Disk-Generation/Ring-Collection Scanning Electrochemical Microscopy: Theory and Application Peter Liljeroth, Christoffer Johans, Christopher J. Slevin, Bernadette M. Quinn,* and Kyo 1 sti Kontturi Laboratory of Physical Chemistry and Electrochemistry, Helsinki University of Technology, P.O. Box 6100, FIN-02015 HUT, Finland. The potential of ring-disk ultramicroelectrodes (RD UMEs) as probes for scanning electrochemical micros- copy (SECM) was investigated both theoretically and experimentally. In particular, the disk-generation/ ring- collection (DG/ RC) mode of operation was considered. In this case, the interaction of two species with the substrate under investigation can be followed simulta- neously from single tip current -distance measurement (approach curve) to the substrate. Theoretical approach curves for DG/ RC were calculated by numerical methods. Such approach curves to both insulating and conducting substrates indicate a strong tip response dependence on the ring radius while the response was relatively insensi- tive to ring thickness and overall tip radius. The RD tip was characterized by fitting experimental approach curves recorded at insulating and conducting substrates to simulated curves for a given tip geometry. DG/ RC SECM was then applied to investigate the partitioning of iodine across a liquid-liquid interface. Scanning electrochemical microscopy (SECM) 1-6 has been used to study charge-transfer phenomena at a wide variety of interfaces, such as polymer films on electrodes, 7 Langmuir monolayers at air-water interfaces, 8-10 phospholipid bilayers, 11,12 and liquid-liquid interfaces, 13-21 in addition to the traditional metal electrode-electrolyte solution interface. One very active area of recent development has been probing spontaneous reactions, such as partitioning at soft interfaces, 14,18,21 adsorption/ desorption processes, 22 and dissolution reactions. 23-26 This has been achieved either by equilibrium perturbation (EP) 5,15,22,25,26 or by double potential step chronoamperometry (DPSC-SECM) experiments wherein the species involved in the spontaneous reaction is first generated at the tip, and subsequently, the unreacted amount is probed by a reverse potential step. 5,14,21 Recently, Unwin et al. introduced a triple potential step technique to probe lateral diffusion in electroactive Langmuir monolayers. 27 We previously reported the use of a ring-disk (RD) micro- electrode as a scanning electrochemical microscopy probe. 28 This approach makes it possible to simultaneously monitor two species with a steady-state measurement, thus eliminating the problems associated with double layer charging and, additionally, simplifying the theoretical treatment. The common mode of operation is the disk-generation/ ring-collection (DG/ RC) in which a redox mediator reacts under diffusion-controlled conditions at the disk and is regenerated at the ring electrode. Conceptually, this mode of operation resembles the DPSC-SECM, wherein a precursor is electrolyzed at the tip during the forward potential step, and then, subsequently, the electrode-generated species is collected during a reverse potential step. As the DG/ RC mode of operation * Corresponding author. Tel: +358 9 451 2572. Fax.: +358 9 451 2580. E-mail: bquinn@ cc.hut.fi. (1) Bard, A. J.; Fan, F. R. F.; Kwak, J.; Lev, O. Anal. Chem. 1989 , 61, 132- 138. (2) Bard, A. J.; Fan, F. R. F.; Pierce, D. T.; Unwin, P. R.; Wipf, D. O.; Zhou, F. Science 1991 , 254, 68-74. (3) Bard, A. J.; Fan, F. R. F.; Mirkin, M. V. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1994; Vol. 18, pp 243-373. (4) Mirkin, M. V. Anal. Chem. 1996 , 68, 177A-182A. (5) Unwin, P. R. J. Chem. 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