The Detection of Solution PHASE Superoxide As a Reaction Intermediate of Oxygen Reduction in Neutral Aqueous Solutions (2016)

Despite decades of research, certain aspects of the mechanism of dioxygen, O 2 , reduction on solid electrodes in aqueous electrolytes still remain poorly understood. 1 Particularly elusive has been the role of solution phase superoxide, a relatively short lived species, as a reaction intermediate. A method is herein described that allows for solution phase superoxide generated via the reduction of dioxygen in neutral aqueous solutions at a rotating disk electrode to be oxidized at a concentric Au ring electrode bearing a covalently linked monolayer of 3-mercapto-1-propanol, a modified surface that blocks the oxidation of solution phase of hydrogen peroxide 2 . Experimental All measurements were performed at room temperature in a conventional three electrode glass cell using either a gold rotating disk electrode (RDE, Pine Instruments, area: 0.164 cm 2 ), Au ring- glassy carbon (GC) disk RRDE (Pine Instruments: AFE7R9GGCAU) as working electrodes. A Au wire and a Ag/AgCl in 3.5M KCl contained within a Pt cracked seal capillary served as counter and reference electrodes, respectively. Experiments were carried out in 0.1M PB of pH 7.4 prepared, respectively, by mixing Na 2 HPO 4 (J.T. Baker Ultrapure Bioreagent 99.7%), and NaH 2 PO 4 (J.T. Baker Ultrapure Bioreagent 99.9%), using ultrapure water (UPW, 18.3 MΩ cm, EASYpure UV system, Barnstead). Shown as inserts in Fig. 1 are typical cyclic voltammetric curves recorded with the Au electrodes employed in this study in deaerated 0.1 PB solutions of pH 7.4 displaying characteristic features associated with the clean surfaces. Such Au electrodes were chemically modified by immersing the RDE or RRDE in a solution 10 mM 3-mercapto-1-propanol (MP, Sigma Aldrich, 95%) in ethyl alcohol (AAPER, 200 proof) for 20 h, as described by Gobi et al. 3 Shown in thick lines in Fig. 1 are dynamic polarization curves recorded with the Au RDE at a scan rate, n = 10 mV/s and a rotation rate, w = 900 rpm, in O 2 -saturated 0.1 M PB aqueous solutions, pH 7.4, before (blue) and after (black) adding 1mM H 2 O 2 . These results clearly indicates that the MP-modified Au electrode blocks the reduction of O 2 and both the oxidation and reduction of H 2 O 2 over the specified potential range. Shown in Fig 2 top panel is the dynamic polarization curves were then recorded at ν = 10 mV/s, for the glassy carbon (GC) disk electrode of the Au|GC RRDE in O 2 –saturated PB aqueous solutions of pH 7.4 and pH 10 at ω = 900 rpm. The results obtained in the experiments in which the Au ring was modified by a covalently linked MP monolayer also yielded an increase in i ring at the onset potential for O 2 reduction on the disk. i ring in this case, displayed a bell shaped curve with a maximum at E disk ca. -0.4 V In agreement with the data shown in Fig. 1, the increase in i ring for the MP-modified Au induced by adding H 2 O 2 to the buffered solution to a concentration of 1 mM amounted to ca. 3 nA for pH 7.4 at 0.4 V, i.e. less than an order of magnitude smaller than those found in lower panels, Fig. 2. It may concluded on this basis that the species being oxidized by the MP-modified Au ring is indeed with a very minor contribution due to solution phase peroxide, which appears to reach a maximum at E disk = -0.4 and thus consistent with its further reduction as E disk becomes from negative. ACKNOWLEDGMENT This work was supported by a grant from NSF, CHE-1412060 REFERENCES Adzic, R. In Electrocatalysis ; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: New York, 1998. Chen, X. J. J.; West, A. C.; Cropek, D. M.; Banta, S. Anal. Chem. 2008, 80 , 9622. Gobi, K. V.; Mizutani, F. J Electroanal Chem 2000, 484 , 172. Figure 1