Katherine B Holt

University College London, Londinium, England, United Kingdom

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Publications (36)140.06 Total impact

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    ABSTRACT: Cyclic voltammetry (CV) of polystyrene nanospheres was carried out after immobilisation onto boron-doped diamond electrodes. Although the polystyrene is insulating, a voltammetric response was obtained. This was attributed to the high surface area of the nanospheres, allowing the redox chemistry of the polystyrene surface to be probed despite the non-conducting nature of the bulk. The polystyrene redox response was found to be strongly dependent on prior mechanical agitation. Centrifuged, sonicated and vortexed polystyrene nanospheres all exhibited significantly higher oxidation currents than the non-agitated polystyrene. Mechanical treatment by sonication and centrifugation was found to bring about changes to surface chemistry of the polystyrene spheres, in particular the introduction of oxygen functionalities. For these samples the CV response is attributed to the presence of surface phenol functionalities. On the non-agitated and vortex treated polystyrene surfaces X-ray photoelectron spectroscopy revealed an absence of oxygen functionalities that could explain the redox response. Repetition of the CV experiment in the presence of a solution spin trap suggest that radical species play a role in the observed response. For the vortexed sample the increased oxidation currents were attributed to significant surface roughening and deformation, as revealed by Transmission Electron Microscopy.
    Physical Chemistry Chemical Physics 11/2014; · 4.20 Impact Factor
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    ABSTRACT: Graphene nanoflakes (GNF) of diameter ca. 30 nm and edge-terminated with carboxylic acid (COOH) or amide functionalities were characterised electrochemically after drop-coating onto a boron-doped diamond (BDD) electrode. In the presence of the outer-sphere redox probe ferrocenemethanol there was no discernible difference in electrochemical response between the clean BDD and GNF-modified electrodes. When ferricyanide or hydroquinone were used as redox probes there was a marked difference in response at the electrode modified with COOH-terminated GNF in comparison to the unmodified BDD and amide-terminated GNF electrode. The response of the COOH-terminated GNF electrode was highly pH dependent, with the most dramatic differences in response noted at pH < 8. This pH range coincides with partial protonation of the carboxylic acid groups as determined by titration. The acid edge groups occupy a range of bonding environments and are observed to undergo deprotonation over a pH range ca. 3.7 to 8.3. The protonation state of the GNF influences the oxidation mechanism of hydroquinone and in particular the number of solution protons involved in the reaction mechanism. The voltammetric response of ferricyanide is very inhibited by the presence of COOH-terminated GNF at pH < 8, especially in low ionic strength solution. While the protonation state of the GNF is clearly a major factor in the observed response, the exact role of the acid group in the redox process has not been firmly established. It may be that the ferricyanide species is unstable in the solution environment surrounding the GNF, where dynamic protonation equilibria are at play, perhaps through disruption to ion pairing.
    Faraday Discussions 08/2014; · 4.19 Impact Factor
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    ABSTRACT: The mixed-valence triiron complexes [Fe3(CO)7-x (PPh3) x (μ-edt)2] (x = 0-2; edt = SCH2CH2S) and [Fe3(CO)5(κ(2)-diphosphine)(μ-edt)2] (diphosphine = dppv, dppe, dppb, dppn) have been prepared and structurally characterized. All adopt an anti arrangement of the dithiolate bridges, and PPh3 substitution occurs at the apical positions of the outer iron atoms, while the diphosphine complexes exist only in the dibasal form in both the solid state and solution. The carbonyl on the central iron atom is semibridging, and this leads to a rotated structure between the bridged diiron center. IR studies reveal that all complexes are inert to protonation by HBF4·Et2O, but addition of acid to the pentacarbonyl complexes results in one-electron oxidation to yield the moderately stable cations [Fe3(CO)5(PPh3)2(μ-edt)2](+) and [Fe3(CO)5(κ(2)-diphosphine)(μ-edt)2](+), species which also result upon oxidation by [Cp2Fe][PF6]. The electrochemistry of the formally Fe(I)-Fe(II)-Fe(I) complexes has been investigated. Each undergoes a quasi-reversible oxidation, the potential of which is sensitive to phosphine substitution, generally occurring between 0.15 and 0.50 V, although [Fe3(CO)5(PPh3)2(μ-edt)2] is oxidized at -0.05 V. Reduction of all complexes is irreversible and is again sensitive to phosphine substitution, varying between -1.47 V for [Fe3(CO)7(μ-edt)2] and around -1.7 V for phosphine-substituted complexes. In their one-electron-reduced states, all complexes are catalysts for the reduction of protons to hydrogen, the catalytic overpotential being increased upon successive phosphine substitution. In comparison to the diiron complex [Fe2(CO)6(μ-edt)], [Fe3(CO)7(μ-edt)2] catalyzes proton reduction at 0.36 V less negative potentials. Electronic structure calculations have been carried out in order to fully elucidate the nature of the oxidation and reduction processes. In all complexes, the HOMO comprises an iron-iron bonding orbital localized between the two iron atoms not ligated by the semibridging carbonyl, while the LUMO is highly delocalized in nature and is antibonding between both pairs of iron atoms but also contains an antibonding dithiolate interaction.
    Organometallics 03/2014; 33(6):1356-1366. · 4.25 Impact Factor
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    ABSTRACT: Modification of an electrode with an immobilised layer of nanodiamond is found to significantly enhance the recorded currents for reversible oxidation of ferrocene methanol (FcMeOH). Current enhancement is related to nanodiamond diameter, with enhancement increasing in the order 1000 nm < 250 nm < 100 nm < 10 nm < 5 nm. We attribute the current enhancement to two catalytic processes: i) electron transfer between the solution redox species and redox-active groups on the nanodiamond surface; ii) electron transfer mediated by FcMeOH+ adsorbed onto the nanodiamond surface. The first process is pH dependent as it depends on nanodiamond surface functionalities for which electron transfer is coupled to proton transfer. The adsorption-mediated process is observed most readily at slow scan rates and is due to self-exchange between adsorbed FcMeOH+ and FcMeOH in solution. FcMeOH+ has a strong electrostatic affinity for the nanodiamond surface, as confirmed by in situ infrared experiments.
    Faraday Discussions 01/2014; · 4.19 Impact Factor
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    ABSTRACT: Fe2(CO)4(μ-dppf)(μ-pdt) catalyses the conversion of protons and electrons into hydrogen and also the reverse reaction thus mimicing both types of binuclear hydrogenase enzymes.
    Chemical Communications 12/2013; · 6.38 Impact Factor
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    ABSTRACT: Here we report the synthesis, electrochemistry and electrocatalytic activity of Fe2(CO)6(μ-SC6F5)2 (1) where the highly fluorinated bridge is electron-withdrawing, resulting in decreased electron-density at the iron–iron bond. Additionally we discuss the related substituted complexes Fe2(CO)5(PPh3)(μ-SC6F5)2 (2) and Fe2(CO)4(μ-Ph2PCH2PPh2)(μ-SC6F5)2 (3). As none of the complexes could be protonated in their neutral form it was found that proton reduction catalysis in the presence of strong acid (HBF4) took place at the potential of the first reduction of complex 1 and 3, following an EC mechanism. Complex 2 was unstable in the presence of strong acid. For 1 the potential at which proton reduction took place represented a relatively mild reduction potential (−1.15 V vs. Fc/Fc+ in acetonitrile) that was comparable to examples of similar complexes in the literature. Complex 1 generated a small concentration of a highly catalytic species after electrochemical reduction, which we attribute to cleavage of the Fe–Fe bond and formation of a mono-nuclear iron species or to Fe–S bond breakage generating a vacant coordination site. The contributions to the catalytic currents were simulated using DigiSim, where it was found that the rate limiting step for 3 was the elimination of H2. It was also found that the highly catalytic species generated after reduction of 1 was more basic than 1− and also that protonation of this species was faster.
    Journal of electroanalytical chemistry 08/2013; 703:14–22. · 2.87 Impact Factor
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    ABSTRACT: The versatile precursors [Ru(CH═CHC6H4Me-4)Cl(CO)(BTD)(PPh3)2] (BTD = 2,1,3-benzothiadiazole) and [Ru(C(C≡CPh)═CHPh)Cl(CO)(PPh3)2] were treated with isonicotinic acid, 4-cyanobenzoic acid, and 4-(4-pyridyl)benzoic acid under basic conditions to yield [Ru(vinyl)(O2CC5H4N)(CO)(PPh3)2], [Ru(vinyl)(O2CC6H4CN-4)(CO)(PPh3)2], and [Ru(vinyl){O2CC6H4(C5H4N)-4}(CO)(PPh3)2], respectively. The osmium analogue [Os(CH═CHC6H4Me-4)(O2CC5H4N)(CO)(PPh3)2] was also prepared. cis-[RuCl2(dppm)2] was used to prepare the cationic compounds [Ru(O2CC5H4N)(dppm)2](+) and [Ru{O2CC6H4(C5H4N)-4}(dppm)2](+). The treatment of 2 equiv of [Ru(C(C≡CPh)═CHPh)(O2CC5H4N)(CO)(PPh3)2] and [Ru(O2CC5H4N)(dppm)2](+) with AgOTf led to the trimetallic compounds [{Ru(C(C≡CPh)═CHPh)(CO)(PPh3)2(O2CC5H4N)}2Ag](+) and [{Ru(dppm)2(O2CC5H4N)}2Ag](3+). In a similar manner, the reaction of [Ru(O2CC5H4N)(dppm)2](+) with PdCl2 or K2PtCl4 yielded [{Ru(dppm)2(O2CC5H4N)}2MCl2](2+) (M = Pd, Pt). The reaction of [RuHCl(CO)(BTD)(PPh3)2] with HC≡CC6H4F-4 provided [Ru(CH═CHC6H4F-4)Cl(CO)(BTD)(PPh3)2], which was treated with isonicotinic acid and base to yield [Ru(CH═CHC6H4F-4)(O2CC5H4N)(CO)(PPh3)2]. The addition of [Au(C6F5)(tht)] (tht = tetrahydrothiophene) resulted in the formation of [Ru(CH═CHC6H4F-4){O2CC5H4N(AuC6F5)}(CO)(PPh3)2]. Similarly, [Ru(vinyl)(O2CC6H4CN-4)(CO)(PPh3)2] reacted with [Au(C6F5)(tht)] to provide [Ru(vinyl){O2CC6H4(CNAuC6F5)-4}(CO)(PPh3)2]. The reaction of 4-cyanobenzoic acid with [Au(C6F5)(tht)] yielded [Au(C6F5)(NCC6H4CO2H-4)]. This compound was used to prepare [Ru(CH═CHC6H4F-4){O2CC6H4(CNAuC6F5)-4}(CO)(PPh3)2], which was also formed on treatment of [Ru(CH═CHC6H4F-4)(O2CC6H4CN-4)(CO)(PPh3)2] with [Au(C6F5)(tht)]. The known compound [RhCl2(NC5H4CO2)(NC5H4CO2Na)3] and the new complex [RhCl2{NC5H4(C6H4CO2)-4}{NC5H4(C6H4CO2Na)-4}3] were prepared from RhCl3·3H2O and isonicotinic acid or 4-(4-pyridyl)benzoic acid, respectively. The former was treated with [Ru(CH═CHC6H4Me-4)Cl(CO)(BTD)(PPh3)2] to yield [RhCl2{NC5H4CO2(Ru(CH═CHC6H4Me-4)(CO)(PPh3)2}4]Cl. As an alternative route to pentametallic compounds, the Pd-coordinated porphyrin [(Pd-TPP)(p-CO2H)4] was treated with 4 equiv of [Ru(CH═CHR)Cl(CO)(BTD)(PPh3)2] in the presence of a base to yield [(Pd-TPP){p-CO2Ru(CH═CHR)(CO)(PPh3)2}4] (R = C6H4Me-4, CPh2OH). Where R = CPh2OH, treatment with HBF4 led to the formation of [(Pd-TPP){p-CO2Ru(═CHCH═CPh2)(CO)(PPh3)2}4](BF4)4. [(Pd-TPP){p-CO2Ru(dppm)2}4](PF6)4 was prepared from [(Pd-TPP)(p-CO2H)4] and cis-[RuCl2(dppm)2]. The reaction of AgNO3 with sodium borohydride in the presence of [Ru(O2CC5H4N)(dppm)2](+) or [RuR{O2CC6H4(C5H4N)-4}(dppm)2](+) provided silver nanoparticles Ag@[NC5H4CO2Ru(dppm)2](+) and Ag@[NC5H4{C6H4CO2Ru(dppm)2}-4](+).
    Inorganic Chemistry 04/2013; · 4.79 Impact Factor
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    ABSTRACT: Reactions of Fe2(CO)6(μ-pdt) (pdt = SCH2CH2CH2S) with aminodiphosphines Ph2PN(R)PPh2 (R = allyl, (i)Pr, (i)Bu, p-tolyl, H) have been carried out under different conditions. At room temperature in MeCN with added Me3NO·2H2O, dibasal chelate complexes Fe2(CO)4{κ(2)-Ph2PN(R)PPh2}(μ-pdt) are formed, while in refluxing toluene bridge isomers Fe2(CO)4{μ-Ph2PN(R)PPh2}(μ-pdt) are the major products. Separate studies have shown that chelate complexes convert to the bridge isomers at higher temperatures. Two pairs of bridge and chelate isomers (R = allyl, (i)Pr) have been crystallographically characterised together with Fe2(CO)4{μ-Ph2PN(H)PPh2}(μ-pdt). Chelate complexes adopt the dibasal diphosphine arrangement in the solid state and exhibit very small P-Fe-P bite-angles, while the bridge complexes adopt the expected cisoid dibasal geometry. Density functional calculations have been carried out on the chelate and bridge isomers of the model compound Fe2(CO)4{Ph2PN(Me)PPh2}(μ-pdt) and reveal that the bridge isomer is thermodynamically favourable relative to the chelate isomers that are isoenergetic. The HOMO in each of the three isomers exhibits significant metal-metal bonding character, supporting a site-specific protonation of the iron-iron bond upon treatment with acid. Addition of HBF4·Et2O to the Fe2(CO)4{κ(2)-Ph2PN(allyl)PPh2}(μ-pdt) results in the clean formation of the corresponding dibasal hydride complex [Fe2(CO)4{κ(2)-Ph2PN(allyl)PPh2}(μ-H)(μ-pdt)][BF4], with spectroscopic measurements revealing the intermediate formation of a basal-apical isomer. A crystallographic study reveals that there are only very small metric changes upon protonation. In contrast, the bridge isomers react more slowly to form unstable species that cannot be isolated. Electrochemical and electrocatalysis studies have been carried out on the isomers of Fe2(CO)4{Ph2PN(allyl)PPh2}(μ-pdt). Electron accession is predicted to occur at an orbital that is anti-bonding with respect to the two metal centres based on the DFT calculations. The LUMO in the isomeric model compounds is similar in nature and is best described as an antibonding Fe-Fe interaction that contains differing amounts of aryl π* contributions from the ancillary PNP ligand. The proton reduction catalysis observed under electrochemical conditions at ca. -1.55 V is discussed as a function of the initial isomer and a mechanism that involves an initial protonation step involving the iron-iron bond. The measured CV currents were higher at this potential for the chelating complex, indicating faster turnover. Digital simulations showed that the faster rate of catalysis of the chelating complex can be traced to its greater propensity for protonation. This supports the theory that asymmetric distribution of electron density along the iron-iron bond leads to faster catalysis for models of the Fe-Fe hydrogenase active site.
    Dalton Transactions 03/2013; · 4.10 Impact Factor
  • Jan Scholz, A James McQuillan, Katherine B Holt
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    ABSTRACT: Attenuated total reflectance infrared spectroscopy is used to monitor nanodiamond surface group transformations in the presence of aqueous IrCl(6)(2-). Electron transfer between the nanoparticle surface and the solution redox species results in oxidation of ∼8.5% of surface alcohol groups, with concomitant formation of unsaturated ketone or quinone-like moieties.
    Chemical Communications 11/2011; 47(44):12140-2. · 6.38 Impact Factor
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    ABSTRACT: Mixed-valence triiron complexes Fe(3)(CO)(7-x)(PPh(3))(x)(μ-edt)(2) (x = 0-2) have been prepared and are shown to act as proton reduction catalysts. Catalysis takes place via an ECEC mechanism with a reduced overpotential of ca. 0.45 V for Fe(3)(CO)(7)(μ-edt)(2) as compared to the corresponding diiron complex.
    Chemical Communications 09/2011; 47(40):11222-4. · 6.38 Impact Factor
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    ABSTRACT: Thioglycolic acid (TA) and urea hydrogen peroxide (urea H(2)O(2)) are thought to disrupt alpha-keratin disulfide links in the nail. However, optimal clinical use of these agents to improve the treatment of nail disorders is currently hindered by a lack of fundamental data to support their mechanism of action. The aim of this study was to investigate how the redox environment of ungual keratin, when manipulated by TA and urea H(2)O(2), influenced the properties of the nail barrier. Potentiometric and voltammetric measurements demonstrated that urea H(2)O(2) obeyed the Nernst equation for a proton coupled one-electron transfer redox process while TA underwent a series of redox reactions that was complicated by electrode adsorption and dimer formation. The functional studies demonstrated that nail permeability, measured through TBF penetration (38.51+/-10.94 microg/cm(2)/h) and nail swelling (244.10+/-14.99% weight increase), was greatest when relatively low concentrations of the thiolate ion were present in the applied solution. Limiting the thiolate ion to low levels in the solution retards thiolate dimerisation and generates thiyl free radicals. It appeared that this free radical generation was fundamental in facilitating the redox-mediated keratin disruption of the ungual membrane.
    Free Radical Biology and Medicine 09/2010; 49(5):865-71. · 5.27 Impact Factor
  • Daren J Caruana, Katherine B Holt
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    ABSTRACT: We propose that redox reactions on the surface of interstellar dust grains contribute to the synthesis of some polyatomic species that have been identified by spectroscopic signatures. Most of the dust is found in clouds along with a rich abundance of molecular and atomic species, creating a thermodynamically distinct region in the interstellar medium (ISM) where chemistry can be supported. Using knowledge of redox process at the solid/liquid interface, a hypothesis is presented for processing mechanisms involving electron transfer between surface adsorbed species and the solid dust grains found in the ISM. The hypothesis is based on the interaction of dust grains with electromagnetic radiation and plumes of ionised gas, which electrostatically charge dust grains leading to an adjustment of the Fermi energy of electrons on the surface of individual grains. This process is equivalent to applying an external electrochemical potential to an electrode, to drive redox chemistry on an electrode surface in electrolysis or dynamic electrochemistry. Here the individual grains act as 'single electrode' electrochemical reactors in the gas phase. In this paper we highlight a gap in understanding of redox reactions at the solid/gas interface, which is potentially a very fruitful and interesting area of research.
    Physical Chemistry Chemical Physics 04/2010; 12(13):3072-9. · 4.20 Impact Factor
  • Katherine B Holt
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    ABSTRACT: This article discusses some of our recent work on the origins of redox activity of undoped nanodiamond (ND) powders, as well as reviewing some properties and applications of this material. The electrochemical activity is attributed to unsaturated bonding at the ND particle surface; hence the most recent understanding of the surface chemistry of these materials is discussed. The implications of the observed redox activity, especially for use in biological applications, are highlighted, along with future avenues of research.
    Physical Chemistry Chemical Physics 03/2010; 12(9):2048-58. · 4.20 Impact Factor
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    ABSTRACT: The homobimetallic ruthenium(II) and osmium(II) complexes [{RuR(CO)(PPh(3))(2)}(2)(S(2)COCH(2)C(6)H(4)CH(2)OCS(2))] (R = CH=CHBu(t), CH=CHC(6)H(4)Me-4, C(C[triple bond]CPh)=CHPh, CH=CHCPh(2)OH) and [{Os(CH=CHC(6)H(4)Me-4)(CO)(PPh(3))(2)}(2)(S(2)COCH(2)C(6)H(4)CH(2)OCS(2))] form readily from the reactions of [MRCl(CO)(BTD)(PPh(3))(2)] (M = Ru or Os; BTD = 2,1,3-benzothiadiazole) with the dixanthate KS(2)COCH(2)C(6)H(4)CH(2)OCS(2)K. Addition of KS(2)COCH(2)C(6)H(4)CH(2)OCS(2)K to two equivalents of cis-[RuCl(2)(dppm)(2)] leads to the formation of [{(dppm)(2)Ru}(2)(S(2)COCH(2)C(6)H(4)CH(2)OCS(2))](2+). The benzoate complexes [RuR{O(2)CC(6)H(4)(CH(2)OH)-4}(CO)(PPh(3))(2)] (R = CH=CHBu(t), CH=CHC(6)H(4)Me-4, C(C[triple bond]CPh)=CHPh) are obtained by treatment of [RuRCl(CO)(BTD)(PPh(3))(2)] with 4-(hydroxymethyl)benzoic acid in the presence of base. Reaction of [RuHCl(CO)(PPh(3))(3)] or [RuRCl(CO)(BTD)(PPh(3))(2)] with 4-(hydroxymethyl)benzoic acid in the absence of base leads to formation of the chloride analogue [RuCl{O(2)CC(6)H(4)(CH(2)OH)-4}(CO)(PPh(3))(2)]. The unsymmetrical complex [{Ru(CH=CHC(6)H(4)Me-4)(CO)(PPh(3))(2)}(2)(O(2)CC(6)H(4)CH(2)OCS(2))] forms from the sequential treatment of [Ru(CH=CHC(6)H(4)Me-4){O(2)CC(6)H(4)(CH(2)OH)-4}(CO)(PPh(3))(2)] with base, CS(2) and [Ru(CH=CHC(6)H(4)Me-4)Cl(CO)(BTD)(PPh(3))(2)]. The new mixed-donor xanthate-carboxylate ligand, KO(2)CC(6)H(4)CH(2)OCS(2)K is formed by treatment of 4-(hydroxymethyl)benzoic acid with excess KOH and two equivalents of carbon disulfide. This ligand reacts with two equivalents of [Ru(CH=CHC(6)H(4)Me-4)Cl(BTD)(CO)(PPh(3))(2)] or cis-[RuCl(2)(dppm)(2)] to yield [{(dppm)(2)Ru}(2)(O(2)CC(6)H(4)CH(2)OCS(2))](2+) or [{Ru(CH=CHC(6)H(4)Me-4)(CO)(PPh(3))(2)}(2)(O(2)CC(6)H(4)CH(2)OCS(2))], respectively. Electrochemical experiments are also reported in which communication between the metal centres is investigated.
    Dalton Transactions 10/2009; · 4.10 Impact Factor
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    ABSTRACT: The electrochemical response of an electrode-immobilized layer of undoped, insulating diamond nanoparticles is reported, which we attribute to the oxidation and reduction of surface states. The potentials of these surface states are pH-dependent; moreover they are able to interact with solution redox species. The voltammetric response of redox couples Fe(CN)(6)(3-/4-) and IrCl(6)(3-/2-) are compared at bare boron-doped diamond electrodes and electrodes modified with a layer of nanodiamond (ND). In all cases the presence of ND modifies the CV response at slow scan rates if low concentrations of redox couple are used. Enhancements of oxidation currents are noted at potentials at which the ND surface states can also undergo oxidation, and enhancements of reduction currents are likewise observed where ND is also reducible. We attribute these observations to electron transfer occurring between the species generated at the underlying electrode during CV and the ND immobilized in the interfacial region, leading to regeneration of the starting species and hence enhancement in currents due to a feedback mechanism. The magnitude of current enhancement thus depends on the standard potential of the redox couple relative to those of the ND surface states.
    Journal of the American Chemical Society 09/2009; 131(32):11272-3. · 11.44 Impact Factor
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    Jingping Hu, Katherine B Holt, John S Foord
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    ABSTRACT: The fabrication of ultramicroelectrodes (UMEs) for analytical electrochemical applications has been explored, using boron-doped diamond as the active electrode material in an insulating coating formed by deposition of electrophoretic paint. Because of the rough nature of the diamond film, the property of such coatings that is normally exploited in the fabrication of UMEs, namely the tendency to retract automatically from sharp protrusions, cannot be used in the present instance. Instead focused ion beam (FIB) sputtering was employed to controllably produce UMEs with well-defined geometry, critical dimension of a few micrometers, and very thin insulating coatings. If the FIB machining is carried out at normal incidence to the diamond electrode surface, significant ion beam damage reduces the yield of successful electrodes. However, if a parallel machining geometry is employed, high yields of ultramicroelectrodes with a flat disk geometry can be obtained very reliably. The electrochemical properties of diamond UMEs are characterized. They show much lower background currents than the equivalent Pt or carbon fiber electrodes but more varied electrochemical response than macroscopic diamond electrodes.
    Analytical Chemistry 07/2009; 81(14):5663-70. · 5.83 Impact Factor
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    ABSTRACT: In this paper we present preliminary electrochemical investigations into the transport properties of free standing ultra-thin surfactant films and the associated meniscus. We describe a new electrochemical cell composed of a 25 mu m diameter gold wire placed through a stable surfactant film which served as the electrolyte. Solutions containing anionic sodium dodecyl sulphate (SDS) or non-ionic Triton-X100 surfactants, with background electrolyte NaCl and with electroactive probe ferrocyanide or ferrocene methanol, were used to create the surfactant films. The electrolyte was an ultra-thin surfactant film creating a two dimensional solution with a thickness between 300 and 1000 nm, and its meniscus at the gold wire, within which the electroactive probe was free to diffuse. Cyclic voltammetry was used to oxidise and reduce the electroactive probe within the surfactant film and meniscus. It was shown that films and the associated meniscus formed from SDS solution almost completely excluded negatively charged ferrocyanide. A finite difference simulation showed that the voltammetry was dominated by the meniscus region, the unusual spatially-varying bounded geometry of which resulted in an unusual dependence on potential scan rate of the peak to peak separation (decreasing with increasing scan rate) and anodic:cathodic peak current ratio (increasing with increasing scan rate). (C) 2009 Elsevier B.V. All rights reserved.
    Electrochemistry Communications 06/2009; 11(6):1226-1229. · 4.29 Impact Factor
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    ABSTRACT: The redox behavior of an undoped nanodiamond (ND) film grown by chemical vapor deposition was investigated using cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM) and redox mediators Fe(CN) 6 3-, Fe(CN) 6 4-, ferrocenemethanol (FcOH), and Ru(NH 3) 6 3+ . CV showed extremely sluggish kinetics for all redox couples, but the reduction of Fe(CN) 6 3-was found to be especially slow when compared to the oxidation of Fe(CN) 6 4-. SECM confirmed this trend, with experimental heterogeneous rate constants, obtained by fitting approach curves to theory, being of the magnitude of 10 -3 cm s -1 . The oxidation of Fe(CN) 6 4-at an overpotential, |η|, of 0.6 V was found to occur 5 times faster than the reduction of Fe(CN) 6 3-at the same |η|. The results are explained by assuming conduction takes place through extended sp 2 (graphitic and defect sites) through the film. The nondiamond component of the film introduces impurity bands into the band gap that allows limited metallic type conductivity. The slow electron transfer was attributed to the very small percentage of the surface that was electrochemically active and hence relatively narrow impurity bands and limited carrier numbers. About 2% of the surface was calculated to be active in the potential range -0.4 to 0.5 V vs Ag/AgCl. At >0.5 V, the active area was found to increase with applied potential up to about 10% at 0.8 V. This increase in active electrode area explains the faster rate constants obtained for the oxidation of Fe(CN) 6 4-at these potentials. It is postulated that the increase in active area is due to oxidation of defect sites of the film to form electron deficient, hence redox active, centers. This results in the widening of the impurity bands in the band gap and hence an increased density of states. Approach curves to a layer of 5 nm ND powder using the same redox couples exhibited a similar trend, with reduction of Fe(CN) 6 3-taking place much slower than oxidation of Fe(CN) 6 4-. Overall, rate constants were about 10 times faster at the powder interface than the film. It is believed that electron transfer at the ND nanoparticle surface takes place at similar sites as on the ND film but that they are present at higher relative concentrations due to the higher surface to bulk atom ratio of the nanoparticles.
    The Journal of Physical Chemistry C 02/2009; 113(7). · 4.84 Impact Factor
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    ABSTRACT: Treatment of cis-[RuCl2(dppm)2] (dppm = bis(diphenylphosphino)methane) with dithiocarbamates, NaS2CNR2 (R = Me, Et) and [H2NC5H10][S2CNC5H10], yields cations [Ru(S2CNR2)2(dppm)2](+) and [Ru(S2CNC5H10)2(dppm)2](+), respectively. The zwitterions S2CNC4H8NHR (R = Me, Et) react with the same metal complex in the presence of base to yield [Ru(S2CNC4H8NR)(dppm)2](+). Piperazine or 2,6-dimethylpiperazine reacts with carbon disulfide to give the zwitterionic dithiocarbamate salts H2NC4H6(R2-3,5)NCS2 (R = H; R = Me), which form the complexes [Ru(S2CNC4H6(R2-3,5)NH2)(dppm)2](2+) on reaction with cis-[RuCl2(dppm)2]. Sequential treatment of [Ru(S2CNC4H8NH2)(dppm)2](2+) with triethylamine and carbon disulfide forms the versatile metalla-dithiocarbamate complex [Ru(S2CNC4H8NCS2)(dppm)2] which reacts readily with cis-[RuCl2(dppm)2] to yield [{Ru(dppm)2}2(S2CNC4H8NCS2)]. Reaction of [Ru(S2CNC4H8NCS2)(dppm)2] with [Os(CH=CHC6H4Me-4)Cl(CO)(BTD)(PPh3)2] (BTD = 2,1,3-benzothiadiazole), [Pd(C6H4CH2NMe2)Cl]2, [PtCl2(PEt3)2], and [NiCl2(dppp)] (dppp = 1,3-bis(diphenylphosphino)propane) results in the heterobimetallic complexes [(dppm)2Ru(S2CNC4H8NCS2)ML(n))](m+) (ML(n) = Os(CH=CHC6H4Me-4)(CO)(PPh3)2](+), m = 1; ML(n) = Pd(C,N-C6H4CH2NMe2), m = 1; ML(n) = Pt(PEt3)2, m = 2; ML(n) = Ni(dppp), m = 2). Reaction of [NiCl2(dppp)] with H2NC4H8NCS2 yields the structurally characterized compound, [Ni(S2CNC4H8NH2)(dppp)](2+), which reacts with base, CS2, and cis-[RuCl2(dppm)2] to provide an alternative route to [(dppm)2Ru(S2CNC4H8NCS2)Ni(dppp)](+). A further metalla-dithiocarbamate based on cobalt, [CpCo(S2CNC4H8NH2)(PPh3)](2+), is formed by treatment of CpCoI2(CO) with S2CNC4H8NH2 followed by PPh3. Further reaction with NEt3, CS2, and cis-[RuCl2(dppm)2] yields [(Ph3P)CpCo(S2CNC4H8NCS2)Ru(dppm)2](2+). Heterotrimetallic species of the form [{(dppm)2Ru(S2CNC4H8NCS2)}2M](2+) result from the reaction of [Ru(S2CNC4H8NCS2)(dppm)2] and M(OAc)2 (where M = Ni, Cu, Zn). Reaction of [Ru(S2CNC4H8NCS2)(dppm)2] with Co(acac)3 and LaCl3 results in the formation of the compounds [{(dppm)2Ru(S2CNC4H8NCS2)}3Co](3+) and [{(dppm)2Ru(S2CNC4H8NCS2)}3La](3+), respectively. The electrochemical behavior of selected examples is also reported.
    Inorganic Chemistry 10/2008; 47(20):9642-53. · 4.79 Impact Factor
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    ABSTRACT: This paper demonstrates the promoting effects of 5 nm undoped detonation diamond nanoparticles on redox reactions in solution. An enhancement in faradaic current for the redox couples Ru(NH(3))(6)(3+/2+) and Fe(CN)(6)(4-/3-) was observed for a gold electrode modified with a drop-coated layer of nanodiamond (ND), in comparison to the bare gold electrode. The ND layer was also found to promote oxygen reduction. Surface modification of the ND powders by heating in air or in a hydrogen flow resulted in oxygenated and hydrogenated forms of the ND, respectively. Oxygenated ND was found to exhibit the greatest electrochemical activity and hydrogenated ND the least. Differential pulse voltammetry of electrode-immobilised ND layers in the absence of solution redox species revealed oxidation and reduction peaks that could be attributed to direct electron transfer (ET) reactions of the ND particles themselves. It is hypothesised that ND consists of an insulating sp(3) diamond core with a surface that has significant delocalised pi character due to unsatisfied surface atoms and C[double bond, length as m-dash]O bond formation. At the nanoscale surface properties of the particles dominate over those of the bulk, allowing ET to occur between these essentially insulating particles and a redox species in solution or an underlying electrode. We speculate that reversible reduction of the ND may occur via electron injection into available surface states at well-defined reduction potentials and allow the ND particles to act as a source and sink of electrons for the promotion of solution redox reactions.
    Physical Chemistry Chemical Physics 02/2008; 10(2):303-10. · 4.20 Impact Factor

Publication Stats

345 Citations
140.06 Total Impact Points

Institutions

  • 2006–2014
    • University College London
      • Department of Chemistry
      Londinium, England, United Kingdom
  • 2011
    • University of Otago
      • Department of Chemistry
      Dunedin, Otago, New Zealand
  • 2001–2009
    • University of Oxford
      • • Department of Chemistry
      • • Physcial and Theoretical Chemistry Laboratory
      Oxford, ENG, United Kingdom