The micro-optical ring electrode: a new and novel electrode system for photoeletrochemistry

Abstract

The design of a novel photoelectrochemical sensor, the micro-optical ring electrode (MORE), is described. Based on a thin-ring microelectrode and using at fibre-optic light guide as the insulating material interior of the ring, the MORE has been deisigned, constructed and developed to permit electrochemical investigation of photochemically generated solution species.
Initial characterisation of the electrode behaviour in the dark has been ccomplished by the use of ferricyanide in conjunction with predictive mathematical models of the time dependence of the current at micro ring electrode. The photocharacterisation of the MORE has been achieved looking at the photochemical response of tris (2,2'biyridine)ruthenium(II) in presence of the quenching agent Fe3+ .
Subsequent application of the MORE has been in the electrochemical investigation of photoactive drugs employed in Cancer Therapy. In the following study, the microelectrochemistry of methylene blue, a dye commonly employed on Photodynamic Therapy (PDT), has been characterised in the dark using, in the first instance, gold disc microelectrodes. The electrochemical behaviour of MB+ on gold disc microelectrodes has than been compared to the results obtained when using the MORE. Exploration of the photoelectrochemical response of the MORE is reported, achieved via the interrogation of the photoelectrochemistry of MB+.
Photocurrent signals obtained during cyclic voltammetric and chronoamperometric studies of MB\ conducted with the MORE under illuminated conditions and in the absence of any deliberately added reducing agent, are attributed to the formation and subsequent detection of 3 MB+ within the diffusion layer of the microring electrode. The data demonstrate that the use of the MORE for direct electrochemical detection of photogenerated species with lifetimes of < 9 x 5 10- s is possible. The electrochemistry of 3MB+ over the applied potential range from -0.4 to +1.0 V versus SCE is elucidated and discussed in the context of the behaviour of photoexcited MB+ in the presence of deliberately added reducing agent Fe3+.
In order to investigate the production of singlet oxygen associated with cancer treatment, an attempt was made to study the MB+/02 system. This part of the project has not been completed, however a preliminary study of the electrochemistry of the MB