Pair Of Voltammetric Peaks Research Articles

Voltammetry and Single‐Molecule In Situ Scanning Tunnelling Microscopy of the Redox Metalloenzyme Human Sulfite Oxidase

AbstractHuman sulfite oxidase (hSO) is a homodimeric two‐domain enzyme central in the biological sulfur cycle. A pyranopterin molybdenum cofactor (Moco) is the catalytic site and a heme b5 group located in the N‐terminal domain. The two domains are connected by a flexible linker region. Electrons produced at the Moco in sulfite oxidation, are relayed via heme b5 to electron acceptors or an electrode surface. Inter‐domain conformational changes between an open and a closed enzyme conformation, allowing “gated” electron transfer has been suggested. We first recorded cyclic voltammetry (CV) of hSO on single‐crystal Au(111)‐electrode surfaces modified by self‐assembled monolayers (SAMs) both of a short rigid thiol, cysteamine and of a longer structurally flexible thiol, ω‐amino‐octanethiol (AOT). hSO on cysteamine SAMs displays a well‐defined pair of voltammetric peaks around −0.207 V vs. SCE in the absence of sulfite substrate, but no electrocatalysis. hSO on AOT SAMs displays well‐defined electrocatalysis, but only “fair” quality voltammetry in the absence of sulfite. We recorded next in situ scanning tunnelling spectroscopy (STS) of hSO on AOT modified Au(111)‐electrodes, disclosing, a 2–5 % surface coverage of strong molecular scale contrasts, assigned to single hSO molecules, notably with no contrast difference in the absence and presence of sulfite. In situ STS corroborated this observation with a sigmoidal tunnelling current/overpotential correlation.

Photo-Gated Intervalence Charge Transfer of Ethynylferrocene Functionalized Titanium Dioxide Nanoparticles

Ethynylferrocene-functionalized titanium dioxide nanoparticles (TiO2-eFc) were synthesized, for the first time ever, by a modified two-phase hydrothermal method. Transmission electron microscopic measurements showed that the nanoparticles were rather uniform in size, with an average diameter of 4.0±0.5nm and well-defined lattice fringes that were consistent with those of anatase TiO2. 1H NMR, FTIR and XPS measurements confirmed the attachment of the ferrocenyl ligands onto the nanoparticle surface, most likely forming Ti−CC−Fc interfacial bonds. The resulting nanoparticles exhibited a bandgap of ca. 3.3eV, and two emission bands in photoluminescence measurements at 351 and 460nm, with the former due to the TiO2 cores whereas the latter from intraparticle charge delocalization between the nanoparticle-bound acetylene moieties under UV photoirradiation. In electrochemical measurements, only one pair of voltammetric peaks were observed in the dark, due to the redox reactions of the nanoparticle-bound ferrocenyl groups. However, when subject to photoirradiation with UV lights (254 and 365nm), two pairs of voltammetric peaks appeared, with a respective peak spacing of 174 and 198mV, suggesting intervalence charge transfer (IVCT) between the ferrocenyl moieties bound on the nanoparticle surface. This arose from photo-enhanced electrical conductivity of the TiO2 cores that served as part of the chemical linkage bridging the ferrocenyl moieties. Significantly, such photo-gated IVCT varied with the photoexcitation energy that dictated the intraparticle charge transfer pathways.

Intervalence Charge Transfer Mediated by Silicon Nanoparticles

AbstractStable silicon nanoparticles (SiNPs) were synthesized by using a wet chemical method with (3‐aminopropyl)triethoxysilane as the silicon source and ethynylferrocene as the capping ligands by taking advantage of the unique chemical reactivity of acetylene moieties with silicon hydride on the nanoparticle surface, forming Si−CH=CH− interfacial bonds under UV photoirradiation. Transmission electron microscopic measurements showed good dispersion of the nanoparticles with an average core diameter of 2.96±0.46 nm. From UV/Vis absorption measurements, the nanoparticle bandgap was estimated to be 2.6 eV. XPS measurements confirmed the successful attachment of ethynylferrocene ligands onto the nanoparticle surface, with a footprint consistent with that of ferrocene. Electrochemically, the nanoparticles exhibited only one pair of voltammetric waves in the dark, suggesting a lack of effective electronic communication between the particle‐bound ferrocenyl moieties, because of the low conductivity of the nanoparticle cores; whereas, under UV photoirradiation (365 nm), two pairs of voltammetric peaks were observed, with a potential spacing of 125 mV, suggesting that the nanoparticles behaved analogously to a Class II compound. This was ascribed to photo‐enhanced electronic conductivity of the nanoparticle cores that facilitated intervalence charge transfer of the particle‐bound ferrocene moieties.

Ferrocene-Functionalized Graphene Oxide Nanosheets: Efficient Electronic Communication between Ferrocene Centers across Graphene Nanosheets

Graphene oxide (GO) nanosheets functionalized with ferrocenyl moieties (GO-Fc) were fabricated through strong covalent CC bonds. The resulting hybrid showed efficient electronic communication between ferrocene centers due to the strong electron delocalization facilitated by the large pi-pi conjugated structure of graphene sheets. The obtained hybrid exhibited two pairs of voltammetric peaks with a large potential spacing of 0.515V and a broad absorption peak in the near-infrared range (∼ 1428nm) at mixed valence. The electrochemical and near IR spectroscopic features suggested a Class II/III behavior of the intervalence charge transfer. This work indicates clearly that strong electronic coupling between ferrocene centers can be easily realized across graphene nanosheets with sp2-hybridized carbon.

The Measurement of the Gibbs Energy of Transfer Between Oil and Water Using a Nano‐Carbon Paste Electrode

AbstractThe use of nano‐carbon paste electrodes for the measurement of Gibbs energies of transfer between oil and aqueous phases is reported. In this method the oil of interest is used as the binder for the nano‐carbon paste electrodes and the molecule of interest is dissolved in the organic or aqueous phase. Voltammetry is performed over a period of time and used to monitor the transfer of the molecule between the two phases. The method is illustrated for the transfer of ferrocenemethanol between water and oil using the ferrocenemethanol / ferroceniummethanol (FcCH2OH/FcCH2OH+) redox couple. Three pairs of voltammetric peaks were observed in a 0.1 M KCl solution when the nano‐carbon paste electrode was modified by dissolution of FcCH2OH in the binder oil: P1 [E=0.23 V, 0.17 V vs. Ag/AgCl (1 M KCl)], P2 [E=0.36 V, 0.32 V vs. Ag/AgCl (1 M KCl)] and P3 [E=0.55 V, 0.46 V vs. Ag/AgCl (1 M KCl)]. These are assigned to the FcCH2OH species existing in the aqueous solution [FcCH2OH(aq)/FcCH2OH+(aq)], originating in the oil (o) [FcCH2OH(o)/FcCH2OH+(aq)] and to oxidation of adsorbed (ads) material on the nano‐carbon [FcCH2OH(ads)] respectively. When supporting electrolyte containing the anions Cl−, NO3− or SCN− was used, an expulsion of the oxidised ferrocene occurred and the difference in midpoint potentials (Emid) between the peaks P1 and P2 observed in these experiments allowed the calculation of the Gibbs energy (ΔG°) of transfer of ferrocenemethanol from water to oil. The average ΔG° value thus obtained was (−12.7±0.2) kJ mol−1. For more hydrophobic anions (X−=PF6−, AsF6−), the electron transfer is coupled to the transfer of the anion into the oil and the ΔG° for the transfer of the ion pair of FcCH2OH+ and X− ions from water to oil was found to be −1.3 and −3.9 kJ mol−1 for PF6− and AsF6− respectively.

Alkyne-Protected Ruthenium Nanoparticles

Stable ruthenium nanoparticles protected by 1-octynyl fragments were synthesized by a wet chemical method. Transmission electron microscopic measurements showed that the resulting particles exhibited an average core diameter of 2.55 ± 0.15 nm with well-defined Ru crystalline lattice fringes. Because of the formation of Ru—C≡ bonds, the C≡C vibrational stretch was found in FTIR measurements to red-shift to 1936 cm−1 from 2119 cm−1 that was observed for monomeric 1-octyne. Interestingly, the nanoparticles underwent ligand exchange reactions with alkynyl lithium (e.g., 5-phenyl-1-pentynyl lithium) for further surface functionalization, as manifested in FTIR as well as 1H and 13C NMR measurements. Optically, whereas UV−vis absorption measurements exhibited only a featureless profile, the Ru nanoparticles displayed apparent photoluminescence with an emission peak at 428 nm, which was accounted for by intraparticle charge delocalization as a consequence of the strong Ru—C≡ bonds and the conducting Ru metal cores such that the particle bound C≡C moieties behaved analogously to diacetylene derivatives. The impacts of the interfacial bonding interactions on intraparticle charge delocalization were further illustrated by Ru nanoparticles functionalized with a mixed monolayer of both octyne and ethynylferrocene ligands. At a ferrocene surface coverage of ca. 13%, electrochemical measurements depicted two pairs of voltammetric peaks with a potential spacing of 265 mV. A new NIR absorption band centered around 1687 nm also started to emerge with the addition of nitrosonium tetrafluoroborate (NOBF4) as the oxidizing reagent and the peak intensity exhibited a volcano-shape dependence on the amount of NOBF4 added. These observations strongly suggested that there existed effective intervalence charge transfer between the particle-bound ferrocene groups at mixed valence, analogous to the observation where the ferrocene moieties were bound onto the particle surface by Ru—carbene π bonds.

Long-Range Interfacial Electrochemical Electron Transfer of Pseudomonas aeruginosa Azurin−Gold Nanoparticle Hybrid Systems

We have prepared a “hybrid” of the blue copper protein azurin (Pseudomonas aeruginosa) and a 3 nm gold nanoparticle (AuNP). The AuNP/azurin hybrid was assembled on a Au(111)-electrode surface in a two-step process. The AuNP was first attached to the Au(111) electrode via Au−S chemisorption of a 4,4′-biphenyldithiol (4,4′-BPDT) monolayer. This was followed by 1-decanethiol modification of the bound AuNP and hydrophobic binding of azurin to the AuNP. The Au(111)/AuNP/azurin system was characterized by atomic force microscopy (AFM), cyclic voltammetry (CV), and in situ electrochemical scanning tunneling microscopy (in situ STM). AFM and STM point to the feasibility of preparing both dense and sparsely populated AuNP monolayers. CV shows two pairs of voltammetric peaks at high scan rates, both around the azurin equilibrium potential. One pair of redox peaks follows closely that of azurin hydrophobically immobilized directly on a Au(111)/1-tetradecanethiol reference surface. The other pair, tentatively assigned to the AuNP/azurin hybrid, shows a 20-fold electron transfer rate enhancement over the reference system. This dual pattern is supported by in situ STM which shows two distinct contrasts. A strong contrast most likely arises either from azurin-free AuNPs or from AuNP-free azurin displaced onto the Au(111)/4,4′-BPDT surface. The other contrast, assigned to the AuNP/azurin hybrid, is weaker and fluctuates in time. Mechanisms of electronic conductivity of the AuNP/azurin system are discussed.

Intervalence transfer of ferrocene moieties adsorbed on electrode surfaces by a conjugated linkage

Effective intervalence transfer occurred between the metal centers of ferrocene moieties that were adsorbed onto a ruthenium thin film surface by ruthenium–carbene π bonds, a direct verification of Hush’s four-decade-old prediction. Electrochemical measurements showed two pairs of voltammetric peaks where the separation of the formal potentials suggested a Class II behavior. Additionally, the potential spacing increased with increasing ferrocene surface coverage, most probably as a consequence of the enhanced contribution from through-space electronic interactions between the metal centers. In contrast, the incorporation of a sp 3 carbon spacer into the ferrocene–ruthenium linkage led to the diminishment of interfacial electronic communication.

Photo-Gated Charge Transfer of Organized Assemblies of CdSe Quantum Dots

The electronic conductivity of tri-n-octylphosphineoxide (TOPO)-protected CdSe quantum dots (QDs) was studied at the air-water interface using the Langmuir technique within the context of photochemical and photophysical excitation. It was found that, upon photoirradiation with photon energies higher than that of the absorption threshold, the voltammetric currents increased rather substantially with a pair of voltammetric peaks at positive potentials. However, the photoconductivity profiles exhibited a dynamic transition, which was ascribed to the strong affinity of oxygen onto the CdSe surface and the consequent trapping of the photogenerated electrons. The resulting excess of holes led to photocorrosion of the particle cores. The oxygen adsorption and photoetching processes were found to be reversible upon cessation of the photoexcitation. In contrast, only featureless voltammetric responses were observed when the particle monolayers were deposited onto the electrode surface and the film conductance was measured in a vacuum (the overall profiles were analogous to that of a Coulomb blockade). A comparative study was also carried out with a CdSe dropcast thick film immersed in acetonitrile, where the photoconductivity profiles were reversible and almost linear. The latter was attributed to the separation of photogenerated electrons and holes which were subsequently collected at the electrodes under voltammetric control. In the dropcast system, the oxygen effects were minimal which was ascribed to the acetontrile medium that limited the access to oxygen and thus the particles were chemically intact. These studies suggest that chemical environment plays an important role in the determination of the chemical stability and electronic conductivity of CdSe QD thin films.

The role of slow surface-atom reordering processes in the underpotential deposition of metals: Silver underpotential deposition on platinum in acid solutions

The role played by slow surface-atom reordering processes in the underpotential deposition of Ag on polycrystalline Pt and polyfacetted Pt single crystal substrates from 10 −3 M Ag salt in aqueous acid solutions has been studied by using potentiostatic and potentiodynamic techniques complemented with Auger spectroscopy. The potential cycling between E r, the reversible potential of the Ag/Ag + electrode reaction, and 1.10 V (SHE) for 24 h resulted in the formation of a Ag + Pt surface alloy and the incorporation of Ag into the second layer of Pt atoms. The Ag electrodeposition/ Ag anodic stripping on the Ag + Pt alloy gives rise to two new pairs of voltammetric peaks in the range 0.93-0.75 V, whereas the complete stripping of Ag occurs at 1.15 V. On the basis of surface-atom reordering processes which initiate through a place exchange mechanism involving Ag and Pt surface atoms followed by a slow Ag atom diffusion into bulk Pt, the entire cyclic voltammetric features and Auger data can be accounted for.