Biomedical subjects

Allen J Bard

Publications and source records attributed to Allen J Bard.

At least 19 recordsLinked to original sources

PubMed2007

Electrogenerated chemiluminescence. 83. Immunoassay of human C-reactive protein by using Ru(bpy)3(2+)-encapsulated liposomes as labels.

Liposomes ( approximately 100-nm diameter) containing Ru(bpy)32+ (bpy = 2,2'-bipyridine) were prepared as an electrogenerated chemiluminescent (ECL) tag for a sandwich-type immunoassay of human C-reactive protein (CRP). Polyclonal human CRP antibodies were introduced onto liposomes and magnetic beads through biotin-streptavidin interaction. The antigen-antibody conjugates formed on addition of a CRP-containing sample were separated from unreacted species magnetically. Addition of 0.1 M tri-n-propylamine and 0.1 M phosphate buffer (pH 7.6) containing 0.1 M NaCl and 1% (v/v) Triton X-100 caused liberation of the Ru(bpy)32+ from the liposome. ECL obtained in this medium showed a detection limit of 100 ng/mL for human CRP with good linearity of ECL intensity versus antigen concentration over the range 100 ng/mL-10 microg/mL.

2,2'-Dipyridyl↗

PubMed2006

Generation and detection of single metal nanoparticles using scanning electrochemical microscopy techniques.

Different pathways towards the generation and detection of a single metal nanoparticle (MNP) on a conductive carbon support for testing as an electrocatalyst are described. Various approaches were investigated including interparticle distance enhancement, electrochemical and mechanical tip-substrate MNP transfer onto macroscopic surfaces, scanning electrochemical microscopy (SECM)-controlled electrodeposition, and the use of selective binding monolayers on carbon fiber electrodes (CFEs) for solution-phase-selective adsorption. A novel SECM technique for electrodepositing MNPs on CFE tips immersed 100-200 nm below the electrolyte level was developed and used to generate single Pt and Ni nanoparticles. Following their generation, we demonstrate electrocatalytic detection of Fe3+ on individual Pt particles with the CFE in a Fe3+/H2SO4 solution. We also describe an approach of attaching MNPs to CFEs by controlling the composition of monolayers bonded to the CFE. By employing a monolayer with a low ratio of binding (e.g., 4-aminopyridine) to nonbinding molecules (e.g., aniline) and controlling the position of the CFE in a colloidal Pt solution with a SECM, we attached a single 15 nm radius Pt nanoparticle to the CFE. Such chemisorbed Pt particles exhibited a stronger adhesion on surface-modified CFEs and better mechanical stability during proton reduction than MNPs electrodeposited directly on the CFE.

Adsorption↗

PubMed2006

Characterization and theory of electrocatalysts based on scanning electrochemical microscopy screening methods.

A strategy for finding new electrocatalysts for the oxygen reduction reaction (ORR) in acidic solutions is outlined and illustrated with results for Pd-Co catalysts. This is based on establishing guidelines for selecting test systems, rapid preparation of arrays, and rapid screening by scanning electrochemical microscopy. Promising candidates are further tested as supported electrocatalysts by larger scale electrochemical methods and in fuel cells, with optimization of the composition and structure. Those that emerge are characterized by a variety of methods, including X-ray diffraction, scanning electron microscopy, and X-ray photoemission spectroscopy. Finally, density functional theory is used for detailed calculations of oxygen adsorption and dissociation on the material and provides better guidelines for further testing.

Journal Article↗

PubMed2006

Ruthenium(II) complexes containing tetrazolate group: electrochemiluminescence in solution and solid state.

In this work, we report the results about the solution and solid-state phosphorescence emission properties of six Ru(II) complexes containing various 5-substituted tetrazolate ligands. The photo- and electrochemiluminescence spectra of all compounds revealed a red shifted emission with respect to the Ru(bpy)(3)(2+). Significant changes to the light emission energy and to the efficiency and sensitivity to oxygen were also determined by varying the nature of the substituent ring of the tetrazolate ligand. Light-emitting solid devices with active layers containing solid films of the same complexes were prepared, and preliminary studies of their electroinduced emission properties were performed. The electrochemiluminescence (ECL) emission intensity of two of the six complexes was of the same order of magnitude as the reference Ru(bpy)(3)(2+).

Journal Article↗

PubMed2006

Carbon nanofiber electrodes and controlled nanogaps for scanning electrochemical microscopy experiments.

The electrochemical behavior of electrodes made by sealing carbon nanofibers in glass or with electrophoretic paint has been studied by scanning electrochemical microscopy (SECM). Because of their small electroactive surface area, conical geometry with a low aspect ratio and high overpotential for proton and oxygen reduction, carbon nanofiber (CNF) electrodes are promising candidates for producing electrode nanogaps, imaging with high spatial resolution and for the electrodeposition of single metal nanoparticles (e.g., Pt, Pd) for studies as electrocatalysts. By using the feedback mode of the SECM, a CNF tip can produce a gap that is smaller than 20 nm from a platinum disk. Similarly, the SECM used in a tip-collection substrate-generation mode, which subsequently shows a feedback interaction at short distances, makes it possible to detect a single CNF by another CNF and then to form a nanometer gap between the two electrodes. This approach was used to image vertically aligned CNF arrays. This method is useful in the detection in a homogeneous solution of short-lifetime intermediates, which can be electrochemically generated at one electrode and collected at the second at distances that are equivalent to a nanosecond time scale.

Carbon↗

PubMed2006

Metal/polypyrrole quasi-reference electrode for voltammetry in nonaqueous and aqueous solutions.

As an alternative to the usual commercial reference and quasi-reference electrodes (QREs), we propose metal (Pt, stainless steel) coated with partially oxidized polypyrrole as a QRE. The electrode is easily fabricated by cyclic voltammetry (CV) with the metal electrode in an acetonitrile or CH2Cl2 solution of 10 mM pyrrole containing 0.1 M Bu4NPF6. These QREs were more stable than the widely used metal (e.g., Ag, Pt) wire QRE as demonstrated by testing in several aqueous and organic media, with the stability checked by CV with ferrocenemethanol as the redox species. The results obtained demonstrate good stability of these QREs over a period of 1 day and relatively reproducible potential in a given solvent and supporting electrolyte. This electrode has the advantage that it does not contaminate the test solution with ions or solvent from a usual liquid electrolyte (e.g., the KCl in an Ag/AgCl). It is also simple to fabricate very small electrodes for use in nanocells, e.g., in scanning tunneling and electrochemical microscopy.

Electrochemistry↗

PubMed2006

Electrochemistry, spectroscopy, and electrogenerated chemiluminescence of silole-based chromophores.

We studied the electrochemical and spectroscopic properties of a series of extended silole-based chromophores to understand the effect of structure on behavior. By changing the substituents attached to the chromophore, we observed large variations in luminescence quantum efficiency (ca. 0-0.6), lambdamax for absorbance and photoluminescence (PL), and radical ion stability. The differences are related to the motion in the 2,5-substituents and the steric protection of both the chromophore and the reactive parts of the substituents. For several compounds the electrogenerated chemiluminescence (ECL) spectrum was also compared to the photoluminescence spectrum. In all cases, the ECL lambdamax and the PL lambdamax were about the same.

Journal Article↗

PubMed2006

Single-molecule spectroelectrochemistry (SMS-EC).

We introduce single-molecule spectroelectrochemistry (SMS-EC), a powerful new technique for studying electrochemical kinetics in highly heterogeneous systems. This technique uses fluorescence single-molecule spectroscopy to indirectly measure electrochemical kinetics one molecule at a time, offering for the first time the distribution of key electrochemical variables, such as the half-wave potential, E1/2, not just the ensemble averages. In SMS-EC, the potential of the working electrode of an electrochemical cell is linearly scanned while simultaneously measuring the florescence intensity, Ifl(t), of individual single molecules as a function of time in a wide-field microscope. SMS-EC is used herein to study the oxidation at an indium tin oxide (ITO) electrode of single molecules of the organic conjugated polymer F8BT. The results reveal both excited singlet state and ground state oxidation of F8BT. The latter process occurs over a narrow distribution of single-molecule half-wave potential values, indicating a relatively uniform electrochemical potential at the electrode.

Journal Article↗

PubMed2006

Scanning electrochemical microscopy of the photosynthetic reaction center of Rhodobacter sphaeroides in different environmental systems.

The present work uses a scanning electrochemical microscopy technique to study systems containing the membrane-bound reaction center protein (RC) from the purple photosynthetic bacteria Rhodobacter spheroides to chromatophores (spherical reorganization of cell membrane following its mechanical rupture) and liposomes (reconstituted membrane systems at lower degree of complexity). Scanning electrochemical microscopy is a useful tool to investigate redox processes involving a RC, because the effective heterogeneous rate constants for the redox reaction with different mediators can be measured. The technique is also able to provide information on the role of the outer cell membrane permeation on the kinetics of the electron-transfer processes and to obtain more insight into the nature of the species involved.

Chromatophores↗

PubMed2006

Chemically imaging living cells by scanning electrochemical microscopy.

Scanning electrochemical microscopy (SECM) is useful in probing and characterizing interfaces at high resolution. In this paper, the general principles of this technique are described and several applications of SECM to biological systems, particularly to living cells, is discussed, along with several example systems. Thiodione was detected and monitored electrochemically during the treatment of hepatocytes with cytotoxic menadione. The antimicrobial effects of silver(I) was followed by SECM through bacterial respiration. Living HeLa cells were shown to accumulate ferrocencemethanol (FcMeOH) and generated positive feedback for FcMeOH oxidation that can be further used to monitor the cell viability. Finally, individual giant liposomes, as cell models, with encapsulated redox compounds were successfully probed by SECM. In general SECM has the advantage of very high spatial resolution and versatility, especially for the detection of electroactive substances.

Biosensing Techniques↗

PubMed2006

Scanning electrochemical microscopy. 56. Probing outside and inside single giant liposomes containing Ru(bpy)32+.

Giant liposomes containing Ru(bpy)(3)(2+) (bpy = 2,2'-bipyridine) were prepared as model systems for biomembranes and cells and studied by scanning electrochemical microscopy (SECM). Conical carbon fiber tips of submicrometer size were used to approach, image, and puncture individual liposomes immobilized on glass substrates. SECM images of the liposomes were obtained, and the leakage of Ru(bpy)(3)(2+) through the lipid membrane was probed. The tip was also pushed into liposomes and characteristic breakthrough transients, corresponding to liposomes with different compartmental configurations, were obtained. Voltammograms were obtained with the tip inside a single liposome after breaking through the membrane, and the influx of mediator and efflux of encapsulant after puncture could be observed.

2,2'-Dipyridyl↗

PubMed2006

A new family of ruthenium(II) polypyridine complexes bearing 5-aryltetrazolate ligands as systems for electrochemiluminescent devices.

A new family of mono- and dinuclear ruthenium polypyridyl complexes containing 5-aryltetrazolate ligands such as the deprotonated form of 4-(1H-tetrazol-5-yl)benzonitrile (4-TBNH) and bis(1H-tetrazol-5-yl)benzene (BTBH(2)) have been synthesized and thoroughly characterized. The reactivity of the mononuclear species toward different electrophiles such as H(+) and CH(3)(+) has been investigated, and the effects of the resulting regioselective electrophilic attacks on the electronic and structural properties of the tetrazolate ligand have been studied by NMR ((1)H, (13)C) spectroscopy and X-ray crystal structures. Absorption and emission spectroscopy, together with an electrochemical and UV-vis-NIR spectroelectrochemical investigation of the uncoordinated ligand and complexes, has been performed, highlighting a rather good luminescence efficiency and a poor bridge-mediated electronic communication between the metal centers of the dinuclear complexes. The electrogenerated chemiluminescence (ECL) of the dinuclear species has been explored, and for one of these, an exceptionally high ECL efficiency has been observed, comparable to that of [Ru(bpy)(3)](2+), which is considered a standard in ECL studies.

Crystallography, X-Ray↗

PubMed2006

Novel carbon-doped TiO2 nanotube arrays with high aspect ratios for efficient solar water splitting.

The photocatalytic splitting of water into hydrogen and oxygen using solar light is a potentially clean and renewable source for hydrogen fuel.(1,2) There has been extensive investigation into metal-oxide semiconductors such as TiO(2), WO(3), and Fe(2)O(3), which can be used as photoanodes in thin-film form.(3-5) Of the materials being developed for photoanodes, TiO(2) remains one of the most promising because of its low cost, chemical inertness, and photostability.(6) However, the widespread technological use of TiO(2) is hindered by its low utilization of solar energy in the visible region. In this study, we report the preparation of vertically grown carbon-doped TiO(2) (TiO(2-x)C(x)) nanotube arrays with high aspect ratios for maximizing the photocleavage of water under white-light irradiation. The synthesized TiO(2-x)C(x) nanotube arrays showed much higher photocurrent densities and more efficient water splitting under visible-light illumination (> 420 nm) than pure TiO(2) nanotube arrays. The total photocurrent was more than 20 times higher than that with a P-25 nanoparticulate film under white-light illumination.

Journal Article↗

PubMed2005

Combinatorial biomimetics. optimization of a composition of copper(II) poly-L-histidine complex as an electrocatalyst for O2 reduction by scanning electrochemical microscopy.

A simple approach to prepare and characterize biomaterial-based electrocatalysts for oxygen reduction was carried out. Poly-l-histidine was used as a matrix and ligand to complex Cu2+ to mimic the active sites of laccases. A modified glassy carbon (GC) electrode with Cu2+-poly-l-histidine complex decreases the oxygen reduction overpotential as compared with the bare GC electrode. An array of Cu2+-poly-l-histidine spots with different compositions was deposited on a GC substrate, and their catalytic activity for oxygen reduction was evaluated by a scanning electrochemical microscopy-based screening technique. The electrocatalytic activities of complexes for oxygen reduction strongly depended on the mole ratio of Cu2+ to poly-l-histidine and the applied potential of the substrate.

Biomimetic Materials↗

PubMed2005

Pd-Co-Mo electrocatalyst for the oxygen reduction reaction in proton exchange membrane fuel cells.

The catalytic activity of carbon supported Pd-Co-Mo for the oxygen reduction reaction (ORR) in a single cell proton exchange membrane fuel cell (PEMFC) has been investigated at 60 degrees C and compared with data from commercial Pt catalyst and our previously reported Pd-Co-Au and Pd-Ti catalysts. The Pd-Co-Mo catalyst with a Pd:Co:Mo atomic ratio of 70:20:10 exhibits slightly higher catalytic activity like the Pd-Co-Au catalyst than the commercial Pt catalyst, but with excellent chemical stability unlike the Pd-Co-Au catalyst. The Pd-Co-Mo catalyst also exhibits better tolerance to methanol poisoning than Pt. Investigation of the catalytic activity of the Pd-Co-Mo system with varying composition and heat treatment temperature reveals that a Pd:Co:Mo atomic ratio of 70:20:10 with a heat treatment temperature of 500 degrees C exhibits the highest catalytic activity. Although the degree of alloying increases with increasing temperature from 500 to 900 degrees C as indicated by the X-ray diffraction data, the catalytic activity decreases due to an increase in particle size and a decrease in surface area.

Journal Article↗

PubMed2005

Interaction of silver(I) ions with the respiratory chain of Escherichia coli: an electrochemical and scanning electrochemical microscopy study of the antimicrobial mechanism of micromolar Ag+.

Electrochemical techniques were used to study the behavior of Escherichia coli on the addition of <or=10 microM AgNO(3). Respiration in the presence of glucose was measured using a Clark ultramicroelectrode to determine the oxygen concentration as a function of time. The rate of respiration increased initially upon the addition of silver(I) because of the uncoupling of the respiratory chain, followed by cessation of respiration. The toxicity of 1 microM AgNO(3), as determined by the time until respiration ceased, increased in the absence of glucose and in the presence of K(+). The uptake of 1 microM Ag(+), measured from the stripping peak height of Ag electrodeposited on a 25 microm Pt ultramicroelectrode, showed a fastest uptake in the presence of glucose, although glucose was not required for uptake. Efflux of Ag(+) from cells was also detected in the presence of glucose. Scanning electrochemical microscopy (SECM) was used to follow the uptake of 1 microM Ag(+) by living and dead E. coli immobilized on a glass slide coated with poly-l-lysine. Ferricyanide was used as an alternative electron acceptor to oxygen to probe the site of inhibition of Ag(+) in the respiratory chain.

Anti-Infective Agents↗

PubMed2005

Pd-Ti and Pd-Co-Au electrocatalysts as a replacement for platinum for oxygen reduction in proton exchange membrane fuel cells.

Fuel cells are appealing for a variety of energy needs, but the high materials and manufacturing costs have hampered their commercialization. The limited availability and the high cost of the currently used platinum catalysts, for example, pose a serious problem in their practical application. We report here non-platinum electrocatalyst systems, such as Pd-Co-Au and Pd-Ti, that are proposed from simple thermodynamic guidelines and selected by a rapid screening technique, which show electrochemical performance in proton exchange membrane fuel cells (PEMFC) similar to that found with commercial platinum catalysts. This finding opens up a new avenue to develop potentially less expensive electrocatalysts.

Alloys↗

PubMed2005

Electrochemistry and electrogenerated chemiluminescence with a single faradaic electrode.

We describe an electrochemical cell containing a single faradic electrode (a Pt ultramicroelectrode) and a blocked (polarized) electrode (Si with an insulating SiO2 film) that served as a capacitive counter electrode. When a bias was applied between the two electrodes, a current was observed from a faradic process at Pt and a capacitive current at the blocking electrode. A steady charging current was obtained when the solution was moved along the insulator to continuously contact fresh surface (emersing a layer of charged ions and electronic countercharge into the gas phase). Electrogenerated chemiluminescence was clearly observed from a system containing Ru(bpy)(3)2+ in this kind of cell under pulsed excitiation, demonstrating that faradaic reactions can be carried out in an electrochemical cell without generation of any products at a counter electrode. The use of such a system for coulometric addition of desired species in nanosystems and in synthesis is suggested.

Journal Article↗