Electroactive Nanostructured Membranes (ENM): Synthesis and Electrochemical Properties of Redox Mediator-Modified Gold Nanoparticles Using a Dendrimer Layer-by-Layer Approach

The layer-by-layer (LbL) self-assembly process has become an important tool in the fabrication of nanostructured devices for electrochemical applications, especially in those cases where control at the molecular level is required. In this paper we present a system based on electroactive nanostructured membranes (ENM) with ITO-PVS/PAMAM-Au LbL electrodes, in which a redox mediator (Me) is electrodeposited around the Au nanoparticles to form an ITO-PVS/PAMAM-Au@Me system. The redox mediators used were Co, Fe, Ni and Cu hexacyanoferrates. The 3-bilayer ITO-PVS/PAMAM-Au@Me system was characterized electrochemically by cyclic voltammetry and electrochemical impedance spectroscopy. All hexacyanoferrate modified electrodes showed electrocatalytic activity towards hydrogen peroxide, thus demonstrating that this new approach can be used in biosensors and nanodevices, where a redox mediator is required

Photo-induced electron transfer in supramolecular materials of titania nanostructures and cytochrome c

In the present paper, we report on the molecular interaction and photochemistry of TiO2 nanoparticles (NPs) and cytochrome c systems for understanding the effects of supramolecular organization and electron transfer by using two TiO2 structures: P25 TiO2 NPs and titanate nanotubes. The adsorption and reduction of cytochrome c heme iron promoted by photo-excited TiO2, arranged as P25 TiO2 NPs and as nanotubes, were characterized using electronic absorption spectroscopy, thermogravimetric analysis, and atomic force microscopy. In an aqueous buffered suspension (pH 8.0), the mass of cytochrome c adsorbed on the P25 TiO2 NP surface was 2.3 fold lower (0.75 μg m−2) than that adsorbed on the titanate nanotubes (1.75 μg m−2). Probably due to the high coverage of titanate nanotubes by adsorbed cytochrome c, the low amount of soluble remaining protein was not as efficiently photo-reduced by this nanostructure as it was by the P25 TiO2 NPs. Cytochrome c, which desorbed from both titanium materials, did not exhibit changes in its redox properties. In the presence of the TiO2 NPs, the photo-induced electron transfer from water to soluble cytochrome c heme iron was corroborated by the following findings: (i) identification by EPR of the hydroxyl radical production during the irradiation of an aqueous suspension of TiO2 NPs, (ii) impairment of a cytochrome c reduction by photo-excited TiO2 in the presence of dioxane, which affects the dielectric constant of the water, and (iii) change in the rate of TiO2-promoted cytochrome c reduction when water was replaced with D2O. The TiO2-promoted photo-reduction of cytochrome c was reverted by peroxides. Cytochrome c incorporated in the titanate nanotubes was also reversibly reduced under irradiation, as confirmed by EPR and UV-visible spectroscopy21974177426CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQCOORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP475235/2010-0; 2011/01541-0Sem informação2008/04849-0; 2009/15558-1; 2011/01541-

Interaction of Fe3+ meso-tetrakis (2,6-dichloro-3-sulfonatophenyl) porphyrin with cationic bilayers: magnetic switching of the porphyrin and magnetic induction at the interface

An organized multilayer was constructed by the layer-by-layer technique in which alternating layers of metalloporphyrin and dioctadecyldimethylammonium bromide bilayers were deposited onto an indium tin oxide surface electrode. The porphyrin molecules that are organized in the different layers showed a strong electroactivity with a well-defined electrochemical process. In LbL, electroactivity could be explained only by the occurrence of electron hoping. Thus, total Kohn–Sham density functional theory (KS-DFT) was performed to better understand the conditions responsible for the electroactivity of the metalloporphyrin layers intercalated by an insulating material. Total KS-DFT theory involves local density approximation energy calculations based on spin-polarized variant of KS-DFT theory. The results revealed a magnetization switching of the metalloporphyrin induced by the interaction with the surfactant bilayer accompanied by spin polarization of the porphyrin-interacting surfactant molecule. Although discrete, the surfactant magnetization had significant repercussions on the electron conductivity. Calculations also demonstrated loss of porphyrin symmetry promoted by a parent surfactant with a shorter hydrocarbon chain, ditetradecyldimethylammonium bromide. The calculation results were corroborated by experimental results obtained by the electron paramagnetic resonance and magnetic circular dichroism techniques.FAPESPCNPqCAPE

Electrochemistry of the [4Fe4S] Cluster in Base Excision Repair Proteins: Tuning the Redox Potential with DNA

Escherichia coli endonuclease III (EndoIII) and MutY are DNA glycosylases that contain [4Fe4S] clusters and that serve to maintain the integrity of the genome after oxidative stress. Electrochemical studies on highly oriented pyrolytic graphite (HOPG) revealed that DNA binding by EndoIII leads to a large negative shift in the midpoint potential of the cluster, consistent with stabilization of the oxidized [4Fe4S]^(3+) form. However, the smooth, hydrophobic HOPG surface is nonideal for working with proteins in the absence of DNA. In this work, we use thin film voltammetry on a pyrolytic graphite edge electrode to overcome these limitations. Improved adsorption leads to substantial signals for both EndoIII and MutY in the absence of DNA, and a large negative potential shift is retained with DNA present. In contrast, the EndoIII mutants E200K, Y205H, and K208E, which provide electrostatic perturbations in the vicinity of the cluster, all show DNA-free potentials within error of wild type; similarly, the presence of negatively charged poly-L-glutamate does not lead to a significant potential shift. Overall, binding to the DNA polyanion is the dominant effect in tuning the redox potential of the [4Fe4S] cluster, helping to explain why all DNA-binding proteins with [4Fe4S] clusters studied to date have similar DNA-bound potentials

Supramolecular architectures in layer-by-layer films of single-walled carbon nanotubes, chitosan and cobalt (II) phthalocyanine

The building of supramolecular structures in nanostructured films has been exploited for a number of applications, with the film properties being controlled at the molecular level. In this study, we report on the layer-by-layer (LbL) films combining cobalt (II) tetrasulfonated phthalocyanine (CoTsPc), chitosan (Chit) and single-walled carbon nanotubes (SWCNTs) in two architectures, {Chit/CoTsPc}n and {Chit-SWCNTs/CoTsPc}n (n = 1-10). The physicochemical properties of the films were evaluated and the multilayer formation was monitored with microgravimetry measurements using a quartz microbalance crystal and an electrochemical technique. According to atomic force microscopy (AFM) results, the incorporation of SWCNTs caused the films to be thicker, with a thickness ca. 3 fold that of a 2-bilayer LbL film with no SWCNTs. Cyclic voltammetry revealed a quasi-reversible, one electron process with E1/2 at -0.65 V (vs SCE) and an irreversible oxidation process at 0.80 V in a physiological medium for both systems, which can be attributed to [CoTsPc(I)]5-/[CoTsPc(II)]4- and CoTsPc(II) to CoTsPc(III), respectively. The {Chit-SWCNTs/CoTsPc}5 multilayer film exhibited an increased faradaic current, probably associated with the supramolecular charge transfer interaction between cobalt phthalocyanine and SWCNTs. The results demonstrate that an intimate contact at the supramolecular level between functional SWCNTs immobilized into biocompatible chitosan polymer and CoTsPc improves the electron flow from CoTsPc redox sites to the electrode surface.FAPEPICNPq (472369/2008-3)CAPESRede NanoBioMednBioNet Films & Sensor

Oxygen reduction and diffusion in electroactive nanostructured membranes (ENM) using a layer-by-layer dendrimer-gold nanoparticle approach

The fabrication of an electroactive nanostructured membrane (ENM) for oxygen reduction, made of layer-by-layer (LbL) films comprising Au nanoparticle-containing amine-terminated G4 PAMAM dendrimer alternated with poly(vinylsulfonic acid) (PVS) layers is reported. Electrochemical impedance spectroscopy and cyclic voltammetry show that electrodes with PVS/PAMAM-Au multiple bilayers are efficient for oxygen reduction and diffusion. A linear increase of oxygen reduction current occurs for up to 3 bilayers, with no further significant increase occurring for more than 3 bilayers. The 3-bilayer PVS/PAMAM-Au electrode, as an Au-ENM, is an attractive new system with potential for building diverse electrocatalytic devices with high molecular control.http://www.sciencedirect.com/science/article/B6TG0-4MYMFYF-9/1/8855a87cb0ee008f2c3da593edfeec5

Enhanced Charge Transport and Incorporation of Redox Mediators in Layer-by-Layer Films Containing PAMAM-Encapsulated Gold Nanoparticles

In this work, we exploit the molecular engineering capability of the layer-by-layer (LbL) method to immobilize layers of gold nanoparticles on indium tin oxide (ITO) substrates, which exhibit enhanced charge transfer and may incorporate mediating redox substances. Polyamidoamine (PAMAM generation 4) dendrimers were used as template/stabilizers for Au nanoparticle growth, with PAMAM-Au nanoparticles serving as cationic polyelectrolytes to produce LbL films with poly(vinylsulfonic acid) (PVS). The cyclic voltammetry (CV) of ITO-PVS/PAMAM-Au electrodes in sulfuric acid presented a redox pair attributed to Au surface oxide formation. The maximum kinetics adsorption is first-order, 95% of the current being achieved after only 5 min of adsorption. Electron hopping can be considered as the charge transport mechanism between the PVS/PAMAM-Au layers within the LbL films. This charge transport was faster than that for nonmodified electrodes, shown by employing hexacyanoferrate(III) as the surface reaction marker. Because the enhanced charge transport may be exploited in biosensors requiring redox mediators, we demonstrate the formation of Prussian blue (PB) around the Au nanoparticles as a proof of principle. PAMAM-Au@PB could be easily prepared by electrodeposition, following the ITO-PVS/ PAMAM-Au LbL film preparation procedure. Furthermore, the coverage of Au nanoparticles by PB may be controlled by monitoring the oxidation current

Enzyme immobilisation on electroactive nanostructured membranes (ENM): Optimised architectures for biosensing

Electroactive nanostructured membranes have been produced by the layer-by-layer (LbL) technique, and used to make electrochemical enzyme biosensors for glucose by modification with cobalt hexacyanoferrate redox mediator and immobilisation of glucose oxidase enzyme. Indium tin oxide (ITO) glass electrodes were modified with up to three bilayers of polyamidoamine (PAMAM) dendrimers containing gold nanoparticles and poly(vinylsulfonate) (PVS). The gold nanoparticles were covered with cobalt hexacyanoferrate that functioned as a redox mediator, allowing the modified electrode to be used to detect H2O2, the product of the oxidase enzymatic reaction, at 0.0 V vs. SCE. Enzyme was then immobilised by cross-linking with glutaraldehyde. Several parameters for optimisation of the glucose biosensor were investigated, including the number of deposited bilayers, the enzyme immobilisation protocol and the concentrations of immobilised enzyme and of the protein that was crosslinked with PAMAM. The latter was used to provide glucose oxidase with a friendly environment, in order to preserve its bioactivity. The optimised biosensor, with three bilayers, has high sensitivity and operational stability, with a detection limit of 6.1 [mu]M and an apparent Michaelis-Menten constant of 0.20 mM. It showed good selectivity against interferents and is suitable for glucose measurements in natural samples.http://www.sciencedirect.com/science/article/B6THP-4SDPX4X-3/1/4f9bf4e20f357feac1d3435f51b9823

A primary battery-on-a-chip using monolayer graphene

We present here a bottom-up approach for realizing on-chip on-demand batteries starting out with chemical vapor deposition-grown graphene. Single graphene monolayers contacted by electrode lines on a silicon chip serve as electrodes. The anode and cathode are realized by electrodeposition of zinc and copper respectively onto graphene, leading to the realization of a miniature graphene-based Daniell cell on a chip. The electrolyte is housed partly in a gel and partly in liquid form in an on-chip enclosure molded using a 3d printer or made out of poly (dimethylsiloxane). The realized batteries provide a stable voltage (similar to 1.1 V) for many hours and exhibit capacities as high as 15 mu Ah, providing enough power to operate a pocket calculator. The realized batteries show promise for deployment as on-chip power sources for autonomous systems in lab-on-a-chip or biomedical applications