The photocurrents generated by a monolayer consisting of a CdS nanoparticles/acetylcholine esterase hybrid system associated with an electrode are controlled by the enzyme inhibitors.
Oppositely charged polyelectrolyte assemblies of poly(acrylic acid) and poly-l-lysine are deposited as
alternate layers on cystamine-functionalized Au surfaces. Microgravimetric, quartz-crystal-microbalance,
measurements and ellipsometric studies reveal a nonlinear increase in the polymer assembly thickness
upon the buildup of the polymer layers. This is attributed to the swelling of the polymer upon the stepwise
assembly of the layered film. The interfacial and intra-assembly properties of the polyelectrolyte systems
were characterized by Faradaic impedance spectroscopy. In the presence of Fe(CN)6
3-/Fe(CN)6
4- as the
redox label, all assemblies that terminate with a negatively charged interface are characterized by a high
interfacial electron-transfer resistance, originating from the electrostatic repulsion of the redox label from
the interface. The interfacial electron-transfer resistance also increases as the number of layers, or assembly
thickness, increases. For assemblies terminated with a positively charged polyelectrolyte, a very low
interfacial electron-transfer resistance for the redox process of the negatively charged redox probe is
detected. This is attributed to a neutralized, porous structure of the polymer assembly. For the positively
charged redox label, protonated N,N-dimethylaminomethyl-ferrocene, similar results are observed for the
assemblies with the opposite dependence on the charge of the terminal layer. The electrodes functionalized
with the polymers were used for the selective oxidation of a mixture consisting of Fe(CN)6
4- and protonated
N,N-dimethylaminomethyl-ferrocene.
A new approach to the construction of functional materials is demonstrated. Gold nanoparticles immobilized in a polymer hydrogel allow external control over the conductivity by switching the gel between its swollen and shrunken states. This reversible cycling (see Figure) controls the interparticle distances between the Au nanoparticles, which may lead to new methods in sensing and catalysis.
A series of single-cysteine-containing cytochrome c, Cyt c, heme proteins including the wild-type Cyt c (from Saccharomyces cerevisiae) and the mutants (V33C, Q21C, R18C, G1C, K9C and K4C) exhibit direct electrical contact with Au-electrodes upon covalent attachment to a maleimide monolayer associated with the electrode. With the G1CÈCyt c mutant, which includes the cysteine residue in the polypeptide chain at position 1, the potential-induced switchable control of the interfacial electron transfer was observed. This heme protein includes a positively charged protein periphery that surrounds the attachment site and faces the electrode surface. Biasing of the electrode at a negative potential ([0.3 V vs. SCE) attracts the reduced Fe(II)-Cyt c heme protein to the electrode surface. Upon the application of a double-potential-step chronoamperometric signal onto the electrode, where the electrode potential is switched to ]0.3 V and back to [0.3 V, the kinetics of the transient cathodic current, corresponding to the re-reduction of the Fe(III)-Cyt c, is controlled by the time interval between the oxidative and reductive potential steps. While a short time interval results in a rapid interfacial electron-transfer, s~1, long time intervals lead to a slow interfacial electron transfer to the Fe(III)k et 1 \ 20 Cyt c, s ~1. The fast interfacial electron-transfer rate-constant is attributed to the k et 2 \ 1.5 reduction of the surface-attracted Fe(III)-Cyt c. The slow interfacial electron-transfer rate constant is attributed to the electrostatic repulsion of the positively charged Cyt c from the electrode surface, resulting in long-range electron transfer exhibiting a lower rate constant. At intermediate time intervals between the oxidative and reductive steps, two populations of Cyt c, consisting of surface-attracted and surface-repelled heme proteins, are observed. Crosslinking of a layered affinity complex between the Cyt c and cytochrome oxidase, COx, on an Au-electrode yields an electrically-contacted, integrated, electrode for the four-electron reduction of to water. Kinetic analysis reveals that the rate-limiting step O 2 in the bioelectrocatalytic reduction of by the integrated Cyt c/COx electrode is the O 2 primary electron transfer from the electrode support to the Cyt c units.
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