Background Emission of Electrogenerated Chemiluminescence during Oxidation of Tri-<italic>n</italic>-propylamine from the Dimeric ¹_g State of O₂

The background electrogenerated chemiluminescence (ECL) emission observed only upon electrochemical oxidation of tri-n-propylamine (TPrAH) on a platinum electrode is a limiting factor in ECL analytical techniques and is poorly understood. We studied this reaction in aerated acetonitrile (MeCN) solution with TPrAH oxidized at a constant potential at the Pt surface and observed ECL spectra with an emission band at 630 nm, which is characteristic of the emission of the dimeric Δg state of O2. No ECL emission was observed when the same solution was deaerated. This background ECL emission is attributed to the reaction between dissolved oxygen and two different products of TPrAH oxidation: the TPrAH• radical that reduces O2 to the superoxide ion and the TPrAH •+ radical cation that oxidizes this species to singlet O2. T (TPrAH) is a widely used coreactant in electrogenerated chemiluminescence (ECL) sensors, e.g., for immunoassay. In this reaction electro-oxidation of TPrAH in an aqueous medium produces a highly reducing intermediate that reacts with the oxidized luminophore, Ru(bpy)3 , to produce intense ECL emission. The TPrAH reaction is complex with both oxidant and reductant intermediates produced. In nonaqueous solvents TPrAH also behaves as a coreactant to produce ECL signals with many luminophores. However, even in the absence of luminophores, like Ru(bpy)3 , electro-oxidation of TPrAH produces a weak ECL signal that, in fact, limits the sensitivity of ECL analytical methods. The nature of this reaction is not understood, e.g., what species in this system is the ultimate ECL emitter and how it is produced. ECL processes that involve oxygen have been studied previously. Our laboratory proposed a mechanism for producing the electronically excited state O2 by the ECL annihilation reaction between the one-electron reduction product of oxygen, superoxide (O2 •−), and ferricenium cation in nonaqueous solution. Moreover, Wang et al. reported that very weak luminescence was produced at a Pt electrode during evolution of oxygen in an aqueous medium, which they attributed to recombination between excited singlet oxygen molecules. Further, Reshetnyak and Kovalchuk observed ECL by the electroreduction of peroxydisulfate on a Pt electrode and proposed a scheme for singlet oxygen formation. In this work we show that the background TPrAH ECL emission involves formation of the dimeric Δg state of O2 and propose a mechanism for the reaction. ■ EXPERIMENTAL DETAILS Chemicals. Tri-n-propylamine (TPrAH), the electrochemical grade of acetonitrile (MeCN), and tetra-n-butylammonium hexafluorophosphate (TBAPF6) were used as received from Sigma-Aldrich. TPrAH was distilled under vacuum conditions and kept in an Ar atmosphere glovebox. Instrumentation and Procedures. A 2 mm diameter Pt disk inlaid in glass was used as the working electrode (WE, surface area = 0.024 cm) and was bent at a 90° angle (L-type electrode) so that the electrode surface faced the detector in the ECL experiments. Before each experiment the WE was polished with 0.3 μm alumina and then sonicated in ethanol and water for 5 min. Pt wire served as a counter electrode and a Ag wire as a quasi-reference electrode (QRE). All glassware and electrodes were dried at 120 °C for 2 h prior to transfer to the Ar atmosphere glovebox (Vacuum Atmospheres Corp., Hawthorne, CA). All solutions were prepared in an Ar atmosphere glovebox and placed in an airtight electrochemical cell with a Teflon cap for measurements completed outside of the box. Cyclic voltammetry (CV) and potential step experiments were performed with an Autolab electrochemical workstation (Eco Chemie, The Netherlands). The ECL along with CV signals were measured simultaneously with a photomultiplier tube (PMT, Hamamatsu R4220p). The PMT was held at −750 V with a high-voltage power supply Kepco (New York, NY). The photocurrent generated at the PMT was converted to a voltage Received: September 19, 2012 Accepted: November 27, 2012 Published: November 27, 2012 Article