Alison J. Downard

Electrochemically Assisted Covalent Modification of Carbon Electrodes

A comprehensive review of the methods, scope and applications of electrochemically assisted covalently modified carbon electrodes is presented. Methods involve the oxidation of amines or arylacetates or the reduction of aryl diazonium cations. For these species it is proposed that a radical formed in solution at the carbon surface couples with the surface via a covalent bond to a surface C. In an alternative strategy, oxidation of GC electrodes in anhydrous alcohols is assumed to produce a surface-based radical which reacts with alcohols giving an ether link with the surface. Some less characterized and/or related methods are also described. A wide range of groups has been covalently linked to carbon surfaces using these methods and further modifications of initially formed monolayers have also been successful. Applications of covalently modified electrodes are reviewed; these range from formation of improved carbon composite materials to preparation of new sensing surfaces and formation of carbon surfaces with controlled structure for studies of electron transfer activity.

Covalent modification of carbon electrodes for voltammetric differentiation of dopamine and ascorbic acid

Abstract Conventional-size glassy carbon electrodes and glassy carbon fibre microelectrodes have been covalently modified with P-phenylacetate groups via a diazonium salt coupling procedure. Using catecholates of different charge, changes in electrode processes and accumulation or exclusion of analyte at the electrode surface are shown to be controlled by electrostatic interactions with surface groups. The modified electrodes have been assessed for the voltammetric differentiation of dopamine (DA) and ascorbic acid (AA). At the macroelectrode, the differential pulse peak current for oxidation of DA is significantly enhanced after modification of the electrode and that for AA is drastically decreased. The detection limit for DA at the modified electrode (measured as 3σ for 0.2 μM DA) is 90 nM, and the linear working range is 0.45–32.0 μM. Under optimized chronoamperometric conditions, the relative sensitivity for equimolar AA: DA is 0.43%. Modification of glassy carbon fibre microelectrodes leads to larger cyclic voltammetric peak currents for DA at fast scan rates and gives excellent resolution between the DA and AA signals in the differential pulse mode.

Detection of two distinct substrate-dependent catabolic responses in yeast cells using a mediated electrochemical method

Abstract. Mediated electrochemical detection of catabolism in prokaryotic cells is well documented; however, the application of this technique to eukaryotic cells has received less attention. Two catabolic substrate-dependent mediated electrochemical signals were detected in the yeast Saccharomyces cerevisiae. The signal using a single hydrophilic mediator (ferricyanide) is small whereas the response using a double mediator system comprising a hydrophilic and a lipophilic mediator (ferricyanide and menadione) is up to three orders of magnitude larger. The behaviour of each response during cell ageing is different: the single mediator response increases whereas the double mediator response decreases. This difference indicates that the two signals originate at different points in the catabolic pathways. In S. cerevisiae the double mediator response is proposed to originate from the reduction of the lipophilic mediator by NADPH produced in the pentose phosphate pathway. The single mediator signal arises from reduction of the hydrophilic mediator by an extracellular redox species produced in response to the presence of glucose.

Reaction of gold substrates with diazonium salts in acidic solution at open-circuit potential.

The reaction of gold substrates with p-nitrobenzene diazonium tetrafluoroborate (NBD) in 0.1 M H(2)SO(4) at open-circuit potential (OCP) is demonstrated to proceed by electron transfer from gold to the NBD cation. Electrochemical, atomic force microscopy, and X-ray photoelectron spectroscopy analyses reveal the formation of multilayer films with the same composition as electrografted films. The film growth characteristics (surface concentration and film thickness vs time) also follow those observed during electrografting, consistent with electron transfer from the substrate to the diazonium cation. The OCP of the gold substrate increases during the period of film growth ( approximately 60 min) and then decreases to close to its initial value. The increase corresponds to accumulation of positive charge as electrons are transferred to NBD; the discharge process is tentatively attributed to slow oxidation of adventitious impurities in the reaction solution. Films formed at OCP or by electrografting from aqueous acid solution are markedly less stable to sonication in acetonitrile than are those electrografted from acetonitrile. Increased amounts of physisorbed material in films prepared in aqueous media or bonding of aryl groups to different gold sites in the two media are tentatively proposed to account for the different stabilities.

Development and application of the diffusive gradients in thin films technique for the measurement of total dissolved inorganic arsenic in waters.

The diffusive gradients in thin films (DGT) technique, utilizing an iron-hydroxide adsorbent, has been investigated for the in situ accumulation of total dissolved inorganic As in natural waters. Diffusion coefficients of the inorganic As(V) and As(III) species in the polyacrylamide gel were measured using a diffusion cell and DGT devices and a variety of factors that may affect the adsorption of the As species to the iron-hydroxide adsorbent, or the diffusion of the individual As species, were investigated. Under conditions commonly encountered in environmental samples, solution pH and the presence of anions, cations, fulvic acid, Fe(III)-fulvic acid complexes and colloidal iron-hydroxide were demonstrated not to affect uptake of dissolved As. To evaluate DGT as a method for accumulation and pre-concentration of total dissolved inorganic As in natural waters, DGT was applied to two well waters and a river water that was spiked with As. For each sample, the concentration obtained with use of DGT followed by measurement by hydride generation atomic absorption spectrometry with a Pd modifier (HG-AAS) was compared with the concentration of As measured directly by HG-AAS. The results confirmed that DGT is a reliable method for pre-concentration of total dissolved As.

Suppression of Protein Adsorption at Glassy Carbon Electrodes Covalently Modified with Tetraethylene Glycol Diamine

Adsorption of high molecular weight species on electrode surfaces can complicate the application of voltammetric techniques to clinical and environmental samples. The performance of tetraethylene glycol diamine as a protective electrode coating has been examined. Glassy carbon electrodes were covalently modified with the diamine via an electrochemically assisted reaction. The voltammetric response of ferrocenemonocarboxylic acid (FCA) at unmodified and modified electrodes in the presence and absence of bovine serum albumin (BSA), lysozyme and ribonuclease (RNase) was used to monitor surface fouling by protein adsorption. Modified electrodes which retained very high sensitivity for FCA oxidation also exhibited excellent resistance to adsorption of all three proteins.

A Simple Approach to Patterned Protein Immobilization on Silicon via Electrografting from Diazonium Salt Solutions

A highly versatile method utilizing diazonium salt chemistry has been developed for the fabrication of protein arrays. Conventional ultraviolet mask lithography was used to pattern micrometer sized regions into a commercial photoresist on a highly doped p-type silicon (100) substrate. These patterned regions were used as a template for the electrochemical grafting of the in situ generated p-aminobenzenediazonium cation to form patterns of aminophenyl film on silicon. Immobilization of biomolecules was demonstrated by coupling biotin to the aminophenyl regions followed by reaction with fluorescently labeled avidin and visualization with fluorescence microscopy. This simple patterning strategy is promising for future application in biosensor devices.

Binuclear manganese(II) and nickel(II) complexes with a potentially octadentate macrocyclic ligand (H4L1): synthesis and characterisation. X-ray crystal structures of [Mn2(H2L1 )(CH3COO)2]·2CH3OH and [Ni2(H2L1 )(H2O)2Cl2]·2H20

Abstract A series of binuclear manganese(II) and nickel(II) complexes of the macrocyctic ligand H4L1 has been prepared and characterised. ([Mn2(H2L1)A2], where A=Cl−, Br−, NO3−, NCS−, CH3COO− or N3−; [Ni2(H2L1)X2Y2], where X=Cl−, Br− or NO3− and Y= H20, [Ni2(H2L1)(NCS)2(C2H5OH)] and [Ni2(H2L1)(CH3COO)2]·2H2O). The macrocycle is formed by Schiff base condensation of two molecules of 2,6-diformyl-4-methylphenol with two molecules of 1,3-diamino-2-hydroxypropane in the presence of Ni(lI) or Mn(II) as template ion. The complexes are all binuclear with the two metal ions bridged by the deprotonated phenolate donors of the macrocycle. The alcohol groups of the macrocyclic ligand do not coordinate to the metal ions. The complexes have been characterised by analytical, spectroscopic, electrochemical and magnetic susceptibility measurements and the single crystal X-ray structures of [Mn2(H2L1)(CH3COO)2]·2CH3OH (triclinic, P 1 , a=8.701, b= 10.258, c=10.612 A, α=62.44, β=67.83, γ= 77.74°, V=776.8 A3, Z=1) and of [Ni2(H2L1)(H2O)2Cl2]·2H2O (rhombohedral, R 3 , a=28.885, c=9.783 A, V=7069 A3, Z=9) have been determined.

Controlled assembly of gold nanoparticles on carbon surfaces

Citrate-capped gold nanoparticles are electrostatically assembled on amine films attached to carbon surfaces. Primary amines are covalently grafted to carbon surfaces by an electrochemically-assisted method which gives easy control of the number of amine functionalities on the surface, and hence the density of the nanoparticle assembly. Further control of nanoparticle assemblies can be gained by choice of the amine modifier, and by adjusting the nanoparticle concentration, assembly time and pH of the nanoparticle solution. This simple and versatile approach for preparation of tethered nanoparticle assemblies should be compatible with any conducting carbon substrate, giving new materials for applications ranging from catalysis to sensing.

Voltammetric determination of aluminium(III) using a chemically modified electrode

Abstract An alizarin-modified electrode for the voltammetric determination of aluminium(III) is reported. The single-use modified electrode is simply prepared by dip-coating a high-density graphite electrode in an N,N -dimethylformamide solution of alizarin. Optimum experimental conditions for aluminium(III) determination include a solution pH of 8.4 ± 0.2, an accumulation time of 1 min and use of the differential-pulse mode for measurement of the oxidation peak of the aluminium(III)—alizarin complex. The detection limit is 1.5 × 10 −7 M, the response is linear up to 1.0 × 10 −5 M and the relative standard deviation for replicate preparations of the electrode [at 7.5 × 10 −6 M Al(III)] is 3.8%. The determination of aluminium in soil samples is reported.