution of L-alanine requires 140 h. A careful and detailed quantitative analysis of each of the key steps involved shows that the enzyme catalyzed oxidation is so thermodynamically uphill that it can be driven efficiently to completion only when both the coenzyme regeneration and the pyruvate reduction are very effective. The first condition is easily fulfilled. Under the best conditions, it is the rate of the chemical reaction producing the imine which controls the whole process kinetically. Copyright 1999 John Wiley & Sons, Inc.

Electrochemical measurement of lateral diffusion coefficients of ubiquinones and plastoquinones of various isoprenoid chain lengths incorporated in model bilayers.

The long-range diffusion coefficients of isoprenoid quinones in a model of lipid bilayer were determined by a method avoiding fluorescent probe labeling of the molecules. The quinone electron carriers were incorporated in supported dimyristoylphosphatidylcholine layers at physiological molar fractions (<3 mol%). The elaborate bilayer template contained a built-in gold electrode at which the redox molecules solubilized in the bilayer were reduced or oxidized. The lateral diffusion coefficient of a natural quinone like UQ10 or PQ9 was 2.0 +/- 0.4 x 10(-8) cm2 s(-1) at 30 degrees C, two to three times smaller than the diffusion coefficient of a lipid analog in the same artificial bilayer. The lateral mobilities of the oxidized or reduced forms could be determined separately and were found to be identical in the 4-13 pH range. For a series of isoprenoid quinones, UQ2 or PQ2 to UQ10, the diffusion coefficient exhibited a marked dependence on the length of the isoprenoid chain. The data fit very well the quantitative behavior predicted by a continuum fluid model in which the isoprenoid chains are taken as rigid particles moving in the less viscous part of the bilayer and rubbing against the more viscous layers of lipid heads. The present study supports the concept of a homogeneous pool of quinone located in the less viscous region of the bilayer.

An electrochemical approach of the redox behavior of water insoluble ubiquinones or plastoquinones incorporated in supported phospholipid layers.

Physiological mole fractions of long isoprenic chain ubiquinone (UQ[10]) and plastoquinone (PQ9) were incorporated inside a supported bilayer by vesicle fusion. The template of the bilayer was an especially designed microporous electrode that allows the direct electrochemistry of water insoluble molecules in a water environment. The artificial structure, made by self-assembly procedures, consisted of a bilayer laterally in contact with a built-in gold electrode at which direct electron transfers between the redox heads of the quinones molecules and the electrode can proceed. The mass balances of quinone and lipid in the structure were determined by radiolabeling and spectrophotometry. A dimyristoyl phosphatdylcholine stable surface concentration of 250 +/- 50 pmol x cm(-2), unaffected by the presence of the quinone, was measured in the fluid monolayer. The mole fraction of quinone was between 1 and 3 mol%, remaining unchanged when going from the vesicles to the supported layers. The lipid molecules and the quinone pool were both laterally mobile, and cyclic voltammetry was used to investigate the redox properties of UQ10 and PQ9 over a wide pH range. Below pH 12, the two electrons-two protons electrochemical process at the gold electrode appeared under kinetic control. Thus all thermodynamic deductions must be anchored in the observed reversibility of the quinone/hydroquinol anion transformation at pH > 13. Within the experimental uncertainty, the standard potentials and the pK(a)'s of the pertinent redox forms of UQ10 and PQ9 were found to be essentially identical. This differs slightly from the literature in which the constants were deduced from the studies of model quinones in mixed solvents or of isoprenic quinones without a lipidic environment.

Electrochemical measurements of the lateral diffusion of electroactive amphiphiles in supported phospholipid monolayers.

Chronocoulometry was used to characterize the fluidity and lateral diffusion coefficient of supported phospholipid bilayer assemblies. The bilayers were formed on the inner surfaces of the microporous template films of aluminum oxide on gold electrodes. The lipid monolayers were formed by adsorption and fusion of phospholipid vesicles on alkylated oxide surfaces. Octadecyltrichlorosilane (OTS) was used in the initial alkylation step. The surface concentration of the lipids in monolayer assemblies was measured by a radioactive assay method. Surface densities corresponding to 48 +/- 10 A2/molecule (DPPC) and 56 +/- 11 A2/molecule (DMPC) were obtained (for exposure times > 120 min) independent of the temperature of the vesicle's fusion (below or above chain-melting transition). Octadecylviologen (C18MV2+) was used as an electroactive probe species. Its limiting lateral diffusion coefficient in DMPC monolayers was 5 x 10(-8) cm2/s, measured as C18MV2+ mole fraction extrapolated to 0 decreasing linearly from 20 to below 1 mol%. Linear Arrhenius plots for C18MV2+ diffusion in DMPC monolayers were obtained with slopes of approximately 40 kJ/mol between 18 and 45 degrees C, demonstrating homogeneity and fluidity of the lipid monolayers. Chronocoulometry was also used to obtain lateral diffusion coefficient of ubiquinone in DMPC/OTS bilayers. A value of 1.9 x 10(-8) cm2/s at 30 degrees C was obtained.

Reconstitution of functional electron transfer between membrane biological elements in a two-dimensional lipidic structure at the electrode interface.

These studies develop a methodology to form supported phospholipid bilayers at an electrode/solution interface that models biological membrane systems. Two kinds of electrode were used, a planar gold electrode and a microporous aluminium oxide electrode on which octadecanethiol or octadecyltrichlorosilane was self-assembled. The supported lipidic structures were produced by transfer of a phospholipid monolayer by the Langmuir-Blodgett technique or by direct fusion of phospholipid vesicles. Ubiquinone was introduced into the lipidic structures during their formation; electrochemical measurements demonstrated the mobility of ubiquinone along the plane of the bilayer. A membrane enzyme, pyruvate oxidase from E. coli, was successfully incorporated into this artificial bilayer and was found to be able to exchange electrons with ubiquinone present in the bilayer.

Potentiometric and voltammetric investigations of H2/H+ catalysis by periplasmic hydrogenase from Desulfovibrio gigas immobilized at the electrode surface in an amphiphilic bilayer assembly.

Interactions of an enzyme with an organized amphipilic bilayer are explored as a general means of enzyme immobilization in electroenzymatic systems. Immobilization of Desulfovibrio gigas hydrogenase at the electrode surface involves hydrophobic interactions of the enzyme with the bilayer assembly consisting of octadecyltrichlorosilane and octadecylviologen (C18MV2+) molecules. Due to a hydrophobic character of the enzyme, these interactions direct the enzyme to occupy a central position in the bilayer's hydrocarbon region and lead to immobilization of 3 pmol/cm2 of the enzyme in the plane of the bilayer. This corresponds to 50% surface coverage. The immobilized enzyme catalyzes H2 oxidation mediated by the C18MW2+/.+ couple. This electroenzymatic scheme functions under steady-state voltammetric as well as potentiometric conditions in the pH range 3.5-10. Coupling of enzymatic activity to the electrode surface is accomplished via lateral diffusion of the octadecylviologen molecules along the bilayer assembly.

Displacement of equilibrium in electroenzymatic reactor for acetaldehyde production using yeast alcohol dehydrogenase.

Electrochemical regeneration of NAD was performed in a bench scale reactor in which yeast alcohol dehydrogenase catalyzed the oxidation of ethanol. By recycling one of the products of the reaction, it was possible to displace the equilibrium and favor the production of acetaldehyde. The flow-through electrode was made of graphite felt and had a specific area of 275 cm(-1). A mathematical model taking into account the enzymatic and electrochemical reaction rates as well as the mass transfer to the electrode was used to analyze the results. The limiting steps in the reactor are the electrochemical reaction for low potentials and the cofactor mass transfer for high potentials.

The effect of electrochemical regeneration upon the enzymatic catalysis of a thermodynamically unfavorable reaction.

The association between enzymatic and electrochemical reactions, enzymatic electrocatalysis, had proven to be a very powerful tooth in both analytical and synthetic fields. However, most of the combinations studied have involved enzymatic catalysis of irreversible or quasi-irreversible reaction. In the present work, we have investigated the possibility of applying enzymatic electrocatalysis to a case where the electrochemical reaction drives a thermodynamically unfavorable reversible reaction. Such thermodynamically unfavorable reactions include most of the oxidations catalyzed by dehydrogenases. Yeast alcohol dehydrogenase (E.C. 1.1.1.1) was chosen as a model enzyme because the oxidation of ethanol is thermodynamically very unfavorable and because its kinetics are well known. The electrochemical reaction was the oxidation of NADH which is particularly attractive as a method of cofactor regeneration. Both the electrochemical and enzymatic reactions occur in the same batch reactor in such a way that electrical energy is the only external driving force. Two cases were experimentally and theoretically developed with the enzyme either in solution or immobilized onto the electrode's surface. In both cases, the electrochemical reaction could drive the enzymatic reaction by NADH consumption in solution or directly in the enzyme's microenvironment. However even for a high efficiency of NADH consumption, the rate of enzymatic catalysis was limited by product (acetaldedehyde) inhibition. Extending this observation to the subject of organic synthesis catalyzed by dehydrogenases, we concluded that thermodynamically unfavorable reaction and can only be used in a process if efficient NAD regeneration and product elimination are simultaneously carried out within the reactor.

A heterogeneous immunoassay performed on a rotating carbon disk electrode with electrocatalytic detection. Mass transfer control of the capture of an enterotoxin.

An ELISA procedure for the determination of enterotoxin A from Staphylococcus aureus conducted on the surface of a glassy carbon electrode is described. The electrocatalytic detection of the immobilized labelled second antibody is based upon the electrochemical reaction and the enzymatic catalysis occurring on the same surface. The indirect quantification of the bound antigen is, therefore, very sensitive (10(-15) mol cm-2). This heterogeneous technique was used to study the kinetics of antigen binding to the immunological solid phase, the mass transfer of the antigen being controlled under well-defined hydrodynamic conditions. The experiments were performed with a rotating solid phase disk in such a way that thickness of the diffusion layer was known. We found that the capture of the antigen by the immobilized monoclonal antibody was solely limited by diffusion. A simple theoretical model permitted the amount of bound antigen and the sensitivity of the method to be predicted as a function of the incubation time, the rotational speed of the solid phase and the volume of the sample. Both the theory and the experimental results indicate that the assay may be performed with the sample volume undefined.

Electrochemical regeneration of NAD in a plug-flow reactor.

Electrochemical regeneration of NAD was performed at a laboratory preparative scale to illustrate both the efficiency and intrinsic simplicity of the electrochemical method. A powerful plug-flow reactor was realized with a flow through graphite felt electrode, the ratio of the effective area of electrode/volume of reactor increased to 380 cm(2)/cm(3). This graphite-felt electrode was able to oxidize NADH coenzyme at a very low overvoltage. On the example of the gluconic acid production catalyzed by glucose dehydrogenase, current as high as 0.1 A was obtained in experience where enzymatic activity was the main limitation. In confirmation of our previous work, the results show that the yield of NADH electrochemical oxidation is better than 99.95%.

Direct electrochemical determination of glucose oxidase in biological samples.

An electrochemical method for the quantitation of glucose oxidase in murine plasma and tissues has been developed. Instead of oxygen, this method uses benzoquinone as an artificial cosubstrate of glucose oxidase. The quantitative detection of the enzymatically produced hydroquinone by controlled-potential amperometry allows measurement of glucose oxidase concentrations in biological samples. The use of an internal standard corrects for all possible interfering effects. We demonstrated a 10-fold increase in sensitivity, as well as the ability to work in turbid media, in comparison to spectrophotometric methods.