Carbon Microsphere – Polystyrene Composite Electrode for Three‐Phase Boundary Oil Analysis: Quinizarin in Methyllaurate

Abstract

Quinizarin (see molecular structure and redox processes in Scheme 1) is water-insoluble in the ambient pH range and has been shown to be electrochemically active. In technical application, quinizarin has been proposed as colorant and anti-corrosion coating for anodised aluminium [11].M olecular quinizarin adsorbed onto carbon has been shown to catalyse the oxygen reduction process [12,13].Q uinizarin has also been shown to act as aM ichael-acceptor when oxidised [14] and this has potential implications in electroanalytical detection of sulphur species.W hen examined as aD NA groove-binding redox reagent, quinizarin has been shown to act as af luorescence reporter in biological samples [15] and it has been investigated as am odel system for anti-cancer drug molecules [16].H owever, the main application of quinizarin is as dye-marker additive in hydrocarbon oils [17,18] for safety and tax reasons.Methyllaurate is am ajor component in bio-fuels [19] and employed here as a" model oil" with quinizarin as a" model redox marker". Then ewly developed electroanalytical method based on ac arbon microsphere composite electrode could be applied to aw ider range of oils containing redox active substances such as anti-oxidants. No electrolyte is added to the oil phase which is deposited directly into ap orous carbon electrode made from ac omposite with glassy carbon microspheres and polystyrene.F igure 1s hows as chematic drawing of the electrode with as mall amount (A) and with ah igher amount (B) of oil deposited. Thee lectrochemical reaction is believed to occur at the three-phase boundary.AFaradaic current is anticipated as long as the electro-analyte (quinizarin in oil) can diffuse towards the three-phase boundary,w here ions (here protons) present in the neighbouring phase can balance the build-up of charge resulting from the electron transfer. From an electroanalytical standpoint, not having to alter or prepare the sample before electrochemical analysis is beneficial and developing as imple "dip-probe" method could be very useful.In this study ac ost efficient and easy-to-make (potentially disposable) porous carbon electrode is fabricated to facilitate three-phase boundary electrochemistry.T he Abstract:O il analysis for the case of quinizarin in methyllaurate is demonstrated with ad isposable carbon microsphere -p olystyrene composite electrode.O xidation and reduction of quinizarin are observed as three-phase boundary processes at the immiscible oil j water j carbon interface.R andomly packed glassy carbon microspheres (2-12 mmd iameter) held together with ap olystyrene binder (ca. 0.1 %b yw eight) are immobilized onto ap encil lead electrode and used for electrochemical oil analysis.P orefilling with oil is optimised for voltammetric detection and fluorescence spectroscopy is employed to support the mechanistic analysis based on voltammetry.F uture applications for aw ider range of oils and additives are proposed.