Ultrasonic enhancement of electrochemiluminescence from arylacetate electrooxidation D.J. Walton*, S.S. Phull, D. Colton, P. Richards, A. Chyla, T. Javed, L. Clarke, J.P. Lorimer and T.J. Mason School of Natural and Environmental Sciences, Coventry University, Priory Street, Coventry CV1 5FB, UK This paper describes the previously unreported electrogenerated chemiluminescence (ECL) from the electro-oxidation of several aryl carboxylates at a platinum flag electrode in aqueous, methanolic and acetonitrile solutions using tetramethylammonium hydroxide as a supporting electrolyte. In most cases electrochemiluminescence was markedly enhanced by simultaneous irradiation with ultrasound during electrolysis. Increase in electrochemiluminescence intensity with electrolysis current and carboxylate concentration was observed. Addition of a radical scavenger or purging of the solution with oxygen lowers ECL emission intensity. In contrast the presence of oxygen enhances the intensity of sonoluminescence, which is a concurrent but weaker process occurring under ultrasonic irradiation alone. Ring- substituted phenylacetates almost always produced less electrochemiluminescence than the unsubstituted parent molecule, though within the subst~tuents studied chloro-substituted phenylacetates produced more ECL intensity than either methoxy- or nitro-substituted derivatives. Keywords: electrochemiluminescence; carboxylate electro-oxidation; ultrasonic enhancement glectrochemiluminescence (ECL), the emission of light from an electrolysis cell, is a relatively uncommon phenomenon because not only must there be sufficient energetics to achieve a suitable excited state, but also a reaction event of very short duration is required to prevent thermal relaxation by bond vibration or other mechanical reorganization of energy 1. Several electrochemiluminescent systems are well- known and these often have complex mechanisms. Thus luminol (5-amino-2,3-dihydro-1,4-phthalhydrazide) produces light from the excited state of the final product 3-aminophthalate. The mechanism in aqueous alkaline solution exposed to oxygen or containing hydrogen peroxide involves electrochemical co-oxidation of both the luminol anion and oxygen-derived species leading through a chain of intermediates to the emitting species 2. Despite these complicated mechanistic pathways there is considerable interest in ECL systems at present because of their potential analytical applications, in particular the analysis of bioactive systems, immunoassays, and the distribution, metabolism and pharmacokinetics of drugs. In these systems an ECL tagged bioagent undergoes a change in emission when it interacts with its target molecule 3,4. There are drawbacks in the development of ECL *To whom correspondence should be addressed systems, arising from the complex reaction characteristics, which cause 'edge effects' at planar or disc electrodes (in which nonuniform diffusion causes greater brightness near the edges), and patchiness across the electrode surface. These factors place restrictions upon cell configuration, and the small electrode areas required for reproducibility result in the emission of low light intensities which limit detection. Furthermore, many ECL systems, particularly those involving electro- oxidations, suffer from electrode fouling after prolonged usage. This is a problem which causes irreproducibility, lessened efficiency, shortened device lifetime and requires cleaning procedures or other methods of reactivation 4. We have previously examined the influence of ultrasound upon electro-organic synthetic reactions, in particular the electro-oxidation of carboxylate anions, and have noted the following effects; enhanced diffusion, increased yields and current efficiencies, altered reaction mechanism, shifts in product distribution, absence of electrode fouling and lessened cell voltage requirement 5- v. These are all of potential benefit to the enhancement of ECL; and we have also shown that simultaneous ultrasonic irradiation substantially enhances electro- chemiluminescence from the well-known ruthenium bipyridine system 8'9. We now disclose the influence of ultrasound on a previously unreported ECL system, namely the electro-oxidation of phenylacetate derivatives. 13504177/94/010023-04 (t 1994 Butterworth-Heinemann Ltd Ultrasonics Sonochemistry 1994 Vol 1 No 1 82.3