The modification of carbon electrodes for biosensor applications

Abstract

This thesis reports upon the covalent attachment of 6 different types of linkers bearing the Boc protecting group to the surface of glassy carbon electrodes, highly ordered pyrolytic graphite with the edge and basal plane orientation and multi-walled carbon nanotube abrasively immobilised onto a glassy carbon substrate by electrochemical oxidation or reduction of the corresponding diazonium salt. Removal of the Boc group allows redox probes, such as anthraquinone-2-carboxyl acid, 4 nitrobenzoyl chloride and 3 and 4 dihydroxybenzoyl chloride, to be coupled to the surface using solid-phase coupling methods. The surface coverage, scan rate and pH effects, as well as the determination of the kinetic parameters, were evaluated using cyclic voltammetry. It was found that the type of carbon electrode and the choice of linkers had a significant influence on the surface coverage of redox probes and the electron transfer rate. A 4-(N-Boc aminomethyl) benzene diazonium tetrafluoroborate salt linker (C6H4CH2NH?) was also spontaneously grafted onto the CNTs by refluxing in the C6H4CH2NHBoc diazonium salt at 60 oC in an acetonitrile solution. After the removal of the Boc protecting group, the anthraquinone (AQ) and nitrobenzene (NB) groups were attached to the benzyl amine linker by solid-phase amide coupling. The grafted CNTs were characterized using FTIR and cyclic voltammetry techniques; the surface coverage and the stability of the tethered functional groups were also investigated. The oxygen reduction reaction was studied on bare and anthraquinone (AQ) modified edge planes, on a basal plane with highly oriented pyrolytic graphite, and on glassy carbon (GC) and multi-walled carbon nanotube electrodes. The anthraquinone modified rotating disc GC electrode results show the two electron oxygen reduction reactions with hydrogen peroxide as the final product. The immobilization of laccase (ThL) onto GC electrodes modified with anthraquinone and anthracene through EDA and C6H4CH2NH– linkers was also achieved by employing high-throughput screening using a multichannel potentiostat for a library of 12 electrodes; the activity of these electrodes to oxygen reduction was examined. The experimental findings demonstrated a successful attachment of laccase to the modified glassy carbon, as well as successful electron transfer between substrate- and enzyme-active sites. DFT calculations were used to investigate the structural properties of the functionalized basal plane of graphene after modification and to discover why the edge site shows higher reactivity to the attachment of linkers than the basal sites and whether the type of linker has an effect on the surface coverage of AQ. On the basis of considering the relationship between binding energy and charge transfer, the root of the effect of the type of linker on surface coverage was investigated to ensure whether it is steric, electronic, or both.

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ORCID for Philip N. Bartlett: orcid.org/0000-0002-7300-6900

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