Research Article Liquid–liquid ion transport junctions based on paired gold electrodes in generator–collector mode Simultaneous electrochemically driven double anion transfer across liquid–liquid inter- faces is demonstrated at a gold–gold junction electrode. In the presence of two closely spaced electrodes (generator and collector), anion uptake into the organic phase (oxida- tion) and anion expulsion into the aqueous phase (reduction) can be combined to result in a generator–collector anion transport system across the liquid–liquid interface. In this report we are employing a paired gold junction grown by electro-deposition to ca. 5 mm gap size with the N,N-diethyl-N 0 ,N 0 -didodecyl-phenylene-diamine water immiscible redox liquid immobilized into the gap to demonstrate simultaneous perchlorate anion uptake and expulsion. The effects of redox liquid volume and scan rate on the magnitude of currents and two mechanistic pathways for ion transport are discussed in the context of micro-electrophoretic processes. Keywords: Anion transport / Generator-collector / Gold junction / Redox liquid / Sensor / Voltammetry DOI 10.1002/elps.200900083 1 Introduction Ion transfer at liquid–liquid interfaces is a well-known process studied with electrochemical methods for many decades [1] and with constantly emerging new applications [2, 3]. The electrochemically driven transfer of ions into non- ion conducting organic media at the triple phase boundary organic liquid–aqueous electrolyte–electrode is a more recent experimental approach [4, 5] based on micro-droplet deposits [6], macro-droplets [7], micro-fluidic two-phase systems [8], or punctured droplet electrodes [9]. A range of triple phase boundary systems based on water-immiscible redox liquids have been studied including redox liquids such as N,N,N 0 ,N 0 -tetrahexyl-phenylene-diamine [10], tetraaryl- phenylene-diamine derivatives [11], nitrobenzene and octa- nol-based redox systems [12], and 4-(3-phenylpropyl)-pyri- dine solutions of porphyrin metal complexes [13, 14]. A diverse range of triple phase boundary ion transfer processes have been reported including the transfer of drugs [15] and peptides [16], carboxylate [17] and a-hydroxy- carboxylate (facilitated by boronic acids) [18], as well as chromate [19] and phosphate [20]. The liquid–liquid transfer of biologically relevant ions [21] and ion transfer at bio- inspired membranes [22] are particularly important as a fundamental process for the development of novel ion separation and detection systems. Although employing a second sensor probe (e.g. in generator–collector mode [23]) usually adds new information and provides enhanced sensor responses, bipotentiostatically controlled ion transfer experiments at liquid–liquid interfaces have rarely been reported [24]. Generator–collector experi- ments in homogeneous phase for interfacial electron transfer without ion transfer have been carried out for example with micro-ring-disc [25], rotating [26], or sono-ring-disc electrode systems [27], using hydrodynamic double channel electrodes [28], SECM systems [29, 30], dual or array band [31], or interdigitated array electrodes [32]. Interfacial ion transfer processes may also be studied by generator–collector type experiments with appropriate electrode geometries to allow diffusion of ions between a generator and a collector electrode. Recently, we have developed a new automated electro- deposition procedure for the formation of paired gold elec- trodes with a well-defined gap size [33]. This work is based on the pioneering studies by Porter et al. [34, 35] and involves the simultaneous growth of the two gold hemispheres, which is stopped automatically when the desired junction gap has been reached [36]. Generator–collector experiments in homo- geneous solution have been demonstrated for the oxidation of nitric oxide [33] and the effect of the diffusion coefficient on the collection efficiency has been determined [36]. Here, a new type of experiment is proposed where a water immiscible liquid (redox liquid) is placed into the junction gap between the two gold electrodes (see Fig. 1A). The resulting triple phase boundary reaction zones are shown schematically in Fig. 1B. An oxidation process at the generator electrode will cause the formation of positive Robert W. French 1 Yohan Chan 2 Philip C. Bulman-Page 2 Frank Marken 1 1 Department of Chemistry, University of Bath, Claverton Down, Bath, UK 2 School of Chemical Sciences and Pharmacy, University of East Anglia, Norwich, Norfolk, UK Received February 10, 2009 Revised February 13, 2009 Accepted February 14, 2009 Abbreviation: DDPD, N,N-diethyl-N 0 ,N 0 -didodecyl-phenylene- diamine Correspondence: Dr. Frank Marken, Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK E-mail: F.Marken@bath.ac.uk Fax: 11225-386231 & 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.electrophoresis-journal.com Electrophoresis 2009, 30, 3361–3365 3361