Joe A. Crayston

A three-enzyme microelectrode sensor for detecting purine release from central nervous system

As the purines, in particular adenosine, are important signaling agents in the nervous system we have devised a new biosensor for directly measuring their production in real time during physiological activity. Our amperometric adenosine biosensor is made by entrapping 3 enzymes (xanthine oxidase, purine nucleoside phosphorylase and adenosine deaminase) in a composite lactobionamide and amphiphillic polypyrrole matrix around a Pt microelectrode. The resulting sensors are small (25-100 microm diameter), fast responding (10-90% rise time, 2+/-0.23 s), sensitive (100-222 mA M(-1) cm(-2)) and stable (100% activity after 5 days). The sensor was used in vivo to demonstrate the spatial localization of release of adenosine from Xenopus embryo spinal cord during fictive swimming.

Synthesis and electrochemical characterisation of poly(tempoacrylate)

Abstract A new polymer containing TEMPO radical groups has been characterised by ESR and cyclic voltammetry; electrodes modified with this polymer display high electroactivity for amine oxidation.

Electrochromic Nb2O5 and Nb2O5/silicone composite thin films prepared by sol–gel processing

Alcoholysis of NbCl5 in EtOH gives a viscous solution of NbClx(OEt)5 –x which may be used to spin-coat optically transparent electrodes (indium tin oxide, ITO). The electrode is converted to Nb2O5 by immersion in 1 mol dm3 H2SO4. Electron microscopic examination of the resulting 5–10 µm thick films revealed extensive shrinkage and cracking on gel drying. These films proved to be electrochromic (λmaxca. 800 nm) but unstable to voltammetric cycling in MeCN–(0.5 mol dm–3)LiClO4. Greater stability (> 30 cycles) was achieved with a composite Nb2On/silicone electrode, prepared from a solution containing Nb:Si:H2O in the molar ratio 5 : 13 : 82. This increase in durability was achieved with no sacrifice in electrochromic efficiency. The electrochromic colouration efficiency (6 cm2 C–1) compares favourably with that for sputtered Nb2O5 films. The colouration–bleaching cycle of the composite is complete in < 40 s, and the electrochromical kinetics of colouration and bleaching follow a diffusional model with DLi= 6 × 10–8 cm2 s–1 for Nb2O5 and 3.2 × 10–8 cm2 s–1 for the composite. The mechanism of the electrochromic response and its decay are discussed.

Covalent binding of redox active centres to preformed regioregular polythiophenes

The synthesis of regioregular head-to-tail poly[3-(6-bromohexyl)thiophene] is reported, together with its reaction with 2-carboxyanthraquinone (Anth) to give an example of a regioregular polythiophene containing pendant functional groups (87% loading). NMR data on the two soluble polymers are reported together with preliminary studies of some of their physical properties. Cyclic voltammetric studies of anthraquinone polymer coated electrodes show that the observed response is coverage dependent: thin films display four redox couples due to the Anth0/–/2– processes and the p- and n-doping of the conjugated thiophene backbone. Thick films are rectifying in the sense that reduction of the Anth groups is inhibited on the negative sweep. Spectroelectrochemical studies confirm the nature of the anodic p-doping process (the film turns red to nearly colourless) and show characteristic changes on reduction (red to black).

Cyclic voltammetry study of the electrocatalysis of carbon dioxide reduction by bis(polyazamacrocyclic) nickel complexes

Abstract Two series of binuclear macrocyclic nickel(II) complexes with varying lengths of the chain linking the two macrocyclic rings were characterised by cyclic voltammetry under argon and CO 2 . The first series consisted of binuclear complexes [Ni 2 L 2–6 ] 4+ containing pentaaza macrocycles with (CH 2 ) n bridges ( n =2, 3, 4, 6) or a p -xylyl linkage (L 6 ). In general, the two nickel sites in the binuclear complexes behave independently with the currents corresponding to the simultaneous transfer of two electrons. The redox potentials are remarkably constant along this series, but the peak separations increase, reflecting slower electron transfer due to more effective adsorption on the electrode. Electrochemical data for the electrocatalytic reduction of CO 2 in MeCN/10% H 2 O revealed catalytic waves for CO 2 reduction with E p c close to −1.7 V and catalytic currents ( i p c ) which are about half those of the mononuclear complex, proposed to be due to steric constraints allowing strong interaction of only one nickel centre of the binuclear one on the surface. The catalytic currents increased slightly as the linking chain length increased as the stereochemical constrains were relaxed somewhat. There was also a splitting in the catalytic peaks of the bismacrocyclic complexes which could reflect two types of adsorbed catalyst sites. In the more sterically crowded series of complex, [Ni 2 L 7 ] 4+ along with the series of linked heptaaza macrocyclic complexes [Ni 2 L 9–11 ] 4+ much more positive redox potentials were observed due to both alkylation of the coordinated nitrogen atoms, which decreases the ligand field, and the introduction of steric barriers to axial coordination. These steric barriers prevented strong electrode interaction and led to a lower catalytic activity. Indeed, the complex [Ni 2 L 7 ] 4+ did not even show any interaction with CO 2 in dry acetonitrile. The complexes showed well separated peaks due to solution and surface catalytic activity, and the surface catalytic currents were now comparable to mononuclear complexes at the same effective concentration. We proposed that the less effective absorption on the electrode arising from ligand steric interactions places far fewer stereochemical constraints on the adsorption of both nickel centres to the same extent as the binuclear complex, and hence the catalytic currents for binuclear complex and mononuclear complex are comparable.

The preparation, chemistry and crystal structure of the nickel(II) complex of N-hydroxyethylazacyclam [3-(2′-hydroxyethyl)-1,3,5,8,12-penta-azacyclotetradecane nickel(II) perchlorate]. A new electrocatalyst for CO2 reduction

Abstract The reaction of formaldehyde and ethanolamine with the nickel(II) complex of 1,9-diamino-3,7-diazanonane (2,3,2-tet) gives the nickel(II) complex of the macrocycle 3-hydroxyethyl-1,3,5,8,12-penta-azacyclotetradecane (L) which can be readily isolated as the perchlorate salt. The crystal structure of [NiL](ClO4)2 has been determined. The nickel atom is in an essentially planar environment with N1, N5, N8 and N12 acting as donors with NiN bond distances in the range 1.930–1.938 A The ligand has a trans III configuration of the sec-NH centres with chair six-membered and gauche five-membered chelate rings. The hydroxyethyl group on N3 is axial. The two perchlorate anions lie in the axial sites but the NiO(4) and the NiO(7) distances of 2.836(3) and 3.028(3) A indicate that there is no bonding interaction between these centres. In aqueous solution the complex is predominantly square planar. Addition of HCl leads to axial addition of chloride while addition of thiocyanate gives the trans- [NiL(NCS)2] complex. The ligand readily folds to give cis-complexes. Thus addition of ethylenediamine to [NiL]2+ gives cis-[NiL(en)]2+ in solution. The electrochemistry of [NiL]2+ has been studied in detail. The complex is a good electrocatalyst for the reduction of CO2. The complex was less active for hydrogen evolution in an acetate electrolyte than [Ni(cyclam)]2+ and thus appeared to be more active than the cyclam complex for CO2 reduction under these conditions.

The constitution of a ferrocenyl diketone: Solution and solid state NMR spectroscopy. Crystal structure of 1-ferrocenyl-3-hydroxybut-2-en-1-one

Abstract The title compound has been shown by 1 H and 13 C NMR spectroscopy to exist in solution as an enol/keto mixture, with an isomer ratio at ambient temperature in chloroform of approx. 17:1. CP-MAS 13 C NMR spectroscopy shows that only the enol form occurs in the crystalline state. This conclusion was confirmed by a single crystal X-ray structure determination. The OCCCO fragment of the molecule is planar, with an asymmetric intramolecular hydrogen bond; there are noi ntermolecular hydrogen bonds.

Cyclic voltammetry and morphology of polyaniline-coated electrodes containing [Fe(CN)6]3–/4– ions

Three strategies for incorporation of [Fe(CN)6]3–/4– into polyaniline (PANI)-coated electrodes have been investigated: A, ion-exchange of [Fe(CN)6]3–/4– into pre-formed PANI-Cl- or PANI-SO4-coated electrodes; B, electropolymerisation of aniline from acidified K4[Fe(CN)6] or K3[Fe(CN)6]; and C, electropolymerisation of aniline from H4[Fe(CN)6]. Method A led to observable ion exchange into PANI-Cl or PANI-SO4 with acidified K4[Fe(CN)6] or K3[Fe(CN)6] solutions. The amount of film-confined [Fe(CN)6]3– corresponds to protonation of ca. 19% of the N atoms in PANI, but the response is not long-lived in hexacyanoferrate(III)-free electrolyte; a partition equilibrium constant of 220 ± 50 is measured. Method B effected [Fe(CN)6]3–/4– incorporation, but only when high concentrations of redox anion were present during PANI growth. The potential of the [Fe(CN)6]3–/4– couple in the polymer is shifted to +0.32 V (ΔEp= 40 mV) for PANI-[Fe(CN)6]3– compared with the standard potential +0.36 V vs. SCE in solution (at a freshly polished glassy carbon electrode). The oxidized form of the redox couple PANI-[Fe(CN)6]3– has a time-dependent voltammogram which collapses to a single broad wave at +0.32 V (ΔEp= 30 mV). The results also show that [Fe(CN)6]4– is exchanged more rapidly when the electrode potential of the film is cycled in HCl rather than in H2SO4, owing to ion-size considerations. Scanning electron microscopy (SEM) analysis on the PANI-[Fe(CN)6] films revealed a fibrous morphology indicative of a slower rate of growth than PANI-Cl. Method C also led to successful incorporation of [Fe(CN)6]3–/4–, whose presence was confirmed by fourier transform infrared (FTIR) spectroscopy.

Hydrolysis of acetonitrile in the presence of NbCl5

Abstract The mechanism of the hydrolysis of acetonitrile to ammonium ion in the presence of NbCl 5 has been investigated. Two possible mechanisms are discussed: metal-assisted (catalysed) hydrolysis and a mechanism based on the addition of alcohol and HCl to the CN triple bond to give initially an imidate hydrochloride salt, which may undergo further hydrolysis to the ester and ammonium ion (Pinner synthesis). Support for this route came from the spectroscopic identification of the alkoxyimminium ion intermediate, CH 3 C(OCH 3 ) = NH 2 + . Although the reaction is not catalysed by Nb, the extent of reaction is strongly related to the amount of Nb present. It is concluded that the Pinner reaction is operating with the amount of HCl available for the reaction being controlled by the Nb concentration.

Studies on the electrochemical deposition of niobium oxide

Various strategies for the electrodeposition of niobium oxide from both non-aqueous and aqueous solution are explored. The methods used were based on the electrochemical manipulation of the pH which leads to hydrolysis of soluble, well defined niobium precursors, leading to oxide deposition at the electrode. The reactions which proceed are believed to be closely related to those involved in conventional sol–gel processing. In non-aqueous systems containing niobium alkoxides, hydroxide ions are generated by the two-electron reduction of tertiary alcohols (at +1.5 V cause the formation of mixed niobate and niobium oxide films on the electrode after >30 min.