The micro-optical ring electrode: Development of a novel thin ring based device for spectrophotoelectrochemistry and its application to the study of rose bengal

Abstract

Electrochemists have at their disposal, for the study of photo- electrochemical systems, a range of sensors and devices such as rotating optical disc ring electrodes, optically transparent electrodes and optically transparent thin layer electrodes.
A microelectrode is an electrode with at least one of its characteristic dimensions of less than 50 pm. This provides a number of advantages to such devices used in analytical techniques: it allows access to hitherto inaccessible media, the low
current associated allows for a low analyte consumption and ohmic polarisation, it also renders possible the detection of short-lived species.
Based on a thin ring microelectrode design and using the electrode shank as a light guide, the Micro- Optical Ring Electrode (MORE) is a novel device that has been designed and constructed to permit electrochemical investigation of photochemically
generated solution species. Thin rings exhibit high rates of material flux to the electrode surface, facilitating the detection of short lived- photo-generated solution intermediates. To exploit this advantage, we have prepared MOREs with ring inner
radius to ring outer radius ratios in excess of 0.999. Electrode behaviour in the dark has been characterised by the use of ferricyanide in conjunction with predictive mathematical models of the time dependence of the current at micro ring electrodes in the dark. Further device improvement has involved the design and construction of a more robust light guide-to-electrode coupling. The resultant increase in the transmitted light intensity and its reproducibility have facilitated the study of the spectral dependence of the photocurrents observed from photo- electro- active systems as well as their correlations to the concentrations of both the photo- active species and the quencher.
By exploiting the properties of discontinuous integrals of Bessel Functions, a mathematical model of the behaviour Of the device has been developed allowing for the generation of asymptotic analytical expressions describing:
(I) The distribution of the photoexcited analyte over the electrode surface;
(ii) The tong illumination time (steady state) photocurrent as a function of electrode dimensions, light intensity and lifetime of the photogenerated species; and
(iii) The time dependence of the tight-on photocurrent transient as a function of
species lifetime.
These expressions can be used to describe and explain the form of the Ru(II)/Fe(III) data described above and, coupled to the analyte-specific use afforded by the light selectivity of the device, make the MORE a powerful analytical tool capable
of potentially calibrationless use.
The spectral dependence of the photocurrent at the MORE has been investigated and found to correspond to the singlet-to-singlet, metal-to-ligand charge transfer band of the UV-Visible spectrum of ruthenium(II) 2,2'-trisbipyridine complex.
The value of the Stern-Volmer constant for the quenching of photoexcited Ru(bipy)3 2 by Fe3 obtained at the MORE (0.358m 3 mor1) compares favourably with the value obtained from fluorescence measurements (0.9 m 3 mol' [Un, 1976]).
The photochemistry of organic dyes is a very important field especially in the understanding of the mechanisms involved in the production of singlet oxygen, with application ranging from pollution remediation processes to photodynamic therapy
(PDT), an advanced cancer treatment. Singlet oxygen can be produced by photoexcitation of an organic molecule whose excess energy may then be passed on to a neighbouring oxygen molecule that is forced into its highly reactive singlet state.
Effectiveness of singlet oxygen production is usually measured either by the phosphorescence of singlet oxygen at 1270nm or by use of singlet oxygen traps. Both these methods have limitations, especially in vivcr therefore we wish to extend the
applications of the MORE to direct electrochemical singlet oxygen detection. To this end, we have investigated the photo-electrochemical properties of a singlet oxygen precursor: rose bengal (RB). The dark electrochemistry of RB was investigated using the Electrochemical Quartz Crystal Microbalance and the Rotating Ring Disc Electrode.
We have identified all electrochemical processes associated with rose bengal, and attempted the detection of photoexcited rose bengal in both anaerobic conditions, as well as direct detection of singlet oxygen.