Electrogenerated chemiluminescence at bare glassy carbon electrode in basic media
Introduction
Glassy carbon electrode (GCE) is one of the most used electrodes in electrochemical studies owning to its unique physical and chemical properties. It is well accepted that the bulk structure of GCE comprises thin, tangled ribbons of cross-linked graphite-like sheets that share sp2 bonding and the basic structure of a six-member ring [1], [2]. The unpolished, polished and electrochemically treated GCEs show different prosperities towards electron transfer and adsorption due to difference in microcrystallite size, microscopic roughness, surface cleanliness and surface functional groups [3], [4]. The polished surface possesses a number of impurities and a microcrystalline structure quite different from that of the bulk carbon [5]. However, after polishing, the exact structure of the GCE surface is not completely clear. In this process the cross-linked six-member rings are possibly broken up at the surface. Moieties connected to the skeleton of GCE were believed to contain alcohol, phenol, aldehyde, ketone (or quinine) and carboxylic acid (or anhydride) [6], [7], [8]. Various techniques, including scanning tunneling microscopy (STM) [2], X-ray photoelectron spectrum (XPS) [9], Raman [10], fourier transform infrared spectrum (FT-IR) [11], ellispsometric and X-ray diffraction (XRD), [12] etc. were used to characterize the surface properties of GCE, indicating that oxide coverage was highly increased within the thin-layer of the surface after electrochemical activation [13]. When a positive potential was applied, radicals such as , OH, O−, CHO, HCOO− might be produced in the vicinity of the electrode surface [14] and evolution of oxygen was observed as it was electrochemically oxidized at 1.8 V vs. Ag/AgCl in 0.1 mol/L NaOH [15].
GCE has also been used in electrogenerated chemiluminescence (ECL) studied for many years. It was either directly used as the working electrode [16], [17] or as the base electrode for modification [18], [19], [20] to investigate the ECL properties of other chemicals in the electrolyte. However, studies on the ECL behavior of GCE itself have rarely been reported [21]. Karatani et al. mentioned the phenomena of weak ECL signals produced at GCE in the detection of alcohols and carbohydrates in basic media [21], and left it as a background. In this work, three ECL peaks generated at bare GCE in basic media with KNO3 as supporting electrolyte were observed and the mechanisms were proposed.
Section snippets
Experimental
GCE (ϕ4 mm, Tianjin Lanli Technology Ltd. Co., Tianji, China) and platinum foil (35 mm2) were used as working electrodes, an Ag/AgCl (3 mol/L KCl) reference electrode and a Pt foil were used as reference and auxiliary electrodes. Cyclic voltammogram (CV) was recorded with a CHI 604B Electrochemical Analyzer (CH Instrument, Shanghai, China). ECL signals were recorded by an Ultra-weak Chemiluminescence Analyzer (BPCL-K, Institute of Biophysics, academia Silica, Beijing, China) controlled by a
ECL behavior at bare GCE in basic media
Typical ECL recorded at bare GCE in the basic media (0.1 mol/L NaOH + 0.1 mol/L KNO3) is shown in Fig. 1A (curve 1). Three peaks are observed seated at ca. 1.36 V, 1.72 V, 2.34 V during positive sweeping (denoted as ECL-I, ECL-II and ECL-III, respectively). In neutral solution (0.1 mol/L KNO3) at GCE (curve 2) and in basic media at Pt electrode (curve 3), ECL-I and ECL-II disappear and only ECL-III exists, implying that ECL-I and ECL-II account for the special surface structure of GCE itself in basic
Conclusions
The mechanisms of ECLs at GCE in basic media were investigated and three ECL peaks during the positive sweeping between 0 and 2.6 V were observed. Mechanisms for those ECLs were proposed. Both ECL-I and ECL-II were closely related to the oxidation process of surface structure (S–R–CH2OH) at bare GCE, and ECL-III to the process of single state oxygen generation and its further conversion. The whole process was found to be totally reversible. The findings would facilitate the understanding of ECLs
Acknowledgments
The authors would like to thank for the financial support from the National Science Foundation of China (Nos. 90407019, 20775015 and 20735002).
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