Katherine B. Holt

Diamond at the nanoscale: applications of diamond nanoparticles from cellular biomarkers to quantum computing

Although nanocrystalline diamond powders have been produced in industrial quantities, mainly by detonation synthesis, for many decades their use in applications other than traditional polishing and grinding have been limited, until recently. This paper presents the wide-ranging applications of nanodiamond particles to date and discusses future research directions in this field. Owing to the recent commercial availability of these powders and the present interest in nanotechnology, one can predict a huge increase in research with these materials in the very near future. However, to fully exploit these materials, fundamental as well as applied research is required to understand the transition between bulk and surface properties as the size of particles decreases.

Multimetallic Assemblies Using Piperazine-Based Dithiocarbamate Building Blocks

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.

Voltammetry at carbon nanofiber electrodes

Carbon nanofibers with diameters in the range 10–500 nm have been evaluated as novel electrode materials for electrochemical applications. Compared with other forms of nanostructured carbons, such as aerogels or activated charcoal, carbon nanofibers exhibit low BET surface areas, 50 vs. , because their surfaces are not readily penetrated by gaseous nitrogen. But somewhat surprisingly, they exhibit higher electrochemical capacitances (ca. 60 vs. ) because the spaces between the fibers are readily penetrated by electrolyte solution. As a result, capacitive currents tend to mask voltammetric currents during cyclic voltammetry. The situation is quite different when the spaces between carbon nanofibers are impregnated by an inert dielectric material, such as high-melting paraffin wax. Then the carbon nanofibers form a high-density composite electrode with good conductivity and low capacitance. Indeed, well-defined voltammetric responses are readily observed for the reduction of Ru(NH3)63+ in aqueous solution, even in the absence of supporting electrolyte. Metal deposition and anodic stripping processes can also be observed for the reduction of Pb2+ in aqueous nitric acid. This suggests that carbon nanofibers represent a new class of material suitable for electroanalytical applications.

Microwave-enhanced anodic stripping detection of lead in a river sediment sample. A mercury free procedure employing a boron-doped diamond electrode

Microwave activation of electrochemical processes has recently been introduced as a novel technique for the enhancement and control of processes at the electrode-solution interface and is employed here to improve the analytical detection of Pb2+. Instead of the conventional mercury-based accumulation and stripping procedure, mercury-free boron-doped diamond electrodes are employed. The deposition and anodic stripping detection by square-wave voltammetry of Pb2+ in a 0.1 M HNO3 solution is shown to be strongly enhanced by microwave activation at boron-doped diamond electrode. The temperature at the electrode-solution interface is calibrated with reversible redox couple Fe3+/Fe2+ (4 mM Fe3+, 4 mM Fe2+) in 0.1 M HNO3 and a standard addition procedure is developed for the sensitive detection of Pb2+ concentrations from 1 µM to 5 µM. The limit of detection by square-wave voltammetry after 20 s deposition time was found to be 0.1 µM and 1.0 µM with microwave activation and without microwave activation, respectively. Then, the Pb content in a water sediment sample detected by anodic stripping voltammetry at boron-doped diamond electrodes is shown to be in good agreement with two other independent analytical procedures based on ICP mass spectroscopy and on sono-cathodic stripping voltammetry.

Reduction of Carbon Dioxide to Formate at Low Overpotential Using a Superbase Ionic Liquid

A new low-energy pathway is reported for the electrochemical reduction of CO2 to formate and syngas at low overpotentials, utilizing a reactive ionic liquid as the solvent. The superbasic tetraalkyl phosphonium ionic liquid [P66614][124Triz] is able to chemisorb CO2 through equimolar binding of CO2 with the 1,2,4-triazole anion. This chemisorbed CO2 can be reduced at silver electrodes at overpotentials as low as 0.17 V, forming formate. In contrast, physically absorbed CO2 within the same ionic liquid or in ionic liquids where chemisorption is impossible (such as [P66614][NTf2]) undergoes reduction at significantly increased overpotentials, producing only CO as the product.

Models of the iron-only hydrogenase: a comparison of chelate and bridge isomers of Fe2(CO)4{Ph2PN(R)PPh2}(μ-pdt) as proton-reduction catalysts

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.

Electrochemistry of undoped diamond nanoparticles: accessing surface redox states.

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.

Lead Dioxide Deposition and Electrocatalysis at Highly Boron-Doped Diamond Electrodes in the Presence of Ultrasound

Boron-doped diamond (BDD) is u versatile and novel electrode material which, due to its mechanical and chemical robustness, wide potential window, low background interference, und ease of chemical modification, is becoming an interesting alternative to conventional electrodes for a wide range of electrochemical applications. It is shown in this study that BDD is a good substrate for the ultrasound-enhanced electrodeposition of lead dioxide, producing strongly adhered electrically conducting deposits. Power ultrasound is used to enhance both the efficiency of the PbO 2 deposition procedure and the rate of the electrocatalytic ethylene glycol oxidation process at the PbO 2 -modified BDO electrode. The presence of high levels of aqueous organic material is shown to interfere with the lead dioxide deposition process. Under optimized insonation conditions the PbO 2 deposit, quantified by using cathodic stripping voltammetry, is shown to be mechanically stable. When used in conjunction with power ultrasound to perform the electrocatalytic oxidation of ethylene glycol two distinct types of oxidation processes at PbO 2 , chemically rate limited and electrochemically rate limited, are observed.

Mechanistic aspects of the sonoelectrochemical degradation of the reactive dye Procion Blue at boron-doped diamond electrodes

Redox processes of the reactive dye, Procion Blue, at polycrystalline boron-doped diamond film electrodes, in buffered aqueous solution are studied as a function of pH, dye concentration and ultrasound treatment. Direct partial oxidation with a four electron transfer, within the solvent window up to 2.5 V vs. SCE in PBS, at pH 2 is possible. However, more extensive degradation of Procion Blue does not occur at potentials below that required for solvent decomposition. Oxidation is most easily achieved in acidic solution and at low dye concentrations, as evidence of fouling of the electrode surface was noted under more alkaline conditions and at higher dye concentrations. The dye is also found to influence redox processes which are directly associated with defect sites on the diamond electrode itself.

Anodic activity of boron-doped diamond electrodes in bleaching processes: effects of ultrasound and surface states

In this study three types of polycrystalline highly boron-doped (ca. 1020 cm−3) diamond electrodes are compared with respect to their activity in anodic bleaching processes. A commercially available boron-doped diamond electrode (from De Beers), a conventional polycrystalline boron-doped diamond electrode grown in a hot filament chemical vapour deposition (HFCVD) process, and an sp2-carbon impurity state-rich polycrystalline boron-doped diamond electrode (grown in the presence of a high methane concentration) are characterized by voltammetry, Raman spectroscopy, and electron microscopy methods. Next, the efficiency of anodic bleaching processes (assumed to be based on hydroxyl radical generation at the diamond electrode surface) is investigated as a function of surface modification and with/without activation by power ultrasound. As a model process, the bleaching of the spin trapping reagent N,N-dimethyl-p-nitrosoaniline (RNO), is employed. Power ultrasound is shown to drastically improve the rate of bleaching by increasing the rate of mass transport at the electrode|solution interface. However, the state of the diamond electrode surface is also important. Boron-doped diamond electrodes rich in sp2 carbon impurity states are initially more efficient in the bleaching process. Reactive intermediates, such as hydroxyl radicals, appear to be formed preferentially in the vicinity of impurity states. However, mass transport is the dominating parameter in controlling the efficiency of the bleaching process.