Peroxidase model electrodes: heme peptide modified electrodes as reagentless sensors for hydrogen peroxide

A peroxidase model electrode was devised for reagentless sensing of hydrogen peroxide (H2O2). A small model molecule, which mimics the vicinity of the reaction center of a redox enzyme, can communicate electrochemically with an electrode. Heme nonapeptide (MW congruent to 1600) having peroxidase activity was adopted as a peroxidase model compound and was covalently immobilized on a tin oxide (SnO2) electrode as a roughly monomolecular layer. The modified electrode thus obtained responded to H2O2 at concentrations down to 10(-6) M without electron mediator or promoter, at a mild potential of +150 or +300 mV vs Ag/AgCl. In a batch system, the response reached a steady state in a few seconds. Measurements were possible also in a flow system with an assay time of 0.5-1.0 min/sample. The steady-state response of the electrode was kinetically analyzed.     

Diamond-modified AFM probes: from diamond nanowires to atomic force microscopy-integrated boron-doped diamond electrodes

In atomic force microscopy (AFM), sharp and wear-resistant tips are a critical issue. Regarding scanning electrochemical microscopy (SECM), electrodes are required to be mechanically and chemically stable. Diamond is the perfect candidate for both AFM probes as well as for electrode materials if doped, due to diamond's unrivaled mechanical, chemical, and electrochemical properties. In this study, standard AFM tips were overgrown with typically 300 nm thick nanocrystalline diamond (NCD) layers and modified to obtain ultra sharp diamond nanowire-based AFM probes and probes that were used for combined AFM-SECM measurements based on integrated boron-doped conductive diamond electrodes. Analysis of the resonance properties of the diamond overgrown AFM cantilevers showed increasing resonance frequencies with increasing diamond coating thicknesses (i.e., from 160 to 260 kHz). The measured data were compared to performed simulations and show excellent correlation. A strong enhancement of the quality factor upon overgrowth was also observed (120 to 710). AFM tips with integrated diamond nanowires are shown to have apex radii as small as 5 nm and where fabricated by selectively etching diamond in a plasma etching process using self-organized metal nanomasks. These scanning tips showed superior imaging performance as compared to standard Si-tips or commercially available diamond-coated tips. The high imaging resolution and low tip wear are demonstrated using tapping and contact mode AFM measurements by imaging ultra hard substrates and DNA. Furthermore, AFM probes were coated with conductive boron-doped and insulating diamond layers to achieve bifunctional AFM-SECM probes. For this, focused ion beam (FIB) technology was used to expose the boron-doped diamond as a recessed electrode near the apex of the scanning tip. Such a modified probe was used to perform proof-of-concept AFM-SECM measurements. The results show that high-quality diamond probes can be fabricated, which are suitable for probing, manipulating, sculpting, and sensing at single digit nanoscale.     

Electrochemical performance of diamond thin-film electrodes from different commercial sources

The electrochemical properties of two commercial (Condias, Sumitomo) boron-doped diamond thin-film electrodes were compared with those of two types of boron-doped diamond thin film deposited in our laboratory (microcrystalline, nanocrystalline). Scanning electron microscopy and Raman spectroscopy were used to characterize the electrode morphology and microstructure, respectively. Cyclic voltammetry was used to study the electrochemical response, with five different redox systems serving as probes (Fe(CN)(6)(3)(-)(/4)(-), Ru(NH(3))(6)(3+/)(2+), IrCl(6)(2)(-)(/3)(-), 4-methylcatechol, Fe(3+/2+)). The response for the different systems was quite reproducibile from electrode type to type and from film to film for electrodes of the same type. For all five redox systems, the forward reaction peak current varied linearly with the scan rate(1/2) (nu), indicative of electrode reaction kinetics controlled by mass transport (semi-infinite linear diffusion) of the reactant. Apparent heterogeneous electron-transfer rate constants, k degrees (app), for all five redox systems were determined from deltaE(p)-nu experimental data, according to the method described by Nicholson (Nicholson, R. S. Anal. Chem. 1965, 37, 1351.). The rate constants were also verified through digital simulation (DigiSim 3.03) of the voltammetric i-E curves at different scan rates. Good fits between the experimental and simulated voltammograms were found for scan rates up to 50 V/s. k degrees (app) values of 0.05-0.5 cm/s were observed for Fe(CN)(6)(3)(-)(/4)(-), Ru(NH(3))(6)(3+/2+), and IrCl(6)(2)(-)(/3)(-) without any extensive electrode pretreatment (e.g., polishing). Lower k degrees (app) values of 10(-)(4)-10(-)(6) cm/s were found for 4-methylcatechol and Fe(3+/2+). The voltammetric responses for Fe(CN)(6)(3)(-)(/4)(-) and Ru(NH(3))(6)(3+/2+) were also examined at all four electrode types at two different solution pH (1.90, 7.35). Since the hydrogen-terminated diamond surfaces contain few, if any, ionizable carbon-oxygen functionalities (e.g., carboxylic acid, pK(a) approximately 4.5), the deltaE(p), i(p)(ox), and i(p)(red) values for the two systems were, for the most part, unaffected by the solution pH. This is in contrast to the typical behavior of oxygenated, sp(2) carbon electrodes, such as glassy carbon.     

Electron emission from nitrogen-doped polycrystalline diamond/Si heterostructures

The field electron emission from polycrystalline diamond/silicon and nitrogen-doped polycrystalline diamond/silicon structures obtained by HF CVD deposition method has been investigated. Electron emission currents from the samples were measured in a chamber at the pressure equal to 2·10^?6 Pa in sphere-to-plane diode configuration with the 5 ??m distance between electrodes. As expected, the results confirm the relation between the structure of diamond films and their emission properties. The type of silicon substrate also influences the value of emission currents and the diamond/n-Si heterostructures exhibit better electron emission than diamond/p-Si ones. The nitrogen doping significantly enhances the electron emission from the heterostructures and their emission parameters. The values of the threshold field between 2 V/??m and 3 V/??m were registered, the values of emission current close to 1 mA/cm² at 5 V/??m for the nitrogen-doped films were obtained. The shape of current-voltage characteristics for nitrogen-doped polycrystalline films may be interpreted in terms of stochastic distribution of diameters of conducting channels which form the emission centers.     

Electron transfer reactions of glucose oxidase at Au111 electrodes modified with phenothiazine derivatives

The catalytic reaction of glucose oxidase (GOx) mediated by 3-(10-phenothiazyl)propionic acid (PT-PA) and phenothiazine-labeled poly(ethylene oxide) (PT-PEO1000) that are covalently bonded to Au(111) electrodes has been investigated. The PT-PA and PT-PEO1000 are reacted with 2-aminoethanethiol (AET), followed by the formation of a self-assembled monolayer (SAM) onto the Au surface. The PT group immobilized on the SAM of AET acts as an effective mediator for the electron transfer (ET) between the electrode and the FAD center of freely diffusing GOx in solution. The ET rate constant estimated from the catalytic current using a newly derived equation is larger by 1 order of magnitude for the PT-PA-modified system (1.1 x 10(5) dm(3) mol(-1) s(-1)) than for the PT-PEO1000 system (1.4 x 10(4) dm(3) mol(-1) s(-1)). The order of the magnitude of the ET rate constant clearly contrasts with the GOx hybrid systems that we previously investigated (Anal. Chem. 2003, 75, 910-917), in which the presence of the PEO spacer enhances the ET reaction rate. The reduction in the apparent PT concentration at the electrode interface due to the high mobility of the PEO chain, leading to low efficiency in the formation of an enzyme-mediator complex, is a possible reason for the lower mediation ability of PT-PEO1000 than that of PT-PA for the ET between the FAD group and PT(+) immobilized on the electrode. Inhibition of the penetration of GOx molecules into the monolayer and of the accessibility of some part of PT groups to GOx molecules could also be reasons for the lower mediation ability of PT-PEO1000 thickly modified on the electrode.     

Carbon nanotubes transfer to diamond by laser irradiation

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Selective detection method derived from a controlled diffusion process at metal-modified diamond electrodes

A novel, selective methodology is derived based on the difference between the diffusion processes at microelectrodes (i.e., hemispherical diffusion) and the macroelectrode (i.e., linear diffusion) in a metal-implanted boron-doped diamond electrode (metal-BDDs). As an example, the selective detection of glucose in a solution containing interference species such as ascorbic acid and uric acid is demonstrated. The electrochemical properties of BDD, which are low background current, extremely high stability, and (especially) inactivity toward glucose, play an important role in realizing these differences in the diffusion characteristics. The present methodology can be applied not only to selective glucose detection by the metal-BDD system but also to other selective detection systems.     

Electrochemical degradation of chlorobenzene on boron-doped diamond and platinum electrodes

In this paper the electrochemical degradation of chlorobenzene (CB) was investigated on boron-doped diamond (BDD) and platinum (Pt) anodes, and the degradation kinetics on these two electrodes was compared. Compared with the total mineralization with a total organic carbon (TOC) removal of 85.2% in 6 h on Pt electrode, the TOC removal reached 94.3% on BDD electrode under the same operate condition. Accordingly, the mineralization current efficiency (MCE) during the mineralization on BDD electrode was higher than that on the Pt electrode. Besides TOC, the conversion of CB, the productions and decay of intermediates were also monitored. Kinetic study indicated that the decay of CB on BDD and Pt electrodes were both pseudo-first-order reactions, and the reaction rate constant (k_s) on BDD electrode was higher than that on Pt electrode. The different reaction mechanisms on the two electrodes were investigated by the variation of intermediates concentrations. Two different reaction pathways for the degradation of CB on BDD electrode and Pt electrode involving all these intermediates were proposed.     

Electron emission characterization of diamond thin films grown from a solid carbon source

Diamond films of various morphologies and compositions have been deposited on silicon substrates by a plasma-enhanced chemical transport (PECT) process from a solid carbon source. Electron emission efficiency of these diamond films is related to their morphology and composition. The electric field required to excite emission in a boron-doped polycrystalline diamond film ranged between 20 to 50 MV m⁻¹. In an undoped conducting nanocrystalline diamond composite film, the field was as low as 5–11 MV m⁻¹. The cold field electron emission of these films is confirmed from the Fowler-Nordhelm plots of the data. Enhancement of electron emission by band-bending and by the nanocrystalline microstructure are discussed. New diamond emitters made of nanocrystalline boron-doped diamond composite are proposed.     

Electrochemical oxidation of histamine and serotonin at highly boron-doped diamond electrodes

The electrochemistry of histamine and serotonin in neutral aqueous media (pH 7.2) was investigated using polycrystalline, boron-doped diamond thin-film electrodes. Cyclic voltammetry, hydrodynamic voltammetry, and flow injection analysis (FIA) with amperometric detection were used to study the oxidation reactions. Comparison experiments were carried out using polished glassy carbon (GC) electrodes. At diamond electrodes, highly reproducible and well-defined cyclic voltammograms were obtained for histamine with a peak potential at 1.40 V vs SCE. The voltammetric signal-to-background ratios obtained at diamond were 1 order of magnitude higher than those obtained for GC electrodes at and above 100 microM analyte concentrations. A linear dynamic range of 3-4 orders of magnitude and a detection limit of 1 microM were observed in the voltammetric measurements. Well-defined sweep rate-dependent voltammograms were also obtained for 5-hydroxytryptamine (5-HT). The characteristics of the voltammogram indicated lack of adsorption of its oxidation products on the surface. No fouling or deactivation of the electrode was observed within the experimental time of several hours. A detection limit of 0.5 microM (signal-to-noise ratio 13.8) for histamine was obtained by use of the FIA technique with a diamond electrode. A remarkably low detection limit (10 nM) was obtained for 5-HT on diamond by the same method. Diamond electrodes exhibited a linear dynamic range from 10 nM to 100 microM for 5-HT determination and a range of 0.5-100 microM for histamine determination. The FIA response was very reproducible from film to film, and the response variability was below 7% at the actual detection limits.     

Selective voltammetric and amperometric detection of uric acid with oxidized diamond film electrodes

Electrochemically anodized diamond film electrodes were used for selective detection of uric acid (UA) in the presence of high concentrations of ascorbic acid (AA) by differential pulse voltammetry and chronoamperometry. Because the oxidation peak potential for AA is approximately 450 mV more positive than that for UA at anodized diamond electrodes, UA can be determined with very good selectivity. By use of chronoamperometry, linear calibration curves were obtained for UA over the concentration range up to 1 x 10(-6) M in 0.1 M HClO4 solution, with the lowest experimental value measured being 5 x 10(-8) M. This is consistent with the fact that a statistical analysis of the calibration curve yielded a detection limit of 1.5 x 10(-8) M (S/N = 3). AA in less than 20-fold excess does not interfere. The practical analytical utility of the method is demonstrated by the measurement of UA in human urine and serum without any preliminary treatment.     

Electron transfer and projectile excitation in single collisions

Studies of electron transfer (capture) and projectile excitation occuring together in a single encounter are reviewed. This combined capture and excitation can result from either a correlated or an uncorrelated interaction. The correlated process is referred to as resonant transfer and excitation (RTE), while the uncorrelated process is called nonresonant transfer and excitation (NTE). RTE is analogous to dielectronic recombination (DR) which occurs in the interaction between an ion and a free electron. Experimental and theoretical works leading to establishing the existence of RTE are reviewed as well as considerations relevant to distinguishing RTE from NTE. The dependences of RTE on projectile atomic number (14≤Z≤23) for Li-like ions, and on projectile charge state for H-like to Ne-like ions have been measured and compared with theory. The effect of the target electron momentum distribution on RTE has been demonstrated by comparing measurements for H₂ and He targets. Measurements of RTE involving L-shell excitation have been conducted and compared with K-shell results. All of the measured RTE cross sections to date for H₂ and He targets are in reasonable agreement with calculations based on theoretical DR cross sections averaged over the target electron momentum distribution; however, systematic discrepancies between experiment and theory exist. Additionally, it has been found that RTE can contribute considerably to total single-electron capture cross sections in the region where RTE is important, accounting for nearly half of the total capture events for calcium ions colliding with H₂.     

Factors controlling stripping voltammetry of lead at polycrystalline boron doped diamond electrodes: new insights from high-resolution microscopy

We report wide-ranging studies to elucidate the factors and issues controlling stripping voltammetry of metal ions on solid electrodes using the well-known Pb/Pb(2+) couple on polycrystalline boron doped diamond (pBDD) as an exemplar system. Notably, high-resolution microscopy techniques have revealed new insights into the features observed in differential pulse anodic stripping voltammetry (DPV-ASV) which provide a deeper understanding of how best to utilize this technique. DPV-ASV was employed in an impinging wall-jet configuration to detect Pb(2+) in the nanomolar to micromolar concentration range at a pBDD macrodisk electrode. The deposition process was driven to produce a grain-independent homogeneous distribution of Pb nanoparticles (NPs) on the electrode surface; this resulted in the observation of narrow stripping peaks. Lower calibration gradients of current or charge versus concentration were found for the low concentrations, correlating with a lower than expected (from consideration of the simple convective-diffusive nature of the deposition process) amount of Pb deposited on the surface. This was attributed to the complex nature of nucleation and growth at solid surfaces in this concentration regime, complicating mass transport. Furthermore, a clear shift negative in the stripping peak potential with decreasing concentration was seen correlating with a change in the size of the deposited NP, suggesting an NP size-dependent redox potential for the Pb/Pb(2+) couple. At high concentrations a nonlinear response was observed, with less Pb detected than expected, in addition to the observation of a second stripping peak. Atomic force microscopy (AFM) and field emission scanning electron microscopy revealed the second peak to be due to a change in deposition morphology from isolated NPs to grain-independent heterogeneous structures comprising both thin films and NPs; the second peak is associated with stripping from the thin-film structures. AFM also revealed a substantial amount of Pb remaining on the surface after stripping at high concentration, explaining the nonlinear relationship between stripping peak current (or charge) and concentration. Finally, the use of an in situ cleaning procedure between each measurement was advocated to ensure a clean Pb-free surface (verified by AFM and X-ray photoelectron spectroscopy analysis) between each run. The studies herein highlight important and complex physicochemical processes involved in the electroanalysis of heavy metals at solid electrodes, such as pBDD, that need to be accounted for when using stripping voltammetry methods.     

Electron chemoemission from metal to semiconductor

A method of monitoring hot electron generation in a metal, accompanying a chemical reaction on the metal surface, is proposed. The phenomenon of electron chemoemission from metal to semiconductor has been observed. The effect of conversion of the energy liberated on the surface of a metal catalyst into the electric energy is found.     

Analysis of direct transformation from graphite to diamond crystal

The lattice parameters of cubic diamond and rhombohedral graphite under the probable direct transformation synthesis conditions have been obtained by means of linear expansion coefficient and elastic constant. Based on the empirical electron theory in solid and molecules, the valence electron structures (VESs) of graphite and diamond, the covalent electron densities (CEDs), and the relative electron density differences (REDDs) of the diamond growth interfaces have been calculated. It has been found that the REEDs of graphite/diamond interfaces were awfully large and the CEDs were discontinuous at the first order approximation. Not any meaningful atomic state of graphite structure, which satisfied the bond length difference formula, existed on the detonation synthetic conditions. Accordingly, it was considered that the direct transformation from graphite to diamond could not come true from the perspective of VES. In addition, the mechanism of synthesis diamond by explosive detonation was discussed based on the VESs of graphite and diamond.     

Experimental and theoretical investigations on the adsorption of 2'-deoxyguanosine oxidation products at oxidized boron-doped diamond electrodes

Electrochemical oxidation of 2'-deoxyguanosine has been performed on boron-doped diamond (BDD) electrodes, resulting in a strong adsorption of the formed oxidized products onto the BDD surface. The adsorption behavior has been investigated by studying the electrochemical behavior of a redox probe ([IrCl6]3-) using cyclic voltammetry. The most probable situations are the formation of (A) an insulating adsorbed film resulting in a partially blocked electrode behavior, (B) a porous film, or (C) an overall conductive film. Different parameters such as the standard rate constant, the charge-transfer coefficient, the electrode/adsorbed products/solution interface resistance, and the formal potential of the redox couple were determined. Through comparison of theoretical current-potential curves obtained by analytical calculations with experimental cyclic voltammograms, we found that the oxidized products of 2'-deoxyguanosine form a continuous conductive film on BDD.     

Monocrystalline diamond paste-based electrodes and their applications for the determination of Fe(II) in vitamins

A new class of electrochemical sensors, namely, electrodes based on diamond paste, was designed using monocrystalline diamond (natural diamond 1 microm and synthetic diamond, 50 microm (synthetic-1) and 1 microm (synthetic-2)) powder and paraffin oil. The characterization of the electrodes was performed using cyclic voltammetry and differential pulse voltammetry. Fe(II) was determined by differential pulse voltammetry (DPV) at 75 mV (vs Ag/AgCl) using all diamond paste-based electrodes. The linear concentration range was between 10(-8) and 10(-4) mol/L for both the natural diamond and synthetic-2 with detection limits of 10(-10) and 10(-9) mol/L, respectively, whereas the linear concentration range for synthetic-1 was between 10(-7) and 10(-3) mol/L with a detection limit of 10(-8) mol/L Fe(II) was determined successfully from four types of pharmaceutical products. The recovery values of Fe(II) in the pharmaceutical products were higher than 98.00% with relative standard deviation values < 5%.     

Entanglement transfer from light to atoms

Abstract

We describe an approach to mapping of a quantum polarization state of free propagating light on a collective spin of an atomic ensemble. Recent experimental results on generation of a macroscopic spin squeezed ensemble of cold atoms via interaction with squeezed light are analysed in detail.     

Defect transfer from nanoparticles to nanowires

Metal-seeded growth of one-dimensional (1D) semiconductor nanostructures is still a very active field of research, despite the huge progress which has been made in understanding this fundamental phenomenon. Liquid growth promoters allow control of the aspect ratio, diameter, and structure of 1D crystals via external parameters, such as precursor feedstock, temperature, and operating pressure. However the transfer of crystallographic information from a catalytic nanoparticle seed to a growing nanowire has not been described in the literature. Here we define the theoretical requirements for transferring defects from nanoparticle seeds to growing semiconductor nanowires and describe why Ag nanoparticles are ideal candidates for this purpose. We detail in this paper the influence of solid Ag growth seeds on the crystal quality of Ge nanowires, synthesized using a supercritical fluid growth process. Significantly, under certain reaction conditions {111} stacking faults in the Ag seeds can be directly transferred to a high percentage of <112>-oriented Ge nanowires, in the form of radial twins in the semiconductor crystals. Defect transfer from nanoparticles to nanowires could open up the possibility of engineering 1D nanostructures with new and tunable physical properties and morphologies.